SAX: Phoswich Detector System (PDS)

This is only a general description of the instrument. Please, refer to the Instrument Home Pages for updated information.

• The SAX PDS has an energy range of 15-300 keV and is dedicated to celestial X-ray source studies in the upper range of the energy band of the narrow field of view instruments.
• To achieve high sensitivity in the discrimination of celestial X-ray sources from the background, the PDS employs:

  

  • four inorganic scintillation crystal units - called phoswiches;
  • two hexagonal cell collimator arrays each with a 1.5 degrees field of view employed as in the HP-GSPC;
  • four lateral active shields and one top active shield.

• Each phoswich contains a thin front primary crystal of Thallium doped Sodium Iodide - NaI(Tl) - and thick rear crystal of Sodium doped Cesium Iodide - CsI(Na). These crystals have substantially different scintillation decay times. X-rays arriving in the instrument from a celestial source under observation will mainly interact, producing scintillation light, in the front primary crystal and can be discriminated against the background arriving from all directions, seen by the rear crystal, by analysis of the shape of the pulse from the photomultiplier tube attached to the crystal unit. The four active lateral shields again made of CsI (Na), and the top active shield of plastic scintillator which detects charged particles, are used to further discriminate against background, increasing the instrument sensitivity.
• A phoswich signal is correlated to an X-ray photon from cosmic sources when: a photon is coming from the top within the field of view, is detected only by one phoswich unit, and with no detection by either the lateral shields, the top shield or the other phoswiches.
• In addition the lateral shields are employed to detect gamma-ray bursts in the 60-600 keV energy range. Apart from the intensity and temporal evolution of these bursts, their rough direction of origin can be determined.

This file was last modified on Wednesday, 02-Sep-1998 17:28:56 CEST by Mauro Orlandini