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What are Lichtenberg figures, and how do we make them?
(Last updated 4/20/14)
What are Lichtenberg figures?
How do we make our Captured Lightning® sculptures?
Video clip of a huge 15 x 20 x 2 inch sculpture being discharged
Lichtenberg figures are fractals
Other interesting effects: fluorescence, solarization, birefringence, and discharge-free zone
"Iced 'bergs" and negative Lichtenberg figures. Do we get curved figures in a magnetic field?
Discharge speed and current measurements... and a paradox
Natural Lichtenberg figures - fulgurites, natural tattoos, and fractal lightning,
Are there practical uses for Lichtenberg figures?
Captured Lightning Sculptures are "fossilized lightning bolts"
Can I make my own Lichtenberg Figures?
How do I get a Captured Lightning Sculpture of my very own? 
More fun with electrons: Glowing rocks, flashing crystals, going to the dark side, and the "Rad-Cam"
References and Further reading
Other Questions? See our Captured Lightning FAQ


DoubleShot

Doubly-Irradiated "Windblown Lightning" Sculpture

This Captured Lightning® sculpture was created by injecting a block of acrylic with trillions of high-speed electrons from a 5 million volt particle accelerator. Electrons were first injected from the left side, the specimen was rotated 180 degrees, and additional electrons were injected through the opposite side. This created two independent layers of electrical charge inside the specimen, each located about one-half inch below the surface. The charge layer on the right side was then manually discharged, creating a miniature flash of "lightning" within the nearest charge layer. Additional electrical discharges then grew from the right layer towards the left layer, forming a complex, beautifully interconnected 3D discharge structure. The entire discharge event occurred in less than 100 billionths of a second! The sculpture is illuminated from below by blue light emitting diodes (LED's). Each of our Captured Lightning sculptures contains an incredibly detailed, fractal discharge pattern. Unlike laser art, every one of our sculptures is a one-of-a-kind treasure. As they branch, the discharge channels become increasingly finer and hair-like, ultimately disappearing at the tips. The smallest discharges are thought to extend down to the molecular level. See our
Frequently Asked Questions (FAQ) or our one-page explanation for a quick overview of how these beautiful objects are created, or learn all the details by reading this web page.
(Sculpture size: 3 x 3 x 2 inches or 7.6 x 7.6 x 5 cm)


What are Lichtenberg figures?
"Lichtenberg figures" are branching, tree-like patterns that are created by the passage of high voltage electrical discharges along the surface, or through, electrical insulating materials (dielectrics). The first Lichtenberg figures were actually 2-dimensional "dust figures" that formed when dust in the air settled on the surface of electrically-charged plates of resin in the laboratory of their discoverer, German physicist  Georg Christoph Lichtenberg (1742-1799). Professor Lichtenberg made this observation in 1777, demonstrating the phenomenon to his physics students and peers. He reported his findings in his memoir (in Latin): De Nova Methodo Naturam Ac Motum Fluidi Electrici Investigandi (Göttinger Novi Commentarii, Göttingen, 1777). The translated title of Lichtenberg's paper is, "Concerning the New Method Of Investigating the Nature and Movement of Electric Fluid". The physical principles involved in forming Lichtenberg figures eventually evolved to become the modern-day science of plasma physics.

Professor Lichtenberg used various high-voltage electrostatic devices to electrically charge the surfaces of various insulating materials including resin, glass, and ebonite (hard rubber). He then sprinkled a mixture of finely powdered sulfur (yellow) and minium ("red lead" - lead tetroxide) onto the charged surface. He discovered that powdered sulfur (which became negatively-charged through friction with its container) was more strongly attracted to the positively-charged regions. Similarly, frictionally-charged red lead powder was found to be attracted to negatively-charged regions. The colored powders made previously hidden regions of stranded surface charge, and their polarity, clearly visible. Charged regions on the insulator surface were deposited by small discharges of static electricity. It is now known that, once deposited, electrical charges will remain stranded on the surface of electrical insulators for quite some time since the insulating material prevents them from easily moving and dissipating. Lichtenberg also discovered that the shapes of positively and negatively charged figures were significantly different. Positive figures tended to be star-like with long, multiply-branched paths, while negative figures tend to be shorter, rounded, and fan or shell-shaped. By carefully pressing a piece of paper onto the dusted surface, Lichtenberg was able to transfer these images onto the paper, demonstrating what eventually became the modern processes of xerography and laser printing. Drawings of some positive and negative figures created by Lichtenberg are shown below.

Positive Lichtenberg Figure
Negative Lichtenberg Figure
Positive Lichtenberg figure Negative Lichtenberg figure

The following demonstration video replicates Lichtenberg's experiments using a mixture of powdered red lead and sulfur to highlight positive (yellow) and negative (red) Lichtenberg figures. Although a more modern Whimshurst electrostatic generator is used as the high voltage source instead of an electrophorus, as originally used by Lichtenberg, the principles are the same. In the video, branching positive Lichtenberg figures are created first, followed by shell-shaped negative Lichtenberg figures.


A number of other physicists and experimenters investigated Lichtenberg's figures over the next two hundred years. Notable 19th and 20th century researchers included physicists Gaston Planté and Peter T. Riess (mid-1850's), French artist and scientist Etienne Leopold Trouvelot, Thomas Burton Kinraide (late 1800's), and professors Carl Edward Magnusson, Maximilien Toepler, P. O. Pedersen, and Arthur Von Hippel (1920's-30's). Modern researchers used photographic film to directly capture the faint light emitted by the electrical discharges. A wealthy English industrialist and electricity researcher, Lord William G. Armstrong, published two beautiful full-color books showing some of the results of his high voltage and Lichtenberg Figure research. Although these books have become quite scarce, an online copy of Armstrong's first book, "Electric Movement in Air and Water, with Theoretical Inferences", was recently made available through the kind efforts of Jeff Behary at the Turn of the Century Electrotherapy Museum. In the mid-1920's, Von Hippel discovered that Lichtenberg figures were actually created through complex interactions between ionized gas (corona discharges and small electrical sparks called streamers) and the underlying dielectric surface. The discharges deposited matching patterns of electrical charge onto the dielectric surface below, where they became temporarily stranded. Von Hippel also discovered that increasing the applied voltage, or reducing the surrounding gas pressure, caused the length of the figures to increase.

Riess discovered that the overall diameter of a positive Lichtenberg figure was about 2.8 times the diameter of an equal-voltage negative figure. The relationships between the length of Lichtenberg figures versus voltage and polarity were utilized in early measuring and recording instruments, such as the klydonograph, to measure the peak voltage and polarity of high voltage impulses. Klydonographs photographically recorded the size and shape of Lichtenberg figures that were generated by abnormal electrical surges on electrical power lines created by nearby or direct lightning strikes. These measurements allowed lightning researchers and power system designers in the 1930's and 1940's to accurately measure lightning-induced voltages, providing critical information about the electrical characteristics of lightning strikes. This information allowed power engineers to create "man-made lightning" with similar characteristics under controlled laboratory conditions in order to test the effectiveness of various lightning-protection approaches. Lightning protection is now a critical part of the design of all modern electrical transmission and distribution systems.  A schematic diagram of the main parts of a klydonograph is shown on the leftmost drawing below, along with examples of "klydonograms" from equally large positive and negative high voltage transients. Notice how the positive Lichtenberg figure is considerably longer than the negative figure even though the peak voltage is of the same magnitude.

Schematic of a klydonograph Klydonograms
Schematic view of a klydonograph showing the position of the
photographic film and high voltage electrode. Light from the high
voltage discharges creates a photographic record of the event.

From W.W. Lewis, "The Protection of Transmission Systems
Against Lightning", John Wiley & Sons, 1950


Lichtenberg figures are now known to occur during electrical breakdown processes within gases, insulating liquids, and solid dielectrics. Lichtenberg figures may be created within billionths of a second (nanoseconds) when dielectrics are subjected to very high electrical stress, or they may develop over a period of years through a progressive series of small, low-energy, partial discharges. Countless partial discharges on the surface or interior of solid dielectrics often create growing, partially-conductive 2D surface Lichtenberg figures and internal 3D "electrical trees". 2D electrical trees are often found along the surfaces of contaminated power line insulators. 3D trees can also form, hidden from view, inside dielectrics due to the presence of small internal defects or voids, or at points where an insulator has been physically damaged. Since these partially-conductive trees will eventually cause complete electrical failure of the insulator, preventing their formation and growth is critical to the long-term reliability of high-voltage equipment. The study of electrical trees and tree formation has been critical to the reliable design of the high-voltage power transmission systems that transfer electrical power to our homes and businesses.

