Microsecond Isomers Research Articles

Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB.

Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with ^{32}Na nuclei. This is the only known microsecond isomer (1 μs≤T_{1/2}<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and abinitio theories. It can be represented as the coupling of a proton hole and neutron particle to ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2^{+} state at 885keV and a low-lying shape-coexisting 0_{2}^{+} state at 1058keV. We suggest two possible explanations for the 625-keV isomer in ^{32}Na: a 6^{-} spherical shape isomer that decays by E2 or a 0^{+} deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.

Production yields of micro-second isomers populated in 252Cf(SF)

A twin Frisch grid ionization chamber loaded with an ultra-thin 252 Cf sample is used in order to detect spontaneous fission events. The chamber is surrounded by 8 HPGe detectors allowing to perform γ-spectroscopic measurements. We are interested in the decay of isomeric states populated in fission fragments. In this contribution we report on preliminary results on production yields of isomeric states with half-lives ranging from few nanoup to few micro-seconds.

Decay of 72Co and microsecond isomers in even-mass neutron-rich nickel isotopes

A new excited state at 2164 keV has been identified in 72Ni with tentative spin and parity of 4+ based on comparison to shell model calculations. This level is suggested to be populated via the β-decay of an isomeric state in 72Co, and is postulated to be one of the key states that play a role in the disappearance of isomers in Ni. The known low-energy levels in 72Ni are compared with results of shell model calculations using the NR78 interaction to follow the seniority of the observed states. In addition, the half-life of the 8+ isomer in 76Ni has been measured as 630(90) ns and the full cascade of transitions from this isomer has been verified experimentally in this work.

Observation of new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon238U

A search for isomeric γ decays among fission fragments from 345 MeV/nucleon 238U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed γ rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 μs following the implantation. We identified a total of 54 microsecond isomers with half-lives of ∼0.1–10 μs, including the discovery of 18 new isomers in very neutron-rich nuclei: 59Tim, 90Asm, 92Sem, 93Sem, 94Brm, 95Brm, 96Brm, 97Rbm, 108Nbm, 109Mom, 117Rum, 119Rum, 120Rhm, 122Rhm, 121Pdm, 124Pdm, 124Agm, and 126Agm, and obtained a wealth of spectroscopic information such as half-lives, γ-ray energies, γ-ray relative intensities, and γγ coincidences over a wide range of neutron-rich exotic nuclei. Proposed level schemes are presented for 59Tim, 82Gam, 92Brm, 94Brm, 95Brm, 97Rbm, 98Rbm, 108Nbm, 108Zrm, 109Mom, 117Rum, 119Rum, 120Rhm, 122Rhm, 121Pdm, 124Agm, and 125Agm, based on the obtained spectroscopic information and the systematics in neighboring nuclei. The nature of the nuclear isomerism is discussed in relation to the evolution of nuclear structure.13 MoreReceived 10 August 2012DOI:https://doi.org/10.1103/PhysRevC.86.054319©2012 American Physical Society

G Factor of the [formula omitted] isomer in 126Sn and first observation of spin-alignment in relativistic fission

We report on the g factor measurement of the isomeric 7− state (E*=2219 keV, T1/2=5.9(8) μs) in the neutron-rich 126Sn nucleus. The nucleus was produced by the fission of a relativistic 238U beam and reaction products were selected by the FRS fragment separator at GSI. For the first time, spin-alignment was observed after relativistic fission. It was used to deduce the g factor of the 7− isomeric state, g(7−)=−0.098(9), from the measured perturbed angular distribution of its γ decay using the RISING Cluster detectors. The observed value confirms the suggested ν(h11/2−1d3/2−1) dominant configuration, which has been proposed for the 7− isomers in neutron-rich Sn isotopes. The failure of the g factor additivity rule and the importance of core polarization evolution with increasing distance from the doubly-magic 132Sn is discussed. The first observation of 18(8)% of spin-alignment produced by the relativistic fission of a 238U beam paves the way to study moments of neutron-rich (sub-)microsecond isomers, which are difficult to align by other means.

