Investigation of (n,t) reactions at 14.6 MeV and an analysis of some systematic trends in the cross-section data

Cross sections for (n, t) reactions at 14.6 ±0.4 MeV on ⁴⁶Ti, ⁵⁶Fe, ^58,60Ni, ⁸⁸Sr, ⁹⁰Zr, ¹⁴¹Pr and ²⁰⁴Pb have been measured by the activation technique in combination with specific radiochemical methods. Some systematic trends in the cross-section data have been observed. An analysis of the gross trend and isotope effect is given.     

The (p, t) and (p, τ) reactions on O at 39.8 MeV

Differential cross sections have been measured at forward angles for (p, t) and (p, τ) transitions from ¹⁷O to the ⁻ ground states and lowest-energy ⁻ states in the ¹⁵O and ¹⁵N mirror nuclei. The data are compared with DWBA calculations using simple single-particle and single-hole wave functions. When the (p, t) and (p, τ) transitions are considered separately, the calculated and experimental ratios of the ⁻ integrated cross sections to the ⁻ integrated cross sections agree to within 30 %; however, the ratios of (p, τ) cross sections to the mirror state (p, t) cross sections are calculated to be about twice as large as actually measured. This experimentally observed reduction of the (p, τ) cross section relative to the (p, t) cross section can possibly be attributed to interference between the S = 0 and S = 1 components of the (p, τ) transitions.     

Isomeric cross sections for the (n, 2n) reaction on 82Se, 81Br,and 45Sc at 15.1 MeV

Activation techniques have been used to measure the cross sections at 15.1 MeV neutron energy for the following reactions: ⁸²Se(n, 2n)^81m+gSe, ⁸¹Br(n, 2n)^80m+gBr, and ⁴⁵Sc(n, 2n) ^44m+gSc. Isomeric cross-section ratios were evaluated by applying the method of least squares to the time behavior of γ-ray activity following the ground-state decay of each isomeric pair. The absolute cross section σ_m for the formation of the metastable state was measured by the mixed-powder method with the ²⁷Al(n, α)²⁴Na reaction as the monitor. The cross section σ_g for the formation of the ground state was then determined by using the isomeric cross-section ratio. The sum of σ_m and σ_g for each reaction is compared with the theoretical value obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.     

Inclusive (p, π) reactions at 201 and 180 MeV

Inclusive proton-induced charged pion production was studied on ¹²C, ⁸⁹Y and ^natPb at 201 and 180 MeV. A QQD spectrometer was used for forward angles and a plastic scintillator range spectrometer for backward angles. The angular dependence, variation with the pion energy, and total cross sections are deduced from doubly differential cross section measurements.     

Isomeric cross-section ratios for (n, 2n) reactions induced by 14.7 MeV neutrons in In and Sb

Isomeric cross section ratios were measured for (n, 2n) reactions induced by 14.7 MeV neutrons in In and Sb isotopes. Experimental results were compared with the theoretical calculations performed with the Huizenga and Wandenbosch method.     

The 206, 204Pb(p, α)203, 201Tl reactions at 35 MeV

The ²⁰⁶Pb(p, α)²⁰³Tl and ²⁰⁴Pb(p, α)²⁰¹Tl reactions have been studied using 35 MeV protons. For both ²⁰³Tl and ²⁰¹Tl approximately 50 levels with excitation energies up to 4.0 MeV were identified and angular distributions of most of these states were measured. Cluster-model DWBA calculations were shown to produce excellent fits to the measured angular distributions. Using these calculations high-spin $\text{(j ≧}\text{15}\text{2}\text{)}$ states were found in both nuclei. The strengths, energies, and number of high-spin states excited in ²⁰¹Tl and ²⁰³Tl are compared with those observed in ²⁰⁵Tl in a previous study.     

Observation of two-hole states at high excitation energy in (p,t) reactions

Bumps of two-hole states at high excitation energies were observed systematically in the triton spectra from (p, t) reactions with 52 MeV protons on nuclei in a broad range of masses. The cross sections of the bumps are almost equal for various targets with the same deep major shells. These cross sections vary discontinuously with variation of the corresponding deep major shell. About 20 to 50 % of the total expected strength is observed experimentally, if the bumps are assumed to arise from two-neutron pickup from the deep major shells. The centres of gravity of the bumps are located at excitation energies of about 7 to 9 MeV in all cases. On the other hand, the widths of the bumps change from about 3 MeV for ⁶⁶Zn to about 9 MeV for ²³⁰Th.     

Alpha-cluster break-up and reaction mechanism in (p, α) reactions on light nuclei

The differential cross sections for the ¹¹B(p, α)⁸Be and ²³Na(p, α)²⁰Ne reactions were measured at bombarding energies between 6 and 24 MeV. The observed angular distributions can be divided into two domains: at low energies the shapes vary rapidly with incident energy indicating a compound nucleus reaction ; at higher energies rather stable diffraction patterns are seen exhibiting a direct reaction mechanism and DWBA calculations are able to describe the shapes. The change from one region to the other is rather abrupt and this behaviour seems typical for reactions having an α-like compound nucleus. The energy at which this change occurs corresponds to an excitation energy in the compound nucleus of about 20 MeV above the α-threshold.     

