Total Photoneutron Reactions Research Articles

Cross sections of partial photoneutron reactions: the problems with reliability and new data

The (𝛾, 1𝑛), (𝛾, 2𝑛) and (𝛾, 3𝑛) reactions cross sections the majority of which were obtained using beams of quasimonoenergetic annihilation photons were discussed. The significant systematic uncertainties of the experimental results far exceeding the achieved statistical accuracy were considered. The results of experiments for about 50 nuclei from 51V to 209Bi were analyzed using objective physical criteria of data reliability. It was shown that in many cases the systematic uncertainties of experimental cross sections were significant and data do not satisfy physical criteria. The experimental theoretical method for evaluation of cross sections satisfying those criteria was described. The systematic uncertainties of several types due to shortcomings of the method of photoneutron multiplicity sorting used in experiments and specific features of neutron detectors used. The new evaluated reliable cross sections of the partial reactions and total photoneutron reaction (𝛾, 1𝑛)+(𝛾, 2𝑛)+(𝛾, 3𝑛) were presented.

Photodisintegration of <sup>127</sup>I: Systematic Uncertainties of Experiments and Data Evaluated Using Physical Criteria

The experimental data for photoneutron reaction cross sections for 127I obtained using beams of quasimonoenergetic annihilation photons and the method of neutron multiplicity-sorting at Livermore (USA) and Saclay (France) were analyzed using objective physical data reliability criteria. It was found that data of both laboratories contain significant systematic uncertainties and therefore are not reliable. New data for partial and total photoneutron reactions cross sections for 127I satisfied physical criteria of data reliability were evaluated using experimental-theoretical method based on both experimental neutron yield reaction cross-section and results of calculation in the combined photonucleon reaction model (CPNRM). The neutron yield reaction cross-section obtained at Saclay (France) was used in evaluation procedure. The newly evaluated cross sections for partial (γ, 1n), (γ, 2n) and (γ, 3n) reactions for 127I were used for discussion in detail the problems of significant disagreements between experimental data for many nuclei obtained at Saclay and Livermore. It was found that systematic uncertainties of experimental data for the (γ, 1n), (γ, 2n), and (γ, 3n) reactions cross sections for 127I obtained at both laboratories are of different nature. One of the reasons of noticeable systematic uncertainties of cross sections obtained are the shortcomings of the procedures used to separate counts into 1n, 2n, and 3n events. At the same time it was shown that the main reason of significant disagreements between new evaluated data and data obtained at Livermore experiment for 127I is the loss of many neutrons from the (γ, 1n) reaction. This situation is analogous to those in Livermore experiments for 75As and 181Ta.

New Data on the Photodisintegration of 140, 142Ce and 153Eu Nuclei

Experimental data for 140, 142Се and 153Eu nuclei are analyzed jointly using the cross sections for total and partial photoneutron reactions obtained with quasimonoenergy photon beams formed during the annihilation of relativistic positrons. The results from an experiment performed on a bremsstrahlung beam are also used for such an analysis for the 153Eu nucleus. Well-known systemic discrepancies between the results of different experiments are analyzed using the objective physical criteria of data reliability, and ways of allowing for them are considered. New data on the cross sections of (γ, 1n) and (γ, 2n) reactions for 140, 142Се nuclei, on the cross section of (γ, 3n) reaction for 153Eu nucleus, and on the cross section of total photoneutron reaction for all three nuclei were obtained additionally the cross sections of partial reactions that satisfy the established criteria of reliability. It was shown that substantial deviations of the experimental reaction cross sections from those evaluated are due to an unreliable sorting of neutrons between the channels with a multiplicity of 1, 2 due to the abovementioned systematic uncertainties.

New Data on Photoneutron Reaction Cross Sections for 76,78,80,82Se Nuclei

The problem of reliability of the cross section data obtained for partial photoneutron reactions on 76,78,80,82Se nuclei in beams of quasimonoenergetic annihilation photons by means of neutron multiplicity sorting is discussed by employing objective physical criteria. It is shown that, because of substantial systematic uncertainties, experimental data on the (γ, 1n) and (γ, 2n) cross sections are unreliable. New data satisfying the reliability criteria are obtained for the partial photoneutron reaction cross sections for 76,78,82Se nuclei by an experimental—theoretical method for evaluating such cross sections and are compared with experimental data and with data evaluated earlier for the isotope 80Se. The evaluated integrated cross sections for the total photoneutron reactions on 76,78,80,82Se nuclei are compared with the predictions of the Thomas-Reiche-Kuhn classical dipole sum rule.

