3.2.5 System Calibration
Flux Calculation, The unscattered flux of gamma rays of energy E at a height h above a smooth
air-ground interface due to an emitter distributed in the soil is given by (see Figure 3-2):
> -| |
0
0
Agr?
ur
a)
dx dz
exp [-(h/p), Pg Tal exP (n/p), ps tgl ° 20 x
where
S,
= the activity per unit volume
.
r= rtr, (cm),
Y/sec
cm
,
2
(h/p),4. (4/p), = the air and soil mass attenuation coefficients (5 ) and
Da Ps = the air and soil density (g/em),
z = depthin soil below the surface
This expression assumes a source distribution which varies only with depth.
A uniform distribution in
the horizontal plane is assumed, which leads to results expressed in terms of an area average over
the field-of-view of the detector. For fallout activity subject only to environmental weathering, the
distribution after a period of time can be reasonably approximated by an exponential distribution
given by:
= §9—°%7
(2)
sy = the activity per unit volume at the surface (vee) and
cm
& = the reciprocal of the relaxation length (em=!).
Detector
h
Air
Soil
rs
Zz
.
x
Source
Element
FIGURE 3-2. GEOMETRY USED IN THE DERIVATION OF CONVERSION FACTORS RELATINGIN SITU
PHOTOPEAK COUNT RATE DATA TO SOURCE CONCENTRATION IN THE GROUND
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