Cross sections of fast neutron induced reactions on molybdenum isotopes

Cross sections of (n, p), (n, α), and (n, 2n) reactions on molybdenum isotopes have been measured, in the neutron energy range from 13 MeV to 17 MeV, and interpreted in terms of evaporation from the compound nucleus and preequilibrium emission.     

Activation cross sections and isomeric ratios in reactions induced by 14.5 MeV neutrons on152Sm,154Sm and178Hf

Cross sections for the reactions¹⁵²Sm(n, p)^152g,m1,m2 Pm,¹⁵⁴Sm(n, p)^154g,m Pm,¹⁷⁸Hf(n, p)^178m,g Lu,¹⁵⁴Sm(n, d)¹⁵³Pm and¹⁵²Sm(n, α)¹⁴⁹Nd were measured at 14.5 MeV neutron energy by the activation method. On the basis of these cross sections, the associated isomeric ratios in¹⁵⁴Pm,¹⁵²Pm,¹⁷⁸Lu and the comparison with the predictions of different compound and precompound models, conclusions are drawn about the role of the preequilibrium processes in 14.5 MeV neutron induced reactions. Calculations for equal angular momentum removal by equilibrium and preequilibrium emitted particles better reproduced the experimental isomeric ratios, than for higher angular momentum removal in the preequilibrium phase. The isomeric ratios may be used as a source of additional information about the spin of the isomeric states in¹⁵²Pm and¹⁵⁴Pm for which the spectroscopic information is uncertain.     

A systematic study of (n,d), (n,n' p) and (n,pn) reactions at 14.7 MeV

Cross sections for [(n, d) + (n, n′p) + (n, pn)] reactions induced by 14.7 ± 0.3 MeV neutrons on ⁴⁴Ca, ⁴⁹Ti, ⁵⁰Cr, ^{67, 68}Zn, ⁹²Zr and ^{97, 98}Mo have been measured by the activation technique using enriched isotopes as target materials, modern radiochemical separations and high-resolution counting methods. A brief summary of the literature data and our own earlier measurements is given. Our activation data are generally in agreement with those deduced from emitted charged particle characterisation. Some systematic trends in the activation cross-section data were analysed. Similar to other (n, charged particle) reactions, the [(n, d) + (n, n′p) + (n, pn)] reaction cross section decreases as a function of (N?Z)/A; the data, however, fall on two curves, one for nuclei with neutron separation energies (S_n higher than the proton separation energies (S_p) and the other for nuclei with S_n < S_p. For nuclei with S_n ? S_p the (n, n′p) process is very important. Detailed Hauser-Feshbach calculations show that in general contributions of statistical processes to the (n, d) reaction cross section are very small.     

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.     

Investigations with two-neutron transfer reactions on the even isotopes of samarium

The (p, t) reaction on the even isotopes of samarium has been investigated at a proton energy of 25.5 MeV. Angular distributions were obtained in the range 18° ≦ θ ≦ 148° with angular steps between 2° and 5°. The experimental energy resolution varied between 35 keV and 50 keV FWHM. Spin and parity assignments are performed by comparing the measured angular distributions to zero-range DWBA calculations. Some difficulties of DWBA calculations for (p, t) reactions are pointed out. The relative cross sections for transitions to different levels of the final nuclei are compared with other (p, t) and (t, p) measurements in the same region of the rare earth isotopes. The dependence of the (p, t) cross sections for different transitions on the neutron number of the final nuclei is discussed. Some 2⁺ states observed in (p, t) and (t, p) reactions are described in the quadrupole pairing vibrational picture.     

Neutron-emission mechanisms in the <Superscript>115</Superscript>In(<Emphasis Type="Italic">α, xn</Emphasis>) reactions

Spectra and angular distributions of neutrons from the ¹¹⁵In(α, xn) reactions were measured at α-particle energies of 16, 18, 27, and 45 MeV. The measurements were performed with time-of-flight neutron spectrometers at pulsed accelerators of charged particles. The data obtained in this way were analyzed within the models of equilibrium, preequilibrium, and direct reaction mechanisms. The exact formalism of Hauser-Feshbach statistical theory was used in the calculations, level densities in residual nuclei excited in the reactions under consideration being found from the neutron evaporation spectra in the (p, n) reactions on tin isotopes. Contributions from equilibrium, preequilibrium, and direct neutron emission were studied in a wide range of α-particle energies.     

Excitation of giant resonances in the Ca isotopes in (π±, π0) reactions

The excitation of the giant isovector dipole and monopole resonances in the even-A Ca isotopes in pion single-charge-exchange reactions is studied theoretically. Transition densities obtained from a sum-rule approach and from a microscopic charge-exchange RPA are employed in DWIA calculations. The relation of the (π^±, π⁰) cross sections to proton and neutron densities is discussed.     

