HAL Id: jpa-00214843
https://hal.archives-ouvertes.fr/jpa-00214843
Submitted on 1 Jan 1971
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
ARE THERE α -DOORWAY STATES IN 28Si ?
P. Charles, R. Ballini, M. Dost, B. Fernandez, P. Fouan, J. Gastebois
To cite this version:
P. Charles, R. Ballini, M. Dost, B. Fernandez, P. Fouan, et al.. ARE THERE α - DOORWAY STATES IN 28Si ?. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-155-C6-156.
�10.1051/jphyscol:1971625�. �jpa-00214843�
JOURNAL DE PHYSIQUE Co11oque C6, supplkment au no 1 1-12, Toine 32, Novembre-Dkcembre 1971, page C6-155
ARE THERE a -DOORWAY STATES IN * Si ?
P. CHARLES, R. BALLINI, M. DOST, B. FERNANDEZ, P. FOUAN and J. GASTEBOIS Departement de Physique NuclCaire
C. E. N. Saclay
Rbume. - Les fonctions d'excitation de la reaction 12C(160, a)24Mg, conduisant a des etats de 24Mg, jusqu'k une excitation de 22 MeV, ont ete mesurees a 15O et 22,S0 (angles dans le labo- ratoire), de Elab = 41,67 MeV a Elrlb = 56 MeV, par pas de 0,33 MeV. Des structures correfees de un k deux MeV de large sont observks. Differentes interpretations possibles sont discutks.
Abstract. - The excitation functions of the reaction 12C(160, a)24Mg*, leading to states of 24Mg up to 22 MeV excitation, have been measured at two laboratory angles, lSOand 22.S0, from Elab = 41.67 MeV to 56 MeV, in steps of 0.33 MeV. One-to-two-MeV wide correlated structures are observed. Possible interpretations are presented.
The observation [I] of only a few strongly excited states in 24Mg by the 160(12c, a)24Mg reaction was suggestive of a simple direct transfer mechanism.
This led to speculations about their structure and motivated various measurements [2], [3], [4].
After finding that the spectra changed quite dras- tically with '('0 bombarding energy we set out to measure the excitation functions of the differential cross section in 12C(160, a)24Mg, thus extending the measurements of Halbert et al. [5] considerably in final nucleus excitation energy (22 MeV) and somewhat in the range of bombarding energy (18 to 25 MeV C. M.). The energy resolution in the a-spectra, taken with conventional E-AE counter telescopes, was about 160 keV, the main contributions coming from target thickness (40 pg/cm2) and kinematic broade- ning. The target thickness corresponds t o at1 energy averaging in the excitation functions of about 100 keV C . M. (the value of the coherence width T in 28Si as determined by Halbert et al. [5] at somewhat lower excitation energy is about 120 keV). In figure l a are shown the excitation functions of ten states in 2 4 ~ g , measured at 15O lab. angle, between 35 and 42 MeV excitation in 28Si, with the resolution indicated above, and in 140 keV C. M. steps. The same data, averaged over I MeV C. M., are shown in figure Ib in order to bring out the wider structure also present.
The fluctuating excitation functions of figure l a exhibit several examples of cross-correlation, the clearest ones for the 15.15, 16.55, and 17.17 MeV states in the region of about 38 MeV excitation in 28Si, and for the 8.12 and 11.88 MeV states near 37 and 39 MeV excitation in 28Si. This is brought out even more strikingly in the curves of figure lb.
Structures of 1-2 MeV width appear in several of the curves at the same energies.
Frci. la. - Excitation functions of the reaction lZC(160, a)24Mg for 10 levels, at e l a b -- 150, averagcd over 100 keV.
According to the results derived from a synthesized, randomly fluctuating excitation function by Singh et al. [6], we estimate the probability for chance occurrence of a 1 MeV wide structure (1 MeV z 8 T) in two 7 MeV ( 2 56 T) long excitation functions within the same 1 MeV interval to be inferior to For the occurrence in three excitation function (as for instance for the 15.15, 16.55 and 17.17 MeV states), however, the estimated probability reduces to < Thus, it is not impossible but unlikely that we are dealing with purely statistical fluctuations of the cross sections.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971625
C6-156 P. CHARLES, R. BALLINI, M, DOST, B. FERNANDEZ, P. FOUAN A N D J. GASTEBOIS
~ ~ 0 6 0 , ~ ) ~ ~ y
EXClTbTlON FUNCTIONS bVERAGE0 W I R I M N
EiClibiioH C l t l G I 1H "s, Bi*b-IS. tlCtFh7ION C R t l i G T I Y 2a~
m 1 12 24 n
f..lY.VI I. 24 C,.UI.Vl I I I . 2'
FIG. Ib. -Excitation functions of the reaction 12C(1b0,1)24Mg
for 10 levels, at Olab = IS0, averaged over 1 McV.
