Isomeric Decay Research Articles

Blurring the Boundaries: Decays of Multiparticle Isomers at the Proton Drip Line

A multiparticle spin-trap isomer has been discovered in the proton-unbound nucleus (73)(158)Ta85 . The isomer mainly decays by γ-ray emission with a half-life of 6.1(1) μs. Analysis of the γ-ray data shows that the isomer lies 2668 keV above the known 9+ state and has a spin 10ℏ higher and negative parity. This 19- isomer also has an 8644(11) keV, 1.4(2)% α-decay branch that populates the 9+ state in (154)Lu. No proton-decay branch from the isomer was identified, despite the isomer being unbound to proton emission by 3261(14) keV. This remarkable stability against proton emission is compared with theoretical predictions, and the implications for the extent of observable nuclides are considered.

Isomer and beta decay spectroscopy in the132Sn region with EURICA

A. Jungclaus et al.; 4 pags.; 1 fig.; Open Access funded by Creative Commons Atribution Licence 2.0

New Isomers in the Neutron-Rich Region Beyond208Pb

The region of neutron-rich nuclei beyond 208Pb has been very difficult to explore due to its high mass and exoticity. However, recent experimental improvements allowed one to perform a quite extended isomer decay spectroscopy of these nuclei.

Recoil-βtagging study of theN=Znucleus66As

An in-beam study has been performed to further investigate the known isomeric decays and to identify T = 1 excited states in the medium-heavy N = Z = 33 nucleus As-66. The fusion-evaporation reaction Ca-40(Si-28,pn)As-66 was employed at beam energies of 75 and 83 MeV. The half-lives and ordering of two known isomeric states in As-66 have been determined with improved accuracy. In addition, several prompt gamma-ray transitions from excited states, both bypassing and decaying to the isomeric states in As-66, have been observed. Most importantly, candidates for the 4(+) -> 2(+) and 6(+) -> 4(+) transitions in the T = 1 band have been identified. The results are compared with shell-model calculations using the modern JUN45 interaction in the pf(5/2)g(9/2) model space.

Decay of 161m1,m2Dy isomers under conditions of a resonance environment (Mössbauer Screen)

The half-lives of the isomers 161m1Dy and 161m2Dy (E = 25.6 keV and T 1/2 ∼ 30 ns for the former and E = 74.6 keV and T 1/2 ∼ 3 ns for the latter) placed in a 160Gd2O3 crystal lattice at T = 300 K and surrounded by stable 161Dy nuclei in the composition of 161Dy2O3 were measured by the method of (β-γ) coincidences in the beta-decay process 161Tb → 161Dy. Nuclei of 161m1,m2Dy were obtained according to the chain 160Gd(n, γ)161Gd → 161Tb → 161Dy from 160Dy2O3 weighted portions irradiated at the PWR-M reactor of the Petersburg Nuclear Physics Institute (PNPI, Gatchina, Russia). The T 1/2 value observed for the isomer 161m1Dy was found to be correlated with the number of surrounding 161Dy nuclei. The presence of this correlation in 161m1Dy can be explained by the multiple resonance scattering of photons from isomer decay within the sample used. No such correlation was observed for 161m2Dy. The half-lives measured for the isomers 161m1Dy and 161m2Dy in the absence of the above environment are 29.2(1) and 3.50(1) ns, respectively.

Preparation of the 178m2Hf isomer used in the induced gamma decay experiment by X-ray from synchrotron radiation facility

