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

Abstract................................... 1

Introduction................................ 1

Acknowledgments........................... 2

Physical design............................ 2

Electronic design........................... 2

Page

Calibration................................. 3

Spectrometry............................... 5

Special applications......................... 5

Future developments ........................ 5

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

A small, light, portable scintillation counter, 

primarily intended for geologic field use, has been 

designed and is now commercially manufactured. The 

instrument embodies a very fast trigger amplifier and 

a compact relaxation-oscillator power supply. The 

circuit takes full advantage of the high counting rate 

that can be obtained from a sodium iodide crystal. 

Another, still smaller and lighter, scintillation counter 

of the total intensity type is now being tested.

INTRODUCTION

From rather primitive beginnings (Rajewsky, 1943; 

Ridland, 1945), the portable gamma-ray detector has 

rapidly developed into a popular geologic tool. Probably 

there are more gamma-ray detectors in use today than 

all other geophysical instruments.

The Manhattan Engineer District expended considerable effort on the design of a portable GeigerMuller survey meter for health physics work. The 

basic design adopted toward the end of the war utilized 

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

various high voltage supplies (dry batteries, oscillators, 

vibrators), and was produced commercially by many 

instrument manufacturers. The instrument was clearly successful and-continues to be manufactured to this

day without major 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 geological applications has revealed that the ordinary 

Geiger-Muller survey meter is not sensitive enough 

to be useful in areas of low levels of radiation, roughly 

of the order of background 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 (Nelson, 1953; Slack, 1949; 

Slack and Whitman, 1951; and many unpublished reports) 

couldbe obtained. However, the large counters (as much as 

40 inches long) were exceedingly clumsy to use in the field.

A portable scintillation counter would not be subject to this handicap. Such an instrument was announced by G.'M. Brownell in Canada (1950), and became commercially available shortly thereafter. 

Brownell's instrument weighed about 15 Ibs., had 

considerable drift with temperature and time, and cost 

more than $1,000 in the United States, so that it could 

not become widely used. Nevertheless, Brownell's 

work showed that a portable scintillation counter could 

be very useful in geologic studies.

About 1950, portable scintillation counters were developed independently at the Oak Ridge National Laboratory
and Los Alamos Scientific Laboratory. Each of these instruments had definite advantages: the Oak Ridge circuit 

(Borkowsky and Dandl, personal communication) was very 

fast and had a good oscillator power supply; the Los Alamos 

circuit (Watts, personal communication) 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.

The instrument that was developed (fig. 1) weighs 

about 7 pounds, stands about 12 inches high, and retails 

for about $500. It has now been produced in quantity by 

three manufacturers and used extensively during the 

1953 field season by many private individuals and personnel of the Geological Survey, the Atomic Energy 

Commission, and the Department of Agriculture. 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.

ACKNOWLEDGMENTS

We are greatly indebted to our colleagues at the 

Oak Ridge National Laboratory and 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 Harshaw 

Chemical Company have been very helpful in discussions 

of the various techniques of crystal mounting. We are 

grateful to the many geolo0sts and engineers who have 

helped us in the practical field evaluation of this instrument. The work here reported is part of a program 

that the U. S. Geological Survey is conducting on behalf of the Division of Raw Materials of the U. S. Atomic 

Energy Commission.

PHYSICAL DESIGN

The outward design of the instrument was determined largely by a committee of field geologists in 

Denver, under the chairmanship of L. R. Page of the 

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 is connected to the probe by a flexible coiled cord.

The probe contains most of the circuitry, the 

sensitive element, and the meter. It must be as light 

as possible, and waterproof for work in mines. For 

obvious reasons, the probe must be very rugged. The 

meter must be so placed that it is easily readable at 

arm's length, and the use of the instrument should require only one hand. The probe fastens to the battery 

case with a spring catch and must be 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 

one cubic inch or larger in volume, is housed in a 

thin spun-aluminum can (fig. 2) 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 face of the crystal in contact with 

the photocathode. The other surfaces of the crystal 

are left rough. Commercially available potted crystals are preferred by one manufacturer, but the silicone 

mount, originally developed at the Chalk River Laboratory (Carmichael, personal communication), 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 such as is available commercially under various trade names. The entire 

assembly of the sensitive element is suspended in the 

probe in sponge rubber, to minimize damage when the 

probe is dropped accidentally. Some manufacturers 

are now producing the instrument in a single box in 

order to reduce the cost and still produce a rugged 

instrument.

