Generalization of α-Decay Cluster-Model to Nuclei Near Spherical and Deformed Shell Closures

The cluster model of α-decay is extended to the regions around doubly magic spherical nucleus 208pb and around deformed shell closure 270Hs, respectively. The effects of spherical shell closures (N = 126 and Z = 82) on α-decay are investigated by introducing an N-dependent α-preformation factor and a Z-dependent one inspired by a microscopic model. Good agreement between the theoretical α-decay half-lives and the measured ones is obtained for the spherical nuclei near the doubly magic nucleus 208 Pb, where the nuclear shell effect is included in the expression of α-preformation factor. The cluster model is also generalized for the decay of deformed nuclei. The branching ratios of a-decays from the ground state of a parent nucleus to the ground state (0 ) of its deformed daughter nucleus and to the first excited state (2 ) are .calculated in the framework of the cluster model. The results indicate that a measurement of c spectroscopy is a feasible method to extract the information of nuclear deformation of superheavy nuclei around the deformed nucleus 270 Hs.     

First principles study of the magnetism driven by cation defects in CeO₂:the important role of O2p states

The magnetism driven by cation defects in undoped CeO 2 bulk and thin films is studied by the density functional theory corrected for on-site Coulomb interactions (DFT+U) with U = 5 eV for the Ce4f states and U = 7 eV for the O2p states. It is found that the Ce vacancies can induce a magnetic moment of the ～ 4 μ B /supercell, which arises mainly from the 2p hole state of the nearest neighbouring O atom (～ 1 μ B on per oxygen) to the Ce vacancy. The effect of the methodology is investigated, indicating that U = 7 eV for the O2p state is necessary to obtain the localized O2p hole state in defective ceria with cation vacancies.     

Identification of a 9/2~-[505] isomer in the neutron-rich ~(193)Os nucleus

The neutron rich nucleus ¹⁹³Os was produced in the ¹⁹²Os(⁷Li, ⁶Li)¹⁹³Os reaction. An isomeric state based on the 9/2^-[505] Nilsson orbital was identified in the present work. The half-life of the isomeric state was extracted and discussed in terms of the K quantum number. A level scheme built on the isomeric state was proposed based on the experimental data.     

Gound State Properties of Nuclei in ^295118 α-Decay Chain

The properties of nuclei belonging to the α-decay chain of superheavy element ^295118 have been studied in the framework of axially deformed relativistic mean field （RMF） theory with the parameter set of NL-Z2 in the blocked BCS approximation. Some ground state properties such as binding energies, deformations, and α-decay energies Qα have been obtained and agree well with those from finite-range droplet model （FRDM）. The single-particle spectra of nuclei in ^295118 α-decay chain show that the shell gaps present obviously nucleon number dependence. The root-mean-square （rms） radii of proton, neutron and matter distributions change slowly from ^283112 to ^295118 but dramatically from ^279110 to ^283112, which may be due to the subshell closure at Z = 110 in ^279110. The α-decay half-lives in 295118 decay chain are evaluated by employing the cluster model and the generalized liquid drop model （GLDM）, and the overall agreement is found when they are compared with the known experimental data. The α-decay lifetimes obtained from the cluster model are slightly larger than those of GLDM ones. Finally, we predict the α-decay half-lives of Z=118, 116, 114, 112 isotopes using the cluster model and GLDM, which also indicate these two models can corroborate each other in studies on superheavy nuclei. The results from GLDM are always lower than those obtained from the cluster model.     

Proton radioactivity half-lives with nuclear asymmetry factor

The dependence of proton emission half-lives on the nuclear asymmetry parameter is investigated using the WKB method and two types of empirical formula.Using the single-folding formalism with asymmetry-depend-ent nuclear radius and surface diffuseness of nuclear matter,the nuclear potential and consequently the half-life are functions of the asymmetry factor.Despite small values of asymmetry in neutron-deficient proton emitters,notice-able changes in the half-lives are observed.The addition of an asymmetry parameter term to the two forms of empir-ical formulas leads to a reduction in the rms error for ground state and isomeric transitions.A noticeable reduction of about 43%is obtained for isomeric transitions in the second form of the empirical formula.Considering ground state transitions in two categories,odd-even and odd-odd emitters,and adopting deformation and asymmetry-dependent empirical formulas,the rms decreases remarkably.The low est values of rms errors,viz.0.1492,0.2312,and 0.1999,are obtained for the aforementioned empirical formulas for ground state transitions of odd-even and odd-odd emit-ters and for all isomeric transitions,respectively.     

