6720 Lecture 2_radiation source

6720 Lecture 2_radiation source - Lecture II 1 2 3 4 5 6...

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1 PHYS 6720 - Lecture II 1 Lecture II Production of Radiations 1. Basic processes in x-ray production 2. X-ray tube: structure and design 3. Power circuitry of X-ray tube 4. Anode related image quality factors 5. Nuclear instability and gamma rays 6. Radioactivity PHYS 6720 - Lecture II 2 II-1 Basic processes in x-ray production Free electron generation from filament This is done by heating filament to a high temperature and can be described by a thermionic emission model ( Richardson-Dushman equation ) where J=electron current density (A/m 2 ) A=material constant ~1 × 10 6 (A/m 2 .K 2 ) T=filament temperature(K) k B =Boltzmann constant=1.38 × 10 -23 (J/K) W=work function of filament 2 exp{ / } B J AT W k T =− PHYS 6720 - Lecture II 3 Example Temprature (K) 0 500 1000 1500 2000 2500 3000 J(A/m 2 ) 0 2x10 3 4x10 3 6x10 3 8x10 3 10x10 3 Tungsten (W): A=0.6x10 6 , W=4.52eV, T m =3680K Cs on W: A=0.32x10 6 , W=1.36eV, T m =? PHYS 6720 - Lecture II 4 Electron acceleration Through an electric field between filament (W) in a cathode block (Ni) and anode Æ a potential difference (tube voltage V T ) of either DC voltage (kV) or rectified voltage (kV p ) needs to be maintained between the cathode and anode: 25 < V T < 150kV II-1 Basic processes in x-ray production PHYS 6720 - Lecture II 5 X-ray generation in anode (target) High energy free electrons converts their energy into x- ray photons mainly through two pathways: Bremsstrahlung + Characteristic radiation II-1 Basic processes in x-ray production universal to all atoms characteristic of atoms PHYS 6720 - Lecture II 6 (i) Bremsstrahlung – “braking radiation” I BS (E)=dP/dE=c 1 *Z*(E max -E) Å unfiltered output where E=hf=photon energy, E max =eV T , Z=atomic number of the anode material, c 1 =constant II-1 Basic processes in x-ray production I BS (E) E
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2 PHYS 6720 - Lecture II 7 (i) Bremsstrahlung – “braking radiation” II-1 Basic processes in x-ray production max 2 1m a x 0 () = / 2 E total BS PI E d E c Z E = Ze E PHYS 6720 - Lecture II 8 (ii) Characteristic radiation II-1 Basic processes in x-ray production Æ Electrons shells are designated K,L,M,N,O… Æ Orbital shells contain 2 electrons in the K, 8 in the L, 18 in the M, 32 in the N shell … Free electrons bombard the atoms on or below anode surface Æ inner shell electrons ejected Æ outer shell electrons fill in the vacancy and emit x-ray photons with energies characteristic of the atomic structure. z z z PHYS 6720 - Lecture II 9 (ii) Characteristic radiation II-1 Basic processes in x-ray production Molybdenum (Mo) Tungsten (W) Z4 2 7 4 E K {17.4keV, 19.8keV} {58.0keV, 67.7keV} E L 2.6keV 11keV T m 2890K 3680K c 250 J/kg.K 125 J/kg.K PHYS 6720 - Lecture II 10 Filtration All x-ray tubes has some filtration to remove low energy photons Æ Extremely important for medical x-ray sources since low- energy photons are strongly absorbed by tissues and thus can cause severe ionization damage – safety hazard Æ Low energy photons has small penetration depth and thus contribute little to detected signals.
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This note was uploaded on 04/25/2010 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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6720 Lecture 2_radiation source - Lecture II 1 2 3 4 5 6...

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