Instruction Book and Maintenance Manual for Radiological Survey Meter, Fcda Item No CD V-720 Model 1, Chatham Model CH-720

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Radiological Survey Meter FCDA ITEM NO. CD V-720 MODEL 1 DIVISION OF GERA CORPORATION LIVINGSTON, NEW JERSEYBEST AVAILABLE COPYBEST AVAILABLE COPY

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I he Chatham CH-720 was designed specifically to fulfill the requirements l 1tdizing three ;anges of 5, 50, and 500 roentgens per hour full scale, it lllcasures gamma ,md x-ray radiation fields from 0.5 to 500 roentgens per hour and detects beta particle radiation e\'en in the presence of such fiefds. \11 air-filled ionization chamber serves as the radiation detecting element. I he chamber has a thin window which permits the transmission of beta particles into the chamber volume, thereby making detection possible. \\' ith the window covered by a movable shutter the instrument responds only to gamma or x-radiation. \Vhen the shutter is placed in the open position, the bet'.l window is exposed and the unit also responds to beta The current produced in the ionization chamber by the action of a radiation field is amplified by a single electrometer-tube circuit and then measured on a meter with an easily-read, linear scale, thereby providing a "isual indication of the magnitude of the radiation field. The power for both the amplifier section and the ionization chamber is obtained from ·l 'hc instrument can be carried easily in one hand by means of the handle or suspended comfortably from the neck or shoulder with the adjustable carrying strap. It is completely ruggedized and designed to operate under adverse field conditions. The accuracy of the instrument is within ± I ;v~ of true dose rate throudiout a temperature range from -20°F to +125°F. It is also witliin ± I 5% for photon energies from .08 l\feV to 1.2 MeV with the beta shutter in either the open or closed positicn. It is unaffected by altitude changes up to 25,000 feet. 2.0 THEORY OF OPERATION \Vhen the instrument is exposed to radiaticn some of the energy of the radiation field is absorbed within the walls of the ionization chamber. As a result, electrons are ejected from the walls into the air contained within the chamber. As these electrons traverse the chamber, they create a con- ~iderable amount of ionization in the air by collision processes. Under the influence of the electric field existing between the chamber electrodes the 10ns move to the electrodes and are collected. Thus, a current which is directly proportional to the magnitude of the radiation field flows in the external circuit connected to the ionization chamber. \Vben the thin beta window of the chamber is exposed, beta particles can penetrate the ch:imber and initiate the process described above. Since the movable beta shutter is sufficiently thick to stop these particles, the instrument will not respond to this fonn of radiation with the shutter in the fully closed position. . 3BEST AVAILABLE COPYBEST AVAILABLE COPY

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The current from the ionization chamber is amplified by a cathode follower type circuit utilizing a CK-5886 clcctromctcr tube. (Sec Figure 2 for simplified circuit). With the SELECTOR SWITCH in the ZERO position, the Hi-Megohn resistor R is removed from the grid circuit so that no signal voltage can be developed as a result of any ioniz.ation chamber current. The quiescent value oE the plate current Ip• is then exactly balanced by the bucking current Ir so that the resultant current through the meter is zero. The bucking current is obtained from the filament battery Er and is adjusted by means of the ZERO ADJUST potentiometer When the SELECTOR SWITCH is placed in a range position, th~ I Ii Megohm resistor R is reinserted into the grid circuit. Current from the ionization chamber therefore produces a positive signal voltage acr~ this resistor which in tum results m an increas~ in the plate current Ip. Thm the resultant current through the meter is no longer zero and the m~tf'T therefore measures the increase in plate current. Since this chance " proportional to the magnitude of the radiation field, the meter scale lAn be calibrated directl y in roentgens per hour. It should be noted that since the input grid resistor is completely remo~cd front the circuit when the instrument is being zeroed, the zeroing process can be accomplished in the presence of a high intensity radiation field.BEST AVAILABLE COPYBEST AVAILABLE COPY

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3.1 Battery Installation and Replacement J'he top cover assembly should first be: remm·cd by loosening the fa~tening ,crew and slowly raising the top cover from the case shell. Tlus should be done carefully to avoid any excessi\'c strdin on the 10:1ization chamber 1.ihlc. ' I he oock end of the top cover should be tiltt:o upwards until the w111E.atio11 chamber cable can be disconnected by gently pulling on the aluminum tubing which shields the cable plug-in connector. The batteries can now tx: installed in the following manner: I. Clmc the 22½ volt battery strap by meam of the snap fastener. ., Inscrt the positive side of the plate supply battery as shown in fi~mc 3 making sure that the pin on the battery contact terminal fits into the hole in the contact plate on the battery. The positive terminals of all three batteries should be toward the + POS. marking on the battery mounting bracket. 3. Press the negative side of the battery into the strap until the contact pin slides into the hole on the negative contact plate of the battery. 4. Repeat steps 2 and 3 to insert the second 22½ volt battery. 5. Close the I½ volt D-eell battery strap by means of the snap fastener.BEST AVAILABLE COPYBEST AVAILABLE COPY

