A New Mechanism for Low Energy Pion Induced Double Charge Exchange Reaction

Using a method developed in theory of double beta decay[1], we propose a new mechanism, namely the two-nucleon pion absorption-emission mechanism for pion double charge exchange reaction at low energies. The calculations are compared with the experimental data of the angular distribution at 50 MeV and the 0° excitation functions of the DCX reactions on ¹⁴C, ¹⁸O with energies < 130 MeV. It is found that the contribution of this new mechanism is very important at low energies.     

Effects of nuclear structure in double charge exchange reactions on 16O and 40Ca

The energy dependence of forward double charge exchange (DCX) reaction is studied for the doubly closed shell nuclei ¹⁶O and ⁴⁰Ca. A common characteristic of the two DCX reactions is a resonance-like peak around 50 MeV pion lab energy. While some authors have interpreted this peak as the evidence of a dibaryon resonance state, we show that it arises naturally in a two-step process in the conventional pion—nucleon system with proper handling of nuclear structure. The effects of nuclear structure are demonstrated through a comparison among different nuclear structure models: the symmetry model SU(4) ? SU(3), the single particle shell model and the «realistic” shell model.     

Contribution of the two-nucleon pion absorption-emission mechanism to the pion-nucleus double-charge-exchange reaction

The two-nucleon pion absorption-emission mechanism 1) and its contribution to the pioni-induced double-charge-exchange (DCX) reaction at energies T_π 300 MeV is studied. By combiing this new mechanism with the conventional sequential mechanism, the 0° excitation functions for the DCX reactions to the isobaric analog states on the ¹⁴C, ¹⁸O, ²⁶Mg, and ⁴²Ca are calculated and compared with the experimental data. We find that the theoretical results are much improved after considering the contribution of the new mechanism, especially at low energies.     

Theoretical Calculations and Analysis of n+208Pb Reaction for Intermediate Energy Neutron

Based on experimental data of total, nonelastic, elastic cross section and elastic scattering angular distributions for n+Pb reactions, a set of neutron optical model potential parameters is obtained in the region of incident neutron energy from 1—300 MeV. The cross sections, angular distributions and energy spectra are calculated and analyzed by optical model, distorted wave Born approximation theory, Hauser-Feshbach theory, exciton model and cascade mechanism inside nuclear. The results indicate that the cross sections can be given for n+ ²⁰⁸Pb reactions which are all in good agreement with experimental data.     

Fast Neutron Capture Cross Section of Natural Molybdenum

The radiative capture cross sections of natural Mo relative to the ¹⁹⁷Au (n,γ) were measured in the 0.7—1.4MeV neutron energy range,using a large liquid scintillator detector and the time-of-flight method.In the 0.01—2.0MeV neutron energy range,the theoretical calculation of (n,γ) reaction cross section for Mo was performed.The result of calculation is compared with the experimental data.     

Calculation and Analysis of Neutron-Induced Reactions on 58Ni Below 150MeV

Based on the experimental data of the total, the nonelastic and the elastic cross sections and the elastic scattering angular distributions for n+⁵⁸Ni reactions, a set of parameters for a neutron optical model potential is obtained in the incident neutron energy region from 0.8—150MeV. Then reaction cross sections, angular distributions, energy spectra, gamma-ray production cross sections and gamma-ray production energy spectra are calculated in terms of the optical model, the distorted wave Born approximation theory, the Hauser-Feshbach theory, the exciton model and the cascade mechanism inside nuclear. The results are analyzed and compared with the existing experimental data and other evaluated data from ENBF/B-6. They are in agreement with each other within error bars.     

Cross Section Calculations for Proton-Induced Reaction on 208Pb with Intermediate Energy up to GeV by Using Quantum Molecular Dynamics

The spallation neutron source induced by high energy protron nucleus interaction is an important link for acceralator driven system. The quantum molecular dynamics (QMD) model is applied to analize the high energy proton induced reactions on ²⁰⁸Pb. The QMD simulations of the double differential cross section of ²⁰⁸Pb (p,xn) reactions with incident energies of 590,800 and 1500MeV are in good agreement with the experimental data,and the results of QMD calculations are better than that of HETC and LAHET.     

