ATOMIC ENERGY COMMISSION
" A PORTABLE RADIATION INSTRUMENT
C. O. Ballou
Published for use within the Atomic Energy Commission. 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,
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.
Date of Manuscript: September 22,
Document Declassified: May 19,
This document consists of 10 pages.
A portable beta and gamma radiation meter of exceedingly small dimensions and weight
has been developed. User acceptance has been more enthusiastic than any previous instru-
ment of its type.
using one Victoreen V-32 tube,
is the simplest electronic circuit possible
radiation work and gives high sensitivity.
Stability exceeds anything of comparable sensitivity which has come to our attention.
e short term stability is due to a circuit which prevents emission before the cathode
Lches operating temperature. Long term stability has been improved by evacuating the
enclosure and switch.
one unit instrument, weighs four pounds two ounces, and is carried with
. a pistol grip. Exclusive of chamber and handle, its dimensions are 3"
wide, 6" long, and
The case is formed of aluminum and is designed to give excellent visibility of the meter.
Three ranges of approximately 50,
500 and 5000 mr/hr have been incorporated in the
The instrument has been named "Cutie Pie" due to its diminutive size.
Digitized by the Internet Archive
in 2011 with funding from
University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation
" A PORTABLE RADIATION INSTRUMENT
Instability and failure to hold calibration have given portable electronic radiation survey
instruments an undersirable reputation with field users of this type of equipment.
Opinions obtained from field users of such instruments have resulted in the following
tabulation of what a satisfactory instrument should be:
Ability to zero the instrument at all times regardless of humidity conditions.
Humidity conditions at Clinton run at saturation during the night. Depending
upon weather conditions, it may remain at saturation during the day or may
drop as low as 30%.
During summer periods instruments have shown particu-
larly bad characteristics due to this severe humidity shift.
2. Inasmuch as each instrument is radiation calibrated, linearity of response is not
a major consideration, although it is desirable.
The instrument must dependably hold calibration within at least 10%.
which can be conveniently read, should be from 10 to 25 mr/hr
up to about 5000 mr/hr.
The instrument must be free from spurious response.
That is: the tube and
its associated grid circuit should not collect ions or electrons when the instru-
ment is subjected to radiation.
The instrument should be of one unit construction and have a chamber not
exceeding three or three and one-half inches in diameter.
The instrument must be reasonably free from instability or short time drift
after a short warm-up period.
The instrument about to be described has fulfilled all the above requirements with the
exception of paragraph three.
effect of humidity and battery voltage changes
are, after two months operation, not fully known.
II CIRCUIT THEORY
If only one tube could be used as a current amplifier, a marked reduction in battery
supply requirements would result.
Provided that the circuit and tube were reasonably
, such an arrangement appeared unusually attractive.
The Victoreen V-32 triodes are rated at 140 micro-mhos conductance, and 1014 ohms
cold input resistance.
An experimental set-up was made and a realizable gain of 80 micro-mhos resulted. An
increment of .25 volt grid potential gave 20 microamperes plate current increment with an
1800 ohm microammeter in the plate circuit.
20 x 10-6
= 80 x 10-6 mhos
Electrode potentials were the same as those given on the appended circuit diagram.
After numerous changes, the circuit given was established as the simplest possible and
incorporated the following desirable features:
It establishes a constant plate current. In this case it is approximately 125
microamperes. The Gm of the tube remains more constant with this arrange-
ment than with fixed grid voltage and variable plate current as is common to
other zero setting circuits.
With the elimination of variable leakage paths,
the required balancing range
has been restricted to such an extent that any marked drop of battery voltages
prevents zero setting.
Thus, the tube must work within very restricted con-
editions and the Gm should remain quite constant.
2. The switch, connected in the zero setting and grid bias circuit at the filament
end, imparts unusual benefits.
When the amplifier is turned on, plate and cathode potentials are simultaneously applied
to the tube.
With normal operating grid potentials, the tube operates under emission limited
conditions until the cathode reaches normal temperature.
This has been shown to be the
cause of de-stabilization of the cathode and resultant amplifier instability.1
However, with this circuit when the amplifier is off, the grid is at -7 volts potential
due to the location of the balancing current switch.
Immediately after turning the amplifier
on, the grid remains at comparatively high bias due to the time constant of the grid resistor
Victoreen Instrument Co. Technical report No.
"Stabilization of tubes having
oxide coated cathodes."
and the distributed capacity of the grid circuit. This space charge limits the space current
until the cathode is at operating temperature. The tube has a mu of 1.75. Consequently,
with 7T volts applied to the plate, the plate current is cut-off until the grid drifts to -4.3
volts, which requires several seconds with a 1011 grid resistor.
mI GRID CURRENT AND LEAKAGE
After completion and satisfactory tests, four instruments were placed in field use. Im-
mediately after being placed in service a protracted period of high humidity was experienced
which caused instability and inability to zero set the instruments. They further displayed
spurious response, which was of sufficient magnitude as to make readings taken by the in-
strument questionable. The first four instruments made did not utilize the vacuum compart-
ment to be described later. Instead, the tube, input resistors and switch were placed in a
shield which was carried at bias potential.
