An array of six wide angle Cerenkov detectors was constructed amongst the scintillator and muon detectors of the CYGNUS II array at Los Alamos National Laboratory to investigate cosmic ray composition in the PeV region through measurements of the shape of Cerenkov lateral distributions. Data were collected during clear, moonless nights over three observing periods in 1995. Estimates of depths of shower maxima determined from the recorded Cerenkov lateral distributions align well with existing results at higher energies and suggest a mixed to heavy composition in the PeV region with no significant variation observed around the knee. The accuracy of composition determination is limited by uncertainties in the expected levels of depth of maximum predicted using different Monte-Carlo shower simulation models

Berley, D.

Hillas, A.M.

Paling, S.

UNT Digital Library

t OG 10.4.13 RESULTS FROM THE CACTI EXPERIMENT: AIR-CERENKOV AND PARTICLE MEASUREMENTS OF PEV AIR SHOWERS AT LOS ALAMOS S. Paling', A. M. Hillas', D. Berley2, S. Biller*, Mei-Li Chen2, P. Chumney3, D. Coyne4, D. Dorfan4, R.A. Ellsworth5, J. A. Goodman2, T. J. Haines6, C. M. Hoffman6, L. Kelley4, R. Schnee4, A. Shoup3, C. Sinnis6, D. Williams4, T. Yang4, G. Yodh3 'Physics Department, University of k e d s ,  k e d s  LS2 9J1: UK 2University Of Maryland, College Park, MD 20742, USA University Of California, Irvine, CA 9271 7. USA 4University Of California, Santa Cmz, CA 95064. USA SGeorge Mason University, Failfax, VA 22030, USA 6Los Alamos National Laboratory, LQS Alamos, NM 87545. USA RfCElVED DEC 1 6  1997 ABSTRACT An array of six wide angle Cerenkov detectors was constructed amongst the scintillator and muon detectors of the CYGNUS I1 array at Los Alamos National Laboratory to investigate cosmic ray com- position in the PeV region through measurements of the shape of Cerenkov lateral distributions. Data were collected during clear, moonless nights over three observing periods in 1995. Estimates of depths of shower maxima determined from the recorded Cerenkov lateral distributions align well with exist- ing results at higher energies and suggest a mixed to heavy composition in the PeV region with no significant variation observed around the knee. The accuracy of composition determination is limited by uncertainties in the expected levels of depth of maximum predicted using different Monte-Carlo shower simulation models. INTRODUCTION An improved knowledge of the composition of cosmic rays in the knee region of the spectrum may help to increase our understanding of the origins and acceleration of high energy cosmic rays and the characteristics of cosmic ray propagation and confinement within our galaxy. Direct measurement of primary cos- mic rays is confined to energies < 1015 eV because of the rapidly diminishing flux. Various indirect methods of determining mass composition through interpretation of the measured electron, muon and pho- ton components of air showers have been investigated, although no consistent and generally accepted results have been pro- vided to date. 100 m *..--s-- -..  0 b'... CACTI ,.--. Cerenkov Detectors 7 0  -***, '8, o :-.. 0 o', 0 .  0 '., 0 A "."," 0 0 0 0 ;,b 0 :  0: :"?-:--I, c..?.."..: _ _ _ _  *' *e _ _ _ _ _ _  s - - . n  Previous investigations suggested that Scintillator the shape of the Cerenkov lateral distri- Array Muon Detector butions (CLDs) of showers is sensitive to height of maximum (XmJ (Patterson Fig. I: A map of the CYGNUS II experiment and the and Hillas, 1983). Simulations show that CACTI &renkov array. showers induced by lower mass primary particles tend to reach maxima deeper in the atmosphere resulting in steeper CLDs, particularly no- ticeable in regions of the CLD close to shower core (eg. < 100m). In regions further from the shower core the Cerenkov light intensity is less sensitive to X, and is closely related to total shower energy. Investigations of X,, (and hence primary mass) from Cerenkov measurements are thought to have DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. the advantage over methods using electron and muon measurements of being less sensitive to core- location errors and statistical fluctuations, and less susceptible to interaction modelling uncertainties in simulation (Paling and Hillas, 1995). The