BSIR 77-1279

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U S. DEPARTMENT OF COMMERCE , National Bureau of Standards

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Cross Sections

and Yields for

High Energy

Neutron Source

Reactions

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July 1977 /

NBSIR 77-1279

Cross Sections and Yields for

High Energy Neutron Source Reactions

Edited by:

M. A. Lone

Chalk River Nuclear Laboratories

Chalk River, Ontario, Canada K.OJ-1JO

L. Stewart

Los Alamos Scientific Laboratory

Los Alamos, New Mexico 87545

A. D. Carlson and C. D. Bowman

Center for Radiation Research

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

July 1977

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

PREFACE

The need for intense sources of high energy neutrons for cancer

therapy and materials radiation damage studies for CTR programs has

led to renewed interest in the measurements of differential and total

neutron thick target yields from charged particle induced reactions over

a wide range of neutron energies. Knowledge of both the absolute neutron

source strength and the neutron spectral distributions is important for

these programs.

The accuracy of the measured differential and total neutron yields

depends on the neutron standards and other data used for calibration of

the neutron spectrometers. Because this is a relatively new field there

is a lack of adequate neutron standards and relevant data in the energy

range of interest. Consequently in some instances there are discrepancies

between the data reported by different groups on the same source reaction.

A session, primarily a workshop, was organized in conjunction with the

International Specialists Symposium on Neutron Standards and Applications

held at the National Bureau of Standards, Gaithersburg, Maryland site from

March 28-31, 1977. The session was scheduled on the third evening of the

Symposium, March 30, 1977, from 8:00 to 10:30 p.m. and was chaired by

Professor H. H. Barschall of the University of Wisconsin. Approximately

sixty delegates attended the special session.

When commercial equipment, instruments and materials are mentioned or

identified in this report it is intended only to adequately specify experimental procedure. In no case does such identification imply recommendation

or endorsement by the National Bureau of Standards, nor does it imply that

the material or equipment identified is necessarily the best available for

the purpose.

M. A. Lone

Chalk River Nuclear Laboratories

Canada

L. Stewart

Los Alamos Scientific Laboratory

Los Alamos , New Mexico

A. D. Carlson and

C . D . Bowman

National Bureau of Standards

Washington, D. C.

Ill
ABSTRACT

Eight papers were presented at a workshop on thick-target yields from

high-energy neutron source reactions on March 30th, 1977 in Bethesda, MD.

Presentations were made on several source reactions including D(d,n),

Li(p,n), Li(d,n), Be(p,n), Be(d,n), and C(d,n) including energy and angular

distributions. Special emphasis was placed on these sources due to the

needs in medical applications and for studies of radiation damage expected in an

operating fusion reactor.

Key words: D(d,n) source, Li(p,n) source, Li(d,n) source, Be(p,n) source,

Be(d,n) source, C(d,n) source, dosimetry, thick-target yields,

neutrons, nuclear reactions, neutron energy distributions, neutron angular distributions.

IV
TABLE OF CONTENTS

CROSS SECTIONS AND YIELDS FOR HIGH ENERGY NEUTRON SOURCE REACTIONS*

Chairman: H. H. Barschall

Page

1. Neutron Spectra From Deuteron and Proton Bombardment of Thick Lithium Targets,

by C. E. Nelson, F. 0. Purser, P. Von Behren, and H. W. Newson 1

2. Neutron Spectral Distributions from Proton and Deuteron Bombardment of Thick

Li & Be Targets at lU.8, 18 & 23 MeV, by M. A. Lone 5

3. 35 and 65 MeV Protons and 35 MeV Deuterons on Lithium and Beryllium,

by M. Awschalom 10

4. Spectral Measurements of 25 to 55 MeV Neutron Beams,

by S. W. Johnsen 15

5. Protons and Deuterons on Be at Ep=100 MeV and E^=80 MeV by G. H. Harrison, C. R. Cox, E. B. Kubiczek, and J. E. Robinson 19

6. Neutron Yields and Dosimetry for Be(d,n) and Li(d,n) Neutron Sources at

E(j=no MeV, by M. J. Saltmarsh, C. A. Ludemann, C. B. Fulmer, and R. C. Styles 2k

7. Neutron Spectra and Doses from Deuteron Bombardment of Light Nuclei,

by H. H. Barschall 30

8. Neutron Spectral Measurements and Interpretation at the Naval Research

Laboratory, by L. S. August, P. Shapiro, and R. B. Theus 31

*0rganized by M. A. Lone, Chalk River Nuclear Laboratories, Ontario, CANADA

V

NEUTRON SPECTRA FROM DEUTERON AND PROTON BOMBARDMENT OF THICK LITHIUM TARGETS

C.E. Nelson,* F.O. Purser, P. Von Behren and H.W. Newson

Triangle Universities Nuclear Laboratory ^and Duke University

Durham, North Carolina 27706, U.S.A.

Neutron energy spectra and yields produced by the bombardment of thick lithium targets

by deuterons and protons have been measured using the time-of-f light method. Measurements were made at angles up to 45° for deuteron energies of 8, 12 and 15 MeV and a proton energy of 15 MeV. The average neutron energy of the (d,n) reactions is shown to vary approximately at .44 Ed

. The (p,n) reaction has En=4.7 MeV.

(energy spectra, yields; neutrons; thick target; 7 Li(d,n) 8 Be, 7 Li(p,n) 7 Be; Ed =8, 12, 15 MeV, E

p=15 MeV; time-of-f light)

INTRODUCTION

Present efforts to develop a hospital based fast neutron source for cancer therapy are concentrated on

the D-T generator1 ). This machine uses the 8 H(d,n) 4 He reaction with several hundred keV deuterons to obtain

an intense monoenergetic source of 14 MeV neutrons.

A possible alternative to the D-T generator for

a hospital based neutron source is a compact medical

cyclotron. Table 1 lists the specifications of some

of the cyclotrons presently installed in hospitals in

the United States. Table 1

Medical Cyclotron Specifications*

Cyclotron Particle Energy

Extracted

Beam

Model No. (MeV) (pA)

CS 15 P 15 100

d 8 100

CS 22 P 22 50

d 12 50

CS 30 P 26 60

d 15 100

Cyclotron Corporation

It is known that at the deuteron energies available to these cyclotrons, a thick beryllium target produces an

intense neutron flux, which has an average energy of approximately .4 E^ 1 ). The resultant neutron beam is,

however, of limited usefulness for radiation therapy

because of its poor penetration into tissue. The

9Be(d ,n) reaction at Ed =16 MeV, for example, has a central axis depth dose curve which falls to 50% of the surface dose at 9.2 cm for a 10 x 10 cm field at 125 cm

TSD1

), whereas Brennan has suggested 10 cm as an acceptable depth for 50% of the surface dose for a 5 x 5 cm

field4 ).

The large positive Q-value (+15.03 MeV) of the

7 Li(d,n) 8 Be reaction suggests that bombarding a thick

lithium target with deuterons could result in a substantial increase in the average neutron energy of the resultant neutron yield. Previous measurements of this average neutron energy for these lower energy deuterons

are in disagreement, indicating an average neutron

energy of .4 Ed to .55 Ed for Ed<15 MeV^^jb).

The 7 Li(p,n) 7 Be reaction is known to be a prolific source of high energy neutrons for thin targets, with

over 60% of the neutrons from this reaction going to the ground or first excited state of 7 Be 7 ). Thick

target spectra have not been measured for proton

energies above Ep=4.0 MeV.

The Triangle Universities Nuclear Laboratory neutron time -of -flight facility has been used to determine the average neutron energy and yield of thick

lithium (d ,n) and (p,n) reactions at deuteron energies

of 8, 12 and 15 MeV and a proton energy of 15 MeV to determine whether these reactions are suitable for fast neutron therapy.

EXPERIMENTAL

Neutron energy spectra were measured using timeof-f light techniques. A pulsed beam of protons or deuterons from the Triangle Universities Nuclear Laboratory Model FN Tandem Van de Graaff accelerator was

used in conjunction with the TUNL neutron time-off light facility. Beam currents of 2-5 nA with a time

resolution of 1.5 to 2.0 nS were used.

The lithium target was enriched to 99.99% 7 Li

and was encapsulated in an aluminum container under a helium atmosphere. The upstream face of the lithium

was covered with a 3.5 pm molybdenum foil and the target length was 1.9 cm, sufficient to stop 15 MeV deuterons

.

The charged particle beam passed through a 3 . 2 mm

tantalum collimator 15 cm upstream of the lithium target. Collimator currents were kept to less than .1 nA. Beam spot sizes on target were kept to less than 5 mm

diameter. Target beam currents, with appropriate electron suppression, were integrated to determine the in- cident charged particle flux.

The lithium target was mounted at the pivot of the main neutron detector shield at the end of the all metal, wide aperture neutron time-of-f light beam line.

The very small collimator currents, combined with the extremely well shielded detector made beam and room

associated background measurements unnecessary.

The neutron detector consisted of a 8.89 cm diameter by 5.04 cm thick NE 218 liquid scintillator coupled to a 56 DVP photomultiplier. Pulse shape discrimination was used to discriminate against gamma ray

events. This detector was housed in a massive (7300

kg) shield to reduce time uncorrelated background

events to a negligible amount8 ). A smaller detector,

located at a fixed angle to the charged particle beam,

monitored the neutron flux.

Standard time-of-f light electronics, including

1
pulse shape discrimination, were used for all datataking which was done with the TUNL DDP 224 computer.

Main detector ADC dead times were kept to less than

20%. Spectra were measured at 0°, 10°, 20°, 30° and

45°.

The efficiency of the detector as a function of energy must be known to convert the time-of-flight

spectra to energy spectra. The previous measurements

of neutron yield have used a detector bias level corresponding to 1.6 to 2.5 MeV neutron energy and have

been required to extrapolate the neutron yield to zero neutron energy. The low energy cut-off of the main

detector for this work was set at 1/10 the ^^Cs compton edge, corresponding to a neutron energy of 200-

300 keV, in order to minimize errors introduced by

such arbitrary extrapolations. Calculations of detector efficiencies are very unreliable with such low

energy thresholds. Thus the relative efficiency of the main detector was measured over the range of

.7 MeV to 24 MeV, using two different techniques.

For neutrons between .7 MeV and 5 MeV, the angular

distribution of neutrons of 7.5 MeV scattered from hydrogen was measured. A 6 mm diameter by 2.55 cm high

polyethylene scattering sample was used and spectra were taken from 35° to 72° in the laboratory system.

The data, when corrected for finite geometry effects

and normalized to the known H(n,n)H differential cross

section, yield a relative efficiency curve. The efficiency from 4.0 MeV to 24 MeV neutron energy was measured by using the well-known T(p,n)^He and T(d,n)^He

reactions at several charged particle energies^ >10)

,

The measured yields, normalized to the differential

cross sections, give a family of curves spanning different, overlapping neutron energy regions. These

curves were overlapped and normalized to the H(n,n)H

measurement to yield the relative efficiency data

shown in fig. 1. The absolute efficiency of the detector was obtained by normalizing this relative efficiency at 1^=4. 0 MeV to a monte carlo calculation of the efficiency of our detector done at Oak Ridge

National Laboratory^) . A least squares fit was then

made to this combined data. This fit was used to obtain the calculated efficiency as a function of neutron energy and is shown as the smooth curve in fig. 1.

Fig. 1 Efficiency of the main neutron detector.

The time-of-flight spectra, after correction for computer dead time and relativistic effects, were converted to neutron energy spectra using this calculated

efficiency curve. The steep slope of the efficiency

curve below En

=1.0 MeV precludes an accurate determination of the neutron yield in this region. All

| subsequent calculations of neutron yields and average

energy, therefore, use En=1.0 MeV as the low energy

cut-off point.

NEUTRON SPECTRA AND YIELDS

The zero degree energy spectra of neutrons obtained with 8, 12 and 15 MeV deuteron bombardment of

a thick lithium target are shown in figs. 2, 3, and 4

Fig. 2 Neutron energy spectra at zero degrees for

8 MeV deuterons on lithium.

12 MeV deuterons on lithium.

2
Fig. 4 Neutron energy spectra at zero degrees for

15 MeV deuterons on lithium.

These spectra are characterized by 1) a broad peak in

the energy distribution at approximately .4 Ej , 2) a very small high energy tail and 3) a low energy tail

which appears to be increasing rapidly below En=1.0 MeV,

the low energy cut off point for these measurements.

Fig. 5 shows the zero degree energy spectrum for

15 MeV proton bombardment of a thick lithium target

.

15 MeV protons on lithium.

It is characterized by 1) a peak in the energy distribution at En=3.4 MeV and 2) a decrease in yield

with neutron energy, varying approximately as 1/En

.

The neutron spectra at each angle were integrated

to obtain the yield versus laboratory angle data shown

in f ig . 6 .

angle

.

The ^Li(d,n) data indicate a peak in the yield at zero degrees, which becomes more pronounced with increasing

deuteron energy, while the ^Li(p,n) data are essentially isotropic.

Table 2 gives the average neutron energies calculated from these neutron yield data and numerical

values of the yields.

Table 2

Thick Target Yields and Average Neutron Energies for En >1.0 MeV

Ed =8 MeV Ed= 12 MeV Ed =15 MeV E

p =15 MeV

6 lab

, E

n s (MeV) (l89 neut

v Trc'=sr >

E

” Yn

En

Y

n

En

Y

n

0 4.4 3.8 5.5 12. 6.5 25. 4.7 4.1

10 4.5 3.4 5.7 9.6 6.6 20. 4.6 4.0

20 4.8 2.7 5.9 6.7 6.6 13. 4.5 4.0

30 5.7 2.0 5.7 4.5 5.9 8.2 4.5 3.8

45 6.0 0.85 5.3 3.0 5.4 5.8 4.5 3.4

3
DISCUSSION REFERENCES

There are several earlier measurements of neutron

spectra from thick lithium targets2»5,6) . Comparisons

of the present data with these measurements are shown

in fig. 7, where both the yield and average neutron

energy are plotted as a function of deuteron energy.

teron energy.

