Nuclear spectrometry

Nuclear spectrometry and U-Th clean laboratory

Clean U-Th chemistry laboratory

The equipment of the nuclear spectrometry laboratory comprises 4 alpha spectrometers with Si detectors (ORTEC^TM), and 2 gamma spectrometers with HPGe detectors (1 BEGe detector, and 1 Ge-well detector, both manufactured by CANBERRA^TM).

Alpha spectrometry is used to measure radionuclides emitting alpha particles. It requires acid-dissolution of solid samples and careful chemical separation of the nuclides of interest. This method allowed the early development of the so-called U-Th dating method, that requires the analysis of U (²³⁸U and ²³⁴U) and Th (²³⁰Th and ²³²Th) isotopes. It is also useful to analyse short-lived nuclides like ²¹⁰Po, sometimes as a substitute for ²¹⁰Pb. All the chemical separations take place in the clean U-Th room, also used when U and Th isotopes are measured through multi-collector mass spectrometry (MC-ICP-MS) with the instrument of the AETE-ISO platform of OSU OREME (french). This method significantly improves the precision on the calculated ages. Recent studies mainly concern U-Th dating of carbonates (coral, mollusk shells, speleothems…), as a tool for paleo-environmental reconstructions.

Gamma spectrometry has the advantage of allowing measurement of gamma-emitting radionuclides directly on powdered solid-samples or solid media used to concentrate radionuclides from water samples. A large number of nuclides of the natural ²³⁸U, ²³⁵U, and ²³²Th decay-series can be analysed by this method (²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra…), together with artificial radionuclides like ¹³⁷Cs or ²⁴¹Am. Applications of these nuclides are numerous, both as dating tools and as tracers of the kinetic of geochemical processes at various timescales from 1 y to several ky. Among them are the determination of sedimentation rates (²¹⁰Pb, ¹³⁷Cs), the ²³⁰Th-²²⁶Ra dating of recent volcanic rocks, the use of the four Ra isotopes as tracers of the origin and mixing of continental waters, the study of radionuclide mobility in phosphate deposits, …

Ge-well detector

Gamma-ray spectrum