Commissioning of a new photon detection system for charge radii measurements of neutron-deficient Ca*

Calcium is unique for its possession of two stable isotopes of ``doubly magic" nuclei at proton and neutron numbers $(Z, N) = (20, 20)$ and $(20, 28)$. Recent charge radii measurements of neutron-rich calcium isotopes yielded an upward trend beyond current theoretical predictions [R. F. G. Ruiz et al., Nat. Phys. 12, 594 (2016)]. At the BECOLA facility at NSCL/MSU, Ca charge radii measurements will be extended to the neutron-deficient regime using collinear laser spectroscopy. A new photon detection system with an ellipsoidal reflector and a compound parabolic concentrator has been commissioned for the experiment. The system increases the signal-to-noise ratio by reducing background, which is critical for the low production rates of the Ca experiment. Details of the system and results of the characterization tests will be discussed.