II





PB 151360

I

^ecknical <

7'2©te

A DOUBLE-PULSE TOTAL-ABSORPTION

FAST NEUTRON SPECTROMETER

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended h\ Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards'; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau's work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope "of activities is suggested by the listing of divisions and sections on the

inside of the back cover.

Reports and Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear cither in the Bureau's own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data lor determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Scries, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau's publications can be found in l\BS Circidar 160, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Go\ eminent Printing Office, Washington 25, D. C.

Inquiries regarding the Bureau's reports should he addressed to the < )ffice of Technical Information, National Bureau of Standards. Washington 25, I). C.
NATIONAL BUREAU OF STANDARDS

technical <vl©te

1

APRIL 24, 1959

A DOUBLE-PULSE TOTAL- ABSORPTION

FAST NEUTRON SPECTROMETER

L. J. NICASTRO AND R. S. CASWELL

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price 75 Cents
National Bureau of Standards OCT 3 1980

103,562,

QC/oo

.U5")53

ABSTRACT C o f3 / 2.

A fast neutron spectrometer utilizing the double-pulse totalabsorption technique has been developed for use in the energy range

1-20 Mev. The total absorption spectrometer uses a hydrogenous

scintillator , in which a fast neutron may lose all its energy,

together with a detector for slow neutrons,, Scintillation detection

of the disintegration caused by absorption of a slow neutron in an element of high reaction cross section serves to identify those

neutrons which lost essentially all their energy in the hydrogenous

scintillator.

Various organic liquid scintillators loaded with natural methyl

borate, and a plastic scintillator with a boron-10 or lithium-6 slow

neutron scintillation detector nearby were all tested with monoenergetic neutrons obtained from the D(d,n) He-^ and T(d,n) He^" reactions.

The best experimental results were obtained with a plastic scintillator

and a Li^lCEu) crystal. Representative data for this spectrometer are given below. For comparison purposes, the results of Monte Carlo

calculations, made by Leiss±/for a phenylcyclohexane liquid scintillator

loaded with methyl borate (B^ enriched), are also included.

1/ —J. E. Leiss, Summary Technical Report for the Atomic Energy Commission

NBS Project No. 3152, 1955-
1 . INTRODUCTION

This report describes an attempt to develop a dauble-^pulse totalabsorption fast neutron spectrometer with moderately good energy

resolution (30$ at 2 Mev to 10$ at 15 Mev) and moderately good

detection efficiency (l0"5 at 2 Mev to 10"*^ at 15 Mev),, This spectrometer was designed to measure the energy distribution of neutrons in

the energy range 1-20 Mev.

There has bean a need for such an instrument in the field of

neutron physics for some time. Swartz^' has given a review of the fast

neutron spectrometers that have been used by various investigators.

Swartz found that the spectrometers commonly in use have either moderately

good energy resolution or moderately good detection efficiency, but not

both.

Recently, however, Murray^' has developed a fast neutron spectrometer

with moderately good energy resolution and moderately good detection

efficiency. His spectrometer, developed at about the same time as the

spectrometer reported here, has the additional advantage that it is

non-directional, that is, f it is not necessary to know the direction from

which the neutrons are coming. ^ Murray's device employs a Li^I (Eu)

crystal, and depends on the Li (n,o;)T reaction. Since this reaction has

a Q of 4.78 Mev, the disintegration products share the energy of the

incoming neutron plus 4.78 Mev. The pulse height is therefore a measure

of the neutron energy. Murray's instrument is probably the most successful

fast neutron scintillation spectrometer developed to date using pulseheight analysis.

k/ Beghian, e_t al~ have used a scheme similar to the one reported here.

Their spectrometer accepted only neutrons which were scattered near 90°

and had undergone a single collision. A time-of-f light technique was utilized to identify these neutrons.

5/ McCord— also has attempted to develop a double-pulse total-absorption

fast neutron spectrometer. His instrument, which utilizes a small plastic

scintillator for the neutron energy absorption and a BF5 proportional

counter surrounding the plastic scintillator for the identification

process, does not give peaks of measurable resolution for monoenergetic

neutrons.

-^C. D. Swartz, NYO-3863 (195*0

.

- R. B. Murray, Nuclear Instruments 2, 237 (1958).

-'Beghian, Allen, Calvert, Halban, Phys. Rev. 86, 4Q^|j|[l952).

^'J. W. McCord, UCRL-3^H S (1956).
2.

2. THE TOTAL ABSORPTION METHOD

A fast neutron incident on a hydrogenous medium usually loses

most of its energy in elastic collisions with hydrogen nuclei,

although other elastic and inelastic collisions may occur. In a hydrogenous scintillator the amplitude of the resultant, light pulse

will, under appropriate conditions, be a measure of the neutron

energy absorbed.

