ORNL-1771 - Oak Ridge National Laboratory

ANP QUARTERLY PROGRESS REPORT
A161 SfATTERlNG OF NEUTRONS
In an effort to understand the air-scattered neutron
intensities which have been measured at the Tower
Shielding Facility, a number of calculations have
been carried out with a variety of conditions and
assumptions. The predominant difference between
the current work and that which WQS done pre-
viously4 is that the interference of the ground has
been taken into account. The ground Scattering is
calculated separately. In an attempt to obtain a
better fit to thc data, such aspects as anisotropy of
scattering and slant penetration at the detector tank
(crew shield) have been taken into account in some
of the work. Furthermore, measurements have been
made of the ongular distribution of the radiation
leaving the reactor shield, and these data have
been used i ~? some of the calculations.
Because the importance of the multiply scattered
neutrons has oppeared to be greater than was antici-
pated, considerable effort lins been made to extend
the calculations to beyond the singly scattered
case. Some results are reported far two scatterings,
and a general method has been developed for all
orders of scattering.
Single Anisotropic Air Scattering in the Presence
of the Ground (Shielded Detectcab
F. H. Murray
For a convenient machine cnlculation, the at-
tenuation in air WQS neglected. The detector was
placed with 10 cm of water between it and the face
of the crew shield which, when extended, contained
the reactor source. The beam from the source was
about an axis through the detector, in theory, and
contained teinis cos a and cos2 q c1 calculation
WQS made separately for the terms 1, cos 4 and
cos2 a in i~ie expansion of the source flux. If D is
the reactor-detector distance and h the height of
both above the g~ound, the flux at the detector is
expressed as the sum of the terms:
and
158
where
g(a + /3) = 1 + 0.3 cos (a t /3) I
/(a) = a, 1; b, cos a; c, cos2 a .
The angle + is the angle between the vertical
plane taken as the face of the crew compartment
and the plane triangle formed by the source, the
scattering volume, and the detector, The results
of this calculation5 will be published later.
Single !sotrapis Air Scattering in the Presence
of the Ground (Unshielded Detector)
J. E. Foulkner
A calculation was made of the reading of an iso-
tropic detector caused by first-scattered neutrons
in air as the distance above the ground was varied.
The following assumptions were made:
1. The source and detector have the same alti-
tude,
2. The ground is an infinite plane,
3. The source is isotropic.
4. The scattering in air is isotropic.
5. There is no energy degradation on air scatter-
Ing.
6. The attenuation is pure inverse square.
With these assumptions the reading on the detector
may be shown to he proportional to
1
D
where D is the separation of source and detector,
h is the altitude, and / may be expressed in closed
4See, for example, H. Goldstein, Chop. 2.9, p 831, in
Reactor Handbook, ed by J. F. Hogerton and R. C.
Grass, Technical Information Service, AEC, 1953.
5-
I-. H. Murruy, Single Anisotropic Air Scattering of
Neutrons in the Presence of the Ground (Shielded
Detector), 'ORNL CF-548-1041 (to be issued).
for O 5 4 5 n/2,