Radioisotope Techniques

Biotech notes for BSc students

1. Dr. Veena M. 1

Radioactivity, Half-Life, Radioisotopes, GM counter, Scintillating
counting, Autoradiography, applications, Biosafety
RADIOACTIVITY
Henry Becquerel discovered radioactivity by accident. Radioactivity is the
phenomenon of the spontaneous disintegration of unstable atomic nuclei to atomic nuclei
to form more energetically stable atomic nuclei. When the nucleus of an atom does not stay
intact it undergoes decay accompanied by release of energy and radiation. It is independent of
external factors like light, electric, magnetism and chemical factors. All elements rounding
from atomic number 83 (Bismuth) to 92 (Uranium) are radioactive. Radioactivity is also
known as radioactive decay, nuclear decay, radioactive disintegration, or nuclear
disintegration.
A material containing unstable nuclei is considered radioactive. Three of the most
common types of decay are alpha decay (α-decay), beta decay (β-decay), and gamma decay
(γ-decay), all of which involve emitting one or more particles.
It is the property of some unstable atoms (radionuclides) to spontaneously emit
nuclear radiation, usually alpha particles or beta particles often accompanied by gamma-rays.
This radiation is emitted when the nucleus undergoes radioactive decay and is converted into
a different isotope which may, according to its number of neutrons and protons, be either
radioactive (unstable) or non-radioactive (stable). This “daughter” nucleus will usually be of
a different chemical element to the original isotope.
Types of radioactive decay
1. Alpha decay (α)
2. Beta decay (β)
3. Gamma decay (γ)
1. Alpha decay - occurs when the nucleus ejects an alpha particle (helium nucleus). Eg.
Radium decays by alpha emission.

They are large sized, slow moving with double positive charge. They collide
with atoms coming across their path. They cause severe ionisation and
excitation. They cannot penetrate deep.
2. Beta decay - occurs in two ways;
Another common decay process is beta particle emission, or beta decay. A beta
particle is simply a high energy electron that is emitted from the nucleus.

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a. Decay by negatron emission
Neutron proton (by emission of –vely charged β-particle)
Neutron proton + negatron
Eg.


b. Decay by positron emission
Proton Neutron + Positron (+vely charged β-particle)
Eg.


3. Gamma decay - a radioactive nucleus first decays by the emission of an alpha or beta
particle. The daughter nucleus that results is usually left in an excited state and it can decay to
a lower energy state by emitting a gamma ray photon.
Eg.

Uses of Radioactivity
1. Americium-241 is an alpha emitter and is used for domestic smoke detectors in the
United States.
2. Gamma rays are used to kill cancerous cells and hence used in radiotherapy.
3. Cobalt-60 is used to destroy carcinogenic cells.
4. Gamma rays are used in scanning the internal parts of the body.
5. Gamma rays kill microbes present in food and prevent it from decay by increasing the
shelf life.
6. Age of the rocks can be studied using radioactive radiations by measuring the argon
content present in the rock.
7. Mutagenesis – mutagenic strains of microorganisms are developed by these isotopes.
8. In molecular biology studies of DNA , RNA sequencing ,replication, transcription , r-
DNA technique etc.,

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9. Metabolic pathway studies.
10. Used in pharmacology.
11. In medicine – radiolabelled drugs eg. Hyperactive thyroid
12. Archaeology C-14 dating
Disadvantages of radioactivity are:
1. High dosage of radioactive radiation on the body might lead to death.
2. Radioactive isotopes are expensive.

HALF-LIFE
Atoms with an unstable nucleus regain stability by shedding excess particles and
energy in the form of radiation. The process of shedding the radiation is called radioactive
decay.
The radioactive decay process for each radioisotope is unique and is measured with a
time period called a half-life. One half-life is the time it takes for half of the unstable atoms
to undergo radioactive decay.


Half-Life is normally defined as the time needed by a radioactive substance (or one
half the atoms) to disintegrate or transform into a different substance.

The rate constant or decay constant for a given decay process is,
t1/2 = 0.693/λ
Where, t1/2 = half-life
λ = decay constant
OR
T1/2 = In (2)/ λ

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Where, In = Natural logarithm of 2

Table gives a list of some radioisotopes.



RADIOISOTOPES
Different isotopes of the same element have the same number of protons in their
atomic nuclei but differing numbers of neutrons. Eg. Protium
1
1H, deuterium
2
1H and
tritium
3
1H.
Radioisotopes are radioactive isotopes of an element. They can also be defined as
atoms that contain an unstable combination of neutrons and protons, or excess energy in
their nucleus. They spontaneously emit radiation in the form of α, β and γ rays.
The unstable nucleus of a radioisotope can occur naturally, or as a result of artificially
altering the atom. In some cases a nuclear reactor is used to produce radioisotopes, in others,
a cyclotron. Nuclear reactors are best-suited to producing neutron-rich radioisotopes, such as
molybdenum-99, while cyclotrons are best-suited to producing proton-rich radioisotopes,
such as fluorine-18.

