Short-lived Isomeric States Research Articles

Search for fission from a long-lived isomer in No250 and evidence of a second isomer

In the present work, a $K$ isomeric state in $^{250}\mathrm{No}$, which is more stable against fission than the ground state, was experimentally studied. The aim was to measure the fission branch of this isomeric state. In total, 780 fission events attributed to the decay of $^{250}\mathrm{No}$ were detected. Among them 133 cases were attributed to the ground-state decay with a half-life of 4.0(4) $\ensuremath{\mu}\mathrm{s}$, which was populated by the deexcitation of the isomeric state via electromagnetic transitions with a half-life of 23(4) $\ensuremath{\mu}\mathrm{s}$. In addition, in two more cases, this long-lived isomeric state was populated in the deexcitation of a hitherto unknown, yet higher-lying and short-lived isomeric state with a half-life of $0.{7}_{\ensuremath{-}0.3}^{+1.4}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. No direct fission from the long-lived isomeric state, i.e., with a lifetime of longer than 40 $\ensuremath{\mu}\mathrm{s}$, was identified. This results in an upper limit of 0.035 for the branching ratio for fission. This is a significantly more strict limit than the previously known value of 0.5. Nonobservation of fission branching of the long-lived isomer is discussed relative to theoretical predictions and within various semiempirical ways, which resulted in an attribution of a lower limit of ${10}^{4}$ for the fission-hindrance factor, caused by the $K$ quantum number. The presences of multiple high-$K$ isomeric states seemingly is a widespread phenomenon in deformed heavy nuclei.

Half-life measurement of the 199-keV isomeric state in Ga76

Background: Isomeric states in atomic nuclei are a sensitive probe of their underlying microscopic structure and can be used to study the evolution of shell structure far from stability. Recent studies have identified and provided detailed spectroscopy of isomers in neutron-rich nuclei with $Z=28\text{\ensuremath{-}}50$. Isomeric states in the odd-odd gallium isotopes have been reported for all gallium isotopes from $A=72$ to $A=80$ with the exception of $^{76}\mathrm{Ga}$.Purpose: The purpose of this experiment was to observe short-lived isomeric states in the vicinity of $^{78}\mathrm{Ni}$.Methods: In-beam fragmentation of a $^{86}\mathrm{Kr}$ primary beam at the National Superconducting Cyclotron Laboratory produced radioactive ions which were delivered to and deposited in a $\mathrm{Ce}{\mathrm{Br}}_{3}$ scintillator coupled to a position-sensitive photomultiplier tube. Beta-delayed $\ensuremath{\gamma}$ rays were measured by ancillary HPGe clover and $\mathrm{La}{\mathrm{Br}}_{3}$ detectors which surrounded the implantation detector.Results: The previously observed ${J}^{\ensuremath{\pi}}={1}^{+}$, 199-keV level in $^{76}\mathrm{Ga}$, populated following the $\ensuremath{\beta}$ decay of $^{76}\mathrm{Zn}$, was identified as isomeric with a half-life of $34{(1)}_{\text{stat.}}{(8)}_{\text{sys.}}\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$. Shell-model calculations suggest this state is formed by the coupling of protons in negative-parity configurations to $1/{2}^{\ensuremath{-}}$ neutron configurations. Transition strengths assuming a ground-state spin of $J=2$ and $J=3$ were determined from the experimental data.

Level structures of the transfermium odd-odd nucleus Md252

Low-energy level scheme of doubly odd transfermium nucleus ${}_{101}^{252}{\mathrm{Md}}_{151}$ is constructed using our well-tested two-quasiparticle rotor model with explicit inclusion of residual n-p interaction contribution. Energy levels thus deduced are examined critically to assign spin-parity ${J}^{\ensuremath{\pi}}K$ and two-quasiparticle (2qp) configurations to the $^{252}\mathrm{Md}$ ground state and other energy levels with ${E}_{x}<400$ keV. Our low-energy level structures are then examined to predict a high-spin $({J}^{\ensuremath{\pi}}={8}^{+})$ long-lived isomer with ${E}_{x}\ensuremath{\approx}125$ keV and possibly also a lower-lying short-lived isomeric state. Comparison with $^{256}\mathrm{Lr}$ $(\ensuremath{\alpha})$ populated $^{252}\mathrm{Md}$ excited states leads to ${J}^{\ensuremath{\pi}}K$ and 2qp assignment to each of these experimentally postulated levels. Further, the five $\ensuremath{\gamma}$'s ascribed to $^{252}\mathrm{Md}$ from $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence experiments are placed in our evaluated levels corresponding to $\mathrm{\ensuremath{\Delta}}I=2$ transition in each case. Additionally, exploration of the upward extended radioactive connection of $^{232}\mathrm{Th}$ heading the $A=4n$ naturally occurring radioactive series to the heaviest known superheavy element with $A=4n$, namely ${}_{111}^{272}{\mathrm{Rg}}_{161}$, has led us to establish landmark position of $^{252}\mathrm{Md}$ in this $^{272}\mathrm{Rg}\text{\ensuremath{-}}^{232}\mathrm{Th}$ extended decay chain.

