Nuclear Radiation refers to processes whereby unstable nuclei become more stable by emitting energetic particles. The three types of nuclear radiation refer to alpha, beta, and gamma radiation. In order to become stable, a nucleus may emit an alpha particle (a helium nucleus) or a beta particle (an electron or a positron). Often, losing a particle this way leaves the nucleus in an excited state. Then, the nucleus releases the excess energy in the form of a gamma-ray photon.
A matter is ultimately made up of atoms. Atoms, in turn, are made up of protons, neutrons and electrons. Protons are positively charged and electrons are negatively charged. Neutrons are not charged. Protons and neutrons reside inside the nucleus of the atom, and protons and neutrons are together called nucleons. Electrons are found in a region around the nucleus, which is much larger than the size of the nucleus itself. In neutral atoms, the number of protons is equal to the number of electrons. In neutral atoms, the positive and negative charges cancel each other, giving a zero net charge.
Properties of Protons, Neutrons, and Electrons
Note that the neutron is slightly heavier than the proton.
- Ions are atoms or groups of atoms that have lost or gained electrons, making them have a net positive or negative charge. Each element is made up of a collection of atoms having the same number of protons. The number of protons determines the type of the atom. For instance, helium atoms have 2 protons and gold atoms have 79 protons.
- Isotopes of an element refer to atoms having the same number of protons, but different numbers of neutrons. For instance: protium, deuterium and tritium are all isotopes of hydrogen. They all have one proton each. Protium, however, has no neutrons. Deuterium has one neutron and tritium has two.
- Atomic Number (proton number) (): the number of protons in the nucleus of an atom.
- Neutron number: The number of neutrons in the nucleus of an atom.
- Nucleon Number (): The number of nucleons (protons + neutrons) in the nucleus of an atom.
Notation for Representing Nuclei
Nuclei of an isotope are often represented in the following form:
For example, hydrogen’s isotopes protium, deuterium and tritium are written with the following notation:
, , .
Sometimes, the proton number is also emitted and only the symbol and the nucleon number are written. e.g.,
, , .
There is no problem with not showing the proton number explicitly, as the number of protons determines the element (symbol). Sometimes, a given isotope may be referred to with the element name and the nucleon number e.g. uranium-238.
Unified Atomic Mass
Unified atomic mass () is defined as the mass of a carbon-12 atom. .
The Three Types of Nuclear Radiation
Alpha Beta and Gamma Radiation
As we mentioned before, the three types of nuclear radiation are alpha, beta, and gamma radiation. In alpha radiation, a nucleus becomes more stable by emitting two protons and two neutrons (a helium nucleus). There are three types of beta radiation: beta minus, beta plus and electron capture. In beta minus radiation, a neutron can transform itself into a proton, releasing an electron and an electron antineutrino in the process. In beta plus radiation, a proton can transform itself into a neutron, giving off a positron and an electron antineutrino. In electron capture, a proton in the nucleus captures an electron of the atom, transforming itself into a neutron and releasing an electron neutrino in the process. Gamma radiation refers to the emission of gamma-ray photons by nuclei in excited states, in order for them to become de-excited.
What is Alpha Radiation
In alpha radiation, an unstable nucleus emits an alpha particle, or a helium nucleus (that is, 2 protons and 2 neutrons), to become a more stable nucleus. An alpha particle can be denoted as or .
For example, a polonium-212 nucleus undergoes alpha decay to become a nucleus of lead-208:
When nuclear decays are written down in this form, the total number of nucleons on the left-hand side must be equal to the total number of nucleons on the right-hand side. Also, the total number of protons on the left-hand side must be equal to the total number of protons on the right-hand side. In the above equation, for example, 212 = 208 + 4 and 84 = 82 + 2.
The daughter nucleus produced by an alpha decay, therefore, has two protons and four nucleons less than the parent nucleus.
In general, for alpha decay, we can write:
Alpha particles emitted during alpha decay have specific energies, which is determined by the difference in masses of the parent and daughter nuclei.
Write the equation for the alpha decay of americium-241.
