03 - Ionization chambers - Novinky · 03 - Ionization chambers ... (proportional counters) ... (Geiger-Muller counter) Electronic signal at the output, pulsed or current regime

03 - Ionization chambers

Jaroslav Adam

Czech Technical University in Prague

Version 2

Jaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 1 / 130

Principle of operation

Charged particle getting through a volume of a gas or noble liquid

Interaction proceed through ionization and end excitation of the molecules, electron-ion pairsare created

Ion can be created directly by the incident particle, or by the δ-electrons, when the energyfrom the primary particle if first transferred to the electron which acquires enough energy tomake further ionization

Electric field is applied by the electrodes, electrons and ions drift to them

If the field is high enough, drifting electrons can also ionize the gas (proportional counters)

After further increase of the field strength, electrons emit UV light on the anode(Geiger-Muller counter)

Electronic signal at the output, pulsed or current regime

Position sensitivity by segmenting one of the electrodes (xy ) and by timing measurement (z)


Ionization detectors without amplification


Number of ion pairs

Minimum energy W to be transferred from the incident particle to create at least one ion pair

Quantified as average energy loss to create the pair

Given by the least tightly bound shell, W = 10 - 25 eV

Non-ionizing energy loss (excitation) makes number of pairs lower

Fully stopped 1 MeV particle produces 30 000 ion pairs

Number of pairs is important for resolution


Energy dissipation per ion pair (the W-Value)


Fano factor

Fluctuation in number of pairs affect the energy resolution

The simplest approach postulate Poisson statistics for number of ion pairs (σ =√

N)

Fano factor makes correction to predicted variance to get observed variance

Fano factor = 0 if all incident energy converted into pairs, no statistical fluctuation

In gases Fano factor < 1, Poisson distribution valid but fluctuations are smaller than√

N

Significant in pulse mode


Principle of ionization chamber

Figure : Planar ionization chamber

Suppose two metallic electrodes at distance D covering volume of a gas or noble liquid

Voltage V applied to anode (thousands of kV)

Number of electrons n− given by the number of minimum-ionizing-particles (mip)

n− =nW

DρdEdx

(1)

ρ is the density and dE/dx is energy loss per g cm−2


Charge carriers in gas/liquid volume

Several processes applies to the ion pairs

Drift movement by external electric field

Diffusion due to random thermal movement

Charge transfer

Recombination


Drift movement

Electrostatic force moves the charges, positive ions opposite to electrons and negative ions

Drift of electrons characterized by drift velocity in electric field E = V/D

vd (e) =dxdt

=µe

pE =

Vµe

pD(2)

Electron mobility µe given in unit of bar cm2 V−1 s−1, p is the gas pressure

Mobility of ions is about 1000 less than of electrons

Description with mobility provides calculation of readout times

ms for ions, µs for electrons


Saturation of the drift velocity

Figure : Electron velocity vs. filed

No increase in drift velocity after reaching it’s maximum, only in some gases

Hydrocarbons, argon-hydrocarbon mixtures

In non-saturation gases, E/p proportionality holds up to high fields


Diffusion

Thermal movement with mean free path of about 10−6 - 10−8 m

More important for electrons since their thermal velocity is bigger

Point-like collection of electrons form Gaussian spatial distribution widening with time

Widening in one direction x , y or z given by diffusion coefficient D

σ =√

2Dt (3)

D is given by kinetic gas theory or the process is described by a transport model


Charge transfer collisions

Electron transfer from neutral gas molecule to positive ion in mutual collision

Significant in mixtures, net positive charge transfered to species with lowest ionization energy

Negative ion can be formed by capturing of free electron (oxygen)


Recombination

Free electron captured by positive ion making ordinary neutral atom

Positive and negative ions recombine, most probable compared to electron and ion case

Original charge is lost, no contribution to final signal

Recombination rate given by density of positive and negative species n+ and n− andrecombination coefficient α

dn+

dt=

dn−

dt= −αn+n− (4)


Columnar (initial) recombination

Electron-ion pairs created in column along particle trajectory

Recombination of electron with it’s parent ion

Mainly for heavy ionizing particles (low energy α), minimal for mip

Independent of interaction rate

Recombination may occur with neighboring ion when electrons are drifting in electric field

