D. Giugni

The silicon pixel tracking system for the ATLAS experiment at the Large Hadron Collider is described and the performance requirements are summarized. Detailed descriptions of the pixel detector electronics and the silicon sensors are given. The design, fabrication, assembly and performance of the pixel detector modules are presented. Data obtained from test beams as well as studies using cosmic rays are also discussed.

We report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV 7Be neutrinos is 49+/-3stat+/-4syst counts/(day.100 ton). The hypothesis of no oscillation for 7Be solar neutrinos is inconsistent with our measurement at the 4sigma C.L. Our result is the first direct measurement of the survival probability for solar nu(e) in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of 7Be, pp, and CNO solar nu(e), and the limit on the effective neutrino magnetic moment using solar neutrinos.

Borexino, a large volume detector for low energy neutrino spectroscopy, is currently running underground at the Laboratori Nazionali del Gran Sasso, Italy. The main goal of the experiment is the real-time measurement of sub-MeV solar neutrinos, and particularly of the monoenergetic (862 keV) 7Be electron capture neutrinos, via neutrino–electron scattering in an ultra-pure liquid scintillator. This paper is mostly devoted to the description of the detector structure, the photomultipliers, the electronics, and the trigger and calibration systems. The real performance of the detector, which always meets, and sometimes exceeds, design expectations, is also shown. Some important aspects of the Borexino project, i.e. the fluid handling plants, the purification techniques and the filling procedures, are not covered in this paper and are, or will be, published elsewhere (see Introduction and Bibliography).

CUORE is a proposed tightly packed array of 1000 TeO2 bolometers, each being a cube 5cm on a side with a mass of 760g. The array consists of 25 vertical towers, arranged in a square of 5 towers×5 towers, each containing 10 layers of four crystals. The design of the detector is optimized for ultralow-background searches: for neutrinoless double-beta decay of 130Te (33.8% abundance), cold dark matter, solar axions, and rare nuclear decays. A preliminary experiment involving 20 crystals 3×3×6cm3 of 340g has been completed, and a single CUORE tower is being constructed as a smaller-scale experiment called CUORICINO. The expected performance and sensitivity, based on Monte Carlo simulations and extrapolations of present results, are reported.

A 4.8 m3 unsegmented liquid scintillation detector at the underground Laboratori Nazionali del Gran Sasso has shown the feasibility of multi-ton low-background detectors operating to energies as low as 250 keV. Detector construction and the handling of large volumes of liquid scintillator to minimize the background are described. The scintillator, 1.5 g PPO/L-pseudocumene, is held in a flexible nylon vessel shielded by 1000 t of purified water. The active detector volume is viewed by 100 photomultipliers, which measure time and charge for each event, from which energy, position and pulse shape are deduced. On-line purification of the scintillator by water extraction, vacuum distillation and nitrogen stripping removed radioactive impurities. Upper limits were established of < 10−7 Bq/kg-scintillator for events with energies 250 keV < E < 800 keV, and < 10−9 Bq/kg-scintillator due to the decay products of uranium and thorium. The isotopic abundance of 14C12C in the scintillator was shown to be approximately 10−18 by extending the energy window of the detector to 25–250 keV. The 14C abundance and uranium and thorium levels in the CTF are compatible with the Borexino Solar Neutrino Experiment.

The techniques researched, developed and applied towards the measurement of radioisotope concentrations at ultra-low levels in the real-time solar neutrino experiment BOREXINO at Gran Sasso are presented and illustrated with specific results of widespread interest. We report the use of low-level germanium gamma spectrometry, low-level miniaturized gas proportional counters and low background scintillation detectors developed in solar neutrino research. Each now sets records in its field. We additionally describe our techniques of radiochemical ultra-pure, few atom manipulations and extractions. Forefront measurements also result from the powerful combination of neutron activation and low-level counting. Finally, with our techniques and commercially available mass spectrometry and atomic absorption spectroscopy, new low-level detection limits for isotopes of interest are obtained.

