1. Modern Instrumentation PHYS 533/CHEM 620
Lecture 13
Magnetic Field and Radiation Sensors
Amin Jazaeri
Fall 2007

2. Basic Principles for Magnetic Field Sensing
Broadly magnetic sensors and actuators rely on only a few basic principles including*:-
the Faraday law of induction,
for magneto-inductive devices
the Ampere force law,
for magnetomechanical sensors
changes in materials properties in a magnetic field,
such as magnetoresistance, magneto-optics or magnetoelasticity
* D.C. Jiles and C.C.H. Lo, “The role of new materials in the development of magnetic sensors and actuators”, Sensors and Actuators. A. Physical, Vol. 106(1-3), pp. 3-7, 2003.

3. Hall effect sensors
Hall effect was discovered in 1879 by Edward H. Hall
Exists in all conducting materials
Is particularly pronounced and useful in semiconductors.
One of the simplest of all magnetic sensing devices
Used extensively in sensing position and measuring magnetic fields

4. Hall effect - principles
Consider a block of conducting medium through which a current of electrons is flowing caused by an external field as shown in Figure 5.30.
A magnetic filed B is established across the conductor, perpendicular to the current (.
The electrons flow at a velocity v
A force perpendicular to both the current and field is established.

5. Hall effect - principle

6. Hall effect - principles
The electrons are pulled towards the front side surface of the conductor (holes in semiconductors move towards the back)
A voltage develops between the back (positive) and front (negative) surface. This voltage is the Hall voltage and is given by:
d is the thickness of the hall plate,
n is the carrier density [charges/m3] and
q is the charge of the electron [C]

7. Hall effect - principles
If the current changes direction or the magnetic field changes direction, the polarity of the Hall voltage flips.
The Hall effect sensor is polarity dependent,
may be used to measure direction of a field
or direction of motion if the sensor is properly set up.
The term 1/qn [m3/C] is material dependent and is called the Hall coefficient KH.

8. Hall coefficient The hall voltage is usually represented as:
Hall coefficients vary from material to material
Are particularly large in semiconductors.
Hall voltage is linear with respect to the field for given current and dimensions.
Hall coefficient is temperature dependent and this must be compensated if accurate sensing is needed.

9. Hall coefficient - cont.
Hall coefficient is rather small - of the order of 50 mV/T
Most sensed fields are smaller than 1 T
The Hall voltage can be as small as a few V
Must in almost all cases be amplified.
Example, the earth’s magnetic field is only about 50 T so that the output is a mere 25 V

10. Hall effect sensors - practical considerations
Hall voltages are easily measurable quantities
Hall sensors are among the most commonly used sensors for magnetic fields:
simple, linear, very inexpensive, available in arrays
can be integrated within devices.
Errors involved in measurement are mostly due to temperature and variations and the averaging effect of the Hall plate size
These can be compensated by appropriate circuitry or compensating sensors.

11. Hall effect sensors - fabrication
A typical sensor will be a rectangular wafer of small thickness
Made of p or n doped semiconductor (InAs and InSb are most commonly used because of their larger carrier densities – hence larger Hall coefficients)
Silicon may also be used with reduced sensitivity)
The sensor is usually identified by the two transverse resistances – the control resistance through which the control current flows and the output resistance across which the Hall voltage develops.

12. Hall effect sensors - applications
In practical applications, the current is usually kept constant so that the output voltage is proportional to the field.
The sensor may be used to measure field (provided proper compensation can be incorporated)
It may be used as a detector or to operate a switch.
The latter is very common in sensing of rotation which in itself may be used to measure a variety of effect (shaft position, frequency of rotation (rpm), position, differential position, etc.).

13. Hall effect sensors - applications
Example is shown in Figure 5.31 where the rpm of a shaft is sensed.
Many variations of this basic configuration: for example, measurement of angular displacement.
Sensing of gears (electronic ignition)
Multiple sensors can sense direction as well

14. Hall element as a rotation sensor

15. Electronic ignition

16. Hall effect sensors - applications
Example: measuring power
The magnetic field through the hall element is proportional to the current being measured
The current is proportional to voltage being measured
The Hall voltage is proportional to product of current and voltage - power

17. Some Hall element sensors

18. A 3-axis Hall element probe

19. Hall sensors used to control a CDROM motor

20. Magnetoresistive sensors
Two basic principles:
1. Similar to Hall elements
The same basic structure is used but
No Hall voltage electrodes. (Figure 5.37)
The electrons are affected by the magnetic field as in the hall element
Because of the magnetic force on them, they will flow in an arc.

