X - RaysCourse: B.Tech

Subject: Engineering Physics

Unit: IV

Chapter: 2

1

Objectives

• Introduction and production of X-Rays

• Properties of X-Rays

• Diffraction of X-Rays

• The Bragg’s X-Ray spectrometer

• Continuous spectra

• Characteristics Radiation

• Moseley’s law

• Absorption of X-Ray

• Compton effect

• Applications of X-Rays

Introduction of X-Rays• Rontgen discovered X-rays in 1985 during some

experiments with a discharge tube.

• He noticed that a screen coated with barium

platinocyanide present at a distance from the

discharge tube. Rontgen called these invisible

radiations “X-rays”.

Finally, he concluded that X-rays are produced

due to the bombardment of cathode rays on the walls

of the discharge tube.

• X-rays are highly penetrating and it can pass through

many solids.

• X-rays occur beyond the UV region in the

electromagnetic spectrum.

• Their wavelengths range from 0.01 to 10 Å.

Production or Generation of X-

raysX-rays are produced by an X-ray tube. The

schematic of the modern type of X-ray tube is

shown in above figure.

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It is an evacuated glass bulb enclosing two

electrodes, a cathode and an anode.

The cathode consists of a tungsten filament

which emits electrons when it heated. The

electrons are focused into a narrow beam with

the help of a metal cup S.

The anode consists of a target material, made of

tungsten or molybdenum, which is embedded in

a copper bar.

Water circulating through a jacket

surrounding the anode and cools the anode.

Further large cooling fins conduct the heat away

to the atmosphere.

• The face of the target is kept at an angle

relative to the oncoming electron beam. A very

high potential difference of the order of 50 kV is

applied across the electrodes.

The electrons emitted by the cathode are

accelerated by the anode and acquire high

energies of order of 105 eV. When the target

suddenly stops these electrons, X-rays are

emitted.

• The magnetic field associated with the

electron beam undergoes a change when the

electrons are stopped and electromagnetic waves

in the form of X-rays are generated.

• The grater of the speed of the electron beam, the

shorter will be the wavelength of the radiated X-

rays. Only about 0.2 % of the electron beam energy

is converted in to X-rays and the rest of the energy

transforms into heat. It is for the reason that the

anode is intensively cooled during the operation of

X-ray tube.

• The intensity of the electron beam depends on

the number of electron leaving the cathode. The

hardness of the X-rays emitted depends on the

energy of the electron beam striking the target. It

can be adjusted by varying the potential difference

applied between the cathode and anode. Therefore,

the larger potential difference, the more penetrating

or harder X-rays.

Properties of X-Rays…

They have relatively high penetrating power.

They are classified into Hard X-rays & Soft X-

rays.

The X-rays which have high energy and

short wavelength is known as Hard X-rays.

The X-rays which have low energy and

longer wavelength is known as Soft X-rays.

X-rays causes the phenomenon of

flouroscence.

On passing through a gas X-rays ionize the

Properties of X-Rays…

They are absorbed by the materials through

which they traverse.

X-rays travel in straight line. Their speed in

vacuum is equal to speed of light .

X-rays can affect a photographic film.

X-rays are undeflected by electric field or

magnetic field.

Diffraction of X-Rays – Bragg’s

lawConsider a crystal as made out of

parallel planes of ions, spaced a distance d

apart. The conditions for a sharp peak in the

intensity of the scattered radiation are:

1. That the X-rays should be secularly

reflected by the ions in any one plane.

2. That the reflected rays from successive

planes should interfere constructively.

• Path difference between two rays reflected

from adjoining planes: 2dsinθ,

• For the rays to interfere constructively,

this path difference must be an integral

number of wavelength λ,

nλ =2dsinθ -------

(1)

Bragg angle is just the half of the total angle by

which the incident beam is deflected.

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The Bragg’s X-Ray spectrometer

• An X-ray diffraction experiment requires,

• X-ray source

• The sample

• The detector

• Depending on method there can be variations

in these requirements. The X-ray radiation

may either monochromatic or may have

variable wave length.

• Structures of polycrystalline sample and single

crystals can be studied. The detectors used in

these experiments are photographic film.

The schematic diagram of Bragg’s X-rayspectrometer is given in above.

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• X-ray from an X-ray tube is collimated by passingteam through slits S1 and S2. This beam is thenallowed to fall on a single crystal mounted on a tablewhich can be rotated about an axis perpendicular tothe plane of incident of X-rays. The crystal behavesas a reflected grating and reflects X-rays. Byrotating the table, the glancing angle θ at which theX-ray is incident on the crystal can be changed. Theangle for which the intensity of the reflected beam ismaximum gives the value of θ. The experiment isrepeated for each plane of the crystal. For first orderreflection n = 1 so that, λ = 2d sinθ; for n = 2, 2λ = 2dsinθ; ……., and so on.

• A photographic plate or an ionization chamber isused to detect the rays reflected by the crystal.

Continuous or Bremsstrahlung X-rays

• "Bremsstrahlung" means "braking radiation"

and is retained from the original German to

describe the radiation which is emitted when

electrons are decelerated or "braked" when

they are fired at a metal target.

• Accelerated charges give off electromagnetic

radiation, and when the energy of the

bombarding electrons is high enough, that

radiation is in the x-ray region of

the electromagnetic spectrum.

Continuous X-rays…

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Continuous X-rays…• It is characterized by a continuous distribution

of radiation which becomes more intense andshifts toward higher frequencies when theenergy of the bombarding electrons isincreased.

• The curves above are who bombarded tungstentargets with electrons of four different energies.

• The continuous distribution of x-rays whichforms the base for the two sharp peaks at left iscalled "Bremsstrahlung" radiation.

