to 1.2 milli rads) per hour, depending on location in the habitable volume, corresponding to an
annual dose range of 44 to 105 milliGy (4.4 to 10.5 rads). The variation in TID with location in
the habitable volume is largely a result of variations in effective shielding mass with location.
No destructive SEE events of any kind have been observed during the first two years of flight.
Only one ISS vehicle equipment item fault that may be uniquely attributed to SEE with a
reasonable level of confidence has been observed.
Detailed consideration of the effects of both the
natural and induced ionizing radiation environment
during ISS design, development, and flight
operations has produced a safe, efficient manned
space platform that is largely immune to deleterious
effects of the low-Earth orbit ionizing radiation
environment. However, model estimates show the
need for more work directed to development of a
practical understanding of secondary particle
production in massive structural shielding for single
event effect (SEE) design and verification. Utilizing
computer-aided design it was determined that total
dose estimates for shielding mass distribution were
reasonable and accurate for the ISS pressurized
elements (Koontz 2005).
At present, the best active dosimeters used for
radiation linear energy transfer (LET) are the tissue
equivalent proportional counter (TEPC) and silicon
detectors. The best passive dosimeters are the
thermoluminescence dosimeters (TLDs), optically
stimulated luminescence dosimeters (OSLDs) for
low LET, and CR-39 plastic nuclear track detectors
(PNTDs) for high LET. TEPC, CR-39 PNTDs, TLDs, and
OSLDs dosimeters were all used to investigate the
radiation environment for ISS Expedition 12, STS-
112, and STS-114, and proven to be successful and consistent in measuring the LET spectra and
all radiation quantities with excellent agreement among different detector types. The sensitivity
fading of CR-39 detectors for long-time exposures was observed, and the method of “internal
LET calibration using GCR iron peak” was developed to correct the sensitivity fading giving final
CR-39 results that are consistent with those measured by TEPC and TLDs/OSLDs. This study
indicates the LET spectrum method using CR-39 PNTDs, the LET calibration for CR-39 detectors,
and the combination method for results measured by passive dosimeters are reliable and will
be continually used in future space missions (Zhou 2007).
ISS015E12111 (June 15, 2007) – View of the
Tissue Equivalent Proportional Counter
(TEPC) Radiation Detector (gold cylinder)
and the TEPC Spectrometer (gold box) in the
US Laboratory/Destiny during Expedition 15.
The TEPC monitors radiation doses at the
cellular level.