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.     

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.     

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.     

Search for narrow resonances in e+e− annihilation at c.m. energies between 29.90 and 31.46 GeV

A search for narrow resonances in e⁺e^? annihilation at c.m. energies between 29.90 and 31.46 GeV provides no evidence for the existence of such states. The 90% confidence upper limit on the integrated resonance cross section is 38 nb MeV, significantly below the value expected for the lowest (t,t?) bound state.     

The reactions 18O(p,p)18O and 18O(p,p')18O1(1.98 MeV) for Ep = 3.8–6.1 MeV

Angular distributions of cross sections and analyzing powers have been measured for ¹⁸O(p, p)¹⁸O and ¹⁸O(p, p₁)¹⁸O¹ (1.98 MeV) in 25 keV intervals for proton energies between 3.8 and 6.1 MeV. A phase-shift analysis of the elastic scattering data was carried out, yielding resonance parameters for 16 levels in ¹⁹F in the excitation energy region 11.6–13.8 MeV. The results generally are in good agreement with previous work. On the basis of spin, parity, excitation energy and a comparison of reduced proton widths with reduced neutron widths of levels in ¹⁹O, an assignment of $\text{T =}\text{3}\text{2}$ could be made to at least five of the levels, including the analog of the broad $\text{3}\text{2}{}^{\mathrm{+}}$ level in ¹⁹O at 5.45 MeV. A Legendre-polynomial analysis of the inelastic scattering data suggests that the cross section for proton energies between 5.0 and 5.5 MeV is dominated by the broad $\text{3}\text{2}$⁺ resonance at E_p = 5.15 MeV.     

20Ne + n → 21Ne(T = 32) resonances

$\text{T =}\text{3}\text{2}$ resonances in ²¹Ne have been studied in measurements of the total neutron cross section of ²⁰Ne using the 190 m neutron time-of-flight facility of the Karlsruhe Isochronous Cyclotron. The high time-of-flight resolution of 6.6 ps/m enabled the study of sharp $\text{T =}\text{3}\text{2}$ resonances in ²¹Ne with an effective energy resolution of up to 4000. Five $\text{T =}\text{case3}\text{2}$ levels have been observed as sharp resonances allowing the precise determination of total width Λ, partial decay with Λ_no and resonance energy E_R. The c.m. resonance parameters of the first $\text{T =}\text{3}\text{2}$ state in ²¹Ne are E_R = 2098.6 ± 0.3 keV, Λ = 2.2 ± 0.5 keV and Λ_no = 0.21 ± 0.05 keV. Upper limits for the partial decay widths are deduced for those $\text{T =}\text{3}\text{2}$ levels which do not appear as resonance anomalies. A search for additional $\text{T =}\text{3}\text{2}$ states was undertaken. The resonance energies are discussed in the framework of the isbobaric mass multiplet equation. The decay widths are compared with shell-model predictions of isospin mixing and the systematics of isospin-non-conserving particle decays.     

Excitation function of giant resonances as doorway states in the 12C(p, p′)12C1 reaction between 19 and 23 MeV

Differential cross section and analyzing power angular distributions for the elastic scattering and inelastic scattering to the 2⁺ state at 4.44 MeV and the 1⁺ state at 12.7 MeV have been measured at incident proton energies varying from 19.15 to 23.34 MeV in 200 keV steps. Elastic scattering data are analyzed using an averaged optical model. Coupled-channel calculations reproduce roughly the 2⁺ data. The rapid variation of the data concerning the 1⁺ state is explained by virtual excitation of giant resonances. For each value of the incident energy, the coupling strength for each resonance is found by fitting the experimental angular distributions. The analysis assuming a weak coupling in the compound system gave a satisfactory fit to the cross section but a poor reproduction of the analyzing power. The assumption of a strong coupling in the ¹³N system allowed a good fit of all data. The angular distributions are dominated by the E1 resonance, whose $\text{1}\text{2}{}^{\mathrm{+}}$ component exhausting more than 37 % of the energy weighted sum rule, explains the isotropy of the cross section below 22 MeV. A $\text{7}\text{2}{}^{\mathrm{+}}$ resonance (15 % EWSR) is located at 19.9 MeV. The $\text{5}\text{2}{}^{\mathrm{?}}$ resonance with its maxima at 20.2 and 21.4 MeV, exhausts about 18 % of the sum rule, which is in good agreement with the results of previous works.     

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.     