When a large enough scintillator is used, practically ail the

neutrons incident on it will have their energy completely absorbed.

If only elastic collisions are considered, the scintillation response

of the detector is due to the recoil nuclei. However, the response

of scintillation detectors to the recoil nuclei is not a linear

function of energy, but depends on both the energy and mass of the

nucleus, (Birks£/). Therefore, even though a neutron may lose all its

energy in the scintillator, the amplitude of the light pulse will not

be a measure of the energy lost*

In order to obtain a scintillation response which is related to

the energy lost, Clelandl/proposed a spectrometer of such small

dimensions that only those neutrons which lose a large fraction of

their energy in the first collision will have a large probability of

having their energy totally absorbed. In this case, only a few of

the incident neutrons will lose all their energy, and most of these

will be neutrons which lose a large fraction of v;heir energy in the

first collision. The rest of the neutrons, after a few collisions,

will leave the spectrometer. Therefore, the scintillation response

will approach a known functional relation to the energy lost. The

relation between scintillation response and energy for protons is

given by Birks—

(

Since any neutron which loses energy in the scintillator will

make a pulse, there must be some method to identify the pulses

associated with neutrons which have lost all their energy. The

absorption of slowed down neutrons in B or Li , and the detection

of the subsequent disintegration products or the detection of a gamma

ray associated with the disintegration process was the method used to

identify neutrons which lost essentially all their energy. The

1 n elements B and Liu have high absorption cross sections for low energy

neutrons, negligibly small absorption cross sections for high energy

neutrons, and disintegrate following the absorption event.

-'j. B. Birks, Phys. Rev. Qk, %k (l95l); 86, 5^9 (1952).

2'M. R. Cleland, NBS Report 2036.
3.

A delayed coincidence technique is employed to identify events

in which the proton recoil pulse is followed by the identification

pulse. When a proton recoil pulse , delayed by the proper amount of

time, coincides with an identification pulse , the height of the proton

recoil pulse is analyzed and is a measure of the neutron energy.

3» THE SPECTROMETER

Several liquid scintillator cells were prepared, all of which

contained 50 percent natural methyl borate, B(0CH^)x; (l8.8 percent

of the boron is B

10

). The scintillator cells were prepared with

50 percent phenylcyclohexane (C12H16) > 50 percent xylene (CqH^q)> or 50 percent toluene (CyHft) as the organic liquid. The primary organic

solute which gives the highest maximum relative pulse height,

2 - phenyl-5(4 - biphenylyl) - 1,3,4 - oxadiazole (PBD)-{ and the

most efficient secondary solute, 1,4 - di 2 - (5 - phenyloxazolyl)

benzene (P0P0P)z( were used as the scintillating materials. The

concentration of these solutes in the particular liquids was the

optimum as given by Hayes, et al

*-*-',

These scintillators were loaded with a fairly high percentage of

B

1

, so that the reaction, B^{n,0i)L± ' , could be used to identify those

neutrons which lose most of their energy in the scintillator. At

thermal neutron energies, this reaction proceeds in two ways: In

5.8 percent of the disintegrations, the Li' nucleus is left in the

ground state and the alpha particle has an energy of 1.77 Mev; in the

other 94.2 percent of the disintegrations, the Li' nucleus decays to

the ground state by the emission of a 478 kilovolt gamma ray and the

alpha particle has an energy of 1.46 Mev.

There are three methods by which the disintegration event can be

distinguished. After the initial large pulse in the spectrometer

caused by the slowing down of the neutron, the disintegration event may

be observed as: (a) a small delayed pulse in the spectrometer due to

the 1.46 or 1.77 Mev alpha particle, (b) a delayed pulse in the spectrometer due to the 478 kilovolt gamma ray, (c) a delayed pulse in a closely

adjacent Nal(Tl) scintillator due to the 478 kilovolt gamma ray. The

schematic diagrams of Figure 1 illustrate these three methods. The

unloaded hydrogenous scintillator with nearby slow neutron detector is

also illustrated. In all cases only one proton recoil is shown, although

there may be several.

ft/ — Hayes, Otto, Kerr, Rogers, Nucleonics 1_3, No. 12 (l955)« - Hayes, Otto, Kerr, Nucleonics l4, No. 1 (l956)„
The above methods were tried with essentially the same electronic

set-up shown in Figure 2, Cleland's calculations showed that for

neutrons which lose essentially all their energy in the liquid

scintillator loaded with 50 percent natural methyl borate, the disintegration event occurs at an average time of two microseconds after

the initial large pulse. Therefore, a three microsecond gate was used.

