Polarization of neutrons from the T(p, n)He3 reaction from 1.9 to 3.5 MeV

Nuclear Physics 51 (1964) 3 9 5 - - 4 0 0 ; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

POLARIZATION OF NEUTRONS F R O M THE T(p, n)He s REACTION F R O M 1.9 to 3.5 MeV C. A. K E L S E Y , B. H O O P , JR., a n d P. V A N D E R

MAATt

University of Wisconsin, Madison, Wisconsin tt Received 10 October 1963

Abstract: Previous m e a s u r e m e n t s in this laboratory o n the polarization o f the n e u t r o n s f r o m the T(p, n ) H e s reaction have been extended to lower energies a n d o t h e r emission angles. A solenoid was used to precess the spin vector o f the n e u t r o n s a n d the polarization was determined f r o m the observed a s y m m e t r y in scattering f r o m helium. T h e results indicate t h a t the polarization is largest n e a r a centre-of-mass emission angle o f 45 ° with a value o f 0.40 between 2.0 a n d 2.4 M e V b o m barding energy.

1. /ntroduetion Previous measurements 1) in this laboratory on the polarization of neutrons from the T(p, n) reaction have been extended to other angles and lower energies. The results of Walter et al.1) indicate that for an emission angle of 33 ° the polarization of neutrons from this reaction rises to about 0.25 at 2.9 MeV. The measurement of Willard, Bair and Kington 2) at 1.46 MeV and 50 ° emission angle is consistent with zero polarization. The only other results available in this energy region are the data of the Notre Dame group 3) at 1.95, 2.25, 2.9 and 3.0 MeV and one point of May et al. 4) at 3.1 MeV. The three lower energy measurements are all consistent with a polarization of 0.30 but the measurement at 3.0 MeV indicates a somewhat smaller value, as does the result of May et al. The present experiment was undertaken to investigate the neutron polarization for bombarding energies below 2.9 MeV. In addition to determining the usefulness of this reaction as a source of low energy polarized neutrons additional polarization data near threshold will also be useful in a theoretical analysis of the T(p, n)He 3 reaction.

2. Experimental Procedure Protons from an electrostatic accelerator were incident on a gas cell 4 cm long containing 1 atm of tritium. The gas cell and foil cooling system have been described in ref. 1). The polarization of the neutrons from the T(p, n) reaction was determined N o w at L o s A l a m o s Scientific Laboratory. *t W o r k s u p p o r t e d by U. S. A t o m i c Energy C o m m i s s i o n . 395

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from the asymmetry in scattering from helium. This method has been discussed previously 1, 5) and will only be briefly mentioned here. The neutrons from the reaction were collimated by an air core solenoid and scattered by a helium analyser located approximately one meter from the neutron source. The scattered neutrons were detected by plastic scintillators 7.5 cm wide, 7.5 cm deep and 2.5 cm thick placed at equal angles 02 above and below the helium cell. The neutron scattering angle was adjusted to give scattered neutrons of highest energy consistent with large analysing power in the helium scattering. The distance from the centre of the detectors to the axis of the helium cell was 15 cm, resulting in an angular acceptance of the neutron detectors of about 30 °.

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Fig. 1. A block diagram of the electronic coincidence circuit. Detailed descriptions of the solenoid and high pressure (150 atm)helium cell used in this experiment have been published previously 5). A block diagram of the circuit used is shown in fig. 1. It is similar to the circuit used in ref. 5). The fast pulses from the two plastic scintillators were mixed and fed into one of the inputs of a fast coincidence circuit. The other input was a fast-rising pulse from the helium cell. The output of this coincidence circuit was used to gate a multi-channel analyser which analysed the linear pulses from the helium cell. In this way only pulses corresponding to neutrons scattered from helium were analysed. The storage location of the accepted pulses was determined by routing pulses from the upper and lower linear amplifiers. The sealers marked " u p " and " d o w n " recorded all pulses from the upper and lower detectors which were in coincidence with the helium cell pulses. These sealers were used as a continuous monitor on the operation of the electronics. The final data were always obtained from the multi-channel analyser.

