Week 4: Radiation detection (part 1)

Question 1

What is the general composition of a radiation detector?

Answer 1

1. Detector
2. Pre-amplifier
3. Amplifier
4. Multi-channel analyser
5. Spectrum display

Question 2

What is energy resolution?

Answer 2

The ability of a detector to quantify the energy of the radiation.

It is measured using the FWHM divided by the location of the centroid.

Question 3

What is pulse mode?

Answer 3

When measurements of the energy of each particle of radiation incident on the detector have to detect each separate signal pulse.

Question 4

What is dead time?

Answer 4

The detector system can only handle pulses arriving at a certain rate. If pulses are produced faster than this, the system cannot process them, so counts are lost.

The dead time is the amount of time required between pulses.

Question 5

What is paralysable dead time?

Answer 5

The arrival of additional pulses during the dead time extends the duration of the dead time, meaning the detector can become unable to respond to any further signals.

Question 6

What is non-paralysable dead time?

Answer 6

The detector is unable to respond to all pulses during the dead time, so the amount of time inactive is constant no matter the true count rate.

Question 7

Derive the non-paralysable dead time formula.

Answer 7

(16)

Question 8

How would you find the true count rate for a non-paralysable dead time?

Answer 8

The rate of missed counts can be equally expressed as the difference between the true rate and the detected rate, meaning we can solve for the true rate.

(17)

Question 9

What is the relation between the detected rate and the true rate for a paralysable detector?

Answer 9

(18)

Question 10

How is the relation between true rate and detected rate for a paralysable dead time derived?

Answer 10

By considering the probability that there is another interaction within the existing amount of dead time. As the true rate increases, the signal lost due to dead time becomes more and more problematic.

Question 11

What is current mode?

Answer 11

If the time between pulses is too small, or they overlap, it is possible to instead measure the current produced at a given time to indicate how much radiation is reaching the detector.

Question 12

What is the equation for the current in current mode? Define each symbol.

Answer 12

I0 = rQ

I0 = average current
r = rate of events
Q = charge produced by each event

Question 13

Derive the current uncertainty for currnet mode.

Answer 13

(19)

Question 14

How can the uncertainty of current mode be used as a source of information about the signal?

Answer 14

(20)

Question 15

What is mean square voltage mode?

Answer 15

If the average current I0 from the detector is blocked, and only the square of the variation of the signal is measured, the average is proportional to the event rate r, but also to Q^2. This is known as MSVM.

Question 16

What is the basic principle of all gas filled detectors?

Answer 16

A gas-filled chamber is subjected to an electric field between an anode and a cathode.

Radiation causes ionisation of the gas and the ions created are attracted to the oppositely charged electrode.

This causes an electric current, which can be measured.

Question 17

Sketch the basic layout of a gas-filled detector.

Answer 17

(21)

Question 18

What are the three types of gas-filled detectors?

Answer 18

Ionisation chamber
Proportional counter
Geiger counter

Question 19

Draw an ionisation chamber schematic.

Answer 19

(22)

Question 20

How does an ionisation chamber operate?

Answer 20

The electric field applied sweeps electrons to the anode and ions to the cathode before they can recombine into neutral atoms or molecules.

Question 21

What conditions do ionisation chambers usually operate?

Answer 21

They usually operate at (relatively) low voltages.

It takes approximately 30 eV to ionise the gas atoms/molecules.

Question 22

What voltage would be induced on the plates by the collection of charge in and ionisation chamber?

Answer 22

V = Q/C

Question 23

How would you find the voltage developed from a single particle in an ionisation chamber?

Answer 23

Dividing the energy deposited by the particle by the energy required for one ionisation gives us the number of ionisations.

Multiplying by the elementary charge tells us the amount of charge of each sign that is generated.

Dividing the charge by the capacitance of the detector gives us the resulting voltage.

Question 24

What is the circuit time constant in an ionisation chamber?

Answer 24

A resistor R is included in the detector circuit. The time constant of this circuit is RC, where C is the capacitance.

Question 25

Why is there a circuit time constant in the circuit in an ionisation chamber?

