Alpha-particle condensation in nuclei

A round-up of the present status of the conjecture that nα nuclei form an α-particle condensate in excited states close to the nα threshold is given. Experiments which could demonstrate the condensate character are proposed. Possible lines of further theoretical developments are discussed.     

Alpha-particle superfluid bands in nuclei

The systematics of α-particle separation energy is studied from the point of view of pairing-rotational structure. Two mass regions are identified where collective pairing bands can exist. The moment of these bands is compared with linear sums of (n-n) and (p-p) bands and an increase in collectivity of factor 10 is deduced.     

On pion condensation in finite nuclei

The critical pionic state leading to pion condensation is investigated for finite nuclear systems. The threshold density for light nuclei, including⁴He, is found to be close to that of nuclear matter. For¹⁶O and⁴⁰Ca, the angular momenta of the most critical states areL=0 andL=2.     

Surface pion condensation in finite nuclei

We discuss here the possible occurrence of surface pion condensation in finite nuclei. We show the argument for this possibility by the recent findings in few-body calculations and the spin-flip experiments. We show the calculated results on various N = Z nuclei using the relativistic mean-field theory with a pion.Received: 30 September 2002, Published online: 22 October 2003PACS: 21.60.-n Nuclear-structure models and methods     

Meson condensation and anomalous nuclei

The possibility of the existence of 1) superdense, 2) neutron and 3) supercharged nuclei, is discussed. The stability of such nuclei can be provided by the π-condensation.     

Unnatural parity states in nuclei and pion condensation

This is a systematic study to assess a lower limit on the threshold density for pion condensation on the basis of empirical nuclear data. It turns out that the isovector unnatural parity states in ⁴He, ¹⁶O and ⁴⁰Ca, and quantities related to them are best suited for a quantitative test. Assuming for the interactions parameters in the nucleon-hole and Δ-isobar-hole channels g′: = g′_NN = g′_NΔ = g′_ΔΔ, a value g′ = 0.7±0.1 is obtained from the measured energy shifts of unnatural parity states. Taking also into account results by Oset and Rho who obtain g′_ΔΔ = 0.6?0.7 from the observed quenching of Gamov-Teller matrix elements of (A±1) nuclei around ¹⁶O and ⁴⁰Ca, one finds the onset of pion condensation in nuclear matter at densities considerably larger than twice normal nuclear density, if at all. We discuss the work of Migdal et al. who prefer the assumption g′_NΔ = g′_ΔΔ = 0 and obtain lower critical densities. Also the question of (pre)critical behaviour in finite nuclei is investigated in terms of a quantitative example. It is found that a genuine signal for proximity to pion condensation would be the appearance of a strongly collective spin-isospin mode which has, however, not been observed experimentally so far.     

Investigation of the pion condensation threshold in finite nuclei

The threshold condition for pionic instabilities in finite nuclei is investigated by evaluating the response function for pion-like nuclear excitations (i.e. isovectorJ ^π =0^?, 1⁺, 2^?, ... excitations). The onset of pion condensation is then defined as the point where the response function for givenJ ^π becomes infinite at frequencyω=0. Using a momentum space representation for the response function which allows to establish clear connections to the nuclear matter case, we find that for angular momentaJ smaller than a maximum value which depends on the size of the nucleus, the threshold condition is almost the same as for infinite matter. We also study the systematics of this phenomenon as a function of nuclear mass number.     

Alpha-particle clustering in nuclear reactions

There is extensive evidence that nucleons in the nucleus can sometimes form clusters and, of these, alpha-particle clusters are the most likely. This is illustrated by considering alpha-particle emission, transfer and capture reactions, and also the interactions of electrons, pions and kaons with nuclei. Present knowledge of the probability of clustering as a function of atomic number and position in the nucleus is summarized.     

Simulation of supersaturated vapor nucleation on molecular condensation nuclei

The results of model calculations of nucleation of di(2-ethylhexyl)sebacate (DEHS) supersatu-rated vapor on FeO molecules based of the simplest structural models of clusters are considered. The earlier conclusion that the allowance for the escape of a molecular condensation nucleus (MCN) from a cluster weakly affects the results of analysis of nucleation on a MCN is confirmed. It is found that the interaction of electric charges of the FeO molecule and the ester groups of the DEHS molecule plays the important role in the DEHS nucleation. The importance of the strength of the bond between the MCN and the first condensate molecule is established. The strength of this bond considerably affects the coefficient of conversion of the MCN into aerosol particles as well as the form of the dependence of this quantity on the spontaneous condensation background.     

Thermodynamics of supersaturated vapor nucleation on molecular condensation nuclei

A key problem in the theory of a supersaturated vapor nucleation on molecular condensation nuclei (namely, the work of formation and the equilibrium concentrations of clusters) is considered. To calculate these quantities using the structural models of clusters, which are better suited for this purpose than the classical droplet model, we derive the equation connecting the work of transfer of a molecular condensation nucleus from the gas phase to a homogeneous cluster with a change in the number of contacts between molecules, occurring in the course of this transfer, and with the work of rupture of individual contacts.     