A large volume (4.8 m3) liquid scintillator detector has been running in Hall C of the Gran Sasso Underground Laboratory since February 1995. This detector is called the “Counting Test Facility” (CTF). The main goal of the detector facility is the measurement of ultralow background levels in scintillators and the development of processes able to purify them at this level. The detector has been designed to have exceptional sensitivity using a variety of methods to identify backgrounds. With the CTF, records were achieved in the domain of low background large volume detectors. Limits of 3.5 ± 1.3 × 10−16 g/g and 4.4−1.2+1.5 × 10−16 g/g for the 238U and 232Th daughters, respectively, and 1.85 ± 0.13 ± 0.01 × 10−18 for the isotopic abundance of 14C relative to 12C were obtained. These results are very encouraging and point towards the feasibility of low energy, real time scintillation detectors for solar neutrinos, such as Borexino.

The 14C/12C ratio in 4.8 m3 of high-purity liquid scintillator was measured at (1.94±0.09)×10−18, the lowest 14C abundance ever measured. At this level the spectroscopy of low-energy solar neutrinos, in particular a measurement of the 7Be neutrino flux, will not be obstructed by the 14C β decay intrinsic to a liquid scintillator detector. A comprehensive study of the deviation of the shape of the 14C β spectrum from the allowed statistical shape reveals consistent results with recent observations and calculations. Possible origins of the 14C in the liquid scintillator are discussed.

The successful deployment of the Borexino solar neutrino detector required assorted physical and chemical operations to produce exceptional pure fluids and fill multiple detector zones. The composition and flow rates of high purity gases and liquids had to be precisely controlled to maintain liquid levels and pressures. The system was required to meet exceptional requirements for cleanliness and leak-tightness. A large scale modular system connecting fluid receiving, purification and fluid delivery processes was developed for Borexino. At the core is a flow control system that delivers scintillator components to plants for purification, and then fills the Borexino detector volumes with ultrahigh purity buffer or ultrahigh purity scintillator. The liquid handling system maintains precise control over the liquid levels and differential pressures between the different volumes of the detectors that are separated by flexible nylon vessels. The preparation, commissioning and operation of the system for filling the Borexino detector with scintillator is described.

The fluorescence light propagation in a large volume detector based on organic liquid scintillators is discussed. In particular, the effects of the fluor radiative transport and solvent Rayleigh scattering are emphasized. These processes have been modelled by a ray-tracing Monte Carlo method and have been experimentally investigated in the Borexino prototype which was a 4.3 ton, 4π sensitive detector. The comparison between the model prediction and the experimental data shows a satisfactory agreement indicating that the main aspects of these processes are well understood. Some features of the experimental time response of the detector are still under study.

This paper describes the evaporative system used to cool the silicon detector structures of the inner detector sub-detectors of the ATLAS experiment at the CERN Large Hadron Collider. The motivation for an evaporative system, its design and construction are discussed. In detail the particular requirements of the ATLAS inner detector, technical choices and the qualification and manufacture of final components are addressed. Finally results of initial operational tests are reported. Although the entire system described, the paper focuses on the on-detector aspects. Details of the evaporative cooling plant will be discussed elsewhere.

The results of background measurements with the second version of the BOREXINO Counting Test Facility (CTF-II), installed in the Gran Sasso Underground Laboratory, were used to obtain limits on the instability of nucleons, bounded in nuclei, for decays into invisible channels (inv): disappearance, decays to neutrinos, etc. The approach consisted of a search for decays of unstable nuclides resulting from N and NN decays of parent 12C, 13C and 16O nuclei in the liquid scintillator and the water shield of the CTF. Due to the extremely low background and the large mass (4.2 t) of the CTF detector, the most stringent (or competitive) up-to-date experimental bounds have been established: τ(n→inv)>1.8×1025 yr, τ(p→inv)>1.1×1026 yr, τ(nn→inv)>4.9×1025 yr and τ(pp→inv)>5.0×1025 yr, all at 90% C.L.

The prototype of the Borexino detector Counting Test Facility, located in the Gran-Sasso laboratory, has been used to obtain a bound on the stability of the electron. The new lower limit on the mean lifetime defined on 32.1 days of data set is τ(e−→νe+γ)⩾4.6×1026 yr (90% c.l.).

We report on the study of a new liquid scintillator target for neutrino interactions in the framework of the research and development program of the Borexino solar neutrino experiment. The scintillator consists of 1,2-dimethyl-4-(1-phenylethyl)-benzene (phenyl-o-xylylethane, PXE) as solvent and 1,4-diphenylbenzene (para-Terphenyl, p-Tp) as primary and 1,4-bis(2-methylstyryl)benzene (bis-MSB) as secondary solute. The density close to that of water and the high flash point makes it an attractive option for large scintillation detectors in general. The study focused on optical properties, radioactive trace impurities and novel purification techniques of the scintillator. Attenuation lengths of the scintillator mixture of 12 m at 430 nm were achieved after purification with an alumina column. A radiocarbon isotopic ratio of C14/C12=9.1×10-18 has been measured in the scintillator. Initial trace impurities, e.g. 238U at 3.2×10-14g/g could be purified to levels below 1×10-17g/g by silica gel solid column purification.

