I. Bergqvist

The giant isovector E2 resonance in calcium observed in radiative neutron capture

Abstract The reaction 40 Ca(n, γ 0 ) 41 Ca has been studied in the neutron energy range 20 28 MeV, where the isovector giant quadrupole resonance is expected. Interference between El and E2 radiation gives rise to a fore-aft asymmetry of the emitted γ-rays. The measured asymmetries are compared with calculations based on the direct-semidirect capture model. Good agreement with the experimental data is obtained assuming an isovector E2 resonance located at 32 MeV with a strength exhausting about 35%, of the isovector sum rule. This corresponds to the full strength of the T component.

Radiative capture of fast neutrons by 89Y and 140Ce

The γ-ray spectra from the reactions 89Y(n, γ)90Y and 140Ce(n, γ)141Ce have been measured in the neutron energy range of 6.2–15.6 MeV. The pulse-height spectra were recorded with NaI(Tl) spectrometers and time-of-flight techniques were used to improve signal-to-background ratio. Capture cross sections were determined for γ-ray transitions to the two 2d52 levels at 0 and 203 keV of 90Y and to the 2f72 ground state of 141Ce as well as integrated cross sections to bound states in these nuclei. The observed γ-ray spectra and partial radiative capture cross sections were compared with predictions of the direct-semidirect capture theory. The resonance behaviour with neutron energy of both the ground-state and integrated partial capture cross sections shows the validity of the semidirect model for 89Y and 140Ce in the region of neutron energy encompassing the giant-dipole resonance. The observed symmetry of the cross sections about the peak of the resonance argues strongly for the complex form of the particle-vibration coupling interaction. A detailed comparison of the predictions of the DSD model using the complex coupling interaction shows that the capture cross sections are relatively insensitive to the real part of the interaction.

Direct-semidirect and compound contributions to radiative neutron capture cross sections

Abstract Gamma-ray spectra from radiative capture of neutrons in calcium, nickel, yttrium and radiogenic lead have been recorded at neutron energies between 0.5 and 11 MeV. The γ-radiation was detected by a large NaI(T1) scintillation detector using time-of-flight techniques to suppress background radiation. Cross sections for capture to bound final states, mainly ground states, were determined. Measured cross sections and data from previous experiments are compared with predictions of the direct-semidirect and compound-nucleus models. For neutron energies below 4 MeV the compound-nucleus model accounts reasonably well for the observed cross sections. Above 7 MeV the direct-semidirect model gives a good description of the experimental data. In the energy region from 4 to 7 MeV the contributions from the two models are of the same order of magnitude and interference between the two capture processes might be important.

Levels in 115In studied by means of inelastic neutron scattering

Abstract Gamma-ray spectra from the 115 In(n, n γ) reaction have been studied using a Ge(Li) detector at neutron energies ranging from 0.78 to 1.80 MeV. The excitation curves for individual levels of 115 In have been compared with the predictions of the Hauser-Feshbach theory and the spins and parities deduced. The possible existence of a K = 1 2 rotational band in 115 In is discussed.

Gamma rays from fast neutron capture in silicon and sulphur

Abstract Gamma-ray spectra from neutron capture in natural samples of silicon and sulphur have been recorded at several neutron energies between 3.2 and 15 MeV. Time-of-flight techniques have been used to improve the signal-to-background ratio and the γ-radiation has been detected by a large NaI(Tl) scintillator. Cross sections have been determined for transitions to individual (or groups of) levels in the final nuclei. Calculations based on the direct-semidirect model show that this model fails to describe the observed energy dependence of the cross sections in the low-energy range of the giant dipole resonance. The inclusion of the compound-nucleus capture process gives a conclusive improvement in this energy region. A discrepancy in magnitude between observed and calculated cross sections for the reaction 32 S(n, γ) 33 S still persists. Furthermore, the experimental results for 28 Si(n, γ) 29 Si show considerable fluctuations which might be attributed to other non-statistical capture processes.

Angular distributions of γ-rays from fast neutron capture in strontium and yttrium

Abstract Gamma-ray spectra from neutron capture in natural samples of strontium and yttrium have been recorded at various angles with respect to the direction of the incident neutron flux. Angular yields have been observed at six neutron energies in the range 7 to 11 MeV using time-of-flight techniques to improve the signal-to-background ratio. The γ-radiation was detected by a large NaI(Tl) crystal placed in a heavy radiation shield. Certain combinations of Legendre polynomial coefficients were extracted for transitions to low-lying single-particle states ( 2 d 5 2 and 3 s 1 2 ) in the final nuclei. The energy dependence of the angular distribution coefficients indicates interference between the electric dipole amplitude and amplitudes of opposite parity. The results are compared with theoretical calculations based on the direct-semidirect model.

Proton capture by 176Yb in the giant dipole resonance region

Abstract The proton capture cross section for the reaction 176 Yb(p, γ) 177 Lu has been measured for incident proton energies between 6 and 24 MeV. The excitation function for this deformed nucleus agrees remarkably well with the results of previous studies on spherical nuclei, e.g. 142 Ce(p, γ) 143 Pr. The results indicate that the giant dipole resonance (GDR) is strongly excited as predicted by the direct-semidirect (DSD) model. It is found that the model describes reasonably well the excitation function. In the low-energy proton range, where the excitation function increases rapidly with proton energy, the observed cross section is significantly higher than the DSD predictions. The difference can only partly be explained by compound nucleus contributions. In the high-energy end, the predicted cross section tends to be too high primarily due to an increasing contribution of direct capture to orbitals with large angular momenta.

Cross sections for 197Au(n, γ)198Au and 115In(n, γ)116mIn in the neutron energy region 2.0–7.7 MeV

Abstract The cross sections for the reactions 197Au(n, γ)198Au and 115In(n, γ)116mIn have been measured with the activation method in the neutron energy region 2.0–7.7 MeV. The influence of background neutrons on the results was studied in considerable detail. The main problems are caused by low-energy neutrons produced by charged-particle reactions in the target material and secondary neutrons from nonelastic reactions in the sample and surrounding materials. The measured capture cross sections are generally lower than previous results and the deviation tends to increase with increasing neutron energy. The data are also compared with calculations based on the compound-nucleus model and quite good agreement is obtained.

The influence of background neutrons on (n,γ) activation cross section measurements in the energy region 2.0–7.7 MeV

Abstract Cross sections for the reactions 197Au(n,γ)198Au and 115In(n,γ) 116mIn have been measured between 2, 0 and 7, 7 MeV with the activation method, with special attention being paid to corrections for background neutrons.

Fast neutron radiative capture in silicon

Using the28Si(n, γ)29Si reaction, transitions to the ground state and first excited state in29Si have been studied in the neutron energy range 3–14 MeV with improved neutron energy resolution (of about 100 keV). The 90° cross sections show considerable structure in the entire neutron energy range. Comparison with theoretical calculations shows that compound-nucleus and direct-semidirect processes account for the non-resonant part (smoothly varying part) of the cross section. A microscopic model is, however, required to describe the resonance structure. Continuum shell-model calculations have proven to be a very promising means towards a better understanding of the capture process in, and below, the giant resonance region in light nuclei. The angular distributions of gamma rays in the neutron energy range 8–14 MeV indicate that the capture reaction is mainly of direct character and that the effect of interference between the electric dipole and isoscalar quadrupole resonance is weak.