purpose of the CACTI experiment has been to investigate the mass composition in the PeV re- gion using measurements of CLDs and the electro-magnetic and muon content of cosmic ray showers. CACTI consisted of an array of six wide-angle Cerenkov detectors built amongst the CYGNUS II ar- ray at the Los Alamos National Laboratory, USA. The experiment was operated during clear, moonless evenings in 1995. This report describes the design and operation of the experiment and summarises the results of the analysis of CLD data and the comparison of recorded data with simulations. Further results of data analysis will be presented at the conference. EXPERIMENTAL DESIGN AND OPERATION The CYGNUS I1 array, situated at an altitude of 2310m (780g~m-~),  consisted of 96 scintillator de- tectors dispersed over 6 x lo4 m2 and a 70m2 muon detector with an energy threshold of - 2GeV (R. Allen et al., 1992). The CACTI experiment was located towards the centre of the CYGNUS I1 array, and consisted of six wide-angle Cerenkov detectors positioned in two distinct groups separated by approximately 150m as shown in figure 1. The design of the CACTI Cerenkov detectors and the data acquisition signal path is shown in figure 2. Each detector consisted of an 8” hemispherical Hama- matsu R1408 photomultiplier tube (PMT) mounted in a steel housing with a 0 . 3 1 ~  aperture. The PMT signals were amplified at the detector to reduce the effects of electronic pickup before traversing 150m of cable. Af- ter being further amplified and stretched to a rise time of - 30ns, the signals were split. One copy was used for triggering (a simple multiplicity trigger) and the other was recorded by LeCroy 2262 waveform digitis- ers. The digitisers continuously recorded the previous 8ps of each signal waveform in 10 bit, 2511s samples, and were read out by a PC controlling the experiment whenever a multiple coincidence of Cerenkov signals had occurred. The Cerenkov detector optics and data acquisition system were designed to reduce the level of noise due ‘t 60cm : 4 1 - 0  an za zw aw to Night Sky Background light (NSB). The PMTs used were operated with a high dynode chain current and a gain of less than < 2 x lo4 to avoid saturation effects Fig. 2: Schematic of the Cerenkov detector design and data acquisition signal path due to operation in high background light conditions. The energy threshold of the array was -0.3 PeV, however the range of energies for which recorded events were clear of trigger biases and waveform digitiser saturation was 1 to 10 PeV. The experiment was operated during three dark periods in 1995. Over 70,000 events were recorded in -100 hours of observation under good night sky conditions (corresponding to a duty cycle of 6%). Individual events were considered appropriate for CLD analysis only if Cerenkov samples were taken over a wide range of core distances and the incident angle was within the full viewing aperture of the detector, resulting in a selection of -15% of the total events recorded. DATA ANALYSIS AND RESULTS The MOCCA-92 Monte-Carlo shower simulation (Hillas, 1997) was used to investigate the perfor- mance of the experiment and develop the techniques used in data analysis. Simulations were also used h signals at 40m (C40) and 140m (C140) from the shower core were determined with an accuracy pally by core-location errors and NSB noise). E - a , .  of -20% and -10% respectively (limited princi- eerenkov intensity far from the core and is used The value of C140 provides an estimate of the to estimate shower energy ( E  = C140°.97). The % ratio of C40 and C140 provides a measure of the $ CLD steepness and is related to the average depth of shower maximum (Xmx = log (a)). 