Comparison of the vector analysing powers of the mirror reactions 3H(d ,n)4He and 3He(d , p)4He

I

2*B

I

Nuclear Physics A173 (1971) 216-224;

@ North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

COMPARISON

OF THE VECTOR

OF THE MIRROR REACTIONS D. HILSCHER

ANALYSING

3H(& n)‘He

t, P. A. QUIN

POWERS

AND 3He(& p)4He

and J. C. DAVIS

tt

Uniuersity of Wisconsin, Madison, Wisconsin ttt Received 17 June 1971 Abstract: The vector analysing power of the reaction 3H(a, n)4He has been measured for laboratory angles between 25” and 155” at deuteron energies of 6 MeV and 10 MeV. For the mirror reaction sHe(& p)4He, measurements were performed at the same deuteron energies for laboratory angles between 105” and 150”. For the angles between 105” and 150” the average ratio of the vector analysing power of 3He(z, p)4He to that of sH(z, n)4He is 1.016&-0.015 at 6 MeV and 1.035f0.020 at 10 MeV. At all angles the measured vector analysing power of the “H(>, n)4He reaction agrees to within +0.025 with results previously obtained at this laboratory for the 3He(G, p)4He reaction. E

NUCLEAR

REACTIONS

3H(polarized

d, n)4He,

3He(polarized

d, p)4He,

E = 6, 10 MeV; measured vector analyzing power iTI (Ed; 6). Enriched targets.

1. Introduction

The assumption of charge symmetry implies that the observables of mirror reactions such as ‘H(d, P)~H and ‘H(d, n)3He and 3He(d, p)4He and 3H(d, n)4He should be identical if electromagnetic effects are small enough. The differential cross sections [refs. ‘-“)I for these pairs of reactions are nearly identical for deuteron bombarding energies above 6 MeV. It has been pointed out 4), however, that although the energy and angular dependence of the nucleon polarizations are quite similar for these mirror reactions, the proton polarizations are larger than the neutron polarizations. The most recent measurements of neutron polarizations for the ‘H(d, i)3He reaction ‘) and 3H(d, G)4He reaction 6*‘) indicate tha t the factor by which the proton polarization exceeds the neutron polarization is 1.15 to 1.20. It has been suggested that the n-u analysing power above 20 MeV that is needed in determining the neutron polarizations might be in error and cause the difference. Use of the reaction ‘H(t, n)4He at back angles instead of 3H(d, n)4He at forward angles in the work of ref. “) reduces the neutron energy to 11 MeV and thus insures that the uncertainty in the analysing power cannot cause the observed differences in the nucleon polarizations from the 3He(d, G)4He and 3H(d, G)3He reactions. In the present experiment the vector analysing powers of the “H(& n)4He and 3He(& p)4He reactions were measured in order to investigate whether a difference t NATO Fellow on leave from Hahn Meitner Institut, Berlin, Germany. tt Present address: Lawrence Radiation Laboratory, Livermore, California. ttt Work supported in part by the US Atomic Energy Commission. 216

3H(z,

217

n), 3He(z, p)

occurs for these mirror reactions similar to that observed in the nucleon polarizations. Although the vector analysing power iTI, and the nucleon polarization PN are different observables, the experimental data “) for the 3He(& p)4He reaction indicate that the relationship iT, 1 x - &I3 Pp holds for deuteron bombarding energies above 6 MeV. The difference seen in nucleon polarizations from mirror reactions thus might also be observed in the vector analysing powers. If a polarized beam is available, the vector analysing power can be measured more easily and accurately than the nucleon polarization since the measurement of the analysing power requires the observation of only one nuclear interaction while nucleon polarization measurements require the observation of two nuclear interactions. Bernstein et al. ‘) have measured the 2H(& P)~H and ‘H(& n)3He vector analysing powers. These measurements were not of sufficient accuracy to determine whether the 15-20 % difference observed in the nucleon polarizations is also seen in the vector analysing powers. The purpose of the present experiment was to measure the vector analysing powers of 3He(d, p)4He and 3H(d, n)4He to sufficient accuracy that such a comparison of the analysing powers should be meaningful. Nucleon polarizations from these two reactions are usually compared at the same lab angle and deuteron energy despite the difference in Q-values for the reactions. We chose to compare the vector analysing powers at the same laboratory deuteron energy to be consistent with the polarization comparisons. 2. Experimental method 2.1.

