A. Likar

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.

Analysis of fast neutron capture data based on the refined direct-semidirect model

Abstract The real incident neutron-target nucleus vibration coupling within the direct-semidirect theory of fast nucleon capture does not lead to a satisfactory reproduction of experimental data on fast neutron capture in heavier nuclei. The agreement between theory and experiment, however, is improved, if the refined DSD theory involving the complex coupling interaction is applied. The uncertainties affecting the reliability of the free parameters in this coupling interaction are considered.

Angular distribution of γ-rays from the radiative capture of fast nucleons

Abstract The angular distribution of γ-rays from the radiative capture of fast nucleons has been calculated according to the direct-semidirect capture model and compared with the available experimental data. Some general features of the angular distribution are discussed. A new measurement of the angular distribution is proposed in order to investigate further the shape and strength of the form factor for the inelastic excitation of the dipole giant resonance.

Integrated cross sections in fast neutron capture in medium weight and heavy nuclei

Abstract The fast neutron radiative capture in chromium, iron, strontium, yttrium, cerium, barium and lead was studied using the recently proposed consistent version of the direct-semi-direct capture model. We present the comparison of calculated excitation functions and integrated cross sections with experimental data. The results of this study and the analysis of a previous work indicate that the integrated cross section at neutron energy of 14 MeV is a smooth function of the mass number. For the nuclei studied the calculated integrated cross section is less variant compared to the experimental values. Some of the discrepancies of the experimental nature are pointed out.

Fast proton capture through a consistent version of the direct-semidirect model

Abstract We present the calculations of excitation functions and angular distributions of γ-rays in the proton capture reactions 142 Ce(p, γ) 142 Pr, 208 Pb(p, γ) 209 Bi and the photo-proton reaction 40 Ca(γ, p 0,1 ) 39 K using the recently proposed consistent version of the direct-semidirect (DSD) capture model. The results are in excellent agreement with experimental data.

Fast proton capture in 208Pb according to direct-semidirect model

Abstract The refined and generalized direct-semidirect model is used to calculate excitation functions for proton capture in 208 Pb populating different single-particle states in 209 Bi. The results are compared with available experimental data. It is shown that good and consistent results can be obtained for direct-semidirect capture through a giant dipole state if proper optical model parameters are used. Excitation functions for proton capture to single-particle states with low orbital angular momenta can always be easily reproduced. This is not the case for capture to states with high angular momenta, where the shapes of excitation functions strongly depend on the optical model parameters used in calculation. The asymmetry of experimentally measured angular distributions of γ-rays can be reasonably well reproduced using a generalized direct-semidirect model which includes excitation of the giant dipole and giant quadrupole resonance. The existence of compact isovector quadrupole resonance at excitation energies around 22 MeV cannot be confirmed from this analysis.

Method to determine the depth of Cs-137 in soil from in-situ gamma-ray spectrometry

We present a method for the determination of the average depth of the activity of Cs-137 in soil, exploiting the information contained in the low-energy part of an in-situ spectrum. The predictive power of the method is evaluated by comparing the results with laboratory measurements of the activity profile and it is shown that the proposed approach yields results of comparable quality in a much shorter measurement time. The method relies on the analysis of the entire spectrum of in-situ gamma-rays from a single spectrum, including the broad peak of scattered photons at the energy between 20 and 80 keV. It is based on a simple notion that the deeper the source is buried beneath the surface, the more numerous the photons scattered in the soil, compared to the corresponding peak of gamma-rays of the original energy. Monte Carlo calculations are necessary for this approach and were based on the GEANT system developed at CERN, Geneva.

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.

The isovector E2 resonance in bismuth excited by neutron radiative capture

Abstract To investigate the isovector giant quadrupole resonance in the A = 208 mass region, gamma-ray spectra from the reaction 209Bi(n, γ)210Bi were recorded at five neutron energies in the energy range 17.7 to 22.0 MeV at 55° and 125°. The measured fore-aft asymmetries of the angular distributions for transitions to bound single-particle states are attributed to interference between radiation from the isovector giant quadrupole resonance and radiation of opposite parity (from the high-energy tail of the giant dipole resonance and direct E1 capture). A comparison of the asymmetry data obtained in the present work with calculations based on the direct-semidirect capture model shows that the data are compatible with an excitation energy of 22.5 ± 1 MeV, a width of 6 ± 3 MeV and complete exhaustion of the energy-weighted sum rule for the isovector giant quadrupole resonance. Ratios of cross sections to single-particle states are also compared with direct-semidirect model calculations. In general, agreement between experimental data and model calculations is observed. For a special case, effects of a possible decay of the isovector giant quadrupole resonance to the low-lying octupole state are investigated.

The radiative capture of 14.1 MeV neutrons in 138Ba, W, Pb and 209Bi

Abstract Spectra of prompt γ-rays due to radiative capture of 14.1 MeV neutrons to bound nuclear states in natural Ba, W, Pb, and Bi, and the corresponding integrated cross sections are presented. The integrated cross sections for 138 Ba, W, Pb, and 209 Bi are 1600 ± 300 μb. 930 ± 170 μb, 1100 ± 180μb and 950 ± 180 μb, respectively. The values agree with previous observations that the cross sections depend smoothly on mass number. The high energy parts of the experimental spectra and recently published fast neutron capture excitation functions are compared with those calculated according to the two approaches of the direct-semidirect theory, namely, the surface peaked particle-nucleus interaction and the volume type interaction. It is concluded that purely on the grounds of presently known experimental data one cannot judge which form of the coupling interaction is more appropriate for the description of the fast nucleon capture process.