Collimators
The field-of-view of the HEXTE phoswich detectors is defined through the passive collimation of
incident x-rays by a honeycomb structure of hollow hexagonal tubes, which are constructed from a
composite of 94% lead and 6% antimony (for strength). The structure is epoxied within a magnetic
shield housing (see Figure 5.2). Due to the manufacturing process the walls of each hexagonal tube
are 0.004 in thick on four of the six edges, and twice that on the other two. Thus, the mean wall thick
ness is 0.005 in. The collimator tubes are 7.14 in long and 0.125 in across as measured from flat-to-
flat of the hexagons. This gives a Full Width at Half Maximum (FWHM) response of about 1° and a
Full Width at Zero Intensity (FWZI) response of about 2.2° diameter; the response as a function of
off-axis angle is shown in Figure 5.3. The open fraction of the collimator assembly is approximately
85%, such that each detector has roughly 225 cm² of face-on open area to the sky when viewed
through its collimator.
The detector crystal housing also contains a 0.2-in thick ring of lead that suppresses high energy x-
rays incident on the crystals from the side. Similarly, the collimator housing incorporates a lead ring
to block radiation entering obliquely close to its base.
Phoswich Detectors HEXTE Clusters