RADIOACIVITY

INTRODUCTION

The smallest unit of matter that cannot be chemically broken up into smaller units is referred to as an atom. As such, atoms form the basic building blocks of all matter. An atom consists a core called  a nucleus.  A nucleus contains  protons which are positively charged and neutrons which carry no charge. Protons and neutrons are collectively known as nucleons. Also part of the atom are electrons which are negatively charged. Electrons revolve around the nucleus in energy levels, also referred to as orbits. 

Electrons stay within specific energy levels due to electrostatic force of attraction between the electrons and the nucleus.  The orbit closest to the nucleus has the lowest energy while that farthest from the nucleus has the highest energy. For an electron to move from one energy level to another, it must loose energy to move to a low-energy orbit, and gain extra energy to move to a higher-energy orbit.  This is because each orbit is associated which a specific amount of energy. The energy of orbital electrons is therefore discrete.

Compared to an electron, a neutron is 1842 heavier while a proton is 1836 times heavier. The nucleus therefore carries nearly all the mass of an atom. The large mass is held within the relatively small nucleus by forces called strong nuclear forces. These forces are attractive in nature and occur between neutrons and neutrons and between neutrons and protons. The attractive nuclear force must balance the electrostatic repulsion force between the protons for the nucleus to be stable.

An atom is said to be stable if it contains equal numbers of protons, neutrons and electrons. Instability of an atom occurs if;

1. Number of electrons is not equal to the number of protons resulting in a charged atom, called an ion.
2. The  forces within the nucleus are unbalanced leading nuclear instability.  Such an atom is called a radionuclide and is said to be radioactive.

Charged atoms or  ions are either positively charged or negatively charged. An atom becomes positively charged when it looses some of its electrons. Consequently, the number of protons exceeds that of the electrons. The atom that gains extra electrons ends up with more electrons than protons. Such an atom is said to be negatively charged. Ions with opposite charge attract while those with similar charge repel. Ions also attract neutral atoms. thus, ions normally bond with oppositely charged ions and atoms to gain stability.  

Nuclear instability occurs if;

1. The nucleus is too heavy, or the number of protons is not equal to that of the neutrons. To be more stable, the nucleus disintegrates (decays) by emitting particles and in the process transforms to a new element. The disintegration process is referred to as radioactive decay or radioactivity. The radioactive decay continues until a final stable element is obtained. 
2. The  nucleus is in an excited state (has excess energy). In this case, the radionuclide decays by shedding off the excess energy in the form of high energy electromagnetic radiation called gamma radiation. This type of radioactive decay does not transform the radionuclide into a new element.
3. The nucleus is too light to be stable. In this case, a relatively lighter radionuclide combines with another (fuses) in pursuit of stability, a process called nuclear fusion.  In the process, energy (nuclear radiation and heat) is emitted.
4. A stable atom  is bombarded with fast moving neutrons making it radioactive. The resulting radionuclide subsequently splits up into two or more new elements with the release of radiation, a process referred to as nuclear fission. 

The following discussion focuses on the nuclear  instability.

RADIOACTIVITY

Radioactivity or radioactive decay is the spontaneous disintegration of unstable atomic nuclei (radionuclides) with the release of energy/radiation in the form of particles or electromagnetic radiation. Where particles are emitted, the decaying radionuclide, called the parent, transforms to a new element, called the daughter. If the daughter is radioactive, it also disintegrates to a daughter. The process continues until a stable (non-radioactive) element is produced. The radionuclides formed before a stable element is produced makes up the decay series, also called decay chain.

Decay modes

Radioactive decay generally falls under three categories (modes)  based on the type of radiation emitted; 

1. alpha decay 
2. beta decay
3. gamma decay. 

Alpha and Beta decays produce particulate radiation hence the parent is transformed to a new element. Gamma decay produce energy in the form of electromagnetic radiation and does not transform the parent into a new radionuclide. 

(1) Alpha decay 

This  occurs in heavy atoms. In alpha decay, the parent (X) transforms into a daughter (Y) and an alpha particle is released. An alpha particle is a helium nucleus (₂⁴He). During the decay process, the atomic number Z (number protons) of the parent reduces by 2 and the atomic mass number A (number of nucleons (protons + neutrons)) reduces by 4;

Alpha particles are heavy, slow, less penetrating (can be stopped by a thin paper), highly ionizing (form thick tracks) and are deflected by both electric and magnetic field. Alpha particles are extremely damaging when they interact with soft tissue such as lungs and can lead to cancer

Beta decay (beta minus) 

This occurs in elements with an excess of neutrons. During beta decay, a neutron is converted to a proton and an electron. The electron is emitted as a beta (beta minus) particle (_-1⁰e) while the proton adds the atomic number by one. The atomic mass number does not change. 