Investigation of (n, 2n) reactions by the evaporation process

The (n, 2n) cross sections for about 80 nuclei ranging fromA=45 to 209 were calculated using the evaporation model. The calculation was performed for incident energiesE _n which satisfy the conditionU _R =E _n +Q _n ,2n=6±1 MeV. For 11 nuclei in the above mass region, (n, 2n) excitation functions were calculated from threshold to 20 MeV. In all calculations a single set of input parameters was consistently used. The influence of the level-density parameter and neutron cross sections on the evaporation model was tested. All calculated results were compared with experimental data. The systematic discrepancy between experimental data and evaporation calculations leads to the necessity of introducing the preequilibrium emission mechanism.     

Measurement of (n,p) cross sections on isotopes of Cd,Sn and Te for 14 MeV neutrons

For several isotopes of Cd, Sn and Te, cross sections for the (n, p) process have been measured by activation relative to ²⁷Al(n, α)²⁴Na. As target materials, natural Cd, Sn and TeO₂ of high purity have been used. Measurements were carried out by γ-detection using a Ge(Li) detector. For measuring short-lived activities, a pneumatic tube system was used. The neutron energy was 14.6±0.2 MeV for all activations. The results obtained for the total (n, p) cross sections show decreasing values with increasing number of neutrons for a given proton number. This trend obviously follows the behaviour of the difference of the proton and neutron separation energies, determining the competition between proton and neutron emission.     

Continuous spectra of protons emitted in α-particle induced reactions

Proton spectra resulting from (α, p) reactions on ⁵⁴Fe, ⁵⁶Fe, ⁵⁹Co, ⁵⁸Ni, ⁶¹Ni and ⁶³Cu were measured at E_α = 23.0 MeV. Energy and angular distributions of protons present two main components due to evaporative and preequilibrium processes. The hybrid model was chosen for analysing the data: no free parameters were allowed in the computations. The analysis is consistent with the one performed for (α, n) reactions on the same target nuclei. Hybrid model calculations succeed in explaining simultaneously both neutron and proton emission in α-particle induced reactions.     

Proton transitions in22Na(n,p)22Ne reaction

The nuclear reactions induced by thermal and 2 keV neutrons on²²Na radioactive nucleus were studied. For the thermal neutrons a more accurate value of²²Na (n,p)²²Ne reaction cross section was obtained. A weak proton transition to the²²Ne ground state was observed. The upper limits of cross sections for thermal neutron induced (n,∞) reaction and (n, p) reaction with 2 keV neut·rons are given.     

Gamma rays and neutrons following preequilibrium 165Ho(p, xcnypγ) reactions at Ep = 60 MeV

Neutrons and γ-rays following 60 MeV proton bombardment of ¹⁶⁵Ho were measured in coincidence with discrete γ-rays characteristic of the reaction channels (residual nuclei). The cross sections for the (p, xcnγ) reactions with x = 2–6, the γ-ray multiplicities, and the energy and angular distributions of the emitted neutrons were analyzed in terms of the preequilibrium and equilibrium deexcitation processes. Characteristic behaviours of the preequilibrium process were found in the (p, 2nγ) and (p, 4nγ) reactions where the sum E_T, = ∑^xE_i of the energies E_i of the emitted neutrons was large, while those of the equilibrium process were typical for the (p, 6ny) reaction with small E_T. The reactions are well reproduced by the expression σ. ≈0.35∑_xσ (2, x?2) + 0.4∑_xσ(1, χ?1)+ 0.25∑_xσ(0, x), where σ(n_p, n_c) stands for emission of n_p neutrons at the preequilibrium stage followed by evaporation of n_c neutrons at the equilibrium stage.     

Cross sections of fast neutron induced reactions on selenium isotopes

Cross sections for^{74, 76, 77, 78}Se(n, p)^{74, 76, 77, 78}As,⁷⁶,Se(n,2n)⁷⁵Se and⁸⁰Se(n, α)⁷⁷,Ge reactions have been measured in the energy range from 13.0 to 16.6 MeV by the activation technique using Ge(Li) detectorγ-ray spectroscopy. Samples of selenium of natural isotopic composition were used as targets. The measurements provide information on the evolution of the (n, p) excitation functions with the increasing target neutron number (isotopic effect). The reaction model combining the compound nucleus and the preequilibrium emission processes is used to interpret these three types of reactions. The isotopic trend for the (n, p) reactions due to the neutron-proton competition in the compound nucleus is satisfactorily reproduced. However, a further experimental and theoretical work is needed for a better understanding of the origins of discrepancies which are suggested to be related to the precompound aspects of the assumed reaction model.     

