135 research outputs found
Development of Silicon Strip Detectors for a Medium Energy Gamma-ray Telescope
We report on the design, production, and testing of advanced double-sided
silicon strip detectors under development at the Max-Planck-Institute as part
of the Medium Energy Gamma-ray Astronomy (MEGA) project. The detectors are
designed to form a stack, the "tracker," with the goal of recording the paths
of energetic electrons produced by Compton-scatter and pair-production
interactions. Each layer of the tracker is composed of a 3 x 3 array of 500
micron thick silicon wafers, each 6 cm x 6 cm and fitted with 128 orthogonal p
and n strips on opposite sides (470 micron pitch). The strips are biased using
the punch-through principle and AC-coupled via metal strips separated from the
strip implant by an insulating oxide/nitride layer. The strips from adjacent
wafers in the 3 x 3 array are wire-bonded in series and read out by 128-channel
TA1.1 ASICs, creating a total 19 cm x 19 cm position-sensitive area. At 20
degrees C a typical energy resolution of 15-20 keV FWHM, a position resolution
of 290 microns, and a time resolution of ~1 microsec is observed.Comment: 9 pages, 13 figures, to appear in NIM-A (Proceedings of the 9th
European Symposium on Semiconductor Detectors
The MEGA Advanced Compton Telescope Project
The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to
improve sensitivity at medium gamma-ray energies (0.4-50 MeV) by at least an
order of magnitude over that of COMPTEL. This will be achieved with a new
compact design that allows for a very wide field of view, permitting a
sensitive all-sky survey and the monitoring of transient and variable sources.
The key science objectives for MEGA include the investigation of cosmic
high-energy particle accelerators, studies of nucleosynthesis sites using
gamma-ray lines, and determination of the large-scale structure of galactic and
cosmic diffuse background emission. MEGA records and images gamma-ray events by
completely tracking both Compton and pair creation interactions in a tracker of
double-sided silicon strip detectors and a calorimeter of CsI crystals able to
resolve in three dimensions. We present initial laboratory calibration results
from a small prototype MEGA telescope.Comment: 7 pages LaTeX, 5 figures, to appear in New Astronomy Reviews
(Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"
Total ionizing dose test with DEPFET sensors for Athena's WFI
The focal plane of Athena's WFI consists of spectroscopic single photon X-ray
detectors that contain arrays of DEPFETs (DEpleted P-channel Field-Effect
Transistor) as well as ASICs that are used for steering, readout and analog
signal shaping. These components have to be examined regarding the effect of
ionizing radiation. A Total Ionizing Dose (TID) test was done with prototype
detector modules with 64x64 DEPFETs and one SWITCHER and VERITAS ASIC each. The
current design of the WFI detector head features a proton shield equivalent to
4 cm of aluminum in order to prevent a strong increase of leakage current in
the fully depleted 450 m thick bulk of the sensor. This keeps the expected
doses and dose rates during the nominal mission relatively low (5 Gy). It
is nevertheless important to study the current system in a dedicated TID test
in order to exclude unforeseen effects and to study any radiation related
changes that can have an effect on the very sensitive readout chain and the
detector performance. The combination of low doses, low dose rates, low
operating temperature (<-60{\deg}C) but high sensitivity on small changes of
the threshold voltages represent somehow unusual boundary conditions in
comparison to TID tests for standard radiation hard electronic components.
