Radiation characteristics in the spherical tissue-equivalent phantom on the ISS during solar activity minimum according to the data from Liulin-5 experiment

Abstract

The Liulin-5 charged particle telescope observes the radiation characteristics in the spherical tissue-equivalent phantom of MATROSHKA-R international project on the International Space Station (ISS). Liulin-5 measures time resolved deposited energy spectra, linear energy transfer (LET) spectrum, flux and absorbed dose rates for electrons, protons and the biologically relevant heavy ion components of the cosmic radiation at three depths along the phantom's radius.We present some new results of Liulin-5 experiment obtained from June 2007 to March 2010.The average quality factor for different time intervals is between 2.7 and 4.4. Due to the heavy ions in LET spectrum of the galactic cosmic rays (GCR), the GCR quality factor is bigger than that of the trapped protons. The absorbed dose rates measured at depths in the phantom corresponding to the depths of blood forming organs in human body are 7.75–9.6μGy/h and the dose equivalent rates are 24.6–36.7μSv/h. Usually the trapped protons contribute about 60% of the total absorbed dose at 40mm depth in the phantom and about 40% at 165mm depth. The rest of the dose comes from GCR. Space Shuttle docking and the change of ISS attitude preformed for that purpose lead to a decrease in the total doses and to decreasing the trapped protons contributions. The doses from GCR are not affected neither by the depth of measurement not by Shuttle docking.At 165mm depth in the phantom the largest fluxes along the ISS orbit are obtained from the trapped protons in the South Atlantic Anomaly (SAA) at L values 1.26–1.27, B∼0.198Gs, geographical longitude ∼−51° to −55°, latitude ∼−28° to −29° and altitudes 361–363km. Minimal values about 0.1 particles/cm2s were recorded at L∼1 from GCR, at L≥4 the GCR flux reaches 2.1 particles/cm2s.