The Lanzhou Heavy Ion Accelerator Cooling Storage Ring(HIRFL-CSR)is an ideal device for studying the decay of highly charged and short-lived isomers.In the lifetime measurement experiment of the short-lived highly charged ion 94mRu44+,we directly observed the decay of 94Ru from 8+isomer to the ground state,and identified 49 decay events in the ob-servation time window of(20 μs,180 μs).In order to identify more decay events,a new method based on amplitude of fre-quency spectrum was studied in this paper.Based on the simulation results,this new method can effectively identify decay events within(15 μs,185 μs).By applying the new identification method to the experimental data processing,54 decay events were identified within(15 μs,185 μs).Based on these 54 decay events,the lifetime of 94mRu44+in the laboratory frame was calculated to be 194(121)μs.The new lifetime result is within the error range of the previous result 218(148)μs.

基于重离子储存环建立的等时性质谱术（IMS）是测量远离稳定线核素质量的有效工具。但是，采用常规IMS测量缺中子一侧的核素质量时，发现T z =-1/2和T z =-1核素的质量测量结果在宽时域范围内存在系统性偏差。本工作利用CSRe直线段上的双飞行时间（TOF）探测器，同时测量了循环离子的周期和速度。利用这些实验信息，对常规IMS质量测量中出现的系统性偏差进行了研究。发现系统偏差是由于储存的离子动量分布不对称以及储存环能量转变参数γ t 非恒定造成的。在离线数据处理时，发现通过限制动量接收度的大小，可以消除常规IMS质量测量中的系统偏差。这一结果对采用常规IMS进行质量测量具有重要参考价值和指导意义。

Using the ground-state mass of 52Ni and two-proton decay energy of 54Zn,the ground-state mass excess of 54Zn is deduced to be-6504(85)keV.This value is about 2 MeV lower than the prediction of the quadratic form of the isobaric multiplet mass equation(IMME).A cubic fit to the existing mass data of the A = 54,T = 3 isospin multiplet yields a surprisingly large d coefficient of IMME,i.e.,d = 18.6(27),being 6.9σ deviated from zero,and the resultant |b/c| ratio significantly deviates from the systematics.This phenomenon is analyzed in this study,and we conclude that the breakdown of the quadratic form of IMME could be likely due to the mis-assignment of the T = 3 isobaric analog state(IAS)in the Tz = 1 nucleus 54Fe or extremely strong isospin mixing.

Isochronous mass spectrometry (IMS) of heavyion storage rings is a powerful tool for the mass measurements of short-lived nuclei. In IMS experiments, masses are determined through precision measurements of the revolution times of the ions stored in the ring. However, the revolution times cannot be resolved for particles with nearly the same mass-to-charge (m/q) ratios. To overcome this limitation and to extract the accurate revolution times for such pairs of ion species with very close m/q ratios, in our early work on particle identification, we analyzed the amplitudes of the timing signals from the detector based on the emission of secondary electrons. Here, the previous data analysis method is further improved by considering the signal amplitudes, detection efficiencies, and number of stored ions in the ring. A sensitive Z-dependent parameter is introduced in the data analysis, leading to a better resolution of ^34Ar^18+ and ^51Co^27+ with A/Z=17/9. The mean revolution times of ^34Ar^18+ and ^51Co^27+ are deduced, although their time difference is merely 1.8 ps. The uncorrected, overlapped peak of these ions has a full width at half maximum of 7.7 ps. The mass excess of ^51Co was determined to be -27332(41) keV, which is in agreement with the previous value of -27342(48) keV.

We are developing a fast counter for energy loss ΔE using a thin inorganic scintillator to realize online selection of atomic number Z of a radioactive isotope (RI) beam in the in-flight fragment separator.We measured the time and charge resolution of a thin yttrium aluminum perovskite (cerium) (YAP(Ce)) scintillation counter by using heavy ion beams.The resultant time resolution is ~25 ps for a 200-MeV/nucleon 84 Kr beam; however, the charge resolution needs further improvement.

We developed a simple position readout method for a plastic scintillation counter used in heavy ion beam diagnostics.We created a prototype detector consisting of 20 plastic scintillator bars coupled with 4 multi-pixel photon counters (MPPCs) through common light guides, and tested it by using a 84 Kr beam of 200 MeV/nucleon.The beam position distribution was obtained successfully.In the x-direction perpendicular to the bar, we identified the scintillator bar where each ion passed through.In the y-direction parallel to the bar, the position resolution was σ = 2 mm.A detection efficiency close to 100% was achieved.