Depends on the angle between incident particle and electric field


Volume recombination

Recombination during drift towards electrodes

Unlike initial/columnar recombination, this depends on irradiation rate

Suppressed by fast charge separation and collection -> high electric field


Ionization current, DC ion chamber

Constant irradiation rate creates constant formation of ion pairs

Steady-state current is measure of the rate

Supposing negligible recombination and efficient charge collection

Figure : Planar ionization chamber


Current-voltage characteristics

Electric field by external voltage

Current in the circuit equal to ionization current at equilibrium

Increasing voltage begins to separate the charges that would recombine

High electric field makes recombination negligible

After ion saturation, all charges are collected, on increase in current when increasing thevoltage

Standard operation of ion chambers, current in the circuit is an indication of the rate of ionpairs formation


Saturation currentRecombination, especially columnar recombination require high voltage

Volume recombination important at high irradiation intensity

Higher voltage required to get true saturation current

More important in neutron measurement, where heavy fragments are detected

With chambers filled by ambient air, recombination depends on humidity


Perturbations in current due to diffusion

Imbalance in steady-state situation supposing uniform production of ions within the chamber

Larger concentration of positive ions close to cathode, opposite for electrons close to anode

Gradient in concentration formed, diffusion opposite to drift

Perturbation in measured current in planar chamber given by

−∆II

=εkTeV

(5)

ε is ratio of average energy of charge carrier, kT/e ≈ 2.5× 10−2 V at room temperature

ε close to one for ions, but several hundreds for electrons


Losses of saturation current due to diffusion

ε close to one for ions, but much larger, several hundreds for electrons

Minimized by high voltage

Columnar recombination not fully eliminated

Separate measurements of ionization current as a function of voltage

1/I as a function of 1/V to determine true saturation current


Operation of DC ion chamber

No special requirement on gas since negative charge can be collected as free electrons aswell as negative ions

Only recombination could affect the amount of charge, suppressed by high enough voltage

Few centimeters and tens of hundreds of volts sufficient to reach saturation

Air for gamma-ray exposure, denser gases like Argon to increase ionization density

Pressure about 1 atmosphere, higher to increase sensitivity


Geometry of ionization chamber

Electric field given by geometry of electrodes

Uniform electric field with planar geometry

Cylindrical geometry with electrical field as E(r) =U0

r ln(ra/ri )

Figure : Cylindrical ionization chamber


Insulators

Small values of ionization current, < 10−12 A

Resistance of insulator at least 1016 Ω to keep leakage current below 1% for U = 100 V

Leakage by moisture absorbed on surface suppressed by the guard rings and smooth surfaceof insulators

Plastics or ceramic for higher irradiation


Measurement of gamma-ray exposure with ion chambers

Amount of charge in air-filled ionization chamber

Charge is measure of exposure, ionization current gives exposure rate

Requires to measure ionization of all secondary electrons, mean free pairs several meters

compensation for secondary electrons needed


Compensation by free-air ionization chamber

Collimated X-rays, compensation in horizontal direction

Compensation of electrons escaping the sensitive volume by electrons emerging elsewhere

Up to gamma-ray of 100 keV


Air equivalent compensation

Reduces space requirements for higher gamma-rays energies, wall thickness less than 1 cm

Exposure rate given by saturation current Is and mass M in active volume

R =IsM

(6)


DC chambers for radiation survey

Radiation monitoring

Saturation current in closed volume of several cm3

Dose measured by charge integration

Initial charge on the chamber, drop in voltage measures total integrated ionization charge


Ionization dosimeter

Cylindrical air capacitor, initially charged at some voltage V0

Discharged by the radiation

Voltage reduction measures absorbed dose


Calibration of radiation source

Saturation current depends only on geometry, long time stability

Current from unknown source compared with standard under same geometry

Chambers with thousands of cm3

Pressured gas for higher sensitivity


Geometry for calibration of gamma-ray sources

Calibration for gamma-ray sources, ionization current for small displacement of the source


Measurement of radioactive gases

Radioactive gas as constituent of fill gas, sampled as continuous flow

Ionization current I given by average energy deposition per gas disintegration E and totalactivity α

I =EαeW

(7)

Principle of smoke detectors where ionization current from internal alpha source decreasesdue to presence of the smoke in sampled air


Remote sensing of ionization

Positive and negative ions in the air survive for minutes before recombination

Flow of air is transported to a chamber outside the source of radiation

LRAD - Long Range Alpha Detector:

