photo multipliers
TRANSCRIPT
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
extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum
useful for light detection of very weak signals multiply the current produced by incident light
by as much as 100 million times enabling individual photons to be detected
when the incident flux of light is very low
ADVANTAGES OVER OTHER PHOTO DETECTORS Large area light detection High gain ability to detect single photons Low noise High frequency response
PARTS OF A PHOTO MULTIPLIER
A. PRIMARY PARTS1.PHOTOCATHODE2.DYNODES3.ANODEB. SECONDARY PARTS4.INPUT WINDOW
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
A. side-on type receives incident light through the side
of the glass bulb employ an opaque photocathode where
electrons are emitted from the illuminated side
B. head-on type received through the end of the glass bulb has a semitransparent photocathode
deposited upon the inner surface of the entrance window where electrons are emitted from the side opposite to the incident light
The cathode can be large (from ten to a few hundred millimetres in diameter) and the window on which it is deposited can be flat or curved.
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
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
5. Bialkali (Sb-Rb-Cs, Sb-K-Cs) These have a spectral response range similar to the
Sb- Cs photocathode, but have higher sensitivity and lower noise than Sb-Cs
Can be used for head-on photocatodes6. High temperature bialkali or low noise bialkali (Na-K-Sb) particularly useful at higher operating temperatures
since it can withstand up to 175°C Application is in the oil well logging industry operates with very low dark current
7. Multialkali (Na-K-Sb-Cs) has a high, wide spectral response from the ultraviolet
to near infrared region can be extended out to 930nm by special
photocathode processing8. Cs-Te, Cs-I are sensitive to vacuum UV and UV rays but not to
visible light and are therefore called solar blind Cs-Te is quite insensitive to wavelengths longer than
320nm, and Cs-I to those longer than 200nm
Photocathode Response
1.quantum efficiency or QE% is the average photoelectric yield per incident photon normally expressed as a percentage the most fundamental unit concerning the
performance of the photomultiplier2. radiant sensitivity defined as the photocathode current emitted per watt
of incident radiation at wavelength at wavelength L and is expressed in mA/W
3. luminous sensitivity most relevant specification for light sources which
have a spectral response corresponding to that of the human eye
cathode luminous sensitivity is the photoelectric current from the photocathode per incident light flux (10-5 to 10-2 lumens) from a tungsten filament lamp operated at a distribution temperature of 2856K
anode luminous sensitivity is the anode output current (amplified by the secondary emission process) per incident light flux (10-10 to 10-5 lumens) on the photocathode
4. spectral response characteristic spectral response characteristics are
determined on the long wavelength side by the photocathode material and on the short wavelength side by the window material.
relationship between photocathode sensitivity(conversion efficiency for photons into photoelectrons) and wavelength
DYNODES
electron multiplier consists of from 8, up to 19 stages of electrodes called dynodes
Amplification is carried out using the dynode chain
Types of Dynode Configurations
1. Circular-cage type generally used for the side-on type of photomultiplier
tube prime features of the circular-cage are compactness
and fast time response
2. Box-and-grid type consists of a train of quarter cylindrical
dynodes widely used in head-on type photomultiplier
tubes because of its relatively simple dynode design and improved uniformity
3. Linear-focused type features extremely fast response time widely used in head-on type photomultiplier
tubes where time resolution and pulse linearity are important.
4. Venetian blind type has a large dynode area primarily used for tubes with large photocathode
areas offers better uniformity and a larger pulse output
current usually used when time response is not a prime
consideration.
5. Mesh type has a structure of fine mesh electrodes
stacked in close proximity type provides high immunity to magnetic
fields, as well as good uniformity and high pulse linearity
6. Microchannel plate (MCP) MCP is a thin disk consisting of millions of micro glass
tubes (channels) fused in parallel with each other Each channel acts as an independent electron
multiplier offers much faster time response than the other
discrete dynodes
7. Metal channel type has a compact dynode costruction achieves high speed response due to its narrower
space between each stage of dynodes
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 region2. 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
160nm4. MgF2 (magnesium fluoride) superior in transmitting ultraviolet
radiation Transmits ultraviolet radiation down to
115nm
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
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.
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
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