oil samples’
d sites
s in
nder such
easure£
various
nferred
ometer
walled
rticular
were of
western
nd birth
II.
CALIBRATION OF NEW 4" x 4" NaI(T1)
DETECTORS
In May of 1965, two new 4" x 4" cylindrical NaI({TL)
detectors for our field gamma spectrometric system were
obtained from Harshaw Chemical Co.
These detectors are
coupled to specially selected 3" photomultiplier tubes.
Each has a resolution better than 8.2% for the 0.66 MeV
137¢s total absorption peak.
with only thin
(.032")
Since the crystals are covered
aluminum windows, cylindrical bake-
lite shields 1/4" thick were fabricated to fit over the
detectors in the field.
These shields provide necessary
B-ray discrimination and also serve to thermally insulate
the crystals minimizing temperature gradient effects on
the crystal response during a measurement.
er of
mine the
rates.
urement
The method of calibration of photopeak areas, energy
bands, and total spectrum "energy" to obtain total and
individual component dose rates is described in HASL-150
(ref.
recision
ata
rom the
vy
cip are
Lation
ariation
section IV
Also
2x
Lbed
> its
in
11S
ita
; well
iamber
mment.
Fr pee oe
3).
We shall
thus only summarize the
results
of the
present calibration using the same notation as in HASL-150.
All the calibration resuits discussed here are for these
4" x 4" detectors with shields.
The calibration of the two
detectors differed by only about 2%, and thus only one set
of results is given.
Photopeak Area Method
In determining the dose rates from particular natural
or fallout gamma emitters in the soil the assumption is made
that the estimated areas of the total absorption peaks shown
in Figure 1 are proportional to the true peak areas and
therefore to the incoming primary flux at the detector.
The
final calibration factor (Np/I), which relates the estimated
peak counts in the field spectrum to the total dose rate for
a particular isotope, is the product of the ratios (N>/¢),
the ratio of estimated absorption peak counts to primary
flux for axial
factor;
rate.
and
incidence:
($/1I),
{Np/No),
the angular correction
the ratio of primary flux to total dose