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The 1H(t, n)3He reaction as a neutron source
NUCLEAR
INSTRUMENTS
AND
METHODS
23 (1963) 305--308; N O R T H - H O L L A N D
PUBLISHING
CO.
THE 1H(t,n)3He REACTION AS A NEUTRON SOURCE W. D E U C H A R S , J. L. P E R K I N and R. B A T C H E L O R
Atomic Weapons Research Establishment, Aldermastion, U.K. Received l D e c e m b e r 1962
T h e m a i n characteristic of this reaction is t h a t all the neutrons are e m i t t e d in the forward direction. A n g u l a r distributions of t h e s e neutrons for incident triton energies of between 3.7 a n d 11.86 MeY h a v e been measured. In the forward direction the n e u t r o n flux obtainable is considerably g r e a t e r t h a n t h a t from
1. Introduction In experiments concerned with the interactions of neutrons with energies in the region 1 to 10 MeV it is usual to employ the 3H(p, n)3He a n d 2H(d, n) 3He reactions. Apart from near threshold energies in the case of the 3H(p, n)3He reaction, neutrons are emitted in all directions from these reactions. These m a y adversely affect any detector used to observe radiation from neutron interactions in an irradiated sample. A source from which neutrons are projected into a narrow cone would be of considerable value in such experiments. Neutrons from the 1H(t, n) 3He reaction have this property for all incident triton energies. In this note an outline of the characteristics of this reaction is given together with an example of the type of experiment where the use of this reaction as a neutron source would be advantageous.
the b o m b a r d m e n t of t r i t i u m b y protons. This a d v a n t a g e is s o m e w h a t offset b y the greater neutron energy variation with the angle of emission. The results of some preliminary measurem e n t s of the g a m m a r a y s from the Fe(n, n'7) reaction are given to illustrate the possible uses of this neutron source.
illustrated in fig. 1. It should be noted that one of the two groups has a relatively low energy and, as will be seen later, a low intensity. Its presence therefore should not be a serious handicap in m a n y experiments. I
600
I
t
t
/" I /
HIGH 400
7
~
ENERGY
G R ~ / / / t ~
~
56 ~
,
~,
/
~ b
ZOO
\
,/
/
--EHERGIES OF NEUTRONGROUPS
5 2
2. General Properties of the 1H(t,n)3He Reaction At the threshold of the 1H(t, n)3He reaction the triton energy (in laboratory coordinates) ET, is 3.06 MeV and neutrons of 0.575 MeV are emitted at 0 °. Above threshold, neutrons are emitted in the forward direction within a cone of semiangle 0 given by sin 0 -~ (E T -- EThreshold) ½E.r-i. As E T increases, 0 approaches the limit of 90 °. The neutron energy at any particular angle and incident triton energy is double valued. These energies have been compiled using a relativistic computer programme for triton energies up to 13 MeV and are given in table 1. The results for neutron emission at 0 ° are
4
6 8 I0 TRITON ENERGY MeV
12
14
Fig. I. Dotted curve; energies of neutron groups e m i t t e d at 0 ° from the 1 H ( t n)SHe reaction. Solid curves; differential crosssections a t 0 °.
From the known information on the differential cross sections for the 3H(p, n)3He reactiont), it is possible to calculate the cross sections for the 1H(t, n)3He reaction. The yields at 0 ° of the two 1) j . L. Fowler and J. E. Brolley, Rev. Mod. Phys. 28 (1956) 103 a n d ' F a s t N e u t r o n Physics (Interscience, New York, 1960) P a r t I. 305
306
W. D E U C H A R S
neutron groups are plotted as functions of triton energy in fig. 1. A comparison of the yield for the high energy group with those yields for neutrons of the same energy produced by either the all(p, n) 3He
el al.