Americium has an atomic number of 95. During the alpha decay, the americium nucleus would emit an alpha particle. The new nucleus produced (“the daughter nucleus”) would have two less protons and four less nucleons altogether. i.e. it should have an atomic number 93 and a nucleon number 237. The atomic number 93 refers to an atom of neptunium (Np). So, we write,
What is Beta Radiation
In beta radiation, a nucleus decays by emitting an electron or a positron (a positron is the antiparticle of the electron, having the same mass but the opposite charge). The nucleus does not contain electrons or positrons; so, first a proton or a neutron needs to transform, as we will see below. When an electron or a positron is released, in order to conserve lepton number, an electron neutrino or an electron antineutrino is also released. The energy of beta particles (which refers to either electrons or positrons) for a given decay could take a range of values, depending on how much of the energy released during the decay process has been given to the neutrino/antineutrino. Depending on the mechanism involved, there are three types of beta radiation: beta minus, beta plus and electron capture.
What is Beta Minus Radiation
A beta minus () particle is an electron. In beta minus decay, a neutron decays into a proton, an electron and an electron antineutrino:
The proton remains in the nucleus while the electron and the electron antineutrino are emitted. Beta minus process can be summarised as:
For example, gold-202 decays by beta minus emission:
What is Beta Plus Radiation
A beta plus () particle is a positron. In beta plus decay, a proton is transformed into a neutron, a positron and a neutrino:
The neutron remains in the nucleus while the positron and the electron neutrino are emitted. Beta minus process can be summarised as:
For example, a phosphorous-30 nucleus can undergo beta plus decay:
What is Electron Capture
In electron capture, a proton in the nucleus “captures” one of the atom’s electrons, giving a neutron and an electron neutrino:
The electron neutrino is emitted. The electron capture process can be summarised as:
For example, Nickel-59 shows beta plus decay as follows:
What is Gamma Radiation
After undergoing alpha or beta decay, the nucleus is often in an excited energy state. These nuclei then de-excite themselves by emitting a gamma photon and losing their excess energy. The number of protons and neutrons does not change during this process. Gamma radiation typically take the form:
where the asterik represents the nucleus in an excited state.
For example, cobalt-60 can decay into nickel-60 via beta decay. The nickel nucleus formed is in an excited state and emits a gamma-ray photon to become de-excited:
Photons emitted by gamma rays also have specific energies depending on the specific energy states of the nucleus.
Properties of Alpha Beta and Gamma Radiation
Comparatively, alpha particles have the highest mass and charge. They move slowly compared to beta and gamma particles as well. This means that as they travel through matter, they are able to strip electons off matter particles that they come in contact with much more readily. Consequently, they have the highest ionising power.
However, because they cause ionisations most easily, they also lose their energy the fastest. Typically, alpha particles can only travel through a couple of centimetres in air before they lose all their energy from ionising air particles. Alpha particles cannot penetrate through the human skin either, so they cannot cause any harm so long as they remain outside the body. If a radioactive material emitting alpha particles is ingested, however, that can cause a lot of damage because of their strong ability to cause ionisation.
Comparatively, beta particles (electrons/positrons) are lighter and can travel faster. They also have half the charge of an alpha particle. This means that their ionising power is less compared to alpha particles. In fact, beta particles can be stopped by a few millimetres of aluminium sheets.
Photons emitted from gamma radiation are uncharged and “massless”. As they pass through a material, they can give energy to electrons that make up the material and cause ionisations. However, their ionising power is much less compared to that of alpha and beta. On the other hand, this means that their ability to penetrate into materials is much greater. A block of lead several centimetres thick could reduce the intensity of the gamma radiation, but even that is not enough to completely stop the radiation.
The chart below compares some of the properties of alpha, beta and gamma radiaton
|Property||Alpha radiation||Beta radiation||Gamma radiation|
|Nature of particle||A helium nucleus||An electron/positron||A photon|
|Relative speed||Slow||Medium||Speed of light|
|Relative ionisation power||High||Medium||Low|
|Stopped by||Thick sheet of paper||Few mm of aluminium sheet||(to some extent) A couple of cm of a block of lead|
Particle Data Group. (2013). Physical Constants. Retrieved July 24, 2015, from Particle Data Group: http://pdg.lbl.gov/2014/reviews/rpp2014-rev-phys-constants.pdf