Photonuclear Reactions on Bypassed Nuclei <Superscript>84</Superscript>Sr and <Superscript>92</Superscript>Mo

The yields of photonuclear reactions on mixtures of natural isotopes of strontium and molybdenum that result in the formation and decay of bypassed nuclei ⁸⁴Sr and ⁹²Mo are measured and compared to theoretical results obtained using the TALYS code and calculations in the combined model of photonucleon reaction (CMPR). The need is demonstrated to consider isospin splitting in cases of photoproton reactions. The yields of the ⁹²Mo(γ,p)⁹¹mNb, ⁸⁴Sr(γ,p)⁸³Rb reactions that make the main contribution to the formation and decay of ⁸⁴Sr and ⁹²Mo in the p-process of nucleosynthesis differ from the TALYS and CMPR results by several hundred percent.     

Photonuclear reactions on molybdenum isotopes

The results of experimentally and theoretically studying the photodisintegration of molybdenum isotopes are presented. The yields of various photonucleon reactions on stable molybdenum isotopes were determined by the gamma-activation method. Use was made of bremsstrahlung photons from an electron accelerator that were characterized by the endpoint energies of 19.5, 29.1, and 67.7 MeV. Reactions involving the emission of up to three nucleons from the target nucleus were observed. The experimental results in question were compared with theoretical predictions based on the combined photonucleon-reaction model and obtained for 15 molybdenum isotopes. The relations between basic photonucleon reactions on the molybdenum isotopes considered in our present study were explained.     

Yields of photonuclear reactions on Sn isotopes at an energy of 29.1 MeV

Yields of photonuclear reaction on natural mixture of Sn isotopes irradiated by bremsstrahlung beam with maximum energy of 29.1 MeV are studied via the residual activity method. Using the TALYS code and a combined model, the cross sections of the photonucleon reaction in stable isotopes of Sn are calculated. Measured yields are compared with the yields calculated using theoretical, experimental, and estimated cross sections.     

Photonuclear reactions on mercury isotopes in the region of the giant-dipole-resonance energy

The induced-activity method is used to measure yields of photonuclear reactions induced in stable mercury isotopes by beams of bremsstrahlung photons whose spectra have the endpoint energies of 19.5 and 29.1 MeV. On the basis of a collective model, the partial cross sections and yields are calculated for photoproton and photoneutron reactions on these isotopes. The yields calculated theoretically are compared with their measured counterparts. The possibility for the production in photonuclear reactions of the bypassed nucleus ¹⁹⁶Hg, which cannot be formed in astrophysical r and s processes, is analyzed.     

Photonuclear reactions on the cadmium isotopes <Superscript>106,108</Superscript>Cd at the bremsstrahlung endpoint energy of 55.5 MeV

The gamma-activation technique was used to measure the absolute yields of photonuclear reactions on the cadmium isotopes ^106,108Cd. The results obtained in this way were compared with the results of the calculations based on the statistical model. For reactions on the isotope ¹⁰⁸Cd, agreement between these theoretical and experimental results is good, but the experimental ratio of the yields of photoproton and photoneutron reactions on the isotope ¹⁰⁶Cd differs substantially from its theoretical counterpart. The results of our present study are discussed from the point of view of the production of bypassed nuclei in the p-process of nucleosynthesis.     

Photodisintegration of Pb isotopes

Yields of photonucleon reactions of different multiplicity of natural isotopic mixture of Pb were measured on a bremsstrahlung photon beam with a maximum energy of 67.7 MeV using gamma-activation methods. Cross-sections of photonucleon reactions of different multiplicity were calculated on stable Pb isotopes within the bounds of the combined model. The calculated cross sections are compared to those measured in experiments on quasimonochromatic photon beams. They are also used to split the measured yields into yields of particular photonucleon reactions.     

Excitation functions of some neutron threshold reactions on isotopes of molybdenum

Cross sections were measured for (n,p) reactions on ^{92, 95, 96, 97, 98}mo, (n, α) reactions on ^{92, 98}Mo, and (n, 2n) reaction on ¹⁰⁰Mo for the first time in the neutron energy range of 5.9 to 9.6 MeV. The quasi-monoenergetic neutrons were produced via the ²H(d, n)³He reaction using a deuterium gas target at a compact cyclotron. Use was made of the activation technique in combination with high-resolution γ-ray spectroscopy. Some systematic trends observed in the excitation functions are discussed. For the various isotopes of molybdenum, with increasing mass of the target nucleus, the thresholds of (n,p) and (n,α) reactions increase and the magnitudes of cross sections near the maxima of the excitation functions appear to decrease. Hauser-Feshbach calculations show that in general the excitation functions of (n, p) and (n, α) reactions are described within a factor of 2 by the statistical model only up to about 8 MeV; the (n,2n) reaction on ¹⁰⁰Mo, however, is reproduced well from threshold up to 15 MeV by this model.     