The present data are in substantial agreement with the previous measurements of Weaver e_t a_1.2 The discrepancy between the present work and Weaver for the average neutron energy (approximately 10%) is consistent

with the different low energy cut off and extrapolation to zero neutron energy used

The large discrepancies between the present work

and that of Jones et_ al_. and Goland are likely the result of similar extrapolation differences.-’’*’ The pre- sent work, because it extends to a neutron energy of 1.0 HeV, is less dependent on extrapolations to zero neutron energy than the previous measurements, some

of which extrapolated from En

'\4.0 MeV to zero neutron

energy^

9.

10 .

11 .

*

Kelsey, C.A., 1975, Medical Physics 1_, 185. Weaver, K.A. , Anderson, J.D., Barschall, H.H. and

Davis, J.C., 1973, Nuclear Science and Engineering

52, 35. Parnell, C.J., 1971, British Journal of Radiology,

44_, 612. Brennan, J.T., 1969, Radiological Clinics of North America, 7_, 365. Jones, D.T.L. and Bartle, C.M., 1974, Nuclear In- struments and Methods, 118 , 525. Goland, A.M., Snead, C.L. Jr., Parker, D.M. and Theus, R.B., 1975, Reported at the Charged Particle

Accelerator Conference, Washington, D.C. IEEE,

NS521 . Jungerman, J.A., Brady, F.P., Knox, W.J., Montgomery, T., McGie, M.R., Romero, J.L. and Ishizaki,

Y, 1971, Nuclear Instruments and Methods 94_, 421. Glasgow, D., Purser, F.O., Clement, J.C., Mack, G.

Stelzer, K., Boyce, J.R., Epperson, D.H., Hogue,

H.H., Bilpuch, E.G., Newson, H.W. and Gould, C.R.,

1975, NBS Special Publication 425, Proceedings of Nuclear Cross Sections and Technology Conference 1, 99. Drosg, M. , 1974, Los Alamos Scientific Laboratory

Report LA-5732-MS.

McDaniels, D.K., Brosg, M. , Hopkins, J.C. and

Seagrave, J.D., 1973, Physical Review 882. Morgan, private communication.

Supported by National Cancer Institute Research

Fellowship #1 F22 CA00332-01

Supported by U.S. ERDA

The present data show that at the deuteron energies available to the smaller cyclotrons, the ^Li(d,n)

reaction is not a practical source of neutrons for fast neutron therapy. The neutron yield compares favorably with the neutron yield from thick beryllium

targets. However, the average neutron energy is substantially lower than 10 MeV for all deuteron energies, varying with deuteron energy approximately as ^=.44 E(j. This is similar to the relationship found

for beryllium targets by Weaver, ^=.42 Ej . Both the average neutron energy and yield of the ?Li(p,n) reaction indicate that it is not suitable for fast neutron therapy at E

p=15 MeV. The average neutron energy

is only 4.7 MeV wnile the yield is only

.4 x 1010 n/yC-sr.

To conclude, the present work indicates that neither the deuteron induced or the 15 MeV proton induced reactions on thick lithium produce a suitable

fast neutron beam for therapy. The average neutron

energy is such as to provide only marginal penetration

in tissue.

j

4
NEUTRON SPECTRAL DISTRIBUTIONS FROM PROTON AND DEUTERON BOMBARDMENT OF THICK Li & Be TARGETS AT 14.8, 18 & 23 MeV

M.A. Lone Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories , Chalk River, Ontario, Canada KOJ 1J0

This report compares the spectral distributions of neutrons of energies greater than 300 keV produced by bombarding thick 7 Li and 9 Be targets with protons and deuterons of energies 14.8, 18 and 23 MeV. The experimental

j technique used to measure the scattered neutron background is described and the data are compared with previously

! reported spectral distributions.

(Nuclear reactions; 7 Li(p,n), 9 Be(p,n); measured neutron spectral

distributions for En >0.3 MeV; 0 = 0°, 10°, 20°, 30°, 40°; E = 14.8, 18, 23 MeV)

Introduction Experiment

The usefulness of neutron sources for cancer theri| apy and materials radiation damage studies for CTR Programs critically depends on the absolute neutron source

strength and the neutron spectral distributions. The

intense neutron fluxes for such studies can be obtained

from the (d,n) and (p,n) reactions on Li and Be targets

at moderate projectile energies. From these reactions

the energy distributions of the neutrons are quite

broad and depend on the target thickness, the incident

| projectile energy and the reaction channels open.

Low energy neutrons are to be expected in these

reactions even with thin targets because of two processes: first, the compound nucleus formation and the subsequent statistical neutron decay to the highly

excited states in the residual nucleus and second, multibody break-up reactions which give rise to a general

continuum of low energy neutrons. Thresholds for several such reactions are given in Table 1.

The measurements were made with a pulsed beam from

the Chalk River Tandem Accelerator using the experimental set-up shown in figure 1. The metallic

Fig. 1. Schematic illustration of the experimental setup.

Table 1

Q VALUES OF NEUTRON PRODUCING CHANNELS

TARGET REACTION Q(MeV) REACTION Q(MeV)

s Be (d,n)'°B +4.4 (P,n) 9 B -1.9

(d,2n) 9 B •4.1 (p , 5 H e

)

5 L

i

-4.4

(d,pn) 9 Be -2.2 (p , na) 5 L

i

-3.5

(d,p2n)2*He -3.8 ( p , pn

)

8 Be -1.7

(d,p)2 5 He -5.7 (p,pa) 5 He -2.5

( d , D -1.0 (p,npa) 4 He -1.6

7 li (d,n) s Be + 15.0 (p,n) 7 Be -1.7

(d,2n) 7 Be -3.9 ( p . 2n

)

6 Be •12 3

(d,3n) 6 Be -14.5 ( P . pn

)

6 L

i

• 7.3

( d , pn

)

7 L j -2.2 (p . pd) 5 He -9.6

(d,a) 5 He + 14.2 (p,

3 He) 5 He -4.

1

( p , na

)

3 H e -3.2

I _ I Q Several measurements of the neutron spectral

distributions from Li and Be targets from proton and

deuteron projectiles have been reported. Most of these measurements have been made with detector thresholds

set above 2 MeV. None of these data extend to neutron

energies below 1 MeV. In particular, the previous com- pilations 8,19-21 of the neutron spectral distributions

from Be+d reactions with thick targets tend to imply

almost negligible neutron yields below 1 MeV neutron

energy. For cancer therapy applications a quantitative

knowledge of the low energy neutron yields is important

in order to estimate the dose to the skin of a patient.

This report, which is based on a measurement^^ at

the CRNL MP Tandem accelerator, compares the yields and

spectral distributions of neutrons above 0.3 MeV from

the bombardment of thick 7 Li and 9 Be targets with deuterons and protons

.

^Li(0.82 g-cm and 9 Be(0.82 g*cm ) targets were mounted on a thin (0.12 mm) stainless steel window at the end of a 12 cm long tube that was insulated from the rest of the beam line and served as a Faraday cup. An unshielded 13.2 cm 3 cubical stilbene crystal was used

for neutron detection. A 60 cm long iron-masonite (50%

iron) block placed midway between the target and the detector was used to block off the direct neutron beam

for the purpose of measuring the scattered neutron and y-ray background for each projectile energy and angle.

A schematic diagram of the electronics is shown in figure 2. The neutron events were separated from the

y-ray events by pulse shape discrimination (PSD) which

separates the pulses according to their rise times determined by the time difference between the leading edge

(measured with a constant-fraction timing unit) and the cross over point of a double-delay-line differentiated

linear signal. The neutron energy was determined by the
time-of-flight (T.O.F.) technique. For each energy and

angle a set of four spectra were recorded (ungated T.O.F. , neutron T.O.F. , y-ray T.O.F. , linear pulseheight or PSD time-to-amplitude converter (TAC) output).

The lower threshold of the single channel analyser (SCA), which provided start signals to the PSD TAC, was set slightly higher than that of the discriminator (DSC)

providing start signals to the T.O.F. TAC. This procedure ensured that the detector bias was determined by

the SCA threshold alone. This threshold was set at 0.04 MeV electron energy and was monitored at intervals with

a

22 Na y-ray source to safeguard against drifts that

would have affected the detector efficiency.

A number of scalers (SCL) were used to monitor the

dead times of the T.O.F. TAC, the PSD circuit, the ana- logue-to-digital converter (ADC) and the computer individually. The overall system dead time was of the order

of 20%.

Background Checks

As mentioned earlier the background events due to scattered neutrons and y-rays were measured by blocking

the direct path between the source and the detector with

a 60 cm long iron-masonite block placed midway in the

flight path. This location of the block minimized its

effect on the background events reaching the detector

from the room walls, the floor and the beam tubes. The

block was sufficiently long that the secondary radiation from it was almost isotropic and consequently,

because the two solid angles involved were small, its

effect on the detector was negligible. For the direct measurements the block was moved well away from the flight path in order to eliminate small angle scattered

events reaching the detector.

Fig. 3. T.O.F. spectra; a) y-rays; b) neutrons with no block in the flight path; c) with block in; d) the difference between b) and c)

The T.O.F. spectra shown in figure 3 demonstrate

the effectiveness of this blocking technique in eliminating the background events from the neutron T.O.F.

spectrum. Figure 3(a) shows the difference of the T.O.F. spectra of y-ray events (gated with the y-ray

peak in the PSD TAC pulse height spectrum, inset figure

3) recorded with block out and block in. The y-ray

events above channel 700 are from the inelastic scat- tering of fast neutrons in the vicinity of the detector. The spectrum (d) in figure 3 is the difference

between the dead time corrected spectra (b) taken without the block and spectrum (c) taken with the block.

As expected the difference spectrum (d) contains no events beyond the prompt y-ray peak, nor at channels

below the T.O.F. corresponding to the neutron detection

threshold which was set at 0.3 MeV neutron energy.

Detector Efficiency

The detector efficiency was calculated with the ah

of the program DETEFF which is based on an earlier ver sion by Kurz 23

. The program includes contributions to the scintillation light output from the (n,n) reaction

in H as well as the (n,p), (n,y), (n,n'y) and (n,n'3a) re- actions in C. Thornton and Smith24 have carried out extensive tests of this program and have found the cal culated efficiencies to be accurate to within ±10%.

O i Fig. 4. Neutron detection efficiency of the 13.2 cm stilbene crystal calculated with the program DETEFF23

.

3 The calculated efficiency of the 13.2 cm stilbencj

crystal is showh in figure 4 for an electron energy

threshold of 0.04 MeV. The dotted lines in the figure

represent the calculated efficiency with the threshold

varied by ±25%. The uncertainty, due to energy calibration and resolution, in setting the threshold rela- i tive to the Compton edge of the 0.511 MeV y-rays from

the 22 Na source was less than 25%.

Results: Spectral Distributions

The spectral distributions were measured at flight

paths of 3m for the (p,n) reactions and 3.76 m for the

(d,n) reactions. The overall time resolution in the

T.O.F. spectra was 2.8 ns. The resultant energy reso- lution was 5.8% at 5 MeV and 11.4% at 20 MeV for the

(p,n) measurements and 4.6% at 5 MeV and 9.1% at 20 MeV

for the (d,n) measurements.

The experimental uncertainties in the spectral

distributions come primarily from three sources. First,

there are the statistical uncertainties which are indicated by error bars in figures 5-8. Second, there is the beam current (at the Faraday cup) integrator uncer- tainty which was less than 3%. Third, there are uncer- tainties due to the detector efficiency. These are difficult to assess but based on previous tests24 these

are' expected to be ±10% for En > 2 MeV. For En < 2 MeV

the uncertainty is shown by the dotted curves in the

inset in figure 4. The resultant overall systematic

uncertainty is estimated to be less than ±15% for

En > 2 MeV.

Li + d

The thick target neutron spectral shapes from 7Li and natural Li (93% 7 Li) targets for deuteron energies

13 to 23 MeV are shown in figure 5. Curves 1, 3 and 5 show our 0° results from 23, 18 and 14.8 MeV deuteron

bombardment. The error bars represent the uncertainty p due to counting statistics alone. The small bumps

6
to 23 MeV are shown in figure 6. Curves 1, 3 and 5 give

our measurements at 0° from 23, 18 and 14. 8 MeV deuteron

bombardment respectively. Curve 2 shows the measure-

[j ments of Daruga and Matusevich5

. They used a stilbene

neutron detector but the neutron yields were extracted

by converting the amplitude distributions into neutron

energy spectra. Curve 4 shows the neutron yields at

j| 3.5° from 18 MeV deuteron bombardment measured by

^Weaver et al. 1 with a Ne213 detector with the detector

^threshold set at 2.3 MeV. Curve 6 shows the spectral

|. distributions from 15 MeV deuteron bombardment reported

j. by Cohen and Falk6

. The neutron spectral distributions

from 15 MeV deuteron capture reported by Meulders et al

,

7

, are also in good agreement with the shapes shown in figh ure 4. However, several other compilations 8,19-21 of

the neutron spectral distributions from the Be+d reactions show a rapid decrease of neutron yields at

En < 0.3 Ed in disagreement with our observations.

Li + p

Fig. 6. Neutron spectral distributions from the 9 Be+d

reactions in thick targets.

Fig. 7. Neutron spectral distributions from the 7 Li+p

reactions

.

The neutron spectral distributions are shown in figure 7. The uncertainty due to the counting statistics is shown by vertical bars. Unlike those from the

(d,n) reactions the neutron yields from the (p,n) reactions show an essentially monotonic decrease with

increasing neutron energy. Our results are in disagreement with the thick target spectral distributions

calculated by Chaudhri et al. 25

Be + pThe neutron spectral distributions from a thick

9 Be target are shown in figure 8. Curve 4 shows the results of Kuchnir et al. 2 measured at 15° with 14.8 MeV protons

.

7
Fig. 8. Neutron spectral distributions from the 9 Be+p

reactions

Low Energy Neutrons and the Average Neutron Energies

1-10

In several previous measurements the spectral

distributions have been measured with neutron detector

thresholds set above ^2 MeV. Figure 9 shows the relative magnitude of the low energy neutrons En < 2.3 MeV

observed in different reactions at 0°. The target

thicknesses were 1.5 cm for 7 Li and 0.45 cm for 9 Be. The dotted curves are simply to guide the eye.

