O J E C T PROGRESS R E P O R T
distance t o f
64 ft. The functional dependence of
t , and should be accounted for i n red dose rate calculations. In the present eriments a range of values of t i s covered for a ingle separation distance t a n d for a single value f p; however, some data were obtained with p changed from 45 to 15 cm, and the effect of t h i s he dose scattering probability i s disTo illustrate the use of the dose probability for a conical shell beam in the prediction of scattered dose rate, assume that the angular distribution of the dose rate emission from a primary reactor shield i s obtained by dose rate measurements a t a distance d from the reactor. This dose rate distribution obtained a t distance d is denoted as Di(t9,d). Then DJ8,d)d2 i s the dose rate which would be obtained on the surface of a unit sphere drawn about the equivalent point source. The scattered dose rate i s obtained by weighting D i ( 8 , d ) d 2 by the scattering probability p"(6) determined from the differential experiments and integrating with respect t o p = cos 8 (first integral i n Fig. 14.21). The second integral i n Fig. 14.21, which i s equivalent to the first, i s i n terms o f the t 2 P s ( 8 ) used in the plots.
.
Effects of Direct-Beam Collimation
~
Figure 14.22 shows the effect of beam collimation on the dose scattering probability averaged over the beam. These results were obtained from single air-scattering calculations. The beam i n the TSF exper iments had approximately a cos4 distribution. The cos8 and cos16 distributions represent an increasing degree of collimation of the beam. It must be remembered that i n the interpretation of the TSF experiments the assumption was made that the dose scattering probability averaged over the TSF experimental beam was equal t o the dose scattering probability for a differential beam having an angle 8 equal t o the angle of the axis o f symmetry of the experimental beam. These results indiate that t h i s assumption i s v a l i d within 10 t o 15% beam angles greater than 30 deg. For beam les between 0 t o 30 deg the averaging of the e scattering probability i s quite sensitive t o beam collimation, and therefore the experimental results obtained are not v a l i d if any sharp wellfined beams are emitted from the reactor shield ge of beam angles. Consideration of cattered neutrons in the calculations tended t o reduce the effect of beam
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Fig. 14.22.
Effect of Beam Collimation on Scattering Function Based on Single-Scatter Calculation.
collimation shown on t h i s figure because the sources of multiply scattered neutrons are of necessity more diffuse than the sources of singly scattered neutrons. Effect of Neutron Energy Spectrum
A small amount of data was also obtained for a water thickness of 15 cm a t the reactor t o obtain an indication of the effect of change i n neutron spectrum on the dose scattering probability. Figure 14.23 presents the angular distribution of the dose rate a t the rear o f the detector tank for p = 15 cm of water. Again, t o obtain the directbeam dose rate, it i s necessary t o subtract out the scattered dose rate. The procedure described for the p = 45 cm case was used, but the results are The calculated directnot nearly so satisfying. beam dose rate has a flatter distribution than the measured direct plus scattered dose rate in the region of 8 = 0 t o 45 deg, where the scattered dose rate should be a relatively small contribution. For all the data obtained a t p = 45 cm, both for the direct-beam and scattered dose rates, a plot of the dose rate vs the beam angle always showed a s l i g h t d i p i n the curve i n the region of 6 = 30 deg. This diD has been attributed to an out-of-roundness
II