The Anti-Coincidence Counter (ACC) within the Alpha Magnetic Spectrometer (AMS) serves as its vigilant guardian, meticulously filtering through the deluge of particles passing through AMS. Among the ten thousand particles per second traversing AMS, the ACC discerningly discards a substantial 8/10, preserving only those particles deemed indispensable for physics analysis.
Why is the ACC Essential?
Cosmic rays bombard AMS from all directions, presenting a challenge for accurate particle analysis. AMS excels in analyzing particles traveling from top to bottom, but those arriving at high incident angles pose difficulties. The ACC steps in to reject such particles, ensuring the integrity of AMS data.
Additionally, the ACC plays a crucial role in eliminating events with unfavorable topologies. High-energy particles interacting with AMS materials can generate numerous secondary particles, confounding Tracker pattern recognition. These events, potentially masking faint antimatter signals, are efficiently rejected by the ACC.
How Does the ACC Operate?
The ACC comprises a barrel of scintillation counters encircling the Tracker. A vertical particle triggers a signal in Time-of-Flight (ToF) but not in ACC, while a horizontal particle may activate ACC but not ToF. The strict event recording condition, TOF AND NOT ACC, ensures data accuracy. Exceptions to this condition occur under specific circumstances:
- ACC Trigger: Events involving incoming ions disable the ACC veto, allowing ions accompanied by electron production (δ-rays) to pass through.
- Electron/Positron Conversion: Electrons or positrons converting in the Electromagnetic Calorimeter (ECAL) produce backsplash particles that may trigger ACC. In such cases, the trigger condition is modified to TOF AND NOT MORE THAN 4 ACC PADDLES FIRED.
The ACC comprises sixteen paddles arranged cylindrically around the Tracker. Wavelength shifter fibers collect light emitted by the scintillating paddles, transmitting it to Photomultiplier Tubes (PMTs) via clear fibers. Positioned approximately 40 cm from the racetrack coils, the ACC PMTs minimize magnetic field interference. The scintillating fibers ensure high efficiency and homogeneity, guaranteeing swift and reliable ACC veto signals for particles at high inclinations.
The ACC’s meticulous filtering ensures the fidelity of AMS data, enabling precise particle analysis and advancing our understanding of cosmic phenomena.