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Manuscript Title: A subroutine package for the fast simulation of air showers and response of surface detectors.
Authors: K. Kasahara, S. Torii
Program title: GENAS V2.1
Catalogue identifier: ABZN_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 64(1991)109
Programming language: Fortran.
Computer: FACOM M780.
Operating system: OS IV F4/MSP, SUN OS, MS-DOS.
RAM: 330K words
Word size: 32
Keywords: Air shower simulation, Surface array, Gamma-ray point sources.
Classification: 11.3.

Nature of problem:
Point sources emitting high energy gamma rays (>10**13 eV) are normally studied with surface air shower arrays which usually employ plastic scintillator detectors. We need a reliable Monte-Carlo code to simulate air shower observation, by which simulation a quantitative comparsion with experiment should be possible for knowing or improving array performance. Every quantity needed for the simulation is, in principle, a function of the primary particle type, energy, zenith angle, age of the shower, etc., and fluctuates largely. Besides these, the detector structure seriously affects the apparent measured feature of air showers. The simulation should be able to anticipate these complex features, and yet be capable of producing 10**5 or more showers in moderate computation time. The full Monte-Carlo method is impractical because of more than 1 hour cpu time for simulating a 10**15 eV shower with a present fastest computer.

Solution method:
We made full and hybrid Monte-Carlo simulations of air shower observation to obtain various distributions of quantities such as particle lateral spread, delay time, photon yield in scintillator, etc. These are expressed in tables or approximate formulas from which we can make a rapid sampling of various quantities needed to simulate air shower observations as if the full Monte-Carlo method was used.

Restrictions:
Muons and hadrons are not included since their contribution in normal observations is negligible. However, at core distances >100m, and primary energies >10**16 eV, muons might become important for the fast timing simulation. At energies exceeding the existing accelerators, quasi-scaling is assumed for hadronic interactions, and complete scaling is assumed for electromagnetic interactions. The detector structures we can support for quantitative simulations are limited to two types: one is 0.1 cm iron + 3.5+-0.5 cm plastic scintillator, and other is 0.5 cm lead + 0.1 cm iron + 3.5+-0.5 cm plastic scintillator.

Unusual features:
The package needs a user supplied uniform (pseudo) random number generator. There are essentially two machine or system dependent features in the package. However, they can easily be changed for a particular system by using Editor commands.

Running time:
When GENAS is employed to simulate air shower observation with an array of 50 fast timing detectors, ~ 1/500 sec is needed for completing 1 event with FACOM M780 (37 MIPS machine).