Evaluation of Reliable Cross Sections of Partial and Total Photoneutron Reactions for the 139La 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.

Photoneutron cross sections for 59Co : Systematic uncertainties of data from various experiments

Data on partial photoneutron reaction cross sections $(\gamma ,1n)$ , $(\gamma ,2n)$ , and $(\gamma ,3n)$ for 59Co obtained in two experiments carried out at Livermore (USA) were analyzed. The sources of radiation in both experiments were the monoenergetic photon beams from the annihilation in flight of relativistic positrons. The total yield was sorted by the neutron multiplicity, taking into account the difference in the neutron energy spectra for different multiplicity. The two quoted studies differ in the method of determining the neutron. Significant systematic disagreements between the results of the two experiments exist. They are considered to be caused by large systematic uncertainties in partial cross sections, since they do not satisfy physical criteria for reliability of the data. To obtain reliable cross sections of partial and total photoneutron reactions a new method combining experimental data and theoretical evaluation was used. It is based on the experimental neutron yield cross section which is rather independent of neutron multiplicity and the transitional neutron multiplicity functions of the combined photonucleon reaction model (CPNRM). The model transitional multiplicity functions were used for the decomposition of the neutron yield cross section into the contributions of partial reactions. The results of the new evaluation noticeably differ from the partial cross sections obtained in the two experimental studies are under discussion.

Reliability of the data on the cross sections of the partial photoneutron reaction for 63,65Cu and 80Se 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 σeval(γ, in)= F theor σexp(γ, xn), where ratios F theor = σtheor(γ, in)/σtheor(γ, xn) = σtheor(γ, in)/σtheor[(γ, 1n) + 2(γ, 2n) + …] are calculated using a combined model of photonuclear reactions, and σexp(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.

Photodisintegration of the isotopes 186,188,189,190,192Os: Similarities and distinctions

In addition to the results obtained earlier for the isotopes 188,189Os, experimental data on the photodisintegration of the isotopes 186,190,192Os are analyzed on the basis of specially introduced objective criteria of reliability of data on the cross sections for partial photoneutron reactions. It is found that the (γ, 1n), (γ, 2n), and (γ, 3n) cross sections for each isotope satisfy differently or, on the contrary, do not satisfy the data-reliability criteria. In many cases, the multiplicity transition functions specified as the ratios Fi = σ(γ, in)/σ(γ, xn) of the cross sections for the (γ, in) partial reactions to the neutron-yield reaction cross section σ(γ, xn) = σ(γ, 1n) + 2σ(γ, 2n) + 3σ(γ, 3n) +... have values that are physically unreliable by definition. It is shown that ambiguities in the dependence of significant systematic uncertainties in experimentally determined neutron multiplicities on the measured kinetic energies is the reason for this. The dependence of these uncertainties on the energy spectra of neutrons is analyzed. For the isotopes 186,190,192Os, new evaluated data satisfying the data-reliability criteria are obtained for the cross sections for partial and total photoneutron reactions.

New data on cross sections for partial and total photoneutron reactions on the isotopes 91,94Zr

Experimental data on 91,94Zr photodisintegration that were obtained in a beam of quasimonoenergetic annihilation photons by the method of neutron multiplicity sorting are analyzed. It is found that the cross sections for the (γ, 1n), (γ, 2n), and (γ, 3n) reactions on both isotopes do not meet the objective data-reliability criteria formulated earlier. Within the experimental–theoretical method for evaluating partial-reaction cross sections that satisfy these criteria, new data on the cross sections for the aforementioned partial reactions, as well as for the (γ, sn) = (γ, 1n) + (γ, 2n) + (γ, 3n) +... total photoneutron reaction, are obtained for the isotopes 91,94Zr.

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.

