Ionic liquids as solvents for Čerenkov counting and the effect of a wavelength shifter

MIRENDA, M.; RODRIGUES, D.; FERREYRA, C.; ARENILLAS, P.; SARMIENTO, G.P.; KRIMER, N.; JAPAS, M.L.

Buenos Aires

Conferencia; 21st INTERNATIONAL CONFERENCE ON RADIONUCLIDE METROLOGY AND ITS APPLICATIONS; 2017

Ionic liquids(ILs) are organic salts that have melting points near to room temperature,taken 100ºC as an arbitrary upper limit [1]. The organic constituent ions givethem distinctive physicochemical properties that clearly differ fromtraditional organic solvents, most of them toxic, volatile and flammable. Theirhigh thermal stability, low flammability and moderate conductivity are some ofthe remarkable features that justify its nascent application, e.g. in nucleartechnology [2].We have been alreadyshown that ILs are suitable solvents for liquid scintillation counting (LSC)[4]. The specific activity of a 18F solution (emitting positrons upto 634 keV) has been standardized by means of the TDCR- Čerenkov method [3],using 1-Butyl-3-methylimidazolium Cloride (BmimCl). This IL (with a refractionindex of ~1.5) allowed a detection efficiency of 7 %. In the presentwork we have synthesized a novel IL, the 1-Butyl-3-methylimidazolium 8-hydroxypyrene-1,3,6-trisulfonate(BmimHPTS), with the aim to enhance the amount of detected Čerenkov-light. The strategywas to use the 8-hydroxypyrene-1,3,6-trisulfonate anion, most commonly referredas pyranine, as a wavelength shifter. This dye is suitable to re-absorb the Čerenkovradiation in the UV region (low photocathode responsivity) and to re-emit it asfluorescence in the blue region (optimum photocathode responsivity). First, weperform an exhaustive photophysical characterization of different BmimHPTS/BmimClmixtures in order to find the optimal concentration of pyranine that gives thehighest fluorescence signal. The analysis of absorption and steady-state emissionspectra enables us to find the adequate balance between fluorescence intensityand self-absorption.Second, we usedthe TDCR system at the LMR-CNEA [5] in order to optimize the pyranineconcentration for differences in vial geometries. As a result, the detectionefficiency for 18F, using the TDCR?Čerenkov method, increases up to15 % for 1.5 × 10-4 M of BmimHPTS in BmimCl. This result shows nosignificant difference compared with the activity concentration of the same 18Fsolution using the classical TDCR method with a commercial scintillationcocktail. Third, weperform several control experiments to safely discard another kind ofluminescence that could eventually cause coincidences in the photomultipliers. Inparticular, no coincidence events above the background were observed formeasurements with 14C in the same experimental conditions. The fact that theactivity of high energy β particles emitters can be quantified in ILs mixtureby TDCR?Čerenkov technique makes them a promising cocktail for future applicationsin LSC measurements, especially for cases where high water solubility cocktailsare required. [1] Weingärtner, H. Angew. Chem. Int. Ed. 2008,47, 654.[2] Sun, X.; Luo, H.; Dai, S. Chem. Rev. 2012,112, 2100.[3] Kossert, K. Appl. Rad. Isot. 2010,68, 1116.[4] Mirenda, M.; Rodrigues, D.; Arenillas, P.;Gutkowski, K. Rad. Phys. Chem. 2014, 98, 98.[5]Arenillas, P.; Cassette, P. Appl. Rad. Isot. 2006,64, 1500.