Monoenergetic neutrons were obtained from the.D(d,n)He^ and T(d,n) He^"

reactions. Severe electronic and background difficulties were encountered,

however, and none of these methods was very successful in giving

consistent results in the experimentally determined monoenergetic neutron

spectra. It was found in methods (a) and (b) that the electronic apparatus

used could not be made fast enough to observe the small disintegration

pulse within three micro-seconds after the large initial pulse. The

electronic difficulties encountered in methods (a) and (b) did not arise

in method (c) but the large number of chance coincidences due to the high

background in the Nal(Tl) crystal, which is highly sensitive to all kinds

of radiation, obscured true coincidences that may have been present.

Instead of developing fast electronics, it was decided that the

simpler approach would be to use an unloaded hydrogenous scintillator for

the slowing down process and a nearby low energy neutron detector for the

identification process (See Figure l). This scheme removed the electronic

difficulties of the previous approach, and also made it possible to use solid hydrogenous scintillators in place of inconvenient liquid scintillators,

The background difficulty encountered previously could also be removed by the

proper choice of slow neutron detector.

The solid hydrogenous scintillator used for the slowing down process

was a plastic scintillator, Pilot Scintillator B. This plastic is

100 percent hydrocarbon and contains diphenylstilbene as the scintillating

material.

Two different slow neutron detectors were used for the identification

process: a boron polyester and a lithium iodide crystal.

The boron polyester, Slow Neutron Detector NE 401, was obtained from

Nuclear Enterprises Limited. This detector, 1.2 mm thick by 2 inches in

diameter, is a milky white, opaque solid which contains 55 percent B

1 by

weight and ZnS(Ag) for the scintillating material. The detection of slow

neutrons by this scintillator depends on the B (n,a)Li' reaction, which

has already been described. By testing the boron polyester with a thermal

neutron source, it was found that the pulse height distribution of the

disintegration products had no peak, most of the pulses being at low

pulse height.
When testing the boron polyester with Pilot Scintillator B,

essentially the same electronic set-up as that shown in Figure 2 was used except that an integral discriminator took the place of the single

channel analyzer, since the disintegration pulse events could not be

resolved. A gate of three microseconds was used. The separation of the

scintillators was one centimeter.

The results showed a distinct peak in the experimental energy , spectrum of monoenergetic neutrons (2.5 Mev) obtained from the D(d,n)He

reaction. However, the background in the boron polyester was very high,

and therefore many of the coincidences were chance coincidences, which

had to be subtracted out of the spectrum. The boron polyester contains

a high percentage of hydrogen, so that the high background could have

been due to proton recoil pulses. For higher energy neutrons (l4.1 Mev)

obtained from the T(d,n)He^ reaction, the background difficulty became

more severe, and a peak in the experimental energy distribution was not

observed.

The second slow neutron detector, a three mm thick by 1.5 inch

diameter Li"l(Eu) crystal, was obtained from the Harshaw Chemical Company.

Since the detection of slow neutrons by this crystal depends on the

Li"(n,Q:)T reaction, the crystal obtained was one in which the lithium was

enriched to 96 percent lithium - 6. This reaction has a Q of 4.78 Mev.

At thermal neutron energies the disintegration products share this much

energy. But, like B

1

, Li" has a high absorption cross section for low

energy neutrons and a low absorption cross section for high energy

neutrons. Lib l(Eu), a transparent crystal, was also tested with a thermal

neutron source. With this crystal, the pulse height distribution of the

disintegration products had a fairly well resolved peak, (lk percent

full width at half maximum).

The experimental arrangement of detectors and the electronic set-up

for a double-pulse total-absorption spectrometer utilizing Pilot

Scintillator B and a Li°l(Eu) crystal are shown in Figure 2. The details

of the electronic equipment and typical experimental data are given in

the following two sections.
6.

k. APPARATUS

Shown in Figure 2 is the block diagram of the apparatus used for

the spectrometer. Two Pilot B plastic scintillators were employed in

various tests (see Figure 2;. One of these plastic scintillators,

1 inch thick by k inches in diameter, was optically coupled to a five inch diameter, Dumont Type 6j>6k, photomultiplier tube. The sec- ond plastic scintillator, 1 inch thick by 1-^/k inches in diameter,

was optically coupled to a two inch diameter, Dumont Type 6292, photomultiplier tube. The Li°l(Eu) crystal, J) mm thick by 1.5 inches in

diameter, was optically coupled to a two inch diameter, Dumont

Type 6292, photomultiplier tube. The coupling fluid was Dow Corning

Silicone Grease. Each photomultiplier tube was followed by a cathode

follower, and a Ghase-Higinbotham type non-overloading linear amplifier

with an integral pulse height selector (PHS). RG 65/U delay cable of

the proper length (23.^-5 feet per microsecond delay) was used to obtain

the indicated delays. The linear gates were coincidence circuits, which

gave an output pulse proportional to the input pulse when a coincidence

between input and trigger pulses occurred. If either an input or a trigger pulse went into the linear gate without making a coincidence,

there was no output pulse. The single channel analyzer, a Hamner

Differential Discriminator, was followed by a decade scaler, which

counted the pulses that went through the differential discriminator

window. Pulse height analysis of the proton recoil spectrum was done

with a University of Denver Model 103 10-channel pulse height analyzer.

