gamma detectors - Med Phys 4R06/6R03 Radioisotopes and...

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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology 2-1 Chapter 2 General Properties of Radiation Detectors Ionizing radiation is most commonly detected by the charge created when radiation interacts with the detector. The definition is, after all, ionizing radiation . Crudely, the efficiency with which a detector measures a particular type of radiation depends on the efficiency with which the radiation type creates charge within the detector. When charged particles like protons or electrons are incident on the detector, they continuously interact with atomic electrons of the detector material and produce electron- ion (or electron-hole) pairs along their tracks. This initial charge produced by radiation is most important information and further processed by a proper signal processing electronics after collection. When neutral radiation fields like those consist of gamma-rays or neutrons are measured, a neutral particle interacts with the detector material and then the product secondary charged particle (electron, proton, alpha etc.) deposits its energy in the detector. 2.1. Operation modes Assume that a radiation interaction with a detector created charge Q within the detector volume. As the collection process of charge Q progresses, a current signal i(t) is induced at the collection electrode. i(t) i t dt Q tc ( ) = 0 t t c : charge collection time tc The detection event rate (or counting rate) is dependent on both radiation field and detector efficiency. If the event rate is not high, each detection event can be well separated and analyzed. Current flowing in detector i(t) time
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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology 2-2 A. Current mode A simple way of measuring a detector signal is current measurement. In this mode, a current meter is connected to the detector output. Since the current level is in pA or nA, a precise meter is required. Given that the current meter has a response time T, the observed current from a sequence of events at time t will be = T T t dt t i T t I ' ) ' ( 1 ) ( The response time is usually longer than the time between individual detection events, so that an average current is recorded at a time t. The current mode is used when event rates are very high, which makes a stable current. B. Pulse mode In the pulse mode, we preserve the information on energy and timing of individual events, i.e. the information on the signal amplitude and time of occurrence is preserved. The signal shape from a radiation detector depends on the electronics to which the detector is connected as well as the detector response. Often, the input stage of the electronics is an RC circuit. In the circuit, R represents the measuring circuit input resistance, C is the summed capacitance of the detector, the cable and the input capacitance of the preamp. V(t) is the time dependent voltage across the load resistor; V(t) is the signal that is produced.
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