Random numbers are often needed for gaming, cryptography, Monte Carlo simulations, and statistical modeling. For non-critical applications, it may be sufficient to use a pseudo-random number algorithm, such as built into Microsoft Excel—which produces a series of numbers that are not truly random because they are predictable and they eventually repeat.

Another approach is to download random numbers from a website, such as www.fourmilab.ch/hotbits/, or to use a table of random numbers. The quality of these published random numbers may be uncertain and they, too, may eventually repeat, or be discoverable by an unfriendly party unless the series is used only once and then discarded. To be certain that numbers are truly random, you must somehow generate them yourself using a process that is secure.

It is hopeless to try to generate random numbers using a digital computer alone, since digital circuits are deterministic. One needs to use some type of stochastic, unpredictable process. Possible approaches have included the timing between manual keyboard strokes, a mouse movement, or digitizing the picture of a continuously moving lava lamp.

Radioactive Decay

The "gold standard" of random-number generators is radioactive decay. Radioactive atomic nuclei are unstable and decay in a random fashion based on quantum mechanical probabilities. One atom decaying at, say, time t = 0, has no impact upon when the next atom will decay, so the timing of disintegrations is unpredictable, unknowable, unaffected by environmental conditions, and the patterns will not repeat in any deterministic manner. Also, each atom of a radioactive element is identical, so each has an equal probability of decaying in, say, the next second; the process is unlike some other natural phenomena, for example the grains of sand falling through an hourglass with the smaller grains tending to drop earlier than larger grains.

As an example of a simple algorithm, consider three sequential radioactive disintegrations, separated by time intervals ∆T₁₂ and ∆T₂₃. If ∆T₁₂ > ∆T₂₃, then assign a binary 0 bit. If ∆T₁₂ < ∆T₂₃, then assign a 1 bit. (If, by chance, ∆T₁₂ = ∆T₂₃, then reject and try again.) By timing many such intervals, a random series of bits can be generated and converted into any desired range of numbers—for example, 8 bits will produce a random integer from 0 to 255.

A Geiger-Mueller tube can be used to detect individual radioactive disintegrations. When radiation hits the tube, a current pulse cascades between electrodes within the tube, until the current is quenched and the tube again becomes quiescent. With suitable electronics, these pulses are detected as audible clicks. A G-M tube detects the decay of individual atoms.

A surplus, low-range Geiger counter can be modified to use as a true random number generator.  These are left over from civil defense uses during the Cold War; they frequently are available for about $50 on eBay, but they usually are sold as-is and may not be working.  Another source is www.surplustuff.com. Avoid the high-range survey meters employing ionization chambers--they are not sufficiently sensitive. The preferred model is CDV-700, which measures radiation less than 1.0 mR/hr. For an introduction to the various incarnations of CDV-700 meters, refer to this Oak Ridge site: www.orau.org/ptp/collection/civildefense/cdv700.htm

The CDV-700 has a speaker jack which produces audio clicks when a disintegration is detected. This can be used as an input to a digital circuit or computer sound card for timing the intervals between disintegrations. This is much preferable to ripping into the detector itself, which has high voltage. One idea for interfacing the CDV-700 to a PC sound card is at: www.geocities.com/revtkatt/

RM-60

A somewhat more straightforward approach is to purchase the Model RM-60 micro-roentgen radiation monitor (about $150) from Aware Electronics: www.aw-el.com

Model RM-60

The RM-60, which plugs into a PC serial port, comes with software for generating random numbers.  Naturally occurring background radiation may be used or, for faster results, a natural material or stone that is slightly radioactive. Another, more hazardous approach, is to use a household ionization smoke detector or a Staticmaster brush used for dusting photographic negatives. If such a radioactive source is used, it should not be removed from its enclosure, should not be handled more than necessary, should not be accessible to children, and should ultimately be disposed of in accordance with the manufacturer’s instructions. The source should not be removed from its enclosure since it would present unnecessary radiation exposure and it may be small enough to be swallowed by a child or pet.

The RM-60 also can be used for measuring home radon levels and for general experiments involving radioactive decay and cosmic rays.