Doppler Shift Attenuation Technique Research Articles

Doppler-shift attenuation lifetime measurement of the Ar3621+ level

At TRIUMF, the TIGRESS Integrated Plunger device and its suite of ancillary detector systems have been implemented for charged-particle tagging and light-ion identification in coincidence with γ-ray spectroscopy for Doppler-shift lifetime studies and low-energy Coulomb excitation measurements. As a test of the device, the lifetime of the first 2+ excited state in Ar36 was measured from the γ-ray line shape of the 21+→0g.s.+ transition using the Doppler-shift attenuation technique following Coulomb excitation. The line-shape signatures, vital for precision lifetime measurements, were significantly improved by enhanced reaction-channel selectivity using a complementary approach of kinematic gating and digital rise-time discrimination of recoiling charged particles in a silicon PIN diode array. The lifetime was determined by comparisons between the data and simulated line shapes generated using our TIGRESS Coulomb excitation code as an input to the Lindhard method, which was then extended and included as a class in geant4. The model-independent lifetime result of 490±50 fs corresponds to a reduced quadrupole transition strength of B(E2;21+→0g.s.+)=56±6e2fm4 and agrees well with previous intermediate energy Coulomb excitation measurements, thereby resolving reported discrepancies in the 21+ level lifetime in this self-conjugate nucleus.5 MoreReceived 13 April 2017Revised 28 June 2017DOI:https://doi.org/10.1103/PhysRevC.96.024305©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsLifetimes & widthsNuclear structure & decaysProperties20 ≤ A ≤ 38TechniquesRadiation detectorsShell modelNuclear Physics

Investigation of chirality in the case of 102Rh

Excited states in 102Rh were populated in the fusion-evaporation reaction 94Zr(11B, 3n)102Rh at a beam energy of 36 MeV using the INGA spectrometer at IUAC, New Delhi. The angular correlations and the electromagnetic character of some of the gamma-ray transitions observed were investigated in details. A new chiral candidate sister band was found. Lifetimes of exited states in 102Rh were measured using the Doppler-shift attenuation technique and the derived reduced transition probabilities are compared to the predictions of the Two Quasiparticles Plus Triaxial Rotor model.

Candidates for Twin Chiral Bands inRh102

Excited states in 102Rh, populated in the fusion-evaporation reaction Zr94(11B,3n)102Rh at a beam energy of 36 MeV, were studied using the Indian National Gamma Array spectrometer at Inter University Accelerator Center, New Delhi. The angular correlations and the electromagnetic character of some of the gamma-ray transitions observed were investigated in detail. A new chiral candidate sister band was found. Lifetimes of exited states in both chiral candidate bands of 102Rh were measured for the first time in the A∼100 mass region by means of the Doppler-shift attenuation technique. The derived reduced transition probabilities are compared to the predictions of the two quasiparticles plus triaxial rotor model. Both experimental results and calculations do not support the presence of static chirality in 102Rh.

Collective and two-quasiparticle excitations in128Te

Excited levels of ${}^{128}$Te to 3.3 in MeV excitation have been studied using $\ensuremath{\gamma}$-ray spectroscopy following inelastic scattering of accelerator-produced neutrons. Spectroscopic information, including transition energies, level spins, $E$2/$M$1 multipole-mixing ratios, and $\ensuremath{\gamma}$-ray branching ratios, was determined from $\ensuremath{\gamma}$-ray excitation functions measured from ${E}_{n}=2.15$--3.33 MeV in 90-keV increments, $\ensuremath{\gamma}$-ray angular distributions measured at ${E}_{n}=2.2$, 2.8, and 3.3 MeV, and $\ensuremath{\gamma}$$\ensuremath{\gamma}$ coincidences measured at ${E}_{n}=3.6$ MeV. Lifetimes of levels in ${}^{128}$Te were deduced using Doppler-shift attenuation techniques. Absolute transition probabilities were determined for many levels and compared to interacting boson model and particle-core coupling model calculations to identify few particle and collective structures; states exhibiting the decay characteristics expected for two-phonon, mixed-symmetry, and quadrupole-octupole coupled states are identified.

