Light Collection Efficiency and Uniformity of Light Guides for the sPHENIX Electromagnetic Calorimeter

A new electromagnetic calorimeter is being designed for the sPHENIX experiment at RHIC which consists of a tungsten powder and epoxy absorber with embedded scintillating fibers that are read out with SiPMs through short acrylic light guides. The light guides must be as short as possible in order that the calorimeter fit inside the spectrometer magnet of the sPHENIX experiment. In addition, a high degree of segmentation is required in order to measure photons and jets in heavy ion collisions, thus leading to a large number of individual towers with approximately 25K light guides. These requirements present numerous challenges in terms of achieving good light collection efficiency and uniformity from the absorber blocks which is necessary to obtain good energy resolution from the calorimeter. We have tested different versions of simple trapezoidal light guides on prototype calorimeters in beam tests conducted at Fermilab, and also utilized a simulation program to study more complicated light guide geometries and compared them to measurements in the laboratory. We will present the results of these studies and discuss our findings on how to implement an optimal light guide design on the sPHENIX electromagnetic calorimeter.