Large 3D Lichtenberg figures were first created inside transparent plastic by physicists Arno Brasch and Fritz Lange in the late 1940's. Using their newly-invented electron accelerator, they injected trillions of free electrons into plastic specimens, causing internal electrical breakdown and the formation of carbonized internal Lichtenberg figures. Electrons are tiny, negatively charged particles that orbit the positively-charged nucleus of the atoms that make up all condensed matter. They used high voltage pulses from a multimillion volt Marx Generator to drive a pulsed electron beam accelerator. An article about their research and their accelerator (which they called a "Capacitron") originally appeared in the March 10, 1947 issue of LIFE Magazine. The Capacitron could deliver a three-million volt pulse, and could generate a powerful beam of free electrons with a peak current of up to 100,000 amperes! The glowing region of ionized air created by the exiting high-current beam of electrons resembled a bluish-violet rocket engine flame. A complete set of previously unpublished B&W pictures, including Lichtenberg figures inside a clear block of plastic, has recently become available online, as has another article with color pictures from the April, 1951 issue of Popular Mechanics. In 1944, Brasch founded the Electronized Chemicals Corporation (ECC), a pioneering researcher of cross-linking monomers and polymers to improve their electrical and physical properties. ECC was eventually purchased by the 3M Company in 1985. 

The first formal scientific study of charge movement and charge trapping within dielectrics was conducted by a Brazilian physicist, Dr. Bernhard Gross, in the early 1950's. Dr. Gross confirmed that internal Lichtenberg figures could be created within a number of different polymers and glasses by injecting them with high-energy electrons. The techniques that we use to make our sculptures are built upon the theoretical work and experimental techniques originally developed by Brasch, Lange, and Gross. 3D acrylic Lichtenberg figures are sometimes called "electron trees" or "beam trees" - we call our state of the art creations Captured Lightning® sculptures.

How do we make our Captured Lightning® sculptures?
Over many years, we have refined our irradiation and fabrication techniques to create a wide variety of beautiful 2D and 3D sculptures. We start with carefully cut and polished specimens made from a clear, glass-like polymer called polymethyl methacrylate (or PMMA). This material, commonly known as "acrylic", is sold under various trade names such as Lucite, Plexiglas, or Perspex (UK). PMMA has a unique combination of high optical clarity and superior electrical and mechanical properties. Besides being an excellent electrical insulator, it's actually clearer than glass! A number of other clear polymers, such as polycarbonate (PC), polystyrene (PS) , polyethylene terephthalate (PET), and polyvinyl chloride (PVC) can also make Lichtenberg figures with varying degrees of success. However, most of these other materials tend to form dark gray or black discharge patterns instead of the attractive, mirror-like figures seen within PMMA. Lichtenberg figures can also be created within glass. However, since glass Lichtenberg figures often explosively shatter upon discharge or, unpredictably, days or weeks later, we no longer make them. 

We inject electrons into our specimens using a 5 million volt, 150 kW particle accelerator called a Dynamitron. The heart of this device is the accelerator tube - a huge three-story high "vacuum tube" that operates at voltages between one and five million volts. At the top of the tube, electrons are emitted by a small, white-hot tungsten filament. The filament is connected to the negative output terminal of a multimillion volt power supply, while the bottom of the tube is connected to ground and the positive terminal of the high voltage supply. This configuration creates a very strong electrical field that accelerates electrons emitted from the filament to a very high velocity as they "fall" though the large potential difference towards ground. The bottom of the vacuum tube has very thin (only 2.3 thousandths of an inch thick!) titanium window that separates the high vacuum on the inside from air, at atmospheric pressure, on the outside. The high velocity electrons pass right through the titanium window, almost as though it wasn't even there! The electrons then emerge from the outside surface of the window, and then travel through another 24 inches of air before crashing into our acrylic specimens on the movable carts below. Although the typical lifetime of free electrons in air is only 11 billionths of a second, that's more than enough time for them to work their magic.

The energy of electrons leaving the accelerator is measured in millions of electron volts (or MeV). Most of our sculptures were created using electrons with energies between 2 and 5 MeV. At these energies, electrons are traveling at relativistic velocities - between 98.5% and 99.6% of the speed of light. During irradiation, these energetic electrons are driven deep inside the acrylic before they come to a stop. The penetration depth is a function of the energy of the electron beam, the target material's dielectric properties, and its atomic density. The higher the energy of the electron beam, the deeper the electrons will penetrate. For example, electrons with an energy of five MeV will penetrate about one-half inch through acrylic, but a sheet of lead only 1/16" thick will completely stop them.

When a thick piece of acrylic is irradiated, huge numbers of electrons accumulate inside the specimen, creating a strongly-charged cloud-like layer called a space charge. Because acrylic is an excellent electrical insulator, injected electrons become temporarily trapped inside the acrylic. By passing specimens through the electron beam in two or more passes, changing specimen orientation between passes or rotating them while they're being irradiated, complex 3-dimensional space charge regions can be created. As the number of electrons accumulate during irradiation, the electrical stress (called the electrical field or "E-field") inside the acrylic dramatically rises, reaching several million volts per centimeter. We normally charge our specimens to just below the point where they'll break down. We then force the charged specimen to release ("discharge") the electrons at the desired location by poking it with a heavily-insulated, pointed metal tool. This creates a small fracture that concentrates the electrical stress at that point. The electrical field at the tip of the fracture overcomes the dielectric strength of the acrylic, initiating electrical breakdown of the acrylic. During breakdown, chemical bonds that hold the acrylic molecules together rapidly break, stripping away free electrons in a process called ionization. These newly-freed electrons become accelerated by the extreme electric field, colliding with, and ionizing additional acrylic molecules. In an instant, portions of the acrylic abruptly become electrically conductive in a runaway process called avalanche breakdown.

Within billionths of a second, a network of branching, white-hot, plasma channels form within the acrylic and, with a bright flash and a loud BANG, the material suddenly undergoes dielectric breakdown. The previously-trapped electrical charges rush out in a river-like torrent as thousands of smaller tributaries dump their share of stored charge into larger channels that eventually merge into a single, brilliant discharge path that exits the acrylic. Although images and videos may appear as though we're injecting high voltage into each specimen, we're actually removing excess charges that were previously trapped inside. Dielectric breakdown occurs with incredible speed - the main electrical discharge within a 4 inch square specimen lasts less than 120 billionths of a second (120 nanoseconds)! Some physicists think that dielectric breakdown within a charge-injected solid may be the most energetic (explosive) chemical reaction known, vastly exceeding that of high explosives.

The following image shows a 12 x 12 x 1 inch specimen being discharged. In the image, a neutral density filter reduced the brilliance of the discharge so that the individual paths can be seen. Note the bright high-current discharge that exits from the discharge point jumping along the top surface of the specimen to the grounded metal table below:

12 x 12 x 1" Square Discharge
(Photo courtesy of Terry Blake)


As the miniature lightning bolts blast their way through the acrylic, they create millions of microscopic tubes and fractures, leaving behind a permanent "lightning fossil" deep inside the acrylic. The peak current within the electrical discharge reaches several hundred, to several thousand, amperes depending upon the size of the specimen. The temperature of the hot plasma in the confined discharge channels may exceed 10,000 degrees, and the combination of heat and high pressure causes nearby acrylic to vaporize and fracture. Higher-current "roots" may even char the surrounding acrylic. The exit point of the discharge creates a small crater on the surface. Surprisingly, although we inject a huge amount of negative charge into our specimens, the electrical discharges originate from points which are more electrically positive (versus the space charge layer), so all of our Captured Lightning® sculptures are actually "positive" Lichtenberg figures!

