Naked Science Forum

Non Life Sciences => Technology => Topic started by: thedoc on 27/11/2012 15:54:19

Title: How do you design for neutron rain?
Post by: thedoc on 27/11/2012 15:54:19

A local election result was altered by neutron rain interacting with an e-voting computer.  How can we design against damage done by neutron rain, to electronic systems like this, and others?
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Title: Re: How do you design for neutron rain?
Post by: Boogie on 30/11/2012 17:05:40

If this is true, then why do hand held radiation detectors last so long? I have a GM detector that is very old that still works just fine. I've used several hand held radiation detectors made in the 80's with solid state electronics still functioning normally, even after being through intentional exposure to neutron, gamma, alpha and beta radiation for years.

It must be mostly SMT modern solid state electronics with very high frequency clocks that have the potential to fail? What good is miniaturization of components if we must use redundancy so electronics have a reasonable lifetime?

Title: Re: How do you design for neutron rain?
Post by: SeanB on 02/12/2012 13:46:22

Mostly size. When you have features on a chip measured in single figure atoms wide a single event can have a big effect. On older electronics a feature will be millions  to trillions of atoms wide, a few changes here and there will have no effect.

Modern electronics is getting to the point where single electrons are getting used to store information, and a charge difference of only a few can be detected. There are commercial meters that can literally count the electrons flowing in a wire, and these can be affected by radiation.

Title: Re: How do you design for neutron rain?
Post by: evan_au on 08/12/2012 18:06:14

The neutrons transmute silicon atoms into Phosphorus, which changes the doping of the semiconductor, and mechanically distort the crystal structure, creating points which allow electrons and holes to recombine prematurely.

This affects properties such as the voltage at which the circuit switches state, and the conductivity of the semiconductor material, which also affects how much current it can drive into adjacent circuits, and how quickly the adjacent circuit can reach its switching voltage. It has a larger impact on older "bipolar" transistor technology (which rely on low concentrations of holes or electrons in the narrow base region) than on "CMOS" transistors, so using CMOS technology improves reliability.

• Designing circuits so they have a high voltage swing, and running them at much less than the maximum clock rate helps. Both of these mean that space-rated computers are much less powerful than ones we use on Earth.
• Building chips with radiation shields and using non-conductive mounting materials like sapphire prevents chips from short-circuiting when exposed to cosmic rays, but increases the cost.
• Using error-detection circuitry allows correction of memory errors, and "watchdog timers" can restart a computer whose software has locked-up.
• Using several computers to cooperate on a single task and vote on the answer improves reliability, but increases hardware cost and software complexity (and how do you recover if radiation damages the voting circuitry?).
  

There are many types of radiation, which have a variety of effects on semiconductors, and can use a variety of protection methods http://en.wikipedia.org/wiki/Radiation_hardening#Resultant_effects