236U及其加速器质谱（AMS）测量

236U是一种长寿命放射性核素, 其半衰期为T_1/2=2.342(3)×107 a, 在自然界的含量极微, 天然铀中 236U/238U的原子比约为～10-14。 目前, 对于236U及其测量的研究报道较少。首先介绍了236U的产生, 并对地球中 236U的总量进行了估算。 概述了236U测定在亿年级中子注量率积分评估、 核环境和核活动监测, 以及地质学等领域中的重要应用。 最后,对国内外加速器质谱（AMS）测量236U的状况进行了综述,报道了中国原子能科学研究院AMS小组通过对同位素干扰鉴别的多方面研究, 初步建立了236UAMS测量方法。 236U is a longlived radionuclide with half life of 2.342(3)×107 a. The ratio of 236U/238U is about 10-14 in the natural Uranium. The origin and production of 236U in globe are introduced and estimated in this paper,respectively. The major applicationsion of 236U as a 100 million year neutron flux integrator,as a “fingerprint” for monitoring nuclear environment and nuclear activity,and as a tracer in geological studies are briefly summarized. The accelerator mass spectrometry(AMS) measurement of 236U in the world and the research on HI 13 tandem accelerator at China Institute of Atomic Energy(CIAE) is also mentioned in this paper.     

236U AMS measurement at CIRCE

In order to measure the isotopic ratio of actinides,the upgrade of the accelerator mass spectrometry system at the Center for Isotopic Research on Cultural and Environmental heritage at the Second University of Naples,Italy,was performed.The beam emittance of 238U and the isotopic abundance sensitivity of 236U were measured on the present beam line.Utilizing a 16-strip silicon detector,the sensitivity of 236U/238U≈ 1×10-11 was obtained.     

Isotope shift and hyperfine structure of the highly excited atomic uranium

The laser induced fluorescence method using atomic beam combined with Doppler-free two-photon absorption technique was applied for the measurement of isotope shifts and hyperfine structures of atomic uranium including ²³⁴U, ²³⁵U, ²³⁶U and ²³⁸U isotopes. The isotope shifts between ²³⁸U-²³⁴U, ²³⁸U-²³⁵U, ²³⁸U-²³⁶U, and the hyperfine structure of ²³⁵U were obtained in the high lying odd levels around 4 eV. Received 3 December 2001 / Received in final form 4 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: oba@analchem.tokai.jaeri.go.jp     

Measurement of uranium and its isotopes at trace levels in environmental samples using mass spectrometry

Actinides have widely entered the environment as a result of nuclear accidents and atmospheric weapon testing. These radionuclides, especially uranium, are outstanding radioactive pollutants, due to their high radiotoxicity and long half-lives. In addition to this, since depleted uranium (DU) has been used in the Balkan conflict in 1999, there has been a concern about the possible consequences of its use for the people and environment. Therefore, accurate, precise and simple determination methods are necessary in order to evaluate the human dose and the concentration and effects of these nuclides in the environment. The principal isotopes of uranium e.g. ²³⁵U and ²³⁸U are of primordial origin and ²³⁴U present in radioactive equilibrium with ²³⁸U. ²³⁶U occurs in nature at ultra trace concentrations with a ²³⁶U: ²³⁸U atom ratio of 10⁻¹⁴. Concentrations of uranium in soil samples were determined using inductively coupled plasma mass spectrometry (ICP-MS) and isotope ratios of uranium were measured using a thermal ionisation mass spectrometer. Radioactive dis-equilibrium of ²³⁴/²³⁸U, depletion of ²³⁵/²³⁸U and significant evidence of ²³⁶U/²³⁸U were noticed in soil samples.      

Side-effects of the space charge field introduced by a hollow electron beam in the electron cooler of CSRm

An electron cooler is used to improve the quality of the ion beam in a synchrotron;however it also introduces a nonlinear electromagnetic field to the accelerator,which causes tune shift,tune spread and may drive resonances leading to ion beam loss.In this paper the tune shift and the tune spread caused by the nonlinear electromagnetic field of a hollow electron beam is investigated,and the resonance driving terms of the nonlinear electromagnetic field are analysed.The differences are presented compared with a solid electron beam.Calculations are performed for ~(238)U~(32+) ion of energy 1.272 MeV stored in the main Cooler Storage Ring(CSRm) at the Institute of Modern Physics,Lanzhou.It is found that in this situation the nonlinear field caused by the hollow electron beam does not lead to serious resonances.     

