Cherenkov and Liquid Scintillator Detectors

R. Svoboda, UC Davis

R.Svoboda, HEPCAT Summer School 2023

Energy Loss in Matter

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free

electron

ionization

energetic

charged

particle

exciton

formation

Nuclear Recoil

All this energy eventually ends up as heat...but on the way It can make free charge and photons!

E_Total = E_ion+ E_exciton + E_heat

How Particles Produce Light

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• Cherenkov radiation
• Happens when charged particle speed exceeds local speed of light
• E&M wave front coherence process in transparent media
• water, oil, glass, gasses,…
• Scintillation
• Happens in some materials due to molecular or crystal excitations
• Liquid scintillators are mostly organic compounds
• Transition radiation
• Happens when charged particle moves through inhomogenous media

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(Note: electrically neutral particles must interact first to produce charged particles)

Many breakthroughs in Neutrino Physics were enabled by the invention of large optical detectors

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Large size for cost, fast timing for background reduction, low threshold, reconfigurable as the field progressed

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Water Cherenkov

Liquid Scintillator

• Excellent Transparency
• large size, low cost
• Directionality
• Particle ID
• Potential for large isotopic loading

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• High light yield
• low threshold
• good energy resolution
• Can be radiologically very clean

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Cherenkov

Emission

charged

particle

Cherenkov photons

refractive

index

The Cherekov angle

depends on the refractive

index of the medium and

the velocity

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Number of emitted optical photons per eV per cm

in terms of the wavelength interval,

Cherenkov light tends toward the

UV/blue end of the spectrum

UC Davis MNRC Reactor

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Blue glow is distinctive!

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gamma rays scattering electrons above the Cherenkov threshold, given by:

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Gas Cherenkov particle tagging

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photosensor

(not to scale)

nitrogen gas

(variable pressure)

Cherenkov

light

200 bars

300 bars

Ring Imaging CHerenkov (RICH) particle ID

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Cherenkov tracking detectors

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θ_C

Tracking via timing

charged

particle

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θ_C

O

charged

particle

Tracking via timing

vertex

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θ_C

O

charged

particle

Tracking via timing

vertex

R.Svoboda, HEPCAT Summer School 2023

θ_C

O

charged

particle

Tracking via timing

vertex

R.Svoboda, HEPCAT Summer School 2023

θ_C

O

charged

particle

Tracking via timing

vertex

Super-Kamiokande

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Showering and Non-Showering tracks

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Super-K atmospheric neutrinos

Liquid Scintillator

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What is scintillation?

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What is scintillation?

• One of the most common techniques in nuclear and particle physics
• Crookes (1903) ZnS screen to count alpha particles, used human eye to record data
• Curran and Baker (1944) coated a photomultiplier tube with ZnS to produce a scintillator with electronic readout
• Packard (1953) organic liquid scintillator (for medical uses)
• Very fast (nanoseconds)
• Good energy resolution
• Can be made as a liquid!

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R.Svoboda, HEPCAT Summer School 2023

Organic Scintillators

• The scintillation property comes from the physics and chemistry of the benzene ring
• Thus, liquid scintillators are all known as aromatic hydrocarbons
• How does this work?

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Liquid Scintillators are based on Benzene

• Carbon ring with a resonance structure of single and double bonds (σ bonds)
• The location of the orbitals above and below the plane leads to a complex set of excitation states (π bonds)

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Carbon Bond Hybridization

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σ and π bonds

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π bond delocalization in benzene

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π bonds

leads to electrons in circular

π bond structure in benzene

to move freely!

Energy levels and de-excitation times

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This leads to energy levels

in the benzene ring that

are Singlet or Triplet

Aromatic Organic Solvents

• Typically, any aromatic solvent can scintillate
• Some are hazardous materials due to either toxicity, flammability, or both

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hazardous

solvents

less hazardous

solvents

LAB is now popular

due to high flash point

and use in soap industry

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�

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unless a PPO molecule can absorb the energy, it will eventually be lost to heat. Thus, increasing the PPO

concentration increases the light yield in general

In order to avoid self absorption of the light produced by scintillation, a fluor (e.g. PPO) is needed.

