Radioactive decay - waves and particles Revision Notes

Exploring Internal Organs with Tracers

Radioactive tracers are used to examine the function and structure of internal organs without invasive surgery. A small amount of a radioactive isotope is introduced into the body, often by injection or ingestion. This isotope then travels through the body and is absorbed by the target organ. The chosen isotope must have a relatively short half-life (e.g., technetium-99m, half-life of 6 hours) so that it decays quickly, limiting the patient's exposure to radiation once the diagnostic procedure is complete. It also needs to emit gamma radiation, which can easily pass through the body to be detected externally by a gamma camera. This camera then produces an image, highlighting areas where the tracer has accumulated, indicating normal function or abnormalities like tumours or blockages.

Controlling and Destroying Unwanted Tissue (Radiotherapy)

Radiotherapy is a treatment that uses high-energy radiation to destroy cancerous cells or control their growth. The radiation, typically gamma rays or X-rays, damages the DNA of the cancer cells, preventing them from multiplying and ultimately causing them to die. This treatment can be delivered externally, by directing a beam of radiation at the tumour, or internally, by placing radioactive sources directly within or near the cancerous tissue. A key challenge is to deliver a high enough dose to the cancer cells to kill them, while minimising damage to surrounding healthy tissues. This requires precise targeting and careful planning of radiation doses. Radiotherapy is a localised treatment, meaning it affects only the part of the body that is being treated.

Properties of Radiations for Medical Use

The type of radiation used for medical purposes depends on its penetration power and ionising ability. For internal imaging (tracers), gamma radiation is preferred because it is highly penetrating, allowing it to exit the body and be detected, and less ionising than alpha or beta particles, reducing internal damage. For therapy (radiotherapy), high-energy gamma rays or X-rays are used, which are highly penetrating to reach deep-seated tumours. While ionising radiation is damaging, this property is exploited to destroy cancer cells. The half-life of isotopes used for tracers must be short to minimise patient exposure after the scan, typically hours. For radiotherapy, radioactive sources might have longer half-lives as they are not inserted into the patient and can be carefully managed.

Balancing Nuclear Equations: The Basics

Nuclear equations represent radioactive decay processes. To ensure these equations are correctly written, two key quantities must be conserved: the mass number (the total number of protons and neutrons) and the atomic number (the number of protons). The mass number is placed as a superscript before the element symbol (e.g., X), and the atomic number as a subscript (e.g.,
X). When a nucleus decays, the sum of the mass numbers of the reactants must equal the sum of the mass numbers of the products. Similarly, the sum of the atomic numbers of the reactants must equal the sum of the atomic numbers of the products. This conservation principle allows us to identify unknown daughter nuclei or emitted particles.

Alpha Decay: Emission of Helium Nuclei

Alpha decay involves the emission of an alpha particle, which is a helium nucleus. An alpha particle is represented by the symbol He (or sometimes α). When an unstable nucleus undergoes alpha decay, its mass number decreases by 4, and its atomic number decreases by 2. For example, if an element X decays into element Y by alpha emission, the equation would be X → Y + He. A common example is the decay of Uranium-238 into Thorium-234: U → Th + He. Note how both mass numbers (238 = 234 + 4) and atomic numbers (92 = 90 + 2) are balanced.

Beta Decay: Electron Emission

Beta decay involves the emission of a beta particle, which is an electron. A beta particle is represented by the symbol
e (or sometimes
β). This process occurs when a neutron within the nucleus converts into a proton, releasing an electron. Consequently, the mass number of the nucleus remains unchanged, but its atomic number increases by 1. For example, if an element X decays into element Y by beta emission, the equation would be X → Y +
e. A common example is the decay of Carbon-14 into Nitrogen-14: C → N +
e. Here, mass numbers (14 = 14 + 0) and atomic numbers (6 = 7 + (-1)) are balanced.

Gamma Emission: Pure Energy Release

Gamma emission involves the release of gamma rays, which are high-energy electromagnetic radiation. Gamma rays are represented by the symbol γ. Unlike alpha and beta particles, gamma rays have no mass and no charge. Therefore, the emission of a gamma ray does not change the mass number or the atomic number of the decaying nucleus. Gamma emission often accompanies alpha or beta decay when the nucleus is left in an excited, high-energy state after emitting a particle. The excited nucleus then releases the excess energy as gamma rays to reach a more stable ground state. For example, X* → X + γ, where X* denotes an excited nucleus.

Unstable Nuclei and Decay

Some atomic nuclei are inherently unstable. This instability arises from an unfavorable ratio of protons to neutrons, or simply due to having too much energy. To become more stable, these nuclei undergo a process called radioactive decay. During this process, they spontaneously emit various types of radiation. The radiation can be in the form of particles, such as alpha particles, beta particles, or neutrons, or as electromagnetic waves, specifically gamma rays. This emission changes the composition or energy state of the nucleus, leading to a more stable configuration.

Alpha Particles

Alpha particles are made up of two protons and two neutrons, meaning they are identical to the nucleus of a helium atom. They carry a positive charge and have a relatively large mass. When an unstable nucleus emits an alpha particle, its atomic number (number of protons) decreases by 2, and its mass number (total number of protons and neutrons) decreases by 4. Alpha particles are relatively large and slow, meaning they have low penetrating power and can be stopped by a sheet of paper or a few centimetres of air. However, they are highly ionising.

Beta Particles

Beta particles are high-energy electrons that are emitted from the nucleus. This occurs when a neutron within an unstable nucleus transforms into a proton and an electron (the beta particle). The emitted electron is then ejected from the nucleus. As a result of this transformation, the atomic number of the nucleus increases by 1 (due to the new proton), while the mass number remains unchanged (a neutron is replaced by a proton). Beta particles are much lighter and faster than alpha particles, giving them greater penetrating power than alpha particles. They can be stopped by a few millimetres of aluminium.

Gamma Rays and Neutrons

Gamma rays are a form of high-frequency electromagnetic radiation, meaning they are pure energy and have no mass or charge. They are often emitted after alpha or beta decay when the resulting nucleus is still in an excited (high-energy) state and needs to release this excess energy to become fully stable. Gamma emissions do not change the atomic number or mass number of the nucleus. They have very high penetrating power and require thick lead or concrete to significantly reduce their intensity. Neutrons can also be emitted by certain unstable nuclei, particularly in processes like nuclear fission or when a nucleus has a significant excess of neutrons. Neutron emission reduces the mass number by 1, but the atomic number remains the same.

Defining Atoms and Elements

An atom is the basic unit of an element. The identity of an element is determined solely by the number of protons in the nucleus of its atoms. This number is called the atomic number (Z). For example, every atom with 6 protons is a carbon atom, and every atom with 8 protons is an oxygen atom. The atomic number is unique to each element.

Composition of the Nucleus

The nucleus of an atom contains protons and neutrons. Protons are positively charged particles identified by the atomic number. Neutrons are neutral particles. Both protons and neutrons contribute to the mass of the atom. The sum of protons and neutrons in the nucleus is called the mass number (A).

What are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron count means they have different nuclear masses (mass numbers). For example, Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons. Both are carbon, but their nuclear masses differ.

Properties of Isotopes

Because isotopes of an element have the same number of protons and therefore the same electron configuration, they exhibit very similar chemical properties. Their physical properties, such as density and melting point, can differ slightly due to the difference in mass. Some isotopes are stable, while others are unstable (radioactive) and spontaneously decay, emitting radiation.