Doppler-shift Lifetime Measurements Research Articles

Enhanced Electric Dipole Strength for the Weakly Bound States in ^{27}Ne.

An enhanced low-energy electric dipole (E1) strength is identified for the weakly bound excited states of the neutron-rich isotope ^{27}Ne. The Doppler-shift lifetime measurements employing a combination of the γ-ray tracking array GRETINA, the plunger device, and the S800 spectrograph determine the lower limit of 0.030 e^{2} fm^{2} or 0.052W.u. for the 1/2^{+}→3/2^{-} E1 transition in ^{27}Ne, representing one of the strongest E1 strengths observed among the bound discrete states in this mass region. This value is at least 30 times larger than that measured for the 3/2^{-} decay to the 3/2_{gs}^{+} ground state. A comparison of the present results to large-scale shell-model calculations points to an important role of core excitations and deformation in the observed E1 enhancement, suggesting a novel example of the electric dipole modes manifested in weakly bound deformed systems.

On the effect of the hyperfine interactions on Recoil distance Doppler-shift lifetime measurements

The hyperfine interactions effect on the results from RDDS experiments is considered. Expressions for the influenced observables are derived within the context of application of the Differential decay curve method for data analysis. In particular, a formula for the lifetime is given where, on the one hand, measured quantities—peak areas of the shifted (fast) components and on the other hand, modeled perturbed angular distribution functions are taken into account. This formula allows for both study of the hyperfine interactions and correct derivation of the lifetime of the investigated level.

Investigation of exotic nuclei with absolute transition probabilities

Transition probabilities are crucial for the understanding of nuclear structure. Deep inelastic reactions, knockout reactions and projectile Coulomb excitation with fast radioactive beams are suited to populate excited states in exotic nuclei. Examples are presented which demonstrate that recoil Doppler shift lifetime measurements can be applied successfully in combination with such reactions to measure level lifetimes.

Lifetimes of intruder states in 186Pb, 188Pb and 194Po

Lifetimes of prolate intruder states in 186Pb and 188Pb and oblate intruder states in 194Po have been determined through recoil distance Doppler-shift lifetime measurements. Deformation parameters of | β 2 | = 0.29 ( 5 ) and | β 2 | = 0.17(3) have been extracted from experimental B ( E 2 ) values for the prolate and the oblate bands, respectively. The present study addresses the phenomenon of shape coexistence typical for the nuclei near Z = 82 and N = 104 , providing information on configuration mixing of intrinsic structures of the nuclei of interest. The results are compared with the available lifetime data and theoretical results for neutron-deficient Po, Pb, Hg and Pt nuclei. Furthermore, new self-consistent mean-field calculations have been carried out for 194Po.

Collectivity in neutron-deficient Pb and Po nuclei

Lifetimes of low-lying yrast states in 186Pb and 194Po have been determined by employing, for the first time, the recoil-decay tagging technique in recoil distance Doppler-shift lifetime measurements. In addition, lifetime measurements of prolate states in 188Pb up to the 8+ state were carried out using the recoil gating method. The deformation parameters $\vert \beta_{2}\vert = 0.29(5)$ and $\vert \beta_{2} \vert = 0.17(3)$ for the prolate (186,188Pb) and the oblate (194Po) bands, respectively, have been extracted from the measured lifetimes. 27.70.+q–

Collectivity and Configuration Mixing inPb186,188andPo194

Lifetimes of prolate intruder states in 186Pb and oblate intruder states in 194Po have been determined by employing, for the first time, the recoil-decay tagging technique in recoil distance Doppler-shift lifetime measurements. In addition, lifetime measurements of prolate states in 188Pb up to the 8+ state were carried out using the recoil-gating method. The B(E2) values have been deduced from which deformation parameters |beta2|=0.29(5) and |beta2|=0.17(3) for the prolate and the oblate bands, respectively, have been extracted. The results also shed new light on the mixing between different shapes.

Gating on the feeding or on the depopulating transition in coincidence recoil-distance Doppler-shift lifetime measurements

Two alternative ways of setting gates and lifetime determination in coincidence RDDS measurements are investigated for the case of two consecutive γ-ray transitions. Assuming a very general perturbed angular correlation function, it is shown that only the approach where the gate is set on the flight peak of the feeding transition gives results which are not influenced by the hyperfine interactions.

