Excited Nuclear Levels Research Articles

Statistical investigation of the angular momentum dependence of the nuclear level density parameter

The nuclear level density (NLD) parameter is crucial for calculating cross-sections in nuclear physics, astrophysics, reactor design, and medical physics. Spin and parity, along with excitation energy, are fundamental properties of an excited nuclear level. Previous investigations into the NLD’s dependence on spin and parity have primarily used approximate methods like parity equidistribution and Gaussian distribution of spins. However, the specific impact of spin and parity on the NLD parameter, a key component in NLD formulation, has not been extensively studied. We examined the spin and parity dependence of the NLD parameter. Our findings demonstrate that the NLD parameter’s dependency on both excitation energy and angular momentum can be accurately characterized by a Laplace distribution, highlighting the complex interplay of these factors in nuclear physics.

New Constraints on the Axion–Electron Coupling Constant for Solar Axions

The resonant excitation of the 83Kr first excited nuclear level (E = 9.4 keV) by solar axions whose fluxes depend on the axion–electron coupling constant gAe is sought. The γ- and X-ray photons and the conversion and Auger electrons from the excited-level relaxation are detected with a gas proportional counter of a low-background detector in the underground Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences). As a result, a new constraint {text{|}}{{g}_{{Ae}}}(g_{{AN}}^{3} - g_{{AN}}^{0}){text{|}} ≤ 1.50 × 10–17 (90% C.L.) has been obtained for the axion–electron and axion–nucleon coupling constants, which corresponds to new constraints on the axion mass mA ≤ 320 eV and mA ≤ 4.6 eV in the KSVZ and DFSZ axion models, respectively.

Exploiting Isospin Symmetry to Study the Role of Isomers in Stellar Environments.

Proton capture on the excited isomeric state of ^{26}Al strongly influences the abundance of ^{26}Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic ^{26}Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical ^{26m}Al(p,γ)^{27}Si reaction. These constraints demonstrate that the rate is at least a factor ∼8 lower than previously expected, indicating an increase in the stellar production of ^{26}Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of ^{26}Al.

Lifetime determination via the particle-γ coincidence Doppler-shift attenuation method

This paper illustrates the principle of the Doppler-shift attenuation method (DSAM) using particle-γ coincidences, a method for determining lifetimes of excited nuclear levels in the range of few femtoseconds up to one picosecond. The coincident detection holds several advantages towards conventional DSAM experiments, such as the elimination of background and feeding transitions. Using the experimental data on 94Zr, the concept of the (p,p’γ) DSAM analysis is presented. Additional experimental results are highlighted.

Nuclear transformations and radioactive emissions: Part II—secondary transitions and post-effects

The present chapter follows part I of a series of two articles, which characterized unstable nuclei and described primary transformation pathways to stabilize a nucleus in terms of its nucleon compositions. Part II focuses on follow-up phenomena of those “primary” transformations. It first illustrates the class of secondary transitions, which all originate from an excited nuclear level of a nucleus, formed in a primary transformation. Next it discusses post-effects, which appear when nuclear transformations and transitions affect the atom as a whole, i.e. also its electron shell. Third, there are several post-effect phenomena when particular or electromagnetic radiation emitted in the course of the above-mentioned processes interact with surrounding condensed matter. Consequently, there are many specific emissions which accompany those secondary and post-effect processes, and which are not directly related to the primary transformations. A prominent example is γ-emissions as emitted in one of the three secondary transition pathways. These γ-rays, however, represent just one of the many emission which altogether are the essence of radioactivity.

