8/15/2011ICRC Beijing1 PRODUCTION OF SECONDARY COSMIC RAYS IN THE UPPER ATMOSPHERE D. Müller P.J. Boyle 1 J.R. Hörandel 2 A. Obermeier 2 THE UNIVERSITY OF CHICAGO, USA 1 now at Mc Gill University, Montreal, Canada 2 now at Radboud University, Nijmegen, NL 32 nd International Cosmic Ray Conference, Beijing, August 2011

8/15/2011ICRC Beijing2 TRACER 2003 Zero Pressure Long-Duration Balloon

8/15/2011ICRC Beijing3 Carbon Nuclei: Interaction probabilities in the ISM in the Atmosphere Carbon Nuclei: Interaction Probabilities in the ISM (Λ 0 =0.7 g/cm 2 ) in the Atmosphere 5 g/cm2 3g/cm2

8/15/2011ICRC Beijing4 TRACER 2006 Altitude, res. atmosphere, vs. time

8/15/2011ICRC Beijing5 GROWTH CURVES OF PARENT NUCLEI O and C N O (X) = N O (0) exp (-X/Λ O ) N C (X) = N C (0) exp (-X/Λ C ) with X = atm. depth (g/cm 2 ); Λ = M Target /σ (g/cm 2 ) From geometric cross sections (e.g. Westfall et al 1979): Λ O = 24.6 g/cm 2 Λ C = 32.1 g/cm 2 (small correction for spallation of O included)

8/15/2011ICRC Beijing6 GROWTH CURVE FOR CARBON BEST FIT Λ C = 38.6 ± 8.4 g/cm 2 (Fit) = 32.1 g/cm 2 (Calculated)

8/15/2011ICRC Beijing7 GROWTH CURVE FOR OXYGEN BEST FIT Λ O = 23.4 ± 7.3 g/cm 2 (Fit) = 24.6 g/cm 2 (Calculated)

8/15/2011ICRC Beijing8 GROWTH CURVE FOR IRON BEST FIT Λ Fe = 11.5 ± 4 g/cm 2 (Fit) = 14.2 g/cm 2 (Calculated)

8/15/2011ICRC Beijing9 GROWTH CURVE OF BORON INCLUDES SPALLATION OF C AND O: λ C and λ O are “differential” path lengths for spallation into boron (e.g.from Webber et al 1990)

8/15/2011ICRC Beijing10 GROWTH CURVE FOR BORON BEST FIT λ eff = 225 ± 102 g/cm 2 (Fit) = 227 ± 44 g/cm 2 (Calc.) Λ B = 28.7 g/cm 2

8/15/2011ICRC Beijing11 THE B/C ABUNDANCE RATIO CORRECTION TERM DOES NOT DEPEND ON ENERGY = (+0.007/-0.004) for X = 5.2 g/cm 2

8/15/2011ICRC Beijing12 MEASURED B/C RATIO

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8/15/2011ICRC Beijing16 CONCLUSIONS Height fluctuations in LDB flight of TRACER lead to growth curves of primary nuclei C, O and Fe which are consistent with geometric cross sections (Westfall et al. 1979). Growth curve of boron affected by spallation production; consistent with partial cross sections compiled by Webber et al Atmospheric secondary boron generates energy-independent background in the measurement of the B/C ratio This background becomes equal to the true B/C ratio at energies of a few hundred to a few thousand GeV per amu.

8/15/2011ICRC Beijing17 CONCLUSIONS, cont’d: Uncertainties in this background from measurements of the growth curves are currently larger than those from quoted errors in the cross sections. This would improve with better control of systematics and longer flight durations. Accurate B/C measurements in the TeV/amu region require exposures of the order of 1,000 m 2 sr days, and energy- sensitivity to 10 TeV per amu. This is very difficult to achieve with currently active or planned instruments. Growth-curve technique should also be useful for evaluating background in balloon measurements of pbar or positrons.