Nuclear Reactions– changing the hearts of atoms

Chapter 7

Nuclear Reactions–
changing the hearts of atoms

She points it to the rock, and the rock turns into gold.

- a legend

Alchemists dreamt of changing worthless mercury into the precious gold and platinum. Chemical reactions never change the identities of element, and alchemists' dream can never be realized. Nuclear reactions change identities of elements and they fulfilled alchemists dreams, however the process costs more than the products.

However, nuclear reactions are not for the purpose of producing precious elements. They are useful in making, for example, radioactive nuclides, new elements, qualitative analyses, quantitative analyses, and weapons. Furthermore, these reactions are employed in fission nuclear reactors and future fusion nuclear reactors. Nuclear reactors are mainly for energy production.

Radioactive decays also change identities of nuclides, but decays need no stimulants. The radioactive nuclei undergo decay (decomposition) by themselves. They may be considered a special kind of nuclear reactions. Nuclear reactions, however, are usually induced by bombarding a sample with energetic subatomic particles or high-energy photons.

In order to understand nuclear reactions, they are studied experimentally under controlled condition. On the other hand, they also occur naturally.

Nuclear-Reaction Experiments

Radioactivity has always been present but it was not discovered until 1896 because the phenomena due to radioactivity cannot be directly detected by human senses. Like radioactivity, nuclear reactions are taking place in nature all the time, but they are not directly observable. Thus, their discoveries are made by deductive minds after careful analyses of various phenomena.

Nuclear Reactions

Nuclear reactions change the identity of elements or nuclides by altering the energy states of atomic nuclei. Changes in states can be in the form of energy, number of nucleons (protons and neutrons) or number of quarks. In contrast, chemical reactions change the identities of compounds, but not identities of elements. Physical reactions change the states (solid, liquid, gas, solution etc) of substances, but not identities of molecules.

• 
  What is a nuclear reaction?
  How are nuclear identities changed in nuclear reactions?
  How can the changes be detected and confirmed?
  What are the reactants and products in nuclear reactions?
  What is the role of energy in nuclear reactions?
  

Particles Used in Nuclear Reactions
	Symbol	Particle
 	photon electron
p or ^1H	proton
n	neutron
d or ^2D	deuteron
t or ^3T	triton
 or ^4He ^nE	alpha other nuclide

I
A nuclide, A, when bombarded by energetic subatomic particles, a, changes to another nuclide is called a nuclear reaction. The energetic particles a either from radioactive decays or from particle accelerators. Often, the products consist of light particles b and another nuclide B. The reaction can be written as

a + A  B + b.

This reaction is often written in a short form,

A (a, b) B,

where a and b may be an , , , neutron, proton, deuterium, a nuclide, or high-energy electron. An exothermic nuclear reaction releases energy, and an endothermic nuclear reaction requires energy. The energy required in an endothermic reaction can be supplied in the form of kinetic energy (of the incident particle a).

Potential Energy of Nuclear Reaction

Ideally, the energetic particle a must approach A within 10 ¹⁵ m for a nuclear reaction to take place, because the strong force will only be effective at this distance. Particles such as protons,  and light nuclides with a positive charge experience a repulsion of the atomic nucleus, due to the electromagnetic force. The repulsion results in a rise of the potential energy called the Coulomb barrier. They must carry enough energy to overcome the Coulomb barrier. Once in contact (10­­^-15 m) with any nucleon or quark of the nucleus, the strong force becomes effective, merging the incident particle with the nucleus. Such an interaction makes the potential energy uniform and low, within the nucleus forming a potential well due to the strong force.

On the other hand, neutral particles (neutrons) approach the target nuclei experiencing no Coulomb repulsion. Once in contact with the nucleus, a neutron becomes part of the nucleus. However, neutrons carrying high kinetic energies will be bounced off or knock other nucleons out of the nucleus. These considerations are given in planing nuclear reaction experiments.

The forgoing consideration suggests that the type of nuclear reaction depends on the target material A, the incoming particles a, and their energies. Particles from an accelerator may have the same energy before they enter the target. Interactions of incident particles with the target atoms alter the energy of particles before they react with A. Due to the range of energies of the incident particles, several modes of nuclear reactions may take place.

Review Questions

1. 
   What are nuclear reactions?
   How are they different from physical and chemical reactions?
   What particles are used to induce nuclear reactions?
   What particles are usually produced in a nuclear reaction?
   
2. 
   What force is responsible for the Coulomb barrier?
   What particles experience it, and what particles will not experience it?
   