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6. Insert the b~tton contact which forms the positive ·terminal of the l½ volt D-cell into the positive battery contact terminal as shown 7. Press the negative side of the battery into the strap until the negative battery contact terminal slides into the recess on the negatl\ e side of the battery. The battery stra_p will also make adequ.itc u111tact with other types of batteries which do not have the recc~, on the negative side. The carrying strap i~ attached to the mstrument by inserting each L11,I of the strap through the respective slot; at the ends of the top co, l I ncl then threading the ends through the adjusting slides. The adjustin~ 11o LS are also used to vary the length of the strap to a suitable carrying p<>\111011 The instrument has been designed so that a minimum of attention 011 thL part of the operator is required .to obtain satisfactory performance. After 6BEST AVAILABLE COPYBEST AVAILABLE COPY

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prop~rly installing the batteries and carrying strap, as dc,cribcd in section 3, the: unit is set into operation by carrying 011t the proccdmc, dcscrihcd The SELECTOR SWITCH should be rotated clockwiM: to the ZERO position and the ZERO control located on the top cover Jdjmted until the meter reads zero. This step can be carried out at any time during the operational life of the instrument and under any conditions ot The SELECTOR SWITCH should now be rotated to the extreme counter-clockwise CHECK position ar:d maintained in that position while the reading is noted. This reading should be above the C IRCUIT CHECK mark on the meter, If at any time during the operation of the unit this reading is below the mark, the batteries should be checked and the defective ones replaced. The operator will notice that upon release, the SELECTOR S\\'ITCH automatically returns to the OFF position so that it i, necessary to mam· tain pressure on the switch to hold it in the CIRCUIT CHECK position. 111is is done to eliminate the possibility :>f inadvertently leaving the instrument on in the CIRCUIT CHECK position. A fresh set of batteries provide a minimum of 1;o hours of continuous service under normal conditiom. 4.4 Determination of Radiation Field The selector switch should now be rotated to the X-100 range. If no deflection is obtained the X-10 range should be selected. Similarly, if no deflection is obtained the switch should be placed on the X-1 ran&e and left on that ra nge until an off-scale reading 1s obtained. When this occurs, the next highe~t range should be selected until the deflection can be read on 011c of the higher ranges. In all cases the magnitude of the radiation field will be given by the product of the meter reading and the scale multiplier. A,\ume a reading of 2.5 is obtained on the middle, or X-10 scale. The radiation field will, therefore, be 2.5 x 10. or 25 roentgens per hour. The operator generally should not attempt any maintenance beyond the replacement of batteries, particularly under field conditions. The instrument is designed so that the batteries can be replaced without exposing any critical circuit elements and without requiring the use of special tools. \ :Vhenever the instrument is to be stored for prolonged periods of time, the batteries should first be removed to prevent any damage due to their deterioration in the equipment. 7BEST AVAILABLE COPYBEST AVAILABLE COPY

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Should the surfaces of the instrument hccome contaminated with radioJctn·e material, they can be deaned with ~cap and water or any convenient cletergc11t. Il <l\\ e\!:r. care should he taken at all times to prC\·cnt the ac gn·cn 111 ,ection 3, with particular care taken to obtain good contact, 1111 ,1)) he 1mpccted for any sigm of deterioration. The instrument should then he ultbrated 111 ,tccorclance with the instructions given in paragraph - 4. 7 .1 Instrument does riot Zero Properly Most of the difficultie, c11countcred on the Chatham CH-720 will orig111ate cithcr in the h,tttery conncctiom or in the tatteries themscl\'es. Therefore, that proper contact is being made. I he battery \(iltages can be measured with a con\'cnti011al 20,000 ohms per rnlt voltmeter. The hearing .ml 7.2 Meter Deflection is Negative on All- RangH pears to be 111ckpcndc11t of the ~ctlmg of the ZERO potentiometer 111d1 catc, in,11ffiurnt rnm:nt in the plate circuit of the cleetrometer tnhc. The filament , should lhc:rcforc he d,ccked . This is most casih done m the the ioni;,.1tion chamber cahlc..\gain, this should be clone ,I,,, h to avoid ,trJ111111g the cable. 2. Rernm r the () ;,in sockct from the glass fced-thru. placed 1trnrdi11~ to the mstructiom gi,en 111 paragraph 7.3. If the fil.1111c11t, art 111 proper workin° order, the SELECTOR S\VJTCH. 8BEST AVAILABLE COPYBEST AVAILABLE COPY

