Energy and angular dependence of the left-right asymmetry of D—D neutrons scattered by carbon

Nuclear Physws 15 (1960) 4 5 2 - - 4 6 3 , (~) North-Holland Pubhsh,ng Co, Amsterdam Not to be reproduced b y p h o t o p n n t or Imcrofflm without written permtsslon from the pubhsher

E N E R G Y A N D A N G U L A R D E P E N D E N C E OF T H E L E F T - R I G H T A S Y M M E T R Y OF D - - D N E U T R O N S S C A T T E R E D BY C A R B O N P

S DUBBELDAMt,

C C JONKER

and H

J. B O E R S M A

Natuurkund*g Laboratorzum der Vr*je Umvers,te,t, Amsterdam R e c e i v e d 2 D e c e m b e r 1959 T h e l e f t - r i g h t a s y m m e t r y in t h e i n t e n s i t y of n e u t r o n s f r o m t h e D - - D reaction, s c a t t e r e d b y carbon, w a s m e a s u r e d for d e u t e r o n energies b e t w e e n 300 a n d 500 keV, a t t w o a n g l e s ~Ilab = 50° a n d 22 ° 30', w~th a gold " d r i v e - r e " t a r g e t T h e p o l a r i z a t i o n v e c t o r w a s t u r n e d b y t h e m a g n e t m field of a solenoid, a n d t h e d e t e c t o r s r e m a i n e d xn a f i x e d positron F r o m t h e c o m p a r m o n of t h e c o m p u t e d e n e r g y a v e r a g e of t h e effect w i t h t h e e x p e r i m e n t a l v a l u e s it follows t h a t t h e a n a l y s i n g p r o p e r t i e s of C 12 d e r i v e d f r o m t h e p h a s e a n a l y s i s of Meier et al ') give a m u c h b e t t e r a g r e e m e n t t h a n t h o s e a c c o r d i n g to Wills et al 18) T h e e x p e r i m e n t s could n o t decide u p o n t h e possible e x i s t e n c e of a sin 401 d e p e n d e n c e of t h e differential p o l a r i z a t i o n of t h e D - - D n e u t r o n s F r o m t h e e n e r g y d e p e n d e n c e of t h e differential p o l a r i z a t i o n t a k e n f r o m o t h e r a u t h o r s it follows t h a t , w i t h i n t h e l i m i t s of t h e d e s c r i p t i o n of t h e D - - D r e a c t i o n b y B e l d u k , P r u e t t a n d K o n o p m s k l , i n d e e d o t h e r n o n - c e n t r a l forces b e s i d e s t h e t e n s o r force a r e p r e s e n t in t h e n u c l e o n - n u c l e o n i n t e r a c t i o n

Abstract:

1. Introduction The polarization of the neutrons originating from the D - - D reaction can be measured b y means of the left-right asymmetry in the intensity that results when one scatters the neutron b y a spin-0 nucleide, usually He 4 or C12 1-13). The relative left-right a s y m m e t r y R is defined as N(~)--N(180°+~) • R(~0) = N(~)WN(180°+~0) '

(1)

N(~0) and N(180°+~0) are the numbers of neutrons counted b y two detectors with equal efficiencies in diametral positions; ~ is the azimuth, or the angle between reaction and scattering plane. In the ideal case of a mono-energetic source, point scatterer, and point detectors the following formula applies, according to the theorem of Wolfenstein. Rth(9 ) =

P1P~cos 9;

(2)

P1 is the magnitude of the polarization of the initial beam of neutrons, which is perpendicular to the reaction plane; P2 is the magnitude of the polarization that would result if an unpolarized beam of neutrons were scattered b y the same t Present address

Phys

Dept

U n of W i s c o n s i n , Madison, W i s c o n s i n 452

ENERGY AND ANGULAR DEPENDENCE

453

nucleide; it is perpendicular to the scattering plane. P1 is a function of the deuteron energy E D and the angle 01 between deuteron and neutron beam, P~ is a property of the scattering material and depends on the neutron energy E n and the scattering angle 02. In general one measures the asymmetry by rotating the neutron detector from one position to the diametral one. We have shown 3) that it is advantageous to turn instead the polarization vector by means of a solenoid. The quoted reference gives the details of our method. With our method we measured the left-right asymmetry at several energies E D t h a t are covered by our neutron generator. We used again a gold "drive-in" target; 02 lsb was 42 °. We performed these measurements at two angles 01 mb ----50 ° and 22030 ' . These angles are such, t h a t the influence of the third term in the formula of Fierz 14) for the differential polarization t of D - - D neutrons I I ( E D , 01) = al(ED)a sin 201+~v/a0a20¢ sin 201+a2(ED) fl sin 201(3 cos201-- 1)

