X-Rays

X-Rays



 X-rays are electromagnetic radiation

of very much short wavelength.

 The wavelengths of X-rays are in the

range 5 – 25 nm.

Production of X-rays (1)



 X-rays are produced when rapidly moving

electrons that have been accelerated

through a potential difference of order 1

kV to 1 MV strikes a metal target.

Evacuated

glass tube









Target



Filament

Production of X-rays (2)



 Electrons from a hot element are

accelerated onto a target anode.

 When the electrons are suddenly

decelerated on impact, some of the

kinetic energy is converted into EM

energy, as X-rays.

 Less than 1 % of the energy supplied is

converted into X-radiation during this

process. The rest is converted into the

internal energy of the target.

X-Ray Tube



 The tube is designed

with a cooling system

to prevent the target

from melting.

 A motor rotates the

anode to keep it from

melting.

 A cool oil bath

surrounding the

envelope also absorbs

heat.

Properties of X-rays



 X-rays travel in straight lines.

 X-rays cannot be deflected by electric

field or magnetic field.

 X-rays have a high penetrating power.

 Photographic film is blackened by X-rays.

 Fluorescent materials glow when X-rays

are directed at them.

 Photoelectric emission can be produced

by X-rays.

 Ionization of a gas results when an X-ray

beam is passed through it.

X-ray Spectra (1)



 Using crystal as a wavelength selector, the

intensity of different wavelengths of X-rays can

be measured.









minimum wavelength

X-ray Spectra (2)



 The graph shows the following features.

 A continuous background of X-radiation in

which the intensity varies smoothly with

wavelength. The background intensity reaches

a maximum value as the wavelength

increases, then the intensity falls at greater

wavelengths.

 Minimum wavelength which depends on the

tube voltage. The higher the voltage the

smaller the value of the minimum wavelength.

 Sharp peaks of intensity occur at wavelengths

unaffected by change of tube voltage.

Uses of X-rays



 In medicine

To diagnose illness and for

treatment.

 In industry

To locate cracks in

metals.

 X-ray crystallography

To explore the structure

of materials.

Continuous X-ray Spectrum



 The free electron is

attracted to the

tungsten atom nucleus.

 As the electron speeds

past, the nucleus alters

its course.

 The electron loses

energy, which it

releases as an X-ray

photon.

Minimum wavelength in the X-ray

Spectra

 When an electron hits the target its entire

kinetic energy is converted into a photon.

 The work done on each electron when it

is accelerated onto the anode is eV.

 Hence the maximum frequency is given

by

eV

f max 

h

Therefore,

hc

min 

eV

The Characteristic Spectrum (1)









 The free electron collides with the tungsten

atom, knocking an electron out of a lower

orbital.

 A higher orbital electron fills the empty position,

releasing its excess energy as a photon.

The Characteristic Spectrum (2)



 Different target materials give different

wavelengths for the peaks in the X-ray

spectra.

 These peaks occur at wavelengths

independent of the tube voltage.

 The exact wavelength of the peak depends

on the higher energy level from which the

electron falls.

 The peaks for any target element define its

characteristic X-ray spectrum. That is, an

element can be identified from the peaks.