1 . 0

0 . 8

0 . 6

R

0. 4

0. 2

0 10 14 18 22 2G 30 34 30

1 ' 1 r V ' NEUTRON YIELD R -

“ —Ey,in*

0.3 NEUTRON YIELD

"1 1 T

THICK TARGETS . 0 DEGREE

A ^

-O Be + p

-• Li + p

' -A ^ " a Be + d " " Li d

J i L

E

<«»V) Fig. 9. Relative magnitude of the low energy En < 2.3 neutron component as a function of the projectile

energy.

I

! The average neutron energy decreases with increase

ing angle. The dependence of the average neutron eneh

gies on the angle is shown in figures 10 and 11.

Fig. 10. Variation of the average neutron energy En with the neutron emission angle for the 7 Li+d and

7 hi+p reactions with thick targets.

8,^(1111

Fig. 11. Variation of the average neutron energy En with the neutron emission angle from the 9 Be+d and

9 Be+p reactions with thick targets.

Angular Distributions

The relative magnitude of the low energy component

decreases at higher projectile energies. However, it should be pointed out that our targets were thicker

than needed to stop the protons and deuterons of energies less than 25 MeV and we have not as yet investigated the effects of this extra thickness on the spectrum

shapes.

The angular distributions are shown in figures 12 and 13. As observed previously 1 ' 2 ' 5 the angular distributions from the (d,n) reactions are much more

strongly forward peaked than those from the (p,n)

8
Fig. 12. Angular distributions of the neutron intensity

from the 9 Be+d and 7 Li+d reactions with thick targets.

ANGLE (DEGREES) Fig. 13. Angular distributions of the neutron intensity

from the 9 Be+p and 7 Li+p reactions with thick targets.

5. V.K. Daruga, E.S. Matusevich and Kh. Uarziev,

Atcmnaya Enerqia, ^_3 (1972) 934.

6. B.L. Cohen and C.E. Falk, Phys. Rev. 84(1951)173.

7. J.P. Meulders, P. LeLeux, P.C. Macq and C. Pirart,

Phys. Med. Biology 20 (1975) 235.

8. C.J. Parnell, Brit. Journ. of Radiology 45 (1972)452.

9. A.N. Goland, C.L. Snead Jr., D.M. Parkin and R.B. Theus, IEEE Transactions on Nuclear Science, Vol

.

NS-22 (1975) 1776.

10. R.B. Theus, R.O. Bondelid, F.H. Attix, L.S. August,

P. Shapiro, R.E. Surratt and C.C. Rogers, Cancer

34(1974) 17.

11. M.J. Saltmarsh, C.A. Ludemann, C.B. Fulmer and R.C. Styles, Report ORNL/TM-5696 Oak Ridge National Laboratory 1976.

12. L. Cranberg, J.P. Barnett, D.S. Cramer and R.D. Wilson, Nucl. Inst, and Meth. 96(1971)493.

13. J . A . Jungerman, F.P. Brady, W.J. Knox, T. Montgomery,

M.R. McGie, J.L. Romero and Y. Ishizaki, Nucl. Inst, and Meth. 94(1971)421.

14. M.W. McNaughton, N.S.P. King, F.P. Brady, J.L. Romero and T.S. Subramanian, Nucl. Inst, and Meth.

130(1975) 555.

15. J.W. Wachter, R.T. Santoro, T.A. Love and W. Zobel,

Nucl. Instr. and Meth. 113 (1973) 185

.

16. C.J. Batty, B.E. Bonner, A. I. Kilvington,

C. Tschalar, L.E. Williams and A.S. Clough, Nucl.

Inst, and Meth. 68 (1969) 273

.

17. M.C. Scott, J. of Nucl. Energy 25 (1971) 405.

18. D.R. Nethaway, R.A. van Konynenburg and T.M. Adams, Measurements of the Neutron Spectrum from the Reaction of 30 MeV Deuterons on a Thick Be Target,

UCRL-52024, Lawrence Livermore Laboratory,

California 1976.

19. K. Schmidt and H. Munzel. Production of Neutrons

through Irradiation of Thick Be Targets with Deu- terons in the Energy Range from 15 to 55 MeV. Report KFK 1288, Gesellschaft fur Kernforschung

M.D.H. Karlsruhe 1970.

20. G. Burger and J.J. Broerse, IAEA/SM-179/35 , Int. Symp. on the Biological Effects of Neutron Irradiation, Neuherberg (Munchen) Germany, 1973 (IAEA

1974) p. 3

.

21. S.M. Qaim, NBS-425. Nuclear Cross Sections and

Technology, Vol. II, p.664; National Bureau of Standards, 1975.

22. M.A. Lone, C.B. Bigham, J.S. Fraser, H.R. Schneider,

T.K. Alexander, A.J. Ferguson and A.B. McDonald,

Nucl. Instr. and Meth. (to appear in 1977).

References

1. K. A. Weaver, J.D. Anderson, H.H. Barschall and J.C. Davis, Nucl. Sci. and Eng. 52 (1973) 35.

2. F.T. Kuchnir, L.S. Skaggs, A.J. Elwyn, F.P. Mooring

and N.A. Frigerio, Cyclotron-1972, Ed. J.J. Bugerjon and A. Strathdee (Am. Inst, of Physics, New York, 1972) p.638.

3. E . A. Kuzmin, A. A. Ogloblin, N.I. Sidorov, A.R. Faiziev and G.B. Yankov, INC Report INDC (CCP) -37/U

in Proc. IAEA Symp. on the Usage of Nucl. Data in Science and Technology, Paris (1973) IAEA Nucl. Data Section Karntner Ring 11, A-1010 Vienna.

4. C.E. Nelson, F.O. Purser, P. von Behren and H.W. Newson, Cont. PH2/L4 to the 1976 Int. Conf. on Interactions of Neutrons with Nuclei, July 6, 1976,

Lowell, Massachusetts, U.S.A.

23. R.J. Kurz, UCRL-11339, Lawrence Radiation

Laboratory (1964)

.

24. S.T. Thornton and J.R. Smith, Nucl. Instr. and

Meth. 96(1971)551.

25. M.A. Chaudhri, S. Zuberi, A.J. Chaudhri and O.J. Chaudhri, 7 Li(p,n)Be Reaction as a Useful Source

of Fast Neutrons for Therapy, reported at the 2nd Meeting on Fundamental and Practical Aspects of the Applications of Fast Neutrons in Clinical Radiotherapy, October, 1973.

9
35 and 65 MeV PROTONS AND 35 MeV DEUTERONS ON LITHIUM AND BERYLLIUM

M. Awschalom

Fermi National Accelerator Laboratory*

P.0. Box 500 Batavia, Illinois 60510 Protons of 35 and 65 MeV and deuterons of 35 MeV were used to bombard beryllium and lithium targets

of various thicknesses. Energy spectra were measured by time-of-f light

(Li, Be, neutron sources, thick-target yields)

IIntroduction

This work is part of a much broader survey

made for the characterization of various neutron beams by collaborators from four laboratories. The full report, Physical Characterization of Neutron Beams Produced by Protons

and Deuterons of Various Energies Bombarding

Beryllium and Lithium Targets of Several

Thicknesses by H. I. Amols , M. Awschalom,

L. Coulson, J. F. Dicello, S. W. Johnsen and

R. B. 'Jheus is available as a Fermilab preprint . While planning a neutron beam irradiation

facility for cancer therapy research at the

Fermi National Accelerator Laboratory, it was deemed necessary to characterize physically a number of potentially useful neutron beams.

The available proton beam could have any

energy between 37.54 and 66.18 MeV. Hence,

the characteristics of the forward scattered

neutron beams created during the bombardment

of the lithium and beryllium targets of

various thicknesses were studied as well as those of neutron beams due to 35 MeV deuterons incident on beryllium. These measurements were done in conjunction with personnel

from a medical accelerator development project

at LASL. The part reported here is that con- cerned only with neutron energy spectra.

Targets

Natural lithium and beryllium were used as targets

The Li targets were sealed under vacuum ir side stainless steel cases. These cases had

one inch inside diameters and 0.02 5 cm stain

less steel entrance windows. They were machined from commercially available Li ex- trusions . The windows caused an energy degradation

of the incident beam. Hence, to equalize incident energies on the lithium and the beryl-!

lium targets, 0.025 cm stainless steel absorbers were also placed on the upstream side

of the beryllium targets.

The thicknesses of the targets were choser

such that protons traversing them and having

incident energies of 65 MeV would loose eitheji

all their energy, 50, 40, 30 or 20 MeV by ior

ization. These thicknesses were calculated

using Janni's tables. 4 The thickness tolerance was such that the actual target would

cause the stated energy loss plus or minus

0.5 MeV.

The various target configurations are sum-- marized in Table 1.Table 1 Although the measurements have produced a large amount of data, they were designed only

as a preliminary study. Hence, the uncertainties associated with the present data may be

unacceptable for some applications.

Experimental Set-Up

These measurements were performed using the

76-inch isochronous cyclotron at the University of California, Davis. 3 The charged particle beams were directed successively onto

eight targets mounted in a target wheel. The

neutrons emanating at 0° from the incident

beam were collimated by a 1.5 m long steel

collimator with a rectangular opening 5.7 cm

by 7.0 cm. The collimator exit was 2.85 m from the upstream face of the targets.

Charged particles exiting the target were

swept out of the neutron beam by a magnet

placed between the target wheel and the collimator. The incident beam current was measured

by placing a 0.012 mm thick aluminum foil upstream of the target wheel in the charged particle beam and measuring the secondary electron emission current produced by the particles traversing the foil. This system was calibrated by comparison of the secondary

electron emission current against a Faraday

cup current. This calibration was repeated

periodically throughout the course of the ex- periment . ^Operated by Universities Research Assn. , Inc.

under contract with USERDA.

Target Lengths

Energy Loss by

Ionization (MEV) 65 50 40 30 20

Be Thickness (cm) 2.412 2.212 1.957 1.603 1.155

± Tolerance (cm) 0.033 0.010 0.015 0.020 0.025

Li

Thickness (cm) 7.993 7.338 6.498 5.325 3.841

± Tolerance (cm) 0.110 0.033 0.051 0.067 0.082

Neutron Energy Spectra

Experimental Technique

Neutron yield and energy spectra were meas ured for 65.4 MeV protons, 35 MeV protons and

35 MeV deuterons incident upon selected targets. The actual accelerated beam energies

were 65.6 MeV and 37.5 MeV for the protons am

39.2 MeV for the deuterons to allow for energf

degradation in the windows.

The detector used for these measurements

was a cylindrical NE213* liquid scintillator,

5.08 cm long by 5.08 cm diameter mounted co- axially in the neutron beam. The efficiency

of the detector as a function of incident

neutron energy had been calculated using a *Nuclear Enterprises, Ltd.

10
modified version of the Monte Carlo code by

Stanton.

5

In a previous experiment, the efficiency of the detector had been measured at

several energies to investigate the accuracy

of the calculated values. ® Pulse shape discrimination techniques were used to distinguish events initiated by gamma rays from

those attributed to neutrons.

The detector was placed 2.90 m downstream

from the target wheel and was fully illuminated by the neutron beam. The neutron timeof-flight was measured relative to a stop

signal from the cyclotron r.f.

The measured target-to-detector distance,

. and the position of the gamma ray peak rela-

! tive to the rest of the time-of-f light spectra were used to calibrate the neutron flight

' times. The longest flight times observed were

limited by the overlapping of successive beam

bursts, giving a threshold of five to nine

MeV.

Results

The measured neutron spectra are displayed

in Figs. 1 through 3. Figures 1 and 2 show

spectra obtained from 65.4 MeV protons incident upon beryllium and lithium targets,

respectively. The energy resolution corresponding to 65 MeV neutron energy is ± 5 MeV.

The overlapping of beam bursts and the finite

time resolution of the detection system, together with phase shifts of the beam bursts

relative to the cyclotron r.f., lead to some uncertainties in the shape of the spectra. For

example, the curves do not pass through zero

at the kinematic limit. The Q-values of the

9 Be(p,n) 9 B and

7

Li(p,n) 7 Be reactions are

-1.85 and -1.64 MeV respectively, hence, we

would not expect neutrons above 63.8 MeV.

These plots show that the thinner targets

produce harder spectra as expected. The in- crease in yield shown in changing from a target which stops the beam to a slightly

thinner one is attributed to the absorption

of neutrons by the thick target. The additional target material used to stop the beam,

produces few neutrons in the energy range measurable, yet attenuates neutrons produced in

the upstream portions of the target.

Figure 3 shows spectra measured for 35 MeV

protons and deuterons incident upon the 1.16

cm beryllium and 3.84 cm lithium targets. The

p-Li and p-Be spectra indicate a large low

energy component of evaporation neutrons, as also evidenced in other work.^ The lithium

- spectrum has a larger high energy component

reflecting a larger neutron production cross

section for the inclusive p-Li reaction than

for p-Be near the kinematic limit. 8 The d-Li

and d-Be spectra display the characteristic

shape associated with stripping reactions. The

0 beryllium data agree with other work9 within

the accuracy of the measurements.

:f The neutron yield for these spectra is

n tabulated in Table II. The overall uncertainty in the yield, including beam current

integration and detector efficiency, is ± 25%.

Table II Integral yield of neutrons produced by bombarding lithium and beryllium targets with protons and deuterons.

Particle

Type

Incident

Energy MeV

Target

Thickness cm Mat'

1

Yield

Neutrons/

uC/sr

Cut Off

Energy MeV

prot 65.4 2.41 Be 8 . 8xl0

10

9.1

II II 2.21 Be 9.4xl010 II

II II 1.96 Be 8.8xl010 II

II II 1.60 Be 7. 0xl010 II

II » 7.99 Li 10.6xl010 II

II II 7.34 Li 10. 9xl010 II

II II 6.50 Li 10. 7xl010 II

II II 5.33 Li 9.4xl010 II

II 35 1.16 Be 1.6xl010 5.1

II II 3.84 Li 1. 7xl010 5.1 deut II 1.16 Be 19.1xl010 5.0

II II 3.84 Li 21. 3xl0

10 5.0

References

. Amols, M. Awschalom, et al. "Physical

Characterization of Neutron Beams Produced By

Protons and Deuterons of Various Energies Bombarding Beryllium and Lithium Targets of

Several Thicknesses", FERMILAB-Pub-76/102-EXP

.