3. 
   What are the advantages of using neutrons to bombard atomic nuclei?
   This is an open-ended question, because the more you know, the more you can give.
   

Discoveries of Nuclear Reactions

Nuclear reactions were discovered in 1919. At that time, tracks of  particles were made visible in cloud chambers. Their discovery was due to the power of mind and a keen observation.

• 
  When were the first few nuclear reactions discovered, and by whom?
  How were they discovered?
  What are the reactions?
  

In 1914, E. Marsden and E. Rutherford studied  particles. In the vicinity of the  particle source, they observed some tracks of positively charged particles that were different from those of  particles.

In the cloud chamber, these particles made longer but thinner tracks than the -particle tracks. Furthermore, these particles gave more point like scintillation images on the zinc sulfide (ZnS fluorescence material) screen than the  particles did. Eventually, they identified them as hydrogen nuclei or protons. At first, they thought the protons came from ionization of water molecules, but they carried out these experiments carefully under water-free conditions. The persistence of the protons around the  source led them to the extraordinary conclusion that "the nitrogen atom is disintegrated under the intense force developed in a close collision with a swift  particle". They considered the hydrogen atomic nuclei so liberated constituents of the nitrogen nuclei. This conclusion led to the observation of the first nuclear reaction in 1919, and they postulated the reaction to be:

¹⁴N + ⁴He  ¹⁷O + ¹H

or in short form ¹⁴N (, p) ¹⁷O, which is often called an (, p) reaction.

At about the same time, F. Joliot and I. Curie bombarded aluminum with alpha particles. After the bombardment, they found the aluminum metal radioactive. The induction of artificial radioactivity by  particle bombardment marks another nuclear reaction,

²⁷Al (, ¹n) ³⁰P ( , ⁺ or EC) ³⁰Si.

The ³⁰P further decay by positron emission or electron capture (EC) leading to a stable isotope of silicon, ³⁰Si. The half-life of ³⁰P is 2.5 min. The short notation ( , ⁺ or EC) indicates a radioactive decay process which involves no incident particles as a reactant.

Another milestone in the study of nuclear reactions took place around 1929 when John D. Cockroft and Ernest T.S. Walton^ devised an accelerator in the Cavendish Laboratory, Cambridge, England. They applied high voltage to accelerate protons and observed the reaction:

⁷Li + p  2 

This was actually a proton induced fission reaction because the lithium nuclei were divided into two halves. However, they called the reaction the smashing of an atom by artificially accelerated particles.

Skill Developing Questions

1. 
   What contributed to the discovery of nuclear reactions?
   This is an open-ended question for discussion, but some factors are keen observation, careful analysis, sound deduction, and bold conclusion.
   
2. 
   Describe the nuclear reactions discovered by Rutherford and Marsden; F. Joliot and I. Curie; and J.D. Cockroft and E.T.S. Walton.
   

Nuclear Reaction Experiments

A typical nuclear reaction experiment requires a source of energetic particles, a target containing atomic nuclei, a shield, detectors, and a data collection and analysis system as depicted here. Furthermore, the complicated data collection and analysis may be helped by the use of computers.

• 
  What particle sources are available and what are the energies these particles?
  What target materials are used?
  How products can be identified?
  What to use to detect the emitted small particle in a nuclear reaction?
  How can a conclusion be reached?
  

In an intended experiment, we usually know the particles and target nuclides used, but usually not the products. The parameters such as the types and energies of the particles and targets are set or known in an experiment, but the products are seldom as predicted. To understand a nuclear reaction, products must be detected and identified. Instruments extend our senses to see the products. Careful analysis of the data helps us to interpret the reaction.

In addition to particles from radioactivity, high-energy particle accelerators provide energetic particles for the study of nuclear reactions^*. Often, charged particles such as protons, alpha particles, atomic nuclei, electrons and positrons are accelerated to energies in keV, MeV, and GeV. They are used in nuclear reactions. After the bombardment, sophisticated detectors are built to detect particles emitted by the target nuclei after the reaction. Energy, charge, and type of emitted particles can be determined by specific detectors. Thus, some of the products can be identified.

The unidentified products can be inferred based on the conservation of charges, particles, and masses.

Research nuclear reactors usually provide neutron sources. Neutrons are captured by many nuclides and the reactions produce radioactive nuclides. Identities of the products can be determined by measuring the types of decay, the energies of the particles, and the half lives. These measurements usually lead to the identification nuclides produced by comparison with properties of known radioactive nuclides.