Coincidence recoil-distance Doppler-shift lifetime measurements in128Ba

Mean lifetimes of 24 excited states in ${}^{128}\mathrm{Ba}$ populated via the ${}^{116}\mathrm{Cd}{(}^{16}\mathrm{O},4n)$ reaction at $E=76\mathrm{MeV}$ were measured by means of the recoil-distance Doppler-shift method in coincidence mode. A new version of the differential decay-curve method which takes into account the velocity distribution of the recoils and the effect of the Doppler-shift attenuation in the stopper was applied for the lifetime determination. The $B(E2)$ values in the ground-state band indicate a reduction of the transition strengths which may be partly explained by the O(6) limiting case of the interacting boson model or alternatively, by rotationally induced changes in the single-particle level structure at spins higher than ${I}^{\ensuremath{\pi}}{=4}^{+}.$ The general collective (or Frankfurt) model is found to describe well the intraband transition strengths in the quasi-\ensuremath{\gamma} band which also exhibit the features characteristic for a $K=2\ensuremath{\Elzxh}$ rotational band. However, none of the collective models applied can describe in a consistent way all observed properties of the low-lying collective states. The newly measured $B(E2,\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}2)$ and $B(M1,\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}1)$ transition strengths reconfirm the configuration $\ensuremath{\pi}{h}_{11/2}{d}_{5/2}$ of the negative-parity semidecoupled bands. The lowest levels of the higher-lying ``dipole'' band in ${}^{128}\mathrm{Ba},$ which is built on the high-K four-quasiparticle prolate configuration $(\ensuremath{\pi}{h}_{11/2}{d}_{5/2})\ensuremath{\bigotimes}(\ensuremath{\nu}{h}_{11/2}{g}_{7/2}),$ are studied.

Candidates for two-phonon octupole excitations in208Pb

Following the identification of the lowest-spin member of the two-phonon octupole quartet in ${}^{208}\mathrm{Pb},$ the additional members of the multiplet have been sought with the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. Based on energy arguments and $E1$ transition probabilities determined from Doppler-shift lifetime measurements, the 5286- and 5216-keV states are suggested as candidates for the ${2}^{+}$ and ${4}^{+}$ two-phonon octupole excitations in ${}^{208}\mathrm{Pb}.$ The difficulties of identifying the ${6}^{+}$ member of this quartet are examined.

Doppler-shift lifetime measurements in 96Zr with the inelastic neutron scattering reaction

Lifetimes of states in 96Zr have been extracted from the Doppler shifts of γ-rays produced by inelastic scattering of accelerator-produced neutrons at incident energies of 3.8 and 4.3 MeV. From these data and the assumption that the Blaugrund theory is applicable, sixteen lifetimes and another seven lifetime limits were determined. Special features of these types of measurements are discussed. For several levels that decay directly to the ground state, spin assignments have been deduced from the γ-ray angular distributions. From measured B( E2) and B(E1) values, support is found for suggested collective quadrupole and octupole structures in this nucleus.

Doppler-shift lifetime measurements of levels in 45Sc and in 46Ti

The lifetimes of 15 levels in 45Sc up to 2.8 MeV and of 12 levels in 46Ti have been measured by the Doppler-shift attenuation method, using the reactions 45Sc(p, p'γ) 45Sc and 45Sc(p, γ) 46Ti. A two-target arrangement for the quasi-simultaneous observation of γ-rays at a forward and a backward angle was used in the measurements. For the extraction of lifetimes from the shifts observed in the (p, p'γ) singles γ-spectra, F( τ) curves averaged over all initial recoil velocities were employed. A correction factor ƒ = 0.9 to the theoretical stopping power, obtained by comparing the lifetimes of four levels in 45Sc with results from resonance fluorescence measurements, was used in the lifetime analysis for both nuclei. The new lifetime results for 46Ti help to clarify discrepancies in previous results.

