Design of wide-energy-range neutron calorimeter for nuclear collisions in the Korea rare-isotope accelerator

We have designed a neutron calorimeter dedicated to the measurement of the nuclear symmetry energy in the planned Korea rare-isotope accelerator. The design was optimized to precisely measure the neutron energy with high efficiency in a wide energy range from about 30 to 300 MeV. The final configuration of the detector is a hybrid of the homogeneous and the sampling calorimeters with a veto counter in the front. The homogeneous part is ideal for measuring low-energy neutrons below ~50 MeV whereas the sampling part is efficient for measuring high-energy neutrons. The detection efficiency is estimated to be about 88% for neutron energies larger than 50 MeV and decreases to ~78% at 30 MeV.We have estimated the energy resolution as a function of the incident energy based on the time-of-flight method. Assuming an ideal detector performance with null time resolution, the relative energy resolution decreases as the incident neutron energy increases. However, under a more realistic situation with a finite time resolution for the detector, the energy resolution monotonically increases with increasing the neutron energy, following a logarithmic function. Imposing an energy resolution of better than 3% at the highest neutron energy, we find the nominal position of the hybrid calorimeter to be 15 m from the target for a time resolution of 1.0 ns, but the detector needs to be free to move closer to or farther from the target, depending on the physics goal.