X-Rays

X-Rays were discovered by Roentzen in 1895. X-Rays are electromagnetic waves of short wavelengths in the range of 10Å to 0.5Å. They are produced when electrons with high speed strike a metal target of high atomic weight. The longer wavelength end of the spectrum is known as “soft x-rays” and the shorter wavelength end is known as “hard x-rays”. X-Rays are also called as Roentzen rays. All of them are not of single wavelength. Different X-rays have different wavelength but lines within the range.

Soft X-Rays:
a. Have long wavelength
b. low energetic and low penetrating power
c. wavelength above 4Å.

Hard X-Rays:
a. having short wavelength
b. more energetic and high penetrating power
wavelength is below 4Å.

 X-Rays Production:

X-rays are produced when fast moving electrons are suddenly stopped by a solid target. A coolidge tube is shown in the figure below:

The tube is exhausted to the best possible vacuum of the order of 10^-5 mm of mercury. The cathode consists of a tungsten filament (F) heated by a low tension battery. Thermoionic electrons emitted by a filament are accelerated towards a target (T) by a high P.D maintained between F and T. The filament is placed inside a metal cup G to focus the electrons on to the target. 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 a hollow copper tube through which cold water is continuously circulated. The target is made of a metal like tungsten or molybdenum having a high melting point and a high atomic number. Metals with high atomic number give more energetic and intense X-rays when used as targets.
In the time of X-rays production, majority percentage of incident power is converted into heat i.e more than 99%. Minority percentage of incident power is converted inro X-rays radiations i.e less than 1%.
In the Coolidge tube, the intensity and frequency of X-rays can be easily controlled.
The intensity of X-rays depends on the number of electrons striking the target per second. The number of electrons given out by the filament is proportional to its temperature, which can be adjusted by varying the current in the filament circuit. Therefore, the intensity of X-rays varies with the filament current.
The frequency of X-rays emitted depends on the voltage between the cathode and the anode(target). Let V be the accelerating potential across the tube, e be the charge on the electron then workdone on the electron in moving from the cathode to the anticathode= eV. The electron thus acquires Kinetic energy (K.E) which is converted into X-rays, when the electron strikes the target. If v(max) is the maximum frequency of the X-rays produced, then hv(max)= eV.
Therefore, the minimum wavelength produced by an X-ray tube= hc/eV.

Properties of X-rays:
a. X-rays are elecromagnetic waves.
b. X-rays are not deflected by fields (electric and magnetic).
c. They move along the straight path with the velocity of light in vacuum.
d. X-rays can cause photoelectric effect.
e. X-rays undergoes reflection, refraction, diffraction, interference and polarization.
f. They produce illumination of fluorescent materials on which they fall.
g. They ionize the gas through which they pass.
h. They can penetrate thin materials like wood, thin sheet etc.
i.  They donot pass through heavy metals and bones
j.  X-rays cast their shadow on the screen.
k. They affect photo-graphic plates.
l.  X-rays fall on the metal surface having high mass number, secondary X-rays are produced.