ELECTRONIC DESIGN

The pulse amplifier (fig. 3) is a trigger pair of 

subminiature tubes (type CK-533 AX) with V-l normally 

conducting. The input sensitivity is varied by means of 

the potentiometer marked "CALIBRATE, " which directly affects the bias on V-2 and to some extent, the 

bias on V-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 f 

resolution period. The pulse length and resolution 

peiiod are independent of range setting, and less dependent on input pulse amplitude and counting rate 

than any other type amplifier tested for portable scintillation 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 

cylindrical crystal one inch thick and IT inches in 

diameter. The ranges are selected by switching resistors in the plate circuit of V-2. A series network 

with preselected 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 C 2 

respectively. The necessary voltage to drive the 

metering circuit is obtained from the range resistors 

in the plate circuit of V-2. This circuit-is essentially 

a vacuum tube voltmeter using a type CK-526-AX. 

A four-volt swing of the grid of V-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 

R 2 in the cathode circuit, which is controlled externally, 

serves as zero balance. Zero drift is extremely small, 

less than one microampere in 40 hours of operation.

The negative 900 volts for the photomultiplier is 

supplied by an oscillator high voltage supply with an 

approximate plate efficiency of 16 percent. The overall
PHOTOMULTIPLIER CRYSTAL

ALUMINUM CUP SPRING

Figure 2. The crystal photomultiplier assembly.

efficiency is about 3.5 percent. The relaxation oscillator (about 100 cycles per second) triggers the grid 

of V-4. The reactive voltage developed by the choke 

in the plate of V-4 is rectified by V-5 and regulated 

by V-6. This voltage is sensitive to oscillator frequency, which drifts as the battery supply fades. The 

frequency is set slightly above the optimum point, 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 feed-back of 

the change of voltage with load at the regulator tube 

through R2 to the grid of V-4. The selection of chokes 

for the high voltage supply is also very important. 

Of a group tested only two 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 such as 

these (fig. 1) are easily knocked over, with obvious 

consequences for the meter.

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 Ra produce 

sufficient light in a 1 x IT inch crystal to be detected 

by the circuit with a photomultiplier tube of minimum 

acceptable sensitivity. The type 6199, 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 in respect to 

the cathode at fixed dynode voltage. A slight jarring 

will cause the tube to "jump" from one stable state to 

another. With dynode voltage held constant, the sensitivity varies over a wide range. Selection of tubes 

from commercial runs is essential. On the average, 

one tube out of four 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 in 7 tubes 

tested). Unfortunately, the necessary overall operating voltage of 1200 volts is somewhat high for portable 

instruments. Much of the difficulty with photomultipliers may be avoided by operating each tube at its 

best voltage. When the regulator tube (V-6) is matched 

to the photomultiplier, 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 1050 volts) 

can be obtained on special request and make possible 

the selection of the proper regulator for the individual 

phototube being used. The matching procedure makes 

it possible to use some photomultipliers otherwise 

discarded, but greatly complicates servicing.

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 is difficult to achieve with any degree of reproducibility in a simple portable instrument.

CALIBRATION

In principle, the procedure for absolute calibration of a scintillation counter is the one used for a 

Geiger-Muller counter. The instrument is brought 

into a known flux of gamma radiation, usually from 

a radium neddle, 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.

up.

In practice, however, various difficulties come 

Our instrument is calibrated by first flight quanta
_ Time Range Constant

C=IOOMMF 

-L each

V 1,2,5 = CK 533 AX 

V3,4 =CK 526 AX

V6 =5841 or 1038

Figure 3. Circuit diagram of the portable scintillation counter.
from an effective point source. Scattered radiation is 

measured separately 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 soft end.

The Geiger-Muller counter is not especially sensitive to soft 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 right 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 photomultipliers and 

the consequent wide variation in sensitivity of the scintillation counters. Given a group of instruments, one has 

the alternative of either tailoring each circuit to fit the 

particular photomultiplier or calibrating all instruments 

at the sensitivity level of the worst one in the group. 

The first alternative is prohibitive in cost, and the 

second 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. Perhaps future experience will modify this view.

