Search

The nucleus ${}^{208}$Pb provides an iconic test of the nuclear shell model because its 82 protons and 126 neutrons are both magic numbers. When excited levels are discovered and characterized, this nucleus provides a means to investigate the mechanisms responsible for the generation of angular momentum and, ultimately, for the development of collectivity. This experiment is a tour de force that reveals extensive new information on ${}^{208}$Pb about high-angular-momentum states, isomeric states, and collective states of octupole nature; thus it serves as a showcase for shell-model descriptions. Many of the states explored show good overall agreement with theory, but others at the highest energies reveal discrepancies, pointing to the need for improved calculations. Above the highest-lying ${28}^{-}$ isomer $\ensuremath{\gamma}$ transitions arising from yet higher spin levels could be identified in two separate experiments as displayed in the figure.

$\ensuremath{\beta}$ decay of neutron-rich nuclide $^{94}\mathrm{Kr}$ was reinvestigated by means of a high resolution on-line mass separator and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ spectroscopy. In total 22 $\ensuremath{\gamma}$-ray transitions were assigned to the decay of $^{94}\mathrm{Kr}$, and a new isomeric state was identified. The new information allows us to build detailed levels systematics in a chain of odd-odd rubidium isotopes and draw conclusions on nuclear structure for some of the observed states. The discussed level structure affects the evolution of $\ensuremath{\beta}$-decay half-lives for neutron-rich selenium, krypton, and strontium isotopes.

Excited states with seniority $\ensuremath{\nu}=3$, 5, and 7 have been investigated in odd neutron-rich $^{119,121,123,125}\text{Sn}$ isotopes produced by fusion-fission of 6.9-MeV/$A\phantom{\rule{4pt}{0ex}}^{48}\text{Ca}$ beams with $^{208}\text{Pb}$ and $^{238}\text{U}$ targets and by fission of a $^{238}\text{U}$ target bombarded with 6.7-MeV/$A\phantom{\rule{4pt}{0ex}}^{64}\text{Ni}$ beams. Level schemes have been established up to high spin and excitation energies in excess of 6 MeV, based on multifold gamma-ray coincidence relationships measured with the Gammasphere array. In the analysis, the presence of isomers was exploited to identify gamma rays and propose transition placements using prompt and delayed coincidence techniques. Gamma decays of the known $27/{2}^{\ensuremath{-}}$ isomers were expanded by identifying new deexcitation paths feeding $23/{2}^{+}$ long-lived states and $21/{2}^{+}$ levels. Competing branches in the decay of $23/{2}^{\ensuremath{-}}$ states toward two $19/{2}^{\ensuremath{-}}$ levels were delineated as well. In $^{119}\text{Sn}$, a new $23/{2}^{+}$ isomer was identified, while a similar $23/{2}^{+}$ long-lived state, proposed earlier in $^{121}\text{Sn}$, has now been confirmed. In both cases, isomeric half-lives were determined with good precision. In the range of $\ensuremath{\nu}=3$ excitations, the observed transitions linking the various states enabled one to propose with confidence spin-parity assignments for all the observed states. Above the $27/{2}^{\ensuremath{-}}$ isomers, an elaborate structure of negative-parity levels was established reaching the ($39/{2}^{\ensuremath{-}}$), $\ensuremath{\nu}=7$ states, with tentative spin-parity assignments based on the observed deexcitation paths as well as on general yrast population arguments. In all the isotopes under investigation, strongly populated sequences of positive-parity ($35/{2}^{+}$), ($31/{2}^{+}$), and ($27/{2}^{+}$) states were established, feeding the $23/{2}^{+}$ isomers via cascades of three transitions. In the $^{121,123}\text{Sn}$ isotopes, these sequences also enabled the delineation of higher-lying levels, up to ($43/{2}^{+}$) states. In $^{123}\text{Sn}$, a short half-life was determined for the ($35/{2}^{+}$) state. Shell-model calculations were carried out for all the odd Sn isotopes, from $^{129}\text{Sn}$ down to $^{119}\text{Sn}$, and the results were found to reproduce the experimental level energies rather well. Nevertheless, some systematic deviations between calculated and experimental energies, especially for positive-parity states, point to the need to improve some of the two-body interactions used in calculations. The computed wave-function amplitudes provide for a fairly transparent interpretation of the observed level structures. The systematics of level energies over the broad $A$ = 117--129 range of Sn isotopes displays a smooth decrease with mass $A$, and the observed regularity confirms most of the proposed spin-parity assignments. The systematics of the $B(E2)$ reduced transition probabilities extracted for the $23/{2}^{+}$ and $19/{2}^{+}$ isomers is discussed with an emphasis on the close similarity of the observed $A$ dependence with that of the $E2$ transition rates established for other $\ensuremath{\nu}=2$, 3, and 4 isomers in the Sn isotopic chain.

Excited states above the seniority $\ensuremath{\nu}=2$ isomers have been investigated in even neutron-rich ${}^{118--128}$Sn isotopes produced by fusion-fission of 6.9 MeV/$A$ $^{48}\mathrm{Ca}$ beams with $^{208}\mathrm{Pb}$ and $^{238}\mathrm{U}$ targets and by fission of 6.7 MeV/$A$ $^{64}\mathrm{Ni}$ beams on a $^{238}\mathrm{U}$ target. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold $\ensuremath{\gamma}$-ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques, including prompt and delayed cross-coincidence methods. As a result, seniority $\ensuremath{\nu}=4$, 15${}^{\ensuremath{-}}$, and 13${}^{\ensuremath{-}}$ isomers were observed and their half-lives determined. These long-lived states in turn served as steppingstones to delineate the isomeric decays and to locate higher-lying states with good sensitivity. As the observed isomeric decays feed down to 10${}^{+}$ and 7${}^{\ensuremath{-}}$ isomers, firm spin-parity assignments could be proposed for most of the seniority $\ensuremath{\nu}=4$ states. Higher-lying, seniority $\ensuremath{\nu}=6$ levels were assigned tentatively on the basis of the observed deexcitation paths as well as of general yrast population arguments. Shell-model calculations were carried out down to $^{122}\mathrm{Sn}$ in the ${g}_{7/2}$, ${d}_{5/2}$, ${d}_{3/2}$, ${s}_{1/2}$, and ${h}_{11/2}$ model space of neutron holes with respect to a $^{132}\mathrm{Sn}$ core. Effective two-body interactions were adjusted such that satisfactory agreement with data was achieved for $^{130}\mathrm{Sn}$. The results reproduce the experimental level energies and spin-parity assignments rather well. The intrinsic structure of the states is discussed on the basis of the calculated wave functions which, in many instances, point to complex configurations. In a few cases, the proposed assignments lead to unresolved issues. The smooth, systematic decrease of the level energies with mass $A$ is accompanied by the similarly regular behavior with $A$ of the reduced transition probabilities extracted from the isomeric half-lives. This $A$ dependence is discussed for the $E1$ and $E2$ transitions in the decay of the seniority $\ensuremath{\nu}=4$ isomers and is compared to that determined in earlier work for the $E2$ transition rates from the $\ensuremath{\nu}=2,3$ isomers.

Neutron-rich sdf shell nuclei have been produced in deep-inelastic processes during collision of 230 MeV {sup 37}Cl ions on a thick {sup 208}Pb target. The in-beam as well as off-beam {gamma}-{gamma} coincidence data yielded new spectroscopic information on some hard-to-access N=18,19 isotones. A new 27 ns isomer at 5583 keV in {sup 32}Si has been located. A candidate for the negative parity state 4{sup {minus}} has been identified above the 200 ns isomer in {sup 32}Al. {copyright} {ital 1997} {ital The American Physical Society}

The low-spin level scheme of ${}^{68}$Ni was investigated with the Gammasphere array following reactions between a ${}^{70}$Zn beam and ${}^{238}$U, ${}^{208}$Pb, and ${}^{197}$Au targets. Spin assignments for some states have been verified through $\ensuremath{\gamma}$-ray angular correlations, including the ${0}^{+}$ assignment for the 2511-keV level. Two previously unknown states at 3302 and 3405 keV have been identified. No evidence was found for a recently reported 216-ns, ${0}^{+}$ isomer at 2202 keV that was attributed to a proton two-particle, two-hole intruder configuration, despite experimental conditions similar to those used in the measurement reporting its discovery.

Excited states in ${}^{67}$Ni were populated in deep-inelastic reactions of a ${}^{64}$Ni beam at 430 MeV on a thick ${}^{238}$U target. A level scheme built on the previously known 13-$\ensuremath{\mu}$s isomer has been delineated up to an excitation energy of 5.3 MeV and a tentative spin and parity of (21/2${}^{\ensuremath{-}}$). Shell model calculations have been carried out using two effective interactions in the ${f}_{5/2}p{g}_{9/2}$ model space with a ${}^{56}$Ni core. Satisfactory agreement between experiment and theory is achieved for the measured transition energies and branching ratios. The calculations indicate that the yrast states are associated with rather complex configurations, herewith demonstrating the relative weakness of the $N=40$ subshell gap and the importance of multi-particle-hole excitations involving the ${g}_{9/2}$ neutron orbital.

The $\ensuremath{\beta}$ decay of the $N=51$ nucleus $^{81}\mathrm{Zn}$ was studied by means of $\ensuremath{\beta}$-$\ensuremath{\gamma}$ spectroscopy and from isotopically pure beams produced at the Holifield Radioactive Ion Beam Facility (HRIBF). We observe several competing $\ensuremath{\beta}$ transitions populating $^{81}\mathrm{Ga}$ that are interpreted as allowed Gamow-Teller decays to positive parity, core excited states, and first-forbidden decays to negative parity states. The measured $\ensuremath{\beta}$-decay pattern suggests an assignment of ${I}^{\ensuremath{\pi}}=5/{2}^{+}$ for the $^{81}\mathrm{Zn}$ ground state. The systematics of core excited states in $N=50$ isotones indicate a strengthening of the $N=40$ subshell gap near $^{78}\mathrm{Ni}$.

Excited states in the N=30 neutron-rich isotope {sup 49}K have been studied using multinucleon transfer reactions with thin targets and the PRISMA-CLARA spectrometer combined with thick-target {gamma}-coincidence data from Gammasphere. The d{sub 3/2} proton-hole state is located 92 keV above the s{sub 1/2} ground state, and the proton-particle f{sub 7/2} state is suggested at 2104 keV. Three other levels are established as involving the coupling to 2{sup +} of two neutrons above the N=28 shell. The measured or estimated lifetimes served to reinforce the interpretation of the observed level structure, which is found to be in satisfactory agreement with shell-model calculations.

Lifetimes of low-lying excited states of the neutron-rich $^{44,46}\mathrm{Ar}$ nuclei, populated via multinucleon transfer reactions, are measured by means of the differential recoil distance Doppler shift method. The extracted electromagnetic transition probabilities are compared with previous intermediate-energy Coulomb-excitation measurements and with large-scale shell-model calculations. The increase in the deduced $B$($E2;{2}^{+}\ensuremath{\rightarrow}{0}^{+}$) transition probability from $^{44}\mathrm{Ar}$ to the closed-shell nucleus $^{46}\mathrm{Ar}$ contradicts the earlier results of Coulomb-excitation experiments. Shell-model calculations using different effective interactions agree with the new measured values.

The yrast structure of the neutron-rich nucleus $^{97}\mathrm{Zr}$ has been studied using fission of targetlike products in the reaction of a $^{48}\mathrm{Ca}$ beam on a thick $^{238}\mathrm{U}$ target. The level scheme, known from previous studies up to an excitation energy and spin-parity of approximately 4619 keV and $23/{2}^{\ensuremath{-}}$, has been extended by about 3 MeV and a few units of angular momentum. Two states fed in the $\ensuremath{\beta}$ decay of the $27/{2}^{\ensuremath{-}}$ high-spin isomer in $^{97}\mathrm{Y}$ were identified at 5570 and 5606 keV. The located level structure can be discussed in terms of shell-model configurations.

Production of nuclei above $^{100}\mathrm{Sn}$ in fusion-evaporation reactions between $^{58}\mathrm{Ni}$ and $^{54}\mathrm{Fe}$ ions was studied at Oak Ridge National Laboratory by means of the recoil mass spectrometer and charged particle detection. The beam energy was varied to optimize the yields for the two-, three- and four-particle evaporation channels. Experimental results verified the predictions of the statistical model code HIVAP. The optimum energy for the $^{54}\mathrm{Fe}$($^{58}\mathrm{Ni}$,$4n$)$^{108}\mathrm{Xe}$ reaction channel that allows one to study the $^{108}\mathrm{Xe}$-$^{104}\mathrm{Te}$-$^{100}\mathrm{Sn}$ \ensuremath{\alpha} decay chain is deduced as 240 MeV.

