Average neutron resonance parameters and radiative capture cross sectins for the isotopes of molybdenum

The neutron capture cross sections of the stable molybdenum isotopes have been measured with high energy resolution (ΔE/E ? 0.2 %), between 3 and 90 keV neutron energy, at the 40 m station of ORELA. Average resonance parameters are extracted for s- and p-wave resonances. The s-wave neutron strength function is close to 0.5 × 10^?4 for all isotopes, but the p-wave strength function exhibits a well defined peak near At~ 95.Both s- and p-wave radiative widths decrease markedly as further neutrons are added to the closed shell. The p-wave radiative widths are generally greater than the s-wave widths showing the presence of non-statistical γ-decay mechanisms.Valence neutron theory fails to explain the magnitude of the p- to s-wave radiative width disparity and doorway state processes are invoked. In particular, the data for ⁹⁸Mo appear to violate the usual valence theory, since the correlations between radiative and neutron strengths are small. Further, the radiative widths are smaller than can be explained on the valence model. An explanation for the loss of valence strength is advanced.Interpolated resonance parameters allow an estimate for the unknown cross section for ⁹⁹Mo(n, γ).     

Deeply bound hole states and isobaric analog states observed by neutron pickup reactions on even tin isotopes

Deeply bound hole states in the odd tin isotopes ^{115, 117, 119, 123}Sn were investigated by using 81 MeV (³He, α) and 52 MeV (p, d) reactions. Excitation of low-spin states was largely suppressed in (³He, α) reactions due to the angular-momentum mismatch so that the excitation energies and widths of the $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ states have been clearly determined. DWBA analyses showed that the deeply bound hole states have spectroscopic factors less than 20% of the sum-rule limit. Isobaric analog states $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{, 2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{and}\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ corresponding to the ground and two lowest excited states of In isotopes were investigated in ^{115, 117, 119, 121, 123}Sn isotopes. Coulomb displacement energies were deduced. DWBA analyses of these states were used to check the calculations for the deeply bound states. The spectroscopic factors of the analog states are in good agreement with the sum-rule prediction.     

Nuclear reactions between oxygen isotopes

Elastic and inelastic cross sections have been measured for the system ¹⁶O + ¹⁷O at c.m. energies from 12.5 to 15.5 MeV, and for ¹⁶O + ¹⁸O at c.m. energies from 12 to 20 MeV, at angles between 60° and 125°. Position-sensitive detectors were employed, using the kinematic coincidence technique. The data have been analyzed with particular attention to the contributions of multiple-exchange processes.     

Excitation functions of alpha particle induced reactions on aluminium and copper

Cross-sections for the reactions with product nuclei²⁴Na,²²Na,⁶⁸Ga and⁶⁷Ga were investigated over the energy range of 30 to 75 MeV for alpha particle induced reactions on natural aluminium and copper, using stacked-foil activation technique. The measured excitation functions were analysed with special reference to their suitability for monitoring beam energy and intensity. The experimental results were compared with the predictions of hybrid model of Blann. The assumption of initial exciton numbern ₀=4(4p0h) best satisfies the measured excitation functions in the present work.     

Investigations with two-neutron transfer reactions on the even isotopes of samarium

The (p, t) reaction on the even isotopes of samarium has been investigated at a proton energy of 25.5 MeV. Angular distributions were obtained in the range 18° ≦ θ ≦ 148° with angular steps between 2° and 5°. The experimental energy resolution varied between 35 keV and 50 keV FWHM. Spin and parity assignments are performed by comparing the measured angular distributions to zero-range DWBA calculations. Some difficulties of DWBA calculations for (p, t) reactions are pointed out. The relative cross sections for transitions to different levels of the final nuclei are compared with other (p, t) and (t, p) measurements in the same region of the rare earth isotopes. The dependence of the (p, t) cross sections for different transitions on the neutron number of the final nuclei is discussed. Some 2⁺ states observed in (p, t) and (t, p) reactions are described in the quadrupole pairing vibrational picture.     

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.     

He particle emission in fast neutron induced reactions

Cross sections for some (n, ³He + dp + n2p) and (n, + 2n2p + n³He + pt + dd + dnp) reactions induced by fast neutrons produced via breakup of 53 MeV deuterons on a Be target (E_n = 4–50 MeV; I_max at 22.5 MeV; FWHM = 15.8 MeV) were measured for isotopes of the elements I, La, Nd, Tb, Ho, Yb, W and Au by the activation technique using high-resolution γ-ray spectroscopy, wherever necessary chemical separation, and in several cases enriched isotopes as targets. Furthermore, ³He/ emission cross-section ratios were measured for Co, Ag, In, Cs, La, Ta, Au and Bi using a quadrupole mass spectrometer. The cross sections decrease as a function of increasing (N−Z)/A of the target nucleus. For target elements with Z 44, the ³He/ emission cross-section ratio increases with increasing Z; for elements with Z > 44, however, the trend is reversed. Hauser-Feshbach calculations suggest that the ³He/ emission cross-section ratios are not explainable by the statistical model.     

