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Re: Dose equivalent rates at the surface of a 10 Ci (239)PuBe?
Although I can't give you the answer you requested, I have some information
that may be useful:
1) Per ICRU Report 10b, "Physical Aspects of Irradiation," (NBS Handbook 85)
1964, the neutron emission from a Pu-239/Be source is approximately 1.5 E6
n/sec/Ci. You can then calculate the flux from the surface area of the
source, and apply the dose factor of 10CFR20.1004(c) - 1 rem = 2.5 E7 n/cm2.
If you want to hazard a guess about the spectrum, there are more accurate
values listed for specific energies. The gamma dose is insignificant compared
to the neutron dose.
2) The above should be treated with caution, since the neutron yield of a
Pu-239/Be source will increase over time, due to the buildup of Am-241, which
is a daughter of Pu-241, which is usually present in small quantities. See
the paper: M.E. Anderson, "Increases in Neutron Yield of Pu-129/Be(alpha,n)
Sources - II, published in "Nuclear Technology," Vol 52, March 1981. This
paper estimates that for a source that originally contained 0.7% Pu-241, the
yield will reach a maximum of approximately 1.4 times the original yield in 71
years. The Am-241 buildup will also tend to increase the gamma dose rate.
3) The source may also contain moderating material, which will affect the dose
rate.
4) I'd be especially concerned about leakage, since the source, apparently,
has not been leak tested in more than 25 years. Before handling the source,
swipe the area around it and check for alpha activity.
The opinions expressed are stricly mine.
Here's to a risk free world, and other fantasies.
Bill Lipton
liptonw@detroitedison.com
Can anyone tell me measured dose (equivalent) rates (both gamma and
neutron) at the surface of a 10 Ci (239)PuBe source (Monsanto corp)? I
am looking for ballpark figures for dose planning purposes.
Mine has been in a drum since 1971 and I don't have record of the
surface dose rates last time they were measured.
Thanks
Noel Montgomery, CHP
Kirtland AFB Radiation Safety Officer