0, i7j Comparison of the average CLDs for the CACTI and MOCCA data show that the recorded data 103 6 2 S a, 3 0- LL 8 : - w A  2 $ A  rs g4  qj A Protons 0 Helium .3 Oxygen 0 Iron CACTlData ! . : 0 5 10 15 20 25 C4QlC14Q The average value of the ratio of C40 and C140 for CACTI data also agrees with MOCCA simulations of iron (as expected from the previous re- sult). However the observed spread in the ratio of C40 and C140 is signifi- cantly larger than would be expected for a pure iron composition, and is consis- tent with a lighter or mixed composi- tion. Figure 4 shows the distributions of the C40/C140 ratios for CACTI and simulated data for events between 1 and 3.2 PeV again normalised to 3.2 PeV and an average incident angle of 12' . It is unlikely that there is a significant component of cosmic rays of mass > Fig. 4: Distributions of the ratio of the Cerenkov signals at 56, consequently these results indicate a 40m (C40) and 140m (Cl4O)frorn the core for MOCCA sim- systematic uncertainty in the steepness ulations and CACTI data noml i sed  to 3.2 PeK of the CLDs predicted by the MOCCA simulation. This would agree with previous investigations of the MOCCA 92 program (Hillas 1997) suggesting an over-estimation of X, due to inaccuracies in the modelling of hadronic interactions. Figure 5 shows the variation in estimated Xmx for the CACTI data with increasing shower energy in comparison to a variety of simulation models: MOCCA 92, SIBYLL (Fletcher et a1.,1994) and QGSJET (Ostapchenko et a1.,1997). Also shown are the results of previous measurements of X,, at higher energies by Fly’s Eye (Bird et al.,1993) and Yakutsk (Dyakonov et al.,1993). The Xmx measurements for the CACTI data appear to agree well with a continuation of the Fly’s Eye and Yakutsk data to lower en- ergies. A significant disagreement in the predic- tion of XMx by each of the simulation mod- els used is clearly seen. Estimations of pri- mary mass from the measurements of Xmx are hence strongly dependant on the simulation model used. Comparison with MOCCA sug- gests an average mass composition of iron for all of the X, measurements between 1015 and > 1019 eV, once again indicating a sys- tematic uncertainty in the MOCCA simula- tion. The SIBYLL and QGSJET simulations appear to agree more closely. Comparison with data suggests a mixed to heavy com- position for the CACTI data with no signifi- cant variation observed around the knee and a mixed composition for the Fly’s eye and Yakutsk data becoming lighter at higher ener- gies. CONCLUSIONS y 850 z:800 > u v 750 700 650 600 550 500 450 E X I 1 0 Yakutsk : e CACTI Simulations - - - m MOCCA SIBYLL ---_ - QGSJET I , , , , I , , , , I , , , , I I I 1 , I o I , I , , , I I I I . I I I I I I I I ~ I  15 15.5 16 16.5 17 17.5 18 185 19 1 Log E (e’ .5 Fig. 5: Depth of shower maximum (X,,) verses pri- mary energy for recorded data and various shower simulation models. A relatively simple array of Cerenkov detectors in conjunction with data from a scintillator array has been used to record event by event Cerenkov lateral distributions and investigate the depth of maxima and primary mass of cosmic ray showers in the PeV region. The depths of shower maxima verses energy determined for recorded data between 1 and 10 PeV ap- pear consistent with a continuation of existing results at higher energies from the Fly’s Eye and Yakutsk experiments. Comparison .with simulations suggest a mixed and heavy average mass composition in the PeV region with no significant variation observed around the knee. Absolute composition deter- mination from depth of maxima measurements is limited by uncertainties in the predictions of Monte- Carlo air shower simulations used. Comparisons of recorded data with MOCCA-92 simulations add to existing evidence for a system- atic uncertainty in the prediction of depth of maxima by the current version of the program. REFERENCES Patterson, J. R. and Hillas, A. M., J. Phys. G:Nucl. Phys. 9(1983) 1433-1452 Paling, S .  and Hillas, A. M., Proc. 24th Int. Cosmic Ray Conf, (Rome) 3 (1995) 408 Allen R. C., et al. Nucl. Instr. and Methods A3 11 (1992) 350 Hillas, A. M., Nuclear Physics B (Proc Suppl.) 52B (1997) 29 Fletcher, R. S. et al. Phys. Rev. D50 (1994) 5710 Ostapchenko, S. et al. Nuclear Physics B (Proc Suppl.) 52B (1997) 113 Bird, D. J. et al., Astr0phys.J. 424 (1993) 491 Dyakonov, M. N. et al., Proc. 23rd Int. Cosmic Ray Conf, (Calgary) 4 (1993) 303 M98001535 I l11 Ill l111l lol lllll llllllllll lllllllllllll %bl. Date (11) Sponsor Code (1 8) J C Category (1 9) 0 E 19980619 058 DOE 

Results from the CACTI experiment: Air-Cerenkov and particle measurements of PeV air showers at Los Alamos

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Results from the CACTI experiment: Air-Cerenkov and particle measurements of PeV air showers at Los Alamos

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