Recoil Distance Doppler Shift Method Research Articles

Lifetime measurements in the yrast band of 187Hg

Lifetimes of excited nuclear states in 187Hg have been measured using the 159Tb (32S, p3n)187Hg reaction at a beam energy of 154 MeV at the Inter University Accelerator Center, Delhi. In the experiment, the lifetimes of the five lowest yrast states have been determined using the recoil distance Doppler-shift (RDM) method. The extracted transition quadrupole moments show a sharp increase with increasing level spin, indicating a major shape transition happening in this nucleus. For a better insight into the predicted shape coexistence picture in 187Hg nucleus, the experimental results are compared with the results of the total Routhian surface calculations. The results indicate the shifting of deformed structures from weakly prolate at low-spins to strongly prolate at high spins in 187Hg nucleus.

Study the structure of the low-lying states of 206Po

The nucleus 206Po was studied in the two proton transfer reaction 204Pb(16O,14C)206Po and the lifetime of the first excited 2+ state was determined by utilizing the Recoil Distance Doppler Shift method. The experimental results are compared with shell-model calculations based on different effective interactions. The calculations qualitatively reproduced the experimentally observed B(E2;21+→01+) value, suggesting that the 21+ state of 206Po exhibits a collective nature. However, the employed effective interactions revealed some limitations, particularly in their description of the 41,2+ states. These results emphasize the importance of understanding the properties of low-lying states, especially their evolution from single-particle dynamics to collective modes, in evaluating various effective nuclear interactions.

Advancements of $$\gamma $$-ray spectroscopy of isotopically identified fission fragments with AGATA and VAMOS++

$$\gamma $$ -ray spectroscopy of fission fragments is a powerful method for studies of nuclear structure properties. Recent results on the spectroscopy of fission fragments, using the combination of the AGATA $$\gamma $$ -ray tracking array and the VAMOS++ large acceptance magnetic spectrometer at GANIL, are reported. A comparison of the performance of the large germanium detector arrays EXOGAM and AGATA illustrates the advances in $$\gamma $$ -ray spectroscopy of fission fragments. Selected results are highlighted for prompt $$\gamma $$ -ray spectroscopy studies, measurements of short lifetimes of excited states with the Recoil Distance Doppler-Shift method, using both AGATA and VAMOS++ and prompt-delayed $$\gamma $$ -ray spectroscopy studies using AGATA, VAMOS++ and EXOGAM.

Lifetime measurements in the yrast band of Po212 with a shell-model investigation

The lifetimes of the first excited ${4}^{+}$ and ${6}^{+}$ states in $^{212}\mathrm{Po}$ were measured using the recoil-distance Doppler shift method. The nucleus of interest was populated in the $^{208}\mathrm{Pb}(^{18}\mathrm{O},^{14}\mathrm{C})^{212}\mathrm{Po}$ reaction. The experimental results are compared to large-scale shell model calculations performed using the recently developed H208 effective interaction. The calculations describe well the low-lying structure of $^{212}\mathrm{Po}$ with an exception of the ${2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$ transition rate which is strongly overestimated. This has been attributed to additional components in the wave function of the ground state.

Development of the multi-purpose Cologne Compact Differential Plunger (CoCoDiff) for the measurement of nuclear level lifetimes with the Recoil Distance Doppler-shift method

A new 3-foil plunger device, the Cologne Compact Differential (CoCoDiff) plunger has been built and commissioned. Due to its compact size, it can be used together with many different spectrometers and auxiliary detectors. As a commissioning experiment, level lifetimes of the 21+ and the 41+ excited states of 50Cr have been measured, using the Differential Decay Curve method (DDCM). A derivation is given on how this method can be applied to a differential plunger measurement, in order to measure distances for lifetimes from two different regions of sensitivity at the same time. The commissioning experiment took place at the Cologne FN tandem accelerator, using the reaction 24Mg(32S,4p2n)50Cr. Lifetimes, deduced from this measurement, agree well with literature values from earlier measurements.