There are many applications for nuclear reactions. For example, some information on the Basics of Boron Neutron Capture Therapy (BNCT) can be found in the URLs: http://www.mit.edu:8001/people/flavor/intro.html. and http://www.mallinckrodt.nl/nucmed/noframes/general/nucmed.htm
Review Questions

1. 
   What are the key requirements in a nuclear reaction experiment?
   
2. 
   What are some of the particle sources?
   Give a short list of them that you know how they are generated.
   
3. 
   When ¹⁰B nuclei are irradiated by neutrons, alpha particles are emitted. What is the reaction?
   

Neutron Sources

Neutrons are ideal bombarding particles for nuclear reactions, because they approach atomic nuclei experiencing no Coulomb barrier as do positive particles.

• 
  What nuclear reactions will produce neutrons?
  Can the production of neutrons be made into convenient neutron sources?
  What are the applications of neutron sources?
  

Mixtures used as Neutron Sources
Neutron

Source Reaction energy / MeV

Ra and Be ⁹Be (, n) ¹²C up to 13

Po and Be up to 11

Pu and B ¹¹B (, n) ¹⁴N up to 6

In 1932 James Chadwick^* bombarded beryllium with alpha particles, and discovered a neutral particle, the neutron. The reaction is now used as a neutron source, and the reaction is


⁹Be (, n) ¹²C.

Further study showed that bombardment of boron by alpha particles also produced neutrons in the reaction, ⁹B (, n) ¹²N.

Since  particles do not travel more than a few centimeters,  emitting radioactive nuclides Ra, Po, and Pu are mixed with beryllium or boron to produce neutrons. Only small fractions (in the order of 0.005% to 0.05% depending on the mixture) of the alpha particles emitted result in the production of neutrons. These mixtures are called neutron sources. The energies of the neutrons so produced are in the order of MeV.


Two-component Neutron Sources
Neutron

Source Reaction energy / MeV

Ra, Be ⁹Be (, n) ⁸Be 0.6

Ra, D₂O ²D (, n) ¹H 0.1

²⁴Na, Be ⁹Be (, n) ⁸Be 0.8

²⁴Na, D₂O ²D (, n) ¹H 0.2

Neutrons are also produced when light nuclides are excited by high-energy photons. Since the emission of gamma rays often follow the emission of  or  rays, excitation by photons requires  sources separate from the light elements to avoid irradiation by  and  particles. Usually, beryllium, Be, and heavy water, D₂O, are suitable target materials. Some well-known neutron sources are listed here. These neutrons are much less energetic than those given earlier.

Neutrons can also be produced using accelerated particles. A d-d reaction,

²D (d, n) ³He,

gives different yields depending on the energy (100 KeV to 2 MeV) of the accelerated deuterium, d (²D). Better yields of neutrons are obtained with the d-t reaction, ³T (d, n) ⁴He. These fusion reactions are well studied, and they will be discussed in Chapter 9 on nuclear fusion.

Another type of neutron source is provided by spontaneous fission. For example, the nuclide ²⁵²Cf decays by 97 % alpha decay and by 3% spontaneous fission. Every fission reaction releases an average of 3.8 neutrons. Nuclear fission reactions are discussed in Chapter 8.

Major sources with very high numbers (intensities or densities) of neutrons (10¹⁵ n cm^-2 s^-1 or higher) are close to the core area of nuclear reactors. More information will be provided for these sources in conjunction with nuclear fission and nuclear reactor technology in Chapter 8.

Skill Building Questions

1. 
   Give some examples of alpha induced reactions that produce neutrons.
   What applications have been made of these reactions?
   
2. 
   Give some examples of neutron sources using gamma ray technology. What are neutrons used for?
   
3. 
   Discuss the d-d and d-t reactions. (Open ended question)
   

Neutron Induced Radioactivity

Neutrons, discovered in 1932, are ideal projectiles for inducing nuclear reactions. Neutrons are captured by most stable nuclides. The increase of neutrons in these reactions produces radioactive materials, mostly beta emitters.

• 
  What are the typical nuclear reactions induced by neutrons?
  How can the products be identified?
  

Emission of light particles , , and  in neutron-induced reactions are often delayed. Half-lives of nuclei produced and their decay energies are determined by experiments, and these data provide identification for the products. Once the products are identified, the reactions are deduced. Almost every element absorbs neutrons, but some more than others.

