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Flight data obtained between 1990 and 1997 from the Cosmic Radiation Environment Monitors CREAM & CREDO carried on UoSAT-3, Space Shuttle, STRV-1a (Space Technology Research Vehicle) and APEX (Advanced Photovoltaic and Electronics Experiment Spacecraft) provide coverage over half a solar cycle. The modulation of cosmic rays and evolution of the South Atlantic Anomaly are observed, the former comprising a factor of three increase at high latitudes and the latter a general increase accompanied by a north-westward drift. Comparison of particle fluxes and linear energy transfer (LET) spectra is made with improved environment & radiation transport calculations which account for shield distributions and secondary particles. While there is an encouraging convergence between predictions and observations, significant improvements are still required, particularly in the treatment of locally produced secondary particles. Solar-particle events during this time period have LET spectra significantly below the October 1989 event which has been proposed as a worst case model.

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.

The Cosmic Radiation Effects and Activation Monitor has flown on six Shuttle flights between September 1991 and February 1995 covering the full range of inclinations as well as altitudes between 220 and 570 km, while a version has flown at supersonic altitudes on Concorde between 1988 and 1992 and at subsonic altitudes on a SAS Boeing 767 between May and August 1993. The Shuttle flights have included passive packages in addition to the active cosmic ray monitor which comprises an array of pin diodes. These are positioned at a number of locations to investigate the influence of shielding and local materials. Use of both metal activation foils and scintillator crystals enables neutron fluences to be inferred from the induced radioactivity which is observed on return to Earth. Supporting radiation transport calculations are performed to predict secondary neutron spectra and the energy deposition due to nuclear reactions in silicon pin diodes and the induced radioactivity in the various scintillator crystals. The wide variety of orbital and atmospheric locations enables investigation of the influence of shielding on cosmic ray, trapped proton and solar flare proton spectra.

Activation induced by cosmic and trapped radiation in /spl gamma/-ray detector materials represents a significant source of background for space-based detector systems. Selection of detector materials should therefore include consideration of this background source. Results are presented from measurements of induced radioactivity in different scintillators activated either as a result of irradiation by mono-energetic protons at accelerator facilities, or flight on board the Space Shuttle. Radiation transport computer codes are used to help compare the effects observed from the scintillators, by identifying and quantifying the influence on the background spectra from more than one hundred of the radionuclides produced by spallation. For the space experiment data, the simulation results also permit determination of the contributions to detector activation from the different sources of radiation in the Shuttle cabin.

The Cosmic Radiation Environment & Activation Monitor (CREAM) was carried in high inclination (57.1 degrees) orbits on Shuttle missions STS-48 in September 1991 (altitude 570 km) and STS-53 (altitude 325 to 385 km) in December 1992. On both occasions the instrument observed an excess of counts due to protons of greater than 30 MeV in energy in the region off of South Africa where field lines of L=2.5 intersect low earth orbit. Meanwhile the Cosmic Radiation Environment and Dosimetry Experiment (CREDO) carried to 840 km, 98.7 degrees orbit on UOSAT-3 has continued to sample the high field portions of the L-shells around L = 2.5 from April 1990 until the present time. When careful subtraction of cosmic-ray contributions is made it can be seen that the March 91 enhancement persisted for approximately 8 months and explains the STS-48 observation. There would appear to have been a further increase produced by the 31 October 1992 flare event and seen by STS-53.

The Cosmic Radiation Environment and Dosimetry experiment (CREDO) on STRV-1a is monitoring the radiation environment in geostationary transfer orbit (GTO) and in conjunction with similar detectors in low earth orbit (LEO) affords coverage of the whole magnetosphere. Results are showing the time variability in the environment and are testing the standard models.

Accurate knowledge of the sources of detector background is essential in /spl gamma/-ray astronomy for correct data interpretation, and for optimising detector design and operation. In the low-to medium-energy range, a major source of background arises from radioactivity that is induced in the detector medium itself by nuclear spallation and neutron capture. Results are presented from the Shuttle Activation Monitor flown in the Space Shuttle middeck area on two missions during August 1989 and September 1991. This experiment has been valuable in the study of induced radioactivity in BGO scintillators resulting from bombardment by cosmic-rays, trapped protons, and their secondaries created by interactions in a heavy spacecraft. Data from mono-energetic proton irradiation experiments have allowed investigation of activation effects in BGO for less complicated irradiation conditions, and the results from these experiments are also presented. Detailed simulations have been performed to determine BGO detector activation and response, and these take into consideration the effects of shielding by the spacecraft structure. The results compare very well to the experimental data, allowing features in the measured spectra to be identified. They also highlight the importance of shielding both in reducing activation by trapped protons, and enhancing cosmic-ray induced activation due to secondaries. >

Calculations to predict the radiation environment for spacecraft in low earth orbit sometimes ignore the contribution from secondary radiation products. However, the contribution of secondaries, particularly neutrons, on heavy spacecraft or in planetary bodies can be of concern for biological systems. The Shuttle Activation Monitor (SAM) and Cosmic Radiation Effects and Activation Monitor (CREAM) experiments provide valuable data on secondary (as well as primary) radiation effects. Comparisons have been made between induced activity from flight-exposed samples, induced activity in a ground-irradiated sample, and Monte Carlo-derived predictions with and without secondaries. These comparisons show that for a flight-exposed sample, predictions which omit the secondary contribution result in a spectrum that is too low by a factor of 2. The addition of the secondaries results in a predicted spectrum that closely matches the measured data.

