Improving the angular resolution of an ultra-high-energy gamma-ray telescope by the addition of thin lead sheets above its detectors

The prediction that the angular resolution of a scintillator array can be improved by placing lead sheets over its constituent detectors has been tested experimentally with the gamma-ray telescope at Haverah Park. We find that the addition of lead significantly improves the prompt timing and density response of detectors, and that the optimum thickness of lead, for detectors at sea level, is about 9 mm (1.6 radiation lengths). The solid-angle uncertainty in the direction of showers found using a small array shielded with this thickness is about 60% less than for directions found using an identical, unmodified, array. We also find that there is a 30% enhancement in the density signal observed for showers falling in the distance range 40 <r< 60 m: further work is required to derive the lateral distribution function which needs to be used with an array of lead-covered detectors. (author)