Decay properties of high-spin isomers and other structures inSb121andSb123

High-spin states populated in the decay of microsecond isomers in the transitional nuclei $^{121}\mathrm{Sb}$ and $^{123}\mathrm{Sb}$ have been investigated in detail in several experiments using $\ensuremath{\gamma}$-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with $^{136}\mathrm{Xe}$ beams and also using the $^{120}\mathrm{Sn}(^{7}\mathrm{Li},\ensuremath{\alpha}2n)^{121}\mathrm{Sb}$ and $^{122}\mathrm{Sn}(^{7}\mathrm{Li},\ensuremath{\alpha}2n)^{123}\mathrm{Sb}$ incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers (${T}_{1/2}~1$ ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the $19/{2}^{+}$ member of the $9/{2}^{+}$ deformed band in $^{121}\mathrm{Sb}$, in competition with the main decay path through the rotational band. This is attributed to mixing between the $19/{2}^{+}$ band member and a $19/{2}^{+}$ spherical state. Both levels are predicted to coincide approximately in energy in $^{121}\mathrm{Sb}$. The fact that a $25/{2}^{+}$ isomer occurs for $A=121$ and the lighter isotopes, while a $23/{2}^{+}$ isomer is observed for $A=123\text{\ensuremath{-}}131$ is explained through a multistate mixing calculation, taking into account the gradual shift of the $2{d}_{5/2}$ and $1{g}_{7/2}$ proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the $N=82$ shell closure. The observed enhancement of the $B(E2;19/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}15/{2}^{\ensuremath{-}})$ values in $^{121}\mathrm{Sb}$ and $^{123}\mathrm{Sb}$ over the $B(E2;{7}^{\ensuremath{-}}\ensuremath{\rightarrow}{5}^{\ensuremath{-}})$ values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.

Decay properties of high-spin isomers and other structures in Sb121 and Sb123

High-spin states populated in the decay of microsecond isomers in the transitional nuclei 121 Sb and 123 Sb have been investigated in detail in several experiments using γ-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with 136 Xe beams and also using the 120 Sn( 7 Li,α2n) 121 Sb and 122 Sn( 7 Li,α2n) 123 Sb incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers (T 1/2 ∼ 1 ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the 19/2 + member of the 9/2 + deformed band in 121 Sb, in competition with the main decay path through the rotational band. This is attributed to mixing between the 19/2 + band member and a 19/2 + spherical state. Both levels are predicted to coincide approximately in energy in 121 Sb. The fact that a 25/2 + isomer occurs for A = 121 and the lighter isotopes, while a 23/2 + isomer is observed for A = 123-131 is explained through a multistate mixing calculation, taking into account the gradual shift of the 2d 5/2 and 1g 7/2 proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the N = 82 shell closure. The observed enhancement of the B(E2;19/2 - → 15/2-) values in 121 Sb and 123 Sb over the B(E2; 7 - → 5 - ) values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.

Optimizing recoil-isomer tagging with the Argonne fragment mass analyzer

A new focal plane detector arrangement for the Fragment Mass Analyzer (FMA) has been built and tested at Argonne National Laboratory. This set-up is particularly sensitive for performing Recoil-Isomer Tagging on nuclei with isomeric states with lifetimes in the microsecond range. Recoiling nuclei from fusion–evaporation reactions at the target position are dispersed by their ratio of mass to charge ( A / q ) by the FMA and stopped in low pressure gas (air) at the focal plane. Subsequent gamma decays from isomeric states in the reaction products are observed using Ge detectors. A constant gas flow through the focal plane chamber efficiently removes longer-lived beta-decaying species from sight of the detectors. This set-up has been commissioned successfully with the microsecond isomer in 80Rb, populated via the 52Cr( 32S, 3pn) reaction at 135 MeV.