First principles study of the magnetism driven by cation defects in CeO2: the important role of O2p states

The magnetism driven by cation defects in undoped CeO2 bulk and thin films is studied by the density functional theory corrected for on-site Coulomb interactions （DFT＋U） with U = 5 eV for the Ce4f states and U = 7 eV for the O2p states. It is found that the Ce vacancies can induce a magnetic moment of the -4 gB/supercell, which arises mainly from the 2p hole state of the nearest neighbouring O atom （-1μB on per oxygen） to the Ce vacancy. The effect of the methodology is investigated, indicating that U = 7 eV for the O2p state is necessary to obtain the localized O2p hole state in defective ceria with cation vacancies.     

Projected Shell Model Description for the Proton Emitters

Proton radioactivity is an important decay mode for nuclei near the proton drip-line. Studies of this decay mode can reveal valuable information on exotic nuclear structure and provide important information on the stucture of nuclei in extreme conditions. The new experimental data can let us understand the interactions in exotic systems, which motivate further theoretical development. The most recent application of the projected shell model (PSM) for proton emitters is represented. We study the rotational bands of the deformed proton emitter 141Ho by using the PSM. The experimental data are well reproduced. Strongly suppressed γ transition from the low-lying IΠ= 3/2+ state makes this state isomeric. Variations in the dynamical moment of inertia are discussed due to band crossings using the band diagram. The calculated results for proton emitter 151Lu shows it is oblately deformed     

Exploring the uncertainties in theoretical predictions of nuclear β-decay half-lives

Nuclear β-decay half-lives are predicted based on an empirical formula and the mass predictions from various nuclear models.It is found that the empirical formula can reproduce the nuclearβ-decay half-lives well,especially for short-lived nuclei with T_1/2<1s.The theoretical half-life uncertainties fromβ-decay energies and the parameters of the empirical formula are further investigated.It is found that the uncertainties of the half-lives are relatively large for heavy nuclei and nuclei near the neutron-drip line.For nuclei on the r-process path,the uncertainties for those with N=126 are about one order of magnitude,which are much larger than the uncertainties for those with N=50 and 82.However,theoretical uncertainties from the parameters of the empirical formula are relatively small for the nuclei on the r-process path,which indicates that the empirical formula is very suitable for predicting theβ-decay half-lives in r-process simulations.     

Generalization of α-Decay Cluster-Model to Nuclei Near Spherical and Deformed Shell Closures

The cluster model of a-decay is extended to the regions around doubly magic spherical nucleus ^208Pb and around deformed shell closure ^270Hs, respectively. The effects of spherical shell closures (N=126 and Z=82) on α-decay are investigated by introducing an N-dependent α-preformation factor and a Z-dependent one inspired by a microscopic model. Good agreement between the theoretical a-decay half-lives and the measured ones is obtained for the spherical nuclei near the doubly magic nucleus ^208Pb, where the nuclear shell effect is included in the expression of α-preformation factor. The cluster model is also generalized for the decay of deformed nuclei. The branching ratios of α-decays from the ground state of a parent nucleus to the ground state (0^ ) of its deformed daughter nucleus and to the first excited state (2^ ) are calculated in the framework of the cluster model. The results indicate that a measurement of α spectroscopy is a feasible method to extract the information of nuclear deformation of superheavy nuclei around the deformed nucleus ^270Hs.     

Charge Density Wave States and Structural Transition in Layered Chalcogenide TaSe_(2-x)Te_x

The structural features and three-dimensional nature of the charge density wave(CDW) state of the layered chalcogenide 1 T-TaSe_(2-x)Te_x(0≤x≤2.0) are characterized by Cs-corrected transmission electron microscopy measurements. Notable changes of both average structure and the CDW state arising from Te substitution for Se are clearly demonstrated in samples with x 0.3. The commensurate CDW state characterized by the known star-of-David clustering in the 1 T-TaSe_2 crystal becomes visibly unstable with Te substitution and vanishes when x=0.3. The 1 T-TaSe_(2-x)Te_x(0.3≤x≤1.3) samples generalily adopt a remarkable incommensurate CDW state with monoclinic distortion, which could be fundamentally in correlation with the strong q-dependent electronphonon coupling-induced period-lattice-distortion as identified in TaTe2. Systematic analysis demonstrates that the occurrence of superconductivity is related to the suppression of the commensurate CDW phase and the presence of discommensuration is an evident structural feature observed in the superconducting samples.     

