Experimental study on the 3p size-resonance in thep- wave neutron strength function

Using a silicon filtered fission neutron beam of an energy width of 20 keV around 143 keV we measured the total cross sections for 37 nuclides and elements having mass numbers between 87 and 140 and determined thep-wave strength functions. The 3P-resonance atA=98 shows no splitting into theP _3/2?P _1/2 doublet. The narrow resonance peak and the following broad distribution of thep-strength function (A=103 to 140) can approximately be reproduced by deformed optical model calculations. The spin-orbit term in the optical potential is consistent with the spin orbit force in the shell model. For nuclei around the closed (N=50) neutron shell a shell effect in thep-wave strength function is indicated.     

Optical model calculation for neutron capture in the mass region A=40–70

Calculations of thes, p andd-wave neutron strength functions based on the spherical optical model have been performed for the mass region A=40–70. The aim was to investigate the radiative neutron capture observed in this region leading to final states with spin J=5^?/2 and J=7^?/2. Results of the calculation indicate thatd-wave neutron capture followed by E1 radiative decay is the most likely explanation of the process.     

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.     

Experimental study onp-wave neutron strength functions for light nuclei

Broad energy distributions in fast neutron beams have been achieved by appropriate filtering of the²³⁶U fission radiation provided from the RENT converter facility at the FRM research reactor. Transmission measurements in such beams result in average cross sections to which resonance reactions and shape elastic scattering contribute. We used a silicon (124.5 cm) filtered beam with a median energy of 143 keV (width 20 keV) and beams with 1.3 MeV (0.55 to 3 MeV) and 2.1 MeV (1 to 5.5 MeV) obtained through different filter combinations of lead and polyethylene. The relative high energies and the broad spectra made it possible to determine experimentally the contributions ofs- andp-wave resonance reactions to the average cross section even for light nuclei. Using the three different beams we determined the average cross sections for the elements in the mass regionA=9 to 65. Analysing the measured cross sections by means of theR matrix formalism provided a complete set ofp-wave strength functions and distant level parameters. Moreover, single particle shell effects in the cross sections were observed. In conclusion we obtained informations on the 2P and the 3S size resonances and about the validity of the optical model for neutron reactions with light nuclei.     

Quadrupole moments and nucleon excitation strengths in even-A Sm isotopes

We present a coherent coupled-channel analysis of 7 MeV neutron and 16 MeV proton elastic and inelastic scattering from ^{148, 152, 154}Sm. The optical potential and nuclear deformation parameters are determined so as to fit not only these elastic and inelastic scattering data but also the low-energy neutron scattering properties and the total cross sections over a wide energy range. This analysis provides evidence of the same excitation strengths for both projectiles in the case of ^{152, 154}Sm, and of a smaller excitation strength for the proton than for the neutron in case of ¹⁴⁸Sm. Moreover the quadrupole moments of these deformed optical potentials are in good agreement with those extracted from Coulomb excitation measurements and from nuclear matter distribution calculations.     

Analysis of average primary gamma-transition intensities and cross sections of the113Cd(n, γ) reaction

A combined analysis of the available data on the primaryγ-ray intensities from the¹¹³Cd(n, γ) reaction atE _n=1.9 and 24.3 keV neutron energies together with the data on¹¹³Cd neutron capture cross sections in theE _n=3–200 keV energy region was carried out. The neutron strength functions were determined asS _n0=(0.260±0.073) 10^?4 and S_n1=(5.06±0.67) 10^?4. No spin-orbit splitting of thep-wave neutron strength function was found. The energy dependence of theE 1 radiative strength function {ie147-01} was fitted by the Kadmenski-Furman model somewhat better than by a standard Lorentzian. TheM 1 giant resonance parameters were obtained as E _G ^{M 1} =8.8±1.6 MeV and Γ _G ^{M 1} = 4.7±2.6 MeV. The neutron capture cross section of¹¹³Cd from its isomeric state ({ie147-02}=11/2^?, E ₁ ^m =263.7 keV) was calculated.     

Nature of parity violation in neutron interaction with lead

The effect of parity violation in the interaction of thermal neutrons with lead was discovered in a number of studies. According to the existing theory, this effect is explained by the mixing of compound states characterized by different parities (s-and p-wave resonances). In view of the absence of a p-wave resonance in the region of thermal neutron energies, it is of importance to reveal a level below the neutron binding energy, a so-called negative resonance. The energy dependence of the cross section for radiative neutron capture on lead was measured in the present study, and it is shown that, for the ²⁰⁷Pb isotope, the results of this measurement deviate from the \({1 \mathord{\left/ {\vphantom {1 {\sqrt E }}} \right. \kern-\nulldelimiterspace} {\sqrt E }}\) law, thereby suggesting the presence of a strong negative resonance. The parameters of this resonance are estimated.     

Cross section of the reaction 6Li(p, γ)7Be

The cross section of the reaction ⁶Li(p, γ)⁷Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E_p from 200 keV to 1200 keV. At E_p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μb. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for ⁶Li(n, γ)⁷Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.     

Proton strength functions in the mass region of 30 to 70 [II]

Theoretical values ofs-,p- andd-wave proton strength functions were calculated for the target mass region between 30 to 70 and for the incident proton energy region of 2–4.75 MeV. They were compared with experimental results. Good agreement was observed for thes- andp-wave proton strength functions.     

