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 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 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
future.
KEYWORDS: Calibration, Sensors, Telescopes, Space telescopes, Monte Carlo methods, Gamma radiation, Germanium, Data conversion, 3D metrology, Polarization
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
mass model.
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 mm3, and 10x10x10 mm3 monolithic pixellated
detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.
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 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.
Recent progress has been made in the development of the Modified Vertical Bridgman (MVB) technique for the growths of 3-inch diameter CZT crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. 40% and 80% of the ingots have the single crystal volumes over 300cm3 and 100 cm3 per ingot respectively. Defects (such as Cd-vacancies, Indium dopants and purity) in CZT have been systematically studied. Detectors fabricated from these CZT ingots showed sharp energy resolution and good uniformity.
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.
Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in3) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.
KEYWORDS: Sensors, Calibration, Germanium, Electrodes, Spectral resolution, Monte Carlo methods, Prototyping, Electronics, Telescopes, Signal detection
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.
A germanium-detector based, gamma-ray imaging system has been designed, fabricated, and tested. The detector, cryostat, electronics, readout, and imaging software are discussed. An 11 millimeter thick, 2 millimeter pitch 19x19 orthogonal strip planar germanium detector is used in front of a coaxial detector to provide broad energy coverage. The planar detector was fabricated using amorphous germanium contacts. Each channel is read out with a compact, low noise external FET preamplifier specially designed for this detector. A bank of shaping amplifiers, fast amplifiers, and fast leading edge discriminators were designed and fabricated to process the signals from preamplifiers. The readout system coordinates time coincident x-y strip addresses with an x-strip spectroscopy signal and a spectroscopy signal from the coaxial detector. This information is sent to a computer where an image is formed. Preliminary shadow and pinhole images demonstrate the viability of a germanium based imaging system. The excellent energy resolution of the germanium detector system provides isotopic imaging.
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.
Said Slassi-Sennou, Steven Boggs, Bryon Philhour, Robert Campbell, Paul Feffer, Robert Lin, Steven McBride, Henry Primbsch, Chris Cork, Fred Goulding, Paul Luke, Norman Madden, Don Malone, Richard Pehl
KEYWORDS: Sensors, Germanium, Gamma radiation, Shape analysis, Electronics, Signal detection, Spectrometers, Atmospheric particles, Monte Carlo methods, Space operations
For actively shielded, narrow aperture germanium spectrometers at balloon or spacecraft altitudes, the beta decay of radio-active nuclei is the dominant source of background in the 0.2 to 2 MeV energy range. This component of the background is internal to the germanium detectors (GeDs) and results from the activation of Ge nuclei by cosmic ray secondaries. The sensitivity of GeD spectrometers can be improved by rejecting beta-decay events, which deposit energy at a single site in the detector, while retaining photon events, which are predominantly multiple site at these energies. Pulse shape discrimination (PSD) techniques can distinguish between single- and multiple-site events by analyzing the shape of GeDs' current pulses. Here we present results of laboratory tests of PSD with a newly developed narrow-inner-bore (0.6 cm diameter) coaxial GeD and compare them to numerical simulations.
Michael Pelling, Paul Feffer, Kevin Hurley, Sharad Kane, Robert Lin, Steven McBride, J. Primbsch, David Smith, K. Youseffi, G. Zimmer, F. Cotin, J. Lavigne, G. Rouaix, Said Slassi-Sennou, Gilbert Vedrenne, Richard Pehl, Chris Cork, Paul Luke, Norman Madden, Don Malone
The elements of a high resolution gamma-ray spectrometer, developed for observations of solar flares, are described. Emphasis is given to those aspects of the system that relate to its operation on a long duration balloon platform. The performance of the system observed in its first flight, launched from McMurdo Station, Antarctica on 10 January, 1992, is discussed. Background characteristics of the antarctic balloon environment are compared with those observed in conventional mid-latitude balloon flights and the general advantages of long duration ballooning are discussed.
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