Revisiting the measurement of absolute foil-separation for RDDS measurements and introduction of an optical measurement method

Uncertainties in absolute plunger distances are an often overlooked component in measurements with the Recoil Distance Doppler-shift (RDDS) method when the Differential Decay Curve (DDC) method cannot be used for the analysis. It is shown how these uncertainties arise when the capacitance method is used to determine absolute distances and how they influence the obtained lifetime values of a plunger experiment. Furthermore, a new approach to obtain absolute foil separations for plunger measurements using a high precision optical distance probe is introduced, which can reduce these uncertainties by a large margin. The performance of this method is demonstrated by comparisons to the capacitance method as well as to a precise measurement of absolute distances using known lifetimes in 181Ta.

Lifetime measurements in the tungsten isotopes W176,178,180

Lifetimes of yrast states in the rare-earth midshell isotopes $^{176,178,180}\mathrm{W}$ have been measured with fast-timing methods using the Cologne iron-free Orange spectrometer and the Cologne HORUS spectrometer and with the recoil distance Doppler shift method using the Cologne coincidence plunger setup. Different fusion evaporation reactions have been used to populated excited states in the investigated nuclei. Lifetimes of the ${2}_{1}^{+}$, ${4}_{1}^{+}$, ${6}_{1}^{+}$, and ${8}_{1}^{+}$ yrast states were measured in all three tungsten isotopes and in addition the lifetime of the ${10}_{1}^{+}$ state was measured in $^{176}\mathrm{W}$. Quadrupole deformation parameters, reduced transition probabilities, and ${\text{B}}_{4/2}$ ratios are extracted and discussed in the context of interacting boson model 1 calculations. The results with newly determined signatures largely confirm the investigated tungsten isotopes to be deformed prolate rotors.

Single-particle and collective excitations in the transitional nucleus 166Os

The mean lifetimes of the lowest energy 2+, 8+ and 9− states in 166Os have been measured using the recoil distance Doppler-shift method in conjunction with a selective recoil-decay tagging technique. These measurements extend studies into the most neutron-deficient mass region accessible to current experimental methods. The B(E2; 2+ → 0+) = 7(2) W.u. extracted from these measurements is markedly lower than those observed in the heavier even-mass Os isotopes. The 8+ and 9− states yield reduced transition probabilities that are consistent with single-particle transitions. While these values may indicate a departure from collective structure, the level scheme and the underlying nuclear configurations can also be interpreted in terms of a simple collective picture. This contrasting behaviour suggests an intriguing dichotomy in the description of heavy transitional nuclei.

Lifetime measurements of excited states in Cr55

Excited states in $^{55}\mathrm{Cr}$ have been populated via the fusion-evaporation reaction $^{48}\mathrm{Ca}(^{11}\mathrm{B},p3n){\phantom{\rule{0.16em}{0ex}}}^{55}\mathrm{Cr}$ at a beam energy of 32 MeV. The Cologne plunger device surrounded by a $\ensuremath{\gamma}$-ray detector array was employed to determine lifetimes with the recoil-distance Doppler-shift method. $\ensuremath{\gamma}$ rays were observed by one Ge EUROBALL cluster detector and five HPGe detectors. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence data were analyzed with help from the differential decay-curve method, and precise lifetimes for the first excited states were extracted from the $5/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}3/{2}^{\ensuremath{-}}$ and the $9/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}5/{2}^{\ensuremath{-}}$ transitions. Reduced transition strengths $B(\ensuremath{\sigma}\ensuremath{\lambda})$ were determined and compared to shell-model calculations employing four interactions KB3G, FPD6, GXPF1A, and GXPF1Br. The calculations were also performed for the $N=31$ isotonic chain from $^{53}\mathrm{Ti}$ to $^{61}\mathrm{Zn}$. Inspection of the wave functions as well as particle-plus-rotor model calculations allow for a detailed understanding of the excited states and the reduced transition strength in $^{55}\mathrm{Cr}$. The interactions GXPF1A and GXPF1Br reproduce well the experimental findings in $^{55}\mathrm{Cr}$ and in other isotones.

