Beta Particle Radiation

Beta Beta

Most of the radionuclides used in a biomedical research environment are beta emitters. Beta particles are high energy electrons, either negatively or positively charged, that are emitted from an unstable nucleus. A positively charged beta is usually referred to as a positron and a negatively charged beta can be referred to as a negatron.

Beta(ß)^- Decay

There are two types of ß decay; ß⁺ and ß^- decay. An excess of neutrons in an atom's nucleus will make it unstable, and a neutron is converted into a proton to change this ratio. During this process, a ß particle is released, and it has the same mass and charge as an electron. The resulting atom and the ß particle have a total mass which is less than the mass of the original atom, and one would think that the ß particles should have the energy equivalent to the mass lost (E = mc²). But ß particles aren't mono-energetic, and have a broad energy spectrum from zero to the maximum energy predicted. So the ß particle is accompanied by virtually massless and chargeless particles called neutrinos, whose kinetic energy makes up for the energy difference still remaining. As a result of ß^- decay, the atomic number of the atom increases by 1.

Negatron Particle Interactions

As beta(-) particles travel through matter their negative charge interacts with the negative charge of orbital electrons, ejecting them from their orbits (producing ion pairs) or causing excitation. This process will continue until the electron has lost enough energy to be captured by a nucleus. Beta particles are lighter, faster, and have a smaller charge than alpha particles. This gives them deeper penetrating power than alphas. Beta particles come in many energies. Some, like the weak beta from tritium (H-3) can't penetrate the dead layer of skin. Others, like Phosphorous-32, can reach the living skin layers and the lens of the eye.

Beta (-) Particle Interactions

ß⁺ Decay

When there is an excess of protons in the nucleus, and it is not energetically possible to emit an particle, ß⁺ decay occurs. This is where the nucleus becomes stable by converting a proton into a neutron. During ß⁺ decay, a positron (a particle with the same mass as an electron but with positive charge), and a neutrino are released. Positrons interact with electrons, causing both to be completely destroyed. Two gamma ray photons with the same energy as the mass of the positron and electron are released.

Positron Interactions

A beta (+) particle will lose its kinetic energy through ionizations and excitations by collision with electrons in a similar fashion as a beta (-) particle. In addition to this type of interaction, a positron will also then combine with an electron. The two particles are annihilated, producing two 0.511 MeV gamma rays called annihilation radiation which in turn may produce more ionizations.

Positron Interactions

Electron Capture

Sometimes it is not energetically feasible to convert a proton into a neutron by emitting a positron (ß⁺ decay). In these cases, electron capture, or K capture occurs. This is where the nucleus captures an electron from an inner orbital, usually K orbital, and converts a proton into a neutron with it. The difference in mass is converted into a gamma ray and a neutrino.

Internal Conversion

In the process internal conversion, a gamma ray is emitted from the nucleus and strikes an orbital electron. The electron absorbs the energy and is then ejected from the atom.

Bremsstrahlung Radiation

When a beta (-) particle approaches the nucleus of an atom, it's velocity increases then decreases as it moves away from the nucleus due to Coulombic attraction of positively charged protons in the nucleus. The acceleration then sudden deceleration of the beta particle produces electromagnetic radiation equal to that of the change in kinetic energy of the beta particle. This type of emission is referred to as Bremsstrahlung radiation (Bremsstrahlung means braking). The energy of the Bremsstrahlung radiation has a continuous spectrum up to a maximum equal to the maximum kinetic energy of the beta particle. The production of Bremsstrahlung increases with the atomic number of the target atom, thus increased Coulombic attraction. Therefore, low Z materials such as aluminum or Plexiglas are used as beta shields not high Z materials such as lead.

Bremsstrahlung Radiation

Why Eject Betas?

So why does the nucleus eject these particles? Well, for the lighter elements, an even ratio of protons to neutrons is the most stable. Different isotopes of an element will have the same number of protons, but differing numbers of neutrons. Some of these isotopes will be very stable, while others have too few protons or too few neutrons to have the ideal 1:1 ratio of protons to neutrons. If the isotope has too few protons, a neutron in the nucleus is converted to a proton, a negatron (beta -), and a neutrino. The proton remains in the nucleus and the negatron and neutrino are ejected from the nucleus. The beta particle and neutrino share the surplus kinetic energy they possess after escaping the atom. Beta particle energies are listed as maximum and/or average since the beta particle energy can vary from a maximum to zero depending on how much energy the neutrino takes away. A good rule of thumb is that the average beta energy is approximately 1/3 the maximum. Remember, neutrino particles are without rest mass or charge. This tiny particle passes through matter at close to the speed of light without interaction. They are not a health concern.

The reverse of the above reaction occurs when a nucleus has an abundance of protons. The proton changes to a neutron, a positron, and a neutrino. The positron and neutrino are then ejected from the nucleus.

Protection

For these radionuclides, we not only have to prevent ingestion and inhalation, but we also have to ensure we have appropriate shielding or minimize our time of exposure to their radiations.

The properties of the specific radionuclide must also be thoroughly understood before working with or cleaning-up the radioactive material. This will help you make decisions on shielding, measuring, and clothing requirements.

The ideal shielding for beta radiation is one with a low Z number to minimize the generation of Bremsstrahlung radiation. We usually use Lucite or Plexiglas for beta shields. If the researcher is using a large quantity of a beta emitter, or a positron emitter, lead can also be used on the outside of the Plexiglas or Lucite. Now, on to gamma rays and x-rays!

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