Development of the time-resolved ion beam luminescence technique and its application to the provenance studies of lapis lazuli

(1) INFN Sezione di Firenze, Via Sansone 1, 50019 Sesto Fiorentino (Fi), Italy
(2) INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy

A setup for time-resolved ion beam induced luminescence (TRIBIL) has been developed at the pulsed beam facility of the 3 MV Tandetron accelerator of the LABEC laboratory. Measurements are performed exploiting the pulsed beamline DEFEL [1], consisting of two electrostatic deflectors operating together to produce a pulsed beam from a continuous one. By controlling the beam current it is possible to obtain a variable and finely controlled number of particles per bunch. The new TRIBIL setup, here described, is based on a photomultiplier tube (PMT) detecting in vacuum the light emitted by the target. Optical filters in front of the PMT window allows for wavelength discrimination. The signal from the PMT is sent to the VME sampling ADC (CAEN/V1724). The digitizer (250 Ms/s sampling rate) was programmed to start acquisition in coincidence with the signal from the DEFEL system, corresponding to the bunch creation. The acquisition time window can be adjusted in the μs-ms range; a user-definable trigger delay can be used to acquire longer signals. With luminescent targets, for each trigger signal we obtain a waveform whose initial part is acquired before the ions hit the target (baseline determination), while the rest of the signal correspond to the light burst induced by the beam bombardment and to its decay after the excitation is turned off.  Pushing forward a study started in 2007 [2, 3], TRIBIL has been applied to the provenance study of lapis lazuli, a semi-precious stone admired for its blue color. Lapis lazuli has been used since the early civilizations as an ornamental stone and it can be found only in a few places (Afghanistan, Siberia, Chile). As shown in previous studies [3], the IonoLuminescence (IL) technique can help to discriminate materials of different origins, being the emitted light dependent on the nature and traces of the minerals composing the stone. We have thus proposed the use of TRIBIL to contribute to finding new markers of lapis lazuli provenance, since it can separate overlapping features not distinguishable through steady-state luminescence. Lapis lazuli of known certified provenances were studied. Since IBIL is dependent on dose and dose rate [4], we used these parameters to characterize the components of the luminescence decay.  Preliminary results show differences in the TRIBIL signals collected from samples of different provenances: it was possible to highlight that the Chilean lapis lazuli is characterized by decay times longer than those of all the other provenances.  IBIL allied with lifetime measurements can thus constitute a strong and innovative method to discriminate among diverse provenances.   

[1] F.A. Mirto and L. Carraresi, 2008, NIM B (266) 2113.
[2] E. Colombo et al., NIM B 266 (2008) 1527–1532
[3] A. Lo Giudice et al., 2009, Anal. Bioanal. Chem. (295) 2211
[4] P.D. Townsend and Y.Wang, 2013, Energy Procedia (41) 64