Pieces of apparatus with critical settings were set as follows:

The Li l(Eu) crystal, mounted on a two inch diameter photomultiplier

tube, was calibrated by placing it close to a thermal neutron source. Pulse height analysis gave a resolution of 1^- percent full width at half

maximum for the disintegration spectrum. The window of the differential

discriminator was then set so that all pulses in the entire "thermal"

peak, and disintegration pulses caused by neutrons with energies up to

about 5 kev would pass through the window. When taking a "fast" neutron

spectrum, the Li"l(Eu) crystal was placed with respect to the plastic

scintillator as shown in Figure 2. Between the face of the crystal and

the edge of the plastic there was a separation of one-quarter inch.

PHS No. 1 and PHS No. 2 were used to indicate that pulses coming

from the two amplifiers were in the proper time sequence. Since the

integral discriminators operate on the pulse rise time, PHS No. 1 and

PHS No, 2 were set at zero volts discrimination level. This choice of

discrimination level was made so that the time delay of pulses going to

Linear Gate No. 1 was not variable. However, constant time delays were

introduced by the various pieces of apparatus, and in order to obtain

the proper time sequence between proton recoil pulses and disintegration

pulses, the delay of O.k microsecond connecting PHS No. 1 and the trigger
7.

of Linear Gate No. 1 had to be inserted. The overall time delay was

such that, if a pulse occurred in the plastic scintillator, and at

0.1 microsecond later a second pulse occurred in the Li"l(Eu) crystals

the two pulses arrived at Linear Gate No. 1 at the same time.

The resolving time / of the electronics was determined by the gate

length of Linear Gate No. 1. By feeding pulses to the amplifiers which

were completely unrelated, this gate length was determined experimentally

5. DATA

The data presented here were obtained by allowing monoenergetic

neutrons to fall on the spectrometer. The sources of monoenergetic

neutrons were, the D(d 9 n)He^ and T(d,n)He 4 reactions. For these data,

the scintillators were set with respect to the incident neutrons as shown in Figure 2. The neutrons incident on the spectrometer were

those that leave the target at an angle of 90° with respect to the

deuteron beam. In each case the bombarding deuterons were accelerated

through a potential difference of 300 kev by a 2 Mev High Voltage

Engineering Corporation Van de Graaff accelerator. At this deuteron

energy, the neutrons that leave the target at an angle of 90° with

respect to the deuteron beam have an energy of 2.5 Mev for the

D(d,n)He-^ reaction and l4„l Mev for the T(d,n)He^ reaction.

Chance coincidences, calculated from the relation

P

CD^- C

C'

have been subtracted from the spectra. Cp(prOton recoil count rate)

is the count rate in the 10-channel analyzer with the analyzer gate out

of the circuit, ^(disintegration event count rate) is the count rate

of pulses that get through the single channel analyzer window, V is the

resolving time of the electronics, and Cq is the calculated count rate

of chance coincidences. With the analyzer gate in the circuit, the

coincidence spectrum, Cq , was obtained. The difference between C^ and

Cc gives the pulse height spectrum of true coincidences. The ratio of

true coincidences to chance coincidences for 2.5 Mev neutrons was about

7 to 1, while for l^-.l Mev neutrons it was about 1 to 1.

The experimental results will be compared with calculations made by

Leissi< It should be noted, however, that the calculations by Leiss

were performed for a right circular cylinder, 2 cm thick by 8 cm in

diameter composed of 50 percent phenylcyclohexane and 50 percent methyl

borate, the boron being enriched to 9k percent B^O, in the. experiment,

Pilot Scintillator B was used in place of the liquid scintillator.

Although these are two different spectrometers, they are comparable in

many respects. For example, the plastic scintillator has approximately
8.

the same hydrogen to carbon ratio and gives approximately the same pulse height for various radiations as the liquid scintillator,,

The dimensions of the large plastic scintillator, 1 inch thick by

k inches in diameter, were slightly larger than those used in the

calculations for the liquid scintillator. Important differences

between the two spectrometers are: The Pilot B-Li°I spectrometer

is expected to have lower efficiency; the time delay between the

initial large pulse and the detection of the slowed down neutron is

much shorter in the B loaded liquid scintillator.