Neutron skin effect of some Mo isotopes in pre-equilibrium reactions

The neutron skin effect has been investigated for even isotopes of molybdenum at 25.6 MeV ^{94 − 100}Mo(p, xn) reaction using the geometry-dependent hybrid model of pre-equilibrium nuclear reactions. Here the initial neutron/proton exciton numbers were calculated from the neutron/ proton densities obtained from an effective nucleon–nucleon interaction of the Skyrme type. Initial exciton numbers from different radii of even Mo isotopes were used to obtain the corresponding neutron emission spectra. In this investigation the calculated results are compared with the experimental data as also with each other. The results using central densities in the geometry-dependent hybrid model are in better agreement with the experimental data.     

GDR Isospin Splitting and Photoproton Reactions on Isotopes of Tin

Photonuclear reactions on a natural mixture of tin isotopes are studied by means of gamma activation. The experiment is performed using the bremsstrahlung γ-beam of an RTM55 racetrack microtron with a maximum electron energy of 55.6 MeV. The measured yields of photoproton and photoneutron reactions are compared to results from other experimental studies, and to calculations using the TALYS code and the combined model of photonucleon reactions (CMPR).     

Yields of photonuclear reactions on Hg isotopes under a bremsstrahlung beam with a maximum energy of 29.1 MeV

Yields of photonuclear reactions on stable Hg isotopes under a bremsstrahlung photon beam with a maximum energy of 29.1 MeV were measured using the induced activity method. Single-neutron and single-proton reactions and the ¹⁹⁸Hg(γ,1p1n) reaction were observed. Photoneutron reaction yields with the production of metastable states were measured.     

Photodisintegration of tantalum

The yields of photonucleon reactions on the isotope ¹⁸¹Ta that are induced by bremsstrahlung photons whose endpoint energy is 67.7 MeV are measured by the residual-activity method. The cross sections for photonucleon reactions on the isotope ¹⁸¹Ta are calculated on the basis of the TALYS code and a combined model. The measured reaction yields are compared with their counterparts rescaled from cross sections obtained in experiments that employed beams of quasimonochromatic and bremsstrahlung photons. The spectra of neutrons emitted from ¹⁸¹Ta nuclei after the absorption of photons having various energies are calculated within the TALYS code and the combined model in question.     

Critical consideration of the statistical model analysis of photonuclear isomeric cross-section ratios

We have measured isomeric cross-section ratios for the photonuclear reactions ¹⁸¹Ta(γ, 3n) ^178m,gTa, ¹⁴²Nd(γ, n)^141m,gNd, ¹¹⁰Pd(γ, n)^{109m₁,m₂,g}Pd, ¹⁰⁸Pd(γ, n)^107m₁,m₂Pd, ¹⁰⁰Mo(γ, n) ^99m,gMo, ⁹²Mo(γ, n)^91m,gMo and ⁸⁹Y(γ, n)^88m₁,m₂Y. The experimental results were compared with statistical model calculations according to the method of Huizenga and Vandenbosch. The applicability and limits of this method, especially the simple decision model for the population of the isomeric state and the ground state of the residual nucleus, were tested and discussed. Additionally, it is shown that for photonuclear reactions on magic nuclei, contributions of a direct reaction mechanism are not negligible.     

Cross sections of multiparticle photonuclear reactions with <Superscript>203, 205</Superscript>Tl isotopes

An experiment on the irradiation of the natural mixture of ^203,205Tl isotopes by the bremsstrahlung beam of pulsed racetrack microtron RTM-70 (Skobeltsyn Institute of Nuclear Physics, Moscow State University) at an electron energy of 67.7 MeV was performed. The yields and integral cross sections of multiparticle photonuclear reactions with ^203,205Tl isotopes were measured.     

Alpha decay of new neutron deficient gold,mercury, and thallium isotopes

In fusion evaporation reactions of a⁹²Mo beam with targets of neutron deficient Rb — Mo isotopes very neutron deficient isotopes of elements between Au and Po have been produced. The new isotopes^{173, 174}Au,^{175, 176}Hg, and¹⁷⁹Tl were identified by alpha spectroscopy. The mass excess value of¹⁷⁶Hg could be linked to known values of theN?Z=16 chain. The location of the new isotopes with respect to the proton drip line is discussed. A new high energyα transition of (7.20±0.02) MeV andT _1/2=(1.4±0.5) ms has been found in the reaction⁹²Mo+⁸⁹Y→¹⁸¹Tl^* at an excitation energy of 37 MeV. It is tentatively assigned to isotopes produced in 2-particle evaporation channels.     