2 H. Amols, et al. "Fast Neutron Dosimetry and

Ion Linear Accelerators", LA-UR-76-1007 (1976).

3 J. A. Jungerman and F. P. Brady, "A MediumEnergy Neutron Facility", Nucl. Instr. Meth.

89_, 167 (1970).

4 J. F. Janni, Calculations of Energy Loss,

Range, Pathlength, Straggling for 0,1 to

1000 MeV Protons", AFWL-TR 65-150 (Sept. 1966).

5M. W. McNaughton, N. S. P. King, F. P. Brady

and J. L. Ullmann, "Improved Predictions of

Neutron Detection Efficiency Resulting from

New Measurements of 12 C(n,P) and 12 C(n,d) Reactions at 56 Mev", Nucl. Instr. Meth. 129 , 241 (1975).

^P. Urone, private communication.

7 S. W. Johnsen, "p-Be Neutron Production at 25

to 55 MeV", to appear in Med. Phys.

g J. A. Jungerman, F. P. Brady, W. J. Knox, T. Montgomery, M. R. McGie, J. L. Romero and Y. Ishizaki, "Production of Medium-Energy Neutrons

from Proton Bombardment of Light Elements",

Nucl. Instr. Meth. 94, 421 (1971).

, Greater detail in the analysis of the un- certainties is presented by S. Johnsen in an accompanying report at this conference.

I

9 J. P. Meulders, P. Leleux, P. C. Maig and

C. Pirat, "Fast Neutron Yields and Spectra

from Targets of Varying Atomic Number Bombarded With Deuterons from 16 6o 50 MeV", Phys.

Med. Biol. 20, 235 (1975)
Fig. 1 Spectra of neutrons produced by bombarding

beryllium targets with 65.4 MeV protons.

The target thicknesses used to produce each

spectrum are: curve A, 2.41 cm; curve B, 2.21 cm; curve C, 1.96 cm; curve D, 1.60 cm. The

ordinate is in units of 10 9 n sr

-1 yC -1 MeV-1

.

12
Fig. 2 Spectra of neutrons produced by bombarding

lithium targets with 65.4 MeV protons. The

target thicknesses used to produce each

spectrum are: curve A, 7.99 cm; curve B, 7.34 cm; curve C, 6.50 cm; curve D, 5.33 cm. The

ordinate is in units of 10 9 n sr -1 pC -1 MeV-1

.

13
N

(E)

Fig. 3 Spectra of neutrons produced by stopping

35 MeV protons and deuterons in lithium and

beryllium. Curves A and B: 35 MeV protons

stopping in beryllium and lithium respectively;

full scale = 1.5 x 10 9 n sr

-

pC-1 MeV-1. Curves C and D: 35 MeV deuterons stopping in

beryllium and lithium respectively; full

scale = 1.5 x 10 10 n sr

-1 pC-1 MeV-1

.

14
SPECTRAL MEASUREMENTS OF 25 TO 55 MeV NEUTRON BEAMS*

S. W. Johnsen

Crocker Nuclear Laboratory

University of California

Davis, California 95616

Energy spectra of neutron beams produced by 25, 35, 45 and 55 MeV protons stopping in beryllium have been measured. The spectra exhibit a large low energy component which

has small angular dependence and a high energy component which is peaked in the forward

direction. The dominant uncertainties in these time-of-fl ight measurements are the un- certainties in detector efficiency and the energy resolution for neutrons with short

flight times.

(Distributions; neutrons; resolution; spectra; uncertainties; yield)

Introduction

Interest in the development of accelerators for

use in neutron radiotherapy has prompted the investigation of neutron production using beams of protons

bombarding various targets. We have measured the energy spectra and yield of neutrons produced by bom- barding thick beryllium targets with protons at energies from 25 to 55 MeV. Such data are needed both

for accurate dosimetry of these neutron beams and to aid in the design of accelerators dedicated to such

use.

Since the results of these measurements are re- ported in detail elsewhere,^ they will be only briefly

outlined herein. The bulk of this paper will discuss

the techniques and tools employed to acquire these

data, and the limitations inherent in these methods.

Experimental Methods

The measurements were performed at the 76-inch

isochronous cyclotron of the Crocker Nuclear Laboratory

at the University of California, Davis. Proton beams

from the cyclotron were transported to a beryllium target placed in a target holder fashioned after that em- ployed at the Naval Research Laboratory, Washington,

2 D.C., radiotherapy facility. The target assembly in- cluded beam current readout capability and electron

suppression to insure accurate beam current measurements.

For each proton energy used, the beryllium target

was at least thick enough to stop the beam. For the

25 MeV incident proton beam the target was 0.46 cm thick while for the other energies the thickness was 3.00 cm. The beryllium was backed on the downstream

side by 3 mm of aluminum.

The detector used for these measurements was a cylindrical NE-213 liquid scintillator, ^ 5.08 cm long

by 5.08 cm diameter, mounted coaxially with respect to the incident neutrons. The detector was placed 2.0 m downstream from the target and was fully illuminated

by the neutrons. There was no collimation used

between the target and the detector.

Standard fast electronics were used to process

each event and the time-of-fl ight (T0F) spectra were

recorded and analyzed on an on-line PDP-1 5/40 computer.

The neutron T0F was measured relative to a signal derived from the cyclotron r.f. Pulse shape discrimination

This work was supported in part by grant PHS CA 17419

and in part by NSF grant 71-03400.

(PSD) techniques were used to distinguish events initiated by gamma rays from those attributed to neutrons.

The longest flight times, and therefore the low- est measurable neutron energies, were limited by the overlapping of successive beam bursts.

The overall performance-of the system has been

tested in another experiment4 using 35 MeV deuterons

on beryllium and the agreement with work by others was within the accuracy of the measurements.

Experimental Results

Figure 1 shows the neutron spectra measured at 0° produced by bombarding a thick beryllium target with

25,35,45 and 55 MeV protons. The spectra for all energies are characterized by a large low energy evap- oration component together with a broad peak near 20 MeV which drops to zero at the kinematic limit. Al- though this peak becomes more prominent as the proton energy is increased, the position of the peak does not change. The low energy tail also increases substantially in magnitude as the proton energy increases.

FIG. 1 . Neutron spectra at 0° produced by stopping

25, 35, 45 and 55 MeV protons in beryllium.

15
10. 20. 30 . 40 . NEUTRON ENERGY CMEVO

FIG. 2. Spectra from 35 MeV protons stopping in a 3.00 cm thick beryllium target at angles from 0° to

50° with respect to the incident proton beam.

As the angle of detection is moved away from

zero degrees the peak near 20 MeV decreases in magnitude whereas the low energy tail exhibits very little

angular dependence. This is illustrated for the 35

MeV case in figure 2.

For all measurements other than those at 25 MeV,

the beryllium target was thicker than that required to stop the protons. Using neutron-beryllium cross

5 section data, the attenuation of neutrons in this excess target length was calculated to be 25%, 20% and

15% for the 35, 45 and 55 MeV yields respectively.

The measured yields together with those corrected for

excess target lengths are tabulated in table 1. Figure 3 shows the corrected yields plotted versus the angle of detection. As discussed below, the overall

uncertainty in these values is estimated at 20%.

Compared with d-Be spectral measurements in the

same energy ranged these p-Be measurements show app- roximately one-twelfth the neutron flux and much less angular dependence of that flux.. The increased yield

as a function of incident proton energy varies approximately as the cube of the energy, similar to the d-Be

case.

Experimental Uncertainties

The uncertainties in these TOF measurements can

be considered as three uncorrelated problems: 1) un- certainties in proton production and measurement,

2) neutron detector efficiency uncertainties, and

3) the finite time resolution of the systems involved.

TABLE 1. P-Be Neutron Yields, E n

> 5 MeV.

Proton Angle Measured Yield Corrected

Energy (deg) Yield for Excess Target

(MeV) (n/Coul/Sr) (n/Coul/Sr.

)

25 0 0.80 • 10

16 0.80 • 1016

II 10 0.74 0.74

II 20 0.59 0.59

II 30 0.49 0.49

II 40 0.43 0.43

II 50 0.36 0.36

35 0 1.7 • 10

16 2.3 • 1016

II 5 1.6 2.1

II 10 1.5 2.0

II 15 1 .4 1.8

II 30 0.9 1.2

II 50 0.6 0.8

45 0 3.3 • 10

16 4.1 • 1016

II 5 3.0 3.8

II 10 2.6 3.3

II 15 2.3 2.9

II 30 1.6 2.0

II 50 0.9 1.2

55 0 5.5 • 10

16 6.4 • 1016

II 5 5.3 6.2

II 10 4.9 5.8

" 15 4.4 5.1

II 30 2.9 3.4

II 50 1.6 1.9

ANGLE C DEGREES

FIG. 3. Angular distributions of p-Be yields of

neutrons above 5 MeV. The data have been corrected

for attenuation of neutrons in excess target length.

16
Proton Beam Uncertainties

For protons in the energy range considered here,

the beam of the Crocker Nuclear Laboratory cyclotron

appears as a succession of bursts, each burst 1 to 1.5

nsec wide with a period of 45 to 85 nsec between bursts

depending on the beam selected. For the 55 MeV measurements the overlapping of neutrons from successive

beam bursts resulted in an effective neutron threshold

of 4.5 MeV. The short flight path required to achieve

a suitably low overlap energy together with the finite

time resolution of the system, as discussed below, compromises the energy resolution for the higher energy

neutrons with short flight times.

The beam current was measured using a well calibrated current integrator. The target assembly has

been tested to verify that the electron suppression is adequate. Uncertainties in beam current measurement

are less than 3 %.

The beam current required to give adequate but not excessive count rate was from .5 to 2 nanoamperes.

Stability of the beam and instrumentation was more than

satisfactory during the approximately 30-minutes run

times required to achieve adequate counting statistics.

3 Due to the approximately E energy dependence of

the neutron flux, the 1% uncertainty of the accelerated

proton energy adds a 3% uncertainty in the neutron flux

determination.

Detection System Uncertainties

The absolute efficiency of the detector was calculated using a code originally written by Stanton.

The code as used has been modified by the inclusion of

g recent cross section data. The accuracy of the modified code has been tested at Davis in measurementSgOf

the efficiencies of several organic scintillators.

Figure 4 compares the measured and computed efficiencies of a 5.1 cm x 7.6 cm x 10.2 cm rectangular NE-213

liquid scintillator at several energies between 11 and

50 MeV. Data are included for three different detector thresholds corresponding to 1 , 2, and 4 MeV electrons. Over the range 11 to 50 MeV the root-meansquare deviation of the code's prediction from the measured values is 25% for the 1 MeV threshold data. Over the limited energy range of 11 to 30 MeV the r.m.

s. deviation improves to 11%. Limited efficiency

measurements of the detector used for the above spectral measurements indicated a comparably accurate efficiency calculation by the code.

For all thresholds tested the code predicts sys- tematically low values for the efficiency at neutron

energies above 30 MeV. In this energy range the neutron-carbon inelastic processes become significant.

The cross sections for these interactions are not well

1 o known, particularly the C(n,n'3a) reaction. Additionally, the light output functions used in the code

for low energy alphas are suspect. Since the alphas

produced in these reactions have low energies, their

detection efficiency is highly dependent upon the threshold.

In general, the use of PSD techniques, when incorporated in the prediction code, improves the accuracy

of the code's predictions.

By weighing the uncertainties in the efficiency by the energy distributions of the measured p-Be spectra,

we estimate the overall uncertainty in the yield measurements due to the efficiency uncertainties to be 20%.

NEUTRON ENERGY CMEV3

FIG. 4. Experimental and code-predicted efficiencies

of a 5.1 cm x 7.6 cm 4 10.2 cm NE-213 liquid scintillator.

This uncertainty dominates the other uncertainties in the yield determinations.

Time Resolution

Figure 5 shows the T0F spectra of gammas and neutrons for the 35 MeV data at 0°. The stop pulses

for the time-to-amplitude converter (TAC) which measures the T0F are derived from every second r.f. cycle

so that the time spectra acquired show nearly two full beam bursts. This allows time calibration of the sys- tem using the well determined cyclotron r.f. frequency

and additionally eliminated any gaps in the spectra

from TAC dead time. The width of the prompt gamma

burst from the target is a good measure of the overall

time resolution of the system, which is better than 3.5

nsec. There are three major contributions to this tim

ing uncertainty.

First, as mentioned above, the intrinsic width of

the cyclotron beam bursts is 1 to 1.5 nsec. This is

a function of the energy and particular tune used.

Secondly, the phase of these bursts relative to the cyclotron r.f. varies slightly. This time jitter

can be as great as ± 1 nsec, again depending on the tune and the stability of the cyclotron. The low beam currents employed in these T0F measurements necessitate

the use of the cyclotron r.f. for timing rather than a beam pick-off unit, which would give a timing signal

that was independent of such phase variations.

17
FIG. 5. Time spectra of 35 MeV p-Be neutrons and

associated gammas.

Thirdly, there is the intrinsic time resolution

of the detection system. For monoenergetic gamma rays

this resolution is better than 1.5 nsec; with a polyenergetic spectrum of mixed particles it is much worse. The use of PSD techniques implies that the timing will

be dependent upon the ionization density of the recoiling particle in the NE-213 scintillator. Using a constant fraction discriminator which triggers when 20% of

the light is collected from an event, the events due to alpha recoils appear late relative to the proton recoil events. This problem is most troublesome at neutron

energies above 30 MeV where a relatively large proportion of the events are due to reactions which produce

alphas.

The uncertainties in the energies of detected

neutrons caused by timing uncertainties are worsened

by the use of relatively short flight paths as necessitated by the overlapping of beam bursts. For the 35

MeV p-Be data shown, the energy resolution at 35 MeV is ±3.5 MeV. For this reason some of the neutrons may

appear to exceed the kinematic limits. The resolution

improves at lower energies as the flight times increase; at 10 MeV neutron energy, the energy resolution is better than ± 1 MeV.

