Phukan, J P

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29th International Cosmic Ray Conference Pune (2005)  6, 305-308  Optical Pulses associated with UHE Cosmic Rays  J.P.Phukana , K.Boruaha and P.K.Boruahb  (a)Deptt. of Physics, Gauhati University Assam, INDIA.  (b)Dept. of Instrumentation and USIC, Gauhati University, Assam, INDIA. Presenter: J.P. Phukan (jp_phukan@yahoo.com), ind-phukan-JP-abs1-he13-poster  Detection of UHE Cosmic Rays ( Ep >1017eV ) provides important  information on characteristics of High Energy nuclear interaction and also helps to solve astrophysical problem of Cosmic Ray origin, beyond the Greisen-Zatsepin-Kuz’min cut off ( > 1020 eV ). Čerenkov Radiation ( CR ) associated with Ultra High Energy Cosmic Rays passing through earth’s atmosphere are detected using a Čerenkov detector consisting of a 5” photo multiplier tube (PMT , type 9792KB make EMI) to be operated in coincidence with the  mini-array of Physics Department., Gauhati University, The Čerenkov detector is calibrated using Čerenkov photons produced in a tank of distilled water through which secondary cosmic rays ( mainly  muons) of known average flux pass. In this paper , we report  Čerenkov pulse spectrum using  Multi Channel Analyser   ( MCA) for calibration and simulated spectrum triggered by mini-array events within the energy window 1017 – 1018 eV.  1. Introduction  The ultra relativistic electrons and positrons in an EAS while traveling down the atmosphere produce Čerenkov Light flash, which can be detected using a PMT in a moonless night  This detector is designed to be operated in coincidence with the existing particle detector array (mini array) for detecting UHE Cosmic Rays of energy 1017 to 1018 eV [Ref.1,2,3] .It is well known that optical pulses bear information about primary mass composition ,which is not yet conclusively established in the UHE range of primary spectrum. The slope of the lateral distribution of Čerenkov radiation is linearly related to the depth of shower maximum [Ref.4], which is different for different primary mass and energy. Hence, measurement of  particles and Čerenkov  components of an EAS provide estimates of primary energy and mass composition.  2. Calibration of  Čerenkov Detector  For calibration, we use a  water tank of  radius 20cm and height 32 cm,  filled with distilled water up to a height of 22cm and its outer surface is coated black to make it light tight. The PMT is placed at the bottom of the water tank with a thin transparent plastic sheet separating the tank. The anode pulses are fed to the emitter follower placed very close to the PMT. This arrangement including emitter follower circuit is finally placed inside a light tight enclosure . The output of the emitter follower is carried via co-axial cable to the polarity inverter and inverted pulses are fed to the Multi Channel Analyser (MCA, Type –NETS-3M/U )for recording. Čerenkov events are due mostly to secondary Cosmic Ray muons of average intensity 180 m-2s-1. Events are recorded for 5 hours with average count rate of 17 per minute   2.1 Method of Calibration  Points of incidence of vertical muons are chosen at random over the cross- section of the  water tank, taking the origin at the centre of the water surface. For vertical muons of average energy 2GeV, θc = 41o (in water ) .The effective path length in water and the area over which Čerenkov photons are distributed are calculated from the geometrical consideration. The number of photons emitted by the effective path length z is  estimated from the number of photons emitted per cm (~200).[Ref.5] Assuming these photons to be distributed uniformly over area A ,the actual number of photons collected by the PMT is estimated from the  306      J.P. Phukan, K.Boruah and P.K. Boruah  fractional area of overlap by the photocathode area  The absorption  of photons  in distilled water is also taken into account. One thousand events are simulated using Monte–Carlo method and frequency distribution of the number of photons collected in each case is plotted using suitable class intervals.[fig.3]                                                                               PMT                                                                                                       DSO      GPIB        PC                                                                    Pulse                                                                                 INV                                                                                                                            MCA                                 Figure 1. Experimental Setup for Calibration of Cerenkov Counter.            2.2 Simulation  of  Čerenkov Spectrum triggered by Mini-array events  The Optical  Čerenkov detector placed at the centre of the  mini-array is calibrated for estimating primary mass composition using simulated  events by Monte Carlo method. The acceptance area of the mini-array is an annular ring of inner radius  determined by the minimum time spread (100nS) and outer radius determined by minimum particle density ( at least 3 particles per 2m2 area ) EAS events are simulated with random core positions within the acceptance area of the mini-array. Two sets of events are simulated for primary energy E=1017eV and 1018eV assuming first pure proton composition. The point of first interaction X1 is simulated using exponential distribution with proton mean free path of λ = 80 g/cm2 . The depth of maximum  tmax ( in unit of radiation length X0) is computed for the first interaction point, using GIL formula [Ref.6].                                  tmax = a + b [ ln (E / Є ) – ln A ]   -------------------- (1)                            where Є = 81 MeV ( critical energy ) , b= 0.76 , a=1.7  Cerenkov Lateral Distribution Function(LDF) is assumed to be a power law, [Ref.4],                                                           Φ ( r) = C ( R + 50 )-δ         ---------------(2)                                             δ = 0.0014 Xmax +1.32  ---------------(3)    Distilled Water    Optical pulses associated with …                        307  For each primary energy and core distance, shower size and particle density are estimated using mini-array reanalysis result using CORSIKA[Ref.7]. The number of photons falling on the Čerenkov  detector PMT is calculated using equation (2) and track length integral  [ Ref.8]. The whole procedure is repeated assuming pure iron composition. Figure 2. Simulated  and MCA Pulse Height Distribution                                            Figure 3. Simulated spectrum of 1017 eV Čerenkov pulse triggered by mini-array events .   308      J.P. Phukan, K.Boruah and P.K. Boruah   Figure 4.  Simulated spectrum of  1018 eV Čerenkov pulse triggered by mini-array  3. Results and Discussion  The experimental pulse height distribution and the simulated frequency distribution for the water Cerenkov, are shown in fig.2.The shape of the two distributions are remarkably similar except for the lowest photon numbers. The position of MCA peaks are correlated with the simulated spectrum and the detector is calibrated for the number of photons. Čerenkov pulse height spectra  obtained from a model detector triggered by mini-array event of Primary energies  1017  eV  and  1018 eV , under the assumptions of pure proton and pure Iron compositions, are shown in fig.3 and fig.4. Results show visible difference at the higher energy,  1018 eV.   4.Acknowledgement  The authors thank the Indian Space Research Organisation (ISRO, Bangalore), Department of Space, Govt. of India, for financial support. UGC Special Assistance Program (SAP) of Department of Physics, GU is gratefully acknowledged for  providing the MCA  for experimental  work.   References  [1] Bezboruah T , Boruah K and Boruah P.K., (1999) , Astroparticle Physics, [2] Phukan JP, Boruah K and Boruah K.B, 27th ICRC , Hamburg ,vol.2, p-407 [3] Bezboruah T , Boruah K and Boruah P.K., (1998),NIM A420,No.2,206. [4] Protheroe R J and Turver K E (1979) – Nuo.Cim 51A,277. [5] Jelly J.V ( 1958) – Cerenkov Radiation and its Applications , Pergamon Press, London. [6] O.Catalano ,G.D.’Ali Staiti ,M.Gabriele and L.La Fata ,Proc.27th ICRC, Hamburg, vol.2 , p-498. [7] Goswami U D , Boruah K and Boruah PK ( 2005) ,Astroparticle Physics 22 ,421-429  [8] Linsley J (1985).Proc.19th ICRC ,LaJolla,7,167. 

Optical Pulses associated with UHE Cosmic Rays

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