Excited Nuclear States Research Articles

Measurement of picosecond lifetimes in neutron-rich Xe isotopes

Background: Lifetimes of nuclear excited states in fission fragments have been studied in the past following isotope separation, thus giving access mainly to the fragments’ daughters and only to long-lived isomeric states in the primary fragments. For the first time now, short-lived excited states in the primary fragments, produced in neutron-induced prompt fission of 235U and 241Pu, were studied within the EXILL&FATIMA campaign at the intense neutron-beam facility of the Institute Laue-Langevin in Grenoble. Purpose: We aim to investigate the quadrupole collective properties of neutron-rich even-even 138,140,142Xe isotopes lying between the double shell closure N = 82 and Z = 50 and a deformed region with octupole collectivity. Method: The γ rays emitted from the excited fragments were detected with a mixed array consisting of 8 HPGe EXOGAM Clover detectors (EXILL) and 16 LaBr3(Ce) fast scintillators (FATIMA). The detector system has the unique ability to select the interesting fragment making use of the high resolution of the HPGe detectors and determine subnanosecond lifetimes using the fast scintillators. For the analysis the generalized centroid difference method was used. Results: We show that quadrupole collectivity increases smoothly with increasing neutron number above the closed N = 82 neutron shell. Our measurements are complemented by state-of-the-art theory calculations based on shell-model descriptions. Conclusions: The observed smooth increase in quadrupole collectivity is similar to the evolution seen in the measured masses of the xenon isotopic chain and is well reproduced by theory. This behavior is in contrast to higher Z even-even nuclei where abrupt change in deformation occurs around N = 90.

Double resonant enhancement in the neutrinoless double-electron capture ofPt190

Background: The observation of neutrinoless double-$\ensuremath{\beta}$ transitions would indicate physics beyond the standard model as the lepton number conservation is violated. For a complete degeneracy in the energy of the initial and final states, the neutrinoless double-electron capture is resonantly enhanced. This shortens the half-life to similar orders of magnitude as the neutrinoless double-$\ensuremath{\beta}$ decay and expands the set of nuclei for the search of neutrinoless double-$\ensuremath{\beta}$ transitions as the observation of either process would be equally likely.Purpose: To clearly identify transitions that are resonantly enhanced, among other parameters the total energy of the decay, ${Q}_{\ensuremath{\varepsilon}\ensuremath{\varepsilon}}$, needs to be measured very precisely. Of the 12 initially identified candidates, the last remaining decay without a precise ${Q}_{\ensuremath{\varepsilon}\ensuremath{\varepsilon}}$ was $^{190}\mathrm{Pt}(0\ensuremath{\nu}\ensuremath{\varepsilon}\ensuremath{\varepsilon})^{190}\mathrm{Os}$.Method: The ${Q}_{\ensuremath{\varepsilon}\ensuremath{\varepsilon}}$ value was determined with the Penning trap mass spectrometer LEBIT by measuring the ratio of the cyclotron frequencies of $^{190}\mathrm{Pt}^{+}$ and $^{190}\mathrm{Os}^{+}$ in a 9.4-T superconducting magnet.Result: The ${Q}_{\ensuremath{\varepsilon}\ensuremath{\varepsilon}}$ value was determined to be 1401.57(47) keV with an uncertainty reduction of an order of magnitude compared to its previously known value. The absolute value is shifted by 17.17(623) keV relative to the previously accepted one. Furthermore, the mass value of $^{190}\mathrm{Pt}$ was found to be shifted by more than three standard deviations. In addition we improved the mass values for $^{186,190}\mathrm{Os}$ and $^{194}\mathrm{Pt}$.Conclusion: Transitions to the two nuclear excited states of $^{190}\mathrm{Os}$ with 1326.9(5) and 1387.00(2) keV energy were identified to be resonantly enhanced within a $1\ensuremath{\sigma}$ uncertainty. The significantly reduced uncertainty of ${Q}_{\ensuremath{\varepsilon}\ensuremath{\varepsilon}}$ confirmed the potential for a resonantly enhanced transition.

