Download SMU-DDE-Assignments-Scheme of Evaluation PROGRAM Bachelor

SMU-DDE-Assignments-Scheme of Evaluation
PROGRAM
SEMESTER
SUBJECT CODE &
NAME
BK ID
DRIVE
MARKS
Q.No
1.
A
2.
A
Bachelor/Diploma in Medical Imaging Technology
I
BMI 103 – Fundamentals of Medical Imaging Technology
B1893
WINTER 2015
60
Criteria
Marks
Total
Marks
Discuss the properties of electromagnetic waves.
(Unit 2;Section 2.3;Pg 25-27)
 Electromagnetic waves are self-sustaining oscillations of electric and
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magnetic fields in space or vacuum.
 Electromagnetic waves can travel through air, as well as empty space.
 They travel through a vacuum at a constant speed of 3.00 x 108 m/s
 Electromagnetic wave travels through material medium too (solids
and liquids). But the speed of these waves changes in the material
medium and it depends upon the permittivity and permeability of the
medium.
 Electromagnetic radiation has properties of wave and particles, a
phenomenon known as wave-particle duality.
 Electromagnetic waves slightly bend as it passes around the edge of
an object.(Diffraction)
 When electromagnetic waves pass from rarer medium to denser
medium, they deviate from their original direction with respect to the
normal.(Refraction)
 When electromagnetic waves are passed through a polarizer, then all
electric field vectors vibrate in the same direction and this
unidirectional property is known as polarization.
 Electromagnetic waves carry energy and momentum like other waves.
 Electromagnetic wave also exerts pressure because it contains
momentum and this pressure is called radiation pressure.
Define electronic emission. Explain the process of electron emission.
(Unit 5;Section 5.3;Pg 75-78)
The process of liberation of electrons from the surface of a material is
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known as electron emission.
4 types of electron emission depending upon the method of liberation of
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electrons:
1. Thermionic emission
2. Field emission
3. Photoelectric emission
4. Secondary emission
Thermionic emission
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 When a metal is heated to a sufficiently high temperature, it enables
the free electrons to leave the metal surface.
 The temperature required for this may be about 2500ºC.
 The number of electrons emitted depends upon the temperature.
SMU-DDE-Assignments-Scheme of Evaluation

The higher the temperature, greater will be the number of free
electrons.
 Used in vacuum tubes.
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A
Field emission
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 In this method a strong electric field (positive high voltage) of the
order of 106 V/m is applied at the metal surface, which forces the
electrons to move out of the metal surface.
 This process does not depend upon the temperature.
 The stronger is the electric field, greater will be the emission. This
type of emission process is used in cold cathode devices, mercury
and rectifier tubes.
Photoelectric emission
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 In this process, the surface of a metal is illuminated by a beam of
light, causing electron emission from the surface of metals by light
photons, provided photon energy is greater than that of the work
function of the metal.
 The greater the intensity of light beam falling on the metal surface,
greater will be the photoelectric emission current.
Secondary emission
2
 In this process, a beam of high-energy electrons strike the metal
surface, causing emission of free electrons from the metal surface
 The intensity of secondary emission depends upon the source
material of high energy electrons, mass and energy of bombarding
electrons.
Define transistor. Explain the different parts and the principle and working of a transistor.
(Unit 10;Section 10.9;Pg 213-217)
A transistor is basically a silicon or germanium crystal containing three
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separate regions. It can either be n-p-n type or p-n-p type.
Transistor consists of following three parts
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1. Emitter, which is moderate-sized, highly doped and its function is to
emit mobile carriers.
2. Base, which is thin-sized and lightly doped and its function is to
transport mobile charge carriers from emitter to collector.
3. Collector, which is large in size and moderately doped, and its function
is to collect the mobile charge carriers.
 A transistor has two p-n junctions. One junction is between the emitter
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and the base and is called emitter-base junction or simply emitter
junction.
 The other junction is between the base and the collector and is called
the collector junction. Thus in a transistor two p-n junctions are
connected back to back.
 In n-p-n type of transistor, n-p part of the transistor is forward-biased
with respect to the battery, i.e. electrons act as charge carriers which
flow through the n (emitter) part of the transistor, whereas the p (base)
part of the transistor neutralizes electrons at the n-p junction.
Principle and working
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 The transistor is only conductive when a large current flows, and the
greater the base current the greater is the collector current.
SMU-DDE-Assignments-Scheme of Evaluation
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A
5.
As the collector current is large compared to the base current the
transistor works as a current amplifier
 A transistor can be biased in four ways: one in which emitter base
junction is forward-biased and collector base junction is reversedbiased. When emitter junction is forward biased then potential barrier
is reduced and space charge region at the junction becomes narrow.
As such majority carriers diffuse across the junction.
 The resulting current consists of electrons travelling from emitter to
the base, and holes passing from the base to the emitter. Only the
electron current is useful in a transistor.
 Therefore electron current is made much larger than the hole current.
 This is done by doping the base region more lightly than the emitter
region.
 The sum of charge movements constitutes the emitter current. Once
the electrons are injected by the emitter into the base they become
minority carriers.
 The transistor may be used as a power, current or voltage amplifier or
as an electronic switch.
Discuss the different types of transformers.
(Unit 11;Section 11.4;Pg 225-229)
The different types of transformers are as follows:
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1. Open core transformer
2. Closed core transformer
3. Autotransformer
4. Step-up transformer
5. Step-down transformer
 Open core transformer: An open core transformer has two separate iron
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cores each for the primary and the secondary windings.
 Closed core transformer: A closed core transformer is made up of a
soft iron core with primary windings on one side and secondary
windings on the other. The core is laminated to avoid losses due to
eddy currents.
 Autotransformer: An autotransformer has the same coil for primary
and secondary windings. The input comes through the primary
windings and the number of windings in the secondary coil can be
changed by changing the location of switch on the secondary side.
 Step-up transformer: A step-up transformer converts low-voltage
current to high-voltage current.
 Step-down transformer: A step-down transformer converts highvoltage current to low-voltage current. The number of turns in the
primary coil is more than the number of turns in the secondary coil.
Explain stationary anode x-ray tube.
(Unit 12;Section 12.3;Pg 253-257)
SMU-DDE-Assignments-Scheme of Evaluation
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Stationary anode x-ray tube consists of a cathode, an anode, a filament
and a focusing cup
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Cathode
 The cathode assembly consists of a tungsten filament placed in a
negatively charged focusing cup.
 The cathode is constructed as a coiled wire filament of reasonably
high resistance so that for a given heating current typically in the
order of 5 A it gives abundant supply of electrons.
 Most x-ray tubes have dual filaments. These filaments lie parallel to
each other in the focusing cup.
Filament
 The filament is made up of tungsten wire 7–15 m long and 1–2 mm
thick, coiled into filament 0.1–0.2 mm wide.
 Tungsten is chosen as filament because it has high melting point of
3370°C, can be drawn into thin wire and does not vapourize easily
when heated to high temperatures.
 The purpose of drawing cathode in filament or coil is to maximize the
surface area of cathode to increase its efficiency.
 When filament current of 10 V and 3–5 A is applied to the filament, it
emits electrons.
 The emission of electrons from the filament when heat is applied is
called thermionic emission.
 When the filament vapourizes, it turns into gas and then solidifies on
the glass of the x-ray tube. It is called suntanning or sun-burning of
the x-ray tube.
 It reduces the output of the tube and causes arcing and tube failure.
Thorium is added to minimize vapourization of the filament.
 The number of electrons released and thus the number of x-rays
produced is directly proportional to the filament current. More the
milliamperes applied more will be the number of electrons released
and more the number of x-rays produced.
 In modern x-ray tubes, there are two filaments in a tube.
 The smaller filament forms a smaller focal spot and thus geometrical
unsharpness is less.
Focusing cup
 When the filament emits electrons, electrons repel other electrons and
tend to diverge. They spread out and form a cloud of electrons near
the filament.
 The focusing cup avoids the electrons from spreading in all directions
and repels them from the filament. Focusing cup is made up of nickel
and has a low negative charge. This negative charge repels electrons
away from the cathode assembly and electrons are converged to focal
spot of required size on the target.
 The filament is embedded in the focusing cup
 In grid-controlled x-ray tube this focusing cup is negatively biased to
about 3 kV (3000 V).
Anode
 The anode is a positively charged electrode, made up of tungsten or
tungsten-rhenium alloy.
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SMU-DDE-Assignments-Scheme of Evaluation

