Resonance neutron capture in 56Fe

The neutron capture cross section of ⁵⁶Fe has been measured with 0.2–0.3% energy resolution from 2.5 keV up to the inelastic neutron scattering threshold. Results are compared with recent total cross-section data and average parameters are derived for s-, p- and d-wave resonances. The low correlation coefficient observed between the s-wave reduced neutron and radiative widths is consistent with the minor contribution of the valence capture mechanism as calculated in the framework of the optical model. Broad E1 and M1 doorway states for s-, p- and d-wave resonances are postulated to explain the cross-section data and γ-ray spectra up to 1 MeV.     

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).     

Neutron capture gamma-rays from the low-lying resonances of 54Fe

Neutron capture γ-rays have been observed from the low-lying resonances of ⁵⁴Fe using the Harwell electron linear accelerator HELIOS as a neutron source. Relative values for the partial radiative widths of resolved lines were obtained by area analysis of the γ-rays peaks over neutron resonance regions, and absolute values were obtained by a shape analysis fit to the yield deduced by comparison with a ¹⁰B(n, αγ)⁷Li measurement. For the 7.8 keV s-wave resonance, a comparison of the partial widths with the predictions of the valence model has been made. In both relative and absolute senses, there is good agreement.     

Analysis of E1 radiative neutron capture based on the optical model

The optical model formula of Lane and Mughabghab for E1 radiative neutron capture has been applied to resonance capture in ⁵⁶Fe and ⁹⁰Zr. It is shown that in general the optical model predicts the contributions to the radiative widths which are correlated with the neutron widths, except for a depletion factor representing the proportion of E1 single-particle strength remaining near the neutron threshold. The degree of correlation between measured and calculated radiative widths is related to the ratio of the variances of these widths. This enables an estimate of the factor . It is shown that the theoretical value for the ratio of the radiative to neutron widths is to a good approximation independent of the imaginary part of the optical potential. The calculated radiative width is thus well defined. The results are compared with those from the valence model and from experiment. The direct background contribution is found to be negligible compared to the average compound cross section. The contribution of d-waves to the radiative capture cross section is discussed.     

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on ⁵⁶Fe and ⁶⁰Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.     

Resonance neutron capture in silicon

Gamma ray spectra from neutron capture by natural silicon have been measured for resonances at 31.7, 38.8, 55.9 and 67.7 keV. Absolute partial radiative widths have been obtained with the 35 keV s-wave resonance in aluminium as a reference standard. Strong single particle effects were observed in the final state correlation. These cannot be accounted for by the valence model of neutron capture. A different single particle mechanism must therefore occur at these energies.     

Properties of nuclear levels excited by neutron capture γ-rays from cobalt

Neutron capture γ-rays from cobalt have been used to photoexcite nuclear levels in the 5–8 MeV region. The decay properties of the 7491 keV level in ⁵⁵Mn and the 6877 keV level in ¹⁴²Nd were studied in detail. Total and partial radiative widths of nuclear levels in several isotopes were determined using nuclear self-absorption, temperature variation, and absolute scattering cross section measurements. The total radiative widths were found to be of the same magnitude as those of unbound levels populated in neutron resonances. The spins and parities of some resonance levels were determined by carrying out angular distribution and polarization measurements, respectively.     

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.     

Measurements of a partial cross section for the reaction 58Ni(n, γ0)59Ni

The partial cross section for radiative neutron capture accompanied by gamma transitions to the ground state of the ⁵⁹Ni nucleus was measured as a function of energy by a new neutron-spectrometry method that employed the shift of a primary gamma transition in response to a change in the energy of the captured neutron. The reaction ⁷Li(p, n)⁷Be was used as source of neutrons for the present measurements. The protons that induced this reaction were accelerated by a Van de Graaff electrostatic generator to energies exceeding the reaction threshold by 60 keV, in which case an appropriate geometry of the experiment permitted irradiation of the sample under study with neutrons whose energy ranged between 10 and 120 keV. The partial widths of some resonances and radiative strength function for hard primary M1 gamma transitions were determined in addition to the above cross sections.     

