Doppler-shift Attenuation Measurements Research Articles

Transition Probabilities inPr134: A Test for Chirality in Nuclear Systems

Exited states in 134Pr were populated in the fusion-evaporation reaction 119Sn(19F,4n)134Pr. Recoil distance Doppler-shift and Doppler-shift attenuation measurements using the Euroball spectrometer, in conjunction with the inner Bismuth Germanate ball and the Cologne plunger, were performed at beam energies of 87 MeV and 83 MeV, respectively. Reduced transition probabilities in 134Pr 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 134Pr underlying the importance of shape fluctuations. Only within a dynamical context the presence of intrinsic chirality in 134Pr can be supported.

Check for chirality in real nuclei

Exited states in 134Pr were populated in the fusion-evaporation reaction 119Sn( 19F, 4n) 134Pr. Recoil distance Doppler-shift and Doppler-shift attenuation measurements using the Euroball spectrometer, in conjunction with the inner BGO ball and the Cologne plunger were performed at beam energies of 87MeV and 83MeV, respectively. The measured B(E2) values within the two chiral candidates bands are not identical while the corresponding B(M1) values have a similar behaviour within the experimental uncertainties.

Background subtraction for Doppler-shift attenuation and angular correlation measurements with multi-detector Ge arrays

A novel background subtraction method for Doppler-shift attenuation and angular correlation measurements with large multi-detector HpGe arrays is presented. The formalism is designed for recently developed data base storage techniques with the event format resembling the original raw data structure. The background spectrum is constructed based on parameters extracted from one-dimensional spectra; the method, therefore, is complementary to currently applied methods which are designed for multi-dimensional, symmetrized histograms.

Proton-neutron mixed-symmetry2ms+and3ms+states in96Ru

The one-phonon mixed-symmetry ${2}_{1,\mathrm{ms}}^{+}$ state and two-phonon mixed-symmetry ${3}_{1,\mathrm{ms}}^{+}$ and ${2}_{2,\mathrm{ms}}^{+}$ states have been identified in ${}^{96}\mathrm{Ru}$ based on intensity branching ratios, $E2/M1$ multipole mixing ratios, upper lifetime limits, and a comparison with the experimental data for ${}^{94}\mathrm{Mo}.$ The observables were obtained from Doppler-shift attenuation measurements and $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlation analyses following the ${}^{95}\mathrm{Mo}{(}^{3}\mathrm{He}{,2n)}^{96}\mathrm{Ru}$ reaction and the ${\ensuremath{\beta}}^{+}$ decay of ${}^{96}\mathrm{Rh}.$ Shell-model calculations performed for ${}^{96}\mathrm{Ru}$ give clear support for the interacting boson model 2 mixed-symmetry assignments of the one-phonon ${2}_{3}^{+}$ state at 2283.8 keV as well as of the two-phonon ${2}_{5}^{+}$ state at 2739.8 keV and ${3}_{2}^{+}$ state at 2897.7 keV.

Characterization of the “three-phonon” region of 100Ru

Lifetimes of excited states in 100Ru were determined from Doppler-shift attenuation measurements following the 100Ru(n,n′γ) reaction. Absolute transition rates or limits thereon were extracted for states in the three-phonon region and were compared with an interacting boson model description of this nucleus. Intruder states were identified, and a candidate for the 2+ mixed-symmetry state is suggested.

Analysis of Doppler-shift attenuation measurements performed with accelerator-produced monoenergetic neutrons

Methods employed for the analysis of Doppler-shift attenuation data, obtained for nuclei excited by the inelastic scattering of accelerator-produced monoenergetic neutrons on large samples are presented. Analytical formulas were derived to describe the influence of two major considerations in the analysis. The reaction mechanism for stopping of recoil nuclei was found to increase the attenuation factor F ( τ ) calculated with the center-of-mass initial velocity, while geometrical effects tend to decrease F ( τ ). It is also shown that the influence of these major factors can be minimized by keeping the neutron energy close to the level threshold and by minimizing the target size. The formulas obtained can be used for any reaction that proceeds via the compound nucleus mechanism and for targets of sufficient size to stop the recoiling nuclei.

