Scintillation R.L. Cooper

What is Scintillation? • In particle physics, particles deposit energy into a detector medium • The medium is “excited” • How do we extract this information? • Last week we heard of a number of ways • Utilize the EM interaction in some way

• Scintillation is the conversion of the excitation energy to photons • Depending on timescale, also referred to luminescence, phosphorescence, fluorescence, etc

Some Scintillators I Have Used 128

Inorganic BGO Crystals (RDK)

Organic Liquids (SciBath) Liquid Noble Gas (COHERENT) !-"#$%:+*(*(>:'%&'(')(*+ =>,&8+1'(16(,&*?34;(,

●*9'%)/:* ;* 1 ms • Impurities or defects can create “energy” traps or metastable states • BGO (among others) tend to have low afterglow out to 3 ms • Many Tl-doped scintillators can be quite high à order-% after 3 ms • COHERENT experiment uses CsI[Na] instead of more traditional CsI[Tl] for this reason

A Table of the Most Common Inorganic Scintillators NaI(Tl)

CsI(Tl)

CsI(Na)

CsI(pure)

BGO

CaF2 (Eu)

Light Yield (photons / keV) 300 K

38

54

41

2

8-10

19

Light Yield at 77 K relative to 300 K

< 1∗

< 1∗

< 1∗

10

3

n/a

Emission wavelength (nm)

415

550

420

315

480

435

Primary Decay Time (ns) 300 K

250

1000

630

16

300

940

Density (g/cm3)

3.67

4.51

4.51

4.51

7.13

3.18

Index of Refraction

1.85

1.79

1.84

1.95

2.15

1.47

Hygroscopic

yes

slightly

yes

slightly

no

no

A more extensive HTML table is available at: http://scintillator.lbl.gov

Scintillation Mechanism for Most Inorganic Crystal Band Structure Inorganic Crystals

excitation

electron

hole

from S.E. Derenzo

Organic Liquid Scintillators

Minimum Amount of Chemistry • Carbon bonds hybridize • sp2 hybridization forms s bonds that are in plane and bond carbons together • The remaining 6p orbitals form p bonds are out of plane and delocalized • Electrons in p bonds “orbit” with both spins in both directions

ersystem crossing can Competing Effects populating the triplet

• Excited state waveT1 and T 1-S0 cannot occur function has some ctly due to angular overlap with ground mentumstate à and parity non-radiative ection rules decay

s a long lifetime since • Intersystem excitation longto theof triplet state à S0 is forbidden

lived phosphorescence is phosphorescence

• T1 ßà S0 forbidden by ed fluorescence from spin / parity (slow)

plet can also occur

+ T1 → S1 + S0 + phonons

http://micro.magnet.fsu.edu/primer/techniques/fl uorescence/fluorescenceintro.html

from http://micro.magnet.fsu.edu/primer/techniques/fluorescence/fluorescenceintro

Making a Liquid Scintillator 1. Start with inexpensive benzene derivative

pseudocumene

xylene

2. Add high-efficiency fluor to non-radiatively transfer energy: tend to be more expensive butyl-PBD

PPO

p-terphenyl

3. Add additional wavelength shifters: bis-MSB, POPOP 4. Dilute for attenuation length (e.g., mineral oil)

Other Popular Solvents • Other issues with benzene derived solvents • Flammability, flash point, carcinogenic, etc • DIN

• LAB

Pulse Shape Discrimination • Some scintillators have different pulse shape due to different ionization / excitation methods (e-, nuclear-recoil)

Quenching from Ionization Recombination • A photon strikes an atomic electronic • Electron carries tremendous energy and creates ionization and excitation in a long track • Ionization and excitation decay back to ground state emitting photons

Quenching from Ionization Recombination • Neutron strikes an atomic nucleus • Recoiling nucleus deposits more energy in a given path length (higher dE/dx than electrons) • Ionization and excitation density higher • More non-radiative recombination may be possible in shortened region

Ionization Quenching: Birks’ Law Ionization Quenching – Birks’ Rule • Assume linear light output to start with linearity at low ionization dE density dL =A energy deposit at low dx dx ionization density let the density of excited molecules be proportional to the dL/dx = A dE/dx ionization density dE B • Also, density of excitation is dx let k be the fraction that is proportional to energy quenched dE A density dL = dx B dE/dX dx 1 + kB dE dx for small dE/dx, approaches • k is the fraction of excitation linearity that get quenched for large dE/dx, approaches saturation dL dE AA dL

dx = dx = kB dx 1 + kB dE dx

from Birks

• for any given scintillator composition, must fit the quenching data to get kB • A is the absolute scintillation efficiency

