The High Energy Telescope (HET) Anti-Coincidence System (HACOS) uses sodium iodide (NaI) crystals coupled with photomultiplier tube detectors to reject events arising from high-energy particle radiation (cosmic rays) that are detected in the HET detectors. Passive shielding is employed to stop low-energy cosmic rays. The HACOS active shielding can also be used to detect gamma rays up to roughly 20 MeV, thereby extending the HET's effective energy range.
When an X-ray photon is absorbed in an HET CZT detector it creates an electrical signal which is processed as a valid X-ray event. Cosmic rays, which are extremely energetic atomic nuclei, can also create electrical signals in CZT detectors that mimic X-ray events. To reduce this particle-induced background noise, EXIST employs both passive and active shielding. A schematic of the shielding on one side of an HET sub-telescope is shown at right. NaI shielding is also used to cover the bottom of each sub-telescope (see sub-telescope schematic). Note that some dimensions are out of date, but the basic design is correct.
Passive shielding is used to stop low-energy cosmic rays before they hit the CZT detectors. EXIST uses layered sheets of lead (Pb), tin (Sn) and copper (Cu). This layered approach is called "Graded-Z" shielding, which refers to the order and atomic number (Z) of those metals (Z equals 82 for lead, 50 for tin, and 29 for copper). Materials with larger Z have greater stopping power, and lead is used for the outermost layer. When a low-energy cosmic ray hits the lead it will be absorbed and ionize a lead atom which then emits an X-ray at the "characteristic energy" of lead, 88 keV, in a random direction. Lead does not absorb its own X-rays very well, so to prevent any of those X-rays from getting through the shielding, a layer of tin comes next. After absorbing the 88-keV X-ray, the tin may then emit a 29-keV X-ray. A layer of copper comes last. The few copper X-rays that reach the CZT detector are too low in energy (9 keV) to cause a problem.
Passive shielding can stop low-energy particles, but impractical thicknesses are needed to stop high-energy cosmic rays. The HET, therefore, also uses active shielding in the form of an anti-coincidence system emplying sodium iodide (NaI) scintillators. When an energetic cosmic ray blasts through the NaI it will ionize atoms all along its path. The NaI will then "scintillate" and emit visible light photons which are detected by light-sensitive photomultiplier tubes. If the cosmic ray is directed such that it also passes through a CZT detector, the CZT and NaI/photomultipler electrical signals will be virtually coincident in time. Coincident signals therefore indicate a cosmic ray event, and the event is discarded.
At photon energies greater than roughly 500 keV, the tungsten masks on the front of each HET sub-telescope and the CZT detectors become increasingly transparent. The HACOS NaI crystals, however, can be used to provide useful spectra up to approximately 20 MeV, with a spatial resolution roughly equal to the collimation of each sub-telescope (~20°). HACOS is therefore very useful in characterizing the high-energy spectra of gamma ray bursts (GRBs), for which accurate positions will be provided by the HET and LET.