The proton optical-model potential near the Coulomb barrier

Proton elastic scattering data from ¹⁹⁷Au, ²⁰⁸Pb and ²⁰⁹Bi at energies near the Coulomb barrier are analyzed. The energy dependences of the real volume and imaginary surface-derivative potential depths V_R and W_SF of a local optical-model potential with fixed geometric parameters are found to be much more rapid than at higher energies. The strong energy dependence of V_Rnear the Coulomb barrier is explained in terms of the non-locality of the nucleon-nucleus interaction.     

A global optical-model analysis of neutron elastic scattering data

Differential cross sections for neutron elastic scattering in the energy range 7–26 MeV from ⁴⁰Ca, ⁹⁰Zr, ⁹²Mo, ^{116, 124}Sn and ²⁰⁸Pb are analyzed in terms of a global optical-model potential. A smooth variation of the real radius parameters with mass number is investigated. Otherwise, optical-model geometrical parameters are kept fixed at values obtained by averaging individual best fits. The energy and isospin dependences of potential strengths and volume integrals per nucleon are presented.     

Calculation of the imaginary part of the nucleon-nucleus optical-model potential

The imaginary part of the optical-model potential for the scattering of nucleons by nuclei is studied in the frame of the shell-model approach to nuclear reactions. Special attention is paid to the one-hole target nuclei. The imaginary part of the optical-model potential in the second order in the nucleon-nucleus interaction is divided into two parts. The first corresponds to the average resonant scattering. The second corresponds to the inelastic scattering leading to the non-collective states of the target nuclei. A local potential equivalent to the non-local theoretical one is constructed in order to facilitate comparison with experiment. Numerical calculations concern the scattering of 14.5 MeV protons by ³⁹K. It is found that the imaginary part depends upon the angular momentum and that its radial variation is governed by strong shell effects. The predicted absorption is approximately 60% of the experimental one. The average resonant scattering contributes to the imaginary part of the optical-model potential as much as the inelastic non-collective excitations of the target.     

Fragmentation of relativistic oxygen nuclei in interactions with a proton

The data on investigation of inelastic interactions of ¹⁶O nuclei in a hydrogen bubble chamber at an incident momentum of 3.25 A GeV/c are presented. Separate characteristics as fragments isotope composition and topological cross-sections of fragmentation channels are given. The processes of formation of light fragments and unstable nuclei, and the break-up of the ¹⁶O nucleus into multicharge fragments are investigated. A comparison between the experimental data and the calculations of the cascade fragmentation evaporation model (CFEM) is made. The observed singularities of the interactions point out the important role of the nucleus α-cluster structure in the formation of the final products. Received: 9 January 2001 / Accepted: 13 June 2001     

Three-body effective interactions in the f72 region

The three-body effective forces in a pure $\text{(0}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{)}{}^{\mathrm{<mtext>n</mtext>}}$ model space are calculated to second order in perturbation theory. The full effect of the $\text{(0f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{1}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}{}^3$ configuration on the effective interaction in the $\text{(0}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{)}{}^3$ model space leading to three-body forces is calculated to all orders in G. The resulting three-body force matrix elements come out much weaker than demanded by phenomenological fits. Their main effect is to push the spectra as a whole up in energy, improving the fit of the calculated ground state binding energies with experiment. The biggest three-body-force contribution appears toward the end of the $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ shell where its main object is to compensate for Pauli violations arising from the two-body diagrams.     

Non-static effects in the one-gluon-exchange potential

The one-gluon-exchange quark-quark amplitude is evaluated explicitly in momentum space using the helicity representation, thus avoiding the non-relativistic limit. Non-static effects are investigated in detail and, for non-strange quark masses, are found to be rather important, especially in the central and tensor part.     

Isobaric analogue resonances in proton scattering on Ba

Extensive experimental data for the resonance scatttering of protons to the ground state and first excited state in ¹³⁸Ba are presented. A Breit Wigner analysis with a background term of the form Σ_n(a_n/Eⁿ) is performed and the extracted background function is compared with optical-model calculations using the parameters given in a detailed investigation by Perey. Neutron spectroscopic factors calculated in different ways from this data are compared with the corresponding factors from (d, p). An accurate determination of resonance energies for the first two resonances reveals a deviation from a constant Coulomb shift of the order of 10 keV.     

Triton emission in the interactions of fast neutrons with nuclei

Cross sections have been measured for the formation of tritium in the interactions with Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, Ca, V, Co, As, Y, La, Pr, Tb, Ho and Tl of fast neutrons produced via break-up of 53 MeV deuterons on a Be target (E_n = 11.5–43.5 MeV; I_max at 22.5 MeV; FWHM = 15.8 MeV). The activation technique in combination with vacuum extraction and low-level gas phase β^? counting of tritium was employed. Furthermore, cross sections were measured for isotopes of the elements Ne, Mg, Ar, Ca, Ti, Cr, Fe, Ni, Zn, Ge, Se, Zr, Mo, Pd, Te, Ba, Pr, Dy, W, Tl and Pb by γ-ray spectroscopic analysis of the radioactive reaction products. A comparison of the two sets of cross sections shows that for nuclei with A > 40 the emission of three single particles (lp2n) is much more probable than the emission of a bound tri-nucleon (³H). The cross-section data obtained via tritium counting show that the (n, t) cross sections for the lightest nuclei are exceptionally large, probably due to direct interactions, and the process competes with other modes of de-excitation; for elements with Z > 20, on the other hand, the cross section is low (< 0.25 % ofσ_n.e.) and practically constant, showing thereby that in the medium and heavy mass regions the probability of emission of a triton is relatively independent of the target nucleus. A comparison of the cross-section systematics at E_n = 14.6 MeV and for the neutron spectrum described above is presented; the trends are somewhat similar.     

