Polarization of neutrons from the 16O(d, n)17F reaction

1

2.G

1

Nuclear Physics A170 (1911) 485-491; Not to be

@ North-Holland Publishing Co., Amsterdam

reproduced by photoprint or microfilm without written permission from the publisher

POLA~ZATION

OF NEUTRONS

FROM THE 160(d, n)“F

REACTION

S. T. THORNTON, R. P. FOGEL and C. L. MORRIS ofPhysics, University of Virginia, Charlof~esvi~~e, VO@kz,

Department

USA t

Received 13 April 1971 The polarizations of neutrons from the 160(d, no)17F and 160(d, n#‘F reactions have been measured at 30” (lab) in steps of approximately 0.15 MeV from Ed = 3.96 to 5.35 MeV. Polarization angular distributions have been obtained at 3.96 and 5.35 MeV. It is determined that the random phase approximation for the scattering amplitudes is not appropriate for the compound nucleus contributions. The analysis indicates probable interference between the compound nucleus and direct interaction reaction mechanisms.

Abstract:

E

NUCLEAR

160(d, n), E = 3.96-5.35 MeV; measured &, 6 = 30°1sb); studied reaction mechanism; gas target.

REACTIONS P(E;

P(E,, c?),

I

1. Introduction

The motivation for the present experiment was to further investigate the reaction mechanism of the 160(d, n)17F reaction. Because of fluctuations in the yield curves of d + I60 reactions and the lack of success in predicting ’ 60(d, p$ 7O polarization results, it was thought that the neutron polarization measurements might help to clarify the situation. We have measured polarization angular distributions at 3.96 and 5.35 MeV and a polarization excitation curve at 30”(lab) in steps of 0.15 MeV from 3.96 to 5.35 MeV for 160(d, n,)“F and 160(d, n,)17F. In recent years a number of experimental measurements have been made for d + “0 single-particle transfer reactions ’ - I1 ). These experiments have proliferated because (i) ’ 6O is an easily available target and the first excited state in I70 and “F can easily be separated in energy from the ground state and (ii) the reactions ’ 60(d, n)’ 7F and ’ 60(d, p)’ 7O should be good single-particle transfer reactions and thus be amenable to direct interaction (DI) analysis. Differential cross sections have been obtained over a wide range of energies for ’ 60(d, n,)’ 7F and 160(d, n,)l’F. Distorted wave method (DWM) analyses (in conjunction with Hauser-Feshbach (HF) compound nucleus (CN) analyses at lower energies) have yielded spectroscopic factors near 0.9. Similar results have been obtained for 160(d, p0)170 and 160(d, p1)170. From the cross-section data the DI reaction mechanism and single-particle nature of the ground and first excited states of I70 and 17F seemed well established. Several theoretical calculations, however, t Work supported in part by the National Science Foundation Center for Advanced Studies (NSF). 485

and by the University of Virginia

486

S. T. THORNTON

et al.

