An Implantation and Detection System for Spectroscopy of 22,24Si

A new decay detection system with high detection efficiency and low detection threshold has been developed for charged-particle decay studies, including -delayed proton, decay or direct proton emission from proton-rich nuclei. The detection system was tested by using the -delayed proton emitter 24Si and was commissioned in the decay study of 22Si produced by projectile fragmentation at the First Radioactive Ion Beam Line in Lanzhou. Under a continuous-beam mode, the isotopes of interest were implanted into two double-sided silicon strip detectors, where the subsequent decays were measured and correlated to the preceding implantations by using position and time information. The system allows to measure protons with energies down to about 200 keV without obvious background in the proton spectrum. Further application of the detection system can be extended to the measurements of -delayed proton decay and the direct proton emission of other exotic proton-rich nuclei.     

Revisit to Two-Proton Radioactivity of 19Mg and Observation of Two-Proton Decay of 30Ar

An experiment aimed to investigate the two-proton (2p) decay of the previously unknown nucleus 30Ar was performed at GSI: By tracking the decay products in-flight with silicon micro-strip detectors, the 2p decays of 30Ar were observed for the first time. For the calibration purpose, 2p decays of 19Mg were also remeasured by tracking the coincident 17Ne+p+p trajectories. By comparing the measured angular p-17Ne correlations with those obtained from the corresponding Monte Carlo simulations, the simultaneous 2p decay of 19Mg ground state and the sequential 2p emission of several known excited states of 19Mg were confirmed. One new excited state in 19Mg and two new excited states in 18Na were observed.     

Foreword

In 2009, we have a fruitful year both in basic research and heavy-ion application. Based on HIRFL, remarkable progresses have been made in nuclear physics and atomic physics, and significant achievements include the precise mass measurement of very proton-rich nuclei, the first observation of X-rays from Radi-ative Electron Capture, the relativistic effect in     

Properties of the β-delayed proton decay of ~(146)Ho

The proton-rich isotope 146Ho was produced via the fusion-evaporation reaction 92Mo (58Ni, 3p1n). The β-delayed proton decay of 146Ho was studied by proton-γ coincidence measurements using a He-jet tape transport system. The γ-transitions in 145Tb following the proton emissions were observed, and the β-delayed proton branching ratios to the final states in the grand-daughter nucleus 145Tb were determined. According to the relative branching ratios, the ground-state spin of 146Ho has been proposed and the possible configuration discussed.     

Systematic Study on Alpha Decay Half-Lives of Superheavy Nuclei

The α-decay half-lives of a set of superheavy nuclear isotope chain from Z = 105 to 120 have been analyzed systematically within the WKB method, and some nuclear structure features are found. The decay barriers have been determined in the quasi-molecular shape path within the Generalized Liquid Drop Model （GLDM） including the proximity effects between nucleons in a neck and the mass and charge asymmetry. The results are in reasonable agreement with the published experimental data for the alpha decay half-llves of isotopes of charge 112, 114, and 116, of the element 294118 and of some decay products. A comparison of present calculations with the results by the DDM3Y effective interaction and by the Viola-Seaborg-Sobiczewski （VSS） formulae is also made. The experimental a decay half lives all stand in between the GLDM calculations and VSS formula results. This demonstrates the possibility of these models to provide reasonable estimates for the half-lives of nuclear decays by a emissions for the domain of SHN. The half-lives of these new nuclei are thus well tested from the reasonable consistence of the macroscopic, the microscopic, the empirical formulae and the experimental data. This also shows that the present data of SHN themselves are consistent. It could suggest that the present experimental claims on the existence of new elements Z =110 - 118 are reliable. It is expected that greater deviations of a few SHN between the data and the model may be eliminated by further improvements on the precision of the measurements.     

