1. Scintillation Detectors

For energies higher than a few tens of keV, the X-ray cross-section of gas-filled detectors becomes too small. Detectors that use scintillating crystals as the absorbing volume have better efficiency. Background rejection is obtained by surrounding the detector with a guard scintillator. Photomultiplier tubes detect the scintillations from both crystals. X-rays from the source being observed enter the detector through a mechanical collimator and do not pass through the guard volume. Events resulting in scintillations from both volumes can be recognised by their pulse shape and can thus be rejected. Such an arrangement is known as a phosphor sandwich or phoswich detector.