AI Unit-IV
AI Unit-IV
AI Unit-IV
Introduction
• Radio-isotopes (isotopes of elements with
unstable atomic nuclei) emit radiation which
can be detected by suitably located detectors.
• The proportion of radioactive atoms in the
volume of material perceived by the detector
can thus be determined by the measurement
of the intensity of such radiation.
Preparation
• In one method, a pure but radioactive form of the
substance to be determined is mixed with the
sample in known amount. After equilibrium, a
fraction of the component of interest is isolated and
the analysis is then based upon the activity of this
isolated fraction.
• Alternatively, activity is induced in one or more
elements of the sample by irradiation with suitable
particles. The measurement of this activity gives
information about the element of interest.
Time Decay of Radio isotopes
Half life/Half period:
• The half-period of a radioactive isotope is the time required for
half of the initial stock of atoms to decay.
• Thus, after one half-period has elapsed, the total activity of any
single radioactive isotope will have fallen to half its initial value;
• After two periods, the activity will be one-quarter its initial value
and so on. After 6.6 half-periods, the activity will be 1% of the
initial activity.
• Decay is a random process which follows an exponential curve.
(Here λ is the decay constant for a particular radio-isotope)
Units of radio activity
• The unit of radioactivity is curie. This was originally
defined to represent the disintegration rate of one gram
of radium.
• It is used as the standard unit of measurement for the
activity of any substance, regardless of whether the
emission is alpha or beta particles, or X or gamma
radiation.
• the curie is defined as an activity of 3.7 × 1010
disintegration. The curie represents a very high activity.
Therefore, smaller units such as millicurie or micro-curie
are generally used.
Energy
• Energy: The basic unit used to describe the
energy of a radiation particle or photon is the
electron volt (eV).
• An electron volt is equal to the amount of energy
gained by an electron passing through a potential
difference of one volt. The energy of the
radiation emitted is a characteristic of the
radionuclide.
Particles Emitted in Radioactive Decay
• The theory of atomic structure proves that some
elements are naturally unstable and exhibit
natural radioactivity.
• On the other hand, elements can be made
radioactive by bombarding them by high-energy
charged particles on neutrons, which are
produced by either a cyclotron or a nuclear
reactor respectively.
• Radioactive emissions: Alpha, Beta and Gamma
rays.
Radio active emissions
• Alpha emissions: Alpha particles are composed of
two protons and two neutrons. They are least
penetrating and can be stopped or absorbed by
air. They are most harmful to the human tissue.
• Beta emissions: These are positively or negatively
charged and are high-speed particles originating
in the nucleus. They are not as harmful to tissue
as alpha particles, because they are less ionising,
but are much more harmful than gamma rays.
Radio active emissions
• Gamma emissions:
Emissions like X-rays constitute electromagnetic
radiation that travels at the speed of light. They
differ from X-rays only in their origin. X-ray
originates in the orbital electrons of an atom.
Gamma rays originate in the nucleus. They are due
to an unstable nucleus. X-rays and gamma rays are
also called ‘photons’ or packets of energy. As they
have no mass, they have the greatest penetrating
capability. Gamma rays are of primary interest in
radiochemical methods of analysis.
Radio active emissions
• Alpha emission is characteristic of the heavier radioactive elements
such as thorium, uranium, etc. The energy of alpha particles is
generally high and lies in the range 2–10 MeV (million electron volt).
• Their penetrating power is low and are completely stopped by foils
and solid materials like aluminium. Due to larger ionising power of
alpha particles, they can be distinguished from beta and gamma
radiations on the basis of pulse amplitude they produce on a
detector.
• Beta emission consists of a very energetic electron or positron
(beta particles that carry a unit positive charge). Their penetration
power is substantially greater than alpha particles, and have energy
range 0–3 MeV.
• Gamma rays are high-energy photons having high penetrating and
low ionising power.
Interaction of radiation with matter
• Beta particles interact primarily with the electrons in the
material through which they pasts. The absorption depends
mainly upon the number of electrons in their path. The
molecules of the matter may be dissociated, excited or ionised.
However, it is the ionisation which is of primary importance in
the detection of beta particles.
• Alpha particles have relatively large mass and higher charge,
the specific ionisation produced by them is much larger than for
beta particles.
• Upon interaction with matter, gamma rays ionise energy by
three modes.
• The photoelectric effect transfers all the energy of the gamma
ray to an electron in an inner orbit of an atom of the absorber.
This involves ejection of a single electron from the target atom.
This effect predominates a low gamma energies and with target
atoms having a high atomic number.
Interaction of radiation with matter
• The Compton Effect occurs when a gamma ray and an
electron make an elastic collision. The gamma energy is
shared with the electron and another gamma ray of
lower energy is produced which travels in a different
direction. The Compton effect is responsible for the
absorption of relatively energetic gamma rays.
• When a high-energy gamma ray is annihilated following
interaction with the nucleus of a heavy atom, pair
production of a positron and an electron results.
• Pair production becomes predominant at the higher
gamma-ray energies and in absorbers with a high
atomic number. The number of ion pairs per centimetre
of travel is called specific ionisation.
