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Nuclear PhysicsB (Proc. Suppl.) 91 (2001) 58-65

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Borexino Gioacchino Ranucci a *, G. Alimonti, C. Arpesella, H.O. Beck, M. Balata, T. Beau, G. Bellini, J. Benziger, S. Bonetti, A. Brigatti, B. Caccianiga, L. Cadonati, F.P. Calaprice, G. Cecchet, M. Chen, A. De Bari, E. De Haas, O. Donghi, M. Deutsch, F. Elisei, A. Etenko, F. Von Feilitzsch, R. Fernholz, R. Ford, B. Freudiger, A. Garagiola, F. Gatti, S. Gazzana, M.G. Giammarchi, D. Giugni, A. Golubehikov, A. Goretti, C. Grieb, C. Hagner, T. Hagner, W. Hampel, E. Harding, F. X. Hartmann, R. von Hentig, H. Hess, G. Heusser, A. Ianni, P. Inzani, H. De Kerret, S. Kidner, J. Kiko, T. Kirsten, G. Korga, G. Korschinek, D. Kryn, V. Lagomarsino, M. Laubenstein, F. Loeser, P. Lombardi, S. Magni, S. Malvezzi, J. Maneira, I. Manno, G. Manuzio, F. Masetti, U. Mazzucato, E. Meroni, P. Musico, H. Neder, M. Neff, S. Nisi, L. Oberauer, M. Obolensky,, M. Pallavicini, L. Papp, L. Perasso, A. Pocar, R.S. Raghavan, G. Ranucci, W. Rau, A. Razeto, E. Resconi, T. Riedel, A. Sabelnikov, P. Saggese, C. Salvo, R. Scardaoni, S. Schnert, K. H. Schuubeck, H. Seidel, T. Shutt, A. Sonnenschein, O. Smirnov, A. Sotnikov, M. Skorokhvatov, S. Sukhotin, R. Tartaglia, G. Testera, R.B. Vogelaar, S. Vitale, M. Woijcik, O. Zaimidoroga, Y. Zakharov aIstituto Nazionale di Fisica Nucleare, via Celoria 16, 20133 Milano - Italy In the exciting arena of the solar neutrino research a new actor is expected to come soon in the game: Borexino. This massive, calorimetric, liquid scintillation detector, in advanced phase of installation in the underground Gran Sasso Laboratory, will be focused towards one of the fundamental issues of this field, i.e. the direct determination of the flux of the neutrinos produced in the 7Be electron capture reaction in the Sun. As a pilot program for the full detector, the Counting Test Facility operated for two years at Gran Sasso, provided the convincing evidence that the fundamental technological challenge of the experiment, the achievement in the scintillator of unprecedented radiopurity levels, can be accomplished successfully,thus opening the way to the realization of the experiment.

1. I N T R O D U C T I O N A long history of 30 years of measurements is at the basis of the well known solar neutrino problem, which from a pure astrophysics puzzle has now become a crucial particle physics issue, with strong hints for scenarios beyond the standard model, in which possible neutrino mass/mixing properties of massive neutrinos are very likely at the origin of oscillation effects among different neutrino flavors. In this framework, Borexino [1], since from the early stages of its proposal, has been conceived as a massive, calorimetric, liquid scintillation detector, to be installed in the underground Gran Sasso Laboratory, focused to the detection of the low energy monoenergetic neutrinos (0.862 MeV) produced in theTBe electron capture reaction in the Sun. The important role played by such a *Speaker on behalf of the Borexino Collaboration

measurement has been pointed out by various analysis of the data of the experiments operated so far, showing that the precise and direct determination of these so called 7Be neutrinos will be a major milestone in the attempt to clarify this long standing unexplained enigma. The construction of a massive detector sensitive to signals in the sub-MeV region is a true herculean task, because of the need to achieve ultralow natural radioactive levels in the detection medium, i.e. the liquid scintillator. With the purpose to demonstrate convincingly that the unprecedented purity levels required for the scintillator can be actually obtained, a 4 tons prototype, called Counting Test Facility (CTF), has been constructed and operated in the Gran Sasso Laboratory. Its results, obtained after two years of operation, have demonstrated that the scintillator can be produced and purified to the extremely low ra-

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G. Ranucci et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 58-65 dioactive level (10-169/g of Uranium and Thorium) needed for the successful accomplishment of the 7Be neutrino measurement, hence opening the way to the next step of the project, the installation of the complete detector, now in progress in the underground Laboratory.

