| a. |
Draw
a diagram for an X-ray tube. Derive an expression for the minimum wavelength.
Explain why cooling is necessary for such an apparatus. |
5
marks |
|
|
|
|
 |
2 |
|
As the electrons reach the target, the kinetic
energy is equal to the loss in electric potential energy, assuming that
the electrons are at rest initially. |
|
|
 |
1 |
|
X-ray photons of the minimum wavelength are
emitted when all the kinetic energy of the electron are converted into
radiation: |
|
|
 |
1 |
|
In practice, only a very small fraction (<
1%) of the kinetic energy of the electrons is converted into X-rays. The
rest becomes internal energy. So, a cooling system is necessary to avoid
melting the target. |
1 |
|
|
|
| b. |
Illustrate
the differences in the production of X-ray and light from a sodium lamp.
State the similarities and differences between their spectra. |
6
marks |
|
|
|
|
X-rays are emitted as the inner shell electrons
of the tungsten target are excited. This requires large amount of energy.
As a consequence, the photons emitted are also of very high energy (thus,
very high frequency and very short wavelength). |
|
|
Light from a sodium lamp are emitted as the
outer shell electrons of the sodium atoms are excited. Their wavelengths
are longer and fall into the visible spectrum. |
|
|
Since the tungsten target is a solid, the excited
atoms in the X-rays tube are very closely packed. They easily exchange
their energy with neighbouring molecules. Thus, a continuous spectrum is
obtained. |
|
|
Light from a sodium lamp is a line spectrum.
This is because sodium vapour are at low pressure. They emit light as soon
as they are excited. |
|
|
X-rays, however, consists of characteristic
lines which have extra high intensity compare to the rest. This reveals
that the X-rays photons are emitted as a result of energy change between
the energy levels of the target element. |
|
|
 |
|
|
|
|
| c. |
Discuss
how X-rays could be detected and state their applications. |
5
marks |
|
|
|
|
|
|
|
Detection of X-rays |
|
|
-
Photographic film is blackened on exposure to X-rays
-
Fluorescent materials glow as X-rays fall on them.
-
Photoelectric emission is produced by X-rays
-
GM tube registers counts as X-ray photons enter the tube
|
2 |
|
Applications of X-rays |
|
|
-
Medical uses in diagnosis
-
Medical uses in therapy
-
X-ray crystallography in analysis of molecular structure
-
Industrial radiography in locating internal imperfections
-
Inspection of suitcases in airport
|
3 |
|
|
|