Presentation on theme: "Dispersive property of a G-M tube HV - + In the proportional region a G-M tube has dispersive properties 0 250 500 750 1 10 2 10 4 10 6 10 810 tube voltage."— Presentation transcript:
dispersive property of a G-M tube HV - + In the proportional region a G-M tube has dispersive properties 0 250 500 750 1 10 2 10 4 10 6 10 810 tube voltage pulse size C
Solid Radiation Detectors Advantage: High density improves the efficiency of radiation absorption. Recombination is not a serious problem. Types: Scintillation detectors. Semiconductors detectors.
Excitation and Recombination in Solids energy Conduction Band Valence Band Crystal structure Electronic Band Structure electronhole defect A D
Scintillation energy Conduction Band Valence Band A D 1) Deliberately added impurities enhance trapping of charge carriers and recombination. 2) Excess energy is released in a form of visible radiation. 3) Due to the size of the band gap, the light is not absorbed in the crystal.
Properties of Selected Scintillators scintillatorDensity (g/cm 3 ) Emission (Å) Decay time (ns) Anthacene1.24440026 p-Terphenyl1.17410010 Stilbene1.1941008 NaI (Tl)3.6741000.25 ZnS (Ag)4.145009 Sodium iodide activated with thallium may produce one electron-hole pair for every 50 eV of radiation energy. The intensity of the emitted light is proportional to the energy of the radiation.
Scintillation Detector _ + 900 – 1500 V dynodes anode output signal scintillator photocathod e Scintillation detectors are suitable for radiation spectroscopy.
Scintillation Statistics channel counts Monoenergetic radiation results in a response distributed about the most probable value. The primary reasons are: The overall response of a scintillator detector obeys Poisson statistics, convergent to normal distribution for large numbers. FWHM random nature of electron-hole pair formation variations in energy distribution between light and other recombination mechanisms uneven efficiency of reaching the PM tube nonuniform distribution of sensitivity at the photocathode and dynodes.
charge collection in semiconductor detectors V - + i Preferably each generated carrier should be collected by its attracting electrode. The signal (fraction of collected carriers) depends on both the lifetime and mobility of the carriers. The equilibrium carrier concentration should be small (low conductivity). Detectors of this type must be cooled (with LN) for proper operation.
depletion layer semiconductor detectors V - + np depletion layer Low intrinsic concentration is achieved in the depletion region. The diode conducts only after electron-hole pairs are generated by the radiation in the depletion region.
surface-barrier semiconductor detectors np Small combined thickness of the n-layer and the gold contact results in very low radiation absorption in these two regions. (Front surface detector.) V - + output signal
Multi Channel Analyzer (MCA) Multi channel analyzer counts separately (in a channel) pulses with amplitudes in a specific range. channel number of pulses When connected to a dispersive detector, each channel resisters radiation particles with energy in a corresponding energy range.
- spectroscopy In a magnetic field, charged particle moves along a circular path determined by the magnetic field, mass and charge of the particle, and its kinetic energy (speed) G-M scaler ++