GEOLOGICAL SURVEY CIRCULAR 427

A FIELD INSTRUMENT FOR 

QUANTITATIVE DETERMINATION OF 

BERYLLIUM BY ACTIVATION ANALYSIS
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

GEOLOGICAL SURVEY CIRCULAK 427

FIELD INSTRUMENT FOR QUANTITATIVE DETERMINATION OF BERYLLIUM

BY ACTIVATION ANALYSIS

By W. W. Vaughn, E. E. Wilson, and J. M. Ohm

Washington, D. C., 1960

Free on application to the Geological Survey, Washington 26, D. C.
CONTENTS

Page Page

Abstract _____________________________________ 1 Instrument operation_______ ___________________ 5

Introduction __________________________________ 1 Sample preparation ___________________________ 6

Acknowledgements.--.-.---__-_____-__________ 3 Calibration __________________________________ 8

Physical design_____-_-_______________________ 3 Possible application __________________________ 9

Electrical design _____________________________ 3 Selected references--------------------------- 9

ILLUSTRATIONS

Page

Figure 1. Beryllium detector __________ ______________________________________________________^_________ 2

2. Photomultiplier tube circuitry _______________________________________________________________ 4

3. Oscilloscope trace with sample removed ______________________________________________________ 5

4. Oscilloscope trace with sample in place _______________________________________________________ 5

5. Characteristics of tube ______ ________________________________________________________________ 5

6. Schematic of detector._______________________________________________________________________ 6

7. Equipment for sample preparation ____________________________________________________________ 7

8. Calibration curve ___________________________________________________________________________ 8
A FIELD INSTRUMENT FOR QUANTITATIVE DETERMINATION OF BERYLLIUM

BY ACTIVATION ANALYSIS

By W. W. Vaughn, E. E. WUson, and J. M. Ohm

ABSTRACT

A low-cost instrument has been developed for quantitative determinations of beryllium in the field by 

by activation analysis. The instrument makes use of 

the gamma-neutron reaction between gammas emitted 

by an artificially radioactive source (Sb12* ) and beryllium as it occurs in nature. The instrument and power 

source are mounted in a panel-type vehicle. Samples 

are prepared by hand-crushing the rock to approximately ^-inch mesh size and smaller. Sample volumes 

are kept constant by means of a standard measuring cup. 

Instrument calibration, made by using standards of 

known BeO content, indicates the analyses are reproducible and accurate to within ±0.25 percent BeO in the 

range from Ito 20 percent BeO with a sample counting 

time of 5 minutes. Sensitivity of the instrument maybe 

increased somewhat by increasing the source size, the 

sample size, or by enlarging the cross-sectional area 

of the neutron-sensitive phosphor normal to the neutron 

flux.

INTRODUCTION

In February of 1958, the U.S. Geological Survey began experimental work with an activation analysis technique using photoneutrons to indicate the presence of 

beryllium in rock samples. The object of the work was 

to develop a low-cost reliable beryllium assay instrument that would require minimum sample preparation 

and analytical time. During the course of the development of the described instrument, the authors became 

aware of a similar development program of an instrument for field exploring by Dr. George M. Brownell of 

the Geology department of the University of Manitoba, 

Winnipeg, Canada, through a visit by Dr. Brownell to 

the Geological Survey offices in Denver, Colorado. A 

description of Dr. Brownell's instrument has been published (Brownell, 1959).

When a gamma source emitting photons with an energy 

of at least 1.66 Mev is placed in close proximity to a 

material containing beryllium, neutrons are released 

from the nuclei of the beryllium atoms. This reaction, 

commonly expressed as Be9 (7, n) Be 8 , has been used 

for at least two decades in applications where radium 

and beryllium in concentrated form are combined to 

provide a source of neutrons for experimental work 

with nuclear-induced reactions. Determinations of the 

Be9 (7,111) Be*, reaction using T- rays from the radium chain 

and from ThC'^Tl 20* ) were summarized by Houtermans 

and Bartz in 1943. In 1948, Russel, and others, described experiments using a small chamber whose walls 

were composed principally of beryllium. The chamber 

was filled with various gamma emitters and the neutron 

flux from the beryllium was measured. Russell noted 

the high efficiency with which neutrons were produced

when antimony and beryllium were placed in close proximity. He concluded that there was probably a sharp 

peak in the yield curve at the photon energy of antimony, 

because at higher photon energy levels the yield declined. Results of this experimental work indicated 

that under favorable conditions, and with a good neutron 

detector, the presence of beryllium in nature could be 

measured if the geometric relation between the irradiating photon flux and the beryllium -bearing material 

were optimum.

