Search

Through the use of the accelerator facilities at home and abroad, the nuclear reaction group of the China Institute of Atomic Energy has made many remarkable achievements in the study of fusion-fission dynamics, fusion-enhancement mechanisms at sub-barrier energies, reaction dynamics induced by exotic nuclei, and the related exotic nuclear structure and proton decay. In this study, some representative achievements are reviewed briefly. (1) The fusion mechanisms at near-barrier energies were investigated systematically, and a self-consistent method to evaluate the coupled-channel effects was proposed. (2) Nuclear deformation parameters were extracted from backward quasi-elastic scattering, which offered evidence for hexadecapole shapes. (3) A surrogate capture method was developed, based on which the first 239Pu(n,2n) excitation function developed in China was derived. (4) Systematic studies of exotic decay spectroscopies for proton-rich nuclei in the sd-shell were performed, following which a β2p decay of 22Si and a large isospin-asymmetry decay were discovered, and a strongly isospin-mixed doublet in 26Si was revealed. (5) Systematic studies of reaction mechanisms induced by exotic nuclei at energies close to the Coulomb barrier were performed, providing evidence for the failure of the dispersion relation in the optical potential of 6He+209Bi, and the reaction dynamics of proton drip-line nuclei of 8B and 17F were investigated. Future research based on the new HiTOF and BRIF facilities is discussed as well.

Academician ZHANG Huanqiao is an outstanding scientist cultivated by the new China in the 1950s. The impoverished and weak war environment of the old China and the hardship era of the new China have nurtured his patriotic determination to serve his motherland. He adhered to the frontline of scientific research and devoted himself wholeheartedly to it. His research fields spanned neutron physics, fission physics and heavy-ion nuclear physics, and he has achieved excellent results under difficult conditions. He committed himself to the country and measured the urgently needed nuclear data to cooperate with nuclear weapon development. He is rigorous and realistic, and repeatedly verifies the experimental results to ensure accuracy. He dares to take the lead and constantly delves into new research fields. All his actions reflect a silent loyalty and love for his motherland and science. He is an inheritor and speaker of the older generation scientists' spirit, and a microcosm of the scientist's spirit and the development of science and technology in the new China. We write this article for sharing on the occasion of academician ZHANG Huanqiao's 90th birthday.

Surrogate reaction method is an important approach to overcome the difficulties meet in the direct measurement of neutron induced nuclear reaction. The current existing surrogate reactions generally employ the peripheral reactions such as inelastic excitation and transfer reaction where the involved angular momenta are much larger than the neutron capture reaction, which causes a difficulty in theoretical correction of spin of compound nucleus. We proposed to use capture reaction of light charged particle as the surrogate reaction, thus the spin distributions of compound nucleus in two reactions are quite similar and therefore the spin correction is not strongly desired. Based on this idea, the 239Pu(n, f) and (n, 2n) cross sections were successfully extracted by using 236U(α,, f) and (α, 2n) reactions as the surrogate reactions. The well coincidence of the present results with the data of ENDFB7 within the error bars shows the reliability of the proposed surrogate capture reaction method.

We will introduce some new results derived recently by the nuclear reaction group at China Institute of Atomic Energy, on the properties of the optical model potentials of neutron-halo 6He system and reaction mechanisms induced by proton-drip line nucleus 17F at energies around the Coulomb barrier. For the study of optical model potentials of exotic nuclear system, we proposed a novel method, i.e., the transfer reaction method. This method has been applied to extract the optical potentials of neutron-halo 6He+209Bi system by measuring the one-proton transfer reactions induced by 7Li on a 208Pb target. A complete picture of threshold anomaly behavior was obtained in the 6He+209Bi system for the first time, where a decreasing trend of the depth of the imaginary potential is observed in the deep sub-barrier region, and the reaction threshold energy is extracted. Moreover, results show thatthe dispersion relation is not applicable for this exotic nuclear system. Reaction mechanisms of proton-rich nuclear systems 17F+89Y and 58Ni were also studied at energies around the Coulomb barrier. Continuum discretized coupled-channels calculations indicate that the coupling effects of the continuum states of 17F+89Y is not significant. Thanks to the employment of a powerful ionization-chamber based detector array, the reaction products over a large Z in the systems of 17F+58Ni can be identified clearly. The data analysis is undergoing.

