Transcript Antonio Ballarin Denti

FISICA
AMBIENTALE 1
Lezioni 27-28
Radioattività:
misure 1
Marie Curie
FISICA AMBIENTALE
RADIATION
DETECTION
AND
MEASUREMENT
Antonio Ballarin Denti
RADIATION DEVICES ARE BASED ON THE PHYSICAL
OR CHEMICAL EFFECTS OF RADIATION
IONIZATION IN GASES
Mainly used as health physics
monitoring instruments
IONIZATION AND EXCITATION IN CERTAIN SOLIDS
Certain crystalline solids exhibit increases in electrical
conductivity and effects attributable to excitation as
scintillation, termoluminescence and photographic effect.
ACTIVATION BY NEUTRONS
Used for neutron detection
CHANGES IN CHEMICAL SYSTEMS
..rather insensitive
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.
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
t
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
BASIC ELEMENTS OF A PROPORTIONAL COUNTER
The outer cathode must also provide a vacuum-tight enclosure
for the fill gas. The output pulse is developed across the load
resistance RL.
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