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.     

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     

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.     

Scheme of the β-decay of <Superscript>234</Superscript>Th

The scheme of the β-decay of ²³⁴Th is still not completely clear. Only the upper bound (10 keV) of the energy of the isomeric transition from level T_1/2 = 1.17 min of the ²³⁴Pa nucleus is known, and the correspondence between gamma quanta of several energies emitted in the β-decay of ²³⁴Th and specific nuclear levels of ²³⁴Pa has not been established. In this work, the isomeric transition energy of 2.6 ± 0.5 keV is determined via the γ-spectrometry of a ²³⁴Th source in γγ-coincidences, and a new ²³⁴Pa level populated in the β-decay of ²³⁴Th is introduced.     

Half-life and α-ray energy of146Sm

Up-to-date α-decay data of the double-even,N=84 isotones are presented, including the presently remeasured values for¹⁴⁶Sm. For this nuclide a half-life,T _1/2 =1.031 (45) ×10⁸ y, and α-ray energy, E_α=2455(4) keV, were measured. The significance of these data for the subshell closure atZ=64 observed in α-decay is discussed.     

Beta and gamma decays of J π = 1+, 24mAl state

A β-γ spectroscopy of the J ^π , 426 keV isomeric state of ²⁴Al (^24m Al) has been carried out by using a ²⁴Al secondary beam with high purity and high isomeric ratio. From the absolute γ-ray and β-particle intensities observed in the decay of the isomeric state, the branching ratio R _B of the isomeric γ decay from ^24m Al to the J ^π = 4⁺, ground state of ²⁴Al have been derived. The obtained R _B value of 69.6(7)% is much smaller than the previously accepted value of 82.5(30)%. The precise half-life for the isomer decay, T _1/2 ^m = 130.9(13) ms, has been also determined in this experiment. Accordingly, the M3 γ-decay strength B(M3) of the ^24m Al decay becomes smaller and the total β-decay branching ratio becomes larger. In particular, the β-decay branching ratio to the ground state of ²⁴Mg becomes 24.3(9)%, which is 2.4 times larger than the previous value of 10.1(28)%. By combining the branching ratio and the half-life, the Gamow-Teller (GT) transition strength B(GT) of 0.0194(7) is deduced for the GT transition from ^24m Al to the J ^π = 0⁺, ground state of ²⁴Mg. This value is in good agreement with the values derived from charge-exchange reactions.     

Location of the181Os isomeric state

The decay of¹⁸¹Ir has been studied on-line with mass-separated sources from the ISOCELE facility. The level scheme of¹⁸¹ Os has been established. The isomeric state has been located at 48.9 keV above the ground state. Thus the isomeric and ground states are identified as the 7/2?(T1/2 =2.7 m) and 1/2?(T1/2=105 m) levels respectively.     

The 130Nd↦130Pr↦130Ce decay chain revisited

An investigation of low-lying energy levels of ¹³⁰Pr and ¹³⁰Ce has been made via β-decay. The radioactive parent nuclei were produced via ^94,96Mo + ⁴⁰Ca reactions at 6.8 MeV/nucleon and transported by a He-jet system. Gamma-γ-t, γ-X-t, γ-e^--t coincidence measurements were performed. Internal conversion coefficients have been measured with a magnetic spectrometer. The new β-decay half-life of ¹³⁰Nd has been determined to be 21(3) s. A partial level scheme of ¹³⁰Pr at low-spin has been constructed. The β-decay of ¹³⁰Pr to ¹³⁰Ce suggests the existence of three β-instable isomeric states in ¹³⁰Pr, while only one half-life T _1/2 = 40(4) s has been measured. Based on the present work and low-lying excited states in neighbouring odd-mass nuclei, the spins and parities of isomeric states in ¹³⁰Pr are proposed to be 2⁺, (4,5)⁺ and (7,8^±). The ground state in ¹³⁰Pr is still undetermined. Received: 20 August 2001 / Accepted: 6 November 2001     

Fine Structure in the alpha decays of 226U and 230Pu

The nuclei ²²⁶U and ²³⁰Pu have been populated via reactions involving ²⁰⁸Pb targets bombarded by ²²Ne and ²⁶Mg projectiles. Fusion-evaporation residues were separated in-flight using a gas-filled recoil separator. A position-sensitive Si-strip detector was employed at the focal plane in order to identify correlated !-decay chains. Two fine structure !-decay lines have been observed. The first, with an energy of 7385(5) keV, is assigned as the ! decay from ²²⁶U to the first excited 2⁺ state of ²²²Th. The second line, observed for the first time in this work, has an energy of 6961(30) keV and is assigned as the ! decay from ²³⁰Pu to the first excited 2⁺ state of ²²⁶U. The excitation energy of the first excited 2⁺ state in ²²⁶U was determined to be 96(25) keV, giving an estimate of the deformation parameter #₂ ­ 0.20.     

Theories of alpha-decay

The derivation of the width for α-decay is examined with particular emphasis on methods which do not involve arbitrary channel radii. A new method of treating the initial decaying states is introduced and the use of ambiguous phenomenological potentials is avoided. This method yields consistent and acceptable quantitative results for the g.s. to g.s. (ground state) transitions in even polonium isotopes and for the branching ratio for the decay of the isomeric state ^212mPo.     

Evidence for low-energy γ-decay above the shape isomer in 239U

Recently, γ-decay with energies between 1.2 and 1.5 MeV feeding an apparent shape-isomeric ground state in ²³⁹U has been reported. The analysis of these high-resolution γ-spectra from resonance neutron-capture in ²³⁸U has now been extended to γ-energies well below 1.0 MeV. In total four previously unobserved transitions between 477 keV and 633 keV show a strong positive correlation with the respective class-II admixture into the observed neutron resonance and may therefore be assigned as γ-decay towards the shape-isomeric ground state.     

57Fe Mössbauer spectroscopy and x-ray diffraction study of some complex metamict minerals

Metamict minerals are a class of natural amorphous materials which were initially crystalline but self-radiation damage mainly from alpha decays of ²³⁸U and ²³²Th series inside the structure can produce partially or fully amorphization (metamictization) of these minerals. This paper reports the results of ⁵⁷Fe Mössbauer spectroscopy, gamma-ray spectrometry and X-ray diffraction (XRD) study of some complex metamict minerals like: davidite-(La), gadolinite, steenstrupine-(Ce), vesuvianite and comparatively epidote. The absorbed α-dose for these minerals varies in wide range from 1.9 × 10¹⁴ α-decay/mg (epidote) to as high as 2.7 × 10¹⁶ α-decay/mg (steenstrupine). The Mössbauer spectra show decreasing IS values (except steenstrupine) for Fe²⁺ components with absorbed α-dose. Rather unexpected feature of these spectra is a noticeable decrease of the spectral line widths with increasing absorbed α-dose both Fe²⁺ and Fe³⁺ components in gadolinite, davidite and steenstrupine.     

Decay of 129Nd to low-lying states in 129Pr

The ¹²⁹Nd activities were produced by bombarding ^92,94Mo targets with 210 or 255 MeV ⁴⁰Ca beams. Mass identification was made using the IGISOL technique. A measured β-decay half-life of 7(1)s was assigned to ¹²⁹Nd. A new low-lying level scheme has been built for ¹²⁹Pr. The 11/2^? state is proposed to be isomeric at 383 keV excitation energy.