Yrast decay schemes from heavy-ion +Ca48fusion-evaporation reactions: IV:Cr53,V54,Co62, andNi61−63

Fusion-evaporation reactions induced by beams of 25-55 MeV $^{9}\mathrm{Be}$ and $^{18}\mathrm{O}$ on an isotopically enriched $^{48}\mathrm{Ca}$ target have been used to populate high-spin yrast levels in $^{53}\mathrm{Cr}$, $^{54}\mathrm{V}$, $^{62}\mathrm{Co}$, and $^{61\ensuremath{-}63}\mathrm{Ni}$. Measurements consisted of $\ensuremath{\gamma}$-ray excitation functions, angular distributions, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, and recoil-distance and Doppler shift lifetime measurements, from which were deduced the energy levels, $\ensuremath{\gamma}$-ray branching ratios, most probable spin-parity assignments, and level lifetimes.NUCLEAR REACTIONS $^{48}\mathrm{Ca}(^{9}\mathrm{Be}, xn, yp)^{53}\mathrm{Cr}$ and $^{54}\mathrm{V}$. $^{48}\mathrm{Ca}(^{18}\mathrm{O}, xn)^{61\ensuremath{-}63}\mathrm{Ni}$ and $^{48}\mathrm{Ca}(^{18}\mathrm{O}, 3n, p)^{62}\mathrm{Co}$. $E=25\ensuremath{-}55$ MeV; measured $\ensuremath{\sigma}(E, {E}_{\ensuremath{\gamma}})$ and coin; deduced levels; measured $\ensuremath{\sigma}({E}_{\ensuremath{\gamma},} \ensuremath{\theta})$; deduced ${J}^{\ensuremath{\pi}}$ for high-spin levels; measured RDM and DSA; deduced ${\ensuremath{\tau}}_{m}$, ${|M(M1)|}^{2}$ and ${|M(E2)|}^{2}$. Enriched targets, Ge(Li) detectors.

Yrast decay schemes from heavy-ion +Ca48fusion-evaporation reactions. III.Fe57,58,Cr54,55, andMn57,58

Fusion-evaporation reactions induced by beams of 25-65 MeV $^{13}\mathrm{C}$, $^{15}\mathrm{N}$, $^{11}\mathrm{B}$, and $^{9}\mathrm{Be}$ on an isotopically enriched $^{48}\mathrm{Ca}$ target have been used to populate high-spin yrast levels in $^{57,58}\mathrm{Fe}$, $^{54,55}\mathrm{Cr}$, and $^{57,58}\mathrm{Mn}$. Measurements consisted of $\ensuremath{\gamma}$-ray excitation functions, angular distributions, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, and recoildistance and Doppler shift lifetime measurements, from which were deduced the energy levels, $\ensuremath{\gamma}$-ray branching ratios, most probable spin-parity assignments, and level lifetimes. Comparision is made with a large-basis shell-model calculation.NUCLEAR REACTIONS $^{48}\mathrm{Ca}(^{13}\mathrm{C},xn,yp,z\ensuremath{\alpha})^{57,58}\mathrm{Fe}$, $^{54,\phantom{\rule{0ex}{0ex}}55}\mathrm{Cr}$, and $^{57,\phantom{\rule{0ex}{0ex}}58}\mathrm{Mn}$; $^{48}\mathrm{Ca}(^{15}\mathrm{N},\ensuremath{\alpha}2n)^{57}\mathrm{Mn}$; $^{48}\mathrm{Ca}(^{11}\mathrm{B},pxn)^{54,55}\mathrm{Cr}$; and $^{48}\mathrm{Ca}(^{9}\mathrm{Be},3n)^{54}\mathrm{Cr}$. $E=25\ensuremath{-}65$ MeV; measured $\ensuremath{\sigma}(E,{E}_{\ensuremath{\gamma}})$ and coin; deduced levels; measured $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},\ensuremath{\theta})$; deduced ${J}^{\ensuremath{\pi}}$ for highspin levels; measured RDM and Doppler shifts; deduced ${t}_{m}$, ${|M(M1)|}^{2}$, and ${|M(E2)|}^{2}$. Enriched targets, Ge(Li) detectors.

