Reliability of the data on the cross sections of the partial photoneutron reaction for <Superscript>63,65</Superscript>Cu and <Superscript>80</Superscript>Se nuclei

The cross sections of partial photoneutron reactions are evaluated for the ^63,65Cu and ⁸⁰Se isotopes. The cross sections are free of systematic uncertainties from shortcomings of the experimental methods for neutron multiplicity sorting based on measurements of neutron energy used in experiments with quasimonoenergetic annihilation photon beams. An experimental-theoretical method is used to evaluate cross sections σ^eval(γ, in)= F_i^theor σ^exp(γ, xn), where ratios F_i^theor = σ^theor(γ, in)/σ^theor(γ, xn) = σ^theor(γ, in)/σ^theor[(γ, 1n) + 2(γ, 2n) + …] are calculated using a combined model of photonuclear reactions, and σ^exp(γ, xn) is the experimental cross section of the neutron yield reaction free from neutron multiplicity sorting problems. The cross sections are evaluated for reactions (γ, 1n) and (γ, 2n) for the ^63,65Cu and ⁸⁰Se isotopes, and for the total photoneutron reaction σ(γ, Sn) = σ[(γ, 1n) + (γ, 2n) + …]. It is shown that noticeable deviations of the experimental cross sections from the evaluated values result from the unreliable sorting of neutrons between the channels with multiplicities 1 and 2.     

Cross sections of the photoneutron reaction for <Superscript>141</Superscript>Pr and <Superscript>186</Superscript>W nuclei,estimated from physical criteria of data reliability

Using objective physical criteria for data reliability, cross sections of partial photoneutron reactions (γ, 1n), (γ, 2n) and (γ, 3n) that are free of the shortcomings of neutron multiplicity sorting methods used on beams of quasimonoenergetic annihilation photons are obtained for ¹⁴¹Pr and ¹⁸⁶W nuclei. Evaluation is performed using the experimental–theoretical method (ETM), based on the experimental cross section of neutron yield reaction σ^exp(γ, xn) = σ^exp(γ, 1n) + 2 σ^exp(γ, 2ⁿ) + 3 σexp(γ, 3n) + … and ratios F _i ^theor= σ^theor(γ, in)/σ^theor(γ, xn) calculated within the combined model (CM) of photonuclear reactions, which stipulates that σ^eval(γ, in) = F _i ^theor σ^exp(γ, xn). It is found that for ¹⁴¹Pr and ¹⁸⁶W, ratios F ^exp _i do not contradict the data reliability criteria only at energies up to ~21 and ~22 MeV, respectively. At the same time, there are notable discrepancies between F _i ^theor and F ^exp _i, and thus between the evaluated and experimental cross sections of reactions. It is shown that the discrepancies between the evaluated and experimental cross sections are due to the assumed unreliable experimental distribution of neutrons in the channels with multiplicities 1, 2, and 3.     

Data on photoneutron reactions from various experiments for <Superscript>133</Superscript>Cs, <Superscript>138</Superscript>Ba and <Superscript>209</Superscript>Bi nuclei

Basic methods for determining cross sections for photoneutron partial reactions are examined. They are obtained directly in experiments with quasimonoeneregetic annihilation photons or from the cross section for the (γ, xn) = (γ, 1n) + 2(γ, 2n) + 3(γ, 3n) +... neutron-yield reaction in experiments with bremsstrahlung photons by introducing corrections based on statistical nuclear-reaction theory. The difference in the conditions of these experiments, which leads to discrepancies between their results because of sizable systematic errors, is analyzed. Physical criteria are used to study the reliability of data on the photodisintegration of ¹³³Cs, ¹³⁸Ba, and ²⁰⁹Bi nuclei. The cross sections for partial and total reactions satisfying the reliability criteria are evaluated within the experimental–theoretical method (σ^eval(γ, in) = F_i^theor × σ^expt(γ, xn)) on the basis of the experimental cross sections σ^expt(γ, xn) and the results of the calculations within the combined model of photonuclear reactions.     

The Reliability of Cross Sections of Partial Photoneutron Reactions for <Superscript>98</Superscript>Mo

The cross sections of partial photoneutron reactions for ⁹⁸Mo were evaluated. These cross sections are free from the shortcomings of various methods for neutron multiplicity determination used at the beams of quasimonoenergetic annihilation photons and bremsstrahlung radiation. New data on the cross sections of reactions (γ, 1n), (γ, 2n), and (γ, 3n) were obtained using the experimental–theoretical method for evaluation of cross sections of partial reactions satisfying the introduced physical reliability criteria. It is demonstrated that considerable deviations of the experimental cross sections from the evaluated ones result from an inaccurate sorting of neutrons between channels with a multiplicity of 1, 2, and 3.     

