S. Fortier

Astrophysical rate of the 11C+p reaction from the Coulomb break-up of a 12N radioactive beam

Abstract The Coulomb break-up technique has been used to determine the radiative width of one excited level in 12 N and the direct capture contribution to 11 C(p,γ) 12 N. The 12 N radioactive beam was produced through transfer and fragmentation reactions of a primary 95.5 MeV/u 14 N beam at GANIL. From the extracted cross section we deduce the rate of the 11 C+p reaction. Consequences for the hot pp chain are analysed.

Neutron-hole strengths and isobaric analog states in 95Zr

Abstract The 96 Zr(τ, α) 95 Zr reaction at 39 MeV incident energy was used to populate both the low-lying and highly excited (up to an excitation energy of 18 MeV) analog states in 95 Zr and some low-lying levels in other Zr isotopes. The main components of the 2d 5 2 . and 1g 9 2 . hole strengths and the analogs of the 2p 1 2 ., 2p 3 2 . and 1f 3 2 . proton hole states in 95 Y have been observed in this work. Angular distributions of transitions to 27 levels in 95 Zr have been extracted and analysed with DWBA calculations to yield spectroscopic factors. Between the excitation energies of 1 and 3 MeV in 95 Zr a number of levels with l = 2, 4 and 5 angular distributions have been identified. These are associated with the 2d 3 2 , 1g 7 2 and 1h 11 2 components already observed in the previous study of the 94 Zr(d, p) and (α, τ) reactions. The total strength and center of gravity for levels assigned to the (1g 9 2 ) −1 configurations in 95 Zr are presented and compared to the results obtained from the studies of neutron pick-up reaction on 91 Zr and 98 Mo. For the isobaric analog states observed between the excitation energies of 15 and 17 MeV, total widths, Coulomb displacement energies and spectroscopic factors are also determined from this experiment and the results compared with the properties of the low-lying proton hole states in the 95 Y parent nucleus.

Spectroscopic study of the N = 27 nuclei by means of the (τ, α) reaction

The (τ, α) reaction on 48Ca, 50Ti, 52Cr and 54Fe target nuclei has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° with a split-pole spectrometer in a large range of excitation energy. A local zero-range DWBA analysis has been carried out, using an isospin-dependent potential for the calculation of the neutron form factor, in order to get a coherent set of spectroscopic factors for both T> and T< levels in different nuclei. Assignments of l-values have been done for a large number of levels, most of them previously unknown, and energy centroids of hole states have been determined. Spectroscopic factors in 39Ca, 47Ti, 49, 52, 53Cr have also been obtained for strongly excited states. A sum rule analysis has been carried out for the N = 27 nuclei: the 1d32 and 2s12T< hole strengths are generally fully exhausted by the observed levels, whereas only a fraction of the 1d52 strength has been evidenced. The 1f72, 1d32 and 2s12 analog states have been observed in all nuclei; in 53Fe, the 1d32 and 2s12 analog states appear to be split in several components. In addition, a CRC analysis has been carried out for some levels with angular distributions not accounted for by a direct pick-up process. These levels are tentatively identified with states resulting from the coupling of one f72 neutron hole with excited states of the target nucleus. In particular, the 52− and 92− members of the [2+ ⊗ f72−1] configuration have been identified in each final nucleus. Unambiguous Jπ assignments are made; and the two-step (τ, α) reaction therefo e appears as a useful spectroscopic tool, especially for investigating high spin states.

D(F-18, p alpha) N-15 reaction applied to nova gamma-ray emission

The 18F(p,alpha)15O reaction is recognized to be one of the most important reactions for nova gamma-ray astronomy as it governs the early E <= 511keV gamma emission. However in the nova temperature regime, its rate remains largely uncertain due to unknown low-energy resonance strengths. We report here the measurement of the D(18F,p)19F(alpha)15N one-nucleon transfer reaction, induced by a 14 MeV 18F radioactive beam impinging on a CD2 target; outgoing protons and 15N (or alpha-particles) were detected in coincidence in two silicon strip detectors. A DWBA analysis of the data resulted in new limits to the contribution of low-energy resonances to the rate of the 18F(p,alpha)15O reaction.

Comparison of low-energy resonances in15N(α,γ)19F and15O(α,γ)19Ne and related uncertainties

A disagreement between two determinations of Gamma_alpha of the astro- physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.A disagreement between two determinations of {Gamma}{sub {alpha}} of the astrophysically relevant level at E{sub x}=4.378 MeV in {sup 19}F has been stated in two recent papers by Wilmes {ital et al.} and de Oliveira {ital et al.} In this work the uncertainties of both papers are discussed in detail, and we adopt the value {Gamma}{sub {alpha}} = (1.5{sub {minus}0.8}{sup +1.5}){times}10{sup {minus}9} eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei {Gamma}{sub {gamma}}({sup 19}F){approx}{Gamma}{sub {gamma}}({sup 19}Ne), {theta}{sub {alpha}}{sup 2}({sup 19}F){approx}{theta}{sub {alpha}}{sup 2}({sup 19}Ne) are discussed, together with the resulting uncertainties on the resonance strengths in {sup 19}Ne and on the {sup 15}O({alpha},{gamma}){sup 19}Ne rate. {copyright} {ital 1997} {ital The American Physical Society}

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.

