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In order to determine the magnitude of the gammaray
dose rate produced by streaming through the
duct, it was necessary to know the dose transmitted
through the lead shield, as well as the effect of
the background produced by adjacent experimental
facilities. Measurements of the gamma-ray dose
rates were therefore made beyond a solid lead
shield in the z,y plane of the source at a distance
z above the top of the lead (see Fig. 5.3.1). The
experimental data (Fig. 5.3.2) are presented in
terms of the ratio of the dose rate with the duct in
position to the dose rate with the duct replaced by
a lead plug as a function of distance from the
duct center line. The ratio of the dose rate beyond
a water-filled hole to that beyond the solid lead is
also shown in Fig. 5.3.2.
The actual ART duct mockup will consist of two
concentric pipes. The inner one, which will contain
the NaK, is not present in these straight-duct
mockups, but it would reduce the dose by about
30%. It should also be pointed out that the lnconel
pressure shell is not part of the mockup shield. If
it were, the dose-rate ratios would-be higher by a
factor of approximately 2 for a 1-in.-thick pressure
she1 I.
A ratio of the dose rate with the aluminum-filled
pipe to that with the air-filled pipe was 0.86 on the
center line, which is in reasonable agreement with
a calculated value. In the calculation the average
energy of the gamma rays was assumed to be 3.5
Mev from the composite source. The ratio was
determined from the following relation:
- D1 ,. = B(p)e-w ,
where
u2
D, = dose rate with pipefilled with aluminum
turnings (with tungsten collar),
D2 = dose rate through empty pipe (no collar),
x t length of duct (12.8 cm),
p = aluminum linear absorption coefficient
= PmPl
pm = mass absorption coefficient (0.034) for
a rays in aluminum,
minum = 0.85 g/cm3,
B(P) = dose rate buildup factor (1.2).
Thus
n
- "l = 0.83
D2
PERIOD ENDlNC JUNE 70, 7956
o x TRAVERSE, z =
A y TRAVERSE, c = 10 5 cm
nemt
ORNL-LR-OW 44896
MINUM-FILLED DUCTS
TRAVERSE,TUNGSTEN COL
RLlNE MEASUREMENT NO COLLAR I = (0.5
-60 40 -20 0 20 40 60
x OR y, DISTANCE FROM DUCT CENTERLINE (cm)
Fig. 5.3.2. Gamma-Ray Dose Rates Beyond
Straight-Thrsugh ART North-Head Duct Mockups
Filled with Air or Aluminum.
as compared with the experimental value of 0.86
mentioned above.
It may be seen in Fig. 5.3.2 that the tungsten
collar offers little attenuation to the dose along
the center line of the pipe. However, the ratio of
the dose rates with and without the tungsten collar
and aluminum turnings, averaged over the emitted
beam, was 1.76. This ratio indicates the effective-
ness of the tungsten collar in this particular con-
figuration only.
In the ART shield design the lead shield is fol-
lowed by 31.5 in. (80.0 cm) of water. The vcater
attenuation of the gamma-ray beam emerging from
the duct mockup (with aluminum turnings and tung-
sten collar) along the z axis is shown in Fig.
275