Modern status of photonuclear data

The reliability of experimental cross sections obtained for (γ, 1n), (γ, 2n), and (γ, 3n) partial photoneutron reactions using beams of quasimonoenergetic annihilation photons and bremsstrahlung is analyzed by employing data for a large number of medium-heavy and heavy nuclei, including those of ^63,65Cu, ⁸⁰Se, ^90,91,94Zr, ¹¹⁵In, ^112?124Sn, ¹³³Cs, ¹³⁸Ba, ¹⁵⁹Tb, ¹⁸¹Ta, ^186?192Os, ¹⁹⁷Au, ²⁰⁸Pb, and ²⁰⁹Bi. The ratios of the cross sections of definite partial reactions to the cross section of the neutron-yield reaction, F _i = σ(γ, in)/σ(γ, xn), are used as criteria of experimental-data reliability. By definition, positive values of these ratios should not exceed the upper limits of 1.00, 0.50, 0.33,... for i = 1, 2, 3,..., respectively. For many nuclei, unreliable values of the above ratios were found to correlate clearly in various photon-energy regions F _i with physically forbidden negative values of cross sections of partial reactions. On this basis, one can conclude that correspondent experimental data are unreliable. Significant systematic uncertainties of the methods used to determine photoneutron multiplicity are shown to be the main reason for this. New partial-reaction cross sections that satisfy the above data-reliability criteria were evaluated within an experimental–theoretical method [σ ^eval(γ, in) = F _i ^theor (γ, in) × σ ^expt(γ, xn)] by employing the ratios F _i ^theor (γ, in) calculated on the basis of a combined photonuclear-reaction model. It was obtained that cross sections evaluated in this way deviate substantially from the results of many experiments performed via neutron-multiplicity sorting, but, at the same time, agree with the results of alternative activation experiments. Prospects of employing methods that would provide, without recourse to photoneutron-multiplicity sorting, reliable data on cross sections of partial photoneutron reactions are discussed.     

Evaluation of Reliable Cross Sections of Partial and Total Photoneutron Reactions for the <Superscript>139</Superscript>La Nucleus

Cross sections of partial photoneutron reactions free of the shortcomings of different ways of determining the multiplicity of neutrons used on beams of quasi-monoenergetic annihilation photons are evaluated for ¹³⁹La. The experimental-theoretical method of evaluation of partial reaction cross sections satisfying proposed data reliability criteria is used to obtain new data on the cross sections of reactions (γ, 1n), (γ, 2n), and (γ, 3n). It is shown that noticeable deviations of experimental cross sections from evaluated values are due to the unreliable sorting of neutrons between channels with multiplicities of 1, 2, and 3.     

Evaluated Cross Sections of Photoneutron Reactions on the Isotope <Superscript>116</Superscript>Sn and Spectra of Neutrons Originating from These Reactions

With the aid of the results obtained by evaluating cross sections of partial photoneutron reactions on the isotope ¹¹⁶Sn and the energy spectra of neutrons originating from these reactions, the possible reasons for the well-known discrepancies between the results of different photonuclear experiments were studied on the basis of a combinedmodel of photonuclear reactions. On the basis of physical criteria of data reliability and an experimental–theoretical method for evaluating cross sections of partial reactions, it was found that these discrepancies were due to unreliably redistributing neutrons between (γ, 1n), (γ, 2n), and (γ, 3n) reactions because of nontrivial correlations between the experimentally measured energy of neutrons and their multiplicity.     

New data on the mechanism of decay of the giant dipole resonance in the <Superscript>58</Superscript>Ni nucleus

New data on the mechanism of decay of the giant dipole resonance in the ⁵⁸Ni nucleus are obtained from an analysis of the experimental cross sections for the photonucleon reactions ⁵⁸Ni(γ, p_i)⁵⁷Co and ⁵⁸Ni(γ, n_i)⁵⁷Ni. The method used in this analysis takes into account both the energy spread of the dipole strength concentrated in various isospin components of the giant dipole resonance and the spread of the spectroscopic strength of the populated nucleon-hole states over the levels of the final nuclei. The entire body of experimental spectroscopic information about the levels of the final nuclei ⁵⁷Co and ⁵⁷Ni is employed. It is found that the probability of the semidirect mechanism of decay of the giant dipole resonance in the ⁵⁷Ni nucleus lies in the range 0.16–0.3. The probability of semidirect processes is much higher in the (γ, n) channel (0.28–0.62) than in the (γ, p) channel (0.07–0.17).     

