Natural radioactivity in sediment of Wei River,China

The concentrations of natural radionuclides in sediment of Wei River of China were measured using γ-ray spectrometry with the aim of estimating the radiation hazard as establishing a database for radioactivity levels of river sediment of China. The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in sediment samples ranged from 10.4 to 39.9 Bq kg⁻¹, 15.3 to 54.8 Bq kg⁻¹ and 514.8 to 1,175.5 Bq kg⁻¹, respectively. The concentrations of these radionuclides were compared with the typical world values and the average activities of Chinese soil and Shaanxi soil. Radium equivalent activity (Ra_eq), external hazard index (H _ex) and representative level index (I _γr) were calculated for the samples to assess the radiation hazards arising due to the use of these sediment samples in the construction of dwellings. All the sediment samples have Ra_eq lower than the limit of 370 Bq kg⁻¹, H _ex less than unity and I _γr close to 1 Bq kg⁻¹. The overall mean outdoor terrestrial gamma dose rate is 64.8 nGy h⁻¹ and the corresponding outdoor annual effective dose is 0.079 mSv. None of the studied location is considered a radiological risk and sediment can be safely used in construction.     

A search for ultra-high energy counterparts to gamma-ray bursts

A small air shower array operating over many years has been used to search for ultra-high energy (UHE) gamma radiation ( 50 TeV) associated with gamma-ray bursts (GRBs) detected by the BATSE instrument on the Compton Gamma-Ray Observatory (CGRO). Upper limits for a one minute interval after each burst are presented for seven GRBs located with zenith angles < 20°. A 4.3 excess over background was observed between 10 and 20 minutes following the onset of a GRB on 11 May 1991. The confidence level that this is due to a real effect and not a background fluctuation is 99.8%. If this effect is real then cosmological models are excluded for this burst because of absorption of UHE gamma rays by the intergalactic radiation fields.     

Room temperature solid-state detectors for Gamma ray burst spectroscopy

We have evaluated several solid state detectors which offer excellent energy resolution at room temperature for soft X-rays. For soft X-rays (< 1 keV to 20 keV), silicon P-intrinsic-N (PIN) and avalanche-mode photodiodes (APD's) have been studied. Using commercially available charge sensitive pre-amplifiers, these photodiodes provide 1 keV resolution without cooling. Their detection efficiencies are limited to about 20 keV and 15 keV, respectively. To overcome this constraint, we have studied thick (1.5 mm) PIN detectors made by Micron Semiconductor Ltd., U.K., as well as compound semiconducting materials with high Z constituents such as CZT and PbI₂. PbI₂ allows high detection efficiencies of photons up to 100 keV with detectors 100–300 microns thick. These new detectors offer the capability to study the low-energy spectral evolution of Gamma ray bursts (GRBs). A matrix of these detectors could be used as an image plane detector with moderate spatial resolution for imaging.     

A GRB model satisfying stringent observational constraints

The temporal behavior of GRBs is quantified using a power spectrum analysis. The power spectrum of great variety of GRBs is well represented by the simple ^–2 behavior. We then study a cosmological GRB model in which relativistic flows interact with dense radiation fields. This mechanism in the densest stellar regions known to exist, surprisingly yields the correct temporal behavior. Other characteristics are also reproduced, including the duration bimodality and the hardness-duration distribution.     

Coalescing neutron stars as gamma ray bursters?

