Gas Detectors

9/15/2010
Contents

Introduction

Types of Gas DetectorsIonization of the Detector GasElectrodesBasic electrode ConfigurationsParallel Plate Electrodes

Coaxial ElectrodesCoaxial ElectrodesMovement of Ion Pairs to ElectrodesBeta Particle Interaction with the DetectorAlpha Particle Interaction with the DetectorPhoton Interaction with the DetectorRadiation Type and Size of Detector SignalModes of Operation
Contents

Current Mode of Operation

GeneralExample

Pulse Mode of Operation

Pulse Characteristics: Voltage (V)

Pulse Characteristics: Current (I)Pulse Characteristics: Current (I)Pulse Characteristics: Charge (Q)Making Pulse Height Related to the Collected ChPulse ShapePulse Rise and the Maximum Voltage (Vmax)

Pulse Tail and the Time Constant (J)
Contents

The Six (or five) Region Curve

GeneralRecombination RegionIonization RegionProportional RegionRegion of Limited Proportionality

Geiger Mueller region

-Mueller region

Continuous Discharge

Know the Difference between These Curves
Introduction
Introduction

Types of Gas DetectorsThere are three types of gas detectors:

1. Ionization chamber

2. Proportional counter3. Geiger Mueller counter
Introduction

Ionization of the Detector GasThey are referred to as gas detectors because the produced when charged particles ionize the gas in detection chamber.

--------------

If the gas is neon (the most common gas in a GM dthe positive member of each ion pair is a neon ion (

while the negative member of the pair is a freed ele

If the detector gas is air, the positive member of the is usually a positively charged nitrogen molecule (N

the negative member of the ion pair is the freed ele

+ + + + + + + + + + + + + +
Introduction

Ionization of the Detector GasThe term “ion” usually refers to the positive membe

pair. Sometimes the electron is also referred to as aIn the absence of an electric field, the ion pairs crea

gas of the detector simply recombinegas of the detector simply recombine. 

However, if the ionization occurs between two oppocharged electrodes, and if the electric field is sufficistrong, the positive and negative members of the iowill separate and be collected at the electrodes.
Introduction

ElectrodesAn electric field is established by creating an potendifference between two electrodes. The potential dithe voltage applied across the electrodes. 

Anode: the positively charged electrod

the positively charged electrod

Cathode: the negatively charged electrod

In most systems, the cathode is grounded. That waindividual touching the cathode won’t receive a shocathode is still negative with respect to the anode h
Introduction

Basic Electrode ConfigurationsThere are two general electrode configurations:

1. Parallel plate electrodes2. Coaxial electrodes

Variations on these configurations are common and situation in a given instrument might only approximthem. Calculating the strength of the electric field is straigfor the parallel plate and coaxial configurations, but far more complicated in a real instrument.
Introduction

Parallel Plate ElectrodesParallel plate electrodes are only used in ionization chambers. Nevertheless, many ion chambers empcoaxial configuration. Parallel plate configurations acommon for the ion chambers used in radiology.As the name implies, the two electrodes are parallearranged as shown in the following diagram.

Cathode

-

Anode

+
Introduction

Parallel Plate ElectrodesThe strength of the electric field is:

ξ is the strength of the electric field (e.g., volts /mV is the potential difference (e.g., volts)d is the distance between the two plates (e.g., m

The strength of this field is uniform – it is the same throughout the space between the electrodes.
Introduction

Coaxial ElectrodesThe coaxial electrode configuration, or a variant thefar more common than the parallel plate configuratiIn the most typical coaxial configuration, the cathodcylinder that serves as the detector chamber wall. Icylinder that serves as the detector chamber wall. I

cases, the cathode is a conductive coating on the inthe chamber wall. The anode consists of a central wire running that ruthe chamber’s axis. 
Introduction

Coaxial Electrodes

ξ is the strength of the electric field (e g olts /m

is the strength of the electric field (e.g., volts /m

V is the potential difference (e.g., volts)

r is the radial distance from the anode center to t

which the electric field strength is calculated (e.g

a is the anode radius (e.g., meters)

b is the cathode radius (e.g., meters)
Introduction

Coaxial Electrodes

r

Cathode >

(-)

a

Anode

(+)

