Volume 25B. number 7 PHYSICS LETTERS 16 October 1967

4. P. Bounin and G. R. Bishop, J. Phys. Rad. 22 (1961) 555; J. H. Fregeau and R. Hofstadter, Phys. Rev. 99 (1955) 1503; H. F. Ehrenberg, R. Hofstadter, U. Meyer-Berkhout, D. G. Ravenhall and S. E.Sobottka, Phys. Rev. 113 (1959) 666.

5. M. Bernheim, T. Stovall and D. Vinciguerra, Nucl. Phys. A97 (1967) 488.

6. Yung-Su Tsai, Phys. Rev. 99 (1955) 1020. 7. R. Hofstadter, Eelectron scattering and nuclear and

nucleon structure, (W. A. Benjamin, Inc. New York, 1963).

*****

EXCITATION OF THE 0.908 MeV LEVEL IN 89Y BY INELASTIC SCATTERING OF 61.2 MeV PROTONS *

A. SCOTT **, M. L. WHITEN University of Georgia, Athens. USA

and

J. B. BALL Oak Ridge National Laboratory. Tennesse. USA

Received 5 September 1967

The differential cross section for excitation of the 0.908 MeV level in 8gY by 61.2 MeV protons was measured over the angular range from 17O to 120 ‘. The results are compared with calculations using the microscopic description of the inelastic scattering process.

This experiment was motivated by the wide- spread current interest in the microscopic de- scription [l-4] of the inelastic scattering of protons by nuclei. In this treatment, the effec- tive interaction is written as a sum of individual interactions between the projectile and the target nucleons.

The first excited state (4’) in 8gY appears to be one of the best examples of a “pure” single particle excitation. This state, at 0.908 MeV, and the ground state (f-) are thought to differ only by the promotion of a single proton from the 2pl shell-model level into the lgg level. Thus the effective interaction giving rise to the scattering would be predominantly that between the projectile proton and the single *extra-core” proton [3,5]. A study of this state, therefore, seemed to offer one of the best ways to calibrate the effective interaction used in these newly developed scattering theories. * Research sponsored by the U.S. Atomic Energy Com-

mission under contract with Union Carbide Corpo- ration and by the University of Georgia Office of General Research.

** Part of this work was performed as an Oak Ridge Associated Universities Summer Research Partici- pant.

Nucleon inelastic scattering above 100 MeV has been well described by the impulse approxi- mation [6]. This experiment was performed at 61 MeV in the expectation that the reaction mechanism would still be much simpler than at lower energies. This is presently the highest proton beam energy available with energy resolution allowing separation of this weakly ex- cited state from contaminant peaks. Comparison of this experiment with those at lower energies [‘I-9] yields evidence on the energy dependence of the effective interaction.

Previous calculations in the microscopic de- scription neglected exchange contributions to the inelastic scattering, but Amos and McCarthy [5] have recently suggested that proper antisymme- trization is necessary and that proton scattering which excites the 0.908 MeV level in 8gY would be a good test of their contention.

Data were obtained with a proton beam from the Oak Ridge Isochronous Cyclotron and the broad-range spectrograph [lo]. Protons scat- tered from a 9.6 mg/cm2 89Y target were de- tected by Ilford 2” X 10” plates with 50 p thick G5 emulsions. The angular range from 170 to 120° was covered except for the region from 26O

463

Voiume 25B, numher 7 ~.___ PHYSICS LETTERS

-p- 16 October 1967

1.6:

80 100 220 140 160 480 200 220

DISTANCE ALONG PLATE (mm)

Pig. 1, Spectrum of inelastic protons at a scattering angle of 33 deg. The elastic cross section at this angle is three orders of magnitude higher than that for the 0.908 MeV state.

