Research activities at the Department of Nuclear Research.
Prompt gamma activation analysis (PGAA or PGNAA) is a nuclear analytical technique for non-destructive determination of elemental and isotopic compositions. The sample is irradiated in a neutron beam and the gamma-rays from the radiative capture are detected. All elements can be analyzed (except helium), without any prior information on the analyte. Contrary to the conventional neutron activation analysis (NAA), the irradiation and the detection is simultaneous. The energies and intensities of the peaks are independent of the chemical state of the material; hence the analytical result is free of chemical compositions. Both neutrons and gamma-rays are highly penetrating, therefore – in contrast to many instrumental elemental analysis techniques – the average composition of the entire illuminated volume is obtained. Every step of the measurement and the evaluation can be described with statistical methods, and the uncertainties of the concentrations can be readily estimated from one measurement.
Geological,
archaeological, environmental samples, artifacts, minerals, metals,
glasses, catalysts, ceramics are routinely analyzed, as well
as samples from the industry. Measurements for material
science, nuclear technology, nuclear astrophysics and nuclear
structure are also regularly carried out.
Neutrons for elemental analysis
Scheme of the radiative neutron capture
Whenever a nucleus captures a neutron, a compound nucleus is formed. The excitation energy is close to the binding energy, i.e. the kinetic energy of the neutron is negligable (in the meV range), when irradiating with slow neutrons. This excitation energy is between 6 and 10 MeV for about 80% of the stable nuclei. The decay of the compound nucleus takes place in about 10–16 s. The nucleus reaches its ground state, typically in 10–9 – 10–12 s, by emitting 2–4 gamma rays in a cascade. Gamma rays are called prompt, if their decay times following the capture, are much shorter than the resolving time of the detection system, which typically is in the range of 10 ns to 10 μs. Prompt gamma radiation is characteristic, i.e. the energy values of the gamma rays identify the nuclide, and their intensities are proportional to the number of the atoms. Most nuclides emit hundreds (sometimes several thousands) of different energy prompt gamma-rays. When the ground state reached after the de-excitation is not stable, radioactive decay radiation (typically beta-decays and electron capture followed by gamma-rays) with a given half-life will also be emitted.
The probability of a reaction is characterized by the cross section. The capture cross section is highly dependent on the neutron energy. For slow neutrons the most important energy dependence is the so-called 1/v law. This dependence continues until the first, so-called resonance in the eV – keV range. In PGAA, worth introducing the quantity "partial gamma-ray production cross section", which is the product of the isotope's natural abundance, the thermal neutron capture cross section for 2200 m/s-neutrons and the emission probability of the gamma-ray with the given energy. This is proportional to the count rate of the analytical peak.
The probability of a reaction is characterized by the cross section. The capture cross section is highly dependent on the neutron energy. For slow neutrons the most important energy dependence is the so-called 1/v law. This dependence continues until the first, so-called resonance in the eV – keV range. In PGAA, worth introducing the quantity "partial gamma-ray production cross section", which is the product of the isotope's natural abundance, the thermal neutron capture cross section for 2200 m/s-neutrons and the emission probability of the gamma-ray with the given energy. This is proportional to the count rate of the analytical peak.
Ongoing research projects and recent achievements
- Development of the PGAA method
- A comprehensive analytical library for elemental analysis with PGAA
- Partial gamma-ray production cross-sections of all stable elements (including non 1/v-elements, low cross-section elements)
- Partial gamma-ray production cross-sections of isotopes
- Partial gamma-ray production cross-sections of decay lines
- Quantitative analysis
- Methodological improvements
- Developments in gamma-ray spectroscopy (peak fitting, nonlinearity, efficiency, concentration calculations) see also Hypermet-PC.
- Instumentation, digital signal processing
- Chopped-beam PGAA
- Introducing new, energy calibration standards
- Neutron guide simulations
- Analytical applications of PGAA
- Archaeometry (flintstones, ceramics, metals, historical glasses)
- Geochemistry (volcanism in the Carpathian basin, comparative studies)
- Material science (metals for hydrogen-storage, thermoluminescent dosimeter materials, fullerenes)
- Industrial applications
- Inactive tracing for industrial technology developement
- Compositions of materials in sealed containers
- Impurities in various materials
- Application to the nuclear technology
- Total neutron capture cross-section measurements (for transmutation studies)
- Safeguards
- Determination of spontaneous fissioning materials (e.g. illicit trafficing)
- Detection of Uranium in lead and iron containers, even in presence of a masking radioactive source
- In-beam Mössbauer spectrometry with cold neutrons
- Greatly extends the number of Mössbauer-nuclides
- Preliminary experiments are in progress. In cooperation with the Department of Catalysis and Tracer Studies.
- Nuclear physics and detectors
- Nuclear structure and level scheme studies
- A comprehensive partial gamma-ray production cross section library for nuclear physics
- γ-γ coincidence measurements
- two-step cascade measurements
- Gamma strength functions
- Nuclear astrophysics
- Study of the response function for HPGe detectors