fig. 2 the energy variation from 0 ° to 10 ° as a function of neutron energy is shown for the three neutron producing reactions under discussion. It can be seen that the IH(t, n)3He reaction is not a
TABLE 1 E n e r g i e s (in MeV) of n e u t r o n s from t h e 1H( t, n) a He r e a c t i o n as a f u n c t i o n of t h e t r i t o n e n e r g y a n d t h e a n g l e of emission of t h e n e u t r o n , 0n, in l a b o r a t o r y c o - o r d i n a t e s Triton energy MeV
High energy group
Low energy group
On
3.1
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0 5 10 15 20 25 30 35 40 45 50 55 60
0.735 0.684 0 0 0 0 0 0 0 0 0 0 0
1.660 1.634 1.556 1.426 1.242 0.988 0 0 0 0 0 0 0
2.477 2.447 2.357 2.209 2.006 1.750 1.441 1.063 0 0 0 0 0
3.260 3.225 3.119 2.947 2.711 2.418 2.073 1.680 1.228 0 0 0 0
4.031 3.990 3.868 3.669 3.399 3.064 2.672 2.232 1.748 1.206 0 0 0
,4.795 4.749 4.611 4.385 4.078 3699 3.257 2.763 2.228 1.655 1.004 0 0
5.556 5.504 5.349 5.096 4.753 4.328 3.834 3.285 2.693 2.069 1.407 0 0
6.315 6.257 6.085 5.805 5.424 4.954 4.408 3.802 3.151 2.469 1.764 0.984 0
0 5 10 15 20 25 30 35 40 45 50 55 6O
0.447 0.481 0 0 0 0 0 0 0 0 0 0 0
0.198 0.201 0.211 0.231 0.265 0.333 0 0 0 0 0 0 0
0.133 0.135 0.140 0.149 0.164 0.188 0.228 0.310 0 0 0 0 0
0.101 0.102 0.106 0.112 0.121 0.136 0.159 0.196 0.268 0 0 0 0
0.082 0.083 0.085 0.0913 0.097 0.108 0.123 0.148 0.188 0.273 0 0 0
0.069 0.069 0.072 0.075 0.081 0.089 0.101 0.119 0.148 0.199 0.328 0 0
0.059 0.060 0.062 0.065 0.069 0.076 0.086 0.100 0.122 0.159 0.234 0
°
l,O [Zo
13.0
7.072 7.009 6.820 6.512 6.094 5.578 4.979 4.315 3.603 2.861 2.103 1.313 0
7.828 7.759 7.554 7 218 6.762 6.200 5.549 4.826 4.053 3.249 2.433 1.605 0
8.584 8.509 8.286 7.923 7.430 6.822 6.116 5.335 4.500 3.634 2.757 1.880 0.932
0.047 0.052 0.047 0.053 0.054 0.048 0.051 0.057 0.054 0.061 0.067 0.059 0.066 0.075 0.076 0.087 0.092 0 105 0.133 0.115 0.187 , 0.157 0.251 I 0 335! 0
0.042 0.043 0.044 0.046 0.049 0.053 0.059 0068 0.081 0.101 0.135 0.205 0
0.038 0.039 0.040 0.041 0.044 0.048 0.054 0.061 0.073 0.090 0.119 0.175 0.353
L°l
Mass v a l u e s used a r e : n = 1.0089861 Mu, aH = 1.0081456 Mu, aH = 3.0170083 Mu, aHe = 3.0169888 Mu.
or 2H(d, n)aHe reactions, shows that the neutron flux available in the forward direction from the IH(t, n)aHe reaction is considerably greater than that from the other two reactions. For example, the cross sections for the production of 5 MeV neutrons from the 1H(t, n)3He, 3H(p, n)3Heand 2H(d, n)3He reactions are 500, 45 and 40 mb.sterad-m, respectively. The neutron energy resolution achieved in an experiment is limited by the variation of neutron energy with angle of emission from the source. In
good source of neutrons if high energy resolution is required. The threshold of the tertiary reaction 1H (t, np) 2H is at 25 MeV. Neutrons from the 1H(t, n)3He reaction can, therefore, be produced up to an energy of 17.5 MeV free of neutrons from the "break-up" reaction. A search by Wilson et al. 2) for the 3H(p, np)2H reaction indicates that the relative intensity of these "break-up" neutrons is low. 2) W. E. Wilson, R. L. W a l k e r a n d D, B. Fossan, N u c l e a r Phys. 27 (1961) 421.
T H E lH(t,d)3He R E A C T I O N AS A N E U T R O N SOURCE 250
i
]
i
i
307
from the experimentally determined angular distribution of neutrons from the 3H(p, n)3He reaction, at an incident proton energy of 3.28 MeV1). The present experimental measurements include the
i
200 Et =Y7MeV
Et = 5 8 MeV
150
o = 7.82 MeV .)
,
: = 9.85 MeV o
I00
~ = I I . 8 6 MeV
20
0
40
60
I00
80
0° LAB Fig. 3. Experimental angular distributions of neutrons from the 1H(t, n)~He reaction.
