Excited Nuclear States Research Articles

Gamma decay of the giant dipole resonance and feeding of superdeformed states in 143Eu

The γ decay of the giant dipole resonance (GDR) built on excited nuclear states has been measured in coincidence with the low-energy γ discrete transitions for the nucleus 143Eu. The reaction used was 110Pd(37Cl, 4n)143Eu at a beam energy of 165 MeV. The EUROBALL spectrometer (for the measurement of discrete γ transitions) coupled with the HECTOR array (for high-energy γ-ray detection) has been used. The high-energy γ-ray spectrum in coincidence with superdeformed (SD) discrete transitions of 143Eu shows an “excess” between 9–12 MeV if compared with the one associated to cascades which do not pass through the SD configurations. Such an “excess” is in the energy region where one expects the low-energy component of the GDR strength function built on a SD state. The measured intensity can be reproduced by the statistical model assuming that the superdeformation survives only few MeV above the yrast line. A similar and consistent scenario has also been obtained by comparing the high-energy γ-ray spectra of 143Eu in coincidence with its spherical (which is fed by the SD configuration) and its triaxial configuration (which is bypassed by the decay of the SD states).

Electromagnetically induced transparency for gamma-quanta using an RF field

We analyze the interference of the transition paths of the nuclear excitation by the resonant gamma quantum. The interference is produced by the radio-frequency (RF) field driving the spin of the excited nuclear state in resonance. It is shown that such driving reduces essentially the interaction of the gamma-quantum with nuclei. The change of interaction results in the RF-induced transparency and in the decrease of the velocity of the gamma-quantum.

X-Ray-Driven Gamma Emission

X-ray-driven gamma emission describes processes that may release nuclear energy in a ‘clean’ way, as bursts of incoherent or coherent gamma rays without the production of radioactive by-products. Over the past decade, studies in this area, as a part of the larger field of quantum nucleonics, have gained tremendous momentum. Since 1987 it has been established that photons could trigger gamma emission from a long-lived metastable nuclear excited state of one nuclide and it appears likely that triggering in other isotopes will be demonstrated conclusively in the near future. With these experimental results have come new proposals for the creation of collective and avalanche-like incoherent gamma-ray bursts and even for the ultimate light source, a gamma-ray laser. Obviously, many applications would benefit from controlled bursts of gamma radiation, whether coherent or not. This paper reviews the experimental results and concepts for the production of gamma rays, driven by externally produced X-rays.

Short-range correlations in low-lying nuclear excited states

The electromagnetic transitions to various low-lying excited states of 16O, 48Ca and 208Pb are calculated within a model which considers the short-range correlations. In general the effects of the correlations are small and do not explain the required quenching to describe the data.

Imperfect Bose System and Its Mixed State Representation. I: Thermal Equilibrium State of Imperfect Bose System

An imperfect spinless bose system is studied with use of the mixed state representation, which was originally developed to study highly excited nuclear states. With this representation, the degrees of freedom are classified into two types: One describes the thermal behaviour of the non-condensed boson and the other mainly describes the dynamical aspects of the condensed boson. The entropy of the system is specified by only the former type of degrees of freedom. Comparison is made with the conventional results for a dilute bose gas with an interaction of the hardsphere type.

Relaxation-stimulated resonances in Mössbauer absorption spectra under radiofrequency magnetic field excitation

A new type of resonant phenomena in the Mössbauer spectroscopy of soft magnetic materials in radiofrequency (rf) magnetic fields is predicted for the rf field frequencies related to those of components of the magnetic hyperfine structure via the parametric resonance condition. Resonant effects of this kind are still unknown in physics; they are realized not at frequencies of transitions between the energy sub-levels of the ground or excited nuclear states, which take place in conventional nuclear magnetic resonance, but at frequencies which are a combination of the frequencies of hyperfine transitions. In this case a separation between the real energy levels can exceed the value of the `resonant' frequency by about 10 orders of magnitude and the resonant rf field sets as if in coherency between these energy levels. Such resonances can only be observed when relaxation processes play an essential role, which permits them to be defined as the relaxation-stimulated resonances. The simplest relaxation model of so-called one-way and localized relaxation is proposed. Within this model an analytical expression for Mössbauer absorption spectra is derived, based on which the specific features of the new resonant effects are analysed in detail.

