Direction Of Gamma Rays Research Articles

Development of an advanced Compton camera with gaseous TPC and scintillator

A prototype of the MeV gamma-ray imaging camera based on the full reconstruction of the Compton process has been developed. This camera consists of a micro-TPC that is a gaseous Time Projection Chamber (TPC) and scintillation cameras. With the information of the recoil electrons and the scattered gamma-rays, this camera detects the energy and incident direction of each incident gamma-ray. We developed a prototype of the MeV gamma-ray camera with a micro-TPC and a NaI(Tl) scintillator, and succeeded in reconstructing the gamma-rays from 0.3 to 1.3 MeV. Measured angular resolutions of ARM (Angular Resolution Measure) and SPD (Scatter Plane Deviation) for 356 keV gamma-rays were 18 ∘ and 35 ∘ , respectively.

Gamma-ray imaging with position-sensitive HPGe detectors

Due to advances in manufacturing large and highly segmented HPGe detectors along with the availability of fast and high-precision digital electronics, it is now possible to build efficient and high-resolution Compton cameras. Two-dimensionally segmented semi-conductor detectors along with pulse-shape analysis allow to obtain three-dimensional positions and energies of individual gamma-ray interactions. By employing gamma-ray tracking procedures it is possible to determine the scattering sequence in the detector and ultimately to deduce the incident direction of gamma rays without the use of a attenuating collimator. These advanced gamma-ray tracking-based Compton cameras are able not only to image gamma-ray sources with higher sensitivity than collimator-based systems but can increase the sensitivity in finding gamma-ray sources over non-imaging detectors, particularly in complex radiation fields. We have implemented a Compton camera built of a single double-sided strip HPGe detector with a strip pitch size of 2 mm . A three-dimensional position resolution of 0.5 mm at 122 keV by using simple pulse-shape analysis is achieved. We have implemented image reconstruction procedures for search scenarios, which are of interest for national security applications. In addition, we have developed reconstruction procedures to optimize image quality which potentially finds applications in other areas as well.

Performances of the CsI(Tl) detector element of the GLAST calorimeter

The CsI(Tl) Detector Element (CDE) is the unit of detection of the calorimeter subsystem of the Large Area Telescope (LAT) of the GLAST satellite (IEEE Nuclear Science Symposium, 2002), which will be launched in September 2006. The LAT consists of 4×4 identical towers, each one including a strip silicon tracker on the top and a CsI(Tl) calorimeter on the bottom. A segmented plastic scintillator anticoincidence detector surrounds the 16 towers. The calorimeter modules, 8 layers of 12 CDEs each, have a hodoscope's shape to allow the measurement of the gamma-ray direction. The CDE main characteristics are its Light Yield, the energy resolution and the tapering, ratio of left/right signals used for the determination of the crossing position of the particle in the crystal. The CDE have to operate in space at about −10°C. We will describe the CDE, the optimization of its main parameter, the L.Y., and finally its variation with temperature.

A Mössbauer Spectral Study of the Hull Steel and Rusticles Recovered from the Titanic

The recent recovery of steel from the Titanic has permitted a 295 K conversion electron Mossbauer spectral study of the Titanic hull plate steel oriented with the gamma-ray direction either perpendicular or parallel to the microstructural banding directions. The two spectra reveal virtually identical average orientations of the magnetization close to the plane of the plate. The hyperfine parameters are virtually identical to those of α-iron, a finding which agrees with the chemical analysis which reveals at most 0.21 wt% carbon corresponding to 3 wt% of cementite in pearlite. A 4.2 to 295 K transmission Mossbauer spectral study of the rusticles reveals small particles of geothite undergoing superparamagnetic relaxation with a blocking temperature of ca. 300 K. In addition approximately two percent of the Mossbauer spectral absorption area corresponds to a quadrupole doublet with hyperfine parameters typical of green rust. The identified iron containing components in the rusticles agree with the powder X-ray diffraction results which reveal the predominant presence of small particles of poorly crystallized goethite and traces of quartz and green rust. An average size of 20±5 nm for the goethite particles is obtained from both the average hyperfine field and the broadening of the X-ray diffraction peaks. The magnetic anisotropy constant of the goethite particles deduced from the hyperfine field and the particle size is 8×103 J/m3.

