Status of the Borexino detector and the CTF results

ELSEVIER

PROCEEDINGS SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 66 (1998) 346-349

Status of the Borexino detector and the CTF results S. Malvezzi ~* aIstituto Nazionale di Fisica Nucleare, Sezione di Milano, Via Celoria 16, 20133 Milano, Italy The Borexino experiment, aimed at measuring the flux of 7Be neutrinos from the Sun, is outlined and its importance is briefly discussed in the context of the present solar neutrino experimental scenario. The low energy of the 7Be neutrinos presents new challenges in low background counting which have never before been reached; a 5-ton prototype Counting Test Facility has been running for about two years in the Gran Sasso Laboratory to demonstrate the feasibility of the experiment; some of the most relevant results are reported. The background requirements and design strategy of the full-scale detector are discussed with the expected final experimental performance.

1. B O R E X I N O A N D 7Be N E U T R I N O S Refined analyses of the existing solar neutrino data have confirmed the long-standing deficit and provided a consistent picture in which the 0.86 MeV 7Be neutrino must be strongly suppressed; concurrently, helioseismology studies have confirmed the SSM validity favouring a scenario where the missing 7Be solar neutrino poses itself as a problem in the fundamental physics of neutrinos. Several theoretical solutions have been proposed to reconcile data and expectations; a viable and elegant explanation is the MSW. The small angle solution fits the data with good significance and provides an energy-dependent effect able to justify strong suppression of 7Be with less suppression of the higher energy SB neutrinos. Different three-neutrino mixing schemes have been recently suggested to accomodate solar, atmospheric and possible LSND oscillation; energy independent solutions and maximal mixing mechanisms are debated. From the experimental point of view the suppression of 7Be neutrinos is only inferred indirectly from existing experiments; the Borexino proposal is to make a direct measurement of the 7Be flux through the elastic scattering reaction v ÷ e --4 v' + e'

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in real time and in a large volume of liquid scintillator (300 ton) down to a few hundreds of keV. The monoenergetic 7Be v line at 862 keV gives rise to a Compton-like edge in the recoil spectrum at 665 keV. This signature puts a premium on observing the spectrum with the best resolution and lowest possible background. The low energy of the 7Be v presents new challenges in low background counting which have never before been reached; a 5-ton prototype liquid scintillator CTF (Counting Test Facility) has been built to study the feasibility of achieving milestones in radiopurity levels. Borexino is designed to be a "high statistics" experiment: in the defined fiducial volume (100 ton) a rate of about 50 events per day in the SSM is expected. Nevertheless, it is a difficult "singles" experiment, recoil electrons from v's cannot be distinguished from ~ decay of internal contaminants, 7-ray interactions are virtually indistinguishable from v signals, and high energy a's in the U and Th decay produce scintillation light in the energy range of interest due to quenching; all backgrounds are a concern. 2. D E S I G N

MAIN

STRATEGY

The stringent background requirements necessitate a very careful selection of materials employed. The construction philosophy adopted in Borexino is that of a graded shield of progressively lower radioactivity materials approaching

S. Malvezzi/Nuclear Physics B (Proc. Suppl.) 66 (1998) 346-349 Table 1 Delayed coincidence examples U chain Z14Bi .__,2~4Po Th chain 2t2Bi .__~2t2Po

the core of the detector, ending in the definition of a fiducial volume, with the outer shell of ultrapure scintillator working as a shield for the innermost active detection region. The crucial choice concerns the sensitive volume material: pseudocumene (PC) has been preferred since metal impurities (U, Th, K) exist as ions and are insoluble in non-polar organic solvents. An on-line purification system is foreseen to guarantee the long-term capability of maintaining the low background levels during the detector operating years. The background control will be attacked through various techniques in the analysis phase: such as Pulse Shape Discrimination (PSD) to suppress internal backgrounds from a's, tagging of delayed coincidences to recognize some segments of the U and Th chains (see Table 1) and consequent statistical subtraction of the parent isotope activity.

T h e C T F lesson and its impact on t h e B o r e x l n o design An important lesson from the CTF experience is that particular attention has to be paid to the cosmic ray background, mainly muons surving the Gran Sasso mountain. A rate of 1 p/ra 2. h is measured at the experimental site; muons passing through the scintillator create a big pulse-height signal and can be easily recognized; in contrast muons passing through the buffer liquids may produce (~erenkov light and can be energetically and spatially reconstructed in the region of interest for the neutrino signal. We have included in the Borexino design a muon veto detector which makes use of the light generated in the buffer water between the outer tank and the Stainless Steel Sphere (see Fig. 1). From the CTF we also learnt that there is a background source to be singled out: Radon, this is a very mobile noble gas. The Rn in the water buffer was measured to be 30 mBq/m 3 rather then the expected 1 mBq/m 3. A variety of possible sources has been considered: the Permatex liner of the water tank, phototubes,

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cables, dust and nitrogen blanket. The elevated radon concentration in the water buffer led to a steady-state flux from the water into the scintillator. Borexino will employ a Rn barrier to help avoid this problem. The previous considerations led to the design for Borexino illustrated in Fig. 1. Borexino Design

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3. T H E C T F R E S U L T S The CTF [5] has been a good laboratory to test many of the solutions that we will a~lopt for Borexino. Although smaller, only a 5-ton liquid scintillator, the CTF design philosophy is similar to that of Borexino. The detector has been running for almost two years in the Gran Sasso Hall C to demonstrate the fesibility of Borexino [6]. Encapsulated sources were used for position and

