Methods and problems in low energy neutrino experiments (solar, reactors, geo-) II G. Ranucci ISAPP 2011 ISAPP 2011 ISAPP 2011 ISAPP 2011 International School on International School on Astroparticle Astroparticle physics physics THE NEUTRINO PHYSICS AND ASTROPHYSICS July 26th - August 5th, 2011 Varenna - Italy
Some examples of scintillator based detectors Borexino Borexino (low energy solar neutrino detector) described in the (low energy solar neutrino detector) described in the following at length as paradigmatic example of a following at length as paradigmatic example of a scintillator scintillator detector detector Chooz Chooz (reactor neutrino detector) (reactor neutrino detector) KamLAND KamLAND (reactor neutrino detector) (reactor neutrino detector) Planned: SNO+ and LENS Planned: SNO+ and LENS
Borexino Borexino A real time calorimetric scintillation detector for low energy solar neutrinos installed at the Gran Sasso installed at the Gran Sasso underground laboratory, aimed at detecting solar neutrinos through the scattering off the electrons of the scintillator
Designed for good performance as instrument precision in -energy measurement -position measurement needs of calibration and Monte Carlo tuning low background low background -choice of construction materials -assay of materials during the assembly -special precautions for installation procedures (clean room, cleanliness of the surfaces) -accurate strategy for liquid manipulation and purification -special issue : particular care for the nitrogen purity -strategy against the cosmic muon: underground, muon veto, tagging of the residual cosmogenic products
Main components • Scintillator • Nylon (inner and outer) vessels • Buffer liquids • Stainless steel sphere • Support of PMT’s • Containment of the buffer (zero buoyancy for the nylon vessels) nylon vessels) • PMT’s • Concentrators • Muon veto • Calibration equipments • Water Tank • Electronics and DAQ
Plants • Storage vessels • Scintillator purification systems • Water extraction • Distillation • Nitrogen sparging • PPO (solute) distillation • Normal nitrogen • High purity nitrogen • High purity nitrogen purified in 39 Ar and 85 Kr • Fluid handling system • Water purification • Clean room • CTF, the initial prototype
Water Tank
Stainless steel sphere
PMT’s on the sphere surface
Vessel before inflation (viewed by CCD cameras)
Vessel after inflation (viewed by CCD cameras)
Detail Detail of of the the south south end end-cap cap of of the vessel and the vessel and of of the last the last mounted mounted PMT’s on the 3 m PMT’s on the 3 m door door of of the the sphere sphere
Muon veto: tyvek (diffusive panels) and phototubes on the external sphere surface
Tyvek on the surface of the Water Tank dome
Electronic racks (cables length more than 50 meters)
Radiopurity construction requirements Thorrn-EMI photomultipliers Detector and plants materials Low radioactivity Shott borosilicate Low intrinsic radioactivity glass (type 8246) 1.1 ns time gitter for good spatial Low radon emanation resolution Chemical compatibility with PC (Al) light cones for uniform light collection in the fiducial volume mu-metal shilding for the earth magnetic field Pipes, vessels and pipes 384 PMTs with no cones for muon Electropolished identification in the buffer region Cleaned with filtered detergents (Detergent-8, EDTA) Pickled and passivated with acids Rinsing with ultrapure water (class Rinsing with ultrapure water (class 20 – 50 MIL STD 1246 ) Leak tightness Leak rate < 10-8 atm cc /s Nitrogen blanketing on critical elements like pumps, valves, big Nylon vessels flanges Good chemical and mechanical Double seal metal gaskets strength (small buoyancy) Low radioactivity (< 1 count/day/100 tons) Contruction in low 222Rn clean Clean rooms room High purity nitrogen storage Mounting room in class 100 Inner detector in class 1.000 Outer detector in class 100.000 Philadelphia - 30 July, 2008 Gioacchino Ranucci - I.N.F.N. Sez. di Milano
Nylon vessels Requirements: Chemical resistance to PC,PPO, DMP, water Mechanical strength (20MPa – 5° � T) Optical transparency (350-450 nm) Low intrinsic radioactivity (U, Th, K) Clean fabrication (<3 mg dust) Low permeability ti Rn Leak tightness Solutions and results: Sniamid Nylon-6 film 125 � m thick film Index of refract. = 1.53 with >90% trasmittance U, Th less than 2 ppt Umidification to decrese the T g glass transition temperature (brittle state) Philadelphia - 30 July, 2008 Gioacchino Ranucci - I.N.F.N. Sez. di Milano
