Galactic Cosmic Ray Protons Research Articles

Simulation of the interaction of galactic cosmic ray protons with meteoroids: On the production of 3H and light noble gas isotopes in isotropically irradiated thick gabbro and iron targets

Abstract— Thick spherical targets, one made of gabbro (R = 25 cm) and one made of iron (R = 10 cm), were irradiated isotropically with 1.6 GeV protons at Laboratoire National Saturne (LNS)/Saclay to simulate the interactions of galactic cosmic ray protons with meteoroids in space. At various depths, both artificial meteoroids contained a large number of high‐purity, single‐element target foils and chemical compounds of up to 28 target elements. In these individual target foils, the elemental production rates of radionuclides and noble gas isotopes were measured. Here, we report the results for the light noble gas isotopes 3,4He, 20,21,22Ne, and 36,38,39Ar for the most cosmochemically relevant target elements as well as for some meteoritic material from Jilin, Farmington, and Cape York. From 3He analyses done several years apart, 3H diffusive losses during sample storage have been obtained, and direct as well as cumulative 3He production rates for O, Mg, Al, Si, Fe, Ni, and the meteoritic material are given. Losses by diffusion of tritium from metallic Mg and Fe are found to occur on time scales of months, while metallic Al, Si, and stone meteorites are much more retentive. The production rate ratios P(3H)/P(3He)d obtained in the simulation experiments are 0.73, 1.28, and 1.16 for O, Al, and Si, respectively. These rates are based on our best knowledge about the 3H and 3He production rates and should, therefore, replace data published earlier (Leya et al. 2000a). The earlier calculations for 4He, 20,21,22Ne, and 36,38,39Ar remain valid.The new modeled correlation 3Hecum/21Ne versus 22Ne/21Ne for chondrites exposed to cosmic rays with an energy spectrum characterized by a modulation parameter of φ = 650 MeV is in fair agreement with the empirical relationship (“Berne plot”). However, for small meteorites and little shielding in larger ones, there are systematic differences that most likely are due to an underestimation of the spallogenic 22Ne/21Ne ratio by ˜2%.

An Interstellar Origin for the Beryllium 10 in Calcium‐rich, Aluminum‐rich Inclusions

Beryllium 10 is a short-lived radionuclide (t1=2 ¼ 1:5 Myr) that was incorporated live into calcium-rich, aluminum-rich inclusions (CAIs) at the birth of our solar system. Beryllium 10 is unique among the short-lived radionuclides in that it is formed only by spallation reactions and not by nucleosynthesis, e.g., in a supernova. Recent work by McKeegan, Gounelle, and others has stated that the high initial abundance of 10 Be in CAIs ( 10 Be= 9 Be � 1 � 10 � 3 ) cannot be attributed to galactic cosmic rays (GCRs) and therefore concluded that the spallation reactions must have occurred within the solar nebula itself, because of energetic particles emitted by the early Sun. In this paper we reexamine this conclusion. We calculate the contributions of GCRs to the 10 Be abundance in a molecular cloud core as it collapses to form a protostar and protoplanetary disk. We constrain the flux of protons and 10 Be GCRs in the Sun’s molecular cloud core 4.5 Gyr ago. We use numerical magnetohydrodynamic simulations of star formation to model the time evolution of the magnetic field strength and column density of gas in a collapsing cloud core. We account for magnetic focusing and magnetic mirroring and the anisotropic distribution of GCR pitch angles in the cloud core. We calculate the rates at which GCR protons and � -particles induce spallation reactions producing 10 Be atoms, and the rates at which GCR 10 Be nuclei are trapped in the cloud core. Accounting also for the decay of 10 Be over the evolution of the cloud core, we calculate the time-varying 10 Be/ 9 Be ratio. We find that at the time of protostar formation 10 Be/ 9 Be � 1 � 10 � 3 , with an uncertainty of about a factor of 3. Spallation reactions account for 20% of the 10 Be in CAIs, while trapped GCR 10 Be nuclei account for the other 80%. The initial abundance of 10 Be in CAIs is therefore entirely attributable to cosmic rays. We discuss the implications of this finding for the origin of other short-lived radionuclides and for the use of 10 Be as a chronometer. Subject headings: cosmic rays — nuclear reactions, nucleosynthesis, abundances — solar system: formation — stars: formation