Alpha-particle states in extended Elliott model

A model of α-particle and quasimolecular states of nuclei based on the microscopic Hamiltonian of broken SU(3) symmetry commutative with the antisymmetrizer is built. The model provides an explanation for all qualitative properties of the spectra and allows one to calculate the spectroscopic factors.     

Alpha-particle unstable states in12C

Alpha particle unstable states in¹²C have been investigated using the three reactions¹⁴N(d, α′)¹²C,¹⁴N(d, α′α)⁸Be and¹²C(α, α′)¹²C. Excitation cross sections and angular distributions have been measured for the reactions¹⁴N(d, α′) at 52 MeV and¹²C(α, α′) at 90 MeV. For a few angle pairs the α-decay of excited states in¹²C have been observed in a¹⁴N(d, α′α) correlation measurement. The reactions selectively excite onlyT=0 states. A previously undetected level with a large α-decay width (Γ=1.2 MeV) has been observed at 15.62 MeV excitation. This level shows up clearly in both reactions and is further distinguished from the nearby 14.08 state in the correlation measurement because of the distinctly different energy distributions of the decay products. On the basis of a particle angular distributions the 15.62 MeV level was assigned spin and parity 4⁺ and the level at 14.08 was assigned 3^?. The latter differs from the value suggest by earlier work. Comparison with DWBA calculations indicates that angular distributions of all other prominent levels are in agreement with their earlier assignments. Two levels at 19.20 and 20.30 MeV (both Γ?0.4 MeV) and three further levels at 21.81, 22.7 and 24.24 MeV also decay predominantly by α-emission.     

Experimental signatures of cosmological neutrino condensation

Superfluid condensation of neutrinos of cosmological origin at a low enough temperature can provide simple and elegant solution to the problems of neutrino oscillations and the accelerated expansion of the universe. It would give rise to a late time cosmological constant of small magnitude and also generate tiny masses for the neutrinos as observed from their flavor oscillations. We show that carefully prepared beta decay experiments in the laboratory would carry signatures of such a condensation, and thus, it would be possible to either establish or rule out neutrino condensation of cosmological scale in laboratory experiments.     

Experimental observation of Bose condensation in high-temperature superconductors

Emission Mössbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn) and ⁶⁷Ga (⁶⁷Zn) isotopes was used to show that for the superconductors Nd_1.85Ce_0.15CuO₄, La_1.85 Sr_0.15CuO₄, and Tl₂Ba₂ CaCuO₈ in the temperature range T > T _c the temperature dependence of the center of gravity S of the Mössbauer spectrum is determined by the second-order Doppler shift, while in the range T < T _c the Bose condensation of Cooper pairs influences the value of S (here T _c is the superconducting transition temperature). The spatial nonuniformity produced in the electron density by a Bose condensate of Cooper pairs was observed for La_1.85 Sr_0.15CuO₄.     

Optoelectronic system of the aerosol photometer in the detector of molecular condensation nuclei

We study the light scattering by aerosol particles and air in the photometer of the molecular condensation nuclei, as well as the sensitivity of the photodector of the photometer. The interference nature of light scattering by aerosol particles is established and is found to be comparable (in order of magnitude) with the scattering of light by air in the photometer. The sensitivity of the photometer can be increased by more than an order of magnitude due to optimization of the optoelectronic part of the photometer. The detection threshold for the target component of the gas analyzer is attained at the spontaneous ionization background level and not at the limiting sensitivity level of the photodetector.     

Alpha-particle decays from excited states in 24Mg

Using a cluster model based on the Woods-Saxon potential, alpha-particle decays from excited states in ²⁴Mg have been systematically investigated. Calculations can in general reproduce experimental data, noticing the fact that the preformation factor P of alpha particle in alpha-decaying nuclei is of order from 10⁰ to 10⁻². This can be the evidence for the α+²⁰Ne structure in ²⁴Mg. Meanwhile, the results also show the existence of other configurations, such as ¹⁶O+2α. Since the calculated decay widths are very sensitive to the angular momentum carried by the outgoing cluster (α particle), our results could serve as a guide to experimental spin assignments.

     

Influence of condensation nuclei on the optical properties of atmospheric aqueous aerosol

The paper presents the results of investigations on the indicatrices and angular dependence of the degree of polarization of scattered radiation by fog in the visible and near-infrared regions of the spectrum. By judicious comparison of the measured results with calculated data a law was established for the influence of condensation nuclei on the optical properties of atmospheric aqueous aerosol.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 128–131, January, 1976.     

Meson condensation and two photon emission in hadron capture by nuclei and heavy-ion collisions

New laboratory probes for the occurence of meson condensation in nuclei are proposed. They involve experiments which detect the simultaneous emission of two photons. Pion condensation stimulates a new mechanism for such emission which depends essentially upon the ratio of the condensate amplitude to the vacuum expectation value of the effective scalar meson field. We exhibit the special features of this mechanism for the photon energy spectrum and angular correlation, and compare with other mechanisms.