An LGAD-based full active target for the PIONEER experiment

PIONEER is a next-generation experiment to measure the charged pion branching ratios to electrons vs muons $R_e/\mu = \frac{\Gamma\left(\pi^+ \rightarrow e^+ \nu (\gamma) \right)}{\Gamma\left(\pi^+ \rightarrow \mu^+ \nu (\gamma)\right)}$ and pion beta decay (Pib) $\pi^+\rightarrow\pi^0e\nu$. The pion to muon decay ($\pi\rightarrow\mu\rightarrow e$) has four orders of magnitude higher probability than the pion to electron decay ($\pi\rightarrow e\nu$). To achieve the necessary branching-ratio precision it is crucial to suppress the $\pi\rightarrow\mu\rightarrow e$ energy spectrum that overlaps with the low energy tail of $\pi\rightarrow e\nu$. A high granularity active target (ATAR) is being designed to suppress the muon decay background sufficiently so that this tail can be directly measured. In addition, ATAR will provide detailed 4D tracking information to separate the energy deposits of the pion decay products in both position and time. This will suppress other significant systematic uncertainties (pulse pile-up, decay in flight of slow pions) to $<$~0.01\%, allowing the overall uncertainty in to be reduced to O(0.01\%). The chosen technology for the ATAR is Low Gain Avalanche Detector (LGAD). These are thin silicon detectors (down to 50~$\mu m$ in thickness or less) with moderate internal signal amplification and great time resolution. To achieve a ~100\% active region several emerging technologies are being evaluated, such as AC-LGADs and TI-LGADs. A dynamic range from MiP (positron) to several MeV (pion/muon) of deposited charge is expected, the detection and separation of close-by hits in such a wide dynamic range will be a main challenge. Furthermore the compactness and the requirement of low inactive material of the ATAR present challenges for the readout system, forcing the amplification chip and digitization to be positioned away from active region.