Excitation functions for complete-fusion and transfer reactions in <Superscript>4</Superscript>He interaction with <Superscript>197</Superscript>Au nuclei

Excitation functions are measured for the fusion reactions ¹⁹⁷Au(⁴He, xn)^201?xn Tl that are induced by alpha-particle interaction with gold nuclei in the energy range 14–36 MeV and in which x neutrons (0 ≤ x ≤ 3) are evaporated. The stack-activation technique was used to record and separate reaction products. Experimental data on the fusion reactions followed by evaporation of one to three neutrons agree with results of previous studies. For the radiative-capture reaction ¹⁹⁷Au(⁴He,γ)²⁰¹Tl, the upper limit on the cross section proved to be much lower. The excitation functions for the reactions subjected to measurements are compared with the results of calculations based on the statistical model and with the results of an experiment performed previously in a ⁶He beam.     

Energy resolution of experiments with quasimonoenergetic annihilation photons and structure of a giant dipole resonance

Reasons behind the known systematic discrepancies between the results of photonuclear experiments performed with different photon beams are investigated in detail. Information about the cross sections obtained for the reactions ⁶³Cu(γ, n)⁶²Cu and ¹⁹⁷Au(γ, xn) at all stages of experiments with quasimonoenergetic photons from relativistic positrons annihilating in flight is studied, and a comparison with the data of experiments with beams of bremsstrahlung gamma radiation is performed. Data obtained in experiments of both types for the reaction ¹⁶O(γ, xn) are used in the present analysis. It is shown that the difference procedure typically used experiments with quasimonoenergetic annihilation photons hinders the estimation of the actual energy resolution substantially, thus leading to a considerable distortion of information about the structure of cross sections for photonuclear reactions.     

Special features of the isospin splitting of the giant dipole resonance in the <Superscript>90</Superscript>Zr nucleus

Data on the proton and neutron channels of the ⁹⁰Zr photodisintegration were analyzed in detail, basic parameters of the isospin splitting of the giant dipole resonance in ⁹⁰Zr being determined by the properties of these channels. New data concerning the cross sections for the partial photoneutron reactions ⁹⁰Zr(γ, n)⁸⁹Zr and ⁹⁰Zr(γ, 2n)⁸⁸Zr and resulting from a simultaneous correction of data from experiments performed in Livermore (USA) and Saclay (France) by using beams of quasimonoenergetic annihilation photons were invoked. Use was made of information about the positions on the energy scale of states characterized by different isospin values in the ⁹⁰Zr nucleus and nuclei neighboring it, which are members of the respective isospin multiplet. New data on the parameters of the isospin splitting of the giant dipole resonance in the ⁹⁰Zr nucleus were obtained on the basis of a global analysis of data on the giant-dipole-resonance states of the ⁹⁰Zr nucleus, which are manifested in the respective photoneutron and photoproton cross sections and in their decay channels involving states of different isospin in neighboring nuclei.     

New data on (γ, n), (γ, 2n), and (γ, 3n) partial photoneutron reactions

Systematic discrepancies between the results of various experiments devoted to determining cross sections for total and partial photoneutron reactions are analyzed by using objective criteria of reliability of data in terms of the transitional photoneutron-multiplicity function F _i = σ(γ, in)/σ(γ, xn), whose values for i = 1, 2, 3, ... cannot exceed by definition 1.00, 0.50, 0.33, ..., respectively. It was found that the majority of experimental data on the cross sections obtained for (γ, n), (γ, 2n), and (γ, 3n) reactions with the aid of methods of photoneutron multiplicity sorting do not meet objective criteria (in particular, F ₂ > 0.50 for a vast body of data). New data on the cross sections for partial reactions on ¹⁸¹Ta and ²⁰⁸Pb nuclei were obtained within a new experimental-theoretical method that was proposed for the evaluation of cross sections for partial reactions and in which the experimental neutron yield cross section σ ^expt(γ, xn) = σ(γ, n) + 2σ(γ, 2n) + 3σ(γ, 3n) + ..., which is free from problems associated with determining neutron multiplicities, is used simultaneously with the functions F _i ^theor calculated within a combined model of photonuclear reactions.     

