Unusual Near-Horizon Cosmic-Ray-like Events Observed by ANITA-IV.

ANITA's fourth long-duration balloon flight in 2016 detected 29 cosmic-ray (CR)-like events on a background of 0.37_{-0.17}^{+0.27} anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous noninverted polarity, a p=5.3×10^{-4} chance if due to background. All anomalous events are from locations near the horizon; ANITA-IV observed no steeply upcoming anomalous events similar to the two such events seen in prior flights.

Observation of an Unusual Upward-Going Cosmic-Ray-like Event in the Third Flight of ANITA.

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight. These events could be produced by the atmospheric decay of an upward-propagating τ lepton produced by a ν_{τ} interaction, although their relatively steep arrival angles create tension with the standard model neutrino cross section. Each of the two events have a posteriori background estimates of ≲10^{-2} events. If these are generated by τ-lepton decay, then either the charged-current ν_{τ} cross section is suppressed at EeV energies, or the events arise at moments when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos.

Characteristics of Four Upward-Pointing Cosmic-Ray-like Events Observed with ANITA.

We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here, we report on characteristics of these three unusual CR events, which develop nearly horizontally, 20-30 km above the surface of Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible τ-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the standard model τ-neutrino cross section.

Observation of ultrahigh-energy cosmic rays with the ANITA balloon-borne radio interferometer.

We report the observation of 16 cosmic ray events with a mean energy of 1.5 × 10¹9 eV via radio pulses originating from the interaction of the cosmic ray air shower with the Antarctic geomagnetic field, a process known as geosynchrotron emission. We present measurements in the 300-900 MHz range, which are the first self-triggered, first ultrawide band, first far-field, and the highest energy sample of cosmic ray events collected with the radio technique. Their properties are inconsistent with current ground-based geosynchrotron models. The emission is 100% polarized in the plane perpendicular to the projected geomagnetic field. Fourteen events are seen to have a phase inversion due to reflection of the radio beam off the ice surface, and two additional events are seen directly from above the horizon. Based on a likelihood analysis, we estimate angular pointing precision of order 2° for the event arrival directions.

New limits on the ultrahigh energy cosmic neutrino flux from the ANITA experiment.

We report initial results of the first flight of the Antarctic Impulsive Transient Antenna (ANITA-1) 2006-2007 Long Duration Balloon flight, which searched for evidence of a diffuse flux of cosmic neutrinos above energies of E(nu) approximately 3 x 10(18) eV. ANITA-1 flew for 35 days looking for radio impulses due to the Askaryan effect in neutrino-induced electromagnetic showers within the Antarctic ice sheets. We report here on our initial analysis, which was performed as a blind search of the data. No neutrino candidates are seen, with no detected physics background. We set model-independent limits based on this result. Upper limits derived from our analysis rule out the highest cosmogenic neutrino models. In a background horizontal-polarization channel, we also detect six events consistent with radio impulses from ultrahigh energy extensive air showers.

Observations of the Askaryan effect in ice.

We report on observations of coherent, impulsive radio Cherenkov radiation from electromagnetic showers in solid ice. This is the first observation of the Askaryan effect in ice. As part of the complete validation process for the ANITA experiment, we performed an experiment at the Stanford Linear Accelerator Center in June 2006 using a 7.5 metric ton ice target. We measure for the first time the large-scale angular dependence of the radiation pattern, a major factor in determining the solid-angle acceptance of ultrahigh-energy neutrino detectors.

Constraints on cosmic neutrino fluxes from the Antarctic Impulsive Transient Antenna experiment.

We report new limits on cosmic neutrino fluxes from the test flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment, which completed an 18.4 day flight of a prototype long-duration balloon payload, called ANITA-lite, in early 2004. We search for impulsive events that could be associated with ultrahigh energy neutrino interactions in the ice and derive limits that constrain several models for ultrahigh energy neutrino fluxes and rule out the long-standing -burst model.

The energy spectrum of cosmic-ray induced neutrons measured on an airplane over a wide range of altitude and latitude.

Crews of high-altitude aircraft are exposed to radiation from galactic cosmic rays (GCRs). To help determine such exposures, the Atmospheric Ionizing Radiation Project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on a NASA ER-2 high-altitude airplane. The primary instrument was a sensitive extended-energy multisphere neutron spectrometer. Its detector responses were calculated for energies up to 100 GeV using the radiation transport code MCNPX 2.5.d with improved nuclear models and including the effects of the airplane structure. New calculations of GCR-induced particle spectra in the atmosphere were used to correct for spectrometer counts produced by protons, pions and light nuclear ions. Neutron spectra were unfolded from the corrected measured count rates using the deconvolution code MAXED 3.1. The results for the measured cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron dose equivalent and effective dose rates, and their dependence on altitude and geomagnetic cut-off agree well with results from recent calculations of GCR-induced neutron spectra.

New calculations of the atmospheric cosmic radiation field--results for neutron spectra.