In 1950, Gaudin and others reported on the use of this 

nuclear reaction in the design of an instrument for 

automatic concentration of beryl. A one -curie radium 

source, emitting approximately 30 percent of its photons with energies above 1.66 Mev, (the threshold level 

for the production of neutrons in beryllium), was used 

as the irradiating source. A boron trifluoride (BFj) 

gas-filled counter tube was used to detect the neutrdhs 

from the beryllium after they are thermalized by paraffin. With this arrangement, a counting-rate ratio 

between the background from the radium gamma source 

and the neutrons originating in beryllium of 1:5 or better 

was possible. Considerable shielding and proper handling precautions are necessary for personnel safety 

when using a one -curie source of radium. The high 

penetration character of gamma radiation minimizes 

the problem of uniform preparation and positioning of 

sample material, which is desirable when quantitative 

results are required.

A nuclear detector for beryllium described by 

Cantwell and others (1958) used the nuclear reaction 

between polonium alphas and the nucleus of a beryllium 

atom,

4Be 9 + aHe 4 

Alpha

2 + 0NJ 

neutron

The alpha -neutron {»,

reaction was discovered by Chadwick in 1932. Combined concentrations of Po and Be are also used for 

commercial neutron sources ranging up to sever*! 

curies in strength. The capture cross section for the 

o, n { reaction is approximately 250 millibarns as compared to 1 millibarn for the gamma -neutron (7, ^reaction. The barn (»o-«* sq cm) in a "cross sectional" 

measurement of the nucleus of an atom and is a measure of its probability of absorbing incident radiation in 

terms of the type and energy of the incident radiation. 

The threshold level for the release of the neutron from 

a beryllium atom by an alpha particle is 3.7 Mev.

Because the probability of a neutron being released 

in beryllium with the a, n reaction is 250 times greater 

than in the 7, n reaction, and selected alpha sources 

have very little interfering gamma activity, the , n| 

reaction would seem more desirable for beryllium 

determinations. However, with the o, .n technique, the 

alpha emitter (usually polonium) must be plated and
 Elevation screw (H)

Signal out

Photo tube(F)

Slow neutron 

phosphor (D)

Paraffin (E) 

Lead (J)

Sample chamber (I)

Rotating lead 

cylinder (B)

Lead cube (C)

Figure 1. Beryllium detector for quantitative determinations.

2
sealed on a surface with a window 1 mill or less in 

thickness and essentially transparent to the alp ha radiation. With the alpha source very close to this window, 

only a thin layer on the surface of a finely powdered 

sample can be analyzed. Because the window containing the polonium is by necessity very fragile, frequent 

monitoring with an alpha-sensitive instrument is required to insure against leakage of the sealed polonium 

source.

The >,« reaction was selected primarily because it 

affords greater flexibility for nonuniform sample preparation in field work. Personnel safety is assured 

through simple dosimetry procedures.

ACKNOWLEDGEMENTS

We wish to thank J. J. Norton, W. R. Griffitts, C. C. 

Hawley, and W. N. Sharp of the U.S. Geological Survey 

for their assistance in standard sample preparation and 

in the field test of the instrument.

PHYSICAL DESIGN

The instrument was designed mechanically to provide 

a simple and rapid means of analyzing berylliumbear ing material in the field (fig. 1) The Sb 124 

source is inserted in an opening (A) on the perimeter 

of, and parallel to, the axis of a lead cylinder (B) approximately 8 inches long and 3 inches in diameter. The 

lead cylinder is then placed in a brass-lined opening in 

a 6-inch lead cube (C). This cylinder can be turned 

through an arc of 150°, allowing the gamma-emitting 

source to be rotated from the shielded position in the 

lead container to the irradiation position for sample 

analysis with a minimum of exposure to personnel. The 

scintillation head containing the boron-zinc sulfide thermal neutron detector (D), the paraffin moderator (E) for 

thermalizing the neutrons from the beryllium, the photomultiplier tube (F), and associated circuitry are 

mounted on a pivot arm (G) above the sample chamber. 

A hand-operated jackscrew (H) provides a means of 

adjusting the height of the sensitive head above the 

sample chamber (I). With this provision a large, irregularly shaped rock can be examined qualitatively 

for the presence of beryllium without crushing the 

specimen.

