Characterization of Lead Tungstate Crystals for Neutral Pion Detection

Abby McShane and Dannie Griggs

Why is PWO important?- Lead Tungstate Crystals (PWO) are ideal for use in a

compact electromagnetic calorimeter

- Small Moliére Radius

- Short Radiation Length = high stopping power

- Higher light yield than other heavy crystals

- In part due to high density

- Radiation hard

Examples: HyCal and CMS ECAL

Why is PWO important?- HOWEVER, recent

measurements have shown considerable variation of crystal properties

- Necessary to measure and understand the origin of variation

Example: crystal radiation hardness

Neutral Particle Spectrometer

“Next QCD Frontier”Quantum Chromodynamics- study gluons and

their “color charge”

Understanding gluon interactions and the formation of mass

Electron Ion Collider

Cross Sectional Measurements of Neutral Particles

Transverse Momentum Dependent Parton Distribution

General Parton Distribution

31x36 matrix, 61x74 cm2 areaPbWO4 Crystal

Calorimeter

Crystal SpecificationsParameter Unit EIC NPS

Light Yield pe/MeV 15 10-15

Transmittance

@420nm

% >60% >60%

dk m-1

Characteristic 1: Longitudinal Light TransmittancePerkin-Elmer Lambda 950

Photospectrometer

Modifications made in second compartment to accommodate crystals

Crystal Preparation

Isopropanol cleansing

3 layers of teflon tape followed by 1 layer of electrical tape on 20cm x2cm faces

Compartment SetupIris diameter of 11.5 mm

Labjack height of 90.79mm

Crystal Placement guides on LabJack

LabJack and Iris fixated Labjack

Integration Sphere

Iris

8.0 mm diameter 11.5 mm diameter 15.0 mm diameter 17.0 mm diameter

:)

0°

Characteristic 1: Longitudinal Light Transmittance

Characteristic 2: Transverse Light Transmittance

Crystal Preparation:

All tape removed

Wiped with isopropanol

Degradation of Transparency- Irradiation → point structure defects, electron traps, holes create color centers

within crystal lattice that absorb light → reduction in transparency

- Degradation of energy resolutionJ23

Zhu, Ren-Yuan. "Precision Crystal

Calorimeters in High Energy

Physics: Past, Present and Future."

AIP Conference Proceedings

(2006): n. pag. Web.

X-ray Irradiation: 6970 R/minFaxitron CP160

160kV, 6.3mA, 70 minutes

Shelf 8, 14.16cm LabJack height

X-ray Irradiation: 6970 R/min

Variation in Radiation Hardness

J23- Less Radiation Hard J27-More Radiation Hard

Quantification of Radiation Hardness: dk at 420 nm

- Closer to 0 = better radiation hardness

Absorption

coefficient=

How do we restore optical properties post-irradiation?- Thermal annealing

- Not practical with crystals while they are in the array (heavy, time consuming, requires lots of energy)

- Instead, supplying energy to the crystals through photons from LED lights

- Blue LED light “optical bleaching” → ionizes color centers by exciting them → frees them from crystal structure

- Restoration of crystal transparency

LED Curing

LED CuringJ16 J21

Establishment of procedure

Inconclusive results- too little data

LED CuringAbsorption

coefficient=

Light Yield Setup: amount of photoelectrons produced per 0.511 MeV

Na22Plastic Scintillator

PWO

22Na → 22Ne +β+ + v + γ

β+ + β- → 2γ

Light YieldCrystal Preparation:

3 layers of teflon tape followed by 1 layer of electrical on 20cm x 2cm faces and on one 2cmx 2cm face

Characteristic 4: Light Yield

Crystal Peak Single Electron Peak

Single Photoelectron Calibration((365-27.6)/(87-27.6))= 6 photoelectrons per photon

(6 electrons)/(.511MeV)=11.3pe/MeV

Variation in Light Yield due to Slow ComponentsFast Component: Electrons return to ground state rapidly after being excited, releasing energy via a photon

Slow Component: Electrons are “metastable” in high energy levels, take longer to return to their ground state/release photons

Ideally, light yield of slow components will not cause overall light yield to fluctuate.

To test, we measured light yield as a function of gate width.

Fast Component

Slow Component

Ideally, ratio

should be close

to 1. Seems to

correlate closely

with a horizontal

line → ✓

100ns= Standard Gate Width

Conclusion- From what we have seen so far, crystals tend to pass longitudinal and transverse

transmittance tests prior to irradiation

- Several of the crystals we measured did not have dk values below 1.5, meaning there would need to be a curing system in place to restore optical properties

- Many crystals also failed light yield specifications for the EIC (LY below 15pe/MeV)

Outlook- Continuing to measure crystals dk values and light yield /troubleshoot problems

- Infrared LEDs for curing

- Comparison of different manufacturers

- Light Yield pre and post irradiation

- Investigate radiation hardness relationship to light yield

AcknowledgementsWe PROFUSELY give thanks to:

Dr. Horn mentoring us and teaching us so much for the past couple weeks

Marco Carmignotto assisting us with machinery, mentoring us, being fantastic overall

Salina Ali Giving us root codes, helping with technology

Salim Roustom, Jake Wacht, Johann Meija-Ott Being awesome