21. The magnetoresistive sensor

22. Magnetoresistive sensors
The larger the magnetic field, the larger the arc radius
Forces electrons to take a longer path
The resistance to their flow increases (exactly the same as if the effective length of the plate were larger).
A relationship between magnetic field and current is established.
The resistance of the device becomes a measure of field.

23. Magnetoresistive sensors
The relation between field and current is proportional to B2 for most configurations
It is dependent on carrier mobility in the material used (usually a semiconductor).
The exact relationship is rather complicated and depends on the geometry of the device.
We will simply assume that the following holds:

24. Magnetoresistive sensors
k may be viewed as a calibration function.
A particularly useful configuration for magnetoresistor is shown in Figure 5.37c.
This is called the Corbino disk
has one electrode at the center of the disk
the second is on the perimeter.
This device has the highest sensitivity because of the long spiral paths electrons take in flowing from one electrode to the other.

25. Magnetoresistive sensors
Magnetoresistors are used in a manner similar to hall elements
Simpler since one does not need to establish a control current.
Measurement of resistance is all that is necessary.
A two terminal device build from the same types of materials as hall elements (InAs and InSb in most cases).

26. Magnetoresistive sensors
Magnetoresistors are also used where hall elements cannot be used.
One important application is in magnetoresistive read heads where the magnetic field corresponding to recorded data is sensed.
Much more sensitive than hall elements

27. Magnetoresistive sensors
2. The second principle: based on the property of some materials to change their resistance in the presence of a magnetic field when a current flows through them.
Unlike the sensors discussed above these are metals with highly anisotropic properties and the effect is due to change of their magnetization direction due to application of the field.
Another name: AMR (anisotropic magnetoresistance)

28. Magnetoresistive sensors - operation
A magnetoresistive material, is exposed to the magnetic field to be sensed.
A current passes through the magnetoresistive material at the same time.
Magnetic field is applied perpendicular to the current.
The sample has an internal magnetization vector parallel to the flow of current.
When the magnetic field is applied, the internal magnetization changes direction by an angle 

29. Magnetoresistive sensor - operation

30. Magnetoresistive sensors - operation
The resistance of the sample becomes:
R0 is the resistance without application of the magnetic
R0 is the change in resistance expected from the material used.
Both of these are properties of the material and the construction (for R0).
The angle  is again material dependent.

31. Properties of magnetoresistive materials

32. Magnetoresistive sensors - properties

33. Magnetoresistive sensors - comments
Used exactly like Hall sensors
Much more sensitive
Common in read heads in hard drives
Used for magnetic compasses

34. Principle of induction

35. Faraday’s law
Given a coil with N turns and a flux F through it. The emf on the coil is:
B is the flux density
S area of the coil
q is the angle between the two

36. Small loop magnetometer
The relations show that the output is integrating (dependent on coil’s area).
This basic device indicates that to measure local fields, the area of the coil must be small,
Sensitivity depends on the size and number of turns
Only variations in the field (due to motion or due to the ac nature of the field) can be detected.
If the field is ac, it can be detected with stationary coils as well.

37. Small loop magnetometer
There are many variations on this basic device.
Differential coils may be used to detect spatial variations of the field.
In other magnetometers, the coil’s emf is not measured. Rather, the coil is part of an LC oscillator and the frequency is then inductance dependent. In these, fields are not measured - the self generated field is monitored for changes
Any conducting and/or ferromagnetic material will alter the inductance and hence the frequency.

38. Small loop magnetometer
This creates a very sensitive magnetometer often used in such areas as mine detection or buried object detectors (pipe detection, “treasure” hunting, etc.)
The simple coil, in all its configurations, is not normally considered a particularly sensitive device
It is often used because of its simplicity
If properly designed and used, can be extremely sensitive
magnetometers based on two coils are used for airborne magnetic surveillance for mineral exploration).

39. Fluxgate sensor
Fluxgate sensors are much more sensitive than coil magnetometers
Can be used as a general purpose magnetic sensor
More complex than the simple sensors described above such as the magnetoresistive sensor.
It is therefore most often used where other magnetic sensors are not sensitive enough.
electronic compasses,
detection of fields produced by the human heart
fields in space.

