Doppler-shift Attenuation Research Articles

Crystallographic and electrical properties of mechanically rolled gold backed niobium target

This work demonstrates an excellent and high purity target film, realized by depositing uniformly thick mechanically rolled niobium (Nb) film over gold (Au) foil using indium (In) as adhesive layer (Nb/In/Au). The materials properties of the resulting foil in three layered Nb/In/Au structure, characterized using microscopic and spectroscopic techniques demonstrate presence of pure forms of the material contents and depict crystal phases of pure Au and Nb in Au and Nb/In/Au foils, respectively. This suggests the chemical inertness of Nb film towards oxidation during mechanical rolling and evidences formation of high purity Nb layer with precise crystallinity. The current-voltage (I–V) characteristics of Nb/In/Au foil measured at room temperature through a lateral device structure configuration with silver (Ag) top contacts exhibit ohmic conduction with surface resistivity of 2.5mΩm, indicating 127.3% increment after mechanical rolling of Nb on Au foil. The fabricated gold backed niobium target has been utilized in a recent nuclear level lifetime measurement experiment through Doppler shift attenuation method.

High-Precision Spectroscopy of ^{20}O Benchmarking AbInitio Calculations in Light Nuclei.

The excited states of unstable ^{20}O were investigated via γ-ray spectroscopy following the ^{19}O(d,p)^{20}O reaction at 8 AMeV. By exploiting the Doppler shift attenuation method, the lifetimes of the 2_{2}^{+} and 3_{1}^{+} states were firmly established. From the γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2_{2}^{+} and 3_{1}^{+} states, the B(E2) and B(M1) were determined. Various chiral effective field theory Hamiltonians, describing the nuclear properties beyond ground states, along with a standard USDB interaction, were compared with the experimentally obtained data. Such a comparison for a large set of γ-ray transition probabilities with the valence space in medium similarity renormalization group abinitio calculations was performed for the first time in a nucleus far from stability. It was shown that the abinitio approaches using chiral effective field theory forces are challenged by detailed high-precision spectroscopic properties of nuclei. The reduced transition probabilities were found to be a very constraining test of the performance of the abinitio models.

Fabrication and characterization of thin 94Zr target with gold backing for nuclear lifetime measurements using DSAM

For lifetime measurements of excited nuclear states in the sub-picosecond range, a thin target with a thick backing of high-Z material is required. To meet these specifications, a thin isotopic94Zr target of thickness 1.14 ± 0.05 mg/cm2, with backing of 4.7 ± 0.2 mg/cm2 thick Au layer, has been fabricated using the cost-effective cold rolling technique at the target laboratory of Inter-University Accelerator Centre (IUAC), New Delhi. The thickness verification and purity check of the fabricated target have been carried out by non-destructive Energy Dispersive X-ray Fluorescence (EDXRF) and Rutherford Backscattering Spectrometry (RBS) techniques. The results of these measurements are in good agreement with each other. The elemental composition, purity and surface morphology have been explored by Energy Dispersive X-ray Spectrometry (EDS) and Scanning Electron Microscopy (SEM) techniques. The fabricated target has been successfully used with a16O ion beam in a recent γ-ray spectroscopy experiment for lifetime measurements using the Doppler Shift Attenuation Method (DSAM) at IUAC, New Delhi.

Lifetime measurement of the Ex = 2485.3 keV level of 25Al populated through 24Mg(p,γ)25Al resonance reaction

Results of our experimental study of the 24Mg(p,γ)25Al resonance reaction at E=plab223 keV are presented. The proton beam energy is varied from 220 to 265 keV. An evaporated Mg target with thick Ta backing is used. We remeasure a mean lifetime of τ=6.04−2.65+3.03 fs for the E=x2485.3 keV level of 25Al, using Doppler shift attenuation (DSA) method. Three most successfully used empirical effective interactions developed for the sd shell by Wildenthal (w), Chung-Wildenthal (cw) and Preedom-Wildenthal (pw), are utilized to calculate energy spectra, spectroscopic factors, beta decay properties, transition probabilities and the lifetime of the resonance state in 25Al within the framework of nuclear shell model. The theoretical results agree reasonably well with the experimental data. However, a detailed study for each set to identify the most preferred interaction for this nucleus is performed. Calculated lifetimes using two of the interactions (w, cw) agree better with the central value of the experimental lifetime of the resonance state measured in the present work.

