The 1H(7Li, n)7Be reaction as an intense MeV neutron source

Nuclear Instruments and Methods 200 (1982) 285-290 North-Holland Publishing Company

285

T H E tH(TLi, n)7Be R E A C T I O N A S A N I N T E N S E M e V N E U T R O N S O U R C E * J.H. DAVE and C.R. GOULD Physics Department, North Carolina State Universi(v, Raleigh, NC 27650, U.S.A. and Triangle Universities Nuclear Laboratory, Duke Station, Durham, NC 27706, U.S.A. S.A. W E N D E R ** Physics Department, Duke University, Durham, NC 27706 and TUNL, U.S.A. S.M. SHAFROTH Physics Department, University of North Carolina, Chapel Hill, NC 27514 and TUNL U.S.A. Received 7 December 1981

Heavy ion bombardment of hydrogen provides a number of new possibilities for mono-energetic and quasi mono-energetic neutron beam production. Close to the threshold, the strong kinematic focussing for negative Q-value reactions gives rise to intense well-collimated beams of MeV neutrons. An experimental study of 0 ° neutron production in the nH(TLi, n) reaction ( Q - - - 1.64 MeV) has been carried out from the threshold energy, 13.15 MeV, up to 22 MeV. Measured cross sections are compared to predictions based on known 7Li(p, n) cross sections. At 13.5 MeV the effective cross section reaches 4 b/sr, and the neutrons are confined within a cone of half angle 9 °. Cross section predictions for 9Be, nIB and n3C bombardment of hydrogen are also presented.

I. Introduction Reactions induced by protons a n d d e u t e r o n s have long been used to make b e a m s of n e u t r o n s for nuclear physics experiments. Two specific examples are the 7Li(p, n) reaction, employed as a n e u t r o n source in the few MeV range It], a n d the 9Be(d, n) reaction, used as a white n e u t r o n source in the 1-20 MeV energy range [2]. In recent years, d e v e l o p m e n t s in heavy ion (HI) source technology have m a d e it possible to accelerate a wide variety of HI b e a m s with reasonable intensity. It is therefore interesting to consider reversing ( p , n ) and (d, n) reactions a n d investigate the possibilities for neutron beam p r o d u c t i o n by HI b o m b a r d m e n t of hydrogen a n d deuterium targets. In this p a p e r we focus primarily on the IH(7Li, n) reaction. We report o n an experimental investigation of its characteristics as a n e u t r o n source from threshold up to E ( L i ) = 22 MeV. The main feature to emerge for a negative Q reaction like this is that close to threshold, the kinematics of the reaction confine the n e u t r o n s to a n a r r o w cone at 0 °. As a result, the n e u t r o n flux is large, a n d well collimated. Such a source m a y be a d v a n t a g e o u s in experiments * Work supported in part by U.S. Department of Energy. ** Present address: Los Alamos National Laboratory, Los Alamos, NM, U.S.A. 0 1 6 7 - 5 0 8 7 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d

where detectors which are sensitive to n e u t r o n d a m a g e are used, for example Si(Li) or Ge(Li) detectors. The large effective cross section for n e u t r o n p r o d u c t i o n near threshold may also be of value in hydrogen d e p t h profiling. Finally, a HI n e u t r o n source could be of value in radiological appliations. Since the C o u l o m b barrier suppresses reactions of the b o m b a r d i n g HI with the high Z material of the gas cell, v-ray production is low a n d the radiation field consists almost solely of neutrons. A preliminary account of this work was presented in ref. 3. A similar proposal has been m a d e i n d e p e n d e n t l y by Drosg [4].

2. Kinematics The kinematics of negative Q reactions close to threshold are summarized in fig. 1. The n e u t r o n s in the laboratory frame are confined within a cone defined by the vector sum of the velocity of the center-of-mass and the velocity of the n e u t r o n in the center-of-mass. A t a given laboratory angle within the cone, two n e u t r o n groups are present. The higher energy group corres p o n d s to neutrons going forward in the center-of-mass frame (n), the lower energy group corresponds to neutrons going back in the center of mass frame ft. The Q-value for the nH(VLi, n)7Be reaction is - i.64

J.lt. Dave et aL / The JH(TLi. n)ZBe reaction

286 'H(rL,,q} ::

8 LAB

,F~ ~ "

n

~

"

ml

3oc~

'~i (HT n.,)

