28
TT
I
T
l
|
T
239
-
20 [-
=
o
22TH, PPM
24
0
|
|
|
L
|
L
|
uR/HR
Fig. 4—Rice University in situ *°Th soilcontent us. HASL energy-band
“2Th dose-rate determinations.
, the theoretical conversion from
222Th parts per million to microroentgens per hour.
terminations of soil concentration may not necessarily be representative of the average concentration in the volume seen by the spectrometer. The scatter of points seen in Fig. 3 reflects this fact as well as
the usual instrumental uncertainties.
Figure 4 shows soil concentrations of 7’Th plotted against the
HASL
”’Th dose-rate e«timates. The curve is again the theoretical
conversion, assuming uniform soil distribution. These soil concentrations were obtained in the field by J. A. S. Adams of Rice University,
using a portable scintillation spectrometer system.’ Both these and the
HASL measurements are zn sifu, and thus no systematic difference in
slope is observed between the line and the points. The results shown
in Figs. 3 and 4 indicate the high degree of reliability of our ‘°K and
*327h dose-rate estimates. Unfortunately, soil concentrations of ?*u
derived by assuming radioactive equilibrium in the decay series, e.g.,
by soil analyses when such equilibrium is often closely approximated,
are not directly comparable to our uranium-series dose-rate esti-
mates due to the reduction in dose rate caused by ***Rn emanation from
the soil in the field situation. The available *U results do indicate,
however, that our uranium-series dose-rate estimates are reasonable,
although not as precise as those for ‘°K and the thorium series.
SURVEY RESULTS
In Table 2 are listed the surveys undertaken by HASL teams during
the past three years. The indicated gamma dose-rate ranges include
80 to 90% of the spectrometer readings for each survey. Most of the