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S. Malvezzi/Nuclear Physics B (Proc. Suppl.) 66 (1998) 346-349

energy calibration measurements. With a light yield of ,,, 300 p.e./MeV, the detector as a spectrometer performed well with an energy resolution of 9% at 825 keV (214Po) and a spatial reconstruction of 12 cm at the same energy. More importantly the a / ~ discrimination was shown to be better than 95 % in rejecting a with a ~ inefficiency of a few percent. The main result of the CTF is the achievement of the following radiopurity milestones:

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The three-dimensional collection of light allowed several novel measurements of the scintillator properties, such as the absorption re-emission of light; 44 % of the total light is inelastically scattered and there is an increase of the scintillation time from 3.5 (in a small scale experiment) to ~ 5 ns. An on-line purification-system has also been tested. The chemical composition of the impurities is not known and the purification methods are chosen for their genera~ separation power; water extraction and distillation by phase separation, filtration and N2 stripping by size selection. In the CTF we took advantage of our reconstruction capability to distinguish the various contributions, ~ 3000 events/day, through their distinctive radial shapes. The internal activity shows a volume dependence, proportional to R 2, while the external activity decreases strongly (exponentially) from the walls of the vessel. This activity is due to 7's from daughters of the external contaminants such as Rrh which produce showers totally or partially contained within the scintillator. Two more contributions are expected: one from the surface due to the exposure of nylon to Rr~ during fabrication and one from Cerenkov events induced by #'s in the buffer water (800 ~ / d a y in the energy range 250-800 keV). Surface events are mainly reconstructed with an energy dependent gaussian error. Our raw data were fit through a combination of all these contributions; the result of 21 ± 47 events/day attributed to the internal activity after the purification is consistent with an expected rate of 30 ± 7 events due to a known residual contamination from Rn er-

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roneously introduced into the scintillator. Thus, the internal activity is consistent with zero within the CTF sensitivity. Before purification a rate of 470 ± 90 events/day was measured. A systematic error due to the radial parametrization functions and to non-gaussian tails of the surface events is estimated at ~ 30 events/day. 4. E X P E C T E D BOREXINO

PERFORMANCE

OF

The CTF experience has shown us that the basic concepts of Borexino are correct and we can proceed with the experiment. The expected rates for Borexino are shown in Table 2 and the neutrino signal is compared to the expected backgrounds in Fig. 3; estimates are based on the CTF values. The rectangular shape of the energy spectrum at 665 keV offers a unique signature for the 7Be neutrinos. The direct measurement of the ¢Be flux in Borexino would give confirmation of v oscillation and, combined with the existing experiments, places a tight indication on which scenario is favoured. From the simple rate mea-

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S Malvezzi/Nuclear Physics B (Proc. Suppl.) 66 (1998) 346-349

Table 2 Expected rate (events/day) in Borexino for different e- recoil energies and fiduciai volumes 250-800 keV 800-1300 keY i00 t 130 t 160 t I00 t 130 t SSM 53 69 85 3.2 4.1 large MSW 23 30 36.5 1.2 1.5 small MSW 11 14.5 18 0.6 0.8

160 t 5.1 1.9 1.0

checked. In addition to the solar signature, the fact that the Earth-Sun distance varies during the year allows a wide range of vacuum oscillationsolutions to be probed. The sensitivity in Borexino is unique due to the fact that the neutrinos are monoenergetic, have low energy and the statis:"I"C.I :~''--, ~., ,.. ¢ 8 tics is relevant. The first real-time measurement i i%i_i ......... of low-energy solar neutrinos might even reveal evidence for some entirely new and unexpected physics. Borexino could be sensitive to the much lower intensity pep line at 1.442 MeV, but this will depend on the ultimate energy resolution and z ~eV) the background levels in the detector; the rate expected in the 800-1300 keV region is shown in Table 2. Further information on the energy deFigure 3. Neutrino signal (SSM), internal and pendence of v suppression would allow a tighter external backgrounds estimated in Borexino. constraint on possible neutrino oscillation solutions [1,2]. A CTF upgrade will be ready soon and Borexino operation is foreseen by the end of 1999. surement, Borexino can distinguish between the small and large mixing angle solutions [3,4]while REFERENCES a measurement of the rate and spectral shape at energies less that 1.5 M e V can empirically iden1. J.N. Bahcall and P.I. Krastev, Phys. Rev. D53 tify whether it is the C N O or pp cycle that is (1996) 4211. responsible for most of the energy production in 2. E. Calabresu, G. Fiorentini and M. Lissia, Asthe Sun [3].A distinctive signature of solar origin troparticle Physics 5 (1996) 205. lies in the the annual variation of the measured 3. J.N. Bahcall, M. Fukugita, and P.I. Krastev rate. Detection of this annual modulation (i/r ~) Phys. Lett. B374 (1996) 1. is an important goal in the physics program. The 4. E. Calabresu, N. Ferrari, G. Fiorentini and magnitude is -~ 7% (peak-to-peak). Given the M. Lissia, Astroparticle Physics 4 (1995) 159. signal-to-background estimates and assuming the 5. G. Alimonti et aL, A large scale low SSM, this effect will have greater than 5 cr signifibackground liquid scintillator detector: the cance after three years of data taking. In order to Counting Test Facility at Gran Sasso, subdemonstrate the annum variation in the signal, it mitted to Nucl. Instr. and Methods. will be necessary to mantain the experiment un6. Ultr~low background measurements in a der very stable operating conditions. The size and large volume underground detector, the the shape of the fiduciaivolume will be controlled Borexino collaboration, in preparation. and monitored. Calibrations of the detector response and background constancy will be often /

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