Effect of nuclear-reaction mechanisms on the population of excited nuclear states and isomeric ratios

Experimental data on the cross sections for channels of fusion and transfer reactions induced by beams of radioactive halo nuclei and clustered and stable loosely bound nuclei were analyzed, and the results of this analysis were summarized. The interplay of the excitation of single-particle states in reaction-product nuclei and direct reaction channels was established for transfer reactions. Respective experiments were performed in stable (6Li) and radioactive (6Не) beams of the DRIBs accelerator complex at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, and in deuteron and 3Не beams of the U-120M cyclotron at the Nuclear Physics Institute, Academy Sciences of Czech Republic (Řež and Prague, Czech Republic). Data on subbarrier and near-barrier fusion reactions involving clustered and loosely bound light nuclei (6Li and 3He) can be described quite reliably within simple evaporation models with allowance for different reaction Q-values and couple channels. In reactions involving halo nuclei, their structure manifests itself most strongly in the region of energies below the Coulomb barrier. Neutron transfer occurs with a high probability in the interactions of all loosely bound nuclei with light and heavy stable nuclei at positive Q-values. The cross sections for such reactions and the respective isomeric ratios differ drastically for nucleon stripping and nucleon pickup mechanisms. This is due to the difference in the population probabilities for excited single-particle states.

Nuclear states with anomalously large radius (size isomers)

Methods of determination of the nuclear excited state radii are discussed together with the recently obtained data on the states of some light nuclei having abnormally large radii (size isomers). It is shown that such states include excited neutron-halo states in 9Be, 11Be, and 13C and some alpha-cluster states in 12C, 11B, and 13C. Among the latter ones, there is the well-known Hoyle state in 12C—the structure of this state exhibit rudimentary features of alpha-particle states.

Recoil distance transmission method: Measurement of interaction cross sections of excited states with fast rare-isotope beams

The possible appearance of nuclear halos in ground and excited states close to the particle-decay threshold is of great importance in the investigation of nuclear structure and few-body correlations at the limit of stability. In order to obtain direct evidence of the halo structure manifested in nuclear excited states, we have considered a new method to measure the interaction cross sections of excited states. The combination of the transmission method and the recoil distance Doppler-shift method with a plunger device enables us to measure the number of interactions of the excited states in a target. Formulae to determine the interaction cross section are derived, and key issues to realize measurements are discussed. Dominant sources of errors are uncertainties in the excited-state lifetimes and γ-ray yields. We examine prototype experiments and perform simulations to study the impact of each uncertainty on the final result. This method provides a novel opportunity to perform cross section measurements on the excited states of rare isotopes.

Reduced γ–γ time walk to below 50 ps using the multiplexed-start and multiplexed-stop fast-timing technique with LaBr3(Ce) detectors

The electronic γ–γ fast-timing technique using arrays consisting of many LaBr3(Ce) detectors is a powerful method to determine lifetimes of nuclear excited states with a lower limit of about 5ps. This method requires the determination of the energy-dependent time walk of the zero time which is represented by the centroid of a prompt γ–γ time distribution. The full-energy peak versus full-energy peak prompt response difference which represents the linearly combined mean γ–γ time walk of a fast-timing array consisting of 8 LaBr3(Ce) detectors was measured using a standard 152Eu γ-ray source for the energy region of 40–1408keV. The data were acquired using a “multiplexed-start and multiplexed-stop” analogue electronics circuitry and analysed by employing the generalized centroid difference method. Concerning the cylindrical 1.5in.×1.5in. LaBr3(Ce) crystals which are coupled to the Hamamatsu R9779 photomultiplier tubes, the best fast-timing array time resolution of 202(3)ps is obtained for the two prompt γ lines of 60Co by using the leading-edge timing principle. When using the zero-crossover timing principle the time resolution is degraded by up to 30%, dependent on the energy and the shaping delay time of the constant fraction discriminator model Ortec 935. The smallest γ–γ time walk to below 50ps is obtained by using a shaping delay time of about 17ns and an optimum “time-walk adjustment” needed for detector output pulses with amplitudes smaller than 400mV.

Nuclear excitation by electronic transition ofU235

Here, nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that can occur in isotopes containing a low-lying nuclear excited state. Over the past 40 yr, several experiments have attempted to measure NEET of 235U and those experiments have yielded conflicting results.