N=127同中子素219U和216Ac的α衰变研究

本工作通过重离子熔合蒸发反应 40Ar+183W，产生了质子滴线附近的轻锕系核素 219U和 216Ac。实验在兰州充气反冲谱仪(SHANS)上开展，目标核产生后从薄靶中反冲出来，在飞行中与大量的本底粒子进行分离并偏转到位于焦平面的探测系统中。探测系统对注入的反冲核和随后的$ \alpha $衰变进行探测，并利用寻找$ \alpha $衰变链的方法对产物进行寻找和鉴别。在本次工作中，219U已知的$ \alpha $衰变数据得到改善，其基态衰变到子核215Th基态的$ \alpha $粒子能量被确定为$E_{\alpha}\!=\!9\ 763(15)$ keV，半衰期为$ T_{1/2} $=60(7) μs。首次发现了219U两个新的$ \alpha $衰变分支，其能量为$ E_{\alpha} $=9 246(17) keV, 8 975(17) keV，并指认它们分别是从 219U 的基态衰变到子核 215Th的低激发态 (5/2–)和(3/2–)。此外，通过对 216Ac的$ \alpha $衰变数据的分析，证实了216Ac存在同核异能态。     

Possible evidence for shape isomeric γ-decay in μ− atoms of 238U

A search for the γ-decay of the shape isomer in muonic ²³⁸U excited by radiationless transitions has been performed. Seven delayed transitions in the energy region of 700 to 3200 keV have been observed with a large Ge(Li) detector. Two transitions with E_γ = 2215 and 3131 keV have been attributed to the decay of the shape isomeric state into levels in the first well. The isomeric shift of the second minimum E₁₁ ≈ 600 keV in the presence of the muon and the decrease of the lifetime of the shape isomer to τ = 12 ± 2 ns give arguments in favour of the connection of shape isomerism with large quadrupole deformations.     

Observation of M3 isomeric transition from <Superscript>156m</Superscript>Pm through the βdecay of <Superscript>156</Superscript>Nd

An M3 transition in a doubly odd nucleus of ¹⁵⁶Pm was identified by internal conversion electron measurement through the β-decay of ¹⁵⁶Nd which was separated from the fission products of ²³⁵U using the on-line mass separator KUR-ISOL. The isomeric state at 150.3keV de-excites to the ground state with the M3 transition, and the spin-parity is considered to be 1^-. Nilsson configurations are also discussed on the basis of the systematics.     

Hardness and elastic modulus of zircon as a function of heavy-particle irradiation dose: I. In situ α-decay event damage

Abstract

For a natural single crystal of zircon, ZrSiO₄, from Sri Lanka, exhibiting zonation in U and Th contents, the hardness and elastic modulus have been determined as a function of α-decay dose using a mechanical properties microprobe (MPM). The zones vary in thickness from one to hundreds of micrometers, and have uranium and thorium concentrations such that the α-decay dose varies between 2 × 10¹⁵ and 1 × 10¹⁶ α-decay events/mg (0.15 to 0.65 dpa, displacement per atom). The transition from the crystalline to the aperiodic metamict state occurs over this dose range. For a traverse of 75 indent pairs across layers sampling a large portion of the crystalline-to-metamict transition (3.7 × 10¹⁵ to 9.7 × 10¹⁵ α-decay events/mg) both the hardness and elastic modulus decrease linearly with increasing α-decay dose. The radiation-induced softening follows a behavior similar to other radiation-induced changes, that is with the expansion of the unit cell parameters there is a decrease in density, birefringence, hardness and bulk modulus.     