;

,

,~ _~_,]

LAB 71,

•

\

//

"

:~

=

il

i ,~'~,sc.,.t

'~/

~

n >

> <

CM

•t Li

~

ii ' z

.~f~'"

20C ,t_

<

I

v :

/i

'

::-~"J' " :

i d

1H a2 < J

'H(rLI,no);'Be

ErR = ' 5 1 MeV

'H ( rLi, n, ) :'Be ~

E.rH : 16 5 MeV

O t .... O

L_

_ 1 ._i 20 INCIDEN ~

I .... ~__ ~ _..1 ..... 40 60 80 E N E R G Y (Mev)

_

Fig. 1. Kinematics for the i H( ~Li, n) reaction close to threshold. At 0 ° in the laboratory the two neutron groups n and fi are associated with forward and backward motion in the centerof-mass.

Fig. 3. Predicted laboratory cross sections at 0 ° for the n o group in the Ill(HI, n) reaction. Cross sections for the inverse HI(p, n) reactions were taken from refs. 1, 5 for 7Li, refs. 6-8 for 9Be, refs. 7-10 for ~IB and refs. I1, 12 for 13C.

MeV which implies a threshold of 13.1 MeV. A b o v e 16.5 MeV, excitation of the 7Be first excited state is also possible, a n d there will be four n e u t r o n groups at 0 °. T w o of the groups are associated with the 7Be g r o u n d state (n o a n d rio) a n d two are associated with excitation of the first excited state (n, a n d r l ) . T h e energies of the 0 ° n e u t r o n groups associated with the 7Be g r o u n d state are shown in fig. 2. This figure also shows kinematics for some o t h e r HI n e u t r o n producing reactions o n hydrogen. Only the energies of the two ground state n e u t r o n groups are considered. T h e b a c k w a r d going group is always quite low in energy, typically below 500 keV. T h e forward going group increases in energy roughly linearly with the HI projectile energy. At threshold the two groups merge a n d the n e u t r o n s have energy given by [ A / ( A + I)I*IQI where Q is the Q-value a n d A is the mass n u m b e r of the projectile.

If the cross sections for the inverse (p, n) reactions are known, it is possible to predict the cross sections for the two n e u t r o n groups seen at 0 ° in the (HI, n) reaction. The (HI, n o ) cross sections are associated with the 180 ° center-of-mass ( p , n ) cross section values. The (HI, r 0 ) cross sections are associated with the usually better k n o w n 0 ° center-of-mass (p, n) cross section values. Fig. 3 shows the predicted cross sections for the higher energy (no) neutrons for 7Li, 9Be, liB a n d 13C. T h e HI(p, n) cross sections were t a k e n from refs. 1, 5 for 7Li, refs. 6 - 8 for 9Be, refs. 7 - 1 0 for liB a n d refs. 11, 12 for 13C. T h e (HI, n) cross sections are plotted for energies from a few MeV above threshold up to energies typically available from electrostatic accelerators. Closer to threshold the cross sections are expected to increase rapidly before going to zero. This is the region of

' "~n ./t)~.-\ L

l

"~16 v

I

[

I

-~---"' ) T -,7- " n ' H ( L,,

-I

IH( HI, n o) ~

7L I

~,

(D U0 no, no

2O (,P Z

O

IO

OLI

15

"

'

20

'

30

INCIDENT

'

40

ENERGY

'

50

60

i

I

i

15

I

I

17

EL,

i

I

19

(MeV)

(MeV)

Fig. 2. Energies of the ground state neutron groups n o and fi0 for 7Li, 9Be, t~B, and 13C induced reactions on hydrogen.

Fig. 4. Maximum angles of emission of the n o and n, groups as function of projectile energy in the I H(~Li. n) reaction.

287

J.H. Dave et aL / The IH(TLL n)ZBe reaction

greatest interest for a n e u t r o n source a n d is discussed later for the (TLi, n) reaction in c o n n e c t i o n with the experimental results. These cross sections are not shown in fig. 3 because the values d e p e n d very sensitively on the exact form of the threshold (p, n) cross sections. which are generally not known. The m a x i m u m laboratory angle at which n e u t r o n s c a n be emitted in the I H(TLi, n)TBe reaction is shown in fig. 4 as a function of projectile energy, At this angle, called the cone angle, the n and fi n e u t r o n groups merge a n d the source is effectively mono-energetic. F o r the (TLi, no) reaction, the n e u t r o n energy at the cone angle is a b o u t 1.5 MeV for all b o m b a r d i n g energies.