Absorbed dose of secondary neutrons from galactic cosmic rays inside the international space station

In this paper, we present the results of Monte-Carlo simulations of the flux and energy spectra of neutrons generated as a result of galactic cosmic ray proton interactions with the material of International Space Station (ISS) inside Zvezda Service Module, the Airlock between Russian and USA segments and one of Russian Research Modules for a full configuration of ISS. Calculations were made for ISS orbit for the energy ranges <10 and >10 MeV for both maximum and minimum of solar activity. To test the accuracy of the calculations the same simulations were made for MIR orbital station and for CORONAS-I satellite and compared with the results of measurements. Calculated and measured fluxes are in reasonable agreement.

The Ulysses fast latitude scans: COSPIN/KET results

Abstract. Ulysses, launched in October 1990, began its second out-of-ecliptic orbit in December 1997, and its second fast latitude scan in September 2000. In contrast to the first fast latitude scan in 1994/1995, during the second fast latitude scan solar activity was close to maximum. The solar magnetic field reversed its polarity around July 2000. While the first latitude scan mainly gave a snapshot of the spatial distribution of galactic cosmic rays, the second one is dominated by temporal variations. Solar particle increases are observed at all heliographic latitudes, including events that produce &gt;250 MeV protons and 50 MeV electrons. Using observations from the University of Chicago’s instrument on board IMP8 at Earth, we find that most solar particle events are observed at both high and low latitudes, indicating either acceleration of these particles over a broad latitude range or an efficient latitudinal transport. The latter is supported by "quiet time" variations in the MeV electron background, if interpreted as Jovian electrons. No latitudinal gradient was found for &gt;106 MeV galactic cosmic ray protons, during the solar maximum fast latitude scan. The electron to proton ratio remains constant and has practically the same value as in the previous solar maximum. Both results indicate that drift is of minor importance. It was expected that, with the reversal of the solar magnetic field and in the declining phase of the solar cycle, this ratio should increase. This was, however, not observed, probably because the transition to the new magnetic cycle was not completely terminated within the heliosphere, as indicated by the Ulysses magnetic field and solar wind measurements. We argue that the new A&lt;0-solar magnetic modulation epoch will establish itself once both polar coronal holes have developed.Key words. Interplanetary physics (cosmic rays; energetic particles; interplanetary magnetic fields)

Ulysses Cosmic Ray and Solar Particle Investigation/Kiel Electron Telescope observations: Charge sign dependence and spatial gradients during the 1990–2000 A &gt; 0 solar magnetic cycle

Ulysses, launched in October 1990, began its second out‐of‐ecliptic orbit in September 1997. From 1991 to mid‐2000, solar polar magnetic field observations indicate a magnetic field pointing outward over the northern polar region (A &gt; 0 epoch). While the first fast latitude scan in 1994/1995 was performed around solar minimum, the second Ulysses orbit covering high latitudes is being performed around solar maximum. We present data of ∼2.5 GV electrons and protons from the Kiel Electron Telescope up to southern heliographic latitudes of 80°, reached by Ulysses in November 2000. Spatial gradients are determined by comparison with a near‐Earth baseline. The radial gradient for an integral proton channel is found to be 2.2 ± 0.6%/AU before early 1998 and 3.5%/AU thereafter. Until mid‐1999 the latitudinal variation is consistent with a zero gradient at low latitudes and an average gradient of about 0.25%/degree for latitudes above about ±25°. In contrast, the latitudinal gradient decreases markedly after mid‐1999, leading to a spherically symmetric intensity distribution with the approach to solar maximum conditions. The electron to proton (e/p) ratio is used to study charge sign dependent modulation. According to drift‐dominated modulation models, galactic cosmic ray protons and electrons respond differently to the global heliospheric magnetic field. Comparison of the e/p ratio over the period 1991 to 2000 with model predictions indicates that drift effects are important over a large part of an 11‐year period with A &gt; 0. With the approach to solar maximum the e/p ratio increases, indicative for the fading out of drift effects and the transition to a diffusion‐dominated modulation.