We describe a combined ultrasonic instrument for gas flow metering and continuous real-time binary gas composition measurements. The combined flow measurement and mixture analysis algorithm employs sound velocity measurements in two directions in combination with measurements of the pressure and temperature of the process gas mixture.

The results of background measurements with the prototype of the Borexino detector (CTF) have been used to obtain an upper bound on the neutrino magnetic moment, μν. The new upper limit for μν from pp and 7Be solar neutrinos is (5.5×10−10)μB (90% c.l.) in the Standard Solar Model scenario. This is the first limit on μν obtained using sub-MeV neutrinos. The sensitivity of the prototype to the neutrino charge radius and the neutrino radiative decay are also presented.

In this article we discuss the results of tests performed on the 8 in. EMI 9351 photube. The measurements were mainly devoted to evaluate pulse height and time resolution of the tube after single photon illumination. The impact of small magnetic fields on these parameters was also carefully investigated.

We report the direct measurement of the 7Be solar neutrino signal rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso. The interaction rate of the 0.862 MeV 7Be neutrinos is 49±3stat±4syst counts/(day·100 ton). The hypothesis of no oscillation for 7Be solar neutrinos is inconsistent with our measurement at the 4σ C.L. Our result is the first direct measurement of the survival probability for solar νe in the transition region between matter-enhanced and vacuum-driven oscillations. The measurement improves the experimental determination of the flux of 7Be, pp, and CNO solar νe, and the limit on the effective neutrino magnetic moment using solar neutrinos.

We investigate and address the performance limitations of the ATLAS silicon tracker fluorocarbon evaporative cooling system operation in the cooling circuits of the barrel silicon microstrip (SCT) sub-detector. In these circuits the minimum achievable evaporation temperatures with C3F8 were higher than the original specification, and were thought to allow an insufficient safety margin against thermal runaway in detector modules subject to a radiation dose initially foreseen for 10 years operation at LHC.

Abstract A multiplexed system of optical fibers has been designed for the photomultiplier calibration of the Borexino experiment at Gran Sasso. Both time and energy calibration are of paramount importance in Borexino for the measurement of the solar 7 Be neutrino flux. Equalization of photomultipliers within 1 ns is required to define a Fiducial Volume to isolate a very pure internal region of observation, and accurate energy determination and resolution are crucial for the spectral shape recognition of the neutrino signal. The size of the detector, its tightness and radiopurity constraints require special care in the material selection and mechanical handling of the system. The solution of multiplexed fiber chains has been realized for the first time in a large underground detector. In this paper we illustrate the PMT calibration system design and the results of various feasibility tests performed.

CUORE is a proposed tightly packed array of 1000 TeO2 bolometers, each being a cube 5 cm on a side with a mass of 750 g. The array consists of 25 vertical towers, arranged in a square of 5 towers by 5 towers, each containing ten layers of four crystals. The design of the detector is optimized for ultralow-background searches for neutrinoless double beta decay of 130Te (33.8% abundance), cold dark matter, solar axions, and rare nuclear decays. A preliminary experiment involving 20 crystals of various sizes (MIBETA) has been completed, and a single CUORE tower is being constructed as a smaller scale experiment called CUORICINO. The expected performance and sensitivity, based on Monte Carlo simulations and extrapolations of present results, are reported.

In this article we discuss the results of the testing of ∼ 50 Thorn Emi 9351 phototubes used for the light detection system of the Counting Test Facility, prototype of the solar neutrino experiment Borexino, presently under installation at the Gran Sasso Laboratory. The tests were aimed to a systematic evaluation of the spread of the main characteristics of the device and to a thorough evaluation of the shielding effectiveness of μ-metal shields of different lengths.

The Silicon Vertex Tracker (SVT) of the BaBar experiment at the PEP-II asymmetric B factory consists of five layers of double-sided, AC-coupled silicon strip detectors. The detectors are readout with a custom IC, capable of simultaneous acquisition, digitization and transmission of data. The SVT geometry is shown and the construction phases of its modules are described in detail, with emphasis on the bending procedures needed for the arch-modules of the outer layers.

