X-Ray
Introduction
What is X-Ray?
Wilhelm Roentgen, Professor of Physics in Wurzburg, Bavaria, was the first person
discover the possibility of using electromagnetic radiation to create what we now
know as the x-ray.
Roentgen was exploring the path of electrical rays passing from an induction coil
through a partially evacuated glass tube. Although the tube was covered in black
paper and the room was completely dark, he noticed that a screen covered in
fluorescent material was illuminated by the rays. He later realized that a number of
objects could be penetrated by these rays, and that the projected image of his own
hand showed a contrast between the opaque bones and the translucent flesh. He later
used a photographic plate instead of a screen, and an image was captured. In this way
an extraordinary discovery had been made: that the internal structures of the body
could be made visible without the necessity of surgery. X-rays are a form of
electromagnetic wave with the wavelengths ranging from 10-8 to 10-10 m.
X-Ray Tube
X-ray tube contains: 1)a source of electrons, 2)a high acceleration voltage and 3)a
metal target as shown in fig1. Target is always water-cooled to prevent its melting,
since most of the kinetic energy of the electrons after colliding the target is converted
in to heat. X-ray tubes contain an anode (the metal target) maintained at ground
potential and a cathode maintained at a high negative potential, so high voltage is
created between the cathode and anode. (Cullity, 1977). If the electrons are suddenly
decelerated upon collision because their energy not sufficient to break interaction
energy between electrons atom of target; these x-ray are commonly called
brehmsstrahlung or "braking radiation" which appear as continuous spectrum.
While Characteristic X-rays are generated when an energetic beam of electrons ( with
energy higher than that inside target atoms) interacts with the inner shell electrons by
inelastic scattering with enough energy to excite inner shell electrons to outer shell
orbitals, leaving inner-shell vacancies. As outer-shell electrons fall to the various
inner shell orbitals ( fill the vacansy), emitting x-ray photons with precise energies
determined by the electron energy levels. This x-ray is called characteristic x-ray
depending on Bragg's law appear as sharp peaks in x-ray pattern.
Fig 1. X-Ray tube
After x-ray goes out it hits target which is putting perpendicular to its path.Filters also
can be used to prevent Kβ from appearing in x-ray pattern and create only kα which is
lower energy and higher wavelength λ than kβ according to the sub level energy
emission as shown in the figure 2 which shows that the x-rays produced by
transitions from the n=2 to n=1 levels are called Kα-alpha x-rays, and those for the
n=3->1 transition are called Kβ-beta x-rays since n=1 corresponds to k sub level, 2 to
L and 3 to M … etc.
Fig. (2): X-ray emission between certain energy level.
Bragg reflection :determining the lattice constants of monocrystals
Objectives:
1-Investagating Bragg reflection at Nacl monocrystal
2-Determinig the lattice constant a0 of NaCl .
Theory
Crystalline materials which repeat themselves in the form of “unit cells”. This
arrangement behaves like a diffraction grating causing the diffraction of X-rays to
occur at angles of 2θ to verify Bragg's condition.
θ
d
d sinθ
Fig 3. Periodic array between diffracted planes
This relation was first formulated by W. L. Bragg and is known as the Bragg's law;
2d Sinθ = n λ.
1
Where n is an integer representing the order of diffraction, d is the lattice spacing and
θ is the incident and reflected angle of diffraction with respect lattice plane and λ is
X-ray wavelength.
The material(target) is placed on one axis of the x-ray device and tilted by an angle θ
while a detector rotates around it on an arm at twice that angle as shown in figure 4.
Fig 4 Position of detector with respect to the material in the x-Ray device.
In Cubic Nacl crystal (Fig.5), the lattice planes run parallel to the surface of the crystal's
unit cell .Their spacing d corresponds to one half of the lattice constant.
ao
d
Fig 5. Three-Dimensional representation of NaCl.
Then from above we note that the interplanner distance d is half lattice constant ao.
d= a0/2
2
substituting equation 2 in equation 1
ao Sinθ = n λ.
3
In other words, to determine we need to measure the glancing angle for a known wave
length and diffraction order n. This method is more precise when the glancing angles are
also measured in higher diffraction orders.
In this experiment, the molybdenum x-rays are used as radiation of a known
wavelengths , kα=71.08 pm and k β=63.09 pm.
Apparatus:
Nacl- monocrystal Muller- counter tube (shown-Geiger- in figure 6) apparatus X-Pc with
X-ray apparatus program.
Fig. 6 X-ray apparatus
a Mains Power panel
b control panel
K Carrying handles
c Connection panel
d Tube chamber (with X-ray tube Mo)
e Experimental chamber
(here with goniometer)
f Fluorescent screen
g Free channel
i Feet
Procedure:
In the next sheet.
Measurments:
Results:
Graph:
-Plot nλ versus sin θ
-Find the slope
-find a0=slope.
-find the percentage error where a0( theoretical) = 564.02 pm
Application of X-Ray
For medical applications, x-rays are usually generated in vacuum tubes by
bombarding a metal target with high-speed electrons and images produced by passing
the resulting radiation through the patient’s body on to a photographic plate or digital
recorder to produce a radiograph, or by rotating both source and detector around the
patient’s body to produce a “slice” image by computerized tomography (CT).
Cells of skin in our bodies consist of small atoms then these cells do not absorb x-ray
in contract of calcium atoms of bones which are large and absorb x-ray. Radiography
an x-ray beam is passed through the body. A portion of the x-rays are absorbed or
scattered by the internal structure and the remaining x-ray pattern is transmitted to a
detector so that an image may be recorded for later evaluation. Areas that are denser,
such as bone, appear white on the x-ray film. Areas that less dense, such as the lungs
or other areas filled with air appear black. Soft tissue, vessels and organs appear as
various shades of gray on an x-ray image, depending upon their composition and
density.
Fluoroscopy is an x-ray procedure in which x-rays are transmitted through the body
onto a fluorescent screen or monitor after. It is beneficial in that function of joints or
organ systems can be observed (e.g., the movement of material through the
esophagus, stomach, and intestines) after injecting wanted organic by contrast media
like barium.
Industry and Technology
From x-ray most of materials are characterized to identify material structure,
thickness, defects, quality, lattice constant, Grain size, study purity of fabricated
films …… etc.
Security by scanning of person, pets and package in air ports.
See figure 7 below.
Fig 7. Application of x-Ray
Risks from ionizied radiation (x-ray)
Radiation that falls within the ionizing radiation" range has enough energy to remove
tightly bound electrons from atoms, thus creating ions. This is the type of radiation
that people usually think of as 'radiation.' We take advantage of its properties to
generate electric power, to kill cancer cells, and in many manufacturing processes.
The energy of the radiation shown on the spectrum below increases from left to right
as the frequency rises.
When x-ray hit an atom its free electrons and create vacancies in inner shell inside
target atom and a lot of outer shell electron transfer to inner shell which increase
positive ionized atom and since these ions are not neutral then un normal chemical
reaction can be caused inside the living cells which can, change DNA, convert cells if
they do not die to cancer cells.