Damage testing of partial reflectors for 157 nm laser calorimeters

We determined the damage thresholds and lifetimes of several materials using 157 and 193 nm excimer lasers and a beam profile technique similar to that described in ISO 11254-2. We made these measurements to select an appropriate absorbing material for use in our primary standard laser calorimeter for 157 nm excimer laser energy measurements. The materials we tested were nickel-plated sapphire, chemically-vapor-deposited silicon carbide (CVD SiC), nickel-plated copper, and polished copper. Applied pulse energy densities (or dose) ranged from 80 to 840 mJ/cm(2). We determined the applied dose from a series of laser beam profile measurements. Silicon carbide had the highest damage threshold: 730 mJ/cm(2) per pulse. For this reason, and because of its high thermal and electrical conductivities, we have chosen silicon carbide as the absorber material for the 157 nm calorimeter. We also conducted long-term exposure studies in cooperation with MIT Lincoln Laboratory at a pulse energy density of 5 mJ/cm2 to simulate typical calorimeter operating conditions. No aging effects or other surface changes were not observed at these dose levels after 500 million pulses, corresponding to a projected calorimeter lifetime of 50 years.