Measurement of cross sections for inelastic cold-neutron scattering in metals and polymers by the method of (<Emphasis Type="Italic">n, γ</Emphasis>) analysis

The results obtained by measuring the cross sections for the inelastic scattering of very cold neutrons for a number of metals and polymers by the method of a neutron-irradiation analysis are presented. The method is based on simultaneously measuring events of inelastic scattering and neutron capture in the sample under investigation via recording gamma radiation with a semiconductor germanium detector. Neutron capture by a nucleus of the sample is accompanied by the prompt radiation of gamma rays having a known spectrum. Upon inelastic scattering, a neutron acquires thermal energy. Upon leaving the sample, this neutron is absorbed in a special converter that contains the isotope ¹⁰B. The capture of the neutron by a ¹⁰B nucleus is followed by the emission of a 477-keV gamma ray. The probabilities of capture and inelastic scattering are proportional to the respective neutron-interaction cross sections, and the ratio of the recorded detector counts corresponding to events of the two types does not depend on the spectrum of the incident flux of very cold neutrons or on the trajectory of neutron motion in the sample. The sought inelastic-scattering cross section at a fixed sample temperature is calculated by using this ratio and the known cross section for neutron capture by the sample isotope having a known gamma-radiation spectrum.     

Measurements of the partial cross section for the reaction 48Ti(n, γ1)49Ti and estimation of the radiative strength functions for E1 and M1 transitions

The partial cross section for radiative neutron capture by ⁴⁸Ti nuclei was measured as a function of neutron energy. The method of neutron spectrometry used is based on the shift in the energy of the primary γ transition in response to a change in the energy of the captured neutron. The reaction ⁷Li(p, n)⁷Be was used as a neutron source. Protons were accelerated by a Van de Graaff electrostatic generator up to energies of 60 keV above the reaction threshold, which provided neutron energies in the range from 10 to 120 keV. The partial widths of some resonances were determined. The radiative strength functions of E1 and M1 transitions to the first excited state were calculated. __________ Translated from Yadernaya Fizika, Vol. 66, No. 1, 2003, pp. 47–50. Original Russian Text Copyright ? 2003 by Voinov, Serov, Popov, Gundorin, Kobzev, Parzhitski.     

The K+π- elastic scattering cross section as observed in K+p→K+π-Δ++ from 3 to 13 GeV/c

The four-body reaction K⁺p→K⁺π^-Δ⁺⁺ is analysed in a sample of 77 300 bubble chamber events of K⁺p→K⁺π^-π⁺p for beam momenta from 2.5 to 12.6 GeV/c to study the K⁺π^- elastic scattering cross section. The relative merits of several analytic continuations to the pion pole are discussed. Maximum likelihood method is used and, for each experiment fits are performed simultaneously over the whole range of the M(pπ⁺), m(K⁺π^-) and t variables. The K⁺π^- cross section is parametrized in terms of the down solution for the s-wave, a p-wave resonating at the K¹(890) and a d-wave resonating at the K¹(1420). Fitted values are obtained for the four-body cross section, the s-wave contribution to the K⁺π^- cross section, and the masses and widths of the K¹(890) and K¹(1420).     

Neutron resonances in 185Re and 187Re

In a search for intermediate structure, neutron time-of-flight spectra corresponding to 1 and 4 MeV capture γ-ray thresholds were measured for the target isotopes ^{185, 187}Re. The data were analyzed from the point of view of level spacings as well as intermediate structure. A method was developed for estimating the neutron widths of the resonances even in those cases where the neutron widths are comparable to the γ-widths. On the basis of this analysis it was decided which resonances were most likely to be due to p-wave capture. While some unusual behavior was observed, there is no conclusive evidence for intermediate structure. The statistics of level spacings agree with the long range ordering described in the theory of Dyson and Mehta, and are inconsistent with an uncorrelated Wigner distribution. Energies, estimated neutron widths and p-wave probabilities (if over 10 %) for 488 resonances in ¹⁸⁵Re and 335 resonances in ¹⁸⁷Re are tabulated for the energy range 24 eV to 2 keV.     

