Pakistan Institute of Engineering & Applied Sciences (PIEAS)

Lectures on Radiation Detection

Delivered at Workshop in PNRA (April, 2008)

Dr. Nasir M Mirza

Deputy Chief Scientist,

Department of Physics & Applied mathematics,

PIEAS, P.O. Nilore, 45650, Islamabad.

Email: nmm@pieas.edu.pk

Ph: +92 – 51 – 9290273 (ext: 3059)

Gas-filled Detectors

Recommended Text Books

1. Glenn F Knoll ‘s Radiation Detection & Measurement (recent edition).

Lecture Two:

Various Types of Radiation Detectors

Effect Type of Instrument Detector

Electrical 1. Ionizing Chamber

2. Proportional Counter

3. GM Tube

4. Solid State Detector

1. Gas

2. Gas

3. Gas

4. Semiconductor

Chemical 1. Film

2. Chemical Dosimeter

1. Photographic Emulsion

2. Solid or Liquid

Light 1. Scintillation counter 1. Crystal or Liquid

Thermo-

luminescense

1. Thermo - luminescense dosimeter

1. Crystal

Heat 1. Calorimeter 1. Solid or Liquid

Gas Filled Detectors

• Ion chamber

• Proportional counter

• GM tube

Ion Chamber

• Ionization Chamber – a detector in which ion pairs (positive ion and free electron) created by interaction with radiation are collected from a gas.

• W-value: the average energy lost by an incident particle per ion pair formed

• Substantially greater than the ionization energy• Weak function of the gas involved, type of radiation, and

radiation energy• Relatively constant in practice• Can be used to calculate number of ion pairs formed

W-value

• The average energy lost to form an ion pair (Typical 30-35 eV)

Ionization Chamber as a Gas Detector

Ionization Processes in Gas

1. Ionization and excitation of gas molecules happened along the particle track and resulting positive ion and free electron

2. If an external electric field is applied, the charges would move to anode and cathode to yield electronic output

3. Various processes are:

• Diffusion

• Charge transfer

• Recombination

Condenser-type pocket dosimeter

and its charger .

The dosimeter measures gamma

and X-rays within 15% for 30keV

to 1.2 MeV in the range 0-200 mR .

Ion-chambers as survey meters

Small amounts of electrical current are measured using sensitive current-measuring devices called electrometers. Two devices consisting of ionization chambers and electrometers in nuclear industry are survey meters and dose calibrators.

Post-ionization

Once an ion pair is created, the following can occur:• Recombination• Charge transfer collisions (positive ion and neutral gas molecule)• Electron captured by neutral particle to form negative ion

Recombination can be minimized by applying an external electric field to the gas, separating the ion pair constituents.

Gas-filled Detectors

In most ionization chambers, the gas between the electrodes is air. The chamber may or may not be sealed from the atmosphere. Many different designs have been used for the electrodes in an ionizationchamber, but usually they consist of a wire inside of a cylinder, or a pair of concentric cylinders.

Pocket Dosimeter is an Ionization Chamber

The voltage change across the capacitor is measured and is related to the amount of electrical charge collected by the ionization chamber electrodes (dQ = dV x C).

The charge stored on the capacitor is Q = V x C. When the chamber is exposed to radiation, electrical charge dQ is collected by the electrodes, discharging the capacitor.

A device that records total charge collected over a period of time is the pocket dosimeter.

Problems with ion-chambers

• A basic problem with ionization chambers is that they are quite

inefficient as detectors for x and gamma-rays. • Only a very small percentage (less than 1percent) of X- or gamma

rays passing through the chamber actually interact with and cause

ionization of air molecules.

• Two additional problems with ionization chambers should be noted.

The first is that for x and gamma- rays, their response changes with

photon energy because photon absorption in the gas volume

• and the detection efficiency and relative penetration of photons

through the chamber walls both are energy-dependent processes

Proportional Counter

• In proportional region the Secondary ionization causes the avalanche to occur.

• Higher output is achieved.

• Can still tell different radiations from their energy

Gas Multiplication and Avalanche in Proportional Detector

anode

cathode

an electron

The avalanche will stop after the electric field reduced to a threshold caused by the space charge of accumulated positive ions in the gas.

Choice of Geometry

d

Vd )(

)/ln()(

abr

Vr

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

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

--

Uses of a Proportional DetectorProportional counter-ion chamber used for

•Detection and spectroscopy of low energy photons

•Neutron detectors

Operated at a lower voltage than a Geiger-Mueller detector

Proportional Gas-filled Detectors

Increasing voltage reduces ion-electron recombination

Each ion-electron pair collected

Avalanches start. Gas multiplication linear with applied voltage

Large number of positive charges reduce applied electric field

Geometry of Proportional Counters

Cylindrical geometry allows for a large applied electric field. E is very large near r = a where electrons are collected. Cannot achieve similar fields with parallel plate construction.

E r V

r lnb

a

E = electric fieldV = applied voltager = distance from anodea = anode wire radiusb = cathode inner radius

Most charge multiplication occurs near anode wire regardless of the location where original ion-electron pair is formed

Design Features

High voltage applied at anode wire•Diameter must be consistent to maintain proportional counting (constant E)•Easier for thicker wire (which drops possible E)•Must balance contradictory design needs

Vacuum seal

Field tube-large diameter wire gives low E and no gas multiplication. This avoids geometry effects near ends of detector and keeps charge amplification proportional throughout (could also have constant diameter wire with lower potential).

Active volume Grounded

cathode

Thin window design for low energy particle detection

Windowless proportional counters• Sample can be introduced into chamber. Valuable for low energy particle detection or for alpha counting where significant energy might be lost through window.