Nuclear Radiation Detectors
• Ionisation Chamber
• Geiger-Muller Counter
• Proportional Counter
• Scintillation Counter
• Semi conductor detector
Radiation Detectors
Ionisation Chamber:
• The fact that the interaction of radioactivity with
matter gives rise to ionisation makes it possible to
detect and measure the radiation.
• When an atom is ionised, it forms an ion pair. If the
electrons are attracted towards a positively charged
electrode and the positive ions to a negatively charged
electrode, a current would flow in an external circuit.
• The magnitude of the current would be proportional
to the amount of radioactivity present between the
electrodes. This is the principle of the ionising
chamber.
Ionisation Chamber
• An ionisation chamber consists of a chamber which is gas filled
and is provided with two electrodes. A material having a very high
insulation resistance, such as polytetrafluoroethylene is used as
the insulation between the inner and outer electrodes of the ion
chamber.
• A potential difference of a few hundred volts is applied between
the two electrodes. The radioactive source is placed inside or very
near to the chamber. The charged particles moving through the
gas undergo inelastic collisions to form ion pairs.
• The voltage placed across the electrodes is sufficiently high to
collect all the ion pairs.
• The chamber current will then be proportional to the amount of
radioactivity in the sample. lionisation chambers are operated
either in the counting mode, in which they respond separately to
each ionising current, or in an integrating mode involving
collection of ionisation current over a relatively long period.
Ionisation Chamber
Ionisation Chamber
• The current is usually of the order of 10−10A or less. It is measured
using a very high input impedance voltmeter, which has a MOSFET in
the input stage.
• The current is indicated on a moving coil type ammeter. Alternatively,
null method can also be used. In this method, the change in voltage
produced across a capacitor by the ionising current is counterbalanced
by an equal and opposite voltage supplied from a potentiometer.
• A potentiometric recorder of the self-balancing type can be used to
record the signal.
• The magnitude of the voltage signal produced can be estimated from
the fact that the charge associated with the 100,000 ion pairs
produced by a single alpha particle traversing approximately 1 cm in
air would be around 3 × 10−14 coulomb. If this average charge is made
to pass through a resistance of 3 × 1010 Ω in 1 s, a difference of
potential of approximately 1 mV would develop across the high
resistance.
Geiger-Muller Counter
• The Geiger counter is commonly called GM tube. This tube
consists of a metal cylinder which acts as a cathode and is
about 1–2 cm in diameter.
• It has an axial insulated wire working as an anode and is
capable of being maintained at a high positive potential of the
order of 800–2,500 V.
• This assembly is placed in a tubular glass envelope containing
a gas or mixture of gases, which is easily ionisable.
• The envelope is gas-tight and is typically filled to a pressure of
80 mm of argon gas and 20 mm of alcohol. Alcohol, butane or
bromine acts as a quenching gas and argon as the ionising gas.
• The tube contains a window of thin mica or other suitable
material, which permits effective passage of beta and gamma
radiation, but not of alpha radiation.
Geiger-Muller counter
E h hc /
Energy Dispersive XFS
• It consists of an excitation source, a sample and a semiconductor
detector.
• The fluorescent X-radiation resulting from irradiation of the
sample reaches a detector, which produces an electrical pulse
proportional to the energy of the X-rays.
• The energy level indicates the element involved, and the number
of pulses counted at each energy level over the entire counting
time is related to the concentration of the element.
Wavelength Dispersive XFS
• In the flat-crystal arrangement, the primary and secondary slits
and the analyser crystal are placed on the focal circle, so that
Bragg’s law will always be satisfied, as the goniometer is rotated.
• The detector is rotated by an angle twice the angular change in
the crystal setting.
Wavelength Dispersive XFS
• Due to absorption of long-wavelength X-rays by
air and window materials, some intensity losses
of X-rays take place, which can be reduced by
evacuating the goniometer chamber; or the air in
the radiation - from the sample surface to the
detector window may be replaced by helium.
• In some cases, vacuum spectrometers are also
used.
Wavelength Dispersive XFS
• The curved crystal arrangement is more suitable for
the analysis of small specimens.
• In this technique, collimators are not required, but
increase in intensity is obtained by focusing the
fluorescence lines.
• The crystal is bent to the diameter of the focusing
circle and its inner surface is ground to the radius of
the focusing circle.
• The radiation of one wavelength diverging from the
entrance slit will be diffracted for a particular setting
of the crystal and converge to a line image at a
symmetric point on the focusing circle.
Wavelength Dispersive XFS
Applications of X-Ray Spectrometers
XFS:
In Medicine for analysis of Sulphur in Protein, Chloride in
Blood Serum, analysis of tissues, bones , body fluids etc.
XDS:
Crystal topography:
Defects (Dislocations, Faults), Stress Analysis, Doped Crystal
Behavior etc. (Ex: silicon and gallium arsenide crystals )
XAS:
Medical: Radiography, Computed Tomography
Defects in welding, Quantity of liquid in closed vessels and
lowing through closed pipes which can’t be broken,.