Microscopic structure of the one-phonon 2+ states of Po208

The lifetimes of the 2 1 + and 4 1 + states of Po 208 were measured in the α -transfer reaction Pb 204 ( C 12 , Be 8 ) Po 208 by γ -ray spectroscopy utilizing the recoil distance Doppler shift method. The newly extracted transition strengths alongside ones of the decay of the 2 2 + state were compared to the results of large-scale shell-model calculations using an effective interaction derived from the realistic CD-Bonn nucleon-nucleon potential. The comparison indicates the importance of the quadrupole isovector excitations in the valence shell for a fine tuning of the two-body matrix elements of the shell-model interaction.

Lifetime measurements of excited states in neutron-rich Ti53 : Benchmarking effective shell-model interactions

Level lifetimes of the yrast (5/2(-)) to 13/2(-) states in the neutron-rich nucleus Ti-53, produced in a multinucleon-transfer reaction, have been measured for the first time. The recoil distance Doppler-shift method was employed and lifetimes of the excited states were extracted by a lineshape analysis aided by GEANT4-based Monte-Carlo simulations. The experiment was performed at the Grand Accelerateur National d'Ions Lourds facility in Caen, France, by using the Advanced Gamma Tracking Array for the gamma-ray detection coupled to the large-acceptance variable mode spectrometer for an event-by-event particle identification and the Cologne plunger for deep-inelastic reactions. Reduced transition probabilities, deduced from the lifetimes, give new information on the nuclear structure of Ti-53, and are used to benchmark different shell-model calculations using established interactions in the f p shell.

Reexamined lifetimes of the low-lying states of Zr86 by recoil distance differential decay measurements

The $^{86}\mathrm{Zr}$ nucleus, which has a low-lying level scheme characteristic of a transitional nucleus, presents, according to the existing electromagnetic transition data, a subunitary ${\mathrm{B}}_{4/2}=B(E2,{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B(E2,{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ ratio, which is anomalously low for a nonmagic nucleus, as it is outside the range of the traditional collective models values. In order to check this anomaly, we performed new measurements of the lifetimes of its low-lying states, with the $\ensuremath{\gamma}$-ray coincident recoil distance Doppler shift method, using the ROSPHERE detector array. New lifetimes were determined for the positive-parity yrast states up to the ${10}^{+}$ one, and for the ${8}_{2}^{+},\phantom{\rule{0.16em}{0ex}}{5}_{1}^{\ensuremath{-}}$, and ${7}_{1}^{\ensuremath{-}}$ states. The newly determined values of the $B(E2)$ values for the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states characterize a nucleus with a moderate quadrupole deformation and are well described by the interacting boson model. The ratio ${B}_{4/2}=1.38(22)$ is no longer anomalous. The low $B(E2)$ value of the ${6}_{1}^{+}$ state indicates a noncollective structure.

Lifetime measurement of excited states in Ce144 : Enhanced E1 strengths in a candidate for octupole deformation

A lifetime measurement of excited states in $^{144}\mathrm{Ce}$ using the $^{142}\mathrm{Ce}(^{18}\mathrm{O},^{16}\mathrm{O})$ reaction with a beam energy of 67 MeV and the recoil distance Doppler-shift method was performed at the Cologne FN Tandem accelerator. Lifetimes of the three lowest yrast states in $^{144}\mathrm{Ce}$ have been measured as well as for the ${3}_{1}^{\ensuremath{-}}$ state and an effective lifetime of the ${4}_{2}^{+}$ state. Reduced $E2$ transition strengths determined using these results have been compared to predictions from recent shell-model calculations. From the interband transitions reduced $E1$ strengths could be determined, which are strongly enhanced.

