Detection of Radiation

Question 1

What the 3 broad types of radiation detector?

Answer 1

Gas-filled detectors, scintillation detectors and semiconductor detectors.

Question 2

Basic principle of gas filled detectors?

Answer 2

E-field through a gas chamber. Radiation causes ionisation of the gas, which then induces a current between anode and cathode.

Question 3

3 types of gas filled detector?

Answer 3

Ionisation chambers, proportional chambers and Geiger counters.

Question 4

Ionisation chambers?

Answer 4

Simple ionisation, current generated at rate proportional to gas energy loss from ionisation.

Question 5

Function of a Frisch Grid?

Answer 5

A mesh placed near the anode in an ionisation chamber that regulates all electron pulses so that they are all identical and not position dependent.

Question 6

Proportional chambers?

Answer 6

Higher voltages mean that electrons produced can trigger more ionisation events that amplify the original signal so that the pulse height is proportional to energy input.

Question 7

Geiger-Muller counters?

Answer 7

Even higher voltages, so intensive further ionisation. Pulse no longer proportional to energy, so acts as a counter rather than a measurer.

Question 8

Function of a quench gas?

Answer 8

To absorb excess electrons released by positive ions so that they don’t re-trigger a Geiger counter.

Question 9

What is dead time in a detector?

Answer 9

The minimum amount of time between pulses. Any signals arriving at a higher rate than this will not be recorded.

Question 10

Two types of dead time?

Answer 10

Paralysable - excess events are not counted but extend the dead time.
Non-paralysable - excess events are only not counted.

Question 11

Some general features of gas filled detectors

Answer 11

They are sturdy and cheap, low density (so inefficient for neutral particles), and efficient for charged particles.

Question 12

Basic principle of scintillation detectors?

Answer 12

High energy photons enter a scintillator, where they create a region of excitation which releases many lower energy photons. These then pass through a photomultiplier tube which multiplies electrons to produce a current pulse proportional to the original photon energy.

Question 13

Organic scintillators?

Answer 13

Energy deposited excites electrons to a higher excitation state, which on de-excitation releases a scintillation photon.

Question 14

Inorganic scintillators?

Answer 14

Deposition of energy promotes electrons from valence band to conduction band in a semiconductor. When electron returns to valence band, photon is released.

Question 15

Advantages of scintillation detectors?

Answer 15

High gain, cheap (generally), fast response (particularly from organic) and efficient for high energy photons.

Question 16

Disadvantages of scintillation detectors?

Answer 16

They are bulky, fragile due to the internal crystals, there is risk of gain drift from B-fields and temperature changes, and relatively low resolution.

Question 17

Principles of semiconductor detectors?

Answer 17

Radiation deposits energy in the depletion region in a p-n junction, creating free charges which are sent to the contacts and producing a charge pulse proportional to the original γ energy.

Question 18

Lithium drifting?

Answer 18

Lithium atoms are forced into the semiconductor material, increasing the size of the depletion region by neutralising impurities.

Question 19

Features of semiconductor detectors?

Answer 19

Have lower efficiency than scintillators, but higher resolution, allowing for sample identification by analysing peak energies.