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 Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument is a balloon-borne telescope designed to study solar- are particle acceleration and transport. We describe GRIPS's first Antarctic long-duration flight in January 2016 and report preliminary calibration and science results. Electron and ion dynamics, particle abundances and the ambient plasma conditions in solar flares can be understood by examining hard X-ray (HXR) and gamma-ray emission (20 keV to 10 MeV). Enhanced imaging, spectroscopy and polarimetry of are emissions in this energy range are needed to study particle acceleration and transport questions. The GRIPS instrument is specifically designed to answer questions including: What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? GRIPS's key technological improvements over the current solar state of the art at HXR/gamma-ray energies, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid modulation collimator, the multi-pitch rotating modulator (MPRM). The 3D-GeDs have spectral FWHM resolution of a few hundred keV and spatial resolution <1 mm<sup>3</sup>. For photons that Compton scatter, usually ⪆150 keV, the energy deposition sites can be tracked, providing polarization measurements as well as enhanced background reduction through Compton imaging. Each of GRIPS's detectors has 298 electrode strips read out with ASIC/FPGA electronics. In GRIPS's energy range, indirect imaging methods provide higher resolution than focusing optics or Compton imaging techniques. The MPRM gridimaging system has a single-grid design which provides twice the throughput of a bi-grid imaging system like RHESSI. The grid is composed of 2.5 cm deep tungsten-copper slats, and quasi-continuous FWHM angular coverage from 12.5-162 arcsecs are achieved by varying the slit pitch between 1-13 mm. This angular resolution is capable of imaging the separate magnetic loop footpoint emissions in a variety of are sizes. In comparison, RHESSI's 35-arcsec resolution at similar energies makes the footpoints resolvable in only the largest ares.
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.
Hard X-ray and gamma-ray emission during solar flares encode information about electron/ion dynamics and provide a proxy to deduce solar atmospheric parameters. Enhanced imaging, spectroscopy and polarimetry of HXR/gamma-ray are emissions over ~20 keV to greater than or approx. equal to 10MeV is needed to study particle transport; the Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS) instrument is designed to meet these goals. GRIPS' key technological improvements over the current solar state of the art in HXR/gamma-ray energies (RHESSI) include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid modulation collimator, the Multi-Pitch Rotating Modulator (MPRM). The 3D-GeDs allow GRIPS to reconstruct Compton-scatter tracks of energy deposition, providing enhanced background reduction and polarization measurements. Each of GRIPS' sixteen detectors has 298 electrode strips, each of which has dedicated ASIC/FPGA electronics. In GRIPS' energy range, indirect Fourier imaging provides higher resolution than focusing optics or Compton imaging techniques. The MPRM grid-imaging system has a single-grid design which provides 2x the throughput of a bigrid imaging system like RHESSI. Quasi-continuous resolution from 12.5 - 162 arcsecs is achieved by varying the grid pitch between 1 - 13mm. This spatial resolution will be capable of imaging the separate footpoints in a variety of flare sizes. In comparison, RHESSI's minimum 35 arcsec resolution at the same energy makes footpoints resolvable
in only the largest flares. We discuss GRIPS' science goals, the instrument overall, and recent developments in GRIPS' detector and imaging systems. GRIPS is scheduled for an engineering flight from Fort Sumner in September 2014, followed by two long-duration balloon flights from Antarctica in 2015/16.
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.
The balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument will provide a near-optimal
combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions
from ~20 keV to >~10 MeV. GRIPS will address questions raised by recent solar flare observations regarding particle
acceleration and energy release, such as: What causes the spatial separation between energetic electrons producing hard
X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they
dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and
why? The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where
each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution
of <0.1 mm3. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungstenalloy
slit/slat grid with pitches that range quasi-continuously from 1 to 13 mm. The MPRM is situated 8 meters from the
spectrometer to provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec
FWHM), sufficient to separate 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by
analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e., as an active scatterer), with an
estimated minimum detectable polarization of a few percent at 150–650 keV in an X-class flare. GRIPS is scheduled for
a continental-US engineering test flight in fall 2013, followed by long or ultra-long duration balloon flights in
The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of gammaray
emission with high spectral resolution, moderate angular resolution, and novel sensitivity to gamma-ray polarization.