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A
It stops the highly accelerated electrons, converting their kinetic
energy into x-rays.
 Potential difference across the tube causes electrons to accelerate
towards the anode assembly.
 Higher the potential difference more will be the acceleration of
electrons and higher the energy of x-rays produced.
 High-energy x-rays are needed for more penetrating areas.
 The material chosen for the anode should satisfy a number of
requirements.
Line focus principle and anode heel effect
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List the different types of radioactive detectors and explain gas filled detectors.
Following are the various types of radiation detectors.
1. Gas-filled detectors
 Ionization chambers
 Proportional counters
 Geiger Muller counter
2. Scintillation detectors
 Solid
 Liquid
3. Other detectors
 Semiconductor detectors
 Films
 Thermo luminescence dosimeters
(Unit 14;Section 14.5;Pg 306-310)
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Ionization chamber
 Simplest type of detector which measures the ionization in a small
volume of air.
 In ionization chambers, two plates or electrodes with an electric
potential between them is placed in a container filled with air.
 When incident radiation falls on the ionization chamber, it will
produce ion pairs in the gas. Positive ions travel towards the plates and
the electrons travel towards the central wire.
 Ionization chamber measures radiation directly and is energyindependent.
 It can be used to measure X-radiation, gamma radiation, beta radiation,
and if the window is thin enough, can also measure alpha radiation.
 Ionization chamber is stable, has high accuracy and relatively low
sensitivity.
Pocket dosimeter
 Works on the principle of ionization chamber which is used to measure
instant exposure during any procedure. Thus it can be used for
personnel monitoring.
 It is basically based on the principle of a gold leaf electroscope.
 When the electroscope is charged, the leaves will diverge because of
repulsion. If the gas around the leaves is then ionized, the ionized
instruments will be discharged.
 The pocket dosimeter is charged by an auxiliary charger to specific
potential as shown by the position of the needle on a calibrated scale
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SMU-DDE-Assignments-Scheme of Evaluation
which may be viewed through tele-microscope.
 When placed in a radiation field the device will be discharged by an
amount proportional to the ionization provided by the radiation and the
pointer will move across the scale.
 This instrument is not accurate, but due to its compact portable device
it is kept in pocket like a pen.
Geiger Muller counter
 Most common detector used.
 It uses an ionization chamber filled with a special gas and has a
greater voltage supplied between its electrodes.
 Applying high voltage between parallel plates the Geiger Muller (GM)
plateau is obtained
 When working on the GM plateau, only the count rate changes slowly
with the applied voltage, so very precise voltage is not necessary.
 When the voltage is very high the primary electrons are accelerated to
produce avalanche.
 They lose exciting energy by emitting X-rays and ultraviolet photons
which liberate outer electrons from other gas atoms creating further ion
pairs by a process of photo-ionization.
 As these events may occur some distance away from initial events,
there is complete ionization occurring in the tube. These electrons
initiate other pulses which recyle into the counter.
*A-Answer
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