Development of fast detectors for synchrotron X-rays

I have worked in developing fast X-ray detectors for synchrotron radiation experiments. Here I would like to introduce the silicon avalanche photodiode (Si-APD) X-ray detector and the scintillation X-ray detector with high-Z nanoparticle-loaded plastic scintillator. Si-APD had been mainly used as a sensor for near infrared region, until 1980s. The depletion layer of a commercially available Si-APD device was typically several 10 micrometers in thickness and the sensitive area size was several millimeters in diameter. At the early stage in the worldwide use of synchrotron radiation, the first X-ray timing detection with a Si-APD started at the Photon Factory of KEK. We confirmed that Si-APDs provided fast response of the nanosecond order for coming X-ray photons using internal electron multiplication. The timing measurements in the nuclear resonant scattering (NRS) experiments were executed using a single Si-APD X-ray detector with a fast amplifier. The sub-nanosecond timing property obtained with the Si-APD detectors is very important especially for time spectroscopy to analyze the quantum beat structure generated by the radiation emitted from the hyper fine splitting nuclear levels. Improvement on the APD device structure resulted also in the electron bunch-purity measurements for the accelerator diagnosis. Stack and array type Si-APD detectors were produced for the measurements of the electron density distribution by X-ray diffraction, and the NRS experiments with improved detection efficiency or with an enlarged solid angle. Detection of the internal conversion electrons with a Si-APD brought a successful precise measurement of nuclear excitation by electron transition (NEET). The NEET probability in the order of 10^-8 to that of the K-shell ionization was experimentally obtained and the energy value that the NEET phenomenon starts at was decided for ¹⁹⁷Au. A pixelated Si-APD detector has succeeded in the linear-array system which has 128 pixels of submillimeter size. The linear array system is equipped with highly integrated circuits, such as Application Specific IC (ASIC) amplifiers and Field Programmable Gate Arrays (FPGA). The system can be useful for X-ray imaging with simultaneous sub nanosecond timing measurements for each pixel. We are still studying to produce a new fast scintillation detector for high-energy X-rays using a plastic scintillator loaded with heavy metal-oxide nanoparticles. Zirconium, hafnium, and bismuth oxide nanoparticles are incorporated into a plastic scintillator, up to 50-60wt% to increase X-ray detection efficiency. A 40wt% HfO₂ nanoparticle-loaded scintillator demonstrated high detection efficiency of > 10% per millimeter at 67.4 keV. Its timing property show a good timing resolution (FWHM) of < 0.3 ns at 67.4 keV and high-rate counting capability of over several 10⁷ s^-1 at 50 keV. This article may be a personal research history in the synchrotron radiation field, but it could give a hint for young researchers to study on various radiation detectors and measurements. (author)