Angular distribution of polarized neutrons from the Li7 (p, n) Be7 reaction

Nu¢tear Physics 40 (1963) 347--352; (~) North-Holland Publishing Co., Amsterdam N o t to be reproduced by photoprint or microfilm without written permission from the publisher

A N G U L A R D I S T R I B U T I O N OF P O L A R I Z E D N E U T R O N S F R O M THE Li ~ (p, n) Be y R E A C T I O N I. MINZATU, N. MARTALOGU, A. CALBOREANU, I. VILCOV and T. M A G D A

Institute for Atomic Physics, Bucharest * Received 7 August 1962 Abstract: The angular distribution o f the polarization of neutrons from the LiT(p, n)Be 7 reaction has been studied in an angular interval ranging from 5° to 70 °, in 5° steps. The incident proton energy was about 4.3 MeV. The measurements were carried out by a time-of-flight method. The experimental data showed that the polarization is positive in this angular interval with a peak in the vicinity of 40 °.

1.

Introduction

Many workers have studied the LiT(p, n)Be 7 reaction. Taschek and Hemmehdinger analysed the angular distribution and the cross-section in the 1.88-2.50 MeV energy range 1). j. H. Gibbons et al. 2' a) have extended the measurements to Ep = 5.5 MeV. A detailed study of the neutrons resulting from the ground state and from the first excited state (430 keV) of the Be7 nucleus was made by P. R. Bevington et aI. 4). The results of the angular distribution and total cross-section measurements allow some interpretations of the Be a compound nucleus level structure. The level diagram of Be8 in the 16-20 MeV energy range is shown in fig. 1.

&5"

"1+

;~Q

.2+0, -)

2.5

.f-

20 "//I/I/lll/lllltlllll/l///lll//l/llll/~ 0

Fig. 1. Diagram o f levels in Bes from L F + p .

The polarization measurements may, to a great extent, contribute to the knowledge of the reaction mechanism and of the various nuclear parameters taking part in the reaction, since the polarization is very sensitive to small variations of these parameters. From a hypothesis of a reaction mechanism involving the formation of a * The abstract of this paper has been sent to the International Symposium on "Direct Interactions and Nuclear Mechanisms, Padua, on the 17th April 1962. 347

348

~. MINZATUet aL

compound nucleus in the regions of resonances, one expects important modifications of the polarization. Polarization measurements have been made and described in numerous papers s-7). Striebel et al. s), j. A. Baicker 9) and L. Cranberg lo), measuring the polarization at 50 ° (lab.) noted no essential change in polarization at the resonances 2.5, 3 and 3.5 MeV, as they had expected. It is only at an energy of about 5 MeV that the sign of the polarization changes, becoming negative (see ref. 9)). Thus, it seems that the interaction with compound nucleus formation no longer predominates here. The angular distribution of the polarization can supply more accurate data in this respect. Such studies have not been made for Li 7 for energies 11) higher than 3 MeV. In this paper, the angular distribution of the neutron polarization was analysed at Ep = 4.3 MeV in the angular range 00-70 °. 2. Experimental Method

The analysis of the neutron polarization was made by measuring the left-right asymmetry that appears at the angles 02 and - 02, as a result of the neutrons scattering on a carbon scatterer. It is well known that if PI is the initial polarization sought, and/'o the polarizing power of the analyser, then L-R P,(O1)P~(02)

-

(1)

L+R

where L is the relative intensity at the angle 02 and R is the relative intensity at the angle - 0 2 . The experimental arrangement is shown in fig. 2.

~ .

L L ~

~ . ~

D,(L)

D2(R)

......__

"~DICL) Ffg.2. Fig. 2. Experimental arrangement showing the lithium target t the carbon scatterer d and the leftright detectors D1, D~. The proton beam of 1 - 2 / ~ A and 4.3 MeV obtained from the U-120 cyclotron of the Institute for Atomic Physics, Bucharest was focussed on a Li target. The resulting neutrons, collimated through a boric acid and paraffin collimator and scattered on the carbon scatterer were then recorded by means of a Stilben crystal (20 × 30) attached

ANGULAR

DISTRIBUTION

OF POLARIZED

349

NEUTRONS

to an RCA-5819 photomultiplier. The neutrons selected according to their energy by the time-of-flight method were successively recorded at the angles 02 and - 0 2 . The block diagram of the electronic arrangement used is shown in fig. 3. The resolving power of the spectrometer used did not alow the two groups of neutrons to be separated and the measurements therefore had an integral character.

Ti: 2:r° I I etector Distributed amplifier i ~

Distributed amplifier

l

Linear amplifier [

I

Time-to- I ] pulse height ~_a eon~erier |

L I

Slow-lineaT~_~ Pulse height [ gate l rI selector ]

Fig. 3. Block diagram of the electronic arrangement.

Since there is no accurate information in the literature concerning the polarization of the excited group, the corrections regarding their intensity have not been made. The polarization Pc of the neutrons elastically scattered from C t2 was calculated by the authors 12), from Wills, Bair and Cohn's phases t3) by means of Simon and Welton's formulae 24). Generally the polarization shows a maximum for an angle of about 60 ° (c.m.) (see fig. 4). For this reason, 02 was chosen equal to 60 ° cm.

;/

o~ o2oo~

-o2o'- 0,~,0-

/,,..._

-C~60-Q~O-

~.#

I

(9

~9

t

i

3A

39

I

enCMe~

Fig. 4. Calculated curve of the variation with energy of the neutron polarization produced in elastic scattering on C ~ at 60 ° (c.m.)

