Daya Bay Calibration System

1
Daya Bay Calibration System Breakout 1, part II

• Underground Detector Commissioning
• Detector monitoring and calibration with
  cosmogenics
• Automated source calibration system
• Source calibration simulations

Jianglai Liu Caltech
LBL Physics Review
Oct 16, 2006
2
Detector Underground Commissioning turning on
procedure

• Check mechanical and thermal properties of the
  detector (Karstens talk)
• Turning on PMTs
• LED source for gain, threshold, and timing
  calibration
• Deploy radioactive source in center. Calibrate
  QE, energy scale, and optical parameters of the
  detector
• Use tagged cosmogenics for continuous monitoring
  of detector performance

3
Road Map to Precise Determination of Energy
response function
energy at detector center

• Basic requirement radioactive sourcesfixed
  energy
• Response function is position-dependent.
• Fiducial Gd-loaded inner scintillator volume
• measured yield is a volume-averaged uniform ?Y?V

Cosmogenic backgrounds distribution energy
uniformly in detector ? can be used to calibrate
the detector uniform response.
4
DT(T-Tm ), DL
5
12BFit lifetime to extract signal
t29.1ms Q13.4MeV
12B
12N
t15.9ms Q17.3MeV
µ
Fit to data shows that 12B12N 1001
6
Summary of Cosmogenics for Daya Bay

• Muon arrival T0
• n-Gd capture ?1/230 ?s, E8MeV
• 12B beta-decay ?1/220 ms, end point energy 13.4
  MeV

So offline coincidences ?n 1,200 ?s,
?12B 200 ?s,100ms

• Spallation neutron gives excellent (absolute)
  calibration of 8 MeV energy scale.
• 12B (mean and end-point) provides relative
  calibration of energy scale and stability

7
Automated Calibration System
Routine deployment (daily or weekly)

• Establish 8 MeV energy scale
• ? neutron efficiency (0.1)
• Determine 1 MeV threshold energy
• ? positron efficiency (0.02)
• Monitor different scintillator regions
• Overall detector health and status
• - optical attenuation
• - scintillation yield
• - reflectivity, transmission of surfaces
• - dead PMTs
• Provide input to corrections
• ? All detectors should have identical,
  constant response

8
Automated System Hardware

• 3 deploy system per module
• Each corresponds to one point in horizontal
  cross-section with full z axis.
• Each capable of deploying four different sources.

9
One step motor unit

• One turn table, 4 step motors.
• Computer automated DAQ using position encoder,
  load cell readings.

10
Calibration System Software Flowchart Subunit VI
(work of M. Inadomi)
(calibration inputs.vi)
Velocity, Acceleration, Source positions,
Wait times, Wheel-pulley non-linearity
constants, Board ID, Axis number, ADC
number
Parameters passed in from higher level program
or read from file
Move while continually checking for errors
(tension out of range, limit switches tripped,
encoder slipping)
Lengths converted from metric distances to motor
steps using the non-linearity constants
(reverseNonLinComp.vi)
Error
No Error
If error, handle it and write to file. Encoder
slipping return source to pig by putting motor
in open-loop mode and telling it to find the
reverse limit. Alert human. Tension too low move
up a little bit, alert human. Tension too high
move down a little, alert human. Forward limit
switch same as tension too low. Reverse limit
switch write to file.
Wait the appropriate length of time
Return to pig
11
Control of a Sub-unit
Stripcharting tention and position
12
Simulation of Calibration With Sources

• Three Radioactive Sources
• 68Ge, 0 KE e ? 1.022 MeV ?s
• 60Co ? 2.506 MeV ?s
• AmBe ? n ? delayed capture on p (2.2 MeV ?) or
  Gd (8.0 MeV ?s)

? e energy threshold
? e energy scale
? neutron threshold and scale

• Detector condition varied
• Attenuation length
• Reflectivity of SS tank
• Scintillation yield (inner/outer relative
  change)
• Detector failure models
• Dirt deposited on acrylic
• Loss of PMTs

Q Can we measure it? Can we fix it?
13
Change of Attenuation Length
Q How do we tell it from the change of
scintillation yield or optical attenuation?
A Based on position uniformity of the yield
So the uniform/center yield ratio can be used
as a measure of the attenuation length. 10,000
neutron events each
14
Change of Steel Tank Reflectivity
Q How to separate it from attenuation or
scintillation yield?
A Use reflected/direct yield ratio
Use multihit TDC data direct ?20 ns
?reflected (L3 m, n1.5)
15
Change of Scintillation Yield
A global change of scintillation yield in both
inner and outer zones is trivial to calibrate
(putting source at the center)
Q What if only the outer scintillation yield
changes?
A Put the source close to the outer layer and
look at the tubes close-by
16
Resulting Neutron Efficiency
0.1 variation for reasonable range of parameters.
17
Resulting Positron Efficiency
68Ge to calibrate the threshold. Use 60Co to set
the scale.
Stable to 0.02!
18
Resulting Positron Spectrum Ratio
No false spectrum distortion (due to the relative
shift in energy scale) between near and far
detectors.
19
Dead PMTs
Easy to detect dead PMTs no hits in N events.
Q If 10 of the tubes are dead, how big a deal,
and how well can we correct for it?
For dead tubes, KAMLAND used the dentist
approach filling the holes with adjacent good
tubes. But KAMLAND is spherical
20
Add 1cm layer of absorbing (l1cm) acrylic at
bottom of central region
21
Dirty Acrylic Effect
Positron efficiency changes 99.8 ? 98.9
22
Dirty Acrylic Effect

• Simple fixes restore positron efficiency to
  99.6,
• (vs. 99.8 w/o dirt)
• Further studies in progress.