Some specimens self-discharge while they are being irradiated. This is usually caused by a small surface scratch, residual manufacturing stress, or an internal defect in the acrylic. A self-discharged specimen will continue to discharge numerous times as long as its being irradiated as the electron beam continues to inject new charge into the specimen. However, unlike the neatly branched structures seen in manually-triggered sculptures, self-triggered sculptures develop a thicker, mat-like tangle of chaotic discharges, or a complex combination of dendritic and chaotic patterns. Because of their complexity, self-discharged specimens are often among some of our most fascinating sculptures. 

Video clip of a huge 15 x 20 x 2 inch sculpture being discharged:
Following is a short video clip showing a huge 15" x 20" x 2" specimen being discharged. The specimen was first charged on one surface by a 5 MeV electron beam. The fully-charged specimen was then (very carefully!) flipped over and irradiated once again on the opposite side. This created two independent charge planes, each located about 1/2" below the large surfaces. Prior to discharging, the estimated potential of these internal charge planes was about 2.6 million volts. Because of the two charge planes and large size, this specimen stored significantly more electrostatic energy than most of our other specimens - more than four kilojoules! Safety precautions were necessary to prevent the possibility of receiving a dangerous electrical shock.

Although the main discharge is quite brief (under 500 billionths of a second for this specimen), the video successfully captured the brilliance of the 4 kilojoule electrical discharge in a single video frame (below). Numerous secondary discharges continued to flash after the main discharge. These continued sporadically for over 30 minutes. This video is courtesy of Bill Hathaway, GCL Laboratories. The resulting sculpture, cradled within a custom walnut light base and illuminated by an array of white and blue LED's, is also shown below.



Main Discharge

Large 15" x 20" x 2" specimen
(Click on above image for high-resolution image)

The resulting Lichtenberg Figure is a series of branching hollow tubes surrounded by conchoidal (shell-shaped) fractures. Conchoidal fractures are characteristic of the way glassy (amorphous) materials fracture when stressed beyond their breaking point. Since the countless fractures behave as tiny mirrors, illuminating a figure through one or more edges causes the entire Lichtenberg figure to glow brilliantly with the reflected colors of the external light source.


Lichtenberg figures are fractals
The branching pattern of a Lichtenberg figure looks similar at various scales of magnification. This "self-similarity" strongly suggests that Lichtenberg figures can be mathematically described through a recently-developed branch of mathematics called Fractal Geometry. Unlike most common geometric forms, fractal objects do not have integral 2D or 3D dimensions, but instead have dimensions that lie between 1 and 2 (for 2D fractals) or between 2 and 3 (for volumetric fractals). It is thought that Lichtenberg figures may be one of the first fractal-like forms created by man. Our branching 2D Lichtenberg figures have a fractal dimension of about 1.5 (for thin, sparse discharges) to 1.9 for our densest discharges. Most of our 2D sculptures have figures with a fractal dimension of about 1.7, and our 3D figures have a fractal dimension of about 2.5. The appearance of Lichtenberg figures depends upon how much charge was injected into the acrylic and when the specimens are discharged. The technical terms for these branching figures are dendritic or ramified (tree-like). If a larger amount of electrical charge is injected into a specimen, very dense dendritic discharges can be created such as in Figure 1 below. These very dense discharges are similar in appearance to fern fronds ("filiciform") or dendritic plume agates. Specimens exhibiting this form were charged to just below the point of self-breakdown and then immediately discharged. If the density of the injected charge is reduced, more classical, lightning-like or tree-like discharges are created (Figure 2). If premature breakdown occurs as we are actively irradiating a specimen, the resulting discharges form a thicker, densely tangled mat of "chaotic" discharges  (Figure 3). In these specimens, after the initial discharge, newly-injected electrons from the accelerator recharge smaller nearby regions, causing them to repetitively discharge in random directions. The rapidly changing internal electrical fields create a much thicker mat of densely chaotic discharges that are reminiscent of interconnected nerve cells and neural networks. Some of the most complex and fascinating patterns occur when a specimen self-discharges about halfway through the charging process, creating dramatic discharges that evolve from densely dendritic to densely chaotic from one end of a sculpture to the other.

Moss Agate Discharges
(Click for larger image)

Figure 1. Dense (filiciform or fern-like) Discharges
Maximum charge density
Fractal dimension ~1.8-1.9
.

Dendritic
(Click for larger image)

Figure 2. Dendritic Discharges
Moderate charge density
Fractal Dimension ~1.5 - 1.8

Chaotic
(Click for larger image)

Figure 3. Chaotic Discharges
Prematurely discharged
while being irradiated)
Unknown fractal dimension

The self-similarity of dendritic discharges can easily be seen in the following sequence of zooms from a 12" x 12" Lichtenberg Figure. Although the branches become finer and hairlike, the overall branching structure remains similar until the finest tips ultimately disappear at the very edges of the discharge structure.

Zooms

Similar fractal patterns are prevalent in nature. They are seen in aerial views of rivers and their tributaries, and organic structures such as branching tree limbs, your body's circulatory system, and within various organs such as lungs, kidneys, and the liver. The satellite view in the left image below shows the fractal pattern of a river drainage system near Grand Junction, Colorado, USA. The right image is a casting of the bronchial tree of a human lung (courtesy Paul Cazeaux, PhD student at Laboratoire Jacques-Louis Lions (LJLL), Paris, France). The similar branching structure of all of these systems may be a consequence of a recently proposed new law of physics, the Constructal Law, which states that Nature tends to develop a hierarchical branching network of paths that result in most efficient flow. The flowing material can be water, air... or even electrons!

Cast of Lung Tissue
View Larger Map

Lichtenberg figures can be mathematically modeled using an iterative growth process called "Diffusion Limited Aggregation" (DLA). A more accurate model, that combines an electric field with DLA, is called the Dielectric Breakdown Model (DBM). The DBM appears to accurately describe the forms of Lichtenberg figures that occur under various electrical field intensities on the surface or within solid, liquid, and gaseous insulators.
Other interesting properties: fluorescence, solarization, birefringence, and discharge-free zone
When acrylic is irradiated by high-energy electrons, it glows brilliantly with a blue-white color. Radiation chemistry studies suggest that this is mainly due to luminescence that peaks at a wavelength of about 435 nm. However, acrylic also generates fainter glows from X-ray fluorescence, and Cherenkov radiation as high velocity electrons interact with acrylic molecules. The detailed light-producing mechanisms for electron-irradiated acrylic are not yet fully understood.

Newly-irradiated specimens develop an amber-colored layer in the region between the surface(s) that were irradiated by the electron beam and the discharge layer. This phenomenon, called solarization, appears to be caused by various interactions between high speed electrons and the acrylic's molecular structure. During irradiation, electrons in the beam are initially traveling at over 99% of the speed of light. As they penetrate the specimen, they collide with acrylic molecules, rapidly coming to a stop within a fraction of an inch. Electrons in the beam have lot of kinetic energy, and as they collide with the atoms in the acrylic they release this energy as heat, x-rays, and gamma rays. In acrylic, most solarization seems to occur in the regions directly hit by the electrons. However, it has recently been found that regions that are intentionally covered by sheet lead (to prevent electrons from hitting some areas of the acrylic) may also exhibit solarization in deeper regions of the acrylic. As electrons crash into the lead mask, they radiate very intense x-rays and gamma rays that often create darker solarization in the acrylic underneath the mask. Energetic collisions with electrons, x-rays, and excess electrons injected into the acrylic's molecular structure stimulate chemical and physical reactions that alter the physical and optical properties of the acrylic. Deeply-trapped electrons may remain stranded within the acrylic for years, creating color centers which also contribute to solarization. While some of these changes last for only minutes or hours, others persist for months or years after irradiation. And, some changes appear to be permanent. Although all of the specific causes of solarization are not completely understood, there is evidence that irradiation creates longer-lived unstable ("metastable") compounds that preferentially absorb light at the blue end of the spectrum (250 - 400 nm). Since part of the blue portion of ambient light is absorbed by the solarized regions, irradiated specimens appear green, brownish, or amber colored when illuminated by white light.

