Radiation Sensors

1. Radiation Sensors • Radiation is emission if either particles or electromagnetic rays from a source. • Particles are usually nuclear particles which can be emitted from a nucleus as a result of the decay of a radioactive material. • Particles • The mass is used to classify particles as baryons (heavy), mesons (medium, and leptons (light). • Radium (Ra) with atomic mass AZ of 226 (Z=88) and decays to produce radon gas (Rn) (AZ=222, Z=86) and an a particle (AZ=4, Z=2): • B particles are energetic particles with the rest mass of an electron that have been emitted by a source (radioactive or otherwise) and have much lower mass, thus they are called leptons .

2. Electromagnetic Rays • They travel at the speed of light c in vacuum and they have a mass equivalent energy ERwhich relates to their frequency n or wavelength l h = 6.63x10-34 Js is Planck’s constant.

3. Radiation Measurands • The kinetic energy of a nuclear particle, Ek, is related to the particle velocity vR via • The power, P, which is the rate of change of energy • Radiation sensors can be subdivided into two classes: nuclear particle and electromagnetic radiation sensors.

4. Nuclear Radiation Sensors: Scintillation Counters • The scintillation counter consists of an active material which converts the incident nuclear radiation to pulses of light, a light-electrical pulse converter (e. g., a photomultiplier tube) and an electronic amplifier/processor. • The active material that scintillates is an inorganic or organic crystal, a plastic floor or liquid.

5. ERis proportional to pulse height. fR is proportional to count rate.

6. Solid State Detectors • The use of semiconductor materials in nuclear radiation sensors is highly desirable especially Si. • Radiation is absorbed by semiconductors and the level of absorption varies with the material and radiation energy. There are three major processes: Low energies Photoelectric effect Medium energies Compton effect High energies Pair production Interactions of x-rays and g-rays with matter

7. Absorption coefficient a is defined as

8. Schematic diagram of a p/n photodiode

9. Ultra-Violet, Visible, and Near-Infrared Radiation Sensors • UV from 0.002 to 0.4 mm; visible from 0.4 to 0.7 mm, and near infra-red (NIR) from 0.7 to 1.7 mm. • The common semiconductors, e. g., Si operate over this region. Range of radiation sensors

10. Classification of Radiation Sensors

11. Photoconductive Cells • They are semiconductor sensors that utilize the photoconductive effect in which light strikes the photoconductive material reduces its resistance. • For CdStp is very short and hence, Basic structure of a photoconductive cell

12. From Ohm’s law d is the thickness and l/w is the aspect ratio of the device Resistance of CdS in a photoconductive cell as a function of illuminance

13. Photodiodes • Photodiodes may be classified as potentiometric radiation sensors because the radiation generates a voltage across a semiconductor junction: a phenomenon known as the photovoltaic effect. • The main types of photodiodes are: • p-n photodiode • p-i-n photodiode • Schottky photodiode • Avalanche photodiode • Photodiodes are used to detect the pressure, the intensity, and wavelength of UV to NIR radiation. • The advantages of photodiodes over conductive cells are: • Higher sensitivity • Faster response time. • Smaller size. • Better stability. • Excellent linearity. • Si photodiodes can detect radiation from UV to NIR (190 to 100 nm) with a peak at 960 nm.

14. Main types of photodiodes

15. Photovoltage at a p-n junction Equilibrium Forward bias (V) • The absorption of a photon creates an electron-hole pair which are driven by the junction field to the doped regions. This creates a photovoltage, V, as shown in the figures.

16. The open-circuit voltage VOC of a photodiode can be measured when the external load resistor RL is high and is given by IL is the photocurrent and IS is the reverse saturation current. Open-circuit voltage Short-circuit current Typical output of a Si photodiode as a function of illuminance at 25 °C

17. Circuits for operating diodes Reverse-bias Virtual earth

18. The PIN diode has a thin insulating layer between the p-type and n+-type material: this means that the depletion region thickness can be modified to optimize the quantum efficiency and frequency response, The lower junction and package capacitance produces a much faster response than that for a typical p-n diode (~ 0.4 ms). • The Schottky type photodiode has an ultra-thin metal film (~ 100 Å) that forms a Schottky barrier with an n-type semiconductor. The metal film enhances the sensitivity of the diode to the UV range where the absorption coefficient in semiconductors is high. It is necessary to use anti-reflection coating such as 500 Å of ZnS but over 95% of the incident radiation (l ~ 633 nm) is transmitted into the Si substrate. • The barrier height is fB = fm – fswhere fm and fs are the workfunctions for metal and semiconductor, respectively. • The current density J is

19. At UV wavelength (hn>Eg) electron-hole pairs are generated inside the Schottky barrier separated by the local field. At longer wavelengths (hn>fB) electrons within the metal are excited enough to cross the barrier into the semiconductor. However, the probability is lower than band-to-band excitation (at lower wavelengths). The two characteristic regions in the spectral response of a Schottky-type photodiode

20. The Avalanche photodiode is operated under a high reverse bias in which the photon-generated carriers are excited to sufficient levels to collide with other atoms and produce secondary carriers: this process occurs repeatedly and is called Avalanche effect. • This leads to sensitivity to low light levels in the visible NIR region. • The condition for Avalanche is Ek ≥ 3/2Eg. Quantum efficiency of a Si Avalanche photodiode

21. Radiation Sensors and Typical Characteristics