Scintillation Counters 

For Geologic Use

 M . __ . 

GEOLOGICAL SURVEY BULLETIN 1052-F

/V report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission
Scintillation Counters 

For Geologic Use

By W. W. VAUGHN, V. C. RHODEN, E. E. WILSON, and HENRY PAUL

EXPERIMENTAL AND THEORETICAL GEOPHYSICS

GEOLOGICAL SURVEY BULLETIN 1052-F

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

"JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

The XT. 8. Geological Survey Library has cataloged this publication as follows:

Vaughn, William Wendell, 1921-

Scintillation counters for geologic use, by W. W. Vaughn 

t and others] Washington, U. S. Govt. Print. Off., 1959.

iv, 213-240 p. diagrs. 25 cm. (U. S. Geological Survey. Bulletin 

1052-F. Experimental and theoretical geophysics)

Bibliography: p.240.

1. Scintillation counters. (Series: U. S. Geological Survey. 

Bulletin 1052-F. Series: U. S. Geological Survey. Experimental 

and theoretical geophysics)

622.15

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. Price 35 cents (paper cover)
CONTENTS

Page

Abstract____________________________________ 213

Introduction.-----.--.-______________________________ 213

Acknowledgments _____________________________________________ 214

Hand-portable scintillation counter__________________________________ 215

Physical design____________________________________________ 215

Electronic design..._______________________________________ 216

Calibration.________________________________ 220

Spectrometry_ __________________________________ 221

Special applications.________________________________________ 221

Airborne-carborne scintillation counter ______ _ _ ____ _ ____ _ _________ 222

Physical design____ ____________________________ 222

Electronic design_______________________________________ 223

Response..______.__________________________ 227

Portable scintillation gamma-ray well-logging instrument--_ ___ ____ 232

Physical design________________________________________ 232

Electronic design..____...___________________________ 234

Calibration. _________._________________________ 237

Selected bibliography...____________________________._ 240

m
IV CONTENTS

ILLUSTRATIONS

Page

PLATE 9. Eight commercially available models of the hand-portable

scintillation counter,______________________________ Facing 220

10. Carborne scintillation counter______________________ Facing 221

11. Portable scintillation gamma-ray well-logging instruments-- Facing 233 

FIGURE 54. Crystal photomultiplier assembly._________________ 216

55. Circuit diagram of the hand-portable scintillation counter____ 217

56. Circuit diagram of airborne-carborne scintillation counter,___ 224

57. Simplified diagram of the diode pump-type ratemeter. _______ 225

58. Graph showing the response of the airborne-carborne scintillation counter with distance from a 24.28-milligram radium 

source at a speed of 10 miles per hour along 660-foot straightline traverses__________________________________-_-____ 228

59. Graph showing the response of the airborne-carborne scintillation counter with distance from a 24.28-milligram radium 

source at a speed of 20 miles per hour along 660-foot straightline traverses_____________-_____-_-___-----___-----___ 229

60. Graph showing the response of the airborne-carborne scintillation counter with distance from a 24.28-milligram radium 

source at a speed of 30 miles per hour along 660-foot straightline traverses__-_________--__--__--_-_-------------__- 230

61. Graph showing the response of the airborne-carborne scintillation counter with distance from a 24.28-milligram radium 

source at a speed of 40 miles per hour along 660-foot straightline traverses__-_-______-___--------------_----------- 231

62. Circuit diagram of the scintillation probe__________________ 234

63. Circuit diagram of the portable scintillation gamma-ray welllogging instrument____-______-_-------_--------------- 235

64. Diagram of simulated ore body for calibrating scintillation

gamma-ray well-logging instrument_____________________ 238

65. Typical calibration chart for scintillation gamma-ray welllogging instrument------------------------------------ 239
EXPERIMENTAL AND THEORETICAL GEOPHYSICS

SCINTILLATION COUNTERS FOR GEOLOGIC USE

By W. W. VAUGHN, V. C. RHODEN, E. E. WILSON, and HENRY FAUL

ABSTRACT

A small light hand-portable scintillation counter, an airborne-carborne scintillation counter, and a portable scintillation gamma-ray well-logging instrument have 

been designed, constructed, and tested, and are now commercially manufactured. 

The amplifier circuits take full advantage of the high counting rate that can be 

obtained from photomultipliers and phosphors. Relaxation-oscillator-type highvoltage supplies provide regulated voltages for the photomultiplier tubes.

The portable-hand scintillation counter was designed to be carried when 

prospecting on foot. The airborne-carborne unit is primarily intended for broadarea reconnaissance from automobiles or light aircraft at altitudes of 500 feet 

or less. This instrument uses a sodium iodide crystal 3 inches in diameter and 

1% inches thick and a photomultiplier tube with a cathode diameter of 3 inches. 

Larger crystals may be adapted. Variations in the geometry of the measurement 

(mass effect), and the degree of radioactive equilibrium of an exposed surface 

make it impossible to calibrate the instruments directly in terms of uranium 

content. However, the instrument can be calibrated in milliroentgens per hour 

if the type of radiation is specified.

The portable scintillation gamma-ray well-logging instrument has a manually 

operated reel and is capable of logging holes 1,000 feet deep. The probe contains 

only the photomultiplier tube and matching circuit. The energy resolution of the 

instrument using a cesium source is approximately 10 percent, measured through 

the 1,000 feet of coaxial cable. The portable scintillation gamma-ray well-logging 

instrument was designed for dual application as an aid in stratigraphic studies 

and to log ore-grade material. The wide range of sensitivity is accomplished by 

adjustments made at the surface. A sample calibration chart for determining the 

grade of material in a drill hole is included with the report.

INTRODUCTION

From rather primitive beginnings (Rajewsky, 1943; Ridland, 1945), 

the portable gamma-ray detector has rapidly developed into a popular 

geologic tool. Many more gamma-ray detectors are in use today 

than all other geophysical instruments.