The strength of the electric field varies throughout tchamber volume. It increases rapidly as we move the central electrode and decreases as we move toouter electrode. 
Introduction

Movement of Ion Pairs to ElectrodesIf the electric field strength is sufficiently large, the iwon’t recombine. The electrons move to the anode positively charged gas ions move towards the cathoThe drift velocity is the velocity at which the ions mo

y y

towards the electrodes. It is given by the following f

ξ is the electric field strength (e.g., V/m)v is the drift velocity (e.g., m/s)u is the mobility factor of the ion (e.g., m2 atm/V s

p is the gas pressure (e.g., atmospheres)
Introduction

Movement of Ion Pairs to ElectrodesPositively charged gas ions take approximately 10 milliseconds to reach the cathode in a typical detecmobility constant is about 1.3 x 10-4 m2 atm/V s. Si

mobility constant for electrons is roughly 1000 time

electrons reach the anode in microsecondselectrons reach the anode in microseconds.

As the positive ions and electrons move to the electhey collide with various gas molecules and sometitransfer their charge to those molecules (depending latter’s electro-negativity). If an electron collides witelectronegative molecule (e.g., O2), the electron wil

captured and the now negatively charged gas molerather than the electron proceeds to the anode..
Introduction

Movement of Ion Pairs to Electrodes over Time

Ne Ne+

Ne+

eNe Ne Ne 

(-) (-) (-) 

O2 O2 O2

eA neon atom is ionized.

The neon ion goes to the cathode and the electron heads towards the anode

The electron coan O2 molecule 

captured. The n

ion goes to the 

(+) (+) (+) 
Beta

IntroductionBeta Particle Interaction with the Detector

The average energy lost by a charged particle per ioproduced is approximately 34 eV. Since a typical electron/beta particle produces 100 ion pairs per cm a 1 Mev electron might have a range of 300 cm.Given the size of a typical gas detector, beta particletransfer a small portion of their energy to the detecto
Alpha

IntroductionAlpha Particle Interaction with the Detector

The average energy lost by a charged particle per ioproduced is approximately 34 eV. A typical alpha paproduces 40,000 ion pairs per cm. As such, a 6 Meparticle will have a range of about 4.5 cm. Given the size of a typical gas detector, it is possible alpha particles to expend all their energy in the dete
Introduction

Photon Interaction with the Detector

Photons might interact directly with the detector gasare more likely to interact with the detector wall (the In some of these interactions, an electron is “knockedetector wall This electron then traverses the gas indetector chamber. This electron might, or might not, sufficient energy to cause further ionizations.
Introduction

Radiation Type and the Size of the Detector SignWith ion chambers and proportional counters, the mof the detector signal depends on the number of ion produced in the gas by the incident charged particleIn general but not always the largest to smallest sIn general, but not always, the largest to smallest s

would be produced by:

Alpha particles (largest signal)Beta particlesPhotons (smallest signal)
Introduction

Modes of OperationA gas detector can operate in one of two modes: 

1. Current Mode

2 Pulse mode2. Pulse mode

Ion chambers usually operate in the current mode btheir pulses are too small to be easily distinguished electronic noise.Proportional counters and Geiger Mueller detectors operate in the pulse mode.
Current Mode of 

Operation
Current Mode of Operation

GeneralIn the current mode, the system measures the currethe detector chamber rather than count the pulses. The detector calibration relates the current to a quaas the exposure rate (e g R/hr)as the exposure rate (e.g., R/hr).

Current (I) is measured in units of amperes (A). 

1 ampere = 1 coulomb per second 

The current from a radiation detector is very small, the order of picoamperes (pA).
Current Mode of Operation

GeneralCurrent is charge per unit time, e.g., coulombs per 

I = Q/t

I is current (amperes)I is current (amperes)Q is the collected charge (coulombs)t is the time (seconds)

Q = n e

n is the number of collected electronse is the charge on each electron (1.6 x 10-19 
Current Mode of Operation

ExampleIf one million electrons are collected by the anode operiod of 10.7 ns (1.07 x 10-8 s), what is the current

I = Q/t

Q = n e

= 10

6 x 1.6 x 10-19

C

= 1.6 x 10-13

C

I = Q/t

= 1.6 x 10-13 C/ 1.07 x 10-8

s

= 1.5 x 10-5C/s= 1.5 x 10-5 amperes
Current Mode of Operation

ExampleIf it took 8 nanoseconds (instead of 10.7) for the elereach the anode, the current is:

