Production and Identification of New Isotope ^265Bh（Z=107）

New Isotope ^265Bh was produced in bombardment of a ^243Am target with 168 MeV 26Mg ions. The experiment was carried out at the Sector Focus Cyclotron of Heavy Ion Research Facility in Lanzhou in 2003. The reaction products were transported and collected by using helium-jet technique and a set of rotating wheel system. Identification was made by observation of correlated a-particles between the new isotope ^265Bh and its ^261Db and ^257Lr daughter nuclei. A parent-daughter searching mode was used to facilitate detection of α-α correlations. A total of four pairs of PIPS detector (200mm^2 active area) were used to measure the kinetic energy of a particles. The a particle energy resolution was about 40 keV for the top detectors and 100 keV for the bottom detectors because of energy degradation in the polypropylene foil.     

新同位素265Bh(Z=107)的合成证据

利用兰州重离子加速器提供的26Mg离子束轰击243Am靶, 产生了新同位素265Bh.实验中用氦喷技术对产物进行传输, 并用一套具有数对探测器组的转轮收集探测系统对产物进行收集和测量.通过观测265Bh与它的衰变子核261Db及257Lr之间的α衰变的关联, 实现了对新核素的鉴别.实验测得265Bh的α衰变能量为(9.24±0.05) MeV, 半衰期为0.94+0.70 -0.31 s.     

Study of shape isomer yields of ~（240）Am in the framework of a dynamical-statistical model

A dynamical statistical model is used to analyze the experimental shape isomer yields data in the reaction d＋240Pu at E=20–29 MeV.The possibility of determining the nuclear dissipation is discussed.Comparison of the experimental data with the calculations leads to a value of the reduced dissipation coeffcient β=0.45×1021 s-1 for the Am isotopes.     

Alpha decay half-lives of heavy nuclei within a generalized liquid drop model

Theoretical α-decay half-lives of the heaviest nuclei are calculated using the experimental Qα value. The barriers in the quasi-molecular shape path is determined within a Generalized Liquid Drop Model (GLDM) and the WKB approximation is used. The results are compared with calculations using the DensityDependent M3Y (DDM3Y) effective interaction and the Viola-Seaborg-Sobiczewski (VSS) formulae. The calculations provide consistent estimates for the half-lives of the α decay chains of these superheavy elements. The experimental data stand between the GLDM calculations and VSS ones in the most time.     

利用α关联衰变链鉴别新同位素_(107)~(265)Bh

主要介绍了合成和鉴别107号元素的新同位素265Bh的实验装置、实验方法以及实验结果.目标核265Bh是由能量为135MeV的26Mg离子轰击243Am靶,通过融合蒸发反应而产生.反应产物首先由He jet系统传输到装有数个探测器对的转轮收集测量系统,然后依靠母子核遗传关系通过观察新同位素和它们已知子核261Db和257Lr之间的α衰变的关联,来实现对新核素的鉴别.实验测得265Bh的α衰变能量为(9.24±0.05)MeV, 半衰期为0.94+0.70-0.31s.从该实验得出的265Bh的α衰变能量和半寿命能够与理论预言一致.     

α particle preformation and shell effect for heavy and superheavy nuclei

The α particle preformation factor is extracted within a generalized liquid drop model for Z = 84-92 isotopes and N = 126,128,152,162,176,184 isotones.The calculated results show clearly that the shell effects play a key role in α particle preformation.The closer the proton and neutron numbers are to the magic numbers,the more difficult the formation of the α cluster inside the mother nucleus is.The preformation factors of the isotopes reflect that N = 126 is a magic number for Po,Rn,Ra,and Th isotopes,but for U isotopes the weakening of the influence of the N =126 shell closure is evident.The trend of the factors for N = 126 and N = 128 isotones also support this conclusion.We extend the calculations for N = 152,162,176,184 isotones to explore the magic numbers for heavy and superheavy nuclei,which are probably present near Z = 108 to N = 152,162 isotones and Z = 116 to N = 176,184 isotones.The results also show that another subshell closure may exist after Z = 124 in the superheavy nuclei.This is useful for future experiments.     

Soft Giant Dipole Modes in Ca Isotopes in the Relativistic RPA

The isovector giant dipole resonance in Ca isotopes is investigated in the framework of the fully consistent relativistic random phase approximation.The calculations are performed in an effective Lagrangian with a parameter set NL3,which was proposed for satisfactorily describing nuclear ground state properties.It is found that a soft isovector dipole mode for Ca isotopes near drip lines exists at energy around 6-7MeV.The soft dipole states are mainly due to the excitation of the weakly bound and pure neutron (proton)states near Fermi surface as well as the correlation of isoscalar and isovector operators.For nuclei with the extreme value of N/Z,the contributions of isoscalar mesons in the isovector mode play a non-negligible role.     

