Modern Instrumentation PHYS 533/CHEM 620 Lecture 13 Magnetic Field and Radiation Sensors Amin Jazaeri Fall 2007
Transcript
Slide 1
Modern Instrumentation PHYS 533/CHEM 620 Lecture 13 Magnetic
Field and Radiation Sensors Amin Jazaeri Fall 2007
Slide 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.
Slide 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
Slide 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.
Slide 5
Hall effect - principle
Slide 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/m 3 ] and q is the charge of the electron [C]
Slide 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 [m 3 /C] is material
dependent and is called the Hall coefficient K H.
Slide 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.
Slide 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 earths magnetic field is only about 50 T so
that the output is a mere 25 V
Slide 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.
Slide 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.
Slide 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.).
Slide 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
Slide 14
Hall element as a rotation sensor
Slide 15
Electronic ignition
Slide 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
Slide 17
Some Hall element sensors
Slide 18
A 3-axis Hall element probe
Slide 19
Hall sensors used to control a CDROM motor
Slide 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.
Slide 21
The magnetoresistive sensor
Slide 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.
Slide 23
Magnetoresistive sensors The relation between field and current
is proportional to B 2 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:
Slide 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.
Slide 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).
Slide 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
Slide 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)
Slide 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
Slide 29
Magnetoresistive sensor - operation
Slide 30
Magnetoresistive sensors - operation The resistance of the
sample becomes: R 0 is the resistance without application of the
magnetic R 0 is the change in resistance expected from the material
used. Both of these are properties of the material and the
construction (for R 0 ). The angle is again material
dependent.
Slide 31
Properties of magnetoresistive materials
Slide 32
Magnetoresistive sensors - properties
Slide 33
Magnetoresistive sensors - comments Used exactly like Hall
sensors Much more sensitive Common in read heads in hard drives
Used for magnetic compasses
Slide 34
Principle of induction
Slide 35
Faradays law Given a coil with N turns and a flux through it.
The emf on the coil is: B is the flux density S area of the coil is
the angle between the two
Slide 36
Small loop magnetometer The relations show that the output is
integrating (dependent on coils 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.
Slide 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 coils 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.
Slide 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).
Slide 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.
Slide 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.
Slide 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.
Slide 42
Fluxgate sensors
Slide 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.
Slide 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.
Slide 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.
Slide 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)
Slide 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.
Slide 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.
Slide 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.
Slide 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.
Slide 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.
Slide 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)
Slide 53
Electromagnetic Radiation Range above UV is characterized by
ionization Frequency is sufficiently high to ionize molecules based
on Planks 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.
Slide 54
Photon Energy One important distinction in radiation is based
on the Planck equation and uses the photon energy to distinguish
between them: h = 6.6262x10 [joule.second] is Planks constant f is
the frequency in Hz e is called the photon energy.
Slide 55
de Broglies 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 Broglies equation (p=mv is the momentum
of the particle):
Slide 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 3x10 16 and the photon energy is 2000 eV. Clearly an
ionizing radiation.
Slide 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)
Slide 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.
Slide 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.
Slide 60
Units An older, non-SI unit of activity was the curie (1
curie=3.7x10 10 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.
Slide 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.
Slide 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
Slide 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.
Slide 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.
Slide 65
W values for gases
Slide 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.
Slide 67
Ionization chamber
Slide 68
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.
Slide 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
Slide 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.
Slide 71
Ionization chambers in a residential smoke detector
Slide 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
Slide 73
A nuclear fabric density sensor
Slide 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 10 6 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.
Slide 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.
Slide 76
Operation of ionization chambers
Slide 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.
Slide 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.
Slide 79
Geiger-Muller counters Because of the very high voltage, a
single particle can release 10 9 to 10 10 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.
Slide 80
Geiger-Muller sensor
Slide 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 radiations
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.
Slide 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.
Slide 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.
Slide 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 k 1 is now multiplied
by the gain of the photomultiplier G which can be of the order of
10 6 to 10 8.
Slide 85
Scintillation sensor
Slide 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.
Slide 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.
Slide 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 - 3-5 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.
Slide 89
Properties of semiconductors
Slide 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.
Slide 91
Semiconductor radiation sensor
Slide 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.
Slide 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.
Slide 94
Semiconductor radiation detectors Other materials that can be
used are the mercuric iodine (HgI 2 ) 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 10 8 to 10 10 .cm
depending on the construction and on doping, if any (intrinsic
materials have higher resistivity)..
Slide 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.
Slide 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).
Slide 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.
Slide 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.
Slide 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.
Slide 100
Scattering of electromagnetic waves
Slide 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.
Slide 102
A-static and bi-static radar
Slide 103
Radar For any object in the path of electromagnetic waves, the
scattering coefficient, called the scattering cross-section or
radar cross-section P s is the scattered power density Pi the
incident power density R isthe distance from source to target
Slide 104
Radar The power received is calculated from the radar equation
is the wavelength the radar cross-section P r the total received
power P rad the total radiated power D is the directivity of the
antenna.
Slide 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/R 4. 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.
Slide 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.
Slide 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:
Slide 108
Doppler radar - principle
Slide 109
Doppler Radar The returning waves 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.
Slide 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
Slide 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.