1. Double Chooz: Outer Veto
Sophie Berkman
Nevis Labs, Columbia University

2. Outline Neutrino Oscillations Double Chooz Outer Veto Some Studies
PMT Characterization
Scintillator Tests
Efficiency
Cross-Talk
Pulse Height vs. Distance

3. 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

4. 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

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

6. 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 -

7. 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

8. Double Chooz Detectors7m 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

9. 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

10. 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

11. 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

12. 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.

13. 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)

14. Scintillator Setup Four stacked strips 1.5m long
Four sets of trigger counters
Wavelength Shifting fibers
Fiber Holder

15. 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

16. 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

17. 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.

18. 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

19. 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

20. 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

21. 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

22. 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)

23. 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

24. 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

25. 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.

26. 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.

28. 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%