The proton shell closure at 14C

Data for the ¹⁴C(³He, d)¹⁵N and ¹⁴C(t, ⁴He)¹³B reactions are compared to DWBA calculations to measure the spectroscopic factors for $\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{and}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ proton transitions. Good fits are found for the stripping data, as well as to similar stripping data on ¹²C. These results lower the previous value for proton $\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ hole strength in ¹⁴C by a factor of two, indicating that the neutron closure of the p-shell has provided a very good proton closed shell at ¹⁴C. Stripping results to low-lying positive parity states in ¹⁵N are interpreted within a Nilsson scheme, which reproduces the experimental results, but which requires a large deformation for the s-d orbitals.     

Interfering proton and neutron transfer in the reaction C(N, N)C

Elastic scattering and one-nucleon transfer reactions induced by ¹⁶O have been investigated in the energy region 45–60 MeV on targets of ²⁶Mg, ²⁷Al, ³⁰Si and ⁴⁸Ca. Angular distributions were measured in the angular range 4°–40°. Optical model parameters were derived from the elastic scattering data and the transfer reaction data were analysed using both no-recoil and full-recoil DWBA codes. In the case of proton transfer reactions on ⁴⁸Ca, good agreement was obtained between the data and the DWBA calculations while the data for the lighter targets could not be satisfactorily reproduced. The oscillatory pattern of the angular distributions is discussed in terms of the three-parameter model of Kahana, and it is found that the model qualitatively explains the observed transition from smooth to oscillatory angular distributions.     

Band structures and proton-neutron interactions in Ta

Six rotational bands in the odd-odd nucleus ¹⁷⁴Ta have been populated with the ¹⁶⁰Gd(¹⁹F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π1/2⁻ [541] ν1/2⁻ [521] band and semi-decoupled π1/2⁻ [541] ν7/2⁻ [633] band are observed, in addition to the high-K couplings of the π9/2⁻ [514] ν7/2⁺ [633], π9/2⁻ [514] ν5/2⁻ [512], π7/2⁺ [404] ν7/2⁺ [633], and π5/2⁺ [402] ν5/2⁻ [512] configurations. The signature splitting of the π1/2⁻ [541] ν7/2⁺ [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i_13/2 neutrons in the π1/2⁻ [541] bands of odd-Z nuclei.     

Quadrupole interactions in pionic and kaonic atoms

We discuss the quadrupole hyperfine structure in mesonic atoms for nuclei with spin I 1. The optical potential is expanded in terms of the non-spherical density contributions. Examples are given for the hyperfine splitting of peripheral and deeper-lying states in pionic and kaonic atoms.     

Elastic proton scattering from He in the forward direction at 200, 350 and 500 MeV

The differential cross section for the elastic scattering of protons from ⁴He has been measured at 200, 350, and 500 MeV over an angular range from 3.5° to 15° in the lab system. In addition, the analyzing power was measured in the same angular range at 350 and 500 MeV. The experiment makes use of a tenuous gas target in which the recoil -energy is measured with solid-state detectors. The proton scattering angle is measured outside the target by a system of multi-wire proportional counters. The differential cross sections have a nearly exponential dependence on momentum transfer in this kinematic range and grow with increasing energy. The analyzing power shows strong positive asymmetry.     

Theoretical investigation of giant resonances by inelastic proton scattering

A fully microscopic, antisymmetrized DWBA analysis of (p, p') spectra in the giant resonance region of ¹⁶O, ⁴⁰Ca and ²⁰⁸Pb is presented. The nuclear structure information is derived from RPA wave functions calculated in large configuration spaces. The theoretical spectra are in general in agreement with the experimental results. A new interpretation follows from our. investigation for the spectrum of the giant dipole region in the heavy nuclei: This resonance structure is mainly due to the breathing mode, whereas the electric dipole contribution is much smaller.     

Emission of deuterons in the interactions of 9 GeV protons with lead nuclei

The emission of deuterons with energies ranging from 17 to 104 MeV in interactions of 9 GeV protons with lead nuclei has been studied using nuclear emulsion. These deuterons constitute about $\text{1}\text{6}$ of the secondary heavily ionising particles (β < 0.7). Observed features of the emission of the deuterons with energies above 28 MeV can be interpreted in terms of the mechanism proposed by Butler and Pearson.     

High-resolution proton scattering on 50Ti

Differential cross sections were measured for ⁵⁰Ti(p, p) at four angles for E_p = 1.83 to 2.97 MeV, with an overall energy resolution of about 350 eV. Spins, parities and total widths were extracted for 212 levels. An energy region near E_p = 1.37 MeV was also examined in order to study the analogue of the ground state of ⁵Ti. Coulomb energies and spectroscopic factors were determined for the analogues of the ground and first excited states of ⁵¹Ti. The latter analogue was highly fragmented. The s-wave spacing and width distributions were analyzed and the number of missing levels estimated. The $\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ proton strength functions were determined.     

Evidence from inelastic proton scattering for a giant quadrupole vibration in spherical nuclei

A systematic study of inelastic proton continuum spectra produced at small angles by bombardment of ²⁷Al, ⁵⁴Fe, ¹²⁰Sn and ²⁰⁹Bi with 62 MeV protons suggests the existence of a collective region in the continuum with properties of a giant quadrupole vibration.