Performances of the CTF experiment in prospect of Borexino

Nuclear Physics B (Proc. Suppl.) 70 (1999) 377-381

Performances Gioacchino

of the CTF experiment

Ranucciaand

Borexino

PROCEEDiNGS SUPPLEMENTS

in prospect of Borexino

Collaborationb

aIstituto

Nazionale di Fisica Nucleare, Via Celoria 16, 20133 Milano, Italy

b G. Alimonti, G. Anghloher, C. Arpesella, M. Balata, G. Bellini, J. Benziger, S. Bonetti, A. Brigatti, L. Cadonati, F. P. Calaprice, G. Cecchet, M. Chen, N. Darnton, A. de Bari, M. Deutsch, R. Dossi, F. Elisei, F. von Feilitzsch, C. Galbiati, A. Garagiola, F. Gatti, M. G. Giammarchi, D. Giugni, T. Goldbrunner, A. Golubchikov, A. Goretti, T. Hagner, F. Hartmann, R. von Hentig, G. Heusser, A. Ianni, J. Jochum, M. Johnson, G. Korga, M. Laubenstein, F. Loeser, P. Lombardi, S. Magni, S. Malvezzi, I. Manno, G. Manuzio, F. Masetti, U. Mazzucato, E. Meroni, M. Neff, S. Nisi, A. Nostro, L. Oberauer, A. Perotti, P. Raghavan, R.S. Raghavan, G. Ranucci, E. Resconi, M. Ruscitti, F. Sacchetti, R. Scardaoni, S. Schijnert, 0. Smirnov, A. Sotnikov, R. Tartaglia, G. Testera, B. Vogelaar, S. Vitale, M. Wojcik, 0. Zaimidoroga The crucial issue of the direct determination of the 7Be solar neutrino flux will be addressed by the Borexino experiment, a real time, massive calorimetric liquid scintillation detector to be installed at the Gran Sass0 Laboratory. The challenge of the extremely low radioactive level allowed in the scintillator for the feasibility of the measurement required the precise and sensitive evaluation of its radiopurity with a dedicated 5 tons prototype, the Counting Test Facility. The main experimental features of the CTF and its unprecedented results in the area of the low activity measurements are described in this paper, together with their implications in the design of the full scale experiment.

1. INTRODUCTION The well known solar neutrino problem established over the last 30 years by the measurements of the solar neutrino experiments operated so far has gained special attention as a possible clue tow?:.4 scenarios of non standard neutrinos, in which possible neutrino mass/mixing properties could give rise to oscillation effects among different neutrino flavors. In this arena the Borexino project [l] is devoted to the development and construction of a massive, calorimetric, liquid scintillation detector, to be installed in the underground Gran Sass0 Laboratory, focused to the detection of the low energy neutrinos (0.862 MeV) produced in the ‘Be electron cap ture reaction in the Sun. Indeed, the most recent analysis of the experimental data show that the precise and direct determination of these so called ‘Be neutrinos can play an important role to distinguish among the various proposed explanation for the solar neutrino puzzle. 0920-5632/98/$19.00 0 1998 Elsevier Science B.V. PI1 SO920-5632(98)00457-5

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The major technological challenge represented by the construction of a massive detector sensitive to signals in the sub-MeV region is the achievement of ultra-low natural radioactive levels in the detection medium, i.e. the liquid scintillator. Hence, in order to test in a realistic way the radiopurity of the scintillator, a prototype, called Counting Test Facility (CTF), has been constructed and operated in the Gran Sass0 Laboratory. Th. II’ ofitained after two years of operation, have established the feasibility of the experiment, showing that the scintillator can be produced and purified to the extremely low radioactive level (lO-‘sg/g of Uranium and Therium) needed for the successful accomplishment of the 7Be neutrino measurement. 2. IMPLICATIONS SUREMENT

OF THE 7Be MEA-

The copious recent analysis of the data of the existing solar neutrino experiments have pointed

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out the paradox of the missing ?Be neutrino, not corresponding to an equivalent deficit of the *B neutrinos as it would be required by the intrinsic logic of the nuclear reaction pp chain fueling the Sun. The exciting scenario that such a situation depicts is that of non standard effects at work, like resonant conversion in the matter (MSW), or vacuum oscillation, as the only possibility for a meaningful interpretation of the data. On the other hand, astrophysical solutions invoked in the past to explain the long standing *B neutrino deficit are essentially ruled out. Hence, the solar neutrino problem has evolved from a mainly astrophysical issue to a particle physic matter, representing a possible way towards the identification of physics beyond the standard model. In this scenario the role of the 7Be measurement that Borexino is expected to carry out has been thoroughly discussed in [2] and [3], where it has been clearly stressed how the precise count rate determination of the 7Be flux can allow to disentangle between the large and small mixing angle solutions in the frame of the MSW effect, originating also strong constraints to the neutrino parameters in the Am’, sin2(8) plane. Furthermore, it has been also demonstrated how the 7Be determination can identify the occurrence of vacuum oscillations through the striking effect of large seasonal variations, enhanced by the monochromatic feature of the 7Be line. It should be pointed out that, in the absence of any other time dependent effect, the 7% annual modulation of the flux due to the yearly variation of the Earth-Sun distance is nevertheless expected, representing a powerful tag to demonstrate the solar origin of the detected signal. 3. BOREXINO Borexino is essentially based on 300 tons of liquid scintillator acting as detection medium, contained in a nylon transparent vessel of 8.5 m of diameter, observed by more than 2000 photomultiplier tubes located on a stainless steel sphere of 13.7 m of diameter. The whole detector, as sketched in figure 1, is enclosed in a cylindrical tank of 18 meter of diameter and 17 m of maxi-