The existence of cross-section fluctuations makes the presence of a strong non-direct reaction amplitude quite evident. The fact of the large a-cross-sections to relatively few final states a t high excitation in 2 4 ~ g can be related to angular momentum. It has been shown by statistical model calculations for heavier nuclei [7] that slightly below the yrast spin the a-emis- sion probability of the compound levels increases rapidly. Nucleons are no longer able to carry away enough angular momentum. In the collision of two strongly absorbed heavy ions like 12C and '% the non- direct amplitude should proceed by the formation of intermediate states with spins close or equal to the yrast spin.
Various resonance-like structures with interme- diate widths have been observed [$I, [9] in the past years in heavy-ion collisions, mostly involving 12C,
1 6 0 or 14N. For systems containing a ''C ion, at C. M. energies sufficiently above the Coulomb barrier to break up I2C into a-particles (only 7.27 MeV).
finer structure generally appears superimposed on the wide bumps now currently attributed to the optical potential. On the contrary, for the 160-160 system,
at C. M. energies less than needed for break-up (14.43 MeV above the barrier), there is usually no structure observed in the excitation function. The non- observation of distinct peaks in the spectrum [lo]
of the 1 4 N ( 1 4 ~ , ~ 1 ) ' ~ M g reaction at 14 MeV C . M.
supports this view, the break up energy for 14N -+ 3 cr + d being 20.40 MeV.
The C. M. energy of the I2C + 1 6 0 system in the present experiment is 18 t o 25 MeV, i. e. about 9 to 16 MeV above the mutual Coulomb barrier, with the main structure occurring at about 13 MeV above the barrier. This energy is sufficient to break 12C into a-particles, but not to do so for 160. It is therefore attractive to recall t h e idea of Michaud and Vogt [ l l ] of a hierarchy of states corresponding to [a] molecular, [b] cc-cluster, and [ c ] nucleon degrees of freedom in the system of colliding ions and its relation to the structure of scattering excitation functions. It is suggestive to imagine that we are in the presence of a type [b]
excitation in "Si, formed by three a-like clusters coupled to the 160 ground state.
For a rough estimate of the energy of such a state in 28Si one can make use of the binding energy systema- tics published [I21 in order to justify the idea of nuclear quarteting. According to ref. [12], the lowest three-quartet excitation in 28Si is the one where three quartets from the (2 s, 1 d) shell are excited up to the (1 f, 2 p) shell, at about 31 MeV. The overlap of such a configuration with the IZC in the entrance channel ((I s ) ~ (1 p)') is, of course. not known. On the other hand, setting the condition to make this overlap as large as possible would require to estimate the excitation (1 P)8 -+ (2 s, 1 d)' plus (1 s ) ~ -+ (2 s, 1 d)4 in *%i. The first contribution here is 35 MeV, the second > 12 MeV (the energy for (1 s ) ~ -+ ( I p)4 has to be added but is not given in ref. [12]). Thus, one obtains an energy > 47 MeV. The envisaged three quartet excitation would therefore fall between 31 and at least about 50 MeV. In this interval, it is easy t o place the 38-39 MeV excitation at which we observe the structure in the excitation functions. This consti- tutes, however, no positive proof.
References [I] MIDDI~ETON (R.) et a]., Phys. Rev. Letters, 1970, 24,
1436.
[2] GOBRI (A.) et al., Phys. Rev. Letters, 1971, 26, 396.
[3] BALAMUTH (D. P.) et al., Phys. Letters, 1971, 26, 1271.
[4] GASTEBOIS (J.) et al., Lettere a1 Nuovo Cinzento, 1971, 2, 90.
[5] HALBERT ( M . L.) et al., Phys. Rev., 1967, 162, 899.
[6] SINGH (P. P.) et al., Intern. Nucl. Phys. Conf. Gatlin- burg Tennessee, 1966, p. 249 (Academic Press 1 967).
[7] DARRAH THOMAS (T.), Ann. Rev. ~Vucl. Sci., 1968, 18, 343.
[8] SEE BKOMLEY (D. A.), Nuclear Reactions Induced by Heavy Ions, Heidelberg 1969; North-Holland Publishing Co., 1970, p. 27, and references therein.
[9] LASSEN (N. 0.) and OLSEN (J. S.), Mat. Fy.7. Medd.
Dan. Vid. Selsk., 1963, 33, 13.
P A ~ E R S O N (J. R.) et al., N~tcl. P/zys., 1971, A 165, 545.
NAGORCKA (B. N.) et al., Nature Phys. Sci., 1971,231, 18.
[lo] MIDDLETON (R.) et al., private communication.
[l l ] MICHAUD ( G . ) and VOGT (E. W.), Phys. Letters, 1969, 30B, 85.
[12] ARIMA (A.) et al., Phys. Rev. Letters, 1970, 25, 1043.
