know about photomultipliers in detail

1. PHOTO MULTIPLIERS
BY, MD ANAS AHMAD

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

4. CONSTRUCTION

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