The propagation of primary cosmic rays through the Earth's atmosphere and the energy spectra of the resulting secondary particles have been calculated using the Monte Carlo transport code FLUKA with several novel auxiliary methods. Solar-modulated primary cosmic ray spectra were determined through an analysis of simultaneous proton and helium measurements made on spacecraft or high-altitude balloon flights. Primary protons and helium ions are generated within the rigidity range of 0.5 GV-20 TV, uniform in cos2theta. For a given location, primaries above the effective angle-dependent geomagnetic cut-off rigidity, and re-entrant albedo protons, are transported through the atmosphere. Helium ions are initially transported using a separate transport code called HEAVY to simulate fragmentation. HEAVY interfaces with FLUKA to provide interaction starting points for each nucleon originating from a helium nucleus. Calculated cosmic ray neutron spectra and consequent dosimetric quantities for locations with a wide range of altitude (atmospheric depth) and geomagnetic cut-off are presented and compared with measurements made on a high-altitude aeroplane. Helium ion propagation using HEAVY and inclusion of re-entrant albedo protons with the incident primary spectra significantly improved the agreement of the calculated cosmic ray neutron spectra with measured spectra. These cosmic ray propagation calculations provide the basis for a new atmospheric ionising radiation (AIR) model for air-crew dosimetry, calculation of effects on microelectronics, production of cosmogenic radionuclides and other uses.

A new dynamical atmospheric ionizing radiation (AIR) model for epidemiological studies.

A new Atmospheric Ionizing Radiation (AIR) model is currently being developed for use in radiation dose evaluation in epidemiological studies targeted to atmospheric flight personnel such as civilian airlines crewmembers. The model will allow computing values for biologically relevant parameters, e.g. dose equivalent and effective dose, for individual flights from 1945. Each flight is described by its actual three dimensional flight profile, i.e. geographic coordinates and altitudes varying with time. Solar modulated primary particles are filtered with a new analytical fully angular dependent geomagnetic cut off rigidity model, as a function of latitude, longitude, arrival direction, altitude and time. The particle transport results have been obtained with a technique based on the three-dimensional Monte Carlo transport code FLUKA, with a special procedure to deal with HZE particles. Particle fluxes are transformed into dose-related quantities and then integrated all along the flight path to obtain the overall flight dose. Preliminary validations of the particle transport technique using data from the AIR Project ER-2 flight campaign of measurements are encouraging. Future efforts will deal with modeling of the effects of the aircraft structure as well as inclusion of solar particle events.

Overview of atmospheric ionizing radiation (AIR) research: SST-present.

The Supersonic Transport (SST) program, proposed in 1961, first raised concern for the exposure of pregnant occupants by solar energetic particles (SEP), and neutrons were suspected to have a main role in particle propagation deep into the atmosphere. An eight-year flight program confirmed the role of SEP as a significant hazard and of the neutrons as contributing over half of the galactic cosmic ray exposures, with the largest contribution from neutrons above 10 MeV. The FAA Advisory Committee on the Radiobiological Aspects of the SST provided operational requirements. The more recent lowering of ICRP-recommended exposure limits (1990) with the classification of aircrew as "radiation workers" renewed interest in GCR background exposures at commercial flight altitudes and stimulated epidemiological studies in Europe, Japan, Canada and the USA. The proposed development of a High Speed Civil Transport (HSCT) required validation of the role of high-energy neutrons, and this resulted in ER-2 flights at solar minimum (June 1997) and studies on effects of aircraft materials on interior exposures. Recent evaluation of health outcomes of DOE nuclear workers resulted in legislation for health compensation in year 2000 and recent European aircrew epidemiological studies of health outcomes bring renewed interest in aircraft radiation exposures. As improved radiation models become available, it is imperative that a corresponding epidemiological program of US aircrew be implemented.

Recent results form measurements of the energy spectrum of cosmic-ray induced neutrons aboard an ER-2 airplane and on the ground.

Crews of future high-altitude commercial aircraft may be significantly exposed to atmospheric cosmic radiation from galactic cosmic rays (GCR). To help determine such exposures, the Atmospheric Ionizing Radiation Project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on a NASA ER-2 high-altitude aircraft. The primary instrument was a sensitive extended-energy multisphere neutron spectrometer, which was also used to make measurements on the ground. Its detector responses were calculated for neutrons and charged hadrons at energies up to 100 GeV using the radiation transport code MCNPX. We have now recalculated the detector responses including the effects of the airplane structure. We are also using new FLUKA calculations of GCR-induced hadron spectra in the atmosphere to correct for spectrometer counts produced by charged hadrons. Neutron spectra are unfolded from the corrected measured count rates using the MAXED code. Results for the measured cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron dose equivalent and effective dose rates, and their dependence on altitude and geomagnetic cutoff generally agree well with results from recent calculations of GCR-induced neutron spectra.

Lunar lava tube radiation safety analysis.

For many years it has been suggested that lava tubes on the Moon could provide an ideal location for a manned lunar base, by providing shelter from various natural hazards, such as cosmic radiation, meteorites, micrometeoroids, and impact crater ejecta, and also providing a natural environmental control, with a nearly constant temperature, unlike that of the lunar surface showing extreme variation in its diurnal cycle. An analysis of radiation safety issues on lunar lava tubes has been performed by considering radiation from galactic cosmic rays (GCR) and Solar Particle Events (SPE) interacting with the lunar surface, modeled as a regolith layer and rock. The chemical composition has been chosen as typical of the lunar regions where the largest number of lava tube candidates are found. Particles have been transported all through the regolith and the rock, and received particles flux and doses have been calculated. The radiation safety of lunar lava tubes environments has been demonstrated.