The Disappearance of the Pfotzer-Regener Maximum in Dose Equivalent Measurements in the Stratosphere

The NASA Radiation Dosimetry Experiment (RaD-X) successfully deployed four radiation detectors on a high altitude balloon for a period of approximately twenty hours. One of these detectors was the RaySure in-flight monitor, which is a solid-state instrument designed to measure ionizing dose rates to air crew and passengers. Data from RaySure on RaD-X show absorbed dose rates rising steadily as a function of altitude up to a peak at approximately 60,000 feet, known as the Pfotzer-Regener maximum. Above this altitude absorbed dose rates level off before showing a small decline as the RaD-X balloon approaches its maximum altitude of around 125,000 feet. The picture for biological dose equivalent, however, is very different. At high altitudes the fraction of dose from highly ionizing particles increases significantly. Dose from these particles causes a disproportionate amount of biological damage compared to dose from more lightly ionizing particles and this is reflected in the quality factors used to calculate the dose equivalent quantity. By calculating dose equivalent from RaySure data, using coefficients derived from previous calibrations, we show that there is no peak in the dose equivalent rate at the Pfotzer-Regener maximum. Instead the dose equivalent rate keeps increasing with altitude as the influence of dose from primary cosmic rays becomes increasingly important. This result has implications for high altitude aviation, space tourism and, due to its thinner atmosphere, the surface radiation environment on Mars