Most acrylic specimens turn a beautiful lime-green color immediately after irradiation. Once discharged, the color changes to brownish-amber, then more slowly fading to a light amber color. The amber region typically fades away over several months to several years. Fading can often be accelerated by gently heating the block in the presence of atmospheric oxygen or by leaving the specimen in bright sunlight for an extended period of time. As oxygen diffuses into the acrylic from the outside surfaces and the porous discharge layer, it slowly bleaches the solarized region, causing the solarized layer in between to gradually become thinner until it eventually disappears. Most Lichtenberg figures older than 2-3 years are completely bleached. Although older specimens may no longer show any solarization, many exhibit various degrees of "fogging" from electron collisions and X-radiation damage to the acrylic's molecular structure. Some PMMA specimens exhibit comparatively little initial solarization, while a small percentage of specimens permanently retain their amber color. Permanently-colored specimens appear to be solarized via a different, deeper penetrating mechanism, such as X-radiation, since these specimens also tend to be uniformly solarized throughout their entire thickness. These differences may be due to subtle variations in the acrylic blends and the specific catalytic agents used by various suppliers to polymerize the acrylic.

It has also been discovered that the solarization layer is often fluorescent. An amateur scientist from Australia, Daniel Rutter, discovered that monochromatic light from a green laser pointer apparently changes color when passed through the solarized layer of a Lichtenberg figure. More recently, we have discovered that the light from a near-ultraviolet source, such as a 405 nm Blu-ray laser or bright blue LED's, also causes the solarized region to fluoresce with a yellow-green color. Both effects appear to be due to one (or more) fluorescent components within the solarization layer. As the solarization fades over time, so does the fluorescence.

Most specimens also exhibit slight changes in the refractive index in the regions near the discharge layer. This may be due to residual mechanical stresses near the discharge fractures. Residual stresses near the Lichtenberg figures can sometimes be seen as multicolored regions near the discharge plane when a sculpture is illuminated by polarized light and then viewed through a second polarizing filter (a configuration called crossed polarizers). When physically stressed, acrylic exhibits a property called birefringence. When viewed through crossed polarizers, stress-induced birefringence causes changes in color that are directly related to the amount and distribution of otherwise hidden stresses. An electrically charged specimen clearly shows internal compressive forces created by the high internal electrical field. These forces are then relieved when the specimen is discharged. Following are images of the same specimen prior to charging, fully charged, and then after discharging. Little internal stress is seen in the initially uncharged specimen. The specimen was then charged by injecting electrons from the left side. The injected charge forms an intensely negative layer of charge near the center of the specimen. At the same time, positive ions are strongly attracted to the external surfaces of the specimen. partially neutralizing the overall external charge outside the specimen. Attraction between the internal negative layer and the positively-charged outer surfaces create significant compressive stresses within the acrylic. For the specimens below, the calculated compressive force created by the internal electrical field was in the range of 400 pounds per square inch (PSI). The compression can easily be seen as colored regions on either side of the center in the middle image. After the specimen is discharged, the electrical stresses are greatly relieved as can be seen in the rightmost image. There are still residual mechanical stresses near the discharge zone due to fracturing. Click on any of the individual images to see full-size images.

Uncharged Specimen Charged Specimen Discharged Specimen
Initially uncharged specimen Fully charged specimen
(electrons were injected from left side)
Discharged specimen

Finally, you may notice that all of our sculptures have a discharge-free boundary along the outside edges. Because acrylic is not a perfect insulator, some of the internal charge leaks away along all the edges. The rate that excess charges leaks away is highest where the internal electrical field is greatest - i.e., in the region between the sharper edge of the internal space charge region and the perimeter of each specimen. In these regions, the amount of stored charge is reduced to the point where it can no longer break down the acrylic, creating a discharge-free perimeter in every specimen.

"Iced 'bergs" and negative Lichtenberg figures. Do we get curved figures in a magnetic field?
From studies done by other researchers, we knew that acrylic specimens could retain their injected charge for weeks, or even months, when chilled, charged, and then maintained at dry ice (-109F/-78.5C) temperatures. One of our team members, Todd Johnson, has christened these as "Iced 'bergs". At room temperature, injected charge leaks away exponentially over a few minutes to a few hours for commercial acrylic. Chilling acrylic significantly reduces the speed that free charges can move inside the acrylic, and this dramatically increases the time that trapped charges can be stored. At dry ice temperatures, trapped charges can apparently be stored indefinitely. We have confirmed virtually full charge retention over several weeks, and other researchers have demonstrated charge storage for up to six months. When later discharged, these specimens behave in a fashion similar to freshly-charged specimens. The lime-green color is also retained in chilled specimens until they are discharged. This suggests that the green color may be related to the higher density of electrons that remain trapped before discharging. Once discharged, chilled specimens rapidly lose their green color, changing to an amber color.

Chilled specimens also develop a heavy layer of frost when exposed to humid air. When we discharge a specimen, we produce a "positive" Lichtenberg figure inside the acrylic. Photographic evidence confirms that the exiting spark "wraps around" and covers the exterior surfaces of the specimen, discharging a layer of positive charges that have attached themselves to the specimen's outer surfaces. The external surface discharge produces a "negative" Lichtenberg figure along the large surfaces of the specimen. However, the negative surface discharges are considerably fainter than the brilliant internal discharges, so they are usually very difficult to see or photograph. We accidentally discovered that, when a charged specimen is coated with frost, the negative discharges along the acrylic surface blast away the frost layer immediately above the discharges, making the paths taken by the negative discharges clearly visible. The following "iced 'berg" was discharged by Todd Johnson and Dr. Timothy Koeth during our 2010 production run.
As can be seen, the resulting negative Lichtenberg figures blasted through the frost show considerably less branching than positive internal figures... just as professor Lichtenberg observed over 200 years ago.

Negative Lichtenberg Discharges captured in Frost

We also wondered if an externally-applied magnetic field might cause discharge paths inside the acrylic to become curved. It was known that Lichtenberg figures created within gases along dielectric surfaces become curved due to Lorentz force acting on the moving charged particles within the electrical discharges. The stronger the magnetic field, the greater the curvature:

Magnetic field effect on Lichtenberg Figures

Since we could chill charged specimens to dry ice temperatures and keep them charged indefinitely, it became possible to perform tests on charged specimens in a more controlled laboratory environment. Following our Fall, 2007 production run, Dr. Timothy Koeth placed a chilled and charged specimen within the poles of a 1.5T (15,000 Gauss) cyclotron electromagnet in his lab and then discharged it while it was within the magnetic field. The blue-white flash of the electrical discharge can be easily seen along the edge of the specimen in the photo below:

Discharge in 1.5T Field

It was found that the paths of the resulting Lichtenberg figure showed no evidence of any curvature - the paths looked completely identical to control specimens discharged with no applied field. It's possible that the 1.5T magnetic field was simply not strong enough to show any effect. Or perhaps the net velocity of the electrons within the discharges inside acrylic is considerably slower than within gases, and the resulting Lorentz force is lower and the degree of curving is much less. Perhaps a future experiment, using a stronger electromagnet, may allow us to create magnetically-curved paths.


Discharge speed and current measurements... and a paradox
During our 2007 and 2009 production runs, we measured and recorded discharge current waveforms for a number of 4" x 4" x 3/4" specimens. We designed a special holding fixture with copper foil plates that made physical contact with the large surfaces of a charged acrylic specimen. A heavily-insulated wire connected the pair of foil plates to a pointed discharge tool. The wire was also passed through the center of a Pearson Model 411 wideband Current Transformer (CT) so that, when the specimen was discharged, the main current pulse flowed through the wire and could be measured via the CT. The CT transformed the discharge current pulse into a voltage pulse, 0.1 volt per amp for this CT, that was captured and stored in a Tektronix TDS3034B 300 MHz digital storage oscilloscope (DSO). An image and schematic of the experimental configuration can be seen below:

Experimental Setup

Current Measurement Setup
Specimens were previously charged by injecting a cumulative charge of ~2.7 microCoulombs/cm2 via an electron beam with a nominal energy of 4.0 MeV.  Charged specimens were then inserted into the test fixture and manually discharged. The discharge current waveforms from one of the specimens is shown below. We found that this main discharge event occurred in less than 120 billionths of a second (120 ns), the peak current reached almost 600 amperes in 45 ns, and the waveform contained four discrete current peaks. Discharges from five subsequent specimens showed similar discharge intervals with three and seven discrete current peaks. Overall, the peak discharge currents declined as the time between irradiation and discharge increased. This decline was expected because the injected charge slowly leaks away, reducing the amount of remaining energy and the peak discharge current. Subsequent peak current measurements on the other specimens ranged from 526 to 404 amperes.