A recommended procedure for estimating the cosmic-ray spectral parameter of a simple power law

A simple power law model with single spectral index α 1 is believed to be an adequate description of the galactic cosmic-ray (GCR) proton flux at energies below 10 13 eV. Two procedures for estimating α 1—the method of moments and maximum likelihood (ML)—are developed and their statistical performance are compared. The ML procedure is shown to be the superior approach and is then generalized for application to real cosmic-ray data sets. Several other important results, such as the relationship between collecting power and detector energy resolution and inclusion of a non-Gaussian detector response function, are presented. These results have many practical benefits in the design phase of a cosmic-ray detector as they permit instrument developers to make important trade studies in design parameters as a function of one of the science objectives.

The production of cosmogenic nuclides by galactic cosmic‐ray particles for 2π exposure geometries

Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for 10Be, 26Al, and 53Mn to measured data from the Apollo 15 drill core. The J0,GCR value of 4.54 cm−2 s−1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for 10Be, 14C, 26Al, 36Cl and 53Mn, as well as for the rare gas isotopes 20,21,22Ne. In addition we present production rates for 36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the 22Ne/21Ne ratio as a shielding parameter is also discussed.

Predictions of secondary neutrons and their importance to radiation effects inside the international space station

As part of a study funded by NASA MSFC to assess thecontribution of secondary particles in producing radiation damage to optoelectronics devices located on the International Space Station (IS), Monte Carlo calculations have been made to predict secondary spectra vs. shielding inside ISS modules and in electronics boxes attached on the truss (Armstrong and Colborn, 1998). The calculations take into account secondary neutron, proton, and charged pion production from the ambient galactic cosmic-ray (GCR) proton, trapped proton, and neutron albedo environments. Comparisons of the predicted neutron spectra with measurments made on the Mir space station and other spacecraft have also been made (Armstrong and Colborn, 1998). In this paper, some initial results from folding the predicted neutron spectrum inside ISS modules from Armstrong and Colborn (1998) with several types of radiation effects response functions related to electronics damage and astronaut-dose are given. These results provide an estimate of the practical importance of neutrons compared to protons in assessing radiation effects for the ISS. Also, the important neutron energy ranges for producing these effects have been estimated, which provides guidance for onboard neutron measurement requirements.

The production of cosmogenic nuclides in stony meteoroids by galactic cosmic‐ray particles

Abstract— We present a purely physical model for the calculation of depth‐ and size‐dependent production rates of cosmogenic nuclides by galactic cosmic‐ray (GCR) particles. besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on only one free parameter—the integral number of gcr particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth‐ and size‐dependent production rates for 10Be, 14C, 26Al, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semi‐empirical estimates by up to a factor of 4 but agree within ∼20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26Al vs. 10Be, 26Al vs. 53Mn, 10Be/21Ne vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.

Differences in the temporal variations of galactic cosmic ray electrons and protons: Implications from Ulysses at solar minimum

According to standard drift dominated modulation models the intensity variations of galactic cosmic ray protons and electrons respond differently to the latitudinal extension of the heliospheric current sheet α. In an A&gt;0 solar cycle intensities of protons should vary weakly with the latitudinal extension, whereas electrons should show a strong response. We investigate this charge dependent variation in the 1990s (A&gt;0) using Ulysses Kiel Electron Telescope (KET) measurements. Proton measurements at 2.5 GV corrected for latitudinal variations show the same time profile as electrons from mid 1994 until the beginning of 1996, and later from September 1997 to the end of 1997. In 1996 and 1997, when α was below ∼25°, two long lasting time periods were found when electrons had a ∼5–10% higher level. These variations are in agreement with our computations indicating that drift effects play an important role in determining the temporal variation of electrons close to solar minimum.