Study of fusion and nucleon transfer channels in the <Superscript>197</Superscript>Au + <Superscript>6</Superscript>He reaction in an energy range of <Superscript>6</Superscript>He to 20 Mev/A

New data on the cross sections of reactions occurring during the interaction of ⁶He nuclei with ¹⁹⁷Au at energies of ⁶He from 40 to 120 MeV are presented. The experiments were performed in the ACCULINNA secondary beam separator of the Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. To identify the reaction products, the activation method for measuring the gamma activity of the target assembly of thin foils was used. The excitation functions for fusion reactions involving the evaporation of up to ten neutrons from the compound nucleus as well as reactions with the emission of charged particles and nucleon transfer in the investigated energy region are obtained. Data analysis was carried out using two codes: ALICE-MP and NRV. The cross sections for the (⁶He, xn) reactions occurring through the compound nucleus are mostly in agreement with the results of model calculations based on the statistical approach. It is shown that, up to the energies of 114 MeV, the cross-section drop in the complete fusion reactions is negligible. The experimental excitation functions of reactions leading to the formation of isotopes of mercury and gold (transfer reaction) indicate that the main contribution to their formation is made by direct processes and that evaporation reactions (⁶He, pxn) and (⁶He, αxn) play a minor role, as is evidenced by a comparison of the measured cross sections with the calculation results.     

Study of <Superscript>179</Superscript>Hf<Superscript><Emphasis Type="Italic">m</Emphasis>2</Superscript> excitation

Isomeric ratios of ¹⁷⁹Hf ^m2,g yields in the (γ, n) reaction and the cross section for the ¹⁷⁹Hf ^m2 population in the (α, p) reaction are measured for the first time at the end-point energies of 15.1 and 17.5 MeV for bremsstrahlung photons and 26 MeV for alpha particles. The results are σ = (1.1 ± 0.11) × 10^?27 cm² for the ¹⁷⁶Lu(α, p)¹⁷⁹Hf ^m2 reaction and Y _m2/Y _g = (6.1 ± 0.3) × 10^?6 and (3.7 ± 0.2) × 10^?6 for the ¹⁸⁰Hf(γ, n)¹⁷⁹Hf ^m22 reaction at Е _ep =15.1 and 17.5 MeV, respectively. The experimental data on the relative ¹⁷⁹Hf ^m2 yield indicate a single-humped shape of the excitation function for the ¹⁸⁰Hf(γ, n)¹⁷⁹Hf ^m2 reaction. Simulation is performed using the TALYS-1.4 and EMPIRE-3.2 codes.     

Role of (<Emphasis Type="Italic">n</Emphasis>,2<Emphasis Type="Italic">n</Emphasis>) reactions in transmutation of long-lived fission products

The conditions under which (n,γ) and (n,2n) reactions can help or hinder each other in neutron transmutation of long-lived fission products (LLFPs) are considered. Isotopic and elemental transmutation for the main long-lived fission products, ⁷⁹Se, ⁹³Zr, ⁹⁹Tc, ¹⁰⁷Pd, ¹²⁶Sn, ¹²⁹I, and ¹³⁵Cs, are considered. The effect of (n,2n) reactions on the equilibrium amount of nuclei of the transmuted isotope and the neutron consumption required for the isotope processing is estimated. The aim of the study is to estimate the influence of (n,2n) reactions on efficiency of neutron LLFP transmutation. The code TIME26 and the libraries of evaluated nuclear data ABBN-93, JEF-PC, and JANIS system are applied. The following results are obtained: (1) The effect of (n,2n) reactions on the minimum number of neutrons required for transmutation and the equilibrium amount of LLFP nuclei is estimated. (2) It is demonstrated that, for three LLFP isotopes (¹²⁶Sn, ¹²⁹I, and ¹³⁵Cs), (n,γ) and (n,2n) reactions are partners facilitating neutron transmutation. The strongest effect of (n,2n) reaction is found for ¹²⁶Sn transmutation (reduction of the neutron consumption by 49% and the equilibrium amount of nuclei by 19%).     

Evaluated Integral Cross Sections of the <Superscript>3</Superscript>H(<Emphasis Type="Italic">t</Emphasis>,2<Emphasis Type="Italic">n</Emphasis>)<Superscript>4</Superscript>He Reaction in the Low-Energy Region Obtained with Regard to Electron Screening

procedure is considered for analyzing ³H(t,2n)⁴He reaction proceeding in a gas environment with regard to electron screening [1–4]. Results from such an analysis are presented. An electron screening potential of 121 eV is obtained. The magnitude of this potential is three times higher than the one given in [5]. Starting with a 100 eV energy of particle interaction the cross sections of ³H(t,2n)⁴He reaction are calculated using the above potential. The reaction rates are calculated using the evaluated cross sections in the lowenergy region. Enhancement factors for cross sections and reaction rates are defined.     