I = 1.6 x 10-13 C/ 8 x 10-9

s

= 2.0 x 10-5 amperes

If it took 16 nanoseconds, the current is:

I = 1.6 x 10-13 C/ 16 x 10-9

s

= 1.0 x 10-5 amperes

In each of these three cases, exactly the same amoionization occurred in the detector gas and exactly charge was collected at the anode.
Current Mode of Operation

ExampleThe following figure shows the current as a function during these three “pulses.” The area of each is the charge. It is the same for al

Charge (coulombs) = current (amperes) x time (Charge (coulombs)

= current (amperes)

x time (

Current

(x 10-5 A)

Time (sec)

12
Current Mode of Operation

ExampleWhen a detector operates in the current mode, the electronics average the current over some period oIn this example, we imagine that the detector integrperiod is 100 nanoseconds (1 x 10-7 s).period is 100 nanoseconds (1

x 10 s).

The averaged current from the detector is:

I = Q/t

= 4.8 x 10-13 C/ 1 x 10-7

s

= 4.8 x 10-6 amperes
Pulse Mode of 

Operation

9/8/2008
Pulse Mode of Operation

Pulse Characteristics: Voltage (V)As long as the detector gas is not ionized, the potedifference between the anode and cathode is const900 volts).During the movement of the electrons and positive During the movement of the electrons and positive 

electrodes, the potential difference decreases. After some period of time, the voltage stops decreareturns to the baseline voltage. This decrease and recovery in the voltage is the pumaximum change in the voltage is the pulse height (amplitude).
Pulse Mode of Operation

Pulse Characteristics: Current (I)The movement of electrons to the anode induces a from the anode during each pulse.The magnitude of the current depends on the pulse and the duration of the pulseand the duration of the pulse. 

I = Q/t

I is the current in amperesQ is the charge in coulombst is the time in seconds
Pulse Mode of Operation

Pulse Characteristics: Charge (Q)The charge (Q) of the pulse is the charge on the elereaching the anode.

Q = n e

Q is the collected charge in coulombsn is the number of electrons reaching the anodee is the charge on an electron (1.6 x 10-19 coulo

The charge is the only pulse characteristic that is arelated to the energy deposited in the detector gas.
Pulse Mode of Operation

Analyzing the PulsesWhen a detector operates in the pulse mode, we m1. Count the pulses, and/or 2. Measure the pulse height (amplitude)

For the pulse height to carry useful information energy deposited in the detector gas, it must brelated to the collected charge. 

As the following discussion indicates, a proper choi

time constant (

J) is necessary for the pulse height t

related to the collected charge.
Pulse Mode of Operation

Making Pulse Height Related to the Collected C

Current

(x 10-5 A)

12

Here we see the detector current as a function of tithree ionizing events. Each event deposits the samin the detector gas. The collected charge (number oelectrons and ions reaching the electrodes) is the seach case. The area of each “pulse” (current x timecollected charge. 

Time (sec)
Pulse Mode of Operation

Making Pulse Height Related to the Collected CThe figure on the following slide shows pulse size (function of time when the time constant is much shothe charge collection time.

Time Constant (

J) << charge collection time

The pulse width indicates the charge collection timeSince the time constant is small, the electronics preresistance to the current. In addition, the system laccapacitance to accumulate a charge. The potential difference (voltage) reflects the instancharge in the circuit rather than the total collected c
Pulse Mode of Operation

Making Pulse Height Related to the Collected C

Pulse Height

(volts)

Time Constant (

J) << charge collection time

The bottom line is that the pulse height doesn’t refleenergy deposited in the detector gas (total collected In other words, the pulse height carries no useful in

(volts)

Time (sec)
Pulse Mode of Operation

Making Pulse Height Related to the Collected CThe figure on the next slide also shows pulse size afunction of time. But in this case, the time constant greater than the charge collection time.