would be most conspicuous (a o, a 1, a 2 are the approach cross sections defined in ref. 15)). In our energy region this term is zero for about 01lab ----50°, and maximum for about 01lab = 22° 30'. This term is important, because its existence would prove, within the limits of the description of the D - - D reaction by Beiduk, Pruett and Konopinski 15), t h a t other non-central forces besides the tensor force are present in the nucleon-nucleon interaction. 2. M e a s u r e m e n t s

and Corrections

The values for R (1 e R(~0) for ~0 = 0 ° or 180 °) from the measurements are given in tables 1 and 2 for 01 lab = 50° and 22°30 ', respectively, at several values of E D, for the two detectors separately, and the average R for both detectors. The values of R have been corrected for the influence of the magnetic field of the solenoid on the photomultipliers (see ref.3)). The values of R are shown in figures 1 and 2. The curves in these figures are discussed m section 3. As to the geometrical corrections, only the finite size of the scatterer and detectors had to be taken into account. We computed their influence following the method of McCormac, Steuer, Bond and Hereford 7). The factor with which the measured R should be multiplied is 1.03. We neglected this correction, in view of the obtained accuracy Because the size of our carbon scatterer is of the same order of magnitude (4 cm) as the scattering mean free path, plural scattenng effects will occur I t is very difficult to compute this correction, because the differential scattering t The differential polarization r/is

defined by

// PI - a(O~)" where a(~l) m the chfferentlal cross section

(3)

454

P S. DUBBELDAM, C C- JONICER AND H. J. BOERSMA

cross section depends also on the azimuth, due to the polarization, and the P~ in (2) for carbon varies strongly with the neutron energy (see below), We tried to find an upper limit for the correction along the following lines. TABLE 1 E x ) e r l m e n t a l v a l u e s of R as a f u n c t i o n of E v a t ~11,b = 50° R m % Upper detector

.g v

(keV)

R m % Lower detector

32 ~0S

3.3±11 19±09 25±11 26~10 34~13

--31±09 --37±06 --19±08 --29±06 --27~09

300 350 400 450 500

Weighed a v e r a g e of R

31 ~05 21 ~07 285~05 295~07

R;. % 4

__-

0

Meier Wills

I

I

I

I

I

100

200

300

400

SO0 E D (keVt

Fig

1 R as a f u n c t i o n of E o a t ~1 l,b = 50°. T h e c u r v e s a r e c o m p u t e d f r o m o t h e r e x p e r i m e n t a l data, taking into account the energy spread.

We computed the influence of the second scattering for unpolarized neutrons, following the method of Blok and Jonker le), taking into account the fact that our scatterer is finite in the direction perpendicular to the primary neutron beam. We determined the ratio of the number of once and twice scattered neutrons that reach the detector (I 1 and Is) to the theoretically important number of neutrons that are scattered once into the direction of the detector (Ith)" We found I 1 / I m = 77 % a n d I~/It~ = 12 %. If we suppose that theratio I s / I S is about the same as I 2 / I 1 etc., we can neglect the higher order scattering in view of our accuracy. Then the factor (I1-FI2)/It~ = 89 % gives the amount b y which the denominator of the definition (1) of R is too small in the measure-

E N E R G Y AND A N G U L A R D E P E N D E N C E

45~

ment. The contribution of the once scattered neutrons to the left-right difference in the numerator, divided b y the theoretical value of the numerator, is 77 %. We do not know the contribution of the twice scattered neutrons to the numerator, b u t it is not to be expected that it has the wrong sign, or is equal to TABLE 2 E x )erlmental values of R as a funciaon of E v a t 01 lsb = 22° 30' Ren % U p p e r detector

ED

(keV) 300 350 400 450 500

R m % L o w e r detector

--1.8±05 --091045 --23104 --15107 --24105

Weighed average of R

10±08 21105 27105 28107 16±07

15 ±04 14 t03 24 103 21 1 0 5 215104

rim% Meier

___ 3

Wdls

-

0

I

I

I

I

I

tO0

200

300

400

SO0 E0

( keY )

