M. Bernas

Réaction (p, d) à 156 MeV et structure des noyaux légers

Abstract Energy spectra and angular distributions were measured with a magnetic spectrometer for the following pick-up reactions at 156 MeV: 6 Li(p, d) 5 Li; 7 Li(p, d) 6 Li; 9 Be(p, d) 8 Be; 10 B(p, d) 9 Be; 11 B(p, d) 10 B; 12 C(p, d) 11 C; 16 O(p, d) 15 O; 28 Si(p, d) 27 Si and 40 Ca(p, d) 39 Ca. It is shown that, at this energy, where the separation between levels is small compared to the energies of the incoming and outgoing particles, the PWBA without cut-off radius, although not being able to reproduce the experimental angular distributions, allows a good extraction of the relative spectroscopic factors F 2 for a given l . The F 2 values agree quite well with those calculated for 1p shell nuclei with the wave functions of Cohen and Kurath and of Boyarkina; the agreement is in general also good with the F 2 extracted by DWBA at various energies. The ratio between the absolute F 2 obtained by PWBA and the theoretical F 2 might be considered as a distortion factor, which is a function of angle θ; it appears that this factor does not vary much throughout the 1p shell. The pick-up in the inner major shell is discussed; the cross section for simple 1s pick-up decreases sharply when going from 4 He to 6 Li target nuclei; a similar effect appears for 1p pick-up between 16 O and 19 F; this behaviour, probably related to the high neutron momenta involved, is not yet well understood.

Exchange effect in the 166 MeV α-particle elastic scattering on 6Li

Abstract The angular distribution of the elastic scattering of 166 MeV α-particles on 6Li nuclei has been measured from 3° to 175° (c.m.). The backward rise of the cross section is explained in terms of two-nucleon pick-up. The symmetry of the entrance and exit channels makes the measurement a good test of the applicability of finite-range DWBA calculations for this exchange effect. This effect is shown to become negligible for heavier nuclei like 40Ca.

Coherent π+ photoproduction on 3He in the region of the Δ1236 resonance

Abstract An experimental study of the 3 He( γ , π + ) 3 H reaction has been performed in the region of the Δ 1236 resonance between 227 MeV and 453 MeV. The experiment shows two main features: at fixed four-momentum transfer q 2 , the resonance is shifted towards lower energies, this shift increasing with q 2 ; at a fixed angle, but variable q 2 , the minimum of the charge form factor observed in the elastic scattering of electrons on 3 He does not appear. Recent calculations explain partly these two effects. This experiment might be used as part of a test of time-reversal invariance in the electromagnetic interaction of hadrons.

Reaction (p, d) À 156 MeV sur l'azote et le fluor et melanges de configurations dans les noyaux 13N, 14N, 18F et 19F

Abstract The (p, d) reaction at 156 MeV on nitrogen and fluorine has been studied to test a configuration mixing description of the target and residual nuclei, and the single interaction pick-up mechanism. At this energy, a simple method is used to extract the relative spectroscopic factors for a given l for the whole spectrum. The comparison has been made using spectroscopic factors calculated for nitrogen and fluorine with wave functions of Ripka and Giraud, and with the 1p wave functions of Cohen and Kurath for nitrogen. For nitrogen there is a good agreement between experimental and theoretical spectroscopic factors, especially when comparing with those calculated by Kurath where agreement is within about 20%. Such an agreement has also been obtained for other nuclei of the 1p shell. For fluoribe, the theoretical description seems qualitatively correct up to about 5 MeV excitation in 18F. Excitation of the observed higher states must be assigned to pick-up in the inner 1p shell or to the contribution of more complex configurations.

Two-nucleon final state interactions and the 4He(p, t)2p, 4He(p, 3he)pn and 4He(p, d)3He reactions at 156 MeV

Abstract The pp and pn final state interaction (FSI) spectra were measured in the 4 He(p, t)2p and 4 He(p, 3 He)pn reactions at 156 MeV. The Watson-Migdal treatment and the known scattering length were used to analyse the spectrum shapes at different small angles: the agreement obtained is quite good; the Watson-Migdal assumptions are indeed better satisfied for higher primary energies. Use of a modified Born approximation for the pp spectrum further improves the fit. When comparing the absolute magnitudes of the three reactions, it appears that the 4 He(p, 3 He)pn spectra should include mainly the singlet contribution. The angular distribution of the reaction 4 He(p, d) 3 He has been measured from 3° to 177° (c.m.). No evidence has been found for excited states of 3 He.

Charge symmetry in the mirror reactions 3H(d, 3He)2n and 3He(d, t)2p at 83 MeV

Abstract The nn and pp final-state interaction spectra were measured at 2.5° and 7.5° in the mirror reactions 3 H(d, 3 He)2n and 3 He(d, t)2p at 83 MeV deuteron energy. The Watson-Migdal treatment does not give a good description of these spectra. Application of the comparison method of Slobodrian et al. gives a mean value for a nn of −15.5 fm, and very large values for r on : the variation of the production form factor with the relative energy of the two nucleons seems also very large. The coherent addition of a constant amplitude to the Watson-Migdal amplitude introduces only a limited number of parameters: this method gives for r on a mean value of 3.3 fm, but the values found for a nn (around −12 fm) are in general smaller than the theoretical predictions. A more rigorous theoretical treatment of these deuteron induced reactions seems necessary. Measurements have also been performed on the mirror reactions 3 H(d, t)pn, 3 He(d, 3 He)pn and 3 H(d, t) 2 H, 3 He(d, 3 He) 2 H.

First study of the (14C, 12C) reaction: Selectivity of the reaction and the energy levels of 28Mg and 30Si

Abstract The first measurements are reported for ( 14 C, 12 C) two-neutron stripping reactions. Energy spectra up to an excitation energy of 12 MeV have been measured at 69 MeV for the 26 Mg( 14 C, 12 C) and 28 Si( 14 C, 12 C) reactions. A strong selectivity of this reaction is observed. Using this selectivity, the comparison of the spectra suggests J π assignments for several 30 Si and 28 Mg states. The results from the other two-neutron stripping reactions, (t, p) and ( 18 O, 16 O) are compared with those of the ( 14 C, 12 C) reaction.

Asymétrie dans la réaction α + d → t+3He à 55 MeV (c.m.) et théorème de Barshay-Temmer

Abstract The complete angular distribution and the forward-backward asymmetry have been measured for the reaction α+d → t+ 3 He at 55 MeV (c.m.). Validity of the isospin concept requires this angular distribution to show up a forward-backward symmetry, following a theorem by Barshay and Temmer. Important asymmetries have been observed previously at lower energies. At 55 MeV a small but significant angle dependent asymmetry appears, which is, however, only about half the one observed at 27 MeV (c.m.). A calculation of the Coulomb effects in a DWBA analysis seems to account for this asymmetry. Other possible effects are also discussed.

Étude expérimentale de la réaction D(p, γ)3He à 156 MeV

Abstract The D(p, γ)3He reaction has been studied at 156 MeV in view of obtaining information on the wave function of 3He. The differential cross section has been measured from θγ(lab) = 30° to 120°.

DIFFUSION QUASI-LIBRE SUR DES SOUS- STRUCTURES α DANS 24Mg ET 40Ca A 156 MeV

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published 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. DIFFUSION QUASI-LIBRE SUR DES SOUSSTRUCTURES α DANS 24Mg ET 40Ca A 156 MeV D. Bachelier, M. Bernas, J. Boyard, T. Hennino, H. Holmgren, J. Jourdain, P. Radvanyi, M. Roy-Stephan