Characteristic X-rays

• Characteristic X-rays are emitted from heavy

elements when their electrons make transitions

between the lower atomic energy levels.

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Characteristic X-rays…

• Characteristic X-rays emission which shown astwo sharp peaks in the illustration at left occurwhen vacancies are produced in the n = 1 or K-shell of the atom and electrons drop down fromabove to fill the gap.

• The X-rays produced by transitions from the n =2 to n = 1 levels are called Kα X-rays, and thosefor the n = 3->1 transition are called Kβ X-rays.

• Transitions to the n=2 or L-shell are designatedas L - shall X-rays (n= 3->2 is Lα, n = 4->2 is Lβ,etc.

Uses of Characteristic X-rays..

• Characteristic X-rays are used for the

investigation of crystal structure by X-ray

diffraction.

• Crystal lattice dimensions may be

determined with the use of Bragg's law in

a Bragg spectrometer.

Moseley’s law and its importance.• The English physicist Henry Moseley (1887-

1915) found, by bombarding high speedelectrons on a metallic anode, that thefrequencies of the emitted X-ray spectra werecharacteristic of the material of the anode.

• The spectra were called characteristic X-rays.

• He interpreted the results with the aid of theBohr theory, and found that the wavelengths λ ofthe X-rays were related to the electric charge Zof the nucleus. According to him, there was thefollowing 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

Z is an integer.

When elements are arranged in lineaccording to their position in the PeriodicTable , the Z value of each element increasesone by one.

Moseley correctly interpreted that the Zvalues corresponded to the charge possessedby the nuclei. Z is none other than the atomicnumber.

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.

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.

Absorption of X-Ray

When the X-rays hit a sample, the

oscillating electric field of the electromagnetic

radiation interacts with the electrons bound in

an atom.

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A narrow parallel monochromatic x-ray beamof intensity I0 passing through a sample of thicknessx will get a reduced intensity I according to theexpression:

ln (I0 /I) = μ x ------- (1)

Where μ is the linear absorption coefficient,which depends on the types of atoms and the densityρ of the material.

At certain energies where the absorptionincreases drastically and gives rise to an absorptionedge. Each such edge occurs when the energy of theincident photons is just sufficient to cause excitationof a core electron of the absorbing atom to acontinuum state, i.e. to produce a photoelectron.

The absorption edges are labeled in the

order of increasing energy, K, LI, LII, LIII, MI,….,

corresponding to the excitation of an electron

from the 1s(2S½), 2s(2S½), 2p(2P½), 2p(2P3/2),

3s(2S½), … orbitals (states), respectively.

Thus, the energies of

the absorbed radiation

at these edges

correspond to the

binding energies of

electrons in the K, L,

M, etc.., shells of the

absorbing elements.8

Compton effect

A phenomenon called Compton scattering,

first observed in 1924 by Compton, and

provides additional direct confirmation of the

quantum nature of electromagnetic radiation.

When X-rays impinges on matter, some of the

radiation is scattered, just as the visible light

falling on a rough surface undergoes diffuse

reflection.

• Observation shows that some of the scattered

radiation has smaller frequency and longer

wavelength than the incident radiation, and

that the change in wavelength depends on the

angle through which the radiation is

scattered.

• Specifically, if the scattered radiation emerges

at an angle φ with the respect to the incident

direction, and if f and i are the wavelength

of the incident and scattered radiation,

respectively, it is found that,

Where, m0 is the electron mass.

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10

• In figure, the electron is initially at rest

with incident photon of wavelength and

momentum p; scattered photon with longer

wavelength f and momentum p and

recoiling electron with momentum P. The

direction of the scattered photon makes an

angle φ with that of the incident photon, and

the angle between p and p is also φ.

called Compton wavelength.

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hc 00243.0

• Compton scattering cannot be

understood on the basis classical

electromagnetic theory.

• On the basis of classical principles, the

scattering mechanism is induced by motion

of electrons in the material, caused by the

incident radiation.

Applications of X-Rays…

X-rays are used in industrial, medical, purescience research and X-ray crystallography etc…

• X-rays are used to detect defects in radio valves.

• X-rays are used to detect cracks in structures.

• X-rays are used to analyses the structures ofalloys and other composite bodies by diffractionof X-rays.

• They are also used to study are structure ofmaterials like rubber, cellulose, plastic, fibresetc…

• X-rays can destroy abnormal internal tissues.

Applications of X-Rays…• X-rays are used in analysis of crystal structure

and structure of complex organic molecule.

• They are also used in determining the atomic

number and identification of various chemical

elements.

• X-rays are used to detect fractures and

formation of stones in human body.

• They are also being used for tumor treatment

and for this purpose hard X-rays are used.

• X-rays are also used in X-ray crystallography for

Laue method, Rotating crystal method, Powder

method, etc….

Image references links

1. http://alphaoneimaging.com/wp-content/uploads/2013/03/x-ray.jpg

2. https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTINZXxYufauFlzdPYTHCq6JR3CDPHLoDphGojZ7iYshMdY07dXnLKASjc

3. https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQXoHJnzqe1dRx96r9vnPZ6tgu5CObfV_CgMChog2K9_L3R352PibF8MlOr

4. https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcQEGAb7sSHYT6vGOwZhb10hzb5vJUZYIm-esldrsOKnKXWQJKJRlmN18G8

5. http://postimg.org/image/jd2eb1od9

6. http://postimg.org/image/i88c5o1wd

7. http://postimg.org/image/9ehfokexp

8. http://postimg.org/image/vnvcv4ae5

9. http://postimg.org/image/45meqowil

10. http://postimg.org/image/l7f8ssbdp