Dependence of 14 MeV radiative neutron capture on mass number

The 14 MeV neutron radiative capture cross section has been measured for six nuclei in the mass range from A = 155 to A = 238. The integrated radiative cross sections are found to have a value near 1 mb. These results are at variance with the results obtained by the activation technique. Detailed captude γ-ray spectra have been measured for one closed-shell and two deformed nuclei to illustrate their similarity in shape and magnitude.     

s- and p-wave neutron spectroscopy Xc. Intermediate structure: 88Sr

Neutron total cross section measurements of natural Sr were made from 50–875 keV using a high resolution proton beam and the ⁷Li(p, n) reaction as a neutron source. These data were analyzed with the help of an R-Matrix code to extract resonance (energies and other) parameters up to about 850 keV. 2p-1h and particle-vibration doorway interpretation of the s-, p- and d-wave resonances is attempted in terms of the sum rule Σγ_n² = γ_d². Predictions based on both of these models agree with the experimental results. As expected the p-wave resonances are stronger than either s- and d-wave structure. Theory accounts for the p-wave strength remarkably well. Possible location of the p-wave s.p. resonance is reproduced with a real potential and its damping due to the imaginary potential is calculated.More fragmentation of the strong p-wave doorways is observed than was expected for a compound nucleus so near ⁹⁰Zr, but a larger strength function is observed apparently due to the p-wave giant resonance.     

s- and p-wave neutron spectroscopy Xf(i). Intermediate structure in the 28Si + n reaction: R-matrix interpretation of experimental data

A multilevel R-matrix analysis of Si neutron cross-section data measured at NBS has been performed up to about 4.5 MeV neutron energy. Only a small fraction of the p- and s-wave s.p. strength is observed, but both exhibit local concentrations of strength indicative of doorway structure around 1 and 0.2 MeV, respectively. Besides the well-known 180 keV, strong, $\text{1}\text{2}{}^{\mathrm{+}}$ resonance, the s-wave resonance structure is of moderate strength and widely distributed. The f- and d-wave assignments are not unambiguous, but J > 3/2 resonances show strong signs of intermediate structure for d-waves. A possible correlation between neutron and gamma decay channels and the connection between the states observed in (n, n), (d, p), (n, γ), and (γ, n) channels is discussed. A coreparticle doorway interpretation for s and p- waves is presented.     

s- and p-wave neutron spectroscopy. Xe. Intermediate structure in 90Y and the generalized R-matrix theory

The resonance structure observed in the ⁸⁹Y(n, n)⁸⁹Y total cross section measurements in the range of 0.3 to 1.2 MeV incident energy was investigated using the generalized R-matrix theory of nuclear reactions and the doorway interpretation of intermediate structure. The energies and wave functions of the doorway resonances were calculated in a 2-particle and 3p-1h basis of the shell model. The model space and the parameters of the model calculation chosen were consistent with other shell model calculations in the mass-90 region. Several strong p-wave doorways with J^π = 0⁺, 1⁺, and 2⁺ were predicted by the model in the energy range studied. This is due to proximity of p-wave giant resonance. The escape widths Γ^↑ and the spreading widths Γ^↓ for these states were evaluated using the model wave functions and the R-matrix formalism. The calculated energy dependence of the total cross section shows that most of the predicted doorways are in general agreement with the observed anamolies with similar relative strength. More significantly, the underlying p-wave gross structure representing a grand average is of very similar shape in both theory and experiment. As expected in the mass 90-region, the s- and d-wave doorways contribute less significantly to the calculated resonance structure.     

Neutron radiative capture in Cu

Radiative neutron capture in natural Cu has been studied in the neutron energy range from 200 eV to 16.5 keV. An area analysis of the data yields information on resonance parameters: For large resonances (Γ _n ?Γ _γ ) with known isotopic assignments values ofΓ _γ or, if the spin of the resonance is unknown, 2g Γ _γ are obtained. In some cases, it was possible to assign the spin of the resonance from the measured 2g Γ _γ assuming a fairly constant radiation width. For a number of small resonances (Γ_n?Γ _γ ) with unknown isotopic assignment, values of 4agΓ _n (a=isotopic abundance) have been determined. The distribution of these values, including those of the stronger resonances known from literature, shows an excess of small widths which is attributed top-wave levels. From this, thep-wave neutron strength function for natural Cu is estimated to be $$S_1 = \left( {0.30_{ - 0.14}^{ + 0.18} } \right)10^{ - 4} $$ .     

Proton strength functions in the mass region of 30 to 70

Theoretical values ofs- andp-wave proton strength functions were calculated for the target mass region between 30 to 70 and were compared with experimental results. A reasonable agreement was obtained for thes-wave proton strength function.     

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.     

Measurement of the total and differential cross sections for the reaction π − p → ηn with the Crystal Ball detector

The first results obtained in 1998 by the Crystal Ball collaboration from a measurement of the total and differential cross sections for the reaction π ^? p → ηn are presented. These new experimental results for the total cross sections are compared with the predictions of the K-matrix model for pion-nucleon scattering. The angular distribution at momenta near the reaction threshold (685 MeV/c) is determined by the S-wave contribution. The P-wave contribution begins to manifest itself from a momentum of 720 MeV/c.     

The role of two particle-two hole excitations in the spin-isospin nuclear response

We investigate the role of the 2p-2h states in the spin-isospin nuclear response function. This is done in the frame of a microscopic approach which includes the meson exchange currents and the nucleon-nucleon correlation. We first test our theory on the transverse response in the inclusive deep inclastic electron scattering, where we achieve a satisfactory agreement with the data for values of the momentum transfer ranging from 1 to 2 fm^?1. We next explore the p-wave pion-nucleus absorptive optical potential. We find that a strong (～3) Lorentz-Lorenz-Ericson-Ericson quenching factor is needed to reproduce in our framework the phenomenological optical potential deduced from π-mesic atom data. We also examine the real photon absorption cross section accounting rather satisfactorily for its behaviour, in particular for the Pauli blocking at small frequencies. Finally, we elucidate the conditions for the existence of a connection between the magnetic photon absorption and the p-wave pion absorption in nuclei.     

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).     

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.