Systematic Study on Alpha Decay Half-Lives of Superheavy Nuclei

The α-decay half-lives of a set of superheavy nuclear isotope chain from Z = 105 to 120 have been analyzed systematically within the WKB method, and some nuclear structure features are found. The decay barriers have been determined in the quasi-molecular shape path within the Generalized Liquid Drop Model (GLDM) including the proximity effects between nucleons in a neck and the mass and charge asymmetry. The results are in reasonable agreement with the published experimental data for the alpha decay half-lives of isotopes of charge 112, 114, and 116, of the element 294118 and of some decay products. A comparison of present calculations with the results by the DDM3Y effective interaction and by the Viola-Seaborg-Sobiczewski (VSS) formulae is also made. The experimental α decay half lives all stand in between the GLDM calculations and VSS formula results. This demonstrates the possibility of these models to provide reasonable estimates for the half-lives of nuclear decays by α emissions for the domain of SHN. The half-lives of these new nuclei are thus well tested from the reasonable consistence of the macroscopic, the microscopic, the empirical formulae and the experimental data. This also shows that the present data of SHN themselves are consistent.It could suggest that the present experimental claims on the existence of new elements Z = 110 ～ 118 are reliable.It is expected that greater deviations of a few SHN between the data and the model may be eliminated by further improvements on the precision of the measurements.     

Properties of the β-delayed proton decay of 146Ho

The proton-rich isotope 146Ho was produced via the fusion-evaporation reaction ^92Mo （^58Ni, 3p1n）. The β-delayed proton decay of 146Ho was studied by proton-γ coincidence measurements using a He-jet tape transport system. The γ-transitions in ^145Tb following the proton emissions were observed, and the β-delayed proton branching ratios to the final states in the grand-daughter nucleus ^145Tb were determined. According to the relative branching ratios, the ground-state spin of 146Ho has been proposed and the possible configuration discussed.     

Proton Emission——-Recent Results and Future Prospects

Considerable progress has been made in the study of proton-emitting nuclei since the first observation of direct proton emission nearly half a century ago. This has led to improvements in our understanding of this rare decay process and provided invaluable nuclear structure data far from the valley of beta stability. This paper reviews the implications of some recent results for exotic iridium, rhenium and tantalum isotopes and considers prospects for future experimental studies of proton-emitting nuclei located at and above the N =82 neutron shell closure.     

Neutron and Alpha Structure in Neutron Deficient Nuclei in Astrophysics

The paper includes discussions on the important role of neutron and alpha configurations in proton-rich nuclei in nuclear astrophysics in terms of nucleosynthesis under extremely high-temperature hydrogenburning conditions. The p-process, which is supposed to take place at the very early epoch of type II supernovae, has considerable neutrons and alphas together with protons. The alpha-induced reactions on proton-rich unstable nuclei in the light mass regions is expected to play a crucial role, but very few of them were investigated well yet because of the experimental difficulties. Specifically, I report our recent experimental effort for the breakout process from the pp-chain region, 7Be(α ,γ )11C(α ,p)14N under the p-process. The neutron-induced reactions on proton-rich nuclei, which is even more a challenging subject, were investigated previously for very few nuclei. One possible experimental method is the Trojan Horse Method (THM). We successfully have applied THM to the 18F(n, )14N reaction study with an unstable beam of 18F.     

New levels and transitions in ^72Ge following thedecay of ^72Ga

The decay of ^72Ga has been investigated by means of γ-ray spectroscopy. The 72Ga nuclei were produced through the ^71Ga(n, γ)^72Ga reaction. The Compton-suppressed spectrometer and high-purity Ge detectors have been used singly and in coincidence, separately, to study γ-rays in the β-decay of ^72Ga to ^72Ge. Ninety-three events of γ-rays were reported, of which 7 were observed for the first time. A decay scheme of ^72Ga including 4 new levels is proposed which accommodates 87 of these transitions. Spins and parities for new levels are proposed from calculated logft values, modes on the observed decay, and some nuclear reaction experimental results.     