The third problem is due to the nonlinearity of the 

circuit at very high counting rates. Even with a dead 

time of only about fifteen 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-Muller 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 V-2 decreases 

as the plate voltage decreases with aging batteries. 

In addition, the oscillator may be adjusted to a frequency higher than the optimum. Thus, the voltage 

on the photomultiplier increases with aging batteries, 

with the net result of stable overall sensitivity. Specifications require that one calibration adjustment be 

sufficient to keep the sensitivity within 10 percent of 

the original value for a period of 25 eight-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 Browriell, 1950; and others). 

We have found that interest in this application is largely 

academic. It is true that a portable scintillation counter 

could distinguish between uranium, thorium, and potassium under specially 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 spectrometrio examination. For 

some of the rarer thorium-uranium minerals, a visual estimate of the thorium-uranium ratio may not be possible. The 

physical determination of this quantity is difficult even with 

fairly complete equipment in the laboratory (Peirson, 1951; 

Eichholz, Hilborn, andMcMahon, 1953) and one should not 

expect too much from portable equipment.

SPECIAL APPLICATIONS

The scintillation counter described here can be used 

in automobiles and small aircraft without modification, as 

its sensitivity is entirely adequate for such uses. A more 

elaborate version constructed in our laboratory uses crystals 

two inches in diameter to increase the sensitivity. For use 

on automobiles, the detectors are shielded with^ inch of 

lead to improve directional sensitivity toward the sides. A 

detector is mounted on each side of the roof of a car (fig. 4) 

and the output of both photomultipliers is fed into a circuit 

very similar to that shown in figure 3, except that the output 

tube V-3 is atype CK 506 AX, whichhas sufficient power to 

drive a strip chart recording milliammeter. A switch permits the use of one or both of the detectors to facilitate 

directional searching.

The samebasic circuit also has been modified for 

gamma-ray logging of holes as deep as 1,000 feet (fig. 5). 

Where it is necessary to transmit pulses over cables even 

just a few feet long, a matching circuit of the type shown in 

figure 6 is found useful. The logging probe contains a photomultiplier with a small crystal and the matching circuit. 

A similar circuit precedes the input in the surface instrument 

which is identical to the carborne or airborne unit discussed 

earlier.

If it is necessary to feed more than one photomultiplier 

into the circuit shown in figure 3 with matching circuits in 

between, special care must be taken to separate the input 

channels or amplify the signals prior to combining them. 

Transitor 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, with the consequence of impaired overall efficiency.

FUTURE DEVELOPMENTS

It is not likely that the size and weight of the present 

instrument can be substantially reduced by future design. 

It is hoped, however, to improve stability and reproducibility 

to a point where the instrument can be calibrated in absolute 

units and can be used for accurate surveys.

Anew design has been in preparation for aboutayear 

and is now reaching prototype stage (fig. 7). The new instrument uses the same high voltage supply, but the counting 

circuit is greatly simplified. This circuit does not "count" 

pulses, but, in effect, measures the total intensity of light 

emitted by the crystal. Since the amount of light is a function 

of both the number and the energy of the incident gamma 

rays, the meter reading of this instrument will indicate more 

nearly the "total dosage rate" rather than the number of disintegrations of a source of radiation.

The circuit (fig. 8) is essentially a d-c amplifier, 

biased to projected cutoff. The meter is in series with 

the plate and is mechanically set to read zero when about
Figure 4. Car-mounted twin scintillation heads, shielded with lead for directional search.
Figure 5. Scintillation logging equipment. 

7
DETECTOR COAXIAL CABLE

0

TO

RATE 

METER

Figure 6. Matching circuit for transmission of pulses over loner coaxial cables.

Figure 7. Experimental model of the integrating (total intensity) scintillation counter.
0-50 uA

50MMF

IOOK

: 10

< Time 

Constant

67.5v.

CK522AX 5.1 M

>

> , >

>

Ra

-*

\

nge 

200M

20 M

2M

*c*

~

' 

X

Li5 ~i

M

5

22.5

4 

<

^«

 ^ < 

<

^25( 

>

>

44 M

900 v.