40. Fluxgate sensor Fluxgate sensors existed for many decades,
were rather large, bulky and complex instruments
specifically built for applications in scientific research.
Lately, they have become available as off the shelf sensors due to developments in new magnetostrictive materials that allowed their miniaturization and even integration in hybrid semicondutor circuits.
New fabrication techniques promise to improve these in the future and, at the same time that their size decreases, their uses will expand.

41. Fluxgate sensor - principle
The idea of a fluxgate sensor is shown in Figure 5.44a.
The basic principle is to compare the drive-coil current needed to saturate the core in one direction against that in the opposite direction (hence the “gate”).
The difference is due to the external field.
In practice, it is not necessary to saturate the core but rather to bring the core into its nonlinear range.

42. Fluxgate sensors

43. FluxGate: Principle The first winding is used to saturate the magnetic
Material. When the magnetic material is saturate,
there is no voltage across the second winding.
Measurement of delay introduce on the voltage
across the second winding will be help to determine the value of the magnetic filed. When there is no magnetic field, the gap between the pulse is constant. When a magnetic field is applied, the gap between the two pulses are different.

44. Fluxgate sensor - principle
The magnetization curve for most ferromagnetic materials is highly nonlinear
Almost any ferromagnetic material is suitable as a core for fluxgate sensors
In practice, the coil is driven with an ac source (sinusoidal or square)
Under no external field, the magnetization is identical along the magnetic path
Hence the sense coil will produce zero output.

45. Fluxgate sensor - principle
If an external magnetic field perpendicular to the sense coil exists, this condition changes and, in effect, the core has now become nonuniformly magnetized
Produces an emf in the sensing coil of the order of a few mV/T.
The reason for the name fluxgate is this switching of the flux in the core to opposite directions.

46. Fluxgate sensor - principle
The same can be achieved by using a simple rod as in Figure 5.44b.
The two coils are wound one on top of the other
The device is sensitive to fields in the direction of the rod.
The output relies on variations in permeability (nonlinearity) along the bar.
A particularly useful configuration is the use of a magnetstrictive film (metglasses are a common choice)

47. Fluxgate sensor - principle
Magnetostrictive materials are highly nonlinear
The sensors so produced are extremely sensitive – with sensitivities of 10 to 10 T quite common.
The sensors can be designed with two or three axes.
For example, in Figure 5.44a, a second sensing coil can be wound perpendicular to the first.
This coil will be sensitive to fields perpendicular to its area and the whole sensor now becomes a two-axis sensor.

48. Fluxgate sensor - principle
Fluxgate sensors are available in integrated circuits where permalloy is the choice material since it can be deposited in thin films and its saturation field is low.
Nevertheless, current integrated fluxgate sensors have lower sensitivities – of the order of 100 T – but still higher than other magnetic field sensors.

49. The SQUID
Squid stands for Superconducting Quantuum Interference Device.
By far the most sensitive of all magnetometers, they can sense down to 10 T
This kind of performance comes at a price – they operate at very low temperatures – usually at 4.2 K (liquid helium).
They do not seem to be the type of sensor one can simply take off the shelf and use.

50. The SQUID
Surprisingly, however, higher temperature SQUIDs and integrated SQUIDs exist (Liquid nitrogen temperatures - 77K)
Even so, they are not as common as other types of sensors.
The reason for including them here is that they represent the limits of sensing
They have specific applications in sensing of biomagnetic fields and in testing of materials integrity.

51. Radiation Sensors
We have discussed radiation in Lecture 9 when talking about light sensors.
Our particular concern there was the general range occupied by the infrared, visible and ultraviolet radiation.
Here we will concern ourselves with the ranges below and above these.
Range above UV
Range below IR.

52. Electromagnetic Radiation
All radiation may be viewed as electromagnetic radiation.
Many of the sensing strategies, including those discussed in Lecture 9 may be viewed as radiation sensing.
We will however follow the conventional nomenclature
Will call low frequency radiation “electromagnetic” (electromagnetic waves, electro-magnetic energy, etc.)
Will call high frequency radiation, simply “radiation” (as in X-ray, or cosmic)

53. Electromagnetic Radiation
Range above UV is characterized by ionization –
Frequency is sufficiently high to ionize molecules based on Plank’s equation.
The frequencies are so high (above 750 THz) that many forms of radiation can penetrate through materials and therefore the methods of sensing must rely on different principles than at lower frequencies.
On the other hand, below the infrared region, the electromagnetic radiation can be generated and detected by simple antennas.
We will therefore discuss the idea of an antenna and its use as a sensor.

