Detectors

Presenting a topic which is entitled: Detectors
Above topic includes:
Types of detector
phototube detector
photomultiplier tubes
silicon photodiodes
photovoltaic cells
advantages
multi-channel photon detectors
linear photodiode arrays
photodiode array
with basics of instrumentation and science technology

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1. Detectors Submitted by: Preeti choudhary MSc(Applied P hysics )

2. After the light has passed through the sample, we want to be able to detect and measure the resulting light. These types of detectors come in the form of transducers that are able to take energy and convert it into an electrical signal that can be recorded, and if necessary, amplified.

3. Types of photo detectors:- Vacuum Phototubes Photomultiplier Tubes S ilicon photodiode Photovoltaic cells Multichannel Photo detectors

4. This detector is a vacuum tube with a cesium-coated photocathode- photoemissive in nature Photons of sufficiently high energy hitting the cathode can dislodge electrons, which are collected at the wire anode. Photon flux is measured by the current flow in the system. Phototube Detector

5. Phototube

6. Photomultiplier Tubes

7. Photomultiplier tube consists of a cathode, anode a few dynodes. Cathode supports a layer of photo emissive material such as an alkali metal or metal oxide, that tends to emit electrons upon being irradiated. The emitted electrons are accelerated toward a dynode maintained at a potential 90V more positive than the cathode. Upon striking the dynode surface, each accelerated photoelectron produces several additional electron, all of which are then accelerated to dynode 2, which is 90V more positive than dynode 1. H ere again amplification occurs. By the time this process has been repeated at each of the remaining dynodes, 10^6 to 10^7 electrons have been produced for each photon. This cascade is finally collected to the anode. The resulting current is then further amplified electronically and measured.

8. Silicon Photodiodes Forward biasing conduction takes place. small depletion region formed. Reverse biasing non-conducting depletion layer is formed. negligible flow of current.

9. Silicon photodiode

10. A reverse biased diode can serve as a radiation detector because UV and visible photons are sufficiently energetic to create additional electrons and holes when they strike the depletion layer of a pn junction. The resulting increase in conductivity is readily measured and is directly proportional to radiant power. A silicon detector is more sensitive than a simple vacuum phototube but less sensitive than a photomultiplier.

11. Photovoltaic Cells

12. It consists of a flat copper or iron electrode upon which is deposited a layer of a semiconducting material (selenium oxide). The outer surface of the semiconductor is coated with a thin transparent film of gold, silver, or lead, which serves as the collector electrode. When radiation is absorbed on the surface of the semiconductor, electron and holes are formed and migrate in opposite directions, thus creating a current. If the two electrodes are connected with a low resistance circuit, the current produced is directly proportional to the power of the incident beam. The current can be measured through a simple microammeter .

13. Advantages:- Rugged Low cost No external power source is required. Disadvantages:- Not as sensitive as other detectors

14. MULTI CHANNEL PHOTON DETECTORS Consists of an array of tiny photosensitive detectors that are arranged in such a pattern that all elements of a beam of radiation that has been dispersed by a grating can be measured simultaneously Three types of such devices- Linear photodiode arrays Vidicons Charge transfer detectors

15. LINEAR PHOTODIODE ARRAYS The flexibility, reliability, and cost-effectiveness of photodiode arrays have taken these devices into an incredibly diverse range of applications, from NASA satellites to clinical lab tests OTHER APPLICATIONS: X-ray baggage scanners Position encoders Particle measurement and counting systems

16. Photodiode array PDA detector consists of a linear array of several hundred photodiodes that have been formed along the length of silicon chip, 1-6cm in length. The individual widths of diodes are 15 to 50 µm The chip contains a capacitor and an electronic switch for each diode Each capacitor is charged to -5V and the radiation impinging on any diode surface causes partial discharge of its capacitor Resulting charging currents are proportional to the radiant power. Data for entire spectra are obtained in 1s or less

17. ADVANTAGES OF MULTI-CHANNEL PHOTON DETECTORS Simultaneous Multi-wavelength measurement Fast scan speed High signal to noise ratio Wavelength precision Minimal stray light effects Ruggedness

18. Thank-you