The gamma ray large area space telescope - an astro-particle mission to explore the high energy gamma ray sky

The Large Area Telescope (LAT), the main instrument on the Gamma-ray Large Area Space Telescope (GLAST) mission under development for launch by NASA in 2006, will survey the sky in the rich yet poorly explored high-energy band between 20 MeV and 1 TeV with unprecedented resolution and sensitivity. The LAT, which benefits from the application of particle physics detection technologies to astrophysics instrumentation, has a pair-conversion silicon-strips tracker and an imaging CsI calorimeter to measure the direction and energies of gamma rays. The relatively very intense background of charged particles will be rejected primarily by an anticoincidence shield of segmented plastic scintillators. In this paper we give an overview of the many physics goals and potential reach of the LAT, describe the instrument design and performance, and report on the status of the tracker construction.

Gamma-ray imaging by silicon detectors in space: the AREM method

We present the Agile REconstruction Method (AREM) for gamma-ray ( 30 MeV – 50 GeV) direction reconstruction applicable to high-resolution Silicon Tracker detectors in space. It can be used in a “fast mode”, independent of Kalman filters techniques, or in an “optimized mode”, including Kalman filter algorithms for track identification. AREM correctly addresses three points of the analysis which become relevant for off-axis incidence angles: (1) intrinsic ambiguity in the identification of the three-dimensional e +/e − tracks; (2) proper identification of the three-dimensional pair production plane and reconstructed direction; (3) careful choice of an energy weighting scheme for the three-dimensional tracks. We apply our method to simulated gamma-rays in the AGILE detector. The excellent spatial resolution obtained by the AGILE Silicon Tracker, providing crucial analog information, makes it possible to improve the angular resolution of previous detectors (e.g. EGRET) by a factor of ∼2 in containment radius at E γ ≳400 MeV . In this paper, we present the results of our 3D-method for a selected sample of photon incidence angles and energies. A more comprehensive and complete discussion including the use of Kalman filter algorithms will be the subject of forthcoming papers.

Ferrimagnetic ordering in nanostructured CdFe2O4 spinel

Nanostructured CdFe2O4 spinel powders with various grain sizes ranging from 60 to 4 nm were synthesized by ball milling the bulk material. The magnetization measurements at 1 and 9 T reveal a spin-glass-like surface structure and the material is found to have a large anisotropy. Mössbauer studies at 10 K in external magnetic fields of 6 and 8.5 T applied parallel to the direction of gamma rays could clearly show that CdFe2O4 behaves like a mixed spinel on reducing the grain size to nanometer level and exhibits ferrimagnetic ordering. Fe3+ spins at both A and B sites exhibit spin canting.

Update of nuclear reaction rates affecting nucleosynthesis in novae

Gamma ray tracking is an efficient detection technique in studying exotic nuclei which lies far from beta stability line. To achieve very powerful and extraordinary resolution ability, new detectors based on gamma ray tracking are currently being developed. To reach this achievement, the neutron–gamma discrimination in these detectors is also an important task. In this paper, by suitable layered feedforward neural networks (LFNNs), we have constructed novel and consistent empirical physical formulas (EPFs) for some highly nonlinear detector counts measured in neutron–gamma discrimination. The detector counts data used in the discrimination was actually borrowed from our previous paper. The counts used here had been originally measured versus the following parameters: energy deposited in the first interaction points, difference in the incoming direction of initial gamma rays, and finally figure of merit values of the clusters determined by tracking. The LFNN–EPFs are of explicit mathematical functional form. Therefore, by various suitable operations of mathematical analysis, these LFNN–EPFs can be used to derivate further physical functions which might be potentially relevant to neutron–gamma discrimination performance of gamma ray tracking.

Mixed spinel structure in nanocrystallineNiFe2O4

Nanocrystalline ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ spinel has been synthesized with various grain sizes by high-energy ball milling. From the high-field magnetization studies and extended x-ray-absorption fine-structure, and M\"ossbauer measurements in an external magnetic field of 5 T applied parallel to the direction of gamma rays, we could observe that ${\mathrm{Ni}}^{2+}$ ions occupy tetrahedral sites on grain-size reduction due to milling. The ${\mathrm{Fe}}^{3+}$ spins have a canted structure and the canting angle increases with grain-size reduction. It is possible that the core ${\mathrm{Fe}}^{3+}$ spins are also canted because of the magnetocrystalline anisotropy introduced by the occupancy of the ${\mathrm{Ni}}^{2+}$ ions in the tetrahedral sites.

