NEUTRON-PROTON ANALYZING POWER DATA AT 7.6, 12.0, 14.1, 16.0, 18.5 MeV

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NEUTRON-PROTON ANALYZING POWER DATA AT 7.6, 12.0, 14.1, 16.0, 18.5 MeV

G. Weisel, W. Tornow, C. Howell, P. Felsher, M. Alohali, Z. Chen, R. Walter, J. Lambert, P. Treado

To cite this version:

G. Weisel, W. Tornow, C. Howell, P. Felsher, M. Alohali, et al.. NEUTRON-PROTON ANALYZING POWER DATA AT 7.6, 12.0, 14.1, 16.0, 18.5 MeV. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-515-C6-518. �10.1051/jphyscol:1990663�. �jpa-00230933�

NEUTRON-PROTON ANALYZING POWER DATA AT 7.6, 12.0, 14- 1, 16 - 0 , 18

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^{5 M ~ V '}

'

G.J. WEISEL, W. TORNOW, C.R. HOWELL, P.D. FELSHER, M. ALOHALI, Z.P. CHEN, R.L. WALTER, J.M. LAMBERT* and P.A. TREADO*

?uke University and TUNL, Durham, NC 27706, U. S. A.

' '

Georgetown University, Washington, DC, U.S.A.

a-

Nous avons mesure le pouvoir d'analyse neutron-proton A, ( 8 ) B 7.6, 12.0.

14 .l, 16.0 et 18.5 MeV. Cette observable stav&re importante pour d6terminer les dephasages 3 ~ , ,

,

^,

,

de la diffusion n-p. Nous comparons les donnees aux predictions de 2 analyses en dephasages (dont l'une incorpore les effects BIC) et aux predictions faites avec les potentiels de Paris et de Bonn.

Abstract

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Neutron-Proton Ay@) measurements have been made at 7.6, 12.0, 14.1, 16.0, and 18.5 MeV. A sensitivity study establishes the importance of Ay(@ in determining the 3~0,1,2 phase shifts in n-p scattering. The data are compared to the predictions of two phase-shift studies (one of which incorporates CIB effects), and the Paris and Bonn NN potenials.

l - Introduction

Recent theoretical findings concerning the development of nucleon-nucleon (NN) potentials (eg:

Paris and Bonn-OBEPQ) demonstrate the importance of high-precision analyzing power Ay@) measurements for low energy n-p scattering. At the center of these developments is the question of how well the 3~0,1,2 phase shifts are known.

Machleidt has used low-energy n-p Ay(€)) data to fine tune the Bonn-OBEPQ potential (Bonn I) /l/. In order to bring the Bonn I predictions into agreement with the data an increase of the o N N coupling constant was necessary, which is equivalent to lowering the 3p0 phase shift and increasing the magnitudes of the 3p12 phase shifts. This version of the potential has been named Bonn II.

The 3P0,1,2 phase shifts are also pivotal in the issue of charge independence breaking (CIB). The Nijmegen group has suggested that the magnitude of the n-p 3P0,1,2 phase shifts is considerably larger than that of the p-p 3~0,1,2 phase shifts through a model-directed phase-shift analysis of n-p and p-p elastic scattering 121. This difference in phase shifts is equivalent to a difference between the charged pion-NN and neutral pion-NN coupling constants

f2c

^and

g

and constitutes a large CIB of the NN system.

Both of these examples indicate a new stage of fine tuning in the development of

NN

potentials.

In the past, 3P0,1,2 phase shifts for n-p scattering were simply taken from p-p scattering and the CIB of the

NN

force was only considered in the ISo interaction.

(l) Talk given by W. Tornow.

(2) Work s u v m ^by the U.S.D.O.E.. Contract No. DE-AC05-76ER01067

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990663

C6-516 COLLOQUE DE PHYSIQUE

2

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Sensitivitv Studv of n-p Observable~

To prove the efficacy of our Ay(€)) measurements, we investigated the sensitivity of n-p scattering observables to variations of the ~ P ~ , J , J phase shifts, using the differences between the Nijmegen n-p and p-p analyses as an indication of the differences that must be experimentally detected. Starting from the

NN

phase-shift analysis FA87 (and computer code SAID) of R.A. Arndt, we replaced the ISo and 3P0,12 phase shifts first by the Nijmegen n-p values and then by their p-p values. The sensitivity of each scattering observable was gauged by the ratio of its value calculated with n-p phase shifts over that calculated with p-p phase shifts.

Fig. 1 displays the sensitivity ratio for the four most promising n-p scattering observables.

Although these observables have 3 ~ ~ , ~ , ~ sensitivity ratios which are greater than experimental uncertainties, there are phase shift ambiguities which must be considered. For example, A, and A,, are highly sensitive to the T=O &l mixing parameter and lP1 phase shift which are not well determined presently. This makes it difficult to unambiguously determine the 3P0,12 In addition, this ambiguity is present in certain angular ranges of Ay and A,,

.