Investigating the intruder states of 83Se via lifetime measurements

Quadrupole interaction involving protons and neutrons drives the nucleus into deformed configurations at low excitation energies. Intruder states appear in N=49 isotones, reaching a minimum at around 500 keV in 83Se. Since 83Se is in the mid of the proton shell (Z=28-40), it is a good candidate to study the properties of particle-hole intruder states lowered in energy by large quadrupole correlations. Moreover, it will also allow one to estimate the degree of N=50 core breaking in the ground state of Se isotopes. The lifetime of the 540-keV 1/2+ state and 1100-keV 3/2+ state of 83Se were measured using the Recoil Distance Method and the Doppler Shift Attenuation Method respectively. A beam of 82Se, with intensity 0.02 pnA, accelerated to 270 MeV from the Tandem accelerator at LNL-INFN, was sent into a deuterated polyethylene foil (C2D4), evaporated on a 6 mg/cm2 gold layer. The GALILEO γ-array was coupled to the SPIDER silicon array, allowing one to obtain the needed channel selectivity through particle-γ coincidence measurements.

Occupancy of orbitals and the quadrupole collectivity in 45Sc nucleus.

In the present work, the Doppler Shift Attenuation method (DSAM) was used to analyze the observed lineshapes of transitions from excited states in 45Sc, populated in the reaction 36Ar + 12C at a beam energy of 145 MeV. The interpretation and comparison of the experimental results have been performed with large-scale shell model calculations, involving different interactions like: GX1A, GX1J, FPD6, KB3 and ZBM2. KB3 and FPD6 (present work) interactions in the negative parity states, and in positive parity states ZBM2 are most pre-eminent in reproducing the results, due to the large configuration space describing strong collective effects. Furthermore, the present work also looks at the details of the shell model helping in improving the understanding for the occupancy of orbitals. The present investigation suggests the observation of stronger collectivity for positive parity states over negative parity states with predicted enhanced collectivity of states in 45Sc nucleus.

Nuclear structure studies in mirror nuclei

The nuclear structure of the A=31 and A=47 mirror couples produced by two fusion evaporation reactions has been elaborated, utilizing the Doppler-shift attenuation method. Excited states in 31P and 31S were populated using the 1p and 1n exit channels, respectively, of the reaction 20Ne + 12C, while in 47Cr and 47V couple excited states were populated based on 28Si + 28Si reaction, as products of 2αn and 2αp exit channels. The A=31 mirror couple was studied utilizing Piave-Alpi accelerator of the Laboratori Nazionali di Legnaro with GASP multidetector array and for A=47 one - with the EUROBALL array using XTU Tandem also in Legnaro. In both cases the lifetime measurements in mirror couples at the same experiment open possibilities for investigations of isospin symmetry. Determined B(E1) strengths in the mirror nuclei 31P and 31S allow to extract the isoscalar component, which can reach up to 24% of the isovector one. The B(E1) values can be modeled by the Equation of motion method. In the case of A=47 mirror couple, the quadrupole moments can be described by shell-model calculations.

First application of Markov chain Monte Carlo-based Bayesian data analysis to the Doppler-shift attenuation method

Motivated primarily by the large uncertainties in the thermonuclear rate of the 30P(p,γ)31S reaction that limit our understanding of classical novae, we carried out lifetime measurements of 31S excited states using the Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility. The 31S excited states were populated by the 3He(32S,α)31S reaction. The deexcitation γ rays were detected by a clover-type high-purity germanium detector in coincidence with the α particles detected by a silicon detector telescope. We have applied modern Markov chain Monte Carlo-based Bayesian statistical techniques to perform lineshape analyses of Doppler-shift attenuation method γ-ray data for the first time. We have determined the lifetimes of the two lowest-lying 31S excited states. First experimental upper limits on the lifetimes of four higher-lying states have been obtained. The experimental results were compared to shell-model calculations using five universal sd-shell Hamiltonians. Evidence for γ rays originating from the astrophysically important Jπ=3/2+, 260-keV 30P(p,γ)31S resonance with an excitation energy of Ex=6390.2(7) keV in 31S has also been observed, although strong constraints on the lifetime will require better statistics.

