Basic principles of x-ray production

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Terence McCoy
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1 Production of X-Rays part 1 George Starkschall, Ph.D. Lecture Objectives Identify what is needed to produce x-rays Describe how a diagnostic x-ray tube produces x-rays Describe the types of interactions an electron can undergo with a target atom to produce x-rays More details are forthcoming in Medical Physics II and Medical Physics III Basic principles of x-ray production High-energy electrons, forced to decelerate, emit radiation Bremsstrahlung Electrons, in going from an outer shell of an atom to an inner shell, emit radiation characteristic x-rays 1

2 Requirements for Bremsstrahlung Source of electrons (filament/cathode) Accelerating potential (high potential difference) High-voltage generator [transformer, diodes, etc] conventional x-ray generator Multi-staged accelerating potential linear accelerator Target (stops electrons and generates x- rays) Requirements for Bremsstrahlung Quantity of radiation produced proportional to tube current Charge/unit time going from cathode to anode Electron source 2

3 Electron source Filament current vs tube current Electrons boiled off at cathode Increase in filament current results in increase in number of electrons boiled off Electron source Does not necessarily result in increase in tube current Cloud of electrons in equilibrium around filament space charge Electrons driven back by repulsion from electrons already there Electron source Region of low accelerating potential Electrons trapped by space charge space charge limited Increase in filament current will not increase tube current 3

4 Electron source Increased accelerating potential Space charge effects overcome and electrons pulled away from cathode Tube current increases with increase in filament current Electron source Higher voltages: Saturation Electrons removed as fast as they are boiled off Diagnostic x-rays - radiography Want burst of electrons in a short time (before patient moves) Operate where there are enough electrons available Above space charge limited region Below saturation 4

5 Diagnostic x-rays tube current Control output, photon fluence, beam intensity Determined by both kv and filament current 2.5% change in filament current [e.g., 4.1 ma to 4.2 ma] results in 25% change in tube current [350 ma 415 ma] Need very stable filament current Tube current vs. accelerating potential Diagnostic x-rays fluoroscopy Operates at lower filament current saturation region Intensity can be adjusted by adjusting filament current 5

6 A digression sinusoidal behavior Electric power is alternating current Westinghouse def. Edison Voltage and current characterized by sinusoidal behavior Need to review sinusoidal behavior Review of sinusoidal behavior Review of sinusoidal behavior Calculate average (over half cycle) voltage - V avg 6

7 Review of sinusoidal behavior Power = I 2 R = V 2 /R so look at V 2 Review of sinusoidal behavior Power based on root mean square Review of sinusoidal behavior To summarize: 7

8 Review of sinusoidal behavior Voltage: V = V 0 sin Current: Nearly linear with voltage below saturation region X-ray prod ~ V 2 Interactions of electrons with target Interactions with valence electrons Collisional energy transfer (electrons) Interactions with core electrons Characteristic x-rays Interactions with nucleus Bremsstrahlung Interactions of electrons with target 8

9 Interaction with valence electrons Dominant interaction Coulomb interaction with loosely-bound electrons Low-energy transfer Ejected electron does not travel far Interaction with valence electrons Energy ultimately deposited as heat At 100 kev ~99% of energy results in heat while 1% results in x-rays Cooling x-ray targets major issue Digression: Continuous slowing down approximation Most interactions are low-energy transfer Passing electron interacts with many atoms Consequently, electrons continuously slow down, losing energy at the same rate as they slow down Gives rise to continuous slowing down approximation (CSDA) Much more on this later in course 9

10 Interaction with core electrons Core electron ejected Incident electron must have sufficient energy to overcome binding energy of electron Outer shell electron fills vacancy Production of characteristic x-rays Only K-characteristic x-rays important in most x-ray applications Interaction with core electrons Characteristic x-ray has energy equal to difference in binding energies of initial and final shells, e.g. h = E K E L II Note that K-characteristic x-rays may come from various L and M sub-shells Interaction with core electrons Binding energies for different shells in tungsten atom: K kev L I kev L II kev L III kev Note: The I,II,III represent different l values h = E K E L 10

11 Interaction with core electrons Consequently, for tungsten we find the following emission lines: Transition Symbol Energy (kev) Relative Number K-N II N III K K-M III K K-M II K K-L III K K-L II K K-L I K forbidden Interaction with core electrons Note: The K 3 transition, from L I to K, involves l = 0, and is quantummechanically forbidden Interaction with core electrons Consequently, for tungsten we find the following emission lines: Transition Symbol Energy (kev) Relative Number K-N II N III K K-M III K K-M II K K-L III K K-L II K K-L I K forbidden 11

12 Interaction with core electrons Characteristic x-rays are very important in the diagnostic x-ray region: As much as 30% of the energy in a 100 kv beam is characteristic x-rays At 200 kv, only a few percent of the energy is characteristic x-rays At megavoltage energies, virtually none of the energy comes from characteristic x-rays Note on mammography For mammography, using a Mo target, the intent is to have as much of the beam as possible be characteristic x-rays (17-20 kev) At 20 kev most of the photon interactions are photoelectric absorption (we ll see this later) Very sensitive to small changes in Z 20 kev is a very good energy to look for small calcifications in soft tissue Interaction with core electrons If the characteristic x-rays are low energy relative the Bremsstrahlung spectrum, they are usually considered to be a contaminant. If the characteristic x-rays are high energy relative the Bremsstrahlung spectrum, the are usually considered to be a benefit. 12

Shell Atomic Model and Energy Levels

Shell Atomic Model and Energy Levels (higher energy, deeper excitation) - Radio waves: Not absorbed and pass through tissue un-attenuated - Microwaves : Energies of Photos enough to cause molecular rotation