A SYSTEMATIC DISTORTED WAVE IMPULSE APPROXIMATION ANALYSIS OF THE CONVENTIONAL MECHANISM FOR PION DOUBLE CHARGE EXCHANGE

In the framework of the distorted wave impulse approximation,a systematic analysis is performed for the contribution from the conventional mechanism to the double charge exchange to the double isobaric analog state.The effects of distortions of pions,nuclear wave functions,short-range correlation functions,parameters of form factors,the spin-dependent term in the πN amblitude etc.are examined.It is shown that all these elements are very important and not negligible for the conventional mechanism.But after including the effects of all these elements,the conventional mechanism is still not able to reproduce the experimental excitation function and agular distribution for the energy region of 0—300MeV,therefore the effects of some other mechanisms should be included.     

DEPENDENCE OF THE MECHANISM AND CROSS SECTION OF THE α PARTICLES EMITTED FROM 12C+64Ni REACTION ON INCIDENT ENERGY

Energy spectra and angular distributions of the particles emitted from ¹²C+⁶⁴Ni reaction at eight energies in the region of 36.5MeV—69.4MeV are measured. The angular distributions of α particles change from forward peaked one to that peaked near by the grazing angle with decreasing incident energy. This result show that the emission mechanism of non-evaporation α particle is different in different energy regions.     

Effect of σ*,φ Mesons on the Neutron Star Matter at Finite Temperature

Within the RMF approach, considering the contribution of σ*,φ mesons and baryon octet {n,p,Λ,Σ^－,Σ⁰,Σ⁺,Ξ^－,Ξ⁰}, some properties of neutron star matter have been investigated in the temperature region between 5—15MeV. It is found that when the contributions of σ*,φ mesons are included, (1) the critical baryon density decreases (but for the Λ hyperon, the effect is not obvious), but the number of hyperons increases; (2) the equation of state becomes soft at higher energy density; (3) the maximum mass decreases while the corresponding radius increases; (4) the central density, the central energy density, and the central pressure all become smaller. At T=5MeV and 10MeV and for Σ⁰,Ξ⁰,Σ⁺, the σ*,φ mesons make critical baryon density greatly decrease; but at T=15MeV, 20MeV, and 25MeV, the effect is not obvious. However, for Λ,Σ^－,Ξ^－ hyperons, at all the above temperatures, the effect is not obvious.     

RELATIVISTIC STUDY OF PROTON-NUCLEUS ELASTIC SCATTERING AT LOWER ENERGIES

The Lorentz invariant relativistic optical potential have been discussed at energies below 300MeV.The Dirac equation with scalar and vector potential is solved by exact partial wave method.The calculated results of proton ⁴⁰Ca at energy region 300—65MeV are presented and compared with the experimental data of differential cross section dσ/dΩ,analyzing power A_y(θ) and spin rotation function Q(θ).It is shown that the impoved relativistic optical potential fits the data well.     

Analysis of Three Annihilation Potential andp+ 12C Inelastic Scattering

The inelastic scattering of antiproton (p_lab=600MeV/c) by ¹²C leading to the excitation of the three normal parity T=0 levels is studied in the framework of Glauber model. The transition densities obtained from fitting to the electron scattering data and the three annihilation potentials derived from analysis of pp scattering data are used in the calculation. The inelastic angular distribution for exciting the three lowest normal parity T=0 states in ¹²C at 4.44MeV(J^π=2⁺), 7.66MeV(J^π=0⁺), and 9.63MeV(J^π=3^-) by 600MeV/c antiproton are calculated by selecting potential parameter suitably. The results are seen to be in fairly good agreement with the available experimental data. The results are given by using the elementary two-body amplitudes comparatively.     

THE CROSS SECTION MEASUREMENT FOR THE 107Ag(n,2n)106mAg REACTION

The cross section for 107Ag(n,2n)^106mAg has been measured by using the activation method relative to the cross section of ²⁷Al(n,α)²⁴Na in the neutron energy range of 13.50—14.73MeV.The values of 469±22,480±22,503±24,538±25,555±25,572±26mb were obtained at the 13.50,13.60,13.90,14.10,14.35,and 14.73MeV neutron energy,respectively.The results are compared with the published data.The neutron energies were determined by activity ratios of Nb and Zr foils.     