When the Sb 124 source is rotated to the exposed position for counting two lobe-shaped cams, which are an 

integral part of the shaft for the lead cylinder containing the source, are aligned on either side of the sensitive head and extend above the plane of travel of the 

bottom of the chamber. The sensitive head with the 

inch-thick lead plate (J) in the lower compartment is 

therefore locked in position while the analysis is being 

run. This precaution prevents the sensitive head and 

the protective lead plate from being swung aside, exposing the operator to the full flux from the Sb iz* 

gamma source. A rocker arm operated by the lateral 

movement of the sensitive head locks the source holder 

in the "safe" position when the chamber is swung aside 

for insertion of the sample in the sample chamber. The 

assembly is fastened securely to a j-inch steel base 

plate which is attached to the floor of the vehicle.

ELECTRICAL DESIGN

In the activation-analysis technique, it is necessary 

that the gamma-activating source be close to the sample material and, except in rare applications, relatively close to the neutron detector.

All neutron detectors are somewhat gamma sensitive. Therefore, it is desirable to have a detector with 

a large neutron to gamma sensitivity ratio. A boronenriched (13 percent B10 by weight) polyester containing zinc sulfide was selected. This phosphor, in addition to being a good neutron detector, permits discrimination against gammas by pulse height selection which 

is an inherent advantage in all scintillation phosphors.

If a linear, light-amplifying device such as the 

photomultiplier tube is used to observe the scintillations occuring in the phosphor, the voltage pulse developed at the anode of the photomultiplier tube will be 

proportional to the intensity of the light flash in the 

phosphor. The light flash, within reasonable limits, is 

proportional to the type and amount of energy dissipated in the phosphor by the incident radiation.

A minimum amount of circuitry is used in the sensitive element. The photomultiplier tube (fig. 2) 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 one megohm resistors. Dynode 10 is 

by-passed to the cathode by a 0.01 microfarad 1.600- 

volt capacitor to maintain a constant voltage across the 

dynode string. The amplification factor of the photomultiplier is determined by the voltage applied, and is 

set at a value that gives the desired response. The 

overall sensitivity of the instrument is therefore adjusted by varying the voltage across the dynode string.

The transparent side of the plastic neutron phosphor 

is mounted directly on the cathode face of the photomultiplier tube to give maximum optical coupling. A thin 

film of clear silicone grease is applied to the face of the 

phojomultiplier before the the phosphor is brought into 

contact with the glass. The two surf aces are then joined 

and revolved slightly to force out captured air bubbles.

The pulse spectrum appearing at the anode of the 

photomultiplier tube as a result of the light flashes in 

the phosphor is shown in figures 3 and 4. Figure 3 is 

a photograph of gamma activity with the Sb 124 source 

in the counting position and the beryllium sample removed from the instrument. Figure 4 is a photograph 

of the gamma and the neutron flux with a sample of 

beryl in the counting chamber. The oscilloscope gaincontrol setting and the film-exposure time were the 

same for both photographs. Neutron activity results 

in many pulses of light of approximately the same 

amplitude as those caused by the gamma radiation. 

There is, however, a sufficient quantity of high-amplitude 

neutron pulses (figs. 5 and 6) to permit the input bias to 

be set at a level at which no gamma pulses will be 

accepted by the counting circuit. Therefore, the use of 

pulse-height-selection techniques makes possible an 

accurate recording and correlation of counting rate, as 

a function of beryllium content of the sample. The only 

interference in this pattern is due to cosmic neutrons 

(the largest pulse in figure 4) which, for the described 

instrument, amounts to approximately six counts per
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5 INCH PHOTOMULTIPLIER TUBE

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Figure 2. Photomultiplier tube circuitry. 

4
Figure 3. Oscilloscope trace of gamma background 

from Sb 12* with beryllium sample removed.

Figure 4. Oscilloscope trace of gamma background 

from Sb'w and neutron flux with flux beryllium 

sample in place.

minute at 6,000 feet above sea level. It is possible to 

eliminate this cosmic background with additional pulse 

height analysis, however, it was preferred to subtract 

the cosmic events from the gross counting rate because 

the number is small and seems to remain rather constant over a period of time.

The photomultiplier tube and its operational characteristics are critical when used with this technique. 

For best instrument performance, the operational 

characteristics of the particular photomultiplier tube 

used should be known. A graph (fig. 5) of counting 

rate as a function of voltage can be plotted without a

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Figure 5. Operational characteristics of a photo - 

multiplier tube.

sample in the counting chamber. This graph shows the 

high voltage value at which high voltage drift has a 

minimum effect on counting rate. The phosphor, or any 

other source of light that might activate the photo - 

multiplier, can then be removed, the high voltage settings 

repeated, yielding a plot of the dark-cur rent noise 

factor, (fig. 5). A logical operating high voltage on 

the plateau and below the dark current region would be 

1,225 volts for the curves shown.