Fast electric dipole transitions in nuclei nearN=82

Doppler-shift attenuation measurements following the inelastic neutron scattering reaction have been employed to determine the lifetimes of a large number of states in $^{146}\mathrm{Nd}$, and many fast $E1$ transitions have been identified. From an examination of this new information and similar data for two $N=82$ nuclei, ambiguities in determining the degree of nuclear collectivity from reduced $E1$ transition rates are illustrated.

Lifetimes in 15N from gamma-ray line shapes produced in the 2H(14N,p gamma ) and 14N(thermal n, gamma ) reactions.

Mean lifetimes of bound states in [sup 15]N were inferred from the Doppler-shift-attenuation (DSA) of [gamma] rays produced in the inverse reaction [sup 2]H([sup 14]N,[ital p][gamma]) and from the primary gamma-ray-induced Doppler broadening (GRID) of secondary [gamma] rays in the thermal-neutron capture reaction [sup 14]N([ital n],[gamma]). Targets for the DSA measurements were prepared by implanting first neon into a gold backing and then deuterium into the same region such that deuterium was trapped at the neon precipitates. To find out experimentally the initial velocity distributions of the recoiling [sup 15]N nuclei, another target was prepared by implanting deuterium into silicon, which is a slow stopping-power medium. Computer simulations with the Monte Carlo method and experimental stopping powers were used in the DSA analysis of the [gamma]-ray line shapes. This analysis yielded the following lifetimes for eight bound levels in [sup 15]N: 43[plus minus]4 fs (5.30 MeV), [lt]12 fs (6.32 MeV), 11[plus minus]2 fs (7.16 MeV), [lt]3 fs (7.30 MeV), 129[plus minus]6 fs (7.57 MeV), [lt]3 fs (8.31 MeV), [lt]10 fs (8.57 MeV), and [lt]4 fs (9.05 MeV). GRID measurements were made with melamine (C[sub 3]H[sub 6]N[sub 6]) and with silicon nitride (Si[sub 3]N[sub 4]) as both targets and slowing-down media.more » Measurements were also made with air (80% nitrogen). The Doppler-broadened [gamma]-ray line shapes were analyzed by molecular-dynamics simulations of the slowing-down process to obtain the lifetime values of 40[plus minus]3 fs (5.30 MeV), [lt]2 fs (6.32 MeV), [lt]3 fs (7.30 MeV), and [lt]3 fs (8.31 MeV) for levels in [sup 15]N. The extent to which the mirror symmetry of levels in [sup 15]N and [sup 15]O is valid is examined in some detail.« less

Transition probabilities between high spin states in 88Mo and 90Mo.

The reactions (Ni(Ar, alpha2p)Mo)-Ni-58-Ar-36-Mo-88 and (Ni(Ar, 4p)Mo)-Ni-58-Ar-36-Mo-90 at 149 and 140 MeV beam energies have been used to carry out Doppler shift attenuation and recoil distance lifetime measurements and to determine DCO ratios in Mo-88 and Mo-90. A singles angular distribution experiment using the reaction (Ni(Cl, 3p)Mo)-Ni-59-Cl-35-Mo-90 has been performed at 120 MeV beam energy to measure spin assignments in Mo-90 and to calibrate the DCO analyses for both nuclei. Reduced transition probabilities for more than 60 transitions in Mo-88, Mo-90 have been deduced. The extensive data set is incorporated in a detailed comparison between the two nuclei and, for Mo-90, with predictions from shell model calculations within a restricted pi(p1/2), pi(99/2), nu(p1/2), and nu(g9/2) configuration space and with different residual interaction parameters. The electromagnetic properties of the negative parity states of Mo-90 are well reproduced by the calculations while the positive parity states show hints of seniority mixing at intermediate spins.

Properties of high spin states in 53Cr and 53Mn.

