Partial Photoneutron Cross Sections Research Articles

Photoneutron cross section measurements on 208Pb

Photoneutron reactions on 208Pb in the Giant Dipole Resonance (GDR) energy range have been investigated at the γ-ray beam line of the NewSUBARU synchrotron radiation facility in Japan. Making use of quasi-monochromatic laser Compton scattering (LCS) γ-ray beams and of a novel flat-efficiency neutron detection system along with associated neutron-multiplicity sorting method, total and partial (γ,xn) photoneutron cross sections with x = 1 to 4 have been measured for 208Pb in a broad energy range covering the neutron threshold up to 38 MeV.

IAEA Photonuclear Data Library 2019

Photo-induced reaction cross section data are of importance for a variety of current or emerging applications, such as radiation shielding design and radiation transport analyses, calculations of absorbed dose in the human body during radiotherapy, physics and technology of fission reactors (influence of photo-reactions on neutron balance) and fusion reactors (plasma diagnostics and shielding), activation analyses, safeguards and inspection technologies, nuclear waste transmutation, medical isotope production and astrophysical applications.To address these data needs the IAEA Photonuclear Data library was produced in 1999, containing evaluated photo-induced cross sections and neutron spectra for 164 nuclides which were deemed relevant for the applications.Since the release of the IAEA Photonuclear Data Library however, new experimental data as well as new methods to assess the reliability of experimental cross sections have become available. Theoretical models and input parameters used to evaluate photo-induced reactions have improved significantly over the years. In addition, new measurements of partial photoneutron cross sections using mono-energetic photon beams and advanced neutron detection systems have been performed allowing for the validation of the evaluations and assessments of the experimental data. Furthermore, technological advances have led to the construction of new and more powerful gamma-beam facilities, therefore new data needs are emerging.We report our coordinated efforts to address these data needs and present the results of the new up-to-date evaluations included in the new updated IAEA Photonuclear Data Library consisting of 219 nuclides. The new library includes 188 new evaluations produced by the CRP evaluators, and one evaluation taken from the JENDL/PD-2016 library, while 20 evaluations were retained from the previous 1999 IAEA Photonuclear Data Library. In most of the cases, the photon energy goes up to 200 MeV. A total of 55 nuclides are new in this library reflecting the progress in measurements but also the developing data needs. In this paper we discuss the new assessment method and make recommendations to the user community in cases where the experimental data are discrepant and the assessments disagree. In addition, in the absence of experimental data, we present model predictions for photo-induced reaction cross section on nuclides of potential interest to medical radioisotope production.

Experimental and evaluated photoneutron cross sections for Au197

There is a serious well-known problem of noticeable disagreements between the partial photoneutron cross sections obtained in various experiments. Such data were mainly determined using quasimonoenergetic annihilation photon beams and the method of neutron multiplicity sorting at Lawrence Livermore National Laboratory (USA) and Centre d'Etudes Nucleaires of Saclay (France). The analysis of experimental cross sections employing new objective physical data reliability criteria has shown that many of those are not reliable. The IAEA Coordinated Research Project (CRP) on photonuclear data evaluation was approved. The experimental and previously evaluated cross sections of the partial photoneutron reactions $(\ensuremath{\gamma},1n)$ and $(\ensuremath{\gamma},2n)$ on $^{197}\mathrm{Au}$ were analyzed using the new data reliability criteria. The data evaluated using the new experimental-theoretical method noticeably differ from both experimental data and data previously evaluated using nuclear modeling codes gnash, gunf, alice-f, and others. These discrepancies needed to be resolved.

A unified understanding of (γ, n) and (n, γ) reactions and direct neutron-multiplicity sorting

We discuss the γ-ray strength function toward a unified understanding of (γ,n) and (n,γ) reactions and propose a novel technique of direct neutron-multiplicity sorting to resolve the long-standing discrepancy between the Livermore and Scalya data of partial photoneutron cross sections.