350

L MINZATUet aL

The following series ,~f measurements was made for every angle: measurements of asymmetry at +01 , measurements of background at +02, measurements of background at - 0 ~ , measurements of asymmetry at - 0~, and measurements of false asymmetry at 0 °. 3. Corrections and Statistics

The main sources of error measuring the asymmetry A are the finite geometry (target, scatterer and detector) and the energy spread of the beam. The errors arising from multiple scattering are altogether negligible in the polarization measurements carried out, since the carbon scatterer had a diameter of only 3 cm. Therefore, in estimating the corrections, only the first two sources of error were taken into account. These have been examined in detail in ref. 15). The asymmetry is, in fact, an average a = AvAv,

(2)

where (P,)A. is the average of the neutron polarization from Li within the given range of their energy spread. Since the distance between target and scatterer is considerable in comparison with their size, the geometrical corrections for P1 are negligible. (P¢>Av is the polarization of the neutrons elastically scattered from C a2, averaged over the angular interval defined by the reciprocal solid angle between the scatterer and the detector and over the energy spread of the neutron beam
(3)

where, in the notation of 15), e(E,) is the crystal efficiency, ~0(En) iS the shape of the beam energy distribution, 0~ is the effective scattering angle of the neutron, • is the angle between the normals of the reaction and scattering planes, ao(O'2E.) is the differential cross-section of C 12 for neutrons, expressed by ao(O'2E,) = ~=oBLP~. (cos 0~), Po(O'2E.) is the polarization power of C 12, do is the elementary volume of the scatterer, dO is the elementary solid angle under which the elementary volume in the detector is seen by the elementary volume dv and dz' is the elementary variation of the detector length. The geometrical corrections influenced the results by 2 - 4 ~o. The energy correction may be very important particularly in the vicinity of the resonances. The statistical errors of the asymmetry were computed by means of the formula I+A 2 o.2_ _ _ [L+R+2(Lf+Rf)], (L+R) 2

where Lf and Rf are the left-rightmeasured background counts.

(4)

ANGULAR DISTRIBUTION OF POLARIZED NEUTRONS

351

4. Results "Ihe measurements have been made every 5 degrees in the angular interval 0 ° - 70 °. The results of our measurements are shown in fig. 5. The values agree with those made by Baicker 9) at 50 ° (Ep = 4.2 and 4.42 MeV). The polarization is positive in the angular interval studied. It has a m a x i m u m in the vicinity of 40 ° where it takes the value of about + 35 %. In the diagram the value at 70 ° is not plotted since this requires a special discussion. At the 70 ° angle the neutron energy is E n = 2.034 MeV, i.e. in the resonance region of C 13 where the value of the

•

t

fb°

.~o"

io" go- ~oo ~,

÷o=~o o

J0o a~

Fig. 5. The angular distribution of the polarization. polarization changes abruptly and any inaccuracy in the determination of the integration limits with respect to energy in the integral (3) will modify P~ essentially. For 15 ° and 30 ° the measurements were made at + 01 and for 40 ° at - 01 only and it was therefore not possible to obtain an average on these two positions. This paper is the first in a series of papers concerned with the analysis of polarized neutrons f r o m the LiT(p, n)Be 7 reaction in the 4 - 6 . 5 MeV energy range. We are grateful to Professor Horia Hulubei for his very helpfulinterest and fruitful discussions. We are also indebted to the crew of cyclotron operators for their assistance.

Note added in proof: Recently, Benenson et al. 16) have published their results concerning the polarization of neutrons from LiT(p, n)Be 7 reaction for several energies from 4 - 1 0 MeV. For 4 MeV and 4.7 MeV they found an angular distribution of the polarization similar with that obtained at 4.3 MeV by us. It is found that the m a x i m u m of the polarization is displaced with the energy. References 1) 2) 3) 4)

R. R. R. P.

Taschek and A. Hemmendinger, Phys. Rev. 74 (1948) 373 L. Maeklin and J. H. Gibbous, Phys. Rev. 109 (1958) 105 L. Macklin and J. H. Gibbous, Phys. Rev. 114 (1959) 579 R. Bevington, W. W. Rolland and H. W. Lewis, Thesis

3~]

I. MINZATIJ et aL

5) 6) 7) 8) 9) t0) 11) 12) 13) 14) 15) 16)

R. K. Adair, S. E. Darden and P. R. Fields, Phys. Rev. 96 (1954) 503 A. Okazaki, Phys. Rev. 99 (1955) 55 H. B. Willard, J. K. Bair and J. D. Kington, Phys. Rev. 95 (1954) 1359 H. R. Striebel, S. Darden and W. Haeberli, Nuclear Physics 6 (1958) 188 J. A. Baicker, Thesis L. Cranberg, Phys. Rov. 114 (1959) 174 S. M. Austin, S. E. Darden, A. Okazaki and Z. Wilhelmi, Nuclear Physics 22 (1951) 451 A. Calboreanu and I. Minzatu, to be published J. E. ~,rills, J. K. Bair and H. P. Cohn, Phys. Rev. 109 (1958) 891 A. Simon and T. A. Welton, Phys. Rev. 90 (1953) 1036 I. Minzatu and I. Calboreanu, to be published. W. Boneuson, T. H. May and R. L. Walter, Nuclear Physics 32 (1962) 510