5-1 Resolution: Percent Full Width at Half Maximum

Typical experimental pulse height spectra obtained with the

spectrometer are plotted as relative number versus channel in

Figures 3> ^-> 5 and 6. With each figure the neutron producing reaction,

the neutron energy, the resolution and the diameter of the Pilot B

plastic scintillator, are given.

Figures 3 and h are experimental pulse height spectra obtained

with 2.5 Mev neutrons. For the first curve, the H

n diameter Pilot

Scincillator B was used, while for the second the 1-3/^-" diameter

Pilot Scintillator B was used. It is seen from these two figures that

the curves have essentially the same shape and resolution. Figures 5

and 6 are experimental pulse height spectra obtained with l^-.l Mev

neutrons. The 4" diameter Pilot Scintillator B was used for the first

curve, while the 1-3/^-" diameter Pilot Scintillator B was used for the

second curve. Again it is seen from these two figures that the curves have essentially the same shape and resolution. One of the results of

Leiss 1 calculations was that the resolution of the spectrometer would

be relatively independent of the area on which the neutrons were incident,

The experimental data indicate this same result.

The data of Figures 3> '+, 5 and 6 are plotted in Figure 7 as resolution, percent full width at half maximum versus neutron energy. The

circles on the graph represent the experimental points. For the purpose

of comparison, the variation of resolution with neutron energy, as calculated by Leiss, is shown by the solid line, it is seen that the energy

resolution obtained experimentally is not as good as might be expected

from calculations. Experiments done by allowing Po^ lu alpha particles

(alpha particle energy 5°3 Mev) to fall on Pilot Scintillator B also

indicated that the energy resolution should have been better.
PI In the alpha particle tests, the Po^ , plated on a silver

planchet, was placed as close as possible to the face of the. plastic

scintillator- With identical conditions of photomultiplier tube high

voltage and amplifier gain, pulses from the 5° 3 Mev alpha particles

had approximately the same height as the highest proton recoil pulses

obtained with 2.5 Mev neutrons incident on the plastic scintillator

.

However, the energy resolution for the alpha particles was l6 percent

full width at half maximum, while for 2.5 Mev neutrons the energy

resolution was 50 percent full width at half maximum. Such a difference in resolution for pulses of approximately the same, height

is unexpected.

The data of Figure h are plotted again in Figure 8 as relative

number versus relative pulse height. Also plotted is the pulse height

distribution calculated by Leiss for the boron-10 loaded liquid scintillator and 2 Mev neutrons. The histograms have been normalized to the same area and an attempt has been made, to put the peaks of the two

distributions at the same relative pulse height. In the same manner,

the data of Figure 6 and the pulse height distribution calculated by

Leiss for the boron-10 loaded liquid scintillator and 10 Mev neutrons

have been plotted in Figure 9 as relative number versus relative pulse

height. It is seen from these two figures that the experimental and

calculated pulse height distributions are not as dissimilar as Figure 7

indicates.

Possible sources of the excessive broadening of the experimental

neutron pulse height spectra are as follows: (a) the targets used were

thick targets, and therefore a spread in the energy of the neutrons

leaving the target at a particular angle resulted. At an angle of 90°

with respect to the deuteron beam, calculations done by Seagrave—' show

that the energy spread for neutrons from the p(d,n)He-5 reaction is

k- percent, and for neutrons from the T(d,n)He + reaction is 1 percent.

However, Seagrave points out that his calculations did not take into

account multiple scattering of deuterons in thick targets, which would

considerably broaden the energy spectra. It would be a major undertaking

to attempt to estimate the magnitude of this effect. (b) Since the

target was necessarily in a vacuum system, the neutrons had to go through

the walls of the vacuum system. It is expected that the additional

spread in energy caused by this factor was small. (c) The plastic

scintillator was fairly close to the target (77 can), so that neutrons

incident on it have left the target at an angle of 90° ± 3»5° with respect

to the deuteron beam. The results of Seagrave show that the energy spread

due to this cause is about 2 percent for neutrons from the B(d,n)He^

reaction and about 0.5 percent for neutrons from the T(d s n)He^ reaction.

—Seagrave, LAMS-2162, 1958.
10.

(d) The calculations of Leiss showed that the resolution of the

spectrometer would become poorer with increasing scintillator

thickness. The experimental pulse height distributions for a 2.5 cm

thick scintillator have been compared with the calculated pulse height

distributions for a 2 cm thick scintillator, and the extra thickness

may be a source of excessive broadening of the experimental pulse

height spectra. (e) All neutrons arriving at the plastic scintillator

are not incident normal to its face. For neutrons not arriving at

normal incidence, the effective thickness of the detector is increased.

