Muons for non-destructive analysis of cultural heritage

1. Muons for non-destructive analysis of cultural heritage GianrossanoGiannini and Franco Zanini University of Trieste Elettra – Sincrotrone Trieste

2. Elemental analysis is one of the most fundamental and essential techniques for all research fields related to cultural heritage (CH). Most elemental analysis methods are highly developed and extremely sensitive and require small amounts of samples, but some of them are destructive. Non-destructive analytical methods are of great benefit to the CH scientist, and some useful methods that leave the sample unharmed have been developed.

3. PROVENENCE CONSERVATION AUTENTICITY TECHNOLOGY

4. For example, 3D micro X-ray fluorescence (XRF) analysis andneutron activation analysis have been established. However, an analytical method that is non-destructive, 3Dposition selective and can detect multiple elements including light ones for a bulk sample has remained elusive.

5. An alternative approach is the use of an intense muon beam to develop a non-destructive technique, which allows analysis deep inside the material with a good spatial resolution. Because of the large muon mass compared to the electron mass, the muonic X-rays have energies very suitable for standard 𝛾-ray spectroscopy, so every element is easily recognized.

6. The characteristic muonic X-rays have energies which are about 200 times larger than that of the characteristic X-rays generated byelectron beam analysis. Muonsalso have a high transmission ability and can penetrate much deeper into materials than protons utilized for example in PIXE.

7. Negative muons are comparable to heavy electrons; they replace an electron in the outer shell of an atom, then travel near to the nucleus through the modified energy states of the atom. Each transition on this path produces X-ray characteristic of the atom in which the muon was absorbed, hence allowing this spectrum to reflect the atomic species of the sample under investigation.

8. A significant advantage of muonic X-rays over those of electronic X-rays is their higher energy (2 keV -10 MeV). These high energy muonic X-rays are emitted from the bulk of the samples without the added complication of photon self-absorption and can be simply detected by a semiconductor detector. In addition, this technique will not activate the sample, unlike prompt gamma-ray analysis by neutron irradiation.

10. Ninomiya at al. constructed in 2010 a new X-ray measuring system in J-PARC muonfacility and performed muon irradiation for Tempo-koban (Japanese old coin) for test experiment of elemental analysis. Muonic X-rays originating from muon transition in muonic silver and gold atoms were identified. The contents of Tempo-koban (Au:61%) was determined by muonic X- ray intensities.

11. The authors were not able to determine the elemental contents of sample material quantitatively only from muonic X-ray intensity because of the molecular effects on the formation process of muonic atoms. MuonicX-ray intensities and muon atomic capture ratios depends on the muon-capturing molecule. Therefore, it is essential to know the relation between muonic X-ray intensities and material contents from muonic X-ray spectra for the standard samples.

13. For quantitative analysis, the authors performed muon irradiation on three standard Au−Ag alloys with 50, 60, and 80 wt % Au. The relationship between the X-ray intensity ratios of μAu(5- 4)/μAg(4-3) is shown in figure. The muonic X-ray intensity of μAu(5-4) increases linearly with the elemental composition of Au in the sample. A proportional relationship is established between the ratios of muonic X-ray intensity and elemental composition.

14. Depth-profiling elemental compositions of the Tempo-koban. The solid line corresponds to the Au composition from a range of calculations taken from the literature.Theopen circles show the analyzed values as determined by muonic X-ray measurements. The Au-rich layer near the surface was clearly identified. The concentration of Au at each depth was quantitatively determined without sample destruction.

17. Using negative muons as a tool for elemental analysis is possible at the ISIS pulsed neutron and muon facility. Indeed, element analysis on a layered sample has shown controlled depth dependence studies are possible and that measurements deep within the sample are feasible. Moreover, this technique is sensi- tive to all elements and, of course, completely non-destructive. The bronze standards have shown the sensitivity of the technique, while only using one detector.

21. The variety of radiations and particles offers nowadays numerous possibilities in imaging. The choice of the source depends on many parameters, in particular the nature of the object and its thickness. If several options exist below typically one meter, like X-rays or neutrons, deeper structures require very high energy and/or penetration capabilities which are often out of reach of artificial sources.

22. For such cases, a particularly penetrating probe is offered by cosmic muons naturally produced in the atmosphere. Thanks to their high energy and large mass, they can indeed cross up to several hundreds of meters of rocks before being absorbed. Their potential for imaging has been known for decades, but the recent progress on particle detection driven by high energy physics has opened the door to a vast range of applications, resulting in a growing interest for this technique.

24. CEA = gas detectors NE1 and NE2 = nuclear emulsion films KEK = scintillation hodoscopes

26. CHALLENGES DETECTION AREA The modest intensity of the cosmic muon flux obviously requires large size detectors to keep the acquisition time reasonable. For large structures (e.g. a volcano) the real limitation is actually not the total number of crossing muons but rather their spatial dispersion. ANGULAR RESOLUTION The most relevant parameter to characterize the accuracy of a muography instrument is the angular resolution. Obviously the resolution requirement is often in contradiction with the need for large area detector, as it implies a very large number of electronic channels.

27. CHALLENGES ACCEPTANCE In the case of big structures, the instrument should have a wide opening angle to probe a large fraction of the object. An interesting alternative is provided by instruments with a 2𝜋 solid angle which can thus image all the surroundings simultaneously. ROBUSTNESS For a wide class of in situ applications the instrument should be transported in harsh conditions, in the air and/or on trails. AUTONOMY AND ACCESS A typical transmission muography experiment routinely lasts several months. With limited, or even absent infrastructure nearby, the apparatus should run on its own.

28. Palazzo della Loggia – Brescia. Renaissance building by Antonio Palladio (1492 – 1574)

29. STABILITY Measurement uncertainty of the order of less than a mm can be achieved.

30. CONCLUSIONS Both cosmic ray radiography and tomography have been widely demonstrated. The advantage of disposing of a permanent, free source of highly penetrating radiation is obvious from a safety as well as from an economical point of view. The present detection technology opens the possibility to investigate large and bulky objects to unveil hidden structures and/or to characterize the contents of sealed human artifacts with a totally noninvasive procedure.

31. CONCLUSIONS Interpretation of data is another problem…