Advanced Compton Telescope

- instrument design -

The principle of Compton Imaging. By measuring the position and energy of each photon interaction, the initial photon direction can be determined to within an event circle - or an event arc - on the sky. 

Instrument Design Considerations

The most promising instrument design for fulfilling these science requirements is a Compton Telescope utilizing recent advances in detector technologies to improve over the Compton Telescope COMPTEL on CGRO. Modern 3-D position sensitive gamma-ray detectors, arranged in a compact, large-volume configuration, will improve efficiency by two orders of magnitude, provide a powerful new tool for background rejection, and utilize high spectral resolution to dramatically improve sensitivity, as well as accessing a new measurement quantity: gamma-ray polarization.

The principle of Compton imaging of gamma-ray photons is illustrated above: An incoming photon undergoes a Compton scatter, creating a recoil electron that induces the signal measured in the detector. The scattered photon then undergoes a series of one or more interactions which are also recorded. The total energy of the incoming photon can be determined, as well as its incident direction (to a circle or arc on the sky, depending on the instrument's capability to record recoil electron tracks). Two instrumental uncertainties contribute to the finite width of the event circle: the uncertainty due to the finite energy resolution, and the uncertainty due to the finite spatial resolution. There is also a fundamental limit on the width of the event circle set by Doppler broadening due to Compton scattering on bound electrons.

In order to be able to achieve the science goals set out above for ACT, the following science requirements were laid down for ACT:

ACT Science Requirements
Energy Range  0.2 - 30 MeV Compton mode
Energy Resolution  < 10keV FWHM @ 1 MeV
Field of View  > 4 steradian 
Angular Resolution  1 deg
Source Localization  5 arcmin for bright sources
Line Sensitivity
in 1.0E6 sec 
1.0E-7 ph/(cm2s) (narrow)
5.0E-7 ph/(cm2s) (broad)
Continuum Sensitivity  1.0E-5 ph/(cm2sMeV) @ 0.5 MeV
Polarization Sensitivity  1%, 2.0E-3 ph/(cm2sMeV)
10%, 2.0E-4 ph/(cm2sMeV)
These science requirements are expected to translate to an instrument with effective area on the order of 1000-3000 cm2, a position resolution in the detectors of 1mm3, energy resolution of 1% (0.5-2.0 MeV) or better, and possibly recoil electron tracking capabilities for electron energies < 0.5 MeV. The concept study will look at a variety of possible detector technologies for implementing such a Compton telescope. Candidate detectors include, but are not limited to, CZT strip detectors, Si strip detectors, Ge strip detectors, liquid Xe, and gaseous Xe (or Ar) microwell detectors.

The Vision Mission concept study will also investigate the mission requirements for ACT given the most feasible instrument implementations, and determine the requirements for on-board and ground data processing.

The ACT Vision Mission Study will result in a final report detailing both a reference concept design for the mission based on quantitative simulations and to lay out a roadmap for development of the technologies and mission elements necessary for flight. The roadmap will identify instrument development technologies and ACT mission support technologies that appear most probable for successfully contributing to an ACT mission. The roadmap will also recommend a schedule and budget for an advanced technology program that NASA should undertake.