Studies of the pattern of light emitted from waveshifting, scintillating, and waveguide fibers used in detectors for particle physics

Abstract

Scintillating, waveshifting, and waveguide fibers are used as particle detectors and light detection and transport elements in particle physics experiments. A study of light emission from such structures is being carried out for the Compact Muon Solenoid (CMS) experiment at CERN. For CMS, the fibers used are polystyrene core with a double-cladding and a diameter of 940 microns and lengths of up to several meters. Currently, the light produced and transported by such structures is detected by a conventional photo-detectors called hybrid photodiodes (HPD). The experiment is planning to replace the HPDs with a new photo-detectors known as a Silicon Photomultipliers (SiPM) with the possibility of each fiber having its own SiPM element for readout. Due to the thermal and electrical characteristics of SiPMs, and specifically their high thermal noise rate, it is best to keep the cross sectional area of the SiPM as small as possible. When light exits a fiber there is a distribution of the photons at various angles caused by: the differences in index of refraction of the core (n=1.59) and outer cladding (n=1.43) of the fiber; how and where in the fiber the initial light was created and the dominant transmission characteristics of the fiber/waveguide. This light distribution sets the size and placement of the SiPM devices. To study this, experimental measurements are being carried out using waveshifting and clear optical waveguide fibers that are used in CMS. Light is produced within such fiber core by exciting them through their cladding using UV light emitting diodes (LEDs). The LED light penetrates into the fiber and is waveshifted. On one end (called the readout end) is placed up against a fiber-optically-coupled CCD camera. The opposite end is either mirrored (with aluminum) or unmirrored and also read out using another CCD. Initial studies of attenuation and the profile of emergent light are discussed.