The average light emitted by the plastic neutron 

phosphor can be raised to such a high level by excessive gamma radiation, that additional pulses from the 

neutron flux have no effect, and the instrument becomes 

inoperative. In practice, it has been found that the 

gamma field from the Sb f? source at the phosphor 

should not exceed 1,000 milliroentgens per hour. The 

phosphor may also be temporarily "saturated" by normal room light. When the entire phosphor is exposed 

to room light for several minutes, the phosphor will be 

energized to a light level that is easily observed with 

the naked eye in a dark room.

Using the circuitry shown in figure 2, the amplitude of,, 

the pulses resulting from neutrons interacting with the 

phosphor range from 10 to 2 50 millivolts, the largest 

number occurring between 20 and 50 millivolts. Pulse 

height of 20 to 50 millivolts are adequate to trigger 

most commercially available sealers or rate meters 

that might be used as counting devices.

INSTRUMENT OPERATION

An operational schematic of the beryllium detector 

is shown in figure 6. The gamma emitter (100 millicuries of Sb 124 ) has a half life of 60 days. Approximately 70 percent of the photons emitted have an energy 

of 1.7 Mev which is sufficiently above the 1.66 Mev 

threshold energy level for the production of neutrons 

in beryllium.
SCALER

PRINTING

INTERVAL

TIMER

PHOTO TUBE

SLOW NEUTRON PHOSPHOR 

^NEUTRON

PHOTON

SAMPLE CHAMBER

VOLTAGE 

REGULATOR

Sb 124 GAMMA EMITTER

LEAD CYLINDER 

LEAD CUBE

Figure 6. An operational schematic of the beryllium detector.

The neutrons originating in the beryllium have an energy 

centered around 0. 24 Mev region, and and are considered to be in the intermediate energy range. Although the neutrons originating in the beryllium cannot 

be considered as truly monoenergetic, the energy 

spread is probably less than 20 Kev. This makes it 

possible to slow down, or thermalize most of the neutrons released from beryllium.

The neutron has great penetrating powerbecause it 

has no charge and therefore does not ionize atoms by 

dislodging electrons from their orbits as charged 

particles do. It will lose energy to atomic nuclei by 

a series of elastic collisions.

Once the progress of the neutron has been slowed 

or moderated to the thermal excitation level of the 

medium traversed, it may either lose or gain energy 

with successive collisions with nuclei until it is captured by the nucleus of an atom.

In the equipment, described paraffin is used as the 

moderator from which the neutrons emerge with about 

0.025 ev energy and are, on the average, captured 

by the nucleus of B10 atom on entering the slow neutron phosphor. On absorbing the neutron, the B10 atom 

emits an alpha particle which reacts with the ZnS in 

the phosphor and creates a flash of light. This scintillation, in turn, excites the photosensitive cathode 

surface of the photomultiplier tube which releases a 

cloud of electrons. Through secondary emission, from 

dynode to dynode, the cloud of electrons is amplified 

until a sufficient current density is realized at the 

anode to form a voltage pulse across the 1 megohm 

anode load resistor.

SAMPLE PREPARATION

The following method has been used and, although 

seemingly crude, has provided consistent results within
Figure 7. Equipment for sample preparation in the field for quantitative beryllium determinations.

7
16

14

12

10

11.75

LJ 

O

cr

LJ 

Q.

'5.88

200 400 600 800 1000 1200 1400 1600 

NET COUNTS PER MINUTE (TOTAL MINUS COSMIC)

1800 2000

Figure 8. Calibration curve for the beryllium detector.

the operation error (approximately j^0.25 percent BeO) 

of the equipment under field conditions. For analysis 

below 1 percent BeO, a more refined system should be 

devised.

Figure 7 shows the equipment for sample preparation in the field. The sample is crushed by a commercially available hand-operated crusher (a) which 

has adjustable jaws for varying the particle size. A 

split of the crushed material is measured with a standard container (b) which hold approximately 70 grams, 

and placed in the plastic sample holder (c). The plastic 

sample holder, made by the vacuum-mold method, is 

0.020-inch-thick acetate sheet, 3 inches in diameter 

and |.-inch deep.

To make the sample holder, a 6-inch square of the 

acetate sheet is held between two frames of aluminum 

hinged in book form and is heated uniformly to 100°C. 

The assembly is then removed from the furnace and 

immediately placed over a machined mold from which 

air is drawn through a series of small holes. The plastic material, while it is still pliable, is quickly drawn 

down and conforms exactly to the mold shape. By inverting the mold, a second container can be made which

serves as the top of the first molding. An ordinary 

vacuum cleaner provides a sufficient vacuum for this 

purpose.

