TRANSFER OF EIGHT NUCLEONS ON SOME LIGHT NUCLEI

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TRANSFER OF EIGHT NUCLEONS ON SOME LIGHT NUCLEI

M. Conjeaud, S. Harar, E. da Silveira, C. Volant

To cite this version:

M. Conjeaud, S. Harar, E. da Silveira, C. Volant. TRANSFER OF EIGHT NUCLEONS ON SOME LIGHT NUCLEI. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-169-C6-170.

�10.1051/jphyscol:1971631�. �jpa-00214849�

JOURNAL D E P H Y S I Q U E

Colloque C6, supple'nzetlt au no 1 1- 12, Tome 32, Novembre-De'cembre 197 1, page C6- 169

TRANSFER OF EIGHT NUCLEONS ON SOME LIGHT NUCLEI

M. CONJEAUD, S. H A R A R , E. D a SILVEIRA and C. VOLANT DCparternent d e Physique NuclCaire, C . E. N. Saclay, France

Rksumk.

La reaction (14N, 6Li) sur les cibles IzC,

et 24Mg a eteetudiee A une energie incidente de

MeV et B quelques angles avant. Les spectres obtenus sont comparks a ceux obscrvks au

des reactions (7Li, t) ou (hLi, d). Les configurations a multiparticules de quel- ques etats des noyaux residuels sont discutees.

Abstract.

The (1 4N,

reaction on

zC, 14C.

and 24Mg targets has been studied at 45 MeV incident energy and at some forward angles. Spectra obtained are compared with those observed by the (7Li, t) or

d) reactions. Multiparticle configurations of some states of residual nuclei are discussed.

Recent work on the (I4N, 6Li) reaction [I] has shown some evidence of the usefulness of this reaction for the direct transfer of eight nucleons. Moreover the comparison between spectra of nuclei excited via the (7Li, t) o r (6Li, d) reactions a n d the (14N, 6 ~ i ) reaction is very promising a s a new a n d powerful spectroscopic probe t o study nlultiparticle excited states.

I n this paper we report on some results obtained from the (l4N, 6 ~ i ) reaction on "C, I4C,

and 24Mg targets. All these experiments were performed

using a 45 MeV 14N(5+) beam from the Saclay F N Tandem Accelerator.

The l2C(I4N, 6Li)20Ne reaction.

Outgoing 6Li particles were detected a t 30,

a n d 100 using a Buech- ner magnetic spectrograph with eight position-sensitive detectors in the focal plane. The overall energy resolu- tion was 40 keV mainly due t o the 10-12 pg/cm2 target thickness. Spectra were also obtained a t

(Fig. 1) and 20° using a AE.E telescope counter.

FIG. I .

6Li spectra obtained at Is0 laboratory

bombarding

I2C target (upper part) and a 14C target (lower part)

with

a nitrogen beam of

incident energy. Known levels from previous works are indicaded. Blackened peaks (lower part) arise from

contribution of

contaminant.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971631

C6-170 M.

CONJEAUD, S. HARAR,

DA SILVEIRA

C . VOLANT Same experiments have been performed recently at

Heidelberg [ l ] and Brookhaven [2] at 52 and 60 MeV incident energy respectively. General features are very similar to those observed in the present experiment at 45 MeV.

The comparison of relative yield of transitions to 20Ne via the transfer of four [3] and eight nucleons reveals the following features :

i) The members of the ground state band are strongly populated in both reactions.

ii) The members of the K

2- band (based on the 4.97 MeV, 2- state) are much more strongly excited by the (14N, 6Li) reaction than by the (7Li, t) reaction.

This suggests that the structure of this band involves an

excited core.

iii) Tewari et al. [4], and Arima et al. [4] have suggested the possible existence of a 8p-4h band or [220] quartet configuration around 7 MeV. The O+

states at 6.72 MeV and 7.20 MeV are two candidates for such structures with the corresponding 2' states a t 7.43 MeV and 7.84 MeV. The (14N, 6Li) reaction does not excite preferentially one of them. But recently strong evidence for such quartet states at 7.20 and 7.84 MeV have been reported by Middleton et al. [5].