Fig 2 R as a functaon of Ev a t '~z lab = 22° 30' T h e curves are c o m p u t e d from o t h e r e x p e r i m e n t a l data, t a k i n g into a c c o u n t t h e energy spread

zero, because, if e.g. the first scattering is preferably to the left, this will also be valid for the second scattering, as the density of the second scattering centres is larger in the left half than in the right half of the scatterer, and the screening effect for the left (twice) scattered neutrons accordingly smaller. Therefore, if we exclude the possibility that the second scattering would give a left-right difference of the opposite sign, the upper limit for the dewatlon of the numerator is found if we put the contribution of the second scattering zero, (i.e. complete symmetry). Then it is possible to say that the correction factor for the measured left-right a s y m m e t r y will not be larger than ~~(~= 87 %). The actual correction factor will be probably closer to 1. Because the upper limit of this correction,

~56

P

S DUBBELDAM, C C JONKER AND H

J

BOERSMA

a difference of 13 %, is smaller than the accuracy obtained, we feel justified not to make the complicated computations t h a t are necessary to find a more accurate value for this correction.

3. Analysis and Discussion of the Results 3 1

ENERGY

SPREAD

OF THE

NEUTRONS

FROM

A THICK

TARGET

Because we used a thick target it is necessary to consider the influence of the energy spread on the measured left-right asymmetry, in order to compare our results with the published data. The spread in the neutron energy is small, relatively, but the polarization is a function of the deuteron energy, an~ this ranges from zero to the value of the energy of the deuterons in the beam. The shape of the neutron spectrum, as a function of the deuteron energy, is given by [ "~dE D -1

d.N (go) =

t,--&-) dex,

where pD(z) is the density of the deuterons in the metal, at a depth z; a(ED, 0x) is the differential reaction cross section, dE`,/dz is the energy loss of the deuterons in the metal per unit length; E`, and z are connected by the range-energy relation for deuterons in the metal in question; C is a constant. The density distribution p`, follows from the diffusion equation for the deuterons in the metal (Fiebiger 17)) J=-~

dz'

(6)

where J is the current of diffusing D-atoms and a the diffusion constant. The D-atoms are supposed to come from a plane source formed by the beam, with strength Q at a depth z = R(ED), where R(ED) is the range of the deuterons with energy ED in the metal. The solution of (6) in the asymptotic case of saturation (t --> oo), where J -- - Q , and for ,t is a constant, is

Q

PD = -- Z.

(7)

If one approximates a(ED, ,91) by a straight line, and takes dED/dg to be constant, the resulting spectrum has the form of a parabola, with two zeros at E D = 0 and E D -- E D max, the energy of the deuterons in the beam. Computing the spectrum numerically with the experimental data for the mentioned factors we found a curve t h a t indeed differed little from a parabola. Therefore we approximated the spectrum by a parabola in the averaging procedure described below.

ENERGY AND ANGULAR DEPENDENCE

457

In the solution of (6) we supposed t h a t ~ is constant. Becaus~ a increases strongly with the temperature this is true only if the temperature does not change in time and with the place in the metal. If ~ were a constant the neutron yield should increase quadratically with the ion current at constant cross section of the beam. However, because of strong local heating of the metal, the temperature, and therefore x, will increase with the ion current. Thus it is understandable t h a t in the experiment described in ref. 3), where the beam was well focussed, we found a maximum in the yield-current dependence instead of a quadratic increase. Another effect occurs at high values of the beam current. Because the developed heat is mainly transported through the bulk of the target metal, a temperature gradient will exist along the track of the deuterons, and will be a function of z. Then the density PD will not be a linear function of z, but it will be appreciably smaller near the surface of the metal. Thus there will be less neutrons t h a t are produced by deuterons with high energy. We shall then still approximate the spectrum by a parabola, but this has now its second zero at a value E , * t h a t is lower than the energy E Dmax of the deuterons m the beam. We will call the place of the top of the parabola the effective deuteron energy E D etI ~

½ED*.

