The most direct means for separating the fall-out
y-radiation from the natural is by means of energy discrimination. Many of the more prominent y-energios
characteristic of the natural emitters lie between 1-1
and 2-6 MeV, while almost all important energies from
fission products are below 1-1 MeV. The properties of the
significant y-emitting fission products are given in Table 1.
The 1-6 MeV y-ray from lanthanum-140 is important in
the upper energy range; but this is prominent only within
a few months of production because of its short half-life.
As a result, relatively simple spectrometric techniques
have -proved valuable in providing detailed information
on the individual contributions to the total dose rate of
the significant natural and fall-out y-emitters.
In June 1962, several field measurements were made
with a 5-in. diameter x 3-in-high sodium iodide (Tl)
detector placed on a small wooden tripod at a height of
3 ft. above the ground. A 50-ft. cable connected this
essentially unshielded detector to a Nuclear Data 256-
channel pulse-height analyser mounted in the rear com-
partment of a Corvan truck and operated directly from the
12-V motor-car battery through a 300-W a.c.-d.c.
inverter.
20-min readings provided adequate statistics,
and two typical results are given in Fig. 1. The prominent
total absorption peaks at 0-5, 0-75, 1-46 and 2:62 MeV
are characteristic of the spectra obtained during 1962
and 1963. Ths most important peaks from the uranium
series, at 0-61, 1-12 and 1:76 MeV,are generally not con-
spicuous and, in the case of the 0-61- and 1-76-MeV peaks,
are usually hidden by more prominent neighbours.
Having determined that the field spectra obtained in
the manner described showed considerable detail, it
remained to find a means of inferring dose rates from the
spectra.
For the natural emitters, this has been accom-
plished using two related techniques. In the first method",
the areas under the total absorption peaks were simply
approximated by subtracting from the field data a
straight-line fit between the continua on either side of
each peak on semi-log paper as representative of the
Compton continuum under that peak. These results were
assumed to be proportional to the true peak areas and
therefore to the incoming primary flux at the detector in
the field situation. This assumption was tested satisfactorily in the laboratory for several y-energies and the
constants of proportionality between the measured areas
and the incident primary fluxes determined. Assuming
uniform source distributions in the ground half-space and
known decay schemes for potassium-40 and the uranium
and thorium series, the flux and angulardistribution of the
primary photons of the energies of interest and the total
dose rate per unit source at 3 ft. above the ground were
calculated::15, Since the response of the detector in terms
of peak counts per unit flux as a function of angle was
2
LG