have indicated that ’ 6O does not act as simply a doubly magic closed shell nucleus strength [refs. i’s ‘“)I and in fact that an appreciable part (20 o/,) of the single-particle in I70 and ’ 7F may be shifted to levels several McV away from the low-lying states [ref. ‘“)I. To date, DWM calculations have uncertainties too large to ascertain this last suggestion. Although successful predictions have been obtained for ~(0) there has not been comparable success for polarization data. One of the present authors “) has previously performed DWM calculations and found good agreement with the 8 MeV vector analysing power 16O( d , pO)l 7O data of Bjorkholm and Haeberli ’ 5), but found poor power data at 8 MeV agreement with both the 160(d, p1)170 vector analysing [ref. ‘“)I and the 160(d, pr)l 7O po i a rization data of Evans ‘) at 7.0, 8.2, and 9.55 MeV. It was suggested that CN effects would probably have to be taken into account [ref. “)I. Using the weakly-bound-projectile (WBP) model, Pearson, Zissermann and Covan I6 ) had success in predicting the vector analysing power data of Cuno et al. “) for near 10 MeV for 160(d, p,)170, but not for ’ 60(d, pr)l 70. Their calculations the polarization data of Evans ‘) were also not completely satisfactory. Recently, Davison et al. “) obtained ’ 60(d, d)160, ’ 60(d, n,)17F, 160(d, nl)” 7F, and excitation 160(d, p0)170 and ’ 60(d, pJ1 7O cross -section angular distributions curves between 4 and 6 MeV. The yield curves indicate fluctuations, and the authors find that the incoherent sum of the DI and CN cross sections is in good agreement with the experimental data. They do, however, point out that yield curves at forward angles for 160(d, nl)i7F seem to indicate an interference between DI and CN reaction mechanisms. It has been shown by Thompson “) that the vector polarization of nuclear reaction products depends only on DI if the CN scattering amplitudes have random phases. The random phase approximation is made in statistical CN calculations by HF methods. Davison et al. “) did extensive fluctuation analysis calculations and convinced themselves that HF theory is capable of producing meaningful results in d+ ’ 6O reactions from 4 to 6 MeV. Perez and Hodgson in ref. 4, came to similar conclusions for energies < 4 MeV as did Cords et al. ‘) in the energy range 4.0-8.5 MeV. Both groups also performed a fluctuation analysis. If we assume the scattering amplitudes have random phases, then 17)

= (P(O) ~(~))~~,

(I)

where < > represents an energy average over a region in which (S) = Soi, where S is the scattering matrix. A suitable energy region would be LIE > r to smooth out the effects of Ericson fluctuations. The right-hand side of eq. (1) is the DI contribution.

2. Experimental

method and results

The University of Virginia 5.5 MV Van de Graaff pulsed accelerator was used in conjunction with the Mobley magnet bunching system to provide a deuteron beam

0

120

180

12c

60 B0 cm,

Fig. 1. The dots are the experimental polarizations measured in the present experiment with uncertainties typically as small as the dots. The lines are the DI predictions: the solid line was calculated with the potential D3 of Davison eb al. 9, and the dashed line is from the potential of Cords et al. 7).

0.0

__LA---L_” a

l

l

l

-3.2

160k.i,n,~7F

0

i -0.4

3.8

I

1 4.0

1

1 4.2

b

1

1 44

1

1 4.6

ENERGY

1

’ 4.8

1

1 50

1

1 5.2

’

’ 54

’

5.6

(Me’d

Fig. 2. The dots are the experimental polarizations measured in the present experiment at a lab angle of 30”. The uncertainties are typically as small as the dots. The line is the DI prediction using the deuteron potential 03 of Davison at al, 9).

S. T. THORNTON

488

et of.

with pulse duration < 1 ns. The experimental details of the liquid helium neutron polarimeter have been described elsewhere I’). The neutron polarizations at deuteron target energies of 3.96 and 5.35 MeV were measured at lab angles of 15”, 30”, 45”, 60”, 75”, 90” and 110”. A polarization excitation curve at 30”(lab) was obtained for approximately 0.15 MeV steps from 3.96 to 5.35 MeV. The target thickness was approximately 100 keV. A flight path of 3.3 m was sufficient to separate ground and first excited state neutrons by the time-of-flight method. The experimental polarizations are shown in figs. 1 and 2. The uncertainties include statistical and background subtraction uncertainties, but no uncertainty due to the analysing power. A neutron polarization analysing angle of 121” was used for all the measurements.

l’“r-T~-- --.7 (a) 160(d.no,‘7F

3.96

MeV

Cc) ‘6cXdq)“F

3.96

Cd) 160(d.n,)17F

5.35

MeV

MA’

‘./’ -0.4

0

I 60

I

I 120

60

e”

I 120

160

c.m. Fig. 3. The dots are calculated with the polarization data from the present experiment and the c.m. cross section data of Davison ef al. g, and Lodin and Nilsson lo). An energy average of 100 keV was used. The lines were calculated with a spectroscopic factor of 0.9 and is the DI prediction. The solid line was calculated with the deuteron potential D3 of Davison et al. 9, and the dashed line with the potential of Cords et al. ‘).