Mössbauer isomer shifts of crystalline antimony compounds

Variations of the 121Sb Mössbauer isomer shift δ are interpreted for different series of Sb compounds by means of a tight-binding calculation of the electron density at the nucleus ρ(0). The value of the change in the mean square nuclear radius between the nuclear excited and ground states Δ〈 r 2〉 is obtained from the correlation between the experimental values of δ and the calculated values of ρ(0) for 40 Sb compounds. A good agreement with the published values is found, thus validating the present theoretical approach. The values of δ for Sb(V), Sb(III) and Sb(0) compounds are then related to the calculated values of ρ(0) and the electronic numbers of Sb 5s and 5p electrons. Finally, a discussion is proposed for the Sb(V) compounds and for a series of Sb(0) compounds in order to relate the observed variations of δ to changes in the first neighbors of Sb.

Thermal-Spike Lifetime from Picosecond-Duration Preequilibrium Effects in Hyperfine Magnetic Fields Following Ion Implantation

The effective hyperfine magnetic fields acting on short-lived excited nuclear states $(27<\ensuremath{\tau}<127\mathrm{ps})$ have been measured for $\ensuremath{\sim}7.5\mathrm{MeV}$ Ir and Pt ions immediately after implantation into iron hosts at room temperature. The observed field strengths decrease with the lifetime of the probe state and are consistent with the hyperfine field being absent for about 6 ps after implantation. As the hyperfine field is quenched while the local temperature exceeds the Curie temperature, these results give a direct measurement of the thermal-spike lifetime.

Lifetimes of nuclear excited states with neutron gated recoil distance method

A motor driven plunger has been constructed for measuring lifetimes of nuclear excited states in pico second region. An array consisting of six neutron detectors was used to clean up γ-spectra obtained with CS-HPGe detectors. Lifetimes of low excited states in neutron deficient nuclei with low production cross-section e.g.81Y,77Kr and78Rb are reported.

Manifestations of nonstatistical effects in nuclei

The experimental and theoretical data giving evidence of nonstatistical effects in the excitation and decay of highly excited nuclear states and resonances of compound nuclei are reviewed. Nonstatistical effects manifested in β decay, delayed processes, and nuclear reactions involving low-energy protons are analyzed. The reasons for the appearance of nonstatistical effects are discussed.

Magnetic moments of picosecond high-spin states: experimental status and future developments

Magnetic moments are among the most sensitive experimental quantities reflecting the structure of individual excited nuclear states because they are able to distinguish between the nature and the spin coupling of the valence particles. To explore the abilities and limitations of different theoretical models of the nucleus, it is therefore very desirable to determine g-factors with the highest possible reliability. From the experimental point of view, the accuracy achievable is limited by the fact that the effect that has to be measured is extremely small. In addition, when heavy-ion fusion-evaporation reactions are used to populate the nuclei of interest, the complex feeding mechanism inherent in this type of reaction leads to further systematic uncertainties. To overcome these difficulties a new experimental technique called recoil distance transient field technique is introduced allowing for the first time to measure g-factors of individual high-spin states with lifetimes as short as a few picoseconds populated in fusion reactions. For this γγ coincidence technique, the use of highly efficient γ-ray spectrometers is mandatory.

Locations of implanted 19F* atoms in Si, Ge and diamond studied through their nuclear quadrupole interactions

The Hartree-Fock cluster procedure was used to obtain the associated electronic distributions for 19F* (I = 5/2, Ex = 197 KeV excited nuclear state of the 19F atom) at possible sites in crystalline Si, Ge and diamond and to calculate nuclear quadrupole coupling constants vQ and the asymmetry parameter η of the electric field gradient at the modelled sites. Lattice relaxation effects have been incorporated by employing a geometry optimization method to obtain minimum energy configurations for the clusters modelling each site. The intrabond (IB), antibonding (AB) and substitutional (S) sites in the bulk and the atop site on the surface were studied. From a comparison with vQ and η values observed in time differential perturbed angular distribution (TDPAD) measurements we were able to identify the high frequency component in Si and Ge with 19F* at the intrabond site. In diamond two high frequency components are observed. These are identified with 19F* at intrabond and substitutional sites. For the low frequency site in Si and Ge the assignment is made to 19F* implants at dangling bonds in the bulk resulting from implantation damage. In diamond none of the sites studied could provide lower frequency nuclear quadrupole parameters close to the observed ones.