The heart of NCT is a compact array of cross-strip germanium detectors allowing for wide-field imaging with excellent
efficiency from 0.2-10 MeV. Before 2010, NCT had flown successfully on two conventional balloon flights in Fort
Sumner, New Mexico. The third flight was attempted in Spring 2010 from Alice Springs, Australia, but there was a
launch accident that caused major payload damage and prohibited a balloon flight. The same system configuration
enables us to extend our current results to wider phase space with pre-flight calibrations in 2010 campaign. Here we
summarize the design, the performance of instrument, the pre-flight calibrations, and preliminary results we have
obtained so far.
The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.2-10 MeV) telescope designed to study
astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of 12 cross-strip
germanium detectors, designed to provide 3D positions for each photon interaction with full 3D position resolution to <
2 mm^3. Tracking individual interactions enables Compton imaging, effectively reduces background, and enables the
measurement of polarization. The keys to Compton imaging with NCT's detectors are determining the energy deposited
in the detector at each strip and tracking the gamma-ray photon interaction within the detector. The 3D positions are
provided by the orthogonal X and Y strips, and by determining the interaction depth using the charge collection time
difference (CTD) between the anode and cathode. Calibrations of the energy as well as the 3D position of interactions
have been completed, and extensive calibration campaigns for the whole system were also conducted using radioactive
sources prior to our flights from Ft. Sumner, New Mexico, USA in Spring 2009, and from Alice Springs, Australia in
Spring 2010. Here we will present the techniques and results of our ground calibrations so far, and then compare the
calibration results of the effective area throughout NCT's field of view with Monte Carlo simulations using a detailed
The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray telescope. Its compact design uses
cross-strip germanium detectors, allowing for wide-field imaging with excellent efficiency from 0.2-10 MeV. Additionally,
the Compton imaging principle employed by NCT provides polarimetric sensitivity to several MeV.
NCT is optimized for the study of astrophysical sources of nuclear line emission. A ten-detector instrument
participated in the 2010 balloon campaign in Alice Springs, Australia, in order to conduct observations of the
Galactic Center Region. Unfortunately, a launch accident caused major damage to the payload, and no flight
was possible. We discuss the design, calibration, and performance of the instrument as well as prospects for its
The bulk leakage current in a semiconductor detector is an important parameter that affects the noise level and energy
resolution of the detector. For detectors operating with ohmic contacts, the bulk leakage current is determined by the
bulk resistivity of the semiconductor material. However, CdZnTe detectors typically utilize Schottky barrier type
contacts, in which case the bulk leakage current is expected to depend on the contact behavior and not on the bulk
resistivity of the material. We have studied the bulk leakage current and noise of CdZnTe detectors made from
materials supplied by different manufacturers of CdZnTe crystals. The results indicate that there is a marked difference
in bulk leakage currents among materials from different manufacturers and among different samples from the same
manufacturer. In some cases, the bulk leakage current shows no correlation with the bulk resistivity of the materials. In
other cases, the bulk leakage currents tend to be lower for lower resistivity materials, which is opposite from the
commonly held expectation based upon ohmic contact device behavior. In this paper we present a summary of our
electrical measurements on CdZnTe devices and present results indicating a possible relationship between leakage and
bulk material properties, but the specific material properties and the mechanism responsible for the leakage current
variation have yet to be determined.
The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling
Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in
homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors
from THM grown CZT. We discuss the performance of such 20x20x10 mm<sup>3</sup>, and 10x10x10 mm<sup>3</sup> monolithic pixellated
detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.
