G. Berrier-Ronsin

Spectroscopic factors from one-proton stripping reactions on sd-shell nuclei: experimental measurements and shell-model calculations

Abstract The (3He,d) reaction has been investigated at 25 MeV on the target nuclei 16O, 18O, 19F, 23Na, 24Mg, 25Mg, 26Mg, 27Al, 28Si, 29Si, 30Si, 31P, 32S, 34S, 35Cl, 37Cl and 39K. Attention is restricted to the few even-parity levels in each final nucleus which are strongly populated. Values of the single-nucleon Spectroscopic factors C2S are obtained from the differential cross sections measured for these levels by analyzing them in the framework of the DWBA theory of nuclear reactions. The DWBA calculations use sets of 3He and deuteron optical-potential parameters which are experimentally validated and internally consistent. The absolute values of C2S thus extracted are rather insensitive to which sets of equivalent optical-model parameters are used in the DWBA analyses (variations of the order of ±15%) and the variations in the relative values for the different transitions are very insensitive to these choices. Most of the sensitivity which does exist can be traced to the specification of the absorptive part of the deuteron potential. The values of C2S are not sensitive to the presence of spin-orbit terms in either the deuteron or the 3He potential. Contrary to the case for optical-model potentials, however, absolute values of C2S are very sensitive to the radius parameter r0 chosen for the potential which binds the transferred proton, although relative values are almost completely insensitive to this choice. The effects of the spin-orbit part of this bound-state potential on j = l + 1 2 and j = l − 1 2 transitions have also been carefully investigated. Evidence is developed that the radius parameter rs.o. of the spin-orbit part of the bound-state potential should be smaller than that of the central part. The values of C2S extracted from the present data both with standard DWBA analyses (r0 = rs.o. and a0 = as.o.) and with analyses which incorporate a modified treatment of the spin-orbit term of the bound-state potential (r0 ≠ rs.o. and a0 ≠ as.o.) are compared with the predictions of the USD shell-model hamiltonian. The ratios between the experimentally based spectroscopic factors and the model predictions exhibit a marked difference between transitions into the 1 d 3 2 orbit and transitions into the 1 d 5 2 orbit when the standard DWBA analysis is used. However, this difference between 1d 3 2 and 1d 5 2 transitions is greatly reduced if, while the conventional values r0 = 1.25 fm and a0= 0.65 fm continue to be used for the central component of the bound-state potential, values of rs.o. = 1.00 fm and as.o. = 0.52 fm are used for the spin-orbit part.

Optical model analysis of 3He elastic scattering from s-d shell nuclei at 25 MeV

Abstract Angular distributions of elastically scattered 3 He particles from 16 O, 18 O, 19 F, 23 Na, 24 Mg, 25 Mg, 26 Mg, 27 Al, 28 Si, 29 Si, 30 Si, 31 P, 35 Cl, 37 Cl, 39 K and 40 Ca nuclei were measured at 25 MeV bombarding energy. The absolute differential cross-section data were analysed in the framework of the standard optical model with either a volume or a surface imaginary part. Three families of parameters were considered. For all these families, the real potential volume integral j r per interacting nucleon pair decreases as the mass number A increases. The family with j r = 380 MeV · fm 3 for 40 Ca and j r = 590 MeV · fm 3 for 16 O has been identified with the unique family obtained at higher energies, and is therefore considered as the “physical” family. The matter and charge radii deduced from the analysis are presented. The charge radii are compared with the ones obtained from muonic X-ray transitions and electron scattering measurements.

Proton occupancies in the even Se ground states via the (d, 3He) reaction

Abstract The spectroscopic factors for the levels of73,75,77,79,81As have been measured in the Se(d, 3He)As reactions at 25.2 MeV. The proton occupation numbers deduced for the even Se isotopes show that the striking change in proton configuration, observed for the Ge isotopes between N ⩽ 40 and N ⩾ 42, does persist for the Se isotopes. This change is not reproduced, either by a model calculation using spectral distribution methods or by proton wave functions recently suggested for the Se isotopes to explain the results of the Se(d, 6Li)Ge reactions.

Cross sections and asymmetries for the p(p,π0)pp reaction from threshold to 1 GeV

Abstract Total cross sections, differential cross sections and asymmetries for the reaction p( p ,π 0 )pp from threshold to 1 GeV have been measured. The outgoing π0 was detected by a 4π neutral meson spectrometer. Our data are compared with the corresponding np→NNπ± reactions. The isoscalar partial cross sections are extracted from the relevant total cross sections. They are significant throughout the whole energy range. Below the Δ-region, energy-integrated differential cross sections show a large anisotropy difference between the neutral and charged pion channels, respectively. A partial-wave fit shows that the initial T = 0 state is dominated by the 3D1 wave in this energy region. Finally, predictions of phenomenological models give reasonable good account of the measured observables but cannot reproduce the sign of the asymmetries.

