Reliable cross sections of partial photoneutron reactions on 188,189Os isotopes free of neutron multiplicity sorting problems

Experimental data on the cross sections of partial photoneutron reactions, obtained for ^188,189Os isotopes using quasimonoenergetic annihilation photon beams and neutron multiplicity sorting method, are analyzed. Using special criteria (transition multiplicity functions F _i = σ(γ, in)/σ(γ, xn), the ratios of the cross section of the corresponding partial reaction to the total neutron yield reaction’s cross section σ(γ, xn) = σ(γ, 1n)+ 2σ(γ, 2n) + 3σ(γ, 3n) free of the problems associated with experimental neutron multiplicity sorting), it is demonstrated that the data contain significant systematic errors. New data are evaluated for cross sections of partial photoneutron reactions (γ, 1n), (γ, 2n), (γ, 3n) and total photoneutron reaction (γ, sn) for ^188,189Os isotopes within an experimental-theoretical approach proposed earlier. It is shown that the significant systematic errors in the experimental cross sections of partial reactions can be attributed to the ambiguity of the relation between the photoneutrons’ multiplicity and their kinetic energy.     

Photoproton cross section and isospin components of the giant resonance in 64Zn

The absolute cross section for the reaction ⁶⁴Zn(γ, p)⁶³Cu using a thin target of 99.6 % isotopically pure ⁶⁴Zn has been measured at four angles using bremsstrahlung from the Los Alamos Electron Prototype Accelerator. The results are compared with the previously measured ⁶⁴Zn(γ, n)⁶³Zn cross section to clearly show the two isospin components of the giant dipole resonance. The measured photoproton cross section is combined with previously measured (γ, n), (γ, 2n) and (γ, np) results to obtain a “total” photodisintegration cross section for ⁶⁴Zn.     

The photodisintegration of 40Ar

High resolution measurements of the (γ, n), (γ, 2n), (γ, p), (γ, np) and (γ, 2p) cross sections of ⁴⁰Ar over a photon energy range of 10 to 28 MeV are reported. From this data, the total photon absorption cross section integrated to 26 MeV is found to be 434 ± 40 MeV mb. The results of a dynamic collective model (DCM) calculation compare favourably with the photoabsorption cross section, supporting the use of the DCM in this mass region. It is confirmed that isospin plays an important role in the decay of the ⁴⁰Ar giant dipole resonance.     

K+ nucleon elastic scattering at 180° between 1.0 and 1.5 GeV/c incident momentum

We have measured the cross section at 180° for K⁺p and K⁺n elastic scattering in the momentum range 1.0 to 1.5 GeV/c. The K⁺n cross section was measured on deuterium and the K⁺p on hydrogen and deuterium. We were thus able to measure directly the difference between free nucleon (proton) scattering and bound nucleon (proton) scattering at large angles. This difference was found to be small and within our experimental accuracy the K⁺p(n) cross section should be equal to the K⁺p (free) cross section at 180°. We found no evidence for an s-channel resonance Z¹ in either the K⁺p or K⁺n system. A comparison of our data and those of other groups with theoretical predictions is given.     

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.     

Absolute photofission cross section of 209Bi in the energy range from 40 to 65 MeV

The absolute photofission cross section of ²⁰⁹Bi has been measured with monoenergetic γ-radiation between 40 and 65 MeV photon energy. Cross-section data have been obtained with an accuracy between 9 and 20%. The experimental result is compared with the excitation function calculated on the basis of the statistical model. In order to reproduce the measured data on σ_γf, the (γ, n) cross section must decrease with increasing photon energy faster than the experimental total (γ, n) cross section. This behaviour can possibly be explained by the assumption that after photon absorption a compound nucleus is formed only for a small, and with photon energy decreasing, fraction of all decays.     

Formation of isomeric states in (γ, γ′) reactions at energies around the giant dipole resonance

The population of isomeric states in (γ, γ′) photonuclear reactions at energies around the giant dipole resonance is considered for ¹¹⁵In and ¹⁸⁰Hf nuclei. The results are compared with previously published experimental and theoretical data. The behavior of the excitation function suggests an increase in the cross section for the formation of high-spin isomeric states in the giant-dipole-resonance region.     

A measurement of collinear and nearly collinear photon pairs produced by e+e− annihilation at the 3100 MeV resonance

We have measured collinear and nearly collinear photon pairs from e⁺e^? annihilation at total energies covering the resonance at 3100 MeV. The observed cross section is in good agreement with the e⁺e^? → γγ cross section expected from quantum electrodynamics. Upper limits for the decay modes (3100) → γγ, π^Oγ and other modes are derived.     

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.     

The photonuclear cross sections of 48Ca

The sum of the photoneutron and photoproton cross sections of ⁴⁸Ca (⁴⁸Ca(γ, n + p)), has been deduced from an activation yield curve measurement using bremsstrahlung. In a separate experiment the ⁴⁸Ca(γ, p) and partial photoproton cross sections to the ground and first excited states of ⁴⁷K have been deduced from measured photoproton spectra recorded from the electro-disintegration of ⁴⁸Ca. The ⁴⁸Ca(γ, n) cross section is estimated from these data. A statistical decay model is used to estimate the ⁴⁸Ca(γ, 2n) cross section. These reported cross sections are used to determine the relative strengths and energy distributions of the isospin components of the GDR states, and to compare these with theoretical predictions.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

Der Kernphotoeffekt am3H

The nuclear photoeffect in tritium was investigated by irradiating a gaseous³H-target with bremsstrahlung of 32,5 MeV endpoint energy in the collimated beam of the Heidelberg betatron. Protons and deuterons from the reactions³H(γ, n)d and³H(γ, p)2n were measured independently at (90±15)° with respect to the γ-beam by adE/dx-E-semiconductor telescope. The cross section of the twobody disintegration³ H(γ, n)d is derived forγ-energies fromE _γ=18 to 31 MeV. The integrated cross section from threshold to 32 MeV is (11.8±1.4) MeVmb. The cross section of the threebody disintegration³ H(γ, 2n)p is obtained by making assumptions regarding the angular distribution and energy split up amongst the three disintegration products, which appear plausible on theoretical grounds. Within experimental errors the results reported here seem to be in agreement with a theoretical calculation byFetisov.     