Determination ofγ-ray widths inN15using nuclear resonance fluorescence

The stable nucleus $^{15}$N is the mirror of $^{15}$O, the bottleneck in the hydrogen burning CNO cycle. Most of the $^{15}$N level widths below the proton emission threshold are known from just one nuclear resonance fluorescence (NRF) measurement, with limited precision in some cases. A recent experiment with the AGATA demonstrator array determined level lifetimes using the Doppler Shift Attenuation Method (DSAM) in $^{15}$O. As a reference and for testing the method, level lifetimes in $^{15}$N have also been determined in the same experiment. The latest compilation of $^{15}$N level properties dates back to 1991. The limited precision in some cases in the compilation calls for a new measurement in order to enable a comparison to the AGATA demonstrator data. The widths of several $^{15}$N levels have been studied with the NRF method. The solid nitrogen compounds enriched in $^{15}$N have been irradiated with bremsstrahlung. The $\gamma$-rays following the deexcitation of the excited nuclear levels were detected with four HPGe detectors. Integrated photon-scattering cross sections of ten levels below the proton emission threshold have been measured. Partial gamma-ray widths of ground-state transitions were deduced and compared to the literature. The photon scattering cross sections of two levels above the proton emission threshold, but still below other particle emission energies have also been measured, and proton resonance strengths and proton widths were deduced. Gamma and proton widths consistent with the literature values were obtained, but with greatly improved precision.

Nondestructive identification of isotopes using nuclear resonance fluorescence

Nondestructive identification of heavy isotopes concealed in a thick iron box has been demonstrated by using nuclear resonance fluorescence. A quasi-monochromatic photon beam produced by the collision of laser quanta with high energy electrons was used for resonant excitation of nuclear levels in (206)Pb and (208)Pb. By measuring the resonant γ rays emitted from (206)Pb and (208)Pb, each of these isotopes were clearly identified. The ratio of the effective thickness, i.e., concentration distribution, of these isotopes was deduced from the relative intensities of the measured nuclear resonance fluorescence strengths.

Brownian Movement of Inorganic Nanoparticles in Sediments

In the seventies of the previous century physicists partly lost their interests in Brownian movement. Now this topic come back to the frontier of research because of its connection to the nano-bio-technology and generally to the life science as the possible explanation of the speeding up of the transport phenomena at the cellular level [1]. The proper description of the nanoparticles mobility is crucial in molecular engineering [2], as well as in pharmacology [3] and catalysis [4]. The most popular and probably most effective experimental technique for the study of mobility of nanoobjects in soft matter is dynamic light scattering [5]. Its main advantage is the wide range of time windows which may be applied and the broad variety of the particles which may be studied. The limitation is that the environment must be transparent and, which is probable the crucial problem, the method supplies the average mobility of different particles in the distinguished localization in the system. The peculiarities of the Mossbauer spectroscopy, which is used in this work, are just opposite. The method is restricted, in practice, to iron bearing particles which displacements are measured in the characteristic time window of 141 ns, the life time of the excited nuclear level of Fe. However, because of the sensitivity to the hyperfine interactions, the method distinguishes different particles. Moreover, it is possible to observe different localizations of the nanoobjects [6], characterized by the local viscosity. The studied specimens may be quite dense [7] and the optical transparency is not required [8]. The individual iron bearing nanoparticle may be treated as a classical thin absorber for which the Mossbauer absorption line has the Lorentzian shape L(vr), where vr is the relative particle — source velocity vr = v − V , where v and V is the source and particle velocity in the laboratory coordination system, respectively. The shape of the experimental absorption line I(v) is the integral over all particles. I(v) = ∫ p(V )L(v − V )dV , (1)

Improved method of analysis for Recoil Distance Measurements of nuclear lifetimes

The Recoil Distance Method is known as a very powerful tool for measuring the lifetimes of excited nuclear levels in the picosecond region. Usually, for each level, coincidences with the shifted part of a feeding transition are used to avoid the bias from the accumulated delay in the higher lying cascade, by means of the Differential Decay Curve Method. Here, an integral version of the Decay Curve Method is proposed and discussed. This technique has several noticeable advantages, particularly when the overall statistics of the results is limited owing to the weak beam current, as expected with radioactive ion beams.

High-resolution neutron transmission and capture measurements of the nucleusPb206

Neutron total and capture reaction cross-section measurements for $^{206}\mathrm{Pb}$ were performed at the GELINA neutron time-of-flight facility in the energy range from 1 to 620 keV. The resonance parameters corresponding to 304 excited nuclear levels in $^{207}\mathrm{Pb}$ were determined from the data using the R-matrix formalism. The results are compared to existing data. From the capture data photon strength functions have been deduced. No evidence has been found for a previously reported enhancement of the $M1$ transition strength, nor for a strong enhancement of the s-wave doorway state in the photon channel. The neutron capture cross section of $^{206}\mathrm{Pb}$ is of importance in stellar evolution calculations of the formation of the elements by neutron capture. Maxwellian-averaged neutron capture cross sections have been calculated at different stellar temperatures ranging up to 100 keV and compared with the results of previous work and evaluated data libraries.

