A Micromachined Radiation-Resistant Dense High Rate Calorimeter Sensor Prototype Using Secondary Emission

We present a prototype of a 20 stage stack assembly 3.8 cm diameter active area x 2.6 cm deep stack of proximity-focused micromachined 0.65 mm thick CuBe(1%) sheets serving as dynode-like secondary emission sensors, separated by a drift distance of 0.6 mm between sheets by ceramic spacers. The stack has a density of ~25-30% of the density of CuBe. Baseline operation is in a modest vacuum ~10^-4 Torr, 4-5 orders of magnitude higher than that needed in cesiated PMT, with the stack voltages provided by in-vacuum wire wound high vacuum resistor chain. A full COMSOL simulation using custom secondary emission subroutines is in reasonable agreement with the device tested. We discuss tests with gamma, beta and neutron sources, cosmic muons and 100 GeV pion MIPs. The pulse risetime is <10ns, and gain is ~few x 10⁵. At -4,400 V in a ~0.3T B-field from a bar magnet at orthogonal directions, the response was < 2% changed. We posit that full scale MEMs stack calorimeter tests may demonstrate that the response to the low energy components (neutron knock-ons, ion fragments, spallation) at low incident kinetic energies in hadronic showers will result in enhanced signals, since the secondary emission yield follows dE/dx. For charged particle velocities beta~0.03 the yield is ~200x to that of MIPs at beta~1. This enhancement may compensate hadronic energy resolution effects that are lost in scintillators in part due to Fano factor suppression in high dE/dx low energy ion fragments, and de-emphasize the response to e-m components. We discuss prospects for using tungsten and other dense metal micromachined sheet dynodes.