54. Photon Energy
One important distinction in radiation is based on the Planck equation and uses the photon energy to distinguish between them:
h = x10 [joule.second] is Plank’s constant
f is the frequency in Hz
e is called the photon energy.

55. de Broglie’s Wavelength
At high frequencies, where particles are concerned, one can view them either as particles or as waves.
The energy in these waves is given by the Planck equation.
Their wavelength is given by de Broglie’s equation (p=mv is the momentum of the particle):

56. Ionizing Radiation
The higher the frequency the higher the photon energy.
At high frequencies, the photon energy is sufficient to strip electrons from atoms –ionizing radiation.
At low frequencies, ionization does not happen and hence these waves are called non-ionizing.
The highest frequency in the microwave region is 300 Ghz. The photon energy is 0.02 eV. This is considered non-ionizing.
The lowest frequency in the X-ray region is approximately 3x1016 and the photon energy is 2000 eV. Clearly an ionizing radiation.

57. X-ray
Some view radioactive radiation as something different than, say X-ray radiation or microwaves
It is often viewed as particle radiation.
One can take this approach based on the duality of electromagnetic radiation, just as we can view light as electromagnetic or as particles – photons.
We will base all our discussion on the photon energy of radiation and not on the particle aspects.
In some cases it will be convenient to talk about particles. (Geiger-Muller counter, for example)

58. Alpha, Beta, Gamma Radiation
Many of the radiation sensors based on ionization are used to sense the radiation itself (detect and quantify radiation from sources such as X-rays and from nuclear sources (and  radiation).
There are however exception such as smoke detection and measurement of material thickness through radiation.
In the lower range, the sensing of a variety of parameters through microwaves is the most important.

59. Units
Units for radiation, except for low frequency electromagnetic radiation are divided into three:
Units of activity,
Units of exposure
Units of absorbed dose.
Also - units for dose equivalent.
The basic unit of activity is the Becquerel [Bq]
Defined as one transition (disintegration) per second.
It indicates the rate of decay of a radionuclide.

60. Units
An older, non-SI unit of activity was the curie (1 curie=3.7x1010 becquerel).
The Becquerel is a small unit so that the [MBq], [GBq] and [TBq] are often used.
The basic unit of exposure is the coulomb per kilogram [C/kg]=[A.s/kg].
The older unit was the roentgen (1 roentgen=2.58x10 C/kg].
The [C/kg] is a very large unit and units of [mC/kg], C/kg] and [pC/kg] are often used.

61. Units Absorbed dose is measured in grays [Gy] which is [J/kg].
The Gray is energy per kilogram and 1[Gy]=1[J/kg].
The old unit of absorbed dose was the rad (1 rad = 100 [Gy]).
The units for dose equivalence is the sievert [Sv] in [J/kg].
The old unit is the rem (1 rem = 100 [Sv]).
Note that the sievert and the gray are the same.
This is because they measure identical quantities in air.
However the dose equivalent for a body (like the human body) is obtained by multiplying the absorbed dose by a quality factor to obtain the dose equivalent.

62. Radiation sensors Will start the discussion with ionization sensors
Then will discuss the much lower frequency methods based on electromagnetic radiation
Three basic types of radiation sensors:
Ionization sensors
Scintillation sensors
Semiconductor radiation sensors
These sensors are either:
Detectors – detection without quantification or:
Sensor - both detection and quantification

63. Ionization sensors (detectors)
In an ionization sensor, the radiation passing through a medium (gas or solid) creates electron-proton pairs
Their density and energy depends on the energy of the ionizing radiation.
These charges can then be attracted to electrodes and measured or they may be accelerated through the use of magnetic fields for further use.
The simplest and oldest type of sensor is the ionization chamber.

64. Ionization chamber The chamber is a gas filled chamber
Usually at low pressure
Has predictable response to radiation.
In most gases, the ionization energy for the outer electrons is fairly small – 10 to 20 eV.
A somewhat higher energy is required since some energy may be absorbed without releasing charged pairs (by moving electrons into higher energy bands within the atom).
For sensing, the important quantity is the W value.
It is an average energy transferred per ion pair generated. Table 9.1 gives the W values for a few gases used in ion chambers.