Study of excited states and observation of collective level structures in the odd–odd nucleus $${}^{194}$$Bi

High-spin states of the odd–odd $${}^{194}$$ Bi nucleus have been studied by means of nuclear spectroscopy for the first time. Two low-lying rotational bands were observed. The positive-parity band built on a short-lived isomeric state 633 keV above the $$\alpha $$ and $$\beta $$ decaying long-lived (10 $$^-$$ ) state is assigned a $${\pi i_{13/2} \otimes \nu i_{13/2 }^{-1}}$$ configuration, while the other band is attributed to the $${\pi h_{9/2} \otimes \nu i_{13/2 }^{-1}}$$ configuration. Both of these bands are assumed to have oblate shape. Three band-like structures composed predominantly of $$\Delta I = 1$$ transitions were also found. One of the shears band candidates is firmly linked to the lower lying level structures whereas for the other two links to lower-lying structures remain missing.

Activity measurement of Fe60 through the decay of Co60m and confirmation of its half-life

The half-life of the neutron-rich nuclide, {\fesixty} has been in dispute in recent years. A measurement in 2009 published a value of $(2.62 \pm 0.04)\times10^{6}$ years, almost twice that of the previously accepted value from 1984 of $(1.49 \pm 0.27)\times10^{6}$ years. This longer half-life was confirmed in 2015 by a second measurement, resulting in a value of $(2.50 \pm 0.12)\times10^{6}$ years. All three half-life measurements used the grow-in of the $\gamma$-ray lines in {\nisixty} from the decay of the ground state of $^{60}\text{Co}$ (t$_{1/2}$=5.27 years) to determine the activity of a sample with a known number of {\fesixty} atoms. In contrast, the work presented here measured the {\fesixty} activity directly via the 58.6 keV $\gamma$-ray line from the short-lived isomeric state of $^{60}\text{Co}$ (t$_{1/2}$=10.5 minutes), thus being independent of any possible contamination from long-lived $^{60\text{g}}\text{Co}$. A fraction of the material from the 2015 experiment with a known number of {\fesixty} atoms was used for the activity measurement, resulting in a half-life value of $(2.72 \pm 0.16)\times10^{6}$ years, confirming again the longer half-life. In addition, {\fesixty}/{\fe} isotopic ratios of samples with two different dilutions of this material were measured with Accelerator Mass Spectrometry (AMS) to determine the number of {\fesixty} atoms. Combining this with our activity measurement resulted in a half-life value of $(2.69 \pm 0.28)\times 10^{6}$ years, again agreeing with the longer half-life.

Measurements of conversion electrons in the s-process branching point nucleus 176Lu

Conversion coefficients, gamma-gamma and gamma-electron coincidences were measured in the s-process branching point nucleus 176Lu . Our goal was to determine the multipolarities of the $ \gamma$ -ray transitions that connect the high and low K states of 176Lu . This 176Lu nucleus has a long-lived ground state ( $ K=7^{-}$ of 37.6Gy, a short-lived isomeric state ( $ K=0^{-}$ at 122.8keV with half-life of 3.6h, as well as a 58μs isomer at 1588keV ( $ K=14^{+}$ . The excitation structure of this nucleus contains bands of intermediate spins of both positive and negative parities. The intermediate states can under certain stellar temperatures completely change the equilibrium between the isomer and ground state of 176Lu and change the abundance of this nucleus. We populated 37 previously known levels in this nucleus via the 176Yb (p, n reaction and measured 42 conversion coefficients for $ \gamma$ -ray transitions including 17 of them for the first time.

