## 2020 Vol. 37, No. 2

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2020, 37(2): 1-2.
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2020, 37(2): 119-135. doi: 10.11804/NuclPhysRev.37.2019060
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Investigation of fusion reaction mechanism is one of important topics in recent years. In comparison with radioactive ion beam, the beam intensities of weakly bound nuclei are orders of magnitude higher. The study of the reaction mechanism induced weakly bound nuclei can further explore the coupling effect of breakup, transfer and other reaction channels on the fusion process. A lot of experimental data have shown that there are many interesting phenomena in the fusion reaction induced by weakly bound nuclei at energies near the Coulomb barrier, such as "enhancement below the Coulomb barrier'' and "suppression above the Coulomb barrier'' of the complete fusion cross section. In this paper, we mainly review the researches of the suppression phenomenon and discuss the possible reasons for the suppression. The main reason for the suppression phenomenon of the complete fusion cross section is that the weakly bound nuclei break up before entering the fusion barrier, thus reducing the incident flux of the complete fusion reaction channel. At the same time, the experimental results show that the degree of suppression may be related to the mass number and structure of target nuclei. There are three kinds of methods to measure the fusion reaction induced by weakly bound nuclei, which are \begin{document}$\gamma$\end{document} ray measurement, charged particle measurement and charged particle -\begin{document}$\gamma$\end{document} ray coincidence measurement. The charged particle -\begin{document}$\gamma$\end{document} ray coincidence measurement has obvious advantages in reaction channel identification. This paper introduces the three measurement methods and the researches using these three methods at home and abroad, including the researches of our groups. In addition, the recent theoretical research work on fusion reactions induced by weakly bound nuclei is also introduced.
2020, 37(2): 136-150. doi: 10.11804/NuclPhysRev.37.2019068
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The nuclear symmetry energy, which describes the energy difference of per proton and neutron in nuclear matter, has been extensively studied within the last two decades. Around saturation density, both the value and the slope of the nuclear symmetry energy have been roughly constrained, its high-density behavior is now still in argument. Probing high-density symmetry energy at terrestrial laboratories is being carried out at facilities that offer radioactive beams worldwide. While relevant experiments are being conducted, we theoretically developed more advanced isospin-dependent transport model including new physics such as nucleon-nucleon short-range correlations and in-medium isospin-dependent baryon-baryon scattering cross section. New sensitive probes of the high-density symmetry energy are provided, such as squeezed-out neutron to proton ratio, photon and light cluster as well as the production of mesons with strangeness or hidden strangeness. The blind spots of probing the high-density symmetry energy by sensitive observable are demonstrated. Model dependences of frequently used sensitive probes of the symmetry energy have been studied thoroughly based on different transport models. A qualitative observable of neutron to proton ratio at high kinetic energy is proposed to probe the high-density symmetry energy qualitatively. The probed density regions of the symmetry energy by some observables are first studied and usually lower probed density regions comparing with maximum compression density are obtained. Nucleon-nucleon short-range correlations usually reduce values of sensitive observables of the symmetry energy. Probing the curvature of the symmetry energy by involving the slope information of the symmetry energy at saturation point in the transport model is proposed. Besides constraining the high-density symmetry energy by using heavy-ion collisions, a lot of neutron-star related observations from heaven may also be used to constrain the high-density symmetry energy.
2020, 37(2): 151-159. doi: 10.11804/NuclPhysRev.37.2020005
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In this paper, the calculated results of elements \begin{document}$Z\leqslant$\end{document}118 based on the dinuclear system model and other models are compared with the experimental data. It is proved that different models are reliable in predicting the production cross sections of superheavy nuclei. The prediction results of superheavy nuclei \begin{document}$Z=119$\end{document} and \begin{document}$Z=120$\end{document} by different models are compared and analyzed. It is found that the optimal projectile-target combinations to synthesize superheavy nuclei \begin{document}$Z$\end{document}=119 and \begin{document}$Z$\end{document}=120 are \begin{document}$^{48}{\rm{Ca}}+ ^{{\rm{252}}}{\rm{Es}}$\end{document} and \begin{document}$^{40}{\rm{Ca}}+^{{\rm{257}}}{\rm{Fm}}$\end{document}, respectively, and it is likely that the new isotopes \begin{document}$Z$\end{document}=119 will be synthesized in the experiment before the new isotopes \begin{document}$Z$\end{document}=120. Because of the lack of target with \begin{document}$Z>$\end{document}100 in the laboratory, attempts are being made to find heavier projectiles than 48Ca to synthesize superheavy nuclei \begin{document}$Z$\end{document}=121 and \begin{document}$Z$\end{document}=122. The superheavy nuclei \begin{document}$Z$\end{document}=121 can be synthesized by reactions V+Cf. However, the production cross sections of superheavy nuclei \begin{document}$Z$\end{document}=122 are very small, which requires us to improve the detection and identification techniques in the future. We hope that the discussion in this paper can provide some guidances for experimental and theoretical nuclear physicists in the future.