Lifetime measurements using a plunger device and the EUCLIDES Si array at the GALILEO [formula omitted]-ray spectrometer

The GALILEO γ-ray spectrometer, installed at the Laboratori Nazionali di Legnaro (LNL), benefits from intense stable beams provided by the Tandem-ALPI-PIAVE accelerator complex and from radioactive beams to be delivered in the near future by the SPES facility. The spectrometer is complemented with a variety of ancillary devices to allow for nuclear structure and reaction studies. The 4π Si-ball array EUCLIDES coupled to the GALILEO γ-ray spectrometer represents one of the commonly used setup for experiments aiming at spectroscopic studies. High-efficiency detection of light-charged particles in a fusion–evaporation reaction guarantees good discrimination of different reaction channels and provides essential information for the kinematic reconstruction. In this paper we discuss a configuration of the EUCLIDES array developed for the lifetime measurements of nuclear excited states populated in a fusion–evaporation reaction. In such a configuration a part of the EUCLIDES detectors is disassembled allowing for the installation of a plunger device in the reaction chamber. The reduced configuration of EUCLIDES provides high detection efficiency necessary for reliable light charged-particle discrimination. We report on the commissioning experiment focused on the 58Ni(58Ni,3p)113I reaction. The lifetimes of 11/2− and 15/2− states were measured by applying the Recoil Distance Doppler Shift method to be equal to 206(20) ps and 7.9(12) ps correspondingly and were in good agreement with the values cited in the literature. Thus, the combination of the GALILEO and EUCLIDES arrays, and the plunger device has resulted in a powerful experimental setup to determine lifetimes of excited states in neutron-deficient nuclei in the picosecond range.

Shape coexistence in neutron-deficient Hg188 investigated via lifetime measurements

Shape coexistence in the $Z \approx 82$ region has been established in mercury, lead and polonium isotopes. Even-even mercury isotopes with $100 \leq N \leq 106$ present multiple fingerprints of this phenomenon, which seems to be no longer present for $N \geq 110$. According to a number of theoretical calculations, shape coexistence is predicted in the $^{188}$Hg isotope. The $^{188}$Hg nucleus was populated using two different fusion-evaporation reactions with two targets, $^{158}$Gd and $^{160}$Gd, and a beam of $^{34}$S, provided by the Tandem-ALPI accelerators complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the $\gamma$ rays were detected using the GALILEO $\gamma$-ray array. The lifetimes of the excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Using the two-bands mixing and rotational models, the deformation of the pure configurations was obtained from the experimental results. The extracted transition strengths were compared with those calculated with the state-of-the-art symmetry-conserving configuration-mixing (SCCM) and five-dimentional collective Hamiltonian (5DCH) approaches in order to shed light on the nature of the observed structures in the $^{188}$Hg nucleus. An oblate, a normal- and a super-deformed prolate bands were predicted and their underlying shell structure was also discussed.

Lifetime measurements of the low-lying excited states of 208Po

In this study we present the preliminary results about the lifetimes of the , states of 208Po and the upper limit of the lifetime of the state. For measuring the lifetimes of the and states the Recoil Distance Doppler Shift (RDDS) method and for the lifetime of the state the Doppler Shift Attenuation method (DSAM) were used. The resulting absolute transition strength reveals the predominant isovector nature of the state of 208Po.

Lifetime Measurements of Low-lying States in \(^{73}\)Ga and \(^{70,72,74}\)Zn Isotopes

Lifetimes of low-lying states in 73Ga and 70,72,74Zn were measured using the Recoil Distance Doppler Shift (RDDS) method. These nuclei were produced in deep-inelastic reactions in inverse kinematics with a 208Pb beam impinging on a 76Ge target. Prompt γ rays were detected using the AGATA tracking array coupled to the VAMOS++ spectrometer. Lifetime of the 5/2- state in 73Ga, measured for the first time, provides additional evidence for the existence of a 1/2-, 3/2- ground-state doublet. The lifetimes of the 4+ states in 70,72,74Zn were remeasured in an attempt to understand the discrepancies observed between earlier measurements. Our results are in agreement with those of previous plunger experiments. (Less)

