Compton-suppressed Ge Research Articles

Onset of band structure in Ga70

Excited states in odd-odd $^{70}\mathrm{Ga}$ were studied using the $^{62}\mathrm{Ni}(^{14}\mathrm{C},\ensuremath{\alpha}pn)$ fusion-evaporation reaction performed at the John D. Fox Superconducting Accelerator Facility at Florida State University with a beam energy of 50 MeV. The depopulating $\ensuremath{\gamma}$ decays were measured in coincidence using a Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. An investigation of these coincidences resulted in the addition of 16 new transitions in the $^{70}\mathrm{Ga}$ level scheme, including some which belong to the onset of a new positive-parity band structure likely based on the $\ensuremath{\pi}{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{9/2}$ configuration. Spins and parities were assigned based on directional correlation of oriented nuclei ratios and linear polarization measurements. The excitation energies predicted by shell-model calculations using the JUN45 effective interaction compare favorably with the experimental ones for the positive-parity states, but are generally about 400--500 keV too low for the negative-parity states. Total Routhian surface calculations for the lowest positive-parity configuration with signature $\ensuremath{\alpha}=0$ predict significant triaxiality $(\ensuremath{\gamma}\ensuremath{\approx}\ensuremath{-}{20}^{\ensuremath{\circ}})$ with competing noncollective excitations developing at a spin $(J=8)$ that corresponds to the onset of the positive-parity band observed experimentally. The calculations for the lowest negative-parity states with $\ensuremath{\alpha}=0$ yielded surfaces that were qualitatively similar.

Structure of collective states built on the 11/2+ isomer in Os187 : Quasiparticle-plus-triaxial-rotor model and interpretation as tilted-precession bands

Background: The shape of most nuclei is described by its quadrupole deformation (showing the deviation from spherical shape) and its triaxiality (showing the deviation from axial symmetry). Nuclei affected by triaxiality show additional collective rotational bands, called γ bands, that appear at low excitation energy. The γ bands can be caused by the precession of a nucleus with triaxial shape, but can also arise from small γ vibrations around an axially symmetric shape. Purpose: The aim of this work is to search for new collective excitations in Os187 in particular related with the γ degree of freedom. Methods: The rotational structures of Os187 were populated by the W186(He4,3n)Os187 reaction at a beam energy of 37 MeV. The measurements of γ - γ coincidences, angular distribution ratios and γ-ray intensities were performed using eleven Compton-suppressed Ge clover detectors. Results: The previously known positive-parity band built on the 11/2+ isomer has been extended up and a new excited positive-parity band built above a 15/2+ state has been observed. The 11/2+ band was assigned a νi13/2 configuration while the new 15/2+ band was associated with a coupling of the valence i13/2 neutron with the γ band of the even-even core. The quasiparticle-plus-triaxial-rotor model calculations provide a good agreement with the experimental data for both bands. They describe the 15/2+ band as a collective excitation with respect to the 11/2+ band that corresponds to a precession of the intermediate nuclear axis similarly to the precession of a rotating top. Conclusions: As shown by the calculations, the new rotational band can be understood as resulting from the three-dimensional rotation of a triaxially-deformed nucleus. However, a description based on the vibrations of a γ-soft nuclear shape should also be investigated in order to firmly establish the nature of the excited positive-parity band. Further studies able to distinguish between these alternative descriptions will be beneficial.8 MoreReceived 11 May 2021Revised 10 June 2021Accepted 5 October 2021DOI:https://doi.org/10.1103/PhysRevC.104.044326©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsElectromagnetic transitionsIsomer decaysNuclear structure & decaysNucleon distributionProperties150 ≤ A ≤ 189Nuclear Physics

γ -ray spectroscopy of Sm150 through the β decay of Pm150 ( T=2.7 h) and Eum150 ( T=12.8 h)

The $^{150}\mathrm{Sm}$ nucleus, populated through the $\ensuremath{\beta}$ decays of $^{150}\mathrm{Eu}^{m}$ and $^{150}\mathrm{Pm}$, was investigated by means of $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlation studies using an array of 20 Compton-suppressed Ge detectors. The study particularly focuses on the determination of weakly populated levels and weak decay branches between established levels. A weakly populated state at 1603 keV is established to have spin-parity ${0}^{+}$ from the angular correlation data; a second weakly populated state at 1786 keV is confirmed to have spin-parity ${0}^{+}$ on a similar basis.

