3. X-ray Flashcards
What is X-radiation?
electromagnetic radiation with photon energies approximately in the 100 eV to 1 MeV range.
What is X-ray tube?
Vacuum tube containing an anode and a cathode that produces an X-ray beam
How does X-ray tube work?
- The tungsten filament of the cathode is heated by electric current → emits electrons by thermionic emission.
- The power of heating modulates the number of emitted electrons → determining the anode current (I) (electric current) between the cathode and the anode
- A high voltage (U ) applied between the cathode and the anode accelerates the electrons towards the anode.
- X-ray is produced when high speed electrons hit metal target in anode
X-ray tube
The cathode is heated (hot cathode) by an electric circuit and emits (1)___ by the (2)___ effect.
The power of heating modulates the number of emitted electrons, thereby determining (3)____, between the cathode and the anode.
A large (4)____ applied between the anode and the cathode accelerates the electrons towards the (5)____.
- electrons
- Thermionic
- Anode current (I)
- accelerating voltage (U )
- anode
X-ray tube
How does X-radiation originate?
kinetic energy of the electrons that hit the anode
(only small fraction is converted into X-ray)
X-ray tube
X-radiation originates from the kinetic energy of the electrons that hit the anode.
However, only a small fraction (less than 1%) of this energy can be converted into (1)____, most of it is released as (2)___
- X-rays
- heat
X-ray tube
The heat produced on the anode should be (1)____ to avoid (2)___.
- dissipated
- melting
X-ray tube
Because of the substantial heat production, the anode is usually made of a material with (1)____ melting point, such as (2)___
- high
- tungsten (W)
Advantages of using tungsten for X-ray tube
- High melting point (3410 oC)
- high atomic number (Z = 74)
Why is high atomic number a benefit for X-ray tube?
The higher the atomic number, the greater the efficiency of converting electron-energy into X-ray-energy
What is Bremsstrahlung?
X-radiation generated by the sudden deceleration of high-speed (large-kinetic-energy) charged particles, e. g., electrons.
It has a smooth, continuous spectrum with a sharp wavelength minimum (λmin).
DUANE-HUNT LAW is based on ___
Bremsstrahlung
State DUANE-HUNT LAW
Gives the minimum wavelength (λmin) of the X-radiation produced by Brehmstrahlung as: λmin = k / U
where U is accelerating voltage and k is a constant.
Bremsstrahlung is generated as electrons are suddenly decelerated in the anode material.
→ Decelerating electrons emit part of their (1)___ in the form of (2)__ (Fig. 2), and the rest of the energy is dissipated as (3)____.
- kinetic energy
- X-ray photons
- heat
Describe the spectrum of Bremsstrahlung.
The spectrum of the X-radiation is a continuous spectrum that ends at a minimum wavelength (cutoff, Fig. 3).
What happen to the the spectrum of Bremsstrahlung upon increasing the accelerating voltage? What is the consequence (photon energy)?
the spectrum of Bremsstrahlung shifts towards shorter wavelengths
→ towards higher photon energies
→ radiation “hardens”
What happen to the the spectrum of Bremsstrahlung upon decreasing the accelerating voltage?
the spectrum shifts towards longer wavelengths
→ radiation “softens”
What happen to the spectrum of Bremsstrahlung if the accelerating voltage is kept constant?
HINT:
- # of electrons
- Radiation intensity
- Energy of each e-
- The shifting of spectrum
the number of emitted electrons is increased
→ radiation intensity increases
→ the energy of the individual electrons remains the same
→ the spectrum does not shift along the wavelength axis
What does the area under the curve in the spectrum of the Bremsstrahlung (Fig. 5) represents?
the total emitted power of X-radiation in the entire wavelength range.
→ it can be calculated as…
According to the following equation. How can the total X-ray power emitted be controlled?
- changing the anode current (heating of the cathode):
- the power changes proportionally, but the shape of the spectrum remains the same;
- changing the accelerating voltage:
- the power change is proportional to the square of the voltage, and the shape of the spectrum also changes.
How can the efficiency of radiation generation by an X-ray tube be calculated?
It can be calculated as the ratio of the total emitted power and the invested electric power Pin = U x I as follows:
How can the efficiency of radiation generation by an X-ray tube be improved?
by using higher voltage and an anode material of higher atomic number
(but the efficiency is still only around 1%.)
What is TOTAL POWER OF BREHMSSTRAHLUNG?
- The power emitted in the entire wavelength range that depends on
- the anode current (I )
- the accelerating voltage (U )
- the atomic number of the anode material ( Z )
the value of the proportionality constant cX is: 1.1 · 10-9 V-1.
What is CHARACTERISTIC X-RADIATION?
X-radiation produced during electronic transitions of inner-shell electrons of the anode made of high-atomic-number materials.
It has a linear spectrum; the position of the lines is characteristic of the emitting atom.
Characteristic X-radiation
If the accelerated electrons have sufficiently large kinetic energy (accelerating voltage is high), they are able to eject inner-shell electrons from the atoms of the anode.