RIKEN 18GHzECR离子源上金属离子的产生

At RIKEN,three ECR ion sources(10GHz ECRIS,18GHz ECRIS and liquid He-free SC-ECRIS) are operated as external ion sources of heavy ion accelerators.In the last year,multi-charged uranium ion beam was produced from 18GHz ECRIS by using UF_6 and the ~(238)U ion was successfully accelerated by the accelerator complex which consists of the RFQ linear accelerator,RIKEN heavy ion linear accelerator(RILAC)and RIKEN ring cyclotron accelerator(RRC).The typical beam intensity of~(238)U~(14 ) was about 2pμA on faraday cup after analysing magnet.~(70)Zn beam was still supplied for the new super-heavy element search experiment with insertion method.Intense beam of~(70)Zn~(16 ) was produced for long term(～43 days)without vacuum break and remarkably low material consumption rate(～100μgr/h).We already supplied Zn beam longer than 200 days for this experiment.~(48)Ca ion was also produced by insertion method using~(48)CaO rod for the nuclear physics experiment.In this contribution,we will present ion source parameter and techniques for production of each of the metal ions.     

Theoretical study on production cross sections of exotic actinide nuclei in multinucleon transfer reactions

Within the dinuclear system(DNS) model, the multinucleon transfer reactions ~(129,136)Xe +~(248)Cm,~(112)Sn+~(238)U, and ~(144)Xe +~(248)Cm are investigated. The production cross sections of primary fragments are calculated with the DNS model. By using a statistical model, we investigate the influence of charged particle evaporation channels on production cross sections of exotic nuclei. It is found that for excited neutron-deficient nuclei the charged particle evaporation competes with neutron emission and plays an important role in the cooling process. The production cross sections of several exotic actinide nuclei are predicted in the reactions ~(112)Sn +~(238)U and ~(136,144)Xe +~(248)Cm.Considering the beam intensities, the collisions of ~(136,144)Xe projectiles with a ~(248)Cm target for producing neutron-rich nuclei with Z =92-96 are investigated.     

强流重离子加速器装置的增强器慢引出系统

中国科学院近代物理研究所承担的强流重离子加速器装置目前已进入了初步设计阶段。增强器作为该装置的主加速器,可利用双向涂抹技术将238U35+束的粒子数累积至1.0×1011,并将其从注入能量为17 MeV/u加速至高能量,引出能量的范围为200-835 MeV/u。为了提供s量级的准连续束以开展辐照实验,增强器中设计了慢引出系统,该系统将采用三分之一共振与RF-knockout的引出方法。同步加速器中有两种不同种类的六极磁铁,用于实现色品校正与共振驱动,并在设计中考虑了两者能同时运行并互不影响。针对增强器中不同引出能量的238U35+束,对其相应的稳定接受度模拟结果进行了比较,并给出了在引出静电偏转板处的光学匹配参数,这将为增强器中重离子束的慢引出及放射性次级束流分离器的入口光学设计提供重要的理论依据。     

~(36)Cl的加速器质谱测量及其应用

长寿命核素36Cl的半衰期为3.01×105a,广泛分布于自然界中。加速器质谱是目前测量36Cl的最灵敏的方法。介绍了36Cl的样品制备及加速器质谱测量方法,重点描述了基于小型化加速器质谱发展起来的测量36Cl的探测方法及其特点,综述了各实验室利用36Cl在相关领域开展的工作及所取得的成果。     

The SPES direct UCx target

A possible solution for a target system aimed at the production of exotic nuclei as a result of high energy fissions in ²³⁸U compounds has been analyzed. The proposed configuration is constituted by a primary proton beam (40 MeV, 0.2 mA) directly impinging on uranium carbide disks inserted within a cylindrical carbon box. This system has been conceived to obtain both a high number of neutron rich atoms (originated from about 10¹³ fissions/s) and a low power deposition in the target. In order to extract the fission fragments, the box has to be hold at 2000^○C. The thermal analysis shows the capability of the thermal radiation to cool the disks with a reasonable margin below the material melting point. Moreover, the analyses of the thermo-mechanical behaviour and of the effusion times confirm the promising features of this target configuration.     

平均场加区分质子和中子的邻近轨道对力模型及对超铀区核的统一描述

利用严格可解的平均场加区分质子和中子的邻近轨道相互作用对力模型,对超铀区部分大形变核进行了统一的描述.计算了^227—233Th,^232—239U,^236—243Pu同位素和同中异质素²²⁸Ra-²²⁹Ac-²³⁰Th-²³¹Pa-²³²U,²³²Th-²³³Pa-²³⁴U-²³⁵Np-²³⁶Pu,²³⁶U-²³⁷Np-²³⁸Puv-²³⁹Am的结合能,对激发能和奇偶能差并与相应核的实验值进行了比较     

