PHY4: X-RAYS

This unit is about X-rays and how they work in physics.

X- RAYS

X-rays are of short wave length (electromagnetic waves which are produced when cathode rays are stopped by heavy metals).

Production of X- rays

XRAY

Mode of operation

A low voltage is applied   across the filament and heats the filament. Electrons are emitted by the filament by thermionic emission. The concave focusing cathode focuses the    electrons from the filament onto the target. The electrons are accolade across  the vacuum  by  applying  a  high  alternating  voltage  between  the  filament  and  the  anode.  During the half cycles when the anode is at a positive potential relative to the cathode, electrons are accelerated across the tube. No electrons flow to the anode when the anode is at a negative potential relative to the cathode.

When the cathode rays [electrons] strike the Tungsten target, 99 % of the kinetic energy of electrons is dissipated into heat while 1% is turned into x-rays.  The heat generated at the target is cooled by means of cooper cooling fins mounted on the copper anode.

Heat is conducted from the target away from the tube by conduction and radiation.

The x – ray tube is covered by a lead shield with a small widow for the x – rays to prevent the leakages of the X rays.

Intensity of x – rays (quality)

The intensity of x ray in an x – ray tube is proportional to be number of electrons coiling with the target .The number of electrons produced of the cathode depend on the filament supply . The greater the heating current, the greater the number of electrons produced and hence more x rays are produced. Therefore the intensity of x – ray is controlled by the filament current.

Penetration of X rays (quality)

Penetration power of x – rays depends on the kinetic energy of the electrons striking the target. The higher the accelerating voltage, the faster the electrons produced. Factor electrons passes higher kinetic energy and shorter wave length’s rays of great penetration power are produced. Hence penetrating power of x rays is determined by the accelerating voltage across the tube.

Hard and soft x – rays

Hard x rays have a high penetrating power. This because they have very short wave lengths. They are produced when a high p.d is applied across the tube. Soft x rays are produced by electrons moving at relatively lowers velocities than these produced by hard x rays. They have less energy; longer wave lengths, hence less penetration power compared to hard x rays. Hard x -rays can penetrate flesh but are absorbed by bones .Soft x-rays are used to show malignant growths since they only penetrate soft flesh. They are absorbed by such growth.

Properties of x – ray

-They travel in a straight line at a speed of light.

-They are not deflected by both magnetic and electric fields. This indicates that they carry no charge.

-They penetrate all matter to extend. Penetration is least in materials with high destiny and atomic number e.g lead.

-They ionize gases through which they pass.

-They affect photographic plates just like light does.

-They cause fluorescence in some materials.

-They cause photographic effect when they are illuminated on certain metal surfaces.

-They are different by crystals leading to an interference pattern.

Uses of x – rays

PHY4: X-RAYS 1

1-Structural analysis, stresses, fractures in solid, castings and welded joins can be analyzed by examining x-ray photograph.

2-Crystallography: orientation and identification of minerals by analysis of diffraction patterns using B rays law.

3-Medical uses;

a) Analytical uses. These include location of fractures, cancer and tumor defective tissue absorbs x-rays differently from normal tissue.

b)Therapeutics use for destroying cancerous cells and tumors

4-Detection of fire arms at international air ports.

HEALTH HAZARDS CAUSED BY X-RAYS.

-Destroy living cells in our bodies especially hard x-rays.

-Cause Gene mutation (genetic charge in our bodies)

-Cause damage of our eye sight and blood.

-Produce deep skin burns.

NOTE:

As highly important to remember that each time you are exposed to x-rays ,your health is also at risk yet we cannot live cannot them.

SAFETY PRECAUTIONS

-Avoid unnecessary exposure to x- rays.

-When exposure is necessary, keep it as short as possible.

-X-rays beams should only be restricted to the body part being investigated.

-A worker should wear a shielding jacket with a layer of lead.

-Exposure should be avoided for unborn babies and very young children.

Thermionic Diode:

Structure

It consists of an anode usually inform of a nickel cylinder which surrounds the cathode in an evacuated glass bulb. In the indirectly headed cathode type, the cathode is nickel tube with a tungsten filament (or heater) inside it. The heater is insulated electrically from the cathode by packing alumina inside the nickel tube. The outside of the tube is coated with a mixture of Barium and strontium oxides.

Symbol of a diode

PHY4: X-RAYS 2

 Thermionic diode characteristics

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Keeping the filament current if constant, the p.d Va between the cathode and the anode is varied. The corresponding anode current anode voltage characteristics. By setting the filament current to other constant values, the corresponding la-va characteristics can be obtained. These features can be shown below.

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For Va = 0, electrons are emitted by the cathode with a range of speeds-A few of the electrons are emitted with sufficient kinetic energy to be able to reach the anode. This leads to a small current if the anode is made negative relative to the cathode; a reserve current exists for negative potential up to about 0.5v and then decrease to zero

Region AB: Have Va is small. Only these electrons emitted with high speed will be able to reach the anode. The majorities of the electrons is emitted with low kinetic energies and are repelled back towards the cathode. The electron distribution around the cathode constitutes a negative space charge, the current la is small.

Region BC: As Va increases the attraction of the space charge by the anode increases. This result in a large anode current .This region is called space charge limited region.

Region CD: the region anode voltage Va is so large that all the electrons emitted per second by the cathode reach the anode. The space charge is overcome; a constant current called saturation current flows. Region CD is also called by the cathode increases, more electrons are emitted per second by the cathode.

A higher saturation current therefore flows.

Applications of the thermionic diode

  1. Half wave rectification.

Suppose a thermionic diode is connected in series with a source of alternating voltage V1 and a load R1.

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During the half cycles when A is positive to C the diode conducts and a p.d Vr appears across the load R1. During the half cycles when A is at a negative potential relative to C, the diode does not conduct and no p.d appears across R1 .The a.c is half ware rectified.

The input and output voltage ware forms are compared in the diagram below.

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Full ware rectification.

  1. i) Using two diodes.

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When p is at negative potential relative to q , diode D conducts whereas D2 doesn’t.

When p is at a positive potential relative to q, diode D2 conducts whereas Ddoesn’t. Current flows in the same direction through the load Rduring both positive and negative cycles of the input voltage V2.

The input and output voltage ware forms are compared in the diagram below.

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Vd1 and Vd2 is output pd due to conduction of diode D1 and D2 respectively.

Vr is the output voltage across load Rb.

ii)Using four diodes.

The following rectifier symbols will be used.

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Forward bias (diode conducts) reverse bias (diode does not conduct)

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During the half cycles when A is at positive potential relative to B , diodes D2 and D3 are forward biased hence they conduct and current flows through the resistor R in the direction p to Q. Diodes D2 and D4 are reverse biased and they do not conduct.

During the half cycles when B is at positive potential relative to A, diode Di and D4 are forward biased and they conduct. Currents flows through resister R in the direction p to q. Diodes D2 and D3 are reverse biased and do not conduct.

The voltage cross R will have the form;

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THIS VIDEO SHOWS MORE ABOUT X-RAYS.

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ASSIGNMENT : X-RAYS assignment MARKS : 10  DURATION : 1 week, 3 days

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