65. W values for gases

66. Ionization chamber Clearly ion pairs can also recombine.
The current generated is due to an average rate of ion generation.
The principle is shown in Figure 9.1.
When no ionization occurs, there is no current as the gas has negligible resistance.
The voltage across the cell is relatively high and attracts the charges, reducing recombination.
Under these conditions, the steady state current is a good measure of the ionization rate.

67. Ionization chamber
Fig 9.1

68. Ionization chamber
The chamber operates in the saturation region of the I-V curve.
The higher the radiation frequency and the higher the voltage across the chamber electrodes the higher the current across the chamber.
The chamber in Figure 9.1. is sufficient for high energy radiation
For low energy X-rays, a better approach is needed.

69. Ionization chamber - applications
The most common use for ionization chambers is in smoke detectors.
The chamber is open to the air and ionization occurs in air.
A small radioactive source (usually Americum 241) ionizes the air at a constant rate
This causes a small, constant ionization current between the anode and cathode of the chamber.
Combustion products such as smoke enter the chamber

70. Ionization chamber - applications
Smoke particles are much larger and heavier than air
They form centers around which positive and negative charges recombine.
This reduces the ionization current and triggers an alarm.
In most smoke detectors, there are two chambers.
One is as described above. It can be triggered by humidity, dust and even by pressure differences or small insects, a second, reference chamber is provided
In it the openings to air are too small to allow the large smoke particles but will allow humidity.
The trigger is now based on the difference between these two currents.

71. Ionization chambers in a residential smoke detector
Fig 9.1x

72. Ionization chambers - application
Fabric density sensor (see figure).
The lower part contains a low energy radioactive isotope (Krypton 85)
The upper part is an ionization chamber.
The fabric passes between them.
The ionization current is calibrated in terms of density (i.e. weight per unit area).
Similar devices are calibrated in terms of thickness (rubber for example) or other quantities that affect the amount of radiation that passes through such as moisture

73. A nuclear fabric density sensor
Fig 9.1y

74. Proportional chamber
A proportional chamber is a gas ionization chamber but:
The potential across the electrodes is high enough to produce an electric field in excess of 106 V/m.
The electrons are accelerated, process collide with atoms releasing additional electrons (and protons) in a process called the Townsend avalanche.
These charges are collected by the anode and because of this multiplication effect can be used to detect lower intensity radiation.

75. Proportional chamber
The device is also called a proportional counter or multiplier.
If the electric field is increased further, the output becomes nonlinear due to protons which cannot move as fast as electrons causing a space charge.
Figure 9.2 shows the region of operation of the various types of gas chambers.

76. Operation of ionization chambers
Fig 9.2

77. Geiger-Muller counters
An ionization chamber
Voltage across an ionization chamber is very high
The output is not dependent on the ionization energy but rather is a function of the electric field in the chamber.
Because of this, the GM counter can “count” single particles whereas this would be insufficient to trigger a proportional chamber.
This very high voltage can also trigger a false reading immediately after a valid reading.

78. Geiger-Muller counters
To prevent this, a quenching gas is added to the noble gas that fills the counter chamber.
The G-M counter is made as a tube, up to 10-15cm long and about 3cm in diameter.
A window is provided to allow penetration of radiation.
The tube is filled with argon or helium with about 5-10% alcohol (Ethyl alcohol) to quench triggering.
The operation relies heavily on the avalanche effect
UV radiation is released which, in itself adds to the avalanche process.
The output is about the same no matter what the ionization energy of the input radiation is.

79. Geiger-Muller counters
Because of the very high voltage, a single particle can release 109 to 1010 ion pairs.
This means that a G-M counter is essentially guaranteed to detect any radiation through it.
The efficiency of all ionization chambers depends on the type of radiation.
The cathodes also influence this efficiency
High atomic number cathodes are used for higher energy radiation ( rays) and lower atomic number cathodes to lower energy radiation.

80. Geiger-Muller sensor
Fig 9.3

81. Scintillation sensors
Takes advantage of the radiation to light conversion (scintillation) that occurs in certain materials.
The light intensity generated is then a measure of the radiation’s kinetic energy.
Some scintillation sensors are used as detectors in which the exact relationship to radiation is not critical.
In others it is important that a linear relation exists and that the light conversion be efficient.