Sample of alpha-contaminated material in container, air flowing though it

Positive and negative ions carried into an ion chamber


Pulse mode operation

Each radiation quantum provides with distinguishable signal pulse

Application in radiation spectroscopy

Alpha spectrometry

Neutron detection


Equivalent circuit of ion chamber

Chamber is represented by it’s capacity C

Voltage V0 on load resistor R

Drifting charges from ionization create induced charge on electrodes

Voltage drops from equilibrium V0, giving output pulse VR

Slow return to equilibrium according time constant RC

If RC > time to collect all charges, amplitude of the pulse measures the original charge


Equivalent circuit of ion chamber

The ion chamber behaves as parallel connection of resistor and capacity, called RC circuit

Time dependence of voltage V given bycapacity C and resistivity R

CdVdt

= I =VR

(8)

Response to initial charge giving voltage V0is

V (t) = V0e−t/RC (9)


Electron sensitive mode

Time to collect electrons ∼ µs, time for ions ∼ ms

Collection of all charges would require RC of orders of >ms

This would mean low pulse rate only and interference from microphonic signals

For electron sensitivity, RC between electron and ion collection time

Negative ions no longer allowed


Pulse shape in planar chamber

Constant electric field E = V/d

Uniform equipotentials parallel to electrode surfaces

Ions supposed to be formed at equal distance x where electric potential is Ex


Pulse shape of electron and ion collection

Derivation of VR(t) based on energy conservation

RC long enough to collect all charges

Energy in capacitance 1/2CV 20 is energy source to move the charges

Drift velocity v− for electrons and v+ for ions, n0 is number of ion pairs

12

CV 20 = n0eEv+t + n0eEv−t +

12

CV 2ch (10)

V0 + Vch ' 2V0 andVch

d'

V0

d(11)

VR =n0edC

(v+ + v−)t (12)

After collecting the electrons (time t− ≡ x/v−)

VR =n0edC

(v+)t (13)



After collecting the ions (time t+ ≡ (d − x)/v+)

VR =n0edC

[(d − x) + x ] (14)

VR =n0eC

(15)

If RC t+, maximum amplitude of the pulse is

Vmax =n0eC

(16)

Independent of position of incident ionization




Pulse shape in electron sensitive operation

Pulse shape given by electrons

Amplitude sensitive to position of incident radiation

V |elec =n0eC·

xd

(17)

Monoenergetic radiation produces a range of pulses

Removed by dividing the ion chamber by adding a grid


Gridded ion chamber

Designed to remove amplitude position dependency

All interactions between the grid (Frisch grid) and the cathode

Grid at intermediate potential and transparent for electrons

Grid-anode voltage drops after electrons pass through the grid


Electron signal in gridded ion chamber


Electron signal in gridded ion chamber

With RC larger than electron collection time and grid-anode spacing d , signal on load resistoris

VR =n0edC

v−t (18)

Maximum voltage is

Vmax =n0eC

(19)

Allows to operate at short RC

Pulse independent of position of incident radiation

But signals of order of < 10−5 V

Can not be measured directly, amplification needed


Statistical limit to energy resolution

Number of charges n0 and fano factor F gives statistical limit

Supposing fully stopped α at energy E = 5.5 MeV in gas with W = 30 eV/ion pair andF = 0.15

n0 =Ed

W= 1.83× 105 ion pairs (20)

Standard deviation as square root of variance

σn0 =√

Fn0 = 166 (21)


Statistical limit to energy resolution

FWHM of Gaussian distribution for n0 and particle energy

FWHM(n0) = 2.35σn0 = 390 (22)

FWHM(E) = 2.35σn0 ·W = 11.7 keV (23)

Relative energy resolution to the deposited energy

R =2.35σn0 W

Ed= 0.123% (24)

Good theoretical resolution can not be achieved due to electronic noise


Charged particle spectroscopy with pulse-type ion chamber

Some advantages vs. semiconductor or scintillator detectors

Arbitrary size and geometry

Gas dielectric cable 3500 m long was used as a beam-loss monitor at SLAC

Gas pressure to adjust stopping power or effective thickness

Radiation resistant, simple design

Low level alpha particles, cross sectional area 500 cm2

Bragg curve spectroscopy: analysis of pulse shape from incident particle parallel to the field,can distinguish particle species


Alpha spectroscopy with ionization chamber

Fisch grid ionization chamber filled by argon allows to separate two uranium isotopes


