Enhanced UV light detection using a p-terphenyl wavelength shifter

TL;DR

Applying a p-terphenyl wavelength shifter to UV-glass photomultiplier tubes significantly enhances their photon detection efficiency below 300 nm, enabling cost-effective improvements for Cherenkov detectors in high-energy physics experiments.

Contribution

This paper demonstrates a practical, cost-effective method to improve UV photon detection efficiency of PMTs using a p-terphenyl coating, validated for specific models and wavelengths.

Findings

Gain factor up to 5.4 at 215 nm

Detection efficiency improved by approximately 40%

Coating quality verified with UV LED testing

Abstract

UV-glass photomultiplier tubes (PMTs) have poor photon detection efficiency for wavelengths below $300\,\text{nm}$ due to the opaqueness of the window material. Costly quartz PMTs could be used to enhance the efficiency below $300\,\text{nm}$. A less expensive solution that dramatically improves this efficiency is the application of a thin film of a p-terphenyl (PT) wavelength shifter on UV-glass PMTs. This improvement was quantified for Photonis XP4500B PMTs for wavelengths between $200\,\text{nm}$ and $400\,\text{nm}$. The gain factor ranges up to 5.4 $\pm$ 0.5 at a wavelength of $215\,\text{nm}$, with a material load of $110\pm10\,\mu\text{g}/\text{cm}^2$ ($894\,\text{nm}$). The wavelength shifter was found to be fully transparent for wavelengths greater than $300\,\text{nm}$. The resulting gain in detection efficiency, when used in a typical Cherenkov counter, was estimated to be of the order of 40\%. Consistent coating quality was assured by a rapid gain testing procedure using narrow-band UV LEDs. Based on these results, 200 Photonis XP4500B PMTs were treated with PT for the upgraded low-threshold Cherenkov counter (LTCC) to be used in the CEBAF Large Acceptance Spectrometer upgraded detector (CLAS12) at the Thomas Jefferson National Accelerator Facility.