P.R. Truscott

Secondary radiation environments in heavy space vehicles and instruments

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.

Calculations and observations of induced radioactivity in spaceborne materials

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. >

Measurement of the radiation environment from LEO to GTO using the CREAM and CREDO experiments

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.

Temporal variation in the new proton belt created in March 1991 observed using the CREAM & CREDO experiments

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.

Activation of space-borne bismuth germanate /spl gamma/-ray detectors

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. >

Observations and predictions of secondary neutrons on space shuttle and aircraft

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.

Comparison of activation effects in /spl gamma/-ray detector materials

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.

Contribution of secondaries to the radiation environment on space missions

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.

Simulation of spacecraft secondary particle emissions and their energy deposition in CCD X-ray detectors

A radiation transport suite has been used to simulate gamma-ray emissions produced in the X-ray Multi-Mirror Mission spacecraft by interactions of cosmic-rays and solar protons. Their energy deposition in charge coupled device detectors is a significant source of background.

The low earth orbit environment observed using CREAM and CREDO

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.