Q.1 Explain what are continuous and
characteristic X-rays and how they are generated. What are main feature of
X-rays? (2003-04)
Ans. Continuous X-ray: -
Let us consider an electron of high energy strikes the
target in X-ray tube. This high velocity electron will be de-accelerate when it
passes near the positively charged nucleus of an atom of the target. During the deflection
of the electron in the strong electric field of the nucleus, a photon of energy
hn is emitted because of change in
the energy of the incoming electron. The electron is slowed down to a velocity
v¢ from the initial velocity v.
Characteristic
X-ray: -
Let high-speed electron strikes the
target, then this electron will penetrate the atom of the target and remove an
electron from the inner shell by collision. Due to this there is the missing of
an electron in the inner shell. An electron from the next shell jumps into the
vacant space emitting a photon of energy hn. again an electron from the higher shell jumps into the new vacant
space. This process continues until the outermost shell is reached. Thus from a
single atom we get X-rays of different wavelengths and this ray is called characteristic
ray.
Main Features of
X-Rays: -
1. X-rays are not deflected by electric and
magnetic fields. This indicates that X-rays do not consist of charged
particles.
2. X-rays affects photographic plates.
3. X-rays produce fluorescence in many substances.
4. X-rays are
highly penetrating. X-rays of very high penetration power are called “hard”
X-rays.
5. They travel
in straight line with the velocity of light.
6. X-rays
consist of electromagnetic waves of very short wavelength. They undergo reflection,
interference, diffraction and polarization like light rays.
7. They produce
photoelectric effect and there by exhibit a corpuscular nature also.
Q.2 Derive Bragg’s equation for reflection of
x-ray by crystal planes.
Related
Questions -
Q. Derive Bragg’s law for the diffraction
of X-rays by crystals. (AKTU. 2005-06)
Q. Describe Bragg’s law. (AKTU.2005-06)
Ans: Bragg’s Law: -
When monochromatic X-rays
impinge upon the atoms in a crystal lattice, each atom acts as a source of
scattering radiation of the same wavelength. The crystal acts as a series of
parallel reflecting planes. The intensity of the reflected beam at certain
angles will be maximum when the path difference between two reflected waves
from two different planes is an integral multiple of l.
Derivation: -
Let us consider there are two parallel rays ABC and A¢B¢C¢. Both rays are reflected by atoms B and B¢ respectively. From figure it is clear that A¢B¢C¢ has a longer path than ABC. Draw normal BT and BS on
A¢B¢ and B¢C¢ respectively. The path difference will be
Dx = TB¢ + B¢S = dsinq + dsinq = 2dsinq
The two rays will
reinforce each other and produce maximum intensity
If, Dx = nl
So, 2dsinq = nl where n = 1, 2, 3,………
This is Bragg’s
equation for reflection of X-ray by crystal planes.
Q.3 Describe, briefly, about Bragg’s
spectrometer. X-rays of l
= 0.3 Å are incident on a crystal with lattice spacing 0.5 Å. Find the angle at
which second and third Bragg’s diffraction maxima are observed. (AKTU. 2002-03)
Related
Questions -
Q. Describe Bragg’s X-ray spectrometer. (AKTU. 2005-06)
Q. X-ray of wavelength 
are incident on a crystal with a lattice
spacing 0.5 Å. Find the angles at which second and third Bragg’s diffraction maxima
are observed. (AKTU. 2006-07)
are incident on a crystal with a lattice
spacing 0.5 Å. Find the angles at which second and third Bragg’s diffraction maxima
are observed. (AKTU. 2006-07)
Q. Describe
Bragg’s spectrometer and explain how it is used to study the structure of
crystals. (AKTU. 2008 - 09)
Ans. Bragg’s Spectrometer: -