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 total,scattering and inelastic Gamma-ray cross sections of yttrium at few MeV energies

Neutron total, scattering and (n; n′,γ) cross sections of elemental yttrium (⁸⁹Y) were measured in the few-MeV region. The neutron total-cross-section measurements were made with broad resolutions from ≈0.5 to 4.2MeV in steps of ?0.1 MeV. Neutron elastic- and inelastic-scattering cross sections were measured from ≈1.5 to 4.0 MeV, at incident-neutron energy intervals of ≈50keV and at ten or more scattering angles distributed between 20 and 160 degrees using neutron detection. Inelastic-scattering cross sections were also determined using the (n; n′,γ) reaction at incident energies from 1.6 to 3.8 MeV at intervals of 0.1 MeV. Gamma-rays and/or inelastically-scattered neutrons were observed corresponding to the excitation of levels at: 909.0±0.5, 1,507.4 ±0.3, 1,744.5±0.3, 2,222.6±0.5, 2,530±0.8, 2,566.4±1.0, 2,622.5±1.0, 2,871.9 ±1.5, 2,880.6±2.0, 3,067.0±2.0, 3,107.0±2.0, 3,140.0±2.0, 3,410.0±2.0, 3,450.0±2.0, 3,504.0 ±1.5, 3,514.0±2.0, 3,556.0±2.0, 3,619.0±3.0, 3,629.0±3.0 and 3,715.0±3.0 keV. The experimental results are discussed in terms of the spherical-optical-statistical, coupledchannels, and core-coupling models, and in the context of previously reported excitedlevel structure.     

Evidence of an isoscalar E3 giant resonance in 16O

Differential cross sections and analyzing powers for the inelastic scattering of polarized protons from ¹⁶O leading to the 2^? state at 8.88 MeV excitation have been measured at incident energies of 31.7, 33.8, 35.8, 36.8 and 39.9 MeV. These data have been analyzed using a distorted wave theory in which the effects of virtual excitation of E1, E2 and E3 giant resonances as doorway states are included explicitly. This analysis shows that the strong energy variation in the data between 30 and 40 MeV may be predominantly due to a new isoscalar E3 resonance with the contributions from the E1 and E2 resonances corroborating earlier findings.     

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).     

Resonant Bhabha scattering at MeV energies

We discuss, on a phenomenological level, the possible appearance of resonances ine ⁺+e ^? scattering at energies in the MeV region. The expected resonance cross section and angular distribution are examined. The observability depends crucially on the attainable energy resolution which is limit by the momentum distribution of the electrons contained in the target.     

Cross section for the subthreshold fission of 236U

The cross section for ²³⁶U fission in the neutron-energy range E _n = 0.001–20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the ²³⁸U(n, f) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of ²³⁶U was confirmed.     

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.     

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.     

Search for narrow baryons in π−p elastic scattering at large angles

Hoping to find resonant structures in the momentum dependence of π^?p elastic scattering we have measured the differential cross section for this reaction at c.m. angles near 90°. An intense pion beam (≈ 10⁷π/s) has been used, together with a high incident momentum resolution (dP/P ≈ 2 × 10^?4), to scan the region of laboratory momenta from 5.75 to 13.02 GeV/c (c.m. energy from 3.42 to 5.03 GeV). The sensitivity attained by the experiment is such that signals would have been seen corresponding to the formation of non-strange baryon resonances having width larger than ≈ 0.1 MeV and elasticity larger than a few per cent. Within these limits no resonances were sighted.     

Measurement of the fast neutron scattering and total cross sections of141Pr

The differential elastic neutron scattering cross sections of¹⁴¹Pr were measured at the incident neutron energies of 1.2, 1.7 and 1.9 MeV in the angular range between 25 and 150 degrees. At 1.7 MeV the differential inelastic neutron scattering cross sections corresponding toQ=?1122 keV, and at 1.9 MeV the ones corresponding toQ=?1122, andQ=?1295 keV were also determined. In a transmission experiment, the total cross section was measured between 0.50 and 2.42 MeV. The total and differential cross sections were calculated using the nuclear optical model. The calculated results were compared with the experimental data.     

High-resolution measurements of analogue states in 59,61,63Cu

Differential cross sections were measured at four angles for proton scattering on ^58,60,62Ni at energies from 3.0 to 4.0 MeV by means of a high-resolution beam from the Tokyo Institute of Technology 4 MV Van de Graaff. An overall resolution of 400 eV was realized using thin solid targets. Five fragmented p-wave analogue states and two fragmented g-wave analogue states were identified in the elastic scattering data. Spectroscopic factors and Coulomb energy differences were extracted for the analogue states. Spins, parities, total and partial widths were obtained for all resonances observed. The s-wave width and spacing distributions were also analyzed.