If one considers alpha-emitting
species, it is known that plutonium and uranium are present in these
devices in macro amounts (kilograms) as fissile fuels. The former is present as so-called
"weapons-grade" plutonium which contains a high percentage of 239py plus a nominal 5-6 atom
percent of 40pu and only minor amounts of the other plutonium isotopes. One might assume that
pee weapons-grade plutonium has a set of isotopic abundances as listed in Table 6-1. (Oetting,
1965
TABLE 6-1. ASSUMED ISOTOPIC ABUNDANCES FOR WEAPONS-GRADE PLUTONIUM.
Isotope
Atom %
238
0.012
Half-Life (Years)
Alpha Activity %
87.8
2.8
239
93.35
24,100
78.3
240
6.06
6,540
18.9
241
0.55
6.10x10° (@)
0.018
14.40
242
0.02
3.87x10°
0.001
The specific activity of this Pu is 1.62 x 10% disintegrations per minute (dpm) per microgram ( 4g).
Most weapons-grade plutonium will contain some americium-241 (241 4m), since the beta decay of
4l py produces this nuclide; beta emission is the predominant mode of decay for 241 py, Even if a
specific chemical separation of americium is made to purify the plutoniun,its
Am content will
again inerease with time following the chemical separation. Thus, although weapons-grade
plutonium may contain 241 Am in concentrations of a few tens or hundreds of parts per million (ppm)
Uranium is often present in the nuclear device as enriched 235U in order to serve as a fissile fuel.
There may be significant amounts of uranium present with other isotopic compositions also, e.g.
components containing uranium with large percentages of the isotope 238y, Given information on
the composition of the uranium and/or plutonium in each device prior to explosion and given
Fal
at the time of detonation, the great majority of the 241 4m observed after 20-30 years hasits origin
in
“*!Pu beta decay. If one assumes a 20-year decay for the above isotopic distribution, the
resultant 241 Am is 0.249 x 10° dpmfrom 1] pg of the original weapons grade Pu.
knowledge of how the isotopes of these elements are transmuted by neutron-induced reactions during
the explosion, one can predict which alpha-emitting nuclides will be most abundant in debris samples
The plutonium fraction represents the most important alpha-emitting species in any survey sample
taken from Enewetak Atoll] that has not undergone some sort of specific chemical treatment. In
these samples, the most abundant plutonium alpha emitter is 239py. Another important
alpha-emitting isotope is 240py, The radioactivity of this nuclide is often linked with that of 239py
since their alpha particle energies are almost identical and cannot be resolved from one another in
"a
eollected during the Atoll surveys.
ordinary alpha pulse height analysis employing solid-state detectors or Frisch-grid ionization
chambers. Two more nuclides, 238py and 24 Am, are present in significant amounts. These four
woe
most important alpha emitters are listed in Table 6-2 along with their half-lives and specifie
activities.
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