７９ Radiation Exposure

７９　Radiation Exposure　【Space Travel】Dialogue with the Universe

７９－１　Cosmic Radiation Exposure
○Constitution of Cosmic Radiation
・Solar Particle Events (SPE's)：hydrogen (protons) 90%, helium (alpha particles) , all of the rest of the elements 
・Galactic Cosmic Radiation (GCR)：hydrogen (protons) 90%, helium (alpha particles) 9%, all of the rest of the elements 1%[1]


Cosmic Radiation - Go Flight Medicine

○The sun’s magnetic field reverses its direction every 11 years (last flip was Jan 2014). Additionally, solar flares occur from time to time, which may increase cosmic radiation dose for a shorter period of time. [1]
○A majority of cosmic radiation never reaches those of us living on the terrestrial earth due to the sun’s magnetic field, the earth’s magnetic field, and the earth’s atmosphere. [1]
○ If the Earth's magnetic field has disappeared (it was several times on the history of the Earth), radiation exposure on the ground is increased about 14%.[2] [3]



７９－２　Radiation Limits
○Radiation weighting factor [4]
：Coefficient representing the degree of impact on the human body of radiation.
Photons(x-rays, gamma rays)・Electrons(beta particles)：１,protons:2, neutrons:2.5～21, alpha particles(a particle identical to a helium nucleus):20
　 
○Radiation effects
1 mSv／year：Effective dose [5]
2.4mSv／year：Average yearly background natural radiation in the world 
20 mSv／year：Occupational effective dose limit [5]
100mSv：Level at which higher raisk of cancer is first noticeable 
　　　　　Missions on the ISS or Russian Mir space station have registered exposures greater than 100 mSv to some astronauts. [1]
1Ｓｖ：Missions to Mars（1.5 years）[1] [6]
4Ｓｖ：Median lethal dose（LD50） [7] 
7Ｓｖ：100% lethal dose [7] 

７９－３　Mitigation of Cosmic Radiation
○Alpha (α) radiation is stopped by a sheet of paper. Beta (β) radiation is halted by an aluminium plate. Gamma (γ) radiation is eventually absorbed as it penetrates a dense material(Concrete with a thickness of 50 cm, in the case of lead 10cm). Neutron (n) radiation is blocked by light elements, like hydrogen, which slow and/or capture them. [8]
○Aggressive shielding measures in the ISS is not taken.
← Since the launch cost of screening agents is significant.

７９－４　Space Weather
○Military, aviation and space agencies like the FAA and NASA pay close attention to space weather and may publish advisories. To keep up to date on your space weather, the National Oceanic and Atmospheric Administration (NOAA) provides an online service. [1]

【Reference】
1. Cosmic Radiation - Go Flight Medicine
http://goflightmedicine.com/cosmic-radiation/
2. 宇宙放射線 - JAXA 宇宙教育センター
https://edu.jaxa.jp/seeds/pdf/2_radiation.pdf
3. 聴いて観て話しメモする！：地球大気
http://watasumu.blog28.fc2.com/blog-entry-44.html
4. Equivalent dose - Wikipedia
5. New ICRP recommendations
http://iopscience.iop.org/article/10.1088/0952-4746/28/2/R02/meta;jsessionid=8B0E711B408C4AFA7AC92FD11FF30D11.c3
6. Southwest Research Institute, "SwRI Scientists Publish First Radiation Measurements from the Surface of Mars"
7. Acute radiation syndrome - Wikipedia
8. Ionizing radiation - Wikipedia