Comparison of level density models in (γ, n) reactions of some lanthanide nuclei

Level densities are required at excitation energies where discrete level information is not available or incomplete to get a reliable theoretical analysis of cross sections, spectra, and angular distributions. The total reaction cross sections of some lanthanides (141Pr, 142Nd, 144Sm, 153Eu, 160Gd, 159Tb, 165Ho, 175Lu) were calculated using TALYS 1.2 code for gamma-induced reactions through the five level density models in the incident gamma energy range from 5 to 30 MeV. All calculations from the present study were compared with each other and with the experimental data obtained from EXFOR library. The total photo-neutron reaction cross-section values obtained from the model calculations and experimental measurements taken from EXFOR confirm the presence of systematical disagreements reported in the literature, except 165Ho case where one can see a satisfactory agreement.

New data for the 197Au(γ, nX) and 197Au(γ, 2nX) reaction cross sections

The results from experimental and theoretical studies of the total and partial cross sections of photoneutron reactions on the 197Au isotope were analyzed. The cross sections for reactions σ(γ, nX) = σ(γ, n) + σ(γ, np) + … + σ(γ, 2nX) = σ(γ, 2n) + σ(γ, 2np) + … were evaluated in the energy range 7 ≤ E γ ≤ 30 MeV using an approach free of the shortcomings of experimental photoneutron multiplicity sorting methods. The total photoneutron reaction cross sections σexp(γ, xn) = σexp(γ, nX) + 2σexp(γ, 2nX) + … = σexp(γ, n) + σexp(γ, np) + 2σexp(γ, 2n) + 2σexp(γ, 2np) + … were used as the initial experimental data. The contributions from the cross sections σ(γ, nX) and σ(γ, 2nX) to the cross sections σexp(γ, xn) were separated using the multiplicity transition functions F 1 theor = σtheor(γ, 1nX)/σtheor(γ, xn) and F 2 theor = σtheor(γ, 2nX)/σtheor(γ, xn), calculated within an updated version of the pre-equilibrium model of photonuclear reactions. New evaluated data for both partial reaction cross sections, i.e., σeval (γ, 1nX) = F 1 theorσexp(γ, xn) and σeval(γ, 2nX) = F 2 theorσexp(γ, xn), were obtained. The cross sections σeval(γ, nX) and σeval.(γ, 2nX) evaluated using the theoretically calculated functions F 1,2 theor are consistent with the Livermore data, but substantially contradict the Saclay data.

Evaluated cross sections of the σ(γ, nX) and σ(γ, 2nX) reactions on 112, 114, 116, 117, 118, 119, 120, 122, 124Sn isotopes

A combined analysis of experimental data on total and partial photoneutron reaction cross sections, obtained using bremsstrahlung γ-radiation and quasi-monoenergetic annihilation photon beams, was performed for nine Sn isotopes. The partial reactions σeval(γ, nX) and σeval(γ, 2nX) cross sections were evaluated using an approach free of the shortcomings of experimental neutron multiplicity sorting methods. This approach involves calculations within the photonuclear reaction model, based on Fermi gas densities and considering the effects of nucleus deformation, the isospin splitting of its giant dipole resonance (GDR), and experimental data on the total photoneutron cross sections σexp(γ, xn) = σexp(γ, nX) + 2σexp(γ, 2nX) = σexp(γ, n) + σexp(γ, np) + … + 2σexp(γ, 2n) + 2σexp(γ, 2np) + …. The evaluated σeval(γ, nX) and σeval(γ, 2nX) reactions cross sections were obtained using the introduced transition multiplicity functions Ftheor = σtheor(γ, 2nX)/σtheor(γ, xn) = σtheor(γ, 2nX)/[σtheor(γ, nX) + 2σtheor(γ, 2nX) + …]; and σeval.(γ, 2nX) = Ftheor.σexp.(γ, xn) = σeval(γ, nX) = (1 − 2Ftheor)σexp(γ, xn). The evaluated partial reaction cross sections were used to assess the total photoneutron reaction cross sections σeval(γ, sn) = σeval(γ, nX) + σeval(γ, 2nX) + … as functions of the mass number A. The GDR features of 112, 114, 116, 117, 118, 119, 120, 122, 124Sn isotopes were studied and are discussed here.