Beta particles are very light, much faster and more penetrating than alpha particles (stopped by aluminium foil), less ionising (tracks not as thick) and are deflected by both electric and magnetic field but in opposite direction to the alpha particles.

Gamma decay 

This type of decay mode does not transform the parent to a new element. When beta and alpha decays occur, the daughter is often left in an excited (excess energy) state. The daughter de-excites (goes to ground state) by shedding off the excess energy in the form of gamma rays. Gamma rays are massless electromagnetic waves unlike alpha and beta particles. They are extremely penetrating (stopped by lead metal), mainly ionize indirectly, and are not deflected by either electric or magnetic field. Gamma rays have many applications: 

In hospitals: 

• Used in the treatment of cancer whereby the radiation kills the cancer cells by damaging their DNA. 
• Sterilize medical equipment by killing germs. 
• Used in imaging body organs.

In industry: 

• Used to check for defects for example in metal pipes. When gamma rays are directed towards a metal pipe, the solid parts of the pipe will block most of the rays while most of the rays will pass through the cracked parts. 
• Used to determine the thickness of metal since thicker metal will block more gamma rays than thinner metal. 
• Used in the food industry for example to sterilize food products (irradiated food) such as potatoes by killing germs (instead of using pesticides) hence prolonging the shelf-life.

Alpha particles, beta particles and gamma rays are collectively referred to as nuclear radiation. Nuclear radiation is ionizing radiation which means that it is capable of removing electrons from atoms. Ionizing radiation can damage cells and should therefore be handled by trained personnel only. Alpha radiation is the most ionizing (hence most dangerous if produced inside the human body), followed by beta and lastly gamma. Gamma radiation on the other hand is the most penetrating, followed by beta particles and lastly alpha particles being the least penetrating. Gamma radiation can easily pass through human skin while alpha radiation may not pass through skin.

Alpha and beta particles are deflected by an electric field while gamma radiation is not. Alpha particles are attracted by the negative plate since they are positively charged while beta particles are attracted by the positive plate since they are negatively charged. Gamma radiation is not deflected since it has no charge.

Decay law

The decay process is not a one-off affair where all radioactive atoms decay at once. Rather, it is a random spontaneous process where atoms decay at different times. Consequently, the number of atoms of parent radionuclide reduce with time. To put it into perspective, imagine a kiondo (basket) full of fresh tomatoes. As time goes by, the tomatoes decay gradually thereby reducing the number of fresh tomatoes. Eventually all the tomatoes in the kiondo decay. 

The decay law states that; the rate of decay (disintegration) of radioactive elements is directly proportional to the number of radioactive elements present at the time. If for example there are N radioactive elements present at time t=0, then;

                                                                           (i)

                                                                 (ii)

where λ is a constant of proportionality called decay constant. The negative sign is used to show that the number of radioactive elements reduces with time.  The rate of decay is referred to as activity.

                          (iii)              

Decay constants are unique to each element. Some elements have a large decay constant while others have a small decay constant. Those with large decay constant disintegrate very fast hence have a higher activity (for example radon) while those with a small decay constant disintegrates very slowly hence have a lower activity (for example uranium). 

Equation (ii) may be expressed as:

                                (iv)                                

If at t=0, N=N₀ and at a later time t=t, N=N, then;

                (v)

                              (vi)

Equation (vi) is often used to represent the decay law.     

Half-life

The activity of radionuclides is usually related to the half-life (T) of the radionuclide. Half-life refers to the time required for half the radionuclides initially present to decay. If for example at some time t=0the number of radionuclides present is N₀, then at time t = T (half-life) the number of radioactive atoms present N will be N₀/2, i.e., 

                                                 (i)

This means that at half-life, the decay law may be expressed as;

                         (ii)

Equation (ii) may be expressed as;

                  (iii)

Equation (iii) shows that radionuclides with a short half-life decay faster (have a high activity) than those with a long half-life (have a low activity). The half-life of radioactive elements ranges from billions of years to a fraction of a second.