Time Constant (

J) >> charge collection time

The large resistance to the current and the large caof the electronics means that the current is “held upcauses the potential difference (volts) to build up to potential maximum voltage.The maximum pulse height now reflects the energy in the detector gas. This is potentially useful inform
Pulse Mode of Operation

Making Pulse Height Related to the Collected C

Pulse 

Height

Time Constant (

J) >> charge collection time

(Volts)

Time

The output pulse from the detector is negative, i.e., voltage decreases. Nevertheless, it is common to ilthe pulse as if it were positive.
Pulse Mode of Operation

Pulse Shape

Pulse Amplitude

Rise Tail

Time Constant (

J) >> charge collection time

(volts)

Time (sec)

The pulse has two components: a rise and a tail.
Pulse Mode of Operation

Pulse Rise and the Maximum Voltage (Vmax

)

The time that the pulse takes to reach its maximum (rise time) is determined by the charge collection tim

When the maximum voltage is directly related to the 

Time Constant (

J) >> charge collection time

When the maximum voltage is directly related to the 

charge (

J >> charge collection time):

Vmax is the maximum voltage decrease (volts)

Q is the charge collected at the anode (coulomb

C is the detector capacitance (farads)
Pulse Mode of Operation

Pulse Tail and the Time Constant (

J

)

The rate at which the pulse returns to the baseline controlled by the electronic characteristics of the sy

S ifi ll th l th f th l t il i d t i 

Time Constant (

J) >> charge collection time

Specifically, the length o

f the pulse 

t

ail is 

d

e

termine

time constant. The time constant is calculated as fo

J = R C

J is the time constant (seconds)

R is the circuitry (e.g., preamplifier) input resistanC is the circuitry and detector capacitance (farads
Pulse Mode of Operation

Pulse Tail and the Time Constant (

J

)

The following equation gives the voltage at any time pulse has reached its maximum amplitude:

Time Constant (

J) >> charge collection time

Vmax is the maximum voltage at time t = 0Vt is the tail’s voltage at time t J is the circuitry time constant

The time constant is analogous to a half life: If the tconstant is short, the tail is short. If the time constathe pulse has a long tail.
The Six (or five) 

Region CurveRegion Curve
Six Region Curve

GeneralThe six region curve is a plot of pulse height as a futhe high voltage applied to a gas detector.It is sometimes depicted as a five region curve with proportional region and the region of limited propor

gg 

merged together.It indicates that theoretically a single detector can fan ionization chamber, proportional counter or Geigdetector depending on the applied high voltage.When interpreting it, it can be helpful to consider thheight as the number of electrons collected at the a
The Six Region Curve

Pulse Height

(# electrons 

collected at

Recombination

Ionization

Proportional

y

collected at

anode)

Applied High Voltage

Betas

Alphas

Limited Proportionalit

Geiger-Mueller

y
Six Region Curve

GeneralThis curve, which applies to a single detector, showGeiger-Mueller region at a higher voltage than the proportional region.In practice, there are design differences between GIn practice, there are design differences between GMueller tubes and proportional counters that cause Geiger-Mueller detectors to operate at lower voltagmost proportional counters.
Six Region Curve

1. Recombination RegionAt the lowest voltages, the strength of the electric fisufficiently low that many of the primary ion pairs crthe gas by the incident charged particle (e.g., alphasimply recombine. If no electrons reach the anode - there is no pulse.
Six Region Curve

2. Ionization RegionIn the ionization region, the electric field is just stronto prevent recombination of the ion pairs produced One electron reaches the anode for each primary iocreated by the incident radiation. The curve (plateacreated by the incident radiation. The curve (platea

because increasing the voltage doesn’t increase thof electrons reaching the anode. The resulting current is referred to as a saturation cSince an alpha particle produces considerably morthan a beta particle, the alpha pulses are corresponlarger.
Six Region Curve

2. Ionization Region

Alpha
Six Region Curve

2. Ionization Region

Pulse Height

(# electrons 

collected at

Ionization

Region

collected at

anode)

Applied High Voltage

Betas

Alphas
Six Region Curve

3. Proportional RegionThe applied high voltage is greater and the electric substantially stronger than in the ionization region.As the electrons of the primary ion pairs move towaanode, they gain kinetic energy. When they get closanode, they gain kinetic energy. When they get clos

to the anode, the electric field is sufficiently strong (

V/m), that these electrons gain enough kinetic enercreate secondary ion pairs.The electrons produced in these secondary ion paiwith the primary electrons, continue to gain energy move towards the anode, and as they do, they prodand more ionizations. 
Six Region Curve