The spread in pulse height due to this cause is expected to be small,

(f) Many of the neutrons which lost essentially all of their energy

were not identified as totally absorbed neutrons. This came about

because neutrons which are slowed down to fairly low energy (10 ev or below) will take a relatively long time (depending on where the neutron

initially lost its energy in the plastic scintillator) to reach the

Li°l(Eu) crystal. By the time, many of these neutrons diffuse to the

crystal, the gate they originally opened will have closed. With the

electronic set-up that was used, the length of this gate (which is the

resolving time of the electronics) could be made no longer than three

microseconds. On the other hand, neutrons that have been slowed down

to energies between 10 ev and 5 Kev have a much better chance of reaching

the Li"l(Eu) crystal while the gate is open. The result is that many

neutrons are slowed down to fairly low energy (10 ev or below), but only

the neutrons which are slowed down to these energies in the part of the

plastic scintillator that is relatively near to the Li°l(Eu) crystal are detected effectively. Since these neutrons are the best ones to count,

not counting most of them may have the effect of broadening the observed

pulse height spectra.

5.2 Efficiency

Efficiency is defined here as the number of true coincidence counts

in the coincidence spectrum divided by the total number of neutrons

incident on the Pilot Scintillator B during the time the coincidence

spectrum was being taken. This latter quantity was obtained by measuring

the total neutron flux during a run by means of a calibrated "long counter",

With the total neutron flux known, the total number of neutrons incident

on the plastic scintillator during the run could be calculated.

The efficiencies obtained with the spectrometer under various conditions are given in Table 1. If lines 1 and 2 of Table 1 are compared, it

is seen that the efficiency obtained for the 1-3A" diameter Pilot

Scintillator B is about twice that of the V

1 diameter Pilot Scintillator B. However, if the differences in resolving time and fraction of area of

plastic scintillator covered by Li l(Eu) crystal are taken into account,

the V1 diameter Pilot Scintillator B is about three times more efficient
11.

than the 1-3/4" diameter Pilot Scintillator B. The same holds true

if lines 3 and k of Table 1 are compared. Another result of Leiss'

calculations was that the efficiency of the spectrometer would increase

with increasing area on which the neutrons were incident. The experimental data indicate this same result.

The efficiency of the spectrometer is plotted against neutron

energy in Figure 10. The circles represent the data of lines 2 and k

of Table 1. Shown by the solid line is the variation of efficiency with

neutron energy as calculated by Leiss „ It is seen from the figure that

the detection efficiency obtained experimentally was not as good as the

calculated efficiency. However, it has been mentioned already that many

of the neutrons which lost essentially all of their energy were not

identified as totally absorbed neutrons. A crude calculation was performed

to estimate the number of these neutrons. The 4" diameter plastic scintillator, with the Li"l(Eu) crystal next to it, was considered to be broken

down into several sections. In the calculation, the following were taken

into consideration: the probability that a neutron would be slowed down

in a certain section; the probability that a neutron, slowed down in a certain section, would reach the Li"l(Eu) crystal; the probability that

the gate (a 1.48 microsecond gate was considered), originally opened by

the slowing down pulse, was still open by the time the slowed down neutron

reached the Li"l(Eu) crystal. The results of this crude calculation

showed that only 10.6 percent of the neutrons that could be identified

as totally absorbed neutrons by the single Lib l(Eu) crystal were put in

the coincidence spectrum. Therefore, the efficiency could be made a factor of 9-4 higher if the gate were long enough. (it has been mentioned

already that the length of the gate could be made no longer than

3 microseconds with the electronic set-up that was used. ) That the result

of the crude calculation may be reasonable is shown in Figure 11 where

the data of lines 4, 5 and 6 of Table 1 are plotted as efficiency versus resolving time. It is seen that the curve is still going up fast at a resolving time of 2.7^ microseconds.

The efficiency of the spectrometer can also be increased by placing

more Li"l(Eu) crystals around the Pilot Scintillator B. It is given in

Table 1 that 3 -45 percent of the area of the plastic scintillator was covered by the Li°l(Eu) crystal. The efficiency calculated by Leiss

and shown by the solid line in Figure 10 was based on 30 percent of the

slowed down neutrons being identified. If 30 percent of the area of the

4" diameter Pilot Scintillator B were covered by Li°l(Eu) crystals, the

efficiency would be increased by a factor of 8.7. This means nine

Li"l(Eu) crystals would have to be used.
12.

The efficiencies that would be expected of the Pilot B - Li°I

spectrometer with a long enough resolving time and nine Li°l(Eu)

crystals are represented by the squares in Figure 10. It is seen

that under the proper conditions the efficiency of the Pilot B - Lib

I spectrometer will approach the efficiency curve calculated by Leiss

for a boron-10 loaded liquid scintillator.

