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The Hard X-ray Detector

X-rays are famous for their ability to pass through objects, for example, human flesh. This ability, however, depends sensitively on the energy of the X-ray photons - the higher the energy, the better they are able to penetrate into objects. Astronomers refer to higher energy X-rays as "hard." So, the Suzaku instrument that measures higher energy X-rays, in this case between 10 and 600 keV or so, is called the Hard X-ray Detector (HXD). The HXD measures about 34 x 34 x 38cm.

Suzaku X-ray Hard X-ray Detector
The Suzaku Hard X-ray Detector. (Click for larger view.)
It is very difficult to focus high energy X-rays (the XRTs used with XRS and XIS won't work). Instead, the HXD uses a combination of detectors to measure X-rays from a source in the sky. One type of detector (made of a crystal called BGO) is used as a vertical shield, forming a series of "wells". Its purpose is to detect X-rays that don't come straight down the wells. At the bottom of the each well are two other types of detectors. At the very bottom is a detector called a GSO crystal, and above that is a detector called a Silicon PIN diode. The Silicon PIN detector is sensitive to X-rays with energies up to about 40 keV. At energies greater than this, X-rays pass through the Si PIN and are absorbed by the GSO crystal. The GSO crystal is sensitive to X-rays up to about 600 keV.

Any X-rays detected by the PIN and/or GSO, but not by the BGO, must have come stright down the well, and thus are coming from the object that the HXD is pointed at. X-rays that strike the walls of the well must have come from another direction, and the instrument electronics rejects them. This technique, called "active collimation," helps give the HXD very high sensitivity.

Publication Date: June 2005

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