Proportional counters


Charge multiplication in the gas

Late 1940s, effect of gas multiplication

More ion pairs compared to ionization chamber

Low energy X-rays

Neutron detection

Pulse mode most common operation


Avalanche formation

Kinetic energy of the electron sufficient to make secondary ionization

Field at least 106 V m−1

Gas multiplication: secondary electrons also ionize after acceleration

Townsend avalanche

dnn

= αdx (25)

α is the first Townsend coefficient of the gas

Depends on electric field

For constant field (parallel plate geometry), solution is exponential increase in electrondensity:

n(x) = n(0) exp(αx) (26)


Townsend coefficient vs. field


Regions of operation


Cylindrical geometry

Field with anode wire radius a, cathode inner radius b and applied voltage V is

E(r) =V

r ln(b/a)(27)


Field as a function of radius


Avalanche from a single electron, MC transport model


Simulation of avalanche

Winkler et al. Am. J. Phys., Vol. 83, 733-740 (2015)


Development of the avalanche


Construction design

Figure : Cross sectional view, dimensions in cm


Beverage can design, Winkler et al. Am. J. Phys., Vol. 83, 733-740(2015)


2π proportional counter


4π proportional counter


Fill gas

Gases without significant electron attachment

Oxygen or electronegative impurities must be removed in flow-counters

Sealed counters more convenient to use, but may have limited lifetime

Polyatomic gas (methane) to prevent photon-induced effects, component called quench gas

Most common proportional gas is 90% argon + 10% methane (P-10 gas)

Penning effect - component with ionization energy lower than excitation energy of principalgas


Quenching in proportional detectors


Gas multiplication factor

Single electron response by avalanche created by one electron outside multiplication region

Total charge by n0 original pairs with gas multiplication factor M is

Q = n0eM (28)

In cylindrical geometry, Townsend coefficient α depends on the radius (and on the field)

ln M =

∫ rc

adrα(r) (29)

α is higher closer to the anode where the field is stronger

Parametrizations of α on the field use experimental data


Gas multiplication factor vs. voltage


Space charge effects

Reduction of the field near anode due to positive ions

Self induced effects resulting from high gas gain, does not depend on pulse rate

General space charge effect is cumulative for different avalanches, also for lower gain

M should just satisfy signal-to-noise requirement to prevent space charge effects

Simple check by varying the voltage and looking for suppression of large pulse amplitudes


Energy resolution

Fluctuations in n0 and in multiplication

Variance of pulse amplitude given by ion pair fluctuation and single-electron multiplicationvariations

Furry distribution of number of electrons in a given avalanche, probability of ionizationdepends only on E

Polya distribution for higher fields, ionization depends on electron’s history, not only on E

Distribution of pulse amplitude approaches Gaussian for large n0 (more than 20)


Energy resolution

Low energy photons have resolution close to statistical limit

Fractional standard deviation of the peak (FWHM/2.35), P-10 gas at 1 atm pressure

Open circles: standard tungsten anode

Full circles: improved uniformity


Systematic variations in gas multiplication factor


Time characteristics of the signal pulseOutput pulse is sum of drift time of free electrons to reach multiplication region andmultiplication time required for the avanlacheDrift time greater than multiplication time


Equivalent circuit of proportional counter seen by developing signal

Derivation of pulse shape utilizes energy conservation of charge moving across the capacitor

Most of the pulse amplitude from positive ion drift

Collection time of all ions long, hundreds ms, but large fraction of signal developed early ofion drift, fraction of µs

Spread in electron drift times (ionizations along radius) makes spread in rise time of outputpulse


Time development of output pulse

Solid curve represents initial ionization at fixed radius (constant drift time)

Dashed line is for uniform ionization along the radius

Rise time minimized by high electric field in drift region and gases with high electron driftvelocity


Shaping of the output pulse

Pulse shape with different time constants

T is total drift time of positive ions from anode to cathode

Sauli, Principles of operation of multiwire proportional and drift chambers, CERN 77-09 (1977)


Electron drift velocity


Variations of pulse rise time

Pulse shaping with short time constant removes slow component of rise time

Amplitude less than of full collection, effect of balistic deficit

Shape of pulse would vary with radial position of original ion pairs

Minimized by shaping time larger than variation in rise time

Can be used to separate signal α events from electron background


Spurious pulses

Satellite afterpulses from secondary processes following desired signal

Removed by amplitude discrimination since correspond to single-electron avalanche

