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Published for use within the Atomic Energy Con-mission. Inquiries for additional copies
and any questions regarding reproduction by recipients of this document may be referred
to the Technical Information Division, Atomic Energy Commission, P. O. Box E, Oak Ridge,
Inasmuch as a declassified document may differ materially from the original classified
document by reason of deletions necessary to accomplish declassification, this copy does
not constitute authority for declassification of classified copies of a similar document which
may bear the same title and authors.
S Date of Manuscrit: December 10, 1945
SDocument Declassified: June 24, 1947
This document consists of 5 pages.
ATOMIC ENERGY COMMISSION
PORTABLE PROBE TYPE IONIZATION METER, MARK II, MODEL 10
G. R. Carlson
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PORTABLE PROBE TYPE IONIZATION METER, MARK I, MODEL 10
By G. R. Carlson
A request was made by Mr. J. E. Rose of the Health Physics section at the Metallurgical
Laborator., or a probe type portable radiation meter that could be used for the localization
of small sources and beams of gamma and beta radiation. One such application would be the
localization of the gamma and beta activity in a chemical separation or preparation carried
out in a glass vacuum system. To be able thus to measure radiation intensity at essentially
a point, it was decided to use a small chamber of 25 cc volume with full scale sensitivities
ranging by factors of ten, from 0.2 r/hr on the most sensitive scale to 200 r/hr on the least
The complete instrument consists then of a probe connected to a battery box by means
of a cable. The probe consists of the ion chamber and the circuit box so that range switch-
ing is done at the probe. The battery box contains the batteries, the meter, and the circuit
components other than the input tube. The on-off switch, zero set, calibration adjustment,
and the feedback controls are on the battery box. The voltage sensitivity of the most sen-
sitive circuit available to be adapted for this use is about 1/4 volt full scale, so that in
order to make sensitivity as high as 0.2 r/hr it was necessary to use an input resistor of
1012 ohms for the most sensitive scale.
A circuit box was built to contain the input tube (Victoreen V-32), a molded polystyrene
switch (used in "Zeus", CP-2452), and the high resistors. The high resistors were connected
in series to the grid of the tube, and the switch connected the chamber to the various junctions
between resistors. A feedback voltage was applied to the chamber in order to speed up the
circuit (see "Zeuto", CP-3168).
This system worked well for betas, but there was a large amount of ion collection in the
circuit box in a gamma radiation field.
It was immediately noted that the major source of spurious ion current was due to an ion
collection because of the difference in potential between the grid and the circuit box. The
feedback voltage was then connected to the circuit box in addition to the chamber, and the
d-c voltage of the circuit box was made adjustable. This made it possible to maintain zero
voltage between the grid and the circuit box.
The above emphasized a second source of undesired current. There is a large voltage
drop across the largest of the input resistors (i.e., 1/4 to 1 volt due to grid current) when
a V-32 tube is used as the input tube. In a strong radiation field this gave rise to a consider-
able current. This collection took place primarily betw-.n the switch arm and the grid.
The effect can be made negligible by switching in the input resistors independently in-
stead of having them in series. This means though that the bias on the input tube has to be
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switched at the same time that the sensitivity is changed. This, of course, complicates the
switching and the zero calibration. One such circuit was built and tested but it was found
to be inconvenient.
To avoid this complication, the circuit was redesigned to use the electrometer tube
(VE-124) which has a negligible grid current. The voltage across a 1012 resistor result-
ing from grid current is less than 1 per cent of that corresponding to full scale, so that
any collection across the resistor would change the bias by a negligible amount. The resis-
tors were no longer connected in series. This made the circuit faster on the less sensitive
scales. This circuit also made the adjustments of feedback and sensitivity independent.
It was decided finally that the more popular Zeuto circuit with the series high resis-
tors should be used. With this circuit the circuit box voltage control does not eliminate
spurious ion current collection, so the switch and circuit box were radically redesigned by
Dr. F. R. Shonka. The present switch has a negligible air volume for ion production (about
0.1 c.c.) and the resistors and tube are embedded in ceresin wax, so that there is no longer
any concern about ion collection between elements within the circuit box because of voltage
differences. Therefore, it is no longer necessary to adjust the d-c voltage of the circuit
box. The feedback to the circuit box was retained to compensate for the increased dielectric
constant of the wax.
The Zeuto circuit requires that the switch be made a non-shorting type, because a short-
ing type switch short circuits the voltage across the high resistors during the switching -
operation. This throws into the circuit a pulse which takes a long time to decay. The switch-
ing action is quite smooth except when going from the 2 r/hr scale to the 0.2 r/hr scale
when a pulse is introduced which takes a few seconds to decay (10 to 30). This may be attri-
butable to the change in voltage occurring across the chamber which must decay through
the 1012 resistor. Going in the other direction, the pulse is negligible, possibly because
the voltage change can decay across the 1012 ohm resistor.
Procedure for calibration of instrument is as follows: with the feedback control set at
minimum (counter-clockwise in the production model), the probe is placed in a uniform
known radiation field. The probe switch is rotated to the set position and the instrument is
set to zero by means of the zero-set control on the front of the battery box. It must be kept
in mind that the circuit is very sluggish with the feedback control at minimum. The instru-
ment should remain at zero for at least ten minutes before it can be assumed that the zero
is set properly. The probe switch may then be rotated to the appropriate scale until the
meter reads properly. This is also a "slow" adjustment. After this adjustment is properly
made, the circuit may be speeded up by adjustment of the feedback control. It is necessary
to go through this procedure every time the instrument is calibrated, because the sensitivity
control affects the feedback adjustment.
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