RECENT RESULTS AT LOW ENERGY

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RECENT RESULTS AT LOW ENERGY

G. Graw

To cite this version:

G. Graw. RECENT RESULTS AT LOW ENERGY. Journal de Physique Colloques, 1990, 51 (C6),

pp.C1-405-C1-410. �10.1051/jphyscol:1990639�. �jpa-00230906�

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Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990

RECENT RESULTS AT LOW ENERGY

G . GRAW

Sektion Physik der Universitdt Miinchen, 0-8046 Garching, F.R.G.

What i s low Energy?

Following t h e organizers of t h i s conference and t h e i r selection, "low energy" covers all t h e activities i n polarization studies of nuclear systems up t o beam energies a t 80 MeV. We t h u s discuss experiments with t h e velocities of t h e polarized projectiles being below or comparable t o t h e velocities of t h e nucleons in t h e nuclei studied. They t h u s address t o t h e investigation of t h e low momentum components of nuclear wave functions and of nuclear transition den- sities.

Devoted t o these kind of questions, concerning nuclear s t r u c t u r e and nuclear reaction dynamics we had a t t h i s conference 92 contributions from various places, referring t o experiments from 14 different accelerators with polarized beam. In addition t o this, low energetic polarized beams h a v e intensely been used t o study i n detail effects of t h e nucleon nucleon interaction (contri- butions t o section A) and of symmetry violations (contributions t o section D). Taking just t h e number of contributions, low energy studies a r e t h e field of strongest activity.

2. Survey

How t o summarize a l l t h i s i n 15 minutes?

First, I would like t o identify main topics and trends.

These are, from my point of view:

(1) Polarization transfer studies i n elastic or inelastic scattering, i n transfer or in break up processes. Here, t h e progress is most impressing. These experiments a t t h e PSI, RCNP, TUNL, Kyushu and Madison facilities use carefully optimized detection systems, t h e y benefit consi- derably from t h e improved intensities of polarized sources. The experiments address predominantly t o questions of t h e reaction dynamics or to effects of t h e spin-spin or of t h e tensor forces.

(2) Nuclear Structure studies i n inelastic scattering and transfer reactions a t magnetic

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990639

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spectrographs. In addition t o t h e high resolution experiments a t RCNP we h a v e now f u r t h e r improved energy resolution (of 5 keV full width half maximum) a t t h e Munich Q3D. The

experiments identify transition s t r e n g t h (for each transferred quantum number separately) over a range of excitation energies and allow comparison with s t r u c t u r e calculations i n several respects.

(3) D-state Effects. Transfer reactions with tensor polarized deuterons a t TUNL, Tsukuba, Kyushu and Miinchen aim t o determine admixtures of a D-state component t o t h e ground s t a t e wave functions of t h e mass 3 , 4 and 6 nuclei. Especially for t h e He isotopes t h e r e is significant progress. In addtion t o this, there a r e studies with respect to t h e action of t h e tensor force i n light compound systems (A = 3, 4 and 5) i n PSI, Seattle, Koln, Giessen, Kyushu.

(4) The optical ~ o t e n t i a l for t h e elastic scattering of nucleons or of complex nuclei (d, 3He, 3H, etc.) was subject of a v a r i e t y of very sophisticated studies. Typical aspects a r e t h e use of dispersion relations t o combine real and imaginary potential terms, t h e inclusion of virtual break up into a discretized continuum t o reproduce tensor effects i n scattering, t h e application of Fadeev techniques t o calculate deuteron scattering, or of resonating group techniques t o reproduce t and 3He scattering data. There a r e further detailed "model independent" phenomenological analysis of high precision deuteron scattering d a t a (from Saclay, Karlsruhe, Munich) and a n attempt t o reproduce t h i s i n a density dependent folding prescription.

(5) For two other very interesting topics, break up into t h e continuum and reactions with polarized heavy ions, t h e contributions a r e predominantly from Sakai and coworkers, and from t h e Heidelberg and t h e Daresbury group, respectively. The progress i n t h e s e areas is well displayed in t h e review t a l k s of Dr. Sakai of and Dr. Fick a t t h i s conference.

(6) This l i s t of topics should also mention s t i l l ongoing activities i n studies of c o m ~ o u n d nucleus formation, both theoretically (polarisation fluctuations) and experimentally (resonance studies in Giessen).

(7) An essential point a r e t h e refined theoretical calculational techniques which stimulate xperimentalists t o compare their d a t a with calculations. For example, I would like t o mention Fadeev calculations for t h e mass 3 unbound system, Dirac calculations for inelastic scattering and shell model calculations in t h e (fp) space.

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There is no chance t o refer t o or t o work out or t o summarize t h e ideas or t h e r e s u l t s of a l l t h e 92 contributions i n 15 minutes. I t is not t h e purpose of t h i s summary t o s u b s t i t u t e reading of t h e original s h o r t contributions B1 t o B92 or of those extended 14 contributions in t h e s e proceedings, which had been presented orally on Wednesday i n t h e parallel sessions. However, t o illustrate progress, I want t o follow my personal t a s t e and discuss a few cases. Thus contributions of comparable or even of higher quality will remain uncited, I h a v e t o apologize for this.

