16O(16O, 12C)20Ne REACTIONS

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16O(16O, 12C)20Ne REACTIONS

P. Singh, D. Sink, P. Schwandt, R. Malmin, R. Siemssen

To cite this version:

P. Singh, D. Sink, P. Schwandt, R. Malmin, R. Siemssen. 16O(16O, 12C)20Ne REACTIONS. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-279-C6-282. �10.1051/jphyscol:1971665�. �jpa-00214883�

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JOURNAL DE PHYSIQUE Colloque C6, suppl&ment au no 11-12, Tome 32, Novembre-Dkcembre 1971, page C6-279

160(160, 12C)20Ne REACTIONS (*) P. P. SINGH, D. A. SINK, P. SCHWANDT

Physics Department, Indiana University, Bloomington, Indiana, U. S. A.

R. E. MALMIN and R. H. SIEMSSEN Argonne National Laboratory, Argonne, U. S. A.

RBsumB. - On a mesure la section efficace diffkrentielle de la rBaction 160(160, 1zC)ZoNe conduisant a l'ktat fondamental et B I'etat 2+ a 1,63 MeV du 20Ne, ainsi qu'a un Btat pouvant &re l'etat 4+ B 4,25 MeV du *oNe ou l'btat 2+ a 4,43 MeV du IzC, ou les deux. Les mesures ont Bte faites entre 17,5 MeV et 30 MeV (c. m.) a environ 90°, 75O et 600 (c. m.). Les fonctions d'excitation montrent a la fois une large structure et une structure fine, dont les largeurs sont respectivement 1,5 MeV et 0,5 MeV. De fortes correlations existent entre les fonctions d'excitation a diffkrents angles ainsi qu'entre differents Btats finals.

Abstract. - Differential cross sections from 17.5 to 30 MeV c. m. energy at about 90°, 75O and 60° (cm) have been measured for 160(160, 12C)20Ne reactions leading to the ground state, the 1.63 MeV 2+ state, and the 4.25 MeV 4+ state of 2oNe and/or the 4.43 MeV ²⁺state of 12C.

The yield curves have both gross and narrow structures of

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^{1.5 and}

-

.5 MeV in width respec- tively. Strong correlations are seen among the yield curves for different angles and also among those for different final states.

The motivation to ,study the 160(160, 12C)20Ne reaction was twofold. First, such a study may throw light on the mechanism for' 160 f 160 elastic scattering. In particular for coupled-channel calculations, which have been carried out recently [I]-[3] to explain the dominant features of the elastic data [4], one needs to know which channels couple strongly to the elastic channel. Second to see if there are any strong resonances a t high excitation which show up in these reactions and which may have special nuclear structure.

The 60 beam from the ANL tandem Van de Graaff was used to bombard

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100 pg/cm2 thick self support- ing A120, targets. The detection of the two associated particles in kinematic coincidence was used as a method to identify the specific reactions. Three pairs of such detectors under computer control were used simultaneously. The measurements consisted of (a) excitation functions from 35 to 60 MeV lab energy in 250 keV steps for reactions leading to the ground state, 1.63 MeV 2+ state, and to the 4.25 MeV 4' state of 'ONe and/or the 4.43 MeV 2' state of 12C (the experi- mental resolution was not good enough to resolve the two states in the latter case) ; and (b) the angular distributions 500 to 90° in 20 intervals a t 11 selected energies in the above range for transitions studied. I n figures 1-3 the excitation functions and angular distributions a t higher energies are presented for each case.

Prominant features of the excitation functions are (1) presence of sharp peaks and gross structures, (2) of strong correlation [5] between the yield curves at various angles (particularly for 1.63 and 4.3 MeV exci- tations) so much so that almost each peak or shoulder in one curve is present as a peak or a shoulder in the other two, and (3) fairly strong correlations [5] among the yield curves belonging to different excited states.

Although one may expect some angular cross-correlation even in the framework of statistical theory if only a few partial waves contribute, the extensive correlations of type (2) and (3) extending over a wide energy range is suggestive of mechanism which is highly localized in angular momentum space. Because there is good angular momentum matching [2] between the incoming (160 ^f160) and outgoing (I2C + "Ne) channels low lying states could be populated by partial waves including the grazing I-values. Strong correlation can only be understood if only a few partial waves are contributing. It is shown below that most proba- bly it is the partial waves near the grazing I-value that are important in these reactions.