It is widely acknowledged that the 178m2Hf nuclide is the most suitable substance to study the decay characteristic of the isomer induced by low-energy X-ray. In order to conduct the experiment on the induced gamma emission, the research group has started producing the 178m2Hf nuclide based on the 176Yb(α, 2n)178m2Hf reaction. After the chemical purification is conducted, the sample is prepared and used in Shanghai Synchrotron Radiation Facility. During the production of isomer, the natural metal Yb target is got through magnetron sputtering. Bombarded by α particles about 27 MeV, the 178m2Hf nuclide reaches about 1012. Yb target prepared in this way is most suitable for the production of 178m2Hf nuclide in the CS30 cyclotron. There are various nuclides in the irradiated target and the main long-lived nuclides are 173Lu, 172Lu, 175Hf, 172Hf and 65Zn. The chemical separation of 178m2Hf is studied and its process is monitored by radioactive tracer. The above result shows that decontamination factors of Zn and Lu are 105 and 103, respectively, and the yield of hafnium is 69%. Under the protection of vacuum filtration technology, the purified 178m2Hf isomers are entirely transferred to the surface of filter paper, in order to form the sample which satisfies requirements of X-ray triggering the 178m2Hf isomer decay experiment in Shanghai Synchrotron Radiation Facility in the future.

Isomeric states observed in heavy neutron-rich nuclei populated in the fragmentation of a208Pb beam

Heavy neutron-rich nuclei were populated via the fragmentation of a E/A = 1 GeV Pb-208(82) beam. Secondary fragments were separated and identified and subsequently implanted in a passive stopper. By the detection of delayed gamma rays, isomeric decays associated with these nuclei have been identified. A total of 49 isomers were detected, with the majority of them observed for the first time. The newly discovered isomers are in Hg-204,205(80), Au-201,202,204,205(79), Pt-197,203,204(78), Ir-195,199-203(77), Os-193,197-199(76), Re-196(75), W-190,191(74), and Ta-189(73). Possible level schemes are constructed and the structure of the nuclei discussed. To aid the interpretation, shell-model as well as BCS calculations were performed.

High-spinμs isomeric states in96Ag

The isomeric and {\beta} decays of the N = Z +2 nucleus 96Ag were investigated at NSCL. A cascade of {\gamma}-ray transitions originating from the de-excitation of a {\mu}s isomer was observed for the first time and was found in coincidence with two previously-known transitions with energies of 470 and 667 keV. The isomeric half-life was determined as 1.45(7) {\mu}s, more precise than previously reported. The existence of a second, longer-lived {\mu}s isomer, associated with a 743-keV transition, is also proposed here. Shell model results within the (p_{3/2}p_{1/2}f_{5/2}g_{9/2}) model space, using the jj44b interaction, reproduced level energies and isomeric decay half-lives reasonably well.

Indene Formation under Single‐Collision Conditions from the Reaction of Phenyl Radicals with Allene and Methylacetylene—A Crossed Molecular Beam and Ab Initio Study

Polycyclic aromatic hydrocarbons (PAHs) are regarded as key intermediates in the molecular growth process that forms soot from incomplete fossil fuel combustion. Although heavily researched, the reaction mechanisms for PAH formation have only been investigated through bulk experiments; therefore, current models remain conjectural. We report the first observation of a directed synthesis of a PAH under single-collision conditions. By using a crossed-molecular-beam apparatus, phenyl radicals react with C(3)H(4) isomers, methylacetylene and allene, to form indene at collision energies of 45 kJ mol(-1). The reaction dynamics supported by theoretical calculations show that both isomers decay through the same collision complex, are indirect, have long lifetimes, and form indene in high yields. Through the use of deuterium-substituted reactants, we were able to identify the reaction pathway to indene.

Isomers in neutron-rich lead isotopes populated via the fragmentation of 238U at 1 GeV A

Neutron-rich nuclei beyond N = 126 in the lead region were populated by fragmenting a 238U beam at 1 GeV A on a Be target and then separated by the Fragment Separator (FRS) at GSI. Their isomeric decays were observed, enabling study of the shell structure of neutron-rich nuclei around the Z=82 shell closure. Some preliminary results are reported in this paper.