For beams where the flux varies slowly with neutron energy, as these p-Be spectra appear to, the timeresolution of the experiment is not crucial. If the spectra exhibit sharp peaks or abrupt changes with

energy, this may be an important consideration. With

out changes in the intrinsic time resolution of the tools used, better energy resolution could be achieved

at the expense of a higher neutron threshold by per- forming the measurements with a longer flight path.

In spite of their limitations, time-of-fl ight

techniques provide a relatively easy and straightforward means of measuring medium energy neutron spectra

from pulsed sources. The means are available to im- prove the techniques illustrated here and improvements

in these spectral determinations should be forthcoming

References

1. S.W. Johnsen, in press, Med. Phys.4:3 (1977)

2. R.B. Theus, LA-5180-C, presented at the PART Conference, Los Alamos Scientific Laboratory,

Oct. 1972, p.82.

Jj 3. Nuclear Enterprises Inc., San Carlos, California.

4. H.I. Amols, J.F. DiCello, M. Awschalom, L. Coulsor

S.W. Johnsen and R.B. Theus. Fermilab-Pub-76/

102-Exp. 1184.000 (1976) (Submitted to Med.Phys.)

See also: H.I. Amols et al . in Basic Physical

Data for Neutron Dosimetry, ed. J.J. Broerse,

Commission of the European Communities, EUR 5629e

(1976), p. 23.

5. I.F. Bubb et al. Can.J. Phys. 52, 648 (1974)

6. J.P. Meulders, P. Leleus, P.C. Macq and C. Pirat.

Phys. Med. Biol. 20, 235 (1975)

7. N.R. Stanton, COO-1545-92 (1971)

8. M.W. McNaughton, N.S.P. King, F.P. Brady and J.L. Ullmann. Nucl. Instr. & Meth. 129, 241 (1975).

9. P. Urone et al. in Progress Report to the N.S.F. UCD-CNL 187 (1976) p. 91.

Conclusions

The determination of neutron spectra produced by

proton beams from a cyclotron striking a beryllium target has been made using TOF techniques. There are two dominant uncertainties in these measurements as described above.

The uncertainties in neutron detector efficiencies

lead to significant uncertainties in the neutron fluxes

and spectral distributions produced. Additional careful measurements of absolute detector efficiencies are

in order. This would not only provide absolute calibration of the particular detectors measured but would

also provide benchmarks for testing the predictions of

codes such as that used for these experiments. These

codes could then be made more accurate in their pre- dictions for other detector assemblies.
PROTONS AND DEUTERONS ON Be AT E

p

= 100 MeV AND EfJ = 80 MeV#

G. H. Harrison, C. R. Cox, E. B. Kubiczek, and J. E. Robinson

Division of Radiotherapy

University of Maryland

School of Medicine

Baltimore, Maryland 21201

The dose rate of neutrons produced by 80 MeV deuterons on a thick Be target was measured

to be 56.0 rad yamp

-1 min-1 at a target-to-surf ace distance (TSD) 1?5 cm and field size 10 cm

x 12 cm. The maximum of the neutron energy spectrum occurred at 32 MeV. The LET spectrum

( L • D ( L ) ) indicates that 33 percent of the absorbed neutron dose is due tc events with L<1G0

KeV/y, and emphasizes the need for measurements of high energy neutron reaction cross sections

of 12 C, 1 4 N, and 16 0. 100 MeV protons on a Be target 62 MeV thick backed with aluminum yielded neutrons at 14.4 rad yamp

-1 min-1 at TSD 100 cm and field size 8 cm x 10 cm. The maximum

of the neutron energy spectrum occurred at 50 MeV.

(dosimetry, energy spectra, high energy, LET, neutrons, yields)

Introduction

Over the past several years we have performed a variety of experiments using high

energy neutrons at the University of Maryland

cyclotron. These experiments were designed

to yield biological and physical data relevant

to the evaluation of these neutron beams for

use in cancer therapy. 1-4 Using 80 MeV deuterons and 100 MeV protons on Be targets, we have demonstrated high neutron dose rates,

depth-dose profiles superior to 60 Co radiation, and a higher proportion of dose due to

high LET events, compared to lower energy

neutron beams. Although our physical data

were obtained in geometries convenient for

radiobiological studies and are expressed in

rads instead of neutron fluence, some of our data are relevant to the purposes of this

Symposium. We will present:

1. LET spectra of neutrons produced by

80 MeV deuterons on beryllium measured

with a Rossi-type spherical proportional

counter

;

2. Dose rates and estimated yields with

a Spokas tissue equivalent (TE) ion

chamber for neutrons produced by Be bombarded by 80 MeV deuterons, 100 MeV protons and also 6l MeV protons for com- parison with results of other investigators ;

3 • Neutron energy spectra produced by

80 MeV deuterons and 100 MeV protons on

Be;

4. A discussion of needed nuclear cross section measurements.

Experimental Set-Up

Charged particle beams from the Maryland

cyclotron pass through a series of focusing

magnets to the neutron production target.

The beams are centered and focused on a beryllium oxide scintillator mounted on the front

of the target and viewed by a television camera. The beam spot diameter is approximately

15 mm for deuterons and 3 mm for protons.

A cross section of the water-cooled

holder for targets 2 cm thick is shown in

Pig. 1. To provide the 4.25 cm thick Be target used in 100 MeV proton irradiations,

the threaded aluminum plug shown in Fig. 1 is

replaced with a threaded aluminum cup containing additional Be.

The possibility of electron emission

from the front surface of the neutron-producing element, causing errors in beam current

integration, 5 is minimized by the geometry of

the surrounding aluminum and the fact that

most of the ionization from high energy

charged particles occurs deep within the target. While high energies may obviate the

problem of significant electron emission, a new factor is introduced: the range straggling occurring with full energy loss could

lead to unintegrated beam current if the

target thickness is only a few mm greater

than the mean particle range. For this reason the neutron-producing target element is

backed by 1.11 cm aluminum beyond the mean

particle range. An additional consequence of

using high-energy deeply penetrating charged

particles to produce neutrons is that the

neutron source is a line source coincident

with the central axis, and may lead to diffiSCALE

Figure 1. Neutron production target.

19
culties in neutron beam collimation and

inverse square corrections of dose.

Ml neutron beams are filtered by the

1.11 cm thick aluminum plug (Fig. 1) and a 0.32 cm thick aluminum plate (not shown).

Lower-energy data 6 show that such filtration

should have little effect on the neutron

energy spectrum; however, the overall attenuation can be included in calculations of the

intrinsic dose rate of each neutron source configuration

The geometry of neutron beam collimation

was changed several times during the course

of the measurements to be reported in this

paper. The collimation geometry is important

in absolute dosimetry, and will be tabulated

along with corresponding dosimetric results.

LET Spectra

We have measured the linear energy

transfer (LET) characteristics of the neutron

beam produced by 80 MeV deuterons using an EG&G Rossi-type gas-filled proportional chamber. This consists of a 12.8 mm diameter

spherical chamber which is filled with a tissue equivalent gas ( 55 % C 3 H e ,, 39.6? C0 2 and

5.4? N .) and surrounded by a 1.3 mm. thick

wall of tissue-equivalent plastic. The chamber was filled to a pressure of 3^.5 mm of

mercury corresponding to a ly sphere of unit

density tissue

Measurements were made in a 40 cm cubic

tissue-equivalent liquid (Frigerio's threecomponent formula with density = 1. 1)"7 with

the center of the detector at depths of 1 and

5 cm. The phantom was positioned at a TSD

of 125 cm and the neutron field was 10 cm x 12 cm. The data were assembled in a pulse

height analyzer and a paper tape record was made for input to a computer program. This

program unfolds the effect of the variable

path length of the detector's spherical geometry and computes the LET distribution.

The data were collected in three segments

because of the widely varying event rate in

different portions of the spectrum. The an- alysis method is similar to that outlined

by Grant et al. 8

Figure 2 shows L’D(L) plotted against

L, where L is LET and D(L) is the dose delivered at a given value of L. With this

choice of variables, equal areas under the curve represent equal doses. The results

show several peaks over a continuum up to

about 800 KeV/y . The peaks are interpreted

as being due to Bragg peaks of protons, alphas and heavier fragments. 33 percent of

the dose is delivered at LET values above

100 KeV/y.

Neutron Dose Rates

Neutron dose was measured with a 0.5 cc Spokas ion chamber constructed of Shonka type

A-150 TE plastic. It is customary to observe

Figure 2. L-D(L) vs. L for 80 MeV deuterons

on Be at the depths 1 and 5 cm in a TE liquid1 phantom TSD = 125 cm. There is no discern! bl; difference in the spectra beyond 100 KeV/y.

that this plastic is not really tissue equiva

lent for neutrons because of the replacement

of oxygen by carbon. This problem is amplified at the high neutron energies reported

here; our LET spectra demonstrate that a large proportion of high-energy neutron dose

in tissue is caused by n- 1

C and n10 reactions .

Dosimetric Methods

Neutron dose at 5 cm depth in TE

liquid phantoms was determined from the

charge collected from the air filled ion

chamber according to the Bragg-Gray principle

as outlined in neutron dosimetry intercomparisons at lower energies. 9 First the mass

of air in the chamber was determined by measuring the chamber response to a known dose

of 6 °Co gamma rays. The mass was 6 . 5 8 x 10 -7 kg at STP , so that the actual chamber

volume is measured to be 0.5 cc to within

one percent.

The ion chamber was then exposed to the

Naval Research Laboratory (NRL) neutron beam.

This beam, generated by 35 MeV deuterons on Be has been carefully calibrated and intercompared with other lower energy neutron facilities.

Using the air mass and the measured charge

collected in our ion chamber, along with NRL

values for neutron stopping power ratios (Sn)

and energy to create an ion pair (Wn) in air, 5 our calculated neutron dose agreed with the

stated NRL dose to within one percent

The next step was to select appropriate

parameters for the calculation of higher energy neutron doses according to the Bragg-Gray

relation

D = 100 Q Wn Sn ( l

)

M C k

20

—
where D = dose to tissue (rads)

Q = collected charge (coulombs)

Wn = average energy from secondary

charged particles to create

an ion pair in air

(ev/ion pair)

Sn = ratio of average stopping

power of secondary charged

particles (TE plastic/air)

M = mass of air in ion chamber (kg)

Ck = Kerma correction factor

(TE plastic-.tissue)

For the most part we have followed the

procedure used at the highest energy neutron

facility for which there exists careful dosimetry with intercomparisons: the Texas A&M

cyclotron (50 MeV deuterons on Be). 10 There

are two alterations. First, in the computation of Wn, the alpha contribution is taken to

increase from 19 to 33 percent based on our

LET spectrum. Second, Ck is taken to be 1.00.

Usually Ck is greater than 1 because of the

: oxygen deficiency in TE plastic compared to

i tissue; but according to ICRU-13 the ratio of

j neutron kerma in oxygen to kerma in carbon

! appears to approach unity as the neutron enerf gy increases to 2.0 MeV, above which energy

there is no data.

Our resulting dosimetric constants are shown in Table I. resulting in the final re- lationship for high energy neutron rads in

tissue measured with our ion chamber: Dntf=

6.46Q (nanocoulombs) after temperature and

pressure correction. Our measurements were

performed in liquid phantoms using Frigerio's

three-component TE formula with density=l . 10

.

All collimation was provided by untapered aper1 tures in steel shielding.

I: Dose Rate Results

Most of the measured dose rates were obtained in geometries inconvenient for compariTABLE II. Neutron dose rates.

TABLE I. Dosimetry constants

Constant Maryland

s °Co

Maryland rTAMVLC

1)

W

alr (ev/ion pair) 33.73 35-55 35.79

o TE plastic

air 1.137 1.157 1.157

r muscle u kTE plastic - 1.00 1.05

son with other yield results. Table II describes the geometries and measured dose rates,

along with estimated dose rates and under

standard conditions of 125 cm TSD, 1 cm depth

in TE phantom (p = 1.10), using unattenuated

neutron beams. The measured dose rates under

the stated conditions have estimated uncertainties of 12 percent. These results are more reliable than the calculated expected

dose rates under standard conditions. These

expected dose rates require additional corrections for neutron attenuation in the 1.45 cm

of aluminum immediately downstream from the

site of neutron production, changes in TSD,

changes in depth within the phantom, and

changes in phantom dimension. The combined

error for these corrections is estimated to be

nine percent, with the total uncertainty 15

percent

.

The dose rate for 80 MeV deuterons on Be

under standard conditions (56 rad min-^pamp" 1 ) conforms to the NRL yield predictions 11

; at

E, = 80 MeV, 1.24 x 10- 4 xEd

2- w (MeV)^ = 60.7 rad

min-1 pamp -1

, while_1.53 x 10 (MeV) = 55.2 rad min -1 pamp

-1

.

Primary

beam

TSD

(cm)

Depth in

TE phantom

(cm)

Field

size

(cm)

Phantom width

height , depth

(cm)

Measured dose rate*3 (rad pA -1min-1

)

Expected standard

dose rate t,c

(rad pA-1min -i

)

61 MeV ,p 262 5 circular 7xl6xl6

10

0.64 3.2

80 MeV d 125 5 10x12 40x40x40 44 56

100 MeV p

a 100 5 8x10 7xl6xl6 13 9.7

aBe target 62 MeV thick backed with aluminum.

b All neutron beams attenuated by 1.45 cm aluminum as explained in text.

c Standard conditions are TSD=125 cm, dose measured at 1 cm depth, and

subtraction of attenuation in aluminum. These corrections include

experimental data on depth-dose curves and deviations from inverse

square law.

