Once the conversion factor relating the net photopeak counts,
Np,
.
;
oa
.
to the activity per unit volume at the surface, sy is obtained for a
source distribution, Eqs. 5 and 6 can be used to arrive at the corre-
sponding conversion factor for the total activity per unit area S,, A? and
for the average activity per unit volume in the top Z centimeters, sé,
By dividing the 54 by the soil density, in ¢ per cm’, the results can be
expressed in units of activity per unit mass.
Table
] shows
the conversion
factors
for 137¢¢ obtained
Enewetak system for several different depth distributions.
for
the
Also shown
in the last column are the corresponding conversion factors for total
external exposure rate, in
R/h, at the 1 meter level.
These results
were obtained directly from the total activity per unit area conversion
factors using data given by Beck, et al.394
Various assumptions must be made to derive these conversion
factors.4:3
distribution.
The most Significant assumption is made for the depth
In general, it is very desirable to perform field meas-
urements to establish the source distribution with depth, and thus, also
allow for a direct measurement of the soil density.
In @ situation
where the depth distribution varies significantly from point to point
within 2 given area, as on many islands at Enewetak,
obtain, or assume,
an.average depth distribution.
islands at Enewetak,
it is necessary to
For the northern
previous data (1) indicate that the average depth
. distribution for !3’cs has a relaxation length on the order of 10 to
15 cm.
In using the data given in Table 1, a reasonable first approach
would be to take the average of the values given for a 10 cm and for a
15 cm relaxation length.
More precise data can be obtained for any
Ziven area if the depth distribution is better known.
- 8-
ME So bed