The observation of gamma ray bursts and terrestrial gamma-ray flashes with AGILE

Since its early phases of operation, the AGILE mission is successfully observing Gamma Ray Bursts (GRBs) in the hard X-ray band with the SuperAGILE imager and in the MeV range with the Mini-Calorimeter. Up to now, three firm GRB detections were obtained above 25 MeV and some bursts were detected with lower statistical confidence in the same energy band. When a GRB is localized, either by SuperAGILE or Swift/BAT or INTEGRAL/IBIS or Fermi/GBM or IPN, inside the field of view of the Gamma Ray Imager of AGILE, a detection is searched for in the gamma ray band or an upper limit is provided. A promising result of AGILE is the detection of very short gamma ray transients, a few ms in duration and possibly identified with Terrestrial Gamma-ray Flashes. In this paper we show the current status of the observation of Gamma Ray Bursts and Terrestrial Gamma-ray Flashes with AGILE.     

Preliminary results on TeV sources search with AGILE

During its first 2 years of operation, the gamma-ray AGILE satellite almost completed a full study of the gamma-ray sky. This paper presents the preliminary results of the systematic study performed on the AGILE data to search for GeV counterparts and to derive flux upper limits of the TeV sources detected by various instruments (MAGIC, HESS, VERITAS, Cangaroo, MILAGRO, ARGO,….).     

Solar flares monitor with Fermi-LAT

Fermi-LAT is performing an all-sky γ-ray survey from 20 MeV to with unprecedented sensitivity and angular resolution. Fermi is the only mission able to detect high energy () emission from the Sun during the new solar cycle 24. Fermi was launched in June 2008, and since then the high energy emission from the Sun was continuously monitored searching for flare events. Upper limits were derived for all the solar flares detected by other missions and experiments (RHESSI, Fermi-GBM, GOES). We present the analysis techniques used for this study and the preliminary results obtained in the first months of this search.     

Dark matter searches with H.E.S.S.

WIMP annihilations are required to reproduce the correct dark matter abundance in the Universe. This process can occur in dense regions of our Galaxy such as the Galactic center, dwarf galaxies and other types of sub-haloes. High-energy γ-rays are produced in dark matter particle collisions and can be detected by Cherenkov telescopes such as H.E.S.S. We report here the search for γ-ray signals from the Galactic center and the nearby Sagittarius and Canis Major satellites. In the absence of a convincing signal, modelling the dark matter density within these objects allows to put constraints on the WIMP parameters such as its mass and annihilation cross-section. Beyond these targeted searches, the wide-field survey of the inner Galaxy with H.E.S.S. is used to constrain sub-halo formation models involving intermediate-mass black holes.     

Low-energy neutron propagation in MCNPX and GEANT4

Simulations of neutron background from rock for underground experiments are presented. Neutron propagation through two types of rock, lead and hydrocarbon material, is discussed. The results show a reasonably good agreement between GEANT4, MCNPX and GEANT3 in transporting low-energy neutrons.     

Resolution function in deep inelastic neutron scattering using the Foil Cycling Technique

New perspectives for epithermal neutron spectroscopy are being opened up by the development of the Resonance Detector (RD) and its use on inverse geometry time of flight (TOF) spectrometers at spallation sources. The most recent result is the Foil Cycling Technique (FCT), which has been developed and applied on the VESUVIO spectrometer operating in the RD configuration. This technique has demonstrated its capability to improve the resolution function of the spectrometer and to provide an effective neutron and gamma background subtraction method. This paper reports a detailed analysis of the line shape of the resolution function in Deep Inelastic Neutron Scattering (DINS) measurements on VESUVIO spectrometer, operating in the RD configuration and employing the FCT. The aim is to provide an analytical approximation for the analyzer energy transfer function, an useful tool for data analysis on VESUVIO. Simulated and experimental results of DINS measurements on a lead sample are compared. The line shape analysis shows that the most reliable analytical approximation of the energy transfer function is a sum of a Gaussian and a power of a Lorentzian. A comparison with the Double Difference Method (DDM) is also discussed. It is shown that the energy resolution improvement for the FCT and the DDM is almost the same, while the counting efficiency is a factor of about 1.4 higher for the FCT.     

An approximate method for linear signal decomposition in -ray tracking detectors

An approximate method for signal decomposition in γ-ray tracking detectors is presented, based on Singular Value Decomposition (SVD). With this algorithm, the CPU time scales linearly with the number of possible interaction sites for any number of interactions per γ ray. The method is also robust to uncertainty in the signal start time and is applicable to any γ-ray detectors with highly segmented crystals as well as any inverse problem that can be reduced to a search among a countable set of precalculated forward solutions.     