Figure 8. Circuit diagram of the integrating scintillation counter.

three microamperes of plate current flow. Bias 

is applied to the cathode by means of a bleeder 

network. The screen is supplied by a separate 

22. 5-volt battery connected directly between screen 

and cathode. This arrangement prevents the falling off 

of screen voltage which would result in non-linear 

response of the meter if the screen were fed by 

means of a dropping resistor or bleeder from B+. 

The signal is applied directly to the grid from 

dynode No. 10, as a positive voltage is required, 

and the sensitivity is controlled by the amount of 

resistance in this circuit. Earlier experimental 

measurements with this type of circuit showed 

rather large and frequent upswings of the meter 

while measuring relatively low background radiation. 

These upswings are attributable to large pulses, 

from 10 to 30 volts in amplitude caused by cosmic 

showers, whereas the average pulse from radium 

has a peak value of about 2 volts. The effect of 

these extremely large pulses is reduced in this circuit, 

because any pulse greater than about 2. 5 volts in 

amplitude drives the grid positive in respect to 

the cathode and is clipped in the grid circuit. In 

addition, the flow of grid current thus caused develops a bias pulse across the 10 megohm grid 

leak-50 micromicrofarad condenser combination to 

counteract the effect of the large pulse on plate 

current.

A potentially useful feature of this integrating 

instrument is the continuously variable time constant of the meter circuit. By adjusting the 

100,000 ohms potentiometer in series with the 

meter, the response time (and therefore the flutter) 

of the meter can be/ made to have any value that 

a situation may demand.

The complete unit, including batteries, is now 

contained in a case 4 x 4^ x 3 inches, with a probe 

2 inches in diameter projecting 5| inches from 

the front (fig. 7). The meter is mounted in the 

back of the case. The total weight of the preliminary model is less than 3^ pounds, and it has 

a battery life of nearly 200 hours. Bench tests 

show negligible drift in zero and sensitivity throughout the life of the batteries.

There are definite drawbacks to this circuit 

in its present form. Only the best of photomultipliers will work satisfactorily; most have insufficient gain at 900 volts; some have too much dark 

current; some have internal leakage which affects the 

zero adjustment. The dark current sets a lower 

limit to the useful response of the instrument. 

A larger crystal will help solve this difficulty, 

but the weight and cost of the crystal becomes 

an item to be considered in the design of a small 

instrument. The high value of range resistors 

(200 megohms on the most sensitive range) may 

make the reading sensitive to humidity changes. 

It is not known how this instrument will compare 

in the field with those already in use and described in the earlier parts of this paper. Because the meter indication depends as much on 

energy as it does on the number of disintegrations, 

it is possible that this instrument will not show 

as good a response to weak anomalies in the field 

as the instruments now in use. The great advantages of small size and weight and slightly lower 

construction cost may overbalance the obvious 

shortcomings of the circuit. Extensive testing, 

now in progress, will be required before the usefulness of the design can be determined.
LITERATURE CITED

Brownell, G. M., 1950, Radiation surveys with a 

scintillation counter: Econ. Geology, v. 45, 

p. 167-174.

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. Met. Eng. Trans., 

v. 178, p. 458-478.

_______1950, Radioactivity methods: Chap. X, 

p. 987-1015, in Jakosky, J. J., Exploration geophysics, Trija Publishing Company, Los Angeles.

Nelson, J. M., 1953, Prospecting for uranium with 

car-mounted equipment: U. S. Geol. Survey Bull. 

988-1.

Peirson, D. H., 1951, Alpha particle assay and the 

measurement of the thorium-uranium ratio in radioactive ores: Phys. Soc. London Proc. B, v. 64, 

p. 876-888.

Pringle, R. W., Roulston, K. I., and'Brownell, G. M., 

1950, Ultra-sensitive portable gamma-ray spectrometer: Nature, v. 165, p. 527.

Rajewsky, B., DasGeiger-Muller-Zahlrohr imDienste 

des Bergbaues [The Geiger-Muller counter applied to 

mining]: Zeitschr. Physik, Band 120, p. 627-638.

Ridland, G. C., 1945, Use of the Geiger-Mueller 

counter in the search for pitchblende-bearing veins 

at Great Bear Lake, Canada: Am. Inst. Min. Met. 

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, p. 867-874.

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, p. 44-48.

10 INT.-DUP. SEC.. WASH., D.CJ0272