Study of high-lying cluster states of nuclei by the method of particle–particle angular correlations

The current status of research on nuclear reactions induced by light and semiheavy ions of energy up to 10 MeV/nucleon on light and intermediate nuclei by measurement of the angular correlation functions of the final products is reviewed. The principal theoretical and experimental studies on particle–particle correlations carried out in recent years are analyzed. The methods of calculating the angular correlation functions and the spin tensors of the reaction-product density matrix for reactions involving excited states using the distorted-wave method with finite interaction range and the modified compound-nucleus model are described. The α−d and α−t correlations in reactions involving 6,7 Li and 14 N ions are analyzed. The highly excited α-cluster states in 12 C , 16 O , 20 Ne , and 24 Mg are studied. The polarizationtensors of the 6 Li nucleus in the 3 + state are calculated and compared with experiment. It is shown that the particle–particle angular correlation functions and the polarizationtensors can provide unique information about the reaction mechanism and about the structure of the wave functions of highly excited nuclear states, including the nature of the radial dependence in the interior of the nucleus and the optical interaction potentials in the entrance and exit reaction channels, and so on. This review can be useful for both theoreticians and experimentalists working on the physics of nuclear reactions involving particles of moderate energy.

Influence of Giant Nuclear-spin Polarisation on Resonant Gamma-ray Absorption and Emission

We propose a new scheme of gamma-quanta amplification without inversion. Laser pumping of electron states creates giant nuclear-spin polarisation via the hyperfine interaction. This results in extreme cooling of the ground-state nuclear spin in a projection which does not absorb both laser pump and gamma-quanta according to selection rules for these transitions. Induced emission from the nuclear excited state is not influenced by the pump. Therefore gamma-quanta travelling inside the pump beam have an opportunity to induce stimulated emission without subsequent quenching by ground state nuclei.

Cavity Modifications of Nuclear Internal Conversion Rates - Abstract

We extend the principles of cavity quantum electrodynamics to nuclei inside low-loss cavities. The resulting nucleus-cavity system exhibits spontaneous emission and internal conversion rates which differ radically from those of the individual components. The quantum mechanical derivations of these rates are given. The internal conversion process occurs via the exchange of a virtual photon between an excited nucleus and an electron bound to that nucleus. Even though the photon is virtual, partial suppression of the internal conversion process is shown to be theoretically possible. With a controlled modification of the internal conversion rate using a high-Q cavity, one can achieve a number of potentially interesting applications including an excited nuclear state containment unit, a nuclear battery, and a neutron detector.

The Giant Dipole Resonance in the superdeformed143Eu nucleus

The γ-decay of the Giant Dipole Resonance (GDR) built on excited nuclear states has been measured for the nucleus143Eu. The reaction110Pd(37Cl, 4n)143Eu at a beam energy of 165 MeV has been employed. This experiment aimed at searching the γ-decay of the GDR built on the superdeformed143Eu states, populated at high spins. High-energy γ-rays were detected in 8 large BaF2 scintillators in coincidence with discrete transitions measured with the NORDBALL array (in the configuration consisting of 17 HPGe detectors and a 2π multiplicity filter). The spectrum of high-energy γ-rays gated by low-energy transitions between states fed by the superdeformed states shows some excess of yield in the 7–10 MeV region with respect to that gated by transitions between states not populated by the superdeformed states. This excess should be due to the γ-decay of the the giant dipole oscillation along the superdeformed axis of the nucleus that is expected to have a frequency corresponding to ≈9–10 MeV (low-energy component of the GDR strength function). The measured excess, in spite of the large error bars, is found to be of the same order as predicted statistical model values.