Search for resonant absorption of solar axions emitted in an M1 transition in 57Fe nuclei

The resonant absorption of solar axions by 57Fe nuclei, which is accompanied by the excitation of the first excited nuclear level: A + 57Fe → 57Fe* → 57Fe + γ(14.4 keV), is sought. To seek 14.4-keV photons, a Si(Li) detector and an enriched 57Fe target are used. The detector and target are placed in a low-background setup equipped with passive and active shieldings. As a result, a new upper limit m A ≤ 360 eV (at 90% C.L.) has been determined for the axion mass.

Partial level density of n-quasiparticle excitations, radiative strength functions and new experimental information on the dynamics of nuclear structure change in the B n range

The most reliable at present values of the level density in the fixed spin window and the sums of radiative strength functions of cascade gamma transitions are obtained from analysis of intensities of two-step cascades excited upon thermal neutron capture for approximately 40 nuclei in the mass range 40 ≤ A ≤ 200. The maximal reliability of these data is provided by the experimental conditions—minimum possible propagation error coefficients and practically unique solution of the problem of determination of gamma decay parameters from measured spectra. The experimental data are approximated by the sum of partial level densities corresponding to excitation of n quasiparticles. Steplike structures in the level density at excitation energies smaller than 3–4 MeV are described with good accuracy as the superposition of two-quasiparticle (three-quasiparticle in odd A nuclei) and vibrational excitations with the coefficient of collective density enhancement K coll ≈ 10−20. They correspond to excitation-energy-correlated maximum enhancement of the radiative strength functions of primary gamma transitions. The level density at larger excitation energies is well reproduced if the breakup of at least two more Cooper pairs of nucleons is taken into account. The increase in the number of excited quasiparticles in the nucleus corresponds to unconditional reduction of the radiative strength functions of primary gamma transitions of the compound state decay. However, the maximum possible value of partial widths of primary transitions increases regularly with decreasing energy. Some ambiguity in the results of approximation and divergence from existing theoretical ideas of the energy dependence of nucleon correlation functions in an excited nucleus point to the possibility of direct extraction from experiment of fundamentally new information on the structure of excited nuclear levels in the range of the neutron binding energy. These are, first of all, the parameters of dependence of nucleon correlation functions on the excitation energy of the nucleus.

CLIC–LHC-based FEL–nucleus collider: Feasibility and physics search potential

The feasibility of a CLIC–LHC-based FEL–nucleus collider is investigated. It is shown that the proposed scheme satisfies all requirements of an ideal photon source for the nuclear resonance fluorescence method. The tunability, monochromaticity and high polarization of the FEL beam together with high statistics and huge energy of LHC nucleus beams will give a unique opportunity to determine different characteristics of excited nuclear levels. The physics potential of the proposed collider is illustrated for a beam of Pb nuclei.

γ-radiation of excited nuclear discrete levels in peripheral heavy ion collisions

A new process of a nuclear excitation to discrete states in peripheral heavy ion collisions is studied. High-energy photons are emitted by the exited nuclei with energies up to a few tens of GeV at angles of a few hundred microradians with respect to the beam direction. We show that a two-stage process, where an electron-positron pair is produced by virtual photons emitted by nuclei and then the electron or positron excites the nucleus, has a large cross-section. It is equal to about 5 b for CaCa collisions. On the one hand, it produces a significant γ-rays background in the nuclear fragmentation region but, on the other hand, it could be used for monitoring the nuclear beam intensity at the LHC. These secondary nuclear photons could be a good signal for triggering peripheral nuclear collisions.

