R2D2 spherical TPC: first energy resolution results

Abstract

Spherical gaseous time projection chamber detectors, known also as spherical proportional counters, are widely used today for the search of rare phenomena such as weakly interacting massive particles. In principle such a detector exhibits a number of essential features for the search of neutrinoless double beta decay (ββ0ν). A ton scale experiment using a spherical gaseous time projection chamber could cover a region of parameter space relevant for the inverted mass hierarchy in just a few years of data taking. In this context, the first point to be addressed, and the major goal of the R2D2 R&D effort, is the energy resolution. The first results of the prototype, filled with argon at pressures varying from 0.2 to 1.1 bar, yielded an energy resolution as good as 1.1% FWHM for 5.3 MeV α tracks having ranges from 3 to 15 cm. This is a milestone that paves the way for further studies with xenon gas, and the possible use of this technology for ββ0ν searches.

Simultaneous scintillation light and charge readout of a pure argon filled Spherical Proportional Counter

“…This prototype was used to assess the energy resolution in Ar at different pressures and further details on the setup can be found in Ref. [6].…”

“…The width of the waveform is indeed proportional to the time between the first and the last drifted electrons which in turn depends on the radial projection of the track. To validate such a feature we compared the width of the avalanche light signal to the rise-time of the central anode signal which is known to provide information on the radial position of the energy deposits as demonstrated by the NEWS-G collaboration and previous R2D2 works [4,6]. The results shown in Fig.…”

“…Photons produced during the avalanche reach the sphere surface and could eject electrons which would be drifted to the central anode producing a second signal. A digital devonvolution of the charge preamplifier output waveform allowed to recover the charge time distribution which was masked by the RC effect of the charge integrator (see [6] for the method). The resulting deconvolved waveform exhibits a bump on the tail of the primary ionization signal, as seen in Fig.…”

“…The use of Spherical Proportional Counters (SPC) in direct dark matter searches has been going on for a decade within the NEWS-G collabo-ration [1,2,3,4]. More recently, the possibility to use such a technology to search for ββ0ν decay has been investigated [5] and an R&D programme is ongoing [6]. The aim is to tackle the various technical challenges to be faced for the construction of a large SPC, up to 1 m radius, filled with 136 Xe at 40 bar and simultaneously to demonstrate that energy resolution of 1% FWHM, and the recognition and radial localization of the two searched-for beta tracks are possible with this very simple setup.…”

“…So far, very encouraging results on the energy resolution (1.1% FWHM) have been obtained in Ar at 1 bar, and a coarse localization of the interactions within the detector seemed possible [6]. In parallel, the NEWS-G collaboration has demonstrated with SPC low-pressure operations that identification and analysis of events initiated by 2 distinct electrons was routinely reached [7,8].…”