Gamma-ray spectrometry is an analytical technique which identifies and quantifies radionuclides by analyzing the energy spectrum produced by a gamma radiation spectrometer. It is an efficient and non-destructive analysis technique and employed in environmental monitoring, nuclear medicine, reactor corrosion monitoring, nuclear forensics, materials testing , mineralogy, and monitoring of industrial processes. cite:@ortec2014

The figure bellow shows a basic gamma spectrometry setup. The interaction between gamma rays originating from the sample and the detector material produces electric charges that are captured and converted into a spectrum.

With regard to the analysis of radionuclides in environmental samples, gamma spectrometry is one of the most widely used analytical methods (NO_ITEM_DATA:ziegler2002). If necessary, the analysis can be done in a non-destructive way (that is, free from physico-chemical preprocessing that structurally modifies the sample) and allows qualitative or quantitative assessments regarding both natural and anthropic radionuclides. cite:@lannunziata2003 According to (NO_ITEM_DATA:buchtela2014), this technique is a viable choice in environmental analyzes if:

• radiochemical separation procedures are undesirable due to their high cost and complexity and;
• if the elements to be analyzed emit gamma radiation in abundance appropriate for the technique and within the energy limits of the detector.

Basic principles

Gamma-ray spectrometry exists due to the interaction between gamma rays from the sample and special materials. There are two types of gamma detectors used for spectrometry: solid scintillators and semiconductors. cite:@knoll2010

The basic principle behind solid scintillators is using a material capable of emitting light under radiation. cite:@lannunziata2003. The scintillating material most commonly used in this type of instrument is the sodium iodide crystal doped with thallium or NaI (Tl). cite:@perez-andujar2004. The photons produced by the scintillator are directed to a photomultiplier tube. This tube is capable of producing electrons in the incidence of light on its surface. Photoelectrons, as they are called, are directed by a series of diodes through the application of high voltage. cite:@knoll2010 The dynodes are plates of a special material whose incidence of one electron produces a cascade of other electrons. These new electrons fall on the next dynodes, and so on. At the end of the process, the electrons produced are collected by an anode and interpreted by digital instruments.

Gamma-ray detectors that use semiconductors work by ionizing these materials under the incidence of gamma rays. cite:@lannunziata2003 Several crystals are used as semiconductors, high-purity germanium (HPGe) being the most used material. cite:@knoll2010 When gamma radiation from a sample is absorbed by these semiconductor crystals, an electron-gap pair is formed. These pairs are transported under an external electric field to the electrodes where a pulse is generated with amplitudes that are directly dependent on the type of interaction (photoelectric effect, Compton scattering or pair production) and the incident photon energy. Such electrical charges are amplified, digitized, filtered and interpreted by the computer software. cite:@lannunziata2003