(γ, p) cross sections and isospin splitting of the giant dipole resonance in N = 50 nuclei

The differential (e, e'p) cross sections of ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr and ⁹²Mo have been measured at θ = 90°. The results are used to obtain the (γ, p) cross sections by correcting with the angular distribution data. The (γ, p) cross sections are compared with the (γ, n) cross sections. A resonance corresponding to the T_< GDR is found and another resonance is separated by fitting a Lorentz line with a width equal to that of the (γ, n) GDR. The ratios σ(γ, p)/σ(γ, n) and σ(γ, p₀)/σ(γ, p) show differences between the two resonances. The two GDR resonances are studied from the isospin splitting point of view. The sum rule and splitting energy of the (γ, p) GDR agree well with theory. However, when the sum of the (γ, p) and (γ, n) experimental data is taken, the results are too large to be explained by the T_> GDR.     

The polarization and differential cross section of fast neutrons scattered from 6Li and an R-function analysis of the interaction below 4 MeV

The analyzing power of ⁶Li for the elastic scattering of polarized neutrons with energies between 2 and 4 MeV, has been measured at six angles ranging from 25° to 150°. The polarized neutrons were generated from the reactions Pb(γ, n) and ¹²C(n, $\text{n}$), and their polarization was measured using the double-scattering method. The neutron energies were determined with a nanosecond timeof-flight spectrometer. In addition, the differential cross section was measured at three angles in the same energy range; the cross section was determined relative to the well-known n-¹²C cross section. The present results have been combined with existing data for the neutron total, (n, α), and differential elastic cross sections, in a reduced R-function analysis. Clear evidence of a p-wave triplet of shellmodel states emerges from the analysis.     

A study of the photoneutron contribution to the giant dipole resonance in doubly even Mo isotopes

By using a variable monochromatic photon beam, the partial photoneutron cross sections σ(γ, n) + σ(γ, pn), σ(γ, 2n) and σ(γ, 3n) are determined in the region of the giant dipole resonance for the doubly even Mo isotopes ⁹²Mo, ⁹⁴Mo, ⁹⁶Mo, ⁹⁸Mo and ¹⁰⁰Mo. Measured integrated photoneutron cross sections are compared with available integrated photoproton cross sections as a function of A. Broadening of the giant dipole resonance as A increases is observed in good agreement with the predictions of the dynamic collective model. A tentative study of some isospin splitting effects is also carried out.     

Untersuchung der Kernphotoreaktionen12C(γ,n),12C(γ, 2n),39K(γ,n) und40Ca(γ,np) bis zur Mesonenschwelle

In order to obtain data on the photon absorption process between the giant resonance and the meson threshold the cross sections of the reactions¹²C(γ,n)¹¹C,¹²C(γ, 2n)¹⁰C,³⁹K(γ,n)³⁸K^g, and⁴⁰Ca(γ,np)³⁸K^g have been determined by the analysis of yield curves at the 140 MeV electron synchrotron of the PTB. Though the (γ,n) cross sections grow small with increasing photon energy they are different from zero up to energies of 60 MeV and above. The cross section of the reaction¹²C(γ, 2n) is extremely small; its highest value amounts to 0.15% of the highest value of the¹²C(γ,n) reaction. The measured⁴⁰Ca(γ,np) cross section is of the order predicted by the naive quasi-deuteron model. The integrated cross sections of the above reactions up to 140MeV are 85±7,0.90±0.10, 139±16, and 76±7MeVmb respectively.     

Note on influence of dipole-quadrupole coupling upon the C12(γ,n)C11 cross section

The cross section for the C¹² (γ, n) C¹¹ reaction has been measured from threshold to 25-MeV with a small energy bin (100 keV). The giant resonance was resolved into a very well defined triplet structure. It seems that the shape of the C¹² (γ, n) C¹¹ cross section is influenced by the dipole-quadrupole coupling.     

Intermediate structure in the neutron scattering cross sections of iron

Differential cross sections for the Fe(n, n), (n, n′), (n, n′γ) reactions were measured at incident energies between 1.43 and 2.15 MeV. Structure with intermediate widths was observed. The result was analysed by the doorway-state model, and the widths of the assumed doorway states were obtained. Spin and parity assignment of $\text{3}\text{2}$(⁺) was made to a doorway-state resonance at about 2.0 MeV.     

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.     

Cross sections of partial photoneutron reactions for the 115In nucleus and the neutron multiplicity sorting

A joint analysis of the experimental data on cross sections of total and partial photoneutron reactions is performed for the ¹¹⁵In isotope. The data are obtained by using the quasimonoenergetic photons generated upon the annihilation of relativistic positrons. Well-known systematic discrepancies between the results of various experiments are analyzed using objective absolute criteria of data reliability and authenticity. Methods for taking these discrepancies into account are considered. New reliable and authentic evaluations for cross sections of the σ(γ, n), σ(γ2n), and σ(γ, 3n) reactions are obtained using the data on the cross section of the reaction of photoneutron total yields σ(γ,xn) in the context of a new experimental-theoretical approach to evaluating the cross sections of partial reactions. These data are free from the shortcomings of experimental methods for the photoneuron multiplicity sorting.