Mössbauer spectra of 167Er and 166Er in pyrogermanate and diglycollate hosts

The hyperfine splittings and the Mössbauer spectra (MS) of the two isomers 166Er and 167Er in the pyrogermanate (or ErPG) and diglycollate (or ErDG) hosts were calculated using the values of the crystal field (CF) parameters and energy levels obtained previously from fitting the magnetic and optical results. The calculated thermal characteristics of the quadrupole interaction parameter P differed prominently for ErDG and ErPG in both the isomers, viz., for 166Er in ErDG the value of P increased from 1.5 to 21.3 MHz between 300 and 4 K, whereas in ErPG the corresponding change is insignificant ∼1 MHz. It was also found that the excited nuclear level of 166Er in both the compounds split into five doublets giving rise to a five finger pattern in the calculated MS as have been observed for Er 3+ in several other compounds. In case of the 167 isomer more complicated MS for ErDG and ErPG are expected. Due to the non-zero value of the hyperfine constant B in case of ErPG the nuclear wave functions have mixed components of m I which affected the intensity of the spectra. The hyperfine magnetic fields H hf were calculated for both the compounds and were found to be nearly the same, being equal to 4.4±0.1 MG. The hyperfine specific heat C hf were calculated for 167Er in both hosts and was found that T 2 C hf attained constant values at millikelvin temperature. This study showed that MS and C hf of ErDG and ErPG are expected to be quite sensitively affected by the anomalous behaviours observed in these compounds at low temperatures.

Observation of the 22.5-keV resonance in (149)Sm by the nuclear lighthouse effect.

We have observed coherent nuclear resonant scattering of synchrotron radiation at the 22.5-keV resonance of (149)Sm. High-speed rotational sample motion led to an angular deflection of the resonantly scattered radiation off the nonresonant primary beam. This allowed us to determine the resonance energy of the first excited nuclear level of (149)Sm to be 22496(4) eV. Because of the angular deflection of the resonant photons, time spectra of coherent nuclear resonant scattering can be recorded as a function of a spatial coordinate. Time resolutions of a few 10 ps can be expected, which are beyond the limits of existing x-ray detection schemes.

Precision Lifetime Measurements Using the Recoil Distance Method.

The recoil distance method (RDM) for the measurements of lifetimes of excited nuclear levels in the range from about 1 ps to 1000 ps is reviewed. The New Yale Plunger Device for RDM experiments is introduced and the Differential Decay Curve Method for their analysis is reviewed. Results from recent RDM experiments on SD bands in the mass-190 region, shears bands in the neutron deficient lead isotopes, and ground state bands in the mass-130 region are presented. Perspectives for the use of RDM measurements in the study of neutron-rich nuclei are discussed.

Slowing down of atoms in metals studied by the Doppler-broadened γ-ray line shapes produced after thermal-neutron capture in Fe and Cr crystals

Molecular-dynamics simulations describing the slowing down of atoms in solids are used to study interatomic potentials in metals. This analysis is achieved by observing the fine structures of Doppler-broadened \ensuremath{\gamma} rays emitted by the recoiling excited nuclei. The recoil of the atom, \ensuremath{\sim}400 eV kinetic energy, is generated by the emission of a preceding \ensuremath{\gamma} ray following thermal-neutron capture. The experiment was performed with oriented single crystals of Fe and Cr as target and high-resolution \ensuremath{\gamma}-ray spectroscopy. Two different nuclear levels for each element were studied and the best interatomic potential among many available in the literature could be deduced from the data. The construction of a different potential was also investigated with this technique. Lifetime values with a much improved precision were obtained for four excited nuclear levels.

Reexamination of the Optical Gamma Ray Decay inT229h

Optical measurements of a clean, $2\ensuremath{-}\ensuremath{\mu}\mathrm{C}$ ${}^{233}\mathrm{U}$ sample were made to verify light emission from gamma ray decay of the first excited nuclear level in ${}^{229}\mathrm{Th}$. The results showed that the light observed in earlier studies was likely to be caused by alpha-particle induced fluorescence of air. In vacuo, no light emission was discernable. The ${}^{229}\mathrm{Th}$ system, therefore, does not appear to provide the level of access for studying atomic-nuclear interactions suggested by the previous measurements.

Nuclear Resonance Beamline at ESRF

The Nuclear Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The sources of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the Mossbauer transition energies between 6 and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the “prompt” electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general lay-out and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results.