2. T H E B O R E X I N O D E T E C T O R 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 2200 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 maximum height. The scintillator is Pseudocumene (PC), 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 pure PC, 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 ultrapure water. The purpose of the two buffer fluids (water and PC) is to suppress the flux of neutrons and gammas coming from the rock. An important element of the design is the radon barrier, located between the nylon scintillator vessel and the stainless steel sphere, whose purpose is to hinder the diffusion toward the vessel of the radon possibly produced by emanation from the elements in the outer part of the detector. The phototubes in the light detection system are grouped into three different classes. The first is that of the devices intended to perform the detection of the light coming from the vessel. They are 1800, equipped with a light concentrator which is designed in such a way to observe only photons coming from the vessel plus a shell of 20 cm thickness surrounding it. The second class is constituted by 400 devices with no cones, that in addition to the light from the vessel are able to detect efficiently the light produced by events in the buffer. The purpose

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of these tubes is that of survey for the light produced, mostly by Cerenkov effect, by the muons passing in the buffer, which can produce pulses suited to be confused with true neutrino signals. The third class of tubes is represented by 200 devices located on the outer surface of the stainless steel sphere. They can identify muons both traversing and not traversing the buffer. For the former, they allow a double tag to identify such events, while for the latter they can give an alert to search afterward for muon induced events, which can reach the vessel even if the precursor muon was only contained in the external water.

Borexino Design

1@m~

8 m x 8rn x l o o m and 4 m x 4 m ;~4c~n

Figure 1. Sketch of the Borexino detector.

3. T H E LIQUID S C I N T I L L A T O R The liquid scintillator is the key element for the success of the experiment. Its high luminosity and extremely high radiopurity are indeed the factors which make the experiment feasible. As already mentioned in the previous paragraph, the scintillator cocktail is realized with Pseudocumene as solvent, and PPO at the concentration

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G. Ranucci et aL /Nuclear Physics B (Proc. Suppl.) 91 (2001) 58-65

of 1.5 g/1 as solute. A thorough series of laboratory tests have been performed over the entire period of research and development to asses the intrinsic optical properties of the scintillator, as well as its radiopurity. The light yield resulted equal to about 11,000 photons/MeV, the attenuation length at 420 nm 30 m, the scattering length at the same wavelength 7 m, the fast component of the decay time 3.5 ns. The mixture resulted also characterized by a good a / ~ discrimination. To asses the radiopurity properties of the scintillator a broad investigation program has been pursued, whose fundamental results are briefly reminded in paragraph 12. 4. P H O T O T U B E S CENTRATORS

AND

LIGHT

CON-

The phototubes that will be used in the detector are the 8" 9351 manufactured by Electron Tubes Limited (former Thorn Emi ETL). They feature a limited transit time dispersion (1 ns), a pronounced peak to valley ratio (2.5), a reduced dark noise rate (1 kHz), a low afterpulsing probability (<3%). In addition, it must be pointed out that these devices have been manufactured with special low radioactivity glass and internal parts, to comply with the overall stringent radiopurity requirements of the experiment. The light cones have been designed following the prescription for non imaging devices intended to enhance light collection. Specifically, they are truncated string cones with a maximum length of 24 cm, realized with anodized aluminum, a material which features good reflectivity in the 400 nm region as well as good capability to withstand Pseudocumene. 5. E L E C T R O N I C S

The electronics starts data acquisition after the occurrence of a multiplicity trigger condition, i.e. upon detection of the firing of a software defined number of PMT's, ranging between 10 and 40, within a time window of 100 ns. For each triggered event the measured quantities are the photomultipliers signals pulse height, and

the photoelectron arrival times, with a time resolution of 0.3 ns. The former information is used to infer the original event energy, while the latter is exploited in order to estimate the event position. In addition, the absolute time of the event is measured and stored, for the cross check of possible astronomical events, like supernova neutrino burst, recorded by other detectors. 6. D E T E C T O R P E R F O R M A N C E S Given the properties listed above of the phototubes, of the scintillator and of the electronics, the resulting detector performances are: coverage: 30% photoelectron yield: 450 pe/MeV energy resolution (@1 MeV): 5% spatial resolution (@1 MeV): 10 cm It must be pointed out that energy and spatial resolution are essential features for the proper interpretation of the data. Indeed, spatial reconstruction is the fundamental tool to disentangle the external background, coming from the detector materials other than the scintillator, while the energy resolution is essential to identify the recoil spectrum of the scattered electrons from neutrinos, via its characteristic edge at about 660 keV. 7. C A L I B R A T I O N S A variety of calibration and monitoring systems are planned to assure a careful understanding of the detector operational conditions during the data taking. To ensure the proper reconstruction of the events, it is needed a precise time measurement of the PMT's signals. This requires a precise determination of the delay time along the whole pmt + electronics channel, which we plan to perform by illuminating each device with a fiber fed by a laser able to generate a fast pulse of the order of 50 ps. Since the tubes will be illuminated at the level of single photoelectrons, it will be also possible to monitor the occurrence of gain shifts of the devices by looking at the laser induced single photoelectron spectrum at the output of each of them.