The occurrence of multiple current peaks suggests that the electrical trees may progress via a series of larger breakdown events. Each current peak reflects a surge of newly conducting channels ("streamers" and "leaders") as newer channels blast their way into previously untapped reservoirs of charge within the acrylic, pause briefly, then surge again, etc. The average discharge velocity was between 8.5 x 105 and 1.3 x 106 meters/second (526 and 790 miles/second, or around 0.3% the speed of light). However, pauses between successive current surges suggest that the peak discharge velocity during the propagation phases are significantly faster. Surprisingly, the average streamer velocity within the specimen was found to be 10-100 times greater than the velocity of positive lightning leaders in air! This is thought to be due to the extreme electrical field (estimated at over 20 million volts/cm) at the tips of the propagating discharges.

Lichtenberg Discharge

The high streamer velocities within PMMA create a paradox, since they are over 800 times the speed of sound within PMMA. This is completely inconsistent with Griffith's theory of crack propagation within solids, which predicts that the maximum speed that cracks can propagate within a solid is limited to the speed of sound within the material (about 1.6 x 103 meters/second for PMMA). The current waveform clearly demonstrated that the breakdown process (the complete formation of chains of cracks and gas channels across the specimen) for our Lichtenberg figures propagated at speeds that were almost 1000 times FASTER than the maximum predicted by classical fracture theory! A series of independent electro-optical measurements were taken by Dr. Timothy Koeth in his laboratory at the University of Maryland. Dr. Koeth measured the time delay between optical (light) emissions at the beginning and ends of propagating discharges within 6" x 6" x 1" specimens. His optical measurements also confirmed streamer velocities ranging between 7.4 x 105 and 1.55 x 106 m/s.

Some insights into this paradox may come from a Russian researcher, Yu N. Vershinin. Dr. Vershinin explored how electrostatic energy is stored and released within solid dielectrics during electrical breakdown. Specifically, he studied how energy is stored within acrylic when electrical charge is slowly injected into the material ("charge trapping"), and the effects of rapidly releasing these trapped charges ("charge detrapping") during electrical breakdown processes. Vershinin proposed that, when a dielectric contains significant trapped space charge, the stored electrostatic potential energy may be rapidly liberated, contributing to explosive formation and growth of crack tips. As chemical bonds in the surrounding material are ruptured, some of the material breaks down into its constituents, liberating high pressure gases that rapidly expanding the channels behind the propagating crack tips, forcing the crack forward. Vershinin speculated (and experimentally confirmed) that for very high internal electrical fields (E-fields), the potential energy initially stored within the E-field was rapidly converted into kinetic and thermal energy that drove crack propagation at hypersonic velocities. When mechanically shocked, a dielectric that is highly stressed by an electrical field can break down, releasing energy previously stored within the electrical field, and causing larger molecules to break into into smaller, gaseous byproducts. Breakdown occurs along rapidly propagating reaction fronts (streamers), accompanied by shock waves.

Vershinin speculated that dielectric breakdown might be closely related to the process of detonation. He found that "electronic detonation" did indeed occur within solid dielectrics, but only for propagating positive discharges within highly divergent E-fields - the same conditions we create when making our acrylic Lichtenberg figures! An American researcher, Dr. Paul Budenstein, independently developed a theory of dielectric breakdown in solids that seems to explain many of these observations at a more fundamental level. Based upon the rate of channel expansion, Budenstein concludes that dielectric breakdown may be the fastest known chemical reaction in nature. 
During the breakdown process, disassociating dielectric molecules create a network of electrically-conductive gaseous channels that initially have nearly the same density as the surrounding solid material. Budenstein estimates that the initial temperature of the gases inside these highly-compressed channels may reach 100,000K before they supersonically expand to create a network of hollow tubules and fractures that eventually form the resulting Lichtenberg figure. The breakdown process along each channel appears to progress in a series of discontinuous steps: gas pressure creates and expands a crack into the virgin material ahead, stops temporarily, then repeats as gas expands in the newly-created crack.

Some evidence for the above theories of breakdown and discharge propagation can be seen within Captured Lightning Lichtenberg figures. Under a microscope, the discharge channels that make up the figures are found to be hollow tubes, surrounded by countless small fractures that scatter light. Some paths clearly exhibit periodic structures along the discharge channel, similar to beads along a string, and higher current paths may also exhibit charring of the surrounding material. These beaded structures are observed during dielectric breakdown of various polymers as well as crystalline ionic salts. The theories predict that the extreme electrical field ruptures the chemical bonds within the acrylic. The resulting electronic breakdown processes liberate gases as some of the insulating material is decomposed into its atomic constituents. Dr. Koeth has confirmed that a significant volume of gas exits from the discharge point when an acrylic specimen is discharged under water. Other researchers have determined that these evolved gases are composed primarily of hydrogen, carbon monoxide, carbon dioxide, and methane. "Beading" appears to be a repetitive sequence of electronic decomposition, evolution of gases under high pressure, and formation of new cracks ahead of the expanding gas zone. Following is an example of a beaded channel captured by Dr. Bill Hathaway (GCL Laboratories).

Beaded discharge

We have experimentally confirmed that "electronic detonation" appears to be 10 to 100 times faster than the detonation velocity of the fastest known chemical explosive.  Because of the large amount of stored electrostatic energy (several kilojoules for larger charged specimens), and the extremely short discharge intervals (less than a microsecond), the instantaneous power liberated when creating a Captured Lightning sculpture can exceed a gigawatt (109 watts)! Not surprisingly, the energetic discharge creates a loud BANG(!), and the brilliant, lightning-like spark channels wreak considerable havoc inside the acrylic as they blast countless fractures and tubes along the space charge layer(s). The abrupt release of previously-trapped space charge (called charge detrapping) is now known to play a major role in the degradation and breakdown of all solid dielectrics when they are subjected to long-term high voltage stresses, sudden voltage changes, or abrupt polarity reversals. In many respects, charge detrapping within a solid dielectric is similar to a high-voltage capacitor discharge that occurs entirely within the insulating material. 

After we discharge a specimen, hundreds of smaller secondary electrical discharges continue to flash throughout the specimen as small pockets of residual charge continue to redistribute themselves. Immediately after the main discharge, large sculptures sparkle and sizzle, making a sound similar to frying bacon, and intermittent sparking has been seen over 30 minutes after the main discharge. Harmless secondary discharges often sting our fingers when we handle recently-discharged specimens. Click on the following image to see some high resolution video captured during one of our production runs that shows primary and secondary discharges.

12" x 12" discjarge
(Photo and video courtesy of Mike Walker and Theodore Gray)
Click on the Above image to see a video clip
of many Lichtenberg figures being discharged

Natural Lichtenberg figures - fulgurites, natural tattoos, and fractal lightning
Occasionally, nature creates also "fossilized lightning", called fulgurites (from the Latin word "fulgur", or lightning). These are hollow, glass-lined tubes that are formed when the powerful electrical current from a lightning strike creates underground discharge channels within poorly-conducting sandy or sandy-clay soils. The intense heat from the arc-like channels melts the surrounding sand and soil particles, forming hollow glassy tubes in the soil. Larger fulgurites often exhibit fractal characteristics as they split into smaller diameter root-like branches at further distances from the site of the main strike.

Fulgurite

Lichtenberg figures, sometimes called "lightning flowers" or "skin feathering", often form beneath the skin of unfortunate humans who have been struck by lightning.
The victim often has one or more reddish radiating feathery patterns that branch outward from the entry and exit points of the strike. Here's an example of an electrical tattoo from a lucky lightning survivor:

Human Lichtenberg Figure
OUCH! A temporary lightning tattoo on a "lucky" survivor
From "Lichtenberg Figures Due to a Lightning Strike" by Yves Domart, MD, and Emmanuel Garet, MD,
New England Journal of Medicine, Volume 343:1536, November 23, 2000

Medical terms for this phenomenon include arborescent lightning burn, arborescent (tree-like) erythema, keraunographic markings, or ferning patterns. Although the exact causes are subject to some debate, they appear to be the result of physical damage to capillaries under the skin, perhaps caused by the flow of electrical current, or by shock wave bruising from external flashovers just above the skin. These reddish marks fade away over a period of hours or days. They are recognized by forensic pathologists as clear evidence that a victim has been struck by lightning. The patient above survived with no permanent injuries, and the lightning flowers completely faded within a few days. A small Lichtenberg figure has also been observed at the entry point where a high voltage spark penetrated the skin of an unfortunate (but surviving) local electrical experimenter who was accidentally zapped by a homemade 60,000 volt
Marx Generator. No... it wasn't me!

