Excited Nuclear States Research Articles

The role of the continuum and the spurious 1− transitions in incoherent μ −−e− conversion rate calculations

By using the Continuum RPA (CRPA) method, the incoherent transition strength of the exotic μ−−e− conversion in nuclei is investigated. The question whether excited nuclear states lying high in the continuum give an important contribution to the incoherent rate is addressed. Results for 40Ca are compared with those obtained previously for 208Pb. For both nuclei we then investigate in detail the admixture of spurious components in the rate coming from 1− excitations, within the self-consistent CRPA with Skyrme interactions as well as within a less consistent version. We employ and compare two methods for removing the spurious strength: the use of effective operators, as done in a previous work for 208Pb, or simply the exclusion of the spurious state appearing close to zero energy. In all cases, the correction achieved is quite large.

Unified model of nuclear mass and level density formulas

Nuclear ground and excited state properties are described by using parameter systematics on mass and level density formulas. The formulas are based on an analytical expression of the single-particle state density by introducing the shell-pairing correlation in a new way. Main features are the shell, pairing, and deformation effects on the droplet model near the ground state, which are washed out at higher excitation energies. The main aim of the paper is to provide in the analytical framework the improved energy dependent shell, pairing, and deformation corrections generalized to the collective enhancement factors, which offer a systematic prescription over a great number of nuclear reactions. The new formulas are shown to be in close agreement with not only the empirical nuclear mass data but also the measured slow neutron resonance spacings and experimental systematics observed in the excitation energy dependent properties.

Four-reflection “nested” meV-monochromators for 20–30 keV synchrotron radiation

We present the design and performance of three silicon high-resolution ( E / Δ E ≈ 2 – 5 × 10 7 ) monochromators for synchrotron radiation that work at energies corresponding to nuclear excited states in 151Eu (21 542 eV), 119Sn (23 880 eV), and 161Dy (25 651 eV). The design of each monochromator is quite similar with the exception of the silicon crystal reflections that are used and the surface asymmetry angles that are chosen. We give the basic design underlying all three monochromators along with a table of the specific parameters used for each one. Performance in terms of resolution and spectral efficiency are measured and compared to theoretical expectations. Measurements of lattice excitations in different solids containing the aforementioned isotopes demonstrate the utility of each monochromator. Possible improvements to performance are presented.

Level mixing induced transparency for gamma radiation

The propagation of gamma radiation through a resonant medium in the case of interfering quantum transition paths is considered. The interference is made possible by a field that mixes the crossing spin levels in the excited nuclear state and splits two degenerate transitions into two V-type transitions. If forward resonant scattering allows for a change of the gamma radiation polarization, then the two V-transitions are coupled, which results in destructive interference. In this case the absorption is reduced in a particular frequency domain.

A new excited nuclear state

A new excited nuclear state

Spectroscopy with radioactive ion beams at the HRIBF for nuclear astrophysics

Proton elastic scattering, proton inelastic scattering, proton transfer reactions and the (d, p) neutron transfer reaction have been measured in inverse kinematics with radioactive ion beams at the HRIBF to determine the properties of nuclear excited states that are important in stellar explosions. The experimental techniques and some initial results are briefly summarized.

Weighing excited nuclear states with a Penning trap mass spectrometer

We report on high-accuracy mass measurements and isomer identification of 187Pb. In this nuclide, two close-lying isomeric states are known from α-decay studies. With the combined use of the ISOLDE resonance ionization laser ion source and the Penning trap mass spectrometer ISOLTRAP the energy difference of 187Pb and 187 m Pb was determined to be E = 33 ( 13 ) keV . This is the lowest isomeric excitation energy ever determined by weighing nuclei.

Software for X- and gamma-ray spectrometry

A computational approach to the true-coincidence summing correction factor evaluating was developed on the basis of the extended version of the MCNP - a general Monte Carlo N-particle transport code. A specially developed utility program generates the MCNP input on the basis of a routinely updated Evaluated Nuclear Structure Data File (ENSDF) library. Necessary information is automatically added to allow accurate simulating of the emission of correlated particles accompanying the decay of a particular radionuclide, including emission of annihilation quanta, K- and L- X-rays, β-particles and conversion electrons. Gamma-ray angular correlations as well as lifetimes of the nuclear excited states are also taken into account. The approach is applicable to correction factor evaluation for ordinary single- and multi-detector spectrometers as well as for Compton suppression systems. The paper describes the developed computational scheme as well as presents the results of its preliminary testing for the case of both point and volumetric sources.