Soon after the discovery of neutrons, the group led by Enrico Fermi^ in Italy worked feverishly. Just two months after I. Curie and F. Joliot announced their discovery of artificially induced radioactivity, Al (, n) P, in France, Fermi claimed the discovery of the following reactions:

¹⁹F (n, ) ¹⁶N
²⁷Al (n, ) ²⁴Na ( , ) ²⁴Mg.

After that, he told his student Segré^ to buy all possible pure elements found in Mendeleyev's periodic table, and then they bombard what they have bought with neutrons. Using a pure element as target material reduced complication due to other elements. They produced radioactive nuclides with various half-lives for the elements iron, silicon, phosphorous, vanadium, copper, arsenic, silver, tellurium, chromium, barium, samarium, gold, neodymium, etc. They identified (n, ), (n, p) and (n, ) reactions. The neutron bombardments gave them many new radioactive nuclides, and Fermi was awarded with the Nobel Prize for Chemistry in 1938 for his identification of new radioactive elements produced by neutron bombardment and his discovery, made in connection with this work, of nuclear reaction affected by slow neutron. After receiving this prize on Dec. 12, he went to the United States directly from Stockholm, fulfilling his wish since the day Italy joined Hitler.

Skill Building Questions

1. 
   Give an example each of the (n, ), (n, ), and (n, ) reactions?
   
2. 
   Why did Fermi's group bombarded samples of pure element rather than samples of any material by neutrons?
   
3. 
   The Nobel Prize for Chemistry in 1938 was awarded to E. Fermi in recognition of what achievements?
   

Nuclear Reactions Induced by Cosmic Rays

T


he primary cosmic rays arriving at the top of the earth's atmosphere consist mostly of positively charged particles, mainly protons (83 %). Most cosmic protons have energy in the range between 1 and 2 GeV (2 giga eV or 10⁹ eV), and a few reach high energies of ~10¹⁸ eV. Other components of the cosmic rays include nuclei of He (0.6 %), C, N, O and most elements of the periodic Table.

• 
  Do cosmic rays induce any nuclear reaction?
  What are the products and what are the reactions?
  

Cosmic rays interact with atomic nuclei in the atmosphere as well as those of liquids and solids. The impact of primary cosmic rays near the top of the atmosphere produces violent nuclear reactions in which many neutrons, protons, alpha particles and other fragments are produced. Some light nuclides such as ³H, ⁴He, ⁷Be, ¹⁰B are also produced. Lithium, beryllium and boron are practically absent in stellar objects, but are abundant in cosmic rays. They are probably produced in interstellar space through collisions of protons and alpha particles with interstellar gases.

One interesting nuclear reaction due to cosmic rays is the formation of ¹⁴C,

¹⁴N (n, p) ¹⁴C

The half-life of the -emitting ¹⁴C is 5730 y. Carbon atoms circulate around the planet Earth forming a carbon cycle. Thus, carbon in systems actively exchange carbon in this cycle contains a certain amount of the radioactive ¹⁴C. This type of carbon has a specific radioactivity (radioactivity per unit weight of say gram) of 14.9 disintegration per minute per gram. This radioactivity is readily measurable. When a carbon-containing sample is isolated from the carbon cycle, no isotope exchange takes place. Its ¹⁴C isotope decay according to a half-life of 5730 y. Thus, the specific radioactivity decreases. Thus, by measuring the specific radioactivity of a sample enables us to determine the age (of isolation) for the sample. This method is called ¹⁴C-dating or carbon dating.

Meteorites are exposed to a high level of cosmic rays. Nuclear reactions generate many radioactive nuclides, and as a result, the radioactivity of meteorites is usually high. Analysis of isotope distribution reveals interesting results of cosmic rays and history of meteorites, but this subject is a spin-off from a general discussion of nuclear reactions.

Skill Building Questions

1. 
   How is carbon-14 produced?
   Why do living organisms contain an equal percentage of radioactive carbon?
   
2. 
   The chemistry and physics of carbon cause the element to undergo a complicated transformation on the planet Earth. This process is called a carbon cycle. This cycle is an important consideration of the carbon dating. This cycle is often covered in schools, but describe the carbon cycle if you can. Otherwise, check out a source and read about it, and then describe it.
   
3. 
   Assume that 10% of body weights is carbon, and that the specific radioactivity of carbon is 14.9 dis min^–1 g^–1, what is the radioactivity of a human body? You need to assume a weight here, but if everyone uses the average mass of 70 kg, then everyone's answer is the same.
   

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