First observation of excited states in theI138nucleus

Excited states in the $^{138}\mathrm{I}$ nucleus, including ${T}_{1/2}=1.3$ $\ensuremath{\mu}s$ isomer decaying by a stretched $E2$ transition of 68 keV, were observed for the first time. The $^{138}\mathrm{I}$ nucleus was populated in the spontaneous fission of $^{248}\mathrm{Cm}$ and studied by means of prompt \ensuremath{\gamma}-ray spectroscopy using the EUROGAM 2 array. The microsecond isomer was populated in the neutron-induced fission of $^{235}\mathrm{U}$ and observed at the LOHENGRIN separator. Excitation scheme consists of a low-spin part and a medium-spin, $\ensuremath{\Delta}I=1$, band based on the ${7}^{\ensuremath{-}}$ state with the $(\ensuremath{\pi}{g}_{7/2}\ensuremath{\nu}{f}_{7/2}){}_{{7}^{\ensuremath{-}}}$ dominating configuration, as predicted by the shell model. The shell-model calculations of $^{138}\mathrm{I}$ provide the optimum reproduction of the experimental scheme when the $\ensuremath{\pi}{d}_{5/2}$ orbital is lowered by 600 keV relative to its position in $^{133}\mathrm{Sb}$. In the calculation the isomeric level has spin and parity ${3}^{\ensuremath{-}}$ and deexcites by an $E2$ isomeric transition to the ${1}^{\ensuremath{-}}$ level, located only 9 keV above the predicted ${0}^{\ensuremath{-}}$ ground state. Considering additional information on the ground-state spin from the literature, we propose that in $^{138}\mathrm{I}$ the ${1}^{\ensuremath{-}}$ level corresponds to the ground state and the ${0}^{\ensuremath{-}}$ is located above. We note, however, that additional measurements are required to resolve this problem.

Excitation energy and deformation of the1/2+[431] intruder band inTc107

The already detailed study of $^{107}\mathrm{Tc}$ nucleus was complemented by a search for microsecond isomers at very low energy. For this purpose, this neutron-rich nucleus was produced by thermal-neutron-induced fission of $^{241}\mathrm{Pu}$. We have found a new 30.1 keV microsecond isomeric state which deexcites to the ground state by a strongly-hindered $E1$ transition. This isomer was identified as the $3/{2}^{+}$ level of the $1/{2}^{+}$[431] intruder band in $^{107}\mathrm{Tc}$ and is also the lowest-lying member of the band. The very low energy of the band head suggests a large quadrupole deformation. From a comparison with $^{105}\mathrm{Tc}$, where more information is known about the intruder band, it is deduced that the $1/{2}^{+}$[431] band has a quadrupole deformation, ${\ensuremath{\epsilon}}_{2}\ensuremath{\geqslant}0.35$ and a possible triaxial shape, $\ensuremath{\gamma}\ensuremath{\approx}{20}^{\ifmmode^\circ\else\textdegree\fi{}}$.

Triaxiality inMo105andMo107from the low to intermediate spin region

The nuclear structure of the odd $^{105}\mathrm{Mo}$ and $^{107}\mathrm{Mo}$ isotopes was reinvestigated in the present work. The excited levels of $^{105}\mathrm{Mo}$ were studied by observing prompt \ensuremath{\gamma} rays emitted after the spontaneous fission of $^{248}\mathrm{Cm}$ with the EUROGAM2 multidetector array. The already well-studied level structure of $^{107}\mathrm{Mo}$ was also completed by a search for microsecond isomers. For this purpose, this nucleus was produced by the thermal-neutron-induced fission reaction at the ILL reactor, in Grenoble. We have shown that the experimental level energies and the \ensuremath{\gamma}-decay patterns are well reproduced by simple particle-rotor calculations, assuming that these nuclei have an asymmetric shape. The shapes of the odd and even Mo are compared in the neutron range $N=62\text{\ensuremath{-}}66$.

