Relativistic laser interactions with preformed plasma channels and gamma-ray measurements

The propagation of a 1-ps laser pulse at intensities exceeding 10(19) Wcm(-2) in a low-density plasma channel was experimentally tested. The channel was produced by a lower intensity preceding pulse of the same duration. Plasma electrons were accelerated during the propagation of the main pulse, and high energy gamma -ray detectors were used to detect their bremsstrahlung emission. The gamma -ray yield was studied for different channel conditions, by varying the delay between the channel forming pulse and the high intensity pulse. These results are correlated with the interferograms of the propagation region into the plasma.

Gamma ray-induced bystander effect in tumour glioblastoma cells: a specific study on cell survival, cytokine release and cytokine receptors.

Recent experimental evidence has challenged the paradigm according to which radiation traversal through the nucleus of a cell is a prerequisite for producing genetic changes or biological responses. Thus, unexposed cells in the vicinity of directly irradiated cells or recipient cells of medium from irradiated cultures can also be affected. The aim of the present study was to evaluate, by means of the medium transfer technique, whether interleukin-8 and its receptor (CXCR1) may play a role in the bystander effect after gamma irradiation of T98G cells in vitro. In fact the cell specificity in inducing the bystander effect and in receiving the secreted signals that has been described suggests that not only the ability to release the cytokines but also the receptor profiles are likely to modulate the cell responses and the final outcome. The dose and time dependence of the cytokine release into the medium, quantified using an enzyme linked immunosorbent assay, showed that radiation causes alteration in the release of interleukin-8 from exposed cells in a dose-independent but time-dependent manner. The relative receptor expression was also affected in exposed and bystander cells.

Spectral gamma-ray logs and palaeoclimate change? Permian – Triassic, Persian Gulf

Spectral gamma ray (SGR) logs are used as stratigraphic tools in correlation, sequence stratigraphy and most recently, in clastic successions as a proxy for changes in hinterland palaeoweathering. In this study we analyse the spectral gamma ray signal recorded in two boreholes that penetrated the carbonate and evaporate-dominated Permian–Triassic boundary (PTB) in the South Pars Gasfield (offshore Iran, Persian Gulf) in an attempt to analyse palaeoenvironmental changes from the upper Permian (Upper Dalan Formation) and lower Triassic (Lower Kangan Formation). The results are compared to lithological changes, total organic carbon (TOC) contents and published stable isotope (δ18O, δ13C) results. This work is the first to consider palaeoclimatic effects on SGR logs from a carbonate/evaporate succession. While Th/U ratios compare well to isotope data (and thus a change to less arid hinterland climates from the Late Permian to the Early Triassic), Th/K ratios do not, suggesting a control not related to hinterland weathering. Furthermore, elevated Th/U ratios in the Early Triassic could reflect a global drawdown in U, rather than a more humid episode in the sediment hinterlands, with coincident changes in TOC. Previous work that used spectral gamma ray data in siliciclastic successions as a palaeoclimate proxy may not apply in carbonate/evaporate sedimentary rocks.

Hydrogen-Poor Superluminous Supernovae and Long-Duration Gamma-Ray Bursts Have Similar Host Galaxies

We present optical spectroscopy and optical/near-IR photometry of 31 host galaxies of hydrogen-poor superluminous supernovae (SLSNe), including 15 events from the Pan-STARRS1 Medium Deep Survey. Our sample spans the redshift range 0.1 ≲ z ≲ 1.6, and is the first comprehensive host galaxy study of this specific subclass of cosmic explosions. Combining the multi-band photometry and emission-line measurements, we determine the luminosities, stellar masses, star formation rates, and metallicities. We find that, as a whole, the hosts of SLSNe are a low-luminosity (〈M_B 〉 ≈ -17.3 mag), low stellar mass (〈M〉 ≈ 2 × 10⁸ M_⊙) population, with a high median specific star formation rate (〈sSFR〉 ≈ 2 Gyr^-1). The median metallicity of our spectroscopic sample is low, 12 + log (O/H) ≈ 8.35 ≈ 0.45 Z_⊙, although at least one host galaxy has solar metallicity. The host galaxies of H-poor SLSNe are statistically distinct from the hosts of GOODS core-collapse SNe (which cover a similar redshift range), but resemble the host galaxies of long-duration gamma-ray bursts (LGRBs) in terms of stellar mass, SFR, sSFR, and metallicity. This result indicates that the environmental causes leading to massive stars forming either SLSNe or LGRBs are similar, and in particular that SLSNe are more effectively formed in low metallicity environments. We speculate that the key ingredient is large core angular momentum, leading to a rapidly spinning magnetar in SLSNe and an accreting black hole in LGRBs.