Total and partial photoneutron cross sections for Pb isotopes

Using quasimonochromatic laser-Compton scattering $\ensuremath{\gamma}$ rays, total photoneutron cross sections were measured for ${}^{206,\phantom{\rule{0.16em}{0ex}}207,\phantom{\rule{0.16em}{0ex}}208}$Pb near neutron threshold with a high-efficiency 4$\ensuremath{\pi}$ neutron detector. Partial $E1$ and $M1$ photoneutron cross sections along with total cross sections were determined for ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb at four energies near threshold by measuring anisotropies in photoneutron emission with linearly polarized $\ensuremath{\gamma}$ rays. The $E1$ strength dominates over the $M1$ strength in the neutron channel where $E1$ photoneutron cross sections show extra strength of the pygmy dipole resonance in ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb near the neutron threshold corresponding to 0.32$%$--0.42$%$ of the Thomas-Reiche-Kuhn sum rule. Several ${\ensuremath{\mu}}_{N}^{2}$ units of $B(M1)\ensuremath{\uparrow}$ strength were observed in ${}^{207,\phantom{\rule{0.16em}{0ex}}208}$Pb just above neutron threshold, which correspond to an $M1$ cross section less than 10$%$ of the total photoneutron cross section.

Experimental photoneutron cross sections of natural zirconium from 8 MeV to 134 MeV

The inclusive partial photoneutron cross sections and the total photoneutron cross section\(\sigma ^{(1)} (E_\gamma ) = \sum\limits_{i = 1}^{i_{\max } } {\sigma (\gamma ,in...)} \) for natural Zr have been measured from 8 MeV to 134 MeV, with monochromatic photons obtained by in flight annihilation of monoenergetic positrons. The integrated total photoneutron cross section up tillEγ=140 MeV is (1.59±0.11)σ0, whereσ0 is the Thomas-Reiche-Kuhn sum-rule. The integrated total photonuclear absorption cross section is evaluated to be (1.73±0.15)σ0. The behaviour of the total photoneutron cross section as a function of Eγ, in the 40 MeV <Eγ<140 MeV energy range, is well described by a modified version of the quasideuteron model.

Photoneutron cross sections forMn55andCo59

Photoneutron cross sections, including sigma((..gamma..,n) + (..gamma..,pn)), sigma (..gamma..,2n), and sigma (..gamma..,3n), were measured for /sup 55/Mn and /sup 59/Co from threshold to 36.5 MeV, with a photon energy resolution which varied from 80 keV at the lowest to 170 keV at the highest energies measured. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for (..gamma..,1n) and (..gamma..,2n) events were determined simultaneously with the cross-section data by the ring-ratio technique. The cross sections exhibit considerable but weak structure. Other nuclear information extracted from the data includes parameters of the giant dipole resonance, integrated cross sections and their moments, and nuclear symmetry energies. A comparison is made with previous experimental data for these nuclei as well as with theoretical predictions based upon hydrodynamic, vibrational, and dynamic collective models. None of these models fits the data for these odd-even nuclei satisfactorily; more theoretical work is needed for this nuclear mass region.

Giant resonance in transitional nuclei: Photoneutron cross sections for osmium isotopes

Photoneutron cross sections, including $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},pn)]$, $\ensuremath{\sigma}[(\ensuremath{\gamma},2n)+(\ensuremath{\gamma},p2n)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},3n)$, were measured for $^{188}\mathrm{Os}$, $^{189}\mathrm{Os}$, $^{190}\mathrm{Os}$, and $^{192}\mathrm{Os}$ from 7 to 30 MeV and for $^{186}\mathrm{Os}$ from 11 to 20 MeV, with a photon energy resolution of about 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for each multiplicity were determined simultaneously with the cross-section data by the ring-ratio technique. Nuclear information extracted from the data includes parameters of the giant dipole and giant quadrupole resonances, integrated cross sections and their moments, nuclear symmetry energies, and nuclear deformation parameters and intrinsic quadrupole moments. No fewer than eight kinds of evidence point to a sudden change of behavior between $^{189}\mathrm{Os}$ and $^{190}\mathrm{Os}$, which could be interpreted as a phase transition from a statically deformed prolate nucleus to a $\ensuremath{\gamma}$-unstable one, in general (but not detailed) agreement with the prediction of a dynamic-collective-model calculation.

