Cosmic-Ray Induced Neutrons: Recent Results from the Atmospheric Ionizing Radiation Measurements Aboard an ER-2 Airplane P. Goldhagen 1, J.M. Clem 2, J.W. Wilson 3, R.C. Singleterry 3, I.W. Jones 3, M. Reginatto 4 1 Environmental Measurements Laboratory, U.S. Department of Energy, New York, NY; 2 Bartol Research Institute, University of Delaware, Newark, DE 3 NASA Langley Research Center, Hampton, VA; 4 PTB, Braunschweig, Germany Conclusions  Calculations agree with measurements  Doses within  8% at ~0.8 GV  Spectrum shape nearly constant for aviation  Big change on ground  High-energy neutrons are significant  25% , 42% H*, 71% E from E n >10 MeV  Polar neutrons > 8  equatorial  H*(10) > Effective dose for cosmic ray neutrons  MNS with metal shells works for high-E neutrons  Useful at high-energy accelerators Flight paths of the AIR ER-2 flights and altitude profiles of three of them. Neutron data from the boxed portions of the flights have been analyzed. Boeing’s proposed Sonic Cruiser would fly 10,000 feet higher than present-day commercial jets. NASA ER-2 taking off for the AIR cosmic-ray measurements. Radiation sensors were carried in the nose, the fuselage behind the cockpit, and the front of both wing pods. Introduction Crews working on present ‑ day jet aircraft are a large occupationally exposed group with a relatively high average effective dose from galactic cosmic radiation (GCR). Crews of future high-speed commercial aircraft flying at higher altitudes will be even more exposed. To help reduce the significant uncertainties in calculations of such exposures, the Atmospheric Ionizing Radiation (AIR) Project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on a NASA ER ‑ 2 high-altitude aircraft.[1, 2] In addition to evaluation of air crew exposures, the results have applications in radiation effects on microelectronics, in determining source terms for cosmogenic nuclides used for atmospheric tracers and geological dating, and in evaluation of the background radiation exposure of the world population. The intensity of the different particles making up atmospheric cosmic radiation, their energy distribution, and their potential biological effect on aircraft occupants vary with altitude, geomagnetic latitude, and time in the sun’s magnetic activity cycle. Neutrons contribute roughly half of the biological exposure and most of the uncertainty in it. The effective dose from neutrons depends strongly on their energy distribution (spectrum). Cosmic-Ray Measurement Flights Five AIR measurement flights were made in June 1997, a time of maximum GCR (solar minimum). The flights covered latitudes from 18° to 60°N (vertical geomagnetic cutoff rigidities GV), with each flight lasting 6½ to 8 hours. The AIR measurements concentrated on altitudes from 16 to 21 km (52, ,000 ft, atmospheric depths g cm-2), but some data were also taken at lower altitudes and on the ground. Neutron Spectrometer The primary AIR instrument was a highly sensitive extended-energy multisphere neutron spectrometer (MNS, or Bonner spheres). Lead and steel shells within the moderators of two of its 14 detectors were used to enhance response at high energies. Detector responses were calculated for neutrons, protons, and pions at energies up to 100 GeV using the new high-energy Monte Carlo radiation transport code MCNPX. The effect of nearby materials, including other detectors, was included in the response calculations, but the effect of the airplane structure was not. The response of the larger MNS detectors to high-energy protons was significant. Correction for counts produced by protons was made using GCR-induced AIR hadron spectra calculated [3, 4] using FLUKA. Cosmic-ray neutron spectra were unfolded from the corrected measured count rates using the MAXED code.[5] What's New for This Workshop New FLUKA calculations of GCR-induced AIR hadron spectra at each measurement location [3, 4] provide:  Proton spectra to correct for proton counts in MNS   Calculated neutron spectra:  To compare with measured spectra  As starting point for unfolding measurements  Computational procedures for a new AIR Model Results Measured spectra of GCR-induced neutrons at 4 flight locations and on the ground, corrected for proton counts, are shown below. Spectra are shown as graphs of energy, E, times fluence rate per unit energy, d  /dE, on a linear scale vs. energy on a logarithmic scale. Calculated spectra for 2 locations are also shown. There is a pronounced “evaporation” peak at around 1 MeV and a high-energy peak around 100 MeV. 25% of the neutrons are above 10 MeV, and they contribute 71% of the effective dose. Total fluence rates and doses derived from the measured spectra are shown in the table. Cosmic-ray neutron spectra measured at low (0.8 GV) and high (12 GV) geomagnetic cutoff at 20 km altitude. The shape of the spectrum changes only slightly, while the neutron total fluence rate is 8 times higher at the north location. The calculated fluence rate at 0.8 GV cutoff is 10% less than the measured rate. Cosmic-ray neutron spectra measured at 3 altitudes. The shapes of the spectra are almost identical. Total fluence rate was almost constant from 16 to 20 km at ~0.8 GV cutoff, but 2.9  lower at 12 km and 4.5 GV cutoff. At 16 km, calculated fluence is 3% less than the measured rate. The shape of the neutron spectrum changed significantly on the ground, which reflects neutrons differently than air does. Total fluence rate was 280 times lower at sea level than at 12 km altitude. Effect of protons on measured neutron spectrum Joint DOE Low Dose Radiation Research Program Workshop II and NASA 12 th Annual Space Radiation Health Investigators’ Workshop June 27-30, 2001, Arlington, VA