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 .

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

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.

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.

ER is proportional to pulse height. fR is proportional to count rate. Material Code Defects Density (gm/cm3) Light Output (%) Decay Time (ns) Remarks NE102A g, a, b, fast n 1.032 65 2.4 General purpose NE104 68 1.9 Ultra-fast counting NE114 50 4.0 Cheaper for large arrays

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

Absorption coefficient a is defined as

Schematic diagram of a p/n photodiode

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

Classification of Radiation Sensors

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

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

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.

Main types of photodiodes

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.

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

Circuits for operating diodes Reverse-bias Virtual earth

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 – fs where fm and fs are the workfunctions for metal and semiconductor, respectively. The current density J is

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

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

Radiation Sensors and Typical Characteristics