# Double Chooz: Outer Veto

## Presentation on theme: "Double Chooz: Outer Veto"— Presentation transcript:

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(1.27m2L/E) Θ=mixing angle m2=difference in squares of neutrino masses L=distance of oscillation E=energy of neutrinos

Neutrino Mixing with 3 flavors
How do you measure delta? By measuring neutrinos and antineutrinos

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 ~100s) 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 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

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 Laser - stabalize for thermal reasons

Before and After Characterization

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
= 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 Require Effic. 3-fold 91% 4-fold 94% 5-fold 96% Require Effic. 3-fold 83% 4-fold 5-fold 90% 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
All four strips at position 3. 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 Strip no T0 small spacer T0 no spacer T0 grease T2 grease T2 grease (gain online) average per strip 1 260.11 127.66 162.52 177.54 160.55 177.67 2 204.9 174.82 220.29 229.37 336.92 233.26 3 281.66 236.95 212.73 209.66 280.37 244.27 4 364.54 302.58 296.6 268.95 255.62 297.66

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 Conclusion: More efficient with more requirements.
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%