(α, n) reactions in cascade γ-transitions

The ratios of the cross sections for the formation of the ground and isomeric states in reactions on¹⁰⁷Ag,¹⁰⁹Ag, and¹¹³In are calculated based on the assumption that at low energies the (, n) reaction proceeds via a) formation of the compound nucleus, b) evaporation of a neutron, and c) cascade -transitions. The number of cascade -quanta and the distribution of the excitation energy between the -quanta and the neutron is not postulated beforehand (as is usually done), but is calculated based on the statistical theory. By comparing with experiment, the parameter of the spin-dependence of the density of nuclear states is found. It is shown that the average number of -quanta in the cascade increases approximately linearly with the energy of the particles. The energy spectrum of the -quanta in the (, n) reaction is calculated. The distribution of -quanta has a maximum at low energies (1 MeV), which, as the energy of the primary particles is increased, moves slowly into the higher energy range while broadening at the same time.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 8, pp. 77–81, August, 1984.     

Triple correlation in the 10B(n, αγ)7Li reaction

The formally T-odd triple correlation between the directions of the momenta of α particle and γ quantum and the polarization pseudovector of a thermal neutron was examined for the ¹⁰B+n=⁷Li+⁴He+γ reaction. Such T-odd correlations can be directly used for checking time reversal invariance in the elastic scattering of particles. In more complex reactions, this correlation can occur as a result of particle interactions in the entrance and exit channels of the reaction and, being a background effect, requires correct theoretical or direct experimental estimation. Our experiments gave an upper limit of 3.2×10^?4 (90% confidence level) for the possible T-odd asymmetry parameter in the reaction under study.     

Investigation of αγ angular correlations in the reaction 15N(p, αγ)12C induced by 7.5-MeV protons

The double-differential cross sections for the reaction ¹⁵N(p, αγ)¹²C induced by 7.5-MeV protons are measured for alpha-particle emission angles from 20° to 160°. All even spin-tensor components of the density matrix for the 2⁺ state (4.44 MeV) of the final nucleus are reconstructed by a model-independent method. For the same state of the carbon nucleus, the populations of magnetic sublevels and the components of the angular-momentum-orientation tensor are analyzed as functions of the alpha-particle emission angle. Our experimental data are compared with theoretical predictions based on the triton-cluster-pickup mechanism treated within the coupled-channel method. It is shown that correlation features are sensitive to the reaction mechanism and to structural characteristics of the nuclei involved. The role of sequential processes is emphasized.     

Excitation of isomeric states in the (γ,n), (n, 2n), and (γ, 2n) reactions on the 113In nucleus

The induced-activity method was used to measure the isomeric ratio of the yields of the (γ, n), (n, 2n), and (γ, 2n) reactions on the ¹¹³In nucleus. The energy dependence of the isomeric ratios of the yields of the photonuclear reactions ¹¹³In(γ, n)^112m,g In and ¹¹³In(γ, 2n)^111m,g In is studied in the energy range of 12–35 MeV.     

Spin correlations in the resonance reactionsāa→ Ψ(Z) → bb

A study is made of the processes of annihilation of an arbitrarily polarized e^–e⁺ pair into a pair of particles of spin 0, 1/2, 1, and 3/2 in the (Z)-resonance region.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 53–59, October, 1978.     

Polarization observables in (γ,NN) reactions

The formalism of ( γ, NN) reactions is given where the incident photon is polarized and the outgoing-nucleon polarization is detected. Sixteen structure functions and fifteen polarization observables are found in the general case, while only eight structure functions and seven polarization observables survive in coplanar kinematics. Numerical examples are presented for the ¹⁶O(γ, np) and ¹⁶O(γ, pp) reactions. The transitions to the ground state of ¹⁴C and ¹⁴N are calculated in a model where realistic short-range and tensor correlations are taken into account for the np pair, while short-range and long-range correlations are included in a consistent way for pp pairs. The effects of the one-body and two-body components of the nuclear current and the role of correlations in cross-sections and polarizations are studied and discussed. Received: 7 June 2001 / Accepted: 26 September 2001     

Charge-exchange processes in the quasi-elastic (e,e′p) and (e,e′n) reactions

Calculations of charge-exchange processes in the final state of the quasi-elastic (e, e′N) reaction have been performed employing the Lane formalism. The Lane equations were decoupled by means of a transformation to the isospin representation. It is found that charge exchange has a small effect on the (e, e′p) cross section, but may play an appreciable role in the (e, e′n) reaction.     