The Manhattan Engineer District expended considerable effort on 

the design of a portable Greiger-Muller survey meter for health213
214 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

physics work. The basic design adopted toward the end of the war 

utilized the Schmitt (1938) trigger circuit as an amplifier, with a 

high-voltage supply (dry batteries, oscillators, or vibrators), and was 

produced commercially by many instrument manufacturers. The instrument was clearly successful and is still manufactured, with minor 

modifications. Although this instrument was designed primarily for 

use in health-physics work, it has been widely used in geologic field 

work (Faul, 1948,1950).

Experience in radiation measurement for geologic applications has 

revealed that the ordinary Geiger-Muller survey meter is not sensitive 

enough to be useful in areas of low levels of radiation. When large 

Geiger-Muller tubes were connected to the survey meter, singly or in 

bundles, it was found that data of great geologic significance could be 

obtained (Nelson, 1953; Slack, 1949; Slack and Whitham, 1951). 

However, the large counters (as much as 2 inches in diameter and 40 

inches long) were clumsy to use in the field, difficult to mount in 

automobiles and small aircraft, and impossible to lower down small 

bore holes. Portable scintillation counters are not subject to these 

handicaps.

A hand-portable scintillation counter was announced by G. M. 

Brownell in Canada (1950), and became commercially available 

shortly thereafter. Brownell's instrument weighed about 15 pounds, 

had considerable drift with temperature and time, and cost more than 

$1,000 in the United States, so that it did not become widely used. 

Nevertheless, Brownell's work showed that a portable scintillation 

counter could be very useful in geologic studies.

About 1950, hand-portable scintillation counters were being developed independently at the Oak Ridge National Laboratory (C. J. 

Borkowsky and R. Dandl) and at the Los Alamos Scientific Laboratory (R. J. Watts). Each of these instruments had definite advantages : the Oak Ridge circuit was very fast and had a good oscillator 

power supply; the Los Alamos circuit was built with subminiature 

tubes and could be made very small and light. Obviously, an instrument combining the advantages of both designs would be of great 

value to the geologic profession, and we decided to attempt its development.

ACKNOWLEDGMENTS

We are greatly indebted to our colleagues at the Oak Ridge National Laboratory and the Los Alamos Scientific Laboratory for their 

kind and helpful assistance during the early phases of this work. 

Particular thanks are due Messrs. C. J. Borkowsky, Ray Dandl, R. J. 

Watts, and F. J. Davis. Dr. Hugh Carmichael of the Chalk River 

laboratory and Mr. John Harshaw of the Harshaw Chemical Co. have
SCINTILLATION COUNTERS FOR GEOLOGIC USE 215

been very helpful in discussions of the various techniques of crystal 

mounting. Carl Bunker supplied the calibration chart for the portable well-logging instrument. We are grateful to the many geologists, geophysicists, and engineers who have helped us in the practical 

field evaluation of these instruments. The work here reported is part 

of a program that the U. S. Geological Survey is conducting on behalf of the Division of Eaw Materials of the U. S. Atomic Energy 

Commission.

HAND-PORTABLE SCINTILLATION COUNTER

The hand-portable scintillation counter that was developed (pi. 9) 

weighs about 7 pounds and retails for about $300 to $500. It has now 

been produced in quantity by several manufacturers and is used extensively by many private individuals and mining companies, personnel of the Geological Survey, the Atomic Energy Commission, the 

Department of Agriculture, and geological surveys of Brazil, France, 

Norway, Peru, Turkey, and other foreign governments. Much remains to be learned about the calibration and optimum energy response 

of scintillation counters for field use, but in spite of their limitations 

they are of great value to the geologist.

PHYSICAL DESIGN

The outward design of the first instruments was determined largely 

by a committee of field geologists in Denver, under the chairmanship 

of L. E. Page, U. S. Geological Survey. It was decided that the instrument should be housed in two boxes, of which one would contain 

little more than the batteries, or roughly half the weight of the device.

The battery box has suitable loops so that it can be worn on the belt, 

and it is connected to the probe by a flexible coiled cord.

The probe contains most of the circuits, the sensitive element, and 

meter. It is as light as possible, and waterproof for use in mines. 

For obvious reasons, the probe is very rugged. The meter is so placed 

that is easily readable at arm's length, and the use of the instrument 

requires only one hand. The probe fastens to the battery case with 

a spring catch and is firmly held, yet easily removed.

The sensitive element is a very fragile assembly and must be 

mounted in the probe with utmost care to prevent damage in normal 

(that is, rather rough) use. The thallium-activated sodium iodide 

crystal, about 2 cubic inches or larger in volume, was originally housed 

in a thin spun-aluminum cup (fig. 54) filled with clear silicone fluid of 

very high viscosity (as much as several hundred thousand centistokes). 

The can slips over the end of the photomultiplier and is attached with 

industrial adhesive tape. A phosphor bronze spring keeps the polished
216 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

face of the crystal in contact with the photocathode. The other surfaces of the crystal are left rough. Various modifications of this 

technique are used by the commercial suppliers. Commercially available potted crystals using magnesium or aluminum oxide reflectors 

are useful if one attempts to make spectral measurements, but the 

resilient silicone mount is less costly and mechanically sturdier. We 

have not observed any detrimental reaction between the crystal, the 

tape, and the silicone oil.

The photomultiplier is magnetically shielded by a thin sheet of 

highly permeable nickel-iron alloy, available commercially under several trade names. The entire assembly of the sensitive element is 

suspended in the probe in sponge rubber to minimize damage if the 

probe is dropped accidentally.

/Crystal

Aluminum cupFIGDEB 54. The crystal photomultiplier assembly.

Spring

Some manufacturers are now producing the instrument in a single 

box in order to reduce the cost and still produce a rugged instrument 

(pi. 9). The one-box design results in an instrument of less welldefined detector geometry, but offers the advantages of greater ruggedness, durability, and lower cost.