New aspects of the low-energy structure of At211

Lifetimes of low-energy states in the semimagic nucleus $^{211}\mathrm{At}$ were measured employing the recoil-distance Doppler shift and the Doppler-shift attenuation methods. The deduced transition probabilities are compared to two shell-model calculations, one using the modified Kuo-Herling interaction in a multi-$j$ model space and the other using a semiempirical interaction for protons confined to the single-$j 0{h}_{9/2}$ orbital. The Kuo-Herling calculations overestimates some of the ground-state transition probabilities, possibly due to contributions not included in the calculated ground-state wave function of $^{211}\mathrm{At}$. A strong underestimation of the $E2$ strengths involving the $7/{2}_{1}^{\ensuremath{-}}$ state is also observed. Therefore, a modification of a single two-body matrix element of the Kuo-Herling interaction is introduced which improves the agreement with the experimental data significantly. The calculations in the single-$j$ approximation agree very well with the measured transition probabilities, indicating that seniority can be regarded as a good quantum number in $^{211}\mathrm{At}$.

Lifetime measurements in 80Br and a new region for observation of chiral electromagnetic selection rules

Level lifetimes for the candidate chiral doublet bands of 80Br were extracted by means of the Doppler-shift attenuation method. The absolute transition probabilities derived from the lifetimes agree well with the M1 and E2 chiral electromagnetic selection rules, and are well reproduced by the triaxial particle rotor model calculations. Such good agreements among the experimental data, selection rules of chiral doublet bands and theoretical calculations are rare and outstanding in researches of nuclear chirality. Besides odd-odd Cs isotopes, odd-odd Br isotopes in the A≈ 80 mass region represent another territory that exhibits the ideal selection rules expected for chiral doublet bands.

Fabrication of [formula omitted]As targets for heavy ion nuclear reactions

Thin uniform targets of 75As isotope with Pb/Au backing layer were fabricated for nuclear structure studies and sub-picosecond lifetime measurements of nuclear excited states using the Doppler shift attenuation method (DSAM) for nuclei populated in the heavy-ion induced fusion-evaporation reactions. The 75As targets of thickness 2.8–4.0 mg/cm2 (4.9–7.0μm) were prepared by in-vacuo sublimation of the As metal powder on to the backing (∼10 mg/cm2) of Pb (9.4μm) or Au (5.2μm), which were prepared by rolling method. The prepared targets were further rolled to improve their uniformity and avoid any interlayer gaps. The amount 75As material required for target preparation could be significantly reduced to ∼80 mg by increasing vapour flux by replacing the open dimpled tantalum boat with the pinhole tubular form. The uniformity, purity and thickness of As and Pb layers in the fabricated targets were assessed using a non-destructive Energy dispersive X-ray fluorescence (EDXRF) technique set-up. The measured thickness values of As and Pb/Au layers were found to agree within ∼3% with those deduced from the weighing method. The prepared targets have been successfully used with 28Si and 31P ion beams and the in-beam γ-ray spectroscopic studies in excited nuclear states in 98,99Rh, 99,100,101Pd and 102Ag isotopes.

Lifetime analysis of Te128,130 via the Doppler-shift attenuation method

Background: The $Z=52$ nuclei $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$ are interesting candidates to study nuclear-structure properties close to the $Z=50$ shell. The knowledge of lifetimes of low-lying states in those nuclei is still scarce.Purpose: The goal of the present work is to extend the experimental database of nuclear level lifetimes of low-spin states in $^{128}\mathrm{Te}$ and $^{130}\mathrm{Te}$.Methods: Nuclear level lifetimes were determined by means of the Doppler-shift attenuation method (DSAM) using $p\ensuremath{-}\ensuremath{\gamma}$ coincidences measured at the SONIC@HORUS setup located at the FN-Tandem ion accelerator of the University of Cologne.Results: For $^{128}\mathrm{Te}$, lifetimes of eleven excited states could be determined, one of these for the first time. For the case of $^{130}\mathrm{Te}$, 18 nuclear level lifetimes were obtained, 15 for the first time. In both experiments, upper limits for lifetimes of several additional levels were determined. Moreover, the nuclear level scheme of $^{130}\mathrm{Te}$ could be extended. One additional level and 13 new transitions for levels above 2.7 MeV have been added. A standard shell-model calculation was performed and compared with the experimental results, showing an overall agreement.Conclusions: DSAM using $p\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences is a powerful tool to determine lifetimes in the subpicosecond range free of feeding contributions. Although both experiments yielded low statistics, numerous lifetimes could be obtained. The use of $p\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences furthermore enables the extension of level schemes.