Multiple band structures in Ge70

High-spin states in $^{70}\mathrm{Ge}$ were studied using the $^{55}\mathrm{Mn}$($^{18}\mathrm{O},p2n$) fusion-evaporation reaction at a beam energy of 50 MeV. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were measured using the Florida State University Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. An investigation of these coincidences resulted in the addition of 31 new transitions and the rearrangement of four others in the $^{70}\mathrm{Ge}$ level scheme, providing a more complete picture of the high-spin decay pattern involving both positive- and negative-parity states with multiple band structures. Spins were assigned based on directional correlation of oriented nuclei ratios, which many times also led to unambiguous parity determinations based on the firm assignments for low-lying states made in previous work. Total Routhian surface calculations, along with the observed trends in the experimental kinematic moment of inertia with rotational frequency, support the multiquasiparticle configurations of the various crossing bands proposed in recent studies. The high-spin excitation spectra predicted by previous shell-model calculations compare favorably with the experimental one determined from this study.

Coexisting single-particle and collective excitations inAs70

High-spin states in $^{70}\mathrm{As}$ were studied using the $^{55}\mathrm{Mn}(^{18}\mathrm{O},3n)$ fusion-evaporation reaction at a beam energy of 50 MeV. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were measured using the Florida State University Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. A reinvestigation of the known level scheme resulted in the addition of 32 new transitions and the rearrangement of 10 others. The high-spin decay pattern of yrast negative-parity states was modified and enhanced extensively. Spins were assigned based on directional correlation of oriented nuclei ratios. Lifetimes of seven excited states were measured using the Doppler-shift attenuation method. The $B(E2)$ rates inferred from the lifetimes of states in the yrast positive-parity band imply substantial collectivity, in agreement with the results of previous studies. Substantial signature splitting and large alternations in the $B(M1)$ strengths were observed in this band as well, supporting the interpretation of an aligned $\ensuremath{\pi}{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{9/2}$ intrinsic configuration for this structure beginning at the lowest ${9}^{+}$ state. Large-scale shell-model calculations performed for $^{70}\mathrm{As}$ reproduce the relative energy differences between adjacent levels and the $B(M1)$ rates in the yrast positive-parity band rather well, but underestimate the $B(E2)$ strengths. The ${g}_{9/2}$ orbital occupancies for the lowest ${9}^{+}$ state predicted by the shell-model calculations provide additional evidence of a stretched $\ensuremath{\pi}{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{9/2}$ configuration for this state.

Higher-spin structures inF21andNa25

Excited states were investigated in $^{21}\mathrm{F}$ and $^{25}\mathrm{Na}$ using the $^{9}\mathrm{Be}(^{14}\mathrm{C},pn\ensuremath{\gamma})$ reaction at 30, 35, and 45 MeV and the $^{9}\mathrm{Be}(^{18}\mathrm{O},pn\ensuremath{\gamma})$ reaction at 35 MeV. Protons were detected and identified in an $E\ensuremath{-}\mathrm{\ensuremath{\Delta}}E$ telescope at ${0}^{\ensuremath{\circ}}$ in coincidence with one or more $\ensuremath{\gamma}$ radiations in the FSU Compton-suppressed Ge detector array. Many new levels and electromagnetic decays were observed, especially among the higher spin states. Angular distributions and mean lifetimes were measured wherever possible in both nuclei. The energy levels of the positive-parity states in the two nuclei agree rather well with shell model calculations using both the USDA and WBP interactions up to the highest spins observed of $13/2\phantom{\rule{0.28em}{0ex}}\ensuremath{\hbar}$. Both a weak coupling approximation and shell model calculations using the WBP interaction generally reproduce the negative-parity states in $^{21}\mathrm{F}$. The shell model calculations reproduce relatively well the measured $M1$ and $E2$ transitions in both nuclei, but overpredict the parity-changing $E1$ transitions in $^{21}\mathrm{F}$, the only nucleus in which negative-parity states were observed in the present experiment.

Competing single-particle and collective behavior in71Se

The high-spin decay of ${}^{71}$Se was studied using the ${}^{54}$Fe${(}^{23}$Na,$\ensuremath{\alpha}pn)$ reaction at 80 MeV and the Florida State University Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. Based on prompt $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences measured in the experiment, the known level scheme was enhanced and extended to higher spin with 19 new transitions. A band that was previously suggested to have positive parity was reassigned as the ``missing'' signature partner of an existing negative-parity band. Spins were assigned based on directional correlation of oriented nuclei ratios. Lifetimes of 17 excited states were measured using the Doppler-shift attenuation method. Experimental ${Q}_{t}$ values imply an intermediate degree of collective behavior for ${}^{71}$Se at high spin. Theoretical ${Q}_{t}$ values determined from cranked Woods-Saxon (CWS) calculations show better agreement with the experimental ones for the positive-parity states than the negative-parity states. Shape competition and $\ensuremath{\gamma}$ softness characterize the low-spin states of the lowest positive- and negative-parity bands based on the CWS calculations. At high spin, triaxial shapes with $\ensuremath{\gamma}g{0}^{\ensuremath{\circ}}$ are predicted.