ELECTRONIC DESIGN

The pulse amplifier (fig. 55) is a trigger pair of subminiature tubes 

(type CK-533-AX) with tube V-\ normally conducting. The input 

sensitivity is varied by means of the potentiometer marked "Calibrate," which directly affects the bias on tube "F-2 and, to some extent, 

the bias on tube F-l, and is adjustable down to a few millivolts, or a 

point just above the dark current of the photomultiplier tube. The 

amplifier has a gain of 20, a pulse length of 6 microseconds, and a 14- 

microsecond resolution period. The pulse length and resolution period 

are independent of range setting and are less dependent on input 

pulse amplitude and counting rate than any other type amplifier 

tested for portable scintillation counters. Specifications call for a
CK533AX CK533AX 

6i

I

6199

6291

Resistance is in ohms 

K=103 (thousand) 

M=106 (million)

VXR 900 

VXR 1200

All resistors l/3 watt

Capacitance is in microfarads 

unless otherwise noted

CK533AX CK526AX

FIQUBB 55. Circuit diagram of the hand-portable 
218 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

minimum rate of 200,000 counts per minute in a field of 1 milliroentgen 

per hour of radium gamma rays with a cylindrical sodium iodide 

crystal 1 inch thick and iy2 inches in diameter.

The ranges are selected by switching resistors in the plate circuit 

of tube F-2. A series network with selected resistor values for the 

range settings was tried and was found to be very satisfactory, but 

procurement in quantity of resistors of the correct values proved 

to be difficult. The present method was chosen as a compromise. Two 

time constants of 1 and 10 seconds are obtained by switching condensers C-i and Cz respectively. The necessary voltage to drive the 

metering circuit is obtained from the range resistors in the plate circuit of tube F-2. This circuit is essentially a vacuum-tube voltmeter 

using a type CK-526-AX tube. A 4-volt swing of the grid of tube 

F-3 will give full scale deflection on the 50-microampere ruggedized 

meter. The grid voltage versus plate current curve is linear over 

the operation range of grid voltages so that the meter deflection is 

linear with respect to the voltage developed across any range resistor. 

Other tubes may be used where higher meter currents are required, as 

in some applications discussed below. The potentiometer ("Zero") 

in the cathode circuit, which is controlled externally, serves as the 

zero balance. Zero drift is small, less than 1 microampere in 40 

hours of operation.

The negative 900 volts for the photomultiplier is supplied by relaxation oscillator high-voltage supply with an approximate plate efficiency of 16 percent. The overall efficiency is about 3.5 percent. The 

relaxation oscillator (about 100 cycles per second) triggers the grid 

of tube F-4. The reactive voltage developed by the choke in the 

plate of F-4 is rectified by F-5 and regulated by F-6. This voltage 

is sensitive to oscillator frequency, which drifts as the batteries 

weaken. The frequency is set slightly above optimum, which gives 

a peak to be reached and traversed during the useful life of the B 

batteries. This, in turn, gives constant voltage for a longer time 

per battery change. Additional compensation is afforded by feedback 

of the change of voltage with load at the regulator tube through a 

resistor to the grid of F-4. The selection of chokes for the highvoltage supply is important. Of a group tested only two makes of 

reactors had sufficient positive impedance and would develop the 

desired voltage. The resistive and hysteresis loss in other chokes 

tested was excessive.

The use of a ruggedized meter may be thought extravagant by 

economy-minded manufacturers, but experience has shown that meter 

damage is a major item of service cost on instruments of this general 

size and shape. Tall narrow instruments are easily knocked over, 

with obvious consequences for the meter.
SCINTILLATION COUNTERS FOR GEOLOGIC USE 219

The heart of the portable scintillation counter is the sensitive element a thallium-activated sodium iodide crystal and photomultiplier tube. The thallium content of the crystal is about 1 percent by 

weight. About 25 percent of the gamma rays from radium produce 

sufficient light in a 1X 1%-inch crystal to be detected by the circuit 

with a photomultiplier tube of minimum acceptable sensitivity. The 

type 6199 tube, although far from ideal for this application, was used 

because it was the only small-window (1.5 inches in diameter) tube 

available when the instrument was designed. This tube is often 

microphonic, and sometimes exhibits two stable states of operation, 

apparently related to the physical position of the dynode structure 

with respect to the cathode at fixed dynode voltage. A slight jarring 

may cause the tube to change from one stable state to another. With 

dynode voltage held constant, the sensitivity varies over a wide range. 

Careful selection of tubes from commercial runs is essential; on the 

average, 1 tube out of 4 can be used in this instrument. The tubes 

tested in our laboratory exhibit a trend in sensitivity that can be 

associated with physical dimensions: generally, the shorter tubes are 

more sensitive. The new type 6291 is not microphonic in ordinary 

handling, has exceptional gain, and is relatively uniform in sensitivity (factor of 4 tubes in 7 tested). Unfortunately, its overall operating voltage of 1,200 volts is somewhat high for portable 

instruments.

Much of the difficulty with photomultipliers can be avoided by 

operating each tube at its best voltage. When the regulator tube 

(F-6) is matched to the photomultiplier tube, the sensitivity will be 

much more uniform. However, commercially available regulator 

tubes cover only a narrow range of voltages. Regulators with a spread 

of voltages (900 to 1,050 volts) can be obtained on special request 

and make possible the selection of the proper regulator for the 

individual photomultiplier tube being used. This matching procedure 

greatly complicates servicing but makes it possible to use some photomultipliers that otherwise would be discarded.

Pulses of all amplitudes come from the photomultiplier. They are 

roughly proportional to the energy dissipated in the crystal, up to a 

point of saturation. The lower limit for pulses that are detected by 

the amplifier is usually determined by the ultimate input sensitivity 

of the circuit rather than the background noise of the photomultiplier. 

Very fine pulse-height discrimination is common practice in the laboratory, but it is difficult to achieve with any degree of reproducibility 

in a simple portable instrument.
220 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

CALIBRATION

In principle, the procedure for absolute calibration of a scintillation counter is the one used for Geiger-Miiller counters. The instrument is brought into a known flux of gamma radiation, usually from a 

radium needle, and the circuit is adjusted to give the correct reading 

in milliroentgens per hour. If all instruments are correctly calibrated, 

their independent readings can be compared directly, regardless of 

the spectrum of the source of the gamma rays measured, and the 

intensity will be in milliroentgens per hour if the source is radium.

In practice, however, various difficulties arise. Our instrument 

is calibrated by first-flight quanta from an effective point source. 

Scattered radiation is measured separately (by interposing a lead 

block between source and detector) and is subtracted from the gross 

intensity. In nature, the source is almost always extended, and much 

of the radiation reaching the detector has been scattered, so that the 

effective spectrum is greatly enriched on the low-energy end.

