Whoops, I made a mistake earlier. I gave the decay rate for a 1 μCi source. For 5 μCi, it's 185,000 per second. Warren, thanks for the reminder about input capacitance, it had completely slipped my mind. And thanks for the suggestion about neon tubes instead of resistors as the noise source. But this reminds me, there are noise diodes designed exactly for the purpose of generating noise. One supplier is noisecom.com (they were at the top of Google search). Their NC104 model provides 3 MHz bandwidth with noise output into a 2200 Ω load of 3 μV/Hz^1/2, i.e. 5 mV RMS if the full bandwidth is usable, or 1.6 mV if filtered down to 300 kHz. If the first amplifier has 7 pF input capacitance, then, together with the 2200 Ω load, the upper cutoff frequency is 10 MHz. But if you make your own circuit, there remains the problem of getting the random numbers into the computer. The Bull Mountain RNG is supposed to be available in the Intel Ivy Bridge processors that are expected out in early 2012. Since these CPUs, which will implement to RDRAND instruction, will be the main processor in PCs, it seems simplest to go this route, taking one out of 512 bytes if you distrust the AES. -- Gene