Spin Crossover, Liesst, and Niesst-Fascinating Electronic Games in Iron Complexes

Coordination compounds of transition metal ions with open-shell electron configurations may exhibit dynamic electronic structure phenomena depending on the nature of the coordinating ligand sphere. The change of spin state with temperature (thermal spin crossover”) and light-induced electron transfer processes leading to long-lived metastable states are among the most fascinating electronic games encountered in transition metal compounds and are presently under intensive study by chemists and physicists. The first part of this lecture will survey briefly some highlights of previous work and present recent results on thermal spin crossover in iron(II) compounds. The second part of the lecture will focus on selected examples of Light-Induced Excited Spin State Trapping” (LIESST), with special emphasis on the occurrence of bistability and cooperative interactions during LIESST relaxation. The last part of the lecture will be devoted to, Nuclear Decay-Induced Excited Spin State Trapping (NIESST), the unique experiment which allows one to make use of the nuclear disintegration of 57Co → 57Fe as an, internal light source” to generate such photophysical aftereffects as aliovalent charge and anomalous spin states, and to detect and characterize such metastable states simultaneously by time-integral and time-differential Mössbauer emission spectroscopy. Results from lifetime measurements by Mössbauer as well as optical techniques prove that the same relaxation mechanism applies to both phenomena, LIESST and NIESST.

The effects of crystal field on Tm 3+ in Tm(BrO 3 ) 3 · 9H 2 O: An experimental and theoretical study

Crystals of thulium bromate enneahydrate were grown and the magnetic susceptibility χ ⊥ and the magnetic anisotropy Δχ(= χ ∥ − χ ⊥ ) were measured in the range 300-14 K and 300-80 K, respectively. χ ∥ and χ ⊥ show widely divergent thermal variation; the χ ∥ versus T plot is nearly flat compared to that of χ ⊥ · Smooth variation of χ ⊥ down to 14 K does not suggest any structural phase transition or symmetry breaking. Our observed data and that of Simizu et al . [ J. Appl. Phys 55 , 2333 (1984)] below 4 K were analyzed for the first time invoking an effective crystal field. Our theoretical values in general provide excellent agreement to the observed data. Very brief comment on the behavior of χ ∥ below 4 K by Simizu et al . is borne out qualitatively by our theory, but their absolute value seems abnormally high, which makes it suspect. The computed crystal field spectra led us to locate the peaks in the electronic heat capacity and to find the quadrupole splitting of the 8.42 keV nuclear excited state of 169 Tm; high magnetic field enhancement at the nucleus suggests complex Mössbauer spectra in the presence of a magnetic field perpendicular to the symmetry axis.

Measurement of Nuclear Quadrupole Interactions using Mössbauer Spectroscopy

Abstract Mössbauer spectroscopy is a useful probe for investigating nuclear quadrupole interactions. It enables the magnitude, sign and asymmetry of the electric quadrupole coupling constant to be determined. It is especially important for its ability to measure these parameters for excited nuclear states. The Mössbauer effect is used to measure excited state quadrupole moments in materials whose electric field gradient (EFG) is known. More commonly however, it is used to determine the EFG from which atomic bonding and electronic structure are determined. The technique also allows temperature dependent orientation of sublattice magnetization to be measured using the direction of the electric quadrupole coupling as a reference.

Nuclear excitation by laser-assisted electronic transitions.

The excitation of nuclear states by electric and magnetic multipole transitions of bound electrons in the presence of a strong laser field is studied. The excitation probability is calculated for general time-dependent electronic states in first-order perturbation theory. An adiabatic description of the dressed electron states in the laser field allows an easy calculation of the excitation function. Ionization effects caused by the laser are considered in a simple manner. The probability for the excitation of the nuclei $_{66}^{161}\mathrm{Dy}$, $_{76}^{189}\mathrm{Os}$, $_{77}^{193}\mathrm{Ir}$, $_{79}^{197}\mathrm{Au}$, $_{92}^{235}\mathrm{U}$, and $_{93}^{237}\mathrm{Np}$ is investigated as a function of the laser intensity and the photon energy. The increase of the excitation probability is limited by the ionization of the atom and the lifetime of the states. The de-excitation of the 3/${2}^{+}$ (3.5\ifmmode\pm\else\textpm\fi{}1.0 eV) state in $_{90}^{229}\mathrm{Th}$ is an example of the laser-assisted discrete internal conversion. \textcopyright{} 1996 The American Physical Society.