(1) Spin-Spin Interactions

The depolarization i n elastic scattering from a target nucleus with spin is a n old-standing topic. For nucleon scattering from a n unpolarized spin 1/2 target a t sufficiently high energy (to avoid compound nuclear contributions) depolarization of t h e normal component of t h e beam polarization provides a direct measure of t h e strength of a projectile-spin target-spin interaction potential. For t h e f i r s t time there a r e now conclusive results: Dr. Henneck discussed PSI and RCNP data: ($,$) from 13C(1/2-) and lSN(1/2-) a t 80 and 6 5 MeV, respectively. The observed depolarization i s strongest near a diffraction minimum of t h e differential cross section a t a scattering angle near 70'. The experimental effects a r e 2% and 8%, respectively, measured with an impressive 0.4 and 1.3% accuracy. Fitting t h e s e d a t a with phenomenological spin-spin or spin-tensor potentials yields VSS = 0.7 MeV, VST

=

0 MeV and VSS z VST Z 3 MeV, respectively. These significantly different observations a r e , however, explained in a n a t u r a l way, if t h e spin dependent interaction i s derived from t h e Cohen-Kurath shell model wave function of t h e target nucleus, folded with v.Gerambls version of a density dependent effective force, derived from t h e Paris potential. Apparently, t h e t a r g e t spin dependent interactions a r e i n no way global effects, b u t microscopic f e a t u r e s of individual nuclei.

(2) Spin transfer i n inelastic scattering and Dirac coupled channel analysis

Nucleon scattering may be described either in t h e Schrodinger or i n t h e Dirac formalism. In t h e first case, one uses complex central and spin orbit potential terms, i n t h e l a t t e r complex scalar and vector potential terms. The Dirac potentials h a v e a modest energy dependence and a r e related in a r a t h e r direct way t o t h e nuclear density distribution. Deriving from t h e Dirac potential a Schrodinger equivalent potential produces several terms with different geome- t r y and significant energy dependence. Dr. Kamigaito from t h e Kyoto group presented inelastic proton scattering d a t a from 24Mg and a coupled channel analysis (Raynal's ECIS 88 code) for

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t h e ground s t a t e rotational band. The d a t a (of 0,+, 2 , + ) for da/dn, Ay(8) and t h e spin t r a n s - f e r coefficient ~ ~ ~ ' ( 8 ) a r e reproduced equally well both i n t h e Schrodinger and in t h e Dirac prescription. T h a t is consistent with t h e demanded equivalence of both procedures. Deriving from t h e s e potentials, however, geometrical quantities a s quadrupole transition moments or rms radii, t h e a u t h o r s show t h a t t h e quantities from t h e Dirac potential a r e i n significantly b e t t e r agreement with electromagnetically determined values t h a n those derived from t h e Schrodinger prescription. The same observation holds for a reanalysis of their

'

661'68Er and 7 4 * 1 76Yb d a t a . The a u t h o r s do not discuss (in t h e Schrodinger case) correction procedures, taking i n t o account t h e density dependence of t h e interaction (being energy dependent), but nevertheless t h e i r s t u d y indicates, t h a t geometrical quantities seem to be more directly correlated with t h e Dirac t h a n with t h e Schrodinger potential.

(3) Transfer and s t r e n g t h distributions

Polarized projectile induced transfer reactions h a v e angular distributions which usually allow t o assign t h e transferred quantum numbers 8 and j = I Z + S I and t o determine t h e spectroscopic factors, t h a t is t h e percentage of single particle strength nEj projected out from t h e wave function of t h e a c t u a l s t a t e . The Kyushu, Milan and Munich groups report on studies ( a t t h e RNCP or Munich magnetic spectrographs) t o resolve and assign most of t h e s t r e n g t h up t o typi- cally 8 MeV excitation energy. They allow very interesting comparisons with s t r u c t u r e calculations:

In t h e Munich 31P(d,p)32P study each of t h e identified eight lowest I + , 2+ and 3+ s t a t e s , respectively, agree both i n energy and spectroscopic strength surprisingly well with (s,d) shell model calculations of Wildenthal, showing t h u s t h e predictive power of t h e s e (seniority "unre- stricted") model calculations.

For t h e Munich 40Ca(d,p)41 study H. Lenkse presented large configuration space quasi-particle RPA calculations. They give nice reproductions of t h e main f e a t u r e s of t h e strength distributions for eight different transferred ( E j ) values. He addressed t o t h e importance of main shell mixing, t h e modification of t h e radial dependence of t h e transition form factorsand also t o t h e significantly strong contributions of higher main shell orbitals t o t h e spectroscopic factors of weakly excited s t a t e s . These effects h a v e t o be considered carefully if strong depletion of closed shell orbitals i s claimed t o be observed.