Photonuclear cross sections forO18

All the major photonuclear cross sections for $^{18}\mathrm{O}$, including $\ensuremath{\sigma}(\ensuremath{\gamma},p)$, $\ensuremath{\sigma}[(\ensuremath{\gamma},n)+(\ensuremath{\gamma},np)]$, and $\ensuremath{\sigma}(\ensuremath{\gamma},2n)$, were measured as a function of photon energy from threshold to 42 MeV. The photon energy resolution was between 200 and 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. The photoproton cross section was determined by counting the delayed neutrons from the $\ensuremath{\beta}$-decay of the residual $^{17}\mathrm{N}$ nuclei. Integrated cross sections were extracted from the data and compared with sum-rule predictions. Resonances in the pygmy-resonance region exhibit substantial decay branching to negative-parity states in $^{17}\mathrm{O}$. Significant differences in the branching of resonances in the giant-resonance region to the various partial cross sections, together with the photoneutron-energy data, allow isospin assignments to be made for all the resonances, with the consequent delineation of the isospin strength distribution for $^{18}\mathrm{O}$. Evidence was found for two weak but broad resonances above the giant-resonance region. Finally, comparison with the corresponding cross sections for $^{16}\mathrm{O}$ shows considerable differences between the two nuclei.

A study of the giant dipole resonance in doubly even tellurium and cerium isotopes

The partial photoneutron cross sections [σ(γ, n) + σ(γ, pn)] and σ (γ, 2n) of 124, 126, 128, 130Te and 140, 142Ce were measured in the giant dipole resonance region by means of the monochromatic photon beam installation at SACLAY. Absolute total photoneutron cross sections, Lorentz line parameters and integrated cross sections are evaluated. The experimental behaviour of the GDR for the above nuclei and in particular its spreading, is then tentatively interpreted in terms of the improved dynamic collective model using the concept of potential energy surfaces.

A study of the photoneutron contribution to the giant dipole resonance of nuclei in the 64 ≦ A ≦ 86 mass region

The partial photoneutron cross sections [σ(γ, n) + σ(γ, pn)] and σ(γ, 2n) of 64Zn, Ga, 70,72,74,76Ge, 75As and 76,78,80,82Se were measured with a monochromatic photon beam and a high efficiency neutron detecting system in the photon-energy range 8 ≦ E ≦ 30 Me V. Integrated cross sections are discussed. The evolution of the GDR as observed through the neutron exit channel only, is tentatively interpreted either in terms of isospin splitting or in terms of the dynamic collective model.

A study of the photoneutron contribution to the giant dipole resonance of s-d shell nuclei

Partial photoneutron cross sections [σ(γ, n) + σ(γ, pn)] and σ(γ, 2n) of s-d shell nuclei were measured in the GDR region by means of a continuously variable monochromatic photon beam and a high efficiency neutron detecting system. In some cases, additional measurements of target associated β + activities made a separation of the (γ, n) from the (γ, pn) reaction possible. It is shown that the mean energy position of the gross resonance shape of the GDR of s-d shell nuclei does not show a systematic dependence on A as is the case for heavy nuclei. Further, the average energy positions of the main strength concentrations of the observed “structure” in these GDR are shown to depend strongly on shell effects. An analysis of the competition between the (γ, n), (γ, pn) and (γ, 2n) exit channels is also carried out. The ratio of the integrated experimental photoneutron cross section σ 0 n to the classical TRK dipole sum rule for A = 4 N nuclei, is shown to be only half as big as the one obtained for the neighbouring A = 4 N−1 nuclei. This ratio shows also a monotonie increase as a function of isospin T for all nuclei.

