MadSci Network: Physics
Query:

Re: What secondary radiation does polyethelene produce from cosmic rays?

Date: Fri Jul 13 12:26:04 2001
Posted By: Scott Kniffin, Nuclear Engineer, Orbital Sciences Corporation
Area of science: Physics
ID: 994176877.Ph
Message:

Kathleen, 

The answer to this lies in what you are trying to shield.  In the space 
environment, radiation is omnipresent and falls into several distinct 
categories.  There are low energy trapped electrons and protons (Van Allen 
Belts), Solar protons and other heavier ions (Solar Wind), and Galactic 
Cosmic Rays (GCRs).  As you have no doubt been told, the best way to shield 
something is to put a material with a high Z number facing the source to 
block the high energy component and put a low Z material behind it to 
absorb the lower energy secondary particles.  In the case of the ISS or the 
Shuttle, you are trying to shield a biological system (i.e. humans), so you 
put polyethylene inside the primary shield to absorb the secondaries 
created by interactions with the skin of the spacecraft.  This reduces the 
amount of radiation reaching the sensitive region (the person) to a 
reasonable minimum.  With the shielding on a manned mission, the exposure 
inside the cabin is minimized the best as is possible for crew safety.  
Most of what gets in are reactions from high energy solar particles and 
GCRs.  Keep in mind that cost is a factor here.  It costs several thousand 
dollars per pound to put something into orbit.  Since people are the most 
important cargo, the expense is justified in putting more shielding in to 
protect them.  For a satellite, you can't usually justify the expense, thus 
the shielding concerns are a little different.  

In a satellite, there is the metal skin of the space craft which blocks out 
(hopefully) all of the trapped electrons and protons, and a reasonable 
fraction of the solar particles.  In the event of a massive solar flare, 
all bets are off, however.  The end result is that there is a low energy 
spectrum of secondary particles present inside the spacecraft all the time. 
 There is NO shield for GCRs, they are simply too energetic and are 
omnidirectional so there is no practical way to shield them.  (You'd need 
feet of shielding as opposed to the 50 to 200 mils of shielding you 
typically get on a spacecraft.)  For electronics, the key things are size 
and weight. Polyethylene shielding would take up lots of room and weigh 
more than the standard ceramic package that most space flight parts are 
packaged in.  Also, the spectrum of energies will be higher inside the 
spacecraft than in a manned vehicle.  Electronics are quite sensitive to 
energetic particles dumping their energy into the sensitive region of the 
part die, so the idea is to block the most low energy stuff you can to 
prevent this from happening.  The ceramic tends to be a mid Z material so 
it is well suited to this specialized task.  

Figuring out how radiation will screw up a part is what I do for NASA.  I 
work in the Radiation Effects and Analysis Group of the Office for System 
Safety and Mission Assurance, NASA Goddard Space Flight Center.  If you are 
really curious about just how involved this is, pick up the IEEE 
Transactions on Nuclear Science for any year and look at #6 in that volume. 
 These are the proceeding of the Nuclear and Space Radiation Effects 
Conference (NSREC), held each year.  As it happens, it's next week in 
Vancouver, BC.  We have 6 posters and 2 papers this year, business is good. 
 

P.S. Don't let radiation intimidate you, it really isn't as scary as 
you've been led to believe.  

Scott Kniffin
Senior Engineer, Orbital Sciences Corporation
NASA Goddard Space Flight Center
Office of Systems Safety and Mission Assurance
Radiation Effects and Analysis Group, Code 562




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