physics notes

Introduction:

Radioactivity is the spontaneous disintegration of atomic nuclei. The nucleus emits particles, � particles, or electromagnetic rays during this process.

Alpha () Decay:

Alpha decay occurs when the nucleus spontaneously ejects an particle. An particle is really 2 protons and 2 neutrons, or an He nucleus. So when an atom undergoes decay, its atomic number decreases by 2 and its atomic mass decreases by 4. particles do not penetrate much material, for they can be stopped by paper. An example of decay is the following: Pu239 U235 + particle (He-4 nucleus)

There is a difference in mass between the original nucleus and the sum of the mass of the particle and resulting nucleus. This lost mass is converted into energy using the formula E = mc²; the energy would equal the kinetic energy of the particle and the recoil energy of the resulting nucleus.

particles are usually mono-energetic, but they can have different energies, as in the case of 226 Ra. This isotope of radium has a small percentage of particles that don't have their full energy; instead the nucleus is left excited and emits gamma rays. Some of these rays will transfer energy to an orbital electron in the process internal conversion.

Beta(�)^- Decay:

There are two types of � decay; �⁺ and �^- 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 � particle is released, and it has the same mass and charge as an electron. The resulting atom and the � particle have a total mass which is less than the mass of the original atom, and one would think that the � particles should have the energy equivalent to the mass lost (E = mc²). But � particles aren't mono-energetic, and have a broad energy spectrum from zero to the maximum energy predicted. So the � 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 �^- decay, the atomic number of the atom increases by 1.

�⁺ Decay:

When there is an excess of protons in the nucleus, and it is not energetically possible to emit an particle, �⁺ decay occurs. This is where the nucleus becomes stable by converting a proton into a neutron. During �⁺ 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.

Electron Capture:

Sometimes it is not energetically feasible to convert a proton into a neutron by emitting a positron (�⁺ 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.

Gamma Radiation:

Gamma ray emission usually occurs with and � emission. Gamma rays have no charge or mass, so their emission doesn't change the chemical composition of the atom. Instead, it results in a loss of radiant energy. Gamma ray emission occurs because the nucleus is often unstable after and � decay. There are cases where pure gamma emission occurs, and this is where an isotope exists in two forms (nuclear isomers). They have the same atomic and mass numbers, but have different nuclear-energy content. So gamma emission occurs when the isomer goes from a higher to a lower energy form. The isotope protactinium-234 exists in two different energy states, and it emits gamma rays when undergoing transition to the lower-energy state.