Anomalous cosmic rays: Observations

The latitudinal gradients of galactic cosmic ray protons and anomalous cosmic ray (ACR) helium appear to be symmetrical about a plane displaced ∼10° south of the heliographic equator, similar to the displacement of the heliospheric current sheet (HCS). The radial gradient of ∼10 MeV/nuc ACR oxygen in the outer heliosphere is correlated with the amplitude of the tilt of the HCS. The form of the correlation depends on the magnetic polarity of the solar cycle. The observed radial gradients are the basis for an empirical model that reasonably describes the modulation of ∼2–3 GV ACR oxygen at low heliographic latitudes over a 25-year period.

Monte Carlo simulation of induced radioactive background in gamma-ray detector materials

Induced activation in detector materials constitutes a major source of background for spaceborne /spl gamma/-ray instruments. Future detector systems will utilise a variety of background reduction techniques and novel detector materials to maximise signal-to-noise and resolution of the measurements taken, and assessment of the merits of different designs will require detailed attention to a wide range of physical processes. The Integrated Radiation Transport Suite (IRTS) has been applied to simulate the effects of spacecraft shielding against Van Allen and galactic cosmic-ray protons, the production of secondary hadrons, and induced radioactivity in and the response of /spl gamma/-ray detector materials. This paper discusses the physics requirements for these simulations, and the degree of success of the models through comparisons with experimental data obtained from Space Shuttle flights and proton-beam irradiation of detector materials.

Latitudinal distribution of &gt;106 MeV protons and its relation to the ambient solar wind in the inner southern and northern heliosphere: Ulysses Cosmic and Solar Particle Investigation Kiel Electron Telescope Results

We present observations and model calculations of the modulated intensities of galactic cosmic ray protons above 106 MeV/n along the Ulysses trajectory. Data are taken by the Cosmic and Solar Particle Investigation (COSPIN) Kiel Electron Telescope (KET) from spring 1993 to fall 1996. During this time period solar activity decreased and galactic cosmic rays recovered. To separate spatial from temporal variations we used the University of Chicago measurements from IMP 8 near Earth and compare the residual spatial variation with time independent modulation models. According to standard drift dominated modulation model, one would expect a radial gradient of ≈2%/AU and a latitudinal gradient of ≈1.2%/degree in both hemispheres. The measured mean radial gradient of «3%/AU is within the uncertainties in good agreement with the model predictions. However, the measured mean latitudinal gradient has a value of (0.33 ± 0.04)%/degree in both hemispheres at intermediate latitudes and is by a factor of 4 lower than expected. In the modified models the latitudinal gradient is in agreement with the measured ones when we increase the diffusion coefficient κθθ perpendicular to the magnetic field in polar direction to a value of 15% of the diffusion coefficient parallel to the magnetic field. The latitudinal gradients &gt;106 MeV protons calculated by modified modulation models are in very good agreement with the measured ones at intermediate latitudes. At lower latitudes, when Ulysses is embedded in the streamer belt, the models predict approximately the same latitudinal gradient than at intermediate latitudes, whereas the measured ones are significant smaller, or even vanishing. The observations support the previous conclusion from Paizis et al. [1995] that a significant latitudinal gradient is only observed when Ulysses is outside the streamer belt. Another remarkable observation is the difference of the radial gradient Gr at ∼3.5 AU in the southern (Gr≈3.5%/AU) and in the northern hemisphere (Gr≈2.3%/AU). Probable reasons for the significantly lower radial gradient in the northern hemisphere can be either a spatial asymmetry of the heliosphere or temporal variations.