3. Backgrounds It was necessary to correct the data for the effects of two types of background. Background counts caused by neutrons not produced in the target gas were determined

NEUTRON

397

POLARIZATION

by evacuating the tritium target. A measure of t h e background arising from chance coincidence counts was obtained by delaying one of the fast coincidence inputs with respect to the other. Recorded backgrounds were usually less than 10 ~o and always less than 20 ~ except for the point at 2.7 M e V and 70 ° emission angle where the background rose to 40 ~ . Fig. 2 shows a typical spectrum obtained at a proton bombarding energy of 2.7 MeV and a neutron emission angle of 33 ° in the laboratory. Also shown in fig. 2 are spectra obtained with the tritium target evacuated and with the analyser gated to 50

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accept all pulses from t h e helium cell. The peak in the gated recoil spectrum corresponds to neutrons which have been scattered through 02 = 100 ° in the laboratory system ( 0 = 114 ° c.m,) and detected in the upper detector. A similar spectrum was obtained for the lower detector. The width of the peak gives an indication of the angular acceptance of the plastic scintillators. The pulses appearing below the peak probably arise from g a m m a rays, lower energy neutrons and chance coincidence counts. Recorded background counts were reduced by accepting only the counts in the peak. The effect of accepting pulses smaller than those in the peak was investigated by employing two different bias positions. One corresponded to accepting neutrons having greater than ½ the energy of a neutron scattered through 02, and the other corresponded to accepting neutrons with greater than ¥3 that energy. Since the data obtained at 75 ~o bias have fewer background counts than the data obtained with 50 ~o bias, a comparison of the two gives an indication of the error introduced by the presence of the low energy tail. No bias effect was observed, indicating that the background did not appreciably affect the results. The results listed in table I were obtained from the 50 ~ bias data.

c.A. KELSEYet al.

398

4. Results and Discussion T h e results o f the present e x p e r i m e n t are presented in table 1. Th e first five c o l u m n s c o n t a i n the p r o t o n b o m b a r d i n g energy in MeV, the n e u t r o n energy in M e V , the emission angle o f the n e u t r o n s in the l a b o r a t o r y system a n d the centre-of-mass system and the l a b o r a t o r y scattering angle o f the neutrons. Th e last three c o l u m n s list the analysing p o w e r o f helium, P2(0), the q u a n t i t y P1P2 an d the final value o f the polarization P1. TABLE 1 Summary of results Proton energy (MeV)

Neutron energy (MeV)

01 lab (deg)

@1 c.m. (deg)

0z lab (deg)

P2

P1P2

P1

0.364-0.03

2.0

1.0

33

47

60

0.58

0.209

2.1

1.1

33

47

80

0.73

0.300

0.414-0.02

2.3

1.5 1.3 1.2 1.1 0.8

16 33 40 50 70

23 47 57 70 95

90 90 80 70 60

0.67 0.70 0.67 0.64 0.77

0.147 0.287 0.275 0.186 -- 0.069

0.22±0.02 0.414-0.02 0.414-0.02 0.29 -4-0.04 --0.09 4- 0.05

2.5

1.5

33

47

80

0.56

0.202

0.36±0.02

2.7

1.9 1.7 1.4 1.2 1.1 0.9

16 33 50 60 70 813

23 46 69 81 93 104

100 100 90 80 80 60

0.73 0.72 0.69 0.68 0.75 0.67

0.110 0.216 0.228 0.088 --0.075 -- 0.087

0.15±0.02 0.304-0.02 0.33 4- 0.03 0.13-4- 0.03 --0.104-0.02 -- 0.13 ± 0.03

2.8

1.8

33

46

90

0.61

0.177

0.294-13.02

2.9

1.9

33

46

100

0.73

0.183

0.25 4- 0.02

3.2

2.1

33

46

100

0.72

0.137

0.194-0.02

3.4

2.3

33

46

110

0.79

0.142

0.184-0.01

3.5

2.6 2.4 2.0 1.6 1.4 1.2

16 33 50 713 80 90

22 46 68 92 103 114

110 110 110 90 80 813

0.80 0.79 0.77 0.66 0.60 0.72

0.056 0.118 0.139 0.053 0.018 --0.058

0.074-0.02 0.15 4-0.03 0.18 4- 0.02 0.084-0,03 0.03 4- 0.02 --0.08+0.03

T h e analysing p o w e r o f h e l i u m listed in c o l u m n 6 was o b t a i n e d f r o m a g r ap h published by Haeberli 6) a n d has been averaged over the an g u l ar spread o f the n e u t r o n detectors an d the energy spread o f the neutrons. This energy spread was a b o u t 190 keV in all cases. T h e positive direction o f p o l a r i z a t i o n has been chosen in a g r e e m e n t with the Basel co n v e n ti o n . T h e uncertainties listed in the last c o l u m n are a c o m b i n a tion o f statistical a n d b a c k g r o u n d uncertainties.

