(Table 1 continued) 
Component Half -life 
A 53y Component 1 
Dee E,t 
8 ize Material tig (cm?/gm) (Mev) 
: ae Other metals 
D 25d Beryllium 2.5h 7.5% 10-5 “ets 
ae Cadmium 60h 5.2 xX 1074 0.40 
3, Copper 2.5h 1X 107-3 1.15 
ro) Lead 2.5h <1077 1.15 
8 1,170 hr in reactor Niobium 115d 6.6 X 1074 1.2 
@ Titanium 2.5h 4.7 X 1075-5 
£ Thorium 27d 7.4X10 0.3 
£ Ferro-tungsten 2.5h 1.4 107% 1.15 
= Zirconium 2.5h 2.8 X 1054 We15 
iu Concrete 
“2 107 Barytes 
= Concrete 2.5h 9.5 X 1055 125 
= Brookhaven 
S Cement 2.5h 9.5 X 10-4 1.15 
one Portland 
2 Concrete 2.5h 9.5 << 105 set 
$ Graphite 
a GBF 15h 2.7X10-8 2.07 
CS 15h 3.4X10°% 2.07 
C-18 15h 4.7 X 107? 2207 
BERYLLIUM 
From typical Be analyses it is deduced 
that the 80-day component is a combination 
of the activities induced in Zn, Fe and Ni. 
It is difficult to resolve this portion of the 
curve due to the large 5.3-year component 
from Co®, 
CADMIUM 
The 60-hour component, due to 58-hour 
Cd'!5 is less than 14 the calculated value and 
is likely due to a flux depression caused by 
the high cross section Cd!3. The 40-day 
component is likely 43-day Cd!15, The 250- 
day component is likely 250-day Zn®. 
COPPER 
Since more than 95 % of the photons from 
0 2000 4,000 
6000 
Decoy Period (hr) 
‘ ~ 
8,000 
ANALYSIS of decay curve for 347 stainless steel. The 2.5-hr component shown 
in Table 1 was determined by an expanded plot of the first 40 hr decay 
ee 
parent isotope atoms (atoms/gm), ¢ = 
isotopic activation cross section (cm?), 
 =.0.693/tis, ti, = half-life (sec), and 
t = irradiation time (sec). 
The activity, in disintegrations per 
second per gram is 
nv = goN(1 — e™®) 
and the specific activity a, in photons/ 
sec per gram, is 
a = ¢oNX(1 — e™) 
where X = photons per disintegration. 
It is convenient to define a macro- 
scopic absorption cross section, Zz, 
such that 2, = oNX cm?/gm of ma- 
terial. Then 
a = $2,(1 — e™) (2) 
Estimating Induced Activities 
Measurements were made on small 
76 
thin specimens so that no correction 
was required either for self-absorption 
or for depression of the neutron flux. 
One seldom has such ideal conditions in 
engineering applications. These cor- 
rections, which must not be neglected 
in the case of large thick specimens, will 
not be discussed here (for an idea of the 
correction, see references 4 and 5). 
Calculation. Where a period of de- 
cay ensues after reactor shutdown, the 
activity for each component radioac- 
tive element is given by 
A = $2,W(1 —e™)e™ = (8) 
where A = photons/sec, W = weight 
of specimen (gm), ¢, = irradiation time, 
t, = decay time, and ¢, 2z, and A are 
defined as previously. 
Conversion to r/hr. Since the prin- 
cipal use for induced-activity calcula- 
tions is for shielding, it is often desir- 
able to convert the gamma flux to 
roentgen/hr. The gamma flux (pho- 
tons/em?/sec) is related to r/hr by the 
absorption coefficient of air for the 
gamma radiation, the energy of the 
radiation, and an energy conversion 
factor 
r/hr = 6EKp (4) 
where @ = gamma flux (photons/em?/ 
sec), E = energy of the photon (Mev) 
K = conversion factor = 0.053 (r, em’, 
sec)/(Mev, hr) = 1.48 X 10-* r, cm?/ 
Mev X 3,600 sec/hr, and » = absorp- 
tion coefficient = about 3.5 X 107° 
cm~! for photon energies from 0.5 to 
2 Mev. 
E, from Table 1 can be used for the 
energy in this calculation. This is an 