Light Output vs. Recoiling Species for Typical Organic Liquid ELJEN TECHNOLOGY SWEETWATER, TEXAS

RESPONSE OF EJ-301 LIQUID SCINTILLATOR SCINTILLATION LIGHT PRODUCED VS. PARTICLE ENERGY 6

NUMBER OF SCINTILLATION PHOTONS PRODUCED

10

105

ELECTRONS

104 PROTONS

103 ALPHAS

CARBONS

102

10

Data Source: V.V. Verbinski et al, Nucl. Instrum. & Meth. 65 (1968) 8-25

0.1

1

10 PARTICLE ENERGY (MeV)

100

Comparing Inorganic vs. Organic Scintillators • Best crystals have 40,000 photons / MeV energy deposit: Best organics produce just over 10,000 • Inorganic crystals have high density (5 g/cm3) whereas organics are similar to water • Inorganic crystals have variable Z for excellent photoelectric effect response vs. organics have Z ~ 6 • Organic scintillators have fast timing on ns timescales vs. 0.1-1 microsecond timing for inorganic scintillators • Radiation damaging a big issue crystals

Liquid Noble Gas Scintillators

The Basics:

The Basic Properties •

Light yield ~ few 10,000’s of photons per MeV (dependences on E field, LNG has high scintillation light yield particle type and purity)

E. Morikawa et al., J Chem

J Chem Phys vol 91 (1989) Phys 91 (1989) 1469. 1469 E Morikawa et al

(10s photons per keV)

•Wavelength Up to 40 for LAr, 25 for LXe of emission is 128nm

• “Fast” and “slow” components in Light with two characteristic time pulse shape •

constants: • 6 ns: 1600 ns for LAr, 3 ns: 27 ns for LXe - fast component, 6 ns VUV light production - slow component,1500 ns

• 128 nm for LAr, 178 nm for LXe Argon is highly transparent to its • Long photon attenuation lengths and own scintillation light. electron transport lengths (> 10 m)

• For Ar, scalable to > 10 kTon detectors

l (nm) = (1240 eV·nm) / E (eV)

Liquid Noble Gas Scintillation Path Self-Trapped Exciton Luminescence

Ar+

Ar+ Ar e-

e-

µAr Particle Collision

Thermal e- & Recombination

Ionization

Ar*

Excitation Recombination Luminescence

Ar*

Ar

Exciton Formation

Ar

g

Ar

Radiative Decay

Adapted from slides by Ben Jones talk on LArTPC Workshop

Exciton Formation and Light

http://www.e15.ph.tum.de/research_and_projects/liquified_rare_gases/

Exciton Formation and Light Calculations of the excimer state energies of xenon, as a function of nuclear separation J. Chem Phys 52, 5170 (1970)

eAr2+

http://www.e15.ph.tum.de/research_and_projects/liquified_rare_gases/

Excitons: Singlets and Triplets Ar*

Ar*

Ar

Exciton Formation

Ar

Singlet Exciton

Ar*

Singlet Scintillation Time Constants of LAr Scintillat Decay Time ~6 ns

An example from: 2010 JINST 5 P06003 (WArP collaboration)

Ar

Triplet Exciton

Triplet Scintillation Decay Time ~1600 ns

Pulse Shape Discrimination: Exciton Formation Path Matters Self-Trapped Exciton Luminescence

Ar

35% Singlet

Exciton Formation

65% Triplet

Ar*

Ar

50% Singlet

Exciton Formation

50% Triplet

Ar* Recombination Luminescence

Ar

g

Ar

Radiative Decay

Adapted from slides by Ben Jones talk on LArTPC Workshop

Quenching Results in Argon: Results from DEAP Collaboration

Mechanisms for Quenching Quenching by Nitrog • Because triplet is long, strongest effect on slow lifetime • 2 ppm N2 concentrations • What about Xenon dimers? • Nitrogen also absorbs photon Competing Excimer Dissociation Process

Ar*

Ar

Ar

Ar

N

N

N

N*

•  First measured by WArP:

Acciarri et al 2010 JINST 5 P06003

Additional Thoughts About Purity: O2 & H2O Is NOT Your Friend • Oxygen highly electronegative à readily absorbs ionization electrons and stunts recombination scintillation pathway • Diatomic excimer competition too? • 21% of air and generally in all cryogenic liquids • 10 ppb levels of oxygen affect scintillation • For TPCs, oxygen will absorb the traveling electrons • Argon can be too pure and leads to high voltage standoff issues à track down more than anecdotal evidence