D. Bachelier

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.

The (3He, t) reaction at intermediate energies: Spin-isospin multipole transitions

Data for the (3He, t) reaction at 900 MeV and 2 GeV on the targets 26Mg, 40Ca, 48Ca, 54Fe, 90Zr and 208Pb are presented. A multipole decomposition for the data at 900 MeV has been made and the different distributions have been analysed. From the L = 0 cross section the Gamow-Teller strength distributions are extracted and compared with (p, n) data. The L = 1 and 2 distributions are analysed in a schematic model which describes the general systematics fairly well. The spectra at 2 GeV and Θ = 4° show for all targets a well-developed quasi-elastic peak. The A-dependence of the cross section is analysed in a simple model.

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.

Quasi-free (p, pα) scattering on 24Mg, 28Si, 40Ca and 58Ni at 157 MeV

Sixfold energy spectra have been measured for the (p, pα) reaction at 157 MeV on 24Mg, 28Si, 40Ca and 58Ni around quasi-free kinematic conditions. For the three s-d shell nuclei the experiment covered a map ranging from 0 to 220 MeV/c in recoil momentum and from 0 to 20 MeV in excitation energy of the final nucleus. Recoil momentum distributions have been obtained for the 0+ ground state and the 2+ first excited state of 20Ne, 24Mg and 36Ar, and also for the states around 4.4 MeV (mainly 4+) of 36Ar. The a spectroscopic factors extracted by a DWIA analysis are about three times larger than those predicted by the SU(3) model; however, they agree quite well in relative magnitude for a number of cases. The disagreement in shape between experiment and theory observed at low recoil momentum for the 2+ states might result from another reaction mechanism. The cross sections for 58Ni are about a factor often smaller than those for 40Ca. The 58Ni(p, pα)54Fe reaction seems to lead mainly to excited states of the final nucleus.

Excitation of the Δ resonance in heavy ion charge exchange reactions

Abstract Results on the Δ excitation by heavy ion charge exchange are presented. 900 MeV per nucleon 12C, 16O, 20Ne and 1100 MeV per nucleon 12C have been used. The Δ excitation strength depends on the projectile — ejectile nature and on the incident energy. The role of the target mass is also discussed. The peak for the Δ in nuclei is energy shifted from the free Δ peak.

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.

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.