Having Your Cake and Eating it too!Reconstructing Direction in a Liquid Scintillator Detector

Lindley WinslowUniversity of California Los Angeles

Cherenkov Light Scintillation Light

Directionality Energy Resolution

➢ Nucleus Z+2Nucleus Z ➢

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Nuclear Process

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I am particularly interested in Neutrinoless Double Beta Decay.

So I really care about the energy resolution.

Neutrinoless Double Beta Decay(Cerenkov Only)

...but some directionality would be nice!

136Xe 2νββ

76Ge 0νββ

Cherenkov threshold is ~0.2MeV.

136Xe 2νββ

76Ge 0νββ

Probably a statistical separation will be needed.

So how are we going to do this?

Number of Cherenkov Photons for a 1MeV e-

The Cherenkov light is still there...

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Retains directional information!

Longer wavelengths travel faster in scintillator and scintillation processes have inherent time constants.

Using the KamLAND index of refraction...

the velocity at 370nm is 0.191m/s and at 600nm is 0.203nm.

Simple KamLAND

Geant4 scintillation simulation (not using GLG4sim) with scintillator properties from KamLAND.

Simple KamLAND

Assumed 100% coverage and quantum efficiency as measured for the Double Chooz PMTs (~20% like KamLAND but a better measurement).

Simple KamLAND

The default photodetector timing is improved to match that of LAPPDs or Hybrid PMTs.

Simple KamLAND

The rise time of a typical scintillator is often not quoted. We are using 1ns as an estimate from timing data for the Double Chooz LS.

Time [ns]30 35 40 45 50

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So if you have good enough timing....

you should be able to separate the scarce Cherenkov from the abundant scintillation light.

arXiv:1307.5813

Time [ns]30 35 40 45 50

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This is the simulation with 0.1ns timing resolution.

Rc/s = 0.63

The LAPPD could provide this!

arXiv:1307.5813

If we put this timing data into basic reconstruction algorithms (from WCsim)...

we can reconstruct vertices and direction at the center of the detector.

arXiv:1307.5813

Reducing the energy we see the expected broadening due to photon statistics.

5.0MeV

2.1MeV

1.4MeV

arXiv:1307.5813

Does this work with current photomultiplier tubes (i.e. the 17-inch PMTs currently used by KamLAND)?

This is a simulation of a 6.5m spherical detector with 1.28ns timing resolution.

Rc/s = 0.25

Time [ns]30 35 40 45 50

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This is a standard large PMT.

arXiv:1307.5813

The separation needs more red light.

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What about a more red sensitive PMT?

This gives beautiful results!

Rc/s = 1.01

Time [ns]30 35 40 45 50

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The problem is it is a 1cm diameter PMT...

arXiv:1307.5813

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What if I could narrow the emission spectrum?

This is the narrowed emission spectrum with traditional PMTs and 0.1ns timing.

Rc/s = 0.86

Time [ns]30 35 40 45 50

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This is the quantum-dot-doped liquid scintillator.

Quantum Dot Doped Scintillator

What are quantum dots?

What are Quantum Dots?

Quantum Dots are semiconducting nanocrystals.

A shell of organic molecules is used to suspend them in an organic solvent (toluene) or water.

Common materials are CdS, CdSe, CdTe...

Isotope Endpoint Abundance48Ca 4.271 MeV 0.187%

150Nd 3.367 MeV 5.6%

96Zr 3.350 MeV 2.8%

100Mo 3.034 MeV 9.6%

82Se 2.995 MeV 9.2%

116Cd 2.802 MeV 7.5%

130Te 2.533 MeV 34.5%

136Xe 2.479 MeV 8.9%

76Ge 2.039 MeV 7.8%

128Te 0.868 MeV 31.7%

Quantum Dot Materials Overlap with Candidate Isotopes!

Quantum dots provide the chemistry for suspending isotope in scintillator.

Why are they so popular?

Because of their small size, their electrical and optical properties are more similar to atoms than bulk semiconductors.

In fact, the optical properties of quantum dots with diameter <10nm is completely determined by their size.

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Their size is easily regulated during their synthesis.

Example CdS Quantum Dot Spectra:

They absorb all light shorter than 400nm and re-emit it in a narrow resonance around this wavelength.

Very Useful for Biology, Solar Cells, and LEDs!

surface states=not good for us

We have found better quantum dots!

JINST 8 (2013) P10015

We have made attenuation length measurements!

We believe filtering is removing aggregated quantum dots.JINST 8 (2013) P10015

The Trilite450 quantum dots are looking good!

JINST 8 (2013) P10015

Progress is being made in understanding the details of the quantum dot chemistry for our application.

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Next Steps:

1m3 Detector -Create a test stand for both the photodetectors and the scintillator and may be go underground....

ν.25mL -Understandthe chemistry

The 1m3 Detector

a possible tank at LANL

Recall you can have Two Neutrino Double Beta Decay:

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With 10g of 116Cd, I expect 1000 events in 6 months.

➢ Nucleus Z+2Nucleus Z ➢

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Nuclear Process

Next Steps: ν.

Take over a cavern and try a first large-scale deployment of LAPPDs and new scintillators.

From: K. Inoue

Far Future Dreams....

Thank you!

Whats new? Precision Spectrum Measurements

Direct excitation of the toluene, understanding the energy transfer in the scintillator.

Whats new? Precision Spectrum Measurements

Direct excitation of the quantum dots. At these high concentrations it seems the UV light can start changing the chemistry of the quantum dots.

Fitting to a three exponential model + PMT response:

Photon Arrival Time [ns]-300 -250 -200 -150 -100

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410 Toluene + 5 g/L PPOSigma-Aldrich 380 nm Dots

NN-Labs 360 nm Dots