关于合成265Bh实验的可行性分析

分析了目前关于超重核研究的现状,结合现有的设备及条件,在合成259Db以后,下一次实验的目标核初步确定为107号元素的新同位素265Bh.描述了对MG转轮收集探测系统的实验检验结果.实验中成功地观察和测量了24Mg+232Th的产物252No的母子体的α衰变谱,为下一步合成265Bh完成了部分预实验工作.另外,也给出了下一个目标核的衰变特性的理论预言值,同时也选择了产生该目标核的弹靶组合及反应道,并对生成截面进行了粗略估计,给出了该核的可观测性产额及可行性分析. The present status of investigation on the surperheavy nuclei are analyzed. After 259Db synthesized, the next objective nuclide is determined to be265Bh, a new isotope of Z=107 element, according to our existing available equipment and conditions.The test result of the MG rotating wheel collection and detection system is described. The α decay spectra of mother daughter of the product252No from24Mg +232Th reaction are observed and measured successfully in this test experiment. It is regarde...     

Signatures of shell evolution in alpha decay across the N=126 shell closure

Within the alpha-cluster model, we particularly investigate the alpha decay of exotic nuclei in the vicinity of the N = 126 neutron shell plus the Z = 82 proton shell. The systematics of alpha-preformation probability(P_α),as an indicator of the shell effect, is deduced from the ratio of the experimental decay width to the calculated one.Through the comparative analysis of the P_α trend in the N = 124-130 isotonic chain, the N = 126 and Z = 82 shell closures are believed to strongly affect the formation of the alpha particle before its penetration. Additionally, the P_α variety in Po and Rn isotopes is presented as another proof for such an influence. More importantly, it may be concluded that the expected neutron(or proton) shell effect gradually fades away along with the increasing valence proton(or neutron) number. The odd-even staggering presented in the P_α value is also discussed.     

Upgrade of RIBLL

The synthesis of the super-heavy nuclei and the study of their properties attract more and more attentions in recent years. The He-jet MG wheel technique with which ^259Db and ^265Bh were synthesized at IMP has reached its limits due to the short life-time and rare amount of the products. In order to identify the nuclei with higher Z, a device for separating the interested nuclei from the heavy background is definitely needed.     

Evidence for new isotopes of element 107: 266Bh and 267Bh

New neutron rich isotopes 267107Bh and 266107Bh were produced in bombardments of a 249Bk target with 117-MeV and 123-MeV 22Ne ions at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. Identification was made by observation of correlated alpha-particle decays between the Bh isotopes and their Db and Lr daughters using a rotating wheel system. 267Bh was produced with a cross section of approximately 70 pb and decays with a 17(+14)(-6) s half life by emission of alpha particles with an average energy of 8.83+/-0.03 MeV. One atom of 266Bh was observed, decaying within 1 s by emission of a 9.29-MeV alpha particle.     

Alpha-decay properties of <Superscript>261</Superscript>Bh

The isotope ²⁶¹Bh was produced in the reaction ²⁰⁹Bi(⁵⁴Cr, 2n)²⁶¹Bh and its \( \alpha\) decay has been remeasured. It was found that it populates by an unhindered transition of \( \approx\) 10 MeV an excited level at E ^* > 350 keV in the daughter nucleus ²⁵⁷Db . The latter decays by internal transitions either into the isomeric state or the ground state. A somewhat improved half-life value of T _1/2 = 11.8^+3.9 _-2.4 ms was obtained for ²⁶¹Bh . The data support the previous assignment of the \( \alpha\) activities ²⁵⁷Db (1) and ²⁵⁷Db (2) to the isomer and to the ground state, respectively. No evidence for an isomeric state in ²⁶¹Bh decaying by \( \alpha\) emission was found. Based on the experimental results and theoretical calculations a partial decay scheme of ²⁶¹Bh including spin and parity assignments of the ground-state and excited levels in the daughter nucleus ²⁵⁷Db populated by the \( \alpha\) decay and succeeding internal transitions have been suggested. ²⁶¹Bh represents so far the heaviest nucleus for which such an attempt has been made. No spontaneous fission (SF) events that could be attributed to ²⁶¹Bh were observed, resulting in an SF branching b _SF < 0.05 . The measured production cross-section is (64±15) pb at E ^* = 22 MeV.     

First Observation for Isotope 265Bh(Z=107)

New isotope ²⁶⁵Bh(Z=107) was produced in bombardment of an ²⁴³Am target with 135MeV ²⁶Mg ions at HIRFL. Identification was made by observation of correlated α particle decays between the new isotope ²⁶⁵Bh and its ²⁶¹Db and ²⁵⁷Lr daughters with four pairs of detectors. A total of 8 correlated decay events of ²⁶⁵Bh and 4 decay events of ²⁶⁴Bh were identified. ²⁶⁵Bh decays with a 0.94+0.70-0.31s half-life by emission of α particle with an average energy of (9.24±0.05)MeV.     