Under these circumstances it was found beneficial to do the test in our own
laboratory with an X-ray source in order to realize irradiation during nominal
operation conditions. Furthermore, it facilitated to take annealing effects
into account. Reasonably accurate dosimetry is achieved by measuring the X-ray
spectrum and intensity with the device under test. After irradiation to a total
dose of 14 Gy and subsequent annealing the threshold voltage of the DEPFETs
were shifted by a mean value of 80 mV, the performance remained unchanged apart
from a slight increase in readout noise by 10%.Comment: 8 pages, 6 figures, to appear in SPIE Proceeding of Astronomical
Telescopes + Instrumentation, 202
Simulation studies of CZT Detectors as Gamma-Ray Calorimeter
We describe the results of detailed 3-D Monte Carlo simulations of a "CZT
calorimeter" that can be used to detect photons in the keV to several MeV
range. Several astrophysics applications require the detection of photons in
the energy range of keV up to several MeV with good position and energy
resolution. For certain applications Cadmium Zinc Telluride (CZT) detectors
might be the detector option of choice. Up to now, CZT detectors have mainly
been used in the energy range between a few keV to ~1 MeV, we describe the
results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can
be used to detect photons in the keV to several MeV range. The main objective
of these studies is to evaluate the feasibility of CZT calorimeters, to study
their performance and detect and understand performance limiting factors. Such
a calorimeter consists of many layers of closely packed pixellated CZT detector
units. Our simulations of single detector units reproduce experimental results,
indicating that our simulations capture the main factors that limit the
performance of a detector unit. Overall the conclusion of our simulation study
is that between 1 cm and 1.5 cm thick detector units can be used to build a
calorimeter with good performance over the energy range from ~20 keV to ~10
MeV.Comment: Accepted for publication in Astroparticle Physics, 20 pages, 14
figure
Single-shot full strain tensor determination with microbeam X-ray Laue diffraction and a two-dimensional energy-dispersive detector
MEGA: A Medium-Energy Gamma-ray Astronomy mission concept
The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4 - 50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory, the SPI and IBIS instruments on INTEGRAL, and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include, among other things, compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA will accomplish these goals using a tracker made of Si strip detector (SSD) planes surrounded by a dense high-Z calorimeter. At lower photon energies (below ⌠30 MeV), the design is sensitive to Compton interactions, with the SSD system serving as a scattering medium that also detects and measures the Compton recoil energy deposit. If the energy of the recoil electron is sufficiently high (\u3e 2 MeV) its momentum vector can also be measured. At higher photon energies (above âŒ10 MeV), the design is sensitive to pair production events, with the SSD system measuring the tracks of the electron and positron. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission
Erratum to: gespeR: a statistical model for deconvoluting off-target-confounded RNA interference screens
The eROSITA X-ray telescope on SRG
eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2-2.3 keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3-8 keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts z > 1 in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements
Specific inhibition of diverse pathogens in human cells by synthetic microRNA-like oligonucleotides inferred from RNAi screens
Systematic genetic perturbation screening in human cells remains technically challenging. Typically, large libraries of chemically synthesized siRNA oligonucleotides are used, each designed to degrade a specific cellular mRNA via the RNA interference (RNAi) mechanism. Here, we report on data from three genome-wide siRNA screens, conducted to uncover host factors required for infection of human cells by two bacterial and one viral pathogen. We find that the majority of phenotypic effects of siRNAs are unrelated to the intended âon-targetâ mechanism, defined by full complementarity of the 21-nt siRNA sequence to a target mRNA. Instead, phenotypes are largely dictated by âoff-targetâ effects resulting from partial complementarity of siRNAs to multiple mRNAs via the âseedâ region (i.e., nucleotides 2â8), reminiscent of the way specificity is determined for endogenous microRNAs. Quantitative analysis enabled the prediction of seeds that strongly and specifically block infection, independent of the intended on-target effect. This prediction was confirmed experimentally by designing oligos that do not have any on-target sequence match at all, yet can strongly reproduce the predicted phenotypes. Our results suggest that published RNAi screens have primarily, and unintentionally, screened the sequence space of microRNA seeds instead of the intended on-target space of protein-coding genes. This helps to explain why previously published RNAi screens have exhibited relatively little overlap. Our analysis suggests a possible way of identifying âseed reagentsâ for controlling phenotypes of interest and establishes a general strategy for extracting valuable untapped information from past and future RNAi screens
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