2020, 37(2): 160-165. doi: 10.11804/NuclPhysRev.37.2020022
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Multinucleon transfer reaction 136Xe+208Pb at \begin{document}$E_{\rm{c.m.}}$\end{document}=617 MeV is simulated by improved quantum molecular dynamics (ImQMD) model with and without considering the effect of \begin{document}$Q$\end{document}-value. The calculation results of ImQMD including the effect of \begin{document}$Q$\end{document}-value (ImQMD+\begin{document}$Q$\end{document}) generally better reproduce the experimental mass distribution and distribution of total kinetic energy lost (TKEL). \begin{document}$Q$\end{document}-value effect is found to drive the transfer of less than ten nucleons and suppress more nucleons transfer. It also decreases the average lifetime of composite systems in the events at lower impact parameters. In the production of more neutron-rich isotopes with neutron number \begin{document}$N$\end{document}=126, \begin{document}$Q$\end{document}-value effect enhances the cross sections for these isotopes in the comparison between the results of ImQMD and ImQMD+\begin{document}$Q$\end{document}.
2020, 37(2): 166-171. doi: 10.11804/NuclPhysRev.37.2020008
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We study the large transverse momentum dilepton produced by the photon-nucleon interactions in the peripheral Au-Au collisions at RHIC and Pb-Pb collisions at LHC. We calculate the dilepton production yield by using the perturbative QCD factorization approach and the Weizsäcker-Williams approximation. The numerical results indicate that the photon-nucleon collision processes is negligible by comparing with the conventional large transverse momentum dilepton production at RHIC energies. However, in the large transverse momentum region, the photon-nucleon collision could be an important large transverse momentum dilepton source in the peripheral heavy ion collisions at LHC.
2020, 37(2): 172-179. doi: 10.11804/NuclPhysRev.37.2020007
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Our article introduces a method to construct the squeeze operator in quantum field theory: consider two free Hamiltonians for the same scalar field with two different masses, through Bogoliubov transformation, we derive a generalized squeeze operator which maps the ground state of one to the other. The efficiency of its operation is verified in both the Dirac representation and also the Schrodinger wavefunctional representation in quantum field theory. We believe that the squeeze operators can be found similarly in any real scalar field theory as long as there are two sets of creation and annihilation operators connected by linear transformations.
2020, 37(2): 180-185. doi: 10.11804/NuclPhysRev.37.2019058
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In the booster ring (BRing) of the High Intensity heavy-ion Accelerator Facility (HIAF), the multi-bunch should be merged into single bunch after the acceleration. To study the influence of the beam loading effects during the bunch merging, the simulations of particle tracking with 238U35+ beam are carried out. According to the simulation results, during the bunching merging, the beam loading effects can result in the growth of the momentum spread and the bunch length as well as the oscillation of the bunch length and the bunch center. The potential well distortion induced by the wake voltage and wake field coupling during the bunch merging are the reasons why the bunch center oscillates and the emittance of the beam grows. To reduce the influence of the beam loading effects, the multi-harmonic feed-forward system is employed to compensate the wake voltage. With the feed-forward system, the beam loading effects can be compensated during the bunching merging. The feed-forward system is able to guarantee the high quality of the beam for extraction in the BRing. The frequency range to be covered and the largest wake voltage to be compensated by the feed-forward system are determined according to the simulation results.
