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Re: Cobalt-60



A prescript.

Have any of you out there attempted to use Al foil as a beta spectrometer with a G-M system?

This was an intermediate lab. expt. for ~ 20 years, 'till an "accuracy" compulsive instructor taught the course. This was a method used early on in radio-a. history for relatively short lived
isotopes that prevented a sufficient accumulation to use a magnetic spectroscope. It's VERY crude, but one can resolve the two betas from Sr-90. I will appreciate any comments or experience with
this method.


bc


Comments interleaved below:

Michael Edmiston wrote:

Here are three comments in response to things others have said.

* * * 1 * * *
Ludwik Kowalski mentions it is not likely a G-M tube will detect both Co-60
gammas at once. This is true... in fact the likelihood of seeing even one
is not high because G-M tubes are not efficient at stopping (detecting)
these high-energy photons. However, Ludwik seems to imply the possibility
of double detection was why I suggested Cs-137 instead of Co-60. I didn't
suggest Cs-137 for that reason. I avoid Co-60 because there are two
energies, and that makes it a bit more confusing what energy to use when
figuring the absorption coefficient for the absorber. If we could tune our
detector to detect just the 1.17-MeV gammas or just the 1.33-MeV gammas we
could choose to count just one energy. That is possible with a NaI
scintillation system, but not with a G-M tube system. Of course, the
absorption coefficients for 1.17-MeV and 1.33-MeV gammas in aluminum are not
very different, so it may not matter much. But I think it would be more
comforting to the students if they didn't have to question the fact that
they have more than one energy present. Additionally, the lower-energy
Cs-137 gamma will show a bigger absorption effect in a low-Z material like
aluminum.


co-incidentally, I just measured the efficiency ratio of alphas ( I suspect for mono-energetic and close is near 100% a bit less for betas) and gammas of an LND 7232 tube (end window ~ 2 mg/cm^2
one + " diameter) Am-241 somewhat closer than distance where tube begins to detect alphas (< 1") measurement with and with out card covering source ( ~ one microC. from $5! smoke detector); assume ~
1 / 3 60 keV L x-ray per alpha. Corrected (for dead time) rates: 680 cpm with card; 150 k cpm w/o, bkgnd 33. this results in ~ 1% The LND supplied data gives gamma sens. 40 cps / MR ph for
Co-60 The relative photon sens is 10 keV - 2; 40 - 3; 60 - 5; 90 - 3; 200 - 1; 1 meV - 1 + according to Bob Lehnert (of LND) it falls sl. to 8 meV their last measurement, also he confirms
that if an alpha gets through the window it's counted same for beta except sl. less as it must travel further (before hitting the wall) to produce sufficient ionization for a count. Further he's
rather certain that their 7232 is the one vernier sells (as I suspected). He is quite surprised to hear that I've found a large (~ 4 X ) variation in dead time with rate.




Also, it is mostly because the efficiency for detecting gammas in a G-M tube
is very low compared to the efficiency for detecting beta radiation that it
is important to block the beta-particles in this experiment.

right on

Otherwise, the
count rate with no absorber (or with thin absorbers) can be almost entirely
beta counts rather than gamma counts. If you are not blocking the betas,
your no-absorber and/or thin-absorber data might not fit the rest of your
curve.

One more advantage of Cs-137 over Co-60 is the longer half-life. If you
teach for 30 years you may need to replace your Co-60 source once or twice.
You'll only need to buy the Cs-137 source once.

* * * 2 * * *
Bernard G. Cleyet & Nancy Ann Seese

I'm bc.


commented about using a lower-Z absorber
to eliminate the betas rather than the lead sheet I suggested. The points
raised are valid... the absorber will cause some bremsstrahlung radiation
and some compton scattering. Subsequent detection of any of these will
create counts we don't want because they are not the same energy as the
gammas for which we are trying to find the absorption coefficient.

However, the use of lower-Z material such as Plexiglas will not block atomic
x-rays unless it is fairly thick. The Cs-137 source has a strong (emitted
often) 32-keV x-ray that we would rather not count if we are determining the
absorption coefficient for the 0.662-MeV gamma.

I assumed that the first sheet of lead would take care of anything the plexi missed, stupidly forgetting that one would want a point at "zero" absorber thickness. Note that the neptunium L is right
at the most sens. energy for the tube (60 keV). This might suggest a good energy to use in addition to Cs (it's @ 600 keV?) (The alphas may stopped by onion skin paper or a couple inches of air.)



If we do this experiment somewhat loosely in a lower-level course, we use a
lead sheet right on top of the source to block the betas and x-rays. We do
not discuss Compton scattering nor bremsstrahlung x-rays. Students in these
courses don't have much trouble accepting that there is a beta and we are
trying to get just the 0.662-MeV gamma.

If we do this experiment in an advanced class we discuss the Compton
scattering and bremsstrahlung radiation. A little thought shows that we can
partially fix the problem by putting the beta/x-ray shield over the
detector rather than over the source. However, a better way to fix the
problem (and more instructive) is to use a "graded absorber." The graded
absorber might be a sandwich of thin plastic, then thin copper, then thin
lead over the source. The plastic stops many of the betas and will make
some low energy x-rays. The copper can stop the rest of the betas and
absorb some of the plastic x-rays. The lead can stop any straggling betas
and absorb the copper x-rays. The lead also absorbs the 32-keV x-ray from
the source. These layers can be pretty thin. Since they are thin, the
cross section for the 0.662-MeV gamma will not be high... this means we
won't have many Compton events. The use of "grade absorbers" in nuclear
science is quite common.

yup!



* * * 3 * * *

Larry Smith asks if the beta-blocker plate should be in place when measuring
background. I think it won't matter much since it is thin and won't stop
many cosmic rays. However, if the beta blocker is separate from the source
I don't see any reason not to leave it in place during background counting.
For our low-level courses we actually tape the beta-blocker to the source.
That way we can define "the source" to the students as a source that
predominantly emits just the 0.661-MeV gamma. Since the beta-blocker
becomes part of "the source" in this case, it is not present when the
background count is taken.


I once measured the ratio of background from above and below -- but have forgotten -- the point is that it wont stop those from "above."



In our upper-level courses we use a graded beta-blocker sandwich between the
source and the aluminum absorber being studied. In this case we leave the
beta-blocker in place when we count background.

You also use scintillator and MCA?



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