21
The dose rate for 61 MeV protons cn De under standard conditons (3.? rad min-1 yamp

1

) compares well with the value measured by Amols

et al 1 2 for 65 . 4 MeV protons (3.** rad min-1 yamp

-1

)

Neutron yields have been estimated for

the standard conditions by use of fluence-torad data in NBS Handbook 107. For neutrons

produced by 61 MeV p, 80 MeV d and 100 MeV p, the calculated yields at 125 cm are 1.1, 20,

and 2.6 x 10 11 neutrons yC -1 sr

-1 _ respectively. The result 1.1 x ID 11 n yC -1 sr

_1 tor

6i MeV protons on Be compares with Awschalom's

experimentally observed value 0.88 x 10 1

n yC

-1 sr

-1 for 65.4 MeV protons. 12 These

numbers may be consistent in view of the 9 MeV

neutron cutoff in Awscholom's yield. Our result 2 x 10 12 n yC -1 sr

-1 for 80 MeV deuterons

on Be can be compared to the yield of 50 MeV

deuterons on Be measured by Meulders et al. 13 by applying the NRL power law ( 5 0/8 0 ) as suggested above. Our yield estimate at 80

MeV, scaled by the NRL power law predicts a yield 5.0 x 10 1

n yC -1 sr

-1

, compared with

5.8 x 10 1

n yC _l sr

-1 measured by Meulders.

t-z

D>

cc<

cc

co

cc<

cozo

ccH

z>

LiJ 2

10 20 30 40 50 60 70 80 90 100 NEUTRON FNFRGY (MeV) Figure 3. Neutron energy spectra.

2. Stopping powers of protons, alphas,

and heavier fragments above 20 MeV in

l H, I2 C, 14 N, and '*0 to permit the

calculation of Sn.

3. W values of protons, alphas, and

heavier fragments above 20 MeV in air.

Neutron Energy Spectra

4Kerma in *H, 12 C, 1

N, and 1

0

between 20 and 100 MeV.

Neutron energy spectra were obtained by

a time-of-f light technique similar to that of

Theus et al.® In the present work, the flight

path lengths were between 3 m and 5 m. Use of

the 3 m flight path permitted a check on the

d-Be energy spectrum down to a lower limit of

5 MeV. Neutrons were detected in a NE102

plastic scintillator. The detector efficiency

as a function of neutron energy was calculated

using a modified verslsn of the Kurz program. 14 The neutron energy spectrum for 80 MeV deuterons, 6l MeV protons, and 100 MeV protons

on beryllium is shown in Fig. 3. For comparison the NRL spectrum from 35 MeV deuteroninduced neutrons® is shown. The 80 MeV deuteron-induced neutron spectrum is the expected

extrapolation from lower energies, or about

32 MeV. The spectrum of neutrons from 100

MeV protons shows a mean energy near 50 MeV.

In this latter case, the target was designed

to maximize the neutron energies at the ex- pense of dose rate: the 4.25 cm of beryllium

does not stop the protons, but slows them

to about 38 MeV, after which they stopped with

reduced neutron production in the aluminum

backing.

Needed Nuclear Cross Section Measurements

Most of the material presented in this

paper suffers in some way from the lack of

basic high energy nuclear data standards. A list of basic data needed for the work reported here includes:

1. Charged particle production cross sections for n- 12 C, and N- 1 ®0 above 20

MeV to evaluate the effect of substituting 1

C for 1 ®0 in the wall of the LET

counter and the ion chamber, and to

determine the spectra of secondary

charged particles in the ion chamber.

5.

Data of the type specified in item 1 for n- 1

C reactions in hydrocarbon

scintillation detection efficiency calculations. (The work of McNaughton et al. 15 represents some progress in this

direction )

Unfortunately, nuclear data other than

n-p cross sections become both increasingly

important and increasingly scanty as neutron

energy increases. In the case of neutron re- actions in tissue, these ideas are demonstrated by the LET spectrum of neutrons from 80 MeV

deuterons on Be (Fig. 2). At this energy at least 33 percent of the dose is due to the

events other than n-p, while similar data from

50 MeV deuterons show that 19 percent of the

dose is due to other than n-p, and at 35 MeV

the figure is only 11 percent

Two factors should be helpful in obtaining the needed high energy nuclear data.

First, all of the needed data listed above

should vary slowly with energy from about 4C

up to 100 MeV; only a small number of incident particle energies need to be studied.

Second, as we have shown previously

3

, protoninduced reactions should give similar results

to neutron-induced reactions above 20 MeV

provided that Coulomb effects can be subtracted. This approximation should be especially

valid for even-even target nuclei 12 C, 14 N,

and 1 °0 . This means that much easier proton

experiments can be performed to obtain needed

neutron cross sections.

* Work supported in part by the American

Cancer Society and NIH National Cancer

Institute

‘G.H. Harrison, Radiat . Res. 59, 124 (1974)

22
2 G.H. Harrison,

Kubiczelc, Brit

J.E.

. J.

, Robinson, and E.B.

Radiol. 48, 409 (1975)

3 G.H. Harrison, Med

,

. Phys. 2 153 (1975).

4 G.H. Harrison, E.B.i Kubiczelc, and J.E.

Robinson, Brit. J. Radiol. 49, 733 (1976).

5 P.H. Attix, et.al., Phys. Med. Biol.

16, 497 (1973).

6 R.B. Theus, et.al., Cancer 3_4, 17 (1974).

7 N. A . Prigerio, R.P. Coley, and M.J. Sampson, Phys. Med. Biol. 37, 792 (1972).

8 W.H. Grant III, G.D. Oliver, and B.A.

Mitchell, Health Phys. 22, 351 (1972).

9 A . R . Smith, et.al., Med. Phys. 2, 195

(1975).

1 °P.R. Almond

,

et.al., Radiat. Res. 54., 24

(1973).

1 ^.S. August

,

et.al., Phys. Med. Biol. 21,

931 (1976).

12 H.I. Amols, et.al., Ferinilah 76/102 (1976).

13 J.P. Meulders, et.al., Phys. Med. Biol.

20, 235 (1975).

1 4 R.J. Kurz, UCRL 11339 (1965).

15M.W. McNaughton et.al., Nucl. Instr.

Meth. 129 , 241 (1975).

23 —
NEUTRON YIELDS AND DOSIMETRY FOR Be(d,n) AND Li(d,n) NEUTRON SOURCES AT E, = 40 McV

M. J. Saltmarsh, C. A. Ludemann, C. B. Fulmer, and R. C. Styles**

Oak Ridge National Laboratory

Oak Ridge, Tennessee 37830

Time-of- flight measurements are reported for the thick-target neutron yields from the Be(d,n) and Li(d,n) reactions over an angular range of 0—90° at a deuteron energy of

40 MeV. Foil activation measurements are also described for the Be(d,n) reaction. A dosimetry technique based on these measurements is discussed.

[Dosimetry: measured thick-target neutron yi

E > 2 MeV; 0 = 0-90°]

n

Introduction

In recent years there has been growing interest in the use of neutron sources based on the Be(d,n) and

Li(d,n) reactions for fusion-related materials studies.

In this paper we report on measurements of the thicktarget neutron yields from these two reactions, measurements which were made using a 40-MeV deuteron beam

from the Oak Ridge Isochronous Cyclotron (ORIC) . Timeof- flight (TOF) data were obtained for both reactions

over the angular range 0 to 90°, and foil activation

measurements were made for the Be(d,n) reaction in the range 0 to 80°. The results were used to develop a dosimetry technique suitable for the existing ORIC

Be(d,n) neutron source, * and to evaluate the performance expected from the recently proposed Li(d,n)

intense neutron sources.^

Time-of-Flight Measurements

The TOF measurements were made using a 40-MeV

deuteron beam from the ORIC. This beam is pulsed at the cyclotron orbital frequency of 12.97 MHz, providing 2-ns-wide pulses every 77.10 ns. With the maximum

available flight path of 3 m the resultant energy resolution varied from at a neutron energy of

5 MeV to ~15% at 40 MeV. However, neutron energies

below 5 MeV could not be reliably measured with this

flight path because of overlapping events from suc- cessive beam bursts. Accordingly most of the measurements were repeated using a flight path of 0.75 m, which allowed the data to be extended down to

E ~ 2 MeV.

n

Ids, Be(d,n), Li (d , n J ; E. = 40 McV;

d

The experimental arrangement is shown in Fig. 1.

2 The deuteron beam passed through a thin (7.5 mg/cm ) gold scattering foil, used in monitoring the beam, and onto the thick beryllium (or lithium) target. Neutrons and gamma rays emitted at an angle 0 were detected in a cylindrical cell of NE213 liquid

scintillatort 37.3 mm in diameter by 34.1 mm long

viewed by a 56AVP photomultiplier tube. The fast timing signal from the photomultiplier tube was used

to start a time-to-amplitude converter, the stop

signal being derived from every second cycle of the

cyclotron RF. A neutron-gamma discrimination circuit

was used to route an event into one of two 400-channel

TOF spectra. Background events, for example neutrons

scattered from the experimental room floor or walls, were measured by placing a 20-cm-thick lead shadow

shield midway between the target and the detector.

When the lead shield was removed, it was displaced by

the smallest possible distance (6 cm) , so that the effect of in-scattering from the lead was similar for both foreground and background runs.

*Research sponsored by Energy Research and

Development Administration under contract with Union

Carbide Corporation.

**0ak Ridge Associated University undergraduate

trainee. Present address: University of Georgia,

Athens, Georgia.

tManufactured by Nuclear Enterprises, Inc., 935 Terminal Way, San Carlos, CA 94070.

ORNL-DWG 76-13772

Fig. 1. The lay-out of the time-of-flight apparatus.

24
The beam current required for these measurements

was small, <50.1 nA. The beam was monitored by

counting deuterons scattered at ±45° from the thin

gold foil. Two detectors were used to eliminate errors

due to movements of the beam spot on the gold foil. This secondary monitor was calibrated at intervals

during the experiment by increasing the beam current

and using the second scattering chamber as a Faraday

Cup.

The neutron detector threshold was set relative

22

to the half-height of the Compton edge from a Na

3 source. Following the definitions of Verbinski et al

.

this corresponds to 0.89 light units (LU) . For the 3-m runs the threshold was set to be 1.00 LU

(E ~ 3.5 MeV) , while for the 0.75-m data a setting of

n 0.266 LU (E^ ~ 1.3 MeV) was used. The detector efficiency at each threshold was calculated for a number

of neutron energies in the range 1—50 MeV, using the

4 Monte Carlo code 05S.

For data analysis the positions of the two gamma

peaks in the appropriate spectrum were obtained by

fitting a gaussian peak to the data. The time calibration was obtained from these peak locations by

assuming a linear dependence of TOF versus channel

number. The neutron spectrum was then converted to

an energy spectrum, defining the flight path to be

the distance from the geometrical center of the target

to the geometrical center of the liquid scintillator.

Allowance was made for neutron absorption in the aluminum walls of the scattering chamber (~3 mm thick)

and the scintillator cell (~1.5 mm thick) as well as the air along the flight path. The measured energy spectra from d + Be at 0 and 90° are shown in Fig. 2, with the error bars showing statistical uncertainties

only. The data from d + Be and d + Li at all angles

studied are shown as smooth curves in Figs. 3 and 4, respectively. The curves for 7° and 45° do not

extend below 5 MeV as no runs were made at these

angles with a 0.75-m flight path. Monitor uncer- tanties for the 0.75-m data were typically 8—15% as against 3-^1% for the 3-m data. Accordingly, the relative normalization for the two data sets was derived from the total neutron yields for 7.5 MeV <

E < 12.5 MeV, and the overall normalization defined

by the monitor counts for the 3-m data.

The experimental uncertainties for these data

come from three sources. First, there are the

statistical uncertainties indicated by the error bars

0 10 20 30 40

£„ (MeV) Fig. 3. Time-of-flight data obtained by drawing

smooth curves through the results typified by those

in Fig. 2.

to

- 10

"

ORNL- DWG 76-13770

NEUTRON YIELDS FROM d + Be

103

10°

10

T T

90<>

- TIME-OF-FLIGHT HDATA

• 3 m FLIGHT PATH

° 0.75 m FLIGHT PATH

. 8=0

°

IT

10 20 30 40 50

En

(MeV)

Fig. 2. Time-of-flight data from 40-MeV deuterons

on a thick Be target, showing representative error bars. The 0.75-m data (open circles) have been normalized to the 3-m data in the energy ranges 7.5 MeV < E^ <

12.5 MeV. The system resolution (3-m data) varies from

«200 keV at E .,= n

5 MeV to «6 MeV at E =40 MeV. n

ORNL-DWG 76-14245

Fig. 4. Neutron yields from 40-MeV deuterons on

a thick Li target.

25
in Pig. 2. These are generally small. Second, there

is a beam monitor uncertainty at each angle, which amounts to ~4% at all angles, except for 45° where it is 6%. Finally, and most important, there are uncertainties due to the errors in the efficiency calculations. The resultant overall normalization uncertainty

is estimated to be ±15%. Errors in the measured shape

of the energy spectrum due to errors in the shape of

the efficiency curve are difficult to assess, but are probably less than 10%.

Foil Activation Measurements

The activation measurements of the Be(d,n) neutron yields required much higher deuteron beam

intensities than did the TOF measurements. Therefore,

the existing ORIC neutron source, which comprises a water-cooled beryllium target, was used; permitting

deuteron beam intensities 20 pA. In order to make

these measurements in a well-defined geometry, the activation foils were placed at a distance from the target which was large compared to the dimensions of

the interaction region within the beryllium target.

The arrangement adopted is shown schematically in

Fig. 5. Rectangular foils, 50 x 120 mm, were mounted

on a support in the shape of a circular arc 76.2 mm in radius, and the whole assembly located with the center of the arc at the center of the d + Be interaction

region. After exposure for 4 hr at a deuteron beam

level of 20 p,A, the foils were removed, allowed to

cool for several days, and 6.3-mm-diam discs were punched from them in the pattern indicated in Fig. 5. The discs were weighed and the gamma activities listed

in Table 1 were measured using a Ge(Li) detector whose

efficiency was calibrated using a standard source.

To convert the measured activities to neutron

yields the spectrum- averaged cross sections for the

ORNL-OWG 76- 14018

Fig. 5. The experimental arrangement used for the activation measurements in a well-defined geometry.

The dimensions of the interaction region were 6 mm

in length, <5 mm in diameter.