We found that only A,,(900), Ay(900), and the zero crossing of Ay are free of the & l - lP1 problem.

Fig. 1 Ratio of 1.0

results calculated for ^5%

A,, A,, A,,, and A, at 25 MeV: their values using the Nijmegen n-p

a"

phase shifts over those 6 3.00

using the p-p phase shifts.

1.00

The measurement of the spin-correlation coefficient AZx(900) requires both a polarized neutron beam and a polarized proton target for which we assume a f2.5% and f4% uncertainty respectively.

In the case of AZx(900) there is a phase-shift ambiguity introduced by 3s1. When the experimental uncertainties and the uncertainty in the phase shift are combined, the total uncertainty in AZx(900) is greater than the effects demonstrated in Fig. 1.

The zero crossing of Ay is due to the Mott-Schwinger interaction. Its location at energies near Elab = 10 MeV is shifted by about 0.40 when the Nijmegen p-p and n-p phase shifts are interchanged.

Although it seems possible to experimentally determine this zero-crossing angle to 0.10, we are again limited by a phase-shift ambiguity, in this case through the 3D1 and 3D3 partial waves.

The Ay(900) does not contain any phase-shift ambiguities. The ratio test indicates a AAy(W) due to CIB of 0.003. The combined uncertainties of neutron polarization (AA, = 0.001) and statistics

Ay(900) is currently the best way to study the n-p phase shifts and any related CIB of the NN system.

TUNL has completed a first series of experiments measuring Ay@) for n-p scattering at 7.6, 12.0, 14.1, 16.0 and 18.5 MeV. In taking the data, polarized neutrons were produced by the 2H(d,n)3~e source reaction and scattered by a plastic scintillator into liquid scintillator neutron detectors. Two dimensional spectra were produced by measuring the recoil proton energy in the center scatterer in coincidence with the neutron time-of-flight between the center and neutron detectors.

The data, corrected for finite geometry and multiple scattering effects, are shown in Fig. 2 in comparison to the Bonn I1 predictions. The statistical uncertainty of the present data is below W.001.

The error bars reflwt only statistical uncertainties except for those of the 18.5 MeV points which include uncertainties of background subtraction (at lower energies, background uncertainty is small compared to statistical uncertainty). One of the finite geometry corrections is particularly sensitive: the

"polarization dependent detector efficiency", which is due to the presence of 12c in the neutron detectors /3/. The uncertainties for this correction range from f .000 to f -002, depending on the energy and angle. These uncertainties will be documented in a later article, pending a more thorough investigation in our next generation of experiments.

Fig. 2 Data for n-p analysing power at E, = 7.6, 12.0, 14.1, 16.0, and 18.5 MeV, compared to Bonn

II

potential predictions.

The Ay@) data for

En

= 12 MeV are shown again in Fig, 3, where they are compared to the predictions of four models. The prediction based on the global (0-1.3 GeV)

NN

phase-shift analysis

SP89

of R.A. Arndt (dash-dotted curve) is too large for angles beyond 100'. The Nijmegen phase- shift analysis prediction, incorporating CIB effects (dashed curve) is higher in magnitude than the data.

C6-518 COLLOQUE DE PHYSIQUE

The Bonn I1 (solid curve) and Paris (dotted curve) NN potential model predictions both give good overall representation of the data. As far as the present data is concerned, the Nijmegen claim of significant CIB in the 3 ~ ~ , ~ , ~ channels seems unpromising. Indeed, it has recently been suggested by de Swart (private communication) that the previously accepted value for the charged pion-NN coupling constant

f2C

is in error. See also abstract 24A contributed to this conference by Amdt et al.

BC.

^m.

(deg 1

Fig. 3 Data for Ay@) at E,,= 12.0 MeV. The curves are predictions calculated from the Bonn I1 (solid) and Paris (dotted)

NN

potentials, the Nijmegen phase-shift analysis (dashed), and Arndt's O-

1.3 GeV phase-shift analysis (dashed dotted).

4

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Future Work

TUNL

has a new series of experiments planned, the object of which is to take definitive n-p Ay@) data at these energies. The following improvements have been made: A new polarized ion source, recently installed at TUNL, enables greater neutron beam intensity and a better determination of neutron polarization. Also, a new experimental area has been assembled which features a shielded neutron source and an array of 12 neutron detectors. Finally, future work will be guided by our experience in correcting the present data for the polarization dependent detector efficiency. This correction requires a detailed knowledge of the n-12c analyzing power. In those energy regions involving a resonance in the n-12c system the effect is large and insufficient n-12C Ay(@ data is available. In future measurements, energies and angles will be chosen where this effect is at a minimum.

REFERENCES

/l/ Machleidt, R., Adv. Nucl. Phys. JP (1989) 189.

121 Stoks, V.G.J. et al., Phys. Rev. Lett. 61 (1988) 1702.

131 Holslin, D. et al, Nucl. Instr. and Meth. (1989) 207, and references therein.