• Solid angle of close to 2 exposed to proportional gas

• Detector must be purged of air after sample is introduced

• Longer lifetime than sealed detectors because loss of fill-gas can end usefullness of a sealed detector.

P-10 which is 90% Ar and 10% methane is a commonly used fill gas for gamma detectors. Fill gas must not attach to electrons. Heavier inert gas better for high efficiency gamma detection.

Pulse shapes

Most ions and electrons are formed near anode. Positive ion drift is fast near anode

Pulse build up slows as positive ions move into smaller field zone of detector

Fast output pulses are promoted by using low gas pressure, high applied voltage, and small anode wires (larger applied field)

Alpha and Beta counting

•Because of differences in range, only a fraction of a beta’s energy is deposited in the gas where all of the alpha energy is deposited in the gas.

•Leads to two plateaus when generating a counting curve

•Can be used to eliminate beta events sent to counting circuit

Graph assumes equal energy particles

Gas multiplication by Avalanche

• To help visualize the avalanche formation in the neighborhood of the wire surface, Figure shows results obtained from a Monte Carlo modeling of the electron multiplication and diffusion processes;

• It was assumed that a single free electron drifted into the vicinity of the wire.

• The resulting avalanche is confined to a small distance along the length of the wire equivalent to only several times its diameter.

• As a consequence, methods for sensing its position along the wire can accurately measure the axial position of the incident electron.

Types of Geiger-Mueller (GM) Tubes

Geiger-Mueller (GM) Tube

• Entire avalanche – full ionization

• Cannot still tell different radiations

• Quenching is necessary

– Electrical quenching : reduce electrical voltage after avalanche

– Chemical quenching : add a little halogen gas

Uses of GM TubesGeiger-Mueller detector

• Simple, low cost, easy to operate

• Pulse type counter that records number of radiation events

• All energy information is lost-no ability to do spectroscopy

• Dead time greatly exceeds any other commonly used radiation detector

Charge pulseGeiger-Mueller detector •Employs gas multiplication to greatly increase the charge represented by the original ion pairs formed by the radiation•At a specific large electric field, one charge avalanche creates a second, ultimately leading to a self-propagating chain reaction•At larger electric fields, the number of avalanches grows exponentially until interactions between avalanches terminate the chain reaction

All pulses from a Geiger tube are of the same amplitude regardless of the number of original ion pairs

Charge pulseGeiger-Mueller detector

•The build-up of slowly moving positive charges around the positively charged anode terminates the discharge by decreasing the electric field•Discharge would start again when positive ion hit cathode and regain electrons•A quench gas is added to the detector that disassociates rather than freeing an electron from the cathode (GM detector lifetime based on lifetime of quench gas).

Charge collectionThe shape of the output pulse collected varies based on the counting circuit design

• For RC=∞, all charge is collected. Fast rising slope corresponding to collection of fast moving electrons. Slower rising slope corresponding to collection of slow moving positive ions.

• Typical time constants are chosen small enough to ignore the contribution of the positive ions. Since all Geiger discharges are approximately uniform in size and shape, all pulses are attenuated by the same fraction in the shaping process and output pulses will remain almost of one amplitude.

Resolving Time

00

1 R

RR

R : True counting rateR0: Observed counting rate

: Resolving time

Dead timeThe slow moving positive charge ensures a considerable time in which a new pulse could not be detected due to positive space charge effects near anode

The size of secondary pulses depend on the state of the initial discharge since finite pre-existing + space charge will allow discharge termination at lower E

The ultimate pulse size accepted as a count is determined by the counting circuit.

Note polarity!

Note dead time is defined as time until a second pulse, regardless of height, can be detected.

Recovery time isthe time interval that must elapse after a pulse has occurred before a full-size pulse can again occur.

Counting Plateau• At low voltage, pulse

height is below discrimination level and no counts detected

• At higher voltage, all counts recorded (giving counting plateau)

• Plateau is not actually flat• Pulses during recovery

• Inadequate quench

• Areas of reduced electric field (corners, end of tube)

Do not operate herePulses during recovery

Typical design

• At low voltage, pulse height is below discrimination level and no counts detected

• At higher voltage, all counts recorded (giving counting plateau)

• Output of GM tube detector: Count rate

• Thinner window required for alpha counting due to shorter range

• Window must maintain a differential pressure (keep out air)

Typical electronics

Cs=capacitance of tube and associated wiring

Cc = coupling capacitor to block high voltage from counting circuit while transmitting pulse

Preamp typically not needed because of large signal

Counting Efficiency

• Charged particles (alpha, beta) - all particles that enter detector active volume triggers full discharge. Efficiency determined by probability that incident particle penetrates window without absorption or backscatter.

• Neutrons-small interaction probability of interaction with typical gas. Could use a gas with a large absorption cross section. Neutron detectors are typically proportional counters.

• Gamma-interacts with cathode (counter wall). Secondary electron emitted triggers pulse.

• Efficiency determined by:• Probability of gamma interaction with

counter wall• Probability that electrons escapes

metal wall and reaches gas.

Fill & Quench gases

• Geiger counters must be prevented to have excessive multiple pulsing.

• External quenching consists of some method for reducing the high voltage applied to the tube, for a fixed time after each pulse, to a value that is too low to support further gas multiplication.

• Then, secondary avalanches cannot be formed and even if a free electron is liberated at the cathode, it cannot cause another Geiger discharge.

• It is much more common to prevent the possibility of multiple pulsing through internal quenching, which is accomplished by adding a second component called the quench gas to the primary fill gas.

• It is chosen to have a lower ionization potential and a more complex molecular structure than the primary gas component and is present with a typical concentration of 5-10%.

• These gases are to absorb UV photons. Also, the quench gas in a Geiger counter serves to prevents multiple pulsing through the mechanism of charge transfer collisions.