Thermoluminescence Dating Laboratory Improvements Tested on an Archaeological Rescue Site in Trino, Vercelli Province, Italy
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2019 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage Florence, Italy, December 4-6, 2019 Thermoluminescence dating laboratory improvements tested on an archaeological rescue site in Trino, Vercelli province, Italy. Laura GUIDORZI1,2, Fulvio FANTINO3, Elisabetta DURISI1,2, Marco FERRERO1,2, Alessandro RE1,2, Luisa VIGORELLI1, Lorenzo VISCA1,2, Monica GULMINI4, Giovanni DUGHERA2, Giuseppe GIRAUDO2, Debora ANGELICI3, Elisa PANERO5, Alessandro LO GIUDICE1,2 1 Dipartimento di Fisica, Università di Torino, Via Pietro Giuria 1, 10125 Torino, Italy 2 INFN Sezione di Torino, Via Pietro Giuria 1, 10125 Torino, Italy 3 TecnArt S.r.l., Via Modena 58, 10153 Torino, Italy 4 Dipartimento di Chimica, Università di Torino, Via Pietro Giuria 5, 10125 Torino, Italy 5 Ministero per i Beni e le Attività Culturali, Musei Reali di Torino, Piazzetta Reale 1, 10122 Torino, Italy Abstract – Thermoluminescence (TL) is a reliable based techniques, such as ionoluminescence [3-5] and X- radiation-based technique for the dating and ray luminescence [6], always pursuing the development authentication of ceramic objects, allowing the of customised instrumentation that could better meet the evaluation of the time elapsed since their last exposure needs of the archaeological and artistic samples. Since to high temperatures (e.g. firing in kiln or later fire 2007, a thermoluminescence (TL) dating laboratory has events). The TL laboratory developed in the last been developed at the Physics Department, in decade at the Physics Department of the University of collaboration with the National Institute of Nuclear Torino, currently operating within the INFN (National Physics (INFN). In the first years of activity it provided Institute of Nuclear Physics) CHNet network, is an excellent support to the archaeological surveys in the presented. The 10-years-long experience in the field nearby Piedmont area [7,8]. The laboratory has been resulted in the enhancement of the procedures, with progressively expanded with new instruments and the development of customised α and β irradiation methods that are presented in this work. systems and the optimisation of sampling approach The employment of the TL phenomena for dating and chemical pre-treatment. In collaboration with purposes was introduced by Aitken and his group in the TecnArt S.r.l., the improved procedures were 60s of the last century [9]. The technique allows to obtain employed for dating two structures from a rescue the age of a clay object by measuring the radiation dose archaeological site in the Vercelli province (Italy). absorbed since its last firing in some of its compositional minerals (e.g. quartz and feldspars); this is due to natural Keywords: Thermoluminescence, dating, customised occurring radioactivity and it is proportional to the instrumentation, rescue archaeological site luminous signal emitted by the material when heated at a few hundred degrees Celsius in laboratory. This means that not only the absorbed dose (paleodose) is to be I. INTRODUCTION known, but also the dose rate at which the artefact has The dating process of an artefact or structure can be of been subjected in time (annual dose), that depends from uttermost importance for the deep understanding of the conservation conditions and varies with the ancient civilizations and their development. Given this geographical area. The age can be consequently need of a certain collocation in the temporal line, the calculated by the simple ratio between the two quantities. absolute techniques have in time completely A lot of measurements are therefore required for a complemented the archaeological relative approach [1,2]. complete dating: first of all, the evaluation of the Luminescence is a very suitable parameter for paleodose by TL signal acquisitions, obtained via characterizing cultural heritage objects, being directly increasing artificial irradiation of samples using α and β proportional to some intrinsic characteristics of the sources. The data needed for the calculation of the annual material. Therefore, out-of-context objects can also be dose are then collected by the measure of the artefact α investigated. The team at University of Torino activity (caused by decay chain events of Uranium and extensively applies in this field various luminescence- Thorium atoms contained in the clay matrix), the 199 2019 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage Florence, Italy, December 4-6, 2019 quantification of the environmental dose rate and the measure of 40K isotope concentration in the material. In addition, further corrections for water absorption, supralinearity and anomalous fading are performed. Thermoluminescence is an invasive technique, and needs the sampling of about 1-2 grams of powdered material for