The Geiger-Miiller counter is not especially sensitive to low-energy 

radiation, and a point-source calibration is reasonably adequate. The 

scintillation counter, however, has good sensitivity down to very low 

energies, and most of the scattered radiation is detected. Consequently, the portable scintillation counters calibrated with a point 

source will not show the correct intensity (in milliroentgens per hour) 

when the measured radiation is scattered. The gross readings are 

still comparable from counter to counter, but the units are essentially 

meaningless in practice.

The second difficulty in wholesale calibration arises from the great 

variation in quality of photomultiplier tubes and the consequent wide 

variation in sensitivity of the scintillation counters. Therefore, in 

a given group of instruments the choice is either to tailor each circuit 

to fit the particular photomultiplier tubes or to calibrate all instruments at the sensitivity level of the worst one in the group. The 

first alternative is usually desirable. If the voltage versus gain relation is known for all the photomultiplier tubes, then the proper value 

of voltage-regulator tube (F-6, fig. 55) can be selected from a random 

lot for a particular photomultiplier tube. In this manner a high standard of sensitivity can be uniformly achieved.

When it is impossible to select and match components, it may be 

desirable to adjust the scintillation counter to optimum sensitivity, at 

the expense of absolute calibration. Uniform calibration tends to remove the greatest advantage of the scintillation counter, its high 

sensitivity. It has been our experience that geologists prefer to have 

their instruments adjusted for optimum sensitivity, at the expense of 

absolute calibration.
GEOLOGICAL SURVEY BULLETIN 1052 PLATE 9

EIGHT COMMERCIALLY AVAILABLE MODELS OF THE HAND-PORTABLE SCINTILLATION COUNTER
GEOLOGICAL SURVEY 

V

BULLETIN 1052 PLATE 10

A

F.

CARBORNE SCINTILLATION COUNTER

A, Detector assembly, electronic circuit, and strip-chart recorder; B, detector mounted on car top; C 

electronic circuit and strip-chart recorder mounted under instrument panel.
SCINTILLATION COUNTERS FOR GEOLOGIC USE 221

The third problem arises from the nonlinearity of the circuit afc 

very high counting rates. Even with a dead time of only about 15 

microseconds, coincidence loss is high at the rates that are observed in 

some phases of geophysical exploration, particularly underground. 

We see no simple way of greatly reducing the dead time of the circuit. 

Geiger-Miiller counters should be used where the intensity is too high 

for linear operation of the scintillation counter.

The stability of the pulse-height acceptance level of this circuit 

is considerably better than one normally expects from portable equipment. This stability is achieved by suitably matching the various 

batteries and their loads so that bias on the grid of tube F-2 decreases 

as the plate voltage decreases with aging batteries. In addition, the 

oscillator is adjusted to a frequency higher than the optimum. Thusy 

the voltage on the photomultiplier increases with aging batteries, with 

the net result that the overall sensitivity is stable. Specifications require that 1 calibration adjustment be sufficient to keep the sensitivity 

within 10 percent of the original value for a period of twenty-five 8- 

hour days (at constant temperature).

SPECTROMETRY

It is frequently suggested that portable scintillation counters be 

used as spectrometers to identify the radiation source by the energy of 

its gamma rays (Pringle, Roulston, and Brownell, 1950; and others). 

We have found that interest in this application is largely academic. 

It is true that a portable scintillation counter could distinguish among 

uranium, thorium, and potassium under especially favorable conditions. However, the required source concentration is so high that 

almost any geologist can identify the mineral long before he has 

assembled enough of it for a spectrometric examination. For some of 

the rarer thorium-uranium minerals, a visual estimate of the thoriumuranium ratio may not be possible, but the physical determination of 

this ratio is difficult even with fairly complete equipment in the laboratory (Peirson, 1951; Eichholz, Hilborn, and McMahon, 1953; Hurley, 

1956).

SPECIAL APPLICATIONS

Occasionally, it may be necessary to make accurate determinations 

at very low radiation levels, such as measuring the radioactivity of 

vegetation, or tracing isorads in areas where radioactivity differences 

are small. Entomologic applications (the tracing of insects tagged 

with radioisotopes) also fall in this category.

If sufficient time is available to allow the needle to reach a stable 

position (a long time constant), the scintillation counter can be modified by increasing the time constant and connecting a suitable battery
222 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

across the range-resistor system, in opposition to the voltage developed 

across these resistors. This change will increase the amount of meter 

deflection with a given increase in radiation flux and lengthen the 

effective time constant of the instrument. Any portion of the original 

range of sensitivity may thus be expanded electronically to extend 

from zero to full-scale reading on the usual meter. However, the modification tends to increase the drift of the instrument.

If it is necessary to connect more than one photomultiplier to the 

circuit shown in figure 55 (as, for example, in automobile installations), special care must be taken to separate the input channels or 

amplify the signals before combining them. Transistor networks are 

very useful for this purpose. If the channels are not separated, some 

of the signal from one detector will be dissipated in the matching networks of the other, or others, and the overall efficiency will be impaired.

AIRBORNE-CARBORNE SCINTILLATION COUNTER

Shortly after development of the portable scintillation counter it 

became apparent that a more sensitive scintillation counter that would 

make a permanent record of radioactivity was needed. An instrument 

of this type could be used for rapid surveying from light aircraft 

and motor vehicles; therefore, the requirements for both types of installation were considered. Plate 10 shows the instrument and complete installation in a station wagon.

PHYSICAL DESIGN

The detector assembly and its coupling circuit are mounted in a 

nickel-plated waterproof steel cylinder approximately 5 inches in diameter and 10 inches high, weighing about 9 pounds. Photomultiplier tube and crystal assembly is mounted in sponge rubber inside the 

detector unit. For carborne use, the unit is usually mounted on the 

roof on a standard luggage carrier and protected from overhanging 

brush and other obstructions by a steel pipe curved to enclose and 

shield the area of the detector.