2020, 37(2): 186-190. doi: 10.11804/NuclPhysRev.37.2020013
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China Spallation Neutron Source(CSNS) beam power will upgrade to 500 kW(CSNS-II), energy gain of H- linac will up to 300 MeV from 80 MeV, and the 648 MHz βg=0.6 5-cell superconducting elliptical cavity will be adopted to accelerate the H- from 150 to 300 MeV. Accompanied with the advantage of high accelerating gradient, simple structure, easy post-processing, weak stiffeness and detuning is the shortcoming of elliptical cavity, this study analysis detuning characteristic of the elliptical cavity. With code COMSOL Multiphysics, helium pressure sensitivity coefficient KP and lorentz force detuning factor KL of bare cavity was calculated under two end fixed conditions, KP=–45.705 Hz/mbar(1 mbar=100 Pa) and KL=1.574 Hz/(MV/m)2. Stiffener ring was designed and optimized to reduce the Helium pressure sensitivity coefficient and lorentz force detuning factor, and double ring was chosen to improve the detuning of elliptical cavity at last by vast calculation and analysis, the position of stiffener ring was optimized at 75 and 120 mm under two end fixed boundary, Helium pressure sensitivity coefficient drop down to KP=6 Hz/mbar, lorentz force detuning factor drop down to KL=0.43 Hz/(MV/m)2. Moreover, elastic boundary was used to analyze helium pressure sensitivity coefficient and lorentz force detuning factor, more realistic results were got under tuner end 30 kN/mm boundary and the other end fixed condition, KP=4.8 Hz/mbar and KL=1.99 Hz/(MV/m)2, meet engineering requirements. In addition, dynamic lorentz force detuning was analyzed qualitatively with code CST. And natural frequency of cavity was calculated by code Workbench, results show that nature frequency is far from RF pulse repetition rate and ambient vibration frequency, no resonance happen.
2020, 37(2): 191-198. doi: 10.11804/NuclPhysRev.37.2020024
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In the readout electronics for the Water Cerenkov Detector Array(WCDA) of the Large High Altitude Air Shower Observatory(LHAASO), both high precision time and charge measurement are required. A front-end readout chip PASC(Pre-Amplifier and Shaping Circuit) ASIC(Application Specific Integrated Circuit)is designed, and will be actually applied in the third water pond of the WCDA. In order to evaluate the performance of the chips after massive production, it is important to design an automatic test system. This paper presents the design of the ASIC test system, which is used to achieve automatic testing of the time and charge measurement performance of the chips. After a brief introduction of the chip under test, the design scheme and structure of the test system is presented, including the hardware circuits and test software. This system has been applied in batch test of the LHAASO project, and 100 chips have been successfully tested. It can communicate with multiple instruments through the central control software to perform instrument control and complete automated testing and data recording. This automated test method is more suitable for performance testing and evaluation of high precision readout chips under a large dynamic range, which greatly simplifies the test process and can greatly improve the work efficiency of a large number of repetitive test steps in batch test. The test results show that the performance of these chips meet the application requirements of the third pond of the WCDA in LHAASO.
2020, 37(2): 199-208. doi: 10.11804/NuclPhysRev.37.2020020
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The high accuracy and reliability of the T0 signals is one of the most important prerequisites for Chinese Spallation Neutron Source(CSNS) target station spectrometer. Based on the CPLD(Complex Programmable Logic Device) and PMC(Peripheral Mezzanine Card) mother-daughter structure, T0 signal fan-out instruments were designed with a distributed architecture, including a special trigger signal receiver for the neutron chopper with 24 V level trigger. Before the first 1.6 GeV proton beam hits the target, the delay of all T0 signals have been adjusted correctly. These T0 signal fan-out instruments have superior flexibility, maintainability, scalability, which have been working for more than three years, and the whole T0 timing system has run smoothly and achieved high reliability. It has accumulated valuable experience for the higher demand of T0 signal fan-out instruments at the second phase spectrometer in the future.
2020, 37(2): 209-216. doi: 10.11804/NuclPhysRev.37.2020015
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In order to provide a basic understanding on the effect of H on He behaviors in the structural materials for the future nuclear systems, the effect of H on He thermal desorption and retention behaviors in SIMP steels was studied using thermal desorption spectroscopy(TDS), scanning electron microscope(SEM), and transmission electron microscopy(TEM) by comparing the results of the alone irradiation of 130 keV He+ and the sequential irradiation of 130 keV He+ and 160 keV H2+. The TDS results show that a major peak of He release occurred in the temperature of 1 198~1 222 K via the mechanism of bubble migration. The peak temperature of He release was lower and the release amount was larger under the sequential irradiation of He+ and H2+ than that under the He+-alone irradiation. This indicates that He desorption was enhanced by the additional H irradiation. In addition, the enhancement effect of H on He thermal desorption depends on the dose of H irradiation. When the peak concentration of H increased from 5% to 50% (atom fraction), this enhancement effect was weakened. Combined with TEM and SEM results, it was found that the presence of H promoted the surface blistering during TDS heating, thus accelerating the release of He by the mechanism of bubble migration.
