Radioactivity and Nuclear Physics: Exploring Atomic Nuclei
X-rays pass through solid materials, can ionize the air, do not refract in glass, and are not deflected by magnetic fields. They are high-frequency electromagnetic waves emitted by energizing the innermost orbital electrons of atoms.
French physicist Henri Becquerel tried to determine whether some elements spontaneously emitted X-rays.
He found that most elements had no effect, except uranium, which produced rays. It was soon discovered that similar rays were emitted by other elements, such as thorium, actinium, and two new elements discovered by Marie and Pierre Curie: polonium and radium. These rays were the result of a spontaneous disintegration of the atomic nucleus: radioactivity.
Types of Radioactive Decay
All elements with atomic numbers greater than 82 are radioactive. They emit three different kinds of radiation: alpha, beta, and gamma.
- Alpha rays: Have a positive electric charge.
- Beta rays: Are negatively charged.
- Gamma rays: Have no charge.
Alpha rays are a flow of helium nuclei.
Beta rays are a flow of electrons.
Gamma rays are electromagnetic radiation. They are part of the electromagnetic spectrum and often have much greater energy than light and X-rays. They originate in the nucleus.
Nuclear Structure
The particles that occupy the nucleus are called nucleons. When they have an electric charge, they are called protons; when they are electrically neutral, they are called neutrons.
Some nuclei are spherical, but most have different shapes, such as ovoid, and some are shaped like doorknobs.
Quarks
Quarks are sub-nuclear particles created in nuclear collisions. Six types of quarks, carrying fractional electric charges, combine to form hadrons like protons and neutrons. Particle physicists have named them:
- Up
- Down
- Charm
- Strange
- Top
- Bottom
An antiquark is the antiparticle corresponding to a quark. The number of types of quarks and antiquarks is the same. They are represented by the same symbols but with a bar over the corresponding letter. For example, if an up quark is represented by u, an up antiquark is written as ū.
An ion is an atom or other particle with a positive electric charge (cation, loses electrons) or negative charge (anion, gains electrons), as exists in a solution.
An isotope is each species of atom of a chemical element having the same atomic number and different mass number.
Nuclear Forces
Why aren’t protons ejected from the nucleus by the strong force of repulsion? Because there is an even stronger force inside the nucleus that binds neutrons and protons together: the nuclear force.
The nuclear force is much more complicated than the electric force and is still being understood. The main part of the nuclear force, the part that holds the nucleus together, is called the strong interaction. It is a force of attraction that acts between protons, neutrons, and particles called mesons. These are all hadrons.
This force only works at very short distances. Thus, while protons in small nuclei are close, the nuclear force easily overcomes the electric force of repulsion. But for distant protons, like those on opposite sides of a large nucleus, the nuclear force of attraction may be small compared to the electric force of repulsion. A higher concentration of protons is not as stable as a smaller one.
Half-Life and Radiation Detectors
Half-life is the time required for a sample to lose half of its initial radioactivity. For example, radium-226 has a half-life of 1620 years. This means that half of any given sample of radium-226 will decay into other elements after 1620 years.
Radiation detectors are devices used to measure the rate of radioactive decay.
- Geiger Counter: Detects radiation by the way it ionizes a gas enclosed in a tube.
- Scintillation Counter: Indicates radiation by light flashes that occur when charged particles or gamma rays pass through the meter.
- Cloud Chamber: Shows the traces left by charged particles moving through supersaturated steam. When the chamber is in an intense electric or magnetic field, the deviation of the trails provides information about the charge, mass, and motion of the particles.
- Bubble Chamber: Particle tracks are tiny gas bubbles in liquid hydrogen. The liquid hydrogen is heated under pressure inside a glass and stainless steel chamber to a temperature just below its boiling point. Suddenly lowering the chamber pressure causes ions produced by passing particles to create thin trails of bubbles along their trajectories. The entire liquid boils rapidly, but before that happens, pictures are taken of the short particle tracks.
- Spark Chamber: Consists of a set of closely spaced parallel plates. Every third plate is grounded, and the intermediate plates are maintained at high voltage (around 10 kV). Ions produced in the gas between the plates as charged particles pass through the chamber cause discharges along the path of the ions, producing visible sparks between pairs of plates. A trace of many sparks indicates the trajectory of the particle.
- Streamer Chamber: A different design with only two plates far apart. The electric discharge, called a streamer, tracks the trajectory of charged particles.