Structure of 3P size resonance in neutron strength functions

Neutron capture cross sections of 21 nuclei have been determined using the activation technique and an antimony-beryllium photoneutron source. The nuclei have been selected in the mass range 60 < A < 140 so as to cover the 3P size resonance region of neutron strength functions. Using the latest available data on the low energy resonance parameters, the P-wave neutron strength functions of the 21 isotopes are extracted, and found to be in general agreement with similar isotopic values reported in literature. The values of experimental P-wave neutron strength functions in the 3P resonance region (60 < A < 140) are compared on the one hand with the theoretical predictions of Fiedeldey and Frahn and on the other with those of Buck and Perey. The shape and structure of the 3P size resonance, as revealed experimentally, are found to agree better with the predictions of Fiedeldey and Frahn, who assumed twice the normal spin-orbit force in their calculations.     

Cross sections of fast neutron induced reactions on molybdenum isotopes

Cross sections of (n, p), (n, α), and (n, 2n) reactions on molybdenum isotopes have been measured, in the neutron energy range from 13 MeV to 17 MeV, and interpreted in terms of evaporation from the compound nucleus and preequilibrium emission.     

Compound-nucleus mechanism for the (p,np̃) reaction near the neutron threshold

A compound-nucleus mechanism is considered for the reaction where the isobaric analogue resonance (IAR) is populated by a (p, n) stage and decays later emitting a proton p?. It is shown that this mechanism leads to an enhancement in the p? spectra near the IAR energy of the type expected from t · T charge exchange. This phenomenon is general, and will occur whenever the analogue state is not populated selectively in the entrance channel. For the case ⁹¹Zr(p, np?)⁹⁰Zr(g.s.) we estimate the contribution to the cross section due to this mechanism. It is found that the model reproduces the shape of the measured cross section and a normalization consistent with the uncertainties of the data. In order to handle fine-structure properties of the IAR, we develop a version of the external-mixing model particularly adapted for these purposes.     

Study of α-particles produced in thermal neutron induced reactions on 235U

The ²³⁵U(n_th, α) reaction and the α-particles emitted in the thermal neutron induced fission of ²³⁵U were measured using a very pure thermal neutron beam of the Grenoble high flux reactor. An upper limit of 0.66 mb was determined for the ²³⁵U(n_th, α) reaction cross section, which is lower than all the previous results. The energy distribution of the α-particles produced by the ²³⁵U(n_th, f) reaction was measured down to 7 MeV with a non-shielded surface-barrier detector telescope assembly. The measured distribution has a quasi-Gaussian shape; it reveals, however, a pronounced deviation from such a shape at lower α-energies. Several possible explanations for this deviation are discussed.     

Study of neutron induced charged particle reactions on 40K: (I). Thermal neutrons

The (n_th, α) (n_th, p) and (n_th, γα) reactions spectroscopical data are reported on ⁴⁰K and ¹⁴³Nd. For ⁴⁰K, values of σ_α = 0.39 ± 0.08 b, σ_p = 4.4 ± 0.9 b, σ_γα = 26 ± 4 mb are obtained and accurate measurements of E_α0 = 3491.7 ± 7 keV and E_po = 2232.9 ± 3 keV have been performed, α₁, and p₁ transitions and the ⁴⁰K(n_th, γp) reaction have not been observed. The ⁴⁰K(n_th, γα)³⁷Cl and ¹⁴³Nd(n_th, γα)¹⁴⁰Ce spectra are presented. The multipolarity of the primary low energy γ-rays is deduced from the γ-α spectra.     

Energy levels in even actinide isotopes from (t,p) reactions

Energy levels in^{232, 234}Th,^{236, 238, 240}U and in²⁵⁰Cm have been measured using the (t, p) reaction. Angular distributions were obtained for the^{234, 238}U targets and evidence for second order effects in the direct reaction mechanism was found.     

The excitation functions and Isomer ratios for neutron-induced reactions on Mo and Zr

Absolute cross sections of the reactions ⁹²Mo(n, 2n)^91m,91gMo, ⁹²Mo(n, p)⁹²Nb and ⁹²Mo(n, α)^{89m, 89g}Zr, relative cross sections of the reaction ⁹⁰Zr(n, 2n) ^89mZr and isomer ratios of the ⁹⁰Zr(n, 2n) reaction have been measured in the neutron energy range 13–15 MeV. The results for the (n, 2n) reactions are in good agreement with those of the previous studies. The present results for the (n, p) and (n, α) reactions are in disagreement with the previous works. The experimental data are analysed by the statistical model to determine the level-density parameter a, the moment of inertia ? and the strength of the γ-ray transition S_l in order to simultaneously reproduce the experimental data on the excitation function and the isomer ratio in the (n, 2n) reaction. The γ-ray competition, the yrast level and the experimental information on the excited levels of the residual nucleus in the (n, 2n) reaction are taken into account. The obtained values of a, ? and S_l are consistent with those deduced from other types of nuclear data.     

