Formation of secondary electron cascades in single-crystalline plasma-deposited diamond upon exposure to femtosecond x-ray pulses

Secondary electron cascades were measured in high purity single-crystalline chemical vapor deposition (CVD) diamond, following exposure to ultrashort hard x-ray pulses (⁠$140fs$ full width at half maximum, $8.9keV$ energy) from the Sub-Picosecond Pulse Source at the Stanford Linear Accelerator Center. We report measurements of the pair creation energy and of drift mobility of carriers in two CVD diamond crystals. This was done for the first time using femtosecond x-ray excitation. Values for the average pair creation energy were found to be $12.17±0.57$ and $11.81±0.59eV$ for the two crystals, respectively. These values are in good agreement with recent theoretical predictions. The average drift mobility of carriers, obtained by the best fit to device simulations, was $μh=2750cm2∕Vs$ for holes and was $μe=2760cm2∕Vs$ for electrons. These mobility values represent lower bounds for charge mobilities due to possible polarization of the samples. The results demonstrate outstanding electric properties and the enormous potential of diamond in ultrafast x-ray detectors.