Memorandum from Raymond L. Murray and A. C. Menius to Dr. Clifford K. Beck
1 p.
September 5, 1951


September 5, 1951

TO: Clifford K. Beck
FROM: Raymond L. Murray and A. C. Menius
SUBJECT: Shim Widths
CC: A. W. Waltner, Newton Underwood, Leon H. Robinson

The latest Los Alamos cadmium shims are 3" wide, 18" long, and 1/32" thick,
and each of the two is worth 13.5 grams. It is clear from the sketches of the
side views of two reactors
that on the average the cadmium is much farther from the surface in their case.
It is proposed that 4" strip area 40 in.² be used here, which will correspond to
about 25 grams of U-235, giving a total of 50 grams for the two.

Some rough justification is given below:

It has been quoted by Las Alamos that the removal of a 4 x 4 graphite block
has a value of 5 grams or 5/16 grams/in², or for our plate 40(5/16) = 12.5 grams.

Since thermal neutrons escaping into the void can still return to the reactor
with a block removed, but cannot escape with a cadmium patch, the cadmium would
be at least twice as effective.