MainPage:Nuclear:Summer2016:PWO

PbWO4 Characterization and Recovery

Abstract

The potential of using Lead Tungstate crystals (PbWO4) in electromagnetic calorimeters like the Neutral Particle Spectrometer (NPS) at 12 GeV Jefferson Lab and in future projects such as particle identification in the endcaps of the EIC (Electron Ion Collider) detector has been researched extensively. The radiation hardness as well as the small Moliere radius of PbWO4 (PWO) crystals make them ideal for use in a compact detector. Additionally, the light yield of these crystals outperforms that of PbF2 and other heavy crystals, lending to even more sensitivity to event resolution. Recent measurements have shown large variations in crystal properties. This is a major concern for the construction of particle identification detectors like the NPS. Testing of the crystal uniformity and understanding the origin of the variation have thus become necessary. The characterization of the crystals includes measurements of the crystal dimensions, optical transmittance, both longitudinal and transverse, the light yield and decay kinetics to identify slow luminescence components, as well as tests of the radiation hardness. Optical clarity after radiation damage can in principle be restored by stimulated recovery with light. Optical bleaching with blue light is the default method, but curing at longer wavelength may be preferable under specific experimental conditions. The choice of the curing system is thus an important factor in the construction of the NPS infrastructure. The results of crystal characterization and effects of radiation on optical properties, as well as the effectiveness and practicality of the LED curing system will be discussed.

Methods

Light Transmittance

The light transmission of each crystal was measured using the Lambda 950 Spectrophotometer. The crystals were placed in the second compartment and a Labjack was installed and secured to place the crystal at the same height as the light beam. On the Labjack, fixed placement guides were put in place to ensure the constant crystal placement across all trials An iris with a diameter of 11.5 mm was also installed and secured in the same compartment to ensure the beam diameter was located within the crystal's dimensions. At machine startup and every five trails, the machine was aut-ozeroed to ensure accuracy of measurements. The spectrophotometer settings were as follows: _____

The Lambda 950 Spectrometer for measurement of light transmission

Compartment within Lambda 950.

Placement guides used to fix crystals during measurements.

Transverse Transmission

Transverse transmission was measured with the beam firing through the middle of each 20 cm crystal. Paper was placed down to prevent scratches and the crystal was put in the placement guides. All crystals were unwrapped, cleaned with isopropanol, and marked with sharpie to indicate orientation of the crystals.

Longitudinal Transmission

To prepare the crystals for longitudinal transmission, each crystal was first wrapped in 3 layers of teflon tape and second 1 layer of electrical tape. To measure longitudinal transmittance, each crystal was placed in the corresponding placement guides.

Light Yield

The crystals were prepared for light yield measurements by placing one layer of electrical tape over three layers of teflon tape on one open face of the crystal. The baseline gate width used was 100ns.

Sample light yield graph for J41.

8/5/16 Presentation

File:Characterization of Lead Tungstate Crystals for Neutral Pion Detection.pdf