G. Ranucci et al./Nuclear Physics B (Proc. SuppL) 91 (2001) 58-65

External thorium sources, located just inside the stainless steel sphere, at the level of the light concentrators entries, will be used to check the stability in time of the detector, an essential feature in order to test with high accuracy the time variation of the measured neutrino signal. Moreover, to study the global response of the detector it is foreseen to locate internal sources in various positions in the inner vessel, with the specific tasks to study the position reconstruction, to determine the energy calibration and to assess the a / ~ discrimination. In addition, a continuous check of the stability of the optical properties of the buffer and of the scintillator will be accomplished by lasers with different wavelength, that will induce photoexciration processes subsequently studied via proper analysis of the output signals of the phototubes. Furthermore, it must be mentioned that an intrinsic mean for calibration will be represented by some specific signals generated by radioimpurities in the scintillator. There are, indeed, special correlated sequences in the decay chains of uranium and thorium, that can be easily tagged, thus providing a way to internally set the energy scale and the position reconstruction capability. Finally, it has to be underlined that the collaboration is actively pursuing an overall detector test via a ~lCr neutrino source, whose purpose will be to demonstrate unambiguously that the detector is really able to detect neutrinos.

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column purification to restore its properties. Hence the purification plant comprises the equipments needed to accomplish these four different procedures. It must be pointed out that the water extraction can be also performed on line during data taking, should any recontamination of the scintillator occur within the operational life of the detector. Other two plants are the water purification system and the liquid handling. The former is targeted to the purification of the outside water, while the latter is the ensemble of connections, pumps and storage tanks for the preparation of the scintillator solution and its transfer into the vessel. The plants system is completed by four storage vessels, which will hold the 320m 3 of pseudocumene needed for the preparation of the scintillator for the inner vessel. The procurement of the inner vessel pseudocumene will be carried out as soon as possible, so to give enough time to the decay of the cosmogenic 7Be (half-life of about 50 days) produced by the cosmic ray in the interactions with the C atoms of the PC. Finally, it must be pointed out that a special building has been positioned just in front of the detector, where the data acquisition electronics will be located, together with the clean room for the cleaning of the components to be mounted within the detector itself.

8. P L A N T S

9. N E U T R I N O BOREXINO

DETECTION

IN

An important role in the Borexino operation is that of the numerous plants located near the detector. The purification system will accomplish the removal of the radio-impurities in the scintillator and the restoring of its optical quality. On the basis of the successful experience of CTF, the strategy currently envisaged for the scintillator is to preserve as much as possible the quality with which it is manufactured at the production plant during the transportation and storing phases, to test in CTF the quality just prior to filling, and then, if needed, to apply a combination of distillation, water extraction, nitrogen stripping and

In Borexino the neutrino detection will occur through the scattering off the electrons of the scintillator u + e- --~ u + e- signalled by the light produced by the scattered electron. The high radiopurity and high luminosity of the scintillator make it possible to achieve a detection threshold as low as 250 keV. This is the basic fact which allows the possibility to detect the monoenergetic 7Be neutrinos (in particular its component at 0.862 MeV), producing in the detector a continuum recoil spectrum, with a maximum energy of 0.66 MeV. According to the standard solar model, the expected event rate should be of 55 ev/day, for a

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G. Ranucci et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 58-65

fiducial volume of 100 tons. On the other hand, the background for the target purities of lO-lSg/g of U and Th and lO-14g/g of natural K, is of the order of 15 ev/day. In figure 2 the expected spectrum of the scattered electrons is reported, together with the prediction for the dominant, internal background. It must be outlined that these numbers apply to a threshold of 250 keV, whose feasibility is conditioned to the achievement of a ratio 14C/12C in the scintillator in the range of 10 -is.

