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Counter – 3
Gas counter
• A gas-filled chamber has a central electrode
(anode, electrically positive) and a conducting
wall (cathode, negative).
• They are maintained at different potential.
• If a charged particle or gamma ray is allowed
to enter the chamber, it may produce a certain
amount of ionization in the gas.
• The resultant positive ions and electrons are
attracted toward the negative and positive
surfaces, respectively.
• A charge moves in the local field E with a drift
velocity vD = µE, where the mobility µ depends
on the time between collisions and the mean
free path.
• If a magnetic field is present, charges tend to
execute circular paths interrupted by
collisions.
• When the voltage across the tube is low, the
charges merely migrate through the gas, they
are collected, and a current of short duration
(a pulse) passes through the resistor and the
meter.
• More generally, amplifying circuits are
required.
• The number of current pulses is a measure of
the number of incident particles that enter
the detector, which is designated as an
ionization chamber when operated in this
mode.
• If the voltage is then increased sufficiently,
electrons produced by the incident radiation
through ionization are able to gain enough
speed to cause further ionization in the gas.
• Most of this action occurs near the central
electrode, where the electric field is highest.
• The current pulses are much larger than in the
ionization chamber because of the
amplification effect.
• The current is proportional to the original
number of electrons produced by the
incoming radiation, and the detector is now
called a proportional counter.
• If the voltage on the tube is raised still higher, a
particle or ray of any energy will set off a
discharge, in which the secondary charges are so
great in number that they dominate the process.
• The discharge stops of its own accord because of
the generation near the anode of positive ions,
 which reduce the electric field there to such
an extent that electrons are not able to cause
further ionization.
…
• The current pulses are then of the same size,
regardless of the event that initiated them.
• In this mode of operation, the detector is
called a Geiger-Mueller (GM) counter.
Neutron detector
• To detect neutrons, which do not create
ionization directly, it is necessary to provide a
means for generating the charges that can
ionize a gas.
• n absorption in boron:
1 + B10  He4 + Li7
n
0
5
2
3
• One form of boron counter is filled with the
gas boron trifluoride (BF3) and operated as an
ionization chamber or a proportional counter.
SCINTILLATION COUNTERS
• The name of this detector comes from the fact
that the interaction of a particle with some
materials gives rise to a scintillation or flash of
light.
• The basic phenomenon is familiar — many
substances can be stimulated to glow visibly
on exposure to ultraviolet light.
• Molecules of materials classed as phosphors are
excited by radiation such as charged particles and
subsequently emit pulses of light.
• The substances used in the scintillation detector
are
– inorganic (e.g., sodium iodide or lithium iodide) or
– organic,
• in one of various forms —
crystalline, plastic, liquid, or gas.