Nuclear Spectroscopy

Exercise 14.4
Nuclear Fission
Several isotopes that you are using, e.g., 137 Cs, 133 Ba,
and 131 I, are isotopes left after the quick beta decay of
fission products from the fast or slow neutron fission
of 235 U, 239 Pu or 241 Pu. If you search the environment
for evidence of recent fission events, then 137 Cs and
133
Ba are the gamma-emitting isotopes to study. What
fission events can you study experimentally? Reactor
accidents (Chernobyl-Ukraine, Three-Mile Island-
USA, and Wind Scale-England), the release of liquid
reactor wastes (White Oak Lake-USA), and aboveground
nuclear weapons detonations, may appear to
be your best opportunities. Obtaining samples from
these events can be challenging, and given the possible
danger of contamination and exposure, you
should seriously question why you need to study these
samples before you attempt to collect them.
One interesting sample that is easy to handle safely
and that is of significant interest (historically and
scientifically) is the material trinitite, pieces of fused
sand from ground zero of the first nuclear weapon’s
detonation on 16 July 1945 in Alamogordo, N.M.
Calculations from alpha spectroscopy, seismic and
acoustic measurements at the time gave energy yields
between 5 and 18.3 kT (kilotons) of TNT (1 kT = 2.61
x 10 25 MeV = 10 12 cal). Enrico Fermi’s famous measurement
of the yield, from a measurement of the
displacement of falling bits of paper, also falls within
this range.
Nuclear fission of 235 U and 239 Pu by fast or slow neutrons
usually produces two or three fast neutrons and
two nuclei, one heavy (A~140) and one light (A~95).
Fast neutron fission produces about 200 MeV per
fission, of which 180 MeV is released immediately.
The remaining 10% of the energy is released slowly in
the radioactive decay of the fission products. Many
different elements of each mass number are produced,
decaying rapidly to longer-lived or stable nuclei.
After a few months, there are only a few isotopes
remaining of the many created. 137 Cs, being the most
predominant gamma emitter, is created in 6.6% of
239
Pu fissions and 6.2% of 235 U fissions.
Your trinitite sample was taken from ground zero, an
area about 1,100 m in diameter, over which 1-2% of all
the fission products from the detonation accumulated.
Along with the fission products there will be
uranium and plutonium isotopes from unfissioned
material or from neutron capture production and the
subsequent radioactive series decay.
OBJECTIVE
A measurement and determination of the absolute
activities of the gamma-emitting isotopes in a trinitite
sample will allow you to estimate the explosive yield of
that detonation. Create a model that relates the
energy of the explosion to the number of fissions to
the expected number of 137 Cs atoms in your sample.
Coupling this with your measurement of the absolute
activity will result in your measurement of the explosive
energy yield of the first nuclear-weapon detonation.
SUPPLIES
•NaI(Tl) detector with MCA
•trinitite (New England Meteoritical Sources, PO
Box 440, Mendon, MA 01756)
SUGGESTED EXPERIMENTAL PROCEDURES
1. Prepare the detector system for pulse-height analysis
for energies up to 1,300 keV.
2. Place the trinitite on the 8th shelf and count the
sample overnight (~20 hours).
3. Count a long background (~4 hours) and save
both spectra.
ANALYSIS
After background subtraction determine the energies
and net counts in each observed photopeak. From the
energies determine the isotopes. Can these be fission
products? If any 137 Cs and 133 Ba isotopes are observed,
determine the absolute activity per surface area of
each isotope. Correct this for decay to determine the
activity in 1945. Create your own model for isotope
deposition, based upon the data given in the objective’s
section. It should relate your measured gamma activities
to the number of fissions that occurred, and the
energy released from the weapon.
How does your yield measurement compare to the
range of values measured in 1945? What are typical
yields of nuclear fission weapons in today’s stockpiles?
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