Zum Kernphotoeffekt am Ar40 und Ar38

Liquid Argon was irradiated with bremsstrahlung from 18 to 31 MeV endpoint energy in steps of 2 MeV. The yields of the reactions Ar⁴⁰(γ, n)+(γ, p) and Ar³⁸(γ, n) were measured by detecting the 269a and the 35d rest activity with a low-level-counter. Cross section curves for the (γ, n)-processes could be found from the yield values, and they allowed together withσ _N, σ(γ, p) andσ(γ, np) a determination ofσ(γ, 2n) and σ_γabs for Ar⁴⁰. The integrated cross section forσ(γ, n) from threshold to 33 MeV yields (200±40) MeVmb for Ar⁴⁰ and (210±40) MeVmb for Ar³⁸, the corresponding value for σ_γabs being (450±60) MeVmb for Ar⁴⁰.     

Theoretical study of photoionization of the isoelectronic sequence Rb<Superscript>+</Superscript>, Sr<Superscript>2+</Superscript>, and Y<Superscript>3+</Superscript>

The photoionization cross sections for the 4p shell of ions of the Kr isoelectronic sequence Rb⁺, Sr²⁺, and Y³⁺ are calculated. The configuration interaction theory and the perturbation theory are used to describe the many-electron effects. The relativistic effects are taken into account in the Pauli-Fock approximation. The calculated resonance structure of photoionization cross sections for the 4p shell in the region below the 4s threshold associated with the autoionization of the 4s-np singly excited states and the 4p4p-nln′l′ doubly excited states reproduces the results of recent measurements of total photoabsorption cross sections for the Rb⁺, Sr²⁺, and Y³⁺ ions. It is found that, as the nuclear charge in the isoelectronic sequence increases, the ratio between the direct and correlation parts of amplitudes of the 4s-(n/?)p transition changes and, as the consequence, the minimum of the photoionization cross section of the 4s shell shifts from the continuous spectrum to the region of states of discrete spectrum. This accounts for the strong changes in the shape of the 4s-np resonances in the photoionization cross sections for the 4p shell of Rb⁺, Sr²⁺, and Y³⁺, as well as the distinction between the shapes of the 4s-6p _1/2 mirror resonance in the partial 4p _1/2 and 4p _3/2 photoionization cross sections for the Y³⁺ ion which do not suppress each other in the total photoionization cross section, as is the case for similar resonances in Rb⁺ and Sr²⁺.     

Microscopic multichannel investigation of the <Superscript>6</Superscript>Li + <Emphasis Type="Italic">n</Emphasis> seven-nucleon nuclear system at low energies

The ⁶Li + n seven-nucleon nuclear system is studied at low energies within a microscopic approach based on the multichannel algebraic version of the resonating group model. The partial and total cross sections for the ⁶Li(n, t)⁴He reaction are calculated. The contributions of the various partial cross sections to the energy dependence of the total cross section are considered. The results of the calculations are found to be in a good agreement with experimental data.     

Resonance Structure of the Charge-Exchange Strength Function and Neutrino-Capture Cross Sections for the Isotopes <Superscript>71</Superscript>Ga, <Superscript>98</Superscript>Mo,and <Superscript>127</Superscript>I

A resonance structure of the charge-exchange strength function S(E) and its effect on the neutrino-capture cross sections for the isotopes ⁷¹Ga, ⁹⁸Mo, and ¹²⁷I are studied within the self-consistent theory of finite Fermi systems. The calculation of the strength function S(E) takes into account Gamow–Teller, analog, and so-called lower lying pygmy resonances. The neutrino-capture cross sections σ(E) for the above three isotopes are calculated with allowance for the resonance structure of the strength function S(E), and the effect of each resonance on the energy dependence σ(E) is analyzed. It is found that all charge-exchange resonances in the strength function S(E) should be taken into account in calculating the neutrino-capture cross section σ(E) for the isotopes ⁷¹Ga, ⁹⁸Mo, and ¹²⁷I. The disregard of even highlying resonances leads to a substantial underestimation of the cross section σ(E), and this may affect the interpretation of respective experimental data.