The
photomultiplier tube(PMT) is the most
sensitive
photoelectric sensor available at this time, capable
of
detecting the lowest light intencity possible, a
single photon. It consists of a vaccum tube with a light
sensitive cathode at one end. When a photon hits this
cathode, it will
emit an
electron. This
electron try to fly
in a straight line through the tube, but the
dynode(Dy1)'s lower
electron charge(more
positive)
will attract it. When the electron strikes the dynode, more
secondary electrons are released. These electrons fly off at an
angle towards the
anode end of the tube. They will be attracted
by Dy2, and a new electron multiplication will happen. This
continues until the electrons finally hits the anode.
By
measuring the bursts of electrons an indication of
the number of photons can be computed.
Simple schematic of a PMT with four dynodes.
.----------------------------------.
| |
#Cathode anode | |
# |-+-.
# | | |
# | || |R6
# Dy1 Dy2 Dy3 Dy4 || |
# ___ ___ ___ ___ | T
| T T T T | |
|-----+-----+-----+-----+----------' |
| _ | _ | _ | _ | _ |
|-|_|-*-|_|-*-|_|-*-|_|-*-|_|--------|
| R1 R2 R3 R4 R5 |
| ---
High GND
voltage
(2000V)
A normal PMT have ten or more dynodes.
An example of the multiplication:
If we have a
tube with 12 dynodes, where each
have a four times multiplication. We will have:
4^(12-1)=4194304
This means that one photon
triggers the flow of over
four
million electrons.
The tube must be
cooled or you might get false readings
(due to photons from heat(
infrared light)). The tubes
accuracy may also be
affected by
magnetic fields, since
magnetic fields
deflects the
electrons, and
may cause them to
miss the dynodes. Also the high
voltage supply
must be very
stable, since
fluctuation might affect the results.
The PMT is expencive and relatively
fragile but is much used in
applications where high sensitivity is needed.The PMTs are available
for
wavelengths between 200nm and 1100nm(from
UV to
IR).