Methods

Alpha spectrometry

Alpha spectrometry is the most sensitive, but also most involved radiochemistry method. It requires sample dissolution and radiochemical separation to isolate the radioisotope of interest. Alpha emitting isotopes are then measured under vacuum by an alpha spectrometer.

Gamma spectrometry

This cost-effective method makes use of the characteristic energy signatures of emitted gamma radiation produced during radioactive decay to identify and quantify the radionuclides present in a sample. Gamma spectrometry has advantages in that it requires minimal sample preparation compared with other methods that require chemical separation. Additionally, high energy gamma rays have strong penetrating power, making it easier to analyze them since they can easily escape the sample and reach the detector. Gamma spectrometry can determine many radionuclides at the same time, but it is limited because it is not able to analyze radionuclides that emit only alpha or beta particles, such as stontium-90.

Gas flow proportional counting

Gas flow proportional counting measures ionizing radiation (alpha and beta particles) using a gaseous ionization detector. In this method a high voltage is applied across a volume of gas, creating an electric field between two electrodes. Ionizing radiation is detected when an alpha or beta particle is released from the prepared sample and collides with a particle of inert gas in the instrument chamber, ionizing it and producing a flow of current between electrodes. The electrical signal is measured to determine the amount of radiation present and can differentiate between alpha and beta particles. This method is used for general screening of samples (gross alpha and gross beta measurement) or for measurement of specific radionuclides if chemical separations are first performed.

Methods

Liquid scintillation counting

In liquid scintillation counting, the sample is prepared and dissolved in an organic solvent containing a scintillant, or “fluor” having luminescent properties. During radioactive decay, energy is released from a beta particle and transferred to an electron in the scintillant, exciting that electron. When the electron in the scintillant returns to its ground state it releases energy as a photon. This light energy is captured by a detector on the liquid scintillation counter instrument. Each photon emitted corresponds to one atomic disintegration. Radioactivity present in a sample is determined by counting the number of photons released per unit time. This method is best for single isotope determination, such as analysis of tritium or C-14.

Neutron Activation Analysis

Neutron Activation Analysis (NAA) is a sensitive, non-destructive nuclear method for measuring the elemental composition of a sample. During NAA, samples are bombarded with neutrons in a nuclear reactor and the elements present are identified and quantified by analysis of the characteristic gamma rays that are emitted during radioactive decay.

NAA is an acknowledged “referee method”, generally free of matrix effects or contamination from laboratory chemicals. It provides total analysis of the major and minor elements present in a sample. An advantage over other methods, such as ICP-MS is that minimal sample preparation is required; samples do not need to be digested and extracted, but only need to be encapsulated in a plastic vial for irradiation in the reactor.