Gamma-ray emission in near critical density plasmas at laser intensities of 1021 W/cm2

We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 1021 W∕cm2 using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW∕50 fs laser pulse at the optimal plasma density (∼1nc ), about 108 photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

Gamma-ray diagnostics of Type Ia supernovae. Predictions of observables from three-dimensional modeling

Context. Although the question of progenitor systems and detailed explosion mechanisms still remains a matter of discussion, it is commonly believed that Type Ia supernovae (SNe Ia) are production sites of large amounts of radioactive nuclei. Even though the gamma-ray emission due to radioactive decays is responsible for powering the light curves of SNe Ia, gamma rays themselves are of particular interest as a diagnostic tool because they directly lead to deeper insight into the nucleosynthesis and the kinematics of these explosion events. Aims: We study the evolution of gamma-ray line and continuum emission of SNe Ia with the objective of analyzing the relevance of observations in this energy range. We seek to investigate the chances for the success of future MeV missions regarding their capabilities for constraining the intrinsic properties and the physical processes of SNe Ia. Methods: Focusing on two of the most broadly discussed SN Ia progenitor scenarios - a delayed detonation in a Chandrasekhar-mass white dwarf (WD) and a violent merger of two WDs - we used three-dimensional explosion models and performed radiative transfer simulations to obtain synthetic gamma-ray spectra. Both chosen models produce the same mass of 56Ni and have similar optical properties that are in reasonable agreement with the recently observed supernova SN 2011fe. We examine the gamma-ray spectra with respect to their distinct features and draw connections to certain characteristics of the explosion models. Applying diagnostics, such as line and hardness ratios, the detection prospects for future gamma-ray missions with higher sensitivities in the MeV energy range are discussed. Results: In contrast to the optical regime, the gamma-ray emission of our two chosen models proves to be quite different. The almost direct connection of the emission of gamma rays to fundamental physical processes occurring in SNe Ia permits additional constraints concerning several explosion model properties that are not easily accessible within other wavelength ranges. Proposed future MeV missions such as GRIPS will resolve all spectral details only for nearby SNe Ia, but hardness ratio and light curve measurements still allow for a distinction of the two different models at 10 Mpc and 16 Mpc for an exposure time of 106 s. The possibility of detecting the strongest line features up to the Virgo distance will offer the opportunity to build up a first sample of SN Ia detections in the gamma-ray energy range and underlines the importance of future space observatories for MeV gamma rays.

Gamma-ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons

Many-body theory is developed to calculate the γ spectra for positron annihilation in noble-gas atoms. Inclusion of electron-positron correlation effects and core annihilation gives spectra in excellent agreement with experiment [K. Iwata et al., Phys. Rev. Lett. 79, 39 (1997)]. The calculated correlation enhancement factors γnl for individual electron orbitals nl are found to scale with the ionization energy I_nl (in eV), as γnl=1+ √A/Inl+(B/I_nl)β, where A≈40  eV, B≈24  eV, and β≈2.3.

Palaeoclimate change recorded in Upper Cretaceous (Albian-Cenomanian) Kazhdumi Formation Borehole spectral Gamma-Ray Logs, South Pars Gas field, Persian Gulf

Th/U and Th/K data from spectral gamma-ray logs obtained from outcrop successions have been used as a rapid and inexpensive proxy for determining possible episodes of humid-arid palaeoclimate change. Such outcrop-based measurements have never been tested using spectral gamma-ray data obtained from wireline logs in subsurface boreholes. Th/K and Th/U ratios have traditionally been used to decipher sequence stratigraphic patterns, at outcrop and in borehole. The possible influence of palaeoclimate on such ratio changes has yet to be proven, especially from borehole data. In this work, we compare borehole-derived Th/K (and to a lesser extent Th/U) to palaeoenvironmental changes inferred from palynology and deduce that both sea level and changing hinterland weathering regimes caused discrete fluctuations observed in the spectral gamma-ray logs. This is the first time such subsurface information has been used in this way. Interpretation of wireline logs in terms of palaeoclimate as well as sea level may now be considered, and the use of such logs in palaeoclimate reconstruction is strengthened.

Electron shell contributions to gamma-ray spectra of positron annihilation in noble gases

Gamma-ray positron annihilation spectra of the noble gases are simulated using computational chemistry tools for the bound electron wavefunctions and plane-wave approximation for the low-energy positron. The present annihilation line shapes, i.e. the full width at half maximum, Delta epsilon, of the gamma-ray annihilation spectra for He and Ar (valence) agree well with available independent atomic calculations using a different algorithm. For other noble gases they achieve moderate agreement with the experimental measurements. It is found that the contributions of various atomic electron shells to the spectra depend significantly on their principal quantum number n and orbital angular momentum quantum number l. The present study further reveals that the outermost ns electrons of the noble gases exhibit spectral line shapes in close agreement with those measured, indicating (as expected) that the measurements are not due to a simple sum over the momentum densities for all atomic electrons. The robust nature of the present approach makes it possible for us to proceed to more complex molecular systems using the tools of modern computational chemistry.

Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated. (C) 2002 American Institute of Physics.