The main aim of the NEXT-DEMO and NEXT-DBDM prototypes built at IFIC and LBNL respectively, is demonstrating NEXT energy resolution at reasonably high energies (e.g, using positron annihilation 511 keV gamma rays, Cs-137 660 keV gamma rays and Co-60 1.1 MeV gamma rays).
As an alternative method of signal multiplication, the Micromegas (micromesh gas structure) technique is being investigated in the NEXT-MM prototypes. Although the decided baseline for the NEXT100 detector is an electroluminescent photosensor readout, the development of Micromegas is still motivated as a backup option or for eventual future extension to larger masses, due to the promising prospects for large areas offered by MPGDs.
The IFIC NEXT-DEMO chamber is a large prototype of the baseline design for NEXT presented in the LOI.
It is an asymmetric SOFT (Separated-Optimized Function for Tracking) TPC, with SiPMs (coated with a wavelength shifter) in the anode and an array of pressure resistant photomultipliers in the cathode. The detector is housed in a 60cm long cylindrical chamber and has an hexagonal fiducial region of 30cm drift length and 16cm diagonal length.
The scintillation light is observed by 19 Hamamatsu R7378A PMTs (capable of withstanding a pressure of 10 bar) which provide the energy measurement and the t0 of the event. An electric field of up to 40 kV drifts the ionization charge to the electroluminescence region. Such a region is defined at the end of the drift region by two grids separated by 5mm with 3.5 kV/cm bar potential drop and is viewed by 248 SiPMs (model S10362- 11-025C). The SiPMs are placed in Daughter Boards (DB) that hold 16 SiPMs each and coated by a wavelength shifting TPB. The DB, in turn, fit into a large Mother Board that channels the traffic of voltages and signals.
In order to improve light transport, the walls of the fiducial volume are made of polytetrafluoroethylene panels. A gas purification/re-circulation system has been designed and built ad hoc.
NEXT-DEMO started operations in November 2010 and plenty of data have been collected.
The LBNL chamber has been optimized to operate at the largest pressures considered for NEXT (20 bar). Its aim is to prove the energy resolution of NEXT, thus only an energy plane was constructed, using pressure-resistant PMTs but no tracking plane.
The gas system is fully operational and is routinely operated near its maximum rated pressure of 17 bar. Five conductive meshes establish the electric potentials in the TPC. The walls of the TPC are made of PTFE, bare, on the internal side to reflect UV photons and with conductive stripes (at intermediate potentials) on the outside to establish uniform electric field lines along the TPC main axis. The drift region is 8 cm long and the maximum HV on the dedicated feedthroughs is both +-20 kV. It then allows one to measure the energy resolution for drift fields from 0 to 4 kV/cm while maintaining a constant EL gain.
The system is operational at LBNL since December 2010 and has already proven an energy resolution of 1.8% FWHM in the Cs-137 peak. Once extrapolated to the Qββ of Xe-136, it converts into ~0.9% FWHM.
The work performed up to now has been focused in establishing the capability of microbulk readouts to work in high pressure Xe, and more specifically to measure their energy resolution in those conditions. For that task two prototypes have been built. The first one, known as NEXT-0-MM, is a stainless steel vessel of 2 litres, with a diameter of approx. 14cm and a drift region of 6cm, and it is devoted to measurements with small scale readouts, to study gain, operation point, and energy resolution with low energy gammas or alphas. The second prototype, NEXT-MM, has a drift of 35 cm and a readout area of 30 cm diameter. It is capable of fully confining a high energy electron track and will therefore probe the detection principle in realistic conditions.