PGAI/NT
Research Nuclear Reactors like the Budapest Research Reactor (Hungary) and FRM-II (Germany) can provide a large flux of thermal neutrons. Neutrons in the thermal energy range (E < 1 eV) can be captured by various isotopes in the sample. Prompt Gamma Activation Analysis (PGAA) is based on the fact that, after neutron capture, many nuclei de-excite with a prompt emission of gamma-rays of characteristic energy. The gamma spectrum can be measured by high-resolution spectrometers, and the intensities of the emission peaks provide quantitative identification of the emitting nuclei. PGAA is a completely non-destructive technique , allowing for studies on cultural heritage objects without the need to take samples, or to remove patina from a metallic surface: the intact object is just placed in the neutron beam.
PGAA is to be considered a bulk method.
In order to obtain space-resolved information, it is necessary to scan the sample with a collimated neutron beam and gamma-ray detector: this method is called Prompt Gamma Activation Imaging (PGAI). In PGAI the information on elemental and isotopic composition is specific of every isovolume, i. e. the region of the crossing of the neutron and gamma-ray collimators lines-of-sights This region can be as small as few mm3, this value giving the limit of the available spatial resolution of the technique. Acquisition times are 200 – 3600 seconds per spectrum.
In order to optimise the possibilities offered by the PGAI, the best approach is to combine it with the Neutron Tomography. Neutron tomography (NT) is a well-established technique, whose importance for archaeology is widely known since many years. However, it has no element sensitivity.
ANCIENT CHARM has developed a method to combine the characteristics of both methods. By using the NT image of a sample, one can select the most interesting regions to be studied with PGAI. Moreover, the well-resolved images obtained with NT are a useful guide for the scanning and positioning of the object with the collimated neutron beam. This combination of the NT with PGAI speeds up considerably the effective scanning of the object.
NRCA and NRT
The neutron cross section of many elements show intense and narrow absorption peaks in the epithermal region (E up to 1 keV). These peaks (resonances) are specific to every isotope and constitute as a useful fingerprint of the sample composition. Neutron absorption is followed by the prompt emission of a gamma-ray cascade, and both the gamma emission and the neutron transmission can be measured.
Use can be made of the time-of-flight (TOF) technique available in pulsed neutron sources to measure the resonances energy. In the epithermal regime the neutrons are non-relativistic and their energy can be expressed by the classical relation
En = ½ mn (L/t)2
where mn = 1.675 x 10-27 kg is the neutron rest mass, L is the neutron flight path from the source to the sample and t the time to cover it. The TOF technique is in general better suited to spallation neutron sources like the ISIS source at the Rutherford Appleton Laboratory (UK) and the GELINA Facility (Belgium). TOF-based techniques do not produce energy but time spectra: the energy of the emitted gamma rays is not the relevant parameter; only the arrival time of the gamma photons on the detectors is tagged and recorded in a TOF spectrum.
ANCIENT CHARM conjugates the two approaches to the neutron resonances analysis: Neutron Resonance Transmission (NRT) and Neutron resonance Capture Imaging (NRCI).
NRT is a space resolved technique, allowing for 2D images of the sample. In fact, the use of a Position Sensitive Neutron Detector specially developed for ANCIENT CHARM allows for 25 mm x 25 mm element-sensitive radiographies of the sample, with a 2.5 mm resolution; a scan of the sample can be performed in order to obtain a 3D image of the whole investigated object with standard tomographic reconstruction techniques. Typical times for a complete NRT scan of objects of the dimensions of a fibula are of the order of 24 hours.
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Supported by the EU through FP5, FP6 and FP7