Study of chemical composition and acceleration mechanism of primary cosmic ray at mountain altitude in India

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It is important to study the chemical composition of primary cosmic rays to know the origin and propagation and acceleration mechanism of them. At a same time the change in slope of energy spectrum around energy range of 10ィイD113ィエD1〜10ィイD116ィエD1eV, so called "knee" region, is also the primary object of our present observation.We have established the huge detector array for Extensive Air Shower (EAS) observation at Ooty (800g/cmィイD12ィエD1). It is named with GRAPES III. It consists of 217 (each 1mィイD12ィエD1) scintillation detectors (SCD) and 16 (each 36mィイD12ィエD1) proportional counter detectors (PCD). PCD can detect the low energy muons energy grater than 1 GeV. We observe the direction of EAS and the lateral density distribution of electro-magnetic components and muon components precisely. We can study the chemical composition of primary cosmic rays though these observations. Our array is designed to optimize to observe EAS size around 10ィイD14ィエD1 to 10ィイD17ィエD1. So far we are operating our apparatus continuously without much problems and obtained the following results.1. We have derived the size spectrum of EAS between 10ィイD14ィエD1 and 10ィイD16ィエD1. Those results showed reasonable agreement within the experimental error with other measurements.2. Our observed lateral density distribution of muons showed good agreement with the results of Monte Carlo simulation, though there are small difference among the model of hadron interaction.3. We have got upper limit for the ratio between diffused gamma rays and cosmic rays and cosmic rays as 3x10ィイD1-3ィエD1.This value gives the best value of upper limit in the energy range 10ィイD114ィエD1〜10ィイD115ィエD1eV at present. We are satisfied to know our observation system is working pretty well as it was aimed so. We are planning to improve the analysis method and accumulate the simulation results. Comparison between those results will surely tell us the chemical composition of primary cosmic rays.