GEOLOGICAL SURVEY CIRCULAR 353 PORTABLE SCINTILLATION COUNTERS FOR GEOLOGIC USE This report concerns work done on behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commission. UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director GEOLOGICAL SURVEY CIRCULAR 353 PORTABLE SCINTILLATION COUNTERS FOR GEOLOGIC USE By E. E. Wilson, V. C. Rhoden, W. W. Vaughn, and Henry Faul This report concerns work done on behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commission. Washington, D. C., 1954 Free on application to the Geological Survey, Washington 25, D. C. Ficrare 1. Commercially available versions of the scintillation counter. PORTABLE SCINTILLATION COUNTERS FOR GEOLOGIC USE By E. E. Wilson, V. C. Rhoden, W. W. Vaughn, and Henry Paul CONTENTS Page Page Abstract................................... 1 Calibration................................. 3 Introduction................................ 1 Spectrometry............................... 5 Acknowledgments........................... 2 Special applications......................... 5 Physical design............................ 2 Future developments ........................ 5 Electronic design........................... 2 Literature cited............................ 10 ILLUSTRATIONS Page Figure 1. Four commercially available versions of the scintillation counter.-............................. ii 2. The crystal photomultiplier assembly.................................................... ... 3 3. Circuit diagram of the portable scintillation counter.......................................... 4 4. Car-mounted twin scintillation heads, shielded with lead for directional search.................. 6 5. Scintillation logging equipment.............................................................. 7 6. Matching circuit for transmission of pulses over long coaxial cables............................ 8 7. Experimental model of the integrating (total intensity) scintillation counter...................... 8 8. Circuit diagram of the integrating scintillation counter .................................... ... 9 ABSTRACT day without major modifications, Although this instru­ ment was designed primarily for use in health physics A small, light, portable scintillation counter, work, it has been widely used in geologic field work primarily intended for geologic field use, has been (Faul, 1948, 1950). designed and is now commercially manufactured. The instrument embodies a very fast trigger amplifier and Experience in radiation measurement for geo­ a compact relaxation-oscillator power supply. The logical applications has revealed that the ordinary circuit takes full advantage of the high counting rate Geiger-Muller survey meter is not sensitive enough that can be obtained from a sodium iodide crystal. to be useful in areas of low levels of radiation, roughly Another, still smaller and lighter, scintillation counter of the order of background radiation. When large of the total intensity type is now being tested. Geiger-Muller tubes were connected to the survey meter, singly or in bundles, it was found that data of great geologic significance (Nelson, 1953; Slack, 1949; INTRODUCTION Slack and Whitman, 1951; and many unpublished reports) couldbe obtained. However, the large counters (as much as From rather primitive beginnings (Rajewsky, 1943; 40 inches long) were exceedingly clumsy to use in the field. Ridland, 1945), the portable gamma-ray detector has rapidly developed into a popular geologic tool. Probably A portable scintillation counter would not be sub­ there are more gamma-ray detectors in use today than ject to this handicap. Such an instrument was an­ all other geophysical instruments. nounced by G.'M. Brownell in Canada (1950), and be­ came commercially available shortly thereafter. The Manhattan Engineer District expended con­ Brownell's instrument weighed about 15 Ibs., had siderable effort on the design of a portable Geiger- considerable drift with temperature and time, and cost Muller survey meter for health physics work. The more than $1,000 in the United States, so that it could basic design adopted toward the end of the war utilized not become widely used. Nevertheless, Brownell's the Schmitt (1938) trigger circuit as an amplifier, with work showed that a portable scintillation counter could various high voltage supplies (dry batteries, oscillators, be very useful in geologic studies. vibrators), and was produced commercially by many instrument manufacturers. The instrument was clear­ About 1950, portable scintillation counters were de­ ly successful and-continues to be manufactured to this veloped independently at the Oak Ridge National Laboratory and Los Alamos Scientific Laboratory. Each of these in­ silicone fluid of very high viscosity (as much as sev­ struments had definite advantages: the Oak Ridge circuit eral hundred thousand centistokes). The can slips over (Borkowsky and Dandl, personal communication) was very the end of the photomultiplier and is attached with in­ fast and had a good oscillator power supply; the Los Alamos dustrial adhesive tape. A phosphor bronze spring circuit (Watts, personal communication) was built with sub- keeps the polished face of the crystal in contact with miniature tubes and could be made very small and light. the photocathode. The other surfaces of the crystal Obviously, an instrument combining the advantages of both are left rough. Commercially available potted crys­ designs would be of great value to the geologic profession, tals are preferred by one manufacturer, but the silicone and we decided to attempt its development. mount, originally developed at the Chalk River Labora­ tory (Carmichael, personal communication), is less The instrument that was developed (fig. 1) weighs costly and mechanically sturdier. We have not ob­ about 7 pounds, stands about 12 inches high, and retails served any detrimental reaction between the crystal, for about $500. It has now been produced in quantity by the tape, and the silicone oil. The photomultiplier is three manufacturers and used extensively during the magnetically shielded by a thin sheet of highly per­ 1953 field season by many private individuals and per­ meable nickel-iron alloy such as is available com­ sonnel of the Geological Survey, the Atomic Energy mercially under various trade names. The entire Commission, and the Department of Agriculture. Much assembly of the sensitive element is suspended in the remains to be learned about the calibration and optimum probe in sponge rubber, to minimize damage when the energy response of scintillation counters for field use, probe is dropped accidentally. Some manufacturers but in spite of their limitations they are of great value are now producing the instrument in a single box in to the geologist. order to reduce the cost and still produce a rugged instrument. ACKNOWLEDGMENTS ELECTRONIC DESIGN We are greatly indebted to our colleagues at the Oak Ridge National Laboratory and Los Alamos Scientific The pulse amplifier (fig. 3) is a trigger pair of Laboratory for their kind and helpful assistance during subminiature tubes (type CK-533 AX) with V-l normally the early phases of this work. Particular thanks are conducting. The input sensitivity is varied by means of due Messrs. C. J. Borkowsky, Ray Dandl, R. J. Watts, the potentiometer marked "CALIBRATE, " which di­ and F. J. Davis. Dr. Hugh Carmichael of the Chalk rectly affects the bias on V-2 and to some extent, the River laboratory and Mr. John Harshaw of Harshaw bias on V-l, and is adjustable down to a few millivolts, Chemical Company have been very helpful in discussions or a point just above the dark current of the photo- of the various techniques of crystal mounting. We are multiplier tube. The amplifier has a gain of 20, a grateful to the many geolo0sts and engineers who have pulse length of 6 microseconds, and a 14 microsecond f helped us in the practical field evaluation of this in­ resolution period. The pulse length and resolution strument. The work here reported is part of a program peiiod are independent of range setting, and less de­ that the U. S. Geological Survey is conducting on be­ pendent on input pulse amplitude and counting rate half of the Division of Raw Materials of the U. S. Atomic than any other type amplifier tested for portable scin­ Energy Commission. tillation counters. Specifications call for a minimum rate of 150,000 counts per minute in a field of one milliroentgen per hour of radium gamma rays with a PHYSICAL DESIGN cylindrical crystal one inch thick and IT inches in diameter. The ranges are selected by switching re­ The outward design of the instrument was de­ sistors in the plate circuit of V-2. A series network termined largely by a committee of field geologists in with preselected values for the range settings was Denver, under the chairmanship of L. R. Page of the tried and was found to be very satisfactory, but pro­ Geological Survey. It was decided that the instrument curement in quantity of resistors of the correct values should be housed in two boxes, of which one would con­ proved to be difficult. The present method was chosen tain little more than the batteries, or roughly half the as a compromise. Two time constants of 1 and 10 sec­ weight of the device. The battery box has suitable onds are obtained by switching condensers C i and C 2 loops so that it can be worn on the belt, and is con­ respectively. The necessary voltage to drive the