The 14C(14N, 6Li)22Ne reaction. - Spectra have been obtained at two angles (100 and 15O) using a AE.E telescope counter. Targets of 25 pg/cm2 were made using a method described elsewhere [6]. The 14C(14N, 6Li)22Ne spectrum figure lb, does not exhibit a strong selectivity as observed in the I2C(l4N, 6 ~ i ) 2 0 N e . The sum of cross sections for all transitions up to 15 MeV excitation energy is 3 times weaker than for ON^.

In the 14C(14N, 6Li)22Ne reaction the same yield of 7Li relative to 6Li is observed. On the contrary, a factor of 10 has been found for the ratio 6Li/7Li in the case of 12C target. This difference can be explained by Q-value considerations (Q

- 23.8 MeV for (14N, 7Li) reaction on 12C and - 3.3 MeV on 14C).

More data are necessary to try a reliable comparison with the 180('Li, t)22Ne reaction [7].

The 160(14N, 6 ~ i ) 2 4 M g reaction. - Spectra at 30, 50,100 have been obtained using the Buechner magnetic spectrograph. The overall energy resolution is about 100 KeV mainly due to the 40-50 pg/cm2 thickness of SiO targets. Some data have been also recorded on nuclear emulsion plates and with a telescope counter.

Peaks arise up to 20 MeV excitation with comparable strengths in the whole spectrum. The study of the 2 0 ~ e ( 6 ~ i , d)24Mg has shown [8] that the transitions up to 8 MeV are much weaker than the transitions at upper energies.

Relative yields of peaks observed in 24Mg by transferring four or eight nucleons are schematically represented in the figure 2 ; they are normalized on the

FIG. 2.

- Diagram of relative yields of the 20Ne(6Li7

reaction (upper

and of the 160(14N,

reaction (lower part).

6.00 MeV 4' state. The comparison reveals the follow- ing features :

i) The ground state band members (up to the 6+

state) are the most prominent peaks of the spectra and their strengths are nearly proportional to (2 J + 1).

The 2 0 N e ( 6 ~ i , d)24Mg reaction does not exhibit such selectivity. The ground state band might have a large overlap with the

0 structure. Akiyama et al. [9] found that, the (84) SU3 representation of the [44] symmetry corresponds to about 75 % of the wave function of the ground state band.

ii) The 0' state at 6.44 MeV and the 2' at 7.35 MeV are described [9] mainly with the (46) SU, representa- tion. These levels are more excited by the transfer of four nucleons than by the transfer of eight. They might have an appreciable overlap with the 2 0 ~ e

1 quartet structure.

The 24Mg(L4N, 6Li)32S reaction. - Some measure- ments have been done for this reaction. Preliminary results indicate that the cross sections integrated over the whole spectrum is roughly one order of magnitude weaker than the one observed in the 12C(14N, 6Li)20Ne.

The drastic decrease of cross sections when increasing target mass is similar to the observations made for the (6Li, d) and ('Li, t) reactions [5]. In both cases, a break up process of the lithium ion could explain this aspect.

References

[I] MARQUARDT (N.) et al., Phys. Letters, 1971, 35B, 37. [4] TEWARI (S. N.), et al., Ph.vs. Rev., 1970, 1, 1156.

[2] NAGATANI (L.) et a]., Submitted to Phys. Rev. Letters, ARIMA (A.), et al., Phys. Rev. Letters, 1970, 25, 1043.

July 1971. [5] MIDDLF.TON (R.), et al., Preprint, August ^1971.

[6] B I A ~ C H I (L.), Rapport d'activite D . Ph. N . 1971.

i31

(A. D.), et B. A. P.

Washingt0n. _{[7] SCHOLZ (W.),} et al., Phys. Rev. Letters, 1969, 22, 949.

1971, AFl.

- - MIDDLETON (R.). Heidelberg Conference, 1969.

BASSANI ( G . ) , Rapport CEA. [91 AKIYAMA ~ . j ,

al.,

Phys., 1969, A 138, 273.