Radioactivity Revision Notes

Early Atomic Models and the Plum Pudding Model

Initially, atoms were thought to be indivisible spheres. J.J. Thomson's discovery of the electron in 1897 showed that atoms had internal structure. He proposed the 'plum pudding' model, envisioning atoms as a sphere of uniformly distributed positive charge with negatively charged electrons embedded within it, like plums in a pudding. The overall charge was neutral.

Rutherford's Alpha Particle Scattering Experiment and the Nuclear Model

Ernest Rutherford performed the alpha particle scattering experiment. He directed positively charged alpha particles at a thin gold foil. Most particles passed straight through, suggesting atoms are mostly empty space. A small number were deflected at large angles, and some even bounced back, indicating a concentrated positive charge and mass within a very small central nucleus. This led to the nuclear model, with electrons orbiting the nucleus.

The Bohr Model

Niels Bohr further refined the nuclear model. He proposed that electrons orbit the nucleus in specific, fixed energy levels or shells, not in a continuous spiral. Electrons can move between these energy levels by absorbing or emitting discrete amounts of energy (quanta). This explained the characteristic line spectra observed for elements and addressed the instability issue of earlier models.

The Mechanism of Beta-Minus (β-) Decay

Beta-minus decay is a nuclear process where an unstable nucleus, rich in neutrons, transforms. A neutron within the nucleus converts into a proton. This conversion leads to the emission of an electron, known as a beta particle, and an antineutrino from the nucleus. The electron is emitted at high speed, carrying away energy and momentum.

Changes During Beta-Minus Decay

When a nucleus undergoes β- decay, its atomic number increases by one (+1), as a neutron changes into a proton. However, the total number of nucleons (protons + neutrons), and thus the mass number, remains constant. The newly formed nucleus belongs to a different element due to the change in its atomic number (number of protons). This decay also involves the release of energy, shared between the emitted electron, antineutrino, and the recoiling nucleus.

The Mechanism of Beta-Plus Decay

Beta-plus (β+) decay is a nuclear transformation where a proton within an unstable nucleus changes into a neutron. This process is driven by the weak nuclear force and occurs in nuclei with a higher proton-to-neutron ratio than is stable. To maintain conservation laws, this conversion is accompanied by the emission of a positron (an antiparticle of an electron, symbolized as β+ or e+) and an electron neutrino. The positron carries a positive charge, balancing the charge change from the proton. The electron neutrino carries no charge and has a tiny mass, helping conserve lepton number and energy.

Changes to the Nucleus and Emission Products

When a nucleus undergoes β+ decay, its atomic number (Z, number of protons) decreases by 1, because a proton converts into a neutron. However, its mass number (A, total number of protons and neutrons) remains constant, as the total number of nucleons (protons plus neutrons) does not change. This change in atomic number means the original element transforms into a different, lighter element. The emitted particles are a positron (e.g., 0/1 e or β+) and an electron neutrino (νe). The general equation for β+ decay is: (A/Z)X → (A/(Z-1))Y + (0/+1)e + νe.

Conservation Laws in Beta-Plus Decay

Beta-plus decay adheres to fundamental conservation laws. The total charge is conserved: the +1 charge of the proton is carried away by the emitted positron, leaving a neutral neutron. The total mass number is conserved, as a proton is replaced by a neutron of similar mass. Lepton number is also conserved: the emitted positron is an antilepton, and the electron neutrino is a lepton. Lastly, energy and momentum are conserved during the decay process, with the emitted particles carrying away the excess energy.

Terrestrial Sources of Background Radiation

Background radiation from Earth, or terrestrial radiation, primarily originates from radioactive isotopes found in the Earth's crust. Key examples include uranium, thorium, and potassium-40. These elements are present in varying concentrations in rocks, soil, and water. A significant contributor is radon gas, which is a decay product of uranium and thorium. Radon can accumulate in enclosed spaces like homes, particularly in areas with underlying granite, which is rich in these radioactive elements. Other materials used in construction, such as bricks, concrete, and plaster, also contain trace amounts of these naturally occurring radioactive substances, adding to the terrestrial background radiation.

Cosmic Sources of Background Radiation

Radiation from space, known as cosmic radiation, is composed of high-energy particles from the Sun and distant stars. These primary cosmic rays interact with atoms and molecules in Earth's atmosphere, leading to the creation of secondary radiation, including muons, electrons, and photons. The Earth's atmosphere acts as a shield, reducing the intensity of this radiation at sea level. However, the dose of cosmic radiation increases significantly with altitude (e.g., during air travel) because there is less atmospheric shielding. The Earth's magnetic field also plays a role, deflecting some of these charged particles, with stronger deflection near the equator compared to the poles, meaning cosmic radiation levels vary with latitude.

Overall Contributions to Background Radiation

Natural sources account for about 85% of an average person's annual radiation dose. Terrestrial and cosmic radiation are the main natural components. The remaining percentage comes from human-made sources like medical X-rays, nuclear power, and other industrial applications. The exact proportion of terrestrial versus cosmic radiation varies geographically; for instance, mountainous regions experience higher cosmic radiation, while areas with high uranium concentrations in the ground experience higher terrestrial radiation.