Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about radioactive sources

  1. Nov 27, 2013 #1
    Hi there,

    I have a few closed radioactive sources (Cs137, Pb210, Na22) that I am trying to use for a project but I am not that knowledgeable about them.

    Their activity is shown on the source but I'm unsure if this is related to the amount ie grams of substance inside the sealed container, or if it relates to how much radiation is actually being emitted from the sealed container. ie. internal absorbtion is accounted for. For beta radiation in particular I would think that a large portion would be re-absorbed by the plastic mold it is kept in.

    I am trying to find the beta efficiency and gamma efficiency of some geiger tubes and am wondering if it is even possible with what I have??

  2. jcsd
  3. Nov 28, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    The activity is related to the mass or number of atoms present, and the decay constant (half-life), and date from which the source was produced. Since the sources were produced, the activity will decrease in time.

    Detection efficiency will include geometry and distance factors.

    The beta and gamma radiation is emitted in all directions, so only some of the radiation will reach the detector (solid angle subtended by the detection chamber), and only some (most) will interact with the detector. Beta particles will be much more attenuated than gamma rays.

    Ostensibly, one is using these sources at an institution licensed to possess these sources.
  4. Nov 29, 2013 #3
    Thanks for your reply,

    So since it is solely based on the number of atoms present, it may not be adequate for my needs. The problem is, beta particles can only travel a few mm in most materials, (correct me if I'm wrong) therefore much of the emitted beta particles will not exit the closed source. Basically I have no way of knowing how many beta particles are leaving the disk source. I am trying to find (somewhat accurately) the sensitivity of these geiger tubes to beta particles. Is there another way to do this? Or is there a way to discover the actual amount of beta particles being emitted?

    Also yes, I have borrowed these sources from my universities physics department. I am in engineering. Most of them are exempt, one beta source is around 4 micro curie.
  5. Nov 29, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Beta detection depends on the source and detector and what lies in between.

    One is correct that betas will interact with atoms between the source and detector, so detection efficiency will be a function of beta energy as well as other factors.

    For each nuclide, the information given is usually the most probable energy, which is roughly one-third of the maximum energy.

    One is using sealed source, and usually, the best way to count beta activity would be to place the source material inside the detector (2π, or hemsiphere). That may not be practical however. Otherwise, one must bring the window of the Geiger counter close to the beta source.

    Does one have instructions on counting beta and gamma activity. Typically one would measure both, then place a shield between the source and detector to exclude betas and measure mostly gamma.
  6. Dec 10, 2013 #5
    Thank you, I think I managed to collect the data necessary. I will report on my findings in a bit Quick question though.

    Suppose you have Cs137 which decays at a rate of 2109 bq. Is it correct to say that from that you have 2109 Beta decays per second from the Cs137 and also 2109 gamma decays per second from the secondary decay?

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook