Ray Excitation Functions Research Articles

Lifetimes in Te124 : Examining critical-point symmetry in the Te nuclei

The Doppler-shift attenuation method following inelastic neutron scattering was used to determine the lifetimes of nuclear levels to 3.3-MeV excitation in $\phantom{\rule{0.16em}{0ex}}^{124}\mathrm{Te}$. Level energies and spins, $\ensuremath{\gamma}$-ray energies and branching ratios, and multipole-mixing ratios were deduced from measured $\ensuremath{\gamma}$-ray angular distributions at incident neutron energies of 2.40 and 3.30 MeV, $\ensuremath{\gamma}$-ray excitation functions, and $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence measurements. The newly obtained reduced transition probabilities and level energies for $\phantom{\rule{0.16em}{0ex}}^{124}\mathrm{Te}$ were compared to critical-point symmetry model predictions. The $E(5)$ and ${\ensuremath{\beta}}^{4}$ potential critical-point symmetries were also investigated in $\phantom{\rule{0.16em}{0ex}}^{122}\mathrm{Te}$ and $\phantom{\rule{0.16em}{0ex}}^{126}\mathrm{Te}$.

New transitions and feeding of theJπ=(8+)isomer inRe186

The spallation neutron source at the Los Alamos Neutron Science Center Weapons Neutron Research facility was used to populate excited states in $^{186}\mathrm{Re}$ via $(n,2n\ensuremath{\gamma})$ reactions on an enriched $^{187}\mathrm{Re}$ target. Gamma rays were detected with the GErmanium Array for Neutron Induced Excitations spectrometer, a Compton-suppressed array of 18 HPGe detectors. Incident neutron energies were determined by the time-of-flight technique and used to obtain $\ensuremath{\gamma}$-ray excitation functions for the purpose of identifying $\ensuremath{\gamma}$ rays by reaction channel. Analysis of the singles $\ensuremath{\gamma}$-ray spectrum gated on the neutron energy range $10\ensuremath{\le}{E}_{n}\ensuremath{\le}25\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ resulted in five transitions and one level added to the $^{186}\mathrm{Re}$ level scheme. The additions include the placement of three $\ensuremath{\gamma}$ rays at 266.7, 381.2, and 647.7 keV which have been identified as feeding the $2.0\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\mathrm{yr}$, ${J}^{\ensuremath{\pi}}=({8}^{+})$ isomer and yield an improved value of $148.2(5)\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ for the isomer energy. These transitions may have astrophysical implications related to the use of the Re-Os cosmochronometer.

Status of vibrational structure inNi62

Measurements consisting of $\ensuremath{\gamma}$-ray excitation functions and angular distributions were performed using the ($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction on $^{62}\mathrm{Ni}$. The excitation function data allowed us to check the consistency of the placement of transitions in the level scheme. From $\ensuremath{\gamma}$-ray angular distributions, the lifetimes of levels up to $~3.8$ MeV in excitation energy were extracted with the Doppler-shift attenuation method. The experimentally deduced values of reduced transition probabilities were compared with the predictions of the quadrupole vibrator model and with large-scale shell model calculations in the $\mathit{fp}$ shell configuration space. Two-phonon states were found to exist with some notable deviation from the predictions of the quadrupole vibrator model, but no evidence for the existence of three-phonon states could be established. $Z=28$ proton core excitations played a major role in understanding the observed structure.

Fragmentation of mixed-symmetry excitations in stable even-even tellurium nuclei

The lowest six excited ${2}^{+}$ levels of the even-even $^{122\ensuremath{-}130}\mathrm{Te}$ nuclei have been investigated using \ensuremath{\gamma}-ray spectroscopy following inelastic neutron scattering. These levels have been identified and their decay properties have been characterized from \ensuremath{\gamma}-ray excitation functions and \ensuremath{\gamma}-ray angular distributions; additionally, lifetimes of these levels have been deduced using the Doppler-shift attenuation method. Electromagnetic transition rates and $E2/M1$ multipole mixing ratios from the ${2}_{x}^{+}[x=2\text{\ensuremath{-}}6]\ensuremath{\rightarrow}{2}_{1}^{+}$ transitions have been examined to identify the lowest mixed-symmetry states in these nuclei. In each nucleus, the mixed-symmetry strength appears to be fragmented between more than one level. The summed $M1$ strength from the ${2}_{x}^{+}$ [$x=2\text{\ensuremath{-}}6$] states to the ${2}_{1}^{+}$ level agrees rather well with neutron-proton interacting boson model predictions in the U(5) or O(6) limits for these Te nuclei.

