The Advanced Compton Telescope (ACT) should provide well over an order-of-magnitude improvement in sensitivity compared to other previous or planned instruments in low-energy gamma-ray astronomy. This will be needed in the study of the nuclear line/MeV region of the gamma-ray spectrum. Such an instrument covers a broad range of science objectives, ranging from the study the 56Ni light curves of supernovae and provide measurements of supernova dynamics, to 26Al, 22Na, and 60Fe maps of the galaxy, and the first gamma-ray polarization observations probing the geometry of the emission regions of a variety of objects such as AGN, pulsars, and gamma ray bursts. These objectives depend critically on the sensitivity that can be achieved. We present a study of the sensitivity that can be achieved by the ACT, considering estimates of backgrounds, position resolution, energy resolution, Doppler broadening, and recoil electron tracking. Efficiency questions are considered that arise from passive materials within the active volume and track reconstruction. A sensitivity estimate for ACT is presented for a reasonable instrument size and configuration.
In addition to high resolving power in the traditional x-ray band, the Constellation X-ray scientific goals require broad bandpass, with response extending to E >= 40 keV. To achieve this objective, Constellation-X will incorporate a hard x-ray telescope (HXT) based on depth graded multilayer- coated grazing incidence optics and position-sensitive solid state detectors. This paper describes the HXT performance requires, provides an overview of the HXT optics and detector technology development efforts, and present example designs.
We have investigated, both experimentally and theoretically, how to reconstruct in 3D the interaction positions for (gamma) -rays penetrating into a double-sized Ge cross trip detector. We found that when a suitable geometry is used, the 3D-reconstruction problem can be reduced to three 1D ones, which greatly simplifies the task. We report measurements on a 10mm thick detector with 2mm strip pitch, showing that at least 2mm position resolution can easily be achieved perpendicular to the detector plane. While the in- plane resolution is presently limited to the strip pitch we present work on progress in developing algorithms to improve this. This includes in particular the expected effects of the electronics and the interstrip capacitance on the signal shapes. Finally, we present captured waveforms that indicate the possibility of reconstructing more complex events such as Compton scattering.
The Monolithic Systems Development Group at the Oak Ridge National Laboratory has been greatly involved in custom mixed-mode integrated circuit development for the PHIENIX detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and position-sensitive germanium spectrometer front-ends for the Naval Research Laboratory (NRL). This paper will outline the work done for both PHENIX and the Naval Research Laboratory in the area of full-custom, mixed-signal CMOS integrated electronics. The PHENIX detector is a large multi-component detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. PHENIX has over 400,000 channels of electronics, most of which is implemented using custom integrated circuits. We presently have responsibility for developing and manufacturing electronics for the event vertexfinding subsystem, the pads tracking subsystem, the electromagnetic calorimeter subsystem, and the muon tracking/identification subsystems. We have developed an architecture utilizing simultaneous read-write analog memories used throughout the detector that allows data to be continuously taken even during event readout (a deadtime-less system). The manufacturing technologies being used range from multi-layer printed-circuit boards to multi-layer, multi-chip modules (MCMs). The germanium spectrometer electronics for the Naval Research Laboratory consist of low-noise preamplifier-shapers-peak stretchers and discriminators. The preamplifiers have been optimized for detector capacitances of approximately 10 pF and shaping times of 5-10 .ts. This paper will present the architectures chosen for the various PHENIX detectors which include position-sensitive silicon, capacitive pixel, and phototube detectors, and performance results for the subsystems as well as a system description of the NRL germanium strip system and its performance. The performance of the custom preamplifiers, discriminators, analog memories, analog-digital converters, and control circuitry for all systems will be presented.
A new balloon instrument, the advanced thin ionization calorimeter (ATIC), is currently under development by an international collaboration involving researchers in the U.S., Germany, Korea, Russia and the Ukraine. The instrument will be used, in a series of long duration balloon flights, to investigate the charge composition and energy spectra of primary cosmic rays over the energy range from about 1010 to 1014 eV. The ATIC instrument is designed around a new technology, fully active Bismuth Germanate (BGO) ionization calorimeter that is used to measure the energy deposited by the cascades formed by particles interacting in an approximately 1 proton interaction length thick carbon target. The charge module comprises a highly segmented, triply redundant set of detectors (scintillator, silicon matrix and Cherenkov) that together give good incident charge resolution plus rejection of the 'backscattered' particles from the interaction. Trajectory information is obtained both from scintillator layers and from the cascade profile throughout the BGO calorimeter. This instrument is specifically designed to take advantage of the existing NASA long duration balloon flight capability in Antarctica and/or the Northern Hemisphere. The ATIC instrumentation is presented here, while a companion paper at this conference discusses the expected performance.
