Alpha particle is alternatively known as alpha rays and alpha radiations. They are produced as a result of alpha decay and have two protons and two neutrons bound together in a nucleus. Alpha particles are represented by the “α” symbol. However, Due to similarities with the helium nucleus, they are sometimes recognized by the symbol He2+.

alpha particle

In 1899, Ernest Rutherford and Paul Villard worked on radiation and separated them into three types alpha rays, beta rays, and gamma rays. Rutherford named alpha particles on the basis of penetration and deflection properties.

Properties of Alpha (α) particles

These are the following properties of alpha rays:


The deflection of alpha rays in an electric or magnetic field shows that they are positively charged particles. However, Rutherford and T. Royds (1909) introduced that an alpha particle has two units of positive charge and has a mass four times greater than a hydrogen atom.


Alpha particles move with a velocity ranging from 1.4 to 2.0 x 107 ms-1.


They have a large mass and move with high speed and have kinetic energy. Due to large energy, they cause ionization when passed through a gas.


Alpha particle cause luminescence when they strike the zinc sulfide and have intense photographic activity.

Penetrating power

They have a smaller penetrating power and can be stopped by a sheet of paper or thin foil of aluminum.

alpha particle penetration power

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Sources of Alpha (α) rays

These are the few sources that produce alpha particles:

  • Alpha decay
  • Ternary fission
  • Accelerators
  • Solar core reactions
  • Cosmic rays

Alpha decay

Alpha decay is a process to generate alpha particles through the decay of heavier atoms. During the process, heavier atoms lose four nucleons and their atomic mass number decreases by four. This is because four nucleons (two protons and two neutrons) remove as an alpha particles from the heavier atom. However, the atomic number of heavier atoms lose by two due to ejecting the two protons.

Examples are the alpha decay of uranium into thorium and radium into radon. During the nuclear transmutation of these elements by alpha decay, alpha particles are produced.

Ternary fission

Ternary fission is also used to produce alpha particles but it is a very rare nuclear reaction and its resultant products are three instead of two. One type of resultant product is the alpha particles that have a 90% probability of production.

The alpha particles produced by ternary fission are called long-range alphas due to their high energy as compared to alpha particles produced by alpha decay.


Accelerators are alternative sources of alpha particles such as cyclotrons and synchrotron accelerators. They produced very high energetic helium nuclei.

Solar core reactions

Reactions that occur in the solar systems such as in stars, etc have helium nuclei involved. They also produce alpha particles in several reactions.

Cosmic rays

Cosmic rays have 10-12% part of alpha particles that have very high energy.

Energy and biological effects of alpha (α) particle

Energy of alpha particle

Energy of alpha particles depends on the half-life and varies with emission. Higher energy alpha particles are emitted by the larger nucleus and their energies are between 3-7 MeV which corresponds to the long and short half-life of alpha particles emitting nuclides.

Alpha particles with an energy of 5 MeV have a speed of 15,000 kilometers per second which is equal to 5% of the speed of light.

Biological effects of alpha particle

Alpha particles have a short range of penetration and absorption. Therefore, they are not dangerous to the skin of human beings. However, they are very dangerous when the alpha-emitting radionuclides or other sources are inhaled or ingested.

This is because they are the most destructive or dangerous form of ionization radiation and cause radiation positioning in the body. In this way, it can damage internal organs and have effects 20 times greater than gamma rays. High-energy alpha particles that are emitted by polonium-210 can cause lung cancer and bladder cancer.

Applications of alpha (α) particles

These are important applications of alpha particles:

  • Alpha particles are used in smoke detectors in which americium-241 is an alpha emitter. Alpha particles play an important role to ionize the air in the smoke detectors. This results in the flow of current through these ionized air particles. When particles of smoke from the fire pass through this air, it reduces the flow of current and sounds the alarm.
  • They are also used as sources for radioisotope thermoelectric generators which in turn are used in heart pacemakers and space probes.
  • Alpha particle emitter is also used in devices like static eliminators.
  • They are used in nuclear reactions for the transmutation of elements.

Use of alpha particles in cancer treatment

Alpha rays are used to treat cancer. These are the three ways to use them for cancerous tumors:

  • One way is to treat with infusible radioactive (Radium-223) that directly targets the specific tissue in the body.
  • Second is the insertion of a radioactive source (Radium-224) in the solid tumor.
  • Third way is targeted-alpha therapy, in which an alpha particle source is attached to the tumor-targeting molecules such as antibodies.


Radium-223 works as an alpha emitter and is naturally attached to the bones in the human body. It is infused into the patient through veins and reaches the place of the tumor, where it emits alpha radiation and destroys the tumor cells within a hundred-micron distance.


Radium-224 is a radioactive atom used in cancer-treating devices such as diffusing alpha emitters radiation therapy (DaRT) where it emits alpha radiation. These devices come with cylindrical tubes in which radium-224 atoms are enclosed.

During the decaying process, each radium-224 atom produces six daughter atoms and emits four alpha particles. These atoms have the potential to diffuse into 2-3 mm in the tissues and destroy the entire tumor.

Targeted-alpha therapy

Targeted alpha therapy is used to attach alpha rays emitting radionuclides to the molecules that are further used to target tumors. For example, they are attached to antibodies.

According to studies, alpha emitter Actinium-225 is used for metastatic prostate cancer. During the process, it is attached to the prostate-specific membrane antigen (PSMA) and reaches the place where tumors are present. It has a short half-life of 10 days and produces four alpha particles during emission.

Concepts Berg

What are the health effects of exposure to alpha particles?

Alpha particle has very low penetration power. Thatswhy, they are not dangerous to the human body from the outside (skin) but it is very harmful when their sources are ingested.

What are some common sources of alpha particles?

Alpha decay is a common source of alpha particles. Other sources such as ternary fission are very rare.

What are some uses of alpha particles?

Alpha particles have great use in the medical field. For example, nowadays they are used in the treatment of cancerous tumors.

What is an alpha α particle composed of?

Alpha particles are composed of 2 protons and 2 neutrons.

What is the alpha particle symbol?

The symbol of an alpha particle is “α”.