ns for separating the fall-out
ral is by means of energy dishe more prominent y-energies
Table 1.
IMPORTANT FALL-OUT y-EMITTERS
Isotope
2r
Half-life
65-0 d.
jural emitters lie between 1-1
IND
eR
35-4 d.
38-7 d.
1-1 MeV. The properties of the
OR
303.1
'4Sbt
60-1 d.
Cg
30 yr.
wha
40-2h
st all important energies from
om products are given in Table 1.
lanthanum-140 is important in
ut this is prominent only within
on because of its short half-life.
imple spectrometric techniques
providing detailed information
utions to the total dose rate of
d fall-out y-emitters.
field measurements were made
3-in-high sodium iodide (Tl)
Hi wooden tripod at a height of
A 60-ft. cable connected this
lector to a Nuclear Data 256yser mounted in the rear comk and operated directly from the
through a 300-W a.c.-d.c.
8 provided adequate statistics,
given in Fig. 1. The prominent
, 0-5, 0:75, 1-46 and 2-62 MeV
spectra obtained during 1962
ortant peaks from the uranium
‘76 MeV, are generally not conf the 0-61- and 1-76-MeV peaks,
re prominent neighbours.
t the field spectra obtained in
howed considerable detail, it
of inferring dose rates from the
emitters, this has been accom»chniques. In the first method!,
absorption peaks were simply
cting from the field data a
the continua on either side of
aper a8 representative of the
‘that peak. These results were
al to the true peak areas and
primary flux at the detector in
assumption was tested satis- for several y-energies and the
ty between the measured areas
fluxes determined. Assuming
18 in the ground half-space and
potassium-40 and the uranium
: and angular distribution of the
ergies of interest and the total
at 3 ft. above the ground were
»sponse of the detector in terms
ux as a function of angle was
2
Ha
y-Energy (MeV)
0-724
0-757
0-768
0-498
0-610
0-613
0-624
0-603
0-645
ce
2-09
0-662
ima d.
G-930
O0-325
0-488
0-815
y/d*
00-6
1-00
0-9
0-06
0-205
0-105
0-98
0-072
Si
0-063
0-94
O38
+25
0-2
0-4
0-2
1
.
1Ce
32:5 d.
0-145
0-70
1Ce
284-5 d.
0-134
0-105
*y-lines with y/d<0-05 are omitted; data from NAS-NRC Nuclear
Data Tables (to date).
+ Effective half-life is that for }°*Ru, or approximately 1 year.
¢ Not a fission product.
obtained from laboratory measurements with standard
radioactive sources, a simple calculation yielded the
calibration constants of the measured peak areas in
terms of dose rate. These results are given in Table 2.
A second method for determining natural radiation
dose rates utilizes a well-known energy band technique
(for example, refs. 16-18), where the total counts in the
spectrum between energy values that bracket significant
peaks are related to the dose-rate contribution from
the radiation that contributes to these peaks. Three bands
were therefore
peaks already
to include the
to include the
chosen to include the three total absorption
calibrated, that is, from 1-32 to 1-60 MeV
1-46-MeV **K peak, from 1:62 to 1-90 MeV
1-76-MeV 24Bi peak, and from 2-48 to 2-75
MeV to include the 2-62-MeV **T] peak.
The band
and peak methods have given essentially identical values
for the inferred natural radiation dose rates. The band
method has the advantage of providing greater precision
for individual measurements and being more easily amen-
able to routine data analysis, although it is sensitive to
the instrument gain drift and to our determinations of
the zero energy channel and energy per channel for the
field spectrum.
The energy-band approach has also been extended to
provide a measure of the total y-dose rate. The total
energy in the spectrum between 0-15 and 3-4 MeV has
been found to correlate very well with total y-dose rate
as measured by a high-pressure ionization chamber’.
Further, the calibration factor determined in the labora-
tory using a standardradium-226 sourceis consistent with
that inferred from the ionization chamber readings'.
The fall-out y-dose rate can be estimated by taking tho
3