A similar phenomenon is sometimes seen when lightning hits a grassy field, as in this picture where lightning struck a golf course flagpole, leaving this beautiful 25 foot Lichtenberg figure on the green:

Golfcourse
(From "Lightning and Lichtenberg Figures" by Cherington, Olson and Yarnell, Injury, Volume 34, Issue 5, May 2003)

Note the similarity between the figure above and the Lichtenberg figure below (illuminated from below by blue LED's):

Disk Figure


High voltage discharges to the surface of water can also create Lichtenberg figures. Some beautiful examples of both positive and negative Lichtenberg figures on water surfaces can be seen on Dr. Colin Pounder's Lichtenberg figures web site. Natural lightning often creates transient "Lichtenberg figures" in the sky. Air is an excellent dielectric and, although the physical breakdown mechanisms for air and Plexiglas are considerably different, the appearance of the branching discharges is quite similar. So it should not be surprising that the branching forms of propagating lightning leaders also have fractal characteristics. This similarity can clearly be observed during "anvil crawler" and horizontal "spider lightning". Spider lightning follows a thin, positively charged cloud layer that sometimes forms in dissipating storms. These slowly propagating discharges can crawl across the sky for 30 - 70 miles - literally spanning from horizon to horizon! On a much smaller scale, transient Lichtenberg figures (sometimes mistakenly called St. Elmo's Fire) sometimes appear on the outer surface of cockpit windows of airplanes as they fly through thunderstorms.

Similar branching fractal patterns also occur when thunderstorms generate electrically conductive leaders that propagate downward from a charged cloud to the ground below. When one of these leaders connects with an unfortunate object on the ground, a high current pulse (called the return stroke) surges back upward through the completed path, resulting in a Cloud-to-Ground (CG) lightning strike. The peak current is typically tens of thousands of amps, and large positive bolts may reach several hundred thousand amps. Exceptional examples of downward propagating positive leaders have been captured by South Dakota lightning researcher, Tom Warner. Using high speed video imaging equipment, he was able to capture the downward progression of leaders and the return strokes from a positive lightning bolt. Positive lightning is a significantly rarer, and considerably more dangerous, form of lightning than negative lightning. Tom's "slow motion" videos show the air breaking down, forming glowing conductive plasma paths (called leaders) that fan downward from a huge reservoir of excess charge within the cloud above. The brightly glowing tips of the positive leaders smoothly propagate, unlike negative leaders which propagate in a series of discrete jumps (called stepped leaders). The first descending leader to finally connect with the Earth below completes the circuit, resulting in a powerful Cloud-to-Ground (CG) lightning discharge. See Tom's web site to see his spectacular gallery of images and videos of positive and negative lightning.

Under special conditions lightning also creates transient upward-growing tree-like Lichtenberg figures. This phenomenon often occurs when broadcast antennas or mountain tops generate positive leaders that propagate upward into heavily-charged negative regions above. As the ground-based positive leaders propagate into the negatively-charged regions, they form densely-branched positive Lichtenberg figures that, except for their massive scale, look quite similar to the positive Lichtenberg figures inside our Captured Lightning sculptures. This fascinating phenomenon has been captured in another slow-motion video by Tom Warner - click on the following image to see the YouTube clip.
Upward Tower Lightning

Lichtenberg figures can also be seen at some high energy pulsed power facilities, especially where deionized water is used as a dielectric to briefly store large amounts of electrical energy. The following photo is from Sandia National Laboratory's mighty Z Machine, the world's most powerful electrical pulse generator. After the completion of a high energy experiment, the water breaks down from the huge electrical stress, becoming an electrical conductor that safely dissipates unwanted residual energy from the system. The filamentary breakdown paths form Lichtenberg figures that dance across the water's surface. If you look closely, you'll notice that many of the paths actually trace out high voltage electrical field lines along the surface of the water. Although impressive, this display is only dissipating "left over" energy after the experiment is over. The discharges below represent ~5% of the initial energy that was used during the previous pulsed-power experiment.

Z Machine Lichtenberg Figures
(Click for a higher resolution image)

Are there practical uses for Lichtenberg figures?
Analysis of the form and origination points of Lichtenberg figures can be a powerful tool for diagnosing, and subsequently preventing, high voltage breakdown of solid dielectrics. By examining these figures in high voltage equipment, experts can diagnose and prevent future electrical faults within a variety of devices including high voltage power transformers, capacitors, and insulators. Historically, Lichtenberg figures (created by HV measuring equipment such as Klydonographs) were a powerful tool for measuring the polarity and magnitude of high voltage surges on power lines caused by lightning strikes. These early measurements were critical for the development of reliable electrical power transmission and distribution systems. Lichtenberg figures are still used as a forensic clue for identifying the cause of injury or death of human and animal lightning victims. Recent studies of Lichtenberg figures and charge detrapping in polymers are revealing important details on the mechanisms that are involved in the degradation and electrical breakdown of solid insulating materials.

There may be future medical applications as well. In 2009, a team of researchers at Texas A&M University proposed using 3D Lichtenberg figures created within various polymeric materials as "templates" for growing blood vessels (vascular tissue). There are significant similarities between branching Lichtenberg figures and animal circulatory systems - a fact not lost on many medical researchers. The hope is that, by creating branching 3D Lichtenberg figures inside a biodegradable polymer, such as polylactic acid (PLA), scientists can then use these as "molds" to support the development and growth of vascular tissue. Vascularization is essential for growing functional replacement tissues and organs. It's quite possible that the 18th century technology of Lichtenberg figures may ultimately play a critical role in organ growth and replacement therapy during the 21st century!

Captured Lightning sculptures are "fossilized lightning bolts"
Captured Lightning is an accurate description for our sculptures, and holding a Captured Lightning sculpture is about the closest you can come to holding a fossilized lightning bolt. Each Lichtenberg figure is unique - a one-of-a kind treasure, sculpted in exquisite detail by the same electrical forces unleashed by natural lightning. Captured Lightning sculptures are completely safe - they are completely discharged and are not radioactive.

Two-dimensional photos cannot begin to capture the beauty and exquisite detail of our 3D Captured Lightning sculptures. Following are a pair of 3D images that can be rotated 360 degrees so that you can more fully appreciate the detail within some of our doubly-irradiated sculptures. Once the images have been completely downloaded, you can drag your mouse over the image to rotate each for a full 360 degree view. [Note: large size - a high speed Internet connection is recommended].

3D Rotatable Image
Quicktime 3-D Movie
3D Rotatable Image
3-D Rotatable Image
"Heavy Weather"
(Courtesy of Theodore Gray)
"Windblown Lightning"
(Courtesy of Theodore Gray)

Very few people have actually seen or held one of these rare objects of scientific art. Far fewer have had the opportunity to own sculptures as beautiful and spectacular as these. Stoneridge Engineering is proud to be the world's most experienced provider of these rare treasures.