17F + p 厚靶实验对18Ne共振态性质的研究

实验通过17F+p共振弹性散射研究了复合核18Ne激发态中与天体核反应14O(α, p)17F相关的共振态性质。利用兰州放射性束流线(RIBLL)对弹核碎裂反应产生的放射性束17F进行了分离和提纯。经过降能后,能量为4.22 AMeV 的17F束流轰击在T2终端的厚氢靶(CH₂)_n上。　两套∆E-E Si探测器望远镜放置在两个不同的探测角度θ_lab≈2.3°和14°上,对反冲质子的角度和能量进行了测量。实验观测到了复合核18Ne的几条质子共振能级。通过R-Matrix理论程序对激发函数进行拟合,得到了共振态的能量、 自旋、 宇称和衰变宽度等共振参数。Resonant properties in18Ne relevant to the stellar reaction of 14O(α, p)17F have been investigated through a resonant elastic scattering of 17F+p. The 17F RI beam was produced via a projectile fragmentation reaction, and subsequently separated and purified by Radioactive Ion beam Line in Lanzhou(RIBLL). After a series of energy degradation,a 4.22 AMeV 17F beam bombarded a thick(CH₂)_ntarget at T2 terminal. Energy and angle of the recoiled protons were measured by two sets of ∆E-E silicon telescope at θ_lab≈2.3°,14° respectively. Several resonances in 18Ne were observed,and their resonant parametersincluding energy,spin parity and decay width have been determined by an R-Matrix analysis of the experimental excitation function.     

Possible shape coexistence in Ne isotopes and the impurity effect of Λ hyperon

The possible shape coexistence in even-even Ne isotopes and the impurity effects of the sΛand pΛ hyperons are explored employing the multidimensionally constrained relativistic-mean-field(MDC-RMF) model with the PK1 parameter set for the N N interaction and PK1-Y1 for the ΛN interaction. The quadrupole deformation potential energy surfaces(PESs), nuclear deformations, nuclear radii, binding energies, and density distributions of the hypernuclei and core nuclei are examined. The possible shape coexistence in ~(24,26,28)Ne is predicted with small energy differences of 140, 336, and 128 keV, respectively, between the two local energy minima. Different impurity effects of the sΛand pΛ hyperons are revealed. The sΛ hyperon exhibits clear shrinkage effects, which reduce the nuclear size and facilitate a spherical nuclear shape. The prolate pΛhyperon on the 1/2~-[110] orbital renders the nuclear shape more prolate, while the oblate pΛ hyperon on the 3/2~-[101] or 1/2~-[101] orbital renders the nuclei more oblate. Moreover, the Λ hyperon can increase the probabilities of the shape coexistence by reducing the energy differences between the two local energy minima, although the shape coexistence may disappear owing to the vanishment of one energy minimum on the flat energy surface.     

Monopole effects and high-spin levels in neutron-rich 132Te

The neutron-rich nuclei near doubly magic ¹³²Sn have attracted considerable interest in both nuclear physics and nuclear astrophysics. For the particle-hole nuclei in this region, the low-lying and high core excitations have been well described by shell model calculations using the extended pairing plus multipole-multipole force model. However, there is a significant difference between experiment and theory in the high-spin level 17⁺ of ¹³²Te. We intend to illust...     

Triaxiality and shape coexistence in the A ～ 30 “island of inversion”nuclei

Understanding the properties of nuclei inside "island of inversion" is still an interesting issue. Based on a simple Nilsson model with a new set of isospin-dependent parameters, and with non-axial deformations considered, we have performed three-dimensional potential-energy-surface calculations for Ne, Na, Mg and Al isotopes that are claimed to be in or nearby the A～30 island of inversion". It is found that shape coexistence and triaxial deformation(or softness) exist in these nuclei. Large deformations are obtained by the improved Nilsson parameters, which explains the observed large electric quadrupole transition probabilities. The large deformations happening in30 Ne,31Na,32 Mg and33Al indicate the quenching of the spherical N = 20 neutron shell gap. The calculations of nuclear binding and two-neutron separation energies have been also improved with the isospin-dependent parameters and the inclusion of the non-axial deformation degree of freedom.     

