Andrew David Tomasch

Cosmic-Ray Electrons and Positrons from 1 to 100 GeV: Measurements with HEAT and Their Interpretation

Measurements of the energy spectra of negative electrons and positrons have been performed with the High-Energy Antimatter Telescope (HEAT) in two balloon flights—1994 May from Fort Sumner, NM, and 1995 August from Lynn Lake, Manitoba. We present the combined data set from these two flights, covering the energy range 1-100 GeV. We compare our data with results from other groups and discuss the data in the context of diffusive propagation models. There is some evidence that primary electrons above 10 GeV and cosmic-ray nuclei exhibit the same energy spectrum at the source, but that the source spectrum becomes harder at lower energy. Within the experimental uncertainties, the intensity of positrons is consistent with a purely secondary origin, due to nuclear interactions in interstellar space.

New measurement of the cosmic-ray positron fraction from 5 to 15 GeV

We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e(+/-) instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.

Measurements of the cosmic ray positron fraction from 1-GeV to 50-GeV

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Fort Sumner, New Mexico, in 1994 and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. We present results on the positron fraction based on data from the Lynn Lake flight and compare these with the previously published results from the Fort Sumner flight. The results confirm that the positron fraction does not increase with energy above ≈ 10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960s. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

MEASUREMENTS OF THE COSMIC-RAY POSITRON FRACTION FROM 1 TO 50 GeV

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Fort Sumner, New Mexico, in 1994 and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. We present results on the positron fraction based on data from the Lynn Lake flight and compare these with the previously published results from the Fort Sumner flight. The results confirm that the positron fraction does not increase with energy above ≈ 10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960s. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

Cosmic-ray positrons: Are there primary sources?

Abstract Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.

Limits on the antiproton/proton ratio in the cosmic radiation from 100 MeV to 1580 MeV

A search for antiprotons (p-bars) in the cosmic radiation with energies below 1580 MeV at the top of the atmosphere was performed using the PBAR balloon-borne magnetic spectrometer. No antiprotons were observed in 124,000 proton events. For the energy interval 100-640 MeV, an upper limit is reported to the p-bar/p ratio of 2.8 x 10 to the -5th at the top of the atmosphere, after correcting for instrumental efficiencies and contributions from secondary particles. No antiproton was observed in the energy interval 640-1580 MeV, which yields an upper limit to the p-bar/p ratio of 6.1 x 10. By combining both data sets, the limits on the p-bar/p ratio can be improved to 2.0 x 10 to the -5th. The detector performance and instrumental efficiencies of the individual detector components are discussed. A detail Monte Carlo calculation was used to evaluate the instrumental efficiency for both antiprotons and protons as a function of momentum. 48 refs.

The cosmic-ray He-3/He-4 ratio from 100 to 1600 MeV/amu

The Superconducting Magnet Instrument for Light Isotopes (SMILI) flew for 19 hours on September 1, 1989, with a residual overburden of 5 g/sq cm. It measured the charge, rigidity, and velocity of 30,000 cosmic-ray helium nuclei, with velocity determined by time-of-flight and Cerenkov techniques. Using these data, the flux and isotopic composition of helium as a function of energy were determined. The observed isotopic composition is consistent with that expected from interstellar propagation models inferred from the secondaries of CNO, in contrast to earlier observations which indicated an overabundance of He-3. We discuss constraints that this result places on cosmic-ray transport and solar modulation models.

Measurement of the Isotopic Composition of Cosmic-Ray Helium, Lithium, Beryllium, and Boron up to 1700 MeV per Atomic Mass Unit

We present data from the second flight of the superconducting magnet instrument for light isotopes (SMILI), which took place on 1991 July 24. This instrument was optimized to determine the isotopic composition of He, Li, Be, and B in the Galactic cosmic rays, up to an energy of 2 GeV amu-1. The abundances of He, Li, and B are found to be consistent with standard models of cosmic-ray propagation. Our measurement of the abundances of the beryllium isotopes suggests an enhancement of the fraction of the isotope 10Be over that found at low energy. Of 26 beryllium events, nine are found to be 10Be. Monte Carlo calculations based on this observation imply the mean lifetime of cosmic rays to be less than 6 Myr at the 97.5% confidence level.

THE HIGH-ENERGY ANTIMATTER TELESCOPE (HEAT) : AN INSTRUMENT FOR THE STUDY OF COSMIC-RAY POSITRONS

Abstract The HEAT (High-Energy Antimatter Telescope) instrument has been developed for a series of observations in cosmic-ray astrophysics that require the use of a superconducting magnet spectrometer. This paper describes the first configuration of HEAT which is optimized for the detection of cosmic-ray electrons and positrons below 100 GeV. In addition to the spectrometer, a combination of time-of-flight scintillators, a transition radiation detector, and an electromagnetic shower counter, provides particle identification, energy measurement, and powerful discrimination against the large background of protons. The instrument was successfully flown aboard high-altitude balloons in 1994 and 1995. The design and construction of the spectrometer and of the detector systems are described, and the performance of the instrument is demonstrated with data obtained in flight.

Measurements of the Cosmic-Ray Positron Fraction from 1 to 50 G[CLC]e[/CLC]V

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Fort Sumner, New Mexico, in 1994 and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. We present results on the positron fraction based on data from the Lynn Lake flight and compare these with the previously published results from the Fort Sumner flight. The results confirm that the positron fraction does not increase with energy above ≈ 10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960s. This effect could possibly be a consequence of charge dependence in the level of solar modulation.