Target mass correction on the 3He polarized structure function

The diffusion-effusion model has been used to analyse the release and yields of Fr and Cs isotopes from uranium carbide targets of very different thicknesses (6.3 and 148 g/cm²) bombarded by a 1 GeV proton beam. Release curves of several isotopes of the same element and production efficiency versus decay half-life are well fitted with the same set of parameters. Comparison of efficiencies for neutron-rich and neutron-deficient Cs isotopes enables separation of the contributions from the primary (p + ²³⁸U) and secondary (n + ²³⁸U) reactions to the production of neutron-rich Cs isotopes. A rather simple calculation of the neutron contribution describes these data fairly well. The FLUKA code describes the primary and secondary-reaction contributions to the Cs isotopes production efficiencies for different targets quite well.     

Transmutation of 129I, 237Np, 238Pu, 239Pu, and 241Am using neutrons produced in target-blanket system ‘Energy plus Transmutation’ by relativistic protons

Target-blanket facility ‘Energy + Transmutation’ was irradiated by proton beam extracted from the Nuclotron Accelerator in Laboratory of High Energies of Joint Institute for Nuclear Research in Dubna, Russia. Neutrons generated by the spallation reactions of 0.7, 1.0, 1.5 and 2 GeV protons and lead target interact with subcritical uranium blanket. In the neutron field outside the blanket, radioactive iodine, neptunium, plutonium and americium samples were irradiated and transmutation reaction yields (residual nuclei production yields) have been determined using γ-spectroscopy. Neutron field's energy distribution has also been studied using a set of threshold detectors. Results of transmutation studies of ¹²⁹I, ²³⁷Np, ²³⁸Pu, ²³⁹Pu and ²⁴¹Am are presented.      

A high resolution AMS-injector for the Pelletron in Lund

A high resolution injector system has recently been installed at the Lund 3 MV tandem Pelletron accelerator. The new injector, designed mainly for ²⁶Al ions, will increase the experimental potential of the Lund AMS facility considerably. High quality energy- and mass-resolution is obtained by using a 90° spherical electrostatic analyzer followed by a 90° magnetic analyzer. The injector is equipped with a high intensity sputtering source with a spherical ionizer. A new analytical technique for acceptance calculations as well as PC-based computational methods have been used in the design of the ion optical system of the new injector. Compared to our old injector system which has a magnetic analyzer with a bending angle of only 15°, the new system has a more than ten times better resolution. The beam optics of the new system is also better designed to match the accelerator acceptance. In this way the ion transmission from the ion source to the detector, for different ions of interest in our AMS programme, has been increased.     

贫铀球壳中D-T中子诱发的铀反应率的测量与分析

为校验次临界能源堆的概念设计,在R19.4/30.0 cm的贫铀球壳装置上采用活化法开展14 MeV中子学积分实验.布放6片贫铀活化片于球壳中与入射D离子束90°方向上的不同位置处活化,用HPGe探测器测量238U(n,γ)反应、238U(n,f)及235U(n,f)反应和238U(n,2n)各反应产物发射的特征γ射线,得到了相应的反应率.238U(n,γ)反应率的不确定度为3.6％-3.7％,238U(n,D和235U(n,f)反应率的不确定度为5.1％-5.9％,238U(n,2n)反应率的不确定为4.3％-4.7％.用MCNP5程序在ENDF66c数据库下进行模拟计算,238U(n,γ)反应率的计算值/实验值(C/E)为0.972-1.034,238U(n,f)和235U(n,f)反应率的C/E为0.983-1.058,238U(n,2n)反应率的C/E为0.979-1.019.     

重离子碰撞复合体系碎化产生超重核可能性的探讨

讨论了通过重核碰撞形成的复合体系破裂产生超重核的问题。评述了两种主要的理论模型:即早期发展的在碎化理论框架下的量子涨落理论和本课题组尝试发展的微观输运理论模型。概括阐述了微观输运理论研究^197Au＋^197Au,^238U＋^238U和^244Pu＋^244Pu等反应的主要结果,即反应中产生的超重碎块的几率与入射能的关系,复合体系和超重碎块的衰变机制以及所形成的超重碎块的结合能和形状的分布。In this paper, the possibility of producing superheavy fragments through composite system breaking up in massive nuclear reactions is investigated. Two main theoretical models, which are the quantum fluctuations with in the fragmentation theory developed at 1980＇s and improved quantum molecular dynamics model developed recently by our group, are briefly reviewed. The dependence of the production probability of superheavy fragments on the incident energy, the decay mechanism of composite system and superheavy fragments, and the distribution of binding energy of Superheavy fragments are discussed for reactions of ^244Pu＋^244Pu, ^238U＋^238U, 197Au＋^197Au based on the improved quantum molecular dynamics model.     

