A shell-model calculation in the continuum for the reaction N15(n,n′) N15

Using the shell-model approach to nuclear reactions, the Lippmann-Schwinger equation for elastic and inelastic nucléon scattering is solved numerically. The procedure is based upon an adaptation of a method due toWeinberg. The differential and total elastic and inelastic cross-sections and the asymmetry of the scattering of a transversally polarised beam of neutrons are calculated. The positions (energies and total widths) and residues (partial widths) of the scattering matrix in the complex plane are determined. The results are discussed and compared with previous treatments.     

Investigation of 16O excited states via the 14N(3He,p) reaction

The reaction ¹⁴N(³He, p)¹⁶O has been investigated at a bombarding energy of 15.0 MeV, using a differentially pumped gas target. Angular distributions were measured for 30 levels below 16.5 MeV in excitation. Data were compared with shell-model calculations of Zuker, Buck and McGrory for states whose correspondence with theory is established. Many states are found to possess a large compound-nucleus reaction component. Several previously unreported levels are observed at high excitation. Angular distributions for all except the weakest levels have been compared with DWBA calculations.     

Proton polarization in the14N(3He,p)16O reaction near 10 MeV and reaction mechanisms

The angular distribution of the proton polarization has been measured between 30° and 110° in the CM system near 10 MeV laboratory energy. It provides a sensitive test of theoretical fits to the cross section angular distribution. The reaction mechanism is largely direct below 70° and evidence has been found for a deuteron-cluster transfer. Nuclear Reactions¹⁴N³(He,p),E=9.8 MeV; measuredP(E,θ), σ(E,θ); DWBA and deuteron transfer analysis; deduced dominant mechanism of forward angle amplitudes.     

Single-particle and core-excited states in 49Sc: (II). The 48Ca(α, t)49Sc reaction

The ⁴⁸Ca(α, t)⁴⁹Sc reaction has been studied at 36 MeV incident energy. About eighty levels have been observed up to 7.5 MeV excitation energy and angular distributions were measured from 6° to 58°, using a split-pole spectrometer. A local zero-range DWBA analysis has been carried out, and the deduced l-assignments and spectroscopic factors are compared with those obtained from the (³He, d) reaction. In the other hand, a large number of angular distributions cannot be reproduced by the DWBA calculations; they have been compared with the results of coupled-reaction-channel calculations, assuming two-step excitation of weak coupling states with a [${}^{48}\text{Ca}{}^1\text{?}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$] structure. Good agreement between experimental angular distributions and two-step predictions is obtained for several ⁴⁹Sc levels, suggesting spin and parity assignments. Moreover, as rather large cross sections are predicted for two-step excitations, it is concluded that, generally, these processes cannot be neglected in the analysis of (α, t) reactions.     

The polarization of the neutrons from the reaction15N(d,n 0)16O atE d=4.35 and 5.50 MeV

In the reaction¹⁵N(d, n ₀)¹⁶O the angular dependence of the neutron polarization and of the differential cross section was measured at average deuteron energies of 4.35 and 5.50 MeV. Parallel features are observed.     

Single-particle and core-excited states in 49Sc: (I). The 48Ca(3He,d)49Sc reaction

The ⁴⁸Ca(³He, d)⁴⁹Sc reaction has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° using a split-pole spectrometer, for about 160 levels located up to 18 MeV excitation energy. A local zero-range DWBA analysis has been carried out, using Gamow functions as form factors in the case of unbound states; l-assignments and spectroscopic factors are obtained for a large number of levels, most of them previously unknown. The summed experimental spectroscopic strengths for the T_<, l = 1 and l = 3 levels are in good agreement with the shell-model sum-rule limits for 1f-2p proton states, and their energy centroids have been determined. The $\text{lg}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ strength in ⁴⁹Sc is strongly fragmented: about 27% of the T_< strength is carried by twenty-three levels located between 6.5 and 13.5 MeV. Spectroscopic factors for analog states are compared with those from previous (p, p), (³He, dp) and (d, p) experiments.     

Semi-microscopic analysis of multi-nucleon transfer and the 40Ca(α, p)43Sc reaction

A semi-microscopic model for analyzing multi-nucleon transfer reactions in the distorted-wave Born approximation is presented. This model, which is shown to be compatible with other semi-microscopic models, employs cluster form factors. The cluster spectroscopic factors are calculated using shell-model wave functions. Data from the ⁴⁰Ca(α, p)⁴³Sc reaction are presented and used to test the model. The relative cluster spectroscopic factors for a number of levels are found to be in agreement with those calculated using wave functions in an $\text{(}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{)}{}^3$ basis.     