Answer 25

The time constant, RC, is chosen to be significantly larger than the pulse duration. This allows the charge to be integrated in the capacitor over the collection period and then discharged through the resistor. This means that the amplitude of the pulse is proportional to the charge produced in the detector, and the decay time of each pulse is determined by the circuit design.

Question 26

What is the result of electrons and ions having a different drift rate in an ionisation chamber?

Answer 26

The pulse shape will have two components, and the shape of the pulse will depend on where in the chamber the particle stops. This is undesirable, as ideally the same pulse shape should be produced by each detection of the same kind of particle.

Question 27

What is a Frisch grid?

Answer 27

It is a fine mesh placed near the anode and held at a potential between the electrodes. This shields the anode from the movement of the electrons, until the pass through the grid. It is used to solve the drift velocity problem.

Question 28

How does the Frisch grid solve the drift velocity problem in an ionisation chamber?

Answer 28

The pulse develops as the electrons cross the small distance between the grid and the anode, which occurs in a few microseconds. The signal is measured solely from the electrons, and the use of the grid removes the position dependence from the pulse height.

Question 29

Sketch the use of a Fisch grid in an ionisation chamber.

Answer 29

(23)

Question 30

How does a proportional counter usually operate?

Answer 30

Usually operate at high applied voltages. The stronger electric field means that the electrons produced by the interaction of radiation are accelerated and can gain enough energy to cause further ionisation. This results in a Townsend avalanche. This provides amplification of the original signal and with the correct set-up the pulse height remains proportional to the energy input.

Question 31

Sketch a typical proportional counter design.

Answer 31

Proportional counters are typically designed as a cylinder, with a wire anode along the axis, and the outer cylinder acting as the cathode. (24)

Question 32

Derive an equation for the voltage in a proportional counter.

Answer 32

(25)

Question 33

What are the typical operating parameters of a proportional counter?

Answer 33

a = 10 um b = 10 mm V = 1kV Therefore the field near the anode is over 10MVm^-1.

Question 34

What is the result of the typical operating parameters of a proportional counter?

Answer 34

They result in a lot of additional ionisation, and because nearly all of it occurs near the anode, the pulse shape is independent of where the initial energy was deposited.

Question 35

What is the result of moving the voltage to higher values for a proportional counter?

Answer 35

Results in a loss of proportionality of the pulse height.

Question 36

What are the main similarities and differences between a Geiger Muller tube and proportional counters?

Answer 36

Similar: design Different: Operate at higher voltages.

Question 37

What is the effect of Geiger counters operating at high voltages?

Answer 37

Results in intense ionisation. De-excitation of atoms releases ultra-violet photons, which ionise other atoms and extend the huge region of ionisation along the wire. This produces a huge discharge made up of multiple avalanches.

Question 38

Why is the Geiger Muller tube a counter rather than a energy measure?

Answer 38

The pulse is no longer proportional to the initial charge.

Question 39

How is the pulse ended in a Geiger counter?

Answer 39

The slow movement of the positive ions means that they build up around the central wire. This decreases the strength of the electric field. This build up of charge eventually stops the avalanches occurring, ending the pulse.

Question 40

What is a 'quench' gas?

Answer 40

A gas that can undergo charge-exchange with positive ions and later dissociate harmlessly. e.g. methane, ethanol.

Question 41

Why are quench gasses used in Geiger counters?

Answer 41

Positive ions drift to the cathode where there is a high probability that an electron will be released, potentially re-triggering the counter. A 'quench' gas is used to stop this.

Question 42

What are the general features of gas detectors?

Answer 42

Rugged Cheap Low-density Efficient for charged particles Low maximum count rates

Question 43

Why are gas detectors inefficient for gamma rays and fast neutrons?

Answer 43

Low density

Question 44

Why are gas detectors efficient for charged particles?

Answer 44

A thin foil window is required to detect alpha particles, otherwise they cannot enter the chamber.

Question 45

How can a gas detector be used to detect neutrons?

Answer 45

If a polythene surround known as a Bonner sphere is used for moderation.

Question 46

List 4 applications of scintillation detectors.

Answer 46

Medicine Diagnostics, computer tomography. Industry Radioactive waste assay, level gauging. Environment Radiation surveys, geological applications. Physics Nuclear, high energy, particle physics.

Question 47

What is the typical arrangement of a scintillation detector?