100

5

2

10-l

5

‘; \” z 2 L

x G 10-Z

5

% 5

;; b

2

to-3

5

2

10-q

I I I

89Y (P, P’)

1 I i

4=‘61.22 MkV

0 = -0.908 MeV :---- : ~~ : 7m

I I E,= 61.22 MeV

Q = -0.908 MeV

--LOVE AND SATCHLER

-~ OIRECT WITH CORE EXCITATION

YUKAWA (1 =I.0 fm-’

I I I c

20 40 60 80 100 120 0 60 80

BC.w. (deg) &.M. (deg)

Pig. 2 (a). Experimental data compared with the direct coupling calculation of Love and Satchler. The bars on the data points represent overall errors including, where appropriate, estimated uncertainties in subtraction of conta-

minate background. (b). Experimental data compared with the calculation of Love and Satchler including both core polarization and di-

rect coupling. 464

Volume 25B, number 7 PHYSICS LETTERS 16 October 1967

to 32’ where the protons scattered by 12C, 14N,

and 160 contaminants obscured the weak 0.908 MeV 8gY state. The overall resolution varied from 39 keV to 55 keV due to energy straggling in the target. An example of the spectra from this study is shown in fig. 1.

The experimental differential cross sections are compared in figs. 2a and 2b with the recent calculations of Love and Satchler [ll]. The curve in fig. 2a is the calculated cross section assum- ing only a direct coupling between the projectile proton and the valence proton. An interesting result is that the strength of the Yukawa inter- action for 61 MeV protons is half the value used in corresponding calculations [4] for scattering of protons with energy near 20 MeV. Thus, the effective interaction appears to be strongly energy dependent.

Fig. 2b shows the result of their calculation with the effective interaction renormalized to allow for polarization of the target core. The core coupling parameters were obtained from the effective charges needed to explain the ob- served electromagnetic transition strength in gOZr [4]. With core polarization, the direct coupling used is the same Yukawa form, with the same range, but with a strength now only 50 MeV, half the value used if the scattering is as sumed to be entirely due to direct coupling. It is possible that a different range for the Yukawa interaction may be more appropriate when the core polarization is included and at a higher proton energy [12].

The two calculations fit the general shape of the angular distribution quite well. The core polarization calculation produces structure at the larger angles similar to the experimental data, while the direct coupling calculation alone does not. Antisymmetrized calculations at 61 MeV without core polarization are in progress [13], but are not yet available for comparison. These

calculations may yield the structure observed in the experimental differential cross section, but it should be more correct to include both the ef- fects of core polarization and antisymmetrization. Another complication is that there may be some contribution to the observed scattering from a spin-flip I = 3 transition.

We thank G. R. Satchler and K. A. Amos for interesting and helpful discussions and W. G. Love and G. R. Satchler for permission to show the results of their calculations. Travel funds were provided by Oak Ridge Associated Uni- versities.

References

,-

3. 4.

5.

6.

7.

8. 9.

10.

11.

12. 13.

*****

1.

2.

H.O. Funsten et al., Phys. Rev. 134 (1964) B117; N. K. Glendenning and M. Veneroni, Phys. Rev. 144 (1966) 839; G. R. Satchler, Nuclear Physics 77 (1966) 481; V. A. Madsen, Nuclear Physics 80 (1966) 177. W.S.Gray et al., Phys. Rev. 142 (1966) 735; M. B. Johnson et al., Phys. Rev. 142 (1966) 748, 154 (1967) 1204. G.R.Satchler, Nuclear Physics A95 (1967) 1. W. G. Love and G. R. Satchler, Nuclear Physics, Al01 (1967) 424. K. A. Amos, V. A. Madsen and I. E. McCarthy, Nuclear Physics A94 (1967) 103; K. A. Amos, Nuclear Physics, to be published. R. M. Haybron and H. McManus, Phys. Rev. 136 (1964) B1730, 140 (1965) B638; R. M. Haybron, to be published in Phys. Rev. M. M. Stautberg, J. J. Kraushaar and B. W. Ridley, Phvs. Rev. 157 (1966) 977. Y.kvaya, Phys: Letters 21 (1966) 75. S.M. Austin et al., Bull. Am. Phvs. Sot. 12 (1967) 550. J. B. Ball, IEEE Trans. Nucl. Sci. NS-13 (4), (1966) 340. W. G. Love and G. R. Satchler, private communica- tion . G. R.SatcNer, private communication. K. A. Amos, private communication.

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