Instrumentation and Detection

Harry M. Johnson, PhD

CancerCare

Winnipeg, Manitoba, Canada

Instrumentation and Detection

• Purpose: To discuss the detection of radiation from the point of view of the classical instrumentation.

Instrumentation and Detection: Outline

• The Basic Gas-Filled Detector• Classes of Gas-Filled Detectors• The Geiger Tube as an Exposure Meter• Choosing a Radiation meter• Scintillation Detectors• Calibrating a Contamination Meter • Neutron Meter• Thermoluminescent Personnel Dosimetry

The Gas-Filled Detector - 1• Gas-filled counting chamber

• Use air as the gas for initial testing

• sealed or unsealed

• coaxial electrodes, well insulated

• variable voltage, defined anode and cathode

• high resistance resistor

• capacitance C

• radiation enters chamber and ionizes the gas

• ions drift towards the electrodes

• Voltage pulse is detected

The Gas-Filled Detector - 2

• Apply low voltage

• a few ion pairs drift to electrodes

• time constant RC is long

• Voltage V= Q/C is detected at output

• Pulse curve has elongated shape

• Difficult to detect successive pulses

The Gas-Filled Detector - 3

• Maintain the same voltage

• “drifting” of ion pairs drift to electrodes is the mechanism of charge transport from ion pair production

• shorten the time constant RC

• Voltage V= Q/C continues to be detected at output

• Pulse curve has “clipped” shape

• Successive pulses are detectable

• Now possible to count the individual pulse

• This is the preferred chamber design

• Possible to calculate maximum pulse rate

Gas-Filled DetectorsGas-Filled Detectors

Air or Other Fill Gas

ElectricalCurrent

Measuring Device

Incident Ionizing

Radiation +

-

Cathode -

Anode +

+ + +

- - -

+ -Voltage Source

Ion Chamber Instrument

• Example of Ion Chamber

• 6 cc Chamber• 180 cc Chamber• Readout unit is

located remote from the detector

The Gas-Filled Detector • The first plateau

region is the “ion chamber” mode

• Typically 300 volts applied voltage

• Pulse size is independent of LET

• No secondary ionizations - which would amplify the pulse height

• Pulse size distinction is a disadvantage

The Ionization Chamber • Radiation of constant flux is applied to chamber

• Vary the voltage in the few-hundreds of volts region

• ionizations in the air of the chamber create ion pairs

• a plateau region is noted - the operating plateau for the ion chamber

• V is just high enough to collect all ions from ionizations

• Ions are not being accelerated - still “drifting” to electrodes

• Pulses are distinguishable - each pulse is a single ionization eventPules size is independent of voltage

• a beta particle produces 1000 ion pairs, output pulse voltage is low millivolts

• Low output voltage is disadvantageous

The Proportional Counter• Adjust the voltage upward beyond the ion chamber plateau

• Ions from the initial ion pairs are accelerated

• electron acceleration is more important than positive ions

• Secondary electrons are produced by collisions by the primary ion-pair products

• Change the gas in the chamber

• These new design features create a gas amplification factor (>1) - called an “avalanche”

• By increasing the tube voltage the avalanche spreads along the anode wire

• Pulse size is proportional to the chamber voltage

• Pulse size depends on the electric field gradient

• Anode (central) diameter is important

The Proportional Counter• This is the “Proportional Counter” Region

• Note slight upward slope of Voltage-Pulse curve in this region

• Pulse size is dependent on LET - giving the ability to discriminate between type of radiation

• Anode wire size is typically 0.02 to 0.1 mm diameter

• Decreasing the gas pressure increases the multiplication, although the tube may be at atmospheric pressure

• High-voltage supply must be very stable

The Geiger Tube - 1

• Basic Readout module

• Two Geiger tubes: “Pancake” type and the “End-Window” type.

• Entrance windows are very thin

The Geiger Tube - 2• Continue to raise the voltage past the “proportional

counter” plateau to a new plateau.