New levels and a lifetime measurement inTl204

The {sup 205}Tl(n,2n{gamma}) reaction was used to populate excited states in {sup 204}Tl. The {gamma}-ray detection was accomplished with the GEANIE spectrometer, a Compton suppressed array of 26 Ge detectors. An energetic beam of neutrons was provided by the pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility. The time-of-flight technique was used to determine the incident neutron energies. {gamma}-ray excitation functions were determined from incident neutron energy of 1 MeV up to E{sub n}=25 MeV. The level scheme of {sup 204}Tl was enriched and the partial level scheme and nuclear structure above the previously known 7{sup +} isomer at 1104-keV excitation energy were established for the first time up to E{sub x}{approx}2.3 MeV. The high-spin part of the level scheme exhibits striking similarities to that of the neighboring {sup 202}Tl isotope, suggesting similarities in the underlying nuclear structure. The half-life of the 7{sup +} isomer was measured with a more precise result (T{sub 1/2}=60.7{+-}1.2 {mu}s), in agreement with literature values. A lower limit for the excitation energy of the {pi}h{sub 11/2}{nu}i{sub 13/2} structure with J{sup {pi}}=12{sup -} is proposed.

New levels and a lifetime measurement inTl202

The $^{203}\mathrm{Tl}$($n,2n\ensuremath{\gamma}$) reaction was used to study excited states in $^{202}\mathrm{Tl}$. The data were taken using the GEANIE spectrometer. The pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided neutrons in the energy range from 0.6 to 250 MeV. The time-of-flight technique was used to determine the incident neutron energies. \ensuremath{\gamma}-ray excitation functions were measured from the beam-on data, whereas half-lives of isomers were determined from the beam-off data. The level scheme of $^{202}\mathrm{Tl}$ has been considerably enriched and the low-spin part of the level scheme exhibits striking similarities to that of the neighboring $^{204}\mathrm{Tl}$ isotope. The previously reported first excited state in $^{202}\mathrm{Tl}$ has been decomposed into two close-lying states in the present measurement. The half-life of the ${7}^{+}$ isomer at 950-keV excitation energy was measured with a more precise result that differs by \ensuremath{\sim}4% from the value adopted in the literature.

States inAu197from the (n,n'γ) reaction

The level structure of $^{197}\mathrm{Au}$ has been studied using the ($n,{n}^{'}\ensuremath{\gamma}$) reaction. A germanium detector array (GEANIE) for \ensuremath{\gamma}-ray detection and the ``white'' neutron source at LANSCE/WNR were used for the measurement. The energy of the incident neutrons was determined using the time-of-flight technique. Partial \ensuremath{\gamma}-ray cross sections were measured for a total of 90 transitions of $^{197}\mathrm{Au}$. A total of 52 new \ensuremath{\gamma} rays were assigned to $^{197}\mathrm{Au}$ and placed on the level scheme based on the partial \ensuremath{\gamma}-ray excitation functions and the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence data. The level scheme of $^{197}\mathrm{Au}$ is now more complete up to ~2 MeV excitation energy. The first excited states of the decoupled band built on the proton-hole ${h}_{11/2}$ configuration were established.

Intruder structures observed inTe122through inelastic neutron scattering

The excited levels of $^{122}\mathrm{Te}$ to $3.3\ensuremath{-}\phantom{\rule{0.3em}{0ex}}\text{MeV}$ excitation have been studied using $\ensuremath{\gamma}$-ray spectroscopy following inelastic neutron scattering. The decay characteristics of these levels have been determined from $\ensuremath{\gamma}$-ray excitation functions, angular distributions at ${E}_{n}\ensuremath{-}1.72,2.80$, and $3.35\phantom{\rule{0.3em}{0ex}}\text{MeV}$, Doppler shifts, and $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences. Electromagnetic transition rates were deduced for many levels, as were multipole-mixing and branching ratios. Level energies and electromagnetic transition rates were compared to interacting boson model (IBM) calculations, both with and without intruder-state mixing, and to particle-core coupling model calculations. The energies of low-lying levels of $^{122}\mathrm{Te}$ are well described by the IBM with intruder-state mixing calculations, and observed transition rates support emerging intruder bands built on ${0}^{+}$ levels. The other models considered do not produce enough low-lying positive parity states; however, U(5) energies to the four quadrupole-phonon level agree very well with observations when states with large intruder configurations are ignored. Mixed-symmetry and quadrupole-octupole excitations have been investigated, but mixing with other configurations and fragmentation of strength prohibit a clear identification of these states.