Experimental Evidence of Two-Proton Emissions from 18Ne Excited State

Experiment ^18Ne＋^197 Au was performed in Heavy Ion Research Facility Radioactive Ion Beam Line at Lanzhou （RIBLL）. The nuclear energy levels of ^18Ne were built by complete kinematical reconstruction of the decay products and diproton decay were observed from ^18Ne excited states. At 6.15 MeV, the experimental relative momentum and angular correlations of the emitted protons are given. The obvious enhancement at ｜qrol｜= 23 MeV/c and θc.m. = 40° shows the experimental criterion of ^2He cluster decay from ^18Ne^q.     

Spectroscopic study of thulium doped transparent glass ceramics

A systematic investigation on fluorescence spectroscopy of trivalent thulium doped in oxyfluoride glass ceramics containing LaF3 nanocrystals has been carried out in a spectral range from 400 to 900 nm under the direct excitation of 1D2 level at a low temperature. Specific optical transitions related to the fluorescence emissions are studied based on experimental measurements in frequency and time domain. Fluorescence emissions from the ions in crystal phase are distinguished from what in glass phase and their spectroscopic properties are explored. The dynamical process shows that the temporal decay of fluorescence emission consists of two parts： a rapid decay from the ions in glass phase and a slower decay from the ions in crystal phase.     

System size effects on probing nuclear dissipation with neutrons

Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 19~Os, 2~~Hg, and 21~po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size.     

Spectroscopy of Low-lying States in Odd-Z Odd-A Nuclei Beyond Lead

Low-lying states in odd-Z odd-mass nuclei at the proton drip-line beyond lead have recently been studied through fusion-evaporation reactions using a gas-filled recoil separator. Isomeric 1/2+ and 13/2+ states have been observed in odd-mass astatine and francium nuclei. The systematic behaviour of the level energies of these states have been studied and a similarity between the 1/2+ state in astatine and francium has been found. Furthermore, the 13/2+ state has been observed in the francium nuclei with an oblate behaviour suggesting a coupling of the i_13/2 proton to the 2p-2h intruder excitation.     

Nuclear Physics Programs for the Future RIBs Facility in Korea

We present nuclear physics programs based on the planned experiments using rare isotope beams (RIBs) for the future Korean Rare Isotope Beams Accelerator facility(KRIA). This ambitious facility has both an Isotope Separation On Line (ISOL) and fragmentation capability for producing RIBs and accelerating beams of wide range mass of nuclides with energies of a few to hundreds MeV per nucleon. Low energy RIBs at Elab = 5 to 20 MeV per nucleon are for the study of nuclear structure and nuclear astrophysics toward and beyond the drip lines while higher energy RIBs produced by inflight fragmentation with the reaccelerated ions from the ISOL enable to explore the neutron drip lines in intermediate mass regions. The planned programs have goals for investigating internal structures of the exotic nuclei toward and beyond the nucleon drip lines by addressing the following issues: how the shell structure evolves in areas of extreme proton to neutron imbalance; whether the isospin symmetry maintains in isobaric mirror nuclei at and beyond the drip lines; how two-proton radioactivity affects abundances of the elements; what the role of the continuum states including resonant states above protondecay threshold in exotic nuclei is in astrophysical nuclear reaction processes, and how the nuclear reaction rates triggered by unbound proton-rich nuclei make an effect on rapid proton capture processes in a very hot stellar plasma.     

EC/β+ decay of six medium-heavy nuclei

Previous experimental results of (EC+β+) decay for the medium-heavy nuclei reported by our group since 1996, including 153Er, 157Yb, 209Fr, 128Ce, 130Ce, and 128Pr have been briefly summarized. The observed low-lying states in their daughter nuclei have been reviewed in a systematic way and compared with different model calculations. Finally, some questions have been put forward for further study and discussion.