Fabrication and characterization of [formula omitted]Tm target for Nuclear Lifetime Measurements

A self-supporting 169Tm target was fabricated by rolling of 99.9% enriched 169Tm foil in the target laboratory of Inter-University Accelerator Centre, New Delhi. The sample was characterized using standard characterization techniques and confirmation of elemental purity was obtained. The XRD characterization shows that the target is crystalline in nature and free of strain. The prepared target was used to measure the lifetime of short lived excited states in 183Ir via Doppler Shift Attenuation Method.

Lifetime measurements and evidence for triaxial nuclear shapes in Cs127

Rotational bands built on single quasiproton excitations in $^{127}\mathrm{Cs}$ have been investigated using the $^{121}\mathrm{Sb}(^{12}\mathrm{C},\ensuremath{\alpha}2n$) reaction and the ROSPHERE detector array at IFIN-HH, Bucharest. The lifetimes in the yrast negative-parity band based on the proton in ${h}_{11/2}$ orbital have been measured by applying the Doppler-shift attenuation method. The lifetime of the ${7/2}^{+}$ 273-keV state, member of the rotational band associated with the proton ${g}_{7/2}$ orbital, has been determined as $\ensuremath{\tau}$ = 1.22(5) ns by the fast timing technique. The experimental data were compared with the calculations performed using the quasiparticle plus triaxial rotor model and deformation parameters ${\ensuremath{\epsilon}}_{2}$ and $\ensuremath{\gamma}$ have been derived for the positive- and negative-parity structures. Evidence for triaxial nuclear shapes in the states described by the proton ${h}_{11/2}$ orbital was provided by both signature splitting and $B(E2,\mathrm{\ensuremath{\Delta}}J=2)$ transition strengths.

Collective and noncollective states in Zn66

Excited states in the $^{66}\mathrm{Zn}$ nucleus were populated via a $^{56}\mathrm{Fe}(^{12}\mathrm{C},2p\ensuremath{\gamma}$) fusion-evaporation reaction at a beam energy of $\ensuremath{\approx}62$ MeV. The deexciting $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array (INGA). The level scheme of the $^{66}\mathrm{Zn}$ nucleus has been updated by placing several new $\ensuremath{\gamma}$ rays as well as by assigning the spin and parity of various excited states from the present spectroscopic results. The microscopic structure of the observed states have been investigated in the light of large shell-model calculations. The shape of this nucleus in the low-spin regime has been studied under the framework of total Routhian surface (TRS) calculations. The lifetime of first ${3}^{\ensuremath{-}}$ state at 2826 keV is experimentally measured using the Doppler-shift attenuation method and the deduced $B(E1)$ value indicates the presence of octupole collectivity in this nucleus.

Experimental evidence of exact E(5) symmetry in Kr82

The low-lying positive-parity structures of $^{82}\mathrm{Kr}$ have been investigated following their population in the $^{76}\mathrm{Ge}(^{9}\mathrm{Be},3n)$ reaction at ${\mathrm{E}}_{\mathrm{lab}}\ensuremath{\approx}31\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and using a large array of Compton suppressed HPGe clovers as the detection setup. Apart from other spectroscopic measurements, level lifetimes of the states have been extracted using the Doppler shift attenuation method. The level energies and the associated transition probabilities exhibit superlative overlap with the calculations in the framework of the interaction boson approximation and the E(5) model. The $^{82}\mathrm{Kr}$ nucleus, thus, exemplifies the critical point of phase (shape) transition from the U(5) (spherical vibrator) to the O(6) ($\ensuremath{\gamma}$-soft rotor) paradigms of nuclear structure.

