What is photon conversion efficiency ?

Our detector has a 3 radiation length of material (Lead + steel) in front of the preshower plane. Since photon is a neutral particle it is made to convert into electromagnetic shower (electron and positron), and these being charged get detected in our gas detector. Low energy photons may get abosorbed hence will not be detected.

Photon conversion efficiency is defined as the ratio of number of photons incident on the convertor material (lead in our case) to the number of photons which get converted into electron and positiron and gave a signal in our detector.

The conversion efficiency puts a limit to photon counting efficiency. That is we can never have a photon counting efficiency above the photon conversion efficiency.

CLICK ON THE FIGURES

The plot shows the conversion efficiency as a function of energy of incident photon. One sees that above an energy of 1 GeV the photon conversion efficiency is about 90% or higher.

In an event, photons are produced in all energy ranges following some distribution. So it is important to know what is the photon conversion efficiency in an event, how it varies with centrality of collision. The figure shows these variations. In 62.4 GeV Au + Au collisions we can have a maximum of 77% photon counting efficiency. It does not have any centrality dependence.