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 dilier 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: August 22, 1945 Document Declassified: May 12, 1947 This document consists of 10 pages. , EP OSlTORV 1 - MDDC - 952 DESCRIPTION OF AND INSTRUCTIONS FOR OPERATION AND MAINTENANCE OF A NEW MODEL GAMMA RAY POCKET SURVEY METER By 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, and 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 with bayonet studs. The cell can, therefore,be replaced in a matter of seconds. 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 control .is 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 collecting electrode. This permits the electrometer, which is mounted in the lower end of the microscope barrel, to discharge when radiation is present. MDDC - 952 The potentiometer is connected across the second of two twenty-seven volt Eveready 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- nected 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 to zero only at infrequent intervals. The potentiometer shaft has a screwdriver slot for 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- 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. 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.* 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 thtuaverage un- skilled person could not readily readjust and repair. The ruggedness of the meter is lim- ited, at present, by the strength of the case, retaining screws, etc. Greater strength in these parts can readily be attained. - 2a- MDDC - 952 I 8 z z i fc 5 iii a: i tf) ,!» > u r x < i U 3 ■ I* K si a ki h «i \ \i u J « I \1 a a Q c 5 \ s q s e a < - 2b MDDC - 952 —1 O O . t_> < < — ►— UJ _] Q_ -5 U UJ LJ C/3 —> < LU CO 2 >- _J 2 dj O >• O UJ UJ O i/> UJ C\J rn T ui CO 1 — CO rxi C\J OJ Cd CV) c\j OJ C\J Q Z UJ O UJ cr O - S UJ ^ UJ O ^ s c>0 O LJ-l CC DC ^ ffiO t/> CO < cc ._ <** o ,, uj ¥ y •- s t± < < uj w Z — I cc o ^ Q. O O w a O u |_ O tr tx z UJ UJ CO £ ^ =2 = =» z o oo ° ^ Z Z o t: tD O o S, CL o-> O 5 = — q ^ - °| go X ° < _i -! 55s " t s - £ Kf LlJ 00 _ < I- a: i2 < i— o < o oj <-> 5 S cb l- °- CD X 558° _j J w X X O UJ co oo ac or ^ < < O (D => -J -J 5 d cc 2 Z z § ^ £ LU S _J CL i— i— lu r5 o — oj rn •& m io r^ co Q z UJ o Ul _J UJ cr o 2 o cr CO or — 3 < a. a 2 o5 3 - MDDC - 952 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 *or 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 rates 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 - 3a MDDC - 952 o o o o o o o o o o 00 (O