A systematic study of the 88(16O, 15N reaction: Evidence for recoil effects

Augular distributions for the ⁸⁸Sr(¹⁶O, ¹⁵N) reaction leading to the $\text{2}\text{p}\text{1}\text{2}$ and $\text{1}\text{g}\text{1}\text{2}$ states in ⁸⁹Y were measured at several incident energies below and above the Coulomb barrier. Comparison with no-recoil DWBA calculations reveals an energy-dependent normalization for the transition of the $\text{2}\text{p}\text{1}\text{2}$ state. This energy dependence disappeared, however, in a recent DWBA calculation made by Tamura and Low, which included recoil effects.     

Microscopic (p,p′) calculations and polarization charges with large basis shell-model wave functions

Techniques have been developed for performing microscopic model DWBA calculations of inelastic nucleon-nucleus scattering using large basis shell-model wave functions to describe the nuclear states involved. For the case of ¹³⁸Ba at a bombarding energy of 30 MeV, we obtain good fits to the data by including the exchange amplitude in the DWBA and assuming a state and multipole independent polarization charge.     

Cross sections and thermonuclear reaction rates for 42Ca(p, γ)43Sc, 44Ca(p, γ) 45Sc, 44Ca(p,n) 44Sc and 45Sc(p,n)45Ti

The yield of γ-rays from the reaction ⁴²Ca(p, γ)⁴³Sc has been measured as a function of bombarding energy over the range 0.63–3.01 MeV, from ⁴⁴Ca(p, γ)⁴⁵Sc over the range 0.775–4.00 MeV, from ⁴²Ca(p, p'γ)⁴²Ca over the range 2.24–3.01 MeV, and from ⁴⁴Ca(p, p'γ)⁴⁴Ca over the range 1.90–5.03 MeV. The cross section of the reaction ⁴⁴Ca(p, n)⁴⁴Sc has been measured from threshold to a bombarding energy of 5.05 MeV by observation of the 1157 keV γ-ray associated with the residual ⁴⁴Sc activity, and the cross section of the reaction ⁴⁵Sc(p, n)⁴⁵Ti has been measured from threshold to a bombarding energy of 4.00 MeV both by observation of the annihilation radiation associated with the residual ⁴⁵Ti activity and by measurement of the total neutron yield with a wide-angle BF₃ tube and paraffin detector. All these data are compared with statisticalmodel calculations and satisfactory agreement is achieved. Thermonuclear reaction rates for the (p, γ) and (p, n) reactions are calculated for the temperature range 5 × 10⁸-10¹⁰K and the significance of these results for explosive nucleosynthesis in stars is discussed.     

15N(p,po)15N and the levels of 16O for Ex = 14.8–18.6 MeV

Angular distributions of cross section and analyzing power for elastic scattering of protons from ¹⁵N have been measured for E_p = 2.7–7.0 MeV. A phase-shift analysis of the data yields spin-parity assignments and level parameters for seventeen states of ¹⁶O in the excitation energy region 14.8–18.6MeV. Three of the resonances have not previously been identified, among them being a broad J^π = 2^? level at E_p = 6.1 MeV which is almost certainly the analog of the 2^? 1p1h state with configuration ($\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{,}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}$) at E_x ∽ 5.0 MeV in ¹⁶N. The broad level previously reported near E_p = 5.0 MeV has been observed and its parameters determined. A resonance analysis of the phase shifts yielded values of E_r, Γ and Γ_p for all of the levels. The J^π assignments are in agreement with previously reported values. For resonances having J = l, the data can usually be fit with a resonant phase shift corresponding to either J = l + 1 or J = l ? 1, in addition to the phase shift for J = l. Which of the two spurious-J solutions occurs seems to depend on whether the partial wave through which the resonant state is formed is $\text{J = l +}\text{1}\text{2}\text{or}\text{J = l ?}\text{1}\text{2}$.     

The reaction12C(α,p 0)15N around 18 mev excitation energy of16O

Proton angular distributions of the reaction¹²C(α, p ₀)¹⁵N have been measured at excitation energies from 17.5 to 18.7 MeV. The shape of the angular distributions changes strongly betweenθ _cm = 26°–162°. The integrated cross section shows a smooth behaviour in contrast to the (α, n) mirror reaction. The results can be explained by an interference of the known 18.18 MeV, 2⁺ state with a strong isospin mixed 18.1, 3^? state in the¹⁶O compound nucleus.     