The electronic circuit is built in a shock-mounted aviation-type instrument case and weighs about 12 pounds. The on-off, range, and 

time-constant switches are on the front panel. Two meters are usually 

provided. One is a simple milliammeter with a large dial, placed 

where it can be seen easily by the driver (or pilot).

The other meter, connected in series, is a strip-chart recording milliammeter carefully shock-mounted. In carborne installations the chart 

drive of the recorder is geared to the speedometer cable of the vehicle 

so that the chart indication is in terms of radiation intensity versus 

distance of travel. The gear assembly consists of a Metron variable-
SCINTILLATION COUNTERS FOR GEOLOGIC USE 223

ratio (30:1 to 800:1) gear-reduction box mounted on the side of the 

recorder case, and a flexible drive shaft to a right-angle worm gear in 

series with the speedometer cable. Commercial gear assemblies for 

different models of cars are also available. In airplanes, the recorder 

is usually clock-driven and equipped with manually operated edge 

markers for reference marking of the chart.

ELECTRONIC DESIGN

In an effort to produce a circuit of maximum sensitivity, special 

attention was paid to small pulses resulting from scattered and hence 

low-energy radiation, at the expense of uniform pulse shaping. An 

instrument that would operate satisfactorily under field conditions 

would have to be stable with respect to temperature, source voltage, 

and time, would permit the detecting element to be at a reasonable 

distance from the electronic circuits without appreciable loss of signal, 

and would be capable of driving either a meter or a graphic recorder, 

or both, to indicate the relative gamma-ray intensity. In addition, 

the usual compromise had to be made to produce a circuit capable of 

fast response to a source of radiation yet with reasonably low statistical fluctuations. The circuit designed to embody these qualities is 

shown in figure 56.

To help attain a high counting rate, a thallium-activated sodium 

iodide crystal 3 inches in diameter and 1^> inches thick is used. The 

crystal is held in an aluminum cup (0.03-inch wall thickness), sealed 

in silicone fluid (100,000 centistokes viscosity) to exclude moisture, 

and is mounted directly on the cathode face of a 3-inch type 6363 

photomultiplier tube to give maximum optical coupling. The surface 

of the crystal in contact with the photomultiplier tube is finished with 

2-0 emery cloth and wiped with absolute alcohol before mounting. 

All other surfaces are left rough.

The photomultiplier tube is operated with the cathode grounded and 

the anode at a high positive potential. The voltages for dynodes 1 

through 10 are supplied from a voltage divider consisting of a series 

of 22-megohm resistors. Dynode 10 is bypassed to the cathode by a 

0.01-microf arad 1,600-volt ceramic capacitor in order to keep a steady 

voltage across the dynodes. The amplification in the photomultiplier 

is determined by the voltage applied, which is adjusted to a value that 

gives the desired response. The overall sensitivity of the equipment 

is adjusted in this way.

The negative pulses developed at the plate of the photomultiplier 

tube are coupled through a high-voltage capacitor to the grid of a 

type 5654 tube connected as a cathode follower. In order to reproduce 

negative pulses with the extremely fast rise time required by the rate-
224 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

tosi
SCINTILLATION COUNTERS FOR GEOLOGIC USE 225

meter circuit, the cathode follower is biased to draw a substantial 

amount of cathode current (10 milliamperes) and uses a 200-ohm load 

resistor.

The amplifier utilizes negative feedback for stability and has an 

overall gain of about 1,000. A high degree of pulse shaping is obtained here because of the short time-constant coupling circuits (2.5 

microseconds input and 5 microseconds interstage) and by clipping in 

the second stage. The cathode follower drives the diode pump without loading the amplifier excessively.

A simplified diagram of the diode pump is shown in figure 57. Its 

operation is as follows. Negative pulses are fed into the input (#* ). 

During each pulse, capacitor Cr is charged through diode D^ At the 

end of the pulse the voltage across the input drops back to normal and 

Cr discharges through diode Dz into capacitor CT. Owing to the 

small capacity of Cr in relation to GT-, only a small charge is placed on 

CT for each discharge of Gr ; therefore, a large number of input pulses 

are necessary to build up an appreciable voltage across CT. CT and 

RT connected in parallel constitute a long time-constant integrating 

circuit which averages the pulses over a period of time and produces 

at the output (E0ut) a d-c voltage proportional to the number and, to 

some extent, the amplitude of pulses that occurred during the averaging time. Eeducing the size of Or increases the number of pulses necessary to produce a given output voltage and vice versa. Reducing 

the size of CT reduces the time constant, or averaging time, of the integrating circuit, and vice versa, but does not affect the value of the 

output voltage.

'.001 jxf

in

RT

1 megohm

FIGUBD 57. Simplified diagram of the diode pump-type ratemeter.
226 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

This basic diode-pump circuit is modified for use in this equipment. 

The whole circuit operates at a potential of about 35 volts above 

ground, except that the bottom of C? is grounded to filter the pulses 

out of the metering circuit. The plate of diode Z>± is connected to an 

adjustable voltage source so that the level at which the circuit begins 

to operate can be selected. This is the discriminator control. Four 

different values of Or are used to give four ranges of sensitivity. 

Three values of C? are used to give three different time constants, and 

provision is made to keep the unused time-constant capacitors charged 

to proper voltage to prevent the indicating meter from "kicking" 

either upward or downward when the time constant is switched.

The metering circuit consists of a stabilized d-c amplifier and a 

cathode follower. The output of the diode pump is applied to the 

grid of the amplifier, and the plate of the amplifier is connected directly to the cathode follower grid. Thus, the amplified output of the 

diode pump appears at the cathode of the cathode follower. About 

half of this voltage is fed back to the bottom of R? in the diode pump, 

and thus to the grid of the amplifier, in order to stabilize the circuit 

against changes in plate voltage, filament voltage, and aging of tubes. 

The indicating meter or recorder is connected between the cathode of 

the amplifier tube and a tap on the cathode load resistor of the cathode 

follower. The position of this tap is adjustable to provide a zero 

adjustment for the indicator.

