Evidence for neutron capture through doorway states in29Si

Theγ-ray spectra following neutron capture in silicon have teen recorded in the neutron energy range 2.7–6.2 MeV and partial cross sections forγ-rays to the 2s_1/2 ground state and 1d_3/2 first excited states in²⁹Si determined. The results indicate considerable fluctuations with neutron energy with a prominent resonance peak at 4.6 MeV in the (n,γ _o) cross section. The existence of fluctuations is predicted in a recent theoretical calculation based on a model designed to include single-particle resonances in nuclear reaction processes.     

Photoneutron cross section of 29Si

The photoneutron cross section of ²⁹Si has been measured over the energy range 8 to 28 MeV using the bremsstrahlung yield curve technique. The giant dipole resonance is observed to be centered at 21.2 MeV and is 5.5 MeV wide. In addition, a pygmy resonance containing much fine structure is observed below 17 MeV. An attempt is made to interpret these observations in terms of the weak coupling of the extra neutron to a ²⁸Si core. However, detailed comparisons of the structure and relative strengths of the ²⁸Si and ²⁹Si cross sections reveal that the coupling of the extra-core nucleon is large and significantly modifies the core wave function.     

Study of the excitation functions of27Al(n, α)24Na,27Al(n,p)27Mg and28Si(n,p)28Al reactions

Cross sections for the reactions²⁷Al(n, p)²⁷Mg and²⁸Si(n,p)²⁸Al were measured by activation method between 13.40 and 14.83 MeV neutron energy. An accuracy of about 4% was achieved using the²⁷Al(n, α)²⁴Na reaction as a reference at 14.1 MeV where the relative excitation function has also been measured. Results obtained were compared to a recent compilation and that calculated by the Hauser-Feshbach model. Using the back-shifted level density formula and taking into account the contribution of the separated levels, the calculations were extended to the energy range from the threshold to 18 MeV. A structure was observed in the²⁷Al(n, α)²⁴Na reaction cross section curve around 14 MeV neutron energy.     

The 26Mg(14N, 10B)30Si and 26Mg(14N, 11B)29Si reactions

Reactions induced by ¹⁴N on ²⁶Mg at bombarding energies of 60–95 MeV have been studied. Angular distributions for states populated in ²⁹Si by the (¹⁴N, ¹¹B) reaction and in ³⁰Si by the (¹⁴N, ¹⁰B) reaction have been compared with Hauser-Feshbach and DWBA calculations to determine the reaction mechanism and to deduce spectroscopic information. The cross sections for the states populated in ²⁹Si and ³⁰Si are in poor agreement with statistical model calculations, indicating a non-compound nucleus mechanism.     

A study of levels in the highly excited29Si nucleus

The results of fast neutron elastic scattering measurements on natural silicon in the energy range 0.57–2.28 MeV were used for comparison with the nuclear resonance theory. Assignment of spin and parity to some highly excited states in²⁹Si was possible.     

Spectroscopy of low-lying positive parity states in 30P from the study of the reaction (3He,d) at 14.0 MeV

The reaction ²⁹Si(³He, d)³⁰P was studied at an incident energy of 14.0 MeV. Spectroscopic strengths for 14 positive parity states up to an excitation of 4.50 MeV have been obtained using DWBA analysis. The incident channel optical-model parameters for the DWBA calculations were extracted from elastic scattering cross sections measured also at 14.0 MeV.     

Proton resonant and direct capture on 28Si

Excitation functions for the ²⁸Si(p, γ)²⁹P capture reaction have been measured in the proton energy range E_p = 1.3–2.3 mMeV. The analysis of the data reveals the presence of the direct capture process to the ground state and first excited state of ²⁹P. The strengths of the resonances at E_p = 1.65, 2.09 and 2.88 MeV and the spectroscopic factors of the ground state and first excited state in ²⁹P are deduced.     