POLARIZED

BEAM

Measurements were performed at 6 MeV and 10 MeV with the vector-polarized deuteron beam from the Wisconsin Lamb-shift polarized ion source lo). Average target currents after acceleration through the EN tandem were between 20 nA and 60 nA. The vector polarization of the beam determined by d-a scattering was typically ’ x 0.5. The maximum possible vector polarization of a beam with zero tensor It11 components is 11) it,, = l/,/3. The beam polarization was continuously monitored during the measurements and varied less than +O.Ol over 48 h periods. For reactions of vector-polarized deuteron beams on unpolarized targets the cross section may be written as ~(0,4)

= o(0)[1+2it,,iT,,

COST],

(1)

where ~(0) is the reaction cross section for unpolarized beams, 6 and 4 are the polar and azimuthal angles, it, 1 is the beam vector polarization, and iT, 1 is the vector analysing power of the reaction. This simple expression holds only if the beam polarization has no tensor components. For this case the analysing power may be determined by measurements of the relative cross section for deuteron spin oriented along and opposite the direction defined by ki” x k,,,. If the measurements are performed with detectors placed symmetrically about the beam axis (4 = 0” and 180” for each angle

218

D. HILSCHER

er al.

0) the effects of detector acceptance and efficiency cancel in the calculations of the analysing powers. LEFT

MO”\IITOR 0,

\

t

\

LEFT \

\

\

\

\

\

\

’

\

/

t

collimator

1:’ / ‘/

/

,’ I /

//

RIGHT

Fig. 1. Experimental arrangement

DETECTOR

T,-gas

cell

Mo-foal

‘/

I’ /’

/’ d

RIGHT

DETECTOR IOcm

MONITQR

for measurement

of 3H(d, n)4He vector analysing power.

I

I

I T(d,n)*He

125 CHANNEL

Ed=10MeV

25

75

NUMBER

Fig. 2. Proton recoil spectrum at 55” and 125” and Ed = 10 MeV. The dashed curve shows the background. The arrow indicates the cut off channel used for calculating iT, 1.

2.2. THE 3H(z, n)4He EXPERIMENT

Fig. 1 shows the arrangement used to study the “H(& n)‘He reaction. The tritium gas cell was 2.2 cm long and contained 1.3 atm of 3Hz. A 2.5 pm MO foil served as the entrance window and a 0.5 mm Au foil stopped the beam at the end of the cell.

3H(& n), 3He(z, p)

219

The asymmetry in neutron yield was measured with two 2.5 cm diamx2.5 cm long NE 218 scintillators placed at equal angles on opposite sides of the beam axis. These scintillators were 40 cm from the gas cell and could be moved through laboratory angles between 25” and 155”. The angular acceptance of the detectors was less than 5”. The high-energy 3H(d, n)4He neutrons were well separated from the continuum of low-energy neutrons resulting from deuteron break-up and other background sources because of the large positive Q-value of the 3H(d, n)4He reaction. The pulse-height resolution of the scintillatom was better than 10 %. Fig. 2 shows proton recoil spectra obtained at two angles at 10 MeV deuteron energy.

SLITS

7\

\

D%Zm

\

RIGHT DETECTORS

\ ~~

Fig. 3. Experimental arrangement for 3He(< p)4He measurements and absolute beam polarization determinations.

Deuteron beam polarization was monitored during the 3H(& n)4He measurements by two NE 213 scintillation detectors placed at a laboratory angle of 135”. The vector analysing power of the 3H(& n)4He reaction is near a maximum at this angle. A large asymmetry in yield was observed even though the detector resolutions were inadequate to permit complete elimination of background neutrons. Pulse-shape gamma discrimination was used with all four detectors. 2.3. THE 3He(z, p)4He EXPERIMENT

The apparatus for measurement of the vector analysing power of the 3He(& p)4He reaction is shown schematically in fig. 3. A 1 mm x 3 mm slit pair placed 5 cm in front of the 3He gas cell collimated the deuteron beam. The gas cell was 1.9 cm in diameter and had entrance and exit windows of 2.5 pm Havar +. The cell was filled to 3 atm with 99.9 y0 pure 3He gas. Two pairs of 2 mm thick silicon surface-barrier detectors were placed symmetrically about the beam axis 14.6 cm from the target cell center. t Havar is a high tensile strength alloy made by Hamilton Watch Company, Lancaster, Pennsylvania.

