Efficient cosmic ray generator for particle detector simulations

Abstract

Traditional cosmic ray simulations commonly employ the Monte Carlo method to randomize the energy and direction of each simulated particle, often employing simplified or uncorrelated distributions. The flux of cosmic rays is modelled as incident particles originating from a plane above the object of interest (e.g., detectors in particle physics or surfaces in dosimetry studies) with experimentally determined angular and energy distributions. This strategy is highly inefficient because a significant number of particles never intersect the detector. This paper proposes a refined Monte Carlo method to generate a sample of events that intersect the target volume, ensuring their angular distribution matches that of the conventional approach. It is based on the projection of a sphere containing the target volume onto a plane tangent to it at a fixed angle; this is termed the Probability Distribution Projection (PDP) method. This configuration allows computation of the probability that a cosmic particle hits the sphere at this incoming angle, with this probability being proportional to the area of the corresponding section of a cylinder. The performance of this method demonstrates enhanced computational speed while yielding identical physical results. It has been implemented within the REST-for-Physics framework and tested using the geometry of a real detector, the IAXO-D0 Micromegas X-ray detector for the future axion helioscope BabyIAXO. The proposed method achieves a 37-fold improvement in efficiency compared to the traditional Monte Carlo scheme for the same accuracy, and is particularly advantageous when the target volume deviates from a spherical shape.