3

2.5

2

1.5 - - :" L._.;-;..._ ;. !, j

0,5

0

0.4

06

08 Evemt Energy

1

1.2

t4

(MeV)

Figure 2. Monte-Carlo simulation of the v signal and background rates: dotted-dashed line = 7Bey events, dotted line - all different sources induced events, dashed line = background events, continuous line = sum of v and background events.

10. T H E SOLAR NEUTRINO PROBLEM AND THE IMPLICATIONS OF T H E 7Be M E A S U R E M E N T

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

out the paradox of the missing 7Be neutrino, not corresponding to an equivalent deficit of the SB 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 SB 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 2, sin20 plane. In particular, it must be mentioned that the expected count rate in the small mixing angle (SMA) region is equal to about 10 ev/day, while in the large mixing angle (LMA) is of the order of 30 ev/day. A similar rate of 30 ev/day is expected in the LOW region, where in addition a strong day/night effect is expected. Should it be detected by Borexino, it would be a spectacular smoking gun to constraint the mass/mixing parameters. A similar striking effect would happen in the vacuum oscillation (V0) regime, represented by a large seasonal variation, 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 variation along the year of the Earth-Sun distance is nevertheless expected, representing a powerful tag to demonstrate the solar origin of the detected signal.

G. Ranucci et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 58 65 11. O T H E R P H Y S I C S C A P A B I L I T I E S Besides the main focus of 7Be neutrinos detection, other interesting measurements are in the range of capability of Borexino. The first to be mentioned is the exploitation of the unique energy window from 1.5 to 5 MeV for the SB neutrinos measurement, which could allow the identification of MSW related spectral distortions, not accessible to the other higher threshold experiments. Furthermore, a broad program of antineutrino science is made it possible via the reaction V+p --+ n + e + characterized by a threshold of 1.8 MeV, followed, with a time constant of 200 #s, by the reaction n + p -+12 H + 7 signalled by the precise gamma energy of 2.2 MeV. By tagging such events via this delayed coincidence, in Borexino it will be possible to search for solar antineutrinos, for geophysical ve from the Earth, for Pe of supernova origin; it will be also possible to attempt a long baseline experiment exploiting the antineutrinos coming from the European reactors.

12. R A D I O P U R I T Y LATOR

OF THE SCINTIL-

Since from the beginning of the Borexino program it has been recognized that the feasibility of the 7Be measurement relied on the achievement of the fantastic purity level of 10-169/9 in terms of 232Th and 23Su in the liquid scintillator. Being these numbers unprecedented radiopurity limits, never reached and measured before, a broad investigation in the R&D phase has been devoted to unravel all the implications and the factors related to such a crucial topic. In particular, in the early stage of the development studies, various tests have been carried out to determine the radiopurity of samples of scintillator at the liter scale. The achieved limits were in the range of 2 + 3 • lO-15g/g, in terms both of Uranium and Thorium, with clear indications that these results were mainly limited by impurities leached from the walls of the drums containing the samples. The next step in the quest for the ultimate attain-

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able radiopurity was the design and construction of the Counting Test Facility [4], a pilot detector aimed at the direct measurement of the scintillator radiopurity at the ton scale level, targeted to a sensitivity of at least 5 • 10-169/9. Designed and constructed following concepts very similar to that of Borexino, with 100 phototubes surrounding an inner vessel of 1 m of diameter, all immersed in shielding water, the Counting Test Facility has represented the fundamental step in the evolution of the entire Borexino program. The CTF results [5] addressed positively the main questions for which it was constructed, concerning not only the 23Su and 232Th levels, but also the contamination of 14C. 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 825 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 95% and a/3 inefficiency of few percent. Specifically the radiopurity levels evaluated in CTF are: 23su ~ 3.5 + 1.3.10-16g/9 232Th <_4.4 + 1.5- lO-16g/g 14C/12C = 1.94 + 0.09.10 -is These never attained before purity levels not only have opened the perspective of the construction of the detector, but also represented a major, fundamental, breakthrough in the broad field of low activity science. As additional outcome, the CTF has also demonstrated the effectiveness of the planned purification methods for Borexino, whose design has been finalized and optimized according to the CTF experience. Recently, an important and valuable confirmation of these impressive results has been obtained in a total independent way by a newly developed neutron activation analysis method, which indicated for Uranium an upper limit of 2. lO-16g/g, perfectly consistent with the relevant CTF result.

