Double Chooz: Outer Veto Sophie Berkman Nevis Labs, Columbia University
Outline • Neutrino Oscillations • Double Chooz • Outer Veto • Some Studies • PMT Characterization • Scintillator Tests • Efficiency • Cross-Talk • Pulse Height vs. Distance
Neutrino Oscillations • In the standard model neutrinos are massless leptons - cannot mix. • BUT - neutrinos oscillate so by the current interpretation: • Neutrinos have mass • Lepton family number is not conserved
What it means that neutrinos oscillate In a 2-neutrino simplification: • Mass states = 1, 2 • Flavor (weak) states = , e Probability of oscillation: P( -> e)=sin2(2θ)sin2(1.27m2L/E) Θ=mixing angle m2=difference in squares of neutrino masses L=distance of oscillation E=energy of neutrinos
Double Chooz • Measure θ13 • Reactor experiment • Look at e from reactors • Disappearance experiment - reactors only produce e • Two Detectors - identical, cancel uncertainties in neutrino flux and cross-section • Near - unoscillated neutrino flux • Far - after oscillation -
Muon Background • Double Chooz looks for inverse beta decay • e+ p n + e+ • Double coincidence of neutron capture and positron signal (within ~100s) • Cosmic muon background • Muon interacts to form neutrons • Neutrons knock protons out of scintillator • Protons emit light as they move through scintillator and neutron captured by gadolinium • Looks like inverse-beta decay signal
Double Chooz Detectors 7m • Target: liquid scintillator, doped with Gadolinium - n capture • Gamma catcher: measure gammas from n capture • Buffer: holds PMTs, shields detector from PMT radiation • Inner veto: reject fast neutron/muon background • Outer Veto: atmospheric muons 7m
Outer Veto • Reject atmospheric muon background • Stacked scintillator strips • Wavelength shifting fibers • Light transmitted to PMT and DAQ • Nevis: developing electronics/software • All tests done in light tight boxes
PMT Characterization • Why Characterize? • Want all pixels to respond in the same way to light • Pulse height of 350 ADC counts • 350ADC counts =10pe * 35 ADC/pe
Characterization Process • Take Baseline with laser off • Turn laser on and allow it to stabilize for 30 min • Adjust HV to get an average pulse height for all pixels to be 350 ADC counts • Adjust gain across preamplifiers to get a mean pulse height of 350 ADC counts across each individual pixel • Turn off the laser and allow it to stabilize for 30 minutes • Take noise data for different DAC thresholds
Before and After Characterization Spread=18% Spread=2.9% Conclusion: characterization process narrows the spread of the pulse height distributions. Use to determine if bad PMTs.
Gain Constant Distribution • Gain Constant = measure of gain adjustment • Gain constant of 16 means adjust by a factor of 1 Conclusion: Centered around 16 (ie. Adjustment by factor of 1)
Scintillator Setup • Four stacked strips 1.5m long • Four sets of trigger counters • Wavelength Shifting fibers • Fiber Holder
Some Standard Modifications • Spacers to protect the face of the PMT • Large spacer = space of 1.27mm • Small spacer = space of 0.48mm • No spacer = space of 0.000mm • Optical Grease
Efficiency Test Trigger on trigger counters and one strip #Entries=326 Efficiency = = 91% #Entries=359 Trigger on trigger counters • Events over 1pe for triggered strip/trigger counter • Repeat with more coincidences
Efficiency Results • Repeated for more coincidences • Large spacer: ~4.3pe • Small spacer: ~5.2pe Large Spacer Small Spacer Conclusion: more efficient with more coincidences, and with smaller spacer.
Cross Talk • Optical Cross talk: the amount surrounding pixels receive light from the illuminated pixel • # pe smaller than expected • Add pulse heights in surrounding pixels to the signal pixel • Can find maximum #pe without crosstalk • Note: different numbers of surrounding pixels for different pixels
PH distribution before and after addition - no spacer, strip 2 Conclusion:cross-talk is on average ~10% and #pe increases to: ~5-8pe in the nearest position
Pulse Height vs Distance Setup • Noticed dependence on distance from previous studies • All strips at all positions • Use optical grease without spacer • Require 5-fold coincidence • 1 photoelectron cut on non signal strip/trigger
Strips at Position 3 PH=281.7pe=8.049 PH=206.4pe=5.897 PH =305.3 Pe=8.723 PH = 221.8 pe=6.337 Conclusion: Four strips have different pulse heights because of polishing of fibers or scintillator
Strip 4 at four different positions Mean =246.8 Pe=7.051 Mean=269.9 Pe=7.711 Mean=355.1 Pe=10.14 Mean=308.3 Pe=8.809 Conclusion: Pulse Height increases as move closer to the PMT because more light will reach the PMT from closer positions. (Higher PH than previous because of Trigger 2)
Trigger Counters at Position 3 Mean=106.4 Pe=3.040 Mean =76.88 Pe=2.197 Conclusion: Trigger counters have lower PH than strips because light will be lost from muons that hit them at the edge Mean=305.3 Pe=8.723
Attenuation Length • Find using PH vs. distance data • Find by fitting plot of PH vs distance to exponential
Conclusion and Thanks • Process for characterizing PMTs works well and will be possible to implement for all outer veto PMTs • Still generally not as many photoelectrons as expected, but we can use optical grease/other trigger modes to increase the number Thanks to everyone I worked with this summer for teaching me so much about physics and for this extraordinary opportunity to work on Double Chooz.
Bibliography/Picture Permissions • Camilleri, Leslie. Slides. • Shaevitz, Mike. Reactor Neutrino Experiment and the Hunt for the Little Mixing Angle. 30 Nov 2007. • Sutton, Christine. Spaceship Neutrino.
Efficiency Test Find the mean of the pulse height distribution in strip 1 when both trigger counters have at least 1pe Find the mean pulse height distribution in strip 1 when both trigger counters and strip 2 have at least 1pe. Efficiency = Second Mean/First mean Require more strips to have 1pe Look at efficiencies with different requirements for events Repeat with large and small spacer Conclusion: More efficient with more requirements. -Large Spacer went from 83-90% -Small Spacer went from 91-96%