PH YS ICAI. HE Vr EW C VOLUME 9, NUMBER 3

Neutron-proton polarization measurements near 20-Mev incident neutron energy*

C. L. Mozris, t T. K. O' Malley, J.W. May, Jz., and S. T. ThorntonDePartment of Physics, University of Virginia, Charlottesville, Virginia 22901

Qeceived 7 September 1973)

Using the H(d, n) He reaction as a source of polarized neutrons, measurements of the n-Ppolarizations have been made at 16.9 and 21.1 MeV. Results are reported at 40, 60, SO, and140 c.m. for both enexgies as well as at 100 and 120' c.m. for 21.1 MeV. Comparisons aremade with predictions of the LRL-X and the YALE-IV phase-shift analysis. The data favorthe LRL-X analysis at forward angles and the YALE-IV analysis at backward angles.

NUCLEAR REACTIONS H(n, n), S =16.9 and 21.1,MeV, measured P(e),

1. INTRGDUCTION II. EXPERIMENTAL PROCEDURE

Although the T = 1 nucleon-nucleon phase shiftscan be accurately determined in the energy regionnear 20 MeV from existing P-p scattering data,the situation for the T =0 phase parameters ismuch more ambiguous. ' These must be deter-mined from relatively sparse and imprecise m-P

data. Also, the recently investigated possibilityof charge-dependent splitting of the T = 1 phase pa-rameters" ' makes more accurate knowledge ofn-p observables desirabl.

In particular, the n-p polarization is sensitiveto the h~~s and 6» parameters as defined by Muteh-ler and Simmons. 3 These represent the spin-orbitsplitting of the T =1 triplet Pwaves and the T=0triplet D waves. In the energy region covered bythis measurement 4~Is ean be determined from p-Pdata alone. Consequently, n-P polarization datacan be used to test for isospin splitting in this pa-rameter rather directly. Such splitting has beenused in the YAI.E-IV analysis' but not in the LRL-X hnalysis. '

One aim of the present experiment is to providemore accurate data for the determination of thetriplet D-wave phase shifts. Ne also hoped topresent data which is accurate enough to discernwhether charge-dependent splitting in the tripletI' waves is important at these energies.

Polarization measurements in this energy regionhave been reported by Langsford ef al.' (20.5 MeVand higher energies}, Benenson, Walter, and May'(16.4 and 23.7 MeV), Perkins and Simmons' (23.1MeV), Mutchler and Simmons' (11.0, 16.6, 23.1,and 29.6 MeV), Garrett et al. ' (16.2 MeV), and byJones and Brooks' (16.3 and 21.6 MeV). The ener-gies of 16.9 and 21.1 MeV, chosen for the presentwork, represent maxima in the P'e polarizationsource intensity parameter for the 'H(d, n)He re-action near 5.35 MeV as reported by Smith andThornton. '

A diagram of the experimental apparatus is pre-sented in Fig. 1. Tritium was contained in a 3-em-long gas cell" at 1.5 atm of absolute pressure.Entrance windows were fabricated from 5- p.m-thiek molybdenum foil. The gas cell was bombard-ed by a 5.35-MeV deuteron beam produced by theUniversity of Vi,rginia 5.5-MV CN Van de Graaffaccelerator. Neutrons of 21.1 and 16.9 MeV wereproduced at 8, =30 and 80'(lab), respectively.Smith and Thornton' have measured the correspond-ing polarizations to be 0.35 and 0.50.

The direction of the spin was rotated by a super-condueting solenoid magnet" through +90' to be nor-mal to the m-p scattering plane. The solenoid was20-em long, with an inside diameter of 4.15 emand an outside diameter of &.41 cm. The maxi-mum central field of 42 kQ was obtained with acurrent of 47.6 A. The neutrons passed througha 4-cm-diam cylindrical vacuum path centered onthe solenoid axis. The sign of the neutron preces-sion angle was changed by rotating the solenoidDewar through 1&0' about a vertical axis throughits center {the two solenoid positions are denotedas CW and CCW}.

The scatterer was a cylindrical NE213 liquidscintillator (referred to as CD for center detector)3.5 cm in diameter and 5 cm in length in which re-coil protons were detected. Neutrons scattered upor down through +8, were detected by two NE102rectangular plastic scintiilators (referred to asside detectors SD, and SD,) whose dimensions were15 by 5 by 7.5 cm. The neutrons entered the 15-by-5-cm side of the detectors with 68, being alongthe 5-cm side. The gas-cell-to-CD distance was75 em for all measurements. The CD and SD dis-tance was 33 cm for 8, =30, 40, and 70 (lab}, and50 cm for 8, = 20 (lab). The side flight path waslengthened at 20' to minimize the kinematic effectsof finite geometry .

