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Description
The cooler storage ring (CSR) external-target experiment (CEE) is a large-scale fixed-target heavy-ion collision experiment under development at the heavy-ion research facility in Lanzhou (HIRFL). A critical subsystem for beam monitoring and primary vertex reconstruction is the beam-particle monitor (BPM). This detector comprises two micro-TPCs integrated within a common gas chamber to simultaneously measure the transverse positions and timing information of individual beam particles. Each micro-TPC is equipped with pixel readout, dedicated to measuring a single transverse coordinate. The design ensures that beam particles do not directly impinge upon the pixel charge sensors. Instead, the sensors detect the ionization electrons drifting in the gas volume, thereby significantly reducing the radiation dose and the fluxes of hadrons and nuclear fragments on the electronics. These sensors are staggered across three rows to allow the easy wire-bonding and to optimize the sensitive area. They are responsible for sensing the drift electrons generated by beam-induced ionization. Through this design, each micro-TPC provides a precise measurement of one transverse coordinate based on the spatial distribution of the charge on the readout plane. Furthermore, by correlating the arrival time information from both micro-TPCs, the system can reconstruct the full 2D transverse profile and the absolute incident time of each individual beam particle.
The ASIC integrates the charge sensor and readout circuitry onto a single silicon die. This monolithic approach eliminates the need for complex interconnections, which reduces the parasitic capacitance at the input node, resulting in better noise performance. Meanwhile, the ASIC is tailored specifically for the BPM to meet the requirements for single particle tracking including the spatial and time resolutions, while optimizing the rate capability to withstand the intensity of heavy-ion beams at HIRFL. The chip is fabricated in a 130 nm CMOS process. It has a total size of 19 mm × 3 mm, comprising a linear array of 180 pixels. Each pixel features a charge collection electrode (CCE), a charge-sensitive amplifier (CSA), a discriminator, a time-to-amplitude converter for measuring charge amplitude via the time-over-threshold method, as well as a time-of-arrival circuit providing the time measurement with a precision of 25 ns. The CEE made of an exposed top-most metal senses the drift electrons in the gas volume, and is directly coupled to the CSA of the pixel. Each CCE has a size of 89 μm × 1 mm, surrounded by a guard ring to mitigate inter-pixel capacitive crosstalk. In each chip, 180 CCEs are arranged in a row with a pitch of 100 μm, resulting in a total sensitive area of 18 mm × 1 mm.
The experimental validation, conducted through both laser and heavy-ion beam tests, confirms that the BPM meets its primary design goals. A single-track arrival time resolution of better than 22 ns was achieved, contributing to an overall incident time resolution of approximately 105 ns. Furthermore, the detector demonstrated excellent transverse position resolutions of better than 50 μm. The BPM fully satisfies the beam particle measurement requirements of the CEE experiment.
