Eliminating Background Radiation in Double Chooz

1
Eliminating Background Radiation in Double Chooz

• Modeling Radiation from the glass in the
  Photomultiplier Tubes

2
Neutrino Source for Double Chooz

• n0  ?    p   e-    ?e

n0
p
?
?e
e
3
Learning the Language My Toy Program

• Takes an initial position and momentum of a
  positron
• Models the deceleration of this positron using
  the Bethe-Bloch Formula
• Finds the point of annihilation of the positron
• Emits two photons traveling in opposite
  directions

4
Bethe-Bloch Formula

• Approximate the Scintillator Fluid to be dodecane
  to get electron density (n) and the mean
  excitation potential (I).
• This is the form of the Bethe-Bloch Formula for
  high energies.
• This tells you how the positron (or any other
  charged particle) will slow down.

5
Electron-Positron Annihilation

• As the positron loses energy, it is likely to
  pick up an electron to form positronium.
• The positron and electron orbit one another, and
  eventually collapse and annihilate
• This annihilation yields two photons of .511 MeV
  travelling in opposite directions
• ?

?
e
e
?
6
Isotropic Distribution
Distribution of the phi coordinate for Photons
emitted from annihilation
Number of Events
Phi in Radians
7
Eliminating the Background

• The photomultiplier tubes are made of glass.
• This glass contains radioactive isotopes
• 232Th 
• 238U 
• 40K

8
Radioactive Decay from Elements in the Glass

• Alpha Decay
• 238U  ?  234Th    a
• 232Th  ?  228Ra    a
• Beta Decay
• 40K ?  40Ca     e-    ?e

9
Alpha Particles in the Scintillator Fluid

• Knock electrons around
• Electrons end up in excited states
• When the electrons change to ground state in an
  atom, a gamma ray is emitted.

10
Beta Particles in Scintillator Fluid

• Inverse ß decay (electron capture)
• Energy p e ? n ?e
• ß ß ? (at least two) ?
• Give off Bremsstrahlung, electromagnetic
  radiation produced by deceleration of a charged
  particle in matter

11
Photomultiplier tube (PMT)
12
The different areas in Double Chooz
13
Simulating Radiation from the PMTs

• Run a macro by entering coordinates for the
  PMTs.
• Count how many gammas were picked up by the
  PMTs.
• Find how many parent events generated these
  gammas.
• Find the ratio of
• (Events above a given threshold)/(parent events)

14
Positions of the Parent Events Generated
z-axis (mm)
x-axis (mm)
y-axis (mm)
15
Positions of the Parent Events Generated
z-axis (mm)
Cylindrical Radius (mm)
16
Positions of the Parent Events Generated
z-axis (mm)
x-axis (mm)
y-axis (mm)
17
Positions of the Parent Events Generated
z-axis (mm)
Cylindrical Radius (mm)
18
Simulating Radiation from the PMTs A Better Way

• Use a function of Geant4 to find all of the glass
  in the buffer area and fill it with my
  radioactive events.
• Count how many gammas were picked up by the
  PMTs.
• Find how many parent events generated these
  gammas.
• Find the ratio of
• (Events above a given threshold)/(parent events)
  
• Approximate the error by
• Events above the threshold / (parent events)

19
Positions of the Parent Events Generated
z-axis (mm)
x-axis (mm)
y-axis (mm)
20
Positions of the Parent Events Generated
z-axis (mm)
Cylindrical Radius (mm)
21

Energy Deposition for gammas in the Target
generated by the decay of 232Th
Number of events
Deposited Energy (in MeV)
22
Energy Deposition for gammas in the Target
generated by the decay of 232Th
Number of events
Deposited Energy (in MeV)
23
Energy Deposition for gammas in the Gamma Catcher
generated by the decay of 232Th
Number of events
Deposited Energy (in MeV)
24
Total Energy Deposition for gammas in the Target
and Gamma Catcher generated by the decay of
232Th
Number of events
Deposited Energy (in MeV)
25
Final Ratios (error in parenthesis)
.5 MeV .7 MeV 1 MeV
232Th .52 e -2 (.10198 e -2) .4 e -2 (.894427 e -3) .3 e -2 (.74597 e-3)
40K 0.999201 e- 3 (0.446856 e- 3) 0.19984 e -3 (0.19984 e -3) 0.19984 e -3 (0.19984 e- 3)
238U .26 e -2 (.72111 e -3) .24 e -2 (.69282 e -3) .1 e -2 (.447214 e-3)
26
Compared to a Similar Simulation

• Dario Motta did a similar simulation of the
  radiation in the PMTs of Double Chooz
• My data, when compared to Dario Mottas, isnt
  close enough once error is taken into account
• One possible reason for error attenuation

27
Position of Parent Events Generated by Dario
Motta in his Simulation
z-axis (m)
Cylindrical Radius (m)
28
Positions of the Parent Events Generated
z-axis (mm)
Cylindrical Radius (mm)
29
Track Length Histogram for gammas generated by
K40 Events
Number of Events
Track Length (mm)
30
Track Length Histogram for gammas generated by
K40 Events
Number of Events
Track Length (mm)
31
Track Length Histogram for gammas generated by
K40 Events
Number of Events
Track Length (mm)
32
Things Left to do to Find Attenuation

• Of the gammas that have a short track length,
  find which ones begin at the back of the PMTs.
• Find which gammas are travelling towards the
  front of the PMTs.
• This is an approximation for the gammas that did
  not make it due to attenuation inside the PMT.

33
SandAnother Possible Source of Similar
Background Radiation

• Problem Double Chooz needs a better way to
  regulate the thermal energy of the scintillator
  fluid.
• Solution Fill the space between the Veto area
  and the rock with sand to achieve thermal
  contact.
• Disadvantage Sand contains radioactive isotopes
  (just like the glass)

34
The different areas in Double Chooz
35
Simulating Radiation from the Sand

• Approximate the area filled by the sand by
  filling the Steel Shielding of the Veto.
• Count how many gammas were picked up by the
  PMTs.
• Find how many parent events generated these
  gammas.
• Find the ratio of
• (Events above a given threshold)/(parent events)
  

36
Positions of the Steel in the Buffer
z-axis (mm)
x-axis (mm)
y-axis (mm)
37
Positions of the Steel in the Buffer
z-axis (mm)
Cylindrical Radius (mm)
38
Positions of the Steel Shielding in the Inner Veto
z-axis (mm)
x-axis (mm)
y-axis (mm)
39
Positions of the Steel Shielding in the Inner Veto
z-axis (mm)
Cylindrical Radius (mm)
40
Total Energy Deposition for gammas in the Target
and Gamma Catcher generated by the decay of 40K
Number of events
Deposited Energy (in MeV)
41
Things Left to do to Find Radiation from the Sand

• Find a way to fill just the inner veto instead of
  the steel shielding.
• This volume isnt readily apparent from the
  Geometry file my simulation uses.
• Also, try to generate enough events to get energy
  deposition in the target and gamma catcher.

42
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