High spin states of {sup 53}Cr and {sup 53}Mn have been studied by means of the reactions {sup 51}V({sup 4}He,{ital pn}{gamma}){sup 53}Cr and {sup 51}V({sup 4}He,2{ital n}{gamma}){sup 53}Mn, respectively, at {ital E}=30 MeV. In-beam {gamma}-ray singles, multiparameter coincidence, angular distribution, and Doppler-shift attenuation measurements have been performed, yielding significant new information. These include the mean lifetimes 0.15{sub {minus}0.06}{sup +0.05} ps for the 5002.4 keV (17/2{sup {minus}}) state in {sup 53}Cr; 0.23{sub {minus}0.07}{sup +0.08}, 0.17{plus minus}0.08, and 0.25{sub {minus}0.06}{sup +0.08} ps for the 4383.2 (17/2{sup {minus}}), 5613.3 (19/2{sup {minus}}), and 7961.2 (25/2{sup {minus}}) keV states, respectively, in {sup 53}Mn; multipole mixing ratios 0.11{sub {minus}0.11}{sup +0.09} for the 17/2{sup {minus}}{r arrow}15/2{sup {minus}} transition in {sup 53}Cr; {minus}0.16{sub {minus}0.08}{sup +0.03}, {minus}0.05{sub {minus}0.11}{sup +0.10}, and {minus}0.05{sub {minus}0.11}{sup +0.12} for the 19/2{sup {minus}}{r arrow}17/2{sup {minus}}, 23/2{sup {minus}}{r arrow}21/2{sup {minus}}, and 25/2{sup {minus}}{r arrow}23/2{sup {minus}} transitions, respectively, in {sup 53}Mn. The present results lend support to the existence of a {ital K}=7/2 band in {sup 53}Cr. A comparison of the experimental data with the available theoretical calculations in {sup 53}Mn shows that yrast states with {ital J}{ge}17/2 are almost pure 1p4h states.

The collectivity and the de-excitation of the yrast superdeformed band in 150Gd

A Doppler shift attenuation measurement has been carried out to determine the collectivity of the superdeformed band in 150Gd. The data was found to be consistent with a constant inband quadrupole moment of 17 ± 3 eb. This corresponds to a quadrupole deformation of ß 2≈0.58. In addition the measurement has resolved important questions regarding the de-excitation of the band, confirming the rapid de-excitation of the superdeformed band in 150Gd with more than 80% of the band intensity being lost over one transition.

On the coexistence of oblate and prolate deformed bands in83Zr

Deformation parameters of the positive parity yrast band and negative parity bands in83Zr are deduced from lifetimes andE2/M1 mixing ratios. Lifetimes of high spin states have been determined from recoil distance Doppler shift and Doppler shift attenuation measurements using the54Fe(32S,2pnγ) 83Zr reaction. Ten lifetimes and five lifetimes limits were determined. The positive parity band, built on theg 9/2 K=5/2 orbital has an average deformation ¦β 2¦=0.28(2), and shows a reduction ofE2 transition strengths in the observed backbend region at Iπ≈21/2+. In contrast, theE2 strengths in the negative parity states show a steady increase up to Iπ≈=15/2−. These states are more strongly deformed than the positive parity states (¦β 2¦=0.33(3)). TheE2/M1 mixing ratios show that the negative parity band hasK=3/2 and is prolate, and favour oblate deformation for the positive parity yrast band. In theK=1/2− band theE2 strength of the 7/2−→3/2− transition yields a deformation ¦β 2¦=0.26(5). The band structure is compared with calculations within the Hartree-Fock Bogoliubov cranking model.

Electronic stopping power of Ta for Z = 11–18 atoms at energies 0–0.8 [formula omitted

The velocity dependence of the electronic stopping power of Ta has been studied for 23Na, 24Mg, 27Al, 29Si, 31P, 34S, 35Cl 38Ar ions in the energy region 0–0.8 MeV/nucleon. By application of Doppler shift attenuation measurements, computer simulations and experimental nuclear stopping power values, the electronic stopping power was deduced in the previously uninvestigated velocity region v/ v 0 = 0–5.8 ( v 0 = Bohr velocity) in the framework of an empirical model. This model was obtained to reproduce the electronic stopping power of Ta for the Z = 11–18 atoms to within an uncertainty of ± 7%.