3. Proportional RegionThe result is that each electron from the primary ion produces a cascade, or avalanche of ion pairs (Towavalanche). For every primary ion pair created in the chamber b

yy 

ionizing radiation, 102 to 104 electrons reach the an

increase in the number of electrons is referred to as multiplication factor.As the high voltage increases, the distance from the which the field strength exceeds 106 V/m increases

avalanches are initiated further away from the anodmultiplication factor (and pulse size) increases.
Six Region Curve

3. Proportional RegionThe greater the number of primary ion pairs the larger the pulse.The higher the voltageThe higher the voltage, 

the larger the avalanches and the larger the pulse.

The figure illustrates a single pulse consisting of simfive avalanches.
Six Region Curve

3. Proportional Region

Beta 

Particle

A
Six Region Curve

3. Proportional Region

Beta 

Particle

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional Region

A
Six Region Curve

3. Proportional RegionThe voltage at which the electric field strength becoenough for avalanches to be produced marks the eionization region and the beginning of the region ofproportionality. The proportional region is sometimes referred to as region of true proportionality because there is a linerelationship between the number of primary ion painumber of electrons reaching the anode.
Six Region Curve

3. Proportional Region

Pulse Height

(# electrons 

collected at

Proportional

Region

Limited 

Proportionalit

Region

collected at

anode)

Applied High Voltage

Betas

Alphas
Six Region Curve

4. Region of Limited ProportionalityThe high voltage and the multiplication factor are lain the proportional region. What is now different is that pulse size is no longer related to the number of primary ion pairs produced related to the number of primary ion pairs produced 

incident radiation. In other words, doubling the numprimary ion pairs, doesn’t double the pulse size. This increases the uncertainty of using pulse size to distinguish different types of radiation (e.g., beta vs
Six Region Curve

5. Geiger-Mueller RegionIn this region, the size of the pulse is totally indepenthe number of primary ion pairs produced by the incradiation.In other words alpha particles beta particles and pIn other words, alpha particles, beta particles and p

produce pulses of exactly the same size. There is a complete loss of proportionality between radiation energy deposited in the detector gas and size.
Six Region Curve

5. Geiger-Mueller RegionAt the high voltages associated with the Geiger-MuRegion, the avalanche spreads along the complete the anode. The pulse shuts itself down when the accumulation The pulse shuts itself down when the accumulation 

ions around the anode reduces the strength of the efield below that required for continued propagation. At a given voltage in the Geiger region, the size of tis the same no matter how many primary ion pairs wproduced by the incident radiation. 
Six Region Curve

5. Geiger-Mueller Region

Pulse Height

(# electrons 

collected at

Geiger-M

Regio

collected at

anode)

Applied High Voltage

Betas

Alphas
Six Region Curve

5. Geiger-Mueller RegionWhy does the avalanche spread along the anode? avalanches not only involve ionization, they also invexcitation of gas molecules. And when excited molean avalanche de-excite, they can emit UV photons.In the Geiger-Mueller region enough UV photons aproduced for some to be absorbed by the gas molethe photoelectric effect. This leads to further ionizatgas and the creation of additional avalanches. By tha single avalanche becomes thousands of avalanchThe gases used in GM tubes are selected to promogeneration and absorption of UV photons.
Six Region Curve

5. Geiger-Mueller Region

Beta 

Particle

A
Six Region Curve

5. Geiger-Mueller Region

A
Six Region Curve

5. Geiger-Mueller Region

A
Six Region Curve

5. Geiger-Mueller Region

A

PE
Six Region Curve

5. Geiger-Mueller Region

A

e-
Six Region Curve

5. Geiger-Mueller Region

A
Six Region Curve

5. Geiger-Mueller Region

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The result:

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The avalanche completely envelops the anode. 
Six Region Curve

5. Geiger-Mueller RegionThe electrons in the avalanche are collected quicklythe positive ions. This creates a positive “space” charge in the vicinity anode

.

