Romero-Wolf, A

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29th International Cosmic Ray Conference Pune (2005) 3, 97100Antarctic Balloon Flight and Data Analysis of TRACERA. Romero-Wolf, M. Ave, P. Boyle, F. Gahbauer, C. Ho¨ppner, J. Ho¨randel ,M. Ichimura, D. Mu¨ller and S. WakelyEnrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637(a) University of Karlsruhe, Germany(b) Hirosaki University, JapanPresenter: A. Romero-Wolf (afromero@uchicago.edu), usa-romero-wolf-A-abs1-og11-oralThe TRACER cosmic-ray detector was successfully flown from McMurdo, Antarctica in December 2003. Theinstrument has a geometric factor of 5 mster and provided measurements of cosmic ray nuclei from oxygento iron (Z=8 to Z=26). The analysis of the data begins with the reconstruction of the trajectory of each nucleusthrough the instrument. Subsequently, the elemental charge Z and the particle energy are measured from 0.5 to10,000 GeV/amu. This process uses known response functions and fluctuations in response of the individualdetector elements, and the procedures are verified with extensive computer simulations. The analysis is able tocleanly select the very rare events at the highest energies without contamination due to low energy backgroundwhich is more abundant by about a factor of  10.1. IntroductionThe concept of the TRACER instrument for the detection of high-energy cosmic-ray nuclei was first demon-strated with a successful test flight from Fort Sumner, NM in 1999 [1]. To obtain data with much improvedstatistics, a long-duration balloon (LDB) flight is required, and was conducted from McMurdo, Antarctica in2003, at the first opportunity when a launch vehicle compatible with the weight of TRACER (  1600kg) wasavailable at that site. The LDB flight yielded a total exposure of 50 mster days at an average altitude of3.9 g/cm. In contrast to Fort Sumner, the high geomagnetic latitude at McMurdo exposes TRACER to alarge flux of low energy particles. The instrument must therefore avoid the mis-identification of low energyparticles while retaining high efficiency for the detection of the rare high-energy events. Keeping this require-ment in mind, the analysis procedure aims at identifying the particle species with single charge resolution andmeasuring the particle energy E, or Lorentz factor  =E/mc, over four orders of magnitude.2. Analysis ProcedureThe data analysis proceeds by first determining the trajectory of each cosmic-ray particle through the detectorsystem, then determining charge Z and energy E for each event, and finally, evaluating all efficiency lossesdue to the various cuts in the data. Monte Carlo simulations are necessary to test and refine the data analysisprocedure. Extensive simulations of the TRACER detector as well as other analytical tools have been developedthat run in parallel with each step in the analysis procedure. Wherever possible, the results of the simulationsare verified with the measured data themselves.2.1 SimulationsThe TRACER simulation code (TSIM) is a GEANT4 [2] based simulation of the detector. The code includesthe geometry of the individual instrument components and uses the “G4hIonisation” model of the energydeposited and  -ray production in each part of the detector. The program generates a simulated cosmic-ray98 A. Romero-Wolf et al.Figure 1. Scatter plot of top scintillator vs. Cherenkovsignal in arbitrary units.Figure 2. Charge histogram for all events measured inight.data set, which is then subjected to the same analysis procedures as the real flight data. In addition, simplersimulations have been developed to model fluctuations in the detector and  -ray effects where TSIM was foundto be impractical.2.2 Trajectory ReconstructionEach particle travels through sixteen layers of 2cm diameter proportional tubes (see figure 1 in [3]). Theseare oriented in two orthogonal directions and thus provide all details about the particle trajectory. Usingthe positions of the centers of all tubes that show signals above threshold, a first estimate of the trajectoryis obtained. Due to the cylindrical shape of the tubes, the pathlength through each tube along the particletrajectory is generally different. However, the signal amplitude measured in each tube is proportional to thepathlength within statistical fluctuations. This fact is applied to determine a refined track fit. The average totalpathlength through all tube layers is 24 cm for particles incident within the acceptance cone of the instrument.Including the geometry of the proportional tube array in the simulation, the tracking accuracy is determined bycomparing the simulated track with the track reconstructed by the analysis routine. Typically, the recontructedtrack has an accuracy of 2mm in the impact parameter, and 8 mm (3  ) for the total pathlength through alltubes. The tracking efficiency is determined by counting the number of simulated events that pass the trackquality cuts. The tracking efficiency is 95  .2.3 Charge IdentificationThe charge of a particle traversing the instrument is determined primarily from the scintillator signals. Notethat the scintillators are only 5 mm thick. Their response exhibits spatial non-uniformities which are mappedwith muons on the ground and with flight data. The response maps, together with the trajectory information,are used to correct the signal for each event. The presence of low energy particles, with signals larger thanAntarctic Balloon Flight and Data Analysis of TRACER 99Cherenkov Signal0 0.05 0.1 0.15 0.2 0.25 0.3Specific Ionization0.060.070.080.090.10.110.120.130.1411010Figure 3. Scatter plot of dE/dx vs. Cherenkov signalsfor iron nuclei. The