Fine structure in the ground-state proton radioactive decay of highly deformed 131 Eu has been identified. In addition to the previously observed ground-state line, measured here with a proton energy of 932(7) keV, a second proton peak with energy 811(7) keV was observed. We interpret this line as proton decay from the 131 Eu ground state to the first excited 2 1 state of the daughter nucleus 130 Sm. Comparing the measured branching ratio with calculations enables the ground-state configuration of 131 Eu to be unambiguously assigned to the 3͞2 1 [411] Nilsson configuration.Ground-state proton radioactivity is a phenomenon associated with heavy nuclei lying beyond the proton drip-line [1]. Proton decay rates are extremely sensitive to the orbital angular momentum of the proton, ᐉ p , and can be used to characterize nuclear configurations at the extreme limit of stability. For spherical nuclei ᐉ p is a good quantum number and the proton decay spectroscopic factors are in general well reproduced by calculations [2,3]. Long-standing exceptions to this rule are the proton decays of 109 I and 113 Cs [4] which can be reproduced only by using a deformed multiparticle calculational approach [5,6] with a quadrupole deformation b 2 ϳ 0.1.In a recent Letter, we reported on the discovery of proton radioactivity from the highly deformed (b 2 ϳ 0.3) nuclei 131 Eu and 141 Ho [7]. The half-lives of these nuclei could not be reproduced using a spherical basis, whereas deformed calculations of the type developed in [5,6] could reproduce the decay rates [7]. The results were consistent with Nilsson configurations and deformations predicted by macroscopic-microscopic calculations [8,9]. These indicate that there is a rapid change to high prolate deformations in the region of the proton drip-line below Z 69 which reaches its apogee around 131 Eu. Highly deformed nuclei are a natural region to search for the new phenomenon of proton decay fine structure, since low-lying first excited 2 1 states in the daughter nuclei may receive significant decay strength relative to the ground state. The decay rate to the 2 1 daughter state will be sensitive to different components of the parent wave function. The present Letter describes the discovery of fine structure in the proton-radioactive decay of 131 Eu, and presents the first nuclear structure information on the daughter nucleus 130 Sm.A 2 pnA beam of 402 MeV 78 Kr ions produced by the ATLAS accelerator facility was used to bombard a 0.77 mg͞cm 2 thick 58 Ni target resulting in a compound nucleus excitation energy of ϳ82 MeV at the center of the target, chosen to optimize production of 131 Eu nuclei. The Argonne Fragment Mass Analyzer [10] was set to analyze A 131 ions with charge states q 32 and 33, with slits being placed at the focal plane to allow only the transmission of these ions into a 60-mm-thick doublesided silicon strip detector (DSSD) system, previously described in Ref. [2].A 4.6 mg͞cm 2 thick Ni foil was available 8 cm in front of the DSSD to degrade the energies of recoiling 13...
We present a simple method for discerning the evolution from vibrational to rotational structure in nuclei as a function of spin. The prescription is applied to the yrast cascades in the A approximately 110 region and a clear transition from vibrational to rotational motion is found.
Rotational bands feeding the ground state and the isomeric state in the proton emitter 141 Ho were observed using the recoil-decay tagging method. This constitutes direct evidence that 141 Ho is deformed. A quadrupole deformation of b 2 0.25͑4͒ was deduced for the ground state from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7͞2 2 ͓523͔ and 1͞2 1 ͓411͔ configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations for b 2 0.25 indicates, however, that 141 Ho may have significant hexadecapole deformation and could be triaxial in the 7͞2 2 ͓523͔ ground state.The domain of nuclei situated far from the line of b stability has always been an arena of numerous experimental pursuits and a testing ground for new theoretical models. With radioactive beams on the horizon, the physics of nuclei with an excess of neutrons or protons has become one of the focal points of nuclear physics. These nuclei define the very limits of nuclear existence and will be susceptible to phenomena associated with low binding energy, such as halos, skins, or mixing between bound and continuum states.The proton separation energy decreases with decreasing neutron number. Proton-rich nuclei, which have a negative proton separation energy and are, thus, situated beyond the proton drip-line, can spontaneously emit protons. The proton decay rate is governed by the energy and orbital angular momentum of the emitted proton. It also depends on the wave function of the proton-decaying state, which is determined by the shape of the nuclear potential and by residual interactions between valence nucleons. Most of the known proton emitters have decay rates consistent with the as
The ground state rotational bands of the N = Z nuclei (72)Kr, (76)Sr, and (80)Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. (72)Kr shows the most marked effect. It has been widely suggested that these "delayed alignments" arise from np-pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation.
Proton radioactivity from the closed neutron shell nucleus 155 Ta has been observed. It was produced via the p4n fusion evaporation channel using a 58 Ni beam on a 102 Pd target. The measured decay properties are E p ϭ1765(10) keV and t 1/2 ϭ12 Ϫ3 ϩ4 s. Spin and parity J ϭ11/2 Ϫ and a spectroscopic factor S p exp ϭ0.58 Ϫ0.15 ϩ0.20 characterize the decaying state.
γ -ray spectroscopy of 132 Te, obtained from β − decay of a 132 Sb radioactive beam at the Holifield Radioactive Ion Beam Facility, was performed using the Clarion array. A significantly revised γ -decay scheme for 132 Te was obtained including a number of new, likely 2 + , states below 2.5 MeV and the removal of a 3 − state at 2280 keV. A simple shell-model interpretation is discussed for the low-lying levels.
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