A VUV detection system for the direct photonic identification of the first excited isomeric state of 229Th

With an expected energy of 7.6(5) eV, $^{229}$Th possesses the lowest excited nuclear state in the landscape of all presently known nuclei. The energy corresponds to a wavelength of about 160 nm and would conceptually allow for an optical laser excitation of a nuclear transition. We report on a VUV optical detection system that was designed for the direct detection of the isomeric ground-state transition of $^{229}$Th. $^{229(m)}$Th ions originating from a $^{233}$U $\alpha$-recoil source are collected on a micro electrode that is placed in the focus of an annular parabolic mirror. The latter is used to parallelize the UV fluorescence that may emerge from the isomeric ground-state transition of $^{229}$Th. The parallelized light is then focused by a second annular parabolic mirror onto a CsI-coated position-sensitive MCP detector behind the mirror exit. To achieve a high signal-to-background ratio, a small spot size on the MCP detector needs to be achieved. Besides extensive ray-tracing simulations of the optical setup, we present a procedure for its alignment, as well as test measurements using a D$_2$ lamp, where a focal-spot size of $\approx$100 $\mu$m has been achieved. Assuming a purely photonic decay, a signal-to-background ratio of $\approx$7000:1 could be achieved.

Current hot questions on the s process in AGB stars

Asymptotic giant branch (AGB) stars are a main site of production of nuclei heavier than iron via the s process. In massive (>4 M⊙) AGB stars the operation of the 22Ne neutron source appears to be confirmed by observations of high Rb enhancements, while the lack of Tc in these stars rules out 13C as a main source of neutrons. The problem is that the Rb enhancements are not accompanied by Zr enhancements, as expected by s-process models. This discrepancy may be solved via a better understanding of the complex atmospheres of AGB stars. Second- generation stars in globular clusters (GCs), on the other hand, do not show enhancements in any s-process elements, not even Rb. If massive AGB stars are responsible for the composition of these GC stars, they may have evolved differently in GCs than in the field. In AGB stars of lower masses, 13C is the main source of neutrons and we can potentially constrain the effects of rotation and proton-ingestion episodes using the observed composition of post-AGB stars and of stardust SiC grains. Furthermore, independent asteroseismology observations of the rotational velocities of the cores of red giants and of white dwarves will play a fundamental role in helping us to better constrain the effect of rotation. Observations of carbon-enhanced metal-poor stars enriched in both Ba and Eu may require a neutron flux in-between the s and the r process, while the puzzling increase of Ba as function of the age in open clusters, not accompanied by increase in any other element heavier than iron, require further observational efforts. Finally, stardust SiC provides us high-precision constraints to test nuclear inputs such as neutron-capture cross sections of stable and unstable isotopes and the impact of excited nuclear states in stellar environments.

Lifetime measurements in mass regions A=100 and A=130 as a test for chirality in nuclear systems

Two odd-odd nuclei from the A ~ 100 and A ~ 130 regions, namely ^102Rh and ^134Pr have been studied in search for chiral doublet bands via ^94Zr(^11B,3n)^102Rh and ^119Sn(^19F,4n)^134Pr reactions, respectively. Two nearly degenerate bands built on the pig_9/2 ⊗ vh_11/2 configuration have been identified in ^102Rh and on the pig_11/2 ⊗ vh_11/2 configuration for ^134Pr. Lifetimes of excited nuclear states were measured using Dopplershift attenuation method and recoil distance Doppler-shift method. The deexciting gamma rays were registered by the Indian National Gamma Array for ^102Rh and using the EUROBALL IV detector array with an inner Bismuth Germanate (BGO) ball for ^134Pr, respectively. Polarization and angular correlation measurements have been performed to establish the spin and parity assignments for these bands. The derived reduced transition probabilities are compared to the predicitons of the two quasiparticles + triaxial rotor and interacting boson fermion-fermion models.

On the time response of background obtained in [formula omitted]-ray spectroscopy experiments using LaBr3(Ce) detectors with different shielding

Employing the γ-γ fast-timing technique with LaBr3(Ce) scintillator detectors allows the direct determination of lifetimes of nuclear excited states with a lower limit of about 5ps. This limit is increased as soon as background is present in the coincidence spectra underneath the full-energy peaks of the γ-γ cascade. Our aim was to identify the components of the γ-ray background by systematic γ-γ fast-timing measurements using different types of γ shielding within a large γ-ray spectrometer. The energy dependent physical zero-time response was measured using background-free full-energy peak events from the 152Eu γ-ray source. This is compared with the time response of the (Compton-) background distribution as obtained using the prompt 60Co γ-ray source. The time response of the typical Compton background is about 15ps faster than the time response of background-free full-energy peak events. Below about 500keV, a second type of background contributes by the detection of Compton-scattered γ rays generated in the materials of the spectrometer around the detector. Due to the additional time-of-flight of the Compton-scattered γ rays, this low-energy background is largely delayed. Compared with a bare cylindrical 1.5in.×1.5in. LaBr3(Ce) detector, the BGO-shielded detector in the Compton-suppression mode improves the peak-to-total ratio by a factor of 1.66(5), while the Pb-shielded detector only slightly reduces the low-energy background.