Transition probabilities in 31P and 31S: A test for isospin symmetry

Excited states in the mirror nuclei 31P and 31S were populated in the 1p and 1n exit channels of the reaction 20Ne + 12C, at a beam energy of 33 MeV. The 20Ne beam was delivered for the first time by the Piave-Alpi accelerator of the Laboratori Nazionali di Legnaro. Angular correlations of coincident γ-rays and Doppler-shift attenuation lifetime measurements were performed using the multi-detector array GASP in conjunction with the EUCLIDES charged particle detector. In the observed B(E1) strengths, the isoscalar component, amounting to 24% of the isovector one, provides strong evidence for breaking of the isospin symmetry in the A=31 mass region. Self-consistent beyond mean field calculations using Equation of Motion method based on a chiral potential and including two- and three-body forces reproduce well the experimental B(E1) strengths, reinforcing our conclusion. Coherent mixing from higher-lying states involving the Giant Isovector Monopole Resonance accounts well for the effect observed. The breaking of the isospin symmetry originates from the violation of the charge symmetry of the two- and three-body parts of the potential, only related to the Coulomb interaction.

Enhanced quadrupole collectivity in doubly-magic 56Ni: Lifetime measurements of the [formula omitted] and [formula omitted] states

Lifetime measurements of excited states in doubly-magic 56Ni have been performed exploiting the Doppler-shift attenuation method in order to determine reduced transition probabilities. For the 41+ and 61+ states, the deduced B(E2) values are compared with results from shell-model calculations employing the GXPF1A and the modern PFSDG-U interactions. In addition, valence ab-initio calculations were performed using a novel realistic Hamiltonian derived from chiral perturbation theory including three-body potential contributions and are confronted with the experimental findings. The new results show maximum E2 strength in comparison with known values along the N=28 chain of isotones. The results corroborate the high collectivity for the double shell closure at N=Z=28 which was anticipated from the large B(E2;21+→0g.s.+) value despite the considerable increase of its excitation energy as compared to neighboring semi-magic nuclei. Based on similarities in the shell structures of the self-conjugate doubly-magic nuclei 56Ni and 100Sn, the new values could be an indication for an expected comparable collective behavior of the 61+ state in 100Sn.

Lifetime measurement in picoseconds for the excited states of 129Cs

The high-spin states of 129Cs were populated using the fusion-evaporation reaction 124Sn(11B, 6n)129Cs. We have studied the nuclear structure properties – signature splitting and reduced transition probabilities (B(E2)) – of many excited states in 129Cs. Lifetimes in picoseconds were measured using the Doppler Shift Attenuation Method (DSAM). We employed two gating procedures, Gating on transition above (GTA) and Gating on transition below (GTB), to obtain the experimental results consistently. We carefully analyzed the data by incorporating the effects related to nuclear reaction, e.g., change in the velocity and cross section of residues when moving in a medium. Theoretical investigations were carried out for two yrast bands – a positive-parity based on πg7/2, and another negative-parity based on πh11/2 – using the single quasi-particle rotor model (PRM). For both bands, we found the same deformation parameter values ϵ≃0.14, |γ|=18∘ (Lund convention) by simultaneously matching the experimental and theoretical results for the signature splitting and B(E2) values. Most of our results are new and reasonably well understood theoretically.

Lifetime measurements of excited states in O15

The CNO cycle is the main energy source in stars more massive than our sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. One of the largest uncertainties in the CNO chain of reactions comes from the uncertainty in the $^{14}$N$(p,\gamma)^{15}$O reaction rate. This uncertainty arises predominantly from the uncertainty in the lifetime of the sub-threshold state in $^{15}$O at $E_{x}$ = 6792 keV. Previous measurements of this state's lifetime are significantly discrepant. Here, we report on a new lifetime measurement of this state, as well as the excited states in $^{15}$O at $E_{x}$ = 5181 keV and $E_{x}$ = 6172 keV, via the $^{14}$N$(p,\gamma)^{15}$O reaction at proton energies of $E_{p} = 1020$ keV and $E_{p} = 1570$ keV. The lifetimes have been determined with the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets with Mo, Ta, and W backing. We obtained lifetimes from the weighted average of the three measurements, allowing us to account for systematic differences between the backing materials. For the 6792 keV state, we obtained a $\tau = 0.6 \pm 0.4$ fs. To provide cross-validation of our method, we measured the known lifetimes of the states at 5181 keV and 6172 keV to be $\tau = 7.5 \pm 3.0$ and $\tau = 0.7 \pm 0.5$ fs, respectively, which are in good agreement with previous measurements.