Lifetime measurements of excited states in 163W and the implications for the anomalous B(E2) ratios in transitional nuclei

This letter reports lifetime measurements of excited states in the odd-N nucleus 163W using the recoil-distance Doppler shift method to probe the core polarising effect of the i13/2 neutron orbital on the underlying soft triaxial even-even core. The ratio B(E2:21/2+→17/2+)/B(E2:17/2+→13/2+) is consistent with the predictions of the collective rotational model. The deduced B(E2) values provide insights into the validity of collective model predictions for heavy transitional nuclei and a geometric origin for the anomalous B(E2) ratios observed in nearby even-even nuclei is proposed.

B(E2) anomalies in the yrast band of Os170

Background: The neutron-deficient osmium isotopic chain provides a great laboratory for the study of shape evolution, with the transition from the soft triaxial rotor in $^{168}\mathrm{Os}$ to the well-deformed prolate rotor in $^{180}\mathrm{Os}$, while shape coexistence appears around $N=96$ in $^{172}\mathrm{Os}$. Therefore, the study of the Os isotopic chain should provide a better understanding of shape changes in nuclei and a detailed scrutiny of nuclear structure calculations. In this paper, the lifetimes of the low-lying yrast states of $^{170}\mathrm{Os}$ have been measured for the first time to investigate the shape evolution with neutron number.Purpose: Lifetimes of excited states in the ground-state band of $^{170}\mathrm{Os}$ are measured to investigate the shape evolution with neutron number in osmium isotopes and compare with state-of-the-art calculations.Methods: The states of interest were populated via the fusion-evaporation reaction $^{142}\mathrm{Nd}(^{32}\mathrm{S},4n)$ at a bombarding energy of 170 MeV at the ALTO facility from IPN (Orsay, France). Lifetimes of the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states in $^{170}\mathrm{Os}$ were measured with the recoil-distance Doppler-shift method using the Orsay universal plunger system.Results: Lifetimes of the two first excited states in $^{170}\mathrm{Os}$ were measured for the first time. A very small $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=0.38(11)$ was found, which is very uncharacteristic for collective nuclei. These results were compared to state-of-the-art beyond-mean-field calculations.Conclusions: Although theoretical results give satisfactory results for the energy of the first few excited states in $^{170}\mathrm{Os}$ and the $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ they fail to reproduce the very small $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$, which remains a puzzle.

Lifetime measurements in the yrast band of Lu167

Lifetime measurements of the lower excited states in the yrast band of $^{167}\mathrm{Lu}$ have been measured using the recoil distance Doppler shift method. The excited nuclear states in $^{167}\mathrm{Lu}$ were populated via $^{159}\mathrm{Tb}\phantom{\rule{4pt}{0ex}}(^{12}\mathrm{C},4n)$ fusion-evaporation reaction at a beam energy of 74 MeV. In the experiment, the lifetime of states from ${13/2}^{\ensuremath{-}}\ensuremath{\hbar}$ to ${29/2}^{\ensuremath{-}}\ensuremath{\hbar}$ spins were obtained. From extracted lifetimes, the reduced transition probabilities $[B(E2)]$ and transition quadrupole moments $({Q}_{t})$ values have been deduced. The large $({Q}_{t})$ values obtained in the experiment indicate high deformation for $^{167}\mathrm{Lu}$ near ground state, a typical characteristic of the mass region. The almost constant values of experimental $B(E2)$ reflects the stability of deformed structure of $^{167}\mathrm{Lu}$ nucleus in the yrast configuration. The experimental results are compared with the results of the cranked Hartree-Fock-Bogoliubov model based total Routhian surface (TRS) calculations. The experimental values are in better agreement with the TRS calculations at low frequencies than at high frequencies.