Photonuclear effects in a NaI(Tl) calorimeter

The branching ratio $R=\Gamma (\pi\rightarrow e\nu+e\nu\gamma)/ \Gamma (\pi\rightarrow \mu\nu + \mu\nu\gamma)$ has provided the best test of the hypothesis of electron-muon universality in weak interactions. The new TRIUMF PIENU experiment aims at improving the precision of this measurement by a factor of $>5$. A key component of the experiment is a NaI(Tl) calorimeter for positron energy measurements. During test measurements, additional peak structures were observed in the positron spectra. Using Monte Carlo simulation, we show that these structures are due to photonuclear reactions.