Lecture17 - ENU 4612C/5615C Nuclear Radiation Detection and...

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Unformatted text preview: ENU 4612C/5615C Nuclear Radiation Detection and Instrumentation Lecture 17 Proportional Counters Gas-Filled Detectors 3. Proportional Counters Proportional Counters Design features Single pulse characteristics Pulse height spectra and counting curves Related detectors Pulse Mode Gas-Filled Detector 2 MeV 1 MeV 1 V 1 mV 1 V Applied voltage Ion saturation Proportional Limited prop. G-M region Typical pulse size (log scale) Simple Proportional Tube cathode anode wire V L R For cylindrical geometry: Electric Field m V / 5.18x10 (0.008) : surface inner At the 1cm b cm, 0.008 a 2000V, V : For 6 = = = = V a b r ( ) a b r V r / ln ) ( = Electric Field Configuration r E(r) r a minimum electric field for avalanche formation multiplication region r r E 1 ~ ) ( Small-diameter anode wire also confines gas multiplication to a small volume region. Therefore, each ion pair generated creates a Townsend Avalanche of equal intensity (energy information is preserved). Electric Field Configuration ionization particle path electron drift region multiplication region Sealed Tube End Design Active Volume Field Tube Cathode Anode Wire Insulator 2 Gas Flow Proportional Counter Anode Sample Gas Inlet Gas Outlet Must preserve free electrons transport (no negative ion information) Should promote only one avalanche per original ion pair (should reabsorb UV photons emitted by excited atoms) Fill Gas photons) UV (absorb ns Hydrocarbo gases Noble ) CH 10% Ar, (90% 10- P : Examples 4 Total charge per pulse: Diethorn model: Gas Multiplication Factor = ) ln( ln ln 2 ln ln ) ln( K a b a p V V a b V M V = applied voltage b = cathode radius a = anode radius p = gas pressure M e n Q = Space Charge Effects e e e e e e e Self-induced: positive ions influence further stages of the same avalanche General: cumulative effect of positive ions from many avalanches...
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Lecture17 - ENU 4612C/5615C Nuclear Radiation Detection and...

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