Subject: Physics
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.
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 45o 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.
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 fmax 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 λmin be the minimum possible wavelength of the –rays produced, then
$$\lambda _{min} = \frac {C}{f _{max}} = \frac {hc}{eV} $$
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
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.
When fast moving electrons strike on a very hard target of high atomic number, e.g. platinum, tungsten, molybdenum etc., X-rays are produced.
X-rays as waves similar to light waves, but of much shorter wavelength about 10-10 m or 0.1 nm.
X-rays having low penetrating power are called soft X-rays whereas X-rays having high penetrating power are called hard X-rays.
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