Production of X-rays and Nature of X-rays

Production of X-rays

When fast moving electrons strike on a very hard target of high atomic number, e.g. platinum, tungsten, molybdenum etc., X-rays are produced. Dr. William Collidge, in 1913, designed a tube for the production of X-rays. This tube is known as collidge tube or modern x-rays tube.

Coolidge X-ray Tube

The Coolidge tube consists of a glass tube G exhausted to nearly, perfect vacuum of about 10^-5 mm of mercury provided with a cathode and the target T. The cathode consists of a tungsten filament (F) heated by a low tension battery. The filament is placed inside a metal cup C to focus the electron on to the target. The target is made like tungsten or molybdenum having high melting point and high atomic weight held at an angle of 45^o to the horizontal. The target T held by a copper rod outside the tube. The anode is connected to the positive and cathode to the negative terminal of the high tension power supply.

Working

The filament F is heated by a passing a suitable current through it. The electrons emitted from the filament are focused at a point on the target with the help of a metal cup C. On account of the extremely high potential difference between the cathode and the anode, the electrons arrive at the target with high speed. The speed of the electrons can be further increased by increasing the accelerating voltage. On striking the anode, the electrons are stopped. Nearly 98% of the energy of the incident electrons is converted into heat. The remaining energy appears in the form of X-rays. However, intense heat is produced, which may melt the target. Therefore, the target must be cooled to remove the heat generated in it by continuous electron bombardment. The usual method is to mount the target material on the hollow copper tube through which cold water is continuously circulated.

Control of Intensity and Quality

Modern X-ray, the tube has the advantage of independent control of intensity and quality.

1. Control of intensity
   The intensity of X-rays depends upon the number of electrons emitted from the filament. This depends upon the electric current flowing through the filament. So by controlling the current with the help of rheostat, we can control the intensity of X-rays. Hence, the intensity of the can be changed by adjusting the filament current.
2. Control of quality
   If V be the potential difference applied across the two electrodes, then the kinetic energy acquired by the electron is given by
   $$eV = \frac 12 mv^2 $$
   where m, e and v be the mass change and velocity of the electron. Therefore, the quality of X-rays depends upon their energy which, in turn, depends upon the kinetic energy of the incident electrons. The kinetic energy of the incident electrons depends upon the potential difference between the anode and the cathode. Hence, quality of X-rays can be adjusted by changing the potential difference across the tube. X-rays having low penetrating power are called soft X-rays whereas X-rays having high penetrating power are called hard X-rays.

Frequency and Wavelength of X-rays Produced

When the whole of the kinetic energy of the electron is converted into the energy of the X-rays produced, then X-ray of maximum frequency is obtained. Therefore, if f_max is the maximum frequency of the emitted X-rays, then we have
\begin{align*} hf_{max} &= \frac 12 mv^2 = eV\\ \text{where, h is a Planck's constant.} \\ \therefore f_{max} &= \frac {eV}{h} \\ \end{align*}

If c is the velocity of light in vacuum and λ_min be the minimum possible wavelength of the –rays produced, then

$$\lambda _{min} = \frac {C}{f _{max}} = \frac {hc}{eV} $$

Nature of X-rays

X-rays as waves similar to light waves, but of much shorter wavelength about 10^-10 m or 0.1 nm. However, the wavelength of visible light is nearly 10³ times more than the wavelength of X-rays. While the wavelength of the visible light ranges from 4000 A to 8000 A, the wavelength of X-rays generally lies between 1 to 3 A. Since wavelength is inversely proportional to frequency, therefore the frequency of X-rays is nearly 10³ of visible light. Again, since the energy of the photon is directly proportional to frequency, therefore, x-ray photons are much stronger than the photons of visible light.

Properties of X-rays

1. X-rays are electromagnetic waves having a wavelength in the range from 1 A to 100 A.
2. X-rays can ionize atoms and molecules of matter.
3. They travel by a velocity of light i.e. 3×10¹⁰ ms^-1 in vacuum or air.
4. They are not deflected by magnetic or electric fields and, therefore, do not possess any charge.
5. X-ray can produce the photoelectric effect.
6. X-rays can affect photographic plates.
7. X-rays can ionize a gas through which they pass.
8. X-rays can be made to reflect, refract and diffract. X-rays show the phenomenon of interference and polarization.
9. X-rays can produce fluorescence in certain metal
10. When certain x-rays fall on certain metals, secondary X-rays are produced.
11. They show particle-like properties in interacting with matter as in photoelectric effect and Compton effect.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.