Rapid Freeze-Quench 57Fe Mössbauer Spectroscopy: Monitoring Changes of an Iron-Containing Active Site during a Biochemical Reaction

Nuclear gamma resonance spectroscopy, also known as Mössbauer spectroscopy, is a technique that probes transitions between the nuclear ground state and a low-lying nuclear excited state. The nucleus most amenable to Mössbauer spectroscopy is 57Fe, and 57Fe Mössbauer spectroscopy provides detailed information about the chemical environment and electronic structure of iron. Iron is by far the most structurally and functionally diverse metal ion in biology, and 57Fe Mössbauer spectroscopy has played an important role in the elucidation of its biochemistry. In this article, we give a brief introduction to the technique and then focus on two recent exciting developments pertaining to the application of 57Fe Mössbauer spectroscopy in biochemistry. The first is the use of the rapid freeze-quench method in conjunction with Mössbauer spectroscopy to monitor changes at the Fe site during a biochemical reaction. This method has allowed for trapping and subsequent detailed spectroscopic characterization of reactive intermediates and thus has provided unique insight into the reaction mechanisms of Fe-containing enzymes. We outline the methodology using two examples: (1) oxygen activation by the non-heme diiron enzymes and (2) oxygen activation by taurine:alpha-ketoglutarate dioxygenase (TauD). The second development concerns the calculation of Mössbauer parameters using density functional theory (DFT) methods. By using the example of TauD, we show that comparison of experimental Mössbauer parameters with those obtained from calculations on model systems can be used to provide insight into the structure of a reaction intermediate.

A microscopic investigation of the transition form factor in the region of collective multipole excitations of stable and unstable nuclei

We have used a self-consistent Skyrme–Hartree–Fock plus continuum-RPA model to study the low-multipole response of stable and neutron/proton-rich Ni and Sn isotopes. We focus on the momentum-transfer dependence of the strength distribution, as it provides information on the structure of excited nuclear states and in particular on the variations of the transition form factor (TFF) with the energy. Our results show, among other things, that the TFF may show significant energy dependence in the region of the isoscalar giant monopole resonance and that the TFF corresponding to the threshold strength in the case of neutron-rich nuclei is different compared to that corresponding to the respective giant resonance. Perspectives are given for more detailed future investigations.

Revised rates for the stellar triple-α process from measurement of 12C nuclear resonances

In the centres of stars where the temperature is high enough, three alpha-particles (helium nuclei) are able to combine to form 12C because of a resonant reaction leading to a nuclear excited state. (Stars with masses greater than approximately 0.5 times that of the Sun will at some point in their lives have a central temperature high enough for this reaction to proceed.) Although the reaction rate is of critical significance for determining elemental abundances in the Universe, and for determining the size of the iron core of a star just before it goes supernova, it has hitherto been insufficiently determined. Here we report a measurement of the inverse process, where a 12C nucleus decays to three alpha-particles. We find a dominant resonance at an energy of approximately 11 MeV, but do not confirm the presence of a resonance at 9.1 MeV (ref. 3). We show that interference between two resonances has important effects on our measured spectrum. Using these data, we calculate the triple-alpha rate for temperatures from 10(7) K to 10(10) K and find significant deviations from the standard rates. Our rate below approximately 5 x 10(7) K is higher than the previous standard, implying that the critical amounts of carbon that catalysed hydrogen burning in the first stars are produced twice as fast as previously believed. At temperatures above 10(9) K, our rate is much less, which modifies predicted nucleosynthesis in supernovae.

Projectilelike fragment emission angles in fragmentation reactions of light heavy ionsin the energy region 200MeV∕nucleon: Modeling and simulations

A new semiempirical model called FRANG for calculation of fragment emission angles in light and heavy ion fragmentation reactions was developed. Contributions from both central and peripheral collisions were investigated, where fragmentation occurs due to nuclear and Coulomb interaction, respectively. For central collisions the reaction was described by a two step abrasion-ablation model, where collision parameters were determined from a simple geometrical model. The fragment emission angles were calculated using a parametrization of longitudinal momentum loss and transverse momentum uptake in the collision of projectile and target atom. For peripheral collisions the Coulomb excitation of nucleon vibration resonances and subsequent decay into fragments was taken into account. Fragment emission angles were calculated from deflection in the electric field and from the amplitude of vibrations in excited nuclear states. The modeled emission angles were in accordance with the experimental values for most projectile-target systems examined and compared. It was established that the model very well reproduces the experimental results in the energy region $<200\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\text{nucleon}$, despite its simplicity, and can be successfully employed in several applications. The model is estimated to be valid in the energy range from a few $10\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\text{nucleon}$ up to few $100\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\text{nucleon}$.

Modification of Nuclear Decay Constant in the Finite Space

The decay constants of nuclei are calculated based on the Cheon's idea, namely, the population of nuclear excited state can be modified by the mechanism that the gamma-ray emitted from the radioactive nuclei in a solid is reabsorbed by the source after reflected on a metal surface. Our results show that the nuclear lifetime is enhanced inside a lead cylinder.

Decay time characteristics of the U-like excited nuclei

The unified energy dependence of the induced fission times obtained by the crystal blocking technique for heavy nuclei with Z=91–94 in the range of initial excitation energy from 5 to 250 MeV was analyzed. It was demonstrated that, for excitation energies of the investigated heavy fissionable nuclei up to 60–70 MeV, the fission times can be described in the framework of the statistical theory of nuclear reactions taking into account the double-humped structure of the fission barrier and the lifetimes of both classes of excited nuclear states realized in the first and second potential wells. However, for excitation energies above 70 MeV, there is a need to consider the dynamical effects in the fission channel.

