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Neutron Activation Analysis


Neutron activation analysis allows the quantitative determination of elements in very different sample materials, some of which could only be decomposed with considerable effort (rocks, alloys), or whose destruction would be undesirable (archaeological artefacts and works of art).


In the so-called instrumental neutron activation analysis (INAA), the analysis is performed non-destructively on the sample, which is, however, radioactive for a certain time. As a multi-element method, INAA is used especially for trace analysis of inorganic materials or for certain elements in organic materials, whereby the element pattern is treated like a fingerprint. The most important areas of application are analyses of ceramics in order to classify them on the basis of material composition and to identify workshops. Rocks can also be examined in this way to determine their geological origin. This was and is particularly successful in determining the origin of obsidian, a volcanic glass that has always been sought after and widely negotiated as a valuable raw material for cutting tools.


Neutron Activation Analysis (NAA) is a very powerful nuclear physics method for the quantitative analysis of element concentrations of samples that are irradiated with neutrons for this purpose. The irradiation takes place in the TRIGA reactor of the Institute of Nuclear Chemistry at the University of Mainz. The interaction of the neutrons with the atoms in the sample material leads to nuclear reactions in which radioactive products are formed, which allows both the qualitative and, by comparison with a standard of known composition, the quantitative analysis of the elements in the samples. This is because the resulting radioactive nuclides decay with characteristic half-lives and emit radioactive radiation with equally characteristic energies. For the analysis, almost exclusively gamma radiation is used, which is measured with semiconductor detectors made of high-purity germanium. After the radioactivity has decayed (usually a few days or weeks) the sample material can be used for further investigations.

The Curt-Engelhorn-Zentrum Archäometrie in Mannheim has a laboratory for gamma spectrometry and a handling permit for radioactive materials. Two gamma detectors are currently in operation. After delivery of the irradiated samples, they are only unpacked and measured individually in special containers without further pre-treatment. In-house reference material (clay powder) is added to each irradiation, which in turn is linked to several international reference materials through multiple analyses. The analytical precision is better than 5% relative for most elements.

Fig. 1: HPGe Gamma spectrometer (Photo: CEZA)


The analysis of the light elements is not possible and some elements important for metal analysis, such as lead and bismuth, are also not recorded. For the analysis of metals, the applicability depends on the main constituent. If this is strongly activated, as is the case with gold and silver, analysis with instrumental NAA without chemical separation is not possible.

Sample properties

For ceramics and rock material, the sample size depends on the homogeneity. Approximately 200 mg of fine-grained material is sufficient, while a few grams of coarse-grained material are pulverized and homogenized. Typically, 100-200 mg are irradiated, for artifacts also much smaller sample quantities of a few milligrams are possible.