We developed a time-of-flight (TOF) detector for mass measurements of rare radioactive isotopes (RIs) with a storage ring, called the Rare-RI Ring, in RIKEN. For successful mass measurements, a time resolution of less than 100 ps and a detection efficiency close to 100% are required. Additionally, the change of ion velocity in the detector should be as small as possible (< 10−4). To satisfy these requirements, the TOF detector utilizes ion-induced secondary electrons emitted from a thin foil and the crossed static electric and magnetic fields to transport the electrons isochronously to the microchannel plate detectors. The TOF detector was tested in both offline test with an alpha source and online test with heavy ions. In the online test with 84Kr ions of 200 MeV/nucleon, a time resolution of 38.6(2) ps in sigma and a position-averaged detection efficiency of 95.2(2)% were achieved in the entire area of 45-mm-diameter aluminum-coated Mylar foil. This good performance is attributed to the electromagnetic field achieved, which is the strongest thus far for a detector with this design.

根据几种常用放射性核素的寿命计算方法,通过模拟数据研究了直接拟合法、对数时间法、极大似然法、观测时间受限时的极大似然法等四种寿命计算方法的适用范围。当观测时间不受限时,研究了在不同计数下寿命计算方法的适用范围。当观测时间受限时,研究了在不同观测时间窗口下寿命计算方法的适用范围。模拟中选用全剥离离子 94m Ru 44+ 作为目标核素,得到了不同计数及不同观测时间窗口下的寿命及其误差,并给出了四种方法的适用范围。 94m Ru 44+ 寿命的模拟结果与在兰州等时性质量谱仪上获得的实验结果在一倍标准偏差范围内一致,从而进一步验证了寿命计算方法的适用范围及模拟数据的可靠性。该模拟结果可为寿命测量实验设计提供理论依据和参考。

To achieve high-resolution selection and separation of mono-isotopic beams through projectile fragmentation or in-flight fission and to efficiently transport the secondary beams to the Rare-RI Ring (R3) for mass determination, the BigRIPS separator combined with the High-resolution Achromatic (HA) beam-line of SHARAQ is served as a two-stage separator. The first stage from F0 to F2 is used for separation of the nuclei of interest through a Bρ-ΔE-Bρ method and the second stage from F3 to S0 is employed for identification of the beam with a Bρ-ΔE-TOF method. With this scheme, all the secondary ions can be well separated on an event by event basis and be utilized for mass measurements by two complementary methods: the conventional magnetic-rigidity-time-of-flight (Bρ-TOF) method employing the BigRIPS-HA beam-line to measure exotic nuclides within 1 μ s and the newly established isochronous mass spectrometry (IMS) approach via the R3 with a measurement duration less than 1 ms. This report presents the technique advances of the BigRIPS-HA-SHARAQ-R3 as a separator/spectrometer and give a brief description of its application to the complementary TOF methods for mass measurements at RIBF.

Isochronous mass spectrometry has been applied to 112Sn projectile fragments at the HIRFL-CSR facility in Lanzhou. To produce short-lived nuclei of interest, we used projectile fragmentation of 112Sn35+ primary beams in a ～10 mm thick 9Be production target. The fragments were selected and analyzed by RIBLL2 and injected into the experimental storage ring(CSRe) every 25 s. To measure revolution times of stored ions,we used a Time-Of-Flight detector installed in CSRe. A new particle identification method was developed to distinguish ions on the measured revolution time spectrum for each injection. Based on this method, the shifts of the revolution time due to instable dipole magnet fields can be corrected and the ground and isomeric states of 101In have been well-resolved. The measured excitation energy is consistent with the theoretical value in the error range of 112 keV. The lifetime of the isomeric states of 101In is more than 200 μs.

The decay of the fully stripped ion 94mRu44+ in the order of one hundred microseconds has been studied for the first time by using the Isochronous Mass Spectrometry (IMS) at the HIRFL-CSR facility in Lanzhou.94mRu44+ waS produced via projectile fragmentation of a 112Sn primary beam bombarding on a 9Be production target.After the in-flight separation with RIBLL2,the ions were injected into the experimental ring (CSRe) and then stored there.The revolution times of the stored ions were measured by a Time-of-Flight (TOF) detector.Due to the mass change of a 94mRu44+ ion caused by its de-excitation to the ground state,hence the revolution time change,the decay process of 94mRu44+ could be directly observed in the CSRe.The sensitive window for detection of the decay events and the measurement precision of the decay time have been determined in this work.At the same time,we measured the mass of short-lived 94mRu44+ with the half-life about one hundred microseconds,which is the shortest among nuclides that have been studied by using storage-ring mass-spectrometry.

Recent results and progress of mass measurements of neutron-rich nuclei utilizing Isochronous Mass Spectrometry (IMS) based on the HIRFL-CSR complex at Lanzhou are reported. The nuclei of interest were produced through projectile fragmentation of primary 86Kr ions at a realistic energy of 460.65 MeV/u. After in-flight separation by the fragment separator RIBLL2, the fragments were injected and stored in the experimental storage ring CSRe, and their masses were determined from measurements of their revolution times. The re-determined masses were compared and evaluated with other mass measurements, and the impact of these evaluated masses on the shell evolution study is discussed.