Mixed-symmetry strength in114Cd

Properties of low-spin states in ${}^{114}\mathrm{Cd}$ have been studied with the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. $\ensuremath{\gamma}$-ray angular distributions and excitation functions have been used to characterize the decays of the excited levels, and level lifetimes have been obtained with the Doppler-shift attenuation method. The one-phonon, mixed-symmetry strength is found to be concentrated in the ${2}_{6}^{+}$ state, which exhibits a strong $M1$ transition to the first-excited ${2}^{+}$ state and a weak $E2$ transition to the ground state. The data agree well with expectations of the neutron-proton interacting boson model.

Experimental evidence for coexisting structures in125I

Properties of the collective bands in ${}^{125}\mathrm{I},$ populated in ${}^{123}\mathrm{Sb}(\ensuremath{\alpha},2n\ensuremath{\gamma}{)}^{125}\mathrm{I}$ reaction at $E=30 \mathrm{MeV},$ have been studied from $\ensuremath{\gamma}$-ray angular distributions and excitation functions. Spins and parities are inferred for several states. New $E2/M1$ mixing ratios are deduced for five transitions. A three-quasiparticle collective band is proposed on the basis of these and earlier results. The shapes of the ground state yrast band and the ${g}_{9/2}$ band are discussed, providing evidence for the coexistence of prolate and oblate collective states.

Structures and lifetimes of states in110Cd

A set of measurements consisting of \ensuremath{\gamma}-ray excitation functions and angular distributions has been performed using the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction on ${}^{110}\mathrm{Cd}.$ Gamma-ray excitation functions allowed us to clarify the level scheme by placing ten new transitions and to establish one new level. From $\ensuremath{\gamma}$-ray angular distributions, the lifetimes for 16 excited states were extracted using the Doppler-shift attenuation method. The experimental $B(E2),$ $B(M1),$ and $B(E1)$ values of transitions from intruder, octupole, and three-phonon states are compared to different theoretical models.

Decay properties of states populated with the207Pb(n,n′γ)reaction and weak coupling in207Pb

The properties of states in ${}^{207}\mathrm{Pb}$ populated with the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction have been studied. \ensuremath{\gamma}-ray excitation functions, angular distributions, and coincidences have been measured, and the lifetimes of many excited states were determined with the Doppler-shift attenuation method. A sextuplet of states near 3.2 MeV is interpreted as resulting from weak-coupling of the $\ensuremath{\nu}{f}_{5/2}^{\ensuremath{-}1}$ first excited state of ${}^{207}\mathrm{Pb}$ with the octupole phonon of the ${}^{208}\mathrm{Pb}$ core. A number of $E1$ transitions have been identified, and their reduced transition probabilities have been examined. The properties of low-lying levels in ${}^{207}\mathrm{Pb}$ have been studied in the framework of the particle-core coupling model.

Structural characteristics of144Ndthroughγ-ray spectroscopy following inelastic neutron scattering

Excited levels in ${}^{144}$Nd below 3.3 MeV have been studied using the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. Electromagnetic transition probabilities, multipole-mixing and branching ratios, and level spins and parities were deduced from measured $\ensuremath{\gamma}$-ray excitation functions, angular distributions, and Doppler shifts. Mixed-symmetry configurations in low-lying excited states were investigated by comparing experimental electromagnetic transition rates with theoretical calculations made using the interacting boson model and with existing calculations from the quasiparticle phonon model, the cluster vibrator model, and the particle-core coupling model. Fragmentation of the ${2}^{+}$ mixed-symmetry mode is clearly observed through strong $M1$ transitions into the lowest symmetric ${2}^{+}$ level and through small $E2/M1$ multipole-mixing ratios. Comparisons with similar measurements on ${}^{142}\mathrm{Ce}$ reveal that these $N=84$ isotones exhibit strong fragmentation of the mixed-symmetry mode, although it appears to be spread into more levels in ${}^{144}\mathrm{Nd}$. Evidence is found to support the assignment of the 2779.0-keV level as the ${3}^{\ensuremath{-}}$ member of the octupole-quadrupole phonon coupled quintuplet in ${}^{144}\mathrm{Nd}$ and to propose that this excited mode is shared with the 2606.0-keV state. Two- and three-quadrupole phonon excitations, as well as other members of the quadrupole-octupole quintuplet, are also examined.

99Tcproduced by the(3He,pnγ)reaction

The nuclear structure of ${}^{99}$Tc was studied using the ${}^{98}$Mo${(}^{3}$He,$\mathrm{pn}\ensuremath{\gamma})$ reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating $\ensuremath{\gamma}$-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included $\ensuremath{\gamma}$-ray excitation functions, $\ensuremath{\gamma}$-ray angular distributions, and $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences. The results were interpreted using a particle-rotor model. The systematics of increasing deformations in odd-$A$ Tc nuclei are discussed.

