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.
Introduction
Lead Tungstate (PWO) crystals have proven practical for use in compact calorimeters such as within the 12 GeV Neutral Particle Spectrometer (NPS) in Hall C of JLab as well as the Electron Ion Collider (EIC). The properties of PWO such as its small Moliére radius, short radiation length, and relatively high light yield in comparison to other heavy crystals mean that in principle, a PWO array would make a compact calorimeter that would provide good energy resolution due to its strong, defined bursts of scintillation photons (Novotny 477). In fact, the response time of PWO is only about 5-14ns, meaning that it can provide a fast signal with timing resolution greater that 100ns2 (Horn 2015).
However, the manufacturing process of PWO crystals has yet to be perfected, and recent measurements have shown considerable variation in crystal properties. For instance, a 1997 University of Giessen, Germany study measured a wide distribution in optical transmission in addition to light output across different crystals from two companies: RI & NC, located in Belarus; and SICCAS, located in China (Novotny 479). Their results for transverse light transmittance for three samples showed not only differences in the values and curvatures of the transmittance spectra of the different crystals, but also showed great differences in the transmittance spectra at different positions of the same crystal. In particular, one of their samples had a difference in transmittance of over 60% around the 350nm wavelength simply based on the crystal’s position (Novotny 477). The inhomogeneity of these crystals reflects differences in crystal properties in part due to problems with quality control in the manufacturing process. Measuring and understanding the origin of such differences is fundamental to future success in creating and selecting the best quality crystals for use in the NPS and EIC.
In the following sections, we describe our progress toward characterizing 28 crystals with dimensions 20x2x2 cm from Shanghai Institute of Ceramics Chinese Academy of Sciences (SICCA), received in December 2015. Among the parameters measured were the crystals’ transmittances (both longitudinal and transverse), their light yield as a function of gate width in order to identify the fast as well as slow luminescence components, their induced absorption coefficients at 420nm (the crystal’s peak emission wavelength), as well as crystal dimensions. Our aim is to identify crystals that are suitable for use in the EIC and NPS, using the following parameters as guidelines:
Future Projects Involving PWO
The upcoming Electron Ion Collider project will focus on providing a 3-dimensional image of nucleon to better understand sea quarks and gluons and the formation of nucleon spin in the field of Quantum Chromodynamics. Generalized Parton Distributions (GPD) and Transverse Momentum-Dependent Parton Distributions (TMD) offer clear pictures of momentum tomography and spatial tomography necessary to deconstruct nucleon structure. The study of exclusive and inclusive reactions necessary to determine GPDs and TMDs requires high luminosity beams and a world-class detector. Within the detector, the PbWO4 crystals will play a pivotal role in Particle Identification (PID) and particle reconstruction and must have good angular resolution, energy resolution, and efficiency as electromagnetic calorimeters. In particular, they will potentially be located in the Barel EM Calorimeter (BEM) and …???.
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: _____
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.
X-ray Radiation
Dimension Measurements
Data Analysis
Conclusion
8/5/16 Presentation
File:Characterization of Lead Tungstate Crystals for Neutral Pion Detection.pdf
Works Cited
Horn, Tanja. "A PbWO 4 -based Neutral Particle Spectrometer in Hall C at 12 GeV JLab." J. Phys.: Conf. Ser. Journal of Physics: Conference Series 587 (2015): 012048. Print.
Novotny, R., W. Doring, K. Mengel, V. Metag, and C. Pienne. "Response of a PbWO/sub 4/-scintillator Array to Electrons in the Energy Regime below 1 GeV." 1996 IEEE Nuclear Science Symposium. Conference Record 44.3 (1997): 477-83. Print.
"Neutral-Particle Spectrometer." (2014): n. pag. Web. <https://phys.cst.temple.edu/qcd/doc/NPS_WP_11262014_final.pdf>.