Emission Of Heavy Ions Research Articles

Three-dimensional multi-fluid model of a coronal streamer belt with a tilted magnetic dipole

Abstract. Observations of streamers in extreme ultraviolet (EUV) emission with SOHO/UVCS show dramatic differences in line profiles and latitudinal variations in heavy ion emission compared to hydrogen Ly-α emission. In order to use ion emission observations of streamers as the diagnostics of the slow solar wind properties, an adequate model of a streamer including heavy ions is required. We extended a previous 2.5-D multi-species magnetohydrodynamics (MHD) model of a coronal streamer to 3-D spherical geometry, and in the first approach we consider a tilted dipole configuration of the solar magnetic field. The aim of the present study is to test the 3-D results by comparing to previous 2.5-D model result for a 3-D case with moderate departure from azimuthal symmetry. The model includes O5+ ions with preferential empirical heating and allows for calculation of their density, velocity and temperature in coronal streamers. We present the first results of our 3-D multi-fluid model showing the parameters of protons, electrons and heavy ions (O5+) at the steady-state solar corona with a tilted steamer belt. We find that the 3-D results are in qualitative agreement with our previous 2.5-D model, and show longitudinal variation in the variables in accordance with the tilted streamer belt structure. Properties of heavy coronal ions obtained from the 3-D model together with EUV spectroscopic observations of streamers will help understanding the 3-D structures of streamers reducing line-of-sight integration ambiguities and identifying the sources of the slow solar wind in the lower corona. This leads to improved understanding of the physics of the slow solar wind.

OBSERVATIONS AND MODELS OF SLOW SOLAR WIND WITH Mg9 +IONS IN QUIESCENT STREAMERS

Quiescent streamers are characterized by a peculiar UV signature as pointed out by the results from the observations of the Ultraviolet and Coronograph Spectrometer (UVCS) on board SOHO: the intensity of heavy ion emission lines (such as OVI) show dimmer core relative to the edges. Previous models show that the structure of the heavy ion streamer emission relates to the acceleration regions of the slow solar wind at streamer legs and to gravitational settling processes in the streamer core. Observations of Mg9+ ion EUV emission in coronal streamers at solar minimum were first reported by the UVCS instrument. The Mg X 625A emission is an order of magnitude smaller than the OVI 1032A emission, requiring longer exposures to obtain statistically significant results. Here, MgX coronal observations are analyzed and compared, for the first time, with the solar minimum streamer structure in hydrogen and OVI emissions. We employ the 2.5D three-fluid model, developed previously to study the properties of O5+ ions in streamers, and calculate for the first time the density, temperature, and outflow structure of Mg9+ ions in the solar minimum streamer. The Mg9+ ions are heated by an empirical radial heating function constrained by observations of the kinetic ion temperature obtained from MgX emission line profiles. The detailed structure of Mg9+ density, temperature, and outflow speed determined by the Coulomb momentum and energy exchange as well as electromagnetic interactions with electrons and protons in the three-fluid model of the streamer. The results of the model are in good qualitative agreement with observations, and provide insights on the possible link between the magnetic structure of the streamer, slow solar wind sources, and relative abundances of heavy ions.

Investigating the velocity structure and X-ray observable properties of simulated galaxy clusters with PHOX

Non-thermal motions in the intra-cluster medium (ICM) are believed to play a non-negligible role in the pressure support to the total gravitating mass of galaxy clusters. Future X-ray missions, such as ASTRO-H and ATHENA, will eventually allow us to directly detect the signature of these motions from high-resolution spectra of the ICM. In this paper, we present a study on a set of clusters extracted from a cosmological hydrodynamical simulation, devoted to explore the role of non-thermal velocity amplitude in characterising the cluster state and the relation between observed X-ray properties. In order to reach this goal, we apply the X-ray virtual telescope PHOX to generate synthetic observations of the simulated clusters with both Chandra and ATHENA, the latter used as an example for the performance of very high-resolution X-ray telescopes. From Chandra spectra we extract global properties, e.g. luminosity and temperature, and from ATHENA spectra we estimate the gas velocity dispersion along the line of sight from the broadening of heavy-ion emission lines (e.g. Fe). We further extend the analysis to the relation between non-thermal velocity dispersion of the gas and the L_X-T scaling law for the simulated clusters. Interestingly, we find a clear dependence of slope and scatter on the selection criterion for the clusters, based on the level of significance of non-thermal motions. Namely, the scatter in the relation is significantly reduced by the exclusion of the clusters, for which we estimate the highest turbulent velocities. Such velocity diagnostics appears therefore as a promising independent way to identify disturbed clusters, in addition to the commonly used morphological inspection.