Proton-proton bremsstrahlung and the 2H(p, 2p)n reaction at 51.8 MeV

Proton-proton bremsstrahlung cross sections were measured at 33°-33° and 8.5°-8.5° with a coplanar symmetric geometry. A comparison between experimental cross sections and theoretical predictions was made. The ²H(p, 2p)n reaction was also investigated at 12.5°–12.5° and 8.5°–8.5°. The cross sections were compared with the values calculated by existing theories.     

The 90,92Zrd(p,p′) reactions at Ep = 800 MeV+

Angular distributions of analyzing powers and differential cross sections have been measured for elastic and inelastic scattering of 800 MeV protons from ⁹⁰Zr and ⁹²Zr. The data have been interpreted using both the collective-model distorted-wave Born approximation and the semimicroscopic distorted-wave impulse approximation with the Love and Franey t-matrix. Clear differences in the data for transitions to both the 2⁺₁ and the 4⁺ states in the two nuclei, attributable to differences in the microscopic structures of the states, have been observed. These differences are only partially explained by the semimicroscopic analyses. Interpreting the A_y data with the help of recent data-to-data relations suggests that the free L · S interaction used at 800 MeV is incorrect.     

Volume form of coupling interaction in semi-direct (n, γ) and (p, γ) reactions

A coupling interaction between the nucleon and the nuclear E1 mode having a volume radial form instead of the usual surface one is used in the semi-direct nucleon radiative-capture theory. The calculated cross sections for the ²⁰⁸Pb(n,γ) and ¹⁴²Ce(p,γ) reactions in the giantresonance region reproduce the measured ones both in shape and magnitude. Satisfactory agreement is achieved in comparing the predicted and detected γ-ray spectra following neutron capture by ²⁰⁸Pb. A detailed analysis of the energy and angular-momentum dependence of the matrix elements when the volume and surface form factors are used, is performed.     

Proton-neutron final-state interaction in the reaction 2H(p,pp)n at 6.4, 10.0, 15.9, 19.85 and 25.8 MeV

This paper presents and analyzes the results of measurements of the proton-neutron final-state interaction (FSI) at 6.4, 10.0, 15.9, 19.85 and 25.8 MeV. At each energy nine angular combinations were measured and for each experiment the cross section was measured along the whole kinematic curve. The data are compared with the predictions of S-wave rank-one and rank-two separable potential models. The potentials used in one of the models reproduce the experimental S-wave N-N phase shifts. Fairly large differences between experiment and theory are found in the FSI regions and in the other regions of the kinematic curves. These disagreements cannot be attributed to off-shell effects. The inclusion of the tensor force and the P-wave N-N interaction in the calculations reduces the disagreements considerably at the higher energies. However, systematic differences of 10% to 20% remain.     

Analyzing powers for the (p, α) reactions on 58, 60, 62Ni at Ep = 22 MeV

Measurements of both the analyzing power and cross section were made for the (p, α) reactions on ^58,60,62Ni at an incident energy of 22 MeV. Data were taken for the strongly populated proton-hole states (0f_{$\text{7}\text{2}$}, 1s_{$\text{1}\text{2}$} and 0d_{$\text{3}\text{2}$}) in the residual cobalt isotopes and for 8 weakly populated low-lying states in ⁵⁵Co and ⁵⁹Co. Angular distributions were taken between θ_lab = 10° and 140° for the ground state and θ_lab = 10° and 80° for the excited states. Both the cross sections and analyzing powers exhibit a similar angular distribution for states having the same J^π values except in the transition to the $\text{3}\text{2}$^? state in ⁵⁹Co at 1.099 MeV. Using the observed J-dependence of the analyzing power, the unknown J^π values for the states at 2.982 MeV in ⁵⁵Co and 3.090 MeV in ⁵⁹Co are assigned to be $\text{9}\text{2}$^?. The shapes of the differential cross sections were well reproduced by the zero-range DWBA calculations using a triton-cluster form factor. However, all the measured analyzing powers could not be reproduced within the framework of such a simple DWBA calculation.     

Direct reactions to many-channel, many-level final states : (II). (d, p)n and (He, d)p

In an earlier paper, the cross section for a direct reaction to a generalised positive-energy final state, described by an R-matrix wave function, was derived. Here a distinction is emphasised between two classes of such a reaction, depending on whether it can or cannot decay back to the target state. A major contribution in the latter case can be via direct break-up. This is computed for the reaction ⁷Li(³He, d)⁸Be(p)⁷Li, treated by a stripping mechanism. The dependence of the cross section on the final-state (⁸Be) channel energy is discussed. The present work uses shell-model wave functions for R-matrix basis states, and this is shown to be appropriate and useful for direct reactions.     

