Sunday, March 23, 2008

Concept Review Ch.44 X-Rays

Production of X-rays

When highly energetic electrons are made to strike a metal target, electromagnetic radiation comes out. A large part of this radiation has wavelength of the order 0.1 nm (appx 1 A) and is known as X-ray.

Continuous and Characteristic X-Rays

If the X-rasy coming from a coolidge tube are examined for wavelengths present, and the intensity of different wavelengths are measurea and plotted, we can observe that there is a minimum wavelength below which no X-ray is emitted.

The wavelength below which no X-rays are emitted is called the cut-off wavelength or the threshold wavelength.

From the plot it can also be observed that at certain sharply defined wavelengths, the intensity of X-rays is very large. These X-rays are called characteristic X-rays.

At other wavelengths the intensity varies gradually and these X-rays are called continuous X-rays.

K X-Rays

X-rays emitted due to electronic transition from a higher energy state to a vacancy created in the K shell are called K X-rays.

Soft and Hard X-Rays

The X-rays of low wave length are called hard X rays and X rays of large wave length are called soft X rays.

In terms of energy, harder means more energy in is each photon.

Moseley's law

Square root of frequency of X rays = a(Z-b)

√(v) = a(Z-b)

where Z = position number of element.
a and b are constants

Bragg’s law

2d sin θ = n λ

d = interplanar spacing of the crystal on which X-rays are incident
θ = is the incident angle at which X-rays are strongly reflected.
n = 1,2,3 …
λ = wave length of X-rays

Application of Bragg’s law:By using a monochromatic X-ray beam (having a single wave length) and noting the angles of strong reflection, the interplanar spacing d and several information about the structure of the solid can be obtained.

Properties of X rays

X rays are electromagnetic waves of short wave lengths.
So they have many properties common with light.

1. They travel in straight lines in vacuum at a speed equal to that of light.
2. They are diffracted by crystals according to Bragg's law.
3. x-rays do not contain charged particles. hence they are not deflected by electric or magnetic field.
4. They effect a photographic plate. The effect is stronger than light.

Properties which are different than light

1. When incident on certain materials barium platinocyanide, X rays cause fluorescence.
2. When passed through a gas, X rays ionize the molecules of the gas.