Photo multipliers

38
PHOTO MULTIPLIERS MARINEL SIGUE JET ARDLEY PO

Transcript of Photo multipliers

PHOTO MULTIPLIERSMARINEL SIGUEJET ARDLEY PO

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

CONSTRUCTION

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

Kinds of PHOTOCATHODE

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

ANODE

After exiting the last dynode, the electron pulse is collected on the anode

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

used to measure interruptions in beams of light. used in research laboratories to measure the intensity and spectrum of

light-emitting materials used in numerous medical equipment designs