Transition strengths and the role of thef7/2orbital inAs71

High-spin states in $^{71}\mathrm{As}$ were studied using the $^{54}\mathrm{Fe}$$({}^{23}$Na,$\ensuremath{\alpha}2p)$ reaction at 80 MeV. Prompt $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences were measured using the Florida State University Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. The existing high-spin level scheme has been verified, and 21 new transitions have been added based on an investigation of weak $\ensuremath{\gamma}$-ray coincidence relations and relative $\ensuremath{\gamma}$-ray intensities. Lifetimes of 16 excited states were measured using the Doppler-shift attenuation method applied to the experimental line shapes of decays in all of the known rotational bands. The $B(E2)$ strengths inferred from the lifetimes indicate that moderate to high collective behavior persists to the highest observed spins in the lowest positive- and negative-parity bands, in qualitative agreement with projected shell-model calculations. The band suggested to be based on the $\ensuremath{\pi}{f}_{7/2}$ orbital shows a similar degree of collectivity within the same spin range, with $B(E2)$ values in good agreement with those predicted by the projected shell model assuming a constant prolate deformation of ${\ensuremath{\epsilon}}_{2}=+0.27$. The experimental ${Q}_{t}$ values in this band are somewhat smaller than predicted by cranked Woods-Saxon calculations.

Linear polarization measurements and negative-parity states inSr80

High-spin states in $^{80}\mathrm{Sr}$ were studied using the ${}^{54}\mathrm{Fe}({}^{28}\mathrm{Si},2p)$ reaction at 90 MeV and the ${}^{58}\mathrm{Ni}({}^{28}\mathrm{Si},\ensuremath{\alpha}2p)$ reaction at 110 MeV. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were measured from the ${}^{54}\mathrm{Fe}({}^{28}\mathrm{Si},2p)$ reaction using an array of 10 Compton-suppressed Ge detectors. $\ensuremath{\gamma}$-ray linear polarizations were measured in both reactions using three clover detectors as Compton polarimeters. Negative parity has been conclusively assigned to one band and is favored for two others based on the polarization measurements and the observation of six new linking transitions between the bands near their band heads. This evidence supports a picture of strong mixing between low-lying states with negative parity, similar to what has been observed in $^{82}\mathrm{Sr}$. Directional correlation of oriented nuclei ratios support the spin assignments made in the most recent in-beam study.

NEW LEVELS AND TRANSITIONS IN 72Ge OBSERVED IN THE 72As(β++EC)72Ge DECAY

The decay of 72 As has been investigated by means of γ-ray spectroscopy. The 72 As nuclei were produced through the 72 Ge (p, n)72 As reaction. The Compton-suppressed spectrometer and high-purity Ge detectors have been used in singles and in coincidence modes respectively to study γ-rays in the β++ EC decay of 72 As to 72 Ge . 79 γ-rays, among which 3 were observed for the first time, were reported. A decay scheme of 72 As including 1 new level is proposed that accommodates 77 of these transitions. Tentative spin and parity assignments are suggested for a number of levels according to the log ft values and γ-branching ratios.

Identification of theν7/2+[404]band in neutron-richRu109

The high-spin spectroscopy of neutron-rich $^{109}\mathrm{Ru}$ is studied by measuring the prompt \ensuremath{\gamma} rays from the spontaneous fission fragments of $^{252}\mathrm{Cf}$ with 102 Compton-suppressed Ge detectors in the Gammasphere detector array. Previous band structures are confirmed and the ground state band is extended. A positive parity band based on a 332.5 keV level is newly identified. This band is proposed as a single-neutron excitation band built on the $7/{2}^{+}[404]$ Nilsson orbital. Some band structural characteristics are discussed.

N=90region: The decays ofEu152m,gtoSm152

The decays of {sup 152}Eu{sup m,g}{yields}{sup 152}Sm have been studied by {gamma}-ray spectroscopy using the 8{pi} spectrometer, an array of 20 Compton-suppressed Ge detectors. Very weak {gamma}-decay branches in {sup 152}Sm were investigated through {gamma}-{gamma} coincidence spectroscopy. All possible E2 transitions between states below 1550 keV with transition energies >130 keV are observed, including the previously unobserved 2{sub 3}{sup +}{yields}0{sub 2}{sup +} 401 keV transition. The results, combined with existing lifetime data, provide a number of new or revised E2 transition strengths which are critical for clarifying the collective structure of {sup 152}Sm and the N=90 isotones.

