The two gamma rays mentioned are in a cascade with the 1.173 occurring
first, followed by the 1.333. The sequence is... beta minus, 1.173 Mev
gamma, 1.333 Mev gamma.
However, the beta-minus decay does not populate the level (from which the
1.173 MeV gamma originates) 100% of the time... although close. The actual
observed rates are... the 1.173 Mev gamma is seen following 99.90% of the
beta decays, and the 1.333 Mev gamma is seen following 99.98 if the beta
decays. That is certainly close enough for the types of experiments being
described.
For measuring thickness of aluminum, are you using a Geiger-Mueller tube?
If so, you ought to block the beta-minus particles. The count rate with no
aluminum can be dominated by detection of the beta-minus, and some
beta-minus might also make it through the thinner pieces of aluminum,
depending upon how thick your thinnest aluminum is. This means the "count
rate" with no aluminum will be "too high," and it might also be "too high"
with the thinner pieces of aluminum.
This is easy to fix. When we do thickness studies of aluminum with gamma
rays, we tape a piece of lead, maybe 0.5 mm thick, over the source. This
blocks the beta-minus and also any nickel x-rays. That way you get mostly
pure gamma coming through the lead, with the exception of some lead x-rays
that are created.
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.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817