Table 1. Reactions Used for Activation Measurements

Reaction Q-Value

(MeV)

EY

(keV) 58 . f ,58„ Ni(n,p) Co 0.39 810.5

59„ . . ,58„ Co(n,2n) Co -10.47 810.5

^ 5Nb(n,2n) y ^m

Nb -8.95 934

three reactions were computed as a function of scat- tering angle by folding the spectrum shapes as measured in the TOP' runs with the excitation functions

shown in Fig. 6. For energies ^20 MeV the available

56 59 93 data3

’ for the Co(n,2n) and Nb(n,2n) reactions appear to be in reasonably good agreement, but considerable scatter of the experimental results is apparent

58 7 for the Ni(n,p) case. Above 20 MeV there are no

data, so the excitation functions were extrapolated

g using the shapes calculated with the code THRESH. The overall uncertainties in the calculated spectrumaveraged cross sections were estimated to be ±10% for

59 93 the Co(n,2n) and Nb(n,2n) reactions, and ±15% for

58 the Ni(n,p) reaction. In Fig. 7 we show the experimental values of neutron yield (E^ > 2 MeV) versus

0RNL-DWG 76-15179

(MeV)

Fig. 6. Excitation functions used to calculate

spectrum-averaged cross sections. The solid lines

5 6 7 represent the energy regions for which data ’

’ were

available; the dotted portions were extrapolated from

Ref. 8.

O 20 40 60 80 100 SCATTERING ANGLE (deg) Fig. 7. Experimental values of neutron yield

obtained in the present work for 40-MeV deuterons on

thick beryllium targets.

26
I

angle obtained from the three activation foils, cor- rected for absorption in the tantalum end plate. The

TOF data are also shown. The error bars indicate

statistical uncertainties only. The solid line represents the mean of the TOF and activation results. The

four sets of data are displaced from this curve by an

59 average of —10% (TOF), +10%, [ Co(n,2n)J, +1% [^Nb (n, 2n) ] , and —15% [ 5 ®Ni(n,p)], in reasonable

agreement with the estimated uncertainties.

In Table 2 the results of four separate measurements of d + Be neutron yields (^ > 2 MeV) are listed,

together with the average values represented by the solid line in Fig. 7.

The total d + Li neutron yields for E^ > 2 MeV

are given in Table 3. From the beryllium data we see that the TOF results are 10% lower than the average

values for the total yields, accordingly the lithium

data should be renormalized upwards by this amount,

as shown in the final column of Table 3. The errors quoted in this column reflect not only statistical

errors, but also those due to uncertainties in the shape of the detector efficiency curves.

Thick-target neutron yields from the d + Be

9 reaction have previously been measured by Schweimer

at = 40 and 53.8 MeV, and by Meulders et al. 10 at

E, = 16, 33, and 50 MeV. Schweimer' s measurement of

Table 2. Thick-Target Neutron Yields (E^ > 2 MeV) from

d + Be, E, = 40 MeV. Units are n/M*C/sr

d

Scattering

Angle

(deg)

Neutron Yields Activation Data Average*

TOF Data 59D lo

(

n , 2n) ^®Ni (n,p) 93‘ Nb(2 ,2n)

0 33.6+1.3 38 . 7±0 .

4

30.9±0.3 35 . 7±0 .4 36.0+1.8

S 33. 7±0 .

4

24 . 8±0 .

3

29.9+0.3 30. Oil .5 7 25. 1±1 .3 26.6+1.3

10 26.0+0.2 18 ,0±0 .

2

23. i±0 .

2

22.2+1.1

15 13. 2 ±0.5 18.S±0.

3

15.4+0.8

20 13 . 4±0.

2

9.51+0.13 11.6+0.2 11.7+0.6

30 6 . 8 ±0 .

2

9.26+0.17 6.59±0. 11 8.19+0.2 8.1±0.4

40 7.29+0.23 5 . 12±0 . 12 6 ,65±0.

2

5 . 9±0 .

3

45 4 . 2±0 . 25 5.05+0.25

SO 4. 86 ±0.15 3. 74±0.06 4 . 36±0 . 16 4 . 30±0. 22 60 2.77+0.

1

3.30+0.13 2 . 79±0 .07 3. 34 ±0.11 3. 14±0 . 16 70 2 . 2 1 ±0 . 1

6

2. 10±0.08 2.31+0.14 2.30+0.12

80 1.66 ±0.16 1 . 68±0 . 06 1.39+0.08 1 ,67±0 .08 90 1 . 26±0 . 04 Scale uncertainty ±15% ±10% ±15% ±10% ±7%

Corresponds to solid line in Fig. 7.

Table 3. Thick Target Neutron Yields

(E > 2 MeV) from d + Li, E, = 40 MeV

n

10 d Units are 10 n/p.C/sr

Scattering

Angle

(deg)

Neutron Yield

TOF Data

a Renormalized

0 36.7 + 1.8 40.9 ± 2.9

7 25.3 ± 1.3 38.2 ± 2.0

15 11.6 + 0.4 13.0 + 0.9

30 6.28 + 0.25 6.99 + 0.50

45 3.73 + 0.17 4.15 + 0.30

60 2.48 + 0.09 2.76 + 0.19

90 1 .43 + 0.05 1.59 ± 0.11 Scale + 15% + 7% uncertainty

Statistical errors only.

the neutron energy spectrum at 0° extends down to a neutron energy of 11.5 MeV, and the shape is in good

agreement with our results, however, his estimate of

the overall yield is approximately 35% lower. Moulders' data include energy spectra at a number of

angles for neutron energies E^ > 3 MeV. No direct

comparison with these data is possible due to the differing deuteron energies, but our data follow the same trends as regards spectral shapes, angular

distributions and overall yields.

Dosimetry for the d + Be Neutron Source

All the yield measurements described in the previous section were made in well-defined geometries.

However, irradiations performed with the ORIC source generally require samples to be placed within a few millimeters of the beryllium target so that a given

position on the sample corresponds to a range of

scattering angles from the target. Furthermore both

the neutron flux and the energy spectrum vary strongly

with distance from the beam axis. Thus the dosimetry

must provide a measure of both quantities with a spatial resolution ~1 mm. The basis of the method is to assume that the neutron energy spectrum at any point on the sample is adequately represented by the neutron spectrum seen far from the target at some effective scattering angle (0^^) • Then the neutron

dose at a given point on the sample can be completely

described by two numbers, the total neutron yield for

E^ > 2 MeV and the spectrum shape parameter

The response of the nickel and cobalt foils in terms of these two parameters is obtained from the yield measurements described in the preceding section. From the activation measurements done in a welldefined geometry, the ratio of the cobalt and nickel

activities at each scattering angle are obtained, and

used to calculate the ratio of the spectrum-averaged

cross sections, a(Co)/a(Ni) , as a function of angle.

The results are shown in Fig. 8. The solid line in

Fig. 7, which is our best estimate of the total neutron yield, is used in conjunction with the same activation measurements to calculate the value of cr(Co)

Fig. 8. Experimental dependence of a(Co)/a(Ni) as

a function of angle for neutrons emitted from a thick

target bombarded with 40-MeV deuterons.
1

aiul cr(.Ni) as function of scattering angle. These

results (a) arc given in Table 4, together with the values (.b) previously estimated from the TOF data and

the published cross sections. [Strictly speaking the

nickel values include a contribution from competing

reactions such as ^Ni (n,t)^^Co, however, the correction is probably <5‘!> and does not affect the present

discussion.] The dosimetry procedure which we use

is to measure the activity induced in cobalt and

nickel foils at a given location, compute the ratio

a(.Co)/a(Ni ) and then obtain 0^^ from Fig. 8. Knowing

0 the value of o[Co) [or ct(Ni)] can be obtained

from Table 4 and the total neutron fluence calculated.

An example of the method is illustrated in Fig. 9. The cobalt and nickel foils were in the form of 25-mm-diam discs, approximately 0.05 mm thick,

placed perpendicular to the beam axis at a distance

of 10 mm from the center of the beryllium target.

After an irradiation lasting ~26 hr, the pair of

foils was removed and allowed to cool for several

weeks. Discs 1.5 nun in diameter were then punched

from the foils in the radial pattern shown on the

insert of the upper part of Fig. 9. Each small disc

was weighed, and the 810-keV y-activity measured with

a Ge[Li) detector. From these data the location of

the center of the neutron spot (i.e., the maximum

activity) was estimated, and the activity of each

disc plotted against its distance r from this center,

as shown in Fig. 9. There is an implicit assumption

in this procedure that the neutron intensity contours

are circular, which appears to be well justified by

the small scatter of the data points in these plots. From the data in the upper part of Fig. 9 the two dose parameters, 0^^. and neutron yield were calculated and plotted as a function of r as shown in the lower part of the figure. The neutron spot is seen

to have a roughly gaussian profile, with a full width

at half maximum of 8.5 mm, and a maximum of

10 n/ cm

-

, corr

,12

. 1 x 10 n/cm"/sec

sponding to an average

during the irradiation

flux

The overall uncertainty in the neutron fluence

is estimated to be ±10°o due largely to the 7% uncer- tainty of the yield measurements used to calibrate

the activation foils. The uncertainty in the definition of spectral shape is difficult to quantify, but

in terms of the effective scattering angle it amounts

to ±2°, as indicated in Fig. 9.

Table 4. Hstimates of the Spectrum-Averaged Cross Sections

(in mbl for Various Reactions as a Function of Scattering

Angle. Values labeled (a) are used for dosimetry (see text). Values labeled (b) were calculated from TOF data and available cross-section data.

Scattering Angle

(deg)

5S

Ni , (n,p)

5iS rCo 59„ , . , Co(n,2n],58-

1 Co

0 5Nb(n,2n) 9.U

CT*

CT(Ni) (a)* (b) a(Co)

(a) (b) a(Nb)

(a) (b)

0 270 516 + 47 424 394 + 39 205 210 + 20

5 270 400 195 270 330 + 50 383 552 ± 33 187 183 + 18

10 272 348 174

15 278 341 ± 51 300 257 ± 26 149 146 + 15 20 283 260 131

30 298 369 ± 55 212 186 ± 19 116 117 + 12 40 313 175 103

4S 520 586 ± 58 161 168 ± 17 98 113 + 11 50 329 148 93

60 545 575 ± 56 124 116 ± 12 S3 83 + 8 ^0 56

101 74 80 380 87 67

90 400 368 + 55 72 76 ± 8 58 60 + 6 *Includes contributions from competing reactions such as

ORNL-DWG 76-15119

Fig. 9. Dosimetry method used for d + Be neutron

source. The upper graphs show the distribution of

58 Co activity for the cobalt and nickel dosimetry

foils. The lower plots show the spatial dependence of

0 e ff and neutron yield inferred from these measurements

28
Conclusions

The neutron yield data described above provide

an adequate basis for assessing the performance of

Be(d,n) and Li(d,n) neutron sources which use a 40-MeV deuteron beam. It would be useful to extend

these measurements to lower neutron energies

(E^ < 2 MeV) , although the contribution of such neutrons to the total radiation damage is not large

(s£“o), at least in the high-flux regions of the neutron sources.

Because of the similarity of the spectral

shapes from the Be(d,n) and Li(d,n) reactions, the dosimetry technique developed for the ORIC Be(d,n) source should be also applicable to Li(d,n) sources.

However, more accurately known neutron activation

cross sections are needed, particularly in the energy region En > 15 MeV. There will also be a need for measurements of transmutation product cross

sections, particularly those for hydrogen and helium

Integral measurements for helium production could be

made with existing Be(d,n) sources and would provide

a useful check on the theoretical predictions.

References

1. M. J. Saltmarsh, C. A. Ludemann, C. B. Fulmer, and

R. C. Styles, "Characteristics of an Intense Neutron Source Based on the d + Be Reaction,"

(submitted to Nucl. Inst, and Meth.).

2. M. J. Saltmarsh and R. E. Worsham (ED.),

ORNL-TM-5223 (1976).

3. V. V. Verbinski, W. R. Burrus, T. A. Love,

W. Zobel, N. W. Hill, and R. Textor, Nucl. Inst,

and Meth. 65, 8 (1968)

.

4. R. E. Textor and V. V. Verbinski, ORNL-4160 (1968)

5. ENDF-B-IV, MAT-1199, BNL 17541 (ENDF-201) (1975).

6. A. Paulsen and R. Widera, Z. Physik 238, 23 (1970)

7. ENDF-B-IV, MAT-1199, BNL 17541 (ENDF-201) (1975).

8. S. Pearlstein, J. Nucl. Eng. 27_, 81 (1973).

9. G. W. Schweimer, Nucl. Phys . A 100, 537 (1967).

10.

J. P. Meulders, P. Leleux, P. C. Macq, and

C. Pirart, Phys. Med. Biol. 20_, 235 (1975).
NEUTRON SPIitn'RA AND DOSES [ ROM

DEUTERON BOMBARDMENT 01- LKJIT NlJCM-1

II.H. Barschal 1

University of Wisconsin, Madison, Wisconsin 53706

Neutron spectra measured by time of flight and doses measured with a tissue-equivalent

ionization chamber have been studied at the Lawrence Livermore Laboratory for deuterons

up to 20 MeV on targets of D, Li, Be, and C. All the results have been previously published in the following references:

Keith A. Weaver, Ph.D. thesis, University of Wisconsin, Madison, available as Report

UCRL-51310 (1973); additional data in UCRL-51310 (addendum).

K.A. Weaver, J.D. Anderson, H.H. Barschall, and J.C. Davis, Nucl. Sci. Eng. 52, 35 (1973).

K.A. Weaver, J.D. Anderson, H.H. Barschall, and J.C. Davis, Phys. Med. Biol. 18^, 64 (1973).

R.H. Howell and H.H. Barschall, Phys. Med. Biol. Zl, 643 (1976).

30
NEUTRON SPECTRAL MEASUREMENTS AND INTERPRETATION

AT THE NAVAL RESEARCH LABORATORY

L. S. August, P. Shapiro, and R. B. Theus

NAVAL RESEARCH LABORATORY, WASHINGTON, D. C. 20375 USA

The thick-target, neutron energy spectra and yields produced at 0° with respect to the

9 10 q

o

i

i incident ion beam for the reaction Be(d,n) B at E , * 35 MeV and the reaction Be( He,n) C

a at E- - 81.4 MeV have been measured by time-of-f light techniques. The same data are anaJHe lyzed dosimetrically in order to extract kerma-to-f luence factors for the NE 213 scintillat9 10 or. The Be(d,n) B results are compared with predictions of the Serber theory as applied

to a thick target. Rossi counter measurements in the phantom have shown that the fast neutron spectral quality does not change significantly along the beam axis from at least 2 cm to

20 cm depth in tissue-equivalent fluid.