I
I
~
I
I
I
I
I
2
3
4
5
6
7
NEUTRON ENERGYMtV Fig. 2. ]Difference i n e n e r g y b e t w e e n n e u t r o n s e m i t t e d a t 0 ° a n d lO ° as a f u n c t i o n o f n e u t r o n e n e r g y a t 0 ° f o r t h e r e a c t i o n s
]H(t, n)3He, all(p, n)aHe and ~H(d, n)3He.
3. Experimental Measurements The angular distributions of neutrons from the 1H(t, n)3He reaction for various incident triton energies between 3.7 and 11.86 MeV were measured with a " l o n g " counter3). The tritons were accelerated by the Aldermaston Tandem Generator and were focussed on to a cell with a nickel entrance foil and filled with hydrogen to a pressure of 2 atmospheres. The results obtained are shown in fig. 3. Each distribution is characterised by a forward peak and a subsidiary peak at larger angles followed by a sharp cut-off. These characteristics are confirmed by the distribution at approximately E T = 10 MeV, shown in fig. 4 which is calculated 3) A. O. Hanson and J. L. McKibben0 l)hy~ Rev. 72 (1947) 673.
intensities of both the high and the low energy neutron groups. It can be seen however from fig. 4 that although the relative intensity of the low energy group increases with angle, it remains quite low except in the vicinity of the m a x i m u m "cut-off" angle. The dotted curves in fig. 3 are the distributions obtained with the hydrogen removed from the gas cell. It can be seen that the intensity of these background neutrons increases with increasing triton 600 ~ L
~
r
40O :
HIGH ENERGYGROUP
200
LO EN GG YY GROUP i L oWwE HEERR GROU ,,'JJ 0
tO
20
30
40
50
60
o "(tAR) Fig. 4. Calculated angular distribution of neutrons from the 1H(t, n)3He reaction at E l- = 10 MeV.
308
W. D E U C H A R S et al.
energy. Measurements with a hydrogenous shadow cone between the gas cell and counter showed that these neutrons originated at, or close to, the gas cell. No change in the background was observed when the nickel entrance foil was replaced with one of tantalum. It is probable therefore that the background is due to (t, n) reactions with light element contaminants, such as carbon, in the path of the triton beam. This view is supported by the fact that the background intensity was sensitive to the degree of alignment and focus of the triton beam. With extreme care it could possibly be reduced below the levels indicat(d in fig. 3.
do not require a particularly good neutron energy resolution. The reaction should be particularly suitable for providing a source of 0.575MeV
// /
cxlo 4
,
jJ
NYOROGENGAS CEL \ /
/ NEUTRON / / CONE / / IRONSCATTERING
4. Application of the lI-I(t, n)3He Neutron Source The detection of 7 rays from inelastic neutron scattering and the measurement of neutron activation cross sections are two of the more obvious types of experiment for which the 1H(t, n)3He source would be useful. To illustrate the first type of experiment, measurements were made of the ~ rays from inelastic scattering by iron. The experimental arrangement and the results obtained are shown in fig. 5. The sample was placed in the forward'cone of neutrons and the detector a NaI crystal - w a s placed outside of the cone. The only shielding required was a block of lead, a few cm long, placed between the gas cell and detector. The curves shown represent the subtraction of runs taken with and without the sample in place. No corrections have been made for the effect of scattered neutrons from the iron sample. High statistical accuracy was obtained from runs of a few minutes duration. To summarise, the IH(t, n)3He reaction is a useful source of neutrons for experiments requiring a directed neutron beam of high intensity and which
2~1o'
I1 t~ \ l~l!
\
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1.0
~
\\\
~"/Z/SODIUM IODIDECRYSTAL
,
.
[ ~/1K__
\
^
,~'~'~o
INCIDENTNEUTRONENERGY14¢V
"'.,./~".\ 4.5 "~'-t"-,~._ 1.5
2.0 GAMMA
Z.S
RAY ENERGY M c V
3.0
3.5
4.0
Fig. 5. NaI scintillation spectra from ~-rays produced by Fe(n, n ' y .
neutrons. This point could not be cheeked in the present expeliments since the low bombarding energy required could not be obtained with the Tandem accelerator.
Acknowledgement We would like to thank Mr. B. E. F. Macefield for supplying the data given in table 1.
