- In addition to 241Am, one might consider two other isotopes of americium, 243m and 242Mam, as
potential sources of alpha activity. The
Am half-life is 7,380 years, which is 17 times greater
than for 241 4m. It is not an important component of americium activity in debris samples. There is
no appreciable production of 43 Am during the explosion; the only production mechanism is via
neutron capture (n, } reactions on 242py which is a minor constituent of plutonium.
In order to caleulate what 243Am alpha activity one might expect, it could be assumed that, in the
Pu described in Table 6-1, sufficient reactions occur to result in neutron capture by 10% of the
Pu and that the
24lbu abundance doesnot change; i.e., as much
24bpy is produced by capture as
is destroyed by fission. From these conditions the composition of an americium fraction after 20
years decay can be calculated. From an initial microgram of weapons-grade Pu, decay will produce
2.53 x 104
dpm 241 Am and neutron capture on 242Am and 242Pu will produce 9.24
dpm 243am,
which is about 0.04% of the total americium alpha activity. The great majority of the americium at
Enewetak will contain 243Am at an abundance close to 0.04%, and a conservative upper limit for
243.4 m alphaactivity is 1%. Similarly, the contribution of alpha activity from 242M Am (tyq = 152
years) is not important. There is no reasonable mechanism for significant production during the
explosion. Also, its large neutron fission cross section leads to rapid destruction during the
explosion. None of the other americium isotopes is long-lived enough to be considered,
After americium, the next heaviest element (Z = 96) is eurium (Cm). One ean detect 242cm alpha
activity in "prompt" debris samples. Its origin is from neutron capture reactions on
Am present
in the plutonium fissile fuel at the time of explosion. Since the half-life of 244cm (ty /2 = 163 days)
is short relative to the time that has elapsed since the cessation of testing, there is no significant
amount of “42cm present in Enewetak debris samples now. A period of 22 years represents almost
50 half-lives; the amount of 242Cm remaining after 50 half-lives is 1 x 10715 of the original amount.
Heavier Cm isotopes, some of which have longer half-lives, are not detected in significant amounts
$09 do not add significantly to the sum of Pu and 24] an alpha activities. Whatever amounts of
9 45m were originally present have decayed to the
Pu daughter. Complete decay of the
Cm produces only a minor change in the amount of 238 pu in the debris.
Some aspects of the preceding analysis were based upon the idea that the fissile fuel in a low
efficiency nuclear explosive does not undergo large changes in isotopic content as a result of the
explosion. Thus, one can discuss the isotopic content of Pu found in the debris in terms of the
isotopie content of typical "weapons-grade" plutonium. On the other hand, in higher-efficiency
ra
devices, fission, neutron capture, and (n, 2n) reactions can cause appreciable changes in the isotopic
composition of the plutonium. Perhaps the most striking change can arise when 238y undergoes
neutron capture. At high enough neutron fluxes, successive capture reactions occur and one finds
$qgtributions to the Pu isotopic inventory from beta chains that originate with 239y, 2405, 24ly,
U, and so on up to rather heavy species, e.g., to atomic mass number 257. (Ghiorso, 1955; Hoff,
1978) At Enewetak the most extreme example of this effect was observed in the debris from the
Mike explosion, a high-yield test (10 megatons) conducted in November 1952. (Diamond, 1960) Since
scientists studying prompt samples from the Mike test were able to detect products up to mass 255
whose presence was ascribed to multiple neutron capture reactions occurring in
8U that had
experienced very high neutron exposure, the plutonium isotopic content of this debris was examined
The isotopic
The specific activity of this plutonium is 2.25 x 10° dpm per microgram. After 20 years decay,
1 yg of this plutonium will produce 1.26 x 10° dpm 241m from the beta decay of 241 py, Thus, even
for the Mike-debris plutonium, which is relatively rich in the higher mass isotopes, the contributions
of 242py and 244Pu to the total Pu plus 241 am alpha activity are extremely low.
157
ee
to see if the results were substantially different from the previous conclusions.
abundances observed in Mike-debris plutonium are listed in Table 6-3.