Memorandum from Newton Underwood to Clifford Beck
1 p.
September 26, 1952

September 26, 1952

To Clifford Beck
From Newton Underwood
Subject - Ion Chambers

The basic specifications for the linear integrated ion
current systems which Dr. Waltner, myself, and the Reactor
Committee arrived at after much discussion and several meetings

1. At 10,000 watts, the neutron flux should produce a
current of approximately 100 micro-amperes. According to
computations of neutron flux at the location of the ion chambers
by Menius and Murray, this maybe translated as about 1 micro-
per 108 neutrons per cm² per sec.

2. The automatic controls should function at as low a power
level as feasible.

3. The calibration should be as stable as possible.

4. The power level safety trips should release the rods
within 20 milliseconds of a 20 to 30 percent overshoot of the
predetermined level.

The lower level limit of power at which the linear systems
will operate satisfactorily is determined by many factors. The
more important ones are.-

We have assumed that the input insulation resistance can be
maintained sufficiently high so that it is possible to use a
maximum input resistance in the amplifier of 109 ohms. In this
case the smallest current that will give full scale deflection is
10-8 amperes. In accord with the previous assumptions this will
correspond to a reactor power of 1 watt.

It is evident that the current in the ionization chamber
caused by residual activity must be negligible in comparison to
10-8 amperes. This implies a very careful choice of constructional
material (Carbon, Magnesium, and lead, and quartz insulation)
and the use of Boron 10 instead of U235 as the surface coating
for the electrodes. The [alpha] current from 1 gm of U235 (about 2
micro curies) spread over 1000 cm² would theoretically be of the
order of 10-9 amperes. Also gas must flow through the chamber to
sweep out activated atmosphere.

If for any reason the ion current obtained when the reactor
is operating at 10 KW should turn out to be less than 100 micro-
then the above numbers would have to be modified according.