Isomer and β-decay spectroscopy of Tz=1 isotopes below the N=Z=50 shell gap

The RISING setup at the GSI-FRS facility was used to investigate the isomer and beta decays in N~Z~50 Cd, Ag and Pd isotopes. A preliminary analysis of the data has revealed new results on the Tz=1, 94Pd, 96Ag and 98Cd isotopes. In 94Pd a new high-spin isomer was observed, whilst in 96Ag 3 new isomeric states were identified, including core-excited states. In 98Cd a new high-energy isomeric γ-ray transition is observed, thus enabling us to confirm the previous spin assignment for the core-excited 12+ isomer.

Decay Schemes of Three-Quasiparticle Isomers in 119,121Sb and 121,123I

The gamma-ray decay of isomers in near spherical Sb-119,Sb-121 and transitional I-121,I-123 have been studied using gamma-ray spectroscopy with Li-7 pulsed beams delivered from 14 UD Pelletron accelerator at the Australian National University. Isomers of J(pi) = 25/2(+), 19/2(-) and 21/2(-) with half-lives of a few nanoseconds to hundreds microseconds have been observed at 2.4 < E-x < 2.9 MeV. Decay schemes of the isomers in these nuclei are presented and their properties are interpreted in terms of three quasiparticle configurations based on the two-neutron (h(11/2))(2) 10(+) and (d(3/2)h(11/2)) 7(-) states in even-A Sn isotones coupled to an extra proton.

High-spin states and isomers in the one-proton-hole and three-neutron-hole204Tl isotope

The high-spin structure of the neutron-rich ${}^{204}$Tl isotope has been studied up to a 11.2-MeV excitation energy and a $I=30$ spin range using the deep-inelastic heavy-ion $\ensuremath{\gamma}$-spectroscopy method with reactions of ${}^{48}$Ca on thick ${}^{208}$Pb and ${}^{238}$U targets. The established structure of yrast levels involves four isomeric states up to ${I}^{\ensuremath{\pi}}={22}^{\ensuremath{-}}$, the highest spin state available for the maximally aligned four valence holes. The observations are interpreted and quantitatively confirmed by shell-model calculations. The rates of the identified M2 and E3 isomeric decays are discussed and a striking analogy is found for the yrast level structures and $\ensuremath{\gamma}$ decays observed in the ${18}^{+}$ to ${22}^{\ensuremath{-}}$ and $45/{2}^{\ensuremath{-}}$ to $53/{2}^{+}$ spin ranges in ${}^{204}$Tl and ${}^{203}$Hg, respectively. In the highest spin part of the scheme, two prominently populated yrast states are tentatively identified as the ${3}^{\ensuremath{-}}$ ${}^{208}$Pb core excitation built on the ${22}^{\ensuremath{-}}$ and ${20}^{+}$ maximally aligned four-hole states. Their energies are reproduced well by using energy shifts observed in experiments for the ${}^{208}$Pb core octupole excitation coupled to simpler intrinsic structures.

Production and dosimetric aspects of the potent Auger emitter 58mCo for targeted radionuclide therapy of small tumors

