Energy resolution

Let n _Φ photons of luminescence emitted by the scintillator,

n _K photoelectrons emitted by the photomultiplier photocathode,

n _A electrons collected on the photomultiplier anode.

It is assumed that fluctuations in n _Φ (for the same energy loss in the scintillator) are poissonian, as are fluctuations in n _K , hence : $v_{\mathrm{K}}=\frac{{\sigma }_{}^{}}{}$