3. PHOTOMULTIPLIERS DEFINED
Called photomultiplier tubes (PMTs)
members of the class of vacuum phototubes
extremely sensitive light detectors providing a
current output proportional to light intensity.
a photoemissive device in which the
absorption of a photon results in the emission
of an electron
5. ADVANTAGES OVER OTHER PHOTO
DETECTORS
Large area light detection
High gain
ability to detect single photons
Low noise
High frequency response
6. PARTS OF A PHOTOMULTIPLIER
A.PRIMARY PARTS
1.PHOTOCATHODE
2.DYNODES
3.ANODE
B.SECONDARY PARTS
1.INPUT WINDOW
7. PHOTOCATHODE
converts the photon to a photoelectron
active area of the photocathode can be as small
as a few millimeters in diameter or as large as a
sphere half a meter in diameter
wavelength range over which one responds to
light can be adjusted by changing the cathode’s
chemical composition
8. PHOTOCATHODE MATERIALS
1.Ag-O-Cs
sensitive from the visible to infrared range (300 to
1200nm)
Mainly used for detection in the near infrared region
with the photocathode cooled.
2. GaAs(Cs)
usually covers a wider spectral response range from
ultraviolet to 930nm
9. 3. InGaAs(Cs)
has greater extended sensitivity in the infrared range
than GaAs
in the range between 900 and 1000nm, InGaAs has
much higher S/N ratio than Ag-O-Cs.
4. Sb-Cs
a widely used photocathode
has a spectral response in the ultraviolet to visible range
Mainly used for side-on photocathodes
10. DYNODES:
electron multiplier consists of from 8, up to 19 stages
of electrodes called dynodes
Amplification is carried out using the dynode chain
After exiting the last dynode, the electron pulse is
collected on the anode
ANODE:
11. INPUT WINDOW
1. Borosilicate glass
frequently used glass material
transmits radiation from the near infrared to approximately 300nm
not suitable for detection in the ultraviolet region
2. UV-transmitting glass (UV glass)
transmits ultraviolet radiation well
widely used
UV cut-off is approximately 185nm.
3. Synthetic silica
transmits ultraviolet radiation down to 160nm
4. MgF2 (magnesium fluoride)
superior in transmitting ultraviolet radiation
Transmits ultraviolet radiation down to 115nm
12. OPERATION
light absorbed on a photocathode generates free electrons, which are
subsequently accelerated with a high voltage (at least hundreds of volts),
generate secondary electrons on other electrodes, and finally a usable
photocurrent.
require maximum voltages in the region of 1 - 2 kV
there are also ordinary phototubes that can be operated with a much lower voltage
of 15V with only two electrodes and therefore much lower responsivity
13. TYPES OF PHOTOMULTIPLIERS
1. silicon photomultipliers
can be obtained with an array containing may
avalanche diodes
photomultipliers can be replaced with avalanche
photodiodes, which also exhibit an amplification
mechanism, but in that case one which occurs within a
solid-state (semiconductor) material, rather than in a
vacuum tube
cheaper and much more compact and robust
exhibit a higher quantum efficiency, but also a
higher amplification noise.
14. 2. hybrid photomultipliers
a vacuum tube with a photocathode and a silicon avalanche
diode
functions similarly to a PMT but with a different mechanism of
amplification.
suitable for a variety of applications including light detection
and ranging
where electrons from a photocathode are accelerated with
several kilovolts to a semiconductor chip similar to that of an
avalanche diode
15. APPLICATIONS
used to detect low-energy photons in the UV to
visible range, high-energy photons (X-rays and
gamma rays)
medical diagnostics including blood tests
medical imaging
motion picture film scanning
high-end image scanners
the basis of night vision devices
