Lecture 7: X-ray Tube

Question 1

Briefly describe how the x-ray tube works.

Answer 1

Electrons are emitted by a cathode and accelerated towards an anode (target). They then collide with the target, releasing x-rays and heat.

Question 2

What are the three products released in x-ray tubes?

Answer 2

• Bremsstrahlung x-rays
• Characteristic x-rays
• Heat

Question 3

What is the equation for the energy gained by an electron in an x-ray tube?

Answer 3

e = electron charge
∆V = potential difference

Question 4

Define bremsstrahlung

Answer 4

Breaking radiation that occurs when an electron is decelerated by a target.

Question 5

What is the equation for the maximum energy of an emitted photon in an x-ray tube?

Answer 5

Question 6

What is the energy range for bremsstrahlung in an x-ray tube?

Answer 6

The range is between 0 (when the electrons miss the target) and e∆V (when all of the energy is transferred to the photon).

Question 7

What are characteristic x-rays?

Answer 7

X-rays caused by collisions with atomic electrons. The incident electron either excites or ionises an atom so the electronic energy level is filled by a downwards transition of an atomic electron. This causes the emission of a photon with a characteristic energy.

Question 8

What is characteristic energy?

Answer 8

An energy equal to the difference between the two energy levels.

Question 9

Describe a characteristic x-ray emission spectrum.

Answer 9

Question 10

What is the equation for the characteristic energy of hydrogen?

Answer 10

E_n = characteristic energy
m_e = mass of electron
q_e = electron charge
n = principal quantum number

Question 11

What is the equation for the characteristic energy of larger atoms?

Answer 11

E_n = characteristic energy
m_e = mass of electron
q_e = electron charge
n = principal quantum number
Z = atomic number

Question 12

What is the equation for the energy difference when electrons transition between shells?

Answer 12

E_n = characteristic energy
m_e = mass of electron
q_e = electron charge
n_i = initial quantum number
n_f = final quantum number

Question 13

What are the two types of characteristic transition?

Answer 13

• k_α: transition when n_i = 2 and n_f = 1
• k_β: transition when n_i = 3 and n_f = 1

Question 14

What are the defining features of the x-ray spectrum?

Answer 14

• Non-linear
• Dominated by peaks from characteristic x-rays rising on top of effects of bremsstrahlung.
• Lower energy at the start from filtering.
• End point at e ∆V.

Question 15

Describe the shape of an x-ray spectrum

Answer 15

Question 16

What two variables determine the energies for x-rays produced in an x-ray tube?

Answer 16

• The target material
• The voltage drop

Question 17

Molybdenum generates ______ energy x-rays so is used for ____ tissue imaging e.g. mammograms.

Answer 17

Low
Soft