The Halo and cluster structure at the ground state of unstable nuclei are among the most exciting phenomena of current nuclear physics. Probing these structures requires a careful selection of reaction tools. In the past twenty years, knockout reactions have been used intensively to investigate spectroscopically the structure of unstable nuclei. In this report we have illustrated the latest development of the knockout reaction tool and have emphasized the recoiled proton tagging method. A quantitative criteria is developed to evaluate the quasi-free feature of the knockout process. The newly discovered ＂towing mode＂ reaction tool is also outlined and its applicability at transit energies is discussed.
Four high-purity germanium 4-fold segmented Clover detectors have been applied in the experiment of neutron-rich nucleus 21N. The performance of those four Clovers have been tested with radioactive sources and in-beam experiments, and the main results including energy resolution, peak-to-total ratios, the variation of the hit pattern distribution in different crystals of one Clover detector with the energy of γ ray, and absolute full energy peak detection effciency curve, were presented.
The β-decay studies of neutron-rich 18,21N isotopes have been performed using β-n, β-γ, and β-n-γ coincidence methods. The 18,21N ions were produced by the fragmentation of the 22Ne and 26Mg beams, respectively, on a thick beryllium target. The time of flight of the emitted neutrons following the β-decay of 18,21N was measured by a neutron detector system with wide energy detection range and low-energy detection threshold. In addition, several clover germanium detectors were used to detect the β-delayed γ-rays. The half-lives of the β-decays of 18N and 21N were determined to be (619±2) ms and (82.9±7.5) ms, respec tively. Several new β-delayed neutron groups were observed with a total branching ratio of (6.98±1.46)% and (90.5±4.2)% for 18N and 21N, respectively. The level schemes of 18O and 21O were deduced. The experimental Gamow-Teller β-decay strengths of 18N and 21N to these levels were compared with the shell model calculations.
HUA Hui, LI ZhiHuan, YE YanLin, JIANG DongXing, LOU JianLing, LI XiangQing & XU FuRong School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
Knockout reaction experiment was carried out by using the 6He beams at 61.2 MeV/u impinging on a CH2 target. The α core fragments at forward angles were detected in coincidence with the recoiled protons at larger angles. From this exclusive meas urement the valence nucleon knockout mechanism and the core knockout mechanism can be distinguished by the relation be tween the polar angles of the core fragments and the recoiled protons, respectively. It is demonstrated that the core knockout mechanism may result in some strong contamination to the real invariant mass spectrum.
Lü LinHui1, YE YanLin1, JIANG DongXing1, HUA Hui1, ZHENG Tao1, LI ZhiHuan1, GE YuCheng1, LI XiangQing1, LOU JianLing1, CAO ZhongXin1, SONG YuShou1, XIAO Jun1, LI QiTe1, QIAO Rui1, YOU HaiBo1, CHEN RuiJiu1, XU HuShan2, WANG JianSong2, GUO ZhongYan2, ZHANG XueYing2, LI Chen2, HU ZhengGuo2, CHEN RuoFu2, WANG Meng2, XU ZhiGuo2, YUE Ke2, TANG Bin2, ZANG YongDong2, ZHANG XueHeng2, YAO XiangWu2, CHEN JinDa2 & BAI Zhen2 1 School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
The latest progress made in the field of radioactive ion beam physics is outlined and the key problems still under investigation are indicated. The focal points are the limit of nuclear existence, shell evolution and new magic numbers, halo and cluster structures, new excitation modes, and strong coupling between reaction channels. This field is still at a starting phase and much more new outcomes are foreseen.
Three large sized glass resistive plate chambers （RPCs） are built and applied to measure the spatial resolution of the detector. The readout strips are collected to a LC delay-line and the time difference is used to determine the position. Cosmic rays are triggered by a set of two scintillation counters and the coincidently measured positions from the three RPCs are used to deduce the position uncertainty. In average a spatial resolution of 0.90 mm （FWHM） is obtained for a single RPC, with a good uniformity across the detection area. This result suggests that large sized glass RPC operating in the avalanche mode is a promising candidate for the muon tomography detection system.
We have tested and analyzed the properties of two-dimensional Position-Sensitive-silicon-Detector （PSD） with new integrated preamplifiers. The test demonstrates that the best position resolution for 5.5 MeV α particles is 1.7 mm （FWHM）, and the best energy resolution is 2.1%, which are notably better than the previously reported results. A scaling formula is introduced to make the absolute position calibration.
The performance test of a CsI(Tl) crystal (70×27×23 mm3) was performed by applying the pulse shape discrimination technique for identification of light charged particles .The crystal is coupled to a photomultiplier tube during an experiment with 6He beam.The pulse waveform is fully recorded by employing a high precision digital oscilloscope.The fast and slow gates are used for the pulse shape discrimination and the best values for the gate widths were determined to be 0.5 μs and 1.67 μs,respectively.The 6He,4He and 3He are successfully discriminated with this technique.
The design principle for a multi-wire proportional chamber with a cathode strip and delay-line readout is described. A prototype chamber of a size of 10 cm×10 cm was made together with the readout electronics circuit. A very clean signal with very low background noise was obtained by applying a transformer between the delay-line and the pre-amplifier in order to match the resistance. Along the anode wire direction a position resolution of less than 0.5 mm was achieved with a ^55Fe-5.9 keV X ray source. The simple structure, large effective area and high position resolution allow the application of a gas chamber of this kind to many purposes.