3. Experiments Experiments were carried out to investigate some of the characteristics of the (TLi, n) reaction discussed in the previous section. T h e m e a s u r e m e n t s were made with the T U N L neutron time-of-flight ( T O F ) facility [13]. Beams of 7Li ions were extracted from a G e n e r a l Ionex sputter ion source a n d were injected into the T U N L F N T a n d e m Van de G r a a f f accelerator. The beams were c h o p p e d a n d b u n c h e d at a rate of 2 M H z using the double drift b u n c h e r system recently developed at T U N L [14]. The b e a m burst was approximately 2.5 ns wide with less than 570 of the b e a m outside the main burst. Because of the small lithium b e a m current, timing for the b e a m bursts was derived from the b u n c h e r oscillator. This resulted in a d e g r a d a t i o n of the timing but did not otherwise affect the accuracy of the measurements. The n e u t r o n s were produced in a 3 cm long gas cell filled with I / 2 a t m of hydrogen gas. The entrance foil was a 3 p.m thick nickel foil a n d the b e a m was stopped in a thin t a n t a l u m disk. A c c u m u l a t e d b e a m on target was 0.7 # C for most of the runs. A suppressor ring in front of the gas cell was biased at - 5 0 0 V to ensure reliable b e a m current integration. A yield curve at 0 ° was taken in 200 keV steps from 15.4 MeV to 17 MeV a n d then in 500 keV steps from 17 M e V to 23 MeV. A n g u l a r distribution m e a s u r e m e n t s were made at 15.6 a n d 16 MeV in 2 ° and 5 ° steps from 0 ° to a b o u t 20 °. Beam energy losses in the e n t r a n c e foil ranged from 1.9 MeV at 15.4 MeV to 1.5 MeV at 23 MeV. Beam energy losses in the gas were m u c h lower, ranging from a b o u t 500 keV at 15.4 MeV to 340 keV at 23 MeV. The n e u t r o n detector was a NE218 liquid scintillator (diameter 8.9 cm, thickness 5.1 cm) situated inside a collimating shield. The detector could be positioned at angles from 0 ° to 160 ° to a n accuracy of 0.1 °. A flight p a t h of 2.15 m was used, at which distance the detector s u b t e n d e d an angle of 2.5 ° . The threshold energy for n e u t r o n detection was a b o u t 300 keV, corresponding to

the valley below the peak from an 241Am y-ray source. C o n v e n t i o n a l pulse shape discrimination electronics were used to distinguish n e u t r o n events in the scintillator from y-ray events. In practice the y-ray c o u n t rate was very low a n d spectra were accumulated b o t h with a n d without pulse shape discrimination. The characteristics of the source are illustrated by the 0 ° n e u t r o n T O F spectra of fig. 5. These spectra were o b t a i n e d without pulse shape discrimination and are for equal a m o u n t s of b e a m current on target. The b o m b a r d ing energies range from 13.5 MeV, just above threshold for the tH(TLi, n)TBe reaction, to 17.1 MeV, above threshold for the JH(TLi, n)TBe * reaction. The time bin size is 0.47 n s / c h a n n e l . As the lithium energy increases the two g r o u n d state n e u t r o n groups n o a n d fi0 become well separated a n d their intensity decreases rapidly. At 17.1 MeV the four possible n e u t r o n groups n 0, rio, nt a n d i'i~ are clearly seen. Using the absolute efficiency [15] of the n e u t r o n detector, the yields can be converted to effective cross sections. A G a u s s i a n peak fitting p r o g r a m was used to extract yields in spectra where the peaks were not well resolved. T h e experimentally determined cross sections are plotted in figs. 6 - 8 and tabulated in table I for the four different n e u t r o n groups. The data points are

[

'

3/

'

e, ob ° o n'o

'

I

(M~V)l --

13 t

1"I I

no

)'

6 0

__1

0

100 200 Chonnel Number

300

Fig. 5. Neutron TOF spectra obtained in the IH(7Li, n) reaction. Two neutron groups are seen below 16.5 MeV. Four groups are seen at 17.1 MeV where the 7Be first excited state is energetically accessible. Effective cross sections for the four groups are plotted in figs. 6-8.