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The best known example of a naturally-occurring radioisotope is uranium. All but
0.7 per cent of naturally-occurring uranium is uranium-238; the rest is the less stable, or more
radioactive, uranium-235, which has three fewer neutrons in its nucleus.
Example of formation of radioisotopes:
1. Beryllium -7 is produced when Boron-10 captures a proton.

2. Magnesium -24 is bombarded by a neutron Sodium-24 can be produced.

Uses of Radioactive Isotopes:
 They are also used to measure the thickness of metal or plastic sheets.
 Radioisotopes can be used as tracers within a living organism to trace what is going on
inside the organism at an atomic level; that is, radioisotopes can be injected or ingested
by the organism, and researchers can trace the internal activities using the radioactivity.
 They are used in medicine, for example, Cobalt-60 is extensively used as a radiation
source to arrest the development of cancer.
 Iodine-131 is found effective in treating hyperthyroidism.
 Radioactive carbon-14 decay could be used to estimate the age of organic materials.
 Positron Emission Tomography (PET) and PETCT make use of radionuclides emitting
positron particle that is injected in to the target cell or tissue.
 Radioactive isotopes are used in industry to detect the leakage in underground oil
pipelines, gas pipelines and water pipes.
The 3 commonly used methods for detection are:
1. Ionisation of gases
2. Excitation of solids and liquids
3. Induction of chemical change.

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GEIGER-MULLER (GM) COUNTER
Geiger and Muller developed a „Particle detector‟ for measuring „ionizing radiation‟
in 1928. They named it as „Geiger Muller Counter‟. Ever since then it has been one of the
most widely used nuclear detectors in the developmental days of Nuclear physics. The
particle detector developed by Geiger and Muller is a gas filled counter.
The main difference between „proportional counter‟ and „Geiger-Muller Counter‟ is in
the formation of the avalanche. In the proportional counter, the avalanche is formed only at a
point whereas in Geiger-Muller Counter it is formed in the central wire. Therefore, in GM
Counter amplification is independent of initial ionization produced by the ionizing particle.
Geiger counter is also called as Geiger tube. This instrument is actually used for
detecting and measuring ionizing radiation like alpha particles, beta particles, and gamma
rays. A Geiger-Müller counter can count individual particles at rates up to about 10,000 per
second and is used widely in medicine and in prospecting for radioactive ores.

Construction of Geiger-Muller counter:



 It consists of a hollow metal case enclosed in a thin glass tube. This hollow metal case
acts as a cathode.
 A fine tungsten wire is stretched along the axis of the tube and is insulated by ebonite
plugs. This fine tungsten wire acts as anode.

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 The tube is evacuated and then partially filled with a mixture of 90% argon at 10 cm
pressure and 10% ethyl alcohol vapours at 1cm pressure.

Working of Geiger-Muller counter
A Geiger counter consists of a Geiger–Müller tube and the processing electronics, which
displays the result.
The tube is filled with Argon gas, and around voltage of +400 Volts is applied to the
thin wire in the middle. When a particle arrives into the tube, it takes an electron from Argon
atom. The electron is attracted to the central wire and as it rushes towards the wire, the
electron will knock other electrons from Argon atoms, causing an "avalanche". The
ionization is considerably amplified within the tube by the Townsend discharge effect to
produce an easily measured detection pulse, which is fed to the processing and display
electronics. Thus one single incoming particle will cause many electrons to arrive at the
wire, creating a pulse which can be amplified and counted. This gives us a very sensitive
detector.

 The fine tungsten wire is connected to positive terminal of a high tension battery
through a resistance „R‟ and the negative terminal is connected to the metal tube.
 The direct current voltage is kept slightly less than that which will cause a discharge
between the electrodes.
 At one end of the tube a thin window of mica is arranged to allow the entry of
radiation into the tube.
Advantages of a Geiger counter
 They can prevent nuclear accidents by always giving a reading of radiation levels.
 They are used to ensure safety in all operations that require working with radioactive
material.
 They are highly sensitive devices, therefore the readings are usually accurate.
 They can be very useful in expanding the scope of nuclear energy to greater levels.

Disadvantages of GM Counter
 GM counters cannot measure energy due to a lack of differentiating abilities.
Uncharged particles like neutrons cannot be detected.
 GM counters are less efficient due to its large paralysis time limits and also
large dead-time.
 Quenching agents used in GM counters often decompose, which leads to the
reduction in a lifetime.