First observation of low-lying excited states in the very neutron-richKr95

Microsecond isomers of neutron-rich nuclei in the mass $A=95$ chain were investigated at the ILL reactor in Grenoble. These nuclei were produced by thermal-neutron induced fission of $^{241}\mathrm{Pu}$. The detection is based on time correlation between fission fragments selected by the Lohengrin mass spectrometer and the \ensuremath{\gamma} rays from the isomers. In this article, the decay scheme of an isomer of half-life 1.4(2) $\ensuremath{\mu}$s in $^{95}\mathrm{Kr}$ is reported. It is the first time that excited states of this nucleus have been measured. These data show that this neutron-rich $N=59$ isotone is spherical at low excitation energy.

Microsecond isomers in 125Sb

The half-lives of two isomeric states in 125Sb have been measured as 5.7(3) and 28.0(7) µs respectively. Conversion coefficients for the transitions depopulating the isomeric states have also been measured and based on these, the 1971.8 keV state is assigned a spin and parity of 15/2− and the state at 2112.7 keV is assigned a spin and parity of 19/2−. These states have been interpreted as the g7/2 proton coupled to the known isomeric 5− and 7− states in the 124Sn core.

Shape coexistence in the very neutron-rich odd-oddRb96

Microsecond isomers of neutron-rich nuclei in masses $A=96$ and 98 were reinvestigated at the Institut Laue-Langevin reactor (Grenoble). These nuclei were produced by thermal-neutron induced fission of $^{241}\mathrm{Pu}$. The detection is based on the time correlation between fission fragments selected by the Lohengrin mass spectrometer and the \ensuremath{\gamma} rays and conversion electrons from the isomers. A new level scheme of $^{96}\mathrm{Rb}$ is proposed. We have found that the ground state and low-lying levels of this nucleus are rather spherical, and a rotational band develops at 461-keV energy. This band has properties consistent with a $\ensuremath{\pi}[431\phantom{\rule{0.3em}{0ex}}3/2]\ifmmode\times\else\texttimes\fi{}\ensuremath{\nu}[541\phantom{\rule{0.3em}{0ex}}3/2]K={3}^{\ensuremath{-}}$ Nilsson assignment and a deformation ${\ensuremath{\beta}}_{2}&gt;0.28$. It is fed by a ${I}^{\ensuremath{\pi}}={10}^{\ensuremath{-}}$ microsecond isomer consistent with a $\ensuremath{\pi}({g}_{9/2})\ensuremath{\nu}({h}_{11/2})$ spherical configuration. It is interesting to note that the same unique-parity states $\ensuremath{\pi}({g}_{9/2})$ and $\ensuremath{\nu}({h}_{11/2})$ are present in the same nucleus in a deformed and in a spherical configuration. The neighboring odd-odd nucleus $^{98}\mathrm{Y}$ presents a strong analogy with $^{96}\mathrm{Rb}$ and is also discussed.

Neutron-rich In and Cd isotopes close to the doubly magicSn132

Microsecond isomers in the In and Cd isotopes, in the mass range $A=123\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130$, were investigated at the ILL reactor, Grenoble, using the LOHENGRIN mass spectrometer, through thermal-neutron induced fission reactions of Pu targets. The level schemes of the odd-mass $^{123--129}\mathrm{In}$ and new measurements of the $\ensuremath{\mu}\mathrm{s}$ half-lives of the odd-odd $^{126--130}\mathrm{In}$ are reported. However, the expected ${8}^{+}$ isomers in the even-mass Cd isotopes were not observed. The comparisons between the experimental $B(M2)$ strengths for In and Sn isotopes are discussed. A shell-model study of the heaviest In and Cd nuclei was performed using a realistic interaction derived from the CD-Bonn nucleon-nucleon potential. The calculation predicts values of the half-lives of the first ${8}^{+}$ states in $^{126,128}\mathrm{Cd}$ of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, which could explain the nonobservation of $\ensuremath{\mu}\mathrm{s}$ isomers. Comparison shows that the calculated levels of $^{130}\mathrm{In}$ and $^{129}\mathrm{In}$ are in good agreement with the experimental values while some discrepancies occur for the lighter In isotopes. The collectivity of $^{126,128}\mathrm{Cd}$ is discussed in the framework of the shell model and in comparison with $^{204}\mathrm{Hg}$.