(4) Inelastic scattering. shell model calculations, phonon excitations

In inelastic scattering, e.g.56~($,p') a t 6 5 MeV, usually collective form factors reproduce

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best p a r t of t h e transitions, but by no means all of them. In t h i s specific case, t h e y f a i l completely t o reproduce t h e 2,+ s t a t e a t Ex = 2.957 MeV. Dr. Fyiiwara compared 2+ transitions with form factors, derived from recent shell model calculations i n t h e (f,p) shell by Otsuka e t al. He obtained for t h e f i r s t time also for t h e 23+ a satisfying description of ^o(e).

For a fully microscopically treatment t h a t is a very encouraging result. The reproduction of Ay(e), however, is relatively poor i n all cases, i t will be interesting t o s e e progress here.

There is renewed i n t e r e s t in t h e s t u d y of two phonon excitations, especially double quadrupole and double octupole excitations. To separate one and two s t e p contributions, Dr. Hertenberger compared d a t a obtained with relatively high (65 MeV) or low (22 MeV) incident projectile energies. The preliminary results indicate, t h a t two phonon quadrupole excitations (in l12Cd) a r e spreaded significantly over a large number of s t a t e s . The two phonon octupole excitation h a s been studied for 96Z, due t o a n outstanding large octupole collectivity t h i s nucleus might be a good candidate. The observed 0+ and 6+ s t a t e s i n t h e energy range of twice t h e 3;

energy, however, bear much less t h a n 50% of t h e double octupole strength. Thus t h e experimental observation of double octupole strength i n scattering s t i l l remains a challenge.

(5) D-State effects i n 'He and 4He (and i n 'Li?)

One approach t o obtain information on t h e D-state of 3He or 4He in t h e ground s t a t e i s t o s t u d y with tensor polarized deuterons one s t e p transfer reactions (d,,He) or (d,ol), projecting t h u s on t h e cluster wave functions d+p or d+d, respectively. This aims t o determine t h e low momentum expansion coefficient D, of t h e r e l a t i v e orbital wave functions in t h e cluster expansion. The problem is t o find reactions where t h e mechanism of t h e reaction is sufficiently well under control and where t h e sizes of t h e calculated D-state effects in t h e tensor ana- lyzing powers a r e large i n comparison t o t h e uncertainties resulting from our incomplete knowledge about t h e reaction mechanism. Dr. Ludwig discussed t h e 3 2 ~ ( d , 3 ~ e ) 3 1 ~ ( 1 / 2 + ) and 58Ni(d,a) 5 6 ~ o ( 7 + ) reactions yielding good evidence for a D, value of 3He being near t o t h e value of and some evidence for a D2 value of 4He i n between - . I 2 fm2 and -.24 fm2 i n agreement with theoretical expectations. We had, i n addition, some speculations by F.D. Santos about t h e sign of a D-state amplitude of 6Li i n a n a+d cluster expansion, which should equal t h e sign of t h e 4He = d+d expansion. His analysis of t h e 6Li(d,a) reaction, however, should be extended t o higher energies t o avoid Compound nuclear distortions.

(6) Few nucleon systems

For few nucleon reactions, especially those of t h e mass 3 compound system, we h a v e a completely changed situation. The availability of realistic Fadeev calculations for low energy

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reactions is a n outstanding progress. Data a r e reproduced i n detail. W. Gruebler discussed D($,$) polarization t r a n s f e r d a t a with respect t o differences between t h e Bonn and t h e Paris nucleon-nucleon potential, especially with respect t o t h e SS,-3D, mixing parameter 8 . Similar results h a v e been obtained by t h e Kdln group.

(7) Neutron clean fusion?

At t h e Osaka meeting we had much discussion about neutron clean fusion, using polarized fuel. The proposal was dependent on t h e assumption, t h a t transition matrix elements, related t o tensor effects i n t h e interactions, a r e small. This h a s been p u t i n question by many authors. We had experimental contributions from Giessen and Koln t o provide additional low energy data. An analysis of a r a t h e r extended d a t a base by Lemaitre, presented yesterday, confirmed once more t h a t q u i n t e t t (S=2)-Singlet (S=O) transitions a r e apparently much larger t h a n needed for a neutron clean fusion process.

4. Summary

The selection, I presented, i s v e r y subjective and incomplete. I did n o t refer t o very beautiful studies about break up processes i n Seattle, TUNL, Birmingham, Ziirich, Koln and RCNP. They address t o a veariety of questions, t h u s it is difficult t o summarize. I also did not address t o heavy ion reactions, which a r e a rich field in i t s own.

I t is my impression, t h a t t h e main stimulus for t h e low energy s t r u c t u r e s t u d i e s with polarized beams results from t h e comparison of t h e d a t a with good calculations. Here we h a v e seen much progress. From t h e experimental point of view, t h e main developments of t h e l a s t f i v e y e a r s a r e t h e spin transfer and t h e high resolution experiments. But it is essential t o s t a t e , t h a t t h e field h a s a variety of aspects, t h e r e a r e many rich and deep phenomena, t h u s I should not t r y too much t o reveal specific points, I b e t t e r recommend t o read t h e 92 original contributions.