We investigate the dynamics and evolution of coalescing neutron stars. The three-dimensional Newtonian equations of hydrodynamics are integrated by the Piecewise Parabolic Method on an equidistant Cartesian grid. The code is purely Newtonian, but does include the emission of gravitational waves and their back-reaction. The properties of neutron star matter are described by the equation of state of Lattimer and Swesty (1991). Energy loss by all types of neutrinos and changes of the electron fraction due to the emission of electron neutrinos and antineutrinos are taken into account by an elaborate neutrino leakage scheme. We simulate the coalescence of two identical, cool neutron stars with a baryonic mass of 1.6M and a radius of 15 km and with an initial center-to-center distance of 42 km. The initial distributions of density and electron concentration are given from a model of a cold neutron star in hydrostatic equilibrium. We investigate three cases which differ by the initial velocity distribution in the neutron stars. The orbit decays due to gravitational-wave emission and after one revolution the stars are so close that dynamical instability sets in. Within 1 ms the neutron stars merge into a rapidly spinning (P 1 ms), high-density body ( 10¹⁴ g/cm³) with a surrounding thick disk of material with densities 10¹⁰ – 10¹² g/cm³ and orbital velocities of 0.3-0.5 c. The peak emission of gravitational waves has a maximum luminosity of a few times 10⁵⁵ erg/s and is reached for about 1 ms. The amplitudes of the gravitational waves are close to 3 10^–23 at a distance of 1 Gpc and the typical frequency is near the dynamical value of the orbital motion of the merging neutron stars of 2 KHz. In a post-processing step, the rate of neutrino-antineutrino annihilation is calculated from the neutrino luminosities generated during the hydrodynamical simulations. We find the integral annihilation rate to be a few 10⁵⁰ erg/s during the phase of strongest neutrino emission, which is too small to generate the observed bursts considering the fact that the merged object of about 3M will most likely collapse to a black hole within milliseconds.     

GRBs in the 5-10 keV regime as seen by BATSE

The BATSE Spectroscopy Detectors (SDs) each have a thin beryllium window which allows detection of photons down to 5 keV. Until recently, exploitation of low-energy SD discriminator data has been held up by a lack of knowledge of its energy coverage. A recent series of in-flight changes in the instrument configuration has allowed the SD discriminators to be calibrated fairly accurately. With this in hand, we have surveyed BATSE GRBs for such features as X-ray precursors and after-glows in the roughly 5-10 keV band. For a large percentage of the BATSE bursts, no emission is observed in this energy band. In some cases where low-energy emission has been observed, ratios of flux histories indicate differential spectral evolution. In addition, we have obtained joint spectral fits between the low-energy data and the regular SD data, showing that a single spectral model is sometimes insufficient to fit the entire energy range.     

Distributions of secondary muons at sea level from cosmic gamma rays below 10 TeV

The Monte Carlo program is used to predict the distributions of the muons which originate from primary cosmic gamma rays and reach sea level. The main result is the angular distribution of muons produced by vertical gamma rays which is necessary to predict the inherent angular resolution of any instrument utilizing muons to infer properties of gamma ray primaries. Furthermore, various physical effects are discussed which affect these distributions in differing proportions.     

Planetary,solar and astrophysical relativistic electrons: Common energization mechanisms?

Direct observations or deduced analysis indicate clearly that formation of intense fluxes of relativistic electrons is an important ingredient in the evolution of numerous active magnetized plasma systems. Examples of relativistic electron energization include the recovery phase of a planetary magnetic storm, post solar flare coronal activity and the afterglow of gamma ray bursts. It is suggested that there exists a universal mechanism, which may explain electron energization at the vastly different magnetized plasma environments. The favorite configuration consists of an inhomogeneous magnetic field anchored at a given magnetic structure and excitation of whistler waves due to external injection of low-energy non-isotropic electrons. The energization proceeds as a bootstrap process due to interaction with the propagating whistler waves along the inhomogeneous magnetic field.     

Three‐parameter discontinuous distributions for hydrological samples with zero values

A consistent approach to the frequency analysis of hydrologic data in arid and semiarid regions, i.e. the data series containing several zero values (e.g. monthly precipitation in dry seasons, annual peak flow discharges, etc.), requires using discontinuous probability distribution functions. Such an approach has received relatively limited attention. Along the lines of physically based models, the extensions of the Muskingum‐based models to three parameter forms are considered. Using 44 peak flow series from the USGS data bank, the fitting ability of four three‐parameter models was investigated: (1) the Dirac delta combined with Gamma distribution; (2) the Dirac delta combined with two‐parameter generalized Pareto distribution; (3) the Dirac delta combined with two‐parameter Weibull (DWe) distribution; (4) the kinematic diffusion with one additional parameter that controls the probability of the zero event (KD3). The goodness of fit of the models was assessed and compared both by evaluation of discrepancies between the results of both estimation methods (i.e. the method of moments (MOM) and the maximum likelihood method (MLM)) and using the log of likelihood function as a criterion. In most cases, the DWe distribution with MLM‐estimated parameters showed the best fit of all the three‐parameter models. Copyright © 2005 John Wiley & Sons, Ltd.