A measurement of the response to 15.1 MeV γ-rays has been made for the Ge cluster detectors in the EUROBALL array. Each cluster detector consists of seven germanium capsules surrounded by a single anticompton shield of BGO. The reaction D(11B,γ)12C+n at Ebeam=19.1MeV has been employed. The “adding-back” of signals simultaneously present in the capsules composing each cluster detector has been made on an event by event basis. The intensity in full-energy peak increases by a factor of three as compared to that of the spectrum obtained by summing the individual spectra of the 7 capsules. The pulse height to energy conversion is found to be very linear from few hundreds keV to 15 MeV. The efficiency is discussed relative to that of large volume BaF2 scintillators.

This paper describes the main features of the proposed low energy solar neutrino detector Borexino, planned to be installed at the Gran Sasso Laboratory. This real time detector is based on a massive, calorimetric, liquid scintillation spectroscopy technique, whose high luminosity is the base for the attempt to achieve a low signal detection threshold. After a description of the main structural components of the detector, of its performances in term of spatial and energy resolution, and of the neutrino reactions occurring in the liquid scintillator, a description of the crucial background topic is given. Finally the main implications of the physics program of the experiment are briefly illustrated.

The BaBar Silicon Vertex Tracker (SVT) is designed to provide the high-precision vertexing necessary for making measurements of CP violation at the SLAC B-Factory PEP-II. The instrument consists of five layers of double-sided silicon strip detectors and has been installed in the BaBar experiment and taking colliding beam data since May 1999. An overview of the design as well as performance and experience from the initial running will be presented.

The status of the BOREXINO experiment is presented. The physics potential of the experiment is reviewed.

Yield maximization in multichip hybrid pixel sensors is a crucial issue in large volume productions planned for future High-Energy Physics experiments. Bump bonding process optimization can guarantee statistical single bump failure rates at the acceptable level of 10–100 ppm; nevertheless, systematic effects connected to process repeatability can affect the functionality of a full chip in a module to a much larger extent. Because of this, the reversibility of the bonding procedure has been investigated. A feasibility study on single chip assemblies for the ATLAS experiment has been successfully completed, proving the possibility of reworking. As a result of it, a dedicated facility has been conceptually designed, engineered and commissioned. The characteristics of the facility in terms of motion, temperature and tensile strength control are outlined, together with the main results.

We report on the study of a new liquid scintillator target for neutrino interactions in the framework of the research and development program of the Borexino solar neutrino experiment. The scintillator consists of 1,2–dimethyl–4–(1–phenylethyl)– benzene (phenyl–o–xylylethane, PXE) as solvent and 1,4-diphenylbenzene (paraTerphenyl, p-Tp) as primary and 1,4-bis(2-methylstyryl)benzene (bis-MSB) as secondary solute. The density close to that of water and the high flash point makes it an attractive option for large scintillation detectors in general. The study focused

The Silicon Vertex Tracker (SVT) for the BaBar experiment at the PEP-II asymmetric B factory is a 5-layer device based on double-sided, AC-coupled silicon strip detectors. It is read out by a custom IC, the AToM chip, that can simultaneously acquire, digitize and transmit data. The main purpose of the SVT is to accurately measure the decay position of the B mesons that are produced, which is essential for extracting CP asymmetries. Here, we report on the SVT design as well as progress on its fabrication and assembly.

The BaBar Silicon Vertex Tracker (SVT) is a five layer device, made from double-sided silicon strip detectors and read out via a custom time-over-threshold circuit, the AToM chip. The SVT is an essential part of the physics program of BaBar, and is able to reconstruct B meson decay vertices with a precision sufficient to measure time-dependent CP violating asymmetries at the PEP-II asymmetric e+e− collider. This report will give an overview of the SVT, with particular focus on the performance of the SVT, which has been taking colliding beam data since May 1999.

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 Sasso 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.