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13. O T H E R B A C K G R O U N D

ISSUES

The achievement of the background conditions needed for the sub-Mev solar neutrino detection requires also to cope with other sources of radioactive contaminations that may hamper the success of the measurement. The external contribution to the background, i.e. that coming from the detector components, has been minimized with proper selection of low radioactive constructing materials. An issue that CTF has proved to be of paramount importance is the radon emanation process, that may act as the ultimate limit in the achievable background. To deal with it, in the design it has been adopted a suitable combination of low emanating materials coupled to the already mentioned radon barrier, i.e. an outer vessel surrounding the inner one, aimed to suppress the residual emanation flux. Furthermore, radioactive gases, mainly radon itself and krypton 85, if dissolved in the scintillator, can produce a substantial count rate. Purging with ultra pure Nitrogen, purified to the extreme limit of 0.5 # B q / m 3 of radon, has been demonstrated to be a powerful mean to eliminate both these activities. Another source of background are the cosmogenic radionuclides, that will be tagged with high efficiency by the double inner and outer muon veto system described before. Dust also is a major factor to be taken under control. For this purpose, all the constructions and installations of the critical items will be carried out in carefully controlled clean room environments, with final thorough rinsing to eliminate completely the residual particulates. Finally, surface contamination by any kind of contaminants, but particularly radon daughters, has to be properly addressed for any container that will hold pseudocumene. After many tests, a standard procedure contemplating pickling, passivation, electropolishing and final precision cleaning has been developed for all the surfaces exposed to PC.

14. STATUS OF T H E PREPARATION

EXPERIMENT

In the Summer 2000 Borexino is in an advanced construction phase, with many systems and subsystems entering the final path toward the installation in the Hall C of the Gran Sasso Laboratory. Here a summary of the status of the critical items is given. For the crucial scintillator components (PC and PPO) the manufacturers have been identified and the relevant contracts signed. The Nylon Vessels are at the final stage of prototyping, being the selection of the material near to be completed. The stainless steel sphere for PMT's support and buffer containment is now fully constructed, with the final surface treatments in progress. The manufacturing of the PMT's with low radioactive glass is at the 90% of the total number of devices. In addition, a special encapsulation method, both PC and water proof, has been developed and thoroughly tested, and it is now ready to be applied to all the devices. The water tank, acting as container of the whole detector, is also fully installed. An item at a very advanced stage of preparation is the electronics and DAQ: the hardware and the software are ready, with the final integration currently in progress. The dedicated hardware (PMT's and electronics) for the muon veto system has been completely designed, and the construction already started. The light concentrators have been fully designed and prototyped, and the mass production is almost completed. The many calibration equipments previously mentioned have been thoroughly designed and some of them also fully tested in a real operational environment, as for example the timing calibration system for the PMT's. The numerous plants which are essential to the Borexino operation are all under installation in the Hall C. Specifically, the storage vessels, which will hold the Pseudocumene for the scintillator, are already in position, as well as most of the purification systems. In addition, the liquid handling plant is being installed in these months,

G. Ranucci et al./Nuclear Physics B (Proc. Suppl.) 91 (2001) 58-65

with completion planned for the beginning of next year. The performances of all these equipments will be tested using the Counting Test Facility, which has been totally refurbished for this purpose. The last element to mention is the clean room in front of the detector, which is now being installed with the goal to have it completed in the fall, ready for the final installation phase of the inner components. 15. C O N C L U S I O N After the impressive results of CTF, demonstrating the capability to achieve unprecedented levels of radiopurity in the liquid scintillator, the design of Borexino has been completed and the construction started, with the goal to begin a reliable measurement of the 7 B e neutrino flux in the first years of the new millennium. Specifically, not only the design of all the experiment subsystems has been finalized, but also major installations in Hall C have been accomplished. Furthermore, the equipments not yet underground are in advanced preparation phase. The goal of the work in progress is to have Borexino ready for filling by the middle of 2001. It must be finally mentioned the important effort done by the Collaboration to put back into operation the Counting Test Facility, that will be used for the quality control of the scintillator and for the thorough functionality tests of the purification systems. 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, N J, 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

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background liquid scintillator detector: the Counting Test Facility at Gran Sasso, Nucl. Instr. and Meth. A406 (1998) 411. . G. Alimonti et al., Ultra-low background measurements in a large volume underground detector, Astrop. Physics 8 (1998) 141.