# U-238 decay chain question

Hi. We are using a Pb-210 needle source for our cloud chamber science project. Are we correct that the tracks we are seeing inside the chamber radiating off the needle are:

1) Beta tracks from Pb-210 decaying into Bi-210 (in the .063 MeV range)
2) Beta tracks from Bi-210 decaying into Po-210 (in the 1.1 MeV range)
3) Alpha tracks from Po-210 decaying into stable Pb-206 (in the 5.4 MeV range)

If that is correct, are the tracks being produced continuously and in equal amounts?

Also, does that mean the radioactive decay chain of U-238 starts happening from the very first moment of the U-238's existence and continues to happen without ever stopping until all the atoms become stable lead?

Thanks! We really appreciate your help.

## Answers and Replies

mfb
Mentor
If that is correct, are the tracks being produced continuously and in equal amounts?
If you start with a pure Pb-210 source, initially you only have Pb-210 decays, with Bi-210 decays (half-life 5 days) quickly growing (within days to weeks) as the first atoms decayed to Bi-210. Po-210 has a half-life of 138 days, that needs longer until it reaches the same activity.

If you start with pure U-238, the same arguments apply, initially you don't get all decays in equal amounts, but after a while this is a very good approximation.

Nothing stops until everything decayed to stable nuclei.

If you start with a pure Pb-210 source, initially you only have Pb-210 decays, with Bi-210 decays (half-life 5 days) quickly growing (within days to weeks) as the first atoms decayed to Bi-210. Po-210 has a half-life of 138 days, that needs longer until it reaches the same activity.

If you start with pure U-238, the same arguments apply, initially you don't get all decays in equal amounts, but after a while this is a very good approximation.

Nothing stops until everything decayed to stable nuclei.

Thank you for replying so quickly!

Based on what you wrote, so if we bought our needle from a company that used pure Pb-210 to coat the needle head, and we began our experiments about two weeks after receiving it, then we are likely seeing beta tracks from both the Pb-210 decay and Bi-210 decay in almost equal amounts?
(During our experiments, we noticed shorter, faint beta tracks that traveled off the needle, and we also noticed brighter betas that traveled as far as 8 inches off the needle. Is that possibly the two different beta decays, one having a higher MeV?)

Also, why do we see alpha tracks coming off the needle if Po-210 needs longer to decay?

mfb
Mentor
Based on what you wrote, so if we bought our needle from a company that used pure Pb-210 to coat the needle head, and we began our experiments about two weeks after receiving it, then we are likely seeing beta tracks from both the Pb-210 decay and Bi-210 decay in almost equal amounts?
Within 15%, yes.
(During our experiments, we noticed shorter, faint beta tracks that traveled off the needle, and we also noticed brighter betas that traveled as far as 8 inches off the needle. Is that possibly the two different beta decays, one having a higher MeV?)
The decay energy in a beta decay is distributed over the neutrino and the electron. The energies you listed are just the maximal electron energy, it can also be lower. In addition, the range of electrons can vary even for the same energy.
Also, why do we see alpha tracks coming off the needle if Po-210 needs longer to decay?
The decay is a probabilistic process. You will see activity all the time, but it will be higher a year after you bought the sample.
Your sample won't be pure Pb-210, other isotopes can contribute to the visible decays.

Within 15%, yes.The decay energy in a beta decay is distributed over the neutrino and the electron. The energies you listed are just the maximal electron energy, it can also be lower. In addition, the range of electrons can vary even for the same energy.
The decay is a probabilistic process. You will see activity all the time, but it will be higher a year after you bought the sample.
Your sample won't be pure Pb-210, other isotopes can contribute to the visible decays.

You rock! Thanks!