• Slight upward slope of Voltage-Pulse curve in this region.

• Counting rate is independent of voltage.

• Change the gas to Argon ( = 15.7 eV; W = 26.4 eV) or Methane ( = 15.2 eV; W = 27.0 eV).

• Add 10% ethyl alcohol to eliminate uv .

• Tube is sealed.

• Chamber pressure may be 10% of atmospheric pressure

• Output pulse (without amplification) height:approx 1 volt

• Little need for amplification

• Output pulses occur independent of size of initiating ionization - no discrimination re LET of the radiation.

The Geiger Tube - 3• This is Geiger (Geiger-Mueller) tube region (1908).

• Voltage is sufficiently high that both ions of the initiating ion pair are accelerated.

• Accelerated ions cause additional ionizations (avalanche).

• Accelerate +ve ions strike cathode (tube shell), cause excitations of cathode molecules, yielding UV production.

• UV is additional source of gas ionization/excitation.

• When intense ionization occurs in the tube the E-Field along the anode wire drops to zero. This causes dead time.

• GM tube can go into continuous discharge when this occurs. Add alcohol to argon gas to quench the discharge.

• Alcohol reduces dead time to 100 micro seconds. (Resolve up to 10,000 pulses/sec).

• GM tube is easy to build, simple electronics, cheap.

The Geiger Tube - 4

• Choose operating voltage for G-M region at 1/3 to 1/2 way up the plateau.

• Alcohol is present to quench the ionizations and absorb the UV produced when accelerated +ve ions strike shell.

• Alcohol molecules dissociate in this process.

• Lifetime of the tube is limited by the alcohol - total lifetime is 108 - 109 ionizations.

• Ifd voltage is raised above the Geiger region, the avalanche spreads and continuous discharge occurs. Gas tube cannot operate as a detector above the Geiger region.

Resolving Time and Dead Time • Two ionizations in G-M

tube in rapid succession may not be resolved.

• The first ionization causes a Dead Time when no new pulse can be detected

• Followed by Recovery Time when a new pulse may not be identifiable

• Resolving Time is sum of Dead and Recovery times

Resolving Time and Dead Time - 2

• Avalanche starts near the anode wire, spreads along anode due to the High Voltage in G-M region

• Electrons move more quickly than +ve ions

• Rate-limiting step is the transit time of +ve ions to the cathode. This transit time defines the collection time

• Resolving time can be defined

• Tube resets itself after recovery

• Collection time may be a few hundred microseconds

• If CT is 250 s, what is limiting rate of detectable ionizations - how many photos per sec is max input?

Advantages/Disadvantages of Gas Detection Tubes

• Ion Chamber: simple, accurate, wide range, sensitivity is function of chamber size, no dead time

• Proportional Counter: discriminate hi/lo LET, higher sensitivity than ion chamber

• GM Tube: cheap, little/no amplification, thin window for low energy; limited life

Geiger Tube as Exposure Meter

• “Exposure” is the parameter measuring the ionization of air.

• Geiger tube measures ionization pulses per second - a “count rate”.

• The number of ionizations in the Geiger tube is a constant for a particular energy but is energy dependent.

Energy Compensation of Geiger Tube

• An energy compensating shield is required to smooth out the energy response

Energy Response for Victoreen 489-4 GM Probe

0.1

1

10

10 100 1000 10000

Photon Energy in keV

Rat

io O

utp

ut

to A

ctu

al E

xpo

sure

R

ate

Bare TubeResponse

Response - BetaShield Closed

Scintillation Detector

• Readout Module plus Detector

• Photomultiplier Tube (P)

• Scintillator Chrystal (C)

