Radioactivity: alpha, beta and gamma radiation

​​In a nutshell

Radioactivity is where an atomic nuclei disintegrates due to instability, and ejects ionising radiation and particles from itself. There are three different kinds of radiation, called alpha, beta, and gamma radiation. In this lesson you will learn the difference between the three, how electric and magnetic fields affect them, and how to complete nuclear equations surrounding radioactivity.

Definitions

Types of radioactivity

Radioactive substances decay and eject ionising radiation and particles from themselves, which means that they have enough energy to tear electrons away from other substances, turning neutral atoms into ions. 

These types of radiation can be detected using a cloud chamber - a chamber filled with vapour-saturated air at a very low temperature. Ionising the air molecules leads to the vapour condensing onto the ions, leaving a visible trail along the line that the radiation travelled.

Alpha radiation

During alpha decay, the atom will eject an alpha particle. This alpha particle takes the form of a helium nucleus, which is a nucleus comprised of two protons and two neutrons. The two protons give it a charge of $$+2e$$, where $$e$$ is the elementary charge.

An alpha particle is emitted in a straight line, but it can be deflected when passing through an electric or magnetic field. 

Through an electric field, the alpha particle is deflected slightly towards the negatively-charged side, due to its positive electric charge. Through a magnetic field, you can use Fleming's left-hand rule to determine which way it deflects - if it were travelling left to right through a field that passes into your screen, then the particle would be deflected up.

Beta radiation

Beta radiation is caused by the weak nuclear force. Beta minus radiation is usually attributed to a nucleus having too many neutrons to be stable, so the weak force causes a neutron to convert into a proton, ejecting particles. Similarly, beta plus radiation is caused when a nucleus has too many protons to be stable, leading to the weak force causing a proton to convert into a neutron, ejecting other particles. The particles ejected depend on the type of beta decay in order to conserve charge.

Beta radiation comes in two forms - beta plus $$\beta^+$$and beta minus $$\beta^-$$. During beta plus decay, a proton within the radioactive nucleus will decay into a neutron, and eject a fast-moving positron and neutrino. The process is similar during beta minus decay, except in this case a neutron will decay into a proton and eject a fast-moving electron and antineutrino.

Beta particles are also emitted in a straight line and will be deflected in the presence of a magnetic or electric field. 

In an electric field, the positron emitted by beta plus decay will deflect in the direction of the negatively-charged side due to its positive charge, but it will be deflected much more than the alpha particle due to its lower mass. In a magnetic field, the same applies as the alpha particle, except once again, it will be deflected more than an alpha particle would.

The electron emitted by beta minus decay will experience the opposite effect. It will be deflected towards the positively-charged side due to its negative charge, and it will be deflected downwards in the example given above, instead of upwards.

Gamma radiation

Gamma radiation takes a different form than both alpha and beta. During gamma decay, only high-energy photons are emitted, with wavelengths less than $$10^{-13}\,m$$. They are emitted as a result of alpha or beta decay leaving a nucleus in an excited, unstable state. 

Photons have no charge and travel at the speed of light, and as such, they are completely unaffected when travelling through an electric or magnetic field.

Nuclear equations

Nuclear equations are important in tracking conserved quantities during radioactive decay. The two important quantities to track during decay are the nucleon number $$A$$ and the atomic number $$Z$$. The nucleon number refers to the amount of nucleons in a nucleus (protons and neutrons), while the atomic number refers to the number of protons in a nucleus.

Alpha decay

The general form of alpha decay is given by the following equation.

​$${A \atop Z}X \to {A-4 \atop Z-2}Y + {4 \atop 2}He$$​​

In this equation, $$X$$ denotes the parent nucleus and its nucleons, which is the nucleus that is undergoing radioactive decay, and $$Y$$ denotes the daughter nucleus and its nucleons, which is the nucleus that is left over. 

The alpha particle is written as $${4 \atop 2}He$$, as it is a helium nucleus that has 4 nucleons, 2 of which are protons. It can also be written as its own symbol, $${4\atop2}\alpha$$. Nucleon number and atomic number are both conserved, as their totals are equal on either side.

Example

What is the balanced nuclear equation for a uranium-238 nucleus undergoing alpha decay?

Uranium-238 has 92 protons, and 238 nucleons in total. Taking away two protons and two neutrons gives you a nucleus with 234 nucleons, 90 of which are protons. The element with 90 protons is known as thorium. Writing this out in the same form as above:

$${238 \atop 92}U \to {234 \atop90}Th + {4 \atop 2}He$$​​

Beta decay

The general form of beta decay is given by the following equations. Beta minus produces the electron, while beta plus produces the positron.

​$${A \atop Z}X \to {A \atop Z+1}Y+ {0\atop-1}e + \overline{\nu}_e$$​

$${A \atop Z}X \to {A \atop Z-1}Y + {0\atop+1}e + \nu_e$$​​

The electron is written as $${0\atop-1}e$$, showing its negative charge, while the positron is written with a positive charge as $${0\atop+1}e$$. Again, both nucleon and atomic number is conserved.

Example

What is the balanced nuclear equation for a potassium-37 nucleus undergoing beta plus decay?

Potassium-37 has 19 protons, and 37 nucleons in total. The total number of nucleons will not change, but due to the beta plus decay, one proton will be converted into a neutron, so the atomic number will decrease by one. The element with 18 protons is known as argon, so you can write out the nuclear equation in the form above:

$${37 \atop 19}K \to {37 \atop 18}Ar + {0\atop+1}e + \nu_e$$​​

Gamma decay

 Gamma photons are emitted after alpha or beta decay, when a nucleus still has too much energy. No nucleons are changed, so the nuclear equation can be written very simply, denoting the gamma photon as $$\gamma$$.​

​$${A \atop Z}X \to {A \atop Z}X + \gamma$$