Radioactive decay of 217Pa

The radioactive decay of ²¹⁷Pa was investigated by means of α-γ-spectroscopy. Fine structure in the ground-state α-decay was established. Ambiguities in the fine structure of the α-decay of the previously known isomeric state could be clarified by α-γ-coincidence measurements. A previously unknown α-transition of E _α = (8306 ± 5) keV was detected and identified by means of delayed α-α- and α-γ-γ-coincidence measurements. A second isomeric state decaying by α-emission was not observed. The quality of the previously reported data of the α-decay fine structure of ²¹⁷Th was improved. Received: 29 April 2002 / Accepted: 17 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: f.p.hessberger@gsi.de Communicated by J. ?yst?     

Isomeric state of 80Y and its role in the astrophysical rp-process

A careful investigation of the isomeric transition of the long-lived state at 228.5 keV excitation energy in ⁸⁰Y has been done. The HIGISOL facility at the Jyv?skyl? isochronous cyclotron has been used. We used the electron magnetic transporter to prepare an appropriate source and to measure the electron spectra in clean background conditions. The measured internal conversion coefficient α_K = 0.50±0.07 allows unambiguous 1^- identification for the 228.5 keV first excited isomeric state in ⁸⁰Y. With a “bare" half-life of 6.8±0.5 s found in this work, this state is strongly populated in the rp-process during X-ray bursts and has therefore to be taken into account in X-ray burst model calculations. However, because of the similarity of the β-decay half-lives of isomeric and ground states, we find a maximum reduction in the effective β-decay lifetime of ⁸⁰Y of only 17±2%. Our results pave the way for a future investigation of the impact of the isomeric state on the “effective" ⁸⁰Y proton capture rate. Received: 14 March 2001 / Accepted: 10 July 2001     

Spontaneous fission and alpha-decay properties of neutron deficient isotopes 257−253104 and 258106

Spontaneous fission and α-decay of^253?257104 and ²⁵⁸106 were investigated in irradiations of ^204,206,208Pb with ⁵⁰Ti and ²⁰⁹Bi with ⁵¹V, respectively. New spontaneous fission activities were identified and assigned to ²⁵³104, ²⁵⁴104, and ²⁵⁸106. The half-lives were measured as T_1/2 = (48 _?10 ⁺¹⁴ ) μs for ²⁵³104, T_1/2 = (23 ± 3) μs for ²⁵⁴104, and T_1/2 = (2.9 _?0.7 ^+1.3 ) ms for ²⁵⁸106. No indication for α-decay of any of these isotopes was found. For the α-decay branching ratios bα limits corresponding to bα ≤ 0.1 for ²⁵³104, bα ≤ 0.015 for ²⁵⁴104, and bα ≤0.2 for ²⁵⁸106 were obtained. These results prove a reduced fission probability for nuclei with neutron numbers N=152 up to Z=106 and a steep decrease of the fission half-lives for neutron numbers N < 152 up to element 104. α-decay data of ²⁵⁷104 and ²⁵⁵104 have been improved. An isomeric state decaying by α-emission was identified in ²⁵⁷104 and attributed to a low lying 11/2- [725] state. A small α-decay branch for the even — even nucleus ²⁵⁶104, indicated in an earlier experiment at SHIP, was confirmed, allowing a better founded extrapolation of experimental masses for even — even nuclei up to ²⁶⁴Hs (Z=108), the heaviest even — even nucleus identified so far.     

γ-Spectroscopy of 89 215 Ac126 and proton pairing inN=126 isotones

Theγ-decay of 5 excited states along the yrast line has been observed in²¹⁵Ac by inbeamγ-spectroscopy. Lifetimes andg-factors of 3 isomeric levels have been measured. The data show that pairing dominates the structure ofN=126 isotones.     

Observation of nucleon clusters in the spontaneous decay of234U

The emission of Ne and Mg nuclei was detected in the spontaneous decay of²³⁴U with probabilities of, respectively, (3.9 ±1.0) × 10^?13 and (2.3 ±0.7) × 10^?13 relative toα-decay. The upper limits of this kind of decay for²³⁵U and²³⁶U have been established. Systematics of half-lives for cluster emission are found to be similar to the Geiger-Nuttall dependence forα-decay. Some evidence for the presence of hindrance to cluster emission from the odd-mass nuclei²³³U and²³⁵U is obtained.