INSTRUMENTS
AND
METHODS
23 (1963) 305--308; N O R T H - H O L L A N D
PUBLISHING
CO.
THE 1H(t,n)3He REACTION AS A NEUTRON SOURCE W. D E U C H A R S , J. L. P E R K I N and R. B A T C H E L O R
Atomic Weapons Research Establishment, Aldermastion, U.K. Received l D e c e m b e r 1962
T h e m a i n characteristic of this reaction is t h a t all the neutrons are e m i t t e d in the forward direction. A n g u l a r distributions of t h e s e neutrons for incident triton energies of between 3.7 a n d 11.86 MeY h a v e been measured. In the forward direction the n e u t r o n flux obtainable is considerably g r e a t e r t h a n t h a t from
1. Introduction In experiments concerned with the interactions of neutrons with energies in the region 1 to 10 MeV it is usual to employ the 3H(p, n)3He a n d 2H(d, n) 3He reactions. Apart from near threshold energies in the case of the 3H(p, n)3He reaction, neutrons are emitted in all directions from these reactions. These m a y adversely affect any detector used to observe radiation from neutron interactions in an irradiated sample. A source from which neutrons are projected into a narrow cone would be of considerable value in such experiments. Neutrons from the 1H(t, n) 3He reaction have this property for all incident triton energies. In this note an outline of the characteristics of this reaction is given together with an example of the type of experiment where the use of this reaction as a neutron source would be advantageous.
the b o m b a r d m e n t of t r i t i u m b y protons. This a d v a n t a g e is s o m e w h a t offset b y the greater neutron energy variation with the angle of emission. The results of some preliminary measurem e n t s of the g a m m a r a y s from the Fe(n, n'7) reaction are given to illustrate the possible uses of this neutron source.
illustrated in fig. 1. It should be noted that one of the two groups has a relatively low energy and, as will be seen later, a low intensity. Its presence therefore should not be a serious handicap in m a n y experiments. I
600
I
t
t
/" I /
HIGH 400
7
~
ENERGY
G R ~ / / / t ~
~
56 ~
,
~,
/
~ b
ZOO
\
,/
/
--EHERGIES OF NEUTRONGROUPS
5 2
2. General Properties of the 1H(t,n)3He Reaction At the threshold of the 1H(t, n)3He reaction the triton energy (in laboratory coordinates) ET, is 3.06 MeV and neutrons of 0.575 MeV are emitted at 0 °. Above threshold, neutrons are emitted in the forward direction within a cone of semiangle 0 given by sin 0 -~ (E T -- EThreshold) ½E.r-i. As E T increases, 0 approaches the limit of 90 °. The neutron energy at any particular angle and incident triton energy is double valued. These energies have been compiled using a relativistic computer programme for triton energies up to 13 MeV and are given in table 1. The results for neutron emission at 0 ° are
4
6 8 I0 TRITON ENERGY MeV
12
14
Fig. I. Dotted curve; energies of neutron groups e m i t t e d at 0 ° from the 1 H ( t n)SHe reaction. Solid curves; differential crosssections a t 0 °.
From the known information on the differential cross sections for the 3H(p, n)3He reactiont), it is possible to calculate the cross sections for the 1H(t, n)3He reaction. The yields at 0 ° of the two 1) j . L. Fowler and J. E. Brolley, Rev. Mod. Phys. 28 (1956) 103 a n d ' F a s t N e u t r o n Physics (Interscience, New York, 1960) P a r t I. 305
306
W. D E U C H A R S
neutron groups are plotted as functions of triton energy in fig. 1. A comparison of the yield for the high energy group with those yields for neutrons of the same energy produced by either the all(p, n) 3He
el al.