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Applications of a Geiger Counter are as follows:
 Detection of radioactive rocks and minerals in mining.
 For first responders such as firemen and hazard management personnel to ensure that
the site is clear of radiation.
 Ensuring that levels of radiation are within permissible levels around nuclear power
plants.
 Detection of radiation in scrap metal processing industries.
 Detection of radiation in erstwhile warzones.
 Ensuring that patients undergoing radiation therapy are not overexposed to radiation.
 Ensuring that uranium mines and surrounding areas do not become overly radioactive.
 To check for irradiated gemstones in the jewellery trade.
 To check the levels of iodine 131 in cancer patients undergoing radiation therapy.

SCINTILLATION COUNTING
A scintillation counter is an instrument that is used for measuring ionizing radiation. It
comprises the scintillator that generates photons in response to incident radiation. The
process of producing a flash of light by striking the scintillation crystals with alpha or beta or
gamma particles is known as Scintillation.
A scintillation counter is used to detect gamma rays and the presence of a particle. It can also
measure the radiation in the scintillating medium, the energy loss, or the energy gain. The
medium can either be gaseous, liquid, or solid. The scintillator counter is generally
comprised of transparent crystalline material such as glasses, liquids, or plastics.
Scintillation crystals are those which produce a brief flash of light each time they are struck
by an alpha or beta or gamma radiation. For example, sodium iodide, anthracene, zinc
sulphide, naphthalene, etc.
Scintillation counter employs this Scintillation principle for detecting and measuring ionizing
radiation. The scintillation counter consists of a scintillator and a photomultiplier tube (PMT).

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Working of Scintillation counter
 Whenever, radiation strikes the scintillation crystals present in the scintillator, a tiny
flash of light is produced.
 The flash of light is amplified with the help of photomultiplier tube.
 Each radiation particle produces a pulse of anode current at the output of the
photomultiplier.
 The output of photomultiplier is connected to an electronic counter which counts each
flash light generated by the scintillation crystals.
 The intensity of radiation can be detected by counting the number of pulses.
Types of crystals used as Scintillators:
1. Cesium Iodide and Sodium Iodide - This is the most commonly used scintillator in the
study of γ rays.
2. Zinc Sulphide - It is used in the study of α rays.
3. Organic Phosphors - These are useful for the detection of γ -particle.
4. Anthracene - These are useful for the detection of β-particle.

Advantages of Scintillation counter:
 The high efficiency of detection.
 Short resolving time and extremely short duration pulses and higher resolution.
 Its counting rate is very fast.
 Ability to accommodate samples of zny type (liquids, solids, suspensions and gels).
 Ease of sample preparation.
 Ability to count separately different isotopes in the same sample.
 It can detect lower levels of radiation.

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Disavantages:
 High voltages applied to PMT causes high background count (noise).
 Temperature affects counting efficiency.
Applications of Scintillation Counter:
1. Scintillation Counters are widely used in radioactive contamination, radiation survey
meters, radiometric assay, nuclear plant safety, and medical imaging, which are used
to measure radiation.
2. There are several counters mounted on helicopters and some pickup trucks for rapid
response in case of a security situation due to radioactive waste or dirty bombs.
3. Scintillation counters are designed for weighbridge applications, border security,
contamination monitoring of nuclear waste, and ports.
4. It is widely used in screening technologies, In vivo and ELISA alternative
technologies, cancer research, epigenetics, and cellular research.
5. It also has its applications in protein interaction and detection, academic research, and
pharmaceuticals.
6. A liquid Scintillation Counter is a type of scintillation counter that is used for
measuring the beta emission from the nuclides.

AUTORADIOGRAPHY
Autoradiography is the bio-analytical technique used to visualize the distribution of
radioactive labelled substance with radioisotope in a biological sample.
It is a method by which a radioactive material can be localized within a particular
tissue, cell, cell organelles or even biomolecules. It is a very sensitive technique and is
being used in a wide variety of biological experiments. Eg. DNA sequencing.
Autoradiography, although used to locate the radioactive substances, it can also be used for
quantitative estimation by using densitometer. It is based upon the ability of radioactive
substance to expose the photographic film by ionizing it.
In this technique a radioactive substance is put in direct contact with a thick layer of a
photographic emulsion (thickness of 5-50 mm) having gelatin substances and silver halide
crystals. This emulsion differs from the standard photographic film in terms of having higher
ratio of silver halide to gelatin and small size of grain.

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It is then left in dark for several days for proper exposure. The silver halide crystals
are exposed to the radiation which chemically converts silver halide into metallic silver
(reduced) giving a dark colour band. The resulting radiography is viewed by electron
microscope, preflashed screen, intensifying screen, electrophoresis, digital scanners etc.

