Nuclear Reactions– changing the hearts of atoms

Applications of Nuclear Reactions

Nuclear reactions are used for nuclide productions, syntheses of unknown nuclides, syntheses of non-existent elements, and syntheses of heavy elements heavier than the heaviest element, uranium, on Earth. These applications are based on new nuclides produced.

Applications based on the properties of nuclide lead to the analyses of materials. For example, minute amounts of metals present in hair capture neutrons and become radioactive. Analyses of this radioactivity enable us to determine the metal present for medical diagnoses. This type of application is called activation analysis. For example, by irradiating a rock and then measure the radioactivity produced enables us to determine the composition of the rock. Such applications have been used for space explorations as well as the analyses of terrestrial samples.

Applications based on Nuclide Productions

Nuclear reactions produce new nuclides. A major application of nuclear reaction is nuclide production.

• 
  What were some of the radioactive nuclides produced and why were they produced?
  What are the radioactive nuclides used for?
  

Nuclear reactions produce new nuclides for scientific research, for medicine applications, and for the extension of our boundary of nuclides. Many nuclides are tailor made using a combination of nuclear reactions.

Not too long ago, there were empty spaces to be filled in the periodic table of elements. The existence of these elements and the reasons for their absence are fundamental to science.

Although e
Pb Bi Po ? Rn

82 83 84 85 86

lements with atomic numbers greater than 83 have no stable isotopes, isotopes with atomic numbers between 84 and 92 have been identified, except the one with Z = 85. Making this group VII element below iodine was a challenge. Dale R. Corson, K.R. Mackenzie and Emilio Segré. bombarded bismuth with alpha particles in 1940 and they anticipated the formation of an element with Z = 85 by the reaction:

²⁰⁹Bi⁸³ (, xn) ^(213-x)At⁸⁵,

where x is an integer 1, 2, or 3. Various modes of reactions have been mentioned earlier and the element is called astatine (At) named after the Greek word astatos meaning unstable. After metallic bismuth sheets were irradiated by  particles, the sheets were heated to a temperature between 300 and 800⁰C. Isotopes of astatine sublimated and condensed on cold surfaces. This method is used to separate astatine, because astatine should have properties similar to iodine, which sublimates when heated. Naturally occurring radioactive astatine isotopes have subsequently been found in minute amounts. About 20 isotopes are known.

Astatine-211 has a half-life of 7.15 h, and decays by two pathways: 40% by alpha and 60% by electron capture (EC). The isotope ²¹⁰At has the longest half-life (8.3 h) of all astatine isotopes. Thus, astatine must be synthesized shortly before it is used. Small quantities of astatine have been made, and its chemical properties established. Its properties are very similar to those of iodine.

O
Cr Mn Fe

24 25 26

Mo Tc Ru

42 43 44

W Re Os

74 75 76
ther missing elements of the old periodic table are technetium (Z = 43 named after Greek technetos artificial), promethium (Z = 61, named from Greek prometheus, god stole fire from heaven for man's benefit), and francium (Z = 87). The nuclide ⁹⁷Tc was first synthesized by Carlo Perrier and E. Segré in 1937 from the reaction:

⁹⁶Mo + ²D  ⁹⁷Tc + n,

using deuterium from a cyclotron. The isotope ⁹⁷Tc has a half life of 2,6000,000 y. Two other long-lived isotopes of technetium are ⁹⁸Tc (4.210⁶ y) and ⁹⁹Tc(2.110⁵ y). Other isotopes of technetium have been produced by the reaction Mo⁴² (n, ) Tc⁴³. Technetium isotopes are also fission products of ²³⁵U, and some kilograms of ⁹⁹Tc ( emitter) have been produced from processing used nuclear fuels.

Samarium has several stable isotopes with mass numbers 144, 147, 148, 149, and 150. One of these undergoes a neutron capture reaction ¹⁴⁴Sm⁶² (n, ) ¹⁴⁵Sm producing an unstable isotope of the same element. It decays by electron capture (EC) with a half life of 340 days producing an isotope of the missing element promethium,

¹⁴⁵Sm⁶² + EC  ¹⁴⁵Pm⁶¹.

However, ¹⁴⁵Pm⁶¹ further decays with a half life of 17.7 years by EC to ¹⁴⁵Nd, which is a stable nuclide. The other two long-lived promethium isotopes are ¹⁴⁶Pm and ¹⁴⁷Pm with half lives of 5.53 and 2.62 years respectively.

By nuclear reactions, three elements with atomic number less than 83 missing on earth have been synthesized and studied. Their study confirmed the prediction and existence of these missing elements on the periodic table.

A common and well known beta and gamma source is ⁶⁰Co, which is a radioactive isotope emitting  particles and gamma () rays. The  particles may be filtered off, and the gamma rays are used for medical examination, cancer treatment, and food treatment. The isotope ⁶⁰Co is made by placing cobalt metal in a nuclear reactor. The neutron bombardment leads to the formation of ⁶⁰Co and ^60mCo,

⁵⁹Co²⁷ + n  ⁶⁰Co and ^60mCo

This reaction produces two isomers of cobalt, the lower energy or ground-state ⁶⁰Co, and the higher energy isomeric state, ^60mCo. The latter will decay with a half life 10.5 min by gamma radiation leading to the ground state, ⁶⁰Co, that emits  particles and  rays (half life 5.24 years) leading to ⁶⁰Ni. The cross sections for thermal neutron capture reactions are 18 b for the formation of ^60mCo and 19 b for the formation of ⁶⁰Co. A little more of ⁶⁰Co nuclides than ^60mCo are produced at the end of irradiation, but ^60mCo decays to give ⁶⁰Co.

Many isotopes used in medical treatment are synthesized by irradiating the element with neutrons in a nuclear reactor. Two examples are given here:

²³Na + n  ²⁴Na

The cross sections for isomeric and ground states are 0.40 and 0.13 barns respectively.

¹²⁷I + n  ¹²⁸I (6.2 barns)

Radioactive isotopes of iodine are used for thyroid examinations.

Skill Building Questions

1. 
   What elements are missing on the planet Earth? Why?
   How are these synthesized, and what is the significance of their syntheses?
   
2. 
   Suggest a method for the synthesis of an At isotope.
   
3. 
   What are the properties of ^60mCo and ⁶⁰Co? (An open ended question)
   
4. 
   What is radioactive iodine used for in medicine?
   How is it produced?
   What are the decay mode and half life of ¹²⁸I?
   How do these properties affect its application?