Gamma and Cosmic Ray Astrophysics

 
 
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NRL Advanced Compton Telescope

The Advanced Compton Telescope (ACT) concept envisioned at NRL uses the 3-Compton approach to significantly extend the scientific investigations initiated with NASA's COMPTON Gamma Ray Observatory.    The objective is to directly observe the high energy nuclear processes occurring in astrophysical sources.   With dramatically improved sensitivity in the 300 keV-30 MeV region, the ACT will provide the first high resolution maps of the Galaxy in several radioactive lines.  ACT will also observe many tens of Type Ia supernovae per year, several novae, and provide order of magnitude improved capabilities for the study of compact galactic sources and active galactic nuclei.  
Advanced Compton Telescope Si(Li) detectors The baseline instrument (pictured above) is built from thick lithium drifted silicon detectors, and measures roughly 1 m x 1 m in frontal area.  The individual detectors are ~7 mm thick, and measure 10 x 10 cm in area using newly emerging technology in crystal growth and lithium drifted silicon, or  Si(Li).  Detectors are assembled in tower structures, each containing a small 4x4 array of detectors and stacked 24 layers deep.  Readout electronics for the detectors are distributed along the four side walls of each tower.  Cooling to an operating temperature of -40C is accomplished through a fluid loop.  Cooling pipes run vertically past the corners of the detectors, and are a structual element in the design.  The full instrument is composed of four identical towers.  The instrument shown here would require a total of 1536 detectors.   

Dramatic improvements in sensitivity over the original ATHENA concept are expected for two basic reasons:

  1. Factor of 10 higher efficiciency for detecting gamma rays
  2. Lower background due to full imaging of each individual interaction
A plot of the estimated ACT sensitivity is provided.

Silicon vs. Germanium

Variants of this design using using germanium strip detectors have also been considered.  There are advantages to either detector material.  Silicon has less "Doppler broadening" than germanium -- a process which adds a small uncertainty to the Compton scattering formula relating scatter angle and energy loss.  Silicon may also provide higher operating temperatures, making the instrument easier to build.  Germanium has the advantage of higher stopping power, providing a significant fraction of events which undergo total energy absorption, and a smaller average number of interaction for each event compared with silicon.  Germanium is also available in larger detector volumes.  Germanium has a slight edge in potential energy resolution.

Alternatives

Time projection chambers using high pressure gas such as Xenon are also being considered for this application by Columbia University.  The advantages are large, monolithic volumes with comparatively fewer electronics channels to process.  The key issues are Doppler broadening, particularly in Xenon, interaction rates, interaction energy threshold, and energy resolution.

Tracking of the recoil electron produced in a Compton scatter is also being studied at some institutions.  Tracking would be achieved using a stack of thin silicon detectors (0.3 to 0.75 mm thick).  The University of California, Riverside, and Max Planck Institute in Garching, Germany (MEGA mission concept) are funded to develop this concept.

A recent workshop held at NRL in May 2000 brought the gamma ray community together to begin an organized discussion and development of the key technologies.  Proceedings from this workshop are available online from the workshop web pages.

Links to:  Last revised: 13Feb 2002