CALIBRATION

Figure 8 shows a typical calibration curve for the 

beryllium detector. Standards for this calibration were 

prepared by mixing weighed amounts of ground quartz 

and beryl; the beryle was taken from a crystal that had 

been chemically analyzed. An average net counting 

rate (total minus cosmic) w as obtained for each sample 

and plotted as a function of the BeO content. The counting rate increases linearly with the amount of beryllium in the sample. In the range 1 to 20 percent BeO, a 

counting time of 5 minutes was sufficient to give satisfactory reproducibility for field samples.

Rock samples may have concentrations of boron, 

lithium, and other elements with a relatively high 

absorption cross section for neutrons. It was realized that presence of these elements might seriously 

affect the instrument readings. However, tests run 

with prepared samples containing 17 percent of boron
by weight gave no appreciable deviation in counting 

rate from a boron-free split of the same sample material. This probably results from the small sample 

size and from the low specific energy loss of the intermediate range neutrons in the sample material.

To study the effects of grain size, a count was taken 

on a measured amount of various size fractions of a 

given sample, ranging from j-inch and all fines down 

to a rather consistent grind passing 100 mesh. There 

was no significant variation in data in the range of sensitivity studied due to grain size.

The half-life of Sb 124 is 60 days. Therefore, assuming 100 percent initial activity, after 60 days 50 percent 

of the activity will remain and after another 60 days, 

half of the 60-day value or 25 percent of the original 

activity, and so on as time progresses. This decay 

must be taken into account in the quantitative calibration of beryllium detectors over the period of time the 

instrument is u^ed. As the activity decreases, the slope 

of the calibraiion curve (asplotted in figure 8) will increase, that is, the number of neutrons produced by a 

given beryllium concentration decreases. Probably the 

simplest way to correct for the half-life decay is to 

establish a new calibration curve each week, or more 

often as circumstances require. This procedure not 

only establishes a calibration directly associated with 

the instrument and its environment, but provides a permanent record of overall instrument operational effectiveness. A 100-millicurie source of Sbm has given 

satisfactory analyses in the range 0.5 to 12 percent. BeO 

over a period of approximately 10 months (5 half live s).

POSSIBLE APPLICATION

The gamma-neutron reaction for beryllium analysis 

using scintillation counting techniques may be adapted 

to prospecting, core scanning, drill-hole logging, and 

to other applications where radiometric methods apply. 

The most extensive use of the technique, however, will 

probably be made in mines and in process control. 

Under optimum conditions and with a large activating 

source, the instrument could provide instantaneous and

continous monitoring of the beryllium content of a 

slurry or fine aggregate. The beryllium content could 

be indicated automatically in the form of voltage or 

current. The monitoring signal could in turn be used 

to actuate an electromechanical device for the automatic control of the process. Another possible application would be in conjunction with air pollution in an 

industrial environment where beryllium is being processed. By passing the air through the appropriate filter paper and then passing the filter paper through the 

counting chamber, an indication of the beryllium contained in the air could be obtained.

SELECTED REFERENCES

Brownell, George M., 1959, A beryllium detector for 

field exploration: Econ. Geology, v. 54, no. 6, p. 

1103-1114.

Cantwell, T., Hawkes, H. E., and Rasmussen, N. C., 

1958, A nuclear detector for beryllium minerals: 

Mining Eng. v. 11, no. 9, p. 938-940.

Gaudin, A. M., Dasher, John, Pannel, J. H., and Freyberger, W. L., 1950, Use of an induced nuclear reaction for the concentration of beryl: Am. Inst. Mining Metall. Petroleum Engineers Trans., v. 187, p. 

495-498.

Houtermans, F. G., Bartz, I., 1943, Phys.Zeits. v. 44, 

p. 167.

Lapp, R. E., and Andrews, H. L., 1954, Nuclear radiation physics: New York, Prentice-Hall, 532 p.

Russell, B., Sachs, D., Wattenberg, A., Fields, B., 

1948, Phys. Rev. v. 73, p. 545.

Vaughn, W. W., Rhoden, V. C., Wilson, E. E., and Barnett, R. H., 1958, Developments in radiation detection equipment for geology: Second United Nations 

International Conference on the Peaceful Uses of 

Atomic Energy, A/Conf. 15/P/1909, USA.

Vaughn, W.W., Rhoden, V. C., Wilson, E. E., and Paul, 

Henry, 1959, Scintillation counters for Geologic use: 

U.S. Geol. Survey Bull. 1052-F.

Way, Katharine, Fano, Lilla, Scott, Millicent and Thew, 

Karin, 1950, Nuclear data: National Bureau of 

Standards Circular 499.

INT.DUP.,D,WCO-7 71S ,