(Neutrons; energy spectra; yields; thick target

Serber theory; Rossi-counter measurements)

Introduction

The Naval Research Laboratory, in collaboration

with a consortium of east coast therapeutic radiologists, is presently engaged in a clinical trial comparing neutrons with gamma rays in the local control of cancerous tumors. The neutron beam is produced by bombarding a thick beryllium target with 35-MeV deuterons.

Many measurements have been performed to study the

j physical characteristics of this neutron beam. These

studies include the spectral distribution of the neutron beam by time-of-f light techniques, and investigations of the change in spectral characteristics as the beam penetrates tissue-equivalent material. Additionally, the time-of-f light data can be analyzed to yield

kerma-to-f luence information as a function of neutron

energy as well as the dose dependency of the neutron

l beam in penetrating tissue-equivalent slabs.

Time of Flight Measurements

In these measurements a 50-ohm capacitive coupler was used to sense the timing of the deuteron beam

burst itself, rather than the radiofrequency (rf) dee

voltage. This method has the advantage of not depending on a constant phase relation between the rf signal

and the extracted beam burst. This beam burst signal

determined the time at which the neutrons left the

beryllium target. The neutron-beam attenuators were

20-cm-square boxes of various thicknesses, constructed

of 3-mm Lucite and filled with TE liquid. Geometrical

considerations are shown schematically in Fig. 1. A

5-cm diameter NE-213 liquid scintillator coupled to a RCA 8575 photo-multiplier tube was used to detect the neutrons at the end of the 3-m flight path. The fast

timing signal from the photomultiplier assembly determined the arrival time of the neutrons. These neutron- flight, start and stop signals provided the input

timing signals for a time-to-amplitude converter (TAC).

The TAC output was fed into an analogue-to-digital

converter (ADC) connected to an EMR-6050 computer.

The scintillation amplitude or linear signal from the

photomultiplier tube base was entered into a second ADC

unit, and neutron gamma pulse-shape discrimination signals were routed into a third ADC. The flight time and

pulse-height signals from the first two ADC's were re- corded in a two-parameter mode, subject to a neutron software gate on the output of the third ADC. This

software gate assured that only those scintillation

events due to neutrons were processed by the computer. A schematic representation of this system is also shown

in Fig. 1. The two-parameter accumulation of data records the number of events, N(Eq1 , Epj ), ln the ith jth

9 10 9 3 11 time-of-f light; Be(d,n) B; *Be( He,n) C;

E n

-

FIG. 1. Time-of-f light spectrometer.

bin. This bin has neutron energy width E . The x-axis

P

is in equal increments of time but not energy; hence

the bin-width dependency on the channel index "i." The neutron energy spectrum f(E ) for a specific number of incident deuterons is defined such that f (E )

• E is n n

the number of neutrons per coulomb-steradian having

energy —in' the bifC^between E and E + 6E . This spec-~V

- - - ^ •- . . . . h n n _ r j

tral distribution is then related to the accumulated

data by the expression:

f(E"> - daj, AEni ^ N<^ W (1 >

in which Q is the integrated target current in coulombs, fi is the detector solid angle, and t , is the ni

detector efficiency which is calculated by the use of the Kurz code. This spectral expression is evaluated

in the computer and is presented on a line printer and

digital plotter together with the average neutron energy given by:

IE 12)

31
In evaluating this average energy the neutron spectra

amplitude is extrapolated to zero for vanishing neutron

energy. A spectrum as determined by these techniques

is shown in Fig. 2 with the differentia^ yields in good

agreement with those of tleulders et al.

NEUTRON ENERGY (MEV)

FIG. 2. Neutron spectral yield at 0° angle.

This spectrum changes very little with TE fluid

thickness, the average energy of the primary and

forward-scattered neutron component being increased by

only 8% in penetrating 24 cm of TE fluid (Fig. 3).

A AIR; E = 143 MeV

• 9 CM TE SLAB, E= 149 MeV

X 18 CM TE SLAB, E= 15 4 MeV

* *

“ t

* •

NEUTRON ENERGY (MeV) FIG. 3. Neutron spectral distributions after the

penetration of 0, 9, and 18 cm of tissue-equivalent

fluid slabs, respectively.

This result implies that the relative biological

effectiveness (RBE) of the beam should be essentially

independent of tissue depth (assuming constant dose

levels), which is desirable from the standpoint of treatment planning. Radiobiological experiments have

verified the constancy of th^ RBE values with depth in

a tissue equivalent phantom.

T i

e-of-F] igh t Dosime try

II the y-axis of the two-parameter data matrix if. calibrated such that E is the energy per unit mass de- posited in the NE-213 detector to produce the recorded

scintillation amplitude, then E . N(E . ,E .) is the

Pi ni p| energy in the ith jth bin. Summing all energy in these

bins, and including a sum over charged panicle type

(alpha or proton) yields:

E - qm ^ ^ ^ ^Pi N(Enj, EPj) (3) P i j

which is the dose (erg g ) deposited in the scintillator per coulomb of deuteron charge on the target.

In this expression, fl is the mass of the NE-213 detector, and E . is selected as the proton recoil energy,

PJ since the majority of recorded events is due to elastic

proton recoils. The integrated quantity E in Equation

(3), which is related to the total energy per gram de- posited in the detector, was determined for different

TE slabs placed adjacent to the Be target. The neutron beam in this case was produced by 35-MeV incident deuterons. For this geometrical arrangement, the dose rate as a function of slab thickness was obtained with

a spherical TE ionization chamber with wall material

sufficiently thick (5 mm) to assure charged-particle

equilibrium. A comparison of these two results is shown in Fig. 4.

05

0 J i 1 i I , I

9 15 21 27 TE SLAB THICKNESS (cm) FIG. 4. Comparison of the attenuation of neutron dose

by tissue-equivalent slabs as measured by an ionization

chamber and recoil proton energy deposited in a scintillator by time-of-f light technique.

The comparative agreement indicates that the time-offlight determinations of total deposited "energy" offer

a good quantitative measure of the depth dependence of the absorbed dose in tissue.

Another quantity of interest in dosimetry is the kerma-to-f luence factor. This factor specifies the kinetic energy transferred in charged particles, in a gram of material per incident neutron per cm of a given energy E. It is easy to show that the kerma-to-

-1 -2 fluence factor, in units of rad neutron -cm relative

to the accumulated spectral distribution, is obtained

by the expression:

A I ? Ej N(Eni , Epj)

,,ni =

f(Eni ) QQMAEn

(4)

32
j

where A is the detector area, and M is the mass of the detector. In equation (A) the units of dose are

expressed as a function of neutron energy, and can be separated into components due to alphas or protons if is omitted. With proper corrections this equaPtion should yield information relative to dose calculations for monoenergetic neutron beams based on neutron elastic and reaction cross sections. In this

effort kerma components are obtained by converting

1 1 scintillation amplitude to particle energy using the appropriate scintillation output relationship for each

particle type, and then summing the energy deposited

by that particle.

By utilizing various neutron-producing reactions,

kerma-to-f luence factors can be obtained over a large range of neutron energies. Consequently, higher

energy spectral distributions were produced by other

cyclotron accelerated particles. An example resulting

3 from 81.4 MeV He particles bombarding a thick Be

target is shown in Fig. 5. This neutron spectrum and

002

O

>

FIC.

10 20 30 40 50 60 70

NEUTRON ENERGY (MEV)

Neutron spectral yield at 0° angle.

the He-Be reaction were used to obtain the kerma-tofluence factors for the NE-213 hydrocarbon scintillator

over a range of energies for which cross section data

does not exist. The results for proton kerma shown in

Fig. 6 ^gree well with calculations due to Bach and

Caswell. The alpha kerma results cannot be compared

with Bach and Caswell because of the high alpha threshold in this experiment.

In an attempt to understand the fundamental

processes which result in the thick target yield of neutrons when deuterons a^e the bombarding particles,

a theory due to Serber was integrated over the

slowing-down process of monoenergetic deutrons incident

on a Be target. This comparison of the Serber Theory

with spectral measurements is shown in Fig. 7. These

calculations also predict the observed ^cubic dose

dependency on the incident deuteron energy.

A further measurement which indicates that the physical characteristics of the absorbed dose is not significantly altered when the neutron beam penetrates

tissue is shown by the invariance with depth of the pulse-heighth distribution obtained with a "Rossi"

proportional counter.

d on Be Data

FIG. 6. Neutron kerma/f luence in a NE-213 scintillator. The kerma due to energy deposited by protons

and by alphas, as inferred from measurements with the

3 d-Be and the He-Be reactions, are compared with calculations based on the work of Bach and Caswell. The

onset of the experimental values for alpha kerma is at

a higher neutron energy than shown in the calculations because of the alpha-detection threshold in the equipment

.

deuterons incident on a thick beryllium target. The crosses are experimental data determined from time-off light techniques. The dashed curve is the result of a calculation using Serber theory.

Fig. 8 provides a quantitative measure of the

energy sensitivity of the Rossi counter to changes in average neutron energy. Two pulse-height spectra were taken with the counter at a depth of 10 cm in the

phantom and on the beam axis. The neutrons were

produced by incident deuteron beams of energies of 30 and 35 MeV, and the spectra are indicated by filled

and open circles, respectively. The spectra are normalized at the maxima of the energetic knock-on

proton peaks. Clearly, for the higher energy beam, more alphas and heavy ions are produced than at

the lower energy. If one takes the ratio of predominantly alpha events to predominantly proton events.

33
FIG. 8. Rossi-counter pulse-height spectra for a depth

in the phantom of 10 cm and incident deuteron energies

of 35 MeV and 30 MeV.

there is an 18% change in this ratio for a 16% change

in either the incident deuteron energy or the average

neutron energy. Data obtained on the center line in the TE phantom at depths of 2 cm and 10 cm are shown

in Fig. 9. The filled circles are for the 2 cm data,

and the open ones for the data at 10 cm. On inspection, there seems to be no difference in the pulse

height spectra. If the same ratios are taken as indicated on the previous figure, they agree to within 1%.

4

•°s o •

10

10

ny

c/> o

10

3 —

10

• 2 cm TE

o 10cm TE

100 200

Channel

300

Data taken at depths of 2 cm and 20 cm are nearl;

identical to these, with the ratios agreeing to 1/2%. We conclude from these data that the fast neutroi

spectrum in the phantom is essentially constant on the center line from at least 2 cm to 20 cm.

HBS-1

Cro Conclusions

Neutron therapy beams can be characterized by means of time-of-f light measurements (T0F) and by event distributions in a "Rossi" proportional counter. T0F measurements furnish spectral information, neutron fluence, kerma-to-f luence factors, and indicate relative energy deposition due to protons and alpha particles. Event spectra from the "Rossi" counter are used to|

indicate the quality of the neutron therapy beam.

These measurements can be used to show changes in relative dose deposited by alpha and protons as a function

of neutron energy as well as to verify beam quality as

a function of depth in a tissue-equivalent phantom. Our results are generally in good agreement with mea- surements reported by other laboratories. Over quite a range of energy, the shape of thick-target, deuteron

produced high-energy neutron spectra may be characterized as of a skewed-gaussian type which reaches a maxi- mum at 0.4 E where F. . is the incident deuteron

d

, .- d energy. Serber theory, as applied to a thick target, can explain reasonably well this shape. Serber theory

describes the gross features of neutron production in the forward direction, and reflects the importance of the internal momentum of the neutron in the deuteron

prior to the interaction.

Acknowledgments

The authors wish to thank Dr. R. 0. Bondelid for his continuing interest in and support of this work.

They also wish to thank Mr. G. E. Miller and his staff

for their help in obtaining the data.

This work was supported in part by the National Cancer Institute under Interagency Grant Number 1401 CO 40001-01.

References

1. R. J. Kurz, "A 709/7090 FORTRAN II Program to Com- pute the Neutron-Detection Efficiency of Plastic

Scintillators for Neutron Energies from 1 to 300 MeV",

UCRL-11339,, University of California (1964).

2. J. P. Meulders, P. Leleux, P. C. Macq, and C. Pirart, Phys. Med. Biol. 20 , 235 (1975).

3. Paul Todd, Carter B. Schroy, F. H Richard B. Theus, Cancer 34 , 33 (1974).

4. R. L. (1965).

Attix, and

Bach and R. S. Caswell, Rad. Res. 25 , 1

5. R. Serber, Phys. Rev. 72, 1008 (1947).

6. L. S. August, F. H. Attix, G. H. Herling, P. Shapiro and R. B. Theus, Phys. Med, Biol. 2

. 931

(1976).

7. H. H. Rossi and W. Rosenzweig, Radiology 64 , 404

(1955).

FIG. 9. Rossi-counter pulse-height spectra for an incident deuteron energy of 35 MeV and depths of 2 cm and

10 cm.

34
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR R K PORT NO. NBSIR-77-1279

2. (iov’t Accession

No.

3. Recipient’s Accession No.

4. TIT LI1' AND SUBTITLK 5. Publication Date Cross Sections and Yields for High Energy Neutron Source

Reactions

July 1977

6. Performing Organization Code

7. author(S) Edi tors

M. A. Lone, L. Stewart, A. D. Carlson, and C. D. Bowman

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projet t /Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U. S. Energy Research and Development Administration,

and others

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Eight papers were presented at a workshop on thick-target yields from

high-energy neutron source reactions on March 30th, 1977 in Bethesda, Md.

Presentations were made on several source reactions including D(d,n), Li(p,n),

Li(d,n), Be(p,n), Be(d,n), and C(d,n) including energy and angular distributions.

Special emphasis was placed on these sources due to the needs in medical applications

and for studies of radiation damage expected in an operating fusion reactor.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Be(d,n) source; Be(p,n) source; C(d,n) source; D(d,n) source; dosimetry; Li ( d , n

)

source; Li(p,n) source; neutrons; neutron angular distributions; neutron energy

distributions; nuclear reactions; t.hirk-t.argpt yjplHs

18. AVAILABILITY

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Ujt F or Official Distribution. Do Not Release to NTIS

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3

Order From National Technical Information Service (NTIS)

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