HW15-2c-4 - 8-2 Radiation Detection Chap. 8 increasesthe...

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Unformatted text preview: 8-2 Radiation Detection Chap. 8 increasesthe detector efficiency for charged particle detection since there is a higher chancea chargedparticle interacts in the tube gas. (c) Increasing the Z number of the wall material decreaseshe probability a t low-energy photon can reachthe interior of the detector through the wall. However, for a "thin" wall through which photons can pass, increasing Z increasesthe production of photoelectronsinto the tube's interior and, thus, the detector's efficiency is increased. 3. For I-MeV photons, sketch how the detector efficiencyof a proportional or GM chambervaries with the thickness of the cylindrical wall. Solution: For very thin walls, the photons will passthrough the detector with little chance of interaction and henceof being detected. As the wall increasesin thickness, more interactions occur in the wall that liberate electrons into the gaseous interior where an event is registered. Thus, the detector efficiency initially increasesas the wall thickness increases. However, for very large wall thicknesses,the wall begins to act as a shield to the tube interior, and fewer incident photons will reach the inner portions of the wall surface, where electron production and subsequentmultiplication via the avalancheprocess leads to a detector response. Thus, fewer photons are detected as the wall thickness increasesto very large values. July 24, 2002 ...
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