X-Ray Tubes PDF

THE X-RAY TUBE RADT 1010  PRODUCTION OF X-RAYS Source of electrons - electrons going in Target - on anode High-voltage - kV...

THE X-RAY TUBE RADT 1010  PRODUCTION OF X-RAYS Source of electrons - electrons going in Target - on anode High-voltage - kV Vacuum  # * TEST TUBE COMPONENTS Loading… I Dark blue line is the envelope  ENVELOPE Pyrex glass or metal 10” long 6” central diameter 2” peripheral diameter Window - allows light to come through Vacuum - All interactions happen within the envelope -Handles high heat most are and made glass matter - up of  PROTECTIVE HOUSING Controls leakage and scatter radiation Isolates high voltages Provides means to cool tube [bc 99 % heat] Loading…  CONTROL OF LEAKAGE RADIATION AND SCATTER RADIATION Housing made of lead-lined cast steel & Leakage radiation limit 100 milliroentgens per hour (mr/hr) at 1 meter (m) doesn't matter  HIGH-VOLTAGE ISOLATION AND TUBE COOLING Dielectric oil -doesn't respond to heat Insulates Promotes cooling Sometimes circulated through heat exchanger Air fan ) ↳ shouldn't be on 24/7 TUBE COMPONENTS housing  cathode is negative CATHODE ASSEMBLY Filament(s) Focusing cup Associated wiring Has an electric current blut Light eates current -creates  FILAMENT Coil of thoriated tungsten High atomic- number 0.1 - 0.2 millimeter (mm) thick 1 - 2 mm wide it matter Do 7 - 15 mm long Filament length and width impact recorded detail (small vs large focal spot) - OID ↳ equivalent focal to small spot Must provide enough resistance to e- flow so that heat O produced will cause thermionic emission (occurs at 2200 C) Boiling off electrons -  FILAMENT MATERIAL Tungsten selected due to: High melting point Difficult to vaporize Loading… Rhenium and molybdenum are also good choices  DUAL FOCUS ARRANGEMENTS ↳ We focus e so et bunch together Focusing Cup  THERMIONIC EMISSION FilamentCurrent = mA Filament heated Causes electrons to be released from filament When high voltage released at exposure, e- are driven toward anode e-just hangout ↳ Thermionic emission  TUBE FAILURE Tube arcing Vaporized tungsten collection on envelope Filament breakage Often due to boost and hold ↳ Don't had exposure button Avoid holding exposure switch halfway down for longer than needed; whenever possible, press exposure switch all the way down at once.  FOCUSING CUP Anode focusing CUD  FOCUSING CUP more negative Composed of nickel (or molybdenum) Low negative potential applied Biased focusing cup (more neg. Than filament) Compresses thermionic cloud Space charge effect increasing - ma , producing more electrons Maximum ma 1,000 – 1,200 - Limited Saturation current ↳ There effect is a electrons point where KV doesn't KV – quality & quantity ↳ 40k and up  ANODE ASSEMBLY Three functions: Target surface Conducts high voltage Serves as primary thermal conductor  STATIONARY VERSUS ROTATING ANODE (pentistry) stationary Rotating STATIONARY VERSUS ROTATING ANODE & be -reality of movement TARGET  ROTATING ANODE ASSEMBLY Components: Anode Instead oO f il Stator Rotor - is what turns Induction motor ↳ parts insideoutside tube  STATOR Located outside envelope Bank of electromagnets stator attracted magnetism is - c re a te s a Force makes it Spin  ROTOR Copper cylinder connected to anode disk by molybdenum stem Turns when stator energized 3200-3600 rpm 10000-20000 rpm high speed anodes Ball bearings STATOR FAILURE - Rotating anode rotate then didn't Failed  ROTATING ANODE Tungsten-rhenium alloy High atomic number High melting point Heat-conducting ability - Doesn't matter Focal track reaches 1000-2000 C during normal use!  ANODE LAYERING Assists with heat loading Backed with molybdenum and/or graphite Cheat is main problem NORMAL ANODE WEAR - constant use  WARM-UP PROCEDURE ↳ Helps Prolong tube life Gradually warms anode Prevents cracking Helps maintain vacuum Stress relieved anode Back of stress relieved anode; more efficient heat dissipation  ANODE HEAT LOADING Rotating anode Revolutions per minute (rpm) 3,200 – 3,600 rpm Loading… Diameter of disk Target material  TARGET AREA Portion of anode that electron stream contacts Target 3 Focus All the same Focal point Focal spot Focal track e - Point source of X-ray photons  FOCAL SPOT A large focal spot is better for the life of the anode A small focal spot is required for image quality This issue is resolved by the line focus principle I Actual versus effective focal spot  LINE FOCUS PRINCIPLE Effective focal spot Controlled by: Actual focal spot all have an angle Target angle - small for detail , large for anode life  - straight up and down = o - can't have an angle of o be of vacuum - Angle stops e- from early interactions - smaller the better (15) Drawing 0 L less intensity ove r thinner body Part = lay Part under ANODE HEEL EFFECT anoge. is the L intensity of X-rays less under anode 1 - less under [more unde a anode