Modeling Charge Cloud Dynamics in Cross Strip Semiconductor Detectors

When a $\gamma$-ray interacts in a semiconductor detector, the resulting electron-hole charge clouds drift towards their respective electrodes for signal collection. These charge clouds will expand over time due to both thermal diffusion and mutual electrostatic repulsion. Solutions to the resulting charge profiles are well understood for the limiting cases accounting for only diffusion and only repulsion, but the general solution including both effects can only be solved numerically. Previous attempts to model these effects have taken into account the broadening of the charge profile due to both effects, but have simplified the shape of the profile by assuming Gaussian distributions. However, the detailed charge profile can have important impacts on charge sharing in multi-electrode strip detectors. In this work, we derive an analytical approximation to the general solution, including both diffusion and repulsion, that closely replicates both the width and the detailed shape of the charge profiles. This analytical solution simplifies the modeling of charge clouds in semiconductor strip detectors.Comment: Accepted for publication in Nuclear Instruments and Methods in Physics Research

SEARCH FOR GRAVITATIONALLY REDSHIFTED 2.2 MEV LINE FROM 4U 1820-30

We have analyzed 1.7 Ms of the INTEGRAL data of the Low Mass X-Ray Binary (LMXB) 4U 1820-30 and searched for the redshifted 2.2 MeV neutron capture gamma-ray line. This source is unique in that it is thought to be accreting pure Helium and might be a powerful 2.2 MeV line source. If detected, this line would strongly constrain the neutron star equation of state, motivating this search. The line is expected to be redshifted to 1.30-1.72 MeV so we scanned the 1-2 MeV region. Although we failed to detect the redshifted 2.2 MeV line, mainly due to the intense background noise to which INTEGRAL is exposed, we placed upper limits on the source’s flux for different line widths. We plan to do analysis on the rest of the data (over 8 Ms) in the future

A Combined Compton and Coded-aperture Telescope for Medium-energy Gamma-ray Astrophysics

A future mission in medium-energy gamma-ray astrophysics would allow for many scientific advancements, e.g. a possible explanation for the excess positron emission from the Galactic Center, a better understanding of nucleosynthesis and explosion mechanisms in Type Ia supernovae, and a look at the physical forces at play in compact objects such as black holes and neutron stars. Additionally, further observation in this energy regime would significantly extend the search parameter space for low-mass dark matter. In order to achieve these objectives, an instrument with good energy resolution, good angular resolution, and high sensitivity is required. In this paper we present the design and simulation of a Compton telescope consisting of cubic-centimeter Cadmium Zinc Telluride (CdZnTe) detectors as absorbers behind a silicon tracker with the addition of a passive coded mask. The goal of the design was to create a very sensitive instrument that is capable of high angular resolution. The simulated telescope showed achievable energy resolutions of 1.68$\%$ FWHM at 511 keV and 1.11$\%$ at 1809 keV, on-axis angular resolutions in Compton mode of 2.63$^{\circ}$ FWHM at 511 keV and 1.30$^{\circ}$ FWHM at 1809 keV, and is capable of resolving sources to at least 0.2$^{\circ}$ at lower energies with the use of the coded mask. An initial assessment of the instrument in Compton imaging mode yields an effective area of 183 cm$^{2}$ at 511 keV and an anticipated all-sky sensitivity of 3.6 x 10$^{-6}$ photons cm$^{-2}$ s$^{-1}$ for a broadened 511 keV source over a 2-year observation time. Additionally, combining a coded mask with a Compton imager to improve point source localization for positron detection has been demonstrated

Analytical Fitting of Gamma-ray Photopeaks in Germanium Cross Strip Detectors

In an ideal germanium detector, fully-absorbed monoenergetic gamma-rays will appear in the measured spectrum as a narrow peak, broadened into a Gaussian of width determined only by the statistical properties of charge cloud generation and the electronic noise of the readout electronics. Multielectrode detectors complicate this picture. Broadening of the charge clouds as they drift through the detector will lead to charge sharing between neighboring electrodes and, inevitably, low-energy tails on the photopeak spectra. We simulate charge sharing in our germanium cross strip detectors in order to reproduce the low-energy tails due to charge sharing. Our goal is to utilize these simulated spectra to develop an analytical fit (shape function) for the spectral lines that provides a robust and high-quality fit to the spectral profile, reliably reproduces the interaction energy, noise width, and the number of counts in both the true photopeak and the low-energy tail, and minimizes the number of additional parameters. Accurate modeling of the detailed line profiles is crucial for both calibration of the detectors as well as scientific interpretation of measured spectra.Comment: Submitted to NIM

