FISICA AMBIENTALE 1

Radioattività:misure 1

Lezioni 27-28

Marie Curie

RADIATION DETECTION AND MEASUREMENT Antonio Ballarin Denti

FISICA AMBIENTALE

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