Studies of single neutron holes in the transitional nuclei N = 83–89:: The 150, 148Nd(d,t) and 150, 148, 146Nd(3He, α) pick-up reactions

The level structures of the ^{145, 147, 149}Nd nuclei up to about 5 MeV excitation energy have been investigated with the (³He, α) reaction at 24 MeV. Additional 17 MeV (d, t) data have been obtained for ^{147, 149}Nd. The angular distributions have been analyzed with standard DWBA calculations, and spectroscopic factors have been deduced. Two groups of states carrying h_{$\text{11}\text{2}$} single-particle strength may be associated with the $\text{9}\text{2}$^? [514] and $\text{11}\text{2}$^? [505] Nilsson orbitals. A considerable amount of high-l single-particle strength may be found in the continuum observed in the (³He, α) spectra above 3 MeV in all the nuclei.     

The 52Cr(t,p)54 reaction at Et = 15 MeV

Using the ⁵²Cr(t, p)⁵⁴Cr reaction at a bombarding energy of 15 MeV, excitation energies have been measured for 30 levels up to E_x = 5.583 MeV in ⁵⁴Cr. Angular distributions were obtained for all but one of these levels; these have been compared with distorted-wave Born approximation (DWBA) calculations to determine the L-transfer (and hence J^π). The measured cross sections have been compared to the predictions of DWBA calculations that use two-neutron transfer amplitudes from a shell-model calculation with the active neutrons restricted to the $\text{(2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{,}\text{If}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{, 2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ orbitals.     

Study of some two-nucleon pick-up reactions in the 1p shell with 39.8 MeV protons

Differential cross sections at a proton energy of 39.8 MeV (lab) have been measured for the following reactions (energies in MeV): ¹²C(p,τ)¹⁰B(g.s., 0.72, 1.74, 2.15, 3.59), ¹⁴C(p, t)¹²C(g.s., 4.43), ¹⁴N(p, τ)¹²C(g.s., 4.43), and ¹⁶O(p,τ)¹⁴N(g.s., 2.31, 3.95). A zero-range DWBA analysis of the data has been performed using the 1p shell wave functions of Cohen and Kurath. The fits we find are overall somewhat worse in shape than those found in the (p, t) survey of the 1p shell performed by Kahana and Kurath, the principal reason being that of the (p, τ) transitions which proceed with both L = 0 and L = 2 components we find several which occur with a much weaker L = 0 strength than the calculations predict. When ratios of experimental integrated cross sections to DWBA integrated cross sections are compared for all transitions, an rms deviation about the mean of 39% of the mean is found, whereas if only ratios for transitions from a given target nucleus are compared, then the rms deviations are considerably smaller.     

The 28Si(7Li, 7Be)28Al reaction at 36 MeV

The ²⁸Si(⁷Li, ⁷Be)²⁸Al reaction has been investigated at E_7Li = 36 MeV. States and groups of states were observed up to 5.3 MeV excitation in the ²⁸Al+⁷Be system. Experimental angular distribution for unresolved doublets of states at ≈ 0.0 and 0.44 MeV excitation, corresponding to ⁷Be in its ground state (⁷Be₀) and first excited states (⁷Be₁) with ²⁸Al in its ground state (3⁺) and first excited state (0.031 MeV, 2⁺) are compared with microscopic distorted wave approximation calculations.     

16O Induced reactions on 116Sn at 64 MeV

Angular distributions have been measured for the elastic and inelastic scattering of 64 MeV ¹⁶O ions on ¹¹⁶Sn targets and analysed through coupled-equation calculations. The one-proton transfer has also been studied and the DWBA analysis gives results consistent with those derived from (³He, d). The two-proton transfer has been analysed in the framework of both DWBA and CCBA; theoretical wave functions for Te isotopes have been tested and the importance of multistep processes is discussed.     

Excited states of the transitional 189Ir and 187Ir nuclei populated by the (α, 2n) reaction

Enriched targets of ¹⁸⁷Re and ¹⁸⁵Re were bombarded with α-particles of energy between 23.0 and 42.8 MeV. The levels characteristic of ¹⁸⁹Ir and ¹⁸⁷Ir produced by the (α, 2n) reaction were studied. The positive parity bands built on the $\text{3}\text{2}{}^{\mathrm{+}}$ ground state and on the $\text{1}\text{2}{}^{\mathrm{+}}$ first excited state in both nuclei were observed and interpreted as bands built on the $\text{1}\text{2}$⁺[402] and $\text{1}\text{2}{}^{\mathrm{+}}$ [400] Nilsson states. The negative parity levels form two independent systems: the first one possesses characteristics typical of a rotation-aligned band and is interpreted as belonging to the h_{$\text{9}\text{2}$} orbital; the second one is connected with the $\text{11}\text{2}{}^{\mathrm{?}}$ isomeric state which originates from the h_{$\text{11}\text{2}$} orbital. The possibility of a triaxial shape for this h_{$\text{11}\text{2}$}. level system is discussed. In ¹⁸⁹Ir a 3.2 ms isomeric state of 2333 keV excitation energy has been found and, as a by-product, some information has been gathered on the 4.1 ms isomer in ¹⁸⁸Ir.