Systematic study of the single-state dominance in 2νββ decay transitions

The single-state-dominance hypothesis (SSDH) states that the decay rates of the two-neutrino double-beta decay are governed by a virtual two-step transition connecting the initial and final ground states through the first 1⁺ state, 1_l⁺, of the intermediate odd-odd nucleus, for those odd-odd nuclei where the 1_l⁺ state is the ground state. To investigate the validity of the SSDH we have performed a systematical theoretical analysis of all known double-beta-decay transitions where the SSDH conditions are fulfilled. the calculations are based on the quasiparticle random-phase approximation (QRPA) and the results have been obtained by using realistic single-particle bases and realistic interactions. We have studied the double β⁻ decays of ¹⁰⁰Mo, ¹¹⁰Pd, ¹¹⁴Cd, ¹¹⁶Cd and ¹²⁸Te and the double electron-capture transitions in ¹⁰⁶Cd and ¹³⁶Ce. The analysis shows that the SSDH is realized either through a true dominance of the first intermediate 1⁺ state or by cancellations among the contributions of higher lying 1⁺ states of the intermediate nucleus.     

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

It is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–10⁴ times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.     

方波激发下Er~(3+)上转换红光激发机理的研究

用方波电源驱动808 nm激光二极管(LD)激发Er3+掺杂的亚碲酸盐氟氧化物玻璃,测量4F9/2能级上转换发光的上升和衰减,根据上升的时间常数确定中间能级的寿命,从而确定4F9/2能级粒子数积累的过程。通过建立速率方程模型分析4F9/2能级的上升和衰减特性与中间能级的关系,确定808 nm LD激发下上转换红光的激发机理,同时提供了一种通过上转换发光,用光电倍增管测量红外能级寿命的方法。     

DD fusion in crystals

The article discusses the mechanism of DD → ⁴He fusion and so-called nonradiative thermalization of the reaction in crystals. The dynamics of this process is considered. The assumption that the decay time of the compound nucleus depends on its excitation energy makes experiments in crystals compatible with the acceleration data.We consider the processes in the crystals that increase the intensity ofDD fusion in comparison to the amorphous media, and the yield of the reaction is estimated.     

Spin-dependent effects in inelastic scattering

The differential cross sections for the inelastic scattering of 10 MeV, 19.6 MeV, 30.4 MeV, 40 MeV and 49.35 MeV protons to the 2⁺ state (1.409 MeV) in ⁵⁴Fe and of 19.6 MeV protons to the 2⁺ state (0.846 MeV) in ⁵⁶Fe are analyzed in conjunction with the available data on the asymmetries and spin-flip probability amplitudes. The scattering amplitudes for both one step (valence plus core polarization) and two step (intermediate resonance) processes are evaluated using an antisymmetrized distorted wave approximation. Collective model representations for both the one step (core polarization) and two step (intermediate resonance) processes are used, and included are the effects of deforming the full Thomas spin-orbit potentials. The one step processes are fixed by the analyses of the scattering of 30.4, 40 and 49.35 MeV protons, with the core polarization contributions being constrained by the B(E2) values for the γ-ray deexcitation of the 2⁺ states. The analyses of the 19.6 MeV data demonstrates the need for an extra (two step) contribution to the reaction process and are consistent with the virtual formation of an L = 3 giant resonance. The 10 MeV data most certainly demonstrate compound nucleus effects but could also have some strength due to the virtual formation of an intermediate L = 2 giant resonance. The resonance parameters are consistent with recent information concerning the mass variation of giant resonances.     

Nuclear excitation by electron capture followed by fast x-ray emission

The resonance strength of the two-step process of nuclear excitation by electron capture followed by γ decay of the nucleus can be significantly increased in highly charged ions if the resonant capture proceeds via an excited electronic state with subsequent fast x-ray emission. For fully ionized ²³⁸₉₂U and ²³²₉₀Th, the x-ray decay stabilizes the system against internal conversion of the captured electron, with an increase of both nuclear lifetimes and resonance strengths of up to two orders of magnitude compared with the case when occupied atomic orbitals prevent the x-ray de-excitation. Applications of this effect to the measurement of the not yet experimentally observed nuclear excitation by electron capture and to dense astrophysical plasmas are discussed.     