The use of exotic heavy ion transfer reactions to study light neutron rich nuclei

The ¹⁸O + ^{207, 208}Pb reaction at E = 93 MeV has been used to measure the mass excesses of ^{21, 22}O ¹⁹N and ¹⁷C. values of 8.095 ± 0.075, 9.29 ± 0.18, 15.96 ± 0.15 and 21.10 ± 0.22 MeV, respectively, were obtained. These results are in good agreement with previous measurements. A comparison with current theoretical predictions is also presented.     

A study of the Li(p,n)Be reaction by the neutron threshold technique

The reaction ⁶Li(p,n)⁶Be has been studied by slow-neutron detection, with particular attention to the ⁶Be ground state threshold. The detector response was calculated by a Monte Carlo technique and verified with a measurement of the ⁷Li(,n)¹⁰B threshold. An analysis of the shape of the ⁶Be threshold indicated that both s- and p-wave neutrons contribute significantly to the reaction. The results depend on what mode of decay is assumed for the ground state of ⁶Be. If a two-stage process is assumed, with either an alpha particle and an unbound diproton or ⁵Li and a proton as intermediate state, the width is found to be 95±28 keV and the Q-value obtained for the ⁶Li(p,n)⁶Be reaction is −5074±13 keV. It was not possible to detertmine which process is predominant. No higher thresholds in the slow neutron yield up to 4 MeV of excitation in ⁶Be were observed. The data above the ground state threshold are consistent with a broad excited state of ⁶Be or with the occurrence of other neutron-producing reactions.     

Neutron total and scattering cross sections of some even Mo isotopes and the optical model

Neutron total cross sections of ⁹²Mo, ⁹⁶Mo, ⁹⁸Mo and ¹⁰⁰Mo were measured at intervals of ? 10 keV from 1.6 to 5.5 MeV with resolutions of ≈ 10 keV. Neutron elastic and inelastic scattering cross sections of these isotopes were measured from 1.8 to 4.0 MeV at intervals of 0.2 MeV. Neutron groups corresponding to the excitation of forty states were identified. The experimental results were examined in the context of optical and statistical nuclear models. It was concluded that the real part of the optical potential includes a term proportional to [(N-Z)/A] and suggested that the imaginary part of the potential was shell dependent with decreasing magnitude as N = 50 is approached. Comparison of measured and calculated inelastic neutron excitation cross sections suggested a number of J^π assignments extending previous knowledge.     

Reorientation measurements on tungsten isotopes

In a particle-γ coincidence experiment, a thick tungsten target, of natural isotopic abundance, was bombarded with a and ¹⁶O beams. From analysis of the deexcitation γ-rays following Coulomb excitation, the spectroscopic quadrupole moment of the second 2⁺ state (the 22' state) was determined for ¹⁸⁶W and ¹⁸⁴W. In a separate Coulomb excitation experiment a thin, isotopically enriched ¹⁸⁶W target was bombarded with ¹⁶O ions. From analysis of projectiles scattered elastically and inelastically the quadrupole moment of the 2^+' state of ¹⁸⁶W was extracted. The results of the two experiments are in good agreement. The quadrupole moment of the 2^+' state is found to be opposite in sign to that of the first 2⁺ state for both isotopes studied. However, its magnitude decreases rapidly in going from ¹⁸⁶W to ¹⁸⁶W, in contrast to the predictions of the rotation-vibration or asymmetric rotor models. The microscopic theory of Kumar and Baranger does predict the experimental trend, qualitatively. Thus the present results are interpreted as being evidence of strong coupling between β and γ degrees of freedom in the tungsten isotopes, which, according to the theory of Kumar and Baranger, is the source of the reduced value of the quadrupole moment.     

Measurement of neutron polarization from (d,n) reactions on Mg,Si and Ca and comparison to DWBA calculations

Angular distributions of the neutron polarization produced in (d, n) reactions on ²⁴Mg and ²⁸Si were obtained in about 300 keV steps from 3.9 MeV down to 2.2 and 2.9 MeV, respectively. Excitation functions of the polarization were measured at 20° and 40° (lab) over these energy ranges. Polarization angular distributions were also measured for (d, n) reactions on ²⁸Si at 8.1 MeV and ⁴⁰Ca at 3.8 MeV. DWBA calculations are compared to the latter distributions as well as to the (d, n) cross-section data. Fluctuations in the low-energy polarization d ata from the ²⁴Mg and ²⁸Si targets made DWBA comparison of questionable value.