The regulated high-voltage supply uses the inductive "kick" principle of generating a high voltage. Briefly the operation is as follows: 

Energy is stored in a choke coil connected in series with the plate of 

a type 5654 drive tube, owing to the current flowing in this tube. The 

current is then suddenly cut off by the application of a negative pulse 

to the grid of the tube. The voltage generated by the inductive kick 

in the choke due to this sudden break in current is rectified and filtered 

in an R-C filter and then regulated. The negative pulses supplied to 

the grid of the driver tube are generated by a relaxation oscillator.

Regulation of the high voltage is accomplished by taking a portion 

of the output high voltage, amplifying it, and feeding this as an 

error voltage to the bottom of the grid resistor of the driver tube. 

In this way the amount of current that can flow in the choke coil, and 

thus the amplitude of voltage generated by the inductive kick, is 

controlled.

The use of a high-voltage regulator tube as part of the high-voltage 

divider improves regulation by reducing the overall high voltage 

without reducing the change in the error voltage available to the regulator circuit. Cathode reference voltage for the feedback amplifier 

and for the driver tube is established by a type KE-7 neon tube. The
SCINTILLATION COUNTERS FOB GEOLOGIC USE 227

amplified error voltage at the plate of the feedback amplifier is transmitted through an NE-2 tube to the bottom of the driver-tube grid 

resistor. The NE-2 gives a constant voltage drop and is used to shift 

the d-c voltage level from that at the plate of the amplifier to the 

voltage necessary at the grid of the driver tube. The 500K ohm 

resistor from the junction of the NE-2 and the grid resistor to ground 

serves to keep the NE-2 conducting.

The B-f voltage is regulated at 150 volts by type OA2 regulator 

tubes. Because of the high current requirements of the circuit (45 

milliamperes) and the wide fluctuations in supply voltage to be 

expected in vehicular installations, the B-f load can be split into 

two approximately equal parts, each of which is regulated by a 

separate dropping resistor and OA2 combination. A more satisfactory method of regulating the B-f voltage is to use a single OA2 

tube to regulate the voltage and a 10-watt 120-volt lamp for the 

dropping resistor. B power is derived from the 6- or 12-volt system 

of the car or plane through a vibrator power supply of standard 

commercial design. Heater power for the vacuum tubes is taken 

directly from the battery in 6-volt installations; in 12-volt installations 

the heaters are connected in series pairs and powered by the 12-volt 

battery.

RESPONSE

The response of the instrument was tested with a point source of 

24.28 milligrams of radium, in equilibrium with its decay products, 

in a platinum-iridmm needle with a 0.5-millimeter-thick wall. This 

amount of radium corresponds to a field of radiation of 204 roentgens 

per hour at a distance of 1.0 centimeter.

The needle was mounted on a wooden pole 7 feet above the ground. 

The needle and pole were placed at points 50 feet apart along a line 

perpendicular to the line of motion of the vehicle. The car was then 

driven past the source. The graphic recorder was adjusted to approximately 18 inches of chart advance per mile of vehicle travel.

The tests were made in a flat field with a uniform background 

value (0.018 milliroentgens per hour), which was determined by 

taking the average of the readings of a group of laboratory-calibrated 

field-survey meters.

The results are shown in figures 58-61, which show the peaks, each 

representing 660 feet of traverse, obtained by driving in a straight 

line past the source at different distances and speeds. The peak 

heights are in uniform arbitrary units of meter deflection, and the 

smooth curve is the calculated radiation intensity from the point 

source as a function of distance, ignoring buildup due to scattering
228 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

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DISTANCE FROM SOURCE, IN FEET

FIGURE 59. Graph showing the response of the airborne-carborne scintillation 

radium source at a speed of 20 miles per hour along 660-foot straight-

setting 30).
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200 - 

.190 - 

.180 

.170 

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.050

.040

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300 250 200 150 

DISTANCE FROM SOURCE, IN FEET

FIGUEB 60. Graph showing the response of the airborne-carborne scintillatio

radium source at a speed of 30 miles per hour along 660-foot straight

setting 30).
0.220

210

.200

.190

.180

.170

.160 

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.140 

.130 

.120

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300 250 200 150 

DISTANCE FROM SOURCE, IN FEET

FIGDEB 61. Graph showing the response of the airborne-carbone scintillatio

radium source at a speed of 40 miles per hour along 660-foot straight

setting 30).
232 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

from the ground and in the air. This curve is common to all four 

graphs.

From these graphs one can obtain a semiquantitative picture of 

the limits of detectability of a point source as a function of distance 

and speed of travel. It will be seen that the source is clearly picked 

up at a distance of 250-300 feet at a speed of 10 mph, and that it is 

still discernible about 200 feet away at a speed of 40 mph. The top 

speed for surveying from a car seldom exceeds 40 mph.

It should be remembered that in the carborne installation the front, 

back, and bottom of the detector are shielded to reduce undesired 

radiation from the road or from ore samples carried in the vehicle. 

Consequently, the peaks appear sharper than they would without the 

shielding.

PORTABLE SCINTILLATION GAMMA-RAY WELLLOGGING INSTRUMENT

The major advantage of the scintillation gamma-ray well-logging 

instrument, as with other types of scintillation counters, is the high 

sensitivity associated with the extremely high counting rates that can 

be obtained from the sodium iodide crystal and photomultiplier tube 

combination and the inherently small statistical fluctuation of this 

counting rate. These virtues are reflected directly in the ability of 

the instrument to detect and record reliably very small variations in 

radioactivity. A second advantage is the possibility of spectral measurements in the hole (DiGiovanni and others, 1953). However, little 

information has been published to show the accuracy of spectral measurements or how useful they could be in practice. Several aspects are 

discussed by Tittle (1954).

The principal usefulness of the scintillation gamma-ray well-logging 

instrument is in holes where high radioactivity (that is, from uranium 

ore) is not expected. Therefore, our portable scintillation gamma-ray 

well-logging instrument was designed to offer maximum sensitivity to 

gamma radiation, primarily for correlating radioactivity with lithology and stratigraphy. However, when ore is found, the sensitivity 

of the instrument can be reduced by as much as 30 times and ore-grade 

material can be logged satisfactorily.