Answer 47

Radiation deposits energy in a scintillator, creating a region of excitation. De-excitation releases many photons. A reflecting shield helps to collect light on the photocathode of a photo-multiplier tube (PMT). The pulse of current received at the anode is proportional to the number of photons, which is proportional to the energy of the original gamma ray.

Question 48

What is the process of a photomultiplier tube work?

Answer 48

The PMT provides electron multiplication. The first photoelectron reaches the first dynode, producing 3-5 electrons from its surface. These electrons are then accelerated to the second dynode by a PD of around 100V. This process then repeats.

Question 49

Sketch the typical layout of a scintillation detector.

Answer 49

(26)

Question 50

What are the two different types of scintillator detectors?

Answer 50

Organic and inorganic

Question 51

Draw a diagram and explain the absorption and generation of photons by organic scintillators.

Answer 51

For each electron excitation state, there are several associated vibrational states. (27)

Question 52

What are inorganic scintillators made from?

Answer 52

Crystals with small amounts of impurities.

Question 52

Draw a diagram and explain the absorption and generation of photons by inorganic scintillators.

Answer 52

They have a valence band containing electrons, which can be excited to the conduction band by the absorption of radiation. The use of activator atoms to create state within the band gap is an important feature. (28)

Question 53

What is the process of photon emission in an inorganic scintillator?

Answer 53

The deposition of energy promotes electrons from the valence band to the conduction band, leaving a hole in the valence band. Electrons and holes can migrate, and will eventually recombine, with the emission of a photon as the electron returns to the valence band.

Question 54

Why are activator atoms needed in inorganic scintillators?

Answer 54

In pure crystals, the photon emitted by de-excitation will have sufficient energy to excite another electron to the conduction band. This means they will not be transparent to their own light.

Question 55

How do activator atoms solve the problem of the crystal not being transparent to nits own light?

Answer 55

Impurity atoms are added to provide new states within the band gap. The impurities create 'stepping stone' ground and excited states inside the forbidden region. Electrons and holes can migrate to the sites of the impurities and then recombine, emitting lower energy photons that do not posses enough energy to promote and electron from the valence band. This means the crystal becomes transparent to light from the activator states.

Question 56

Why are organic scintillators transparent to the majority of their own light?

Answer 56

Excited states above S10 deexcite to S10 quickly without emitting radiation. The S10 state deexcites by dropping to one of the S0 states. The radiation produced in this deexcitation will only have enough energy to re-excite another electron if the transition is from S10 to S00. This means that organic scintillator materials are transparent to the majority of their own light.

Question 57

What are common properties of inorganic scintillators and list common materials used?

Answer 57

High atomic number High density Sodium iodide with thallium iodide activator. Lanthanum bromide with cerium activator. Bismuth germanate (BGO).

Question 58

Why are organic scintillators poor at gamma radiation detection?

Answer 58

Low atomic number and density. The lower density gives a lower chance of interaction. The interactions that occur will often be Compton scattering, and the low probability of the scattered photon interacting means that there will be relatively few events that deposit the full photon energy.

Question 59

List the advantages of scintillator detectors.

Answer 59

High gain, large signal from PMT. Cheap. Inorganic scintillators are efficient for X-rays and gamma rays. Fast response, especially from organic scintillators.

Question 60

List the disadvantages of scintillator detectors.

Answer 60

Bulky. Crystals can be fragile. Risk of gain drift (due to the influence of temperature, magnetic fields on the PMT).

Question 61

Explain why a Frisch grid is used in an ionisation chamber.

Answer 61

The Frisch grid is used so that the pulses produced by a given particle of radiation are always the same, no matter where in the chamber the ionisation occurred.

Question 62

List the operating regimes of gas-filled detectors in order of increasing applied voltage.

Answer 62

Ion saturation, proportional region, limited proportionality, Geiger-Muller region.

Question 63

Give an example of a quench gas used in Geiger-Muller counters.

Answer 63

Methane Ethanol

Question 64

Briefly explain why actinium impurities are a problem for using lanthanum as a scintillator material.

Answer 64

Actinium isotopes are radioactive and part of long decay chains. This means that if they are used in a scintillator, the material generates a signal of its own that can interfere with the measurement.