6. CONCLUSIONS

It has been mentioned that the efficiency of the Pilot B - Li°I

spectrometer can be improved by the use of longer resolving time and

more Li°l(Eu) crystals. It should be pointed out, however, that

although the efficiency of the Pilot B - Li"I spectrometer in its

present form is low, it possesses the advantage that very large area detectors may be used with consequent gain in count rate.

The most important drawback of the Pilot B - Li°I spectrometer

as it now stands is the relatively poor energy resolution. The calculations of Leiss and the Po^-LU alpha particle experiments indicate

that the resolution should be better. Probably the most important

sources of excessive broadening of the observed pulse height spectra

are: use of thick targets, multiple scattering of deuterons in the

thick targets, and counting only a few of the neutrons which have been

slowed down to fairly low energy (10 ev or below). With the Pilot B - Li°I spectrometer as it is, the first and second possible sources of

broadening can be eliminated by making use of thin gas targets. The

third possible source of broadening can be eliminated by making the

resolving time of the electronics longer. (This also is necessary to

increase the efficiency.

)

It is felt, however, that probably the best approach for further

work would be to use a boron-10 loaded plastic scintillator. A system

such as this would involve the use of fast electronics to detect the

alpha particle from the disintegration event. This method would have

the advantages of the boron-10 loaded liquid scintillator, such as high

probability of absorption of slowed down neutrons by B and short

resolving time, but would not have the disadvantages of preparing and

containing boron-10 loaded liquid scintillators. This method would

also remove the disadvantages of the Pilot B - Li"I spectrometer, such

as,the necessity of having a long resolving time and using several

Li^l(Eu) crystals. Surrounding the Pilot Scintillator B with several

Li"l(Eu) crystals could also result in a spreading of the pulse height
13.

spectra because of elastic scattering from the large amount of matter

in the vicinity of the plastic scintillator. To test a boron-10

loaded plastic scintillator, it would be best to use thin gas targets,

so that any spreading of the pulse height spectra due to the sources

themselves would be eliminated.

7. ACKNOWLEDGMENT

The authors wish to express their gratitude to L. Costrell, Chief

of the Nucleonic Instrumentation Section, for his assistance with the

many electronic problems that were encountered in this work.
e3

"O

(U

!mCO> u O

•H o U

W n

CO o

O i-H 4J 3

O. (0 w

O 'W H H

•H O -HMD

4-1 -U -H O (8 ChJ

CCj CU «H M H O >,

a

PIai •Ho

•H

4-1 m

00c

•H>

rHo <u

CD g

CU -H

OS H

PIo

U bl 4J M

3 0)

a) pi 53 W

PQU

O

4J

cri rH

rH J-l •H <U U 4-1 C CO

•i-l £

O 3

CO -H

4JO

PL,

>1—

o

oo

LT\ LTN LT\ ir\ -=t -4" -=h -=J- hTS 00 K^ K"\ K> O r-l O O O

D \o LTN LT\ LTN Lf\ Oi-l o1—

1

o1—1 orH Oi—l OrH X X X X X X

00

rH

rHO

rH o

CO CM

rH OJ

00

00

-3"

LfN 00

-d- -4- OJ

LTV CM OJ

M3

OJ

OJ

to*

i I -4- -4"
§ 5 § 3

wx wo£

>frtruj XOOQ < — mcr

20 WUJ DO)ZZ X

IUJXQ

LUQ<O

_lcro

1-<

to t

_l o

=> Ia. o

i-P Z Ul

UJ

O >

UI Z

ifI- 3 < UJ

tr z

UJ£

i- o

52

UJ

I-<

cro

CD o

CO9a

eno

LUOo

cra

>-X

r<

<• • < L <

uj in o°< oZ >-

CD

O z

oc 00 .. uj (r Q CD £

UJ < 5= I" >£ o <

uj <r

a o . . <

LUQ

IUJ>

UJo

I-<

a:o

UJ1-z

enQ

^2s 2e z z

oc1-3

_J z

CCh-UJz

Ul

rr <

Ul

ocz

>-<

UJ

Ulz ?• Ul

or

_l »- I- < O Ul 1— * 1- z u 2

p^ (/)

</> Ul 2

or =^ <t _l oc _l <

<^ u. <r> a. Ul < _I is-I UJ < tr i

43u

<w

cogO

<+-)

CO3O

•H

l-l CO>

4-1O

CO€u

bO

rt • •H u

TJ QJ U

0) •H S

•U

CO S-i B u

a> .c a)

CJ

a

c/>

w
in

r o

1-oe Willi 3

(E UJ 5 z 3

UJz<a

ui

CVJ oz<

_)U)(T

^. zN ae OUI . i_i_j £- o—zo

<cn ^

ut c/> co . h- a < o

»-UI - IrrUJ u.

u.O

1°

_l <n (0 <7 o < Ui * o 1- o z z u

tu 3 X K o z

is 1- o o

a. -*1 oo

(EOT Ult- JZ i- <=> 1 oo r* ' too z *

UI 3

o-4 t => o o

CE Ui

i- < o 3 Ui <S> °-« z z

>2

JE

O

UIU1 UIZN - Iz z OT«< T - * O

UI > 10 <

_1 =1

UJ

_

Q tf>

(0

—

ft *

4J

CtJ lu

ctj aa

CtJ

S-i 60

ctj •iHUo

I-l pp

CM

•H

[CSh < < »-= -= m° * o<zo - qUJ 7J0: >*.! -J Q-O-cn-^-ajOo-CL
2 3

Fig. 3.