Measurement and analysis of the 238U(n, 2n) reactionrate in depleted uranium/polyethylene shells

In order to check the conceptual design of the subcritical blanket in a fusion-fission hybrid reactor, a depleted uranium/polyethylene simulation device with alternate shells has been established. The measurement of the ²³⁸U(n, 2n) reaction rate was carried out using an activation technique, by measuring the 208 keVγ rays emitted from ²³⁷U. The self-absorption of depleted uranium foils with different thicknesses was experimentally corrected. The distribution of the ²³⁸U(n, 2n) reaction rate at 90° to the incident D⁺ beam was obtained, with uncertainty between 5.3% and 6.0%. The experiment was analyzed using MCNP5 code with the ENDF/BVI library, and the calculated results are all about 5% higher than the measured results.     

贫化铀/聚乙烯球壳交替系统中铀-238中子俘获率的测量与分析

为校验次临界能源堆的概念设计,建立了贫化铀/聚乙烯球壳交替系统, 采用活化法测量²³⁸U的中子俘获率. 贫化铀片置于系统内与入射D离子束成90^o的方向上活化 ,用HPGe探测器测量²³⁸U俘获中子衰变产生的²³⁹Np 衰变产生的277.6 keV特征γ射线计数,实验修正了贫铀片对277.6 keV γ射线的自吸收, 得到了交替系统中²³⁸U (n, γ)反应率的径向分布,反应率的相对不确定度为3.5%-3.7%, 并计算得到系统上整个贫铀区中²³⁸U的总中子俘获率为2.24 ± 0.09. 用MCNP5程序在常用ENDF库下进行了模拟计算, ²³⁸U (n, γ)反应率分布计算与实验一般在5%以内符合, 总俘获率在1%以内符合.     

BRing中电荷交换引起的束流损失分布模拟计算

在强流重离子加速器运行中,带电粒子与真空管道中的残余气体分子相互作用发生的电荷交换反应是影响重离子束流寿命的关键因素。这种电荷交换过程导致的束流损失将解吸出真空管壁上吸附的气体分子,进而引起真空压力的动态变化,将严重影响加速器的稳定运行和最终束流引出流强。中国科学院近代物理研究所将在广东省惠州市建造的强流重离子加速器装置（High Intensity heavy-ion AcceleratorFacility,简称HIAF）利用增强器（Booster Ring,简称BRing）提供束流流强高达2×1011 ppp的238U35+用于核物理及原子物理等实验研究。对强流重离子加速器BRing中238U35+束流发生电荷交换反应,损失一个电子成为238U36+的过程进行了追踪模拟,计算得到了U36+损失前的运动径迹和全环粒子损失位置分布,模拟结果显示U36+受到色散元件的影响,将集中损失在位于二极磁铁后的漂移节区域中。基于模拟结果,在束流损失位置处设计安装由低解吸率材料制作的准直器,优化设计后的准直效率高达95%以上;并模拟计算了有无准直器时真空压力和束流流强的变化,安装准直器后BRing的平均真空度变化小于10%,将确保BRing加速器的稳定运行。During heavy ion accelerator operation, the charge exchange effect between ions and residual gas molecules is the key factor to influence beam lifetime. The charge exchange process has ions lost on the wall and leads to a dynamical vacuum change, which will seriously affect the accelerator operation and reduce the extraction beam intensity. The Institute of Modern Physics' future project, called High Intensity heavy ion Accelerator Facility (HIAF), will be built in Huizhou city, Guangdong Province, China. The Booster Ring (BRing) will provide 2×11 ppp 238U35+ for nuclear physics experiments. This article studies the track of particle U36+ before impacting on the wall, which is the reference particle U35+ losing one electron, and gets the U36+ loss distribution along the BRing. The simulation result shows that U36+ will be influenced seriously by dispersion elements, and will be lost in the drift sections after the dipoles. Collimators made out of materials with low desorption will be installed in the particles lost positions. The collimator efficiency after optimization can be larger than 95%. It also shows BRing average pressure change and beam intensity change between collimators on and off. The result points out that the BRing average pressure change will be less than 10% with collimators on, which makes BRing operate stably.     

Effect of differences in proton and neutron density distributions on fission barriers

The neutron and proton density distributions obtained in constrained Hartree-Fock-Bogolyubov calculations with the Gogny force along the fission paths of 232Th, 236U, 238U, and 240Pu are analyzed. Significant differences in the multipole deformations of neutron and proton densities are found. The effect on potential energy surfaces and on barrier heights is studied under an additional constraint by imposing similar spatial distributions to neutrons and protons, as assumed in macroscopic-microscopic models.