NEUTRON-PROTON POLARIZATION MEASUREMENTS. . . 925

Electronics for the experiment consistent of afast-slow n-y discrimination circuit for the CDand a neutron time-of-flight system (TOF) betweeneach SD and the CD. Both the digitized TQF signaland the CD pulse height were written on magnetictape with a 1024 channel analog-to-digital conver-sion gain by a ND512 buffer tape system. Incomingdata were continuously monitored on a ND3300 mul-tichannel analyzer in two 32 by 64 channel arrays.

Typical beam currents were 4 pA. The solenoidwas rotated after every 3 X 10 ' C of integrateddeuteron beam charge in order to average beamfluctuations and electronic drifts over both CW andCCW runs.

Two tests of the apparatus were made to insurelong term stability and to insure that the photo-multiplier tubes were sufficiently well shieldedfrom fringe magnetic fields. First, a "Na sourcewas positioned between SD, and SD„singles countrates were scaled with the solenoid in both its CWand CCW positions, and the asymmetry was calcu-lated. The resultant asymmetry was zero to within0.001 over a period of several hours. Secondly,the asymmetry of n-P scattering was measuredusing neutrons from the 'H(d, n)'He reaction with8, =0 and 8, =40 where the neutron polarization

The experimental. data at each angle and solenoidposition and for each detector were summed intotwo-dimensional spectra of CD pulse height vs SDTOF. A low-energy tail apparent on the linearpeak was identified as a background extending un-der the peak. Its magnitude was estimated by fit-ting a straight line to the backgrounds in each ofthe individual SD TOF spectra. By integratingboth the background and the peak, a CD pulseheight spectrum and a corresponding backgroundspectrum were generated for each two-dimensionalspectrum. The measured asymmetry e was cor-rected to obtain the "real" asymmetry e, as fol-lows:

e, =e (1+f),(he), '=[Be (l+f)j'+ [e (hf)j',

(l)

(2)

where f is the ratio of the background to realcounts, and Ae, . and Ae are the correspondinguncertainties in the asymmetries. The assump-tion has been made that the background is unpo-larized.

is zero. This resulted in an asymmetry of 0.0004a 0.0010.

III. DATA REDUCTION

g—Pb

LHe

SOI

GI S OUT F PLANEl SOLEOOIO I

DEUTERON TRITIUMBEAM |-ELL

WATER

P~r

SOp

FIG. 1. Schematic of the experimental apparatus.

MORRIS, O'MA L LE Y, MA Y, AND THORN TON

0,08I I

~ PRESENT RESULTS l69MeV

LCAPETOWN ()973) l6.3MeV

+AUCKLAND()972) 16. IMeV

+LASL (I97() I 6 BMeV

0.08

0.06—

I I

+ PRESENT RESULTS 2l. l MeV

Q CAPE TOWN (l97 Ã

0 Oq YALE ~—&»/'

0.04

0.02 0.02

0.00 0.00

-0.02 I

30I

60 908c m(deg)

(20 I50 )80-0.02

30I I

60 908c ~(deg)

I20 I50

FIG. 2. e-P polarization angular distribution at 16.9MeV. The data sholem are those of LASL (Ref. 3), Auck-land (Ref. 7), and Capetown (Ref. 8).

This procedure for determining the backgroundcontribution can be justified if one considers itsprobable origins. These include inseattering orlow-energy neutrons from the source, multipleneutron scattering in the CD, and neutron-inducedreactions with carbon in the CD. The possible con-tributory n-"C reactions have been discussed inRef. 3; these are listed below:

(1) 12g(n nl y)imp

(2) "C(n, n')3a,

(3) 12C(n P)12' 1RC + P

(4) "Q(n, 2n)"Q.

The contribution from reaction (1) (already shownto be small') is further reduced by n ydiscrimi-na-tion in the CD and by 2 cm of lead placed in theside flight path. Reaction (2) does not generateenough light in the CD to be detected. The leadprevents P particles from reaction (3) being de-tected The hi.gh threshold of reaction (4) preventsthose neutrons from being detected. Pulsed beamexperiments indicated that low-energy neutronscould only account for a very small fraction of thetotal background. Consequently, we believe the

TABLE I. e-p polarization at 21.1 MeV.

FIG. 3. n-P polarization angular distribution at 21.1MeV. References are the same as those in Fig. 2.Here the results of Ref. 6 and of J. J. Malanify, 'P. J.Bendt, T. R. Roberts, and J. E. Simmons, Phys. Rev.Lett. 17, 481 (1966) as summarized by Ref. 3 are in-cluded in the LASL data.

preponderant source of background is multiplescattering from carbon and hydrogen in the CD.One would expect this background to exhibit asmooth TOF distribution extending under the TOFpeak. From the TOF spectra it appeared that astraight liny was a reasonable approximation tothis distribution. Estimates obtained for f in thisway are believed to be accurate to within 20%.