The neutrinoless double-beta decay: A test for new physics

The neutrinoless double-beta decay is not allowed in the Standard Model (SM) but it is allowed in most Grand Unified Theories (GUTs). The neutrino must be a Majorana particle (identical with its antiparticle) and must have a mass to allow the neutrinoless double-beta decay. Apart of one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for the neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, one discusses the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters.     

Resonance internal conversion as a way of accelerating nuclear processes

A theory of resonance conversion (RC) is presented. It is shown that by resonance conversion being a natural extension of traditional internal conversion, into the subthreshold domain, in a number of cases, it strongly affects nuclear de-excitation. Moreover, as it concentrates transition strength in narrow bands corresponding to atomic spectral lines, it is a unique tool for accelerating nuclear processes. Along with the wellknown process of non-radiative nuclear excitation through electron NEET transition and the inverse RC process, resonance conversion provides convenient mathematics for a number of crossing-invariant processes involving a nucleus and electrons, excitation and de-excitation of nuclei, by a hyperfine magnetic field, spin mixing of nuclear states via an electron shell, a hyperfine interaction and magnetic anomalies in an atomic spectra, and the excitation of nuclei in collisions accompanied by the ionization of an electron shell, in muon decay in the orbit, etc. Mechanisms of isomer pumping via a laser-radiation-induced RC and of isomer energy triggering in a resonance laser radiation field are considered. An especially strong effect can be obtained in hydrogen-like ions, with practically no RC damping. The theory is also generalized to the case of discrete Auger transitions.     

Majorana neutrino masses and the neutrinoless double-beta decay

Neutrinoless double-beta decay is forbidden in the Standard Model of electroweak and strong interaction but allowed in most Grand Unified Theories (GUTs). Only if the neutrino is a Majorana particle (identical with its antiparticle) and if it has a mass is neutrinoless double-beta decay allowed. Apart from one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow one to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity-violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present work, one discusses the accuracy of the present status of calculating of the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters. The text was submitted by the author in English.     

Angular distribution of electrons in neutrinoless double-beta decay and new physics

For neutrinoless double-beta decay caused by the exchange of light Majorana neutrinos, an expression for the differential width with respect to the angle between the final-electron momenta is obtained on the basis of a Lorentz-invariant effective Lagrangian of the general form. The shape of this angular distribution is analyzed within various extensions of the Standard Model that allow this process—in particular, within theories that involve Majorana super partners and (or) right-handed currents. The angular correlation coefficient for electrons as a function of the mass of the right-handed W boson and the effective Majorana neutrino mass in the decay of the ⁷⁶Ge nucleus is considered within the model involving left—right symmetry.     

Neutrinoless Double-Beta Decay to Excited 0+ States

A considerable effort has been invested in studies of the two-neutrino double-beta decay to the first (and some others, too) excited 0⁺ states. These decay transitions have been considered favourable in terms of the nuclear matrix elements and the possibility to use -coincidence techniques in cleaning the background of the spectrum. A drawback in these studies is the diminished phase space for the emitted leptons due to the smaller Q value of the process. One positive result, namely the decay rate of ¹⁰⁰Mo to the first excited 0⁺ state in ¹⁰⁰Ru, already exists. Encouraged by this result one could argue that a similar situation could prevail also for the neutrinoless double-beta decay to excited 0 ⁺ states. In this article the situation has been studied for several nuclear systems with potential 0 decay to the low-lying 0⁺ states by using the quasiparticle random-phase approximation with realistic nucleon-nucleon interactions.     

