GAMMA-EMISSION DATA FOR THE CALCULATION OF EXPOSURE RATES FROM NUCLEAR DEBRIS. VOLUME I. FISSION PRODUCTS Page: 85 of 96
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state Ge75 decays feed this level, we find 0.0041 x 0.33 = 0.001 gamma
photons of 0.628 Mev emitted per disintegration. A similar calculation
yields 0.003 gammas of 0.427 Mev per disintegration.
Actually, it is generally better to normalize on the basis of a
more abundant ray when possible. In the case under consideration, we
note that the second level in As75 is fed exclusively (in this decay)
by the 0.92 beta ray (11.4 % of the Ge75 decays), and the level is de-
populated by the 0.265 and 0.066 gammas. The relative abundances of
these two rays are about 1000 and 25, respectively, and both are con-
verted. The relative gamma abundances do not accurately represent the
relative transition probabilities, since the two transitions are con-
verted to different extents. Each relative abundance must be multiplied
by the factor (1+4O) to convert it to a relative transition probability
25 x (1+0.4) =35
1000 x (1 + 0.02) = 1020
Hence, about 3 6 of the atoms decaying to this level de-excite by the
0.066 transition while 97 % make the 0.265 transition. These figures,
along with the 11.4. % abundance figure for feeding the level, yield the
absolute numbers of each transition per disintegration. These numbers
are then distributed between gamma photons and X-ray photons according
to the values of a.
For more complicated decay schemes, the calculation may become con-
siderably more cumbersome, but the principle remains the same; i.e.,
the number feeding a level must equal the number depopulating it.
Needless to say, both beta branching percentages and relative abundances
of the gamma rays are required for the calculation, although very weak
rays can frequently be neglected. Failure to account for conversion
can sometimes strongly affect the results. It should be emphasized that
the whole normalization procedure depends upon the availability of a
logical decay scheme for the nuclei involved. The mass-chain 75 exam-
ples just discussed are fairly typical of the kind of calculations re-
quired to obtain gamma photon abundances from the beta decay information
in the literature. The Nuclear Data Sheets currently being published
(the compilation is continuously revised and updated) often include
absolute transition probabilities marked on the decay-scheme diagram,
as is the case in Figure 1. However, many of the older sheets give
only relative gamma abundances.77
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Crocker, G.R. & Connors, M.A. GAMMA-EMISSION DATA FOR THE CALCULATION OF EXPOSURE RATES FROM NUCLEAR DEBRIS. VOLUME I. FISSION PRODUCTS, report, June 10, 1965; San Francisco, California. (https://digital.library.unt.edu/ark:/67531/metadc1029861/m1/85/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.