Purpose:Based on theoretical calculations, the Auger emitter58mCo has been identified as a potent nuclide for targeted radionuclide therapy of small tumors. During the production of this isotope, the coproduction of the long‐lived ground state 58gCo is unfortunately unavoidable, as is ingrowth of the ground state following the isomeric decay of 58mCo. The impact of 58gCo as a β+‐ and γ‐emitting impurity should be included in the dosimetric analysis. The purpose of this study was to investigate this critical part of dosimetry based on experimentally determined production yields of 58mCo and 58gCo using a low‐energy cyclotron. Also, the cellular S‐values for 58mCo have been calculated and are presented here for the first time.Methods:58mCo was produced via the 58Fe(p,n)58mCo nuclear reaction on highly enriched 58Fe metal. In addition, radiochemical separations of produced radio‐cobalt from natFe target material were performed. The theoretical subcellular dosimetry calculations for 58mCo and 58gCo were performed using the MIRD formalism, and the impact of the increasing ground state impurity on the tumor‐to‐normal‐tissue dose ratios (TND) per disintegration as a function of time after end of bombardment (EOB) was calculated.Results:192 ± 8 MBq of58mCo was produced in the irradiation corresponding to a production yield of 10.7 MBq/μAh. The activity of 58gCo was measured to be 0.85% ± 0.04% of the produced 58mCo activity at EOB. The radio‐cobalt yields in the rapid separations were measured to be &gt;97% with no detectable iron contaminations in the cobalt fractions. Due to the unavoidable coproduction and ingrowth of the long‐lived ground state 58gCo, the TND and the potency of the 58mCo decrease with time after EOB. If a future treatment with a 58mCo labeled compound is not initiated before, e.g., 21 h after EOB, the resulting TND will be approximately 50% of the TND of ‘pure’ 58mCo as a result of the increased normal tissue dose from the ground state.Conclusions: The Auger emitter 58mCo is a potent radioisotope for targeted radionuclide therapy, and the production of therapeutic quantities should be achievable using a small biomedical cyclotron. However, the unavoidable coproduction and ingrowth of the long‐lived ground state 58gCo requires fast radiochemical processing and use of future 58mCo‐labeled radiopharmaceuticals in order to exploit the high achievable TND of 58mCo.

Structure of the proton emitter 117La studied by proton and γ-ray spectroscopy

Proton radioactivity from 117La was re-investigated with much improved statistics and precision. Only the ground-state proton decay (Ep=813(3) keV, T1/2=20.1(25) ms) was observed, no evidence for a previously reported isomeric proton decay was found. Prompt γ rays in 117La were identified using the Recoil–Decay Tagging method. Overall the data indicate that the proton-emitting state is associated with a Kπ=3/2+ configuration, in agreement with all the published proton-decay calculations and predictions for the ground-state configuration of 117La calculated in the framework of either adiabatic or non-adiabatic particle models. However, this is not in agreement with the most recent state-of-the-art quasi-particle non-adiabatic model calculations which predict a Kπ=7/2− configuration.

Search for a 2-quasiparticle high-Kisomer inRf256

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in Rf256, which suggest that a long-lived, low-energy 8− isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a Rf256 nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-μs isomer was identified. However, its low population, ~5(2)% that of the ground state instead of the expected ~30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z=104, which describes nuclei with Z=92–103 very well.Received 25 February 2011DOI:https://doi.org/10.1103/PhysRevC.83.064311©2011 American Physical Society

Characteristics of the [formula omitted] isomer in 93Mo: Toward the possibility of enhanced nuclear isomer decay

To discuss whether an enhanced isomer decay is a preferred process in a plasma environment it is required to know the structure of the isomer as well as the nearby states. The spin-21/2, 6.85-hour high-spin isomer in Mo93 is investigated within a shell model which well describes nuclei in this mass region. By using the obtained wave-functions which correctly reproduce the observed B(E2), B(E4), and B(M1) transitions, characteristics of the isomer are shown in comparison with the isomeric states in neighboring nuclei. Calculations suggest that these high-spin isomers are formed with almost pure single-particle-like configurations. The 93Mo 21/2+ isomer has the predominant configuration π(g9/2)82⊗νd5/2 lying below the 15/2+, 17/2+, and 19/2+ states due to neutron–proton interaction, which is the physical origin of its long lifetime. The key E2 transition that connects the 21/2+ isomer to the upper 17/2+ level is predicted to be substantial (3.5 W.u.), and therefore there is a real prospect for observing induced isomer deexcitation.