Thermonuclear reaction rates for proton induced reactions on 41K and neutron induced reactions on 41Ca

The yield of γ-rays from the reaction ⁴¹K(p, γ)⁴²Ca has been measured as a function of bombarding energy over the range 0.68–2.48 MeV and from the reaction ⁴¹K(p, αγ)³⁸Ar over the range 1.20–2.48 MeV, and the yield of neutrons from the reaction ⁴¹K(p, n)⁴¹Ca has been measured from threshold to a bombarding energy of 2.48 MeV. The energy dependence of the cross sections is compared with statistical-model calculations with global optical-model parameters in all particle channels. The calculations seriously overestimate the cross section for the neutron channel and underestimate those for the other channels. A reduction in the imaginary well depth in the neutron channel leads to good agreement with all the data. Statistical-model calculations with this modified set of parameters are then carried out to provide cross sections for the astrophysically interesting reactions ⁴¹Ca(n, p)⁴¹K, ⁴¹Ca(n, α)³⁸Ar, and ⁴¹Ca(n, γ)⁴²Ca. Thermonuclear reaction rates are calculated for all six reactions over the temperature range 5 × 10⁸–10¹⁰K which includes the range of temperatures of interest in nucleosynthesis calculations.     

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 $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ levels at 0 and 203 keV of ⁹⁰Y and to the $\text{2}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ 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.     

Study of the giant dipole resonance of 232Th and 238U using elastic and raman photon scattering

Differential cross sections for elastic and Raman scattering of photons from ²³²Th and ²³⁸U targets were measured. Eight photon energies in the range 7.9–11.4 MeV were used and were obtained from thermal neutron capture in Fe, Cr, Cu and Ni. The angular distribution of the elastic and Raman scattered radiation from ²³²Th was measured. The results are compared with calculations of the simple rotator model and the dynamical collective model of the giant dipole resonance after incorporating the effect of Delbruck scattering.     

Strength functions of primary transitions following thermal neutron capture in strontium

The primaryE1,M1 andE2γ-radiation in^87,88,89Sr observed after thermal neutron capture was compared with the predictions of single particle and giant resonance models. The nuclei feature a wide range of neutron binding energies between 6.3 and 11.1 MeV, which makes a 5.5 MeV spectrum of primary transition energies available for investigation. The (n, γ) reaction was used to estimate the parameters of the spin-flip M1 giant resonance in strontium. The total energy weightedM1 strength of this resonance exceeds the results of shell model and random phase approximation calculations for⁹⁰Zr by a factor of 3–4. TheE1 strengths were found to agree with the established giant dipole resonance model. The few data on primaryE2 transitions do not allow to differentiate between the giant quadrupole resonance and the single particle models.     

Study of the28Si(d,p)29Si reaction at 3·22 MeV

The differential cross section of the²⁸Si(d,p)²⁹Si reaction at deuteron energy 3·22 MeV has been measured and analyzed including transitions to final states in²⁹Si up to the 5·809 MeV level. Angular distributions are compared with the DWBA and Hauser-Feshbach calculations. Satisfactory agreement is obtained for most of the levels. An attempt is made to explain the angular distribution leading to the excitation of the 4·078 MeV level as a two-step process.     

25Mg (α, n)28Si and 26Mg(α, n)29Si as neutron sources in explosive neon burning

The yields of neutrons from the reaction ²⁵Mg(α, n)²⁸Si and of γ-rays from the reaction ²⁵Mg(α, nγ)²⁸Si have been measured as a function of bombarding energy over the range 1.8–6.3 MeV, and the yield of neutrons from ²⁶Mg(α, n)²⁹Si has been measured over the range 1.8–6.0 MeV. Cross sections for ^{25, 26}Mg(α, n)^{28, 29}Si were extracted from the data and compared with global statistical-model calculations. The agreement is very good. Thermonuclear reaction rates under stellar conditions appropriate for explosive neon burning are calculated and their significance for the nucleosynthesis of rare neutron-rich nuclei is discussed.     