Deeply bound hole states and isobaric analog states in 89Zr

Abstract The 90 Zr(τ, α) 89 Zr reaction, at 39 MeV incident energy, was used to populate hole states up to 13.5 MeV excitation energy in 89 Zr. A split-pole spectrometer was used for particle analysis and detection. In the excitation energy range between 0 and 3.5 MeV, about twenty well-resolved states were observed in 89 Zr, many for the first time. These states contain almost all of the spectroscopic strengths of the 1g 9 2 , 2p 1 2 , 2p 3 2 and 1f 5 2 neutron Subshells. The 30–35 keV energy resolution of the present study revealed a fine structure in the bump previously observed around 4.5 MeV and tentatively identified as the main part of the 1f 7 2 . hole orbital. About thirty levels or groups of levels were excited between 3.5 and 7.0 MeV excitation energy and angular distributions have been extracted for each individual group as well as for the gross structure itself. A DWBA analysis of the data was carried out and shows that the main part of the observed fine structure is populated by l = 3 angular momentum transfer. The full sum rule strengths obtained for the states below 3.5 MeV for the pick-up of the 1g 9 2 , 2p 1 2 . 2p 3 2 and 1f 5 2 nucleon lead to the identification of this highly fragmented bump with the 1f 7 2 shell. We found between 50 to 70 % of the 1f 7 2 hole strength in this region. Four sharp and strongly excited peaks are observed at higher excitation energy (8–10 MeV) above a continuous background. They are identified with the known isobaric analog states of the first four excited levels in the 89 Y parent nucleus. Around 13 MeV a weakly excited peak is observed which has an l = 3 angular distribution. This level is proposed as the IAS of the first component of the 1f 7 2 . proton hole orbital located around 5 MeV in 89 Y. In order to compare the spectroscopic strengths of the IAS and of their parent states, the neutron form factor has been calculated by solving the Lane coupled-channel equations. This method is shown to produce an overall agreement between the strengths of the parent analog pairs of levels.

Spectroscopic study of 55Co via the 54Fe(τ, d)55Co and 54Fe(τ, dp̃)54Fe reactions

Abstract The 54 Fe(τ, d) 55 Co reaction has been studied at 25 MeV incident energy with a split-pole spectrometer. About one hundred levels have been observed in 55 Co up to 10 MeV excitation energy. Angular distributions have been measured and analyzed with DWBA and Gamow functions as form factors for unbound levels. The 54 Fe(τ,dp) 54 reaction has been investigated at 24 MeV incident energy. The angular distributions of the emitted protons were measured in coincidence using method 2 of Litherland and Ferguson, with 0 detection of deuteron groups. Spins, population parameters, branching ratios and proton partial widths for the transitions to the ground and excited states of 54 Fe were determined from the analysis of the angular correlation data. The results of these two experiments provide a large number ofspectroscopic properties of unbound proton states and in particular of analog states of 55 Fe low-lying levels. The IAS of the 3 2 − ground state of 55 Fe is observed to be split between two individual levels. The amplitude of neutron coupling to the first 2 + excited state of 54 Fe is obtained for the lg 9 2 and 2 d 5 2 low-lying parent statfes in 55 Fe. summed spectroscopic factors and the centroid energies of the proton states in 55 Co are obtained. A comparison is made with previous (τ, d), (d, p) and (p, p) results.

Proton states in 59Cu and structure of the 59Cu-59Ni analog pair

The 58Ni(τ, d) stripping reaction has been studied at 25 MeV incident energy for 59Cu levels located above the proton threshold with a split-pole spectrometer. Seventy-four states between 3.8 and 9.0 MeV excitation energy were observed in 59Cu. The main components of the 2p32, 2p12, 1f52, 1g92 and 2d52 analog states of 59Ni levels were identified. Angular distributions of transitions to 67 unbound levels in 59Cu have been extracted and analysed with DWBA and Gamow functions as form factors to yield absolute spectroscopic strength. The 58Ni(τ, dp) was performed at 24 MeV incident energy. The angular distributions of the emitted protons were measured in coincidence using method II of Litherland and Ferguson with 0° detection of deuteron groups. Spins, population parameters and branching ratios of proton partial widths to the ground and excited states of the target were determined from the analysis of the angular correlation data. The results of these two experiments provide a large number of spectroscopic properties of proton states in 59Cu and in particular of analog levels of 59Ni low-lying states. The p32, g92 and d52 IAS are observed to be fragmented into at least two individual levels. The summed spectroscopic factors and the centroid energies of the proton states in 59Cu are obtained. A comparison is made with the previous (τ, d), (p, γ), (p, p) and (d, p) results on the same target. The amplitudes of neutron coupling to the first 2+ excited state in 58Ni are also determined for the g92 and d52 isobaric analog states in 59Cu.

The γ-decay of the 1g92 and 2p inner-hole states in 111Sn via the 112Sn(3He, αγ) reaction at 32 MeV

Abstract The γ-decay of the deeply-bound hole states in 111Sn has been investigated at 32 MeV incident energy by means of the 112Sn(3He, αγ) reaction. The α-particles emitted near 0° were detected in a Si counter located at the image plan of the superconducting solenoidal spectrometer SOLENO. The γ-rays in coincidence with the α-particles were detected by two Ge(Li) detectors located at 90° and 142° with respect to the beam direction, respectively. Energies, spins and decay schemes have been established for the low-lying states up to 2.5 MeV excitation energy in 111Sn. The γ-decay of the broad bump, located around 4.2 MeV and previously attributed to neutron pick-up from the inner 1 g 9 2 , 2 p 1 2 , and 2 p 3 2 neutron. Subshells, reveals the importance of quasiparticle-phonon m the spreading mechanism of the inner-hole strengths. The 1 g 9 2 and 2p strength functions have been deduced from the α-decay of the enhanced structures (3 ≦ Ex≦ 8 MeV). They are compared to the ones measured in previous inclusive neutron pick-up experiments and to those calculated in the framework of the quasiparticle-phonon nuclear model.