Biomedical Engineering Reference
In-Depth Information
10
6
Mostly
protons
1/2 g/cm
2
shielding (Al)
10
5
Dose Rate
(rads/year [Si])
Mostly
electrons
10
4
3 g/cm
2
shielding (Al)
10
3
10
2
10
2
10
3
10
4
10
5
10
6
Altitude (km)
FIGURE 3-3 Radiation environment for circular equatorial orbits. SOURCE: Helvajian, H.,
and S.W. Janson. 1999. Microengineering space systems. Pp. 29-72 in Microengineering
Aerospace Systems, H. Helvajian, ed. Reston, Va.: American Institute of Aeronautics and
Astronautics, Inc. Figure reprinted with permission of the Aerospace Corporation.
centimeter silicon) shielding give lifetimes of roughly 11 days and 3 years, re-
spectively. The significance of Figures 3-2 and 3-3 is that while CMOS circuitry
can be used for low-altitude LEO missions, more radiation-resistant technologies
may be necessary for other orbits.
Specialized radiation-hard devices are available, but they are expensive and
are about two to three technology generations behind their commercial counter-
parts.
44
Fortunately, commercial CMOS foundry processes, in general, have in-
creasing total dose hardness as device feature sizes decrease. The increased hard-
ness apparently results from decreased gate oxide thickness.
45
One commercially
available 0.25-micrometer process has an apparent total dose limit of greater than
100 kilorads without design changes and greater than 500 kilorads with the
addition of guard bands, etc.
46
Latchup and SEUs must still be dealt with, how-
ever.
Finally, another back-to-the-future approach may prove useful for radiation-
hard electronics. Fifty years ago, magnetic core memory composed of small
(about 1/30 inch in diameter) ferrite toroids was used to provide non-volatile
memory.
47
Information was stored as a magnetization state of the ferrite core,
resulting in an inherently radiation-hard system that could also withstand high
current transients. Variants of this technology were used to create simple logic
circuits, which were integrated into B-52 bombers, Minuteman launch control
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