82. Scintillation sensors
Materials used should exhibit fast light decay following irradiation (photoluminescence) to allow fast response of the detector.
The most common material used for this purpose is Sodium-Iodine (other of the alkali halide crystals may be used and activation materials such as thalium are added)
There are also organic materials and plastics that may be used for this purpose. Many of these have faster responses than the inorganic crystals.

83. Scintillation sensors
The light conversion is fairly weak because it involves inefficient processes.
Light obtained in these scintillating materials is of light intensity and requires “amplification” to be detectable.
A photomultiplier can be used as the detector mechanism as shown in Figure 9.5 to increase sensitivity.
The large gain of photomultipliers is critical in the success of these devices.

84. Scintillation sensors
The reading is a function of many parameters.
First, the energy of the particles and the efficiency of conversion (about 10%) defines how many photons are generated.
Part of this number, say k, reaches the cathode of the photomultiplier.
The cathode of the photomultiplier has a quantuum efficiency (about 20-25%).
This number, say k1 is now multiplied by the gain of the photomultiplier G which can be of the order of 106 to 108.

85. Scintillation sensor
Fig 9.5

86. Semiconductor radiation detectors
Light radiation can be detected in semiconductors through release of charges across the band gap
Higher energy radiation can be expected do so at much higher efficiencies.
Any semiconductor light sensor will also be sensitive to higher energy radiation
In practice there are a few issues that have to be resolved.

87. Semiconductor radiation detectors
First, because the energy is high, the lower bandgap materials are not useful since they would produce currents that are too high.
Second, high energy radiation can easily penetrate through the semiconductor without releasing charges.
Thicker devices and heavier materials are needed.
Also, in detection of low radiation levels, the background noise, due to the “dark” current (current from thermal sources) can seriously interfere with the detector.
Because of this, some semiconducting radiation sensors can only be used at cryogenic temperatures.

88. Semiconductor radiation detectors
When an energetic particle penetrates into a semiconductor, it initiates a process which releases electrons (and holes)
through direct interaction with the crystal
through secondary emissions by the primary electrons
To produce a hole-electron pair energy is required:
Called ionization energy eV (Table 9.2).
This is only about 1/10 of the energy required to release an ion pair in gases
The basic sensitivity of semiconductor sensors is an order of magnitude higher than in gases.

89. Properties of semiconductors

90. Semiconductor radiation detectors
Semiconductor radiation sensors are essentially diodes in reverse bias.
This ensures a small (ideally negligible) background (dark) current.
The reverse current produced by radiation is then a measure of the kinetic energy of the radiation.
The diode must be thick to ensure absorption of the energy due to fast particles.
The most common construction is similar to the PIN diode and is shown in Figure 9.6.

91. Semiconductor radiation sensor

92. Semiconductor radiation detectors
In this construction, a normal diode is built but with a much thicker intrinsic region.
This region is doped with balanced impurities so that it resembles an intrinsic material.
To accomplish that and to avoid the tendency of drift towards either an n or p behavior, an ion-drifting process is employed by diffusing a compensating material throughout the layer.
Lithium is the material of choice for this purpose.

93. Semiconductor radiation detectors
Additional restrictions must be imposed:
Germanium can be used at cryogenic temperatures
Silicon can be used at room temperature but:
Silicon is a light material (atomic number 14)
It is therefore very inefficient for energetic radiation such as  rays.
For this purpose, cadmium telluride (CdTe) is the most often used because it combines heavy materials (atomic numbers 48 and 52) with relatively high bandgap energies.

94. Semiconductor radiation detectors
Other materials that can be used are the mercuric iodine (HgI2) and gallium arsenide (GaAs).
Higher atomic number materials may also be used as a simple intrinsic material detector (not a diode) because the background current is very small (see chapter 3).
The surface area of these devices can be quite large (some as high as 50mm in diameter) or very small (1mm in diameter) depending on applications.
Resistivity under dark conditions is of the order of 108 to 1010 .cm depending on the construction and on doping, if any (intrinsic materials have higher resistivity). .

95. Semiconductor radiation detectors - notes
The idea of avalanche can be used to increase sensitivity of semiconductor radiation detectors, especially at lower energy radiation.
These are called avalanche detectors and operate similarly to the proportional detectors
While this can increase the sensitivity by about two orders of magnitude it is important to use these only for low energies or the barrier can be easily breached and the sensor destroyed.

