Precision mass measurements of short-lived nuclides at HIRFL-CSR in Lanzhou

In recent years, extensive short-lived nuclear mass measurements have been carried out at the Heavy- Ion Research Facility (HIRFL) in Lanzhou using Isochronous Mass Spectrometry (IMS). The obtained mass values have been successfully applied to nuclear structure and astrophysics studies. In this contribution, we give a brief introduction to the nuclear mass measurements at HIRFL-CSR facility. Main technical developments are described and recent results are summarized. Furthermore, we envision the future perspective for the next-generation storage ring facility HIAF in Huizhou.     

Mass Measurements on Short-Lived Nuclides with ISOLTRAP

Penning trap mass spectrometry has reached a state that allows its application to very short-lived nuclides available from various sources of radioactive beams. Mass values with outstanding accuracy are achieved even far from stability. This paper illustrates the state of the art by summarizing the status of the ISOLTRAP experiment at ISOLDE/CERN. Furthermore, results of mass measurements on unstable rare earth isotopes will be given. This revised version was published online in July 2006 with corrections to the Cover Date.     

Technical Improvements in the Experiment of Isochronous Mass Measurement of 58Ni Projectile Fragments

 The combination of in-flight fragment separator and the isochronous mass spectrometry(IMS) in storage rings have been proven to be a powerful tool for the precision mass measurements of shortlived exotic nuclei. In IMS, the mass-over-charge ratio is only related to the revolution period of stored ions, and the relative mass resolution can reach up to the order of 10−6. However, the instability of the magnetic field of storage ring deteriorates the resolution of revolution period, making it very difficult to distinguish the ions with very close mass-over-charge ratio via their revolution periods. To improve the resolution of revolution periods, a new method of weighted shift correction (WSC) has been developed to accurately correct the influence of the magnetic field instabilities in the isochronous mass measurements of 58Ni projectile fragments. By using the new method, the influence of unstable magnetic fields can be greatly reduced, and the mass resolution can be improved by a factor up to 1.7. Moreover, for the ions that still cannot be distinguished after correcting the magnetic field instabilities, we developed a new method of pulse height analysis for particle identification. By analyzing the mean pulse amplitude of each ion from the timing detector, the stored ions with close mass-over-charge ratios but different charge states such as 34Ar and 51Co can be identified, and thus the mass of 51Co can be determined. The charge-resolved IMS may be helpful in the future experiments of isochronous mass measurement even for N =Z nuclei.     

Shell Evolution Study for New Magic Number N =32 via Isochronous 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.     

BEPC储存环束流振荡频率测量

在加速器的运行和机器研究过程中,需要经常对束流振荡频率进行测量.BEPC储存环束流振荡频率测量系统包括一台频谱仪,两个条形电极装置,两个电压放大器和一个束流振荡检测器.本文介绍了经过改进的BEPC储存环束流振荡频率测量系统.     

On the masses of A = 54 isospin septet and the isobaric multiplet mass equation

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 \begin{document}$ A=54 $\end{document}, \begin{document}$ T=3 $\end{document} isospin multiplet yields a surprisingly large d coefficient of IMME, i.e., \begin{document}$ d=18.6(27) $\end{document}, being \begin{document}$ 6.9\sigma $\end{document} deviated from zero, and the resultant \begin{document}$ |b/c| $\end{document} ratio significantly deviates from the systematics. This phenomenon is analyzed in this study, and we conclude that the breakdown of the quadratic form of IMME could be likely due to the mis-assignment of the \begin{document}$ T=3 $\end{document} isobaric analog state (IAS) in the \begin{document}$ T_z=1 $\end{document} nucleus ⁵⁴Fe or extremely strong isospin mixing.     

基于HIRFL-CSR测量丰中子重核质量的建议

丰中子重核区有大量原子核质量未知,迫切需要实验测量。我们建议,利用兰州重离子加速器研究装置HIRFL-CSR上的等时性质谱术,高精度测量204Pt等丰中子重核的质量。CSR质量测量实验中,在目标核产额尽可能高的前提下,需要每次注入都有多个离子同时储存到实验环CSRe中,才能针对逐次注入修正磁场晃动的影响,得到高精度测量结果。但在丰中子核区,当目标核产生截面非常低时,每次注入能储存到环中的离子数目太少。为了解决这个问题,我们提出了一种"混合厚度靶"的方法,在不明显改变目标核产额的情况下,显著增加同时储存在CSRe中的离子数,满足实验要求。模拟计算表明,在CSRe测量丰中子重核质量是可行的,并推荐了实验的设置。     