Can I make my own Lichtenberg Figures?
Unfortunately, since electrons must be injected deep into the acrylic, it takes a multimillion-volt electron accelerator to make 3D Captured Lightning sculptures. Even my patient, understanding spouse won't let me install one of these at home. However, 2D Lichtenberg figures can be made on the surfaces of some materials, such as carbonized Lichtenberg figures on wood or cardboard, or as dust figures on the surfaces of some plastics. Some artists have used this technique to make 2D works of art. To make carbonized figures, a high voltage (HV) power source, such as an ignition coil or neon sign transformer (NST), is required. The experiment should be done outside since it generates a significant amount of smoke and some small flames. Two nails or pins are driven into the wood with a gap of 4 - 10 inches. The surface is then lightly sprayed with a saltwater solution to make it partially conductive, and the high voltage source is connected across the two nails. When high voltage is applied, carbonized paths begin to form near the nails. Accompanied by lots of smoke, they begin branching as they grow towards each other. The heat from the process dries out the nearby surface, causing the branches to continuously change direction, often heading away from the opposite nail. The carbonized paths eventually grow to form Lichtenberg figures with "roots" at each nail.  This technique must be done VERY carefully, since it involves using dangerously high voltages and water together. A method to adjust the voltage (such as a variable autotransformer) helps to control the discharge process and will improve the shape of the resulting figure. The following video clip shows this technique using a 9,000 volt 30 mA NST as the high voltage source:



How can I get a Captured Lightning sculpture of my very own?
We offer a wide selection of Captured Lightning sculptures that range in size from affordable 2 inch specimens through museum-quality two inch thick blocks as large as 15 inches by 20 inches. Please visit Gallery 1 or Gallery 2 to select a sculpture at the right price for you. We also offer a wide variety of lighted bases with white, blue, and multi-color color changing options. Many of these are also available with UK, Australian, or EC power options. Our light bases illuminate the delicate patterns within, causing the discharge channels to glow so that even the finest hair-like details become visible. We also offer a variety of attractively-priced factory 2nd sculptures discounted to 50% off regular price.  Be sure to visit our Eye Candy page to see some of the best artistic and experimental work by Stoneridge Engineering and some of our very creative friends.

Style 19
Magnabrite
Style 24
Spiral Galaxy
Gallery 1 Gallery 2 Factory 2nds "Eye Candy"

More fun with electrons: Glowing rocks, flashing crystals, going to the dark side, and the "Rad-Cam"
High-energy electrons, x-rays, and gamma rays cause many fascinating effects within crystalline solids. One particularly interesting phenomenon is called thermoluminescence (TL).  In thermoluminescent materials, high energy electromagnetic radiation (such as x-rays) may temporarily drive some atoms within the crystalline structure into higher, semi-stable energy states. When these excited atoms revert back to their normal state, they radiate electromagnetic energy, often within the visible light spectrum. Thermoluminescent materials are usually triggered into releasing their stored energy by applying heat. Sensitive light detectors, such as photomultipliers, are used to detect the faint light emitted by most specimens. The light output versus temperature is called a "glow curve" and its shape tells much about the nature of the material and its cumulative radiation history. The emitted radiation is often in the infrared (IR) or ultraviolet (UV) portion of the electromagnetic spectrum, and thus not directly visible. Many thermoluminescent materials also require the application of relatively high temperatures to release their thermoluminescence. Some minerals radiate visible light at room temperature. An outstanding example is the mineral Calcite (CaCO3), which often glows brilliantly after being irradiated by a powerful electron beam or X-rays. However, the TL glow is not from the calcite itself, but from traces of impurity elements such as manganese. Manganese is one of many known  activators, and is responsible for the characteristic yellow-orange glow seen in many calcite specimens. Other common TL activators include lead, copper, cobalt, magnesium, iron, nickel, and silver. Very clear calcite (called "Iceland Spar) typically shows relatively little thermoluminescence since the purer crystals contain fewer impurities (i.e., fewer activator atoms). 

The degree of luminescence is proportional to the amount of cumulative radiation seen by the specimen. Passing a manganese-activated calcite crystal through a high-energy electron beam several times will cause the specimen to glow brightly for several hours at room temperature. Although the glow curve for electron-irradiated calcite peaks at about 110 degrees Celsius, significant light is emitted at room temperature. The amount of light rapidly decreases as the temperature of the calcite is lowered, and virtually disappears below 0 degrees Celsius. So, if we irradiate a frozen specimen of calcite, its thermoluminescent properties will not be immediately obvious. However, if we keep it cold and then warm it up to room temperature some time later, it will then glow brightly. The following image shows Dr. Timothy Koeth admiring a spectacular glowing calcite crystal brought by Dr. David Speck during our 2010 Lichtenberg run.

Thermoluminescent Calcite

Common table salt (NaCl) is also thermoluminescent. However, unlike calcite, it doesn't glow at room temperature. When irradiated with high-energy electrons, NaCl changes to a cinnamon color due to the trapping of electrons in defects, called F-Centers. These are vacancies inside the crystalline lattice that makes up salt crystals. Irradiated salt will remain this color as long as it is kept cool, dry, and protected from UV light. When irradiated salt crystals are dropped onto a hot surface (250 C or above), each emits a brilliant green flash as it changes back to its normal (clear/white) color. When dropped into distilled water, the cinnamon color also disappears, and the dissolving salt emits a pale bluish-green glow (called aquoluminescence) as previously-trapped electrons liberate their energy. After exposure to light, irradiated salt changes from cinnamon to a dark blue or dark purple color. This is thought to be caused by trapped F-center electrons combining with Na+ ions, reducing them to atoms of metallic sodium. The resulting colloidal dispersion of atomic sodium throughout the crystal causes the color change. When dissolved in distilled water, the purple color also disappears. We may offer small amounts of irradiated salt ("Flashing Crystals") to interested amateur experimenters and physics instructors in the future.

Another interesting salt is potassium chloride (KCl). This material is normally a white powder. However, when subjected to high-energy electrons or X-radiation, it changes to a dark purple color. Applying heat or UV (sunlight) turns it back to its normal color. This property is actually a relatively rare phenomenon in nature. KCl is a "scotophor". Unlike a phosphor, which emits light when excited by ionizing radiation, a scotophor darkens when irradiated. This process is called reversible photochromism or tenebrescence. Another example are photochromic eyeglass lenses that automatically darken from UV in sunlight, and then bleach back to their normal transparency in lower light. KCl was used in some early radar cathode ray tube (CRT) displays since it could be quickly "written" by an electron beam, creating an image with very long persistence. The recorded image could then be erased by applying a bit of heat to the material. Only a few minerals exhibit reversible photochromism - these include hackmanite, scapolite and tugtupite. The following image shows KCl and NaCl that have changed from being white powders (before irradiation) to dark purple and cinnamon-colored crystals after irradiation by multiple passes through a 5 MeV electron beam.

KCl and NaCl after irradiation


During an experimental run in June 2013, Andrew Seltzman, a guest and graduate-level physics student from the University of Wisconsin, built a protective "cave" for his older Sony Cyber-Shot DSC-P72 3.2 MP camera using sheet lead topped with a one inch thick piece of high-density polyethylene (HDPE) to reduce production of gamma rays. Andrew mounted his camera inside the cave and positioned it so that it could record a "cart's-eye view" of various objects being irradiated with the 3 to 5 million volt electron beam. We were thinking that the first pass through the beam might be a one-way trip, since the scattered electron and radiation levels were so intense. We were amazed to discover that the "Rad-Cam" not only survived, it faithfully recorded its journey through the beam - not just once, but for a dozen trips! The only obvious problem was that the microphone seemed to become less sensitive after every pass. However, the camera seems to have fully recovered a few days after the run.

The following video clip shows boxes of various minerals, and a large calcite crystal becoming energized by ionizing radiation from the beam. The crystal brilliantly fluoresces with a yellow-orange color while being irradiated, then softly glows from orange thermoluminescence afterwards. The acrylic sphere in front of the calcite fluoresces blue-white as the beam hits it, then briefly flashes as it self-discharges a short time later. Speckles in the image and static in the audio are caused by high-energy electrons, x-rays, and gamma rays impacting the camera's image sensor and audio electronics. The loud buzzing sound is from high-energy x-rays directly interfering with the camera's electronics as the beam scans across the width of the cart at 100 cycles per second. The camera's video and audio systems finally become overwhelmed as the camera passes directly under the electron beam. Although the radiation and ozone levels in the room would be lethal to living things, this tough little camera just shook off all the abuse and came back for more!



