What Are Radioisotopes

7/29/2019 What Are Radioisotopes

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Radioisotopes

Radioisotopes or radionuclides are artificially produced, unstable atoms of a chemical element, which have a

different number of neutrons in the nucleus, but the same number of protons and the same chemical properties.

Many live for only minutes. Their existence is measured in “half -lives,” how long it takes for half of the isotope todisappear. To produce radioisotopes, a stable isotope is bombarded with fast neutrons that are produced in a nuclear

reactor or a particle accelerator. The stable isotope is transmuted into an unstable isotope of the same element.

Smaller proton linear accelerators (linacs), which can be located near a medical facility are also under development,

such as that of the Advanced Medical Isotope Corporation in Washington State. The fusion program of

theUniversity of Wisconsin at Madison is investigating a new method of producing isotopes in a small fusion

reactor. A1-watt fusion source has already demonstrated that it could provide very short-lived radioisotope doses for

use with a PET (positron emission tomography) scanner. From the time of the Manhattan Project, scientists had

realized that nuclear fission would provide an unlimited amount of “tracer and therapeutic radioisotopes.” The first

major use of a radioisotope was iodine-131, for diagnosis and treatment of thyroid disease. It was found that the

thyroid specifically absorbs iodine. Now, five decades later, isotope technology has developed to a high degree,

defining which unique properties of radioisotopes are best at particular tasks. There are now

about 200 radioisotopes in use.

2. Production of Radioisotopes in a Research Reactor

Radioisotopes are produced in a nuclear reactor 1,2 by exposing appropriate target material to the neutrons in the reactor,

thereby causing a nuclear reaction to occur which leads to the production of desired radioisotope. The factors which decide

the type of nuclear reaction that takes place andthe rate of production are:

1. The energy of the neutrons and theneutron flux.

2. The characteristics and quantity of the target material.

3. The activation cross-section for the desired reaction.

Neutron flux is the product of neutron density and the average speed of the neutron and is expressed as n/cm2/sec. The

neutron interaction with the nucleus of the target material can be expressed quantitatively in terms of nuclear cross section.

It is a measure of the probability that a given nuclea terms of an imaginary cross-sectional area presented by nucleus

around the nucleus to the beam of neutrons, perpendicular to the beam such that if and only if the neutrons pass throughthis area, the nuclear reaction takes place. The value of the cross-section varies with the energy of the interactingneutrons

and from nucleus to nucleus. The maximum value of the cross-section is for thermal neutrons. Higher the cross-section;

higher the probability of radioisotope formation.

Characteristics of the Target Material

Substances which are explosive,pyrophoric, volatile, etc. are notpermitted to be irradiated in reactor.

Targets should be stable underirradiation conditions.

Isotopically pure target gives high specific activity radioisotopes

The physical form of the target should be such that the neutron flux depressionis minimum.

The target should be in a suitable chemical form for post irradiation processing. Usually target in metallic

form or oxides are preferred.

If the target is hygroscopic, it is preferable to preheat the target prior to encapsulation

3. Production of certain Medical Isotopes:(a) IODINE – 131: This isotope is used in the diagnosis and treatment of thyroid disorders. It is obtained by the

irradiation of natural Te in the reactor.

A wet process of oxidation with H2CrO4 + H2SO4 mixture followed by reduction with oxalic acid is utilized for the

preparation of this isotope from the target. 131I released is absorbed in Na2SO3 solution and supplied as Na131I in alkaline

sulphate solution. This is produced on a weekly basis and is in good demand for use as diagnostic and therapeutic

agent for thyroid disorders

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(b) Phosphorous – 32:

This isotope is used in bone metastasis to control the pain, in agriculture and synthesis of nucleotides. This is obtained by

the neutron irradiation of natural sulphur. Sulphur, after irradiation is distilled under vacuum to leave phosphorous in the

flask. This phosphorous is leached with dil. HCl and then purified for removal

of cationic impurities by passage through Dowex 50x 8. The product 32P is supplied as H32PO4 solution (Ortho phosphoric

acid). Normally about 2-3 Ci 32P is produced fortnightly from 200g Sulphur depending on the time of irradiation, flux etc.

In terms of weight about 10-15μg of phosphorous is separated from bulk of sulphur.(c) Chromium – 51:

This isotope is produced in the reactor by Szilard-Chalmer’s process during the irradiation of potassium

chromate. The difference in the valency state of 51Cr produced (+3) is utilized in effecting a chemical separation of the

product from the target material.



















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What Are Radioisotopes