fig. 2 the energy variation from 0 ° to 10 ° as a function of neutron energy is shown for the three neutron producing reactions under discussion. It can be seen that the IH(t, n)3He reaction is not a
TABLE 1 E n e r g i e s (in MeV) of n e u t r o n s from t h e 1H( t, n) a He r e a c t i o n as a f u n c t i o n of t h e t r i t o n e n e r g y a n d t h e a n g l e of emission of t h e n e u t r o n , 0n, in l a b o r a t o r y c o - o r d i n a t e s Triton energy MeV
High energy group
Low energy group
On
3.1
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0 5 10 15 20 25 30 35 40 45 50 55 60
0.735 0.684 0 0 0 0 0 0 0 0 0 0 0
1.660 1.634 1.556 1.426 1.242 0.988 0 0 0 0 0 0 0
2.477 2.447 2.357 2.209 2.006 1.750 1.441 1.063 0 0 0 0 0
3.260 3.225 3.119 2.947 2.711 2.418 2.073 1.680 1.228 0 0 0 0
4.031 3.990 3.868 3.669 3.399 3.064 2.672 2.232 1.748 1.206 0 0 0
,4.795 4.749 4.611 4.385 4.078 3699 3.257 2.763 2.228 1.655 1.004 0 0
5.556 5.504 5.349 5.096 4.753 4.328 3.834 3.285 2.693 2.069 1.407 0 0
6.315 6.257 6.085 5.805 5.424 4.954 4.408 3.802 3.151 2.469 1.764 0.984 0
0 5 10 15 20 25 30 35 40 45 50 55 6O
0.447 0.481 0 0 0 0 0 0 0 0 0 0 0
0.198 0.201 0.211 0.231 0.265 0.333 0 0 0 0 0 0 0
0.133 0.135 0.140 0.149 0.164 0.188 0.228 0.310 0 0 0 0 0
0.101 0.102 0.106 0.112 0.121 0.136 0.159 0.196 0.268 0 0 0 0
0.082 0.083 0.085 0.0913 0.097 0.108 0.123 0.148 0.188 0.273 0 0 0
0.069 0.069 0.072 0.075 0.081 0.089 0.101 0.119 0.148 0.199 0.328 0 0
0.059 0.060 0.062 0.065 0.069 0.076 0.086 0.100 0.122 0.159 0.234 0
°
l,O [Zo
13.0
7.072 7.009 6.820 6.512 6.094 5.578 4.979 4.315 3.603 2.861 2.103 1.313 0
7.828 7.759 7.554 7 218 6.762 6.200 5.549 4.826 4.053 3.249 2.433 1.605 0
8.584 8.509 8.286 7.923 7.430 6.822 6.116 5.335 4.500 3.634 2.757 1.880 0.932
0.047 0.052 0.047 0.053 0.054 0.048 0.051 0.057 0.054 0.061 0.067 0.059 0.066 0.075 0.076 0.087 0.092 0 105 0.133 0.115 0.187 , 0.157 0.251 I 0 335! 0
0.042 0.043 0.044 0.046 0.049 0.053 0.059 0068 0.081 0.101 0.135 0.205 0
0.038 0.039 0.040 0.041 0.044 0.048 0.054 0.061 0.073 0.090 0.119 0.175 0.353
L°l
Mass v a l u e s used a r e : n = 1.0089861 Mu, aH = 1.0081456 Mu, aH = 3.0170083 Mu, aHe = 3.0169888 Mu.
or 2H(d, n)aHe reactions, shows that the neutron flux available in the forward direction from the IH(t, n)aHe reaction is considerably greater than that from the other two reactions. For example, the cross sections for the production of 5 MeV neutrons from the 1H(t, n)3He, 3H(p, n)3Heand 2H(d, n)3He reactions are 500, 45 and 40 mb.sterad-m, respectively. The neutron energy resolution achieved in an experiment is limited by the variation of neutron energy with angle of emission from the source. In
good source of neutrons if high energy resolution is required. The threshold of the tertiary reaction 1H (t, np) 2H is at 25 MeV. Neutrons from the 1H(t, n)3He reaction can, therefore, be produced up to an energy of 17.5 MeV free of neutrons from the "break-up" reaction. A search by Wilson et al. 2) for the 3H(p, np)2H reaction indicates that the relative intensity of these "break-up" neutrons is low. 2) W. E. Wilson, R. L. W a l k e r a n d D, B. Fossan, N u c l e a r Phys. 27 (1961) 421.
T H E lH(t,d)3He R E A C T I O N AS A N E U T R O N SOURCE 250
i
]
i
i
307
from the experimentally determined angular distribution of neutrons from the 3H(p, n)3He reaction, at an incident proton energy of 3.28 MeV1). The present experimental measurements include the
i
200 Et =Y7MeV
Et = 5 8 MeV
150
o = 7.82 MeV .)
,
: = 9.85 MeV o
I00
~ = I I . 8 6 MeV
20
0
40
60
I00
80
0° LAB Fig. 3. Experimental angular distributions of neutrons from the 1H(t, n)~He reaction.