Fusion evaporation residues and fission fragments have been measured, respectively, at energies around the Coulomb barrier for the 28Si+174Yb and 32S+170Er systems forming the same compound nucleus 202Po. The excitation function of fusion evaporation, fission as well as capture reactions were deduced. Coupled-channels analyses reveal that couplings to the deformations of targets and the two-phonon states of projectiles contribute much to the enhancement of capture cross sections at sub-barrier energies. The mass and total kinetic energy of fission fragments were deduced by the time-difference method assuming full momentum transfer in a two-body kinematics. The mass-energy and mass-angle distributions were obtained and no obvious quasi-fission components were observed in this bombarding energy range. Further, mass distributions of fission fragments were fitted to extract their widths. Results show that the mass widths decrease monotonically with decreasing energy, but might start to increase when Ec.m./VB < 0.95 for both systems.

Taking the anthracite coal in Fanzhuang and Zhengzhuang blocks as the research object, the occurrence, migration and output path of gas and water in coal reservoir and its model were analyzed based on the permeability and nuclear magnetic resonance imaging experiments. The results show that the coal seam water in macropore and microscopic fracture with the pore diameter 10 μm; micropore and mesopore are the main occurrence spaces of CBM, and mesopore, microfracture and endogenous fracture are the main channels for migration and output of CBM, which are very important for migration and production of CBM. Among them, the development of pore and microfracture with the pore diameter >0.5 μm in coal reservoir of Fanzhuang and Zhengzhuang blocks is relatively weak, which restricts the gas production effect.

Taking the anthracite coal in the southerm of Qinshui Basin as the research object, the pore-fracture network models of anthracite coal were established and the key parameters of pore-fracture network were extracted based on the X-ray CT （computed tomography） and focused ion beam scanning electron microscopy（FIB-SEM） imaging experiments. Then, the characteristics of pores and fractures in anthracite coal and their connection relationships were quantitative studied. The results show that the pores of anthracite coal in the southern Qinshui Basin are mainly mesopores with pore size less than 50 nm. Macropore content in anthracite coal is small, and microscopic fractures and cleats make a great contribution to pore volume in anthracite coal. The cross-section is predominantly irregular in shape, which means that the capillary resistance is large. The connectivity of the pores is well with multiple connected paths, which is beneficial to gas migration and output. With the decrease of the scale of pores and fractures, the connectivity and permeability of pore and fracture decrease. Mesopores, microscopic fractures, and cleats play an important role in the connectivity of anthracite coal. The connectivity and permeability of micrometer and millimeter scale pores and fractures in anthracite coal were improved by microscopic fractures and cleats, while the connectivity of nanoscale pores in anthracite coal was dominated by mesopores, which limited the connectivity of anthracite coal.

Two-proton emissions from the excited states of Ne, P and S were investigated experimentally by the radioactive beams bombarding on the Au target via the Coulomb excitation. The complete-kinematics measurements were actualized by the stacks of silicon-strip detectors and CsI+PIN array detectors. The invariant mass of final systems as well as the relative momentum, opening angle, and relative energy of the two emitted protons was reconstructed under the framework of relativistic kinematics. Visible proton-proton correlations were observed in these systems. The mechanisms of two-proton emission were analyzed in a simple schematic model, in which the extreme decay modes like He cluster emission, three-body phase-space decay, and two-body sequential decay were taken into account. With the help of Monte-Carlo simulations, the two protons emitted from the 6.15 MeV excited state of Ne and the excited states in the energy region of 9.6-10.4 MeV of S, respectively, exhibited prominent features of He cluster decay while for the other cases, no obvious diproton emissions were observed. [ABSTRACT FROM AUTHOR]