Yrast decay schemes from heavy-ion +Ca48fusion-evaporation reactions. II.Fe59−60andCo59−60

Fusion-evaporation reactions induced by beams of 25-55-Mev $^{15}\mathrm{N}$ and $^{18}\mathrm{O}$ on an isotopically enriched $^{48}\mathrm{Ca}$ target have been used to populate high-spin yrast levels in $^{59\ensuremath{-}60}\mathrm{Fe}$ and $^{59\ensuremath{-}60}\mathrm{Co}$. Measurements consisted of $\ensuremath{\gamma}$-ray excitation functions, angular distributions, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, and recoil-distance and Doppler-shift lifetime measurements, from which were deduced the energy levels, $\ensuremath{\gamma}$-ray branching ratios, most probable spin-parity assignments, and level lifetimes. The results of $p\ensuremath{-}\ensuremath{\gamma}$ correlation measurements in the $^{58}\mathrm{Fe}(t,p\ensuremath{\gamma})^{60}\mathrm{Fe}$ reaction are also reported.NUCLEAR REACTIONS $^{48}\mathrm{Ca}(^{15}\mathrm{N},xn,yp)^{59,60}\mathrm{Fe}$ and $^{59\ensuremath{-}60}\mathrm{Co}$ and $^{48}\mathrm{Ca}(^{18}\mathrm{O},xn,z\ensuremath{\alpha})^{59,60}\mathrm{Fe}$. $E=25\ensuremath{-}55$ MeV; measured $\ensuremath{\sigma}(E,{E}_{\ensuremath{\gamma}})$ and coin; deduced levels; measured $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},\ensuremath{\theta})$; deduced ${J}^{\ensuremath{\pi}}$ for high-spin levels; measured RDM and DSA; deduced ${\ensuremath{\tau}}_{m}$, ${|M(M1)|}^{2}$ and ${|M(E2)|}^{2}$, $^{58}\mathrm{Fe}(t,p\ensuremath{\gamma})^{60}\mathrm{Fe}$; measured $p\ensuremath{-}\ensuremath{\gamma}(\ensuremath{\theta})$; deduced ${J}^{\ensuremath{\pi}}$. Enriched targets, Ge(Li) detectors.

Yrast decay schemes from heavy ion +Ca48fusion-evaporation reactions. I.Mn54−56,Cr56, andV52−53

Fusion-evaporation reactions induced by a beam of 25-50 MeV $^{11}\mathrm{B}$ on an isotopically enriched $^{48}\mathrm{Ca}$ target have been used to populate high-spin yrast levels in $^{54\ensuremath{-}56}\mathrm{Mn}$, $^{56}\mathrm{Cr}$, and $^{52\ensuremath{-}53}\mathrm{V}$. Measurements consisted of $\ensuremath{\gamma}$- ray excitation functions, angular distributions, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, and recoil-distance and Doppler shift lifetime measurements, from which were deduced the energy levels, $\ensuremath{\gamma}$-ray branching ratios, most probable spin-parity assignments, and level lifetimes.[NUCLEAR REACTIONS $^{48}\mathrm{Ca}(^{11}\mathrm{B},xn,yp,z\ensuremath{\alpha})^{54\ensuremath{-}56}\mathrm{Mn}$, $^{56}\mathrm{Cr}$, and $^{52\ensuremath{-}53}\mathrm{V}$. $E=25\ensuremath{-}50$ MeV; measured $\ensuremath{\sigma}(E,{E}_{\ensuremath{\gamma}})$ and coin.; deduced levels; measured $\ensuremath{\sigma}({E}_{\ensuremath{\gamma}},\ensuremath{\theta})$; deduced ${J}^{\ensuremath{\pi}}$ for high-spin levels; measured RDM and Doppler shifts; deduced ${t}_{\frac{1}{2}}$, ${|M(M1)|}^{2}$ and ${|M(E2)|}^{2}$. Enriched targets, Ge(Li) detectors.]