Heavy-ion emission from short-pulse laser-plasma interactions with thin foils

A Thomson parabola ion spectrometer, implemented on the Laboratory for Laser Energetics Multi-Terawatt laser facility, has been used to study heavy-ion acceleration from ultra-intense laser-plasma interactions. These studies were conducted using 20 μm-thick flat foil targets with on-target intensities of 4−5×1019 W-cm−2. Several charge states of energetic heavy ions were accelerated from the rear of Al and Cu foils in the presence of hydrocarbon contaminants. In contrast to previous work, we show that the mean and maximum energies of each heavy-ion species scale linearly with only the charge of the ion, and not the charge-to-mass ratio, implying that the space-charge is not readily depleted as ions expand. It has been shown previously and observed here that the maximum heavy-ion energies are lower than that of protons, though a fundamental explanation for this discrepancy has not been provided. We show how a two-temperature electron distribution, observed in these experiments, explains these observations.

Preformation of clusters in heavy nuclei and cluster radioactivity

Within the preformed cluster model approach, the values of the preformation factors have been deduced from the experimental cluster decay half-lives assuming that the decay constant of the heavy-ion emission is the product of the assault frequency, the preformation factor and the penetrability. The law according to which the preformation factors follow a simple dependence on the mass of the cluster was confirmed. Then predictions for some of the most possible cluster decays are provided.

Radioactive decay by the emission of heavy nuclear fragments

Radioactive decay of nuclei by the emission of heavy ions of C, N, O, F, Ne, Na, Mg, Al, Si and P isotopes (known as exotic decay or cluster radioactivity) is re-investigated within the framework of a semiempirical, one-parameter model based on a quantum mechanical, tunneling mechanism through a potential barrier, where both centrifugal and overlapping effects are considered to half-life evaluations. This treatment appeared to be very adequate at fitting all measured half-life values for the cluster emission cases observed to date. Predictions for new heavy-ion decay cases susceptible of being detected are also reported.

Potential energy surfaces for cluster emitting nuclei

Potential energy surfaces are calculated by using the most advanced asymmetric two-center shell model allowing to obtain shell and pairing corrections which are added to the Yukawa-plus-exponential model deformation energy. Shell effects are of crucial importance for experimental observation of spontaneous disintegration by heavy ion emission. Results for 222Ra, 232U, 236Pu and 242Cm illustrate the main ideas and show for the first time for a cluster emitter a potential barrier obtained by using the macroscopic-microscopic method.

Search for heavy ion emission from the decay of230U

The exotic nuclear decay of ${}^{230}\mathrm{U}$ by emission of heavy ions has been investigated by use of ${}^{230}\mathrm{Pa}$ $\stackrel{{\ensuremath{\beta}}^{\ensuremath{-}}}{\ensuremath{\rightarrow}}$ ${}^{230}\mathrm{U}$ sources and polyester track detectors. After a run of 248 d, two cases of decay via heavy ion emission, which were most probably for the neon decays of ${}^{230}\mathrm{U},$ were observed for the first time. The preliminary branching ratio for the emission of heavy ions relative to $\ensuremath{\alpha}$ particles from ${}^{230}\mathrm{U}$ comes out to be $(1.3\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}.$

Heavy-ion emission in spontaneous decays of249,252Cfnuclei

Theoretical possibilities for exotic cluster emissions from ${}^{249}\mathrm{Cf}$ and ${}^{252}\mathrm{Cf}$ parents are explored on the basis of the very much used preformed cluster model of one of us (R.K.G.) and collaborators. The calculated \ensuremath{\alpha}-decay half-life time for ${}^{252}\mathrm{Cf}$ match the experiments exactly and the same for ${}^{249}\mathrm{Cf}$ within an order of 3. The most probable heavy cluster decay for both the parents is predicted to be ${}^{46}\mathrm{Ar}$ or ${}^{48}\mathrm{Ca},$ with the predicted decay half-life times far more than the presently available experimental upper limiting values. In other words, the predicted heavy-ion emission probabilities for both the parents are so small that there seem to be very little chance for their exotic cluster decays to be observed in the very near future.

Evidence for Heavy Ion Emission of 230U

Radioisotope of 230Pa was synthesized by proton irradiation of ThO2 powder targets with thickness of about 2g/cm2. Pure 230Pa was extracted and deep purified from thorium and fission products by the radiochemical method. Thin 230Paβ-→230U sources were prepared for measuring the cluster decay of 230U. Two cases of decay via heavy ion emission, which were most probably for the neon decays of 230U, were detected by using solid-state track registration detectors. The preliminary branching ratio relative to α-decay comes out to be (1.3±0.8)×10-14.

The absorption edge technique at the Super-EBIT

In an experiment at the Super-EBIT in Livermore, the absorption edge technique has been applied for the investigation of the X-ray emission of heavy ions in a trap. The aim of the measurement was to resolve the Radiative Recombination transitions into the L-shell of the different ion species which are simultaneously present in the trap in order to measure directly their contribution to the charge state distribution. The energy resolution required was obtained by using a lead absorber foil in front of a solid state X-ray detector which constitutes a considerably improved resolution with respect to standard germanium detectors.