“A physical experiment which makes a bang is always worth more than a quiet one. Therefore a man cannot strongly enough ask of Heaven: if it wants to let him discover something, may it be something that makes a bang. It will resound into eternity.”
– Lichtenberg, Georg Christoph, and Albert Leitzmann. 1906. "Georg Christoph Lichtenbergs Aphorismen: nach den Handschriften.
Drittes Heft, 1775-1779," page 326. O
riginally in German, from Lichtenberg's Booklet F, Aphorism 1138, Oct 11-13, 1778.
 (Thanks to Fermilab scientist and author, Bill Higgins for researching the source of the above quote)


"Thunder is good; thunder is impressive. But it is the lightning that does the work."
- Mark Twain

References and Further Reading:
1. Gross, Bernard, "Irradiation Effects in Plexiglas", Journal of Polymer Science, Volume 27, 1958, Issue 115, Pages 135 - 143
2. Hashishes. Yuzo, "Two Hundred Years of Lichtenberg Figures", Journal of Electrostatics, Volume 6, Issue 1 , February 1979, Pages 1-13
3. Chadwick, K. H., "The Effect of Light Exposure on the Optical Density of Irradiated Clear Polymethylmethacrylate", 1972 Phys. Med. Biol. 17, Pages 88-93
4. Chadwick, K. H., and Leenhouts, H. P., "Fading of radiation-induced optical density in polymethylmethacrylate on oxygen diffusion", Phys. Med. Biol. 15 No 4 (October, 1970), Pages 743-744
5. L. Niemeyer, L. Pietronero*, and H. J. Wiesmann, "Fractal Dimension of Dielectric Breakdown", Phys. Rev. Lett. 52, Pages 1033–1036 (1984)
6. Gardner, Donald G., et. al., "Radiation-induced changes in the index of refraction, density, and dielectric constant of poly(methylmethacrylate)", Journal of Applied Polymer Science, Volume 11, Issue 7,July 1967, Pages 1065-1068
7.Akishin, A.A.; Tseplyaev, L.I., "Edge effect in radiation-charge dielectric materials", Physics and Chemistry of Materials Treatment, v 31, n 1, Jan.-Feb. 1997, p 30-1. A similar paper is also contained within the book "Effects of Space Conditions on Materials", Akishin, A. I., Nova Science Publishers, 2001, ISBN 590330285
8. Fothergill, J.C.; Dissado, L.A.; Sweeney, P.J.J., "A discharge-avalanche theory for the propagation of electrical trees. A physical basis for their voltage dependence", Dielectrics and Electrical Insulation, IEEE Transactions on, Volume 1, Issue 3 , June 1994, Pages 474 - 486
9. R. A. Galloway, T. F. Lisanti and M. R. Cleland, "A new 5 MeV – 300 kW Dynamitron for radiation processing", Radiation Physics and Chemistry, Volume 71, Issues 1-2, September-October 2004, Pages 551-553
10. Sessler, G.M.. "Charge distribution and transport in polymers", IEEE Transactions on Dielectrics and Electrical Insulation, Volume 4, Issue 5, Oct. 1997, Pages 614 - 628
11. Karczmarczuk, Jerzy, "Dendrites in Nature and in Computer", Foton 84/SPECIAL ISSUE, Spring 2006
12. C. M. Foust, General Electric Review: Instruments for Lightning Measurements (Includes Klydonograph and Lichtenberg Figures), Volume 34, #4, April, 1931, Pages 235 - 246
13. Watson, Alan and Dow, Julian, "Emission Processes Accompanying Megavolt Electron Irradiation of Dielectrics", Journal of Applied Physics, December 1968, Volume 39, Issue 13, pages 5935-5940
14. Fujimori, S., "Fractal properties of breakdowns", Properties and Applications of Dielectric Materials, 1988. Proceedings, Second International Conference on Properties and Applications of, 12-16 Sept. 1988, Volume 2, Pages 519 - 522
15. Domart, Yves, M. D., Garet, Emmanuel, M.D., "Lichtenberg Figures Due to a Lightning Strike", New England Journal of Medicine, Volume 343:1536, November 23, 2000, Number 21, Images in Clinical Medicine
16. H. Hiraoka, "Radiation Chemistry of Poly(methacrylates)", Radiation Chemistry, March 1977, pages 121-130
17. Brown, R. G., "Time and Temperature Dependence of Irradiation Effects in Solid Dielectrics", Journal of Applied Physics, September 1967, Volume 38, Issue 10, pages 3904 - 3907
18. Yu. S. Deev, M. S. Kruglyi, V. K. Lyapidevskii and V. I. Serenkov,"Mechanism underlying the formation of dendritic or tree-like channels in a dielectric irradiated with charged particles", Atomic Energy, Volume 29, Number 4, October, 1970, pages 1037-1040
19. Ebert, Ute and Arrayas, Manuel, "Pattern Formation in Electric Discharges", p. 270 - 282 "Coherent Structures in Complex Systems", eds.: D. Reguera et al., "Lecture Notes in Physics" 567 (Springer, Berlin 2001)
20. Yu.N. Vershinin, S.V. Barakhvostov, "Electron Processes in the Pulse Breakdown of Solid Dielectrics", 3rd International Conference on “Technical and Physical Problems in Power Engineering”, (TPE-2006), May 29-31, 2006 - Gazi University, Ankara, Turkey (covers detonation theory of high field breakdown in solid dielectrics)
21. Vershinin, Yu. N., "Parameters of Electronic Detonation in Solid Dielectrics", Technical Physics, Vol. 47, No. 12, 2002, pages 1524–1528. Translated from Zhurnal TekhnicheskoÏ Fiziki, Vol. 72, No. 12, 2002, pp.39–43, ISSN: 10637842
22. Budenstein, P.P., "Dielectric Breakdown in Solids", Technical Report, US Army Missile Command, October 16, 1975, DTIC accession #ADA018550
23. Budenstein, P. P., "Toward Developing a Dynamic Theory of Electric Breakdown in Solids", Conduction and Breakdown in Solid Dielectrics, Proceedings of the 3rd International Conference on, Publication Date: 3-6 July 1989, pp 522-527.
24. C. M. Cooke, E. R. Williams and K. A. Wright, "Space Charge Stimulated Growth of Electrical Trees", Proc. Intl Conf on Properties and Applications of Dielectric Materials, Xian, China, 1985, Pages 1-6
25. C. M. Cooke, "Space-Charge-Induced Breakdown in Dielectrics", Contract: Grant AFOSR-84-0107 for the Air Force Office of Scientific Research, June 1, 1984 to September 30, 1985, by MIT Laboratory for Electromagnetic and Electronic Systems High Voltage Research, DTIC Accession #ADA176969
26. N. I. Kuskova, "Spark Discharges in Condensed Media", Technical Physics, Volume 46, Number 2, 2001, pages 182-185
27. Theodore Gray, "Theo Gray's Mad Science: Experiments You Can Do At Home - But Probably Shouldn't", Black Dog & Leventhal Publishers, 2009, ISBN 978-1579127916
28. Jen-Huang Huang, Jeongyun Kim, et al, "Rapid Fabrication of Bio-inspired 3D Microfluidic Vascular Networks", Journal of Advanced Materials, Vol 21,Issue 35, pages 1-5, DOI: 10.1002/adma.200900584
29. Bejan, Adrian and Zane, J. Peder, "Design in Nature: How the Constructal Law Governs Evolution in Biology, Physics, Technology, and Social Organization", Doubleday Publishing, 2012, ISBN-13: 978-0385534611
30. R. Güler Yildirim, V. Emir Kafadar, et al, "The Analysis of Thermoluminescent Glow Peaks of Natural Calcite after Beta Irradiation",  Radiation Protection Dosimetry (2012) doi: 10.1093/rpd/ncs020.
31. Claudio Furetta, "Handbook of Thermoluminescence", World Scientific, 2003, ISBN 9812382402
32. Health Physics Society Science Support Committee, "Irradiated Salt Demo"
33. Don Lancaster, "Thermoluminescence: Theory and Applications"
34. Dr. George R. Rossman, "Colors from ionizing radiation", California Institute of Technology, Mineral Spectroscopy Web Site
35. P. Uhlig, J. C. Maan, P. Wyder, "Spatial Evolution of Filamentary Surface Discharges in High Magnetic Fields", Physical Review Letters, Volume 63, Number 18, October 30, 1968

Got other questions? See our Captured Lightning FAQ
One-page summary explanation (377 kB PDF file)
One-page summary in German (translated by Harry Meier - Thanks, Harry!)
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