The Nuclear Compton Telescope (NCT) is a balloon-borne soft
gamma-ray (0.2MeV-10MeV) telescope designed to study astrophysical
sources of nuclear line emission and polarization. A prototype
instrument was successfully launched from Ft. Sumner, NM on June 1,
2005. The NCT prototype consists of two 3D position sensitive
High-Purity-Germanium (HPGe) strip detectors fabricated with
amorphous Ge contacts. The novel ultra-compact design and new
technologies allow NCT to achieve high efficiencies with excellent
spectral resolution and background reduction. Energy and positioning calibration data was acquired pre-flight in Fort Sumner, NM after the full instrument integration. Here we discuss our calibration techniques and results, and detector efficiencies. Comparisons with simulations are presented as well.
We flew a prototype of the Nuclear Compton Telescope (NCT) on a high altitude balloon from Fort Sumner, New Mexico on 2005 June 1. The NCT prototype is a soft gamma-ray (0.2-15 MeV) telescope designed to study, through spectroscopy, imaging, and timing, astrophysical sources of nuclear line emission and gamma-ray polarization. Our program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope satellite, while studying gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. The NCT prototype utilizes two, 3D imaging germanium detectors (GeDs) in a novel, ultra-compact design optimized for nuclear line emission (0.5-2 MeV) and polarization in the 0.2-0.5 MeV range. Our prototype flight was a critical test of the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes.
We are developing a 2-detector high resolution Compton telescope utilizing 3D imaging germanium detectors (GeDs) to be flown as a balloon payload in Spring 2004. This instrument is a prototype for the larger Nuclear Compton Telescope (NCT), which utilizes 12-GeDs. NCT is a balloon-borne soft γ-ray (0.2-15 MeV) telescope designed to study, through spectroscopy, imaging, and timing, astrophysical sources of nuclear line emission and γ-ray polarization. The NCT program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope, while studying γ-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. NCT has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. The prototype flight will critically test the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes. In this paper we present the design and preliminary results of laboratory performance tests of the NCT flight electronics.
Our collaboration is developing a 2-detector prototype high resolution Compton telescope utilizing 3D imaging germanium detectors (GeDs) for a test balloon flight in Spring 2003. This instrument is a prototype for a full 12-GeD instrument, the Nuclear Compton Telescope. NCT is a balloon-borne soft gamma-ray (0.2-15 MeV) telescope designed to study astrophysical sources of nuclear
line emission and polarization. The NCT program is designed to develop and test the technologies and analysis techniques crucial for the Advanced Compton Telescope, while studying gamma-ray radiation with very high spectral resolution, moderate angular resolution, and high sensitivity. NCT has a novel, ultra-compact design optimized for studying nuclear line emission in the critical 0.5-2 MeV range, and polarization in the 0.2-0.5 MeV range. This prototype flight will critically test the novel instrument technologies, analysis techniques, and background rejection procedures we have developed for high resolution Compton telescopes. We present the design and expected performance of this prototype NCT instrument.
We have developed germanium detector technologies for use in the Nuclear Compton Telescope (NCT) - a balloon-borne soft γ-ray (0.2-10 MeV) telescope to study astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of twelve large volume cross strip germanium detectors, designed to provide 3-D positions for each photon interaction with ~1mm resolution while maintaining the high spectral resolution of germanium. Here we discuss the detailed performance of our prototype 19x19 strip detector, including laboratory tests, calibrations, and numerical simulations. In addition to the x and y positions provided by the orthogonal strips, the interaction depth (z-position) in the detector is measured using the relative timing of the anode and cathode charge collection signals. We describe laboratory calibrations of the depth discrimination using collimated sources with different characteristic energies, and compare the measurements to detailed Monte Carlo simulations and charge collection routines tracing electron-hole pairs from the interaction site to the electrodes. We have also investigated the effects of charge sharing and loss between electrodes, and present these in comparison to charge collection simulations. Detailed analysis of strip-to-strip uniformity in both efficiency and spectral resolution are also presented.