Figure 1. Schematic view of the Borexino detector.

mum height. The scintillator is Pseudocumene, an aromatic compound in which a small content of PPO, a high scintillation efficiency solute, is dissolved. The region between the nylon vessel and the stainless steel sphere is filled with PC alone, in such a way to avoid the buoyancy force on the vessel itself. On the other hand, the zone between the sphere and the external tank is filled with high purified water. The purpose of the two buffer fluids (water and PC) is to suppress neutrons and gammas coming from the rock. The neutrino detection will occur through the scattering off the electrons of the scintillator, signailed by the light produced by the scattered electron. The expected rates for Borexino are shown in table 1, for the Standard Solar Model prediction and for the small and large MSW mixing angle solutions. The true technological challenge of the experiment is the ultra low radioactivity level required for all the construction materials, particularly for the scintillator, which must be characterized by a radiopurity never reached before. Being this the key of the experiment, it is obvious that most of

G. Alimontiet al. /Nuclear Physics B (Pmt. Suppl.) 70 (1999) 377-381

Table 1 Expected rate (events/day)

SSM large MSW small MSW

100 t 53 23 11

in Borexino for different e- recoil energies and fiducial volumes 800-1300 keV 250-800 keV 100 t 130 t 130 t 160 t 3.2 4.1 69 85 1.2 1.5 30 36.5 0.6 0.8 14.5 18

379

160 t 5.1 1.9 1.0

through the construction of the Counting Test Facility [4]. 4. MAIN CHARACTERISTICS OF THE COUNTING TEST FACILITY

Figure 2. Sketch of the CTF apparatus.

the research and development activity for Borexino has been devoted to demonstrate the possibility to reach in the scintillator a purity at the extremely low level of lo-“g/g of U and Th, and lo-l8 for the ratio 14C/12C, needed to ensure the feasibility of the measurements. Particularly, various laboratory tests have been carried out on numerous samples, of size from 1 to 100 liters, which have demonstrated that, through suitable preparation and handling procedures of the material, radiopurity level of the order of 10-15g/g can be attained, with clear indication that this limit is mainly due to recontamination of the liquid from leaching of impurities on the drums surfaces. The next step of the investigation has been the check of the purity in a condition as close as possible to that of Borexino of a sample of at least some tons of scintillator,

Figure 2 shows the main features of the detector. The core of the apparatus is constituted by a sphere, made out of a thin ultrapure nylon membrane, containing about 5 tons of liquid scintillator. The phototubes are 100, each equipped with a light concentrator to increase the collection efficiency of the scintillation photons; they are located on a spherical supporting structure constructed with stainless steel. The detector is placed inside a cylindrical tank, 10 m of height and 11 m of diameter, filled with purified water, which shields neutrons and gammas from the rocks. All the materials have been carefully selected to assure low radioctive contamination, while during the construction special working procedures have been adopted to minimize the risk of dust contamination, performing the final cleaning and mounting of the detector in a high class clean room environment. Of special importance has been the purification system, intended to bring the scintillator to the incredible low radioactive level which represented the main goal for which CTF was targeted. The design of this system has been based on several preliminary laboratory tests. Since the chemical composition of the background impurities in the scintillator are not known, the purification methods studied were chosen for their general separator powers. These were: distillation (by vapor pressure), water extraction (by solubility), nitrogen stripping (by vapor pressure), and filtration (by size). The full scale purification plant installed at

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G. Alimontiet al. /Nuclear Physics B (Pmt. SuppI.) 70 (1999) 377-381

Table 2 Delayed coincidences measured in CTF 214Bi -b214 PO U chain ZlZBi ~212 po Th chain