Spin alignment and polarisation in the (16O,15N) transfer reactions

The spin alignment of the¹⁵N ₃ ^2/* (6.33 MeV) state was measured in the⁸⁸Sr(¹⁶O,¹⁵N)⁸⁹Y reaction atE _L =96 MeV using theγ-recoil method. Angular distributions of excited states in¹⁵N and⁸⁹Y were measured with high accuracy. The analysis in terms of DWBA shows that the spin alignment is correctly described by the usual reaction models. The polarisation of the outgoing¹⁵N_1/2(GS) and¹⁵N ₃ ^2/* (6.33 MeV) is discussed. It is shown that cross section differences for transitions to final states with different configurations are sensitive to a spin-orbit potential of¹⁵N. The strength and sign of the spin-orbit potential for¹⁵N is determined.     

First measurements of the 16O(e, e'pn)14N reaction

This paper reports on the first measurement of the ¹⁶O(e, e'pn)¹⁴N reaction. Data were measured in kinematics centred on a super-parallel geometry at energy and momentum transfers of 215MeV and 316MeV/c. The experimental resolution was sufficient to distinguish groups of states in the residual nucleus but not good enough to separate individual states. The data show a strong dependence on missing momentum and this dependence appears to be different for two groups of states in the residual nucleus. Theoretical calculations of the reaction using the Pavia code do not reproduce the shape or the magnitude of the data. An erratum to this article is available at.     

On the resonant behavior of the16O+15N reaction

The ¹⁶ O+ ¹⁵ N reaction products have been studied by the γ-ray detection method in the CM energy range 15.5 to 36.1 MeV and by the kinematical coincidence method at energies ranging from E _CM =20.6 to 33.5MeV. The γ-ray yield excitation function of the ¹⁶O 3^? inelastic channel shows the existence of resonant structures. Two structures with ～ 1.6 MeV width are observed in the large angle elastic scattering excitation function, they are correlated with the resonances seen in the inelastic channel. Angular momentum assignments were made from the elastic backward angular distributions.     

Cross section measurements and thermonuclear reaction rates for 48Ca(p, γ)49Sc and 48Ca(p,n)48Sc

Absolute cross sections have been measured for the reaction ⁴⁸Ca(p, γ)⁴⁹Sc for 0.579 MeV ≦ E_{p, lab} ≦ 2.670 MeV and for the reaction ⁴⁸Ca(p, n)⁴⁸Sc for 0.956 MeV ≦ E_{p, lab} ≦ 2.670 MeV. Substantial competition effects in the cross section for ⁴⁸Ca(p, γ)⁴⁹Sc were observed at the thresholds for neutron emission to the 623 keV (3⁺), 1143 keV (2⁺) and 1402 keV (2^?) excited states of ⁴⁸Sc. Thermonuclear reaction rates were calculated from the measured cross sections for 0.1 ≦ T₉ ≦ 10.0. The new rates differ considerably from those used in earlier calculations of the production of the rare, neutron-rich intermediate mass nuclides during explosive carbon burning. In particular, the new rates may change the predicted abundances for ⁴⁸Ca, ^{49, 50}Ti and ⁵⁰V substantially. The good agreement between current global Hauser-Feshbach models and the experimental data indicates that Hauser-Feshbach calculations can provide sufficiently reliable rates for astrophysical calculations in cases where experimental data are non-existent.     

The 18O(p, α)15N reaction at stellar energies

The ¹⁸O(p, α)¹⁵N reaction has been investigated in the energy range E_p = 72–935 keV. The three known resonances above E_p = 620 keV have been confirmed and four new resonances have been found below E_p = 340 keV. All observed resonances correspond to known compound states in ¹⁹F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 10⁹ K. The energy averaged astrophysical reaction rates are compared with predictions.     

Direct and sequential processes in the 16O(7Li, 7Be)16N reaction

Calculations are presented for the ¹⁶O(⁷Li, ⁷Be)¹⁶N charge exchange reaction. For the direct reaction, the tensor interaction leads to form factors with large spin and angular momentum transfer which are important for the unnatural parity states. The (⁷Li, ⁶Li)(⁶Li,⁷Be) sequential process exhibits a strong J-dependence and is the dominant contribution to the natural parity states.     

A shell-model treatment of (0+1) ħω states in A=4–16 nuclei (II)

The results of spurious-state free calculations in a complete (0+1)?ω model space on theA=4–16 nuclei are presented. The calculated spectra as well as electromagnetic properties of the normal- and nonnormal-parity states are compared with the experimental data. In general good agreement is obtained, while in particular the magnetic-dipole and electric-quadrupole moments are very well reproduced. The present results for the normal-parity states are similar to those of Cohen and Kurath. Several nonnormalparity states expected at rather low excitation energies and properties of very neutronrich nuclei have not been observed experimentally so far.