[PPT] - Quantum Dot Liquid Scintillators T. Wongjirad (MIT) for A. Elagin PowerPoint Presentation

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Quantum Dot Liquid Scintillators

photo: plasmachem

T. Wongjirad (MIT) for
A. Elagin (U. Chicago), D. Gooding (MIT),  
B. Naranjo (UCLA), J. Ouellet (MIT),
R. Schofield (UCLA), L. Winslow (MIT)

FroST, 3/16/2016

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Quantum dot nanocrystal are an

intriguing material for the use in scintillator experiments

Controllable optical properties
Can be suspended in both water

and organic scintillator

Made out of elements that can

be used in neutrino-less double beta decay searches

Outline

Introduction

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What are quantum dot nanocrystals?
Past and current work on quantum dot liquid

scintillators (QD LS)

NuDot — a prototype directional liquid scintillator

detector for R&D on LS such as including a QD LS

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Introduction

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Quantum Dots

Quantum dots are nanometer-scale

crystals of semi-conductor with interesting optical properties

They are suspended in both organic

solvents and water through the use of surface coordinating ligands

These ligands are also

critical in the synthesis of the QDs

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Growing Quantum Dots

Cd

MeCd + TOPSe →CdSe + monomers

Inject TOPSe into hot (225⁰C) MeCd solution

QDs made by

growing crystals in solution

Start by mixing

metal components and capping ligands

capping ligand

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Growing Quantum Dots

Cd

MeCd + TOPSe →CdSe + monomers

Metals nucleate

and begin forming a crystal

Capping ligands

coordinate on the surface, controlling the reaction rate, preventing agglomeration, and keeping the crystal in suspension

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QDs made of 0νββ Isotopes

Elements used to make

QDs are also elements with which to look for neutrino-less double beta decay

QD Examples:
CdSe/ZnS
CdTe
Nd2O3
ZrO
QDs are a way to load

scintillator, but also come with great optical properties that we can take advantage of!

Isotope Endpoint Abundance

48Ca

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%

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Optical Properties

Like bulk semiconductor, QDs

can absorb/emit photons through the creation/ annihilation of an exciton

But because crystals are

small, the exciton’s wave function is confined - similar to a particle in a box

confinement separates energy

levels and widens band gap (compared to bulk crystal)

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Optical Properties

result: QDs absorb broadly in the UV and emit in a

narrow band of wavelengths blue curve: absorption red curve: emission

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Furthermore, because
f the confinement

effect, the color emitted is directly dependent on the size

f the QD

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Optical Properties

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Optical Properties

duration

f synthesis

time

The size to which they

grow can be controlled

Can make the QDs

that best fit your goals!

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QD Uses

QDs are used in
Biological labeling
Improved solar cells
TVs
Admittedly, QDs are still

expensive ($100 to $10K per gram), but with commercial uses, there is hope that production increases and prices goes down

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QDs for Detectors

How do QDs help scintillators (apart from loading

metal)?

One way is that it could help in the gathering of

directional information in large-scale scintillator detectors

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Directional Information

Number of Cherenkov Photons for a 1MeV e-

absorbed by scintillator

Cherenkov

photons provide info on direction of particles that can be used to separate signal and background

However, only long

wavelengths can avoid absorption by the scintillator

JINST 7 (2012) P07010

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Directional Information

Number of Cherenkov Photons for a 1MeV e-

absorbed by scintillator

Still, there is

some region where Cherenkov photons can escape and still be detected

JINST 7 (2012) P07010

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Directional Information

Quantum Dots have a tunable absorption and emission spectra
Want a narrow emission spectra and absorption spectra that

ends at low wavelengths allowing more long-wavelength Cherenkov photons to pass

emission [a.u.]

20 40 60 80 100 120 140

wavelength [nm]

200 300 400 500 600 700

(DC T arget) KamLAND (emission.dat) normalized at peak to KamLAND emission.dat normalized and shifted by 80nm

KamLAND emission spectrum QD Cytodiagnostics spectrum (peak at 461nm) same as red, shifted by -77nm

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Directional Information

With fast enough electronics, one could identify

Cherenkov from Scintillation photons

Rc/s = 0.86

JINST 9 (2014) P06012

assuming 0.1 ns resolution

black: Cherenkov photons red: scintillation photons

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Work in Progress

Work is being done to explore using directionality and

QDs in LS. Past published results can be found here:

Some quick highlights

JINST 8 (2013) P10015 arXiv:1307.4742 JINST 7 (2012) P07010 arXiv:1202.4733 JINST 9 (2014) P06012 arXiv:1307.5813

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Past Measurements

JINST 8 (2013) P10015 arXiv:1307.4742

emission [a.u.]

20,000 40,000 60,000 80,000 100,000 120,000

wavelength [nm]

350 400 450 500 550 600 650

CdS400 CdS400 + 5g/l PPO T

luene + 5g/l PPO

emission [a.u.]

