BAT Technical Description

The BAT's 32 768 pieces of $4 \times 4 \times 2$ mm CdZnTe (CZT) form a $1.2 \times 0.6$ m sensitive area in the detector plane. Groups of 128 detector elements are assembled into $8 \times 16$ arrays, each connected to 128-channel readout Application Specific Integrated Circuits (ASICs). Detector modules, each containing two such arrays, are further grouped by eights into blocks. This hierarchical structure, along with the forgiving nature of the coded aperture technique, means that the BAT can tolerate the loss of individual pixels, individual detector modules, and even whole blocks without losing the ability to detect bursts and determine locations. The CZT array will have a nominal operating temperature of 293 K, and its thermal gradients (temporal and spatial) will be kept to within one K. The typical bias voltage is $-200$ V, with a maximum of $-300$ V.

The BAT has a D-shaped coded aperture mask, made of approximately 54 000 lead tiles ( $5 \times 5 \times 1$ mm) mounted on a 5 cm thick composite honeycomb panel, which is mounted by composite fiber struts 1 meter above the detector plane. The BAT coded-aperture mask uses a completely random, 50% open-50% closed pattern, rather than the commonly used Uniformly Redundant Array pattern. The large FoV requires the aperture to be much larger than the detector plane. The detector plane is not uniform due to gaps between the detector modules. The mask has an area of 2.7 m$^2$, yielding a half-coded FoV of $100 \times 60$ degrees, or 1.4 steradians.

A graded-Z fringe shield, located both under the detector plane and surrounding the mask and and detector plane, reduces background from the isotropic cosmic diffuse flux and the anisotropic Earth albedo flux by approximately 95%. The shield is composed of layers of Pb, Ta, Sn, and Cu, which are thicker toward the bottom nearest the detector plane and thinner near the mask.

A figure-of-merit algorithm resides within the BAT flight software and decides if a burst detected by the BAT is worth requesting a slew maneuver by the spacecraft. This is described in detail in the Figure of Merit section of this Technical Handbook of the Swift Technical Handbook.