Transient fieldgfactor and mean-life measurements with a rare isotope beam of126Sn

Background: The $g$ factors and lifetimes of the 2${}_{1}^{+}$ states in the stable, proton-rich Sn isotopes have been measured, but there is scant information on neutron-rich Sn isotopes.Purpose: Measurement of the $g$ factor and the lifetime of the 2${}_{1}^{+}$ state at 1.141 MeV in neutron-rich ${}^{126}$Sn (${T}_{1/2}=2.3\ifmmode\times\else\texttimes\fi{}{10}^{5}y$).Method: Coulomb excitation in inverse kinematics together with the transient field and the Doppler shift attenuation techniques were applied to a radioactive beam of ${}^{126}$Sn at the Holifield Radioactive Ion Beam Facility.Results: $g({2}_{1}^{+})=\ensuremath{-}0.25(21)$ and $\ensuremath{\tau}({2}_{1}^{+})=1.5(2)$ ps were obtained.Conclusions: The data are compared to large-scale shell-model and quasiparticle random-phase calculations. Neutrons in the ${h}_{11/2}$ and ${d}_{3/2}$ orbitals play an important role in the structure of the 2${}_{1}^{+}$ state of ${}^{126}$Sn. Challenges, limitations, and implications for such experiments at future rare isotope beam facilities are discussed.

New lifetime measurements in the stable semimagic Sn isotopes using the Doppler-shift attenuation technique

Precise measurements of lifetimes in the picosecond range of excited states in the stable even-A Sn isotopes 112,114,116,122Sn have been performed using the Doppler shift attenuation technique. For the first excited 2+ states in 112Sn, 114Sn and 116Sn the E2 transition strengths deduced from the measured lifetimes are in disagreement with the previously adopted values. They indicate a shallow minimum at N = 66 in contrast to the maximum at mid-shell predicted by modern shell model calculations.

Evidence for reduced collectivity around the neutron mid-shell in the stable even-mass Sn isotopes from new lifetime measurements

Precise measurements of the lifetimes of the first excited 2+ states in the stable even-A Sn isotopes 112–124Sn have been performed using the Doppler shift attenuation technique. For the isotopes 112Sn, 114Sn and 116Sn the E2 transition strengths deduced from the measured lifetimes are in disagreement with the previously reported values and indicate a shallow minimum at N=66. The observed deviation from a maximum at mid-shell is attributed to the obstructive effect of the s1/2 neutron orbital in generating collectivity when near the Fermi level.

Question of dynamic chirality in nuclei: The case ofPr134

Lifetimes of exited states in $^{134}$Pr were measured by means of the recoil distance Doppler-shift and Doppler-shift attenuation techniques. The branching ratios and the electric or magnetic character of the transitions were also investigated. The experiments were performed at IReS, Strasbourg, using the EUROBALL IV spectrometer, in conjunction with the inner bismuth germanate ball and the Cologne coincidence plunger apparatus. Exited states in $^{134}$Pr were populated in the fusion-evaporation reaction $^{119}Sn(^{19}F, 4n)^{134}$Pr. The possible chiral interpretation of twin bands was investigated in the two-quasiparticle triaxial rotor and interacting boson-fermion-fermion models. The analysis of the wave functions has shown that the possibility for the angular momenta of the proton, neutron, and core to find themselves in the favorable, almost orthogonal geometry, is present but is far from being dominant. The structure is characterized by large $\beta$ and $\gamma$ fluctuations. The existence of doublets of bands in $^{134}$Pr can be attributed to weak chirality dominated by shape fluctuations.

Properties ofCd112from the(n,n'γ)reaction: Lifetimes and transition rates

Lifetimes of levels below 4 MeV in $^{112}\mathrm{Cd}$ have been measured using the Doppler shift attenuation technique following inelastic scattering of monoenergetic neutrons. Reduced transition rates are determined using the results of previous studies and the current lifetimes. The electromagnetic properties of $^{112}\mathrm{Cd}$ are outlined, and together with results from previous nucleon-transfer studies and inelastic scattering, the levels in $^{112}\mathrm{Cd}$ are interpreted in terms of single-particle configurations and collective excitations, assuming a vibrational model with intruder states. The collective states and their \ensuremath{\gamma}-ray decays are compared with IBM-2 model calculations that allow for the mixing between the normal phonon states and intruder configurations. Levels below 1.5 MeV are reproduced reasonably well, whereas at higher excitation energy the calculations fail to reproduce the data in detail.