A new model to study galactic cosmic ray modulation in an asymmetrically bounded heliosphere

We developed a two‐dimensional steady state numerical model that can describe the modulation of galactic cosmic rays in an arbitrarily shaped heliospheric outer modulation boundary. The model is based on the well‐known Parker transport equation for cosmic rays and incorporates: (1) gradient and curvature drift effects including drift along a wavy neutral sheet, (2) adiabatic energy changes, (3) diffusion parallel and anisotropic diffusion perpendicular to the heliospheric magnetic field, and (4) outward convection due to a latitude‐dependent expanding solar wind plasma. The model is described together with results illustrating the effects of an asymmetric heliospheric boundary on the spatial and energy variations of galactic cosmic ray protons throughout the heliosphere. The cosmic ray distribution in the inner heliosphere was only marginally altered, but significant changes were found beyond ∼30 AU, especially in the upwind direction. Concerning the energy dependence of these effects the maximum cosmic ray redistribution occurred at ∼50 MeV. At energies of &gt; 5 GeV the effects became insignificant. Assuming a highly asymmetric heliosphere, we calculated a longitudinal gradient of 0.4 % deg−1 for 1 GV protons at 50 AU, which may be considered an upper limit.

A self‐consistent determination of the heliospheric termination shock structure in the presence of pickup, anomalous, and galactic cosmic ray protons

A self‐consistent time‐dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

Transverse cosmic ray gradients in the heliosphere and the solar diurnal anisotropy

We describe a new scheme for computing the asymmetric part of the cosmic ray particle density gradient. The method makes use of the ecliptic components (Ar, Aϕ) of the cosmic ray solar diurnal anisotropy observed with a detector on spinning Earth and the interplanetary magnetic field sector structure in the heliosphere. We apply this methodology, to compute the magnitudes and directions of the asymmetric (Gθa) particle density gradient in the heliosphere for selected intervals of time during the 1965 to 1993 period. Data from six detectors of the global network are used for this purpose. Their median rigidity of response (Rm) covers the following range: 10 GV ≤ Rm ≤ 300 GV of the galactic cosmic ray (GCR) spectrum. We find that Gθa is well behaved over an intermediate rigidity range (10 GV ≤ Rm ≤, 67 GV) where it is inversely proportional to the rigidity (R) of the GCR protons. At higher rigidities (R ≳ 100 GV), it may fall off more steeply and even reverse sign at large (≳ 1 AU) off‐ecliptic distances. The direction of Gθa changes consistently immediately after each observed solar polar field reversal. Sudden, conspicuous, short‐lived (∼ 1 year) reversals of Gθa are observed during the declining phases of solar activity cycles 20(1974) and 21(1984). They may be caused by a sudden burst of solar activity late in the cycle. We have determined Gθa values from Deep River neutron monitor data (Rm = 16 GV) for the 1965 to 1993 period. Extreme computed values range from (3.0 ± 0.8)%/AU in 1968 to (−2.4 ± 0.8)%/AU in 1987.

Energetic proton irradiation history of the howardite parent body regolith and implications for ancient solar activity