The structure of the $^{56,58,60}\mathrm{Cr}$ nuclei was investigated at Gammasphere using both deep inelastic and fusion-evaporation reactions. As a result of this work, expanded level schemes are now available for these three neutron-rich, even-even isotopes. Experimental signatures were found for both single-particle and collective excitations whose relative importance varies with neutron number. Together with recent structural information on the odd-$A$ $^{55,57,59}\mathrm{Cr}$ neighbors, the present data highlight the changing role of the ${g}_{9/2}$ orbital with neutron number. Comparisons with shell-model calculations using the recently developed GXPF1A interaction and with total Routhian surface calculations are presented.

High-spin excited states have been identified in the N=Z+1 nucleus {sup 91}Rh with the reaction {sup 54}Fe({sup 40}Ca,p2n{gamma}) at 130 MeV, using the GASP array, the ISIS silicon array, and the n-ring detector system. Two structures have been observed, the positive-parity yrast sequence above the (9/2{sup +}) ground state and a sequence assigned to negative parity that is built on a possible (1/2{sup -}) isomeric state. The observed structures are compared with various shell-model calculations in the (p{sub 1/2},g{sub 9/2}) space.

The gamma-ray decay of the giant dipole resonance (GDR) in the compound nucleus {sup 216}Rn formed with the reaction {sup 18}O+{sup 198}Pt at the bombarding energy of 96 MeV was investigated. High-energy gamma-ray spectra in coincidence with both prompt and delayed low-energy transitions were measured. The obtained GDR width at the average temperature {approx_equal}1 MeV was found to be larger than that at T=0 MeV and to be approximately constant as a function of spin. The measured width value of 7 MeV is found to be consistent with the predictions based on calculations of the nuclear shape distribution using the newest approach for the treatment of the fission barrier within the liquid drop model. The present study is the first investigation of the giant dipole resonance width from the fusion-evaporation decay channel in this nuclear mass range.

Gamma rays from neutron-rich nuclei in the vicinity of $Z=20$, $N=28$ have been studied at Gammasphere using deep-inelastic reactions induced by a $330\text{\ensuremath{-}}\mathrm{MeV}$ $^{48}\mathrm{Ca}$ beam on a thick $^{238}\mathrm{U}$ target. The yrast $\ensuremath{\gamma}$-ray cascade of $^{56}\mathrm{Ti}$ was identified for the first time and the location in energy of the ${2}^{+}$, ${4}^{+}$, and ${6}^{+}$ states was determined. The low-spin $^{56}\mathrm{Ti}$ yrast structure does not support the presence of a subshell closure at $N=34$ as suggested on the basis of other data on nuclei in the region as well as shell model calculations with a recently proposed interaction.

High-spin excited states have been identified for the first time in the $N=Z+1$ nucleus $^{89}\mathrm{Ru}$ with the reaction $^{54}\mathrm{Fe}(^{40}\mathrm{Ca},\ensuremath{\alpha}n\ensuremath{\gamma})$ at 130 MeV, using the GASP array, the ISIS Silicon array, and the $n$-Ring detector system. The observed structure is discussed within systematics of the $N=45$ isotones and is compared with shell model calculations.

The yrast band of the $N=Z$ nucleus ${}^{84}\mathrm{Mo}$ has been extended up to the ${10}^{+}$ state. Its moment of inertia varies smoothly up to this state (rotational frequency 0.6 MeV). The new data confirm the systematic delay of the particle alignment frequency in $N=Z$ nuclei with respect to the neighboring $NgZ$ nuclei, which has been suggested as a signature of the neutron-proton pairing interaction. Projected shell model calculations performed for the heaviest $N=Z$ nuclei studied so far, ${}^{84}\mathrm{Mo}$ and ${}^{88}\mathrm{Ru},$ predict that the confirmation of a possible enhancement of the neutron-proton residual interaction requires the observation of still higher spins in these nuclei.