KAHIBARO
Table of Contents
Identity and Basic Properties
Alpha particles are one of the most important examples of heavy charged particles in nuclear physics. An alpha particle is the nucleus of a helium atom. It contains two protons and two neutrons, so it has mass number 4 and electric charge $+2e$.
Because it is relatively massive and carries a double positive charge, an alpha particle interacts strongly with matter as it travels through it. This makes its behavior very different from that of electrons or photons. In matter, alpha particles lose energy quickly and travel only a short distance before stopping.
An alpha particle is a helium nucleus, not a neutral helium atom.
Its key properties are
$$A = 4, \qquad Z = 2, \qquad q = +2e.$$
Because of its large mass and charge, it produces strong ionization and has a short range in matter.
Where Alpha Particles Come From
Alpha particles are commonly emitted by unstable heavy nuclei. In alpha decay, a parent nucleus emits an alpha particle and becomes a different nucleus with atomic number reduced by 2 and mass number reduced by 4. The decay process itself belongs to the topic of radioactive decay, but here the important point is that alpha particles are often produced with kinetic energies of a few MeV.
These energies are large on an atomic scale, yet alpha particles still stop very quickly in solids, liquids, and gases because they interact intensely with the electrons and nuclei of the material.
Motion Through Matter
As an alpha particle moves through matter, it repeatedly interacts with atoms along its path. The most important effect is Coulomb interaction with atomic electrons. Since the alpha particle is positively charged, it attracts electrons and transfers energy to them. This leads to ionization and excitation of atoms.
The alpha particle usually travels in a path that is nearly straight, especially when its energy is high. Its large mass means that collisions with light electrons do not deflect it very much. However, it steadily loses kinetic energy until it eventually comes to rest.
Ionization Produced by Alpha Particles
Alpha particles are strong ionizers. Since they have charge $+2e$ and move relatively slowly compared with light particles of the same energy, they transfer energy efficiently to the atoms they pass.
This means they create many ion pairs per unit length. For this reason, alpha radiation is easy to detect if the source is close enough to the detector and not blocked by air, paper, or other material.
A simple comparison is helpful.
| Radiation type | Charge | Mass | Ionization strength | Penetration |
|---|---|---|---|---|
| Alpha particle | $+2e$ | Large | Very high | Very low |
| Beta particle | $\pm e$ | Very small | Moderate | Higher than alpha |
| Gamma ray | 0 | 0 | Indirect | Very high |
Why the Energy Loss Is Large
The rate of energy loss depends strongly on the particle's charge and speed. For heavy charged particles such as alpha particles, the stopping power is large. A qualitative form of the dependence is
$$-\frac{dE}{dx} \propto \frac{z^2}{v^2},$$
where $z$ is the charge number of the particle and $v$ is its speed.
For an alpha particle, $z=2$, so the factor $z^2 = 4$ already makes the energy loss much stronger than for a singly charged heavy particle moving at the same speed. Also, as the alpha particle slows down, the factor $1/v^2$ tends to increase the energy loss further.
For heavy charged particles, the energy loss per unit distance generally increases as the particle slows down.
Qualitatively,
$$-\frac{dE}{dx} \propto \frac{z^2}{v^2}.$$
For alpha particles, the double charge makes ionization especially strong.
Short Range of Alpha Particles
Because alpha particles lose energy so quickly, their range is very short. In air they may travel only a few centimeters, and in solids or biological tissue the range is typically much smaller, often only micrometers to tens of micrometers depending on energy.
This short range has an important practical consequence. Alpha radiation is usually not dangerous outside the body because it can be stopped by a sheet of paper or the outer dead layer of skin. However, if an alpha-emitting substance enters the body through inhalation, ingestion, or a wound, the radiation can be very damaging locally because of the intense ionization.
Interaction with Atomic Nuclei
Most of the alpha particle's energy loss comes from interaction with electrons, but it can also interact with atomic nuclei. Since nuclei are much heavier than electrons, collisions with nuclei can change the direction of the alpha particle more noticeably. This is called scattering.
Large-angle scattering is less common than many small interactions with electrons, but it is important in nuclear physics. Historically, alpha particle scattering played a key role in revealing the existence of the atomic nucleus.
Shielding of Alpha Radiation
Alpha particles are very easy to shield. A thin barrier can stop them completely. Common shielding materials include paper, plastic, glass, or a few centimeters of air.
| Material | Ability to stop alpha particles |
|---|---|
| Paper | Usually enough |
| Outer skin layer | Usually enough |
| A few cm of air | Often enough |
| Metal sheet | More than enough |
The main issue with alpha radiation is therefore not deep penetration, but contamination by alpha-emitting materials.
Alpha particles have low penetrating power but high ionizing power.
External alpha radiation is usually easy to stop, but internal exposure can be very hazardous.
Typical Track Characteristics
In detectors that make particle tracks visible, alpha particles produce dense, short tracks. This happens because they deposit a large amount of energy in a short distance. Compared with electrons, their tracks are straighter and more heavily ionizing.
This dense ionization is one of the signatures used to identify alpha particles in radiation measurements.
Importance in Radiation Physics
Alpha particles are important because they show clearly how a heavy charged particle interacts with matter. They are a classic example of strong ionization, short range, and localized energy deposition. These features make them useful in detectors, in some medical and industrial applications, and in understanding radiation hazards.
Their behavior also prepares the way for the study of stopping power and the Bragg peak, where the energy loss becomes especially large near the end of the path. That topic is treated separately, but alpha particles are one of the clearest particles for observing it.