Low Energy Gamma-Ray Imager (LEGRI)
The latest (as of December 4, 1997)
From Fernando Ballesteros,
Universidad de Valencia, Spain
LEGRI was successfully launched on April 21st, 1997, on a Pegasus XL rocket.
The instrument was activated on May 19, 1997. The technical evaluation as
well as the scientific analysis is ongoing. One of the technical mission
goals is the measurement of the particle and gamma induced background levels.
These are higher than anticipated beforehand. At the moment less than a quarter
of the detector array is fully operational which hampers the analysis of
image data. Nevertheless, here is a preliminary deconvolved image of an
observation of the Crab field, generated by Paul Connell of the University
of Birmingham, UK:
Courtesy Fernando Ballesteros,
Universidad de Valencia, Spain
LEGRI was proposed in response to the Spanish Instituto Nacional de Technicas
Aerospaciales (INTA) 1993 call for proposals for the first MINISAT mission
(MSAT1). The aim of the proposed mission is the demonstration of the
technological feasibility of the construction of new generation gamma-ray
telescopes, optimized for low energy (10-100 keV) astronomy, using 80 HgI2
detectors and 20 CZT pixels. This material provides an excellent response in the
10-100 keV range, with the best presently achieved efficiency/volume ratio.
The 10-100 keV region contains unique, not fully explored, astrophysical
information regarding nuclear excitation processes, radioactivity,
cyclotron emission and absorption processes
The MURA aperture pattern employed for LEGRI (courtesy Dr. G. Skinner, B'ham).
LEGRI is a prototype of a gamma-ray telescope with high resolution imaging
(20 arcmins source location capability), medium resolution spectroscopy
(4 keV at 30 keV), and high continuum and broad line sensitivity in the
10-100 keV spectral region. The estimated continuum sensitivity of
10 mCrab at 30 keV allows performing continuous surveys of selected
areas and deep measurements of specific fields.
The detector plane is a a mosaic of 10X10 HgI2 crystals and CZT elements,
the crystals each 0.5 cm thick
with an active area of 1 square cm. A coded aperture mask located 54 cm
from the detector plane provides the high quality imaging capability. The
main detector elements are shielded on the bottom and sides by a tbd box
and above by a tantalum collimator and aluminum window.
The data supplied by LEGRI will allow the definition of the range of
application of HgI2 detectors in astrophysics, based on the HgI2 semiconductor
stability, radiation damage, and survival capability under space conditions.
Furthermore, they will provide a good scientific output in the very
interesting low energy gamma-ray/hard X-ray spectral regions.
Characteristics of LEGRI
Detector type HgI2 + CZT
Active Detector area 100 cm**2
Distance Aperture-Detector 54 cm
Field of View 24 (PCFOV), 11 degrees (FCFOV)
Mask element size 24 X 24 mm**2
Angular Resolution (FWHM), on-axis 2.5 degrees (20 arcmins SLA)
Active photon energy range 10-200 keV
Photon energy resolution (FWHM) 13% at 30 keV
Continuum sensitivity 3 mCrab @ 30 keV and 10**5 s (3sigma)
Orbit LEO 600 km and 28 degrees inclination
LEGRI is a collaboration between the
University of Valencia (Spain) (http://www.uv.es),
University of Alicante (Spain),
Centro de Investigaciones Energeticas, Medioambientales y Technologicas
Instituto Nacional de Technica Aerospacial (INTA, Spain),
University of Southampton (UK) (http://www.phys.soton.ac.uk)
,University of Birmingham (UK) (http://www.sr.bham.ac.uk) and
Rutherford Appleton Laboratory (UK) (http://star-www.rl.ac.uk/). The PI of LEGRI is Dr. V. Reglero (firstname.lastname@example.org).
The above info is an excerpt from the
Instrument Definition Document (Version 2),
edited by V. Reglero (PI), and from a paper by Ballesteros et al., in
Proceedings of a workshop on "Imaging in High_energy Astrophysics",
Capri, September 1994 (preprint).
Go to LEGRI home page at University of Valencia (http://pollux.uv.es/legri.htm).