48
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
THEORETICAL CALCULATIONS OF THE GAMMA RADIATION SPECTRUM, ETC.
49
Decay Scheme of wl?
'
‘
Energy
Relative
1)
2)
10.816
9.156
1.0
0.09
5
Jol
Photon
B,(Mev)
Number n,
Te
8.278
3)
(6)
(8)
(9)
=
15
;
\
eg
0.90
o| @
5.287
iy 485
2.3
0.8
tit
To
mmBy = 468.46 Mev
Ein,
.
-——
6.318
5.554
(7)
0.19
1.5
En, = 7.53
4
4
5.2%, excluding cascade
photons (6) and (8).
+
- ft 4
ct
q
4
|
i
|
4
Figure 9.-Decay scheme of excited N*.
E(Mee)
Fraure 11.—Initial gamma differential dose spectrum at 1,500 yards,
2. The second point of interest is that the
spectrum at 1,500 yards is harder than the
spectrum at 1,000 yards: That is, its most
energetic components are relatively more
prominent.
Previously it was shown that
with moenoenergetic gamma point sources,
spectra become “‘softer’’ or less energetic
with increasing distance, due to the generation of secondary scattered photons of lower
energy than the unattenuated source photons.
Consequently, one must. conclude that with
the polyencrgetic bomb source spectrum,
filtration of low energy components occurs
more rapidly with increasing distance in
this range than does degradation of the more
energetic source photons. This would give
rise to a net “hardening” of the spectrum
with increasing distance.
At 3,000 yards the “hardening” effect of
Fravre 10,—Initial gamma differential dose spectrum at 1,000 yards,
increasing distance is even more apparent, ag
is seen in Figure 12. In this spectrum thesingle
most prominent dose contribution is made by
the very most energetic constituent; namely,
the 10.8 Mev gammaphotons from the nitrogen
radioactive capture sources.
The filtering effect of distance on the very
energetic gamma spectra resulting from initial
bomb gammaradiation is emphasized in Figure
13. Here there are presented simultaneously the
integral dose spectra corresponding to the
differential dose spectra just reviewed. [tf is
seen thal with increasing distance there is a
decreasing fraction of total dose contributed by
photons less than-any one given energy. For
example: at 1,000 yards about 73 percent of the
total dose is delivered by photons of 5 Mev or
less, but at 3,000 yards such photons contribute
less than one-half of the total dose.
In summary, this theoretical calculation of
extremely hard initial gamma radiations is of