Nd loaded liquid scintillator to search for 150Nd neutrinoless double beta decay

Abstract

The 150Nd is considered one of the most attractive candidate for searching neutrinoless double beta (0νββ-) decay, thanks to its high Q-value (3.367 MeV), that makes the external background issue less significative respect to other isotopes, and favorable computed matrix elements. The isotopic abundance of this isotope in natural neodimium is only 5.6% and up to now, it has been investigated only in low mass experiments. The next step is to increase the sensitivity of the experiments using larger mass of neodymium (10 ton-1 kton). This could be possible with a Nd loaded liquid scintillator (LS). At the Gran Sasso National Laboratory (LNGS), a joint INFN (Istituto Nazionale di Fisica Nucleare) and INR (Institute for Nuclear Research of Moscow) working group has been carrying out since 2001 an R&D activity aiming to develop organic liquid scintillators (LS) doped with metals. The achieved know-how and the satisfactory results obtained both with In and Gd allowed to face the development and production of Nd doped LS. The development of metal doped LS is challenging because the metal has to be embedded in a proper organic system that makes it soluble in an organic solvent minimizing the impact of the metal-organic compound on the optical and scintillation properties of the LS. A further challenge in the case of Nd is the presence of absorption bands of this element in the optical region with a transparent region around 400 nm, which is about at the maximum of the scintillator emission spectrum. A 2.5 1 Nd loaded LS has been produced diluting an originally developed Nd-Carboxylic (Nd-CBX) salt in pseudocumene (PC), the solvent of the Borexino liquid scintillator. The measured light yield, at [Nd] = 6.5 g/1 and [PPO] = 1.5 g/1, is ∼ 75% of pure PC at the same fluor concentration (~ 10000 ph/MeV). The Nd doped LS has been tested in a 2 1 quartz cell (wrapped by VM2000 reflector film) having dimensions 5x5x100 cm3. The light propagates in the cell by total internal reflection at the external quartz/air interface and is reflected by the VM2000 foil. The light is trasmitted at the windows of the cell where 3-inch PMTs (one for each end) are coupled. Two fillings have been performed: distilled PC (PPO-1.5 g/1) and Nd loaded LS (Nd-6.5 g/1, PPO-1.5 g/1, MSB-30 mg/1). In both cases, the cell is irradiated with a 137Cs source, scanning different positions along the cell axis. For each position the Compton Edge (CE) in the spectrum has been fitted. The optical attenuation length measured with this methodic is: 210±10 cm (only stat error, systematics not yet investigated). The measurements performed by ICP-MS of the radiopurity of the Nd2O3, that is the compound from which the Nd-CBX salt is synthesized, and of the content of optically active impurities show that a purification procedure is needed. Further investigations, at the ppt level, on the Th content in Nd2O3 is planned with the Borexino CTF.