LAT Hardware

Key LAT hardware consists of:

• ACD - Anti-Coincidence Detector (covered by a Thermal Blanket)
• TKR - Tracker
• CAL - Calorimeter

Charged Particle Detection vs. Backsplash. Charged particles striking the LAT produce "tracks", or signals that are "lined up" in the segments of the TKR and CAL. If an event is caused by a charged particle, the track will also point towards the ACD segment that was struck.

However, when a high-energy gamma-ray strikes the LAT, it can produce secondary photons that "splash" out of the CAL and can trigger an ACD file, and the track from this "backsplash" will not line up with the ACD tile that was struck by the gamma-ray. Therefore, the coincident presence of backsplash may be the valuable signature of a high-energy gamma-ray, in which case, the veto signal issued by the ACD whenever a tile is struck is ignored, end the event is read out of the LAT, stored in the LAT's solid-state recorder (SSR), and ultimately transmitted to the ground software for further analysis.

Note: If the ACD were not segmented, valuable, high energy gamma-rays that produced the backsplash signal would be "self-vetoed".

A Thermal Blanket covers the Anti-Coincidence Detector, protecting the detectors against temperature fluctuations in space. In order to maintain transparency to gamma-rays by minimizing pair conversion, the thickness of the GLAST blanket is 0.3 g/cm², or approximately 0.8% of a radiation length.

Tracker (TKR)

The TKR, a precision Si-strip tracker:

• measures photon direction, and
• performs Gamma ID.

It is used to precisely identify the impact point and the path length of the ions in the plastic scintillators of the ACD and the CsI logs of the CAL.

The TKR consists of 16 identical tower modules arranged in a 4x4 array. Each tower has a 36x36 cm surface area and is equipped with 18 x and 18 y planes of silicon-strip detectors and tungsten foils acting as gamma-ray converters. The detectors are supported by a carbon composite structure composed of a stack of 19 panels called trays. Thickness of the tungsten foils is 3% radiation length for the upper 12 trays (light-converter trays), and 18% for the next 4 trays (thick-converter trays). The last 3 trays do not have tungsten foils.

Each tray is rotated 90 degrees with respect to the one above or below. Detectors on the bottom of a tray combine with those on the top of the tray below to form a 90 degree stereo x,y pair with a 2 mm gap between them and with the tungsten converter foils located just above.

All but the top and bottom trays of the tower have silicon strip detectors on both the upper and lower faces. Hence, charged particles pass through up to 36 layers of position-sensitive detectors, leaving behind tracks pointing back toward the origin of the gamma-ray.

Calorimeter (CAL)

The CAL is a Hodoscopic CsI calorimeter consisting of a 4x4 array of CsI(Tl) crystals in 8 layers mounted beneath each of the 16 TKR towers (12 crystals in each of 8 layers X 16 towers = 1536 crystals total. The CAL:

• measures photon energy, and
• images the shower,

thereby assisting in track correlation for background rejection, and improving energy measurement by shower profile fitting.

Calorimeter crystals are arranged in alternating layers. In the top layer, the long axis of each crystal is parallel to the X-axis, in the next, the long axis of each crystal is parallel to the Y-axis, on so on. The former are referred to as X layers and the latter as Y layers.

Each calorimeter module consists of a segmented thallium-doped cesium iodide, CsI(Tl), scintillation crystal calorimeter segmented into discrete detector elements. Light from an energy deposition in a crystal is read out by PIN photodiodes mounted at each end of the crystal, and each crystal end has its own ADC so that the required 192 analog-to-digital conversions are performed simultaneously. The spectral response of the PIN photodiodes provides a large primary signal (~5,000 electrons collected in 1.5 cm diode per MeV deposited), yielding a high intrinsic spectral resolution. The digitized energy measurements are transmitted to the Tower Electronics Module (TEM) mounted on the base plate of the calorimeter modules.

The difference in light levels seen at the two ends of the crystal also provides a determination of the position of the energy deposition along the CsI crystal. The position resolution of this imaging method ranges from a few millimeters for low energy depositions (~10 MeV) to a fraction of a millimeter for large energy depositions (>1 GeV).

The design takes advantage of the fact that Hadronic interactions tend to be diffuse and spread among many crystals, while electromagnetic showers are compact [typically a Moliere radius wide (~2 cm in CsI)]. The energy pattern of cosmic rays tend to have wider deposition patterns than photon-induced showers, and can be eliminated.

The Data Acquisition (DAQ) system and computers located below the calorimeter modules, and an aluminum structural support Grid supports the 16 towers, as well as the DAQ. (See Silicon Tracker Readout Electronics of the Gamma-Ray Large Area Space Telescope.)

Calorimeter calibrations are intended to allow the calculation of various calorimeter responses
which, in turn, allow conversion of measurements in “instrument units” such as ADC units into
“physical units” (e.g., MeV). This is accomplished by performing measurements that produce
known input and measuring the response. The primary quantities that require calibration are the:

• Energy scale (i.e., what deposited energy corresponds to a given output in ADC units).

• Event position scale (i.e., where along a crystal was the energy deposited).

• Trigger threshold performance (i.e., position and efficiency of the trigger threshold).