Decay Of Isomeric States Research Articles

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.

Study of shape transition in the neutron-rich Os isotopes

The neutron-rich isotopes of tungsten, osmium and platinum have different shapes in their ground states and present also shape transitions phenomena. Spectroscopic information for these nuclei is scarce and often limited to the gamma rays from the decay of isomeric states. For the neutron-rich even-even osmium isotopes 194 Os and 198 Os, a shape transition between a slightly prolate deformed to an oblate deformed ground state was deduced from the observed level schemes. For the even-even nucleus lying in between, 196 Os, no gamma ray transition is known. In order to elucidate the shape transition and to test the nuclear models describing it, this region was investigated through gamma-ray spectroscopy using the AGATA demonstrator and the large acceptance heavy-ion spectrometer PRISMA at LNL, Italy. A two-nucleon transfer from a 198 Pt target to a stable 82 Se beam was utilized to populate medium-high spin states of 196 Os. The analysis method and preliminary results, including the first life-time measurement of isomeric states with AGATA, are presented.

The population of metastable states as a probe of relativistic-energy fragmentation reactions

Isomeric ratios have been measured for high-spin states in Po84198,200,206,208, At85208,209,210,211, Rn86210,211,212,213,214, Fr87208,211,212,213,214, Ra88210,211,212,214,215, and Ac89215 following the projectile fragmentation of a 1 A GeV 238U beam by a 9Be target at GSI Helmholtzzentrum für Schwerionenforschung. The fragments were separated in the fragment separator (FRS) and identified by means of energy loss and time-of-flight techniques. They were brought to rest at the centre of the RISING gamma-ray detector array and intensities of gamma rays emitted in the decay of isomeric states with half-lives between 100 ns and 40 μs and spin values up to 55/2 ℏ were used to obtain the corresponding isomeric ratios. The data are compared to theoretical isomeric ratios calculated in the framework of the abrasion–ablation model. Large experimental enhancements are obtained for high-spin isomers in comparison to expected values.

Structure of80Ge revealed by theβdecay of isomeric states in80Ga: Triaxiality in the vicinity of78Ni

The decays of two long-lived low-lying isomeric states of ${}^{80}$Ga were studied at the PARRNe mass separator of the ALTO ISOL facility. Over the 75 $\ensuremath{\gamma}$ rays previously attributed to the ${}^{80}$Ga decay, the decay time of 67 individual $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ activities were measured. This allowed the determination of the decay time of these two recently reported long-lived---actually $\ensuremath{\beta}$-decaying---states as well as to partially disentangle the two decay schemes. Thanks to the relatively high spin difference between these two ${}^{80}$Ga isomers spin assignments of the daughter ${}^{80}$Ge states could be further constrained rendering the comparison with calculations easier. From this comparison it appears that the suspected maximum of collectivity at $Z=32$ along the $N=50$ line should express itself through the coexistence of spherical and collective $\ensuremath{\gamma}$-soft structures.

Isomeric states in 253No and 253Fm

Nuclear structure and decay of isomeric states in 253No were investigated. The isotope was produced by the reaction 48Ca + 207Pb . The excitation energy of the known single-particle isomeric state ( 5/2+[622] was measured by delayed coincidences between γ-rays and implanted evaporation residues and was placed into the level scheme. In addition, decay of a high-lying multi-quasiparticle isomer in 253No was studied using e −-γ coincidence measurements. A rotational band populated by its de-excitation was identified in 253No . A new isomeric state in 253Fm was observed and a partial-level scheme for this isotope is suggested. In addition γ transitions from excited levels are reported for 253Md .

19/2+ isomer of 125Cd and realistic shell-model calculations of neutron-rich Cd isotopes

The decay of a T1/2 = 19(3) μs isomeric state from 125Cd has been observed using γ-ray spectroscopy at the Lohengrin mass spectrometer of the Institut Laue-Langevin, Grenoble. Two coincident γ rays were observed to be emitted from the decay of this isomeric state, contradicting recently published data on this nucleus. Realistic shell-model calculations have been performed to interpret the decay scheme, allowing a spin and parity of 19/2+ to be assigned to the isomeric state. All configurations contributing to this isomeric state have amplitudes less than 7 %. Experimental data have recently been published on the decays of μs isomeric states in 127,128,130Cd. Realistic shell-model calculations for these nuclei are presented which reproduce the known experimental decay schemes reasonably well.

High-spin, multiparticle isomers inSb121,123

Isomers in near-spherical $Z=51$, antimony isotopes are reported here for the first time using fusion-fission reactions between $^{27}\mathrm{Al}$ and a pulsed $^{178}\mathrm{Hf}$ beam of energy, 1150 MeV. \ensuremath{\gamma} rays were observed from the decay of isomeric states with half-lives, ${T}_{1/2}=200(30)$ and $52(3)\ensuremath{\mu}$s, and angular momenta $I=(\frac{25}{2})$ and ${I}^{\ensuremath{\pi}}={\frac{23}{2}}^{+}$, in $^{121,123}\mathrm{Sb}$, respectively. These states are proposed to correspond to $\ensuremath{\nu}({h}_{\frac{11}{2}}){}^{2}$ configurations, coupled to an odd ${d}_{\frac{5}{2}}$ or ${g}_{\frac{7}{2}}$ proton. Nanosecond isomers were also identified at ${I}^{\ensuremath{\pi}}={\frac{19}{2}}^{\ensuremath{-}}$ [${T}_{1/2}=8.5(5)$ ns] in $^{121}\mathrm{Sb}$ and ${I}^{\ensuremath{\pi}}=({\frac{15}{2}}^{\ensuremath{-}})$ [${T}_{1/2}=37(4)$ ns] in $^{123}\mathrm{Sb}$. Information on spins and parities of states in these nuclei was obtained using a combination of angular correlation and intensity-balance measurements. The configurations of states in these nuclei are discussed using a combination of spin/energy systematics and shell-model calculations for neighboring tin isotones and antimony isotopes.

