A SITE WHERE YOU CAN GET DIVERSE MATERIALS ON EDUCATION, INSPIRATION,RELIGIOUS AND GENERAL LIFE....GET ACADEMIC ARTICLES, THESIS, RESEARCH PAPERS AMONG OTHERS. FULLY RE-LOADED TO BE AT PAR WITH THE NEEDS AND THE DEMANDS OF THE REVOLUTIONIZED EDUCATION IN THE 21ST, CENTURY.

Tuesday, December 25, 2012

X RAYS

X RAYS
Introduction
v    X-Rays is an electromagnetic radiation.
-                      It was discovered by Wilhelm Rontgen. {A German}. He was conducting a research on cathode rays.
v    X- Rays owe their name to the fact that at the time of their discovery their nature was unknown.
v    X-rays are now known that they are produced when fast moving electrons are suddenly stopped by matter.
Production of x- rays
-                      X-rays are produced in an x-ray tube. This is when fast moving electrons suddenly hit a metal target.
-                      When a current flows through the filament, electrons are produced through thermionic emission.
-                      Question using the kinetic theory of matter explain how electrons are produced when cathode is heated.
-                      The emitted/dislodged electrons are accelerated towards the anode by the high potential difference of the order 100kv between the cathode and the anode.
-                      The cathode is concave in shape so as to focus the electron beam on to the target.
-                      The target is usually made of tungsten/molybdenum embedded onto copper.  The copper is a good conductor of heat that facilitates efficient dissipation of heat.
-                      Tungsten has a high melting point .This makes it or (molybdenum) to withstand high temperatures.
-                      Most of the kinetic energy possessed by the fast moving electrons is converted to heat (99.5%). Only 0.5 % is converted to x-rays.
-                      Cooling in the tube if further enhanced by the cooling fins outside the tube and the circulating oil.  
-                      The recent x-rays are made of a rotating anode to change the point of impact with electrons. This reduces the wear and tear on it.
-                      The tube is made of a strong glass which is highly evacuated. This ensures that the electrons do not loose energy as they interact/collide with the air particles.
-                      The target is set at an angle (i.e. 45°) to the electron beam to direct the x-rays out of the tube through an opening on the lead shield.
-                      The off- focus (stray) x-rays are normally absorbed by the lead shield.
-                      The step up transformer provides the high potential difference (high voltage).
-                      NB: 1. during the half cycle when the anode is positive with respect to the cathode, the produced electrons produce x-rays when bombardment takes place at the target.
-                      2. During the reverse half cycle, the anode is negative with respect to the cathode. Electrons do not move to the target and hence no bombardment takes place.  X-rays are not therefore produced in the reverse half cycle.
-                      The production of x-rays only takes place during the half cycle when the anode is positive with respect to the cathode. But due to the high frequency of the a.c., production of x-rays appears to be continues.
DIAGRAMS ---X-RAYS
Figure 1.1
            Focusing Cup
• The focusing cup helps control electron cloud. The electrons repel each other & want to spread out. The focusing cup forces the electrons to form a small stream as they move toward the target material.
–Made of nickel
– Has a low negative charge
     Figure 1.2
Rotating Anode Assembly
Figure 1.3


Properties of x-rays
1.      They are neither deflected by the magnetic or electric field. They are not therefore charged.
2.      They penetrate matter with the least amount of penetration occurring in substances with the highest density e.g. lead.
3.      X-rays affect photographic emulsions. This property is used in x-ray photography.
4.      When moving through air, they knock off electrons of molecules on their path. They therefore ionize the air molecules increasing the electrical conductivity of the gases.
5.      They cause photoelectric emission.
6.      They cause fluorescence in certain substances e.g. zinc sulphide.
7.      They are electromagnetic radiations of shorter wavelength than the visible light.  They can be plane-polarized and are diffracted. They are therefore waves in nature.
8.      They travel in straight lines at the speed of light (3.0 x 108 m/s).

