: If one happened to have access to beam time, what would be the proper
: procedure for producing a Lictenberg figure?
: Also, how long should a piece of acrylic be placed in the beam? Is there
: a formula based on beam energy and properties of the acrylic?
The electron beam energy needs to be chosen so that the effective penetration depth is ideally about half the thickness of the specimen (or less). For example, for 1" thick PMMA, this is around 5-6 million electron volts (MeV). The duration depends on the type of accelerator (pulsed or DC), and the available beam current. Bulk PMMA can only hold about 1 microcoulomb/square cm before spontaneously breaking down, so plan your dosage accordingly. You'll ultimately need to use trial and error, since the dielectric strength and electrical resistivity of otherwise identical PMMA specimens can vary considerably between vendors and even from batch to batch from the same vendor. Especially "leaky" material may be difficult to charge with low current pulsed accelerators.
If all specimens are self-discharging as they're being irradiated, reduce the beam current or irradiation time. If you get no discharges or sparse trees when you manually discharge your specimens, increase beam current or irradiation time. If you intend to use beam energies above 10 MeV, you may induce temporary radioactivity in your PMMA specimens through neutron activation, so you may need to let them "cool down" a while (days - weeks). Very hard X-rays are created during irradiation, so make sure you use adequate shielding for the beam energy you're using, and adequate/real-time radiation monitoring. The X-rays can also cause imaging "speckles" and, at higher energies, disrupt the normal operation of nearby digital cameras or video equipment. Scattered electrons and X-rays can even cause semi-permanent darkening (solarization) of nearby camera lenses.
Be especially careful if you attempt to make larger specimens since charged specimens pack a relatively large amount of stored electrostatic energy. Treat charged specimens with respect - they are, effectively, charged high voltage capacitors. Always use a well-insulated hammer and discharge tools, avoid handling larger charged specimens, and NEVER hold any specimen while discharging it. Secondary discharges (after the main pulse) can "nip" you, but are usually otherwise harmless. The initial voltage on the inner charge plane of a 12" x 12" x 1" specimen can exceed 2.2 million volts and the stored energy can approach 1000 joules. This is a serious amount of energy - about 3X the maximum energy setting on a medical defibrillator. You definitely don't want to accidentally become part of the discharge path, since the peak discharge current can be in the range of 2000-5000 amperes.
For a variety of technical, safety, and warranty reasons, do not use an in-service medical LINAC to make Lichtenberg figures. Major LINAC vendors will void the warranty of abused machines, and they will also terminate you (if you are one of their employees) if you abuse one of their machine by trying to make Lichtenberg Figures. Some folks have made figures using LINAC's that were being decommissioned, but this involves major modifications and bypassing of many safety features and interlocks - extremely dangerous activities. Always play safely - there are a multitude of radiation and shock hazards associated with making Lichtenberg figures.