By drying the instruments with heat, the instability and zero setting characteristics were
restored to normal. The spurious response remained, however.
The spurious response,
Considerable time spent on
been vexing for sometime.
leakage, instead of being 10
this leakage existed between
which had been observed
this problem yielded res
A check of all component
14 ohms, was in extreme
n the grid and plate.
on previous instruments, was annoying.
ults which answered problems that had
s used, finally revealed that the tube
cases less than 1011 ohms. Most of
The net result was, that due to this grid to plate leakage, the potential of the grid was
made more positive than the bias voltage, by an amount which was proportional to the grid
resistor and leakage resistance. This permitted electron collection of ionized air by the
Inasmuch as the troubles above referred to are common to practically all electronic
instruments of this type, methods for eliminating these troubles were sought.
IV VACUUM COMPARTMENT
The use of desiccant was attempted and was satisfactory for about four days, at which
time the desiccant became saturated. The replacement of the desiccant at such short in-
tervals was out of the question. The only practical approach to the leakage problem appear-
ed to be enclosing the circuit in a vacuum.
Inasmuch as the cabinet and chamber design were received with more enthusiasm than
anything previously offered, it seemed advisable to design a vacuum compartment which
could be installed in the already designed cabinet.
The greatest stumbling block to such design was a vacuum seal for the rotating switch
shaft in such space as was available.
Suggestions offered by W. A. Adcock led to the present design, in which one end of a
short length of neoprene tubing was secured vacuum tight to the rotating shaft.
end of the tube was secured vacuum tight to a bushing which communicated with the vacuum
Thus, the neoprene tube was flexed in torsion and a vacuum tight seal effected.
The enclosure of the circuit in a vacuum has accomplished the following two results:
It has eliminated variations in electrical leakage which have in the past con-
tributed to unsatisfactory performance.
It has eliminated spurious response due to ionization of air in a chamber,
composed of an enclosure and any circuit elements which might be at po-
tentials different from the enclosure, and associated with the input circuit.
Although the enclosure is 54 volts positive with respect to the grid, a 10 mg Ra source
placed directly against the enclosure produces no reading on the meter with a vacuum of
Hg. A curve of pressure vs. instrument reading is linear,
giving 20 microamperes de-
flection at 16 inches vacuum with the 10 mg source.
a reliable check of vacuum can
be made by switching the input switch to zero set and exposing the instrument to a strong
Provisions have been made for pumping the chamber down after air leakage has occurred.
The valve which is of very simple construction is so placed that a vacuum line can be con-
nected to it through a 3/8"hole in the bottom of the cabinet.
is such that only a moderate vacuum can be obtained. Cons
The construction of this valve
equently, the unit should not be
connected to a system in which high vacuums are intended. Lubricating the valve screw
with heavy Apezion or Silicone grease is required'to obtain 29
after the valve
is closed, it is vacuum tight.
V CHAMBER AND SENSITIVITY
A 2" meter can be read with some degree of confidence at 1/4
was decided that the instrument, to fulfill the requirement of paragraph four, should read
between 50 and 100 mr/hr full-scale.
The maximum value of grid resistance, unless ex-
cessive time constants can be tolerated, is 1011 ohms.
Chamber design must therefore be such that approximately 4
duced with 50 mr/hr radiation.
x 10-12 amperes are pro-
current from an ion chamber is expressed by:
V = Volume in c.c.
per 8 lurs.
With the above given values,
the volume of
the chamber required is 864 c.c. for 50 mr hr.
Chamber dimensions of 3
selected and gives
of 662 c.c.
volume is less
than the computed value.
However, the Dentalab re
used'ran from 1.1 to 1.4
x 10l1 ohms
so the sensitivity was
The chamber has a .001
nylon window in the front for beta measurements.
can be covered with a hinged 1
for gamma measurements in the pre
With 54 volts
applied to the chamber, lack o[ saturation begins
to appear at
Future instruments will be equipped with
These batteries will
the new Type 412
permit 90 volts of chamber
present 45 volt
hearing aid batteries.
The B battery has a usefu
life of well over 600 hours.
and the A battery over 100 hours.
Stability is many,
many times better than any previous
enclosing the circuit in a vacuum the drift after 16 hours
was more than 20 microamperes.
After vacuum enclosure, the 16 hour drift is only 3:'4 microampere.
pliiler on, a slow but steady drift occurs.
With new batter
age this time increases.
sensitivity at full
le is between 40 and 70 mr 'hr up to 5,000 mr* hr and is
dependent upon the grid re
Calibration is not linear
, but does not deviate from linearity
sufficiently to prevent
No guard circuit has been used on the Kovar or Amphenol 93-C connector.
If these crit-
ical areas are thoroughly and completely cleaned, leakage sufficient to affect the instrument
Leakage trouble in previous instruments has no doubt occurred in areas which
have not or cannot be thoroughly cleaned after assembling the parts into a unit.
Acknowledgments are hereby made to G. W.
design and also to D.
Parker for suggestions leading to the case
R. Luster for chamber design, and to W. A. Adcock for suggesting the
vacuum seal used on the switch shaft.
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