The physics of diagnostic X-rays(A-level only) Flashcards – AQA A-level Physics (7408)

How are electrons released from the cathode in a rotating-anode X-ray tube?

The heated cathode releases electrons by thermionic emission.

What accelerates electrons toward the anode in an X-ray tube?

A high potential difference across the evacuated tube.

Where are X-rays produced in a rotating-anode tube?

At a small region of the anode, where focused electrons interact with the target.

What happens to most of the electron energy at the X-ray tube target?

It is transferred to the target as thermal energy.

Why is the anode rotated in an X-ray tube?

Rotation spreads heating over a larger track, allowing high tube loading while retaining a small effective focal spot.

How does tube current affect X-ray output?

It controls the number of electrons reaching the target per second, and therefore the X-ray output.

How does the tube potential difference affect the maximum X-ray photon energy?

It controls the maximum energy gained by each electron, and hence the maximum photon energy.

Besides changing maximum photon energy, what does increasing the tube potential difference change?

The X-ray output and its energy distribution.

For a given X-ray output, what does exposure time control?

The total number of photons produced.

How does filtration change the energy distribution of an X-ray beam?

It preferentially removes lower-energy photons.

How are continuous-spectrum X-rays produced at a target?

Fast electrons are deflected and decelerated in the electric fields of target nuclei, emitting bremsstrahlung photons.

Why do bremsstrahlung photons have a range of energies?

Electrons can lose different amounts of energy in their interactions with target nuclei.

When does a bremsstrahlung photon have the maximum possible energy?

When an electron transfers all the kinetic energy gained through the accelerating potential difference to one photon.

What equation relates the maximum X-ray photon energy to the accelerating potential, maximum frequency and minimum wavelength?

$E_{\max}=eV=hf_{\max}=\frac{hc}{\lambda_{\min}}$

How does increasing the accelerating potential affect the maximum photon energy and short-wavelength cutoff of the continuous X-ray spectrum?

The maximum photon energy increases, and the minimum wavelength decreases.

How is a characteristic X-ray photon produced in a target atom?

An incoming electron removes an inner-shell electron. An electron from a higher energy level fills the vacancy, emitting a photon.

What determines the energy of a characteristic X-ray photon?

The energy difference between the two electron energy levels involved in the transition.

Why does the characteristic X-ray spectrum contain lines at particular energies?

Atomic energy levels are discrete, so transitions produce photons at particular energies.

What determines the energies of the characteristic X-ray lines in a target?

The target element.

How are characteristic X-ray lines positioned relative to the continuous spectrum?

They are superimposed on the continuous spectrum.

What energy condition must an incident electron meet to produce a characteristic X-ray?

It must have enough energy to create the relevant inner-shell vacancy.

What is patient dose in X-ray imaging related to?

The energy deposited in the patient by ionising X-rays.

How can exposure time and tube current be adjusted to reduce patient dose?

Limit both while retaining a diagnostically useful image.

How can the X-ray beam be restricted to reduce patient dose?

Restrict it to the region being examined.

Why does X-ray filtration reduce patient dose?

It removes low-energy photons that are readily absorbed by the patient and contribute relatively little to the image.

What two photon and detector factors affect the exposure needed for an X-ray image?

Photon energy and detection efficiency.