Lifetime measurements of microsecond isomers in theN=48nucleiZr88andMo90using recoil-isomer tagging

The recoil-isomer tagging technique has been used to study the isomeric states of nuclei in the vicinity of the N=50 shell closure. The nuclei of interest were separated from the projectilelike nuclei and other evaporation residues and transported to the focal plane of a recoil separator. The decay of the tagged isomer was studied at the focal plane using a high-purity Ge detector. The prompt transitions feeding the isomer were detected in an array of Clover Ge detectors placed around the target. The measured half-lives of the 8{sup +} isomers in {sup 88}Zr and {sup 90}Mo are 1.41{sub (-0.09)}{sup (+0.12)} {mu}s and 1.17{sub (-0.07)}{sup (+0.19)} {mu}s, respectively. The corresponding transition probabilities are in reasonable agreement with the predictions of shell model calculations. The systematics of effective charges (e{sub p} and e{sub n}) for N=48 nuclei are qualitatively understood in these calculations.

Angular momentum population in the projectile fragmentation ofU238at750MeV∕nucleon

A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of U-238 at 750 MeV/nucleon has been undertaken at the SIS/FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around A similar to 190 were identified and the corresponding. isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.

High-spin microsecond isomers in129Inand129Sb

In this work the microsecond isomers in ${}^{129}\mathrm{In}$ and ${}^{129}\mathrm{Sb}$ were investigated. These nuclei were produced by the thermal-neutron-induced fission of ${}^{241}\mathrm{Pu}.$ The detection is based on a time correlation between the fission fragments selected by the LOHENGRIN spectrometer at the ILL (Grenoble) and the $\ensuremath{\gamma}$ rays or conversion electrons from the isomers. The decay schemes of the new ${17/2}^{\ensuremath{-}}$ isomer in ${}^{129}\mathrm{In}$ and ${23/2}^{+}$ isomer in ${}^{129}\mathrm{Sb}$ are reported. A shell-model study of these two nuclei was performed using a realistic effective interaction derived from the CD--Bonn nucleon-nucleon potential. Comparison shows that the calculated energy levels and electromagnetic transition rates are in very good agreement with the experimental data.

Evidence for an isomer in 76 Ni

In the experiment performed at the LISE2000 spectrometer at GANIL neutron-rich nickel isotopes were studied by microsecond isomer spectroscopy. Evidence for an isomer in 76Ni is found, consistently with the shell model prediction of an 8 + state of \(\nu (g_{9/2})^2\) structure.

Low-lying structure of light radon isotopes

The excited states in the neutron-deficient isotopes Rn have been populated using the Er(Ar,4n), Er(Ar,4n), and Er(Ar,4n) reactions at beam energies of 175, 182, and 177 MeV, respectively. Evaporation residues were selected using an in-flight gas-filled separator and implanted at the focal plane into a 16-element position-sensitive, passivated ion-implanted planar silicon detector. Prompt γ rays were observed at the target position using an array of Compton-suppressed germanium detectors. Correlation with the subsequent radioactive decay of associated recoiling ions in the silicon detector, recoil-γ and recoil-γ-γ coincidences were used to construct decay schemes of light radon isotopes. Measurements of delayed y rays at the focal plane have also been made, and microsecond isomers have been observed in Rn, but not in Rn. Comparison of the results with those for polonium isotopes indicate a common mechanism for the onset of deformation. Candidates have been found in Rn for deformed intruder states which coexist with the spherical ground-state shape.