Decay time characteristics of the U-like excited nuclei

The unified global energy dependence of the induced fission times obtained by the experimental crystal blocking technique for nuclei with Z=91–94 in the range of initial excitation energy from 5 to 250 MeV was analyzed. It was demonstrated that for the energies up to 60–70 MeV the fission times can be described in the frame of the statistical theory taking into account the double-humped structure of the fission barrier and the lifetimes of both classes of excited nuclear states realized in the first and second potential wells. However, for the excitation energies above 70 MeV it is needed to consider the dynamical effects in fission channel.

Control of the atom (nucleus) lifetime in the excited state by means of alow-frequency external field

The radiative decay dynamics of an atomic (nuclear) excited state split into twoneighbouring sublevels are considered for the case of interaction with alow-frequency electromagnetic field. The conditions of cancellation of thespontaneous emission in this system are analysed beyond the rotating waveapproximation.

Estimates of weak and electromagnetic nuclear decay signatures for neutrino reactions in Super-Kamiokande

We estimate possible delayed $\ensuremath{\beta}$ decay signatures of the neutrino induced reactions on ${}^{16}\mathrm{O}$ in a two-step model: the primary neutrino $(\ensuremath{\nu},l)$ process, where l is the lepton in the final state, is described within the random phase approximation, while the subsequent decay of the excited nuclear state in the final channel is treated within the statistical model. We calculate partial reaction cross sections leading to $\ensuremath{\beta}$ unstable nuclei. We consider neutrino energies up to 500 MeV, relevant for atmospheric neutrino detection in Super-Kamiokande, and supernova neutrino spectra.

Internal conversion between bound states and the Pauli exclusion principle

We present the results of an investigation of the contribution to decay of the nucleus by an unexplored mode of internal conversion in which an excited nuclear state deexcites with promotion of an electron to an intermediate filled bound state. This process, which apparently violates the Pauli exclusion principle, may make a significant contribution to the nuclear decay rate when very close matching of the atomic and nuclear transitions is achieved. For an $M1$ transition, we show that Pauli-forbidden bound internal conversion (PFBIC) transition may occur by excitation of an inner electron to an initially occupied outer orbital. Numerical calculations of the PFBIC process are presented for the decay of the 77.351 keV level of ${}^{197}\mathrm{Au},$ in the neutral atom and in ions up to ${69}^{+}.$ The PFBIC process is found to have a maximum probability in ${}^{197}\mathrm{Au}$ ions with charge state of approximately ${40}^{+},$ due to close energy matching of the nuclear transition to the $1\stackrel{\ensuremath{\rightarrow}}{s}3s$ electronic excitation. The expected dependence of the nuclear lifetime on PFBIC is analyzed.

Perturbed γ–γ correlations from oriented nuclei and static moment measurements I: formalism and principles

A formalism to interpret the perturbations of γ–γ directional correlations from nuclei oriented by heavy-ion induced reactions is described. Applications to multidetector arrays are considered and aspects of experiments designed to measure magnetic dipole moments of excited nuclear states are considered.

Comparison of ab initio and density functional calculations of electric field gradients: The Fe57 nuclear quadrupole moment from Mössbauer data

The difficulty in accurate determination of the nuclear quadrupole moment of the first I=3/2 excited nuclear state of Fe57 from electronic structure calculations of the iron electric field gradient combined with Mössbauer measurements of the nuclear quadrupole splitting in the isomer shift is addressed by comparing ab initio with density functional calculations for iron pentacarbonyl, Fe(CO)5, ferrocene, Fe(C5H5)2, and the Δg5 electronic ground states of FeCl2 and FeBr2. While the ligand field gradient tensor components change relatively little with the method applied, the iron electric field gradient is sensitive to the specific density functional used. Single reference many-body perturbation theory for electron correlation also performs poorly for the iron electric field gradient and shows extreme oscillatory behavior with a change in the order of the perturbation series. Even with larger basis sets and coupled cluster techniques a precise value for the iron electric field gradient could not be determined from electronic structure calculations due to limitations in the theoretical procedures. In order to avoid uncertainties in the measured isomer shift which enters into the nuclear quadrupole coupling constant we determined the Mössbauer spectrum of Fe(C5H5)2 between temperatures of 4.2 and 295 K. In this range two phase transitions are observed, but the quadrupole splitting is not very dependent on the solid state structure in each phase. Solid state effects for the Fe(CO)5 are determined by comparing the iron electric field gradient of the isolated molecule with the value obtained from first principle solid state calculations at various levels of theory. These calculations show that the influence of near neighboring effects to the iron electric field gradient is small. Fully relativistic Dirac–Hartree–Fock calculations for Fe(CO)5 reveal that relativistic effects for the iron electric field gradient are small as well. Fe(CO)5 is therefore an ideal test molecule for the determination of an accurate nuclear quadrupole moment from electronic structure calculations if combined with an experimental nuclear quadrupole coupling constant. Our best estimate for the Fe57 nuclear quadropole moment is 0.14(2) barn in reasonable agreement with recent nuclear structure calculations.