the preparation of samples by the fine grain method [9]. Clay material from archaeological objects is often contaminated by organic matter and carbonates (the latter enhanced by long time-spans spent in highly-humid burial grounds). This requires a chemical pre-treatment of the powder before the fine grain, to eliminate these sources of spurious luminescence. In the following sections the equipment and procedures adopted are presented; the results of a recent case study Fig. 1. On the left: α irradiation system with the view for a complete dating will then be illustrated. of inner sample holder wheel. On the right: β irradiation system and its internal structure scheme. II. THE TL LABORATORY AT THE UNIVERSITY OF TORINO A steel chamber contains in the upper part a 241Am For the setup of the TL laboratory in the Physics source, with a 4 MBq activity and an active surface of 16 Department of the University of Torino it was decided cm2. The α source is shielded on the top and sides; the not to use an all-in-one TL analyser, to be able to adapt active surface, facing down towards the sample, is the specific components (TL reader, irradiation systems, protected by a 2 μm Au-Pd layer. In addition, a steel and etc.) to the arising needs of analysis and samples. lead shield is situated between the source and the sample The main instrumentations currently used in the holding wheel, with an automated shutter that opens only laboratory are described in the next sections. All the during measurements. Since the penetration range of α equipment is operated under red light, as well as the particle in air is just a few centimetres, vacuum is needed sampling procedures. If necessary, sampling is followed to reach the samples. With an optimal pressure of 170 by a chemical pre-treatment of the powder, that is mbar the particles get to the sample with a mean energy outlined in section G. of about 2.9 MeV, providing a dose of 2.0±0.1 mGy/s. A. TL measurement apparatus C. Beta irradiation system The instrumental setup for thermoluminescence For the determination of the paleodose, β irradiations measurement consists in a sample holding chamber, a are performed with a ⁹⁰Sr/⁹⁰Y source with nominal 9235QA Electron Tubes photomultiplier powered at - activity of 1.48 GBq and a dose rate of 13.2±0.1 mGy/s. 1100 V by an HiVo voltage supply and the IPSES- The irradiation apparatus has been developed by INFN. TL2000 control unit, regulated by its dedicated software. The source is installed in a brass box irradiating Between the sample and the phototube two filters are downwards and facing the sample holder; this is a wheel inserted: a KG5 filter that cuts the IR radiation and a with four slots that is accessible through the external BG25 filter that selects only photons in the blue range. brass case by a hole with screw top. Additional shielding The sample is put in the chamber on a heating metal is provided by lead plates contained in a plexiglass case. stripe; a thermocouple allows to monitor the temperature The whole system is fully automated and four irradiations and to control the heating rate (kept constant at usually 5- in a row can be set up without reopening the case. Also, a 10 °C/s). During measurements, vacuum is generated in quartz window with 8 mm thickness works as a shutter the chamber by a rotative pump and nitrogen is inserted between the source and the sample wheel when no up to a pressure of 0.4-0.6 bar, to avoid spurious sample has to be irradiated. luminescence and to obtain a homogeneous heating for The high repeatability of irradiations was tested and the sample. The glow curves, as the TL signals are confirmed on (LiF:Mg,Ti) TLD-100 dosimeters with a commonly called, are created registering the TL signal 0.5 Gy dose. every 1 second between 50 and 450 °C. D. Alpha activity measurements B. Alpha irradiation system Uranium and Thorium atoms contained in the clay A customised α irradiation system (fig. 1) was matrix of the object decay with α particle emission and designed, realized and optimised to evaluate the α contribute to the annual dose. The total α activity can be particles efficiency for each analysed object. easily determined by covering a ZnS scintillator with a 200 2019 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage Florence, Italy, December 4-6, 2019 homogeneous layer of ceramic powder (at least 600 mg): obtained via ICP-OES analysis performed at the the scintillator emits a photon each time it is struck by the Chemistry Department of the University of Torino with a α particle and these photons are collected by a Perkin-Elmer Optima 2000 spectrometer. The resulting photomultiplier powered at -1100 V. The generated percentage by mass for K is then used to directly electric signal is transferred to an Amptek MCA 8000A calculate the contribution to the annual dose of his multichannel analyser and the real time spectrum of α radioisotope 40K, up to some multiplicative constants [9].