Description of and instructions for operation and maintenance of a new model gamma ray pocket survey meter

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Material Information

Title:
Description of and instructions for operation and maintenance of a new model gamma ray pocket survey meter
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
10 p. : ill. ; 27 cm.
Language:
English
Creator:
Landsverk, O. G
Argonne National Laboratory
U.S. Atomic Energy Commission
Publisher:
Atomic Energy Commission
Place of Publication:
Oak Ridge, Tenn
Publication Date:

Subjects

Subjects / Keywords:
Gamma ray detectors   ( lcsh )
Nuclear counters -- Handbooks, manuals, etc   ( lcsh )
Genre:
federal government publication   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Restriction:
Date Declassified: May 12, 1947
Statement of Responsibility:
O.G. Landsverk.
General Note:
Manhattan District Declassified Code
General Note:
Date of Manuscript: August 22, 1945

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005023611
oclc - 465267914
System ID:
AA00009294:00001


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MDDC 952






UNITED STATES
ATOMIC ENERGY COMMISSION
OAK RIDGE
TENNESSEE






DESCRIPTION OF AND INSTRUCTIONS FOR OPERATION AND MAINTENANCE
OF A NEW MODEL GAMMA RAY POCKET SURVEY METER

by

O. G. Landsverk


Argonne National Laboratory


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,
Tennessee.

Inasmuch as a declassified document may diner materially from the original classified
document by reason of deletions necessary to accomplish declassification, this copy dpes
-o not constitute authority for declassification of classified copies of a similar document which
imay bear the same title and authors.


Date of Mahuscript: August 22, 1945

D document Declassified: May 12, 1947

This document consists of 10 pages.

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DESCRIPTION OF AND INSTRUCTIONS FOR OPERATION AND MAINTENANCE
OF A NEW MODEL GAMMA RAY POCKET SURVEY METER

SBy O. G. Landsverk


A quartz fibre type of instrument has recently been developed which is light in weight, of
convenient pocket size and very rugged. Because of its ability to stand rough handling it
is particularly adapted to field work. However, it will do accurate work if it is carefully
calibrated and used with a stop watch. It has an almost linear scale and will measure rates
of radiation up to ten roentgens per hour with no observable loss of collection. Five units
have been completed and tested. They have proved very satisfactory.

DESCRIPTION

The case is made of aluminum tubing of one thirty-second of an inch thickness. This is
shaped as shown in the complete assembly 1 of Figure 1. Aluminum plates 2 and 18 are
fitted into the top and bottom ends of the tube. The dimensions are six by two and fifteen-
sixteenths by one and one-quarter inches. The weight of the completed instrument is only
eleven and one-half ounces.

Mounted on the lower end of plate 2, Figure 1, aqd along the axis of the microscope and
electrometer assembly 7 is a number 222 Mazda self-focusing lamp which is held in a
bakelite insulated miniature screw base socket 3. An insulated phosphor bronze contact
strip 4 is electrically connected to the shell of the socket and makes contact with insulated
contact screw 5 when the instrument is in position in the case. The contact screw is con-
nected to one side of SPST push button switch 13. The other side of the switch is
connected to the positive end of flashlight cell 10 by phosphor bronze spring contact 12. The
cell is held securely in place by holder 11. This has a knurled head and locks into position
S- with bayonet studs. The cell can, therefore,be replaced in a matter of seconds.

S. When the microscope and electrometer assembly has been slipped into its housing 17
and secured with a set screw, the collecting electrode is bent so as to be in the most
favorable position for complete collection of whatever ions are produced in the ionization
chamber. The chamber walls include a part of the end plate 2 and of the instrument case
as well as the end of the microscope housing and the parts of the switch housing that are
marked 8. The collecting electrode, the chamber walls and the walls of the electrometer
chamber are covered with a non-flaking coat of conducting plastic paint to insure that the
response to soft gamma rays is not excessive.