This positive charge reduces the strength of the ele

near the anode.The reduced electric field strength shuts down the psince further ionization is impossible.
Six Region Curve

5. Geiger-Mueller Region

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The result:

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The avalanche completely envelops the anode. 
Six Region Curve

5. Geiger-Mueller Region

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ + + + + + + + + + + + + + + + + + + + + + + + 

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +-

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The electrons are collected quickly, but not the pos
Six Region Curve

5. Geiger-Mueller Region

+ + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + +

This positive charge reduces the strength of the elenear the anode which shuts down the pulse. Furtheionization is impossible.
Six Region Curve

5. Geiger-Mueller RegionIn the Geiger-Mueller region, the plateau still slopes As the high voltage increases on the plateau, it takepositive ions to produce a large enough space charovercome the increased field strengthovercome the increased field strength.
Six Region Curve

6. Continuous DischargeIn this region, arcing will occur that can result in a cbreakdown of the gas. Any extended operation in this region will destroy th

Applied High Voltage

Pulse 

Height

(Volts)

Continuou

Discharge
Six Region Curve

Where the Avalanche OccursWe want all the avalanches to be produced within a volume around the anode, i.e., within a fraction of a millimeter.This way, the primary ion pairs produced by the radbe created outside the region in which the avalanchproduced. This is necessary if the pulse size is to renergy deposited in the gas.If primary ion pairs were produced inside the avalanregion, their avalanches would be smaller than thosproduced by primary ion pairs formed outside this rThis is because the avalanches would begin closer anode.
Six Region Curve

Where the Avalanche OccursThis figure shows the effect of primary ion pair beininside the region where the avalanche occurs.The pulse on the right is smaller than it should be.
Know the Difference 

Between these CurvesBetween these Curves
Know the Difference Between these 

Pulse 

Height

(Volts)

Pulse 

Height

(Volts)

Time

Applied Hig

Number

Of

Pulses

Applied High Voltage Pulse He

Number

Of

Pulses
Know the Difference Between these 

Pulse 

Height

(Volts)

Time

This is a single pulse, more or less as might be seean oscilloscope.
Know the Difference Between these 

Pulse 

Height

(Volts)

Applied High Voltage

This is the five/six region curve. Is used to help explain the difference between ionizchambers, proportional counters and Geiger Muelldetectors.
Know the Difference Between these 

Number

Of

Pulses

Applied High Voltage

This is a sometimes known as a characteristic curvIt is produced to help determine the appropriate opvoltage for a gas detector. They are occasionally gfor scintillation detectors as well.
Know the Difference Between these 

Number

Of

Pulses

Pulse Height

This is a spectrum. The peaks on the spectrum are formed by many pusimilar height. The pulse height associated with the identifies the energy of the radiation (gamma or alpnumber of counts in the peak (peak area) is related activity of the source.
Basic Characteristics 

of Gas Detectorsof Gas Detectors
Basic Characteristics of Gas Detect

Ion Chambers

• operate in the ionization region• can use both parallel plate and coaxial electrod

• usually operate in the current mode• primarily used to measure exposure rates due t

rays and x-rays

• usually poor response at low exposure rates (< • capable of measuring high exposure rates (1–1• energy independent (flat relative response curv• air often used as the fill gas• might be sealed or open to the atmosphere• if open to atmosphere: they use a desiccant (sil

dry the air; their response is affected by ambienpressure and temperature
Basic Characteristics of Gas Detect

Proportional Counters• operate in the proportional/limited proportionality • use coaxial electrodes or variant• almost always operate in the pulse mode• primarily used to count alpha and beta sources a

surface contamination

• can distinguish small beta pulses from larger alp• much shorter dead time than GMs, therefore bet

count rates

• might be sealed or operate in gas flow mode

• gas flow mode allows operation at one atmosphe

use of larger an thinner window which improves 

• P-10 is common fill gas: 90% argon and 10% me
Basic Characteristics of Gas Detect

Geiger-Mueller Detectors• operate in the Geiger Mueller region• use coaxial electrodes• usually operate in the pulse mode• used to measure exposure rates or count beta so

surface contamination

• exposure rate measurements ca. 0.1 mR/hr to 1 • energy dependent: when measuring exposure ra

over respond at low photon energies (ca. < 200 k

• all pulses are the same size, hence no discrimina

between alpha and beta and gamma pulses.

• usually sealed with gas at a fraction of an atmos

• neon fill gas with halogen (Cl

2) quench gas is co

• sometimes helium fill gas with organic quench ga• large dead times (e.g., 100 us)