Abstract— Previous studies have shown that the Kapoeta howardite, as well as several other meteorites, contains excess concentrations of cosmogenic Ne in the darkened, solar‐irradiated phase compared to the light, non‐irradiated phase. The two explanations offered for the nuclear production of these Ne excesses in the parent body regolith are either from galactic cosmic‐ray proton (GCR) irradiation or from a greatly enhanced flux of energetic solar “cosmic‐ray” protons (SCR), as compared to the recent solar flux. Combining new isotopic data we obtained on acid‐etched, separated feldspar from Kapoeta light and dark phases with literature data, we show that the cosmogenic 21Ne/22Ne ratio of light phase feldspar (0.80) is consistent with only GCR irradiation in space for ∼3 Ma. However, the 21Ne/22Ne ratio (0.68) derived for irradiation of dark phase feldspar in the Kapoeta regolith indicates that cosmogenic Ne was produced in roughly equal proportions from galactic and solar protons. Considering a simple model of an immature Kapoeta parent body regolith, the duration of this early galactic exposure was only ∼3–6 Ma, which would be an upper limit to the solar exposure time of individual grains. Concentrations of cosmogenic 21Ne in pyroxene separates and of cosmogenic 126Xe in both feldspar and pyroxene are consistent with this interpretation. The near‐surface irradiation time of individual grains in the Kapoeta regolith probably varied considerably due to regolith mixing to an average GCR irradiation depth of ∼10 cm. Because of the very different depth scales for production of solar ∼Fe tracks, SCR Ne, and GCR Ne, the actual regolith exposure times for average grains probably differed correspondingly. However, both the SCR 21Ne and solar track ages appear to be longer because of enhanced production by early solar activity. The SCR/GCR production ratio of 21Ne inferred from the Kapoeta data is larger by a at least a factor of 10 and possibly as much as a factor of ∼50 compared to recent solar particle fluxes. Thus, this study indicates that our early Sun was much more active and emitted a substantially higher flux of energetic (&gt;10 MeV/nucleon) protons.

Cross sections for the production of residual nuclides by low- and medium-energy protons from the target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au

Cross sections for residual nuclide production by p-induced reactions were measured from thresholds up to 2.6 GeV using accelerators at CERN/Geneve, IPN/Orsay, KFA/Jülich, LANL/Los Alamos, LNS/Saclay, PSI/Villigen, TSL/Uppsala, LUC/Louvain La Neuve. The target elements C, N, O, Mg, Al, Si, Ca, Ti, V, Mn, Fe, Co, Ni, Cu, Sr, Y, Zr, Nb, Ba and Au were investigated. Residual nuclides were measured by X- and γ-spectrometry and by Accelerator Mass Spectrometry (AMS). The measured cross sections were corrected for interfering secondary particles in experiments with primary proton energies above 200 MeV. Our consistent database covers presently ca 550 nuclear reactions and contains nearly 15000 individual cross sections of which about 10000 are reported here for the first time. They provide a basis for model calculations of the production of cosmogenic nuclides in extraterrestrial matter by solar and galactic cosmic ray protons. They are of importance for many other applications in which medium energy nuclear reactions have to be considered ranging from astrophysics over space and environmental sciences to accelerator technology and accelerator-based nuclear waste transmutation and energy amplification. The experimental data are compared with theoretical ones based on calculations using an INC/E model in form of the HETC/KFA2 code and on the hybrid model of preequilibrium reactions in form of the AREL code.

The Influence of Pickup, Anomalous, and Galactic Cosmic-Ray Protons on the Structure of the Heliospheric Shock: A Self-Consistent Approach

A self-consistent time-dependent model is used to study the modification of the heliospheric shock in the upwind direction under the influence of three suprathermal particle populations, namely, pickup, anomalous, and Galactic cosmic-ray protons. By ensuring that the resulting modulated cosmic-ray proton spectra are consistent with those observed by the Voyager and Pioneer spacecraft during the solar activity minimum in 1987, two alternative modifications of the heliospheric shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and is mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

Prediction of LDEF exposure to the ionizing radiation environment

Predictions of the LDEF mission's trapped proton and electron and galactic cosmic ray proton exposures have been made using the currently accepted models with improved resolution near mission end and better modeling of solar cycle effects. An extension of previous calculations, to provide a more definitive description of the LDEF exposure to ionizing radiation, is represented by trapped proton and electron flux as a function of mission time, presented considering altitude and solar activity variation during the mission and the change in galactic cosmic ray proton flux over the mission. Modifications of the AP8MAX and AP8MIN fluence led to a reduction of fluence by 20%. A modified interpolation model developed by Daly and Evans resulted in 30% higher dose and activation levels, which better agreed with measured values than results predicted using the Vette model.