Lightest isotope of Bh produced via the 209Bi(52Cr,n)260Bh reaction

The lightest isotope of Bh was produced in the new 209Bi(52Cr,n)260Bh reaction at the Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Positive identification was made by observation of eight correlated alpha particle decay chains in the focal plane detector of the Berkeley Gas-Filled Separator. 260Bh decays with a 35(-9)(+19) ms half-life by alpha particle emission mainly by a group at 10.16 MeV. The measured cross section of 59(-20)(+29) pb is compared to model predictions. The influence of the N=152 and Z=108 shells on alpha decay properties is discussed.     

Separation and implantation of relativistic86Kr-fragments at the FrS; half-life measurement by ion-β time-correlations

Neutron-rich Co and Fe isotopes produced by⁸⁶Kr projectile fragmentation at 500 MeV/u were separated and identified using the fragment separator (FrS) in a bunched energy mode.⁶⁶Co and⁶⁵Fe ions were selectively implanted in a double PIN-diode array where the-decay signals were measured. The half-lives were deduced from time correlations between implantation and-decay signals. The remeasurement of the⁶⁶Co half-life confirms the isotope identification. The value of the⁶⁵Fe half-life was found to be 0.45±0.15 s.     

Investigation of the fusion reaction27Al+236U→263105 at excitation energies of 57 MeV and 65 MeV

The neutron-deficient isotopes^257,258105 were produced in the reaction²⁷Al+²³⁶u in 6n and 5n evaporation channels, respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and from products of different nuclear reactions by the electrostatic separator VASSILISSA [1]. The isotopes were then implanted into position-sensitive silicon detectors and identified using the --correlation method. The measured production cross-section is (5n)=(0.45±0.20)nb atE _P =154 MeV and (6n)=(0.075±0.055) nb atE _P =163 MeV. These cross-sections are compared with data measured for the same isotopes in the more symmetrical reaction⁵⁰Ti+²⁰⁹Bi.     

Half-lives andQ β measurements for new nuclei of89Tc and89mTc

Neutron deficient isotopes of⁸⁹Tc and^89mTc have been produced through the⁶⁰Ni(³²S,p 2n)⁸⁹Tc at 95 MeV and the⁵⁸Ni(³⁵Cl, 2p 2n)⁸⁹Tc reaction at 135 MeV. A rotating catcher foil system was used to collect the activities and transfer them to a measuring position for- spectroscopy. The half-lives of⁸⁹Tc and^89mTc were measured to be 12.8±0.9 s and 12.9±0.8 s, respectively. The total decay energy, Q_EC, for⁸⁹Tc was determined to be 7.51±0.21 MeV.The authors would like to express their thanks to technical staff of the Tandem Accelerator Center at the University of Tsukuba for their operation of the accelerator and assistance during the experiment. This work is partly supported by a Grant-in-Aid from the Japanese Ministry of Education, Grant No. 01790180.     

Discovery of doubly magic 48Ni

In an experiment at the SISSI/LISE3 facility of GANIL, we used the projectile fragmentation of a primary 58Ni26+ beam at 74.5 MeV/nucleon with an average current of 3 &mgr;A on a natural nickel target to produce very neutron-deficient isotopes. In a 10-day experiment, 287 42Cr isotopes, 53 45Fe isotopes, 106 49Ni isotopes, and 4 48Ni isotopes were unambiguously identified. The doubly magic nucleus 48Ni, observed for the first time, is the most proton-rich isotope ever identified with an isospin projection T(z) = -4. It is probably the last doubly magic nucleus with "classical" shell closures accessible for present-day facilities. Its observation allows us to deduce a lower limit for the half-life of 48Ni of 0.5 &mgr;s.     

A scaling phenomenon for the cross section of fragment produced in projectile fragmentation reactions

A scaling phenomenon has been found for the cross section of a fragment, which is defined as a "square" cross section(SCS).This phenomenon can unify the cross sections of fragments in projectile fragmentation(PF) reactions. An empirical formula is proposed to calculate SCSfor a fragment with parameters determined by an extensive investigation of measured reactions ranging from Fermi energies to relativistic energies. The scaling phenomenon of SCShas been verified using different techniques,showing that the scaling of SCSgenerally exists in PF reactions. The concept of SCS, which can be assumed as a standard value for a fragment, is shown to accurately predict the cross sections of isotopes in PF reactions with incident energies ranging from tens of A MeV to approximately 1000 A MeV.