Evidence for a change of structure in the heavy mercury isotopes around 200Hg

Abstract The (p, t) reaction has been performed on natural Hg and separated 196,198–202,204Hg targets, using a 25 MeV proton beam from the Orsay MP tandem accelerator and a split-pole spectrometer. In an excitation energy region below 2 MeV, new excited 0+ levels have been found. The enhancement factors of the populated 0+ states have been extracted. The resulting set of data exhibits striking discontinuities. In particular, both the energy and the (p, t) cross section of the 02+ level show a pronounced extremum in 200Hg. This observation, together with the previous experimental evidence, suggests a quite dramatic change in intrinsic structure of the Hg-isotopes at A=200. Microscopic calculations based on the deformed-shell-model approach suggest an explanation in terms of the Subshell closures at Z=80 and N=120 at weakly deformed oblate shapes, and a possible oblate-to-spherical shape transition. Qualitative theoretical arguments are confronted with the properties of pairing excitations calculated within a variant of the one broken-pair method. Within this approach the lowest excited 0+ states are interpreted as pairing vibrations. Energy positions of 0+ states and the pair transfer matrix elements are compared with the available experimental data.

One-nucleon pickup reactions on 32S: Experimental results and shell-model calculations

Abstract The 32S(d,3He)31P and 32 S ( 3 He ,α) 31 S reactions have been investigated at incident energies of Ed = 27 MeV and E3He = 25 MeV. The experimental values of excitation energies (0 ⩽ Ex(MeV) ⩽ 8) have been compared with the predictions of a complete sd-shell space, shell-model calculation. Spectroscopic factors obtained for 22 levels of 31S through the DWBA analysis of the experimental angular distributions have also been compared with the shell-model predictions. In order to reconcile the experimental and shell-model predicted values of the spectroscopic factors, the geometrical parameters of the spin-orbit part of the transferred nucleon potential are required to be smaller than those of the central part as it was previously observed in studies of the one-proton (3He,d) stripping reaction. The experimental fragmentation of the 1d 5 2 and 2s 1 2 strengths is correctly reproduced by the shell-model calculations. Twenty pairs of levels were identified as mirror states in the 31S and 31P nuclei and the ambiguities concerning the Jπ-values of eleven 31S levels could be removed.

(p,t) strengths in the Ga isotopes

Abstract The (p, t) reaction on the two stable Ga isotopes has been performed at 25 MeV, with 11 keV energy resolution. Levels up to 3.5 MeV in 67Ga and 4.3 MeV in 69Ga were measured. Differences observed between the distribution of the L = 2 (p, t) strength and the distribution of the B(E2)↑ strength may be explained by a mixed character of the 7 2 2 − level wave function. The distribution of the L = 0 strength indicates that the striking change in the ground-state structure shown between N = 40 and 42 already begins, although weakly, between N = 38 and 40.

States inPt193using the (p,t) reaction

Levels in /sup 193/Pt were populated by the ( p,t) reaction using 25 MeV protons and an isotopically enriched /sup 195/Pt target. A number of low-lying negative parity states are observed. By far the most strongly populated is the 1/2/sup -/ ground state. Some of these states are qualitatively interpreted assuming Coriolis coupling and rotation-vibration coupling between the 1/2/sup -/vertical-bar530vertical-bar, 3/2/sup -/vertical-bar532vertical-bar, and 5/2/sup -/vertical-bar532vertical-bar Nilsson states and an oblate core. L=0 angular distributions for states in /sup 193/Pt appear to be associated with the excited J/sup ..pi../=0/sup +/ states in the even-even Pt cores.

Large anisotropy difference in the NNπ system for π° and π± channels

Cross sections and beam asymmetries for the reaction p(pπ0)pp have been measured for incident polarized proton energies from the threshold to 1 GeV, and compared with those for the charged pion channels in p(n,π±)NN reactions. The observed anisotropy difference around 540 MeV has been ascribed to the 3D1 partial wave contribution to the np initial state. (AIP)

States in /sup 193/Pt using the ( p,t) reaction

Levels in /sup 193/Pt were populated by the ( p,t) reaction using 25 MeV protons and an isotopically enriched /sup 195/Pt target. A number of low-lying negative parity states are observed. By far the most strongly populated is the 1/2/sup -/ ground state. Some of these states are qualitatively interpreted assuming Coriolis coupling and rotation-vibration coupling between the 1/2/sup -/vertical-bar530vertical-bar, 3/2/sup -/vertical-bar532vertical-bar, and 5/2/sup -/vertical-bar532vertical-bar Nilsson states and an oblate core. L=0 angular distributions for states in /sup 193/Pt appear to be associated with the excited J/sup ..pi../=0/sup +/ states in the even-even Pt cores.