3. X-ray

Question 1

What is X-radiation?

Answer 1

electromagnetic radiation with photon energies approximately in the 100 eV to 1 MeV range.

Question 2

What is X-ray tube?

Answer 2

Vacuum tube containing an anode and a cathode that produces an X-ray beam

Question 3

How does X-ray tube work?

Answer 3

• 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

Question 4

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)____.

Answer 4

1. electrons
2. Thermionic
3. Anode current (I)
4. accelerating voltage (U )
5. anode

Question 5

X-ray tube

How does X-radiation originate?

Answer 5

kinetic energy of the electrons that hit the anode

(only small fraction is converted into X-ray)

Question 6

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)___

Answer 6

1. X-rays
2. heat

Question 7

X-ray tube

The heat produced on the anode should be (1)____ to avoid (2)___.

Answer 7

1. dissipated
2. melting

Question 8

X-ray tube

Because of the substantial heat production, the anode is usually made of a material with (1)____ melting point, such as (2)___

Answer 8

1. high
2. tungsten (W)

Question 9

Advantages of using tungsten for X-ray tube

Answer 9

1. High melting point (3410 oC)
2. high atomic number (Z = 74)

Question 10

Why is high atomic number a benefit for X-ray tube?

Answer 10

The higher the atomic number, the greater the efficiency of converting electron-energy into X-ray-energy

Question 11

What is Bremsstrahlung?

Answer 11

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).

Question 12

DUANE-HUNT LAW is based on ___

Answer 12

Bremsstrahlung

Question 13

State DUANE-HUNT LAW

Answer 13

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.

Question 14

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)____.

Answer 14

1. kinetic energy
2. X-ray photons
3. heat

Question 15

Describe the spectrum of Bremsstrahlung.

Answer 15

The spectrum of the X-radiation is a continuous spectrum that ends at a minimum wavelength (cutoff, Fig. 3).

Question 16

What happen to the the spectrum of Bremsstrahlung upon increasing the accelerating voltage? What is the consequence (photon energy)?

Answer 16

the spectrum of Bremsstrahlung shifts towards shorter wavelengths

→ towards higher photon energies

→ radiation “hardens”

Question 17

What happen to the the spectrum of Bremsstrahlung upon decreasing the accelerating voltage?

Answer 17

the spectrum shifts towards longer wavelengths

→ radiation “softens”

Question 18

What happen to the spectrum of Bremsstrahlung if the accelerating voltage is kept constant?

HINT:

1. # of electrons
2. Radiation intensity
3. Energy of each e-
4. The shifting of spectrum

Answer 18

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

Question 19

What does the area under the curve in the spectrum of the Bremsstrahlung (Fig. 5) represents?

Answer 19

the total emitted power of X-radiation in the entire wavelength range.

→ it can be calculated as…

Question 20

According to the following equation. How can the total X-ray power emitted be controlled?

Answer 20

• 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.

Question 21

How can the efficiency of radiation generation by an X-ray tube be calculated?

Answer 21

It can be calculated as the ratio of the total emitted power and the invested electric power Pin = U x I as follows:

Question 22

How can the efficiency of radiation generation by an X-ray tube be improved?

Answer 22

by using higher voltage and an anode material of higher atomic number

(but the efficiency is still only around 1%.)

Question 23

What is TOTAL POWER OF BREHMSSTRAHLUNG?

Answer 23

• 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.

Question 24

What is CHARACTERISTIC X-RADIATION?

Answer 24

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.

Question 25

**_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).

Answer 25

1. fill the vacancy 2. excess 3. X-radiation

Question 26

Describe the spectrum of **characteristics X radiation**

Answer 26

Because characteristic X-radiation is caused by a quantum transition, its spectrum is linear.

Question 27

Why is characteristics X radiation “characteristics”?

Answer 27

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

Question 28

The spectrum of the characteristic X-radiation of molybdenum is shown in Fig. 7. Characteristic radiation is superimposed on (1)\_\_\_, which is always (2)\_\_.

Answer 28

1. Bremsstrahlung 2. present

Question 29

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.

Answer 29

1. the accelerating voltage 2. anode current 3. intensity (height of the lines)

Question 30

**_Principal of diagnostic application of X-radiation_** the **attenuation coefficient** *μ* of the different tissues is \_\_\_

Answer 30

different

Question 31

**_Principal of diagnostic application of X-radiation_** The **attenuation coefficient** *μ* of the different tissues is different. **_What are the 2 reasons for this?_**

Answer 31

1. the different densities 2. the different atomic compositions of the various tissues

Question 32

**_Principal of diagnostic application of X-radiation_** How to calculate The attenuation coefficient of X-radiation?

Answer 32

The attenuation coefficient is proportional to the **density** of the absorbing material ( *ρ*): * μ = μ*m *· ρ ,* * where μm denotes the **mass attenuation coefficient***

Question 33

**_Principal of diagnostic application of X-radiation_** What does the contrast of the X-ray image depend on?

Answer 33

1. the density differences between tissues 2. the differences in the mass attenuation coefficient.

Question 34

**_Principal of diagnostic application of X-radiation_** what is mass attenuation coefficient *μ*m?

Answer 34

* 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

Question 35

**_Principal of diagnostic application of X-radiation_** What are the 2 important absorption processes?

Answer 35

1. Compton scattering 2. Photoelectric effect

Question 36

Do the mass attenuation coefficient of Compton scattering (*σ*_m) and the photoelectric effect (*τ*_m) depend on the atomic number (Z)?

Answer 36

While the mass attenuation coefficient of Compton scattering (*σ*_m) is **independent** of the atomic number, that of the photoelectric effect (*τ*_m) **depends** strongly on IT

Question 37

Calculate the mass attenuation coefficient of the photoelectric effect

Answer 37

It is proportional to the third power of both the wavelength and the atomic number of the absorbent (*C* is a proportionality factor). ## Footnote *τ*_m *= Cλ*³*Z*³

Question 38

Why is the strong dependence of *τ_m* on the atomic number of the absorbent important?

Answer 38

Because it contributes to reasonable image contrast even for small differences between the atomic numbers.

Question 39

Why are elements of high atomic number (e.g., lead) good absorbers?

Answer 39

Because they are often used to shield X-radiation.

Question 40

**_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)**

Answer 40

**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

Question 41

**_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)**

Answer 41

* 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

Question 42

Which material is used for **Bragg-diffraction grating?**

Answer 42

**NaCl crystal**

Question 43

**_Bragg-diffraction_** **Radiation power is measured** based on (1)\_\_\_, **by using a (2)\_\_\_**

Answer 43

1. ionization 2. **Geiger–Müller counter**

Question 44

How do we plan for the experiment of X-ray

Answer 44

**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.

Question 45

2 mechanisms of X-ray production

Answer 45

(1) Characteristics radiation (2) Braking radiation

Question 46

3 things that photon energy depends on

Answer 46

(1) Kinetic energy of incoming electron (2) Distance of closet approach to nucleus (3) atomic number of target material

Question 47

What is Spectrum of Bremsstrahlung

Answer 47

* 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

Question 48

Why does Braking radiation have continuous spectrum?

Answer 48

**because the photon energy which is emitted during this kind of X-ray production is discrete ( quantized )**