Fast X-ray reflectivity measurement using a cooling PIN photodiode

It is well known that the photon-counting method is effective for measuring a weak x-ray beam, but ineffective for intense synchrotron radiation (SR). On the other hand, the linearity of a PIN photodiode is more than ten figures when x-ray intensity is measured using the photovoltaic mode. Since the signal-to-noise ratio (S/N) of the photovoltaic mode is worse than that of photon counting, the major challenge when using photodiodes for SR experiments is the suppression of the dark current when measuring weak x-ray signals. We developed a PIN photodiode equipped with a thermo-electrical cooling device and performed a series of x-ray reflectivity measurements. When cooling the photodiode from room temperature to -15 degrees C, the dark current can be suppressed from 1.9 pA to 3.4 fA, which reduces the background x-ray photon noise from 8000 cps to 15 cps.