Photodisintegration of Hg isotopes

Yields of photonuclear reactions of several Hg isotopes are measured on a bremsstrahlung photon beam with a maximal energy of 19.5 MeV using a gamma-activation analysis. Cross sections of photonuclear reactions on stable Hg isotopes are calculated within bonds of the collective model taking isospin splitting of giant dipole resonance into account. The experimental results are compared with the results of theoretical calculations and earlier experimental studies.     

Isomeric ratios in photonuclear reactions of molybdenum isotopes induced by bremsstrahlung in the giant dipole resonance region

We have determined the isomeric ratios of isomeric pairs ^97m,gNb, ^95m,gNb and ^91m,gMo produced in ⁹⁸Mo(γ, p)^97m,gNb, ⁹⁶Mo(γ, p)^95m,gNb and ⁹²Mo(γ, n)^91m,gMo photonuclear reactions in the giant dipole resonance (GDR) region by the activation method. The results were analyzed, discussed and compared with the similar data from literature to examine the role of excitation energy, neutron configuration, channel effect and direct and pre-equilibrium processes in (γ, p) photonuclear reactions. In this work the isomeric ratios for ^97m,gNb from 14 to 19 MeV, for ^195m,gNb from14 to 24 MeV except 20 and 23.5 MeV and for ^91m,gMo at 14 and 15 MeV are the first time measurements.     

Photodisintegration of heavy nuclei in the energy region above the giant dipole resonance

Experimental data on yields of multiparticle photonuclear reactions (involving the emission of up to seven neutrons from the nucleus involved) on ¹⁹⁷Au, ^203,205Tl, and ²⁰⁹Bi nuclei in the region extending from the giant dipole resonance to an energy of 67.7 MeV are presented. These data are compared with the results of modern theoretical calculations that take into account both the excitation of a giant dipole resonance (GDR) in a nucleus and the photodisintegration of quasideutrons (QD) in it. By and large, experimental data confirm the results of theoretical calculations—that is, only upon taking simultaneously into account both alternative photodisintegration mechanisms (GDR excitation and QD photodisintegration) can one describe these experimental data. The contribution of QD photodisintegration grows with increasing photon energy and neutron multiplicity and becomes dominant for reactions involving the emission of not less than five neutrons from the nucleus being considered. The integrated cross sections for the processes in question were estimated on the basis of simultaneously employing experimental yields of multinucleon photonuclear reactions and the respective cross-section shapes calculated theoretically.     

Results of search for heavy neutron clusters in nuclear fission

Experiments on search for heavy neutron clusters in the reaction of induced fission of ²³⁵U nuclei by neutrons have been carried out on the nuclear reactor, using the activation method. Two hypothetical reactions have been investigated: ⁹²Mo, ⁹⁴Mo, ⁹⁵Mo, ⁹⁶Mo, ⁹⁸Mo, ¹⁰⁰Mo(^xn, (x ? k)n)¹⁰⁵Mo → ¹⁰⁵Tc → ¹⁰⁵Ru → ¹⁰⁵Rh → ¹⁰⁵Pd, with a minimum transfer of 5 to 13 neutrons to activated Mo isotopes, and ¹²²Te(^xn, (x ? k)n)^{122 + k} Te → (β^?) → ^{122 + k} I, with a minimum transfer of 10 neutrons to activated ¹²²Te isotope. Radiochemical methods for selecting technetium isotopes from molybdenum and iodine from tellurium were used. For the first reaction, the upper limit on the probability of neutron cluster formation was found to be P _k ≤ 10^?8 per fission. For the second reaction, indications to the existence of heavy neutron clusters were obtained.     

Refinement of the giant dipole resonance parameters with allowance for all exit channels of photonuclear reactions

The contribution from various channels of photonuclear reactions is considered to obtain parameters of the giant dipole resonance (GDR). It is pointed out that the channel with emission of charged particles is very important for some nuclei. GDR parameters for the ⁶⁰Ni, ⁶³Cu, and ⁶⁴Zn nuclei are evaluated with allowance for all exit channels of photonuclear reactions. The EMPIRE-II code is used for theoretical calculations.