Isomeric decay studies around 204Pt and 148Tb

Relativistic energy projectile fragmentation of Pb-208 has been used to produce a range of exotic nuclei. The nuclei of interest were studied by detecting delayed gamma rays following the decay of isomeric states. Experimental information on the excited states of the neutron-rich N = 126 nucleus, Pt-204, following internal decay of two isomeric states, was obtained for the first time. In addition, decays from the previously reported isomeric I=27h and I=(49/2)h states in Tb-148 and Gd-147, respectively, have been observed. These isomeric decays represent the highest spin discrete states observed to date following a projectile fragmentation reaction, and opens further the possibility of doing 'high-spin physics' using this technique.

Measurement of multiplicity and total energy of gamma radiation accompanying fission of235U by resonance neutrons

It is well known that nuclear fission is accomplished by the emission of neutrons and y-rays. Spectroscopic studies of this radiation yield information about the energy distribution in the fission process, including the energy of excitation of the fragments. The y-rays that are emitted at the stage of formation of the compound system and immediately at the moment of dispersion of the fragments as well as from nuclei of fragments with excess neutrons after evaporation of the last neutron over a time T<I0-' sec are customarily regarded as prompt. The decay of isomeric states of the fragments over a time T~ (I0-9-~ i0 -3) sec leads to emission of y-rays, called delayed y-rays [i]. In the integrated form the prompt and delayed y-rays are characterized by the total energy Ey, the total number of y rays, i.e., the multiplicity Ny referred to one fission act, and the average energy per emitted y ray e = Ey/ Ny. These integral characteristics are of physical interest as parameters of the fission process. Accurate values of these parameters are also required to compute the optimal parameters of nuclear installations.

The [formula omitted] triplets in 190–196Hg

Decays of isomeric states in the even 190–196Hg isotopes are studied by in-beam conversion electron spectroscopy using the 181Ta( 14N, 5n) and 192, 194, 196Pt(α, 4n) reactions. A superconducting solenoid was used to transport conversion electrons emitted by the recoiling excited nuclei to cooled Si(Li) detectors and singles e − as well as e −e − coincidence spectra were measured. The missing 12 + → 10 + transitions in 190, 192, 194Hg were observed with transition energies of 24 keV, 28 keV and 52 keV. The B(E2) rates of the 12 + → 10 + and the 10 + → 8 + transitions are discussed.

In-beam γ-ray studies of complex band structures and of isomeric states in 152, 153Dy nuclei

The high-spin level structures of 152Dy and 153Dy were studied experimentally with 154, 155Gd(α xnγ) in-beam reactions, and for 152Dy also with 144, 146 Nd( 12C, x nγ) reactions. The experiments included measurements of singles γ-ray and conversion-electron spectra, γ-ray angular distributions and E γ - t and E γ - E γ - t coincidences. A multiplicity filter set-up was used to study the feeding and decay of isomeric states in 152Dy. In 152Dy about twenty so far unknown levels were found, including two high-spin isomeric states with T 1 2 ≈ 60 and ≈ 13 ns at excitation energies E x ≈ 5.04 and 6.08 MeV, respectively. These states are compared with recent calculations on yrast traps. The level scheme of 153Dy contains 28 levels up to E x = 4.1 MeV and J π = ( 37 2 +) . Band structures in both nuclei are discussed in comparison with other N = 86 and N = 87 isotones.

Possible evidence for microsecond shape isomerism in Tl isotopes

The decay of isomeric states in 201mTl, 202mTl and 204mTl is investigated with a Ge(Li) detector. The isomeric states are produced with LINAC bremsstrahlung (24–32 MeV). Life-times and precise energies have been determined. In the 201mTl case, shape isomerism is proposed, based on calculations of the total potential energy surfaces and calculations of the lifetime in a Nilsson model.

Isomeric States inFm250andNo254

A preliminary report on the discovery of isomeric states in $^{250}\mathrm{Fm}$ and $^{254}\mathrm{No}$ was included in a recent article on $\ensuremath{\alpha}$-emitting isotopes of element 104. The existence and assignments of the 1.8 \ifmmode\pm\else\textpm\fi{} 0.1-sec isomer to $^{250}\mathrm{Fm}$ and the 0.28 \ifmmode\pm\else\textpm\fi{} 0.04-sec isomer to $^{254}\mathrm{No}$ have now been confirmed by cross-bombardment techniques. Isomeric ratios based on measurements of collection efficiency of recoil atoms from the decay of isomeric states are given. An interpretation of the even-even isomers as high-spin two-quasiparticle states is discussed.