Resonances above the proton threshold inSi26

$^{26}\mathrm{Al}$ remains an intriguing target for observational $\ensuremath{\gamma}$-ray astronomy, thanks to its characteristic decay. The $^{25}\mathrm{Al}(p,\ensuremath{\gamma})^{26}\mathrm{Si}$ reaction is the crucial link in a sequence that bypasses the production of the observable $^{26}\mathrm{Al}^{g}$ in favor of the short-lived isomeric state; determining its astrophysical reaction rate across a range of stellar environments has been the focus of many studies. A reanalysis of previous work, utilizing recent improvements in excitation energies and ground state masses, is presented to reduce the ambiguities in the literature and provide focus to future measurements.

Capture and fission with DANCE and NEUANCE

A summary of the current and future experimental program at DANCE is presented. Measurements of neutron capture cross sections are planned for many actinide isotopes with the goal to reduce the present uncertainties in nuclear data libraries. Detailed studies of capture gamma rays in the neutron resonance region will be performed in order to derive correlated data on the de-excitation of the compound nucleus. New approaches on how to remove the DANCE detector response from experimental data and retain the correlations between the cascade gamma rays are presented. Studies on 235U are focused on quantifying the population of short-lived isomeric states in 236U after neutron capture. For this purpose, a new neutron detector array NEUANCE is under construction. It will be installed in the central cavity of the DANCE array and enable the highly efficient tagging of fission and capture events. In addition, developments of fission fragment detectors are also underway to expand DANCE capabilities to measurements of fully correlated data on fission observables.

Decay of201–203Raand200–202Fr

Decay properties of the neutron-deficient nuclides ${}^{201--203}\mathrm{Ra}$ and ${}^{200--202}\mathrm{Fr}$ were investigated using $\ensuremath{\alpha}$- and $\ensuremath{\gamma}$-decay spectroscopy. The nuclei were produced in fusion-evaporation reactions of ${}^{56}$Fe projectiles with enriched ${}^{147}$Sm and ${}^{149}$Sm targets at the velocity filter SHIP at GSI in Darmstadt (Germany). The $\ensuremath{\alpha}$ decay from the (3/2${}^{\ensuremath{-}}$) state in ${}^{201}$Ra was identified with an energy ${E}_{\ensuremath{\alpha}}=7842(12)$ keV and half-life ${T}_{1/2}={8}_{\ensuremath{-}4}^{+40}$ ms. Ambiguous decay properties for ${}^{202}$Ra from previous measurements were clarified by remeasuring with significantly improved precision, resulting in values of ${E}_{\ensuremath{\alpha}}=7722(7)$ keV and ${T}_{1/2}=3.{8}_{\ensuremath{-}0.8}^{+1.3}$ ms. New short-lived isomeric states were identified in ${}^{200}$Fr and ${}^{201}$Fr with half-lives of 0.6${}_{\ensuremath{-}0.2}^{+0.5}$ $\ensuremath{\mu}$s and 0.7${}_{\ensuremath{-}0.2}^{+0.5}$ $\ensuremath{\mu}$s, respectively. A tentative spin and parity of 13/2${}^{+}$ were assigned to the latter. One event attributed to $\ensuremath{\beta}$-delayed fission of ${}^{200}$Fr was observed.

Evidence for an isomeric 3/2- state in 53Co

The fragmentation of a 550MeV/u primary beam of 58Ni on a 9Be target has been used to measure time- and energy-correlated γ decays following the implantation of event-by-event discriminated secondary fragments into a 9Be stopper plate. A new isomeric γ decay with T 1/2 = 14( $ \begin{smallmatrix}6 4\end{smallmatrix}$ ) ns and E γ = 646.2(2) keV is observed and attributed to the decay of the yrast 3/2- state in 53 27Co26 . This short-lived isomeric state has been populated by means of nuclear reactions during the stopping process of the secondary fragments. The experimental findings are discussed in the framework of large-scale spherical shell model calculations in conjunction with isospin symmetry-breaking residual interactions for the A = 53 , T z = ±1/2 mirror nuclei 53Co and 53Fe .