Scintillator Solvent: Pseudocumene Solute: PPO (1.5 g/l) Light yield: 11000 ph/MeV Attenuation length (@ 420 nm): 30 m Scattering length (@420 nm): 7 m Scattering length (@420 nm): 7 m Decay time (fast component): 3.5 ns Good α/β properties
Photomultipliers 8” Electron Tubes Limited (ETL) 9351 type P/V : 2.5 (measure of the single electron resolution) Transit Time Spread: 1ns ( σ ) Dark Count Rate: 1kHz (typical rate at 20 °C) Afterpulsing < 5% (for single electron pulses) Low radioactive glass and internal parts (main contributors to the external background) Light concentrators Truncated string cone design Truncated string cone design Optimized to collect the light from the inner vessel and 20 cm beyond it Material: anodized aluminum selected for low radioactivity Electronics ADC and TDC circuits Good single electron resolution Time resolution better than 0.5 ns
D etector fully filled on May 15 th, 2007: DAQ starts May 2007 End October 2006 LAKN – Low Argon and Krypton Nitrogen Ultra-pure water Ultra-pure water March 2007 Liquid scintillator Ultra-pure water Photos taken with one of 7 CCD cameras placed inside the detector
Neutrino Detection in Borexino Neutrino Detection in Borexino Detection through the scattering reaction (as in Superkamiokande and in SNO-third method) ν + → ν + e e off the electrons of the scintillator The high luminosity (50 times more than the Cerenkov technique) and high radiopuri (huge challenge: fight the natural radioactivity and high radiopuri (huge challenge: fight the natural radioactivity below 3 MeV) ty of the scintillator lead to a low detection threshold: analysis threshold about 200 keV, acquisition threshold about 60 keV It is possible therefore to detect the recoil electrons produced by the monoenergetic (0.862 MeV) 7 Be neutrinos - maximum recoil energy: 0.66 MeV Other components of the solar spectrum are detectable, as well - flexibility of the detector
Other capabilities � 8 B solar neutrinos in the unique energy window 2 - 5 MeV � Antineutrino science v Geophysical from the Earth e v from type IIa Supernovae e v Long baseline from European reactors e v Investigation of from the Sun e � Other components of the solar spectrum : pep, CNO, pp
Measured quantities The electronics measures and provides for each triggered events: • The photomultipliers pulse height energy measurement • The photoelectrons arrival times (better than 0.5 ns precision) position identification The absolute time of the event Expected detector perfomances Effective coverage 30% Photoelectron yield 500 pe/MeV Energy resolution @ 1 MeV 5% Position resolution @ 1 MeV 10 cm
The light yield has been evaluated Light Yield also by taking it as free parameter in a global fit on the total spectrum ( 14 C, 210 Po, σ 210Po , 7 Be ν Compton edge ) 14 C spectrum ( β − decay(156 keV, end The Light Yield has been point) evaluated fitting the 14 C spectrum, (Borex. Coll. NIM A440, 2000) and the 11 C spectrum 11 C spectrum( β + decay(960 keV) C spectrum( β decay(960 keV) Light Yield = 500 +( 12 p.e./MeV The energy equivalent to the sum of the two quenched 511 keV gammas: E 2 γ( 511) = 0.83 +( 0.03 MeV. The 11 C sample is selected through the triple Energy resolution: 10% at 200 keV coincidence with muon and neutron. We 8% at 400 keV limited the sample to the first 30 min of 11 C time profile, which reduces the random 5% at 1 MeV coincidence to a factor 1/14. NO-VE April 15-18, 2008
The time and the total Position reconstruction charge are measured, and the position is reconstructed for each event . Absolute time is • Position reconstruction algorithms also provided (GPS) – Base on time of flight fit to hit time distribution – developed with MC, tested and validated in CTF – cross checked and tuned in Borexino on selected events ( 14 C, 214 Bi- 214 Po, 11 C) 14 C C Radius (m) ��������������������������������������� � � ��������������������������� Spatial resolution: 16 cm at 500 keV − 1/ 2 ���� σ σ ������������������������������� N p . e . σ σ (scaling as ) �������������������������������� NO-VE April 15-18, 2008
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