Radiation transport calculations are compared with observations of induced radioactivity obtained from the OSSE experiment on the Compton Observatory and the CREAM experiment on Space Shuttle. These results illustrate the importance of secondary particles. >

The Shuttle Activation Monitor (SAM) experiment was flown on the Space Shuttle Columbia (STS-28) from 8 – 13 August, 1989 in a 57°, 300 km orbit. One objective of the SAM experiment was to determine the relative effect of different amounts of shielding on the gamma-ray backgrounds measured with similarly configured sodium iodide (NaI) and bismuth germante (BGO) detectors. To achieve this objective twenty-four hours of data were taken with each detector in the middeck of the Shuttle on the ceiling of the airlock (a high-shielding location) as well as on the sleep station wall (a low-shielding location). For the cosmic-ray induced background the results indicate an increased overall count rate in the 0.2 to 10 MeV energy range at the more highly shielded location, while in regions of trapped radiation the low shielding configuration gives higher rates at the low energy end of the spectrum.

A 7.6-cm*7.6-cm BGO (bismuth germanate) detector was flown in the middeck of the Space Shuttle Columbia August 8-13, 1989, as part of the Shuttle Activation Monitor (SAM) experiment. One of the goals of this experiment was to compare the performance of the BGO to that of NaI in the same environment. Twenty-four hours of data in 5-min time bins were recorded with each detector in each of two locations in a high-inclination orbit (57 degrees , 300 km). Pre- aid post-flight low-background counting was preformed for identification of activities induced by the space radiation environment. A number of isotopes attributed to induced activity from exposure to the space radiation environment have been tentatively identified. >

Secondary radiations produced by the interactions of primary cosmic rays and trapped protons with spacecraft materials and detectors provides an important, and sometimes dominant, radiation environment for sensitive scientific instruments and biological systems. In this paper the success of a number of calculations in predicting a variety of effects will be examined. The calculation techniques include Monte Carlo transport codes and semi-empirical fragmentation calculations. Observations are based on flights of the Cosmic Radiation Environment and Activation Monitor at a number of inclinations and altitudes on Space Shuttle. The Shuttle experiments included an active cosmic-ray detector as well as metal activation foils and passive detector crystals of sodium iodide which were counted for induced radioactivity soon after return to earth. Results show that cosmic-ray secondaries increase the fluxes of particles of linear energy transfer less than 200 MeV/(gm cm−2), while the activation of the crystals is enhanced by about a factor of three due to secondary neutrons. Detailed spectra of induced radioactivity resulting from spallation products have been obtained. More than a hundred significant radioactive nuclides are included in the calculation and overall close agreement with the observations is obtained.

An X2/2B level solar flare occurred on 12 August, 1989, during the last day of the flight of the Space Shuttle Columbia (STS-28). Detectors on the GOES 7 satellite observed increased X-ray fluxes at approximately 1400 GMT and a solar particle event (SPE) at approximately 1600 GMT. Measurements with the bismuth germanate (BGO) detector of the Shuttle Activation Monitor (SAM) experiment on STS-28 showed factors of two to three increases in count rates at high latitudes comparable to those seen during South Atlantic Anomaly (SAA) passages beginning at about 1100 GMT. That increased activity was observed at both north and south high latitudes in the 57 degrees, 300 kilometer orbit and continued until the detector was turned off at 1800 GMT. Measurements made earlier in the flight over the same geographic coordinates did not produce the same levels of activity. This increase in activity may not be entirely accounted for by observed geomagnetic phenomena which were not related to the solar flare.

Radioactivity induced in detectors by protons and secondary neutrons limits the sensitivity of spaceborne gamma-ray spectrometers. Three-dimensional Monte Carlo transport codes were used to simulate particle transport of cosmic rays and innerbelt protons in various representations of the Gamma-Ray Observatory Spacecraft and the Oriented Scintillation Spectrometer Experiment. Results are used to quantify accurately the contributions to the radioactive background, assess shielding options, and examine the effects of detector and spacecraft orientation in anisotropic trapped proton fluxes. >

The Cosmic Radiation Environment and Dosimetry experiment (CREDO) has been operational on board the Advanced Photovoltaics & Electronics Experiment Spacecraft since August 1994. Extensive measurements of cosmic ray linear energy transfer spectra (using data to January 1996) and total dose (using data to November 1994) have been made, and compared with predictions of standard models. Detailed consideration of spacecraft shielding effects have been made. Predictions are shown to overestimate the measured linear energy transfer spectra. The CREAM experiment was flown on STS-63 in the SpaceHab module. Results show penetration of high energy electrons into the SpaceHab module.