The TRIple PLunger for EXotic beams TRIPLEX for excited-state lifetime measurement studies on rare isotopes

A new device, the TRIple PLunger for EXotic beams (TRIPLEX), has been developed for lifetime measurement studies with rare isotope beams. This plunger device holds up to three metal foils in the beam path and facilitates the recoil distance Doppler-shift technique to measure lifetimes of nuclear excited states in the range of 1ps to 1ns. The unique design allows independent movement of the target and the second degrader with respect to a fixed first degrader in between, enabling advanced experimental approaches, such as the differential recoil distance method and the double recoil distance method. The design and control of the device are presented in this paper, together with simulated performances of the new applications. As an example of actual experiments, results from the lifetime measurement of the neutron-rich 17C isotope performed at the National Superconducting Cyclotron Laboratory are shown.

Observation of the competitive double-gamma nuclear decay.

The double-gamma (γγ)-decay of a quantum system in an excited state is a fundamental second-order process of quantum electrodynamics. In contrast to the well-known single-gamma (γ)-decay, the γγ-decay is characterized by the simultaneous emission of two γ quanta, each with a continuous energy spectrum. In nuclear physics, this exotic decay mode has only been observed for transitions between states with spin-parity quantum numbers J(π) = 0(+) (refs 1-3). Single-gamma decays-the main experimental obstacle to observing the γγ-decay-are strictly forbidden for these 0(+) → 0(+) transitions. Here we report the observation of the γγ-decay of an excited nuclear state (J(π) = 11/2(-)) that is directly competing with an allowed γ-decay (to ground state J(π) = 3/2(+)). The branching ratio of the competitive γγ-decay of the 11/2(-) isomer of (137)Ba to the ground state relative to its single γ-decay was determined to be (2.05 ± 0.37) × 10(-6). From the measured angular correlation and the shape of the energy spectra of the individual γ-rays, the contributing combinations of multipolarities of the γ radiation were determined. Transition matrix elements calculated using the quasiparticle-phonon model reproduce our measurements well. The γγ-decay rate gives access to so far unexplored important nuclear structure information, such as the generalized (off-diagonal) nuclear electric polarizabilities and magnetic susceptibilities.

Effects of aNcc¯*resonance with hidden charm in theπ−p→D−Σc+reaction near threshold

We study the effect of a hidden charm nuclear excited state $N^*_{c\bar{c}}$ in the $\pi^- p \to D^- \Sigma_c^+$ reaction near threshold using an effective Lagrangian approach. We calculate the background contribution of the $t$ and $u$ channels by the $D^{*0}$ vector meson exchange and $\Sigma_c^{++}$ intermediate state, respectively. We show that the consideration of a $N^*_{c\bar{c}}$ resonance provides an enhancement of the total cross section close to the reaction threshold. We also evaluate the differential cross section for different energies and we study the angle dependence. It is expected that our model calculations will be tested in future experiments.

First-Principles Study of Arsenic Atom, Its Irons and Molecule

In the present work, we have performed the ground state energy calculations for arsenic atom, its ions and molecule using Hartree-Fock (HF) cluster approximation. The correlation effects in the HF calculations have been taken into account by considering the Møller-Plesset second order perturbation (MP2) and truncated form of Configurational Interaction (CI) that includes single, double, and quadruple excitations, also known as QCISD implemented by the Gaussian 03 sets of program. Our study shows that the ground state of arsenic atom is a quadrate state i.e., charge zero and multiplicity four with ground state energy - 60796.66 eV in MP2 levels of calculation with cc-pVDZ basis set. We have performed the first-principles calculation to study the first electron affinity and ionization energies of arsenic atom up to tenth level. The first-principles calculation has been also carried out to study the equilibrium configuration of arsenic molecule (As2). The bond length and binding energy of arsenic molecule (As2) is found to be 2.15 Å and 3.65 eV in MP2 levels of approximation with basis set 6-311G(3df). Our study has been extended to calculate electrostatic potential for arsenic molecule (As2), whose values at global maxima and minima are found to be 0.30 eV and - 0.20 eV respectively. The calculation of HOMO-LUMO energy gap for the arsenic molecule (As2) is almost independent of choice of basis sets as well as levels of approximation. The HF, MP2, and QCISD calculations also have been carried out to estimate the electric field gradient (EFG) parameters for the excited nuclear state in arsenic molecule (As2). Our results show that the HF, MP2, and QCISD values for the EFG parameters do not differ significantly, indicating that electron correlation effects do not contribute for the determination of EFG parameter.The Himalayan Physics Year 5, Vol. 5, Kartik 2071 (Nov 2014)Page : 17-21

