No fluorescence in the optical simulation

[lihe2013]

Describe the bug
vGate9.0， when running the example（GateContrib-master/imaging/Optical）， there is no fluorescence yield if the incident particle is gamma. If change the particle from gamma to electron, there is fluorescence yield. Could someone help to figure out what is the possible reason?
Desktop (please complete the following information):

• OS: [Ubuntu 18.4]
• Gate version or commit hash: [vGate9.0]
• Geant4 version: [10.6]
• Root version: [ev6-14-00]
• compiler version: [gcc 7.5.0]