399

NEUTRON POLARIZATION

Fig. 3 shows the present results at a laboratory emission angle o f 33 ° together with other available data for angles near 33 ° . The results o f the N o t r e D a m e group indicated by open circles were obtained with a carbon scatterer as a polarization analyser. I

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Fig. 4. Polarization of neutrons from the T(p, n)He s reaction as a function of c.m. emisson angle for four different proton energies of 2.3, 2.7, 2.9 and 3.5 MeV. The data at 2.9 MeV were obtained from Walter et al. 1).

A high pressure helium cell similar to the one used in this experiment was e m p l o y e d to obtain the result indicated by the open triangle. The agreement with previous data is very g o o d except at 2.25 M e V where the present data are in disagreement with the previous measurement. In fig. 4 the results o f the polarization angular distribution

400

c.A. KELSEYet al.

m e a s u r e m e n t s o b t a i n e d in the present e x p e r i m e n t are shown together with an a n g u l a r d i s t r i b u t i o n o f W a l t e r et al. 1) in the same energy region. The m a x i m u m in the curve o f p o l a r i z a t i o n versus angle is seen to occur at larger angles a n d to decrease in m a g nitude as the b o m b a r d i n g energy is increased. T h e D + T interaction has been e m p l o y e d to s t u d y the nucleus He* b o t h in this l a b o r a t o r y 7, 8) a n d elsewhere 9). There is evidence for at least one a n d p e r h a p s several levels in this excitation region. I n fact the large m a x i m u m in the t o t a l cross section near 3 M e V has been cited 1o) as evidence for the existence o f a level in He* n e a r 22 M e V excitation energy. F r a n k a n d G a m m e l 11), W e r n t z 12) a n d others 13) have a t t e m p t e d phase shift analyses o f p - T a n d p - H e 3 scattering d a t a in this energy region. T h e y all find evidence for a 0 + level j u s t b e l o w the T ( p , n) threshold. W e r n t z 12) a n d B a l a s h k o a n d K u r e p i n 13) also find evidence for a s p i n - o r b i t d e p e n d e n c e in the i n t e r a c t i o n at p r o t o n energies j u s t a b o v e t h e threshold. T h e sin 20 d e p e n d e n c e o f the p o l a r i z a t i o n n e a r 2.3 M e V c a n be i n t e r p r e t e d in t e r m s o f a 1 - level in He*. The a u t h o r s w o u l d like to t h a n k Professor S. M. A u s t i n for suggesting this exp e r i m e n t a n d Professor H. H. Barschall for his interest a n d e n c o u r a g e m e n t .

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

R. L. Walter, W. Benenson, P. S. Dubbeldam and T. H. May, Nuclear Physics 30 (1962) 292 H. B. Willard, J. K. Bair and J. D. Kington, Phys. Rev. 95 (1954) 1359 S. F. Trevino and S. E. Darden, private communication T. H. May, R. L. Waiter and H. H. Barschall, Nuclear Physics 45 (1963) 17 P. S. Dubbeldam and R. L. Waiter, Nuclear Physics 28 (1961) 414 W. Haeberli, in Fast neutron physics, Part 2, ed. by J. B. Marion and J. L. Fowler (Interscience Publishers, New York, 1963) chapt. V H. W. Lefevre, R. R. Borchers and C. H. Poppe, Phys. Rev. 128 (1962) 1328; C. H. Poppe, C. H. Holbrow and R. R. Borchers, Phys. Rev. 129 (1963) 733 C. Werntz, Phys. Rev. 128 (1962) 1336 G. F. Bogdanov, N. A. Vlasov, S. P. Kalinin, B. V. Rybakov and V. A. Sidonov, JETP 30 (1956) 981 A. I. Baz and I. A. Smorodinski, JETP 27 (1954) 382 R. M. Frank and J. L. Gammel, Phys. Rev. 99 (1955) 1406 C. Werntz, to be published Yu. G. Balashko and A. B. Kurepin, JETP 44 (1963) 610