average energy that was determined by 
the decay scheme (3, 4) of the isotope 
or (if the isotope was not identified) 
was estimated by an absorption meas- 
urement. For greater accuracy one 
should, of course, make a separate cal- 
culation for each individual gamma-ray 
energy. In Table 2 the pertinent iso- 
topes are listed with their photon 
energies and yields (4). The number 
of photons per disintegration is also 
listed. 
Experimental 
Specimens were prepared by cutting 
chips or shavings small enough to have 
negligible self-absorption. Irradi- 
ations were made in the ORNL Graph- 
ite Reactor for periods from 150 to 
15,000 hr. 
Upon removal from the reactor, the 
decay of gamma activity was followed 
with a high-pressure ionization cham- 
ber (6) (Fig. 1) until the remaining 
activity had a half-life greater than a 
year. The decay of some materials 
was recorded for over 20,000 hr—most 
for at least 8,000 hr. Interest here is 
centered on the long-lived isotopes, and 
measurements were not begun until 3 
or 4 hr after reactor shutdown. 
To assist in establishing the average 
energy of the radiations, absorption 
measurements (1) were made during 
the decay period by interposing sheets 
of lead between the specimen and a 
G-M tube. 
Decay curves for the irradiated 
materials (1) were analyzed into com- 
ponent activities, as shown in the illus- 
tration. The identities of most of the 
component activities—and therefore 
their average energies—can be estab- 
lished from their half-lives and chemi- 
cal analysis of the materials. Activ- 
ity, a, was then calculated from the 
calibration of the ionization chamber 
(6). If the component was not identi- 
fied, the photon energy was estimated 
by absorption measurements. Theab- 
sorption cross section 2, was calculated 
using Eq. 2. (The reactor flux was 
taken from reference 7.) 
BIBLIOGRAPHY 
1. C. D. Bopp, ORNL-1371 (1953) 
2. C. D. Bopp, O. Sisman, ORNL-525 (1950) 
8, K. Way, et al., Circular 499 (National Bureau 
of Standards, U. 8. Department of Commerce, 
Washington, D. C., 1950) 
4. The Reactor Handbook, Vol. 1, Physics, 
AECD-3645 (1955) 
6, J. 8. Levin, D. J. Hughes, Nucieonics 11, 
No. 7, 8 (1953) 
6. J. W. Jones, R. T. Overman, MonC-399 
(1948), AECD-2367 
7. H. Jones, et al., CP-2602 (1945) 
78 
TABLE 2—Photon Yield and Energy for Isotopes of Table 4 
Disintegra- 
Photon tions yteld- 
Half- energies ing a photon 
Isotope life (Mev) (%) 
Na?4 14.8h 1.38 100 
2.75 100 
Sc4s 85d 0.88 100 
1.12 100 
Se48 44h 1.33 100 
0.98 100 
1.00 100 
Cr®1 26.5d 0.32 8 
Mn‘s 2.59h 0.84 100 
1.81 25 
2.13 15 
Co58 72d 0.81 ~100 
Fe? 46d 1.10 50 
1.30 50 
Co® 5. 3y 11 100 
1.33 100 
Nis 2.56h 1.01 14 
1.49 2? 
Cu* 12.9h 0.51 37 
Zn6 250d 1.12 45 
Zr*®-Nb% 65d t 0.75 100 
0.72 99 
0.23 1 
Zr%7 17h 0.75 100 
0.67 100 
Mo* 67h 0.14 10 
0.18 90 
0.37 ? 
0.74 10 
0.78 10 
Cd115 58h 0.34 58 
0.36 42 
0.52 42 
Cdus 43d 1.30 (2)? 
0.95 94 
0.48 12 
Ba!40-La 140 12.8dt 2.9 (6)? 
2.51 5 
1.60 95 
0.82 4 
0.52 51 
0.32 1l 
0.16 70 
0.09 5 
0.03 100 
Tals82 113d 1.13 37 
1.22 57 
W387 24h 0.69 48 
0.55 8 
0.48 25 
0.13 8 
0.21 25 
t i 
Pa233 27d 0.40 70 
0.08 30 
* Photons of very low energy are omitted. 
Photons* 
per 
disintegra- 
tion (X) 
2.00 
2.00 
3.00 
_ 
a 
+ Rate-determining half-life for the mother-daughter combination. 
Average 
energy, * 
Mev 
(E:) 
2.07 
1.00 
1.10 
0.40 
0.9 
0.9 
0.5 
0.30 