This phenomenon of the shift in effective energy, due to the temperature conditions in the metal (in our case gold) can be used for the explanation of a discrepancy between the result for R given in ref. 8), viz. R ---- (4.6~0.2) % at E D ~ 450 keV, with the present results, which give R ---- (2.85~0 5) °/o at E D = 450 keV. Apparently the strong heating of the gold in the former experiment, due to the well-focussed beam, caused a temperature gradient, and consequently a shift in the spectrum to lower energies. The solid curve in fig 1, described below, shows that a shift of the second zero of the parabola, ED*, from 450 keV to 250 keV, and thus a shift in the effective energy from 225 keV to 125 keV could give a value for R that is very near to the value measured in the former experiment. These conchtions did not exist in the present experiment. For now the focussing of the beam was bad, due to a leak in the vacuum system, as could be seen by watching the beam when shooting with protons. As a consequence the local heating of the target was much less. This showed up in the fact that no maximum was reached in the yield-current dependence, and that one needed a finite time to reach the maximum in the yield when shooting on a fresh gold surface (the time needed to reach saturation of deuterons in a metal is inversely proportional to the diffusion constant). The agreement between the sohd curves and the experimental points in figs. 1 and 2 indicate that now no apparent shift of the effective energy occurs. 3 2 THE ENERGY AVERAGE OF THE LEFT-RIGHT ASYMMETRY

The curves in figs 1 and 2 are computed from published values for P1 and P~

458

P

S

D U B B E L D A M , C. C

J O N K E R AND H

J

BOERSMA

by averaging R over the energy spectrum in the following way. As the expression for the differential cross section for the (n, C zs) scattering has the form =

o(Od O + P 1 P , . cos

(5)

we could write for the average R

fo V~u ao(En, v~)pl(Ev,

Oz)p~(E. ' 02 ) sin ~ a1 N(ED) d E v

R =

"VEn

(6)

f : v max ao (02)N (ED)dE D We derived the different factors as follows, for the case 0 t ,~b = 50° o0(E., 03) P2 in

°/o 80

AlPl

~0

/\

_ 40

..,..

I\

//~

_80

I 26

I

I 30

I

I 31. En

(MeV)

F i g 3. P t as a functaon of E n for C 'z, c o m p u t e d f r o m t h e coefficients of Meier 0), a n d t a k e n f r o m t h e figure of Wills is), h i lab = 45°.

from the graphs for the n - - C lz differential cross section of Meier s); Px (ED, 0l) from the low energy polarization experiments of Pasma 10), approximated by PI( in %) = ~/0.18ED(keV); P2 from the work of Meier 9) and of Wills is), as shown in fig. 3. Thus we find two values for R to be compared with our experiments. N (ED) is approximated by a parabola, with its two zeros at E D = 0 and at the energy E D mix of the deuterons in the beam. The current through the solenoid, I, was so near to the optimum value, that the sine is practically 1. To obtain the values for R in the case that 01 lab = 22° 30', we have to know P1 (22o 30'). We suppose, to start with, t h a t the differential polarization can be described by a sin 20 angular dependence. Then we can compute P1(22°30 ')

E N E R G Y AND A N G U L A R D E P E N D E N C E

459

from the Pl(50 °) that is taken from experimental data, as discussed before, according to sin 201 a(ED, 0'1) PI(ED, 01 lib = 22°30') ~ PI(ED, 01 l~b -~- 50°); (7) sin 20' t a(ED, 0(')

O't and Ol" are the c.m.s, angles that correspond to the lab-angles 50 ° and 22°30 ', respectively. We neglect here the transformation of the solid angle. We took the values of 6(ED, 0) from the experiments of Chagnon and Owen 19) combined with the experiments of Fuller, Dance and Ralph s0). With this P1(22°30 ') the same averaging method was applied. The results are given as solid curves (Meier) and as dashed curves (Wills) in figs. 1 and 2 One sees that the positive peaks in fig. 3 result in dips in the R versus E D curves. The dip shifts to a lower value for E D at Ot lab = 22° 30', because the same neutron energy is reached for a lower value of E D. The curves, computed with the Ps of Meier are in much better agreement with the experiments than those computed with the P~ of Wills. Because the curves in figs. 1 and 2 that were computed on the base of a sin 20 dependence of the differential polarization give a very satisfactory agreement of the computed R with the experimental points, there is no need within the accuracy of the experiment of adding a term with sin 40 dependence to the expression of the differential polarization like in Fierz's formula (4). One cannot conclude from this that this term does not exist, because in the energy region in question D-waves are not yet very important, which is necessary for the occurrence of this term. There is one paper in which the authors reach the conclusion that the phase analysis of Wills is preferable to that of Meier, contrary to our opinion: Steuer et al. 13) report a value for the left-right asymmetry of --(7.34-0.7) %, at E D = 750 keV and ~s ~b = 135°. This is the mean value of the result of an experiment with a thick heavy ice target, using a coincidence technique, and of a result obtained in the same laboratory, with a "drive-in" target One wonders, in the first place, whether it is allowed to average over two values obtained for two spectra that are probably very different. For, as we have seen, the neutron spectrum from a metal target as a function of the deuteron energy is approximately a parabola, because the density of the deuterons increases linearly with the depth from a value zero at the surface. In the same approximarion we could represent the spectrum from a heavy ice target b y a straight line passing through the origin, because the density of the deuterons is constant Steuer says that t93 computed for their energy distribution gives a value P1(53 °) = --(8.44-0.8) %, when using the phase shifts of Meier, and a value Pi(53 °) = --(14 54-1.4) ~/o, when using the phase shifts of W ~ s . The latter value then should be in better agreement with the results of Levintov et al. 6) and Pasma 10). We found the value for P1 from the curve drawn b y Pasma through his and others' results; at the energy in question (E D = 750 keV) it is