P C

Scintillation DetectorsScintillation Detectors

Sodium-IodideCrystal

Photocathode

Optical Window

-Pulse

MeasuringDevice

Light PhotonPhotomultiplier Tube

Dynode Anode

Incident Ionizing Radiation

Scintillation Detectors

• Construction of Crystal and PMT assembly

• Design of basic electronics

• Principle of scintillation

• Principle of operation

Scintillation Detection System• Design of Basic Pulse

Height Analysis System

• Amplitudes of voltage pulses are sorted by PHA

• PHA counts number of pulses for various voltages (energies)

• Display is a histogram of pulse heights

Scintillation Detector Materials - 1

Phosphor Rel PulseHeight

Decay Timenanosec

NaI (Tl) 210 250

Cs I 55 1,100

Plastic 28-48 3 - 5

‘Liquid” 27-49 2 - 8

Scintillators - 2 (Derenzo et al 1992)

Phosphor Pulse Heightphoton/MeV

Decay Timenanosec

Na I (Tl) 38,000 230

NE 102A 10,000 2.4

ZnWO4 10,000 5,000

CsI (pure) 59,000 800

CsI (Tl) 2,300 16

CdS(Te) 190 18

Analysis of Scintillator Peak - 1

• NaI (Tl) scintillation peak for Cs-37: 662 keV

• Large crystal: 10x10 cm

• Only photons that lose all energy (i.e. Compton events + final photoelectric event) contribute to the “Total Energy Peak”

Analysis of Scintillation Peak -2

• “Continuous Compton Distribution” arises from light from Compton events and Secondary photons escape from the crystal before the photoelectric event occurs.

• “Compton Edge” (478 keV) is the energy of the maximum recoil electron for h = 662 keV.

• Tmax = 2 h /(2 + mc2/ h ) = 478 keV; This occurs

when the Compton scattering angle (for the secondary Compton photon) is 180 degrees.

Analysis of Scintillation Peak -3• “Backscattered” photon has energy 662-478 = 184 keV.

The backscattered peak is visible in the spectrum.

• Relative area under the Total Energy Peak (photopeak) depends on the size of the crystal.

• Ratio of areas under the “Total Energy” peak and the “Compton Distribution” in a small detector is approximately equal to ratio of photoelectric to Compton cross-section in the crystal material.

Analysis of Scintillation Peak -4• Escape Peak: when the detector is small, the Escape

Peak may be visible.

•

• This peak arises when a K-shell vacancy occurs in iodine (of the NaI material) following a photoelectric event. A characteristic 28 keV x-ray is emitted.

Alpha Detectors

• Proportional Detectors: counting with discrimination from beta-gamma ionizations.

• Scintillation Detector - Zinc Sulphide with discrimination against beta-gamma ionizations by pulse height control and by thin detector efficiency.

ZnS Alpha Detector

• This alpha detector uses a thin scintillator of zinc sulphide on thin plastic, aluminized to keep out light.

• The detector is connected to a rate meter with pulse height discriminator. It senses only alpha radiation & rejects beta and gamma.

Neutron Detectors - Choices

• The dose equivalent detector: a “rem meter”

• Activation foils: cadmium

• Bubble detectors

Neutron Rem Meter• A gas detection tube

(BF3) is located at the centre of a polyethylene sphere with a thin cadmium filter.

• Sphere moderates neutrons to permit detection by BF3 tube

• Energy range 0.025 eV to 10 MeV

• Gamma radiation is rejected

TLD Personnel Dosimetry

• Themoluminescent crystals of LiF are well suited to personnel dosimetry.

• Ionizing radiation creates electron dislocation that remains until heated.

• Light output on heating is proportional to dose.

Choosing a Meter

• Contamination or Radiation?

• X-ray, Gamma, Alpha or Neutron?

• Energy Dependence?

• Response Time: Fast or Slow?

• Sensitivity: Low doses or high doses?

• Fixed or Portable?

• Calibration?

Summary

• Beta- Gamma Gas detectors• Contamination or Radiation• Scintillation Detectors• Analysis of PHA histogram of energy spectrum• Alpha Detectors • Neutron detectors• Personal Dosimetry Methods• Choosing a Meter