LIFETIME MEASUREMENTS IN 134Pr AND CHIRALITY IN NUCLEI

Lifetimes of exited states in 134 Pr were measured be means of the recoil distance Doppler-shift and Doppler-shift attenuation techniques. The experiments were performed at IReS, Strasbourg using the EUROBALL IV spectrometer, in conjunction with the inner BGO ball and the Cologne coincidence plunger apparatus. Exited states in 134 Pr were populated in the fusion-evaporation reaction 119 Sn (19 F , 4 n )134 Pr . Reduced transition probabilities in 134 Pr are compared to the predictions of the two quasiparticle+triaxial rotor and interacting boson fermion-fermion models. The experimental results do not support the presence of static chirality in 134 Pr underlying the importance of shape fluctuations. Only within a dynamical context the presence of intrinsic chirality in 134 Pr can be supported.

Role of 4π charged-particle detector arrays in lifetime measurements by Doppler-shift attenuation methods: the Microball

The effects introduced by 4π charged-particle detector arrays such as the Microball on lifetime measurements by Doppler-shift attenuation techniques are discussed. The effect originates from the lower efficiency for detecting charged particles at backward angles. Procedures for correcting for such “bias” are described.

Collectivity at high spins in156Dy

Mean lifetimes of five levels in the S band and three levels in the ground-state band of ${}^{156}\mathrm{Dy},$ populated via the ${}^{124}\mathrm{Sn}{(}^{36}\mathrm{S},4n)$ reaction at $E=145\mathrm{MeV},$ were measured by the Doppler-shift attenuation technique using a line-shape analysis. The differential decay-curve method was applied for a lifetime determination. In contrast with earlier data, the present $B(E2)$ values in the yrast band do not indicate a reduction in the collectivity after the crossing between the S band and the ground-state band, and confirm the results of theoretical calculations using mean-field approaches.

Evidence for shape coexistence in 77Br

The nucleus 77Br has been studied in the reaction 65Cu( 16O, 2p2n) at E=75 MeV. Lifetimes have been measured for several states using the Doppler-shift attenuation technique. The B(E2) values for the inband transitions, deduced from the lifetime results, reflect strong collectivity for both the positive-parity yrast band (band 1) and the ground-state negative-parity band (band 2). States with spin above 17/2 + in band 1 are found to have a large average quadrupole deformation of β 2=0.35. The experimental results for band 2 suggest that the 25/2 − and 29/2 − states are somewhat more strongly deformed than the lower-spin states. A comparison of the experimental results with particle-rotor-model calculations provides evidence for shape coexistence in 77Br, favouring a prolate shape for the g 9/2 yrast band and an oblate shape for the lowest negative-parity band.

High-spin structure and electromagnetic transition strengths in104Cd

The recoil distance Doppler shift and Doppler shift attenuation techniques were employed to determine for the first time lifetimes of high spin states in Cd-104in the time range 0.3 ps less than equal to tau less than 1.2 ns. The new experimental results are discussed in the frame of the interacting boson plus broken pair model.

Proton–neutron structure of low lying collective quadrupole excitations in 126Xe

Low spin states of the nucleus 126Xe were studied by means of γ-spectroscopy using the fusion evaporation reaction 123Te( α,n) 126Xe performed at the Cologne fn tandem accelerator. The γγ coincidence and singles spectra were measured with the osiris-cube spectrometer. Beside the ground and quasi-gamma bands many other low lying states were observed. γγ directional correlations from oriented states (DCO) were analyzed to determine the multipolarities of the γ-transitions. We observed dominant E2-transitions in the quasi-gamma band and from the gamma- to the ground band but also a large M1-fraction in the 6 + 2→6 1 + transition. We additionally found a strong M1-decay depopulating the 2 + 4-state at 2064 keV. The lifetime of this state was measured using the Doppler shift attenuation technique. The experimental data are compared with a calculation using the Proton–Neutron Interacting Boson Model (IBM-2). Good agreement is reached in the vicinity of the O(6)-limit. The observed M1-transition of the 2 + 4 state is interpreted as due to a fragment of the lowest 2 + mixed-symmetry state predicted by the IBM-2.

