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SPI

Principal investigators
J.-P. Roques CESR Toulouse France	R. Diehl MPE Garching Germany

SPI

The spectrometer SPI (Spectrometer on INTEGRAL) measures gamma-ray energies with exceptional precision. It is 100 times more sensitive than the previous high spectral-resolution space instruments. SPI performs spectral analysis of gamma-ray point sources and extended regions over an energy range between 18 keV and 8 MeV with an unprecedented energy resolution of 2.2 keV (FWHM) at 1.33 MeV.

This is accomplished using an array of 19 hexagonal high purity germanium detectors cooled by a Stirling cooler system to an operating temperature of -188 °C (85 K). It makes the Spectrometer extremely heavy with a mass of 1300 kilograms.

SPI Mask

The total detection area is 500 square centimetres. A hexagonal coded aperture mask is located 1.7 m above the detection plane in order to image large regions of the sky (fully coded field of view = 16 degrees) with an angular resolution of 2 degrees. It is made of 3 centimetre thick tungsten and consists of 127 hexagonal elements, of which 63 are opaque, 64 transparent. The construction allows the imaging of large regions of the sky.

SPI Veto System

To reduce the background radiation, the detectors are shielded by bismuth germanate oxide crystals, these act as a veto (anticoincidence) system and extend around the bottom and side of the detector almost completely up to the coded mask. The aperture (and hence contribution by cosmic diffuse radiation) is limited to ~ 30°. A plastic veto is provided below the mask to further reduce the 511 keV background.

Table Showing Predicted Performance

Parameter	Value
Energy range	18 keV - 8 MeV
Detector area	500 cm²
Spectral resolution (E/ΔE @ 1 MeV)	~ 450 (i.e. 2.33 keV FWHM @ 1.33 MeV)
Field of view (fully coded)	14° flat-to-flat 16° corner-to-corner
Zero Coding	32° flat-to-flat 35° corner-to-corner
Angular resolution (point sources)	2.5° FWHM
Point Source Positioning	<1.3°
Narrow-line sensitivity - 3σ in 10⁶ s, @ 1 MeV - 3σ in 10⁶ s, @ 511 keV	2.4 x 10^-5 phs^-1cm^-2 4.6 x 10^-5 phs^-1cm^-2
Continuum sensitivity (3σ in 10⁶ s, @ 1 MeV)	8.8 x 10^-4 phs^-1cm^-2MeV^-1
Timing accuracy (3σ)	0.129 ms
Resources (following EID-A allocation)
Mass	1309 kg
Power (sun/eclipse)	385/110 W
Data rate	45 kps

	Overview
	IBIS

Last Update: 06 Sep 2011

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