From: firstname.lastname@example.org (Jordin Kare)
Subject: Re: CCD Radiation Protection Strategies
Date: Sun, 14 May 2000 23:25:35 GMT
In article <email@example.com>,
firstname.lastname@example.org (CLVANCIL) wrote:
> Hi All,
> I know that several deep space missions used a "box" of tantalum and a mirror
> as the radiation shielding of the CCD camera is there any simpler method of
> protection? On mass critical missions (like a Mars plane) it seems lead
> (or any of the other dense metals) shield would be a liability. Why not
> use the batteries already onboard as the shield?
Fitting together a spacecraft is already enough of a 3-D jigsaw puzzle.
For shielding, you need to bury the CCD inside a shield with a certain
minimum thickness (otherwise the radiation through the "thin spot" will
dominate the dose rate). Batteries tend not to be convenient shapes to
arrange as shielding, and are not especially uniform in density-- they
need to have various terminals, gas expansion spaces, etc.. The problems
are even worse for modern nickel-hydrogen batteries, which are pressure
vessels and therefore don't pack together well at all. Remember that a
continuous "box" surrounding a 2-cm-wide CCD may be a lot less massive
than even adding shielding to a few spots on a 30-cm-wide battery pack.
Batteries also generate substantial heat, and have significant temperature
swings, which is definitely *not* what you want near your sensor.
Batteries need radiators to get rid of the heat, which further constrains
where you can put them. Modern batteries (Li, Ni-H) don't have any heavy
metals (and the choice of shielding materials is fairly critical, since
you want to minimize cascades from high energy cosmic rays). And finally,
deep space missions in particular often don't have all that many batteries
to begin with, since they spend most of their time in constant sunlight.
All that said, clever designers do tend to try to take advantage of
surrounding structure as extra radiation shielding, especially in LEO
where the bulk of the radiation dose is low-energy electrons.