Rotational Quadrupole Moment of Thermally Excited High Spin States inY164b

The lifetime of the unresolved $\ensuremath{\gamma}$ transitions forming the rotational quasicontinuum spectrum has been measured for ${}^{164}\mathrm{Yb}$ using the Doppler shift attenuation technique. The fractional Doppler shifts of transitions along the diagonal valley of ${E}_{{\ensuremath{\gamma}}_{1}}\ifmmode\times\else\texttimes\fi{}{E}_{{\ensuremath{\gamma}}_{2}}$ spectra are extracted for the first time using a new technique based on the fluctuation patterns of counts. The rotational collectivity was found to persist with full strength also when the rotational bands are strongly mixed. For the high spin $(I>20\ensuremath{\Elzxh})$ and energy $(0<{E}_{\mathrm{ex}}<2\mathrm{MeV})$ regions there is agreement between the discrete and the damped transitions and with ${Q}_{t}\ensuremath{\approx}5.5e\mathrm{b}$.

In-band M1 and E2 transition rates and collective structures in 128Ba

Subpicosecond mean lifetimes of eight excited states in 128Ba populated via the 96Zr( 36S,4n) reaction were measured by the Doppler-shift attenuation (DSA) technique using a line-shape analysis. The differential decay-curve method (DDCM) was applied for the lifetime determination. The B(E2) values in the yrast band indicate that the first band-crossing is with a proton S-band. The configuration πh 11 2 d 5 2 of the negative-parity semi-decoupled bands is confirmed by the measured B(E2, I → I − 2) and B(M1, I → I − 1) transition strengths. The higher-lying “dipole” band in 128Ba can be described as a high- K four-quasiparticle band built on the prolate configuration (πh 11 2 d 5 2 ) ⊗ (νh 11 2 g 7 2 ) .

Lifetime measurements of the rotational quasi-continuum transitions in 163 Tm

Abstract The lifetime of the unresolved transitions forming the rotational quasi-continuum spectra has been measured in 163 Tm using the Doppler shift attenuation technique. Spectra associated to different values of the coincidence folds, angular anisotropies and the dynamical moment of inertia were also obtained from the present experiment. The Doppler shift has been measured for discrete transitions, for the ridge structures in E γ 1 × E γ 2 spectra and for the rotational E2 bump in one dimensional spectra. The data were compared to the theory using a simulation model which makes use of microscopically calculated levels and transition probabilities. Above 1300 keV the results indicate collectivity with lifetime faster than 10 −14 s, while at lower E γ the results are well described by predictions with B (E2) = 400 WU.

Lifetime measurement of excited states in 121,123Te.

The levels up to 1.341 MeV in $^{121}\mathrm{Te}$ and up to 1.037 MeV in $^{123}\mathrm{Te}$ were excited via the $^{121,123}\mathrm{Sb}$(p,n,\ensuremath{\gamma}) reaction. For the first time, lifetimes of the energy levels at 758, 807, 811, 912, 944, 1149, and 1341 keV in $^{121}\mathrm{Te}$ and 784, 895, and 1037 keV in $^{123}\mathrm{Te}$ have been measured using the Doppler shift attenuation technique. For the 831 keV level in $^{121}\mathrm{Te}$ we have given a lower limit to the lifetime.

Ridge lifetimes in 167,168Yb

A Doppler shift attenuation technique has been applied to the quasi continuum ridge structure of the deformed nuclei 167,168Yb populated by the reaction 124Sn + 48Ca. The Doppler shift of the ridge is analysed and compared to a simple rotational model for the feeding transitions and to more detailed Monte Carlo simulations. The data show a reduction of the rotational strength compared to the low spin E2 decay and point to strong alignment effects around spin 40ħ.