50,000 100,000 150,000

wavelength [nm]

350 400 450 500 550 600 650

Trilite450 Trilite450 + 5g/l PPO T

luene + 5g/l PPO
Demonstrated QDs act as tertiary wavelength shifter

addition of PPO as intermediate wavelength-shifter helps light yield

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Past Measurements

emission [a.u.]

1e+06 2e+06 3e+06 4e+06

wavelength [nm]

400 500 600 700

CdS400 (exc. at 360 nm) Trilite450 (exc. at 425 nm) CdS380 S1 (exc. at 360 nm) CdS380 S1 (exc. at 360 nm) with Fluoromax 3

Emission spectra varies on type of dots and who made

them — we want as narrow as possible

state-of-the-art are core/shell QDs with high quantum yield

and smaller long wavelength emission tail (QYs > 0.7)

JINST 8 (2013) P10015 arXiv:1307.4742

Trilite is a core/shell QD

Shell of different semi-conductor elements shields core’s surface and improves stability, QY

Rest are core-only

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Past Measurements

absorbance

0.2 0.4 0.6

wavelength [nm]

400 500 600 700 800

2 2

CdS380 S1 (June 18, 2013) CdS380 S1 (June 18, 2013) filtered

absorbance

0.05 0.1 0.15

wavelength [nm]

400 500 600 700

CdS380 S1 (March 13, 2013) CdS380 S1 (March 25, 2013) CdS380 S1 (June 11, 2013) CdS380 S1 (June 18, 2013) CdS380 S1 (June 18, 2013) filtered CdS380 S2 (June 11, 2013)

= 0.54 m

= 1.08 m L L

Absorbance tail at long wavelengths can grow
ver time — but can be cleaned up via filtering

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Outline

Current Measurements

Avoiding overlap

between the absorbance and emission spectra is important in maximizing light yield

The overlap of the the absorption spectrum

f quantum dots and the emission spectrum
f quantum dots in toluene doped with 2.0

[g/L] of PPO. (Left) The area of the overlap vs light yield of the quantum dots. (Above)

(this and other studies in preparation for publication)

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Further R&D

Many open questions still. Some examples:
What is the absolute light yield for a sizable

detector?

What is the absorption and scattering lengths at

different wavelengths?

Will new types of QDs improve the above?
How to work with QD LS (e.g. purification)?

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Synthesis

We’re starting to learn how synthesize our own dots in

rder to have samples for testing

Diana Gooding, trained chemist turned physicist

Our first batch of QDs

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BG Rejection using Topology

f Cherenkov/Scintillation Photons

Simulation details

l 6.5m radius detector, scintillator model from KamLAND simulation l TTS=100 ps, 100% area coverage, QE 12-23% l Light within a pre-defined time window to capture early light

Key parameters determining separation of 0νββ-decay from 8B:

l Scintillator properties (narrow spectrum, slow rise time) l Photo-detector properties (fast, large-area, high QE)

Cherenkov PEs Scintillation PEs

two-tracks (double-beta decay signal) vs single-track (8B solar neutrino background)

Single 2.53 MeV electron Differences that are hardly seen by eye can be reconstructed by pattern recognition e.g. spherical harmonics analysis

Power spectrum (rotation invariant: works well in spherical geometry e.g., SNO+, KamLAND )

(Publication in preparation)

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Further R&D

NuDot: A Prototype Directional Liquid Scintillator

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NuDot

A 1 m3 scale tank of scintillator

surrounded by PMTs

Readout by PSEC4:

10 GS/s, 1.5 GHz bandwidth

Goals:
study scintillators, including

QD LS

measure two-neutrino

double-beta decay

2.17 meters

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Flat Dot

PMTs

15cm

Quartz Vial NaI Detector

Demonstration of Cherenkov/scintillation separation using tagged Compton source.

Also, system for testing new data acquisition system and small batches of different scintillator cocktails

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PSEC4

30 Channel PSEC4 Card 10 GS/s digitization

Central Readout Card

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Flat Dot

Setting up data acquisition
Testing PMTs
Balloon making

Built SiPM muon paddle

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Summary

We’re exploring the use of colloidal quantum dot

nanocrystals as a new type of liquid scintillator — with an aim towards the search for neutrino-less double-beta decay

QDs can be made out of elements useful in neutrino-less

double beta decay search (Cd, Se, Te)

The absorption and emission spectra of QDs can be tuned
QDs a very active field — lots of different QDs to explore
Developing QD LS and other LS technology through a 1 m3

prototype, NuDot

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Backup

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Backup

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Quantum Dots

Quantum dots are nanometer-scale crystals of semi-

conductor with interesting optical properties

not to scale

image credit: nanoaxis

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Ligands can be exchanged

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Outline

Growing Quantum Dots

Post-synthesis,

capping ligands passivate and protect the surface of the crystal, influencing stability and the quantum yield of the crystal

The ligands, being

loosely bound can be exchanged — in

rder to change

solvents, for example

Ligands protect the surface Ligands can be exchanged