Effect increases with high value of gas multiplication

Optical photons by excited atoms within the avalanche, low energy electron at cathodesurface through photoeffect


Operating voltage

Signal pulse for every particle with enough energy deposit

Sensitivity to more than one avalanche to suppress space charge effects and spurious pulses

Lower values of multiplication factor, some number of original ion pairs to create detectablesignal

Counting curve to select appropriate operating voltage (counting rate vs. voltage at samesource condition), looking for flat region


Alpha counting

Monoenergetic charged particles of range less than dimensions of the chamber


Beta counting

Range greater than chamber dimensions, smaller pulses compared to alpha

4π counter for absolute beta activity

Greater range advantage for mechanical source backing in 4π counter


Mixed alpha and beta sources

Two readout channels for alpha and beta

Separation by amplitude

Background from cosmic rays or ambient gamma (most sources) does not affect alphawindow


X-ray and gamma-ray sources

Fraction of incident photons absorbed in 5.08 cm of proportional gases at 1 atm pressure

Full absorption of low energy X-ray photons, full energy peaks in pulse spectrum

Small signal (∼ keV photons, W = 25− 35 eV) enhanced by internal gas multiplication


Response function for low-energy X-rays

Characteristic X-rays after photoabsorption of primary radiation may escape withoutinteraction

Suppressed with K-shell energy above incident X-ray energy

Requirements to entrance window, beryllium or aluminum 50 - 250 µm


Fluorescence gating

Electronic separation of photoelectric absorption of incident photon and absorption ofconsequent characteristic X-ray

Proportional counter as series of independent cells, X-ray absorbed in different cell

Two pulses in time coincidence corresponding to full absorption, escape peak avoided


Parallel plate avalanche counters

Heavy charged particles which would impose radiation damage to solid state detectors

Parallel plate electrodes, proportional gas at low pressure

Particle species separation by specific energy loss

For gap of 1 mm, fast component from electrons in ns with high field, time resolution 160 ps


Position-sensitive proportional counters

Avalanche occurs around small portion of anode wire length

Position of avalanche is indicator of axial position in cylindrical geometry


Charge division method

Anode with significant resistance per unit length

Two amplifiers at the ends, sum related to conventional pulse, proportion to the position of theinteraction

Alternatively relative rise time could be analyzed, longer rise time for pulse far from amplifier


Multiwire proportional counters (MWPC)

Cathode plates and anode wires positive vs. cathode

Electrons are collected at one of the wires, avalanche at a particular wire

Large negative pulse at anode where avalanche was collected, smaller positive pulses atneighboring anodes


Electric field lines in MWPC

Field by equidistant grid of anode wires between two parallel cathode plates

High field region only in immediate vicinity of the anodes


Enlarged view of the field around the anode wires in MWPC

Wire spacing 2 mm, wire diameter 20 µm

Sauli, Principles of operation of multiwire proportional and drift chambers, CERN 77-09 (1977)


Readout of MWPC

Each wire is connected to preamp and processed independently

Theoretical position resolution is spacing/√

12, but with pulse amplitude for each wire aroundtrajectory of the particle, it’s possible to reconstruct centre-of-gravity and improve theresolution


Two-dimensional MWPC

Cathodes divided into perpendicular strips

Centroid of discharge located by center-of-gravity technique


Multicell proportional counter

Cylindrical independent cellsCoincidence measurement for process selection or background rejectionJaroslav Adam (CTU, Prague) DPD_03, Ionization chambers Version 2 93 / 130

Microstrip gas chamber (MSGC)

Narrow anode metallic strips (10 µm) on substrate

Quick collection of ions, clearing the space charge


Two-dimensional MSGC

Ionization in fill gas between anodes and drift plane, avalanches by electrons close to anode

Most of signal generated by drift of positive ions away from the anode


Gas electron multiplier (GEM)

Preamplification in gas to allow lower voltage readout of microstrip detector

GEM foil of insulating material, both surfaces covered by metallic cladding

Hole density 50 per mm2

Holes of 140 µm pitch and 70 µm diameter


GEM foil

Insulating material, both surfaces covered by metallic cladding


Simulation of GEM

Light lines - electrons, dark lines - ions, spots - ionization occurence


Cascade of GEM foils

Gain up to 104 - 105, sensitivity to single electron


Micromegas (micro-mesh gaseous structure)