black line is the average responseobtained from simulations. The units are arbitrary.Figure 4. Scatter plot of TR vs. dE/dx signal for ironnuclei. The units are arbitrary.minimum ionization, requires to combine the scintillator signals with an energy scale; otherwise the chargeresolution from scintillator measurements becomes smeared out. This scale is provided by the Cherenkovcounter signal. Figure 1 shows the correlation plot of top scintillator signal vs. Cherenkov signal. From thisscatter plot lines of constant charge are identified. By summation along these lines the charge histogram shownin figure 2 is obtained. The charge resolution (1  ) is 0.3 charge units for oxygen and 0.5 charge units for iron.2.4 Energy MeasurementThe energy measurement results from the combination of three types of detector covering a range of 0.5GeV/amu to 10,000 GeV/amu: The Cherenkov counter measures the energy of a particle from 0.5 GeV/amuup to saturation around 10 GeV/amu. For higher energies the energy is obtained from the measurement of thespecific ionization and its relativistic rise in the top eight layers of proportional tubes (dE/dx), or the detectionof x-ray transition radiation in the bottom eight layers of proportional tubes (TRD). (For details of the config-uration see [3].) To find the average dE/dx the signal 	 in each tube along the particle trajectory is takenalong with the corresponding pathlength 	 in the tube. The summation dE/dx= 		 gives theoverall value of the specific ionization. In this summation, the tubes with small pathlengths, 	ﬁﬀ 1cm, areexcluded since fluctuations in the signal increase for shorter pathlength. The average TR signal is found in thesame manner using tubes in the TRD layers.With simulated data generated by TSIM the signal fluctuations in the proportional tube array are determinedand compared to the fluctuations measured from the data. For instance, for iron the measured dE/dx fluctuationsare 3  while the simulated fluctuations are 2  . The increase in the measured value is due to the tracklengthuncertainty (  3  , see section 2.2). Hence, simulations and data agree quite well.The energy deposit in a gas proportional tube is described by the Bethe-Bloch formula. It is degenerate inenergy: a signal larger than minimum ionization could either be generated by a low-energy particle, or could100 A. Romero-Wolf et al.be elevated due to the relativistic rise in dE/dx with energy. To break the degeneracy the energy scale providedby the Cherenkov counter is used. Figure 3 shows a scatter plot of dE/dx vs. Cherenkov signals for ironnuclei. The correlation follows closely the pattern predicted by simulations [3]. With the combined signals ofdE/dx and Cherenkov the minimum ionization point in the dE/dx signal is identified, and the correspondingCherenkov signal distinguishes between low and high energy particles. The Cherenkov signal provides anenergy measurement for particles below the minimum ionization energy.Simulations have been used to study the effect of  -rays generated while the particle traverses the detector. Itis found that  -ray contributions on proportional tube signals are negligible; however, they produce increasedsignals in the large-area scintillator and Cherenkov counters at the bottom of the instrument [4]. The effects of -rays are taken into account in the analysis and are verified with the data.To measure the energy of a particle at higher energies, above 10 GeV/amu, the combined responses of TR anddE/dx are used. Figure 4 shows the correlation of the TR signal with the dE/dx signal for iron data. For eventswith energies below 400 GeV/amu, the onset of transition radiation, the signals are well correlated along thediagonal. The energy assigned to these events is obtained from a parameterization of the relativistic rise in theenergy deposited in a gas tube [3]. Signals at energies higher than 400 GeV/amu have a significant contributionfrom transition radiation and are identified on the scatter plot in figure 4 by their location above the diagonal.The events are assigned an energy according to a parameterization of the TR response curve [3]. Note thatthese rare events at the highest energies (representing a fraction of the order of 10 ﬂof all detected iron nuclei)stand out in the scatter plot without any significant contribution of background.The signal fluctuations for each component of the detector depend on the charge of the particle and its energy.For example, the corresponding energy resolution of the Cherenkov counter measurement for iron is 20  at 3GeV/amu, but 40  with dE/dx at 20 GeV/amu and ﬀ 10  for the TRD at 1,000 GeV/amu.3. ConclusionsThe long-duration balloon flight of TRACER has provided a statistically significant data set of cosmic raynuclei from oxygen to iron up to TeV/amu energies, or even beyond. High energy events are clearly discrimi-nated against the low energy background. The measurements cover four decades in energy with single chargeresolution. The resulting energy spectra are presented in [5].We gratefully acknowledge the services of the National Scientific Balloon Facility and of the NSF AntarcticProgram. This work has been supported by NASA grants NAG5-5305 and NN04WC08G.References[1] Gahbauer, F.H., et al., Astrophysical Journal, 607:333-341, 2004.[2] Agostinelli, S., et al., Nucl. Instrum. Methods Phys. Res., 506, 250, 2003[3] Ho¨ppner, C., et al., 29th ICRC, Pune (2005), OG1.1[4] Gahbauer, F.H., et al., 28th ICRC, Tsukuba (2003), OG1.5, 2245[5] Boyle, P., et al., 29th ICRC, Pune (2005), OG1.1

Antarctic Balloon Flight and Data Analysis of TRACER

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