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Re: Cobalt-60



correction: I suppose obvious, any way, Compton scattered radiation is not x-rays (unless the original photon was an x-ray. But my error also reminds me that another reason for using a low Z material
is that x-rays resulting from photo-electrons will have overflow energy, and, therefore, not be an added background if a non spectral analyzer detector is used.

"Bernard G. Cleyet & Nancy Ann Seese" wrote:

One will also get compton scattered gamma's no matter what one does. However, as MDE suggests Cs-137 "makes life easier." However, using a high Z absorber to eliminate the betas is not as good as
using a low Z such as a sheet of plexi. This way one may absorber the beta's w/o creating as many hi-energy x-rays. The best way to do the xpt. is to use a scintillator (and MCA) and integrate the
peak, thereby eliminating Comptons. However, this "begs the ?" as to the definition of absorption, what one is measuring, etc. This "exercise" is one of the advanced "labs" at UCSB wherein the
student is expected to spend about ten hours collecting and down-loading data and about as much time thinking about it (considering the above problems, etc.) before during and after. Then they spend
another number of hours in fitting, integrating peaks, etc. (they even are supposed to show that the peaks are Gaussian with chi-square testing -- in the Mössbauer and scattering xpts. the peaks are
NOT Gaussian), finally they "write up" using APS standard. Three (varies depending on difficulty of xpt.) in ten weeks! The dept. considers this their more, if not most, impt. course.

bc

Michael Edmiston wrote:

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