Measurement of Excitation Function and Analysis of StatisticalProperty in 27Al+27Al mssipative Reaction

The angular distribuhon in the range of 0_cm=50°—90° and excitation funchon of the reachon products in heavy ion dissipahve reachon ²⁷Al+²⁷Al were measured, in the incident energy region from 114MeV to 120MeV with 200keV step. The statishcal property of the fluctuation of the excitation funchon in heavy ion dissipative reaction was discussed based on the integration of the macro outgoing channels.     

Investigation of Intermediate Mass Fragments Emitted in the Reaction of 294MeV 20Ne+159Tb

Intermediate mass fragments(2Z≤11) emitted from 294 MeV ²⁰Ne+¹⁵⁹Tb reaction have been measured by △E-E telescope counters at the angle of 10°—150°region.The energy spectra of the fragments at back angle(>90°) have bell shape,and their peaks locate near the Coulomb barrier of exit channel.The analysis of the energy spectra has been done in terms of moving source and indicate that all the fragments are emitted from a common source,the compound nuclei formed in incomplete fusion process.The charge and angular distributions of the fragments originated from the asymmetric binary decay of the formed compound nuclei have been calculated based on statistical GEMINI code,and the results are in well agreement with the experimental data.     

The Cross-Section Measurements and Evaluations for 58Ni(n,p),60Ni(n,p),62Ni(n,α) and 54Fe(n,p) Reactions

Activation techniques have been used to measure the cross-section for ⁵⁸Ni(n,p),⁶⁰Ni(n,p) and ⁶²Ni(n,α) reactions in the neutron energy range 13.6—17.8MeV.The uncertainty of measured data has been aculated from 3%—7%.The covariance matrixes for measurement error are calculated.Evaluations for ⁵⁸Ni(n,p),⁶⁰Ni(n,p),⁶²Ni(n,α) and ⁵⁴Fe(n,p) crpss-section are made.     

The Angular Dependence of the Coherence Energy in Dissipative Reaction of 19F+51V

Excitation function fluctuations for projectile-like fragments from ¹⁹F+⁵¹V dissipative reaction within the energy region of 102.25—109.50MeV are reported.The statistical method is applied to the analysis of energy coherence in the cross rection fluctuations and the strong cross correlation between exit channels is obtained.The dependences on charge number and on mass number are presented.The relation between angular velocity damping and the rotational energy dissipation is discussed.     

Theoretical Analysis of Double Differential Cross Sections for n+ 238U Reaction

Based on the new measurements of total, nonelastic, elastic cross section and elastic scattering angular distributions for n+U reactions, a set of neutron optical model potential parameters is obtained in the region of incident neutron energy from 0.1 to 20 MeV. The cross sections, angular distributions, energy spectra and double differential cross sections are calculated and analyzed by optical model, nuclear fission theory, distorted wave Born approximation theory, coupled channel theory, the unified Hauser-Feshbach theory, as well as exciton model. The results indicate that our theoretical model can reasonably analysis n+ ²³⁸U reaction data with neutron energy lower than 20 MeV.     

Study of Recoil Properties of Target Residues From the Interaction of Copper With 20─46MeV/u 12C Ions

By using off- line γ-ray spectroscopy in conjunction with thick target-thick catcher foil techniques,the mean forward recoil ranges have been measured for the residues produced from interaction of copper with 20—46MeV/u ¹²Cions.From these data,linear momentum transfer for central collisions has been extracted. The linear momentum transfer per incident nucleon attains a maximum value of 155MeV/c at approximately 27.5MeV/u.The maximum excitation energy of the composite system is found to be 2.5MeV/u.     

Bi—dimension Position Sensitive Detector Used in the Study of Fusion Fission of Heavy Ions at the Sub—Coulomb Barrier Energy Region

The structure,operation principle and performance of the bi—dimension position sensitive avalanche chamber (BPAC) used in the study of fusion fission induced by heavy ions at the near and sub-Coulomb barrier energy region are describerd.
The fold angle distribution of fragments in different angle region for 84MeV(E_cm)¹⁶O+²³²Th reaction system was obtained by using BPAC,from which the angle distribution of transfer—fission fragments has been distinguished from that of compound mucleus fission fragments.It is thereby certified that transfer—fission is not the reason of anomalous anisotropies of fragment angle distribution.Meanwhile experimental results supported the preequilibrium fission model,in frame of which the anomalous anisotropies of fragment distribution was explained.