101Tc produced by the(3He,pnγ)reaction

The nuclear structure of ${}^{101}$Tc was studied using the ${}^{100}$Mo${(}^{3}$He,$\mathrm{pn}\ensuremath{\gamma})$ reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating $\ensuremath{\gamma}$-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included $\ensuremath{\gamma}$-ray excitation functions, $\ensuremath{\gamma}$-ray angular distributions, and $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences. The results were interpreted using a particle-rotor model.

Excited states in neutron-deficient 198Bi.

The high-spin states of $^{198}\mathrm{Bi}$ have been studied via the reaction $^{187}\mathrm{Re}$${(}^{16}$O,5n${)}^{198}$Bi at $^{16}\mathrm{O}$ energies from 85 to 105 MeV. In-beam measurements of \ensuremath{\gamma}-ray excitation functions, \ensuremath{\gamma}-\ensuremath{\gamma}-t coincidences, and \ensuremath{\gamma}-ray angular distributions were carried out with 6 BGO(AC)HPGe detectors and one intrinsic Ge planar detector. A level scheme for $^{198}\mathrm{Bi}$ with 26 \ensuremath{\gamma} rays was established for the first time, including a ${15}^{+}$ isomer with a measured half-life of 8.0\ifmmode\pm\else\textpm\fi{}3.6 ns. The level structure of $^{198}\mathrm{Bi}$ was qualitatively interpreted by using semiempirical methods. \textcopyright{} 1996 The American Physical Society.

97Tc produced by the (3He,pn gamma ) reaction.

The nuclear structure of $^{97}\mathrm{Tc}$ was studied using the $^{96}\mathrm{Mo}$${(}^{3}$He,pn\ensuremath{\gamma}) reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating \ensuremath{\gamma}-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included \ensuremath{\gamma}-ray excitation functions, \ensuremath{\gamma}-ray angular distributions, and \ensuremath{\gamma}-\ensuremath{\gamma} coincidences. The results were interpreted using a particle-rotor model. \textcopyright{} 1996 The American Physical Society.

Evidence for weak coupling in 145Pm.

States in {sup 145}Pm were studied in the reaction {sup 130}Te({sup 19}F,4{ital n}) at a beam energy of 75 MeV using techniques of in-beam {gamma}-ray and conversion electron spectroscopy. Measurements were made of {gamma}-{gamma} coincidences, {gamma}-ray angular distributions, conversion electrons, and excitation functions between 70 and 85 MeV. The reduced {ital B}({ital E}3) transition strength for the 795 keV (11/2{sup {minus}}{r arrow}5/2{sub g.s.}{sup +}) transition was determined to be 7.8(18) Weisskopf units, supporting the interpretation of the 11/2{sup {minus}} state as a {ital h}{sub 11/2} proton state. A negative-parity sequence built on the 11/2{sup {minus}} state as well as eleven new states above the 1503 keV 15/2{sup +} state were found. A weak coupling model can explain the structure of the deduced decay scheme up to an energy of 2.5 MeV.

Yrast states of217Fr and the onset of static intrinsic reflection asymmetric shapes in the light actinide region

Yrast states in 217Fr have been populated by bombardment of a 210Pb radioactive target with 11B ions at energies in the range 52-68 MeV. Measurements of gamma -ray excitation functions, gamma -ray angular distributions and gamma - gamma coincidences were made. The proposed level spectrum exhibits at medium angular momenta alternating positive and negative parity levels strongly coupled by E1 transitions. A survey of the neighbouring nuclei leads to the conclusion that in the actinide region 217Fr and 217Ra are the lightest nuclei presenting properties which are generally related to static intrinsic reflection asymmetric shapes.

111Cd from the (3He,2n gamma ) reaction and a rotational interpretation.

The structure of $^{111}\mathrm{Cd}$ was studied using the $^{110}\mathrm{Pd}$${(}^{3}$He,2n\ensuremath{\gamma}${)\mathrm{}}^{111}$Cd reaction, which populated a number of new nonyrast states. The experiments included \ensuremath{\gamma}-ray excitation functions, \ensuremath{\gamma}-ray angular distributions, and \ensuremath{\gamma}-\ensuremath{\gamma} coincidences. Even though $^{111}\mathrm{Cd}$ is only two protons away from the closed Z=50 shell, there is evidence that it is slightly deformed. A symmetric particle-plus-rotor model has been used to interpret the results, and shows that rotational features are present.