5 6 7

CHANNEL

D(d,n)He5, 2.5 Mev neutrons, resolution 50$,

k" diameter Pilot Scintillator B.
Fig,

9 10 II 12 13 14 15 16 17 18 19 20

CHANNEL

k. D(d,n)He-5, 2.5 Mev neutrons, resolution 50$

1-5/4" diameter Pilot Scintillator B.
5 6 7 8

CHANNEL

10

Fig. 5. T(d,n)He\ 1^.1 Mev neutrons, resolution 22$,

k" diameter Pilot Scintillator B.
2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21222324

CHANNEL

Fig. 6. T(d,n)He\ 14.1 Mev neutrons, resolution 20$,

1-3/4" diameter Pilot Scintillator B.
3456789 10

NEUTRON ENERGY, Mev

Fig. 7» Resolution vs. neutron energy, experimental

and calculated.

O Resolution obtained experimentally with

Pilot B-Li°i spectrometer.

Resolution calculated by Leiss for

Boron-10 loaded liquid scintillator.

3 14
UJ

a:

8

a:UJ

CD 5

> 4

'I ' ,lO|

gin

IP

._J

„J

(o) —

.J

r-J

1Mb)

r-J

__J

^

i--j

i -j

L_.„

L-.

.2 .6 .8 1.0 RELATIVE PULSE HEIGHT

1.4

Fig. 8, Pulse height distributions, experimental and

calculated.

Curve (a) Pulse height distribution obtained

experimentally with Pilot B-Li°I spectrometer

and 2.5 Mev neutrons.

Curve (b) Pulse height distribution calculated

by Leiss for Boron-10 loaded liquid scintillator

and 2 Mev neutrons.
lO'r—

.J

,o Ol

|o Ol

8

or 6

Ld

CD

Ld

>

n

LU

(T.

(0) (b).N r

._J

L_.

L.,

ZL

RELATIVE PULSE HEIGHT

Fig. 9. Pulse height distributions, experimental and

calculated.

Curve (a) Pulse height distribution obtained

experimentally with Pilot B-Li"l spectrometer

and lk. 1 Mev neutrons.

Curve (b) Pulse height distribution calculated

by Leiss for Boron-10 loaded liquid scintillator

and 10 Mev neutrons.
Fig. 10

4 6 8

NEUTRON ENERGY.

10 12 14

M ev Efficiency vs, neutron energy, experimental

and calculated.

O Efficiency obtained experimentally with

Pilot B-Li"l spectrometer, 5" diameter Pilot B,

1.U8 microsecond resolving time.

D Efficiency expected with Pilot B-Li°I

spectrometer with longer resolving time and

more Lib I crystals. See text for explanation.

Efficiency calculated by Leiss for Boron-10

loaded liquid scintillator.
3.0 x ro

5

0.5 1.0 1.5 2.0 2.5

RESOLVING TIME, MICROSECONDS

3.0

Fig. 11. Efficiency vs. resolving time, D(d,n)He5,

2.5 Mev neutrons, k" diameter Pilot Scintillator B.

February 26, 1959

USCOMM-DC- 53, 228

U. S. DEPARTMENT OF COMMERCE

Lewis L. Strauss, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C, and its major laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical work. In general, each section carries out specialized

research, development, and engineering in the field indicated by its title. A brief description of

the activities, and of the resultant publications, appears on the inside front cover. WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical In- struments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instru- mentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Engine Fuels. Free Radicals Research.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid

State Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory.

Radioactivity. X-rays. High Energy Radiation. Nucleonic Instrumentation. Radiological

Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical

Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass

and Scale. Capacity, Density, and Fluid Meters. Combustion Controls. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and

Specifications. Polymer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Concreting

Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. ' Fire Protection. Air Conditioning, Heating,

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Anolog Systems. Application Engineering.

• Office of Basic Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular Propagation Services. Sun-Earth Relationships. VHF Research. Ionospheric Communication

Systems.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Radio

Systems Application Engineering. Radio-Meteorology.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High

Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave

Circuit Standards.
NBS