Measuring the Neutral Current Event Rate in SNO Using 3 He(n,p)t All Neutron Backgrounds (Estimates) Photodisintegration Background U in D 2 O(20.0 fg/g)160 Th in D 2 O(3.7 fg/g)365 U in NCDs(4.0 pg/g)< 40 Th in NCDs(4.0 pg/g) Co in NCDs(after 200 days u.g.)< 150 Muons (tagged)14000 U Fission(20.0 fg/g U)9 D( ,n  )p(20.0 fg/g U)22 (3.7 fg/g Th)5 17 O( ,n) 20 Ne(20.0 fg/g U)0.5 (3.7 fg/g Th) Po m -2 d Antineutrinos CCP< 22 CCD11 NCD16 Total Expected Background< 1000 Photodisintegration Background  + 2 H  p + n, E  > 2.22 MeV Sources: 232 Th chain 208 Tl  208 Pb, E  = MeV 238 U chain 214 Bi  214 Po, E  = MeV 56 Co 56 Co  56 Fe, E  > MeV (31%) SNO Detector  238 U, 232 Th in water   ’s from PMT’s and their support structure  ( ,p  ) and ( ,n  ) at PMT’s and support structure NCD Detectors  238 U, 232 Th, 56 Co in NCD bodies Diagnostic Techniques  Observation of Cerenkov light from associated  ’s  Radioassay techniques  Estimate from NCD  signal MEASURING THE NEUTRAL CURRENT EVENT RATE IN SNO USING 3 He(n,p)t R.G.H. Robertson 1, T.J. Bowles 4, T.V. Bullard 1, S.J. Brice 4, M.C. Browne 4, P.J. Doe 1, C.A. Duba 1, S.R. Elliott 1, E.I. Esch 4, R. Fardon 1, M.M. Fowler 4, A. Goldschmidt 3, R. Hazama 1, K.M. Heeger 1, A. Hime 4, K.T. Lesko 3, G.G. Miller 4, R.W.Ollerhead 2, A.W.P. Poon 3, K.K. Schaffer 1, M.W.E. Smith 1, T.D. Steiger 1, R.G. Stokstad 3, J.B. Wilhelmy 4, J.F. Wilkerson 1, J.M. Wouters 4 1 Department of Physics, University of Washington, Seattle, WA University of Guelph, Physics Department, Guelph, ON N1G 2W1, Canada 3 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 4 Los Alamos National Laboratory, Los Alamos, NM 87545, USA Sudbury Neutrino Observatory (SNO) Water target D2O 1000 t H2O 1,700 t (sensitive volume) Principle Reactions e - + e  e - + e (ES) 2 H + e  p + p + e MeV (CC) 2 H + x  p + n + x MeV (NC) Expected Signal CC4350 yr -1 pure e SK flux yr -1 BP 1998 SSM NC2030 yr -1 pure e SK flux 4610 yr -1 BP1998 SSM 13,600 yr -1 pure  SK flux Neutral Current Detection via 3 He(n,p) 3 H The neutrons produced by the neutral-current dissociation of deuterium can be detected via the 3 He(n,p) 3 H reaction. An array of 3 He–filled proportional counters is being built for installation in SNO. The parameters of the Neutral Current Detector array are:  775 m total length  300 Ni CVD detectors (2 inch diameter)  96 vertical strings on a 1 m square grid  Estimated neutron capture efficiency  37% Motivation  Separation of charged-current (CC) and neutral-current (NC) events in real time by use of 3 He proportional counters  Signal/Background is determined simultaneously  Observation of secular variations and supernovae Status of Construction (June 2000)  290 out of 300 counters constructed  233 counters at Sudbury in cooldown underground  Radioassay of construction components complete SNO Physics Goals SNO is a high count rate detector, sensitive to e, ,  Search for Flavor Change NC rate and CC/NC ratio Energy Spectrum Distortion Due to Oscillations CC 8 B+hep energy spectrum Time Dependent Solar Flux Observation of 7% orbital eccentricity Day-night asymmetry Solar magnetic field effects Search for Supernova Flavor sensitivity Direct neutrino mass Relic neutrinos High Energy Neutrinos  SNO can resolve the Solar Neutrino Problem, independent of solar models e flux / total flux e flux / e SSM flux (BP98) 8 B Flux Depressed Oscillation Hypothesis SuperK Proportional Counter Signals Neutrino Signal: Neutron from NC interaction Neutrons capture via 3 He(n,p) 3 H in the NCD and produce 573 keV p keV t ionization tracks. Distinguishable Backgrounds:  Tritium in 3 He 3 H decays deposit on average 6 keV in the gas but pile-up can produce proportional counter signals above threshold. Low-temperature purification of the 3 He has resulted in negligible background levels.  Surface and Bulk Alpha Activity 232 Th and 238 U chains in the NCD walls, along with 210 Po surface activity, produce  ’s that underlie the neutron capture peak. These events can be rejected by event by event analysis of digitized pulses. (see “Event Identification by Pulse Shape Analysis”)  Electrons and Gammas  ’s and  ’s from the 232 Th and 238 U chains can only deposit 764 keV through extensive multiple scattering. Less than 2x10 -4 fall into the neutron window.  High Voltage Microdischarges HV induced surface discharge at the endcap can produce pulses. However, all components have undergone extensive high voltage testing and 100% discrimination is expected by pulse shape analysis. Indistinguishable Backgrounds:  Photodisintegration Background Gamma rays with E  >2.22 MeV can disintegrate deuterons and liberate neutrons. This background is indistinguishable from the neutral-current signal and so must be measured and subtracted. (see “Photodisintigration Background”) To determine the neutron capture rate on 3 He it is necessary to discriminate spurious events. A first cut can be made by measuring the energy of the event (right). This leaves a substantial background. Typical signals (below) look very different. Additional parameters such as rise-time or pulse width help distinguish between pulses. A “background free” window can be drawn in a pulse width vs. energy parameter space. This window rejects alpha and beta events with an approximately 50% cut in the efficiency for detecting neutrons.  or n parallel to wire  perpendicular to wire n perp. to wire micro- discharge Event Identification by Pulse Shape AnalysisData Acquisition and Electronics 96 NCD `strings' connect to current preamplifiers that produce signals that go to the electronics. The noise level is approximately 2 mV rms in a 30-MHz bandwidth, and the largest signal the preamplifier can deliver is 2.5 V. NCD Event Rates are dominated by neutrons and alpha particles. Neutrons from muon interactions and NC events are expected to be detected at a rate of 15 per day, and alphas per day. The longest duration of the signal (apart from the ion tail) is about 3  s, corresponding to the drift time across a detector. Preamplifier Signals enter 2 parallel buffer amplifiers, one that drives 20-m long cables to the shaper-ADCs that reside in VME, and the other that drives a delay line and a discriminator. The delay line provides a delay of 320 ns. Pulse Digitization is done with two Tektronix 754A 4- channel oscilloscopes. Each scope services all 96 inputs, with the equivalent scope inputs connected in parallel to 24 multiplexed channels. Scopes provide one level of buffering and permit digitization of pairs of events closely correlated in time. NCD String (1 of 96) VME Controller DAQ Computer Current Preamp Log Amp VME Bus GPIB Shaper ADC 32 Bit differential digital I/O GTID Counter DACs & ADCs NCD MUX/Trig Controller Card SNO MTCD VME ECPU 12 Multiplexer ~300ns Delay VME GBIP Controller Tek 754 Summing Junction NCD DAQ is fully object-oriented, based on the same coding structure as used in the main SNO DAQ. NCD DAQ is currently running on a Macintosh platform, but will soon run on Linux as well. Neutrons/year