γ-ray irradiation in the early Solar System and the conundrum of the 176Lu decay constant

When recent geological calibrations of the 176Lu decay constant are used, the 176Lu– 176Hf ages of chondrites are consistently 4% too old (∼4.75 Ga). Here, we suggest that this discrepancy reflects the photoexcitation of the long-lived 176Lu ground state to the short-lived isomeric state ( T 1/2 = 3.7 h) by γ-rays irradiating early condensates. Irradiation may have been of solar origin and taking place at the inner edge of the nebular disk. Alternatively, the source of γ-rays could have been one or more supernova(e) exploding in the vicinity of the solar nebula. Such photoexcitation has been experimentally observed, but requires γ-ray photons that have energies in excess of 838 keV. At this stage, we cannot assess whether the Hf isotope composition of the Bulk Silicate Earth differs from that of chondrites, eucrites, and the 4.56 Ga old Martian meteorite ALH84001, and therefore, whether the precursor material for these different planetary bodies received comparable fluences of γ-rays.

New short-lived isomers inY84

Low-lying short-lived isomeric states have been investigated in $^{84}\mathrm{Y}$ by the $^{84}\mathrm{Sr}$($p,n$) reaction at ${E}_{p}=13.5$ MeV. The g factors were measured by applying the time-differential perturbed angular distribution method in an external magnetic field. The properties determined for the new isomers are ${E}_{x}=112.4$ keV, ${I}^{\ensuremath{\pi}}=({4}^{+}),{T}_{1/2}=79(2)$ ns, $g=+0.578(7);{E}_{x}=210.4$ keV, ${I}^{\ensuremath{\pi}}=({4}^{\ensuremath{-}}),{T}_{1/2}=292(10)$ ns, $g=+0.234(6)$. On the basis of the measured g factor, the $\ensuremath{\pi}1{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}1{g}_{9/2}$ configuration has been assigned to the ${4}^{+}$ isomeric state. The low-lying states in $^{84}\mathrm{Y}$ were discussed within the two-quasiparticles plus rotor model. The ${4}^{\ensuremath{-}}$ isomeric state has been interpreted by the pure $\ensuremath{\pi}[301]3/2\ensuremath{\bigotimes}\ensuremath{\nu}[422]5/2$ configuration, which reproduces the experimental g factor at a prolate quadrupole deformation of ${\ensuremath{\varepsilon}}_{2}\ensuremath{\approx}0.16$.

Nuclear structure studies by means of short-lived isomers at intermediate energies

Short-lived (Tl,z about 0.1 to 100~s) isomeric states have been studied for over forty years. Among the methods used for their production thermal neutron induced fission played initially the most, important role (see for example [l]). However, the mass A and atomic number Z of excited fission fragments usually were not directly measured in these experiments limiting the possibility for a search of weakly produced isomers. Presently, the most important, source of spectroscopic information on the structure of short-lived isomers is in-beam spectroscopy applied to fusion-evaporation, deep-inelastic and fission reactions at several MeV per nucleon. Strongly decreasing production cross sections very far from the line of stability put severe constrains on the search for new isomers using in-beam spectroscopy methods. In many cases this difficulty can be overcome using fragmentation-like reactions at several tens or hundreds MeV per nucleon. The study of short-lived isomeric states at intermediate and relativistic energies combines an unambiguous and fast event-by-event identification of nuclei far from stability with

Newμs isomers inTz=1 nuclei produced in the112Sn(63AMeV)+natNi reaction

The short-lived isomeric states in the T{sub Z}=1 nuclei, {sup 94m}Pd [T{sub 1/2}=0.6(1) {mu}s] and {sup 96m}Ag [T{sub 1/2}=0.7(2) {mu}s], were identified among the fragmentation products of the {sup 112}Sn (63A MeV) + {sup nat}Ni (93.5 mg/cm{sup 2}) reaction at GANIL. The separation and identification of the reaction products was done by means of the Alpha and LISE3 magnetic spectrometers combined with time-of-flight, energy-loss, and total kinetic energy measurements. Evidence for isomeric states in {sup 80}Y, {sup 98}Cd, and {sup 102}Sn was also obtained. {copyright} {ital 1997} {ital The American Physical Society}

Identification of μs-isomers produced in the fragmentation of a 112Sn beam

Decays of over forty short-lived ( T 1 2 from ≈ 50 ns to 70 μs) isomeric states including a new isomer 66mAs produced in the fragmentation of a 112Sn beam (58 A·MeV, 63 A·MeV) on a natNi target were observed at the final focus of the LISE3 spectrometer at GANIL. Their detection, based on the slow (≈ 10 μs) time correlation of identified ions with the characteristic γ-radiation, represents a novel method to search for new isomers and can be used for unambiguous isotope identification for projectile fragment separator experiments. Isomeric yields and isomer-to-total production ratios were determined.