Cross section for the subbarrier fission of <Superscript>244</Superscript>Cm

The cross section for ²⁴⁴Cm fission induced by neutrons of energy in the range between 0.07 eV and 20 keV was measured by using the lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (Russian Academy of Sciences, Moscow). The parameters of the resonance areas were determined for the lowest eight s-wave neutron resonances, and the respective fission widths were evaluated. Also, the parameters of the intermediate structure in the cross section for the subbarrier fission of ²⁴⁴Cm nuclei were evaluated. The results were compared with available data and recommendations based on evaluations.     

A generalized r-matrix interpretation of the 89Y + n total cross section at low incident energies

The resonance structure observed in the ⁸⁹Y(n, n)⁸⁹Y total cross-section measurements in the range of 0.9 to 1.2 MeV neutron energy is investigated using a comprehensive theory of nuclear reactions. A shell-model calculation which formed the initial stage of this study predicts satisfactorily the energies of the negative-parity states that contribute to the observed anomalies. The neutron decay widths for these resonances are evaluated using the model wave functions. The general trends in the energy dependence of the total cross section are satisfactorily reproduced by the theory. The factors that could contribute to the discrepancies between theory and experiment are discussed. The theoretical estimates of the damping widths for the two 1^? anomalies that occur in this region were within 20 to 25% of the experimental values and support the view that these are intermediate-type resonances. Their configurational structure as predicted by the model calculation suggests that they are the parent states of the T_> components of the giant dipole resonance near 21.0 MeV in ⁹⁰Zr. The distribution of E1 widths calculated for a proposed 1^? → 2⁺ (at 0.78 MeV) transition in ⁹⁰Y indicates that an anomaly corresponding to these 1^? states can also be expected in the (n, γ) reaction.     

Strahlungseinfang langsamer Neutronen im thermischen und Resonanz-Bereich

Within the framework of the many-levelR-matrix formalism the theory ofLane andLynn allows one to calculate the partial cross sections for radiative capture of thermal and resonance energy neutrons. If the cross section for capture to a given final state of the product nucleus has been measured for a number of neutron energies in the thermal and resonance regions, then the variation of the above cross section with incident neutron energy can be completely determined. This calculation has been carried out for the reaction Mn⁵⁵(n, γ) Mn⁵⁶ for two distinct final states of Mn⁵⁶, where enough data are available to fix the parameters for resonance internal capture.     

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.     

恒星能量下核子俘获反应率的晕核效应

恒星能量下俘获截面很难直接测量。¹⁰Be（n，γ）¹¹Be俘获反应涉及到非均匀宇宙大爆炸核合成，无直接测量实验截面数据。利用转移反应¹⁰Be（d，p）¹¹Be的渐近归一化系数（ANC）方法，计算了¹⁰Be（n，γ）¹¹Be俘获反应截面和反应率。¹¹Be是中子晕核。研究表明，在恒星能量下俘获到晕态的截面和反应率显著增大。     

The (nth, α) reaction on 147Sm, 151, 153Eu and Yb isotopes

The (n, α) reaction has been studied using the highly pure thermal neutron beam from the 87m curved neutron guide at the Grenoble high flux reactor. The ¹⁴⁷Sm(n, α)¹⁴⁴Nd reaction showed up five lines corresponding to the ground and the first four excited states of the final nucleus. It is shown that ≈53 % of the 581 μb (n, α) cross section comes from the neutron capture by a bound level of the ¹⁴⁸Sm compound nucleus. The 8.7 ± 3 μb cross section of ¹⁵¹Eu(n, α)¹⁴⁸Pm seems to consist principally of at least two lines corresponding to the ground and the second excited states of ¹⁴⁸Pm. The ¹⁵³Eu(n, α)¹⁴⁹Pm cross section for thermal neutrons is ≦ 1 μb. The lower limits of (n, α) thermal neutron cross section values on ytterbium isotopes are ≈ 20 to 40 times lower than the published data.     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.