The Borexino Counting Test Facility (CTF for short) was a scaled-down prototype of the Borexino detector. It consisted of 4.3 metric tons of liquid scintillator shielded by 1000 metric tons of high purity water contained in a large cylindrical steel tank.
One hundred photomultiplier tubes (PMTs) installed on a steel support structure – known as the “Open Structure” – was used to detect the photons emitted by the scintillator. The PMTs were equipped with mu-metal cones to shield them from Earth magnetic field and concentrators to increase light collection efficiency.
The scintillator – a mixture of an aromatic solvent (pseudocumene, “PC” for short) and 1.5 gr/liter of PPO as a fluor – was contained in a 0.5 mm thick nylon balloon.
The apparatus was coupled to a water ultra-purification facility to produce the low radioactivity water for the shield and to a scintillator purification system. The scintillator was purified via water extraction, distillation and gas stripping.
A 1.8 m2 muon detector positioned on the top of the water tank was used to test the muon identification capabilities of the CTF detection system.
Main goal
The CTF experimental activities started in 1994 (installation) and continued throughout ’95 (data taking) and ’96 (source calibration program). The results of the Counting Test Facility in terms of U, Th and 14C contamination (internal background) confirmed that the radiopurity goals of the experiment had been reached:
- Uranium chain contamination (more appropriately 226Ra) has been measured to be at most 3.5(∓1.3)*10-16
- Thorium contamination was found to be 4.4(∓1.5)*10-16
- Carbon-14 contamination was measured to be 1.94(∓0.09)*10-18 (14C/12C)
Those results indicated that the extreme radiopurity required by Borexino could be reached on the necessary large scale.
Later activities
After the achievement of its main goal the CTF was drained, renewed and improved. PMTs and inner vessel were replaced, the latter with a concentric two-vessel design similar to that adopted for the Borexino detector. This allowed to further reduce the contamination within the fiducial volume.
The renewed detector has been operated for several more years to conduct various other tests and measurements(¹). Among these there have been the test of a possible alternative liquid scintillator, PhenylXylylEthane (AKA “PXE” for short).
Having served all its purposes (and more), the CTF detector was eventually decommissioned after the start of Borexino data taking.
(¹) After the renewal and during the late activities it was sometimes referred to as CTF2 (or CTF-II) and CTF3 (or CTF-III).
Video
Here you can watch a nice time-lapse video of “CTF2” filling operation (May 22 – June 18, 2001), as viewed from the CTF internal CCTV camera.
For more details please refer to these article:
A large-scale low-background liquid scintillation detector: the Counting Test Facility at Gran Sasso
See also: /tag/ctf/