Dual threshold X-ray photon counting pixel array detector

The design, manufacture, and test of a two-dimensional pixelated dual-threshold (DT) X-ray photon counting (XPC) detector is described. The DT-XPC detector consists of a 48 × 48-element silicon detector array hybridized to a custom readout integrated circuit (ROIC) using flip-chip bumping and bonding hybridization. The 520-micrometer-thick deep-depleted silicon detector elements are optimized for detection of X-rays in the range of 5 keV to 20 keV. In each 130-μm pitch unit cell, PIN detectors are connected to low-noise pre-amplification and shaping circuits. These circuits convert the charge generated in the photodiode-due to X-ray absorption-into a voltage that is proportional to the energy of the absorbed X-ray photon. The dual-bandwidth pre-amplifier is designed to operate at frequencies of 10 MHz and 100 MHz, allowing X-ray photon counting of X-rays bunched, i.e., 11.4 ns and 153 ns, respectively. The amplified voltage signal is fed to two in-pixel discriminators that determine whether the pulse exceeds one or both of the experiment's user-defined thresholds. The output signal, from both comparators is input to two multipurpose digital counting circuits that can be configured as two separate 15-bit counters, or integrated as a single 30-bit counter. When configured for operation with 15-bit counters, one for each comparator, the device either supports energy-resolving modes of operation or counts only X-rays that fall within a defined energy range. Each counter is individually enabled with a user-programmable gate signal, allowing the XPC to operate in pump/probe mode where external gating signals are used to direct reference counts to one 15-bit counter and signal counts to the other 15-bit counter. To reduce experiment dead time, counters in the pixel are sampled with a 30-bit output latch before readout. This allows the ROIC to operate in integrate-while-read mode.