Working of Autoradiography:

1. The sample (tissue/cell) is taken in a slide.
2. The radioactive sample is covered with the photographic emulsion.
3. Incubate in dark room for specific time.
4. The radioactive part of the sample activates the silver halide crystals nearby.
5. This results in reduction of Ag+ ions to Ag atom leaving dark colour bands.
6. Radioisotopes in the sample emit radiation.
7. The slide is then washed away by fixers to get insoluble Ag atom only.
8. The autoradiogram can further be viewed and observed under the microscope.

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Types of autoradiography:
Based on the presence of sample in the emulsion.



Factors affecting autoradiography:

1. Energy of emitter: Higher the energy longer is the track length and so it‟s difficult to
localize the points in the low density region of the same track. Further very low
energy radiation also creates a poorer resolution image on the film. Therefore weak
beta-emitting isotopes (
3
H,
14
C and
35
S) are most suitable because the energy of
radiation is in between gamma and alpha radiations.
2. Distance and Thickness of sample: If either the sample is very thick or the sample is
far away from the emulsion film, resolution will be lost.
3. Grain size and amount of silver halide crystals: The grain size should be smaller so
that there is more availability of AgX crystals. Also concentration of gelatin should be
less in emulsion as compared to AgX crystals.
4. Thickness of emulsion: The emulsion thickness affects the efficiency of
autoradiography with different emitters. For β-emitters the thickness of the emulsion
should be less.
5. Exposure time: An autoradiogram must be exposed for a sufficiently long time for
proper exposure to view pattern of the track length.

Uses of Autoradiography
 Autoradiography is used to study the synthesis, turnover, transport and localization of
micromolecular constituents in the cell.
 It has been used for studying the kinetics of DNA, RNA, proteins and carbohydrate
synthesis. To find and investigate the various properties of DNA
 To find the location and amount of particular substance within a cell including cell
organelle, metabolites, etc.

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 Radioactive labelling of various molecules enables the binding of these molecules (as
markers of other molecules) to be accurately monitored by radioisotope
cytochemistry. e.g: enzyme inhibitors, antibodies, nucleic acid probes.
 Radioactive isotopes are also used to track the distribution and retention of ingested
materials. To locate the metabolic activity site in the cell.
 Tissue localization of radioactive substance.
 To find the site and performance of targeted drug.

BIOSAFETY
Sources of Radiation Elements such as thorium, uranium, radium, RN-222, and K-40 are
naturally occurring radioactive elements that can be found in our everyday lives. These
elements can be found in: – rocks, soil and building materials – food and water. Some sources
are a result of ground nuclear testing, which is not naturally occurring.
Ionizing radiation is produced by the natural decay of radioactive material. Beta, gamma, and
x-rays are forms of ionizing radiation that are often used in research. Beta, gamma, x-rays
remove electrons from atoms (Ionization).
X-rays and gamma rays can penetrate the body and irradiate internal organs. Exposure can
result in external and internal doses. Internal exposure can occur when rays are ingested,
inhaled, or absorbed through the skin.
Radiation safety and protection:
 Minimum possible time should be spent near the radiation zone. The
experiment/procedure to should be pre-planned to minimize exposure time.
 Doubling the distance from the source can reduce the exposure intensity by 25%.
 Use of forceps, tongs, and trays to increase the distance from the radiation source.
 Move the item being worked on away from the radiation area if possible.
 Know the radiation intensity where work is performed, and move to lower dose areas
during work delays.
 Lead shielding will reduce the intensity of x-rays and gamma rays being emitted from
a source of radiation.

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 To reduce exposure by a certain desired percent, lead shielding must be a certain
thickness for each type of emitter.
 Dosimeter should be worn by persons who work with or near the radiation.
(Dosimeter is the instrument that measures exposure of radiation over a given period).
 Smoking, eating, and drinking are not permitted in radionuclide laboratories. Food
and food containers are not permitted in the laboratory.
 Radionuclide work areas shall be clearly designated and should be isolated from the
rest of the laboratory.
 All work surfaces shall be covered with absorbent paper which should be changed
regularly to prevent the build up of contamination.
 Protective clothing shall be worn when working with radioactive materials. This
includes laboratory coats, gloves, and safety glasses.
 All containers of radioactive materials and items suspected or known to be
contaminated shall be properly labelled with tape or tagged with the radiation logo
and the word "RADIOACTIVE”.
 All contaminated waste items shall be placed in a container specifically designed for
radioactive waste.
 Warning labels: all items used to manipulate or store radioactive material should be
labeled.
 Warning label requirements: Labels must provide sufficient information on the
container to minimize exposure and to make sure all proper precautions have been
taken. Radionuclide (s) estimated activity and date should be mentioned.
 Radioisotopes waste disposal: radioactive waste should be collected in proper
containers. Containers should be closed and secured unless waste is added. A tag
should be kept on the waste container at all times.

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