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INTERACTION OF POLARISED 7 Li WITH 54Fe AT 70 MeV
O. Karban, G. Kuburas, C. Blyth, H. Choi, N. Davis, S. Hall, S. Roman, G.
Tungate, I. Turkiewicz
To cite this version:
O. Karban, G. Kuburas, C. Blyth, H. Choi, N. Davis, et al.. INTERACTION OF POLARISED 7 Li WITH 54Fe AT 70 MeV. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-435-C6-438.
�10.1051/jphyscol:1990646�. �jpa-00230916�
COLLOQUE DE PHYSIQUE
Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990
INTERACTION OF POLARISED 7 ~ i WITH 5 4 ~ e AT 70 MeV
O. KARBAN, G. KUBURAS, C.O. BLYTH, H.D. CHOI, N.J. DAVIS*, S.J. HALL, S. ROMAN, G. TUNGATE and I .M. TURKIEWICZ* *
School of Physics and Space Research, University of ~ i m f n g h a m .
$B-Birmingham B15 2TT. Great-Britain
Department of Physics, University of Edinburgh, GB- dinb burgh E H ~ 352, :feat-Britain
Institute for Nuclear Studies, Hoza 69, 00-681 Warsaw, Poland
RCsumE
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On a choisi le systime 7Li+
54Fe pour tester le modtle de la syme'trie marEmotrice dans la re'gion Fraunhofer d e la diffusion Elastique et pour rechercher la sensibilite' des puissances analysantes dans la voie d e re'action aux caracteres des Stats rEsiduants du noyau. Les rapports des effets du forme de'crivent d' un f a ~ o n assez bien les donne'es de la dispersion mais une analyse plus rigoreuse des voies couplEes soit nEcessaire pour la interpretation des parametres observables. Les valeurs T T ~ ; pour les transitions 112- dans la voie de re'action ( 7 ~ i , 6 ~ i ) sont en moyen 50% plus grandes que les transitions 312-. Ces caractEristiques sont donne'es par les calculs EFR-CRC.Abstract
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The 7Li+
54Fe system was chosen to test the tidal symmetry model in the Fraunhofer scattering region and to investigate the sensitivity of analysing powers in the reaction channel to the characteristics of the residual nucleus states. The scattering data approximately satisfy the shape effect relations but a full coupled-channel analysis is necessary to interpret the observables. F o r the 312- and 112- transitions in the (7Li.6Li) reaction channel, the T T ~ O values f o r the latter are on average 50% larger. These features are reproduced by the full finite range coupled-channels calculations.1
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INTRODUCTIONExperiments performed during recent years with polarised heavy ions have revealed a number of interesting, often unexpected features of analysing powers in scattering and reaction processes.
The polarisation observables are particularly sensitive to details of the interaction mechanism and also to the shape and internal structure of the projectile. Unlike the case of light polarised particles, the results seem to be generally insensitive to the classical spin-orbit interaction.
interpretation of these data stimulated considerable theoretical interest and led, for exan~ple, recently to the introduction of the Tidal Symmetry concept 11.21 which was used to explain the experimentally observed relations between the tensor analysing powers in the elastic and inelastic scattering. Based on the isocentrifugal approximation this model predicted the validity of the so called "shape effect relations" not only in the Fresnel scattering region but also in certain systems where there is a significant interference between the nearside and farside trajectories.
One of such systems was studied in the present experiment with the aim to test the Tidal Symmetry model in the Fraunhofer region. In addition it was expected to obtain new information on the behaviour of analysing powers in single-nucleon transfer reactions and their sensitivity to the spectroscopic characteristics of both the projectile and residual nuclei. The latter consideration led to a choice of the s4Fe target, providing well resolved states in both 5 5 ~ e and j j C o nuclei.
2
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EXPERIMENTAL DETAILSThe experiment was performed at the NSF in Daresbury using a 70 MeV 7Li beam frorrl the Polarised Heavy Ion Source 131. The reaction-products were detected with position-sensitive detector telescopes placed in the scattering chamber symmetrically around the beam axis. The tensor beam polarisation, typically 2 0.40, was monitored absolutely in a O0 polari~neter using the l H ( Y ~ i , 4 H e ) ~ H e reaction. In the vector mode operation, the relative polarisation was related ro the 4He-particle asymmetry of the above reaction at 2 210 assuming the same depolarisarion a \ rite tensor beam. The mass and charge separated energy spectra were obtained in the angular
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990646
C6-436 COLLOQUE DE PHYSIQUE
range from 90 to 400 (centre of mass). Data were taken in three different orientations of the bean1 spin symmetry axis in the tensor mode and one in the vector mode, yielding measurements of the differential cross sections, TTZO, T20, T21 and i T l l analysing powers for the elastic scattering, inelastic scattering to the first excited states of both the projectile and target, as well as to a number of: states of the 55Fe and 55C0 nuclei.
3
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THE ELASTIC AND INELASTIC SCATTERINGThe four experimental analysing powers corresponding to the elastic scattering, 7Li excitation to the 0.48 MeV, 112- state and 54Fe excitation to the 1.41 MeV, 2+ state are shown in Figure 1 . A peak corresponding to mutual excitation was observed in some spectra but statistics was insufficient to extract the data. Smooth curves drawn through the TTZ0 data point were used i n the shape effects relations to calculate the T20 and T2i analysing powers. The agreement for the OC and 2+ states is quite good, particularly for the Tzl analysing power, but for the projectile excitation the shape effect relations are apparently not a good approximation. Furthermore, in all three cases the Tidal Symmetry model prediction of zero iT11 analysing powers i s not satisfied.