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.

A study of the photofission and photoneutron processes in the giant dipole resonance of 232Th, 238U and 237Np

Partial photoneutron cross sections σ(γ, n) and σ(γ, 2n), together with photofission cross sections σ(γ, F) of 232Th, 232U and 237Np were measured using a monochromatic photon beam in the giant-resonance region. A new data analysing technique has been applied so as to make a direct separate and simultaneous evaluation of the above cross sections possible. Thresholds, absolute cross-section values, Lorentz line parameters, intrinsic quadrupole moments and integrated cross sections are evaluated. A special study of the competition Γ n/ Γ f between the fission and neutron exit channels is also presented. Finally, a determination of the “non-statistical” neutron contribution and an evaluation of the nuclear temperature of 237U are also given.

Giant dipole resonance in N = 82 nuclei

Abstract Total photoneutron cross sections for 56 Ba, 139 57 La, 58 Ce, 141 59 Pr, 60 Nd, using a monochromatic photon beam are obtained by measuring directly and simultaneously the partial photoneutron cross sections σ(γ, n)+σ(γ, pn) and σ(γ, 2n). In particular the modulation of the giant dipole resonance of 141 Pr is compared with previously published results. A tentative analysis of the observed structure within the framework of the particle-hole as well as the collective models is also attempted.

Photoneutron cross sections of 208Pb and 197Au

Partial photoneutron cross sections σ γ, n , σ γ, 2 n , σ γ, 3 n and σ γ·4 n of 208Pb and 197 Au were measured using a monochromatic photon beam and a neutron detecting system capable of analysing directly and simultaneously interactions of the type ( y, xn) in the photon energy range 6 MeV ≦ E ≦ 35 MeV. Nuclear information extracted from these data includes threshold values, integrated cross sections and giant dipole resonance parameters obtained by means of Lorentz line fitting. A tentative analysis of the structure observed on the rising shoulder of the giant resonance of 208Pb is presented. The level density parameters a as well as the non-statistical neutron contributions obtained from a study of the competition between the (γ, n) and (γ, 2n) decay modes, are also given.

Systeme de detection de photoneutrons utilisant un scintillateur a grandes dimensions permettant l'etude simultanee des reactions ( γ, xn)

A detection system capable of analyzing simultaneously (γ, n), (γ, 2n), (γ, 3n) and (γ, 4n) interaction is described. The photoneutron detector is a 1 m dia. sphere containing a gadolinium loaded liquid scintillator viewed by eight photomultipliers. The essential characteristics of the detector, as adapted to this apecific photonuclear work, such as neutron detection efficiencies, time distribution of neutron capture probabilities and pile-up probabilities are discussed. Experimental and theoretical results concerning these parameters show reasonable agreement. The main disadvantaga in this type of detector being its high background sensitivity, the physical and electronic means used to obtain the best signal to noise ratio are also discussed. We finally present the mathematical data analysis required to convert the rough experimental data into the different photonuclear cross sections α( γ, xn) and give the partial photoneutron cross sections of 181Ta as an example.

Photoneutron cross sections of La, Tb, Ho and Ta

Partial photoneutron cross sections (γ, n), (γ, 2n) and (γ, 3n) of 139La, 159Tb and 165Ho have been measured up to 30 MeV using quasi-“monochromatic” photons resulting from annihilation-in-flight. For 181Ta, measurements including (γ, 4n) reactions up to 36 MeV have been taken. An analysis of the experimental photoneutron multiplicity data shows that for 17 MeV < E γ < 21 MeV the “direct effect” neutron contribution varies between 17 and 30%. All photoneutrons from (γ, 2n) and (γ, 3n) reactions are to be classified as “boiled” off neutrons. Quadrupole moments as well as Lorentz line parameters have also been determined.