Optimization of WLS fiber readout for the HERD calorimeter

Abstract

A novel 3-D calorimeter, composed of about 7500 LYSO cubes, is the key and crucial detector of the High Energy cosmic-Radiation Detection (HERD) facility to be installed onboard the China Space Station. Energy deposition from cosmic ray in each LYSO cube is translated by multiple wavelength shifting (WLS) fibers for multi-range data acquisition and real-time triggering. In this study, various methods of surface finish and encapsulation of the LYSO cube were investigated to optimize the amplitude from the WLS fiber end with the aim of improving the signal-to-noise ratio of Intensified scientific CMOS (IsCMOS) collection. The LYSO cube with five rough surfaces and a specular reflector achieves the maximum amplitude at the low-range fiber end, which is increased by roughly 44% compared to the polished cube with PTFE wrapping. The non-uniformity of amplitude at different positions on the LYSO cube surface was measured by X-ray and the positional correlation factor was derived for the entire cube. A simulation based on HERD CALO was conducted, which revealed that both the LYSO cube with five rough surfaces and the cube with rough bottom face exhibit superior energy resolution for electrons compared to the other two configurations.

The HERD experiment: new frontiers in detection of high energy cosmic rays

“…It is homogeneous, isotropic, 3D segmented, deep (about 55 radiation length and 3 hadronic interaction length), with a very large geometric factor and a good energy resolution (about 2.5% for electrons and less than 30% for protons). The calorimeter has a double read-out, indeed the scintillation light produced by every crystal is detected by two independent systems: one based on the use of WaveLength Shifting Fibers coupled to Intensified Scientific CMOS (WLSF system) [11], and one based on the use of two photodiodes with different active areas (PD system) [12] [13] [14]. Both read-out systems should have an incredibly high dynamic range: bigger than 10 7 , in order to acquire signal from the minimum ionizing particle for in-orbit calibration, up to 250 TeV, necessary in order to measure showers induced by PeV protons.…”