J.H. Dave et al. / The IH(ZLi, n)ZBe reaction

288

Io41:

'

1

,

'

IH (7Li,n) 713e

Table 1 Effective cross sections at 0 ° in the laboratory for the i H(7 Li, n) reaction. The 7Li energies are average values at the center of a 3 cm long gas celled filled with 1/2 arm of hydrogen. Scale errors are -'- 15%.

--4

I

~e

Average ~Li energy (MeV)

103

"s b

x~ 1 0 2

.

L_ :O t [

12

I

I t16 18 2'0 14 Inciden! Energy(MeV)

22

Fig. 6. Measured laboratory cross sections for the n o group at 0 ° in the I H(TLi, n)TBe reaction. Vertical error bars on selected points represent statistical uncertainties in the measurements. The horizontal error bars represent the spread in energy associated with beam energy loss in the gas cell. The solid and dashed lines are the cross section predictions based on the known 7Li(p, n) cross section. The solid line is the exact cross section for each energy. The dashed line includes the effects of averaging over the energy loss in the gas cell and the energy spread introduced by the nickel entrance foil.

Cross sections ( m b / s r )

13.24 13.46 13.67 13.89 14.10 14.33 14.54 14.76 14.98 15.51 16.05 16.59 17.12 17.69 18.17 18.71 19.25 19.75 20.29 21.35

no

rio

286 a 2317 2400 I110 825 749 692 655 621 433 198 75 71 78 80 81 75 75 72 60

286 ~ 1640 1686 401 197 116 81 64 61 90 147 118 69 46 31 21 19

nl

fil

29 28 29 34 40 45 52 51

23 ll 8 6 6 5 5

a Not resolved, total yields are divided equally between the two groups. p l o t t e d at average l i t h i u m b e a m energies at the c e n t e r of the gas cell. Selected d a t a p o i n t s are p l o t t e d with vertical a n d h o r i z o n t a l e r r o r bars, r e p r e s e n t i n g the statistical

102 ,

I

I

'H ('/U,n)rBe"

I

' n,

tH(TLi,n)TBe

10 I ~o 10 3

°0

b 101

o b 102 XD

io~ 16 i01 12

/

1

i

[

14 16 18 2o lnciden! Energy(MeV)

22

Fig. 7. Measured and predicted cross sections for the low energy it o group in the 1H(TLi, n)VBe reaction.

I

~

i

~8 20 22 24 Incident' Energy (MeV)

Fig. 8. Measured and predicted cross sections for the n, and fil groups in the 'H(TLi, nl)TBe * reaction, leading to the 0.43 MeV state.

J.H. Dave et al. / The IH(TLi, ,)7Be reaction

4 (2) E

ff'o ~

IH(TLi,n )TBe ELi=lS.9MeV

'o

3

g2

r-

~. 2 t,o

O

_d

1

0o

'

,bo

'

2;0

'

3;0

Chonnel Number Fig. 9. Neutron TOE spectra at angles from 0 ° to 15° in the JH(7Li, n) reaction at 13.9 MeV. Cross sections obtained from the~ spectra are plotted in fig. 10.

uncertainty in the yield and the spread in the beam energy respectively. Overall accuracies in the yields are estimated to be about 15% when uncertainties in beam current integrator and bunching efficiency are included. The solid lines in figs. 6 - 8 are the predicted laboratory cross sections for (7Li, n) based on the known

f04~ i_

i , i IH (tLi,n) 78e

5

=13.9 MeV I0 3

,o i

i'\

\

i@I

I°°o

1 5

I

15 10 Olob (deg)

20

Fig. 10. Cross sections as a function of angle for the sum of the n o and fi0 groups in the IH(7Li, n) reaction at 13.5 and 13.9 MeV. The yields drop by three orders of magnitude within 2° of the expected maximum angles of neutron emission (9 ° and 13° respectively).

