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Re: Cobalt-60



Michael Edmiston wrote:

Actually, we prefer to use a Cs-137 source (also with the
lead to block the beta) because its 0.662 Mev gamma is
alone.

If my memory can be depend upon, the efficiency of a typical
Geiger, for counting gammas is usually a small fraction of one
percent. Thus the probability of detecting two gammas from
the same decaying nucleus is negligibly small, even if both
were emitted in the same direction.

Next imagine the unrealistic case of 100% efficient Geiger
counter with the 4*Pi geometry. You would still detect only
one event per decay, either for Co-60 or Cs-137. But it is
true that beta rays can complicate an experiment whose goal
is to measure the linear coefficient of absorption of gammas.

Why do I use such words as gammas and betas? First because
they will be well understood on this list and in this context.
Also because I do not like to say rays. Electrons and photons
would be OK except that not all photons and not all electrons
can be recorded with a typical Geiger counter. Some people
say beta and gamma particles; but photons are not particles.

Suppose betas are shielded. Is the efficiency of counting
decays from a Cs-137 source higher or lower than from the
Co-60 source? I do not know. On one hand the energy is
lower for Cs-137 and this helps the efficiency. On the other
hand Co-60 emits two photons per decay and this also
helps the efficiency. The answer depends on how fast the
efficiency goes down when the energy is increasing. If I
had to guess I would say efficiency is higher for Cs-137.
There is lot of good physics to learn while using a Geiger
counter.
Ludwik Kowalski