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.     

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.     

On the mechanism of (HI, α x n) reactions

An expression is obtained for the cross sections of (Heavy Ion,α x n) reactions by considering two possible mechanisms: compound nucleus formation and massive transfer. In the latter, an alpha particle is emitted in the very early stage of the reaction while the remnant of the projectile fuses into the target forming an excited nucleus which deexcites by neutron evaporation. A good agreement with the experimental excitation functions of¹⁵⁴Sm(¹⁶O,α x n) reactions is obtained which supports the massive transfer hypothesis.     

High-spin levels in138Ce and140Nd Observed in the (α, 4n γ) reactions

Levels of¹³⁸Ce and¹⁴⁰Nd have been studied using the¹³⁸Ba(α,4nγ)¹³⁸Ce and¹⁴⁰Ce(α, 4nγ)¹⁴⁰Nd reactions. Singleγ-ray spectra,γ-γ coincidence spectra, angular and time distributions with respect to the beam bursts have been measured. A number of higher excited states with excitation energies up to about 5 MeV and with spin value up to 12 are populated in both nuclei. The lower states with spins and parities 7^?, 5^?, 6^? and 10⁺ can be explained by two-quasiparticle neutron configurations of the types (h _11/2 ^?1 ,d _3/2 ^?1 ) 7^? , (h _11/2 ^?1 ,S _1/2 ^?1 ) 5^?, 6^? and (h _11/2 ^?2 ) 10⁺. Several high-spin states observed in¹³⁸Ce and¹⁴⁰Nd can be explained qualitatively as four-quasiparticle states with two-proton-two-neutron configurations. The 3^? state at an energy of 2,137.4 keV is observed in¹³⁸Ce. The evidence for the existence of the low-lying 3^? states in¹⁴⁰Nd at 2,124.0 keV is discussed. Beside the known 9.6 ms (7^?) isomeric state in¹³⁸Ce another state at 3,538.5keV (10⁺) with a half life of about 200 ns has been observed. The observed levels in the¹³⁸Ce and¹⁴⁰Nd nuclei are compared with theoretical predictions using delta force interaction.     

Particle-number fluctuations and correlations in transfer reactions obtained using the Balian-Vénéroni variational principle

The Balian-Vénéroni (BV) variational principle, which optimizes the evolution of the state according to the relevant observable, is used at the mean-field level to determine the particle-number fluctuations in fragments of many-body systems. For fermions, the numerical evaluation of such fluctuations requires the use of a time-dependent Hartree-Fock (TDHF) code. Proton, neutron, and total nucleon number fluctuations in fragments produced in collisions of two (40)Ca are computed well above the fusion barrier. For deep-inelastic collisions, the fluctuations calculated from the BV variational principle are much larger than standard TDHF results, and closer to experimental fluctuations. Correlations between proton and neutron numbers are determined. These correlations are shown to be larger with exotic systems where charge equilibration occurs.     

Complication effects in highly-excited state structure of the (γ, n), (e,e′) and (Μ−, γ) reactions

The complication effects in the highly-excited state structure (impurity of the complex configurations of the 2p2h type) of the (, n), (e, e) and (^–, ) nuclear reactions with the⁴⁰Ca nucleus have been studied in terms of the ph-approximation including the following effects: (a) the presence of states of the two particles — two holes type, (b) rigidity loss by nucleus in excited state.     