Doppler-shift lifetime measurements in 46Ti and 50Cr

Abstract The lifetimes of nine states in 46Ti and fourteen states in 50Cr have been measured through the Doppler-shift attenuation method. The states were populated by the 46Ti(p, p′γ) and 50Cr(p, p′γ) reactions at Ep = 7 MeV. Thick targets (≈ 300 keV) were used to provide adequate averaging over many compound-nucleus levels. Singles γ-ray spectra were obtained with 33 and 55 cm3 Ge(Li) detectors at θγ = 0°, 90° and ≈ 160°, and lifetimes were extracted from the observed centroid shifts. An alternative method of analysis, involving a simulation of profiles of Doppler-shifted peaks with six free parameters gave considerably more accurate results. Both methods were tested by measuring known lifetimes in 58Ni. In addition, the lifetimes of three states in 52Cr were determined. A comparison of the energy spectra and electromagnetic decay properties of these nuclei to shell-model calculations revealed the failure of the simple ( f 7 2 ) 6 picture. A startling example of this failure is the fact that the 22+(E2)21+ rate in 46Ti appears to be a factor of ⪆ 25 faster than that in the cross-conjugate nucleus 50Cr.

A versatile data acquisition system for general gamma ray spectroscopy

A description is given of the associated hardware and instrumentation which have been designed and adapted for use in general gamma-ray spectroscopy studies involving nuclear reactions where high bombarding energies, typically available from tandem electrostatic accelerators, are employed. The function of the system is illustrated by experimental data taken from recent particle-gamma-ray angular correlation and Doppler shift lifetime measurements involving utilization of the laboratory on-line IBM 360/44 data acquisition system.

The use of implanted targets in Doppler-shift lifetime measurements

The possibility of using targets, in which the ions are implanted into the backing material, in Doppler-shift attenuation measurements of lifetimes of nuclear states excited in proton capture reactions has been investigated. The effect of various uncertainties in the stopping power values on the results has also been computed. The analysis is applied to the determination of the lifetime of the 2641 keV state in 23Na and the lifetime of the 4617 keV state in 28Si.

Lifetime and Angular Distribution Measurements from theP31(p, γ)S32Reaction

The $^{31}\mathrm{P}(p, \ensuremath{\gamma})^{32}\mathrm{S}$ reaction has been studied at several resonances with a 20-${\mathrm{cm}}^{3}$ Ge(Li) detector. A level at 6.664 MeV has been populated which may be the same as one found at 6.671 MeV from (${\mathrm{He}}^{3}$, $d$) experiments. This state is found to branch almost equally to the states at 3.775 and 2.230 MeV. Measured branching ratios for the first six excited states are in agreement with earlier work except for a ground-state transition for the 5.006-MeV level which supports an odd-parity assignment for this state. New branching ratios are assigned for the levels at 5.544 and 6.673 MeV. Doppler-shift lifetime measurements for the states at 3.775, 4.280, 4.694, 5.006, 5.410, 5.544, 6.226, and 6.623 MeV are consistent with other measurements. The lifetime of the level at 6.664 MeV is found to be ${0.054}_{\ensuremath{-}0.009}^{0.013}$ psec. An upper limit of 0.01 psec is set for the mean life of the 7.952-MeV level. Angular distributions have been measured with the Ge(Li) detector for the 1.248- and 1.438-MeV resonances. Spins deduced from combining these results with the lifetime measurements confirm previous assignments for levels at 6.623, 6.226, and 5.410 MeV. The spin of the 4.459-MeV level is limited to 3 or 4, consistent with other recent work and with the hypothesis of Goswami et al. that this is the ${4}^{+}$ level of a rotational band based on the ${0}^{+}$ ground state. Mixing ratios are given for a number of transitions.

Doppler-shift lifetime measurements in 20Ne, 24Mg and 28Si

The ( 3He, dγ) reactions has been used to study the γ-decay of levels in 20Ne, 24Mg and 28Si. Lifetimes of three states in 20Ne, five states in 24Mg and eight states in 28Si were measured using the Doppler-shift attenuation method with a 10 cm 3 or a 15 cm 3 Ge(Li) detector operated in coincidence with the outgoing deuterons. In addition, the energies of 43 transitions in these nuclei were determined; most of the energy assignments had smaller estimated errors than previously published values. The present results are used to assess rotational properties of these nuclei. In addition, the lifetime results in 28Si are used as an aid to interpret Hartree-Fock calculations which have been published.