A GEANT4 Monte-Carlo simulation code for precision spectroscopy

The measurement of the β asymmetry parameter in nuclear β decay is a potentially very sensitive tool to search for non V–A components in the charge-changing weak interaction. To reach the required precision (percent level) all effects that modify the emission pattern of the β radiation, i.e. the geometry of the setup, the effect of the magnetic field on the trajectories of β particles as well as (back)scattering in the source, on the sample holder and on the detector, have to be correctly taken into account in the analysis of the data. A thorough study of these effects and a new method based on detailed GEANT4 Monte-Carlo simulations that was developed for this purpose is presented here. The code was developed for β asymmetry measurements by means of the Low Temperature Nuclear Orientation (LTNO) method, but can in principle be generalized to other experimental setups using other polarization techniques.     

Measurements of high-energy -rays with detectors

The full-energy peak efficiency calibration and the energy resolution measurements of the LaBr₃ γ-ray detector are presented for γ-ray energies in the 700 keV–17.6 MeV range. Measurements were done using a combination of proton-capture nuclear reactions on , , , and for high-energy γ-rays, and radioactive sources such as and for the lowest energies. At high energies, two γ-rays in a cascade from proton resonance capture were employed using Al, Na₂WO₄, K₂SO₄ and LiBO₂ targets. The obtained results were compared to the simulations performed using a GEANT4 code.     

Use of the nuclear reaction for diagnostics of energetic particles in burning plasmas

Application of reaction-produced γ-rays to diagnostics of energetic particles in burning plasmas is analyzed. Particularly, we focus on 0.981 MeV γ-rays emitted in the nuclear reaction solely governed in the plasmas by energetic tritons. It is shown that these γ quanta can serve as a promising tool to diagnose α knock-on tritons and α-particles confined in burning DT plasmas. Key parameters of the α knock-on triton population and the α-particle confinement property can be obtained by comparing the experimental γ-ray yield and spectrum with theoretical slowing-down calculations. Even if the γ-ray spectral shape cannot be acquired, one can monitor densities of these tritons and α-particles at energies of 0.6–1.8 MeV and 2.0–3.5 MeV, respectively, in a nearly steady-state plasma. The 0.981-MeV photons also would help to display time evolution of the α-particle population in experiments of deuterium plasmas with pulsed tritium beam shots.     

Crosstalk corrections for improved energy resolution with highly segmented HPGe-detectors

Crosstalk effects of 36-fold segmented, large volume AGATA HPGe detectors cause shifts in the γ-ray energy measured by the inner core and outer segments as function of segment multiplicity. The positions of the segment sum energy peaks vary approximately linearly with increasing segment multiplicity. The resolution of these peaks deteriorates also linearly as a function of segment multiplicity. Based on single event treatment, two methods were developed in the AGATA Collaboration to correct for the crosstalk induced effects by employing a linear transformation. The matrix elements are deduced from coincidence measurements of γ-rays of various energies as recorded with digital electronics. A very efficient way to determine the matrix elements is obtained by measuring the base line shifts of untriggered segments using γ-ray detection events in which energy is deposited in a single segment. A second approach is based on measuring segment energy values for γ-ray interaction events in which energy is deposited in only two segments. After performing crosstalk corrections, the investigated detector shows a good fit between the core energy and the segment sum energy at all multiplicities and an improved energy resolution of the segment sum energy peaks. The corrected core energy resolution equals the segment sum energy resolution which is superior at all folds compared to the individual uncorrected energy resolutions. This is achieved by combining the two independent energy measurements with the core contact on the one hand and the segment contacts on the other hand.     

Depth of interaction detection for -ray imaging

A novel design for an inexpensive depth of interaction capable detector for γ-ray imaging has been developed. The design takes advantage of the strong correlation between the width of the scintillation light distribution in monolithic crystals and the interaction depth of γ-rays. We present in this work an inexpensive modification of the commonly used charge dividing circuits which enables the instantaneous and simultaneous computation of the second order moment of light distribution. This measure provides a good estimate for the depth of interaction and does not affect the determination of the position centroids and the energy release of γ-ray impact. The method has been tested with a detector consisting of a monolithic LSO block sized and a position-sensitive photomultiplier tube H8500 from Hamamatsu. The mean spatial resolution of the detector was found to be for the position centroids and for the DOI. The best spatial resolutions were observed at the center of the detector and yielded for the position centroids and for the DOI.     

A radiation tolerant phototube power supply for the electromagnetic calorimeter of the HERA- experiment

The design of the phototube power supply for the HERA-B  Electromagnetic Calorimeter is presented. A choice of the solution on the basis of the Cockcroft–Walton voltage multiplier is validated. Schematics developed are discussed. A special section describes the behaviour of the crucial components under irradiation since radiation tolerance becomes one of the most important items in the severe HERA-B  radiation environment. Finally, performance achieved is presented.     

