The OPAL Monte Carlo program GOPAL was an essential tool for analysing data. The aim of GOPAL was to simulate the response of the OPAL detector to any given set of particles. The first picture above shows a view of the detailed geometry of the OPAL detector which has been coded in GOPAL.
The next picture shows an area of the endcap electromagnetic calorimeter in GOPAL. A 10 GeV electron has been fired at this area, and the shower can be seen. The red tracks are electrons and positrons, and the blue ones are photons.
You can click on these images to get larger versions.