The coplanar-grid as well as other electron-only detection techniques are effective in overcoming some of the material problems of CdZnTe and, consequently, have led to efficient gamma-ray detectors with good energy resolution while operating at room temperature. The performance of these detectors is limited by the degree of uniformity in both electron generation and transport. Despite recent progress in the growth of CdZnTe material, small variations in these properties remain a barrier to the widespread success of such detectors. Alpha-particle response characterization of CdZnTe crystals fabricated into simple planar detectors is an effective tool to accurately study electron generation and transport. We have used a finely collimated alpha source to produce two-dimensional maps of detector response. A clear correlation has been observed between the distribution of precipitates near the entrance contact on some crystals and their alpha-response maps. Further studies are ongoing to determine the mechanism for the observed response variations and the reason for the correlation. This paper presents the results of these studies and their relationship to coplanar-grid gamma-ray detector performance.
Gamma-ray imaging with position-sensitive germanium detectors offers the advantages of excellent energy resolution, high detection efficiency, and potentially good spatial resolution. The development of the amorphous-semiconductor electrical contact technology for germanium detectors has simplified the production of these position-sensitive detectors and has made possible the use of unique detection schemes and detector geometries. We have fabricated prototype orthogonal-strip detectors for gamma-ray imaging studies using this contact technology. With these detectors, we demonstrate that a gamma- ray interaction event in the detector can be located in three dimensions. This more accurate determination of the interaction event position should ultimately lead to better image resolution. We have also taken advantage of the bipolar blocking nature of the amorphous-semiconductor contacts in order to investigate the use of field-shaping electrodes. The addition of such electrodes is shown to improve the spectroscopic performance of the detectors by substantially eliminating charge collection to the inter-electrode surfaces. In addition, we demonstrate that this incomplete charge collection process can also be reduced by adjusting the properties of the amorphous-semiconductor layer. In this paper, we summarize the development of these position- sensitive detectors and present the results of our studies with the detectors.
Preliminary results of experiments to investigate charge collection in CdZnTe detectors are presented. The experiments support the development of semiconductor- modeling tool for device engineering that will be used to design large volume CdZnTe detectors for gamma ray spectroscopy. Improved diagnostic methods are described, including an automated alpha particle scanner for charge pulse mapping. Semiconductor modeling techniques are presented along with methods to visualize charge transport. Experimental results are compared to a physical model that has been used routinely in research on room temperature devices for gamma ray detection.
Novel electrode configurations, such as coplanar grids, have been successful in mitigating the effects of poor hole transport in CdZnTe gamma-ray detectors. However, poor material uniformity remains a major problem preventing the widespread application of such detectors in gamma-ray spectroscopy. Uniform electron transport is critical for achieving good gamma-ray detection performance in the coplanar-grid configuration. We have investigated the use of alpha-particle response as a quick and simple electron transport uniformity screening technique for material selection, and as a method to study other spectral broadening mechanisms in coplanar-grid detectors. The method consists of uniformly illuminating, with an alpha-particle source, the cathode side of the CdZnTe crystal in either a planar or a coplanar-grid detector configuration. In the planar geometry, the variation in the measured pulse height is dictated in large part by the uniformity of the electron transport. An alpha-particle spectrum that has a single sharp peak with little background indicates uniform electron transport and, consequently, that the CdZnTe crystal should result in a coplanar-grid detector with good gamma-ray detection performance. In the coplanar geometry, the measured pulse-height variation provides information on additional sources of spectral broadening. In this paper we present the results of our study to measure the correlation between these simple alpha-particle measurements and the coplanar-grid gamma-ray detector response.
The coplanar-grid technique provides substantial spectral performance improvement over that of conventional detector designs and electronics when applied to gamma-ray detectors based on compound semiconductors. The technique realizes this improvement by measuring the difference between the induced charge signals from two interdigitated coplanar-grid electrodes. By adjusting the relative gain between the two grid signals prior to subtraction, the difference signal can be made less sensitive to the poor carrier transport properties of the detector material and thus improve the spectral response of the detector. In this paper, we discuss a variation of the coplanar-grid method in which the signal from only one grid electrode is read out. The signal response is optimized by changing the relative areas of the two grid electrodes and the bias applied across the detector. In this scheme, only one preamplifier is needed and signal subtraction is not necessary. This eliminates the electronic noise contribution from the additional preamplifier used in the normal coplanar-grid implementation, and conventional single- amplifier detector electronics can be used. Experimental results using CdZnTe detectors are presented.