Two-stage parallel plate avalanche chamber

Ionization above the mesh which is transparent to electrons

Amplification below the mesh in a high field

Avalanche ions collected by the mesh, fast clearing of the space charge


Resistive Plate Chamber (RPC)

Thin gas volume between electrodes of high electrical resistivity


Construction of RPC

Spacers between plates each 10 cm against electrostatic attraction (constant gap size)


Readout of position sensitive RPC

Capacitive coupling of strips to preamps


Time Projection Chamber (TPC) of the ALICE experiment

3D position and dE/dx using volume of 90 m3 of Ne/CO2/N2 (90/10/5) divided into twosectors of 2.5 m

Voltage 100 kV on central electrode, drift field 400 V m−1

Electron drift velocity 2.7 cm µs−1, maximum drift time 92 µs

Readout by MWPC with cathode pad readout mounted in trapezoidal sectors at each endplate, active area of 32.5 m2


MWPC readout of ALICE TPC, cross sectional view along the wires

Grid of anode wires above pad plane, cathode wire plane and gating grid

Wire geometry and pad size dependent on radial track density, 560 000 readout pads

Anode to cathode distance 2 and 3 mm, gain up to 20 000

Gate opens for electrons from the drift volume by the collision trigger (6.5 µs after thecollision) for the duration of drift time, 92 µs, prevents the space charge of positive ions fromdrifting back from multiplication region


MWPC readout, cross sectional view perpendicular to the wires


Particle identification by dE/dx in TPC

Simultaneous measurement of ionization losses and momentumParametrization of Bethe-Bloch formula for several particle species


Upgrade of TPC

Expected rate 50 kHz of Pb-Pb, 5 interactions within maximal electron drift time, 10 eventssuperimposed from past+future time window

Untriggered readout without use of gating grid

Readout by GEM foils instead of MWPC, field 50 kV/cm in the hole

Suppression of back flowing ions

One side segmented to reduce total charge on the foil, segments coupled by resistors


Geiger-Mueller Counters


G-M counter

Introduced in 1928, one of the oldest detectors

Higher field than in proportional counter

One avalanche can create at least one another avalanche, chain reaction

All pulses have same amplitude, 109 - 1010 ion pairs

Pulse amplitude in volts, no need for external amplification

Counting rate limited by dead time


Geiger discharge

Excited molecules of neutral gas in Townsend avalanche, together with positive ions

They emit visible or UV light, may be absorbed by photoeffect at any place of the tube, newfree electron created

Or free electron created by absorption at cathode

New avalanche from the free electron after reaching the multiplication region around anode

Subcritical proportional tube: n′0p 1 with n′0 number of excited molecules in typicalavalanche and p the probability of photoabsorption

Geiger discharge with single avalanche multiplication 106 - 100 has n′0 bigger, criticalityachieved as n′0p ≥ 1


G-M region


Geiger discharge propagation

New avalanches created close to the original one, new electrons have to reach anode region

Spread of Geiger discharge along the anode wire with propagation velocity 2− 4 cm µs−1

over entire length of anode, few µs after initiating event

Avalanches at random positions around anode, electrons collected by anode, secondarypositive ions around the multiplication region

Field reduced by space charge of the ions, multiplication reduced, Geiger dischargeterminated

Amount of needed space charge independent of initial ionization, the reason for same pulseamplitude


Amplitude of Geiger discharge

Amplitude increases with higher applied voltage

More space charge needed to reduce the field below criticality

Overvoltage defined as voltage above minimum required for Geiger discharge initiation


Fill gases

Same requirements as for proportional counters since charge multiplication is needed (nonegative ions...)

Noble gases, helium, argon, quenching component

n′0 increases with E/p (electric field to gas pressure), G-M as sealed tube with pressure lessthan atmospheric

Voltage 500 - 2000 V for anode of 0.1 mm diameter


Quenching

Slow drift of positive ions after Geiger discharge termination to the cathode, neutralizationwith electron from cathode surface

Energy needed = sum of gas ionization energy and energy to extract the electron fromcathode (work function)

If (gas ionization energy) = 2 × (work function), another free electron emerges from thecathode surface

At least one free electron from cathode with large amount of positive ions

Will drift towards anode to make new Geiger discharge

Cycle would repeat, continues output of multiple pulses

No problem for proportional counters, number of ions smaller, only spurious pulses


External quenching

Needed any method to prevent excessive multiple pulsing

Reduction of high applied voltage for some fixed time after the pulse below gas multiplication