On the validity of the discontinuous stopping technique in Doppler shift attenuation measurements

Monte Carlo calculations have been performed to investigate some quantitative features of the slowing process underlying nuclear lifetime determination in Doppler Shift Attenuation Method (DSAM). Kregar et al’s numerical method developed for a simplified description of the process was found to be applicable in strongly limited intervals of recoil velocity and lifetime only.

Yrast decays in87,89Zr,87,88,89,90Y and86Sr from74,76Ge(18O,xn,yp,z ) reactions

Gamma-ray data from 74,76Ge+18O fusion-evaporation reactions are analysed to provide data on yrast decay schemes in 87,89Zr, 87,88,89,90Y and 86Sr. The data include excitation functions, angular distributions, gamma - gamma coincidences, linear polarisation measurements and lifetimes deduced from both Doppler shift attenuation and recoil distance measurements. Shell-model calculations are presented for 88,90Y and 86Sr.

The effect of the properties of slowing media on nuclear lifetimes derived from DSA measurements

Apparent lifetimes determined from Doppler Shift Attenuation (DSA) measurements are known to depend on the atomic number Z2 of the stopping medium. It is shown here that inaccuracies in the data of the medium result in additional fluctuation as the function of Z2.

High-spin state spectroscopy of 88,90Zr.

The high-spin states of $^{88}\mathrm{Zr}$ and $^{90}\mathrm{Zr}$ were studied up to \ensuremath{\sim}12-MeV excitation via the $^{74}$,76Ge${(}^{18}$O,4n\ensuremath{\gamma})${}^{8}$ 8,90Zr fusion evaporation reactions. Decay schemes were constructed on the basis of gamma-ray excitation functions, angular distribution and linear polarization measurements, \ensuremath{\gamma}-\ensuremath{\gamma} coincidences, and level lifetimes determined from both Doppler shift attenuation and recoil distance measurements. An effort was made to make as definite spin-parity assignments as possible and to delineate the reliability of these assignments. Probable spins up to \ensuremath{\sim}20\ensuremath{\Elzxh} were formed in both nuclei. The experimental results are compared to preliminary results of a large-scale shell model calculation. The yrast spectra of both nuclei can be interpreted as arising from intrinsic modes of excitation with no evidence for collective behavior.

The yrast bands in 77Kr and 76Kr

High-spin states of both parities in 77Kr have been studied in the reaction 63Cu( 16O, pn). On the basis of γγ- and nγ-coincidence experiments, we extended the yrast bands up to spin I π = 25 2 + and 21 2 t-. Doppler-shift attenuation and recoil-distance measurements have been performed for some fifteen states. In addition, lifetimes of some yrast states in 76Kr have been determined in the reaction 63Cu( 16O, p2n). Transition energies and E2 strengths in 76Kr have been interpreted with the IBA-2 and different collective and microscopic triaxial rotor models, those in 77Kr with the triaxial rotor plus quasiparticle model.

Nuclear stopping in polycrystalline materials: Range distributions and Doppler-shift attenuation analysis

We describe a comprehensive computer program COSIPO, computer simulation in polycrystals, which uses the binary-collision approximation to simulate the slowing down of heavy ions in the keV region in crystalline targets. The atomic scattering is governed by a potential which is chosen as an option. Inelastic losses are either impact-parameter-dependent or friction type. Thermal vibrations of the target atoms can be included. Various ways to treat polycrystallinity are described. The range distributions are compared with results based on existing codes and the significance of various parameters is studied. The effect of crystalline structure on the results of Doppler-shift attenuation measurements [line shapes and $F(\ensuremath{\tau})$ values] is calculated in various ways. Both range distributions and line shapes are found to be in agreement with experiment. Nuclear stopping during the slowing-down process in polycrystalline materials can be either smaller or larger than in amorphous materials. Some possible applications of the results are discussed.

A gas cell for Doppler-shift attenuation measurements up to 107 Pa

A cell suitable for Doppler-shift attenuation measurements in gas at high pressures is described. The variation of the gas density as a function of the beam current is studied for argon and krypton gases. Examples of lifetime measurements in the picosecond range are given.