=O(d, n)“F

489

3. Analysis The cross-section data of Lodin and Nilsson lo ) were used to determine PO, except for ~(0) at 5.35 MeV where the data of Davison et al. ‘) were used. The results for (PO) are shown in figs. 3 and 4 for an energy average of 100 keV. What is shown for is P
38

4.0

4.2

44

4.6

4.8

ENERGY

(MeV)

so

52

Fig. 4. The dots are calculated with the present polarization data and the c.m. cross-section data of Lodin and Nilsson lo) at a lab angle of 30”. An energy average of 100 keV was used. The line is the DI prediction with a spectroscopic factor of 0.9 using the deuteron potential D3 of Davison et al. g).

We have used the computer program DWUCK 19) to calculate the DI contributions by the DWM. Several neutron optical model potentials 2o--23) were used, and the final calculations were made with the potential of Rosen ‘O). The bound-state proton potential had a geometry similar to Rosen’s 20) with a spin-orbit term 2. = 25. The calculated polarizations were insensitive to the neutron and proton pzentials. Several deuteron potentials were used. The potentials D3, D4 and D5 of Davison et al. ‘) gave similar results. Potentials of deForest 3), Thornton 6), Bjorkholm and Haeberli 15), and Cords et al. ‘) were also used. The latter three potentials were obtained at energies > 6 MeV and included polarization elastic scattering data in their search. We found all three gave similar results for the (d, n) neutron polarizations.

490

S. T. THORNTON

et al.

We have made extensive optical model searches using the program SNOOPY 2 [ref. “)I for the 4.0 and 5.25 MeV ’ 60(d, d)’ 6O elastic scattering data of Davison et al. “) and have found several new potentials. The DWM calculations for the present P(0) were made at 3.96 and 5.35 MeV for all the deuteron potential sets. The potential D3 of Davison et al. “) was judged to give the best overall agreement. We conclude, therefore, that a potential averaged over a wide energy range is better than a potential at a given energy. The effects of CN are smoothed out. Predictions for D3 and the potential of Cords et al. ‘) are shown in figs. 1 and 3. The excitation curves displayed in figs. 2 and 4 were calculated for D3. The only satisfactory predictions are for the ground state excitation function and P(B) at 5.35 MeV. The first excited state agreement is poor. Thej-dependence of the ’ 60(d, no)17F polarizations is clearly exhibited in P(6) at forward angles for 3.96 and 5.35 MeV. The values of r obtained by previous investigators from cross-section data ranged from 70 to 160 keV. Since in the present experiment the energy resolution was 100 keV, we did not expect to see detailed fluctuations in the excitation functions. For the first excited state there is a broad marked change for P(30”) and P(30”)0(30”) over the energy range from 3.96 to 5.35 MeV. In order to smooth out effects of fluctuations and to compare with Thompson’s result, PO should be averaged over an energy region where (S) = Sn,. After averaging over 500 keV the ground state P(30”) and P(30”) ~(30”) is in good agreement with DI calculations. Averaging will not help with the first excited state results since the polarization has the wrong sign over much of the energy region. 4. ConcIusions It is apparent that we have experienced the same difficulty with the ’ 60(d, ni)’ 7F polarization data that has occurred with 160(d, p1)170. There seems to be a fundamental problem over a wide range of energies. If indeed there are significant CN effects, it is surprising that they occur at such high energies (up to 10 MeV for 160(d, pr)l’O). Better optical model potentials could provide an explanation, but extensive searches have been made even including polarization data. The trouble might be with the DWM, but neither the present results nor the WBP model calculations 16) have been successful. Since Thompson’s result depends only on the random phase approximation, we conclude that the phases of the CN scattering amplitudes are not random for d + ’ 6O reactions below 6 MeV., We conclude that the levels in the compound nucleus ‘*F near 11 MeV are not overlapping to the extent that r > D. If the phases are not random, the CN polarizations are not necessarily zero. There can also be significant interference between DI and CN for the cross-section and polarization data. It is likely that some of the fluctuations in ~(0) and P(O) are due to interference between DI and CN as well as CN Ericson fluctuations. The P(0) for 160(d, n)17F at 3.96 and 5.35 MeV are similar except for the large polarizations at forward angles at 3.96 MeV.

