Eliminating Background Radiation in Double ChoozModeling Radiation from the glass in the Photomultiplier Tubes

Neutrino Source for Double Chooz•n0  →    p+ +  e−  +  νe

→e

νe

p+n0

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

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.

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

•γe

e

γ

γ

Isotropic Distribution

Number of Events

Phi in Radians

Distribution of the phi coordinate for Photons emitted from annihilation

Eliminating the Background

•The photomultiplier tubes are made of glass.

•This glass contains radioactive isotopes:▫232Th ▫238U ▫40K

Radioactive Decay from Elements in the Glass

Alpha Decay:238U  →  234Th  +  α232Th  →  228Ra  +  α

Beta Decay:40K →  40Ca  +   e−  +  νe

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.

Beta Particles in Scintillator Fluid•Inverse β decay (electron capture):

Energy + p + e n + ѵe

•β + β (at least two) γ

•Give off Bremsstrahlung, electromagnetic radiation produced by deceleration of a charged particle in matter

Photomultiplier tube (PMT)

The different areas in Double Chooz

Simulating Radiation from the PMT’s• Run a macro by entering coordinates for the

PMT’s.

• Count how many gammas were picked up by the PMT’s.

• Find how many parent events generated these gammas.

• Find the ratio of:

(Events above a given threshold)/(parent events)

y-axis (mm)

x-axis (mm)

z-axis (mm)

Positions of the Parent Events Generated

Cylindrical Radius (mm)

z-axis(mm)

Positions of the Parent Events Generated

Positions of the Parent Events Generated

z-axis (mm)

x-axis (mm) y-axis (mm)

Positions of the Parent Events Generated

z-axis(mm)

Cylindrical Radius (mm)

Simulating Radiation from the PMT’s 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 PMT’s.

• 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)

Positions of the Parent Events Generated

z-axis(mm)

x-axis (mm)

y-axis (mm)

Positions of the Parent Events Generated

z-axis(mm)

Cylindrical Radius (mm)

Deposited Energy (in MeV)

Number ofevents

Energy Deposition for gammas in the Target generated by the decay of

232Th

Deposited Energy (in MeV)

Number ofevents

Energy Deposition for gammas in the Target generated by the decay of

232Th

Deposited Energy (in MeV)

Number ofevents

Energy Deposition for gammas in the Gamma Catcher generated by the decay of 232Th

Deposited Energy (in MeV)

Number ofevents

Total Energy Deposition for gammas in the Target and Gamma Catcher generated by the decay of 232Th

.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)

Final Ratios (error in parenthesis)

Compared to a Similar Simulation

•Dario Motta did a similar simulation of the radiation in the PMT’s of Double Chooz

•My data, when compared to Dario Motta’s, isn’t close enough once error is taken into account

•One possible reason for error: attenuation

Position of Parent Events Generated by Dario Motta in his Simulation

z-axis(m)

Cylindrical Radius (m)

Positions of the Parent Events Generated

z-axis(mm)

Cylindrical Radius (mm)

Track Length (mm)

Number of Events

Track Length Histogram for gammas generated by K40 Events

Track Length Histogram for gammas generated by K40 Events

Number of Events

Track Length (mm)

Track Length Histogram for gammas generated by K40 Events

Number of Events

Track Length (mm)

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 PMT’s.

•Find which gammas are travelling towards the front of the PMT’s.

•This is an approximation for the gammas that did not make it due to attenuation inside the PMT.

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)

The different areas in Double Chooz

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 PMT’s.

• Find how many parent events generated these gammas.

• Find the ratio of:(Events above a given threshold)/(parent events)

Positions of the Steel in the Buffer

z-axis(mm)

x-axis (mm)

y-axis (mm)

z-axis(mm)

Cylindrical Radius (mm)

Positions of the Steel in the Buffer

z-axis(mm)

x-axis (mm) y-axis (mm)

Positions of the Steel Shielding in the Inner Veto

z-axis(mm)

Cylindrical Radius (mm)

Positions of the Steel Shielding in the Inner Veto

Deposited Energy (in MeV)

Number ofevents

Total Energy Deposition for gammas in the Target and Gamma Catcher generated by the decay of 40K

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 isn’t 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.

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