Near-yrast, medium-spin, excited states ofRb91,Rb93, andRb95

The medium-spin structure of the nuclei $^{93}\mathrm{Rb}$ and $^{95}\mathrm{Rb}$ is studied following the neutron-induced fission of $^{235}\mathrm{U}$ at the PF1B neutron guide, using the FIFI spectrometer, and at the Lohengrin mass spectrometer of the Institut Laue-Langevin Grenoble. These nuclei, plus $^{91}\mathrm{Rb}$, are also studied following the spontaneous fission of $^{248}\mathrm{Cm}$ and $^{252}\mathrm{Cf}$ sources, using the EUROGAM-II and Gammasphere detector arrays, respectively. A high-spin isomeric state, with a half-life of 111(11) ns, is found in $^{93}\mathrm{Rb}$ at an excitation energy of 4422.4 keV, which most likely corresponds to the fully aligned $[\ensuremath{\pi}({g}_{9/2})\ensuremath{\bigotimes}\ensuremath{\nu}({g}_{7/2}{h}_{11/2})]{}_{{27/2}^{\ensuremath{-}}}$ configuration. An analogous configuration is proposed for the 5297.9-keV level observed in $^{91}\mathrm{Rb}$. A new $E3$ decay branch of the 1133.9-keV isomer in $^{91}\mathrm{Rb}$ is found, for which the rather low transition rate of $B(E3)=3.8(10)$ W.u. is determined. The energy of the isomeric state of $^{95}\mathrm{Rb}$ is now proposed to be at 810.6 keV, with a spin of ($9/{2}^{+}$), and its half-life determined to be ${T}_{1/2}=94(7)$ ns. A cascade of prompt transitions is observed on top of the 810.6-keV isomer in $^{95}\mathrm{Rb}$. The near-yrast structures of $^{91}\mathrm{Rb}$, $^{93}\mathrm{Rb}$, and $^{95}\mathrm{Rb}$ are compared to the results of shell-model calculations, which support the proposed $27/{2}^{\ensuremath{-}}$ interpretation of states in $^{91}\mathrm{Rb}$ and $^{93}\mathrm{Rb}$. An analogous $27/{2}^{\ensuremath{-}}$ state is expected to occur in $^{95}\mathrm{Rb}$, as a long-lived isomer at 3.24 MeV. No such isomeric decay could be observed in a measurement using the Lohengrin spectrometer, which shows that, if it exists, its population, following the fission of $^{235}\mathrm{U}$, is at least four times lower than that of the analogous $27/{2}^{\ensuremath{-}}$ isomer in $^{97}\mathrm{Y}$.

High-K multi-quasiparticle states in 254No

We report results from an experiment on the decay of the high-K isomers in 254No. We have been able to establish the decay from the known high-lying four-quasiparticle isomer, which we assign as a Kπ=16+ state at an excitation energy of Ex=2.928(3) MeV. The decay of this state passes through a rotational band based on a previously unobserved state at Ex=2.012(2) MeV, which we suggest is based on a two-quasineutron configuration with Kπ=10+. This state in turn decays to a rotational band based on the known Kπ=8− isomer, which we infer must also have a two quasineutron configuration. We are able to assign many new gamma-rays associated with the decay of the Kπ=8− isomer, including the identification of a highly K-forbidden ΔK=8 E1 transition to the ground-state band. These results provide valuable new information on the orbitals close to the Fermi surface, pairing correlations, deformation and rotational response, and K-conservation in nuclei of the deformed trans-fermium region.

High spin states and isomeric decays in doubly-odd 208Fr

Neutron deficient isotopes of francium ( Z = 87 , N ∼ 121 – 123 ) as excited nuclei were produced in the fusion–evaporation reaction: 197Au( 16O, xn) 213 − x Fr at 100 MeV. The γ rays from the residues were observed through the high sensitivity Germanium Clover detector array INGA. The decay of the high spin states and the isomeric states of the doubly-odd 208Fr nuclei, identified from the known sequence of ground state transitions, were observed. The half-lives of the E γ = 194 ( 2 ) keV isomeric transition, known from earlier observations, was measured to be T 1 / 2 = 233 ( 18 ) ns . A second isomeric transition at E γ = 383 ( 2 ) keV and T 1 / 2 = 33 ( 7 ) ns was also found. The measured half-lives were compared with the corresponding single particle estimates, based on the level scheme obtained from the experiment.