160(d,

n)“F

491

Ruh and Marmier 25) have found disagreement with the polarizations predicted by the statistical theory. They measured the polarizations of protons elastically and inelastically scattered from 24Mg, 52Cr and ‘*Ni. They postulate that doorway state excitation could probably explain the measured polarizations. Although no doorway state calculations have been performed in the present investigation, an explanation by doorway states seems attractive especially for the first excited state results. Much more theoretical and experimental work is needed before the reaction mechanism of 160(d, n,)l’F and 160(d, p1)170 may be understood. Polarization excitation measurements with good resolution and fine energy steps would be useful. More polarization measurements where only the statistical theory is expected to be applicable are needed to test the statistical theory, and a fluctuation analysis of polarization data would be helpful. The authors gratefully acknowledge a computer Virginia Computer Science Center. Acknowledgment States Navy for providing helium gas.

grant from the University of is also given to the United

References 1) J. E. Evans, Phys. Rev. 131 (1963) 1642 2) J. L. Alty, L. L. Green, R. Huby, G. D. Jones, J. R. Mines and 5. F. Sharpey-Schafer, Nucl. Phys. A97 (1967) 541 3) F. G. DeForest, Ph. D. Thesis, University of Wisconsin (1967) 4) 0. Dietzsch, R. A. Douglas, E. Farrelly Pessoa, W. Gomes Porto, E. W. Hamburger, T. Polga, 0. Sala, S. M. Perez and P. E. Hodgson, Nucl. Phys. All4 (1968) 330 5) I. M. Naqib and L. L. Green, Nucl. Phys. All2 (1968) 76 6) S. T. Thornton, Nucl. Phys. Al37 (1969) 531 7) H. Cords, G. U. Din and B. A. Robson, Nucl. Phys. Al34 (1969) 561 8) C. J. Oliver, P. D. Forsyth, J. L. Hutton, G. Kaye and J. R. Mines, Nucl. Phys. Al27 (1969) 567 9) N. E. Davison, W. K. Dawson, G. Roy and W. J. McDonald, Can. J. Phys. 48 (1970) 2235 10) G. Lodin and L. Nilsson, Z. Phys. 233 (1970) 181 11) H. H. Cuno, G. Clausnitzer and R. Fleischmann, Nucl. Phys. Al39 (1969) 657 12) G. E. Brown and A. M. Green, Nucl. Phys. 75 (1966) 401 13) A. P. Zucker, B. Buck and J. B. McGrory, Phys. Rev. Lett. 21 (1968) 39 14) G. E. Brown, J. A. Evans and D. J. Thouless, Nucl. Phys. 45 (1963) 164 15) P. J. Bjorkholm and W. Haeberli, Bull. Am. Phys. Sot. 13 (1968) 723, and private communication 16) C. A. Pearson, D. Zissermann and J. M. Covan, Nucl. Phys. A152 (1970) 449 17) W. J. Thompson, Phys. Lett. 25B (1967) 454 18) S. T. Thornton, C. L. Morris, J. R. Smith and R. P. Fogel, Nucl. Phys., Al69 (1971) 131 19) P. D. Kunz, University of Colorado, private communication 20) L. Rosen, Proc. 2nd Int. Symp. on polarization phenomena of nucleons (Birkhauser Verlag, Basel, 1966) p. 253 21) F. Perey and B. Buck, Nucl. Phys. 32 (1962) 353. Tabulated by L. Rosen, ref. 20) 22) B. A. Watson, P. P. Singh and R. E. Segel, Phys. Rev. 182 (1969) 977 23) F. D. Becchetti, Jr. and G. W. Greenless, Phys. Rev. 182 (1969) 1190 24) P. Schwandt, private communication 25) A. Ruh and P. Marmier, Nucl. Phys. Al51 (1970) 479