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THE STRUCTURE OF 15N INVESTIGATED BY HEAVY ION INDUCED TRANSFER REACTIONS
U. Schlotthauer-Voos
To cite this version:
U. Schlotthauer-Voos. THE STRUCTURE OF 15N INVESTIGATED BY HEAVY ION INDUCED TRANSFER REACTIONS. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-271-C6-273.
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JOURNAL DE PHYSIQUE ColZoque C6, supplkment au no 11-12, Tome 32, Novembre-Dkcernbre 1971, page C6-271
THE STRUCTURE OF 15N INVESTIGATED BY HEAVY ION INDUCED TRANSFER REACTIONS
U. C. SCHLOTTHAUER-VOOS
Physikalisches Institut der Universitat, Marburg, Germany and Max Planck Institut fiir Kernphysik, Heidelberg, Germany
RbumC. - La structure de 15N a 6te Btudiee par des reactions de transfert de proton, triton et alpha induites par ions lourds. Les reactions de transfert A plusieurs nuclCons donnent quelques indications sur les configurations (sd)-l@ 1 6 0 b a n d e rotat. et (p3/z)-I @ 1 6 0 b a n d e rotat. des etats de parite respectivement positive et negative.
Abstract. - The structure of IsN was investigated by heavy ion induced proton-, triton- and alpha-transfer reactions. The multi-nucleon transfer reactions give some indication to explain the observed positive and negative parity states as having the configuration (sd)-1 @ 16Or,t. band and ( ~ 3 / 2 ) - ~ @ 1 6 0 r o t . band respectively.
In recent time 1 5was the subject of many investi- ~
gations, experimental and theoretical ones [I], [2], [3].
However, until now only the structure of some low lying states could be sufficiently explained [2], [3].
The configurations of states above 11 MeV are not yet known. Many of them are expected to have complicated many-particle many-hole structure and therefore should be experimentally best investigated by heavy ion induced transfer reactions. Thus we studied : the proton-transfer 11B(160, 1sN)12C, the alpha-transfer I1B(l60, 12C)15N and the triton- transfer 12C(19F, l60)I5N at incident energies between 27 and 70 MeV. At the high bombarding energies states in 5N up to about 16 MeV excitation energy are observed.
The experiments were performed at the Heidelberg MP tandem van de Graaff accelerator. The reaction products were analyzed by the conventional AEIE tech- nique, the AE detector being a gas filled proportional counter, the E detector a silicon surface barrier counter.
The total energy resolution in the experiments was 150-200 keV for light reaction products as Li-C and about 400 keV for heavy elements as Ne.
Figure I shows spectra of the investigated reactions.
As expected, in the proton-transfer only the two proton hole states in the 160-ground state of I5N are populated. They are the ground state (Jn = 112-) and the state a t 6.32 MeV (Jn = 312-) with the configuration (pl,,)-I O 160,, and (p3/2)-' 0 I6OgS res- pectively. The angular distributions of these transitions were described by DWBA calculations [4] following the method of Buttle and Goldfarb 151 (Fig. 2).
The parameters of the calculations were obtained by the optical model analysis of the elastic scattering data 161. The extracted spectroscopic factors are in good agreement with those obtained by conventional reactions and shell model considerations and are inde- pendent of the incident energy [4].
The triton-transfer reaction mainly populates
O-m
O O N
500 1 1 ~ ( 1 6 0 , 1 5 ~ ) 1 2 ~
-excitation energy channel number--+
FIG. 1 . - Spectra of the proton-, triton- and alpha-transfer reactions leading to 15N.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971663
U. C. SCHLOTTHAUER-VOOS
FIG. 2.
-
Angular distribution of the p-transfer llB(160, 15N)12C together with DWBA calculations following the method of Buttle and Goldfarb.positive parity states around 7, 8, 10 MeV and very selectively at higher energies. An extremely strong transition is observed to the two states a t 5.28 MeV (1" = 512') and 5.31 MeV (J" = 112') or one of them. Although the transferred I-value corresponding to these two states is different, the angular distributions give no indication which state is really populated.
At the higher energies a difference between the I = 0 and I = 2 angular distribution is expected from calcu- lations to occur at cm angles around 5O-lo0, which, however, cannot be measured. This is demonstrated in figure 3, where angular distributions are shown together with DWBA calculations in the fixed range model [7].