A bivariate optimal replacement policy for a multistate repairable system

In this paper, a deteriorating simple repairable system with k+1 states, including k failure states and one working state, is studied. It is assumed that the system after repair is not “as good as new” and the deterioration of the system is stochastic. We consider a bivariate replacement policy, denoted by (T,N), in which the system is replaced when its working age has reached T or the number of failures it has experienced has reached N, whichever occurs first. The objective is to determine the optimal replacement policy (T,N)^* such that the long-run expected profit per unit time is maximized. The explicit expression of the long-run expected profit per unit time is derived and the corresponding optimal replacement policy can be determined analytically or numerically. We prove that the optimal policy (T,N)^* is better than the optimal policy N^* for a multistate simple repairable system. We also show that a general monotone process model for a multistate simple repairable system is equivalent to a geometric process model for a two-state simple repairable system in the sense that they have the same structure for the long-run expected profit (or cost) per unit time and the same optimal policy. Finally, a numerical example is given to illustrate the theoretical results.     

Energy transfer phenomena of keV fullerenes during grazing scattering from an Al(0 0 1) surface

Angular distributions for grazing scattering of fullerenes with energies of up to some 10 keV from an atomically clean and flat Al(0 0 1) surface are studied. Scattering proceeds in the regime of surface channeling where the motions of projectiles parallel and normal to the surface are widely decoupled. At low energies for the motion with respect to the surface normal, the clusters are scattered nearly elastically, whereas for larger energies a substantial amount of normal energy is lost. The results are compared to trajectory simulations using the Tersoff potential for the cluster and 3D- as well as 1D-rigid-wall representations for the surface. We find that, despite the large mass of C₆₀, the surface can be considered as a 1D- rigid wall and that the exact form of the interaction potential with the surface does not influence the normal energy loss. Therefore, properties of the fullerene and its interaction with the surface can be studied under well-defined conditions. The energy loss is transferred to internal excitations of the fullerenes.     

Glued detectors for the CRESST-II experiment

The Cryogenic Rare Event Search with Superconducting Thermometers Phase II (CRESST-II) at the L.N.G.S in Italy is searching for Dark Matter using low-temperature calorimeters. These detectors allow to discriminate different particles by simultaneous measurement of phonons and scintillation light. The sensors used consist of superconducting tungsten thin-film thermometers, which measure the thermal effect of the phonons created in an attached absorber crystal. It has been observed that the scintillation of the CaWO₄ absorber degrades during the process of depositing the tungsten film. In order to prevent this, a new technique for producing the detectors was investigated. This technique might also be valuable by expanding the range of scintillator materials suitable for producing a Dark Matter detector.     

First-principles optical calculations of AsNMg3 and SbNMg3

The structural, electronic and optical properties of antiperovskite semiconductors AsNMg₃ and SbNMg₃ have been studied using the full-potential augmented plane wave plus local-orbital (APW+lo) method. The exchange correlation has been treated using the generalized gradient approximation (GGA). The equilibrium lattice constant and other structural properties have been determined. The AsNMg₃ is found to be a direct band gap semiconductor while SbNMg₃ is indirect. The results for electronic band structures, density of states, charge densities as well as optical properties are presented and discussed.     

A complementary note on Gegenbauer polynomial approximation for random response problem of stochastic structure

Recently, Gegenbauer polynomial approximation was proposed for solving the evolutionary random response problem of a random structure with bounded random parameters under evolutionary random excitations. The bounded random parameters used there are supposed to be proportional to a random variable with λ-PDF (probability density function). For this kind of random parameter, the Gegenbauer polynomial approximation is the unique correct choice for transforming a random structure into its deterministic equivalent system, which plays a central role in solving the response problem. Actually, the Gegenbauer polynomial approximation bridges the gap between the random structural response problem and the conventional methods. Just through its deterministic equivalent system, the random response problem of a random structure can be solved by any available, analytical and numerical method developed for deterministic systems. But the simple assumption on proportionality to λ-PDF may bring some unnecessary limitation on symmetry. Since λ-PDF is symmetrical about its center axis, so are these random parameters. However, not all random parameters have this kind of symmetry. Then, what can we do to lessen the limitation, if random parameters are non-symmetric per se? Besides, the graph of λ-PDF (see Fig. 1 in the text) shows its features’ dependence on λ, that is, the smaller the λ, the more dispersive the PDF. Then, what is the influence of different values of λ on the evolutionary random responses of the stochastic system? Moreover, is there any qualitative information about the dispersion in response with respect to every individual random parameter? This note is devoted to answering the above two questions and giving a supplementary note on Gegenbauer polynomial approximation. Our study shows that the symmetric limitation can be partly lessened by putting the random parameter into a polynomial function, even a quadratic one, of a random variable with λ-PDF. On the other hand, the second-order moment of the random responses of the stochastic structure relative to that of a nominal one can be used to describe qualitatively their relative dispersion with respect to each individual random parameter.