At this point, atomic clocks are old news. They’ve been quietly keeping our world on schedule for decades now, and have been through several iterations with each generation gaining more accuracy. They generally all work under the same physical principle though — a radio signal stimulates a gas at a specific frequency, and the response of the gas is used to tune the frequency. This yields high accuracy and high precision — the spacing between each “tick” of an atomic clock doesn’t vary by much, and the ticks cumulatively track the time with very little drift.
All of this had [alnwlsn] thinking about whether he could make an “atomic” clock that measures actual radioactive decay, rather than relying on the hyperfine transition states of atoms. Frustratingly, most of the radioactive materials that are readily available have pretty long half-lives — on the order of decades or centuries. Trying to quantify small changes in the energy output of such a sample over the course of seconds or minutes would be impossible, so he decided to focus on the byproduct of decay — the particles being emitted.
He used a microcontroller to count clicks from a Geiger-Müller tube, and used the count to calculate elapsed time by multiplying by a calibration factor (the expected number of clicks per second). While this is wildly inaccurate in the short term (he’s actually used the same system to generate random numbers), over time it smooths out and can provide a meaningful reading. After one year of continuous operation, the counter was only off by about 26 minutes, or 4.4 seconds per day. That’s better than most mechanical wristwatches (though a traditional Rubidium atomic clock would be less than six milliseconds off, and NIST’s Strontium clock would be within 6.67×10-11 seconds).
The end result is a probabilistic radiometric timepiece that has style (he even built a clock face with hands, rather than just displaying the time on an LCD). Better yet, it’s got a status page where you can check on on how it’s running. We’ve seen quite a few atomic clocks over the years, but this one is unique and a great entry into the 2025 One Hertz Challenge.
Thanks for the writeup! If you decide to check the status page, it’s only got an uptime of a couple days; there was an extended power outage this weekend. Got to start counting all over again. It was only up for a couple weeks before that though, so doesn’t hurt quite as much as losing a months long run (which has happened)
Looking at your “one minute” video, if I saw a clock ticking that way, I wouldn’t even notice the inconsistency. I’m guessing this is because the number of counts per second is high enough that it really doesn’t vary all that much. If you used a smaller sample (or placed it farther from the Geiger tube), you’d get a lower calibration count, and more people saying, “what’s with your clock?”
Yes, that’s right. The actual rate is about 73.8 geiger clicks/second, and I’m sure there’s some statistics I don’t know off the top of my head that will tell you how much it varies from second to second. Main goal with this was to go for the long term, like weeks or months, to see what happens, and I figured my best shot at that was to have a higher click rate.
I wonder how much decay changes the rate over time and how that might impact it. Or if there could be any extra statistical magic that could help squeeze more out of the clock.