Decay law in terms of half-line

Consider a radioactive element of half-life T and decay constant λ. If N₀ be the number of radioactive atoms initially present and N the elements present at time t=t, then by decay law;          

                                                                                                      (i)

                                                                                            (ii)

But;

                                                                                                          (iii)

Using equation (iii) in equation (ii) leads to;

                                                                                    (iv)

                                                                                     (v)

Obtain exponential on both sides of equation (v) to obtain

                                                                                      (vi)

Hence

                                                                                                   (vii)

It follows from equation (vii) that if:

In general, if t=nT (n^th half-life), then;

                                                                                                     (viii)

Activity

Activity is defined as rate of decay, i.e

                                             (i)

Since

follows that;

                                                                             (ii)

But 

                               (iii)

Hence

                                                                                (iv)

Equation (iv) shows that the activity of a radioactive element reduces exponentially with time.

 Nuclear fission 

This refers to the situation whereby an element disintegrates into more than one daughter elements, emitting radiation (alpha, beta, gamma and neutrons) in the process.  Nuclear fission for the most part is artificially induced inside nuclear reactors by bombarding atoms with fast moving neutrons. There are many applications of nuclear fission. For example;

(i) Nuclear fission is used to produce radio-isotopes (mainly short-lived) for use in medicine and industry. 

• In medicine, the radioisotopes are used for example in cancer treatment as well as in imaging. During cancer treatment, the radioisotopes are introduced to or near the area being treated for instance through injection. The radiation released when the radioisotopes decay destroys the cancer cells. 
• In industry, the radioisotopes are used for instance to trace pollution transfer e.g. liquid sewage. The radioisotopes are mixed with waste and the radiation emitted monitored. 
• Radioisotopes are also used in the oil industry to monitor leaks in pipes. Oil is spiked with the radioisotopes and the intensity of gamma radiation along the path of the pipeline monitored. In regions where oil is leaking, the intensity of gamma radiation will be higher as the gamma penetration will be higher (not blocked by the pipe) than in regions where there is no leakage.

(ii) During nuclear fission, a great amount of heat is generated. The heat is used in nuclear power plants to convert water to vapour which is then used to turn turbines in magnetic fields leading to generation of electricity. Electricity generated using heat from nuclear reactions is called nuclear power.  

Advantages of nuclear power i(over fossil fuels like petrol and paraffin) are:

• It is a low carbon source hence clean 
• It helps in reducing the precursors of climate change
• It is reliable and cost-effective in the long run

Disadvantages of nuclear power are:

• Expensive to build a nuclear power plant
• Security threats (e.g. terrorism related)
• Adverse environmental impact of improperly disposed nuclear waste
• Adverse effects in the event of nuclear accidents                                  

NOTE:

Gamma radiation is extremely penetrating and requires lead shielding to stop it from escaping. (Radiation sources must be shielded to stop radiation formed during radioactive decay from escaping). Nuclear radiation is ionizing and capable of destroying cells (can cause harm). Radiation sources must therefore be stored n such a way that they do not contaminate the environment.

Ionization is the process through which atoms gain or lose electrons. If an atom gains an electron, it becomes negatively charged (an anion), and if it loses an electron, it becomes positively charged (a cation). Ionization process therefore converts a neutral atom into an ion-pair.  

Beta (electrons, negatively charged) and alpha (helium nuclei, positively charged) particles carry energy in the form of kinetic energy. When these charged particles collide with air molecules, some of their kinetic energy is used to dislodge electrons from atoms. Each alpha and beta particle has sufficient kinetic energy to remove electrons from multiple atoms (though alpha particles are more ionizing than beta particles). Alpha and beta particles therefore cause direct ionization. As the charged particles knock off electrons, their KE energy reduces. 

Gamma radiation is not a particle but an electromagnetic wave with energy E dependent on the frequency f (E=hf where h is Planck’s constant). Unlike the charged particles where a single particle can cause multiple direct ionization, a single photon causes only one direct ionization (photoelectric effect). The entire photon is absorbed and a single electron ejected. If the photon is energetic enough, the emitted photoelectron is emitted with sufficient kinetic energy and goes on to cause ionization just like a beta particle would. Gamma radiation is therefore said to cause indirect ionization.

EXAMPLES

Dr. Margaret W. Chege