460

P

$. D U B B E L D A M ,

C. C

JONKER

AND

H.

J

BOERSMA

12 %. We have computed the value for the average left-right asymmetry for a linear neutron spectrum at 0, lab = 135° and E D = 750 keV with the method described before. We then find R = 1.2 %, when using the P l of Wills and /~ = 5.2 % when using the P2 of Meier. Though neither value falls within the limits of Steuer's value for R, the last one is much nearer to it, and therefore it seems that also on the basis of this experiment the phase analysis of Meier is preferable. .91n% 10

J _S

_10

I 7

I 30

I 33

I 36 E n ( MeV )

F~g 4 Experimental values for R, according to Baumgartner, compared w~th the curve computed on the basis of Meler's phase analysis

It seems that also the results of Baumgartner and Huber 1) point towards the positive peak in the P~(En) curves. To check this we transformed the leftright ratio into the R and computed P,. for 03 lab = 78° from the coefficients of Meier. (The corresponding values of Wills could not be computed, because he does not give the relevant coefficients.) We multiplied this Pe with the corresponding P1 to obtain the left-right asymmetry for a thin target that can be directly compared with the experimental points. This is shown in fig. 4. We see that indeed the computed curve represents very well the general behaviour of the experimental points. Only the energy scale for the computed curve is about 90 keV lower than the scale for the experimental points. There are several possible causes for this, as well on the experimental side, as in the

461

ENERGY AND ANGULAR DEPENDENCE

computation of P2, which especially at angles near 90 ° is very sensitive to details in the phase shifts. 4.

The

Differential

Polarization

of

D--D

Neutrons

It was not possible to conclude from our experiments to the existence of the third term in the formula of Fierz for the differential polarization of D - - D neutrons (4). We tried to find this term from the angular dependence of H. This term has also an influence on the energy dependence of H because the approach cross sections a i and a s behave quite differently as a function of the deuteron energy The energy dependence of the second term is given by A/aoa 2 and this expression has about the same energy dependence as a 1 at low energies. Therefore we c o m p u t e d / / f r o m the published measurements of P1 made with a thin target. The differential polarization follows from P i according to the formula (3). TABLE 3 Values of P i , cr(~l) a n d / / =

Reference

Plff(~l) as a f u n c t i o n of the d e u t e r o n energy for different angles, according to several a u t h o r s

~1 l a b

(keY)

(%)

a(01) (mb)

-// 0794-002 1284-008 1614-009 1834-011

-- P1

L e v i n t o v et al 6)

49 °

900 1200 1500 1800

1374-04 1384-14 1584-09 1684-10

5 76 93 102 109

P a s m a x0)

47 °

200 300 350 400 450 500

584-25 904-23 82~24 87±11 95±13 91~08

1 87 2 78 318 3 55 3 90 4 22

0 0 0 0 0 0

Meier et al o)

49 °

600

106+1 1

457

0484-005

B a u m g a r t n e r at al i) and B u d d e et al 2)

45 °

600

18 04-7 0

4 99

0 904-0 35

114-0 254-0 264-0 314-0 374-0 384-0

05 06 08 04 05 03

In table 3 we give the values of P1 for thin target measurements and add the values of a (01) obtained by combining the experiments of Chagnon and Owen 19) with those of Fuller, Dance and Ralph 20), and the values o f / / f o u n d by multiplying P i and a(01). It is true t h a t the values o f / ) 1 are not all measured at precisely the same angle v~i but in our opinion this will not influence our conclusions We wish to remark, however, that the angle is very near to the value