Because this condition is highly unstable, another electron from an outer shell will (1)___ (3w) while the (2)___(lost/excess) energy is emitted as (3)___ (see Fig. 6).
- fill the vacancy
- excess
- X-radiation
Describe the spectrum of characteristics X radiation
Because characteristic X-radiation is caused by a quantum transition, its spectrum is linear.
Why is characteristics X radiation “characteristics”?
The emission lines are organized in several series.
→ . The position of these lines in the spectrum is characteristic to the energy- level structure of the atoms of the anode
The spectrum of the characteristic X-radiation of molybdenum is shown in Fig. 7. Characteristic radiation is superimposed on (1)___, which is always (2)__.
- Bremsstrahlung
- present
The spectrum of the characteristic X-radiation of molybdenum is shown in Fig. 7.
Lines Kα and Kβ are characteristic for molybdenum.
The position (wavelength) of these lines is independent of (1)___ or the (2)___.
Upon changing these parameters, only their (3)____ may change.
- the accelerating voltage
- anode current
- intensity (height of the lines)
Principal of diagnostic application of X-radiation
the attenuation coefficient μ of the different tissues is ___
different
Principal of diagnostic application of X-radiation
The attenuation coefficient μ of the different tissues is different.
What are the 2 reasons for this?
- the different densities
- the different atomic compositions of the various tissues
Principal of diagnostic application of X-radiation
How to calculate The attenuation coefficient of X-radiation?
The attenuation coefficient is proportional to the density of the absorbing material ( ρ):
- μ = μ*m · ρ ,
- where μm denotes the mass attenuation coefficient*
Principal of diagnostic application of X-radiation
What does the contrast of the X-ray image depend on?
- the density differences between tissues
- the differences in the mass attenuation coefficient.
Principal of diagnostic application of X-radiation
what is mass attenuation coefficient μm?
- It can be calculated as the ratio of attenuarion coefficient and density of absorbing material
- It has 2 components which are attenuation coefficients of Compton scattering and photoelectric
Principal of diagnostic application of X-radiation
What are the 2 important absorption processes?
- Compton scattering
- Photoelectric effect
Do the mass attenuation coefficient of Compton scattering (σm) and the photoelectric effect (τm) depend on the atomic number (Z)?
While the mass attenuation coefficient of Compton scattering (σm) is independent of the atomic number,
that of the photoelectric effect (τm) depends strongly on IT
Calculate the mass attenuation coefficient of the photoelectric effect
It is proportional to the third power of both the wavelength and the atomic number of the absorbent (C is a proportionality factor).
τm = Cλ3Z3
Why is the strong dependence of τm on the atomic number of the absorbent important?
Because it contributes to reasonable image contrast even for small differences between the atomic numbers.
Why are elements of high atomic number (e.g., lead) good absorbers?
Because they are often used to shield X-radiation.
Separation of the different wavelengths of X-radiation, measurement of the x-ray power and spectrum
What is the principle of Bragg-diffraction? (for Monochromatic incident X-ray)
Monochromatic incident X-rays scatter on the atoms of the parallel planes of the crystal according to the Huygens principle.
→ scattered X-rays interfere, and directions of constructive and destructive interference arise
If the angle of the incident rays is equal to the angle of the first-order diffraction maximum is as well (Fig. 8), just like in optical reflection
Separation of the different wavelengths of X-radiation, measurement of the x-ray power and spectrum
What is the principle of Bragg-diffraction? (for polychromatic X-radiation)
- For polychromatic X-radiation (e.g., Brems-strahlung) the reflection angle of the first-order diffraction maximum is wavelength dependent → the reflected X-rays will spread in a fan shape according to wavelength (Fig.9).
- The spectrum can be recorded by rotating the detector. This arrangement is similar to the monochromator that uses optical grating
Which material is used for Bragg-diffraction grating?
NaCl crystal
Bragg-diffraction
Radiation power is measured based on (1)___, by using a (2)___
- ionization
- Geiger–Müller counter
How do we plan for the experiment of X-ray
First, we will record at different settings of accelerating voltage and anode current the Bremsstrahlung and the characteristic spectra emitted by an X-ray tube with molybdenum anode.
The second measurement is a demonstration about the absorption of the X- radiation to verify equation (6). We will place different absorbent foils in the X-ray beam, and carry out the measurement at a fixed crystal position, thus at a given wavelength.
2 mechanisms of X-ray production
(1) Characteristics radiation
(2) Braking radiation
3 things that photon energy depends on
(1) Kinetic energy of incoming electron
(2) Distance of closet approach to nucleus
(3) atomic number of target material
What is Spectrum of Bremsstrahlung
- It is the graph that power of emitted photons of given energy as function of photon energy.
- It is a continuous spectrum with wavelength minimum
Why does Braking radiation have continuous spectrum?
because the photon energy which is emitted during this kind of X-ray production is discrete ( quantized )