PHYSICAL DESIGN

The reel is operated manually and has a maximum capacity of 1,500 

feet of RG-59/U coaxial cable. This cable is 0.245 inches in diameter 

and has a tensile strength of approximately 200 pounds. It is wound
GEOLOGICAL SURVEY BULLETIN 1052 PLATE 11

PORTABLE SCINTILLATION GAMMA-RAY WELL-LOGGING INSTRUMENTS

SCINTILLATION COUNTERS FOR GEOLOGIC USE 233

on a reel of light construction. Two versions of the instrument are 

shown in plate 11.

The design of a suitable reel is, to a large extent, a matter of personal 

choice. Only a few aspects are troublesome. The miearta-insulated 

vertical slip-ring assembly is enclosed in a cast-aluminum housing 

and sealed with felt washers. Each slip ring has three spring-loaded, 

wiping contacts. The contacts and slip ring are made of brass; after 

being "run in" on a lathe, the noise level of this moving connection is 

below the noise level of the photomultiplier tube. Two drives for the 

reel are usually provided, one driving the reel directly, the other with 

a mechanical advantage of about four. A metering wheel located on 

a tripod over the bore hole, or directly on the reel frame, provides a 

direct means of measuring the hole depth by reading a revolution 

counter geared to the wheel. The metering wheel also provides a 

means of driving the chart movement of the recorder, providing a continuous log of radioactivity in the bore hole. The diameter of the 

metering wheel is cut to give a convenient relation between the advance of the chart per foot of hole logged. The error in depth measurement is surprisingly small usually less than 0.1 percent.

The probe has an outside diameter of 1% inches and is approximately 14 inches long. Contained within the probe are the 

photomultiplier tube and dynode voltage dividing system, the thallium-activated sodium iodide crystal (l^ inches in diameter and 2 

inches long), and one-half of the impedance-matching circuit necessary 

for transmitting the pulse through the coaxial cable. Figure 62 

is a schematic diagram of the circuit in the probe.

The crystal is immersed in silicone fluid (200,000 centistrokes viscosity) in an aluminum cup with 0.020-inch wall thickness and is 

spring loaded against the window of the photomultiplier tube. The 

silicone fluid provides optical coupling between the surface of the 

crystal and the cathode of the photomultiplier. The photomultiplier 

is shielded magnetically. The part of the aluminum cup containing 

the sodium iodide crystal extends beyond the magnetic shield. The 

skirt of the cup fits over the glass envelope of the photomultiplier 

tube and makes possible a firm connection between tube and crystal. 

The entire assembly crystal, photomultiplier tube, socket, magnetic 

shield, and matching circuit is fitted snugly into the probe shell. 

The probe is then sealed and made waterproof by a pressure gland 

consisting of three O-rings with a compression nut. A probe with 

a brass shell of 0.030 inches wall thickness was tested for strength in a 

triaxial kerosene-filled pressure chamber and collapsed at 600 psi.
234 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

RG59/U

Nal 

crystal

FIGURE 62. Circuit diagram of the scintillation probe.

The amplifier-ratemeter, high-voltage supply, driving circuit for 

the recorder, the surface part of the matching circuit, and the battery 

complement are contained in a suitable case attached to the back of 

the recorder. The external controls are the on-off, range, and timeconstant switches, and the zero and sensitivity controls. The case is 

sealed against moisture and dust for use under the adverse conditions 

common to logging.

ELECTRONIC DESIGN

The sensitivity of the probe is such that when placed in a field 

of 1 milliroentgen per hour of radium gamma rays, a minimum of 

300,000 counts per minute will be recorded. Much higher counting 

rates are possible, but the value quoted here is a compromise which is 

necessary if a number of instruments are to be made uniform in 

response and the calibration maintained over a long period of time.

The electrical impulse from the photomultiplier tube is fed into the 

coaxial cable through the 1000:1 impedance-matching transformer, 

T! (fig. 62). The coaxial cable has a characteristic impedance of 

73 ohms, a capacity of 21 micromicrofarads per foot, is insulated for 

2,300 volts, and has an attenuation constant of 3.8 decibels per 100 feet 

at 100 megacycles. A second impedance-matching transformer, Ta 

(fig. 63), identical to TI and electrically reversed, is located in the 

ratemeter at the surface. The pulse at the high-voltage side of transformer Tz is in the form of a damped oscillation; however, the
SCINTILLATION COUNTERS FOR GEOLOGIC USE 235
236 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

damping factor is large enough so that only the first wave will 

trigger F-l.

The high voltage is connected to the dynode system of the photomultiplier tube through the same electrical path matching transformers and coaxial cable that returns the electrical impulse to the 

counting ratemeter. The high-voltage oscillator tube (F-4) is a subminiature tetrode type CK-526-AX. F-4 is triggered by a sawtooth 

voltage provided by a relaxation oscillator consisting of NE-2, a 

0.005-microfarad condenser, and a 5-megohm resistor. The frequency 

of oscillation is approximately 100 cycles per second and can be controlled by adjusting either the value of the 0.005 microfarad capacitor 

or the 5-megohm resistor. The high voltage, which is affected by the 

frequency of oscillation, is generated by the inductive kick in the 

choke when the current in F-4 ceases abruptly. This voltage is rectified and regulated. The high-voltage supply used in the scintillation gamma-ray well-logging instrument has 2 regulator tubes (F-6 

and F-T) which regulate at 1,200 and 900 volts respectively. The desired voltage is selected by an external high-voltage selection switch 

at the amplifier-ratemeter. The 1,200-volt setting is used for high 

sensitivity, when low-level activity is measured, while the 900-volt 

setting is used for logging bore holes containing ore-grade materials. 

The high voltage can be adjusted over a range of 120 volts by the 

high-voltage adjustment control. This control of counting rate by 

adjusting the high voltage along with an adjustment of the input 

sensitivity control on the instrument makes possible a wide range of 

sensitivity. Therefore, the instrument can be used for logging oregrade materials with the same linearity and reproducibility as in logging for stratigraphic correlation. Linearity and reproducibility are 

extremely important when the instrument is to be calibrated for quantitative measurements.

