Multiplying the expression in the brackets in Equation (6) by the soil density (in g/em®*) leads to
the conversion factor
Np/(Sy/e)
normally given in units of
cps
pCi/g
For the Enewetak cleanup, surface contamination was defined as the average concentration within
the top 3 cm of soil. In general, the average concentration in the top z em, Ss? , for a source
distributed exponentially with depth is given by:
z
sf -+f Sj e°? dz =
°
v
{i-e77)
oz
(7)
Oo
Combining Equations (6) and (7) leads to the final expression for the conversion factor used at
Enewetak:
(Sy/p) _ (1-e°%) B
Np
in units of
oz
1/2
Ag Ps |
2
-1
R (8) tan 6 exp [-{h/p), Pp, h sec @] 46
ta)
at {u/p), p, sec 6
-
pCi/g
.
tees
cps , where B converts Y/sec to pCi for a specific isotope.
Results. In order to evaluate Equation 8, it was necessary first to determine A, and R (@) for each
detector which was used, in its normal field configuration. Aj was determined by placing a known
source directly below the detector at a distance great enough to simulate a parallel beam of photons
at the detector face. In determining Ag it is important to utilize the same method for determining
the net counts in the photopeak as that used in the field. A total of six detectors were calibrated for
the Enewetak program. Although two of these detectors were purchased for another program, all six
were used at one time or another during the course of the cleanup project.
Table 3-1 summarized
the initial 241Am results for these detectors. The detectors were periodically reecalibrated at
Enewetak to correct for efficiency changes which occurred during the course of the cleanup project.
R (6) was measured in detail for gamma ray energies between 60 keV and 2600 keV using detector
#386. The detector was mounted inside the container used at Enewetak. Measurements were made
with and without the Pb-Cd collimator.
Calibrated sources were placed at a fixed distance of 1 m
from the detector face at angles from 0° to 90° (0° being directly below the detector).
Measurements were made at 10° intervals except between 50° and 65° when the collimator was in
place, where 2° intervals were used. In order to account for any azimuthal asymmetries which might
exist in the detector, the source was rotated about the detector at a rate of 4 rpm during each
measurement. Figure 3-3 shows the results for 24. am. The R (6) data were fitted with a Fourier
series to the 10th order and folded into Equation (8) for derivation of the conversion factors.
Although these measurements were made in detail only for detector #386, the results were checked
for “47am using several other detectors: no significant difference was observed.
To evaluate Equation (8), it is necessary to obtain experimentally or make some assumptions on the
source depth distribution and certain properties of the soil. Table 3-2 gives results for 24l am with
the following parameters:
Tat
Photons per disintegration
Woe
TE
Effective area (A,)
Detector height (h)
Depth distribution (¢)
Soil density ( ps)
Wout
Air density ( pg)
Soil mass attenuation coefficient, (p/p )s
Air mass attenuation coefficient, (./p )g
90
0.359
19.0 eps/(y/em2 - sec)
800, 450, 100 em
0.33, 0.10, 0.05 em=]
2.0, 1.5, 1.0 g/ems
1.30 (107»; 1.15 (1073), 1.0 (1073) g/em3
0.333 em /e (for 60 keV gamma rays)
0.188 em2/g (for 60 keV gamma rays)