A Measurement of the Cosmic-Ray Antiproton Flux and a Search for Antihelium

Creators: Buffington, Andrew; Schindler, Stephen M.; Pennypacker, Carlton R.

Abstract

A balloon-borne instrument has measured the cosmic-ray antiproton flux between 130 and 320 MeV and searched for antihelium between 130 and 370 MeV per nuclear. These particles were selected from the background of normal-matter cosmic rays by combining a selective trigger with a detailed spark chamber visualization of each recorded event. Antiprotons are identified by their characteristic annihilation radiation. Residue from background processes meeting the selection criteria is small. The observed 14 antiprotons yield a measured differential flux of 1.7±0.5X 10^(-4) antiprotons m^(-2) sr(-1) s^(-1)i Mev^(-1) at the top of the atmosphere. The corresponding antiproton/pro-ton ratio is 2.2±0.6X10^(-4), only slightly smaller than the ratio observed by other experiments at higher energies. Thus the antiprotons have a spectral shape similar to the protons, at least down to about 100 MeV. The expected flux of these particles can be calculated under the assumption that they were created by collisions of high-energy cosmic rays with the interstellar gas. Calculations using the standard leaky box model for propagation in the Galaxy predict a flux two orders of magnitude smaller than that observed. A small low-energy flux is predicted due to a kinematic suppression of the production of low-energy antiprotons. The discrepancy between calculations and experiment may be evidence that cosmic-ray protons have passed through substantially more than 5 g cm^(-2) of material during their lifetime. In addition, the combined results from this experiment and previous ones may be evidence for stochastic, energy-changing processes in interstellar space which act upon the secondary antiprotons after their creation. The search for cosmic-ray antihelium sets a 95% confidence level upper limit on the He /He ratio of 2.2 X 10^(-5).

Additional Information

© 1981. The American Astronomical Society. Received 1981 February 4; accepted 1981 March27. This work was supported by grants NGR 05-003-553 and NGR 05-002-160 from the National Aeronautics and Space Administration and by the University of California's Lawrence Berkeley Laboratory, supported by the Department of Energy. The balloon flight operation was conducted by members of the National Scientific Balloon Facility in Palestine, Texas. Project authority for the Canadian balloon flight, together with liaison support, was provided by the National Research Council of Canada, Space Research Facilities Branch. Use of the mountaintop laboratory was possible through the hospitality of New Mexico State University at Las Cruces. Logistical and technical support was provided during the mountaintop calibration by members of Sacramento Peak Observatory. We would like especially to acknowledge the devoted labor of John Yamada, John Gibson, and Hal Dougherty of the Space Sciences Laboratory at Berkeley who carried out much of the construction, test, and field operation support for the experiment. The importance of deuterons and tritons as a potential trigger background was first pointed out to us by Ray Hagstrom. George F. Smoot, Phil M. Lubin, and Charles D. Orth all made important contributions to the early design of the apparatus. We thank L. W. Alvarez, R. W. Birge, A. G. Opp, and P. B. Price for much support and encouragement. Finally, we are pleased to thank R. A. Mewaldt, T. A. Prince, E. C. Stone, R. E. Vogt, and other members of the Space Radiation Laboratory at Caltech for careful reading of the manuscript and much advice and criticism received during the data analysis phase of this experiment.

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