Transition strengths and degree of deformation inSr79

High-spin states in $^{79}\mathrm{Sr}$ were studied using the $^{54}\mathrm{Fe}$($^{28}\mathrm{Si}$,$2p$$n$) reaction at 90 MeV, with a thick 14-mg/${\mathrm{cm}}^{2}$ $^{54}\mathrm{Fe}$ target used to stop all recoils. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were detected using the Florida State University Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. The most recent $^{79}\mathrm{Sr}$ level scheme has been confirmed in three separate band structures up to a spin as high as the $({\frac{37}{2}}^{+})$ yrast state based on \ensuremath{\gamma}-ray coincidence relations, intensity and effective lifetime measurements, and directional correlation of oriented nuclei ratios. Lifetimes of 33 excited states were measured using the Doppler-shift attenuation method, with the experimental line shapes obtained at two separate observation angles and by gating from above the transitions of interest whenever possible. Transition quadrupole moments ${Q}_{t}$ inferred from the lifetimes indicate a high degree of collectivity and deformation over a rather wide range of spins in all three observed bands, with evidence for modest reductions in the values with increasing spin. The changes in ${Q}_{t}$ are attributed to the onset of quasiproton alignment and are supported qualitatively by the predictions of the projected shell model and cranked Woods-Saxon calculations in conjunction with the cranked shell model. Lifetimes measured in a band based on the [431]${\frac{1}{2}}^{+}$ intrinsic Nilsson configuration suggest a large quadrupole deformation (${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.41$) associated with this band, providing another example of the strong deformation-driving properties of the ${d}_{5/2}$ intruder orbital in the mass 80 region.

Complete spectroscopy of the 162Dy nucleus

States in 162Dy have been populated by ( n , γ ) , ( n , e − ) , and ( α , 2 n ) reactions in order to characterize a complete set of low K bands below 2 MeV. The ( n , γ ) and ( n , e − ) measurements were made using high precision curved crystal and β − spectrometers. Gamma–gamma coincidence and DCO measurements were made following the ( α , 2 n ) reaction using an array of Compton-suppressed Ge detectors. Using all these data and employing the Ritz combination principle, a level scheme was developed which represents one of the most complete and extensive spectra of levels available for comparison with nuclear models. The observed positive and negative parity bands are discussed in terms of collective quadrupole and octupole vibrational excitations as well as 2 quasi-particle excitations.

Application of ultra-fast timing techniques to the study of exotic and weakly produced nuclei

Ultra-fast time-delayed techniques have been recently applied in a number of studies where exotic nuclei were identified using advanced selection techniques. These include large Compton-suppressed Ge arrays, in-flight separators or recoil separators. Some of the new results are discussed in this presentation. Besides the results for 32Mg and 96Pd, they include the first determination of the half-life of the 8+ state in 80Ge, T1/2 = 2.95(6) ns, and significantly more precise results for 51Mn (3680 keV level) and 48V (421 keV level), T1/2 = 1760(40) ps and T1/2 ⩽ 135 ps, respectively. Development of new scintillators will steadily improve precision and sensitivity of future measurements.

Multiparticle configurations inN=84isotones located at the proton drip line

Excited states in the proton-rich $N=84$ isotones ${}_{72}^{156}{\mathrm{Hf}}_{84},\phantom{\rule{0.3em}{0ex}}{}_{73}^{157}{\mathrm{Ta}}_{84}$, and ${}_{74}^{158}{W}_{84}$ were observed using $^{102}\mathrm{Pd}(^{58}\mathrm{Ni},\phantom{\rule{0.3em}{0ex}}\mathit{xp}2n)$ reactions at 270 MeV. \ensuremath{\gamma} rays were detected with the Gammasphere array of Compton-suppressed Ge detectors coupled with the Argonne fragment mass analyzer and were assigned to individual reaction channels using the recoil-decay tagging method. Prompt \ensuremath{\gamma}-ray cascades were associated with the \ensuremath{\alpha} decay of both the ground state and the ${8}^{+}$ isomeric state in $^{156}\mathrm{Hf}$, the ${h}_{11/2}$ state in $^{157}\mathrm{Ta}$, and the ${8}^{+}$ isomeric state in $^{158}\mathrm{W}$. The level schemes constructed for $^{156}\mathrm{Hf},^{157}\mathrm{Ta}$, and $^{158}\mathrm{W}$ are compared with these of lighter $N=84$ isotones and are discussed within the framework of the shell model.

