H. Postma

Neutron resonance spectroscopy for the characterization of materials and objects

The resonance structure in neutron induced reaction cross sections can be used to determine the elemental compositions of materials or objects. The occurrence of resonances is the basis of neutron resonance capture analysis (NRCA) and neutron resonance transmission analysis (NRTA). NRCA and NRTA are fully non-destructive methods to determine the bulk elemental composition without the need of any sample preparation and resulting in a negligible residual activity. They have been applied to determine the elemental composition of archaeological objects and to characterize reference materials used for cross section measurements. For imaging applications a position sensitive neutron detector has been developed within the ANCIENT CHARM project. The detector is based on a 10 × 10 array of 6Li-glass scintillators mounted on a pitch of 2.5 mm, resulting in a 25 × 25 mm2 active area. The detector has been tested at the time-of-flight facility GELINA and used at the ISIS spallation source to study cultural heritage objects.

Neutron-Resonance Capture Analysis of Materials.

In this paper neutron resonance capture analysis (NRCA) is explored as a new method to analyse the elemental composition of materials and objects using a pulsed beam of epithermal neutrons and a time-of-flight system to recognize resonances of isotopes in the energy range from about 1 to 10 keV. Some test experiments have been carried out with bronze artefacts. Advantages, as compared to instrumental neutron activation analysis (INAA), are the low activation of the objects and the direct availability of results after the measurement.

Imaging of cultural heritage objects using neutron resonances

Neutron resonances are the signature signals of a non-destructive elemental and isotopic analysis technique in archaeological sciences. We report on Neutron Resonance Transmission Analysis and its capabilities as a bulk elemental imaging technique to test the homogeneity of samples and to localize elements of interest in archaeological samples and museum objects. A high neutron flux is required for imaging in order to achieve reasonable spatial resolution and to keep measurement times within realistic limits. A modular system for neutron resonance transmission analysis has been designed and installed at the INES beamline of the ISIS spallation neutron source as a part of the ANCIENT CHARM project. The main component is a neutron position sensitive transmission detector which is based on a 10 × 10 array of 6Li-glass crystals mounted on a pitch of 2.5 mm, resulting in a 25 × 25 mm2 active area. Transmission spectra are obtained by a measurement of the flight time of epithermal neutrons passing through an object. The transmission dips observed in a time-of-flight spectrum can be used to identify and quantify specific nuclides. In this paper the technique is described together with the data reduction and analysis procedures. In addition, preliminary results obtained from measurements on cultural heritage samples are discussed.

A New Position-Sensitive Transmission Detector for Epithermal Neutron Imaging

A new neutron resonant transmission (NRT) detector for epithermal neutron imaging has been designed and built for the ANCIENT CHARM project, which is developing a set of complementary neutron imaging methods for analysis of cultural heritage objects. One of the techniques being exploited is NRT with the aim of performing bulk elemental analysis. The 16-pixel prototype NRT detector consists of independent crystals of 2 × 2 mm pixel size, which allow for 2D position-sensitive transmission measurements with epithermal neutrons. First results obtained at the ISIS pulsed spallation neutron source are presented.

Quantitative neutron capture resonance analysis verified with instrumental neutron activation analysis

The newly developed elemental analysis technique Neutron Resonance Capture Analysis (NRCA) was verified by analyzing a prehistoric bronze arrowhead with both NRCA and Instrumental Activation Analysis (INAA). In NRCA, elements are identified through their neutron resonance capture energies as determined through detection of prompt capture gamma-rays as a function of time of flight. The quantification is obtained from the resonance peak areas. Corrections are required for neutron-energy-dependent dead time and self-shielding, the latter also depending on Doppler broadening. The analysis program REFIT, of which the intended use is the determination of the resonance parameters, was used to this end. The agreement observed between INAA and NRCA results indicates that the NRCA results obtained are accurate.

Neutron Resonance Spectroscopy for the Analysis of Materials and Objects

The presence of resonances in neutron induced reaction cross sections is the basis of the Neutron Resonance Capture (NRCA) and Transmission (NRTA) Analysis techniques. Since resonances can be observed at neutron energies which are specific for each nuclide, they can be used as fingerprints to identify and quantify elements in materials and objects. Both NRCA and NRTA are fully non‐destructive methods which determine the bulk elemental composition, do not require any sample preparation and result in a negligible residual activation. In this text we review the technique and present an analysis procedures including one based on a more methodological approach which relies on a full Resonance Shape Analysis (RSA) and accounts directly for the neutron self‐shielding, multiple scattering, Doppler broadening and instrumental resolution.

Some Applications of Neutron Resonance Capture Analysis

The probability for nuclei to capture neutrons reveals sharp peaks, so‐called “resonances,” which occur at neutron energies specific for each element. These resonances are very suitable for identifying and quantifying elements in objects and materials. They are the basis of an analytical method called “Neutron‐Resonance‐Capture‐Analysis” (NRCA). This is a fully non‐destructive method applicable to almost all stable isotopes, which determines the bulk elemental composition, and does not require any sample preparation and results in negligible residual activity. Up to now NRCA has been mostly applied for archaeological applications. In this paper we review the technique and discuss the applicability of the technique in the biomedical field and in material science.