Radiation Detectors: Types and Functionality

• An instrument for detecting and measuring radiation comprises a detector, converting radiation energy into an electrical pulse, and an electronic circuit, amplifying and recording the pulses
• Detectors can be divided into three classes:

Detector classes

• Gas ionization detectors
• Scintillation detectors
• Solid-state detectors

Gas Ionization Detectors

• A gas is normally non-conducting due to the absence of charge carriers.
• Radiation can produce ion pairs, approximately one for every 30 eV of energy expended.
• If electrons and positive ions are collected by two electrodes before recombination, a small electrical pulse is observed.
• Methods based on gas ionization are subdivided according to the electric field gradients

Pulse Size Variation

• Plotting the electrical pulse size against the potential applied between electrodes results in a characteristic curve.
• Without a potential difference, no pulse is produced.
• As potential rises, more electrons and positive ions are collected until all are collected; increasing potential no further increases the current
• This is the ion chamber region.

Increasing Potential

• Increasing the potential even more causes the electrons to accelerate
• They collide with atoms or molecules to cause secondary ionization
• Some secondary ionization electrons create further ionisation

Ion chamber region

• May count individual particles, or B, y dosimeters measuring cumulative result
• Vary in design, made of two parallel plates, one often a thin metal foil
• Gas between is a special fill, dry air or argon
• Potentials range from 50 to 500 volts, quickly sweeping ion pairs without causing secondary ionisation

Alpha Particles

• Individual alpha particles and fission product fragments can produce sufficient ionization to be detected individually.
• An alpha particle of 5 MeV has a range comparable to ion chamber dimensions at atmospheric pressure.

Typical electrical capacity

• Voltage Pulse = charge/capacity
• Typical value is 10^-11 farad, for electrical capacity of the chamber
• Viltagr pulse = 2x10^-3
• Two millivolt pulse that can be amplified with 10x amplifier to be measured by scaler or ratemeter

Large chamber

• Large enough for a-particle to dissipate all energy for pulse size as a function of initial energy,
• Aids in measuring a-particle energies.
• In a neutron sensitive modification, an internal deposit of a fissionable nuclide is used where presence shown by ejected heavy charged fission fragments into detector.
• Beta particles produce smaller pulses that are within amplifier noise levels.
• Individual B-particles can not be detected by the ion chamber

Integrated effect of particles measured by ion chamber in circuit with series resistor

• Resistance x capacity = time constant of the circuit
• To produce a steady ionization current: Constant MUST be great than interval between the arrival of consecutive particles
• Value of R plays important roles
• 1. it provides circuit with long time so the pulses integrate into a d.c. current
• 2. it creates current through R based off Ohm's Law
• Minute current creates measurable voltage to be applied to amplifier tube

unusual ionisation chamber

• Gold leaf electroscope used by early workers
• Rate of collapse of leaves dependant on degree of ionisation, hence activity, was measured
• A modern version is the pocket dosimeter, same size as pen
• After power pack charging, it is worn, radiation measured by the drift of a quartz fibre

The proportional region

• At increased electric field gradients, electrons accelerate to produce secondary ionization
• The degree of which is dependant on the the gas amplification factor, which is the number of electrons with respect to the anode
• Range of 10^0 to 10^4 - every electron produces 100 - 10000 at anode
• Higher gradients result in higher potential between plates and the wire anode within the cathode forms non-uniform electric field