unit-4 radiation

UNIT IV LIGHT AND RADIATION DETECTORS

Light Detectors: Photo diodes – photo transistor – photo resistor – cooled detectors – CCD and CMOS image sensors – thermal detectors – optical design – gas flame detectors

Radiation Detectors: scintillating detectors – ionization detectors – cloud and bubble chambers

Ionization DetectorsThese detectors rely on the ability of some

gaseous and solid materials to produce ion pairs in response to the ionization radiation.

Then, positive and negative ions can be separated in an electrostatic field and measured.

Ionization happens because charged particles upon passing at a high velocity through an atom can produce sufficient electromagnetic forces, resulting in the separation of electrons, thus creating ions.

Ionization is a Process in which an atom or molecule acquires a positive charge (by losing electrons) or negative charge(by gaining electrons)

Ionization Detectors typesIonization ChambersProportional ChambersGeiger–Meuller CountersSemiconductor Detectors

1)Ionization Chambers

A volume of gas is enclosed between the electrodes, which produce an electric field.

An electric current meter is attached in series with the voltage source E and the electrodes.

There is no electrical conduction and no current under the no ionization

conditions.Incoming radiation produces,

in the gas, positive and negative ions that are pulled by the electric field toward the corresponding electrodes forming an electric current.

Ionization Chamber

•Ionization chambers have wider range of physical shape (parallel plates)

•Air-filled ion chambers are used in portable survey meters

2)Proportional Chambers

The proportional chamber is a type of a gas-filled detector that almost always operates in a pulse mode and relies on the phenomenon of a gas multiplication. This is why these chambers are called the proportional counters.

Proportional counters

Must contain a gas with specific properties such as: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr)

Operates at higher voltage than ionization chamber

3)Geiger–Meuller CountersThe Geiger–Meuller (GM) counter is usually

fabricated in the form of a tube with an anode wire in the center .

The tube is filled with a noble gas such as helium or argon.

GM countersGM counters also must contain

gases with specific propertiesGas amplification produces

billions of ion pairs after an interaction – signal from detector requires little amplification

Often used for inexpensive survey meters

GM counter

Long-handled InstrumentsEnable high radiation fields near unshielded sources to

be assessed from a safe distance

4)Semiconductor DetectorsThe operating principle of these radiation

detectors is the same as that of the semiconductor light detectors. It is based on the transition of electrons from one energy level to another when they gain or lose energy.

Valence Band

Energy Gap

Conduction Band

Electron - hole pair

Gamma Ray (photon)

A good solid-state radiation detector should possess thefollowing properties:

Excellent charge transportLinearity between the energy of the incident

radiation and number of electron–hole pairs Absence of free charges (low-leakage

current) Production of a maximum number of

electron–hole pairs per unit of radiation High detection efficiency Fast response speedLarge collection areaLow cost

Cloud and Bubble Chambers

The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation.

cloud chamberIn its most basic form, a cloud chamber is a

sealed environment containing a supersaturated vapor of water or alcohol.

When a charged particle (for example, an alpha or beta particle) interacts with the mixture, it ionizes it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation).

The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle.

cloud chamberThese tracks have distinctive shapes (for

example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight).

When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge.

cloud chamber

cloud chamberA cloud chamber is composed of a jar of gaseous

alcohol that is cooled at the bottom with dry ice. When a highly energetic charged particle or

photon (a 'particle' of light) moves through this gas it removes electrons from the atoms it passes leaving them positively charged.

Small amounts of the neutral atoms in the gas then condense around them and along the path of the charged particle.

The energy and charge of the particle can then be determined from the shape of the track which could be photographed

Bubble ChambersA bubble chamber is similar to the cloud

chamber except that a liquid is used instead of vapor

Bubble Chambers

Bubble Chambers

Bubble Chambers Interestingly, a glass of champagne or beer is a kind of a bubble

chamber where tiny bubble formations are triggered by the ionizing radiation coming from the environment and outer Space.

For the physical experiments, a bubble chamber is filled with a more prosaic and much colder liquid, such as liquid hydrogen.

It is used for detecting electrically charged particles moving through it.

The bubble chamber is normally made by filling a large cylinder with liquid hydrogen heated to just below its boiling point.

As particles enter the chamber, a piston suddenly decreases its pressure, and the liquid enters into a superheated metastable phase.