The only operating controls push button 9. When it is partly depressed, the microscope
field is illuminated. Additional pressure on the button causes push rod 14 to move forward
and disconnect the potential of the ten megohm potentiometer 15 from the inner end of the
i: collecting electrode. This permits the electrometer, which is mounted in the lower end of
the microscope barrel, to discharge when radiation is present.










number 17-122 midget batteries 16. The batteries are in series. The potentiometer is
set to supply about forty-five volts to the electrometer in order to put the image of the fibre
at the upper end of the scale. The negative terminal of the batteries is, of course, con-
hected to the electrometer housing and ionization chamber. The life of the batteries under
these conditions is equal to shelf life. It should, therefore, be necessary to reset the fibre Aj"":.i
to zero only at infrequent intervals. The potentiometer shaft has a screwdriver slot for ''I
this purpose. If desired, this adjustment can be made quickly without removing the instru-
ment from its case by providing an opening in the case (not shown) directly over the shaft
of the potentiometer. A knockout disc is used to cover this opening. Such an opening is
recommended.

The electrometer and microscope are mounted in a separate unit (see Figure 2 for de-
tails). This makes it convenient to assemble, adjust, install, repair, or replace. The
unit resembles the pocket dosimeter. In fact, the eyepiece and scale holder assemblies are
identical. However, the total length is only four inches. The microscope gives satisfactory
magnification in spite of its two and three-quarter inch total length by the use of an ob- 1
jective lens system that consists of two plano-convex lenses whose combined focal length
is about one-quarter of an inch. The molded polystyrene insulator is three-quarters of an
inch in length. Its lower end projects out of the barrel about one-eighth of an inch. The
upper end has a spherical surface of one-half inch radius. This concentrates the light on ;
the fibre and gives sharper definition of the fibre image on the scale. It is seen that the
phosphor bronze wire that forms the electrometer support frame continues on through the
insulator and serves as the collecting electrode. 1

The electrometer is of the pocket dosimeter type. This is very rugged and dependable for
use in portable meters. The meters have been dropped a number of times on wood and con-
crete floors from a height of three feet without affecting the electrometer in the slightest.1
The platinum coated quartz fibre is five-eighths of an inch long and two and one-half microns
in diameter. Both ends are soft-soldered to the electrometer frame so the fibre has a
semi-elliptic shape before voltage is applied. It is repelled from one side of the plane in
which it lies by electrostatic forces from the electrometer frame. It is attracted from the
other side by the wall of the one-half inch diameter electrometer chamber. The geometry
is such that the motion of the fibre image over the scale is very nearly linear with voltage.



1 Some mechanical troubles did appear. On two occasions the objective lens system -:
slipped slightly and the fibre went out of focus. This was serious since refocusing is not
readily done without a little experience. The entire microscope assembly was, therefore,
shock-proofed. On one occasion the fibre was forced against its support frame and was
caught. Tapping the case directly over the electrometer position readily brought the fibre '
back to its previous position. It is unlikely that the electrometer and microscope system .
will need mechanical attention. This is the only-part of the meter that theaverage tm-
skilled person could not readily readjust and repair. The ruggedness of the meter is liin-n .,
ited, at present, by the strength of the case, retaining screws, etc. Greater strength in I
these parts can readily be attained.


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The error from this source due to using either full scale for a reading or any small part of
it, will average about three per cent.

The meter is about one-sixth as sensitive to gamma rays as the L & W Gamma-Beta Ray
Survey Meter. The main reason for this is that the ionization chamber has a volume of
only forty cubic centimeters as against two hundred cubic centimeters. Electrometer
capacity and voltage sensitivity are about the same in the two cases. (About 1.8 mmf. and
10 divisions per volt.)

Rates of radiation from one milliroentgen per hour to ten roentgens per hour (one divi-
sion per minute and full scale in 0.7 seconds, respectively) are within the range of the
meter. Figure 3 shows, on log-log paper for a typical case, the time required for the fibre
to move full scale for various gamma ray radiation rates. The graph is very nearly a
straight line. This indicates that ion collection is satisfactory and that reliable measure-
ments can be made evei. it a rate of ten roentgens per hour.

Because of the desire to keep its size and weight to a minimum, this meter is not equip-
ped with a built-in timer. The thought is that, in a rough and tumble meter for field use,
sufficient accuracy can be had by counting off the seconds. A little practice will enable a
person to keep his error down to less than ten per cent. One hand is then left entirely free
for other duties.