Mass measurements of relativistic projectile fragments in the storage ring ESR

Two experimental methods of measuring masses of exotic nuclei in the storage ring ESR are presented. Bismuth and nickel fragments were produced via projectile fragmentation, separated and investigated with the combination of the fragment separator FRS and the ESR: (i) Direct mass measurements of relativistic projectile fragments were performed using Schottky mass spectrometry (SMS), i.e., exotic nuclei were stored and cooled in the ESR. Applying electron cooling, the relative velocity spread of circulating low intensity beams can be reduced below 10⁻⁶. Under this condition a mass resolving power of up to m/Δm=6.5·10⁵ (FWHM) was achieved in a recent measurement. Previously unknown masses of more than 100 neutron-deficient isotopes have been measured in the range of 60≤Z≤84. Using known Q _α values the area of known masses could be extended to more exotic nuclei and to higher proton numbers. The results are compared with mass models and extrapolations of experimental values. In a second experiment with ²⁰⁹Bi projectiles the area of the measured masses was extended to lower proton numbers. Due to various improvements at the ESR the precision of the measurements could be raised. (ii) Exotic nuclei with half-lives shorter than the time needed for SMS (present limit: T _1/2 ≈ 5 sec) can be investigated by time-of-flight measurements whereby the ESR is operated in the isochronous mode. This novel experimental technique has been successfully applied in first measurements with nickel and neon fragments where a mass resolving power of m/Δm=1.5·10⁵ (FWHM) was achieved.     

Recent Results on Ne and Mg from the MISTRAL Mass Measurement Program at ISOLDE

The MISTRAL experiment (Mass measurements at ISOLDE with a Transmission and Radiofrequency spectrometer on-Line), conceived for very short-lived nuclides, has reached the end of its commissioning phase. Installed in 1997, results have been obtained consistent with all aspects of the projected spectrometer performance: nuclides with half-lives as short as 30 ms have been measured and accuracies of 0.4 ppm have been achieved, despite the presence of a systematic shift and difficulties with isobaric contamination. Masses of several nuclides, including ^25–26Ne and ³²Mg that forms the famous island of inversion around N=20, have been significantly improved. This revised version was published online in July 2006 with corrections to the Cover Date.     

Consideration of Femtosecond Radiation Generation on NewSUBARU Storage Ring

We studied on realization of short pulse gamma ray and X-ray simultaneously induced by a femtosecond laser on NewSUBARU storage ring. Based on the fact that the transverse dimensions of electron beam are much shorter than the longitudinal one, the laser light is arranged to collide the electron beam at a right angle to generate femtosecond pulse gamma ray, furthermore, the modulated part of the electron bunch gives rise to short pulse X-ray by synchrotron radiation from a downstream bending magnet. The temporal characteristic of the radiation is analyzed in this paper, as well as the performances are estimated.     

储存环HIAF-SRing等时性模式的非线性磁场影响研究

高精度环形谱仪SRing（Spectrometer Ring）是强流重离子加速器装置（HIAF）的重要组成部分,其等时性模式为远离β稳定线的短寿命原子核质量和寿命的精确测量提供国际领先的科研条件。为了扩大短寿命原子核质量测量精度和范围,SRing等时性模式设计了两种光学:γ_t=1.43和1.67。质量分辨是衡量等时性储存环的最重要参数。二极磁铁的高阶场以及磁铁的边缘场能强烈地引起束流光学高阶畸变,对质量分辨产生影响,因此需要高极磁铁对其进行校正。介绍了SRing等时性模式的线性计算,对非线性磁场的影响进行了详细研究。应用六极磁铁和八极磁铁对非线性场和发射度的影响进行了校正后,离子的循环时间标准偏差σ（T）/T达到3.5×10-7,质量分辨△m/m达到1×106。The isochronous mode of the Spectrometer Ring at the High Intensity heavy-ion Accelerator Facility (HIAF) project in China offers the capacity of measuring the mass and half-life of short-lived nuclides. The transition energy settings of the SRing are 1.43 and 1.67, which have been calculated in the same injection scheme. The resolution of mass or revolution time is the most important parameter of the isochronous mode design of a storage ring. The nonlinear magnetic field errors, including high-order magnet field of dipole, fringe field of magnets, have strongly effect to the resolution of revolution time. High-order corrections are required to improve the resolution of revolution time and mass. In this paper, the SRing linear isochronous optical were shown. The influence of nonlinear magnetic field errors on the revolution time resolving power were investigated. With 3 sextupole families and 1 octupole family corrections, the relative variation of revolution time reaches 4.6×10-7 with the momentum spread of ±0.2%. The relation of the relative standard revolution time deviation σ(T)/T and revolution turns was researched. With corrections of high-order isochronous condition and emittance influence by 3 sextupole families and 1 octupole family, one can reach a resolution of up to σ(T)/T=3.5×10-7, which corresponds to the mass resolution of △m/m=1×106.     