Gran Sasso incorporates vacuum distillation, counter-current water extraction, gas-phase nitrogen stripping, and filtration. It has been designed to process 50 L scintillator/hour in the water extraction mode, and 20 L scintillator/hour in the distillation mode. The data analysis crucially relied on the features of the electronics, which has been designed to acquire for each phototube the charge and the time of firing with respect to the occurrence of the trigger. The first information is directly related with the energy of the event, while the latter is exploited to estimate its spatial location. Additional features are the acquisition and storage of the waveshape of the overall pulse, obtained through the analog sum of the outputs of all the PMTB, the pulse shape discrimination capability, used to distinguish between a! and p induced events, and the measurements of the elapsing time between two susbsequent triggers. It should be added that in the period in which the electronics is busy in the acquisition of a signal, auxiliary channels are armed to be ready to acquire an event occurring during the dead time. This capability has been added to the system to make it possible the detection of special correlated events, originated by the decay of some short lived radioactive nuclei in the 238U and 232Th secular chains, whose identification allows the determination of the activity of the progenitors. These important decay sequences are shown in table 2. The water used to shield the detector against the neutron and gamma flux coming from the rock must be very pure, due to its large mass and its proximity to the scintillator. The maximum radioactive level which can be tolerated in the water is 10-13g/g for U and Th, and 10W1’for natural K. To achieve and maintain such a level a system has been built constituted by two parts, one for the initial production and the other for the

t$/2) = 164~~s +1/2) = 0.3/.bus

on line recirculation. The former performs on the raw water the operations of reverse osmosis, ultrafiltration and continuous deionization. In the latter it is included a ionic exchanger, a stripping tower, to remove radon, and further filtration units. The levels of 238U and 232Th measured in the shielding water ranged around 2. lo-14g/g. 5. RESULTS The CTF results [5] addressed positively the main questions for which it was constructed, concerning the 14C the 238U and 232Th levels. The good quality of the data was ensured by the performances of the detector which, with 300 photoelectrons per MeV, an energy resolution of 9% at 751 keV (214Po line) and a resolution of 12 cm for spatial reconstruction at the same energy, performed in agreement with the expectations. Also the a//3 discrimination hardware proved to be very effective, with an a identification efficiency of - 97% and a fi misidentification of N 2.5% percent. Specifically the radiopurity levels evaluated in CTF are: 238u = 3.5 f 1.3 - lo-‘6g/g

(I)

232Th = 4.42;:; - 10-16g/g

(2)

i4C/12C = 1.85 f 0.13 f 0.01. lo-18g/g

(3)

Furthermore, several measurements have been carried out to unravel the main feature of the light propagation in a large scintillation detector. Of special interest has been the evaluation of the absorption re-emission process, which resulted to affect around 44% of the detected light, with a corresponding increase of the scintillation time from 3.5 to - 5 ns. The space reconstruction capability, due to the three dimensional read-out of the detector, has been largely exploited to identify other possible backgrounds. In particular, a large contribution

G. Alimonti et al. /Nuclear Physics B (Pnx. Suppl.) 70 (1999) 377-381

of external background has been identified, produced by 7’s originating in the decay of the 222Rn dissolved in the shielding water at a concentration of about 30 mBq/m3. Additionally, before the operation of the scintillator purification equipment it was detected also the presence in the scintillator of internal activities at a rate of 470 f 90 events/day. After the operation of the purification, the residual internal activity has been measured at the level of 21 f 47 events/day, fully consistent with the expected rate of 30 f 7 events due to a known Rn contamination. Hence, the purification brought the internal contamination to a level consistent with zero within the CTF sensitivity. Some other observations made in CTF played an important role in the definition of the deThe first concerns the sign of the detector. need to suppress with high efficiency the muon events. Indeed the muons, featuring the rate of l/m2h at the underground laboratory, while passing through the buffer liquid can create pulses of Cerenkov light mimicking the signals due to the neutrino interaction. Such an effect has been clearly detected in CTF, requiring the development of special pattern recognition algorithms. On the basis of this finding, in Borexino it has been included a high efficiency muon veto, which will tag each muon event. The second implication for Borexino coming from CTF stems from the understanding of the crucial importance of a tight shielding against Radon. Such a concept has been included in the detector through the adoption of a Radon barrier (shown in figure l), which will stop the Radon diffusion toward the core of the detector, where the maximum radiopurity must be attained.

6. CONCLUSION The impressive results of CTF, demonstrating the capability to achieve unprecedented level of radiopurity in the liquid scintillator, open the way to the construction of Borexino, in the perspective of a reliable measurement of 7Be flux to be started within the year 2000.

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REFERENCES 1. Borexino at Gran Sasso - Proposal for a real time detector for low energy solar neutrinos, eds G. Bellini, M, Campanella and D. Giugni, Dept. of Physics of the Universitity of Milano, INFN - Milano, via Celoria 16 - 20133 Milano, Italy; R.S. Raghavan, AT&T Bell Laboratories, Murray Hill, NJ, USA. 2. E. Calabresu, N. Ferrari, G. Fiorentini and M. Lissia, Astroparticle Physics 4 (1995) 159. 3. E. Calabresu, G. Fiorentini and M. Lissia, Astroparticle Physics 5 (1996) 205. 4. G. Alimonti et al., A large scale low background liquid scintillator detector: the Counting Test Facility at Gran Sasso, to be published on Nucl. Instr. and Meth. A. 5. G. Alimonti et al., Ultra-low background measurements in a large volume underground detector, to be published on Astroparticle Physics.