The excitation functions of complete and incomplete fusion reactions of 6Li with Pt nuclei

Experimental results from measuring the energy dependences of cross sections of fusion and transfer reactions for ⁶Li beams and Pt targets are presented. The experiments were performed using the MSP-144 magnetic analyzer; stacks of platinum foils were installed at the focal plane of this analyzer. In the energy range 22.5–42.5 MeV, the energy resolution of the beam hitting the target stack was not worse than 0.25 MeV and that of the transmitted beam was not worse than 0.40 MeV. The yields of products of neutron and deuteron transfer reaction on target nuclei were measured using the γ activity induced in the platinum targets. Thus, excitation functions for transfer reactions were obtained in a wide energy range, including near the Coulomb barrier. It was shown that the basic reaction channel is the deuteron capture from ⁶Li. In this case, the maximum of the excitation function for ⁶Li breakup and subsequent deuteron capture lies near the Coulomb barrier of the reaction.     

Giant M1 resonance in the direct-semidirect model for nucleon radiative capture

The direct-semidirect model for nucleon radiative capture proceeding via giant E1 and E2 resonance states is extended to account for capture through collective M1 excitation. The ²⁰⁸Pb(N, γ₀) and ¹⁴⁰Ce(N, γ₀) reactions are investigated in the 2–10 MeV energy interval. For the M1 and isoscalar E2 neutron capture processes, calculations provide cross section values of the same order of magnitude, as well as comparable effects on γ-ray angular distributions. The model proves to be an appropriate framework for discussing the E1-M1 and E1-E2 interference processes, giving useful suggestions as to effects arising from the presence of the M1 and E2 collectivities.     

Possible resonance structure in the elastic scattering of neutrons by Y near En = 1 MeV

The total cross section as well as the differential cross section and polarization in the elastic scattering of 0.8–1.4 MeV neutrons by Y have been measured with neutron beams of energy spread less than 20 keV. Rather weak structure with widths ≈ 50 keV was observed at a few energies within this range. The data were analyzed by use of a model in which the scattering process is described in terms of resonance amplitudes superimposed on an optical-potential background. Although not completely definitive, this analysis indicates the existence of three intermediate-width resonances (two 1^? and one 1⁺) at neutron energies between ≈ 1.0 and 1.2 MeV. The properties of the 1^? resonances suggest that these are the parent states of the proposed T_> components of the El giant resonance observed near 21 MeV excitation energy in ⁹⁰Zr produced in the ⁸⁹Y(p,γ₀) reaction. The resolved resonance structure in this energy region is in reasonable agreement with a recent calculation of the energies and widths of negative-parity states in ⁹⁰Y.     

The reaction 26Mg(3He,n)28Si at 23 and 45 MeV

Angular distributions of neutron groups from the ²⁶Mg(³He, n)²⁸Si reaction have been measured at bombarding energies of 23.1 and 45.5 MeV. The experimental results are compared with DWBA and coupled-channel calculations. The zero-range DWBA calculation using shell-model two-nucleon spectroscopic amplitudes explains rather well most of the experimental data of the lowest four states in ²⁸Si, but it is necessary to take two-step processes into account to reproduce all of them satisfactorily. A strong neutron group observed at about 6.9 MeV in residual excitation is described as one composed of unresolved four groups, of which three are attributed to positive-parity states in ²⁸Si predicted in this energy region by the shell-model calculation and one to the 6.88 MeV 3⁻ state in ²⁸Si.     

(γ, γ′) reactions in 89Y, 115In and 197Au at intermediate energies

The yields of isomeric states in ⁸⁹Y, ¹¹⁵In and ¹⁹⁷Au produced by the (γ, γ') reaction have been measured in the energy range 100–800 MeV by the activation method. From the yields the cross sections have been deduced. Large cross sections around the first pion resonance are found. The experimental results are compared to calculations based on the impulse approximation.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Fast Neutron Capture Cross Section of Natural Molybdenum

The radiative capture cross sections of natural Mo relative to the ¹⁹⁷Au (n,γ) were measured in the 0.7—1.4MeV neutron energy range,using a large liquid scintillator detector and the time-of-flight method.In the 0.01—2.0MeV neutron energy range,the theoretical calculation of (n,γ) reaction cross section for Mo was performed.The result of calculation is compared with the experimental data.