Identification of 100Sn and others proton drip-line nuclei in the reaction 112Sn(63 MeV/nucl.)+natNrmNi

Six new neutron-deficient nuclei 103 Sb, 104 Sb, 98 In, 91 Pd, 89 Rh and 87 Ru have been identified among the products of quasi-fragmentation of a 112 Sn beam (63 MeV/nucl.), in addition to the previously reported 100 Sn and 102 Sn. Decays of over forty short-lived ( T 1 2 from ≈50 ns to 70μ s ) isomeric states including new isomer 66 m As were observed in the same reaction.

Reinvestigation of the decay scheme of the short-lived isomeric states in112In

The two short-lived isomeric states in112In have been reinvestigated by means of the112Cd(d, 2n) and109Ag (α, n) reactions. Measurements of delayed X- andγ-ray, lifetime and TDPAD spectra have been performed. A 6.32(3) keVγ-ray has been identified in the isomeric decay and anM1 multipolarity has been assigned to it on the basis of the measured internal conversion coefficients:α L =196(39) andα tot=231(35). TheE1+M2(δ =+0.24(4)) andE2 multipolarities have been determined for the 263 and 188 keV transitions deexciting the 8−(T 1/2=2.82(2) μs) and 7+ (T 1/2=0.93(2) μs) isomeric states, respectively. A revised isomeric decay scheme is presented.

Formation of isomers in the reaction 238U(p,3n)236Np.

The energy dependence of the production cross sections in long-lived (l) and short-lived (s) isomeric states in $^{236}\mathrm{Np}$ together with the isomeric ratio of the reaction $^{238}\mathrm{U}$(p,3n${)}^{236}$Np near the reaction threshold was measured. The yield of $^{236}\mathrm{Np}^{\mathrm{s}}$ was determined from the alpha activity of $^{236}\mathrm{Pu}$ and the yield of $^{236}\mathrm{Np}^{\mathrm{l}}$ from analysis by thermal neutron fission. From the energy dependence of the isomeric ratio it follows that the long-lived state is the ground state in $^{236}\mathrm{Np}$. Theoretical calculations of the yields support these results.

Efficiency and delay of the fission ion guide for on-line mass separation

Efficiency and time distribution of ions passing through an ion guide system, specially designed for on-line isotope separation of products of proton-induced fission, have been studied. This system, unlike the standard ion guide, consists of a separate, plasma-free thermalizing chamber for stopping the isotropically emitted fission fragments. This arrangement reduces the rate of the recombination losses of thermalized ions by the plasma electrons produced by the beam and results in very good and stable operating conditions. The total efficiency of the order of 10 −3 has been determined, nearly independent of the atomic number of the separated nuclide. The main reason for the limited efficiency is the inadequate stopping of the fragments in helium. The delay time distribution of the separated ions is measured to be broad, ranging from about 0.1 ms to 10 ms, the lower limit being set by the flight time through the separator. This has made possible the spectroscopy of short-lived beta-decaying nuclei and the discovery of new short-lived isomeric states, with ms half-lives, populated directly in fission.

Nuclear photoactivation cross sections for short-lived isomeric states excited with a 6 MeV linac

Photoactivation cross sections for short-lived (T 1 2 < 20 s) isomeric states in 167Er, 79Br, 191Ir, 183W, 197Au, 77Se, 137Ba and 89Y were determined using bremsstrahlung from a 6 MeV medical therapy linac. A portable NaI(Tl) spectrometer and pneumatic transfer system was designed to avoid impacting normal use of the linac. Comparison with data obtained at the PITHON flash X-ray source (1.5 MeV endpoint energy) indicates that cross sections for populating these isomers through gateway states in the range 1.5–6 MeV are more than three orders of magnitude greater than corresponding cross sections below 1.5 MeV. The results are significant for the development of photoactivation techniques to measure short (<100 ns), nonreproducible bremsstrahlung pulses.