The 7Li(n, p)7He reaction

i 2.B: 2.L I I

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Nuclear Physics A199 (1973) 513--516; (~) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permission from the publisher

T H E ~Li(n, p)THe R E A C T I O N RICHARD H. LINDSAY, WILLIAM TOEWS and J. J. VEIT Department of Physics, Western Washington State College, Bellingham, Washington Received 2 October 1972 Abstract: The proton energy spectra at several angles from the 7Li(n, p)THe reaction confirm earlier studies using the 7Li(t, 3He)THe reaction where 7He was found to be unbound to neutron emission by 4204-60 keV. The cross section at 0.... = 0 ° is 0.404-0.1 mb/sr, and at 0c.m. = 30° a value of 0.35 +0.I0 mb/sr is obtained. Also observed in this study are the deuteron, triton and 6I-[e spectra from the 14.8 MeV neutron bombardment of 7Li. E [

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NUCLEAR REACTIONS 7Li(n, p), (n, d), (n, t), E = 14.8 MeV; measured a(Ev, 0), a(Ed, 0), o'(Et).

1. Introduction The g r o u n d state o f 7He was first observed by Stokes and Y o u n g 1) using the 7Li(t, 3He)VHe reaction. The results showed a width of 170___40 keV for the particle unstable 7He nucleus and a primary decay channel o f 7He ~ 6 H e + n + 4 2 0 keV. N o peaks suggesting excited states o f 7He were observed. We have studied the charged particles f r o m 7Li f r o m neutron-induced reactions at 14.8 MeV and at several forward angles. While a structureless continuum is observed in the 6He spectra, the p r o t o n spectrum clearly shows a reaction resonance t h r o u g h the 7He g r o u n d state. It is not possible to decide on any other 7He resonance state above the g r o u n d state f r o m these data. The width o f the g r o u n d state is < 200 keV after correction for target and neutron beam broadening.

2. Experimental Enriched 7Li metal targets of about 2 mg/cm 2 were used in a triple-coincidence counter telescope consisting of twin gas proportional A E counters and a 390 /lm silicon detector 2). Energy spectra were observed for the protons, deuterons, tritons and 6He particles at several angles. Fig. 1 shows the p r o t o n spectra at 0 = 0 °. The protons observed from the 7Li target had energies below 4 MeV. At 0 = 0 °, a peak near 3.8 MeV m a x i m u m energy appears and, using the u n b o u n d energy o f 420 keV measured previously 1) for 7He, the c o m p u t e d m a x i m u m p r o t o n energy would be 3.81 MeV for this angle. All the forward angle spectra are consistent with kinematic expectations for this reaction, confirming the 6He-n final-state interaction for the 7He nucleus. In fig. 2 we show the p r o t o n spectra at three angles. 513

I

I

P-4 Ep

BO

e=

I

RESOLUTION

O*

7O 7L~ { n, p )'tHe

6O

5O tO I--

Z4o 0 (.)

50

tt

20

10

I

I

I

I

2

3 4 Ep {MeV)

I 5

6

Fig. 1. Proton energy spectrum from the 7Li(n, p) reaction at 0 = 0 ° with 14.8±0.1 MeV neutrons

PROTON ENERGY SPECTRA 7 L i ( n , p ) 7 H e 8LAB = 0 °

70

8LA B = 12 °

8LA B = 2 0 °

60 (rm,p)ZHe 50

z

(n,p)THe

( n, pi7He

_I-

f-

40

y

0¢,,) ;30

10 3.8 MeV

' 32

~ 4

I 48

I

. 56

~,.6 MeV

3.7 MeV

.

. 64

.

.

~ 52

40

48

l 56

l 64

12

l 40

CHANNEL NUMBER

Fig. 2. The 7Li(n, p) proton spectrum at several angles.

,1

7Li(n , p)

515

~ 7Li(n,d)6Hee.s.

1.5-

+ 7Li(noi~SHee.s. 1.0-

E bC~ 0.5"

0.0'

1

!

o

io

!

,~o

80

%. Fig. 3. Differential cross sections for the 7Li(n, d)6He=.,, group a n d the lower energy p r o t o n group s h o w n in fig. 1.

160 -

7L; (n,t)

5Hes.s.

1

140"

120, 7LI (n,d)6HeQ.s. I00' >I-"

tO 80Z LtJ I--

~ 6o 40

20-

0 20

s'o

,~o

n

/ L ,~o

CHANNEL

Fig. 4. Triton a n d deuteron spectra at 0 = 0 °.

,~o

516

R . H . LINDSAY et al.

A second peak occurs in the proton spectra at an energy of about 2.2 MeV. This is the energy at which one expects to find singlet (1So) deuterons from a pick-up reaction with the p-state proton in 7Li, since the bound (3S1) deuterons from the reaction 7Li(n, d)6Heg.s, have an energy of 6.7___0.1 MeV. Cohen et al. 3, 4) detected singlet deuterons in proton-induced reactions with 9B and 7Li, and according to stripping theory the angular distributions (n, d) and (n, d) should be very similar except for intensity. The (n, d) cross section should be larger. In fig. 3 we show the measured differential cross sections for the 7Li(n, d)6Heg.s, compared with the proton (d?) group at 2.2 MeV (fig. 1), and we note that the angular distributions are similar (both peaking at 0c.m. "~ 20 °) but the (n, d) is higher in intensity. This leads us to speculate that the lower energy group in the proton spectra is not due to another 7He resonance state. The differential cross section for the 7Li(n, p)THegs reaction is measured to be 0.44-0.1 mb/sr at 0 = 0 ° and 0.354-0.1 at 0 .... = 30 °. We have checked this cross section using the 7Li(n, t)SHe~s, reaction, which has a well-known differential cross section at 0 = 0 ° of 15 mb/sr. The triton spectrum at this angle is shown in fig. 4 with the deuteron spectrum.

References 1) 2) 3) 4)

R. H. Stokes and P. G. Young, Phys. Rev. Lett. 18 (1967) 611 R. H. Lindsay and J. J. Veit, Phys. Rev. 157 (1967) 933 B. L. Cohen, E. C. May and T. M. O'Keefe, Phys. Rev. Lett. 18 (1967) 962 E. C. May, B. L. Cohen, T. M. O'Keefe and C. L. Fink, Bull. Am. Phys. Soc. 13 (1968) 83