Characteristics measurement of plastic scintillators and performance evaluation of cosmic ray measurement for the ALPAQUITA experiment

The ALPACA experiment is an extensive air shower array located in Bolivia, South America, consisting of 401 detectors each with an effective area of 1 𝑚2, covering a total surface area of 83,000 𝑚2. In addition, an underground muon detector system is installed, comprising detectors placed 2 metersbelowground levelwitha total sensitivearea of3,600 𝑚2 . Measurements of the scintillator characteristics were performed under conditions closely repli- cating the actual setup used in the ALPAQUITA experiment. Specifically, we measured the scintillationlightyield(charge),lightcollectiontime,andsignalrisetime,aswellastheirposition dependence, using the same type ofscintillatorsdeployed in ALPAQUITA. The results showed that the light yield near the corners was approximately 20% lower than at the center. In terms of signal timing, a delay of about 0.2 ns was observed for particles entering near the cornerscomparedtothose enteringnear thecenter. AsaprototypearrayfortheALPACAexperiment,theALPAQUITAexperiment—withone-quarter thearealcoverage—commenceddataacquisitioninApril2023. IntheALPAQUITAsurfacearray, dataarerecordedundertriggerconditionscategorizedasany1throughany4,correspondingtothe numberofdetectorsregisteringcoincidentparticlehits. Theobservedtriggerrateswere78,000Hz for any1, 4,800 Hz for any2, 600 Hz for any3, and 300 Hz for any4. Among these, the true rate for any2, corrected for accidentalcoincidences, was determined tobe 1,400Hz. In addition, we compared several reconstructed shower parameters between the Monte Carlo simulations and the ALPAQUITA experimental data. This included the even-odd opening angle, which serves as a proxy for the angular resolution of the detector system. The even-odd opening angle measured in the ALPAQUITA surface array was 1.95°, closely matching the value of 1.76° obtained from the simulation. These results demonstrate that the Monte Carlo simulations incorporating the measured scintillator characteristics accurately reproduce the performance of the ALPACA and ALPAQUITA experiments. This, in turn, indicates that the energy spectra of gamma-ray sources observed with ALPACA and ALPAQUITA can be estimated with high precision.