Statistical density of nuclear excited states

Statistical density of nuclear excited states

Nuclear resonant scattering of synchrotron radiation byOs187

We performed nuclear forward and inelastic scattering of synchrotron radiation by elemental Os utilizing the nuclear excited state of $^{187}\mathrm{Os}$ which is otherwise inaccessible using any practical radioactive decay process. The lifetime of the excited state, $3.06(8)\phantom{\rule{4.pt}{0ex}}\text{ns}$, and the energy of the transition, $9.778(3)\phantom{\rule{4.pt}{0ex}}\text{keV}$, are refined. The nuclear quadrupole moment of the excited state, ${Q}_{3/2}=1.46(10)\phantom{\rule{4.pt}{0ex}}\text{b}$, is determined. The density of phonon states for elemental Os, which is herein experimentally determined, suggests that the Os lattice is a model for the lattice dynamics of hcp-Fe. The combination of the low energy of the nuclear transition and the large nuclear mass leads to a high recoil free fraction, ${f}_{LM}=0.95(1)$, at room temperature, a large value that strongly supports the viability of nuclear resonance scattering as a reliable method to study electronic, magnetic, and elastic properties of Os compounds, including Os organometallics.

Spin precession mapping at ferromagnetic resonance via nuclear resonant scattering of synchrotron radiation.

We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance by nuclear resonant scattering of synchrotron radiation at the 14.4keV resonance of ^{57}Fe. The precession of the magnetization leads to an apparent reduction of the magnetic hyperfine field acting at the ^{57}Fe nuclei. The spin dynamics is described in a stochastic relaxation model adapted to the ferromagnetic resonance theory by Smit and Beljers to model the decay of the excited nuclear state. From the fits of the measured data, the shape of the precession cone of the spins is determined. Our results open a new perspective to determine magnetization dynamics in layered structures with very high depth resolution by employing ultrathin isotopic probe layers.

Nuclear response theory with multiphonon coupling in a covariant framework

Background: Nuclear excited states within a wide range of excitation energies are formally described by the linear response theory. Besides its conventional formulation within the quasiparticle random phase approximation (QRPA) representing excited states as two correlated quasiparticles (2q), there exist extensions for 4q configurations. Such extended approaches are quite successful in the description of gross properties of nuclear spectra, however, accounting for many of their fine features requires further extension of the configuration space. Purpose: This work aims at the development of an approach which is capable of such an extension as well as of reproducing and predicting fine spectral properties, which are of special interest at low energies. Method: The method is based on the covariant density functional theory and time blocking approximation, which is extended for couplings between quasiparticles and multiphonon excitations. Results: The covariant multiphonon response theory is developed and adopted for nuclear structure calculations in medium-mass and heavy nuclei. The equations are formulated in both general and coupled forms in the spherical basis. Conclusions: The developed covariant multiphonon response theory represents a new generation of the approaches to nuclear response, which aims at a unified description of both high-frequency collective states and low-energy spectroscopy in medium-mass and heavy nuclei.

Precision measurement of sub-nanosecond lifetimes of excited nuclear states using fast-timing coincidences with LaBr3(Ce) detectors

Precision measurements of electromagnetic (EM) transition rates enable tests of models of internal nuclear structure. Measurements of transition rates can be used to infer the spin and parity differences between the initial and final discrete nuclear excited states via which the EM transition takes place. This short conference paper reports on developments of detection systems for the identification of discrete energy gamma-ray decays using arrays of halide-scintillation detectors acting in coincidence mode, which can be used to determine electromagnetic transition rates between excited nuclear states in the sub-nanosecond temporal regime. Ongoing development of a new multi-detector LaBr3(Ce) array for studies of exotic nuclei produced at the upcoming Facility for Anti-Proton and Ion Research (FAIR) as part of the NUSTAR–DESPEC project are presented, together with initial results from pre-NUSTAR implementations of this array for nuclear structure studies of neutron-rich fission fragment radionuclides at ILL-Grenoble, France and RIBF at RIKEN, Japan.