RADIATION DEVICES ARE BASED ON THE PHYSICAL OR CHEMICAL EFFECTS OF RADIATION
IONIZATION IN GASESMainly used as health physics monitoring instruments
CHANGES IN CHEMICAL SYSTEMS..rather insensitive
IONIZATION AND EXCITATION IN CERTAIN SOLIDSCertain crystalline solids exhibit increases in electrical conductivity and effects attributable to excitation as scintillation, termoluminescence and photographic effect.
ACTIVATION BY NEUTRONSUsed for neutron detection
IONIZATION CHAMBER
The most widely used radiation detectors are devices that respond to ionizing radiation by producing electrical pulses
The pulses are generated by the imparting of energy to electrons by the ionizing particles
in the sensitive volume of the counter
There are 2 major modes of signal production:
CURRENT MODE: the magnitude of the output pulse is proportional to the amount of energy deposited in the detector
PULSE MODE: the deposited energy serves to trigger an output pulse of constant form every time the interaction occurs.
t
The assumed current output from a hypothetical detector
The signal voltage V(t) for the case of small time constant load circuit
The signal voltage V(t)for the case of a large time constant load circuit
IONIZATION CHAMBERS: to measure exposure rates
In health physics instruments the chamber is usually filled with air and is constructed using low atomic number materials
PROPORTIONAL COUNTERS
As the electric field in an ion chamber system is increased the freed e- are accelerated and achieve sufficient kinetic energy to cause additional ionizations within the detector
The outer cathode must also provide a vacuum-tight enclosure for the fill gas. The output pulse is developed across the load resistance RL.
BASIC ELEMENTS OF A PROPORTIONAL COUNTER
The different regions of operation of gas filled detectors. The observed amplitude is plotted for events depositing
2 different amounts of energy within the gas
GEIGER-MÜLLER COUNTER
If the applied voltage is further increased, gas amplification is so great that a single ionizing particle produces a ionization
Avalanche. Each output pulse of current has the same magnitude and no longer reflects any properties of the incident radiation.
Mechanism by which additional avalanches are triggered in a Geiger discharge