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Why cant rads be converted to cpm?

  1. Jun 24, 2010 #1

    i have a geiger counter which measures radiation in rads, but not many people use rads anymore (it is almost obselete i think),

    am i correct it saying that rads are a measure of the number of particles being emitted AND the energy of the particle, but cpm is merely a measure of the number of particles being emitted?

    if i had a radioactive source, (e.g. a lump of uranium ore), and i measure the radiation using a geiger counter that measures in rads and the measurement = 1 rad,
    and i measure the same radioactive source with a geiger counter which measures in cpm, and the measurement = 1000 CPM, does that mean that 1 rad will always equal 1000 CPM? or does that only apply to a radioactive source which emits particles of the same energy as the aforemetioned radioactive source?

    it would be really helpful if someone would measure the radioactivity of common radioactive items (e.g. am. 241 smoke alarm source,fiesta ware plate,lantern mantle) using a geiger counter that measures in rads, and a geiger counter which measures in cpm, and provide a rough conversion chart.

    lolol i tried to make this as non-confusing as possible but i think i failed miserably haha

    thanks for reading my long spiel
  2. jcsd
  3. Jun 24, 2010 #2

    Andy Resnick

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    One reason there are all these units in radiation physics is because of the multiple effects radiation has on matter. There's the number of emitted particles, the energy those particles deposit into matter, and the amount of ionization caused by the passage of those particles. Furthermore, different particles deposit different amounts of energy, and in biology, different tissues are more or less sensitive to the effects of radiation.

    Today, we have a reasonably unified view of all these effects. But around 1900, no such understanding existed: hence, all the different units.

    This site might help you:

    This may help answer some questions also:
  4. Jun 24, 2010 #3


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    You are almost correct. Here is a breakdown of the different units:

    CPM/DPM/Becquerels/Curies: measures number of radiations, regardless of type. CPM is slightly different in that it is dependent on the efficiency of the radiation detector, while the latter three are quantities of the actual radiation source.

    Rad: A measure of absorbed radiation energy (Joules per Kg)

    Roentgen (R): A measure of a combination of radiation intensity and energy (Coulombs per Kg)

    REM (Roentgen equivalent man): same as R, but with a weighting factor depending on the radiation type (alpha/beta/gamma/neutron). This is the most common unit used in the US.
  5. Jun 25, 2010 #4

    thanks ill have a look at those websites soon, thanks for your help xx :)
    could you maybe reply to this thread too please?


    thanks again
    Last edited by a moderator: Apr 25, 2017
  6. Jun 25, 2010 #5

    thanks for your help i greatly appreciate it, can rems be converted to rads, or vice-versa?
    could you maybe reply to this thread please?

  7. Jul 10, 2010 #6
    thanks for all of your help, i greatly appreciate it :)
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