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Ionising Radiation Physics > Detector I > Flashcards

Detector I Flashcards

1
Q

What does a detector capture?

A

Ionisation of incoming radiation causes electrons to be measured at electrode

depends on distance and charge mobility

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2
Q

How many eV are needed to produce electrons in detector?

A

Average energy to create an electron ion pair generally a few electron volts in a semiconductor to 10’s of eV in a gas.

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3
Q

What does one interaction of electron with detector produce?

A

1 interaction produces given amount of charge
and electrons arrive at electrode over a short time period

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4
Q

What does the area under the current-time graph equal to?

A

Total amount of charge from the interaction

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5
Q

What happens in the interaction?

A

Different energies deposited in the interaction and different amounts of charge generated (causing events to occur)

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6
Q

What is the Current mode?

A

Use ammeter to measure current

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7
Q

What is the Pulse mode?

A

Records each individual event

Capacitor discharges across resistor and voltage is measured at resistor

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8
Q

What is the time constant equation which is used to find the detector charge collection time?

A

𝜏 = RC

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9
Q

What is the equation for V_max?

A

V_max = Q/C

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10
Q

What would be the properties of a perfect detector?

A

Use a source of monochromatic radiation

Every photon creates the same quantity of charge in the detector

The electronics always measure and record the same V_max

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11
Q

What is V_max converted to?

A

A digital number which results in a pulse height (H)

Histogram is created of each H value in to pulse height spectrum

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12
Q

What is the pulse height (H) proportional to and what does it depend on?

A

H ∝ energy of incident radiation

It depends on the number of electrons produced

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13
Q

What does every photon create and what is it described by?

A

Every photon creates an average quantity of charge (Q) in the detector

Described by Poisson statistics:

Mean number of electrons produced = N

Standard deviation = √N

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14
Q

What is the equation for energy resolution (R) at pulse height?

A

R = FWHM / H_o

H_o = central/mean value of peak

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15
Q

What do the Poisson stats lead to?

A

Gaussian response

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16
Q

What is the equation for average H value and the standard deviation?

A

H_o = kN

σ = k √N

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17
Q

What is Full Width Half Maximum equation?

A

FWHM = 2.35 σ

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18
Q

What is the Poisson limit to resolution

A

R = FWHM / H_o = 2.35 / √N

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19
Q

What does the Fano Factor (F) account for and what is the equation?

A

It accounts for variance

R = 2.35 √ (F/N)

for scintillators: F ≈ 1
for semiconductors: F &laquo_space;1

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20
Q

What does the real measurement of energy resolution (R) include?

A

It includes other factors:

statistical fluctuations

electronic noise

temporal drift

FWHM^2 = FWHM^2_stat + FWHM^2_noise + …

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21
Q

What does measuring the FWHM define?

A

ability to distinguish between two nearby energies (two different peaks in pulse height)

Separation between the two has to be less than FWHM to distinguish peaks

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22
Q

What is the absolute efficiency equation?

A

ε_abs = no. of pulses recorded/ no. of radiation quanta emitted by source

(includes geometry of the source and detector)

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23
Q

What is the intrinsic efficiency equation?

A

ε_int = no. of pulses recorded/ no. of radiation quanta incident on detector

(related to how good detector is at absorbing radiation

24
Q

What is required in order for a detector to record 2 separate pulses?

A

A minimum amount of time between 2 events to record 2 separate pulses (otherwise pulses get summed and

25
Q

What is dead time?

A

A period after each detection event during which the detector is unable to register another event (at high count rates dead time losses can be high)

26
Q

What are the two models of dead time behaviour?

A

Paralysable: each new event occurring within the dead time resets the dead time, effectively “paralyzing” the detector (distorted spectrum or shut down at high rates)

Non-paralysable: if an event occurs during the dead time of a previous event, it is simply ignored, and the dead time is not extended or reset (fixed dead time)

27
Q

What is the purpose of the detector?

A

Absorbs particles (including photons) and outputs a quantity of charge which is proportional to the energy of the absorbed particle. (then voltage or current)

28
Q

What are the stages of signal production?

A

Detector -> amplifier -> multi channel analyser -> PC

29
Q

What do the amplifier and multi channel analyser do?

A

Amplifier: Shapes the pulse to make it more suitable for further electronic processing and filters noise

Multi-Channel Analyser: Sorts pulses into bins (channels) according to their amplitude – ‘energy’ histogram

30
Q

What is produced from a real pulse height spectrum?

