NEET Physics Notes Optics- Matter Waves -X-Rays
X-Rays
X-rays were discovered by Roentgen. X-rays are produced when fast moving electrons strike a metal of high atomic weight and high melting point.
The phenomena of thermionic emission is used to produce electrons in coolidge tube. A high order of 10-6 Hg cm is to be maintained to produce hard X-rays. Intensity of X-rays is directly proportional to the square of the strength of the current in the filament which heats the cathode.
Quality of X-rays is measured in terms of their penetrating power and depends upon the potential difference applied to the X-ray tube.
X-rays are diffracted by crystals in accordance with Bragg’s Law.
Properties of X-Rays
- X-rays are electromagnetic waves with wavelength range 0.1 Å to 100 Å
- X-rays are invisible.
- X-rays carry no charge, so they are not deflected by electric and magnetic fields.
- They travel in straight line with speed 3 x l08ms_1 through vacuum.
- They obey phenomenon of interference, diffraction and polarisation of light.
- They ionise gases.
- They effect photographic plate.
- They can pass through flesh and blood but not through bones.
- They produce photoelectric effect and compton effect.
- They are not used in RADAR as they are not reflected by the target. .
- rays can be used to detect diseases and to cure them.
Types of X-Rays
X-rays are classified into two types on the basis of penetrating power
Soft X-rays
- These types of X-rays produced when the potential difference across the cathode and target is less than 20000 V.
- They have low penetrating power.
- These are used in the field of medicine.
- These are having large wavelengths.
Hard X-rays
- These type of X-rays produced when the potential difference across the cathode and target is more than 30000 V.
- These rays are more penetrating than soft X-rays.
- They have low wavelength of the order of 1Å
- These are used in the field of science and industry.
- These are having low wavelengths.
X-rays spectra are of two types
- Continuous X-ray spectrum
- Characteristic X-ray spectrum
Continuous X-Ray Spectrum
- X-rays of all wavelengths but having different intensities are emitted by the tube.
- As incident electron loses its energy continuously, due to collisions with atoms of the target, the loss of energy is found as X-rays. Hence, X-rays of all wavelengths are produced.
- The continuous X-ray spectra consist of all wavelengths over a given range.
- These wavelength are of different intensities.
The maximum frequency or minimum wavelength limit is due to the loss of the total energy of electron during a single collision.
Characteristics X-Rays Spectrum
- Few of fast moving electrons having high velocity penetrate the surface atoms of the target material and knock out the tightly bound electrons even from the inner most shells of the atoms.
- If the electron striking the target eject an electron from X-shell of the atom, a vacancy is created in the X-shell. An electron from one of the outer shell say L-shell jumps to X-shell, emitting an X-ray photon of energy equal to the energy difference between the two shells.
- Similarly, if an electron from the M-shell jumps to the X-shell, X-ray photon of higher energy is emitted. The X-ray photons emission gives
lines of the X-series of the spectrum. - If the electron striking the target ejects an electron from L-shell of the target atom, an electron from the M, N… shells jumps to the L-shell, so that X-rays photons of lesser energy are emitted. These photons form the L-series of spectrum.
- In similar way, the formation of M-series, N-series etc., may be explained.
- Intensity wavelength graph is given by
Moseley’s Law
Moseley studied the characteristic X-ray spectrum of a number of a heavy elements and concluded that the spectra of different elements are very similar and with increasing atomic number, the spectral lines merely shift towards higher frequency.
If v denotes the emitted frequency of a substance of atomic number Z, then
nuclear screening constant values of (a) and (b) vary from one series to another series.
- For X-series b = 1
- For L-series b= 7.4
- For M-series b= 19.2
The intercept along + X-axis denotes the constant b.
The atomic number of Cu, Ag and Pt were established to be 29, 47 and 78, respectively.
This law led to the discovery of new elements like prometheum (61), hafnium (72) and rhenium (75) etc. Moseley’s law is in accordance with Bohr’s theory of spectral lines of atoms.
Wavelength of characteristic spectrum,
and energy of X-ray radiations