2020, 37(2): 217-224. doi: 10.11804/NuclPhysRev.37.2019050
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In carbon ion radiotherapy, secondary particles produced through the interaction of carbon ion beam with the treatment head devices and the patient's body can reach many areas in the patient's body. Among the secondary particles, the yield of neutrons and \begin{document}$\gamma$\end{document}-rays is largest. Without affecting the beam delivery function, reducing secondary neutrons and \begin{document}$\gamma$\end{document}-rays produced in carbon ion radiotherapy is very important to decrease the normal tissue complications and secondary cancer risk after radiotherapy. In the present work, the Monte Carlo method was used to calculate the dose and spatial distributions of secondary neutrons and \begin{document}$\gamma$\end{document}-rays deposited in water when multi-leaf collimators (MLC) made from different material leaves were adopted to form a typical 10 cm×10 cm square irradiation field for 400 MeV/u carbon ion beam. The simulation results showed that the secondary neutrons produced were mainly distributed at the incident end in the water phantom when the carbon ion beam passed through the MLC, while the secondary \begin{document}$\gamma$\end{document}-rays were uniformly distributed in the entire water phantom and more dose of the secondary \begin{document}$\gamma$\end{document}-rays appeared in the plateau when the irradiation field with spread-out Bragg peak (SOBP) travelled in the water phantom. The selection of MLC leaves should be determined according to the actual conditions of MLC leaves thickness and secondary particle equivalent dose requirements. The simulation study presented in this paper provides scientific evidence for the material selection of MLC leaves and other components in carbon ion radiotherapy using passive beam delivery system.
2020, 37(2): 225-232. doi: 10.11804/NuclPhysRev.37.2019065
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Ultrafast laser system based on High Harmonic Generation (HHG) technology can not only accurately make atoms and molecules to the target quantum state, but also control the target quantum state by controlling the pulse time accurately. The reaction microscope can accurately measure the differential cross section in 4π solid angle in the quantum few-body collision, which greatly improves the measurement accuracy and the efficiency of experimental study on quantum few-body collision dynamics. The combination of these two advanced technologies will greatly expand the field of quantum few-body dynamics. At present, a single frequency selection of HHG is very important, and the resolution of the reaction microscope is mainly affected by the vacuum degree and the dispersion degree of the cold target. The Institute of Modern Physics (IMP) of Chinese Academy of Sciences(CAS) adopts the design of multistage differential system and titanium vacuum target chamber, which makes the vacuum degree of the spectrometer reach 10–11 mbar. This design greatly reduces the influence of background noise. Moreover, the supersonic cold target system is upgraded to realize the free control of the target thickness, which improves the accuracy of event recorded by the detector. Meanwhile, the XUV ultrafast laser system and the reaction microscope were successfully combined by using the multi-order XUV pulse monochromatic technology of HHG. The XUV pulse energy generated by this experimental platform ranges from 20 to 100 eV, so atomic and molecular dynamics processes with ionization or dissociation energy below 100 eV can be studied.
2020, 37(2): 233-239. doi: 10.11804/NuclPhysRev.37.2020003
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By analyzing the experimental data of \begin{document}${\rm{\gamma }}+^{50,52}$\end{document}Cr, the divergence between the absorption cross section and measurements of neutron and proton emission cross sections was clarified, the correction of neutron emission cross section of 52Cr was given. The EGLO model of photon strength function was selected to describe the absorption cross section. Combined with the quasideuteron contribution, the evaluation of photon absorption cross section was given. MEND-G code has been newly developed for calculating gamma induced medium heavy nuclear reactions. By optimizing the theoretical parameters, the cross sections of n, p, \begin{document}${\rm{\alpha }}$\end{document}, etc. in the range of 200 MeV were given. The results of this work were in good agreement with the main measurement of neutron and proton emission below 30 MeV. All data of \begin{document}${\rm{\gamma }}+^{50,52,53,54}$\end{document}Cr have been output in ENDF/B-6 format, which was convenient for nuclear engineering.
2020, 37(2): 240-248. doi: 10.11804/NuclPhysRev.37.2020010
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This paper presented a study of flowability of the inclined dense granular-flow target for a neutron irradiation facility, which is capable of providing high-energy neutrons suitable to advance fusion reactor material research. The results from simulations and theoretical analysis described the constancy and stability of the flow, which was rarely mentioned before. It was found that 25° was acceptable for a steady and stable accelerating flow and all the results supported the availability of this granular flow for a potential choice of the heat medium.