Course: Diploma

Subject: Applied Science Physics

Unit: V

Chapter: I

Production of X-rays

X-rays are produced when rapidly moving electrons

that have been accelerated through a potential

difference of order 1 kV to 1 MV strikes a metal target.

Evacuated

glass tube

Target

Filament

Production of X-rays

Electrons from a hot element are accelerated onto a target anode.

When the electrons are suddenly decelerated on impact, some of the kinetic energy is converted into EM energy, as X-rays.

Less than 1 % of the energy supplied is converted into X-radiation during this process. The rest is converted into the internal energy of the target.

Minimum wavelength in the X-ray Spectra

When an electron hits the target its entire kinetic

energy is converted into a photon.

The work done on each electron when it is

accelerated onto the anode is eV.

Hence hƒ = eV and the maximum frequency

h

eVf max

Therefore,

eV

hcmin

Ex.1 : A X ray tube operates at 30 keV.

Calculate wavelength of X rays

produced

h= 6.62 x 10-34 Js

C=???????

eV

hcmin

Continuous X-ray Spectrum Bremsstrahlung X-rays can be produced at any

projectile electron energy. In diagnostic radiography most of the x-rays are Bremsstrahlung x-rays.

The Bremsstrahlung x-ray energies range from zero to a peak and back to zero.

This is referred to as the Continuous X-ray Spectrum.

The majority of the useful x-rays are in the continuous spectrum.

The maximum energy will be equal to the kVp of operation.

This is why it is called kVp (peak).

Characteristic X-ray Spectra

Different target materials give different wavelengths for the peaks in the X-ray spectra.

The peaks are due to electrons knock out inner-shell electrons from target atoms.

When these inner-shell vacancies are refilled by free electrons, X-ray photons are emitted.

The peaks for any target element define its characteristic X-ray spectrum.

Properties of X-rays

X-rays travel in straight lines.

X-rays cannot be deflected by electric field or magnetic field.

X-rays have a high penetrating power.

Photographic film is blackened by X-rays.

Fluorescent materials glow when X-rays are directed at them.

Photoelectric emission can be produced by X-rays.

Ionization of a gas results when an X-ray beam is passed through it.

Properties of X rays Affect photographic film.

They cause the phenomenon of fluorescence.

They are classified in hard X-rays(High energy, short

wave length) & soft X-rays(low energy, long wave

length).Soft are dangerous compared to hard x-rays.

Absorbed by material through which they travels.

They travel in straight line. Speed is equals to light.

They undergo reflection & refraction.

Applications of X rays X-ray photons carry enough energy to ionize atoms

and disrupt molecular bonds. This makes it a type

of ionizing radiation and thereby harmful to living

tissue.

A very high radiation dose over a short amount of

time causes radiation sickness, while lower doses can

give an increased risk of radiation-induced cancer.

In medical imaging this increased cancer risk is

generally greatly outweighed by the benefits of the

examination.

The ionizing capability of X-rays can be utilized

in cancer treatment to

kill malignant cells using radiation therapy. It is also

used for material characterization using X-ray

spectroscopy.

Attenuation length of X-rays in water showing the

oxygen absorption edge at 540 eV, the energy-

3 dependence of photo absorption, as well as a

leveling off at higher photon energies due to Compton

scattering.

Hard X-rays can traverse relatively thick objects

without being much absorbed or scattered. For this

reason X-rays are widely used to image the inside of

visually opaque objects.

The most often seen applications are in

medical radiography and airport security scanners,

but similar techniques are also important in

industry (e.g. industrial radiography and industrial

CT scanning) and research (e.g. small animal CT).

The penetration depth varies with several orders of

magnitude over the X-ray spectrum.

This allows the photon energy to be adjusted for the

application so as to give sufficient transmission through

the object and at the same time good contrast in the

image.

X-rays have much shorter wavelength than

visible light, which makes it possible to probe

structures much smaller than what can be seen

using a normal microscope.

This can be used in X-ray microscopy to

acquire high resolution images, but also in X-

ray crystallography to determine the positions

of atoms in crystals.

Moseley’s law and its importance.The English physicist Henry Moseley

(1887-1915) found, by bombarding high

speed electrons on a metallic anode, that

the frequencies of the emitted X-ray

spectra were characteristic of the material

of the anode. The spectra were called

characteristic X-rays.

Moseley’s law and its importance.

He interpreted the results with the aid

of the Bohr theory, and found that the

wavelengths λ of the X-rays were

related to the electric charge Z of the

nucleus. According to him, there was

the following relation between the

two values (Moseley’s law; 1912).

1/λ = c(Z - s)2

where c and s are constants

applicable to all elements, and Z is an

integer.

When elements are arranged in line according

to their position in the Periodic Table , the Z

value of each element increases one by one.

Moseley correctly interpreted that the Z values

corresponded to the charge possessed by the

nuclei. Z is none other than the atomic

number.

It was found that the characteristic X-ray of an unknown

element was 0.14299 x 10-9 m. The

wavelength of the same series of the characteristic X-ray

of a known element Ir (Z = 77) is 0.13485

x 10-9 m. Assuming s = 7.4, estimate the atomic number

of the unknown element.

2

9

9

1c Z s

1c Z s

1c 77 c

0.13485 10

c 1222

11

69.6

222 Z0.14299 10

75

7.4

7.4

Importance of Moseley’s law

Atomic no. is more important than Atomic weight as it is

equals to charge of nucleus.

Difference between Ni,Co,Te & I etc., is explained when

periodic table was constructed with atomic no.

Moseley predicted the existence of elements with atomic

no. 43,61,72 & 75.Thus X ray spectrum analysis new

elements can be discovered.

REFERENCE BOOKS AUTHOR/PUBLICATION

ENGINEERING PHYSICS S S PATEL (ATUL PRAKASHAN)

MODERN ENGINEERING

PHYSICSA S VASUDEVA

ENGINEERING PHYSICS K. RAJGOPALAN

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