Adding a Gamma-Ray Lens to ACT - Looking Deeper into the Fires of Creation

Details Adding a Gamma-Ray Lens to ACT - Looking Deeper into the Fires of Creation Adding a Gamma-Ray Lens to ACT - Looking Deeper into the Fires of Creation

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004head....8.4102w&link_type=abstract

American Astronomical Society, HEAD meeting #8, #41.02; Bulletin of the American Astronomical Society, Vol. 36, p.1206

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Scientific paper

The ACT required gamma-ray line sensitivity of 10-7ph/(cm2s) is driven entirely by ACT's cornerstone goal - the systematic study of 56Co emission from supernovae of type Ia out beyond the Virgo cluster. Even with the best detector technologies available today, aiming for this sensitivity is ambitious in the background-dominated MeV regime, where increasing the detector volume always incurs a corresponding increase in the background level that has to be subsequently suppressed. Concentrating the photons from the source region using a collection area of several hundred cm2 onto a small detector surface area, on the order of a cm2, without compromising the primary Compton telescope's field of view and energy range, is the way out of this dilemma. At least two methods for focusing MeV gamma-rays exist: Laue lenses and Fresnel phase-shift lenses. Laue lenses use diffraction on crystal planes in the volume of a crystal (similar to Bragg diffraction on its surface) to concentrate the photons onto a small detector area. Focal lengths are on the order of 100 m, and a small prototype Ge crystal lens (CLAIRE) has been tested on a balloon at CESR; other crystal materials are under study. Concepts for Laue lenses with 100 keV-wide energy bands exist. Fresnel lenses have the potential of near-100% efficiency and micro-arcsecond resolution, but the disadvantage of focal lengths on the order of a million km, and have not yet been demonstrated at MeV energies. Either lens concept would, in addition to increasing ACT's point source sensitivity for a given object in certain energy bands by decoupling photon collection area and detector area, also overcome the Compton telescope's intrinsic angular resolution limit (on the order of 1 deg) which is due to Doppler broadening.

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