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Re: NUMEC Pu-Be sources
I used Bonner Spheres to evaluate a number of neutron spectra and
encountered Am-241 buildup in a set of 1-Ci Pu-239Be sources from Monsanto
Research Corporation (source data sheet dated 9-05-61). The following is
information found in my thesis (Neutron Spectra Measurements with the
Bonner Sphere Spectrometer, 1995) discussing possible buildup of Am-241 in
Pu-239Be neutron sources:
An evaluation of the suspected Am-241 ingrowth in the KSU Pu-239Be neutron
sources is given in Table 26.The Am-241 buildup is due to B- decay of
Pu-241 (half-life = 13.2 y). The Pu-241 is present as an impurity during
source fabrication. Measurement results were evaluated using the Cf-252
calibration factor (flux) of 1.57 and the Am-241Be calibration factor
(flux) of 1.28. These results suggest that the source emission rate of the
KSU Pu-239Be sources have increased a factor 1.38 times that calculated
using source documentation (95 percent confidence interval of 1.21 to
1.55). This compares favorably to the theoretical value 1.30 calculated
based upon 0.7 percent Pu-241 being present at the time of source
fabrication. The Pu-241 initially present results in a 2 percent per year
increase in the neutron yield due to the buildup of Am-241 over a 30 year
period. A recent newsletter from the Nuclear Regulatory Commission (1993),
using the same initial conditions, reported the source emission rate in
Pu-239Be exhibiting Am-241 ingrowth for 69 years would be about 1.33 times
the initial source emission rate. Calculations made during this study shoe
that the source emission rate would increase by a factor of 1.34 after 69
years. The source emission rate depends upon the Pu:Be atom ratio, powder
mixing method, powder mixing time, and the degree of compaction. There is a
high degree of uncertainty in trying to predict the yield of a Pu-239Be
neutron source, and the NRC recommends that the neutron yield be determined
by direct measurement.
Note that the measurements using the calibration factor (flux) for Am-141Be
alone indicated that the source emission rate had increased by a factor of
1.24.
Ref:
Knoll, G.F., Radiation Detection and Measurement, 2nd edition 1979, pages
30-31
Here is the information from the newsletter (this was found at
http://www.nrc.gov/NRC/FEDWORLD/NRC-SSD/nl941.txt):
INCREASE IN NEUTRON YIELDS OF PLUTONIUM-BERYLLIUM SOURCES
U.S. Atomic Energy Commission contractors such as Mound Laboratories and
other national laboratories made several
thousand plutonium beryllium sources (Pu-238 and Pu-239). Neutron yields of
these sources are increasing because of the buildup of Am-241, an alpha
emitter, from the beta decay of Pu-241 which is present as a contaminant in
the sources. The isotope composition of these sources is not known;
however, based on a limited study, using mass spectrographic analysis, it
is believed to be about 80 percent of the principal plutonium isotope,
either Pu-238 or Pu-239. It must be noted that neutron yield for each
source is a function of plutonium/beryllium (PU-BE) atom ratio, powder
mixing method and time, and degree of compaction of the powder.
Understandably, neutron yield for each source capsule varies significantly.
Increased neutron emission occurs as a result of increased alpha activity,
and subsequent increased interaction with the beryllium nucleus. Edward
Anderson predicts the increase in neutron yields in an article that
appeared in Nuclear Applications in March 1968. The article gives
equations to predict neutron yields. For example: given an amount of
Pu-241, originally present at time of source fabrication, of approximately
0.7 percent, the initial rate of increase in neutron yield would be about 2
percent per year. The maximum yield would occur after about 69 years and
would be about 1.33 times the original yield. Some PU-BE sources are about
30 years old. The Anderson equation can be used to predict the neutron
yields; however, the accuracy with which a prediction may be made is
questionable, because of uncertainties in source fabrication. The neutron
yield is best determined by direct neutron measurement. This issue should
be considered during licensing and device reviews.
DJWhitfill
Opinions expressed are mine and do not reflect official policies or
positions of the Kansas Department of Health and Environment.
denison.8@osu.edu
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radsafe@romulus.eh <radsafe@romulus.ehs.uiuc.edu>
s.uiuc.edu cc:
Subject: NUMEC Pu-Be sources
02/20/00 10:49 PM
Please respond to
radsafe
Looking for information on the plutonium isotope composition of NUMEC Pu-Be
neutron sources. The documentation I've found (including copies of old
reports to the AEC) lists the Pu-239 fraction, but says nothing about the
rest of the Pu mass. I need to know the Pu-241 fraction in order to
calculate the increase in neutron production due to Am-241 ingrowth (see
note below).
I know that Babcock and Wilcox bought out NUMEC back in the 1970's, but I'm
not getting anywhere trying to contact them directly. Their 800 number
referred me to the Lynchburg? Virginia facility, whose engineering
department forwarded me to their legal office, which hasn't called back.
I would appreciate any assistance you folks can give me in locating a
source for this information. Thanks!
Eric
Note for those who haven't played with a Pu-Be source (lately):
Pu-241 beta decays with 14.4-yr half life to Am-241, and Am-241 alpha
decays with a half-life of 432 years and energies in the same range as
Pu-239. Be captures alphas and emits neutrons, regardless of whether the
alphas come from Pu-239 or somewhere else. Since Am-241 is longer-lived
than Pu-241, it builds to an equilibrium level, and since Am-241's decay
constant is 55 times greater than Pu-239's, the strength of the source
grows over time in proportion to the original Pu-241 fraction. If all of
the "non-239" Pu in our source is Pu-241, the neutron output will be 4
times greater than if we have no Pu-241 at all. Makes one hell of a
difference in my dose rate calculations!
J. Eric Denison
Nuclear Engineering Program
The Ohio State University
2030 Robinson Laboratory
206 West 18th Avenue
Columbus OH 43210
(614) 292-3681 or -1074
denison.8@osu.edu
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