电子储存环中离子俘获不稳定性的强-强模型模拟

 电子储存环中,由于被束流势阱俘获的离子会引起束流不稳定性。研究这种不稳定性的产生机制和抑制方法对提高机器的性能有重要理论和现实意义。介绍了用强-强模型对合肥光源（HLS）电子储存环中离子俘获不稳定性产生机制进行的模拟研究。模拟结果可用于理解在合肥光源（HLS）储存环上观察到的离子俘获现象。     

响应矩阵分析方法在BEPC储存环上的应用

由于储存环中各种元件误差的存在, 机器的实际运行模式与设计模式有一定的偏差. 目前广泛开展的响应矩阵方法研究, 可以分析出磁铁元件以及束流位置测量元件的误差, 使束流基本参数得到校正. 介绍了用响应矩阵分析方法, 在BEPC储存环上进行的局部轨道校正的实验研究, 以及BEPC储存环束流参数校正的模拟研究.     

Mass Measurements of 114–124,130Xe with the ISOLTRAP Penning Trap Spectrometer

The masses of the xenon isotopes with 114≤A≤123 were directly measured for the first time. The experiments were carried out at the ISOLTRAP triple trap spectrometer at the on-line mass separator ISOLDE/CERN. A mass resolving power of the Penning trap spectrometer of m/Δm≈500 000 was chosen and an accuracy of δm≈12keV for all investigated Xe isotopes was achieved. An atomic mass evaluation was performed and the results of this adjustment are compared with theoretical predictions. The new results for the xenon isotopes and their effects on neighboring nuclides are discussed within the two-neutron separation energy picture. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of Charged Particles in a Storage Ring to Gravitational Waves

The influence of gravitational waves on the charged particles in a storage ring is studied. It shows thatthe gravitational waves might be directly detected by monitoring the motion of charged particles in a storage ring. Theangular velocity of the charged particles is continually adjustable by changing the initial energy of particles and thestrength of the magnetic field. This feature is very useful for finding the gravitational waves with different frequencies.     

Response of Charged Particles in a Storage Ring to Gravitational Waves

The influence of gravitational waves on the charged particles in a storage ring is studied. It shows that the gravitational waves might be directly detected by monitoring the motion of charged particles in a storage ring. The angular velocity of the charged particles is continually adjustable by changing the initial energy of particles and the strength of the magnetic field. This feature is very useful for finding the gravitational waves with different frequencies.     

Effect of Wigner energy on the symmetry energy coefficient in nuclei

The nuclear symmetry energy coefficient(including the coefficient a_(sym)~((4)) of the I~4 term) of finite nuclei is extracted by using the differences of available experimental binding energies of isobaric nuclei.It is found that the extracted symmetry energy coefficient a_(sym)~*(A,I) decreases with increasing isospin asymmetry I,which is mainly caused by Wigner correction,since e_(sym)~* is the summation of the traditional symmetry energy e_(sym) and the Wigner energy ew.We obtain the optimal values J = 30.25±0.10 MeV,a_(ss)=56.18±1.25 MeV,a_(sym)~((4)) = 8.33±1.21 MeV and the Wigner parameter x= 2.38 ±0.12 through a polynomial fit to 2240 measured binding energies for nuclei with20 ≤ A ≤ 261 with an rms deviation of 23.42 keV.We also find that the volume symmetry coefficient J■ 30 MeV is insensitive to the value x,whereas the surface symmetry coefficient a_(ss) and the coefficient a_(sym)~((4)) are very sensitive to the value of x in the range 1≤x≤4.The contribution of the a_(sym)~((4)) term increases rapidly with increasing isospin asymmetry I.For very neutron-rich nuclei,the contribution of the a_(sym)~((4)) term will play an important role.     

电子储存环中闭合轨道和色散函数的校正

电子储存环中,由磁铁元件制造误差和安装误差导致的闭合轨道和色散函数的畸变,通常会引起一系列的负面效应,介绍了以合肥光源储存环为模型,用响应矩阵的奇值分解法对闭合轨道和色散函数进行校正的可行性研究结果.     

Precision Measurement of the Charged Pion Mass by High Resolution X-Ray Spectroscopy

A new experiment for a high-precision measurement of the pion mass at a 1 ppm level is presented. It combines an improved cyclotron trap that produces pionic and muonic atoms in a small volume with a doubly focusing crystal spectrometer to measure the corresponding exotic X-ray transitions with high accuracy and a novel type of CCD detector. The muonic X-rays lines serve as highly accurate calibration lines. The measurement has been accomplished recently. A detailed analysis of the data is on the way. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mass measurements in the endpoint region of the rp-process at SHIPTRAP

The Penning-trap mass spectrometer SHIPTRAP was designed for precision mass measurements of radionuclides produced in fusion–evaporation reactions. The latest measurement campaign covered heavy nuclei (A>90) related to the astrophysical rapid proton capture process. The masses of 34 neutron-deficient radionuclides have been measured since February 2006 with relative uncertainties between 5×10⁻⁸ and 1×10⁻⁷. Furthermore, the use of an octupolar RF excitation for the time-of-flight ion-cyclotron-resonance technique was investigated and an increase of the resolving power by a factor of ten was observed in agreement with simulations. This will allow to resolve isomeric states with excitation energies of a few 10 keV only.