Although n-"C elastic scattering exhibits largepolarizations in this energy region, "the TOF andlinear peak tails showed no significant polariza-tions. This can be attributed to the averaging ef-fects of multiple scattering. Consequently thebackground polarization has been assumed to bezero.

IV. RESULTS AND DISCUSSION

The results are presented in Tables I and D andare compared with previous measurements inFigs. 2 and 3. It should be noted that the errorsreported in Table I and II are only statistical anddo not include absolute errors introduced by un-certainties in beam polarization. These are esti-mated to be 6% at 16.9 MeV and 3% at 21.1 MeV.The work of Smith and Thornton' and that of Mutch-

TABLE II. n-p polarization at 16.9 MeV.

406080

100120140

0.0166+ 0.00170.0213+0.00120.0227+ 0.00180.0177+0.00370.0086 + 0.00500.0029+ 0.0012

1.121.081.041.071.101.10

0.0373 + 0.00380.0460 + 0.00270.0472 + 0.00370.0380+ 0.00800.0190+0.0110.0065 + 0.0027

406080

140

-0.0105+ 0.0018-0.0137+0.0014-0.0129+ 0.0014-0.0010+ 0.0015

1.111.051.051.10

0.0234+ 0.00800.0288+ 0.00290.0273+ 0.00300.0021+ 0.0034

NEUTRON-PROTON POLARIZATION MEASUREMENTS. . . 927

TABLE IG. Comparison of A and 8 fsee Eq. (3)l from fits to the present polarization datacompared to phase-shift predictions.

16.9 MeVA (fm) B (fm)

21.1 MeVA(f ) B (fm2)

(Y-IV)pp+„p n-PP-P

LRL-XPresent results

0.1600.1500.1330.101+ 0.004

0.12

0.050.10+ 0.01

0.1580.1440.1560.136+ 0.005

0.13

0.060.12+ 0.01

ler, Broste, and Simmons" has been used to ob-tain these error estimates.

At the low energy the present results agree withthose reported by Garrett et al. and with the re-sults of the 90' measurement of Mutchler and Sim-mons. At 21.1 MeV the present data agree excel-lently with that of the Los Alamos Scientific Lab-oratory (LASL) group when the LASL results at23.1 MeV are appropriately scaled for the energydifference. However, there is noticeable disagree-ment with the results of Jones and Brooks at bothenergies in both magnitude and shape.

Results of a comparison of the present resultswith the LRL-X and YALE-IV analyses are simi-lar, but more conclusive, than those of Mutchlerand Simmons. ' The data agree in magnitude withthe LRL analysis but in shape with the YALE anal-ysis.

For L „=2 the polarization data combined withthe cross section data can be fitted with the func-tion':

P(8)o(8) =A sin(8) + Bsin(8} cos(8),

where o(8} can be approximated using Gammel'sformula"

o(8}=4

(1+bcos'8)/(1+-,' b)

b = 2(E„b/90) ' .Here e~ is the total n-P cross section, "and E»is the laboratory neutron energy. Then for A and

Bwe have

2

A = 0' 126+4K2 Ls

Results of this fit along with phase-shift predic-tion are given in Table III.

The results given in Table III indicate a lowervalue for A than is given by either set of phaseshifts, especially at 16.9 MeV. The low value ofA at the lower energy seems to be due to the pointat 140 (c.m. ). The results at 21.1 MeV agree withthose predicted by the (Y-IV}»,„~ P-P scatteringparameters better than with the n-p parameters,although no such conclusion can be drawn at 16.9MeV.

It is hoped that including our data in a multiener-gy phase-shift search will help clarify the impor-tance of charge splitting in the triplet I'-wave state.

ACKNOWLEDGMENTS

The authors would like to acknowledge W. Dean,R. Fogel, R. Karlowicz, and D. Gustafson fortheir valuable help in operating the Van de Graaff;R. A. Amdt for useful discussion and for providinghis phase-shift observable program; and the Uni-versity of Virginia Division of Academic Comput-ing for providing computing time. We also appre-ciate the excellent work done by D. Hills in main-taining the Van de Graaff facility.

*Work supported in part by the University of VirginiaCenter for Advanced Studies (NSF), the UniversityResearch Policy Council, and the Research Corpora-tion.

/Present address: MP-10, Los Alamos Scientific Lab-oratory, Los Alamos, New Mexico.

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MORRIS, O'MALI. EY, MAY, ANl3 THRGNTON

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~~Magnet obtained from American Magnetics, Inc. , OakRidge, Tennessee.

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