462

P. S. DUBBELDAM, C. C JONKER AND H. J. BOERSMA

for which (3 cosl~x - 1) is zero, and thus not favourable for the evidence of this term. The values o f / 7 as a function of E D are shown in fig. 5. The solid curve in this figure is proportional to the first term of Fierz' formula, ala sin 2~1. Above E D ---- 1 MeV the curve is dashed, because the analysis of Fierz is only valid to about this value. One sees t h a t up to E v about 500 keV there is good --17 mb

18

Pasma l evmtov

06 Meier Budde

O6

F i g 5. V a l u e s o f / / a s

a function of

Ev. The

I

I

12

18

I 24 E'D ( MeV )

s o l i d c u r v e r e p r e s e n t s o"t a s m 2 ~ 1 f i t t e d t o e x p e r i m e n t a l points.

agreement with the experimental points. At higher energies it is impossible to make a fit to the points without introducing the next approach cross section a s. This can only supply a new term to the expression for H if other than tensor forces are present. Therefore we conclude that, within the limits of B P K ' s description of the D---D reaction, it is necessary to assume that other noncentral forces besides the tensor force are present in the nucleon-nucleon interaction. If one allows t h a t the curve has an inflection point above 1 MeV, the points at higher energy cannot influence the curve at energies up to 1 MeV, where the analysis of Fierz is still valid. Then our conclusion is based only on the point of Levintov eta/. at ED ---- 900 keV. It will be useful to have more experimental d a t a in this energy region. I t is better, however, to have a series of values o f / 7 at ~x ~b = 22° 30' where the relevant term has a maxinmm. A more detailed description of this research is given in a thesis ~1). We wish to thank S. S. Klein for his help in performing the measurements and computa-

ENSRGY AND ANGULAR DEPENDENCE

463

tions, one of us (H. J. B . ) is indebted to the Foundation of Fundamental Research of Matter (Stichting F.O.M.) for a s t u d y grant. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) :) ,4) 15) 16) 17) 18) 19) 20) 21)

E. B a u m g a r t n e r a n d P Huber, Helv Phys Acta 26 (1953) 545 R Budde a n d P Huber, Helv. Phys. Acta 28 (1955) 49 P S. Dubbeldam, C C J o n k e r and F. J Heemskerk, Nuclear I n s t r u m e n t s 4 (1959) 234 P. P Kane, Nuclear Physics 10 (1959) 429 I I Levmtov, A V. Mdler a n d V. N Shamshev, Nuclear Physics 3 (1957) 221 I. I Levmtov, A V Miller, E. Z Tarumov, and V. N Shamshev, Nuclear Physics 3 (1957) 237 B M McCormac, M F Steuer, C D Bond and F L. Hereford, Phys Rev. 104 (1956)718 B. M McCormac, M F Steuer, C. D Bond and F. L Hereford, Phys Rev. 108 (1957) 116 R W Meier, P Scherrer and G. Trumpy, Helv. Phys Acta 27 (I954) 577 P J Pasma, Thesis, Gronmgen, 1958 P J Pasma, Nuclear Physics 6 (I958) 141 II. Rlcamo, Nuovo C1mento 10 (1953) 1607 M F Steuer, W P Bucher and F. L Hereford, Comptes Rendus du Congr~s International de Physique Nucldalre, Dunod, Paris (1958) 545 M. Flerz, Helv. Phys. Acta 25 (1952) 629 F. M Belduk, J 1~ P r u e t t and E. J. Konopinskl, Phys. Rev 77 (1950) 622 J Blok and C. C Jonker, Physlca 18 (1952) 809 K. Fleblger, Z. angew Phys 9 (1957) 213 J E Wills, J K Bait, H. O. Cohn and H B. Wdlard, Phys. Rev 109 (1958) 891 P. R Chagnon a n d G. E Owen, Phys. Rev 101 (1956) 1798 J. C. Fuller, W E Dance and D. C. Ralph, Phys. Rev. 108 (1957) 91 P. S. Dubbeldam, Measurement of the polarization of D - - D neutrons b y a solenmd, Thes~s Free Umverslty, Amsterdam, 1959