We present an instrument concept called GIPSI that uses germanium strip detectors in an imaging system to provide narrow line sensitivity less than 8.0 multiplied by 10-6 gamma cm-2s-1 at 100 keV in a 2 week exposure (3 sigma), and which has a point spread function (spatial resolution of approximately 20 arc minutes rms. The germanium strip detectors also make an excellent polarimeter by capitalizing on the angular dependence of the Compton scattering cross section. Gamma-ray polarimetry in the energy band around 60 - 300 keV is an interesting area of high energy astrophysics where observations have not been possible with the technologies employed in current and past space missions. We have tested a prototype detector with polarized beams and have measured a modulation factor of approximately 0.8 at 100 keV. A sensitive instrument can be realized on a modest space mission or a long duration balloon flight. Linear polarization can be detected in sources such as the Crab Pulsar, Cen A, Cyg X-1, and solar flares down to less than 5% of the source flux. The proposed instrument would have a collecting area of 400 cm2.
An advanced thin ionization calorimeter (ATIC) will be used to investigate the charge composition and energy spectra of ultrahigh energy primary cosmic rays in a series of long- duration balloon flights. While obtaining new high priority scientific results, this balloon payload can also serve as a proof of concept for a BGO calorimeter-based instrument on the International Space Station. The ATIC technical details are presented in a companion paper at this conference. Here we discuss the expected performance of the instrument based on a GEANT code developed for simulating nuclear- electromagnetic cascades initiated by protons. For simulations of helium and heavy nuclei, a nucleus-nucleus interaction event generator LUCIAE was linked to the GEANT based program. Using these models, the design of the ATIC detector system has been optimized by simulating the instrument response to particles of different charges over the energy range to be covered. Results of these simulations are presented and discussed.
Burst locations with an arc second telescope (BLAST) is a new mission concept being studied for NASA's medium explorer (MIDEX) mission opportunities. The principal scientific objectives of the BLAST mission are (1) to localize gamma- ray burst (GRB) positions to arcsec accuracy; (2) to search for enhancements in the rate of GRBs toward M31; and (3) to conduct the most sensitive sky survey to date of x-ray sources in the 7 - 200 keV regime. These objectives are achieved using a large array of position-sensitive scintillation detectors with a total area of 17,000 cm2. This array is combined with a large field of view telescope (greater than 1 steradian) comprising two separate imaging systems. A coded aperture telescope provides arcminute source localization. For low energy x-rays (less than 50 keV), the aperture is also defined by phase modulation grids with provide complementary arcsecond information. The grid system consists of two aperture planes with 'checker board' patterns of slightly different pitch. The beating between the two grid pitches casts a broad interference pattern on the detector plane. Determining the phase of this interference pattern in both coordinates gives the location of a point source source in the sky, with aliased positions at approximately 1 arcmin spacing. The arcmin ambiguity is resolved by the coded aperture image. BLAST has a sensitivity to bursts of 0.03 photons cm-2 s-1, almost ten times more sensitive than BATSE. We expect to position 20 bursts per year to better than 2 arcsec accuracy and 35 bursts per year to better than 5 arcsec. BLAST will provide an all sky survey in hard x-rays with a sensitivity of 0.2 milliCrab at low energies.
The Hard X-Ray Telescope was selected for study as a possible new intermediate size mission for the early 21st century. Its principal attributes are: (1) multiwavelength observing with a system of focussing telescopes that collectively observe from the UV to over 1 MeV, (2) much higher sensitivity and much better angular resolution in the 10 - 100 keV band, and (3) higher sensitivity for detecting gamma ray lines of known energy in the 100 keV to 1 MeV band. This paper emphasizes the mission aspects of the concept study such as the payload configuration and launch vehicle. An engineering team at the Marshall Space Center is participating in these two key aspects of the study.
We present a space mission concept for a low energy gamma-ray telescope, ATHENA, which is under investigation as the next major advance in gamma-ray spectroscopy following the current COMPTON Gamma Ray Observatory and the planned INTEGRAL missions. The instrument covers the nuclear line emission energy domain with dramatically improved sensitivity and spectral resolution. The baseline configuration combines a high resolution Compton telescope constructed from Ge planar strip detectors for the 0.3 - 10 MeV energy region with a coded-aperture system for the 10 - 200 keV domain. The Ge Compton telescope provides a broad field of view with exceptional spectral and imaging resolution. The requirements, capabilities and simulations of ATHENA are discussed.
Germanium strip detectors combine high quality spectral resolution with two-dimensional positioning of gamma-ray interactions. Readout is accomplished using crossed electrodes on opposite faces of a planar germanium detector. Potential astrophysics applications include focal plane detectors for coded-aperture or grazing incidence x-ray mirror imagers, and as detection elements of a high resolution Compton telescope. We report on test results of two germanium strips detectors, one with 2 mm position resolution, the other with 9 mm. We discuss general device performance in terms of energy and position resolution, crosstalk effects, potential applications, and a demonstration of imaging properties.