The meter can, of course, do very accurate work if it is carefully calibrated, set on a
vibration-free support and used with a stop watch. Quartz fibre instruments are unsur-
passed in this respect. A simple clamp can readily be devised to hold the switch button
down in order to make accurate measurements of low r4tes of radiation. It did not seem
advisable to clutter the meter with a locking switch for this purpose.

Perhaps it is well to discuss the one outstanding fault of the electrometer. If the meter
is rotated about any axis that is perpendicular to the microscope axis, the fibre image will
shift a considerable distance on the scale. This is a gravitational effect and does not mean
that the calibration has changed since the linearity of the electrometer with voltage extends
a considerable distance beyond each end of the scale. It does, however, mean that the meter
must not be rotated while a reading is being made. To do so will cause the fibre shift to be
added to or subtracted from the fibre motion that was caused by radiation. However, it will
be found that such a rotating motion is readily controlled or eliminated by the operator.
Ordinary quaking of the hand, which is involuntary and hence potentially a more serious
source of annoyance if not of erroneous readings, will be found to affect the fibre only
very slightly because of the cushioning effect of the air. A sudden sidewise move of some
magnitude will cause the air to displace the fibre temporarily but again such movement is
usually voluntary and under control. The chance of an error from this source is slight.

The meter can readily be designed to measure beta rays and/or alpha rays by providing
, a sliding door at the edge of the case on which the ionization chamber is located. If pre-
ferred, the door may be omitted and a coarse mesh screen placed over the opening. A


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suitably thick conducting film could be placed underneath the screen to protect the insula-
tor from dust and other foreign matter.

MAINTENANCE

Mechanical.

To remove the mechanism from the case, remove the two cover screws and the screw
which is directly opposite the switch button and which secures the microscope housing to
the case. Remove also the flashlight cell holder and the microscope eyepiece. Remove the
push button by turning it off the screw which secures it. (In the first units that were as-
sembled the push button could not be removed, but had to be depressed so it would slide
inside the case with the meter.) Use-the openings in the cover as finger holes and pull the
meter out of the case. If considerable force is necessary to start, a hooked tool or heavy
hooked wire may be applied under the inner edge of the flashlight cell holder opening.

The mechanical features of the meter are so simple that no further discussion of them
is necessary.

Electrical.

1. If the microscope light ceases to function, the lamp may have come loose in its socket.
This can readily be determined by removing the lower end plate. If the lamp is loose or
must be replaced it should be turned until it sets solidly. Cement should then be applied
between the top edge of the socket and the base of the bulb to insure that shocks will not
Loosen it. Only about one-half of the Mazda number 222 bulbs that are presently available
have their beams sufficiently well directed to be useable.

S 2. The S. P. S. T. switch contact and the electrometer release wire must be so adjusted
that:

Sa. The switch requires sufficient force to close so that is will not readily discharge
S the flashlight cell while the meter is carried in the pocket.

b. Additional force is required to cause the electrometer voltage to be disconnected.
This condition will permit the operator to view the fibre while it is at rest and so to 'get
set' to make the reading properly. This becomes very important for high rates of radiation.

3. If the microscope field is illuminated when the push button is released, the push button
is stuck or the switch arms are bent so that contact is continuous.

THE ELECTROMETER

SThe electrometer is the heart of the instrument and the only part that will normally give
S serious trouble. It has, therefore, been made as accessible as possible. The sources of


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5 MDDC 952


trouble are quite limited in number. They are listed below with suggestions on how to
remedy them.

1. The fibre image has disappeared from the microscope scale. !

This normally means that potential is no longer being applied to the electrometer or that
the potential is too high or too low. It may be caused by an accidental total discharge of the
electrometer (in use the electrometer is never more than partly discharged). Some-of the
fibres will stick to the support frame after complete discharge. Proceed as follows: Rap
the edge of the case sharply with a smooth object at a point that is directly over the electro-
meter. If necessary, any vigor short of such as will dent the case is permissible. The push
button should not be depressed during the operation thus leaving the electrometer charged.
Fibres that are stuck to the support frame can usually be freed by this method. It may be
well to note that frequent repetition of this procedure will, in some cases, cause abrasion "
and damage to the platinum coating of the fibre.