220

D. HILSCHER

et al.

Limiting apertures placed between the detectors and the cell prevented deuteron scattering into the detectors from the foil windows. The angular acceptance of the detector system was 4”. The deuteron beam polarization was monitored by a 4He filled polarimeter I’) placed 60 cm beyond the target cell. To calibrate the polarimeter the 3He cell was replaced with an identical cell filled with 4He. Absolute values of the beam polarization determined from d-a scattering in good geometry were used to relate the observed yield asymmetry in the polarimeter to a known beam polarization. Calibration points were chosen from values of the vector analysing power of d-a scattering obtained previously at this laboratory 13). The 10 MeV calibration points was iT,, = 0.284+ at 0.013 at O,ab= 45”. The 6 MeV calibration points were iT,, = -0.294+0.010 6rab = 32.2” and iTI = -0.296+0.010 at 13,~~= 26.8”. Several times during the 3H(& n)4He measurements the beam was switched from the beam line with the 3Hz cell to the beam line with the scattering chamber to determine the absolute beam polarization from d-a scattering using the calibration points listed above. No differences greater than the statistical uncertainties of the measurements were observed. 2.4. PROCEDURE

The angular distributions of the vector analysing power of the 3H(& n)4He reaction were first measured to allow comparison with results for 3He(& p)4He previously obtained at this laboratory “). Determinations of the left-right yield asymmetry for alternate orientations of the deuteron beam polarization were performed and the vector analysing power was calculated from the asymmetries assuming that the beam had no tensor moment. Results of this work have been reported previously r4). A new analysis of the d-a scattering data similar to that described by ref. “) indicated that a tensor moment rzO = -0.06 was present in the polarized beam. The d-a tensor analysing powers were taken from ref. 16). The corrections required for the tensor moment z2,-, are discussed in the next section. To reduce the sensitivity of the comparison of the 3H(& n)4He and 3He(& p)4He vector analysing powers to errors in the beam polarization, the analysing powers were measured for both reactions for angles between 105” and 150” lab in the following manner. A run was taken on the 3He target, then the beam was switched to the 3Ht target and a run taken for the same deuteron spin direction. The beam spin was then reversed and another run taken on the ‘Hz target. Finally the beam was returned to the 3He target and the second run with reversed spin orientation taken to complete the analysing power measurement for 3He(& p)4He. The comparison of the vector analysing powers thus obtained is independent of the absolute determination of the beam polarization provided that fluctuations in the beam polarization are small in the time interval required for the measurements. For these measurements the beam tensor polarization z2,, determined from d-a scattering was less than 0.01.

3H(&

n), 3He(& p)

221

2.5. CORRECTIONS

The effect of background neutrons in the 3H(& n)4He measurements was determined in two ways. Neutrons from deuteron break-up and reactions on collimating apertures, entrance foil and beam stop were observed by taking runs with the 3Hz gas evacuated from the target cell. These background spectra are shown at two angles in fig. 2. The cut-off channel used in the data analysis is also indicated. This channel is the lowest channel for which the requirement holds that the calculated asymmetry remains the same within the statistical errors if a higher cut-off channel was used in the calculation. Above this bias the background with the cell evacuated was M 1 “/o of the foreground. As the background spectrum above the cut-off channel had the same shape as the spectrum with the cell filled, most of the neutrons probably resulted from reactions with tritium remaining in the cell or adhering to the entrance foil and beam stop. No correction was applied for this background. The background from room-scattered neutrons was measured by inserting a 30 cm Fe shadow bar between the target cell and each detector. Above the cut-off channel this background was less than l-2 y0 of the foreground. The measured asymmetries were corrected for this background assuming that there was no asymmetry in the room scattered background itself. For the “H(& n)4He measurements taken with a beam tensor moment of r2e = -0.06, corrections are necessary because this tensor moment caused errors in the analysis of the neutron yield asymmetries. These corrections had not been applied to the data of ref. 14). As the tensor analysing powers of the 3H(& n)4He reaction are not known, we assumed them to be the same as the 3He(d, p)4He tensor analysing powers determined by the ETH group at Zurich I’). The tensor term to be added to of the tensor eq. (1) is r5) -r20(3T20-t-~J3T22). F or some angles the combination analysing powers has values as large as unity so that corrections of as much as 6 y0 to the values of iT,, were required. Because relatively large adjustments were necessary for the 3H(d, n)4He measurements taken with a large beam tensor moment the uncertainties in these data are larger than those for the 3H(& n)4He and 3He(& p)4He data taken with small beam tensor moments. 3. Results Values of the vector analysing powers of the 3H(& n)4He and 3He(& p)4He reactions obtained in this work are shown in fig. 4. For comparison the Legendre polynomial fits to the data of Plattner and Keller “) for the 3He(& p)4He reaction are also shown. The uncertainty shown includes the statistical error, and uncertainties in background corrections, the beam polarization, and the correction for beam tensor moment contributions. The vector analysing power of the 3H(& n)4He reaction agrees with that of the 3He(& n)4He reaction to within kO.025 at all angles. Results of the simultaneous measurements of the vector analysing powers at back angles are also shown as the ratio of (iT, &, for the “He@, p)4He reaction to (iTI 1)”