Double-beta decay: Recent advances in experimental studies

The current situation in experiments studying double-beta decay is surveyed. The amount of experimental information about the two-neutrino mode of the process has grown considerably over the last decade. The two-neutrino double-beta decay of ten nuclei (⁴⁸Ca, ⁷⁶Ge, ⁸²Se, ⁹⁶Zr, ¹⁰⁰Mo, ¹¹⁶Cd, ¹²⁸Te, ¹³⁰Te, ¹⁵⁰Nd, and ²³⁸U) was observed in direct and geochemical experiments. However, the main fundamental question—that of neutrinoless double-beta decay, which has not yet been recorded, although the sensitivity of present-day facilities featuring germanium detectors is higher than 10²⁵ yr—remains open. The constraint on the effective Majorana mass on the basis of these results is 〈m _v〉<(0.4–1.1) eV. Further advancements in searches for neutrinoless double-beta decays must rely on developing fundamentally new experimental facilities, since the potential of those that already exist has been exhausted to a considerable extent.     

0+ analogue state in 118Sb from 117Sn(p,nγ) reaction

The analogue of the 0⁺ ground state in ¹¹⁸Sn has been observed in the compound nucleus ¹¹⁸Sb through ¹¹⁷Sn(p,n γ) ¹¹⁷Sb reaction. The neutron decays of this analogue resonance have been studied from the deexciting γ-rays of the residual nucleus ¹¹⁷Sb. From off resonance excitation functions, spin assignments have been made to states in ¹¹⁷Sb, on the basis of Hauser-Feshbach formalism. The resonance parameters of the isobaric analogue resonance have been determined, including the total, proton and neutron decay widths.     

Exact inclusion of the Coulomb field in the photobeta decay of a nucleus and problem of bypassed elements

The probability of the endothermic β^? decay of nuclei that is stimulated by an electromagnetic field of Planck frequency spectrum (photobeta decay) is calculated, the effect of the Coulomb field on a relativistic electron and a virtual relativistic positron being exactly taken into account in this calculation. It is shown that the inclusion of Coulomb effects is of paramount importance and that the results of the calculations may differ by an order of magnitude from those that were obtained previously in the plane-wave approximation, depending on the energy range being considered. A model for the synthesis of bypassed elements in the interior of massive stars is proposed on the basis of the mechanism of the photobeta decay of stable elements that originate from s and r processes.     

Magnetoluminescence studies of excitonic scattering processes in high purity GaAs

The broad emission line at about 1.512 eV, which dominates the emission spectra of pure GaAs at intermediate excitation levels (1–300 kW/cm²) and low temperatures (<40 K) is investigated in magnetic fields up to magnetic flux densities of 10 T. The shift of the emission maximum in the magnetic field is exactly the same as recently reported for the free exciton. This demonstrates that at high excitation levels exciton-electron scattering is the dominant mechanism for the radiative decay of free excitons.     

Biphotonic photochemistry of benzophenones in dimethylsulphoxide: a flash photolysis EPR study

Benzophenone and its derivatives in dimethylsulphoxide (DMSO) exhibit biphotonic photochemistry under 355 nm laser photolysis. Flash photolysis electron paramagnetic resonance experiments demonstrate that a single laser pulse is capable of producing and exciting benzophenone triplets, which can sensitize dimethylsulphoxide and subsequently lead to photodecomposition. In decafluorobenzophenone, electron transfer is the dominant process of the highly excited triplet state. Despite the rapid radiationless decay of 2-hydroxybenzophenone (2OHbenzophenone) in non-polar solvents, radical signals are observed from the photoexcitation of 2OHbenzophenone in DMSO. This is attributed to the sufficiently rapid excitation of the triplet state, which competes with the radiationless decay process, aided by the unique solvent properties of DMSO. It is concluded, in contrast to literature data, that the excited triplet state of DMSO is reactive, and can produce methyl radicals that show triplet mechanism polarization via the biphotonic photoexcitation of benzophenone.     