Charged particles create an ionization track, around which the liquid vaporizes, forming microscopic bubbles. Bubble density around a track is proportional to a particle’s energy loss.

Bubble Chambers

UNIT IV LIGHT AND RADIATION DETECTORS

Light Detectors: Photo diodes – photo transistor – photo resistor – cooled detectors – CCD and CMOS image sensors – thermal detectors – optical design – gas flame detectors

Radiation Detectors: scintillating detectors – ionization detectors – cloud and bubble chambers

Thermal DetectorsA resistive element which measures

electromagnetic radiation by absorbing it and converting it into heat. The increase in its temperature is used to measure the radiant energy.

operating principle: sequential conversion of thermal radiation

into heat, and then, conversion of heat magnitude or heat flow into an electrical signal by employing conventional methods of heat detection.

A typical infrared temperature sensor consists of the followingsensing elementsupporting structureHousingprotective window

Thermal Detectors1) Golay Cells2) Thermopile Sensors3) Pyroelectric Sensors4) Bolometers5) Active Far-Infrared Sensors

1)Golay CellsThe operating principle of the cell is based

on the detection of a thermal expansion of gas trapped inside an enclosure.

these detectors sometimes are called thermopneumatic detectors.

Golay Cells

an enclosed chamber having two membranes :

upper and lower. The upper membrane is

coated with a radiative heat absorber (e.g., goldblack).

The lower membrane has a mirror surface (e.g., coated with aluminum).

The mirror is illuminated by a light source. The incident light beam is reflected from the mirror and impinges on a position-sensitive detector (PSD).

The upper membrane is exposed to infrared radiation that is absorbed by the coating and elevates temperature of the membrane.

This, in turn, warms up gas that is trapped inside the sensor’s chamber.

Gas expands and its pressure goes up. The increase in the internal pressure

deflects the lower membrane that bulges out.

A change in the mirror curvature deflects the reflected light beam

2)Thermopile Sensors

Thermopiles belong to a class of PIR detectors. Their operating principle is the same as that of a thermocouple.

A single thermocouple is a low-sensitivity device responding

with tens of microvolts per 1oC change.A thermopile is a chain of serially connected

thermocouples, typically 50–100 junctions.application : thermal detection of light in the mid- and far-

infrared spectral ranges.

A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series.

3)Pyroelectric Sensors

3)Pyroelectric SensorsA pyroelectric sensor belongs to a class of

PIR detectors.The pyroelectric element consists of three

essential components: The pyroelectric ceramic plate and two

electrodes deposited on the opposite sides of the plate.

CharacteristicsHigh Sensitivity Low cost Stable against atmospheric changes

ApplicationsIntrusion sensor Light control Temperature measurement Automatic door switch Visitor detector Home security

4)Bolometersbolometer is a device for measuring the

power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance.

Bolometers are miniature RTDs or thermistors or other temperature sensitive resistors that are mainly used for measuring rms values of electromagnetic radiation over a very broad spectral range from mid infrared to microwaves.

Applicationsinfrared temperature detection and

imaging,measurements of local fields of high power, the testing of microwave devices, RF antenna beam profiling, testing of high power microwave weapons, monitoring of medical microwave heating.

conversion steps in a bolometer are1)An ohmic resistor is exposed to

electromagnetic radiation. The radiation is absorbed by the resistor and converted into heat.

2)The heat elevates resistor’s temperature above the ambient.

3)The temperature increase reduces the bolometer’s ohmic resistance

voltage-biased-bolometerIt consists of a bolometer (a temperature-

sensitive resistor) having resistance R,

a stable reference resistor R0 a bias voltage source E. • The voltage V across R0 is

the output signal of the circuit.

It has the highest value when both

resistors are equal.

Sensitivity of the bolometer to the incoming electromagnetic

(EM) radiation can be defined as

5)Active Far-Infrared Sensorsa PIR sensing element, whose temperature

depends on both the ambient and object’s temperatures, the AFIR sensor’s surface is actively controlled by a special circuit to have a defined temperature Ts that in most applications is maintained constant during an entire measurement process.

To control the sensor’s surface temperature, electric power P is provided by a control (or excitation) circuit .

To regulate Ts, the circuit measures element’s surface temperature and compares it with an internal reference.