Nuclear techniques for tracing the provenance of artefacts

In addition to dating, tracing the provenance can help with authenticating objects. This consists of providing contextual and circumstantial evidence for the object’s original production by establishing the chronology of the ownership, custody or location of a historical object. It refers to a complete record of ownership from a piece’s creation or its discovery to the present. This is usually a matter of documentation; however, different techniques can support and verify provenance investigations.

Figure 1 – Leonardo da Vinci's Mona Lisa (between 1503 and 1506).

The provenance of an object can be assessed by analysing the elemental signature: by determining the presence or absence, or relative abundance of the elements in samples it is possible to identify their geographical or manufacturing provenance. The so-called fingerprint of the elements in a given object depends on the location where the object was produced, and also on the technological process used in its manufacture.
Nuclear techniques can help with that. Keeping in

mind the importance of preserving as much as possible of the object, neutrons are ideal probes for non-destructive tests! Since neutrons have no charge, they can enter deep into the bulk of a material. They interact with the nuclei, ignoring the chemical form of the sample. When a neutron is absorbed into the atomic nucleus a nuclear reaction called neutron capture occurs. The atomic nucleus enters an excited state: it becomes radioactive. Then, the radionuclide decays emitting characteristic (prompt or delayed) gamma photons. By measuring the energies and intensities of this characteristic gamma radiation the elemental isotopic composition of a sample can be determined. The energy of the gamma photons is useful for qualitative analysis while the activity for quantitative analysis.

Figure 2 – Neutron capture by a target, followed by the emission of gamma rays.

The technique based on this principle is called Neutron Activation Analysis (NAA). It was discovered by Hevesy and Levi in 1936 and it has been recognized as the method of choice for archaeological provenance investigations since the 1970s.

Figure 3 – NAA at MLZ/TUM: Irradiation channels in the reactor pool of the FRM II, credits: Bernhard Ludewig.

How is it performed? The sample is irradiated with neutrons: they interact with elements through the

neutron capture reaction (n, γ) leading to the production of radioactive isotopes. This is the activation of the sample. In this case the emitted gamma rays are defined as prompt.

${}_{Z}^{A}X+{}_{0}^{1}n \rightarrow {}_{\ \ \ Z}^{A+1}X+\gamma_{Prompt}$

These isotopes, with an extra neutron will typically decay by emitting negatively charged beta particles. The beta spectrum is continuous, making the measurement difficult. However, beta decay is frequently followed by the emission of gamma photons of discrete energy which are characteristic of a specific isotope. These gamma rays are defined as delayed.

${}_{\ \ \ Z}^{A+1}X \rightarrow {}_{Z+1}^{A+1}Y^{\ast}+e^{-}+\bar{\nu}_{e}$

${}_{Z+1}^{A+1}Y^{\ast} \rightarrow {}_{Z+1}^{A+1}Y+\gamma_{Delayed}$

Conventional NAA is based on the measurement of delayed gamma rays, and so it is performed some time after irradiation. Indeed, analyses are often performed over days, weeks or even months: this allows short-lived radionuclides to decay effectively eliminating interference and thus improving sensitivity for long-lived radionuclides. It is applicable to most elements that form artificial radioisotopes under neutron irradiation. One of the advantages of NAA is that it is possible to detect many elements simultaneously, both light and heavy, with very small sample sizes (between 1 and 200 mg). The technique enables us to analyse all of the bulk of the sample. The sensitivity is determined by the number of radioactive nuclei formed: it is between 0.001- 0.1 ppm.
Generally, relative measurements are performed: the samples are irradiated at the same time with a standard sample of composition similar to the expected one. Then the measurements are performed under the same conditions as well. With the information collected by performing NAA a database of archaeological materials has been developed over time and thus is currently used to support investigations of artefacts of unknown provenance.

NAA can exploit neutrons of different energies (Table 1), on which the neutron capture probability depends. For thermal or sub-thermal neutrons with typical energies of milli-electron-volts the process is called thermal neutron capture. When the energies are higher (up to 0.4 eV for epithermal neutrons) the process is called resonance neutron capture. Thermal neutrons from nuclear reactors are the most commonly applied in inducing (n, γ) reactions within NAA.

Table 1 – Neutron energy range names [Kónya, 2012].

In addition, it is also possible to detect prompt gamma rays emitted by neutron capture reactions. In this case the technique is called prompt gamma neutron activation analysis or PGNAA. It is performed during neutron irradiation with the disadvantages of dealing with a high background, and it is in general an expensive method. PGNAA detects the prompt gamma photons formed in the (n, γ) nuclear reaction (within 10-14 seconds after neutron capture). It is generally applied to

elements which decay too rapidly to be measured by conventional NAA. It is also used for elements with low gamma ray intensities or that produce only stable isotopes and elements with extremely high neutron capture cross-sections.

Figure 4 – PGNAA at MLZ/TUM: Neutron guide hall with PGNAA, credits: Bernhard Ludewig.

PGNAA is particularly sensitive for light isotopes (low Z) such as hydrogen and boron, for which NAA cannot be used. It is used for cadmium, mercury and rare earth elements. Some bulk elements such as carbon, nitrogen and sulphur can also be quantified.
Several types of NAA have been developed over time for specific applications. For example, in some cases to eliminate interference NAA can be supplemented with radiochemical separation (RNAA), however this becomes destructive analysis.
X-Ray Fluorescence Analysis, or XRF, is another classical activation method used to analyse the elemental signature. The sample is irradiated with high energy X-rays, generated by a controlled X-ray tube. An incident X-ray of sufficient energy causes the photoelectric effect: an electron from the inner shells, K or L, is emitted and the atom enters an excited state. It regains stability when

an outer electron fills the vacancy, leading to the emission of characteristic X-ray photons. The fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source is measured.

Figure 5 – Atomic model for the X-Ray Fluorescence Analysis method.

Each of the elements present in a sample produces a set of characteristic fluorescent X-rays ("a fingerprint") that is unique for that specific element and allows its discrimination. XRF is useful for elements of atomic number higher than 20. Indeed, the X-rays of elements with Z from 10 to 19 are absorbed in air and measurements have to be performed under vacuum.
Thanks to the peculiar features of nuclear activation techniques, they result in a very versatile tool for tracing provenance of several different kinds of artefacts. They have become one of the reference techniques in cultural heritage, complementary to classical analytical tools.