Detector I Flashcards
What does a detector capture?
Ionisation of incoming radiation causes electrons to be measured at electrode
depends on distance and charge mobility
How many eV are needed to produce electrons in detector?
Average energy to create an electron ion pair generally a few electron volts in a semiconductor to 10βs of eV in a gas.
What does one interaction of electron with detector produce?
1 interaction produces given amount of charge
and electrons arrive at electrode over a short time period
What does the area under the current-time graph equal to?
Total amount of charge from the interaction
What happens in the interaction?
Different energies deposited in the interaction and different amounts of charge generated (causing events to occur)
What is the Current mode?
Use ammeter to measure current
What is the Pulse mode?
Records each individual event
Capacitor discharges across resistor and voltage is measured at resistor
What is the time constant equation which is used to find the detector charge collection time?
π = RC
What is the equation for V_max?
V_max = Q/C
What would be the properties of a perfect detector?
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
What is V_max converted to?
A digital number which results in a pulse height (H)
Histogram is created of each H value in to pulse height spectrum
What is the pulse height (H) proportional to and what does it depend on?
H β energy of incident radiation
It depends on the number of electrons produced
What does every photon create and what is it described by?
Every photon creates an average quantity of charge (Q) in the detector
Described by Poisson statistics:
Mean number of electrons produced = N
Standard deviation = βN
What is the equation for energy resolution (R) at pulse height?
R = FWHM / H_o
H_o = central/mean value of peak
What do the Poisson stats lead to?
Gaussian response
What is the equation for average H value and the standard deviation?
H_o = kN
Ο = k βN
What is Full Width Half Maximum equation?
FWHM = 2.35 Ο
What is the Poisson limit to resolution
R = FWHM / H_o = 2.35 / βN
What does the Fano Factor (F) account for and what is the equation?
It accounts for variance
R = 2.35 β (F/N)
for scintillators: F β 1
for semiconductors: F «_space;1
What does the real measurement of energy resolution (R) include?
It includes other factors:
statistical fluctuations
electronic noise
temporal drift
FWHM^2 = FWHM^2_stat + FWHM^2_noise + β¦
What does measuring the FWHM define?
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
What is the absolute efficiency equation?
Ξ΅_abs = no. of pulses recorded/ no. of radiation quanta emitted by source
(includes geometry of the source and detector)
What is the intrinsic efficiency equation?
Ξ΅_int = no. of pulses recorded/ no. of radiation quanta incident on detector
(related to how good detector is at absorbing radiation
What is required in order for a detector to record 2 separate pulses?
A minimum amount of time between 2 events to record 2 separate pulses (otherwise pulses get summed and
What is dead time?
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)
What are the two models of dead time behaviour?
Paralysable: each new event occurring within the dead time resets the dead time, effectively βparalyzingβ 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)
What is the purpose of the detector?
Absorbs particles (including photons) and outputs a quantity of charge which is proportional to the energy of the absorbed particle. (then voltage or current)
What are the stages of signal production?
Detector -> amplifier -> multi channel analyser -> PC
What do the amplifier and multi channel analyser do?
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 β βenergyβ histogram
What is produced from a real pulse height spectrum?
Number of events vs ADU
(Calibration is required from ADU to Energy
What components are seen on the real pulse spectrum?
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
What happens in a gas detector?
Charged particles interact with gas molecules
Create ion pairs (electron & positive ion)
Basis of electrical signal (number of ion pairs) = output of detector
What is the Ionisation energy/potential and W-value?
Ionisation energy/potential: Energy required to create ion pair
W-value: average energy to create ion pair (number of pairs β energy deposited)
What happens during the processes of diffusion, charge transfer and recombination?
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)
What are the elements of a basic gas detector?
Two electrodes
Gas filled volume
Applied voltage (which creates electric field)
Charge collection
What are the three types of gas detectors?
Ionisation chambers
Proportional counters
Geiger-Mueller tubes
(differ in magnitude of applied voltage and construction)
What is the Ionisation/saturation region and proportional region?
Ionisation region: Charge created by ionisation collected
Proportional region: Charge is multiplied by factor proportional to detector bias
What is the G-M region?
Uncontrolled multiplication creates avalanche of charge
What happens in ionisation chamber?
Operates in ionsation/saturation region
Low electric field
(recombination is negligible)
What does the electric field superimpose and what is the equation?
Superimposes a drift velocity on thermal velocity/diffusion
v = ΞΌE/p
ΞΌ = mobility
E = electric field strength
p = gas pressure
What happens in a proportional counter?
Operates in pulse mode
High electric field induces avalanche
Cylindrical geometry used to create high field
What happens to the electric field in proportional counter?
Electric field strength increases towards anode wire
Avalanche region is very small (all electrons multiply equally)
What is the gas multiplication factor?
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
What affects the Multiplication factor?
M increase rapidly with V
Multiplication factor is constant
What can be achieved by controlling the gas multiplication factor?
Detectors can achieve the desired sensitivity and signal quality for various applications
In proportional counters, what does each electron give rise to?
One avalanche (each avalanche is independent)
What does gas de-excitation in proportional counter result in?
in UV photon emission and the absorption of UV by primary gas could result in additional avalanche
What is added to reduce additional avalanches in proportional counters?
Quench gas: complex molecular gas to absorb UV
It dissipate energy through processes that do not release electron
What is the recorded current of the proportional counter proportional to?
Number of original ion pairs created
How does Geiger-Mueller Tube work?
Construction same as proportional counter
Operated in pulse mode
Very high E field β increase intensity of avalanches
Each avalanche can create another avalanche β chain reaction
What happens during avalanche of Geiger counter?
Secondary ions are produced which excite molecules
Dexcitation occurs via UV photon emission which interacts with gas and frees another electron (creating avalanche)
When does the the Geiger discharge stop?
When electric field strength is reduced below critical point (due to low positive ion mobility at the anode)
What remains the same after Geiger discharge?
Same signal amplitude regardless of number of ion pairs created originally
What happens after Geiger discharge termination?
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)
What does the quench gas prevent?
New discharge due to ion neutralisation (charge transfer collisions) at cathode