Ground state
Fig. 1
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Angular distributions of the experimental differential cross sections and the analysing powers T20, TT20. T21 and iT11 for the elastic scattering, 7 ~ i excitation to the 0.48 MeV state and S4Fe excitation to the 1.41 MeV state. The lines represent the results of coupled-channel caiculations with the poten~ial C in Table 1.As the first step in the quantitative analysis the elastic scattering cross section was fitted using the optical-model search code HIOPTlM 141 yielding a set of parameters A given in Table 1.
These were used in coupled-channel calculations, performed with the code FRESCO 151.
assuming first only the ground state re-orientation with a coupling strength corresponding to the known spectroscopic quadrupole moment of 7Li and a first derivative shape of the form factor.
Such calculations reproduce well the ground state T T ~ ~ but fail to predict the observed T21 values.
A substantial improvement in the analysing powers fits resulted from the inclusion of the 7Li excitation channel, but this led to a deterioration of the elastic scattering cross section fit and unsatisfactory description of the 112- state cross section. It is well known that in the presence of a strong coupling to inelastic states the interaction potential has to be modified from the optical- model potential. Thus, a grid search in the parameter space was undertaken resulting in the potential B of Table 1. With the exception of the elastic scattering iT11, all the observables are now well reproduced.
Table 1
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The 7Li+
54Fe interaction potentialFinally, the target excitation channel was included explicitly in the calculations using a 5 4 ~ e deformation length /6/ PR=O. 6 2 fm. As above a lack of fit to the 2+ state cross section required further modification of the interaction potential (set C , Table I ) , decreasing the strength and diffuseness of the imaginary part. This procedure caused some deterioration of the fit, particularly to the elastic scattering analysing' powers, overpredicting the oscillation magnitude.
The results for all 15 sets of observables are compared with the data in Figure 1. Although the agreement is acceptable, more complex coupling schemes, involving e.g. the 712- s t a t e of 7Li, should be tested in further analysis.
4
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THE 54Fe(7Li36Li)55Fe REACTIONThere is a considerable number of studies devoted to o n e nucleon transfer reactions induced by 7Li, but information on polarisation effects in such processes is limited. The latter were reported recently by I. M. Turkiewicz et a1 171 using a 4 4 MeV 7Li beam incident on 2 6 and 120Sn ~ ~ targets. In the case of 27Mg, both 5/2+ and 3/2+ states were populated but no striking j- dependence was observed in any of the measured analysing powers.
In the present experiment, data were obtained for several well resolved, strongly populated states in 5 5 ~ e involving P-transfer of 1, 3 and 4. The results for the ground and 0.41 MeV states of the 312- and 112- partners of the 2p shell are shown in Figure 2. Large effects were observed for both T T ~ ~ and iT1l analysing powers, with the former displaying a clear j-dependence: the TT20 values f o r the j=[ -112 transition are on the average 50% larger than those for the j= P +1/2 transition. On the other hand, the remaining analysing powers are approximately the same for both transitions.
The data were compared with CCBA calculations using the full finite range code FRESCO in a similar way as described in ref. 171. Coupled channel approach involving re-orientation and coupling to the 0 . 4 8 MeV 7 ~ i state was used in the entrance channel together with a non-local transfer from both states, represented by a coherent mixture of the l p 1 / 2 and lp3/2 transition amplitudes. The corresponding spectroscopic amplitudes were taken from ref. 181. Solid lines in Figure 2 show results of these calculations which reproduce all the main features of the data, including the experimentally observed j-dependence of the TTZO analysing power. This rather complex treatment of the reaction process seems to be essential as can be seen in Figure 2, where the dashed lines represent calculations neglecting the l p l / 2 transfer amplitude: the fit to the T T ? ~ data is completely lost.
I n conclusion, the full coupled-channel treatment of the 7Li
+
54Fe partition together with finite range CCBA reaction calculations which take into account spectroscopic characteristics of thc projectile and the final nucleus, can succcssfully reproduce the inultitude of observable\COLLOQUE DE PHYSIQUE
g s , 312- 5 L ~ e 1 7 ~ . 6 ~ i 1 5 5 ~ e 0.~1 MeV 112-
Fig. 2
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The angular distributions of the experimental cross sections, the T20, T21, TTzO and i T l l analysing powers of the 5 4 ~ e ( 7 ~ i , 6 L i ) 5 5 ~ e f o r the ground and 0 . 4 1 MeV states compared with CCBA calculations using the code FRESCO. T h e dashed lines represent calculations neglecting the 7 ~ i l p 1 / 2 transfer amplitude.measured in the present experiment. T h e calculations will be further extended to include more complex coupling in both entrance and exit channels.
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( 1 9 8 8 ) 6 0 9 .131 Karban, O., Hardy, W. C., Connell, K . A., Darden, S. E., Blyth, C. O., Choi, H. D., Hall, S . J . , R o m a n , S. and Tungate, G., Nucl. Instr. Meth.
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(1979) 4 1 3 .171 Turkiewicz, I. hl., hloroz, Z., R u s e k , K . . T l ~ o ~ n p s o n . I. J . , Butch, ti., Krclrner, D . , O I I , W . . Steffens, E., Tungate, G . , Decker, K . , Blatt, K . , Jansch, H . J . , Leucker, ti. and Ficli, D . , Nucl. P h y s . &6 (19SS) 153.
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