The observed states below 11 MeV are known to have the structure 1-p-2 h and 3 p-4 h [2], [3] and may be interpreted by the weak coupling of a (sd)-hole to the 4 p-4 h and 2 p-2 h components of the first rotatio- nal band in 160. As to the wave functions given by
Zuker [2] the 5.31 MeV state has the structure s-I @ 1 6 ~ 6 . 0 6 1 0 + . On the other hand 1606.,, corres- ponds to the ''Ne,, coupled to four holes inTthe pIl2-shell. The I9Fg, has the main configuration s-' C3'ONe,,. So the 1/2+ state of 15N at 5.31 MeV corresponds exactly to the 19F-ground state (see left side of Fig. 4).
These considerations lead to the conclusion that at 5.3 MeV mainly the 1/2+ state is excited. The very high cross section is then due to the fact that the initial and final configurations are identical [8].
Similar to the positive parity states some of the negative parity states should have the configuration p,/,-hole (or if possible p,,,) coupled weakly to the
160 rotational band, giving states with spin and parity 112-
...
1112-, five around 12 MeV, four around 16 MeV (see right side of Fig. 4). These states should preferen- tially be populated in a reaction I'B,,+
alpha.Indeed the most striking feature in the spectrum of
THE STRUCTURE OF 'sN INVESTIGATED C6-273
FIG. 3. - Angular distributions of the t-transfer reaction 12C(19F, 160)lSN together with DWBA calculations in the
Fixed Range model.
the alpha-transfer is the strong population of highly excited states above 9 MeV. Two groups may be distinguished, one around 11 MeV consisting of five states, one around 15 MeV consisting of four states.
The first strongly excited state at 9.16 MeV (J" = 312-) should therefore have a large component
4-nucleon transfer 3 - nucleon transfer 4 - nucleon transfer
- 1
on15N onI60 on160 on12C I
-
an12C on"6FIG. 4. - Correspondence between states in lsN, 160, l9F
and 2oNe.
( P ~ / ~ ) - ' @ 1606,06. Indeed this state is the first in 15N for which Lie and Engeland [3] predict a fraction of 4 p-5 h, although a very small one.
The investigated heavy ion induced transfer reactions give some indication how to explain the structure of the observed states in 1 5 (summary in Fig. ~ 4).
However, the experimental energy resolution is insufficient for more detailed information. More complicated experimental techniques as magnetic and time of flight methods should be used for further experiments. Nevertheless there are several reasons why to prefer heavy ion induced transfer reactions to Li induced reactions, which show less experimental difficulties. In heavy ion induced transfer reactions :
(1) the contributions from compound nuclear reactions are negligible,
(2) there is no break up of the projectile because of the stronger binding energies ; both effects may disturb Li induced reactions,
(3) special configurations of the transferred nucleons in the final nucleus may be selected by the choice of the projectile,
(4) there is no limitation in the number of trans- ferred nucleons for the study of even more complicated states.
Acknowledgments. - This work has been performed in cooperation with Prof. R. Bock and Dr. W. von Oertzen. The help of H. G. Bohlen, Dr H. H. Gutbrod and K. D. Hildenbrand during the measurements is gratefully acknowledged.
References
[I] AJZENBERG-SELOVE (F.), Nuel. Phys., 1970, A 152, 1. 151 BUTTLE (P. J. A.) and GOLDFARB (L. J. B.), NucI.
[2] ZUKER (A. P.), BUCK (N.) and MCGRORY (J. B.), Phys., 1966,78,409; N m l . Phys., 1968, A 115,461.
Phys. Rev. Letters, 1968, 21, 39. [6] PUHLHOFER (F.), RITTER (H. G.), BOCK (R.), BROM-
MUNDT (G.), SCHMIDT (H.) and BETHGE (K.), [3] LIE (S.), ENGELAND (T.) and DAHLL (G.) Nucl. Phys.,
1970, A 156, 449. Nucl. Phys., 1970, A 147, 258.
[7] Voos (U. C.), VON OERTZEN (W.) and BOCK (R.), LIE (S.) and ENGELAND (T.), Nucl. Phys., 1971, A 169, Nucl. Phys., 1969, A 135, 207.
617, for further references on lSN see ref. 1, 2, 3. [8] VON OERTZEN
(w.),
GUTBROD (H. H.), MULLER (M.), [4] SCHLOTTHAUER-VOOS (U. C.), VON OERTZEN (W.) Voos (U. C . ) and BOCK (R.), Phys. Lette~s,and BOCK (R.), submitted to Nucl. Phys. 1968, 26 B, 291.