Time greater than transit of positive ions to the cathode (hundreds of ms) and transit time offree electron (few µs)

Value of R selected high enough (108 Ω), so RC is of ms

Several ms for anode to return to nominal operating voltage, low counting rate


Internal quenching

Component of quench gas added to primary fill gas, concentration 5 - 10 %

Smaller ionization potential, more complex molecular structure

Multiple pulsing prevented by charge transfer collisions, positive charge of primary gas iontransfered to quench gas molecule, original ion neutralized before reaching cathode

Ions of quench gas at cathode with some concentration of quench component

At cathode, the excess energy dissociates the more complex molecule, emission of freeelectron suppressed, no additional avalanches

Organic molecules, ethyl alcohol, ethyl formate

Limit of 109 counts due to consumption of quench gas

Problem avoided by halogens as quench gas (chorine, bromine), spontaneous recombinationafter dissociation

Limit to lifetime then by contamination of reaction products from the discharge and polymerdeposition on anode surface


Shape of the output pulse

Signal of multiple avalanches from secondary electronsSpread in time for them to reach multiplication regionRise time of few µsElectric field distorted from the space charge of positive ionsFast rise and slow rise from ions drift, time constant less than 100 µs to eliminate slowcomponentLoss in amplitude tolerated due to large amount of chargeDelay in response by drift of original electrons to multiplying region, applies for timing withG-M as time uncertainty


Dead time

Drift of positive ions outwards, recovery of field in multiplying regionAfter drift to some distance, the field permits new Geiger discharge, lower intensity in thebeginningFirst pulses reduced in amplitudeFull field and full amplitude after all ions neutralized at the cathodeDead time = time between initial pulse and second Geiger discharge, regardless of themagnitude, 50 - 100 µsResolving time = time for pulse of amplitude high enough to be registered, dead time mayinclude also recovery time in practical applicationRecovery time = time to return to the original state, pulse of full amplitude


Dead time behavior


Geiger counting plateau

Dependence of measured counting rate on voltage with constant rate from radiation source

Counter threshold Hd , no pulses if amplitude is below


Onset of continues discharge

Corona discharge by irregularities on anode wire or quenching failure

Decrease of voltage needed

Operating voltage just sufficient for the flat region, reduces quench gas consumption

Low amplitude tail causes some slope of the counting curve

Hysteresis in counting curve due to charges on insulators

Linearity of the plateau measures quality of G-M

Organic quenched tubes have slope of 2 - 3 % with change in voltage by 100 V

Halogen quenched tubes have greater slope, but larger lifetime and lower operating voltage


End-window Geiger tube

Fine anode wire, less requirements on uniformity of the wire and of the electric field

Cylindrical cathode of metal or glass with metallized inner surface

Thin window for short-range particles, differential pressure

More robust window for beta or gamma


Other design of Geiger tube

Wire loop anode in cathode of arbitrary volume

Needle counters - sharply pointed needle, field as 1/r2 (1/r for cylindrical geometry), usedfor smaller active volume

Source in counting volume for low-energy particles14C detection by gas mixture of CO2 with counter gas for high counting efficiency


Counting electronics

R is load resistance for signal development

Cs is capacitance of the tube and wiring, parallel with R

RCs ∼ few µs for fast-rising component

Coupling capacitor Cc to block high voltage on the tube, only signal pulse transmitted, RCclarger than pulse duration


Counting efficiency for charged particles

100 % for any charged particle entering the active volume since a single ion pair triggers fullGeiger discharge

Effective efficiency given by the window of the tube, absorption or backscattering

For alpha counting window with 1.5 mg/cm2


Efficiency for neutrons

Rare application for neutron detection

Low capture cross section for Geiger gases

Gamma-ray discrimination not possible, all pulses have same size

Neutron-induced reactions detected by proportional counters, large amplitude from suchreactions


Counting efficiency for gamma rays

Interaction in solid wall of the counter, secondary electron needs just to emerge into activevolume1 - 2 mm of innermost layer of the wallEfficiency increased with high-Z material of the cathode (bismuth Z = 83)Low-energy gamma may interact with fill gas, xenon and krypton (high-Z) enhance theefficiency up to 100 % for photon energies 10 keV


Counting efficiency for gamma rays


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03 - Ionization chambers - Novinky · 03 - Ionization chambers ... (proportional counters) ... (Geiger-Muller counter) Electronic signal at the output, pulsed or current regime