Search for polarization from the prompt gamma-ray emission of GRB 041219a with SPI on INTEGRAL

Measuring the polarization of the prompt γ-ray emission from gamma-ray bursts (GRBs) can significantly improve our understanding of both the GRB emission mechanisms as well as the underlying engine driving the explosion. We searched for polarization in the prompt γ-ray emission of GRB 041219a with the SPI instrument on INTEGRAL. Using multiple-detector coincidence events in the 100-350 keV energy band, our analysis yields a polarization fraction from this GRB of 98%+/-33%. Statistically, we cannot claim a polarization detection from this source. Moreover, different event selection criteria lead to even less significant polarization fractions, e.g., lower polarization fractions are obtained when higher energies are included in the analysis. We cannot strongly rule out the possibility that the measured modulation is dominated by instrumental systematics. Therefore, SPI observations of GRB 041219a do not significantly constrain GRB models. However, this measurement demonstrates the capability of SPI to measure polarization, as well as the techniques developed for this analysis

POLARIZATION MEASUREMENT OF GRB 041219A WITH SPI

Measuring the polarization of the prompt gamma-ray emission from GRBs can signicantly improve our understanding of both the GRB emission mechanisms, as well as of the underlying engine driving the explosion. We searched for polarization in the prompt gamma-ray emission of GRB 041219a with the SPI instrument. Using multiple detector coincidence events in the 100–350 keV energy band, our analysis yields a polarization fraction from this GRB of 98 +-33%. Statistically, we cannot claim a polarization detection from this source. We cannot strongly rule out the possibility that the measured modulation is dominated by instrumental systematics. Therefore, SPI observations of GRB 041219a do not significantly constrain GRB models. However, this measurement demonstrates the capability of SPI to measure polarization, and the techniques developed for this analysis

X-RAY OBSERVATIONS OF SN 1006 WITH INTEGRAL

We have used 1000 ksec of on-source, and 500 ksec of 10 degrees off-source INTEGRAL data of SN 1006 to characterize the synchrotron emission, and attempt to detect non-thermal bremsstrahlung, using the combination of IBIS and JEM-X spatial and spectral coverage. With the earlier 750 ksec on source data, we have detected the limbs of SN 1006 with JEM-X between 2.4 and 8.4 keV bands. The source was not detected with either ISGRI orSPI above 20 keV. The ISGRI upper limit is about a factor of four above current model predictions, but conrms the presence of steepening in the power-law extrapolated from lower energies (< 4 keV)

Developing a second generation Laue lens prototype: high reflectivity crystals and accurate assembly

Laue lenses are an emerging technology that will enhance gamma-ray telescope sensitivity by one to two orders of magnitude in selected energy bands of the \sim 100 keV to \sim 1.5 MeV range. This optic would be particularly well adapted to the observation of faint gamma ray lines, as required for the study of Supernovae and Galactic positron annihilation. It could also prove very useful for the study of hard X-ray tails from a variety of compact objects, especially making a difference by providing sufficient sensitivity for polarization to be measured by the focal plane detector. Our group has been addressing the two key issues relevant to improve performance with respect to the first generation of Laue lens prototypes: obtaining large numbers of efficient crystals and developing a method to fix them with accurate orientation and dense packing factor onto a substrate. We present preliminary results of an on-going study aiming to enable a large number of crystals suitable for diffraction at energies above 500 keV. In addition, we show the first results of the Laue lens prototype assembled using our beamline at SSL/UC Berkeley, which demonstrates our ability to orient and glue crystals with accuracy of a few arcsec, as required for an efficient Laue lens telescope.Comment: Published in the proceedings of the SPIE conference held in San Diego in August 201