Superasymmetric two-center shell model for spontaneous heavy-ion emission

The single particle levels for the heavy-ion emission process are computed. This decay mode is treated like a superasymmetric fission process. The nuclear shape parametrization is characterized by three degrees of freedom. The difficulties encountered in the microscopic determination of the energy scheme at these very large mass asymmetries are presented. Thereby, a new version of the two-center model, especially designed for very large mass asymmetries, is described. The $^{14}\mathrm{C}$ heavy-ion spontaneous emission from the parent nucleus $^{223}\mathrm{Ra}$ is treated in the frame of this model. The principal trends of the variations obtained for the energetic levels during this superasymmetric nuclear decay are discussed. Mainly, for this kind of decay mode, the levels with lower values of the angular momentum projection \ensuremath{\Omega} show more pronounced variations than those with higher \ensuremath{\Omega}. Also, a qualitative explanation for the favoring of the first excited states in the fine structure in this radioactive process is given. \textcopyright{} 1996 The American Physical Society.

Highly asymmetric binary fission

In this article, a review of the work on the low-yield products formed in the highly asymmetric binary fission (70≥A≥160) of heavy elements (HABF) is made. HABF may be viewed as an extension of the usual asymmetric fission process. A few interesting features of this mode of fission which are apparent from the data available in the literature are identified: The process (a) is rare, (b) seems to depend on the excitation energy of the fissioning system, (c) is highly influenced by shell effects. A closely related phenomenon, viz the recent discovery of spontaneous emission of heavy ions (A>4, Z>2) from heavy nuclei is also examined briefly. It is speculated that HABF may be an important decay mode for the very heavy and the yet-undiscovered superheavy elements. Suggestions for future experiments to further pin down this fission mode are also given in this article.

Decay of 226Ra by 14C emission.

Previous observation of heavy-ion emission from $^{226}\mathrm{Ra}$ has been confirmed. Charge and energy of the emitted fragment were measured using thin $^{226}\mathrm{Ra}$ sources and polycarbonate track-recording films. Decay by $^{14}\mathrm{C}$ emission could be identified unambiguously. The track-detector was calibrated with tandem-accelerated $^{14}\mathrm{C}$ and $^{16}\mathrm{O}$ ions and tested by observing the $^{14}\mathrm{C}$ emission from $^{223}\mathrm{Ra}$ yielding a branching ratio of (5.0\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$. In the case of $^{226}\mathrm{Ra}$ our result for the $^{14}\mathrm{C}$/\ensuremath{\alpha} ratio is (2.3\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$. Estimates of partial half-lives of Ra isotopes for $^{14}\mathrm{C}$ emission are discussed.

Coulomb-induced α decay and cluster emission within the superasymmetric fission model for the collision of238U on238U

The alpha decay and the exotic radioactive emission of heavy ions are assumed to be superasymmetric fission processes. These processes are studied as transitions from Coulomb-excited states of the parent nucleus into excited states of the daughter nucleus. For their description the authors use the superasymmetric fission model and generalise it. Within this generalised model they calculate the cross section for the Coulomb-induced alpha decay in collisions of 238U on 238U below the Coulomb barrier and estimate the cross section for the Coulomb-induced cluster emission. The influence of prompt Coulomb-induced alpha decay is also discussed.

On the 14C content in radioactive ores

U and Th ores have been found to contain significant levels of 14C. The levels of this isotope can be as high as 14 C α =1.7 · 10 −9 , and can be explained in terms of nuclear reactions and some contribution from heavy-ion emission or asymmetric fission. The levels of 14C are compared to other nucleogenic isotopes that have been found in these ores, such as 10Be, 26Al and excesses of rare-gas isotopes.

Cold fission as heavy ion emission

The last version of the analytical superasymmetric fission model is applied to study cold fission processes. Strong shell effects are present either in one or both fission fragments. A smooth behaviour is observed when the proton or the neutron numbers are changed by four units. IncreasingZ andN, in the transuranium region, a sharp transition from asymmetry with a large peak-to-valley ratio to symmetry atZ=100 and/orN=164 is obtained. The transition toward asymmetry at higherZ andN is much smoother. The most probable cold fission light fragments from234U,236U,239Np and240Pu are100Zr,104Mo,106Mo and106Mo, respectively, in good agreement with experimental data. The unified treatment of alpha decay, heavy ion radioactivities and cold fission is illustrated for234U — the first nucleus in which all three groups have been already observed.

Towards a unified description of asymmetric nuclear shapes in structure, fission and cluster radioactivity

The authors propose the description of odd intrinsic multiple moments in nuclear ground states, cluster configurations and fission by emission of heavy ions in a unified way through the introduction of a suitable collective asymmetry coordinate.

Radioactive decay into excited states via heavy ion emission

The exotic radioactive emission of heavy ions from very heavy nuclei is studied with regard to transitions into excited states of the daughter nucleus. The influence of these transitions on the total decay rates and branching ratios is discussed.

Half lives for spontaneous emission of heavy ions from atomic nuclei

Abstract The analytical superasymmetric fission model is used to estimate the half lives for spontaneous emission of heavy clusters from atomic nuclei. One gets a unified description of the new radioactivities, alpha decay and fission processes. Life-times shorter than 10 30 s are found for the emission of more than 140 different clusters with 2–24 proton numbers and 3–31 neutron numbers. Even the “stable” nuclides with Z>40 are metastable with respect to several new decay modes. Solid state nuclear track detectors allow a good discrimination against other disintegration processes.