New Low-Spin Levels in 72Ge and Discussion of Its Yrast Band Structure*

The decay of 72 Ga has been investigated by means of γ-ray spectroscopy. The 72 Ga nuclei were produced through the 71 Ga(n, γ) 72 Ga reaction. The Compton-suppressed spectrometer and High-purity Ge detectors have been used singly and in coincidence, respectively, to study γ-rays in the β - decay of 72 Ga to 72 Ge. 93 γ-rays, among which 7 were observed for the first time, were reported. A decay scheme of 72 Ga including 4 new levels is proposed which accommodates 87 of these transitions. Spins and parities for new levels are proposed from calculated log f t values, modes on the observed decay, and some nuclear reaction experiment results. Combining with the high-spin states observed by the in-beam γ-ray spectroscopy of a previous decay work, the structure of the excited positive-parity yrast states of 72 Ge is discussed in the framework of the projected shell model.

Decay of76Br and Its Daughter's Level Structure*

The decay of 76 Br has been investigated by means of γ-ray spectroscopy. The 76 Br nuclei were produced in two reactions, i.e., 75 As(α,3n) 76 Br and 76 Se(p,n) 76 Br. The Compton-suppressed spectrometer and High-purity Ge detectors have been used singly and in coincidence, respectively, to study γ-rays in the β + +EC decay of 76 Br to 76 Se. 37 γ rays were observed for the first time among 138 γ-rays reported. A decay scheme of 76 Br including 15 new levels is proposed which accommodates 120 of these transitions. Spins and parities for new levels are proposed from calculated log f t values, modes on the observed decay, and some nuclear reaction experiment results. Combining with the high-spin states observed by the in-beam γ-ray spectroscopy of a previous decay work, the level scheme were compared with results from calculations in the framework of the projected shell model.

Isomer decay tagging in the heavy nuclei:Ra210andRa209

Excited states in $^{210}\mathrm{Ra}$ and $^{209}\mathrm{Ra}$ were populated using the $^{184}\mathrm{W}(^{30}\mathrm{Si},xn)$ reaction at $148\phantom{\rule{0.3em}{0ex}}\text{MeV}$ beam energy. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Prompt $\ensuremath{\gamma}$ rays were detected using Compton-suppressed Ge detectors from the YRAST Ball array surrounding the target. Delayed $\ensuremath{\gamma}$ rays, following isomeric decays, were detected at the focal plane of SASSYER with a smaller array of Ge detectors. The decay energy and lifetime for the ${8}^{+}{(\ensuremath{\pi}{h}_{9∕2})}^{n}$ isomer of $^{210}\mathrm{Ra}$ were determined; values for the yrast $B(E2;{8}^{+}\ensuremath{\rightarrow}{6}^{+})$ in $^{210}\mathrm{Ra}$ and neighboring nuclei are interpreted within the seniority scheme. This isomer was also used to select $\ensuremath{\gamma}$-rays deexciting levels above the isomeric state in $^{210}\mathrm{Ra}$. In addition, transitions in $^{209}\mathrm{Ra}$ were identified for the first time. Two high-spin isomers are suggested to exist in this isotope.

Rotational bands and isomeric states inLu175

Rotational bands in $^{175}\mathrm{Lu}$ have been extended through investigation with the $(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. Spallation neutrons bombarded Lu samples, and the resulting $\ensuremath{\gamma}$ rays were detected in a large-scale Compton-suppressed Ge detector array. Prompt- and delayed-$\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences have been used to extend the $5∕{2}^{+}[402]$ band to spin $17∕2$, the $3∕{2}^{+}[411]$ band to spin $11∕2$, the $1∕{2}^{+}[411]$ band to spin $15∕2$, and the $1∕{2}^{\ensuremath{-}}[541]$ and $9∕{2}^{\ensuremath{-}}[514]$ bands to spin $21∕2$. A new band, based on the $7∕{2}^{\ensuremath{-}}[523]$ configuration, is tentatively assigned from its bandhead to spin $13∕2$. The three-quasiparticle ${K}^{\ensuremath{\pi}}=19∕{2}^{+}$ isomer is confirmed and its half-life determined to be $984\ifmmode\pm\else\textpm\fi{}13(\text{stat}.)\ifmmode\pm\else\textpm\fi{}30(\text{sys}.)$ $\ensuremath{\mu}\mathrm{s}$, in agreement with previous results.