The ratemeter proper is essentially analogous to the circuit described earlier (fig. 55). A 1-milliampere recorder is driven by tho 

potential developed across the resistance in the cathode circuit and 

F-3. The circuit dead time of the trigger pair is approximately 8 

microseconds, the pulse time 4 microseconds. Four ranges are calibrated for full-scale readings of 0.04, 0.12, 0.4, and 1.2 milliroentgens 

per hr (unscattered radium gamma rays). These readings correspond 

respectively to 12,000, 36,000,120,000 and 360,000 counts per minute. 

The two potentiometers for adjusting the electrical zero and instrument sensitivity are controlled externally. Two time constants (1 

and 5 seconds) are normally available. The 1-second time constant is 

commonly used. It is possible to reduce the time constant to 0.2 second by modifying the circuit. With such a fast time constant and a 

logging speed of 5 feet per minute, 90 percent of a maximum reading
SCINTILLATION COUNTERS FOR GEOLOGIC USE 237

may be recorded during the interval needed for the probe to travel 

about %-inch. Such rapid response is not normally needed in a portable logging instrument. For all practical purposes, the record of 

radioactivity as a function of depth is not significantly affected by 

variations in logging speed of a foot or two per minute when the 

1-second time constant is used.

CALIBRATION

In order to use the equipment to its full capabilities it should be 

calibrated in terms of grade and thickness of the ore. Although several methods have been attempted, the simplest and most accurate 

involves the use of simulated drill holes (fig. 64). The holes can be 

built up in corrugated-iron culvert liners not less than 4 feet in diameter. An aluminum casing of about 16-gauge thickness is used in the 

hole. In our calibration procedure, the ore thickness was varied from 

0.4 to 2.0 feet. Several grades of radiometrically analyzed ore in the 

0.01-1.0 percent equivalent uranium range were used.

Each grade and thickness combination was logged approximately 

25 times in order to obtain a mean value. The results are shown in 

figure 65.

In order to use the calibration chart to determine the grade of 

material in an exploratory drill hole, it is necessary to know the height 

of the deflection registered on the chart and assume the thickness of 

the postulated ore layer. An indication of the thickness of the material, for layers more than a few inches thick, is obtained by measuring 

the width of the peak at approximately 70 percent of the maximum 

deflection.

Field experience with the described equipment has shown that the 

fast time constant and a logging speed of 10 feet per minute will give 

very good results for both determining lithology and estimating grade 

and thickness of radioactive material. In order to maintain the calibration in the field, a radioactive source should be carried with the 

instrument and the instrument response to the source at fixed geometry 

should be checked daily. In this way any drift due to battery fatigue 

will be recognized and the batteries can be replaced or the necessary 

adjustments can be made to compensate for the drift.
238 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

2-in.-diameter aluminum casing.

Sand

Separator 

(masonrte and paper)

Separator 

{masonite and paper

Sand

FIGCBB 64. Diagram of simulated ore body used In calibrating scintillation gamma-ray

well-logging instrument.
2

.01

^§^

,Y>

0.1 1.0

EQUIVALENT URANIUM, IN PERCE

§5. Typical calibration cjiart for scintillation gam
240 EXPERIMENTAL AND THEORETICAL GEOPHYSICS 

SELECTED BIBLIOGRAPHY

Brownell, G. M., 1950, Radiation surveys with a scintillation counter: Econ. 

Geology, v. 45, p. 167-174.

Davis, F. J., Barter, J. A., Reinhardt, P. W., and Harris, D. F., 1954, Scintillation detector for carborne and airborne use: Nucleonics, v. 12, no. 12, p. 46- 

47.

DiGiovanni, H. J., Graveson, R. T., and Yoli, A. H., 1953, Scintillation unit for 

drill-hole logging: Nucleonics, v. 11, no. 4, p. 34-39.

Eichholz, G. G., Hilborn, J. W., and McMahon, C., 1953, The determination of 

uranium and thorium in ores: Canadian Jour. Physics, v. 31, p. 613-628.

Faul, Henry, 1948, Radioactivity exploration with Geiger counters: Am. Inst. 

Min. Metall. Eng. Trans., v. 178, p. 458-478.

 1950, Radioactivity methods; in Jakosky, J. J., (editor) Geophysical prospecting : Los Angeles, Trija Publishing Co., p. 987-1015.

Hurley, P. M., 1956, Direct radiometric measurement by gamma-ray scintillation 

spectrometer: Geol. Soc. America Bull., v. 67, no. 4, p. 395-412.

Nelson, J. M., 1953, Prospecting for uranium with car-mounted equipment: U. S. 

Geol. Survey Bull. 988-1, p. 211-221.

Peirson, D. H., 1951, Alpha particle assay and the measurement of the thoriumuranium ratio in radioactive ores: Phys. Soc. (London) Proc., sec. B, v. 64, 

p.876-888.

Pringle, R. W., Roulston, K. L., and Brownell, G. M., 1950, Ultrasensitive portable 

gamma-ray spectrometer: Nature, v. 165, no. 4196, p. 527.

Rajewsky, B., 1943, Das Geiger-Miiller Zahlrohr im Dienste des Bergbaues [The 

Geiger-Mtiller counter applied to mining]: Zeitschr. Physik, Band 120, 

p. 627-638.

Ridland, G. C., 1945, Use of the Geiger-Miiller counter in the search for pitchblende-bearing veins at Great Bear Lake, Canada: Am. Inst. Min. Metall. 

Eng. Trans., v. 164, p. 117-124.

Schmitt, O. H., 1938, Thermionic trigger: Jour. Sci. Instruments, v. 15, p. 24-26.

Slack, H. A., 1949, Radioactivity measurements in the Kirkland Lake area, 

northern Ontario, Am. Geophys. Union Trans., v. 30, no. 6, p. 867-874.

 1952, The application of recent counting techniques to geophysical research Part I, Field measurement of the radioactivity of rocks: Am. Geophys. Union Trans. v. 33, no. 6, p. 897-901.

Slack, H. A., and Whitham, K., 1951, A further investigation of the radioactivity 

of the Round Lake and Elzevir batholiths: Am. Geophys. Union Trans., 

v. 32, no. 1, p. 44-48.

Tittle, C. W., 1954, Gamma-ray and neutron logging in the petroleum industry, 

in Faul, H., (editor), Nuclear geology: New York, John Wiley and Son, 

p. 241-250.

O