Double-beta decay in gauge theories

Neutrinoless double-beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view, it pops up in almost every model, giving rise among others to the following mechanisms: (a) the traditional contributions like the light neutrino mass mechanism as well as the j _L –j _R leptonic interference (λ and η terms), (b) the exotic R-parity-violating supersymmetric (SUSY) contributions. Thus, its observation will severely constrain the existing models and will signal that the neutrinos are massive Majorana particles. From the nuclear physics point of view, it is challenging, because (1) the nuclei, which can undergo double-beta decay, have complicated nuclear structure; (2) the energetically allowed transitions are suppressed (exhaust a small part of all the strength); (3) since in some mechanisms the intermediate particles are very heavy one must cope with the short distance behavior of the transition operators (thus novel effects, like the double-beta decay of pions in flight between nucleons, have to be considered; in SUSY models, this mechanism is more important than the standard two-nucleon mechanism; and (4) the intermediate momenta involved are quite high (about 100 MeV/c). Thus one has to take into account possible momentum-dependent terms of the nucleon current, like modification of the axial current due to PCAC, weak magnetism terms, etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino, the effect is much larger and model-dependent. Taking the above effects into account, the needed nuclear matrix elements have been obtained for all the experimentally interesting nuclei A=76, 82, 96, 100, 116, 128, 130, 136, and 150. Then, using the best presently available experimental limits on the half-life of the 0νββ decay, we have extracted new limits on the various lepton-violating parameters. In particular, we find 〈m _ν〉 < 0.3 eV/c ², and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R-parity-violating parameter λ′₁₁₁<4.0×10^?4.     

Electron excitation of hydrogen atom by ions impact in the energy range 20–1000 keV/amu

We have calculated the electron excitation cross sections of hydrogen atom by the impact of protons, alpha particles and He⁺ ions using the boundary corrected continuum intermediate state approximation in the intermediate and high energies. The calculated results are compared with other theoretical and experimental results. The angular influence of excitation to the H atom at the intermediate energy is also discussed. The distortion effects due to the projectile charges in reactions of electron excitation to bound states of the target H atom are shown in the intermediate and high collision energy.     

Investigation of the reaction mechanism for the four-particle photodisintegration of a carbon nucleus

The four-particle photodisintegration of a carbon nucleus in the reactions ¹²C(γ, p)³H2α and ¹²C(γ, n)³H2α is investigated by a method that employs a diffusion chamber in a magnetic field. It is shown that these reactions proceed according a sequential-type scheme: excited states of ¹¹B and ¹¹C nuclei decay to weakly excited states of ⁸Be, ⁷Li, and ⁷Be nuclei. It is concluded that nucleons are knocked out from the s shell. In the excitation curve for the 2α system in the reaction ¹²C(γ, p)³H2α, a resonance is found between the maxima corresponding to the ground and the first excited state of the ⁸Be nucleus, and this resonance is identified as a ghost anomaly. The branching fractions of the decay modes are determined. The angular distributions of nucleons in the reaction c.m. frame are measured. The energy dependence of the asymmetry coefficient for the angular distributions is obtained. A fast increase in this coefficient is observed in the energy range 38–40 MeV. It is concluded that the asymmetry coefficient depends on the excitation energy of the final nucleus in the region of intermediate photon energies.     

Fine structure of the strength function for the β<Superscript>+</Superscript>/EC decay of the <Superscript>160<Emphasis Type="Italic">m</Emphasis></Superscript>Ho (5.02 h) isomer

The strength function for the β⁺/EC decay of the deformed nucleus of the ^160m Ho (5.02 h) isomer is obtained from the experimental data, The fine structure of the strength function S _β(E) is analyzed. It has a pronounced resonance structure for Gamow-Teller transitions and is found to exhibit a resonance structure for first-forbidden transitions. It is shown that for some excitation energies of the 160Dy daughter nucleus the probability of first-forbidden β⁺/EC transitions in the decay of the ^160m Ho isomer is comparable with the probability of Gamow-Teller β⁺/EC transitions.