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the t~p co,·cr ·enclosure and set up externally so thaf complete voltage and continuity checks can be carried out conveniently. Figure 5 shows the complete circuit, except for the ionization chamber, operating outside the cnclo,urc. Assuming steps I, 2, and 3 have been carried out, the procedure lo n·mm·c this ,uh-,•sscmhly is as follows: l. Runo\'e the k11ob~, rubber hoot, lockwashers, and locking nuts from tltc SELECTOR SWITCH and ZERO potentiometer. 1 Rc:mo\'c the fi,·e screws which fasten the CAPSULE ASSEMBLY to tlu. top cover enclosure. , 1'11<,h down gently on the shafts of the switch and potentiometer and :... de the entire suh-a~~embly out of the enclosure. The capsule itself ,, llot rigidly attached to the SELECTOR S\VITCH and it is therefore po~\Jblc for the capsule to separate from the switch shaft while this is beill~ done. However. the switch shaft and studs can be re-inserted mto the capsule after the assembly ha~ been relJ!oved from the encl1,,urc . .'\ small amount of pressure will then be required to hold them -f. Reconnect the 9-pin socket to the glass feed-thru, and by means of z test lead, connect one of the switch studs to a ground point on the top co\'er, preferably on the battery mounting bracket. 9BEST AVAILABLE COPYBEST AVAILABLE COPY

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5. Replace the cqntrol knobs to facilitate switching and adjusting the ze~o. The instrument is now completely operable except for the ionization chamber, which can be reconnected if desired. If the chamber is connected, the case shell should be connected to the top cover by some means such . as a test lead so that a common ground point is obtained. The unit should then be put in the ZERO position so that the \'Jrious voltages mJy be checked. The normal values are ~ivcn in the schematic diagram in figure -; Before carrying out any resistance measurements, however. the batteries should fir5t be removed. 7.3 Replacing the Electrometer Tube already been removed from the enclosure, the procedure to be foll<)\\ cc.I 111 replacmg the clcctrometcr tube 1\: l. UnsoTder the screen and pbtc leads of the electrometer tube. which come through the capsule near the ~lass fced-thru, from the butt connector, located on the yellow and pmk leads from the center deck of 2. Umolder from the center deck of the SELECTOR SWITCH the red and blue leads which extend up th10ugh the capsule cover. ~'hen this 1s accomplished. the SELECTOR SWITCH can be 1110\·ed aside to provide easier :1ccess to the capsule. 3. Remove the four screws which fasten the capsule cover to the capsule. 4. Slide the c-apsulc co,,er off slowly to ave.id damaging the red and blue leads winch were previously disconnected. \\' ith the CO\'er removed the interior of the capsule is exposed Jnd any of the high impedance components can easily be replaced. 5. Unsolder the grid lead from the Teflon feed-thru located inside the 6. Unsolder the butt connectors from pins 8 and 9 of the glass feed-thru. thereby prMiding easy acces, to the filament leads. 7. Umolde1 the two filament leads from the glass feed-thm. TI1e electro meter tulx \hould now be completely free. 8. Imert the new elcctronH'tcr tube and rc,er,e the Jrocedure outlined above.The msulating sleeve O\'er the tube leads shoul be replaced before the le.1ds are ,oldered into place. Extreme care must be taken to avoid any dirt or grime accumulating near the base of the electrometer tube Such conta111in.1tion will interfere with the operation of the instrument \Vhene\'er it hccomes necessary to expose or handle any of the high impedance component~. the technique used must be Aawlessly clean. Fingerprints and similar ('Ontamination will result in large error, and improper operation of tht unit. rad1atio11 fields ,1s high as 500 roentgens per hour. In order to obtain "good geometf) the imtrument should he located at least 15 inches from the radiation ,ourcc with the Jxis of the beam perpendicular to the BETA SHUTTER. -\t th1~ distance the radiation field must be fairly uniform over an area apprmomately 5 inches in diameter. The center of the ionization chamber 1, located l½.6 inches from the bottom edge of the case shell JOBEST AVAILABLE COPYBEST AVAILABLE COPY

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and directly behind the center of the indentation ori the BETA The calibration is carried out in the following manner: "1. l\tove the BETA SIIUITER to the fully closed position. 2. Turn the SELECTOR S\VITCH to the ZERO tosition and set the imtrument to ze10 by means of the ZERO contro . 3. 'I urn the SELECTOR SWITCH to the X-100 range and position the u111t so that the center of the ionization chamber is in an accurately knmm field of at least 250 roentgens per hour. ➔ . If the unit does …