A new analysis method for reconstructing the arrival direction of TeV gamma rays using a single imaging atmospheric Cherenkov telescope

We present a method of atmospheric Cherenkov imaging which reconstructs the unique arrival direction of TeV gamma rays using a single telescope. The method is derived empirically and utilizes several features of gamma-ray-induced air showers which determine, to a precision of 0.12°, the arrival direction of photons on an event-by-event basis. Data from the Whipple Observatory's 10 m gamma-ray telescope are utilized to test selection methods based on source location. The results compare these selection methods with traditional techniques and three different camera fields of view. The method will be discussed in the context of a search for a gamma-ray signal from a point source located anywhere within the field of view and from regions of extended emission.

Gamma ray polarimetry using a position sensitive germanium detector

Imaging gamma-ray detectors make sensitive polarimeters in the Compton energy regime by measuring the scatter direction of gamma rays. The principle is to capitalize on the angular dependence of the Compton scattering cross section to polarized gamma rays and measure the distribution of scatter directions within the detector. This technique is effective in a double-sided germanium detector between roughly 50 keV and 1 MeV. This paper reviews device characteristics important to the optimization of a Compton polarimeter, and summarizes measurements we have made using a device with a 5×5 cm active area, 1 cm thickness, and strip-electrodes on a 2 mm pitch.

The electronics read out and data acquisition system for a liquid xenon time projection chamber as a balloon-borne Compton telescope

A 10 l liquid xenon time projection chamber with a drift gap of 7 cm, was developed for imaging cosmic gamma-rays in the energy band 0.3–10 MeV. It was used for the first time as a balloon-borne instrument in two recent flights from Palestine, TX. The detector uses both the ionization and the primary scintillation signals produced by gamma-ray interactions in the liquid for an accurate measurement of the three spatial coordinates and for the energy deposition in each interaction. With this information, the arrival direction of gamma-rays fully absorbed in the liquid xenon after one or more Compton scatterings can then be reconstructed. Here we describe the electronics readout system used for the initial balloon flights of the LXeTPC operating as a Compton telescope. Data from the laboratory and cosmic gamma-ray sources acquired with this custom-designed system are presented.

Inferring the Spatial and Energy Distribution of Gamma‐Ray Burst Sources. II. Isotropic Models

We use Bayesian methods to study anisotropic models for the distribution of gamma ray burst intensities and directions reported in the Third BATSE Catalog (3B catalog) of gamma ray bursts. We analyze data obtained using both the 64 ms and 1024 ms measuring timescales. We study both purely local models in which burst sources (``bursters'') are presumed to be distributed in extended halos about the Galaxy and M31, and mixed models consisting of a cosmological population of standard candle bursters and a local population distributed throughout a standard Bahcall-Soneira dark matter halo with a 2 kpc core. We find that the purely local models we have studied can account for the 3B data as successfully as cosmological models, provided one considers halos with core sizes significantly larger than those used to model the distribution of dark matter. We infer core sizes for the halo distribution that are smaller than one might expect based on popular semiquantitative arguments, and show why such arguments can lead to unwarranted conclusions. We also find that the 3B data do not constrain the width of power-law luminosity functions for burst sources. Our analysis of mixed models finds two families of models that can successfully account for the data: models with up to 20% of observed bursts in a bright local population visible to ~ 50 kpc; and models with up to 50% of observed bursts in a dim local population visible only nearby (to less than a disk scale height). These models fit as well or better than purely cosmological models. They indicate that a surprisingly large local, anisotropic component could be present whose size is comparable to the sizes of hypothetical classes of bursts inferred from analyses of temporal and spectral characteristics.

The concept of the angular differential dose of ionizing radiation and its measurement

Radiation-induced effects often depend not only on the absorbed dose but also on the distribution of directions from which it is delivered. This distribution can be described by using the angular differential dose, whose integral over all angles equals the integrated dose. A directional sensor was developed to measure this quantity. The design of the sensor is based on a carbon fiber, self-reading, electrometer dosimeter. Its sensitivity to the direction of incident gamma rays or x-rays makes it possible to measure dose or dose rate as a function of the angle of incident radiation. Experiments to demonstrate the properties of this sensor (Directional Dosimeter, DD) were conducted, using point sources of 137Cs. Results show that the sensor can detect and locate multiple 137Cs sources and determine the angular differential dose or angular differential dose rate in physical units of gray (tissue) per degree or grays per degree per minute. The DD was also used to scan and image the back-scatter from a water target of monodirectionally incident 137Cs 662 keV gamma rays.