96. Semiconductor radiation detectors - notes
Semiconducting radiation sensors are the most sensitive and most versatile radiation sensors
They suffer from a number of limitations.
Damage can occur when exposed to radiation over time.
Damage can occur in the semiconductor lattice, in the package or in the metal layers and connectors.
Prolonged radiation may also increase the leakage (dark) current and result in a loss of energy resolution of the sensor.
The temperature limits of the sensor must be taken into account (unless a cooled sensor is used).

97. Microwave radiation sensors - introduction
Microwaves are often employed in the sensing of other quantities because of the relative ease of generating, manipulating and detecting microwave radiation.
Use in speed sensing, in sensing of the environment (radar, doppler radar, mapping of the earth and planets, etc.) are well known.
All of these applications and sensors are based on the properties – especially the propagation properties of electromagnetic waves.

98. Microwave sensing
Sensing with microwaves is based on four distinct methods, some more useful than others:
1. Propagation of waves
2. Reflection of waves
3. Transmission of waves
4. Resonance
These may be combined in a sensor to affect a particular function.

99. Microwave sensing - RADAR
RADAR - RAdio Detection And Ranging.
Best known method of microwave sensing
In its simplest form it is not much different than a simple flashlight (source) and our eye (detector)
Shown schematically in Figure 9.9.
The larger the target and the more intense the source of waves, the larger the signal received back from the target.

100. Scattering of electromagnetic waves
Fig 9.9

101. Microwave sensing - RADAR
Reception may be by the same antenna (pulsed-echo radar), or (a-static radar)
Reception may be continuous by a separate antenna (bi-static radar)
Both are shown in Figure 9.10.
The operation of radar is based on the reflection of waves by any target the incident waves encounter.

102. A-static and bi-static radar
Fig 9.10

103. Radar
For any object in the path of electromagnetic waves, the scattering coefficient, called the scattering cross-section or radar cross-section 
Ps is the scattered power density
Pi the incident power density
R isthe distance from source to target

104. Radar The power received is calculated from the radar equation
λ is the wavelength
σ the radar cross-section
Pr the total received power
Prad the total radiated power
D is the directivity of the antenna.

105. Radar Directivity is a property of the antenna
It is an indication of how directive the radiation is
Depends on the type and construction of antennae.
Radar is a short range device because of dependency on 1/R4.
It is one of the most useful sensing systems capable of sensing distance as well as size (radar cross-section) of objects.
In more sophisticated systems the position (distance and attitude) may be sensed as well as the speed of the target but these are obviously as much a function of the signal processing involved as they are of the radar itself.

106. Doppler Radar
A different approach to radar sensing is based on the doppler effect.
In this type of radar, the amplitude and power involved are not important (as long as a reflection is received).
Rather, the doppler effect is taken advantage of.
This effect is simply a change in the frequency of the reflected waves due to the speed of a target.

107. Doppler Radar
Consider a target moving away from a source at a velocity v as shown in Figure 9.11.
The source transmits a signal at frequency f.
The reflected signal arrives back at the transmitter after a delay 2t where t=S/v.
This delay causes a shift in the frequency of the received signal as follows:

108. Doppler radar - principle

109. Doppler Radar
The returning wave’s signal is lower the higher the velocity of the vehicle.
If the motion is towards the radar source, the frequency increases (negative velocity).
Measuring this frequency gives an accurate indication of the speed of the vehicle.
Used in police speed detectors
The same can be used to detect aircraft or tornadoes – all relying on speed detection.
Doppler radar is totally blind to stationary targets.

110. Doppler Radar
Doppler radar is also actively pursued for anti collision systems in vehicles (rudimentary systems exist in trucks for side collision detection) and for active cruise control.
Radar relies heavily on good antennas and on directivity of these antennas.
Practical radar sensor operate at relatively high frequencies – from about 10GHz to 30 GHz
Systems for collision avoidance operate in excess of 80 GHz

111. Radar There are many other types of radar.
One is the into the ground radar (also called ground penetrating radar).
Operates at lower frequencies for the purpose of penetrating and mapping underground objects.
For space exploration and for mapping of planets, - SAR (Synthetic Aperture Radar)
This method makes use of moving antennas and signal processing to increase the range and apparent power of radar.