8.2.3.1. Alpha Particles

Table of Contents

Nature of Alpha Particles

Alpha particles are one of the main types of radiation emitted by unstable atomic nuclei. An alpha particle is not a single elementary particle. It is a small bundle made of two protons and two neutrons. This means it is exactly the same as the nucleus of a helium atom, specifically helium, 4.

Because it contains two protons, an alpha particle has electric charge $+2e$, where $e$ is the magnitude of the elementary charge. Because it contains four nucleons in total, its mass is about $4$ atomic mass units, often written as $4u$. In symbols, an alpha particle is often written as

$$
\alpha \equiv {}^{4}_{2}\mathrm{He}
$$

Sometimes it is also written as ${}^{4}_{2}\mathrm{He}^{2+}$ to emphasize that it is positively charged and has no electrons attached.

An alpha particle is the nucleus of a helium atom, made of 2 protons and 2 neutrons.
It has charge $+2e$ and mass number $A=4$.

Composition and Basic Properties

The structure of the alpha particle is especially stable. The two protons and two neutrons are tightly bound together by the strong nuclear force. This strong binding is one reason alpha particles appear so often in nuclear decay processes of heavy nuclei.

A useful summary is shown below.

Property	Alpha particle
Symbol	$\alpha$, ${}^{4}_{2}\mathrm{He}$
Number of protons	2
Number of neutrons	2
Mass number	4
Atomic number	2
Electric charge	$+2e$
Approximate mass	$4u$

Since it is positively charged and relatively heavy compared with beta particles, it behaves in a distinctive way when moving through matter and through electric or magnetic fields.

Why Alpha Particles Are Common in Nuclear Physics

Alpha particles occur in many heavy radioactive nuclei. In very large nuclei, the balance between the attractive strong nuclear force and the repulsive electric force between protons becomes delicate. Emitting an alpha particle can move the nucleus toward a more stable state.

When a nucleus emits an alpha particle, its atomic number decreases by 2 and its mass number decreases by 4. The daughter nucleus is therefore a different element.

The general form of alpha emission is

$$
{}^{A}_{Z}X \rightarrow {}^{A-4}_{Z-2}Y + {}^{4}_{2}\mathrm{He}
$$

For example,

$$
{}^{238}_{92}\mathrm{U} \rightarrow {}^{234}_{90}\mathrm{Th} + {}^{4}_{2}\mathrm{He}
$$

This chapter focuses on the emitted particle itself, not the full decay mechanism.

In alpha emission, the parent nucleus changes according to
$$
A \to A-4, \qquad Z \to Z-2
$$
because the emitted alpha particle carries away 2 protons and 2 neutrons.

Motion and Deflection

Because alpha particles carry positive charge, they are deflected by electric and magnetic fields. Their deflection shows that they are charged particles, not neutral radiation.

Compared with lighter charged particles, alpha particles are harder to bend because of their relatively large mass. For the same kinetic energy, they usually move more slowly than very light particles such as electrons.

A simple sketch of an alpha particle moving between charged plates is shown below.

Rendering scientific sketch…

Deflection of an alpha particle in an electric field

The electric field points from the positive plate to the negative plate. Since the alpha particle has positive charge, the electric force acts in the direction of the field.

Ionization Ability

One of the most important features of alpha particles is their strong ionizing power. As they pass through matter, they interact strongly with atoms and molecules, removing electrons from them and creating ions.

This happens because the alpha particle has a relatively large charge, $+2e$, and because it moves through matter with enough energy to disturb atomic electrons. Along its path it loses energy quickly, producing many ion pairs in a short distance.

This high ionization means alpha radiation can be very damaging to biological tissue if the source is inside the body. Outside the body, however, alpha particles are often less dangerous than other radiation types because they do not penetrate deeply.

Alpha particles are strongly ionizing because they are heavy and carry charge $+2e$.
They lose energy rapidly in matter and create many ion pairs along a short path.

Penetrating Power

Although alpha particles are strongly ionizing, they have low penetrating power. They are usually stopped by a sheet of paper, the outer dead layer of human skin, or just a few centimeters of air.

This combination can seem surprising at first. The reason is that strong interaction with matter makes them lose energy quickly. So alpha particles do a lot of ionization, but only over a short distance.

The contrast is useful:

Property	Alpha particles
Ionization ability	High
Penetration in matter	Low
Typical shielding	Paper, skin, short air path

Speed and Energy

Alpha particles emitted in radioactive decay usually have kinetic energies of a few mega electron volts, written as MeV. This is much larger than typical atomic energies, which are often a few electron volts, but much smaller than the rest energy of the particle.

If the kinetic energy is $K$, then in many introductory cases their speed can be estimated using the nonrelativistic formula

$$
K = \frac{1}{2}mv^2
$$

so that

$$
v = \sqrt{\frac{2K}{m}}
$$

This formula is often good enough because alpha particles from ordinary alpha decay usually move at a few percent of the speed of light, not close to it.

Identification in Experiments

Historically, alpha particles were identified by their charge, mass, and behavior in fields and matter. Their relatively short range and strong ionization made them distinguishable from beta particles and gamma rays.

They also played a central role in early nuclear physics. For example, alpha particles were used as projectiles in scattering experiments that helped reveal the existence of the atomic nucleus.

Comparison with Other Common Radiations

A short comparison helps place alpha particles in context.

Radiation type	Nature	Charge	Relative mass	Ionization	Penetration
Alpha	${}^{4}_{2}\mathrm{He}$ nucleus	$+2e$	Large	High	Low
Beta	Electron or positron	$\mp e$, $+e$	Small	Medium	Medium
Gamma	Electromagnetic photon	$0$	Zero rest mass	Low directly	High

This table is only for orientation. The details of beta and gamma radiation belong to their own chapters.

Key Facts to Remember

An alpha particle is a very stable cluster of two protons and two neutrons. It is the nucleus of a helium atom, with charge $+2e$ and mass number $4$. Because it is heavy and positively charged, it ionizes matter strongly but travels only a short distance before stopping. These properties make alpha particles easy to identify and very important in nuclear decay and nuclear physics experiments.

Key facts about alpha particles:
$$
\alpha = {}^{4}_{2}\mathrm{He}
$$
Charge: $+2e$
Mass number: $4$
Composition: 2 protons and 2 neutrons
Behavior: strong ionization, weak penetration

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