PAST KCSE QUESTIONS ON THE TOPIC X-RAYS

1.   An X-ray tube is operating with an anode potential of 10kV and a current of 15.0 mA.
            a)         Explain how the
                        i)          Intensity of X-rays from such a tube may be increased.
ii)         Penetrating power of X- rays from such a tube may be increased
            b)         Calculate the number of electrons hitting the anode per second.
            c)         Determine the velocity with which the electrons strike the target.
            d)         State one industrial use of X-rays.
2.   a)         For a given source of X-rays, how would the following be  controlled.
      i)          Intensity
      ii)         The penetrating power
      iii)        The exposure to patients
b)         An accelerating potential of 20kv is applied to an X-ray tube.
i)    What is the velocity with which the electron strikes the target?
ii)   State the energy changes that take place at the target.
3.   Explain why X-rays are appropriate in study of the crystalline structure materials.
4. Name the metal used to shield X-rays operators from the radiation. Give reasons why it is used.
5.   State the properties of X-rays, which makes it possible to detect cracks in bones.
6. State one difference between hard X-rays and soft X-rays.                (1mk)

7.   A target was bombarded by electron accelerated by a voltage of 106 V.  If all the K.E of the electrons was converted to X-rays, calculate:-
      a)   The K.E of the electrons
      b)   The frequency of the photons emitted.
8.   An X-rays tubes gives photons of 5.9 x 10-15 J of energy.  Calculate:-
      a)   The wavelength of the photons.
      b)   The accelerating voltage
      c)   The velocity of the electrons hitting the target.
9.   If accelerating voltage in an X-ray tube is 40kV, determine the minimum wavelength of the emitted X-rays.  (Electronic charge = -1.6 x 10-19C, planks constant = 6.6 x 10 -34Js, velocity of electromagnetic waves = 3.0 x 108ms-1)
10. State the purpose of cooling fins in the X-ray tube.
11. X-rays are produced by a tube operating at 1 x 104V.  Calculate their wavelength.  (Take h= 6.6 x 10 -34 Js, e= 1.6 x 10-19C, c= 3x108ms-1)
12. State and explain the effect of increasing the EHT in an X- ray tube on the X-rays produced.

                                                                                Answers
                                                            X-RAYS
1.       
2.       (a)      (i)         Heater current or Filament current
(ii)        Anode Potential or operating potential
(iii)       Covering with protective materials where X- rays are not required
o   Minimize exposure time as much as possible
o   Reduce no of exposures as much as possible
(b)        (i)         ½ MeV2 =  eV
V= €√ (2eV)
              Me
= √ 2 x 1.76 x 109 x 20 x 103
= 6.39 x 107 m/s
(ii)        KE- Heat or internal energy and energy of x- rays or radiation.
3.                 X- Rays have wavelengths of the order of the lattice spacing; and therefore they can be diffracted; (Diffraction due to short wavelengths of x- rays). In calculation the atomic separation is equal to slit separation- or grating separation. Lead because it is very dense, has high atomic mass.
4.                 (a)        (i)         Increase the filament current
(ii)        Increase the anode potential
                        (b)        Q= it = 15 x 10-3 A x is = 15 x 10-3C
                                    Electron charge = 1.6 x 10-19 C
                                    No. of electrons in 15 x 10-3C
                                    = 15 x 10-3 = 9.38 x 1016 e/s
                                    1.6 x 10-19
                        (c)        ½ mv2 = ev
                                    V= √ 2eV
                                            Me
                                    (2 x 1.6 x 10-19 x 10 x 103) ½
                                                9.1 x 10-31
                                                = 5.9 x 107 m/s
                        (d)       -           Detecting flows/ fault in metals or other structures
                                    -           Quality control of manufactured items e.g. tyres, thickness  of sheets, Paper.
            -           Analysis of gem stones
5.                 Highly penetrating in matter
6.                 Hard X- rays are more penetrating than soft X- rays due to their higher frequency.
7.      (a)       1.6 x 10-13
(b)       2.424 x 1020Hz
8.      (a)       3.35 x 10-11m
(b)       36,875V
(c)       1.3 x 1016 m/s
9.      3.1 x 10-11m.
10.  The fins are used to cool the copper rod which conducts heat away from the target when electrons hit the target
11.  K.E on input = e.v
= 1.6 x 10-19 C x 1 x 104V
= 1.6 x 10 – 15 Houles
Energy of x- rays is hf
Where f=     c
                  λ mm

 hc = 1.6 x 10-15 Joules
λ mm

λ mm = 6.6 x 10-34 x 3 x 108
                  1.6 x 10­-15

λ mm = 1.24 x 10-10m

12.  Hard x- rays produced higher EHT results in faster electrons hence higher energy x- rays.

Posted by at
Labels:

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)