D. HILSCHER

222

et al.

for the 3H(& n)4He reaction in fig. 5. The uncertainty of the ratio includes the statistical error and the uncertainty in background corrections only. At 10 MeV the ratio

‘A _

IO MeV

0.4 -

0.2 -

P.K. 1969

-

-

0 cm.

Fig. 4. Angular distribution of the vector analysing power iT 11 of the reactions 3H(d, n)4He (open symbols) and 3He(& p)‘He (filled circles) at 6 and 10 MeV. The solid lines are Legendre polynomial fits to the jHe(d, p)4He measurements of Plattner and Keller *).

8 LAB Fig. 5. Ratio r = (iT~l),/(iTll),

at 6 and 10 MeV. p and n refer to the reaction 3He(i, p)4He and 3H(z, n)4He respectively.

3H(& n), 3He(G, p)

223

increases slightly with increasing angle. This increase of the ratio might result from the increasing room-scattered background not properly corrected for in the 3H(& n) 4He data. This background was larger at 10 MeV than at 6 MeV. Averaging over all back angles we find the ratio of the “He@, p)4He to 3H(& n)4He vector analysing power to be 1.016+0.015 at 6 MeV and 1.035f0.020 at 10 MeV. When the same reactions are initiated by unpolarized deuterons the ratio of proton polarization to neutron polarization is found to be z 1.15 to 1.20 over a wide range of energies and angles. The difference observed in the comparisons of nucleon polarization and vector analysing powers from the 3H(d, n)4He and 3He(d, p)4He reactions may arise from electromagnetic effects and differing reaction Q-values and does not necessarily require a violation of charge symmetry. We have attempted to investigate whether such effects would influence the nucleon polarization more than the vector analysing power by carrying out DWBA calculations ’ “) for these reactions. The results of the calculations show very similar effects for the nucleon polarization and for the analysing power. For both these quantities there was a 10 to 20 % difference for the mirror reactions, i.e., there was no evidence for a greater sensitivity of the nucleon polarization than of the vector analysing power to the Coulomb interaction and different Q-values. In the absence of a quantitative theoretical understanding of the 3H(d, n)4He and 3He(d, p)4He reactions we cannot determine a limit on the validity of the hypothesis of charge symmetry from our present knowledge of the cross sections, nucleon polarizations and analysing powers for these reactions. The results of the present work imply any violation of charge symmetry is small. The authors wish to thank Professors H. H. Barschall and W. Haeberli for their interest and advice throughout the course of this experiment. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12)

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13) L. G. Keller and W. Haeberli, Nucl. Phys. Al56 (1970) 465 14) J. C. Davis, D. Hilscher and P. A. Quin, Polarization phenomena in nuclear reactions (University of Wisconsin Press, Madison, 1971) p. 515 15) P. Schwandt and W. Haeberli, Nucl. Phys. All0 (1968)585 16) V. Kbnig, W. Grtiebler, P. A. Schmelzbach and P. Marmier, Nucl. Phys. Al48 (1970) 380 and 391 17) W. Griiebler, V. Konig, P. A. Schmelzbach and P. Marmier, Helv. Phys. Acta 43 (1970) 445 18) P. D. Kunz, Programme DWUCK, private communication