289

(p, n) center-of-mass cross sections. The agreement is good except at the low bombarding energies where the cross sections are changing rapidly. The beam energy spread arising from straggling through the gas cell has a substantial effect on the measured yields near threshold. Averaging through the foil and over the length of the gas cell yields the dashed lines in figs. 6 and 7. The agreement is much improved. For these calculations the beam energy profile at any point was assumed to be Gaussian having a width equal to 10% of the total energy loss up to that point. The neutron T O F spectra obtained for 13.9 MeV at angles from 0 ° to 15 ° are shown in fig. 9. This is 700 keV above threshold. At angles grealer than 0 ° the n o and fi0 groups merge, and the yield is relatively constant within the cone. This is shown clearly in fig. 10 where the effective cross sections for the sum of the n o and fi0 groups are plotted for 13.5 and 13.9 MeV. The expected kinematic focussing is clearly realized even in the relatively crude experimental configuration used here. In both cases the neutron flux drops by over three orders of magnitude within 2 ° of the expected maximum angle of emission.

5. C o n c l u s i o n HI induced reactions on hydrogen provide interesting new possibilities for mono-energetic and quasi mono-energetic neutron beam production. The strong kinematic focussing near threshold gives rise to intense, self-collimated beams of neutrons. For the case of the t H(TLi, n) reaction studied here, the effective cross section for 0 ° neutron production at 13.5 MeV is about 4 b / s r , the neutrons having energies from about 1.2 to 1.7 MeV. The fact that the neutron beam is confined within a narrow cone may have significant advantages in experimental configurations involving detectors sensitive to neutron damage, or in situations where the use of large amounts of shielding is inconvenient or not feasible. The v-ray flux is low in comparison to the neutron flux even at energies well above threshold. The radiation field consists almost solely of neutrons, making such sources of potential interest in radiation therapy. The kinematic focussing is present for all negative Q, HI induced reactions. Since the neutron energy at threshold is essentially equal to the magnitude of the Q-value, different (HI, n) reactions could be selected to provide neutron sources with different energies. Reactions induced by 12C, 1 6 0 , 28Si or 5SNi, for example, could be used to produce collimated neutron beams with energies around 16.7, 15.4, 14.6 or 9.2 MeV, respectively. Because the (p, n) cross sections for T = 0 nuclei are small, the neutron source intensity for these reactions would be much lower than for the (TLi, n) reaction discussed here. The required HI beam energies

290

J.H. Dave et al. / The IH(TLi, n)ZBe reaction

would not in general be available in electrostatic accelerator laboratories. A b o v e threshold the (TLi, n) reaction provides two or four n e u t r o n groups with distinct energies. While less useful as a source, such a feature could be used to calibrate the efficiency of a n e u t r o n detector over a wide energy range. T h e 7Li(p, n) cross sections are generally well k n o w n a n d the backward going groups N 0 a n d fi~ are essentially u n c h a n g e d in energy, thus providing a reference point for m e a s u r e m e n t s m a d e at different b e a m energies.

References [1] H. Liskien and A. Paulsen, At. Nucl. Data Tables 15 (1975) 57. [21 G.F. Auchampaugh, S. Plattard and N.W. Hill, Nucl. Sci. Eng, 69 (1979) 30. [3] S.M. Shafroth et al., Bull. Am. Phys. Soc. 26 (1981) 551. [4] M. Drosg, Los' Alamos National Lab. Internal Report LA-8842-MS (1981).

[5] J.H. Gibbons and R.L. Macklin, Phys. Rev. 114 (1959) 571. [6] C.A. Kelsey, G.P. Lietz, S.F. Trevino and S.E. Darden, Phys. Rev. 129 (1963) 759. [7] R.D. Albert, S.D. Bloom and N.K. Glendenning, Phys. Rev. 122 (1961) 862. [8] B.D. Walker, C. Wong, J.D. Anderson and J.W. McClure, Phys. Rev. 137 (1965) B1504. [9] L. Van der Zwan and K.W. Geiger, Nucl. Phys. A306 (1978) 42. [10] J.C. Overley and R.R. Botchers, Nucl. Phys. 65 (1965) 165. [11] O. Dietzsch, Nucl. Phys. 85 (1966) 689. [12] P. Dagley, W. Haeberli and J.X. Saladin, Nucl. Phys. 24 (1961) 353. [13] D.W. Glasgow et al., Nucl. Sci. Eng. 61 (1976) 521. [14] S.A. Wender et al., Nucl. Instr. and Meth. 174 (1980) 341. [15] L.W. Seagondollar et al., Proc. Int. Conf. on Nuclear cross sections for technology (Knoxville, TN) NBS publication 594 (1979) p. 537.