Within its first year of operation, the BaBar Silicon Vertex Tracker (SVT) has accomplished its primary design goal, measuring the z vertex coordinate with sufficient accuracy as to allow the measurement of the time-dependent CP asymmetry in the neutral B-meson system. The SVT consists of five layers of double-sided, AC-coupled silicon-strip detectors of 300μm thickness with a readout strip pitch of 50–210μm and a stereo angle of 90° between the strips on the two sides. Detector alignment and performance with respect to spatial resolution and efficiency in the reconstruction of single hits are discussed. In the day-to-day operation of the SVT, radiation damage and protection issues were of primary concern. The SVT is equipped with a dedicated system (SVTRAD) for radiation monitoring and protection, using reverse-biased photodiodes. The evolution of the SVTRAD thresholds on the tolerated radiation level is described. Results on the first-year radiation exposure as measured with the SVTRAD system and on the so far accumulated damage are presented. The implications of test-irradiation results and possible future PEP-II luminosity upgrades on the radiation limited lifetime of the SVT are discussed.

Silvia BONETTI University and INFN Milan, Via Celoria 16, 20133 Milan, Italy Borexino * , planned to operate in the Laboratori Nazionali del Gran is a future real time detector that, providing high luminosity, radiopurity and low energy threshold, will give unique information solar neutrinos specifically from the 7 Be source . + Borexino Collaboration : C . Ar esella',A . Donati l ,D . Franciotti l ,R . Tartagli l ,M . Deutsch2 ,F . Von Feilitz q ch3 ,S . Schoe ert 3 ,G . Manuzio 4 ,S . Pakvasa P . Trincherini 6 ,G . Bellini S . Bon~tti M . C~mpanella 7 ,M . Giammarchi 7 ,D . Giugni 7 ,P . Inzani 7 ,I . Manno ,E . Meroni F . Ragusa 7 ,G . Ranucci 7 ,p . Ullucci 7 ,T . hovacs 8 ,3 . Mitchell s ,P . Raghavana ,R .S . RaghavanB ,G . Cecchet 9 ,A . De Bari 9 ,A . Perotti 9 ,R . Steinbergl° 1 Laboratori Nazionali del Gran Sasso, 2 Massachusetts Inst . of Technology, 3 Technical Univ . Munich, ~Genova Univ . and INFN, Univ . of Hawaii at Minoa, 6 Euratom ISPRA, 7 M~lano Univ . and INFN, 8 AT&T Bell Lab ., Murray Hill, Pavia Univ . and INFN, ° Drexel Univ . Philadelphia 1 . INTRODUCTION The two experiments running from longtime' with the goal to measure the flux of neutrinos born inside the Sun have emphasized a puzzling situation : the detected flux is lower than the expected one, evaluated on the basis of the SSM, and the indication of a time-variation found by one of them, is not confirmed by the other . A decisive answer is expected from the two Gallium experiments 2 in the neutrino energy region . The neutrino high energy region, will be deeply investigated by the SuperKamiokande and SNO experiments, that are now in project . BOREXINO 3 is an experiment that has been conceived to perform a neutrino spectroscopy with the unique feature of a real-time sensitivity to the low energy 7 Be monochromatic solar source (E,=0.860 MeV) . Significative results are expected also in the 8 region . The detector will provide high luminosity, low detection threshold and very p-p 8 B

A recently published article by Klapdor-Kleingrothaus et al. critiques the limit on the stability of the electron obtained by the Borexino collaboration. We respond here to the criticisms raised by Klapdor-Kleingrothaus and his colleagues, and re-establish that our result is based on very conservative premises and that the "indication of a signal of 1.4 $σ$" for the decay of the electron in the $γ+ν$ channel, reported by Klapdor-Kleingrothaus and colleagues, is excluded by the Borexino result.

The Cryogenic Underground Observatory for Rare Events (CUORE) will be a large-scale, multi-purpose observatory that is planned for construction in the Gran Sasso underground laboratory in Italy. Its primary focus will be on a search for the neutrinoless double-beta decay of 130Te. The source/detector will be composed of one thousand 5-cm×5-cm×5-cm single crystals of TeO2 all housed in a common dilution refrigerator. Attached to each crystal will be one or more neutron-transmutation-doped (NTD) germanium thermistors that will measure the small temperature rise produced in a crystal when radiation is absorbed. The high natural isotopic abundance of 130Te (33.8%) makes the use of isotopically enriched material unnecessary. In addition, the high Q-value for the double-beta decay of 130Te means that the peak in the summed electron energy spectrum expected from the neutrinoless mode will occur in an energy region of very little natural background. Once constructed, CUORE can also be used in searches for dark matter, solar axions, and other rare decay processes in nuclear physics.