The time it takes to transfer energy

to the PPO gets shorter as concentration increases.

Stokes Shift not enough! Usually need Fluors

Förster Resonance Energy Transfer

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FRET makes the process of transferring

the excitation energy from the solvent

to the fluor fast and efficient

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Z. Guo et al. (2019)

Liquid Scintillator Detectors

• In use since 1950's
• Organic liquid with waveshifting fluor converts ionization to photons in 400-700 nm range
• Detectors with 0.2 MeV threshold up to ~1 kton have been built
• Fast timing, free protons, can be loaded with neutron capture agents like gadolinium

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Disadvantage: Moderately expensive, combustible, semi-toxic,

hazardous waste, light is isotropic – track reconstruction difficult

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Daya Bay

KamLAND

SNO+

Recent Liquid Scintillator Detectors

NOVA

BOREXINO

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Nova

JUNO

(under construction)

RENO

THEIA (proposed)

Plastic Scintillator

• In use since 1960's
• Can be shaped to fit into large detectors like MINOS or in specialized configurations like MINERVA
• Easier to handle than liquid scintillator
• Still fast with free protons

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Disadvantage: Expensive, loading with neutron capture agents

difficult, needs segmented readout

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R.Svoboda, HEPCAT Summer School 2023

R.Svoboda, HEPCAT Summer School 2023

R.Svoboda, HEPCAT Summer School 2023

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R.Svoboda, HEPCAT Summer School 2023

PSD = Pulse Shape Discrimination

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R.Svoboda, HEPCAT Summer School 2023

R.Svoboda, HEPCAT Summer School 2023

Water-based Liquid Scintillator

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~10 nm

LS

surfactant

molecules

Liquid scintillator forms small (~10 nm scale) droplets called micelles in water that are stabilized by surfactant molecules with a hydrophilic head and hydrophobic tail. Micelles form under controlled chemical conditions and are shown to be stable over year time scales.

Advantages: Disadvantages

Cheaper than LS Radiological cleanliness more difficult

Non-combustible Lower light yield than pure LS

Ease of loading Li, Te, etc

Environmentally friendly

Oxygen nuclei instead of Carbon

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Ch/LS light separation, tunable LY

at low cost, environmentally safer than LS, non-combustible, …

Onken, Moretti, Caravaca, Yeh, Orebi Gann, Bourret Mat. Advances 1 (2020)

What can WbLS Do?

LS

WATER

Li, Te, Gd, Ca, …

put the ion of

your choice here

Instead of trying to dissolve desirable ions

in liquid scintillator, one can dissolve them

in water (much easier) to try and open up

new areas of physics

WATER

WATER

WbLS Concentration Can Be Tuned to Physics Needs

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Double Beta Decay

Proton Decay

Solar Neutrinos

Diffuse SN

Galactic SN burst

LS

Concentration

Mass Hierarchy

CP Violation

Geo-neutrinos

A single facility could have a high discovery potential and

a very long useful life due to flexibility and broad program

Sterile Neutrinos

Note: an estimate only!

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Water-based

Liquid Scintillator

Theia-25 detector�at SURF�

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THEIA-25 was proposed as the 4^th DUNE module at the Sanford Underground Research Facility (SURF) at the Module Of Opportunity Workshop at BNL in November 2019

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Funding agencies in Germany, U.S., and U.K.

are now supporting R&D activities for Theia.

Askins, et al. EPJC 80 416 (2020)

Theia White Paper

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This paper explores the potential scientific sensitivity of a hybrid optical detector across a broad range of interest.

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SUMMARY

Optical detectors using Cherenkov

and/or Scintillation are a major

part of HEP technology

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Fast timing and ability to make

radiologically pure are key

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Low construction and operational

costs contribute to widespread usage

in many areas

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New hybrid detectors now being

developed will be a game-changer!