If the fibre image does not reappear on the scale, remove the meter from the case. See
if the small brass electrometer contact wire is touching the collecting electrode when the -..
switch button is released. If not, the trouble is mechanical and its source can readily be
located and proper remedies applied. ;;

The next step is to illuminate the microscope field by pointing the microscope at a
source of light. Vary the fibre potential by turning the potentiometer with a screw driver.
If this does not bring results, return the potentiometer setting to its original position.
Check the battery by connecting the negative lead of a high resistance voltmeter to the
meter frame and the positive lead to the two outer terminals of the potentiometer. The
readings should be about twenty-five and fifty volts (forty-five volts is sufficient to set
most electrometers to the top of the scale). If the battery is good, move the positive ter-
minal of the voltmeter to the collecting electrode. Because of the drop of voltage in the
potentiometer, the reading will be much too low except when a very high resistance volt-
meter is employed, but any reading whatever should signify that the electrometer is re-
reiving proper potential. If, however, the voltmeter now shows no variation in reading as .i
the potentiometer is turned, the latter has an open circuit and must be replaced.

Should the aforementioned procedure fail, it is felt that anyone who is not familiar with
fine quartz fibre work should not attempt to remove the electrometer,itself, but should re-
turn the entire meter to the Ryerson Shops for inspection and repair: The shipment should


be accompanied by an official request for repairs to be made.


2. The fibre image is not distinct.

One should first determine if the microscope lenses are loose. Shaking the meter while
it is held close to the ear is effective. If the l.ses are loose, push the clamping rings
back into place. A bit of lacquer or any similar substance flowed between the ring and its
retaining wall will prevent a repetition of the trouble. The cause may also be a loose or
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MDDC 952


defective microscope lamp. Again, lacquer or some good cement should be applied between
the top edge of the socket and the base of the bulb to prevent the lamp from coming loose.
If the electrometer insulator lock nut has come loose, the fibre will not only become indi-
stinct, but the fibre will rotate so its image will cross the scale at an improper angle.
Finally, the objective lens holder may have been jarred out of its normal position against
a shoulder in the microscope tube. The remedy is to remove the eyepiece and the scale
holder and to push the objective lens holder back against its shoulder. The scale holder
is removed and replaced with a special tool that is -furnished for the purpose.

3. The electrometer discharges when it is disconnected from its source of potential
even when no radiation is present.

There are two conditions that produce this effect. Both are a form of insulator leakage.
If a piece of lint or dirt accidentally assumes such a position that it forms a conducting
path around the insulator, the discharge is likely to be rapid and frequently erratic. The
obvious and usually successful procedure is to blow the offending material away with a
blast of dry air. The objectional matter is usually located at the lower end of the insulator.
This is readily accessible.

The second and more serious cause of trouble is leakage along the surface of the insu-
lator. Such leakage usually appears only when humidity is high. On the other hand, a clean
molded polystyrene insulator has been known to retain its insulating properties for long
periods even when humidity was so high that small droplets of condensed moisture adhered
to the insulating surface. The leakage is supported by contaminating materials that have
been deposited on the insulator. By brushing the flat portion of the insulator that surrounds
the collecting electrode lightly with a clean camel's-hair brush, which has been dampened
with clean absolute alcohol, one may be able to effect a cure. If the insulator still leaks,
it is recommended that the meter be returned to the Ryerson Shops for repair.






















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ACKNOWLEDGMENT-':



In the course of the-development of this instrument Dr; J. E..Rose of the HealthDivision .
of the Argonne National Laboratory has given much valuable advise and made many sugges-
tions. Much creditalso-is due T. J. O'Donnel,. former director of the shops of the Argonne
National Laboratory, to Joseph Getzholtz and Hatry Phelps, shop -foremen and to Walter -
Norlander who were-largely instrumental in developing the. process of molding polystyrene-
insulators and finally to the personnel of.thequartz shop.







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