In particular the attention is directed towards a large and broad (- 3 MeV) resonance like peak centered a t 53 MeV (lab) which dominates the yield curves for the 4.3 MeV excitation and is also present in the yield curves for the 1.63 MeV state and somewhat in those for the g. s. The fact that the angular distribution for

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971665

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C 6-280 P. P. SINGH, D. A. SINK, P. SCHWANDT, R. E. MALMIN AND R. H. SIEMSSEN

FIG. 1. - Excitation functions at 62.9O, 75.9O, and 89.6O in the c. m. (left) and angular distributions at higher energies (right) for the ground state of 20Ne.

the ground state (Fig. 1) in the vicinity of the broad resonance (see those a t 50.5, 52.5 and 54.5 MeV) are very alike and can be fitted with ( aP, + bPL+, l2

polynomial expansion, with best fit obtained with L = 16, confirms the dominance of grazing partial waves in these reactions. The Iack of structure in the angular distribution for the 1.63 and 4.3 MeV excitation (Fig. 2-3) rules out a diffraction phenomena as the cause of the pronounced resonance like structure at 53 MeV. However, a resonance formed with a specific I-value could lead to structureless angular distribution since many outgoing I-values (3 for the 2' and 5 for the 4+ final states) can contribute. It is interesting that the angular distributions for 1.63 MeV and 4.3 MeV exci- tations show most structure a t -- 49 MeV which is off the broad resonance as if this structure is due t o an underlying background. The large width of the structure a t 53 MeV coupled with strong correlations of the type indicated above and relatively simple angular distributions would tend to rule out statistical fluctua- tion as the origin of this peak (and perhaps for most other peaks seen in the excitation functions). Is is tempting to identify this broad peak with a shape

resonance in either the entrance or the exit channel.

The probable spin of 16-18 is consistent with the fact that a I w 18 resonance is indicated [6] in the optical model analysis of the ¹⁶⁰+ ¹⁶⁰elastic data near the peak energy. A resonance in 1 = 16 is also indicated in the analysis of the elastic data using a molecular- type potential [I].

Finally the cross section a t 900 for the three channels and for the 160(160, 160)160* to the cluster of states between 6 and 7 MeV are comparable and each is a significant fraction (10 %) of the 900 elastic cross sections. In coupled channel calculations, therefore, these channels should not be ignored.

In summary these studies indicate that a-transfer channels should be included in the coupled-channel analysis of 160 + ¹⁶⁰elastic data. The yield curves for 160(160, '2 C ) 2 0 ~ e reactions appears to be dominated by resonances which contribute to all three channels studied. Whether these resonances belong to the compound nucleus 32S, or to a molecular system of two 16 ions or represent cluster states is not clear yet. But it seems that these resonances are of high spin and may have simple nuclear structure properties.

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1 6 0 ( 1 6 0 , 12C)ZoNe REACTIONS

FIG. 2. -Excitation functions at 62.2O, 7 5 . 7 O and 89.8O in the for the 1.63 MeV

6 # 0 ~ 2 c ] 2 0 ~ e * 4.3 McV

r I

c. m. (left) and angular distributions at higher energies (right) 2+ state of 2oNe.

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'"I

¹^.o

-

S2[

^.7

2

::I

.o

.7

FIG. 3. - Excitation functions at 62.3O, 76.80 and 87.4O in the c. m. (left) and angular distributions at higher energies (right) for the 4.3 MeV excitation.

19

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C6-282 P. P. SINGH, D. A. SINK, P. SCHWANDT, R. E. MALMIN AND R. H. SIEMSSEN

References Work supported in part by National Science Foundation and U. S. Atomic Energy Commission.

[I] SCHEID (W.), GRINER (W.) and LEMMER (R.), Phys.

Rev. Letters, 1970, 25, 176.

[2] SHAW (R. W.), VANDENBOSCH (R.) and MEHTA (M. K.), Phys. Rev. Letters, 1970, 25, 176.

[3] RAWITSCHER (G. H.), proceeding of the Argonne Symposium on Heavy Ion Scattering, March 25- 26, 1971. To be published.

[4] SIEMSSEN (R. H.), MAHER (J. V.), WEIDINGER (A.) and BROMLEY (D. A.), Phys. Rev. Letters, 1967, 19, 968.

[5] A preliminary statistical analysis of the excitation functions supports these conclusions.

[6] SIEMSSEN (R. H.), FORTUNE (H. T.), MALMIN (R.), RICHTER (A.), TIPPIE (J. W.) and SINGH (P. P.), Phys. Rev. Letters, 1970, 25, 536.