Excitation of the analogue of the T> giant dipole resonance of90Zr via the (n,p) reaction

Proton spectra have been measured at lab angles ofθ=0° to 100° for the reaction Zr(n, p)Y at 21.8 MeV incident neutron energy. Spectra over an angular spread of 80° were accumulated simultaneously using a detection system consisting of proportional counterΔE detectors and a curved plastic scintillator as energy detector. Resonant behaviour is observed in the proton spectra for the Zr(n, p)Y reaction but not for the Fe(n, p)Mn reaction. The resonant structure is attributed to the excitation of the analogues ofT _> giant dipole states of Zr. The resonant structure is shown to correspond in position, gross and fine structure, and, in strength and angular dependence of the cross sections, with expectations for the⁹⁰Zr(n, p)⁹⁰Y reaction. DWBA cross sections are derived and fitted to the experimental data with a Lane potential of 150 MeV.     

Cross section of the reaction 6Li(p, γ)7Be

The cross section of the reaction ⁶Li(p, γ)⁷Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E_p from 200 keV to 1200 keV. At E_p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μb. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for ⁶Li(n, γ)⁷Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.     

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.     

Probing the structure of exotic nuclei by transfer reactions

Low energy single nucleon transfer reactions are proposed as a tool to investigate the structure of nuclei far off stability. Experimental concepts and conditions are discussed, in particular high resolution γ-ray spectroscopy after single nucleon pickup reactions. Nuclear structure is described by Skyrme Hartree-Fock calculations including pairing. As representative examples, binding energies, radii and wave functions for Mg and Sn isotopes are calculated. In the neutron deficient Mg isotopes a proton skin is found. At the neutron driplines the Mg and Sn isotopes develop extended neutron skins. The nuclear structure results are used in DWBA and EFR-DWBA transfer calculations. Single nucleon transfer reactions of ^32,36Mg and exotic Sn beams on targets ranging from ²H to ²⁴Mg in inverse kinematics are explored. The one-nucleon transfer cross sections decrease strongly for high-Z targets. An impact parameter analysis shows that the transfer process is selective on the tails of the wave functions. The largest cross sections are obtained for ²H and ⁹Be targets at incident energies of E _lab = 2-5 MeV/u. The energy-momentum dependence is closely related to the special properties of wave functions of weakly bound states. Two-neutron (p,t) stripping reactions are studied for a ⁶He projectile. A strong competition of sequential and direct processes is found at low energies. Received: 1 October 1997 / Revised version: 25 November 1997     

Precompound emission in the reaction 65Cu(p,xn) and the nucleon-nucleon scattering mechanism

The reaction ⁶⁵Cu(p, xn) has been studied with E_p = 26.7 MeV for angles 3° ? θ ? 177°0. Whereas the angle-integrated spectrum can be explained by a semiclassical preequilibrium model, the nucleon-nucleon scattering approach fails for the preequilibrium neutron emission at far backward angles. The failure is shown to be due to principal shortcomings. In contrast, the quantummechanical statistical multistep calculations give good agreement with the main contribution for residual energies U < 9 MeV coming from the SMDE component.     

Measurement of the 2H(p,n) Breakup Reaction at 170 MeV and the Three-Nucleon Force Effects

The effects of three nucleon force (3NF) have been actively studied via the nucleon–deuteron (Nd) scattering states. The differential cross sections and the vector analyzing powers A _y of the ²H(p, n) inclusive breakup reaction at 170 MeV were measured for the study of 3NF effects in the intermediate energy region. The polarized proton beam of 170 MeV was injected to the deuterated polyethylene (CD₂) target and the energy of scattered neutrons were measured by using TOF method. The data were compared with the Faddeev calculations based on modern nucleon–nucleon (NN) forces with and without the 3NF. Concerning the differential cross sections, we can see large discrepancies between the data and the calculations in the region of scattered neutron energies are low, which is similar to the results of the ²H(p, p) inclusive breakup reaction at 250 MeV.     

Target mass correction on the 3He polarized structure function

The diffusion-effusion model has been used to analyse the release and yields of Fr and Cs isotopes from uranium carbide targets of very different thicknesses (6.3 and 148 g/cm²) bombarded by a 1 GeV proton beam. Release curves of several isotopes of the same element and production efficiency versus decay half-life are well fitted with the same set of parameters. Comparison of efficiencies for neutron-rich and neutron-deficient Cs isotopes enables separation of the contributions from the primary (p + ²³⁸U) and secondary (n + ²³⁸U) reactions to the production of neutron-rich Cs isotopes. A rather simple calculation of the neutron contribution describes these data fairly well. The FLUKA code describes the primary and secondary-reaction contributions to the Cs isotopes production efficiencies for different targets quite well.