A portable gamma camera for radiation monitoring

A compact gamma camera capable of imaging high energy gamma rays (up to 2 MeV) is currently being developed for nuclear industrial applications. This camera uses a three inch square position sensitive photomultiplier tube (PSPMT) [Hamamatsu, R-2487-05] coupled to a NaI(Tl) scintillator. The incoming gamma ray direction is determined by a segmented collimator. The camera exhibits energy resolutions of 17% FWHM at 122 keV and 12% FWHM at 662 keV. With the collimator design used, an angular resolution at the source plane of 6/spl deg/ FWHM was measured for 412 keV. Images have been obtained using gamma ray sources up to 1 MeV using this system. The image reconstruction technique used is presented in detail. Corrections for the non-uniformities across the face of the PSPMT are implemented to improve the spatial and spectral information content in the final image. >

A high angular and high energy resolution gamma ray telescope for observations between 1 GeV and 1 TeV

We propose a compact gamma ray telescope to explore the 1 GeV to 1 TeV energy band with both high angular and high energy resolution. This telescope would use drift chambers to measure the direction of the cosmic gamma rays with high accuracy and a calorimeter made of dense inorganic scintillator layers (BaF 2 or CsI(Tl)) coupled to photosensitive gaseous detectors. It would reach an angular resolution of 5–10 arcminutes and an energy resolution of about 1.0–2.5%.

A new telescope for wide-band gamma-ray astronomy: The Silicon Compton Recoil Telescope (SCRT)

A new prototype gamma-ray telescope is described which is sensitive from 0.3 to 30 MeV as a Compton telescope and to 100 MeV as a pair detector. The Silicon Compton Recoil Telescope (SCRT) uses multilayers of silicon strip detectors as a Compton gamma-ray converter. Recoil electrons are tracked with the silicon strip detectors, and their energy losses and directions are measured. The direction and energy of the Compton-scattered gamma rays are measured with CsI(Tl)-photodiode detectors. Thus unique directions and energies are found for each incident gamma ray for the first time and without the background of overlapping rings. SCRT is the first Compton telescope to image the gamma-ray sky directly. It can also detect electron-positron pairs from gamma rays above 5 MeV, extending SCRT's sensitivity to above 100 MeV. Typical resolutions are 3% (FWHM) in energy at 2 MeV and 0.5 deg (1 sigma) in angle. The proposed prototype SCRT instrument has a sensitive area of 650 sq cm, a detection efficiency of 3%, a size reduction by about an order of magnitude, and a sensitivity of 15 millicrab for a typical Compton Observatory exposure. SCRT can also measure the polarization of the incident gamma rays, especially at low energies and large scattered angles. Simulation calculations and a discussion of results with a laboratory model are presented.

Polarized Mössbauer study of magnetic structure of Fe nanocrystalline

Fe nanocrystalline is a new type of material, developed recently. In this paper, the polarized recoil-free gamma rays were used to determine the distribution of magnetic moment within Fe nanocrystalline. The Mössbauer spectra show that the intensity of lines 2 and 5 decreases obviously at room temperature and varies with external magnetic fields. It indicates that the magnetic structure of Fe nanocrystalline is different from that of α-Fe. There is a preferred orientation after the material is pressed during its formation. The average angle between the magnetic moment direction and gamma ray propagation direction is about 40.3°, and that of the projection of the moment on the plane with the polarization direction of gamma ray is about 57.8°. It means that the suppress process affects the magnetic structure of Fe nanocrystalline.

A source reconstruction method for multiple scatter Compton cameras

A new method for finding the direction of gamma rays incident on a multiple scatter Compton camera has been proposed. The method uses the polarization dependence of double Compton scattering to reduce the azimuthal ambiguity normally associated with interpreting Compton scattering events. The CYLTRAN photon Monte Carlo code was modified to include the polarization effects of gamma rays undergoing Compton scattering in the 150-1000-keV energy range and used to test the method. Results for 150-keV gamma rays incident upon a multiple Compton scatter imaging camera are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiple Compton scattering gamma ray imaging camera

With the recent advances in position sensing detectors, a compact, multiple layered, Compton scattering gamma ray imaging camera has been investigated for technological feasibility. Beginning from Kamea and Hanada's recent work, the multiple Compton scattering camera design has been investigated and refined over the energy range 150–1000 keV. Our Monte Carlo simulations yielded reconstruction efficiencies of 7–14% for Si and 16–28% for Ge detectors. Including anticipated experimental errors, we obtained angular errors of ∼2–5° in the reconstructed incident gamma ray directions over the energy range of interest.