Measurement of Nuclear Composition of Primary Cosmic rays

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The nuclear composition of primary cosmic rays is clue information for the investigation of the cosmic ray origin. In the energy range above 10^<14>eV,however, no direct observation can be done because of their low intensities. Observed primary energy spectra show the well known bending around 10^<15>eV,the knee of the spectrum, and this bending is thought to be related to some changes of acceleration mechanism of primary cosmic rays. In the present research, we have measured the arrival time distributions of air Chrenkov light associated with air showers at Mt.Chacaltaya in Blivia. The primary nuclear composition reflects deeply on the longitudinal development of corresponding primary cosmic rays especially at upper atmospheric depth, and then on the arrival time distribution of air Cherenkov light associated with corresponding air showers. On the basis of a result of our Monte-Carlo simulation, we chose the rise time of integrated Cherenkov pulses as a parameter which characterizes the arrival time distribution. Observations are made for every two weeks around new moons from June to October in 1996. The observed rise times are classified according to air shower sizes and incident zenith angles of their associated air showers. Then the distribution of classified rise times are compared with those calculated with a Monte-Carlo technique. The result will be obtained after more two year observations, but the preliminary result on the nuclear composition at the knee region is as followings :(1) the average atomic number of primary cosmic rays is 9 to 15,(2) the upper limit fraction of proton ant that of iron is 25 and 50%, respectively,(3) the lower limit fraction of (He+CNO+Ne-S) is 40%.In anyway, our result is inconsistent with such models of cosmic ray origin that proton is dominant in the primary composition above the knee region.