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
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.
To study the time evolution of a molecular state in an ultra-fast chemical reaction,the use of shorter pulses with higher photon energy and narrower bandwidth for both pump and probe is necessary.However,quick and precise measurement of their detailed time structures is a challenge.Over the last decade,great efforts have been made to measure an attosecond extreme ultraviolet (XUV) pulse.To date,several methods have been developed to measure the pulse duration and completely reconstruct it.The attosecond spectral phase interferometry for direct electric field reconstruction (SPIDER) and attosecond frequency-resolved optical gating (FROG) techniques are often used.However,these methods use state-of-the-art experimental set-ups and complicated data analysis procedures.To develop attosecond metrology for practical use (e.g.timing,measurement,evaluation,calibration,optimization,pumping,probing),we propose a quick and analytical method to precisely observe an attosecond XUV pulse with laser-assisted photo-ionization.The method is based on determining the laser-related phase of each streaked electron and using a transfer equation for one-step pulse reconstruction without any time-resolved measurements,iterative calculations,or data fitting procedures.Temporal errors of the pulse reconstruction are calculated from the XUV bandwidth.Because the transfer equation establishes a direct connection between the XUV pulse properties,the crucial laser parameters (peak intensity,phase,carrier envelope phase),the atomic ionization potential,and the measured photoelectron energy spectrum,we can use it to study any one of these properties from other known information and probe the dynamic processes of an ultra-fast reaction.
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.
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.
A knockout reaction experiment was carried out by using the 6He beam at 82.5 MeV/nucleon impinging on CH2 and C targets. The a core fragments at forward angles were detected in coincidence with the recoiled protons at larger angles. From this exclusive measure- ment the valence nucleon knockout mechanism and the core knockout mechanism are separated. This study provides a basis for the exclusive spectroscopic investigation of the exotic nuclei.
An overall irradiation and calibration technique was introduced and applied to a test scintillation detector array. An integral conversion method was used to reduce the nonlinearity of the time difference spectrum, and to improve the position determination especially for positions close to the two ends of a long scintillation bar. An overall position resolution of about 3.0 cm （FWHM） was extracted from the residual analysis method and verified by a direct measurement. Energy calibration was also realized by selecting cosmic rays at different incident angles. The bulk light attenuation lengths for the four test bars were also determined. It is demonstrated that these methods are especially efficient for calibrating large and complex detector arrays
