The Compton Spectrometer and Imager (COSI) is a compact Compton telescope which is inherently sensitive to gamma-ray polarization in the energy range of 0.2-2.0 MeV. A long duration gamma-ray burst, GRB 160530A, was detected by COSI during its 2016 COSI’s balloon flight. The polarization of GRB 160530A was constrained based on the distribution of azimuthal scattering angles from each incident photon inside COSI’s germanium detector array.<sup>1</sup> In order to determine COSI’s polarization response and to identify systematic deviations from an ideal sinusoidal modulation, the polarization performance of COSI was validated in the laboratory prior to the 2016. A partially polarized beam was created by scattered emission from a radioactive source off a scintillator. In addition, measurements and simulations of unpolarized radioactive sources were compared to validate our capability of capturing the instrument systematics in the simulations. No statistically significant differences exist between the measured and simulated modulations and polarization angle, where the upper bound on the systematic error is 3%-4%.<sup>2</sup> In this talk, I will present the measurements used to validate COSI’s polarimetric performance. Furthermore, I will use these results to estimate the minimum detectable polarization levels for current and future COSI missions.
The Compton Spectrometer and Imager (COSI) is a medium energy gamma ray (0.2 - 10 MeV) imager designed to observe high-energy processes in the universe from a high altitude balloon platform. At its core, COSI is comprised of twelve high purity germanium double sided strip detectors which measure particle interaction energies and locations with high precision. This manuscript focuses on the positional calibrations of the COSI detectors. The interaction depth in a detector is inferred from the charge collection time difference between the two sides of the detector. We outline our previous approach to this depth calibration and also describe a new approach we have recently developed. Two dimensional localization of interactions along the faces of the detector (x and y) is straightforward, as the location of the triggering strips is simply used. However, we describe a possible technique to improve the x/y position resolution beyond the detector strip pitch of 2 mm. With the current positional calibrations, COSI achieves an angular resolution of 5.6 ± 0.1 degrees at 662 keV, close to our expectations from simulations.
The Compton Spectrometer and Imager (COSI) is a balloon-borne soft gamma-ray (0.2-5 MeV) telescope designed to perform wide-field imaging, high-resolution spectroscopy, and novel polarization measurements of astrophysical sources. COSI employs a compact Compton telescope design, utilizing 12 cross-strip germanium detectors to track the path of incident photons, where position and energy deposits from Compton interactions allow for a reconstruction of the source position in the sky, an inherent measure of the linear polarization, and significant background reduction. The instrument has recently been rebuilt with an updated and optimized design; the polarization sensitivity and effective area have increased due to a change in detector configuration, and the new lightweight gondola is suited to fly on ultra-long duration flights with the addition of a mechanical cryocooler system. COSI is planning to launch from the Long Duration Balloon site at McMurdo Station, Antarctica, in December 2014, where our primary science goal will be to measure gamma-ray burst (GRB) polarization. In preparation for the 2014 campaign, we have performed preliminary calibrations of the energy and 3-D position of interactions within the detector, and simulations of the angular resolution and detector efficiency of the integrated instrument. In this paper we will present the science goals for the 2014 COSI campaign and the techniques and results of the preliminary calibrations.
We report on the status of the Laue lens development effort led by UC Berkeley, where a dedicated X-ray beamline and a Laue lens assembly station were built. This allowed the realization of a first lens prototype in June 2012. Based on this achievement, and thanks to a new NASA APRA grant, we are moving forward to enable Laue lenses. Several parallel activities are in progress. Firstly, we are refining the method to glue quickly and accurately crystals on a lens substrate. Secondly, we are conducting a study of high-Z crystals to diffract energies up to 900 keV efficiently. And thirdly, we are exploring new concepts of Si-based lenses that could further improve the focusing capabilities, and thus the sensitivity of Laue lenses.
The Nuclear Compton Telescope (NCT) is a balloon-borne soft γ-ray (0.2-10 MeV) telescope designed to perform
wide-field imaging, high-resolution spectroscopy, and novel polarization analysis of astrophysical sources. NCT
employs a novel Compton telescope design, utilizing 12 high spectral resolution germanium detectors, with the
ability to localize photon interaction in three dimensions. NCT underwent its first science flight from Fort
Sumner, NM in Spring 2009, and was partially destroyed during a second launch attempt from Alice Spring,
Australia in Spring 2010. We have begun the rebuilding process and are using this as an opportunity to update
and optimize various aspects of NCT. The cryostat which houses the 12 germanium detectors is being redesigned
so as to accommodate the detectors in a new configuration, which will increase the effective area and improve the
on-axis performance as well as polarization sensitivity of NCT. We will be replacing the liquid nitrogen detector
cooling system with a cryocooler system which will allow for long duration flights. Various structural changes
to NCT, such as the use of an all new gondola, will affect the physical layout of the electronics and instrument
subsystems. We expect to return to flight readiness by Fall 2013, at which point we will recommence science
flights. We will discuss science goals for the rebuilt NCT as well as proposed flight campaigns.
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 ~100 keV to ~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.