HotBits uses a 5 microCurie Cs-137 source. This is 37,000 disintegrations per second. It takes 4 consecutive pulses, at times A, B, C, D, to generate 1 bit. The time differences T1 = B - A and T2 = D - C are compared. If T1 < T2 a zero is output, if T1 > T2 a one is output, and if T1 = T2 no bit is output. After the detection efficiency and timing processing, 100 bytes per second are provided to you, but maybe also to the CIA, KGB, and Mossad.
I actually sent the hotbots guy a note, below, but he responded in a very annoyed way, further below. WDS --> John Walker. About hotbits, you are not getting nearly as many bits per second as you could. Let C be the mean time between clicks on your rad detector, and assume that two clicks separated by less than B is bad news since the rad detector has trouble recovering that fast. Finally let A be the time-resolution of your time measuring device. I assume 0<A<<B<C. [C will change gradually as your rad source runs out of atoms, you need to keep its value updated.] Let a click occur that is time T after the preceding click. If T>B then compute Q = exp( (B-T)/C ) else wait for next click. Then Q will be (in principle) a uniformly distributed real number in (0,1) because T should be exponentially distributed.T Because A>0 we cannot get an infinite number of random bits out per click, but it ought to be safe to output the most significant log(C/A) bits of Q. I'm guesstimating that C=300 microsec, B=1 microsec, A=1 nanosec. If so, then each click ought to safely give you 11 more random bits. I gather this would be an improvement over your current bit rate by a factor of about 44. Walker --> WDS: This is complete nonsense. I have put my my own design of a random number generator by radioactive decay out for public scrutiny for more than a decade and every single quibble has been argued from every possible direction. I have a collection of letters from tenured professors in statistics which argue: A) I am crazy to measure two separate intervals delimited by four decay events, and B) I am crazy not not measure the intervals between the first and second and second and third of three decay events. To these, I say, thank you profs, but you can't claim A) has any correlation, and I'd rather not debate the partisans of B). Your suggestion makes vastly more assumptions than can be justified based upon the properties of the generator and the detector. Random event generation encourages radical conservatism in design as much as engineering of earth satellites where one "earns out" the engineering fee only if the bird functions in orbit more than 10 years. You can get a whole lot of bits per second by running an oscillator at, say, 50 MHz, and then latching it at each decay and passing that to the computer. Sounds great, until you think about whether the duty cycle of the oscillator was precisely 50/50 or worry about metastability in the latch. Try putting up a generator with your design and see what E-mail you get from people who critique your design. If I'd had it to do over, I'd never have done HotBits. I do not wish to discuss this further. --end.