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What is Radioactivity?

#Radioactivity is the process whereby unstable atomic nuclei release energetic subatomic particles. The word radioactivity is also used to refer to the subatomic particles themselves. This phenomenon is observed in the heavy elements, like uranium, and unstable isotopes, like carbon-14.

#Radioactivity is a random process, meaning that it is physically impossible to predict whether or not a given atomic nucleus will decay and emit radiation at any given moment. #Rather, radioactivity is quantified using half-life, which is the period of time it takes for half of the given nuclei to decay.

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Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, after which the SI unit for radiation, the Becquerel, is named.

Becquerel discovered that uranium salts were able to blacken a photographic plate placed in the dark, even through a paper barrier.

Subsequent experiments distinguished three distinct types of radiation -- alpha particles, beta particles, and gamma rays. These are positively charged, negatively charged, and neutral, respectively.

In the United States, human exposure to radioactivity is measured in rads, where one rad represents 0.01 joule of energy absorbed per kilogram of tissue.

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• Radionuclide:• A radionuclide is an atom with an unstable nucleus,

which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created radiation particle within the nucleus, or else to an atomic electron.

• The radionuclide, in this process, undergoes radioactive decay, and emits a gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides may occur naturally, but can also be artificially produced.

• Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotope.

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Isotope Activity Half Life Energies (KeV)

Barium 133 1uCi 10.7 years 81.0, 356.0

Cadmium 109 1uCi 453 days 88.0

Cobalt 57 1uCi 270 days 122.1

Cobalt 60 1uCi 5.27 years 1173.2, 1332.5

Europium 152 1uCi 13.5 years 121.8, 344.3, 1408.0

Manganese 54 1uCi 312 days 834.8

Sodium 22 1uCi 2.6 years 511.0, 1274.5

Zinc 65 1uCi 244 days 511.0, 1115.5

List of commercially available radionuclides: Gamma only:

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Beta only:Isotope Activity Half Life Energies (KeV)

Strontium 90 0.1uCi 28.5 years 546.0

Thallium 204 1uCi 3.78 years 763.4

Carbon 14 10uCi 5730 years 49.5 (average)

Iron 55 100uCi 2.73 years

Alpha only:Isotope Activity Half Life Energies (KeV)

Polonium 210 0.1uCi 138 days 5304.5

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Radiography Radiography is the visualization of the pattern of

distribution of radiation.

In general, the radiation consist of X- ray, gamma or beta rays and the recording medium is a photographic film.

For classical X- ray, the specimen to be examined is placed between the source of radiation and the film, and the absorption & scattering of the specimen produces its image on the film.

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HISTORY OF AUTORADIOGRAPHY:The autoradiography was obtained accidently

around 1867 when a blocking was produced on emulsion of silver chloride and iodine by uranium salt.

Such study and the work of the curies in1898 demonstrated autoradiography before and contributed directly to, the discovery of radioactivity.

The development autoradiography as a biological technique really started to happen after world war II with the development of photographic emulsion & then stripping film made of silver halide.

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RADIOISOTOPES:Naturally occurring radioisotopes are because of

their instability, but radioactive atoms can be produced in nuclear reactors by bombardment of stable atom with high energy particles.

Radioactive isotopes also known as radioactive tracers.

There are many types of detectors which are used for the determination of detection and assay of radioisotopes-labeled compounds are: (H3, C14, S35, P32, I131).

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Detectors are :

1) Geiger-Muller counter 2) Autoradiography3) Scintillation counter4) Liquid scintillation media5) Gas-ionization chamber 6) Bernstein-Ballentine counter7) Mass spectrometry8) NMR spectrometry.

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what is Autoradiography: #Autoradiography is an important technique for the location of radioactive isotopes in the biological system.

#The method involves the use of suitable photographic emulsion such as silver bromide crystal in a gel or suspension in the form of X-ray sensitive films.

#The autoradiograph gives the distribution pattern of the labeled radioisotopes in the specimen.

#Autoradiographic technique can be applied in cases of morphological plant part or to microscopical sections of the specimen.

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In case of liquid emulsion method gel is poured over the specimen separately to distinguish between the beta-particles emitted from C14 & H3.

In yet another process known as radio paper chromatography is used in tracer studies of metabolic pathways involving the analysis of amino acids or carbohydrates.

This technique is also applied for the location of radiolabeled P32 product in the study of ATP & ADP.

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Definition: AutoradiographyAutoradiography is the process of localization and

recording of a radiolabel isotopes within a solid specimen, with the production of an image in a photographic emulsion.

These emulsions are composed of silver halide crystals suspended in gelatin.

When a beta-particle or a gamma-ray from a radiolabel passes through the emulsions, silver ions are converted to metallic silver atoms.

This result in the development of a visible image which can be easily detected.

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Types of Autoradiography:Two types: 1) Direct autoradiography: It is ideally suited for the detection of weak to

medium strength beta-emitting radionucleosides (H1, C14, S35).

In this technique the sample is placed in direct contact with the film.

The radioactive emission result in the development of black areas.

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2) Indirect autoradiography: For the detection of highly energetic beta-

particles (e.g. P32, I125),direct autoradiography is not suitable.

This is because these emission pass through and beyond the film, and a major part of the energy gets wasted.

It is useful for the detection of highly energetic beta-particles.

In this technique, the beta-particle energy is first converted to light by a scintillator, which then emits photon on exposure to photographic emulsion.

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Autoradiography method: Living cells are briefly exposed to a ‘pulse’ of a specific

radioactive compound. The tissue is left for variable time. Samples are taken fixed & processed for light or electron

microscopy. Section are cut & overload with a thin film of photographic

emulsion. Left in in the dark for days or weeks while the radioisotope

decays. The photographic emulsion is developed. Counterstaining e.g. with toluidine blue, show the

histological details of the tissue. These autoradiographs provides permanent record.

04/07/2023 20Fig:Autoradiography of rat liver cell

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Application of autoradiography:Autoradiography is closely associated with blotting

techniques for the detection of DNA, RNA and proteins.Colony and Plaque blotting:It is a process of hybridization for the specific identification and

purification of colonies. e.g. bacterial clones.The desired bacteria are grown as colonies on an agar plate.When a nitrocellulose filter paper is overlaid on the agar plate, the

colonies get transferred.They are permanently fixed to the paper by heat.On treatment with alkali (NaOH), the cells lyse and the DNA gets

denatured.When the DNA prints are exposed to specific probes(radiolabel),

hybridization occurs.The hybrid complex can be localized & detected by autoradiography.

04/07/2023 22Fig: Colony and Plaque blotting

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References:Dr. Rangari Vinod, “Pharmacognosy & Phytochemistry Part-

I", Career publication, First edition- March 2002, Reprint September 2005, pp no. 177-191.

Satyanarayana U., “Biotechnology”, Published by Books and Allied(P)Ltd., Kolkata, First published2005, pp no…………….

Evans W.C., “Trease and Evans Pharmacognosy”, Published by Elsevier, First printed in India 2005, Reprinted 2006, 07,08, pp no. 147-149.

Albert, Ground Miranda, “Autoradiography”,pp no. 174-176.

Rogers A.(1979), “Techniques of Autoradiography”, 3rd edition, Elsevier, North Holland, pp no. 429.

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