A

Number of events vs ADU

(Calibration is required from ADU to Energy

31
Q

What components are seen on the real pulse spectrum?

A

photon emission energy peak: entire energy of the incoming photon

Compton Spectrum: broad and continuous distribution due to the range of energies transferred to electrons at various scattering angles.

Compton Edge: represents the maximum energy that the scattered electron can receive from a single Compton scattering event

32
Q

What happens in a gas detector?

A

Charged particles interact with gas molecules
Create ion pairs (electron & positive ion)
Basis of electrical signal (number of ion pairs) = output of detector

33
Q

What is the Ionisation energy/potential and W-value?

A

Ionisation energy/potential: Energy required to create ion pair

W-value: average energy to create ion pair (number of pairs ∝ energy deposited)

34
Q

What happens during the processes of diffusion, charge transfer and recombination?

A

Diffusion: Random thermal motion of spreading of the ionized electrons and positive ions

Charge transfer: Ions and electrons drift toward under the influence of an electric field the respective electrodes to create a measurable current or pulse.

Recombination: Free electrons recombine with positive ions, neutralizing them (reducing signal)

35
Q

What are the elements of a basic gas detector?

A

Two electrodes

Gas filled volume

Applied voltage (which creates electric field)

Charge collection

36
Q

What are the three types of gas detectors?

A

Ionisation chambers

Proportional counters

Geiger-Mueller tubes

(differ in magnitude of applied voltage and construction)

37
Q

What is the Ionisation/saturation region and proportional region?

A

Ionisation region: Charge created by ionisation collected

Proportional region: Charge is multiplied by factor proportional to detector bias

38
Q

What is the G-M region?

A

Uncontrolled multiplication creates avalanche of charge

39
Q

What happens in ionisation chamber?

A

Operates in ionsation/saturation region

Low electric field

(recombination is negligible)

40
Q

What does the electric field superimpose and what is the equation?

A

Superimposes a drift velocity on thermal velocity/diffusion

v = μE/p

μ = mobility
E = electric field strength
p = gas pressure

41
Q

What happens in a proportional counter?

A

Operates in pulse mode

High electric field induces avalanche

Cylindrical geometry used to create high field

42
Q

What happens to the electric field in proportional counter?

A

Electric field strength increases towards anode wire

Avalanche region is very small (all electrons multiply equally)

43
Q

What is the gas multiplication factor?

A

Quantity of change produced event

represents the factor by which the original ionisation charge is multiplied due to the avalanche process

Q = n_0 eM

Q = total charge generated

n_o = number of original ion pairs

e = electron charge

M = multiplication factor

44
Q

What affects the Multiplication factor?

A

M increase rapidly with V

Multiplication factor is constant

45
Q

What can be achieved by controlling the gas multiplication factor?

A

Detectors can achieve the desired sensitivity and signal quality for various applications

46
Q

In proportional counters, what does each electron give rise to?

A

One avalanche (each avalanche is independent)

47
Q

What does gas de-excitation in proportional counter result in?

A

in UV photon emission and the absorption of UV by primary gas could result in additional avalanche

48
Q

What is added to reduce additional avalanches in proportional counters?

A

Quench gas: complex molecular gas to absorb UV
It dissipate energy through processes that do not release electron

49
Q

What is the recorded current of the proportional counter proportional to?

A

Number of original ion pairs created

50
Q

How does Geiger-Mueller Tube work?

A

Construction same as proportional counter

Operated in pulse mode

Very high E field → increase intensity of avalanches

Each avalanche can create another avalanche → chain reaction

51
Q

What happens during avalanche of Geiger counter?

A

Secondary ions are produced which excite molecules

Dexcitation occurs via UV photon emission which interacts with gas and frees another electron (creating avalanche)

52
Q

When does the the Geiger discharge stop?

A

When electric field strength is reduced below critical point (due to low positive ion mobility at the anode)

53
Q

What remains the same after Geiger discharge?

A

Same signal amplitude regardless of number of ion pairs created originally

54
Q

What happens after Geiger discharge termination?

A

Positive ions drift away from anode

Replenish neutrality with electron from cathode

Energy can be released (if energy is great enough additional electron is released)

55
Q

What does the quench gas prevent?

A

New discharge due to ion neutralisation (charge transfer collisions) at cathode

Ionising Radiation Physics (11 decks)

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