Energy levels in159Ho from159Tb(α, 4nγ)159Ho and159Tb(3He, 3nγ)159Ho reactions

The reactions¹⁵⁹Tb(, 4n)¹⁵⁹Ho and¹⁵⁹Tb(³He, 3n)¹⁵⁹Ho have been used to populate states in¹⁵⁹Ho. Gamma-ray spectra in single and coincidence modes have been studied. Assignments have been made for the bands built on the Nilsson states 7/2^–[523], l/2⁺ [411] and l/2^–[541]. The results are discussed in terms of rotational models.     

The influence of the spin-orbital bond on direct photonuclear reactions (γ, n) and (γ, p)

The Courant model for direct photonuclear reactions is modified by using the wave functions of a shell model with spin-orbital bond. It is shown that the introduction of spin-orbital interaction does not influence the cross-section. The changes caused by the spin-orbital bond are apparent only on the angular distribution, which has the Courant forma+bsin² , but with a different value ofb/a, i.e. the anisotropy coefficient. New selection lawsj j, j j±1 are found. The introduction of spin-orbital interaction permits the negative value of the anisotropy coefficient, found in some experimental papers, to be explained in a natural way.

---

- - (, n) (, )

: - . , . , - , , a+bsin² , , b/a, . . . j j, j j±1. - , .

---

---

Excerpt from diploma work done at Faculty of Technical and Nuclear Physics in Prague.

---

In conclusion the author thanks lecturer J. Kvasnica for the choice of subject and valuable remarks.     

Complete spectroscopy of51,52V via the50,51V(n, γ) reactions

The gamma-ray spectra emitted after thermal neutron capture in highly enriched⁵⁰V and^natV targets have been studied using in-pile targets at the ILL high flux reactor and pair and germanium detectors. The neutron binding energies in⁵¹V and⁵²V were determined to beB _n (⁵¹V)=11051.11(17) keV andB _n (⁵²V)=7311.22(26) keV. The thermal neutron capture cross-section in⁵⁰V was measured to be 21 _–2 ⁺⁴ b. From 724 lines attributed to⁵¹V, 330 transitions, comprising 90% of the-ray flux, were placed into the level scheme. Fifty-nine primary dipole transitions from the 11/2⁺ or 13/2⁺ capture states in⁵¹V were established from which the E1 strength distribution was deduced. The energy scaling of these primary transitions was found to follow the E1-giant dipole resonance dominance. Many new levels were established; a number of states proposed in previous (d,p) and (n,) work were confirmed from their primary population and decaying secondary radiations. The density of levels in the high spin (9/2I15/2) region was parametrized with the Fermi gas model. The spin distributions of⁵¹V were analyzed and a spin cut-off parameter =2.8 (3) was deduced. — A nearly complete level scheme of⁵²V up to 3.5 MeV excitation and similar results concerning the level density and the primary -ray spectrum were obtained in the⁵¹V (n, )⁵²V study.Supported by Deutsches BMFT under contract 06GÖ141     

The isomeric ratios in some photonuclear reactions (γ, n), (γ, p), (γ, 2n), and (γ, np) induced by bremsstrahlungs with end-point energies in the Giant Dipole Resonance region

We have determined the isomeric ratios in photonuclear reactions ¹¹⁶Cd(γ, n)^{115m, g} Cd, ¹³⁸Ce(γ, n)^{137m, g} Ce, ¹⁵³Eu(γ, n)^{152m, g} Eu, ⁹⁶Mo(γ, p)^{95m, g} Nb, ¹¹⁸Sn(γ, p)^{117m, g} In, ⁸⁹Y(γ, 2n)^{87m, g} Y, ¹⁰⁶Cd(γ, np)^{104m, g} Ag, and ¹¹²Sn(γ, np)^{110m, g} In induced by bremsstrahlungs with end-point energies in the Giant Dipole Resonance (GDR) region. The targets were irradiated at electron accelerator Microtron MT-25 of the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna. Gamma spectra of irradiated samples were measured with a spectroscopic system consisting of 8192-channel analyzer and high-energy resolution (180 keV at gamma ray 1332 keV of ⁶⁰Co) semiconductor detector CANBERRA. The results were discussed and compared with those of other authors. The article is published in the original.