I
I
~
I
I
I
I
I
2
3
4
5
6
7
NEUTRON ENERGYMtV Fig. 2. ]Difference i n e n e r g y b e t w e e n n e u t r o n s e m i t t e d a t 0 ° a n d lO ° as a f u n c t i o n o f n e u t r o n e n e r g y a t 0 ° f o r t h e r e a c t i o n s
]H(t, n)3He, all(p, n)aHe and ~H(d, n)3He.
3. Experimental Measurements The angular distributions of neutrons from the 1H(t, n)3He reaction for various incident triton energies between 3.7 and 11.86 MeV were measured with a " l o n g " counter3). The tritons were accelerated by the Aldermaston Tandem Generator and were focussed on to a cell with a nickel entrance foil and filled with hydrogen to a pressure of 2 atmospheres. The results obtained are shown in fig. 3. Each distribution is characterised by a forward peak and a subsidiary peak at larger angles followed by a sharp cut-off. These characteristics are confirmed by the distribution at approximately E T = 10 MeV, shown in fig. 4 which is calculated 3) A. O. Hanson and J. L. McKibben0 l)hy~ Rev. 72 (1947) 673.
intensities of both the high and the low energy neutron groups. It can be seen however from fig. 4 that although the relative intensity of the low energy group increases with angle, it remains quite low except in the vicinity of the m a x i m u m "cut-off" angle. The dotted curves in fig. 3 are the distributions obtained with the hydrogen removed from the gas cell. It can be seen that the intensity of these background neutrons increases with increasing triton 600 ~ L
~
r
40O :
HIGH ENERGYGROUP
200
LO EN GG YY GROUP i L oWwE HEERR GROU ,,'JJ 0
tO
20
30
40
50
60
o "(tAR) Fig. 4. Calculated angular distribution of neutrons from the 1H(t, n)3He reaction at E l- = 10 MeV.
308
W. D E U C H A R S et al.
energy. Measurements with a hydrogenous shadow cone between the gas cell and counter showed that these neutrons originated at, or close to, the gas cell. No change in the background was observed when the nickel entrance foil was replaced with one of tantalum. It is probable therefore that the background is due to (t, n) reactions with light element contaminants, such as carbon, in the path of the triton beam. This view is supported by the fact that the background intensity was sensitive to the degree of alignment and focus of the triton beam. With extreme care it could possibly be reduced below the levels indicat(d in fig. 3.
do not require a particularly good neutron energy resolution. The reaction should be particularly suitable for providing a source of 0.575MeV
// /
cxlo 4
,
jJ
NYOROGENGAS CEL \ /
/ NEUTRON / / CONE / / IRONSCATTERING
4. Application of the lI-I(t, n)3He Neutron Source The detection of 7 rays from inelastic neutron scattering and the measurement of neutron activation cross sections are two of the more obvious types of experiment for which the 1H(t, n)3He source would be useful. To illustrate the first type of experiment, measurements were made of the ~ rays from inelastic scattering by iron. The experimental arrangement and the results obtained are shown in fig. 5. The sample was placed in the forward'cone of neutrons and the detector a NaI crystal - w a s placed outside of the cone. The only shielding required was a block of lead, a few cm long, placed between the gas cell and detector. The curves shown represent the subtraction of runs taken with and without the sample in place. No corrections have been made for the effect of scattered neutrons from the iron sample. High statistical accuracy was obtained from runs of a few minutes duration. To summarise, the IH(t, n)3He reaction is a useful source of neutrons for experiments requiring a directed neutron beam of high intensity and which
2~1o'
I1 t~ \ l~l!
\
,/ )~
1.0
~
\\\
~"/Z/SODIUM IODIDECRYSTAL
,
.
[ ~/1K__
\
^
,~'~'~o
INCIDENTNEUTRONENERGY14¢V
"'.,./~".\ 4.5 "~'-t"-,~._ 1.5
2.0 GAMMA
Z.S
RAY ENERGY M c V
3.0
3.5
4.0
Fig. 5. NaI scintillation spectra from ~-rays produced by Fe(n, n ' y .
neutrons. This point could not be cheeked in the present expeliments since the low bombarding energy required could not be obtained with the Tandem accelerator.
Acknowledgement We would like to thank Mr. B. E. F. Macefield for supplying the data given in table 1.