CTF and low background facility at Gran Sasso

A. Iannia, M. Laubensteina and Y. Suvorova

a INFN, Gran Sasso Laboratory, Assergi (AQ), Italy

The Counting Test Facility of Borexino (CTF) is a 4-ton unsegmented liquid scintillator detector located at the GranSasso underground Laboratory. CTF is equipped with 100 PMTs and an active muon veto system. The CTF at present is filled with PC (C9H12) inside a 500m thick nylon vessel. A secondnylon vessel is present outside the scintillator vessel to decrease Rn diffusion toward the core of the detector.1000 tons of High Purity Water produced at the Gran Sasso Laboratory shields the scintillator against neutrons from the surrounding rocks.

Spectrum of prompt events in the scintillator of CTF

Low background facility at the Gran Sasso Laboratory

CTF:4-ton liquid scintillator

A blank nitrogen layer is present inside the CTF tank and above the water shielding. The nitrogen is continuously flushed in and realised, in order to have a constant over-pressure of about 20mbar. Thisnitrogen layer is present to limit Rn diffusion into the CTF tank. In the Hall C of the Gran Sasso Laboratory, where the CTF is located, the Rn in the air is measured at the level of 50Bq/m3.With the aim to monitor the Rn in the nitrogen an electrostatic chamber(30kVolts) of 0.42m3 has been installed on top of the water tank. A dedicated set-up samples nitrogen from the CTF blank, perform a deumidification and pushes the gas inside the chamber. Here an alpha detector is at work to measure the decays of ’s in the Rn chain and determine the activity. This set-up can reach a sensitivity of the order of 0.5mBq/m3. The Rn in the CTF blank is measured at the level of 15mBq/m3. This system can be used to monitor Rn in the air during important installation works and has been used during the inflation of the Borexino vessels.The 210Po peak seen in the detector spectrum is a build-up effect after more than 3 years of operations.

204 days of livetime

The screening of water, the active muon-veto and the Gran Sasso depth allow a zero-bacgkground spectrum above 3 MeV. The main source of background above1 MeV is shown to be Rn from the shielding water. The activity of Rn in the shieldingWater has been measured at the level of a few mBq/m3. Rn emanation and diffusionThrough the nylon vessel is measured at the level of 10Bq/m3.

Whole 4-ton mass

Radial cut at 70cm for Reconstructed events

Radial cut at 60cm for Reconstructed events

1 MeV

Rn monitoring set-up for the CTF210Po

214Po218Po

PresentPresent: : on one floor in a service tunnelon one floor in a service tunnel

FutureFuture: : in one dedicated three-storey building in hall in one dedicated three-storey building in hall AA

At present the facility is equipped with 10 HpGe detectors (equipped with acrylic cover with continuous nitrogen flux for Rn shielding and low activity 210Pb, < 20 Bq/kg), 1 Si-PIPS detector and 1 ultra low background liquid scintillator counter. The main purpose of the facility is material selection for low counting experiments at Gran Sasso and background characterization. In the future the new facility aims to:•Radiocarbon and tritium measurements•Rn monitoring in air•Scientific collaboration with experiments

Ultra low liquid scintillator spectroscopy