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Radiation Shielding
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Space radiation is different from the kinds of radiation we experience here on Earth, such as x rays or gamma rays.
Space radiation is comprised of atoms in which electrons have been stripped away as the atom accelerated in interstellar space to
speeds approaching the speed of light. Eventually, only the nucleus of the atom remains. Space radiation also has very different effects on human DNA, cells and tissues. This is due largely to the increased ionization that takes place near the track a particle of space radiation takes through a material. Ionizing radiation has so much energy it can literally knock the electrons out of any atom it strikes ionizing the atom.
This effect can damage the atoms in human cells, leading to future health problems such as cataracts, cancer and damage to the central nervous system.
Space radiation is made up of three kinds of radiation, all representing ionizing radiation.
-- particles trapped in the Earth’s magnetic field
-- particles shot into space during solar flares (solar particle events). When a solar flare or a coronal mass ejection occurs (the two often occur at the same time, but not always), large amounts of high-energy protons are released, often in the direction of the Earth. These high-energy protons can easily reach Earth in less than 30 minutes. Because such events are difficult to predict, there is often little time to prepare for their arrival.
-- galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system. Galactic cosmic rays include heavy, high-energy ions of elements that have had all their electrons stripped away as they journeyed through the galaxy at nearly the speed of light. Cosmic rays, which can cause the ionization of atoms as they pass through matter, can pass practically unimpeded through a typical spacecraft or the skin of an astronaut. Galactic cosmic rays are the dominant source of radiation that must be dealt with aboard the International Space Station, as well as on future space missions within our solar system. Because these particles are affected by the Sun’s magnetic field, their average intensity is highest during the period of minimum sunspots when the Sun’s magnetic field is weakest and less able to deflect them. Also, because cosmic rays are difficult to shield against and occur on each space mission, they are often more hazardous than occasional solar particle events. They are, however, easier to predict than solar particle events.
Space radiation also can produce more particles, including neutrons, when it strikes a spacecraft or an astronaut inside a spacecraft - this is called a secondary effect.
The absorbed dose of radiation is the amount of energy deposited by radiation per unit mass of material. It is measured in units of rad (radiation absorbed dose) or in the international unit of Grays (1 Gray = 1 Gy = 1 Joule of energy per kilogram of material = 100 rad). The mGy (milliGray = 1/1000 Gray) is the unit usually used to measure how much radiation the body absorbs. However, because different types of radiation deposit energy in unique ways, an equivalent biological dose is used to estimate the effects of different types of radiation. Equivalent dose is measured in milliSieverts (mSv). The mSv, therefore, takes into account not only how much radiation a person receives, but how much damage that particular type of radiation can do - the greater the possibility of damage for the same dose of radiation, the higher the mSv value. Crews aboard the space station receive an average of 80 mSv for a six month stay at solar maximum (the time period with the maximum number of sunspots and a maximum solar magnetic field to deflect the particles) and an average of 160 mSv for a six-month stay at solar minimum (the period with the minimum number of sunspots and a minimum solar magnetic field). Although the type of radiation is different, one mSv of space radiation is approximately equivalent to receiving three chest x rays. On Earth, we receive an average of two mSv every year from background radiation alone.
Crew members could receive higher doses of space radiation during space walks.
Aboard the space station, improving the amounts and types of shielding in the most frequently occupied locations, such as the sleeping quarters and the galley, has reduced the crew’s exposure to space radiation.
Materials that have high hydrogen contents, such as polyethylene, can reduce primary and secondary radiation to a greater extent than metals, such as aluminum.
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The search for these materials is underway by the Radiation Shielding Program -- part of a strategy of the NASA Office of Biological and Physical Research to limit space crews' radiation exposure.
The Radiation Shielding Program is examining new shielding materials that not only block and/or fragment more radiation than aluminum -- the material currently used to build most spacecraft structures -- but also are lighter than aluminum. Spacecraft designers have to be able to shape shielding materials to make various parts of the spacecraft. The material must protect the crew from radiation, and it must also deflect dangerous micrometeoroids. The shielding must be durable and long lasting -- able to stand up to the harsh space environment.
Polyethylene is a good shielding material because it has high hydrogen content, and hydrogen atoms are good at absorbing and dispersing radiation.
One development that the team is testing is reinforced polyethylene. "Since it is a ballistic shield, it also deflects micrometeorites," Kaul says. "Since it's a fabric, it can be draped around molds and shaped into specific spacecraft components." Kaul makes bricks of the material by cutting the fabric and layering 200 to 300 pieces in a brick-shaped mold in his laboratory at the Marshall center. He then uses a vacuum pump to remove air and prevent bubbles in the material, which would reduce its strength. The material is "cooked" in a special oven called an autoclave, which heats the material slowly to 200 degrees Fahrenheit while putting it under pressure of 100 pounds per square inch using nitrogen gas. The combination of heat and pressure causes the chemical reaction that bonds the layers together to form a brick weighing about half as much as a similar piece of aluminum. "Fiber is the secret of the material's strength, " explains Kaul.
If too much shielding material is used, the spacecraft becomes way too heavy to get off the ground.
http://www.spaceflight.nasa.gov/spacenews/factsheets/pdfs/radiation.pdf
http://www.nasa.gov/vision/space/travelinginspace/radiation_shielding.html
http://spaceresearch.nasa.gov/research_projects/radiation.html
http://www.spaceflight.nasa.gov/station/science/bioastronautics/
RE Gold
"Gold protects astronauts on the moon and in outer space. As a high energy radiation shield, gold goes into space suits and on capsules. A mere 0.000006 of an inch of gold coats rocket engines, electronic systems, helmets, visors, tether lines; yet because gold is such a marvelous reflector, that is enough to deflect the burning heat of the sun."
http://www.gold.org/discover/knowledge/aboutgold/industrial_uses/index.html
".. the lunar modules of the Apollo programme that put men on the moon were shrouded with gold foil acting as a radiation shield."
madsci
".. Gold would be slightly less effective than Lead as a gamma/x-ray shield."
ask astro
tpub
Demron
"The new standard in personal radiation protection... currently produced as full body suits, gloves and boots. Demron protects against particle ionizing/nuclear radiation and against X-ray and low-energy Gamma emissions. Non-toxic and Lead-free.
Demron suits are constructed from a unique nanotechnology that surpasses the effectiveness of current nuclear-biological-chemical suits that only protect against radioactive particulate sources."
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