Name: Moseley’s Law
Uploaded: 2019-07-19
Duration: 7 min 40 s
Description: Moseley’s law is an empirical law concerning the characteristic x-rays that are emitted by atoms. Let's watch this video to learn about Moseley's law.

Question

Draw a labelled diagram of a Coolidge tube and explain Why is the target made of a heavy element and what is the relation between the minimum wavelength of the continuous spectrum and the voltage applied across the X-ray tube? Give necessary formula.

Accepted Answer

Coolidge tube is used for the production of X-rays. It has a hard-glass bulb having a high vacuum of about cm (mercury). Two tubes are connected to it. One of the tubes has a tungsten filament F through which current is passed by means of a battery B. The filament, on being heated, emits thermionic electrons whose number per second depends upon the temperature of the filament. Around the filament is a molybdenum cylinder C which is kept at a negative potential relative to the filament. It concentrates the electrons emitted by the filament in the form of a fine beam.

Tungsten is the most commonly used target material in the anode because it has a high atomic number which increases the intensity of the X-rays, and because it has a sufficiently high melting point that it can be allowed to become hot.

Let's assume applied voltage across the X-ray tube is $V$
Thus, for an accelerating voltage, maximum X-ray photon energy is given by
$h ν_{m a x} = e V$ where $ν_{m a x}$ is the maximum frequency of X-ray produced.
The minimum wavelength corresponding to this maximum frequency $ν_{m a x}$ is given by
$λ_{m i n} = \frac{c}{ν_{m a x}}$ where c is speed of light.
Hence, $λ_{m i n} = \frac{h c}{e V}$
This minimum wavelength is inversely proportional to the accelerating potential $V$ .

Draw a labelled diagram of a Coolidge tube and explain Why is the target made of a heavy element and what is the relation between the minimum wavelength of the continuous spectrum and the voltage applied across the X-ray tube? Give necessary formula.

Important Questions on X-Rays

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