TABLE 6-2. MOST IMPORTANT ALPHA EMITTERS IN DEBRIS AT ENEWETAK ATOLL
Alpha
Nuclide (In Order of
Decreasing Abundance)
Specifie Activity
of Pure Isotope
(odpm/ pg)
ti/2
Half-Life (Yrs)
239 py
24,100
1°38 x 108
240 py
6,540
5.06 x 10°
241 am
433
7.60 x 106
238py
87.8
3.80 x 107
It is clear that 239Pu and 249Pu must be present in larger absolute amounts than the shorter-lived
lAm and 238py since, in spite of their lower specific activities, the former are the predominant
alpha-emitting species,
The half-lives of these species are all long compared with the 20-30 years that have elapsed since
tests were conducted at Enewetak and yet are short compared with those of 235) (ty /sg= 7.1 x 10
yrs), 238p (t)/9 = 4.5 x 10° yrs), and other uranium isotopes. Thus, uranium is judged not to present
a significant hazard by virtue of its alpha radioactivity at Enewetak; accurate analytical analyses for
uranium in survey samples have confirmed this prediction (Hoff, 1973).
What other alpha-active nuclides might be present in the Enewetak samples and how important will
their contribution to total transuranic alpha radioactivity be?
vm
Among the Pu isotopes, 241 py will be a minor constituent; see Oetting where it is reported at an
abundance of 0.55 atom pereent. Other than its importance as the beta decay parentof 24lam, this
isotope does not contribute significantly to the potential biological dose rate of Pu because its
alpha-to-beta branching ratio is quite low (e/g = 2.4 x 1075) and because it has a low beta energy
(maximum energy of 0.021 million electron volts (MeV)). Another minor constituent of
reactor-produced plutonium is
Pu. Since it is longer-lived than either 239pu or 240py and is
present as a minor component, it does not contribute significantly to the total activity of plutonium
in Enewetak samples. in the plutonium
discussed by Oetting, 242pu occurs at about 0.02 atom
percent which corresponds to 1.1 x 107°% of total alpha activity. The same comments apply to the
question of 244Pu (t]/g = 8.27 x 10’ yrs) alpha activity in Enewetak samples.
longer half-life and is even more rare than 242py,
This nuclide has a
During the production of plutonium in a nuclear reactor, 244py is isolated from the regular neutron
capture sequence in Pu because of the rapid beta decay of five-hour 243py. The only other
long-lived Pu isotope that has not been discussed is 236pu (t 1/2 = 2.15 yrs).
Based upon the analysis
of prompt samples, this isotope is not present in sufficient quantities to contribute significantly to
total Pu alpha activity.
Among the isotopes of neptunium (Np), only 237 Np (t)/g= 2.1 x 106 yrs) and the 236Np (tyjg = 12x
10° yrs) isomer are long-lived enough to be of interest. Neither isotope is present in quantities large
enough to contribute importantly to overall alpha activity either before or after the nuclear
explosion. Other Np isotopes are not important, although at early times one may observe very large
quantities of 239Np, a product of neutron capture reactions on 238U,
in debris samples.
Its
significance is that it decays by beta emission with a 2.35 d half4ife to 239Pu. In the debris from
U have been exposed to neutrons, the
from neutron capture reactions and subsequent decays of
contribution from 239Pu originally present in the device (Noshkin, 1974).
156
Pu resulting
2390 and 233Np can outweigh any
roe
nuclear explosives where larger amounts of