Editing Soft error (section)

=== Cosmic rays creating energetic neutrons and protons ===
Once the electronics industry had determined how to control package contaminants, it became clear that other causes were also at work. [[James F. Ziegler]] led a program of work at [[IBM]] which culminated in the publication of a number of papers (Ziegler and Lanford, 1979) demonstrating that [[cosmic ray]]s also could cause soft errors. Indeed, in modern devices, cosmic rays may be the predominant cause. Although the primary particle of the cosmic ray does not generally reach the Earth's surface, it creates a [[Air shower (physics)|shower]] of energetic secondary particles. At the Earth's surface approximately 95% of the particles capable of causing soft errors are energetic neutrons with the remainder composed of protons and pions.<ref name="Ziegler1996">
{{cite journal |last1=Ziegler |first1=J. F. |title=Terrestrial cosmic rays |journal = [[IBM Journal of Research and Development]] |volume=40 |issue=1 |pages=19–39 |date=January 1996 |doi=10.1147/rd.401.0019 | issn = 0018-8646 }}</ref>
IBM estimated in 1996 that one error per month per 256&nbsp;[[MiB]] of RAM was expected for a desktop computer.<ref name="cosmicRayAlert" />
This flux of energetic neutrons is typically referred to as "cosmic rays" in the soft error literature. Neutrons are uncharged and cannot disturb a circuit on their own, but undergo [[neutron capture]] by the nucleus of an atom in a chip. This process may result in the production of charged secondaries, such as alpha particles and oxygen nuclei, which can then cause soft errors.

Cosmic ray flux depends on altitude. For the common reference location of 40.7°&nbsp;N, 74°&nbsp;W at sea level ([[New York City]], NY, USA), the flux is approximately 14 neutrons/cm<sup>2</sup>/hour. Burying a system in a cave reduces the rate of cosmic-ray-induced soft errors to a negligible level. In the lower levels of the atmosphere, the flux increases by a factor of about 2.2 for every 1000&nbsp;m (1.3 for every 1000&nbsp;ft) increase in altitude above sea level. Computers operated on top of mountains experience an order of magnitude higher rate of soft errors compared to sea level. The rate of upsets in [[aircraft]] may be more than 300 times the sea level upset rate. This is in contrast to package decay-induced soft errors, which do not change with location.<ref name="GordonGoldhagen2004">{{cite journal |last1=Gordon |first1=M. S. |last2=Goldhagen |first2=P. |last3=Rodbell |first3=K. P. |last4=Zabel |first4=T. H. |last5=Tang |first5=H. H. K. |last6=Clem |first6=J. M. |last7=Bailey |first7=P. |title=Measurement of the flux and energy spectrum of cosmic-ray induced neutrons on the ground |journal=IEEE Transactions on Nuclear Science |volume=51 |issue=6 |date=2004 |pages=3427–3434 |issn=0018-9499 |doi=10.1109/TNS.2004.839134 |bibcode=2004ITNS...51.3427G|s2cid=9573484 }}</ref>
As [[Moore's law|chip density increases]], [[Intel]] expects the errors caused by cosmic rays to increase and become a limiting factor in design.<ref name="cosmicRayAlert">{{cite magazine |last=Simonite |first=Tom |date=March 2008 |title=Should every computer chip have a cosmic ray detector? |url=https://www.newscientist.com/blog/technology/2008/03/do-we-need-cosmic-ray-alerts-for.html |magazine=[[New Scientist]] |archive-url=https://web.archive.org/web/20111202020146/https://www.newscientist.com/blog/technology/2008/03/do-we-need-cosmic-ray-alerts-for.html |archive-date=2 December 2011 |access-date=26 November 2019}}</ref>

The average rate of cosmic-ray soft errors is ''inversely'' proportional to sunspot activity. That is, the average number of cosmic-ray soft errors decreases during the active portion of the [[sunspot cycle]] and increases during the quiet portion. This counter-intuitive result occurs for two reasons. The Sun does not generally produce cosmic ray particles with energy above 1&nbsp;GeV that are capable of penetrating to the Earth's upper atmosphere and creating particle showers, so the changes in the solar flux do not directly influence the number of errors. Further, the increase in the solar flux during an active sun period does have the effect of reshaping the Earth's magnetic field providing some additional shielding against higher energy cosmic rays, resulting in a decrease in the number of particles creating showers. The effect is fairly small in any case resulting in a ±7% modulation of the energetic neutron flux in New York City. Other locations are similarly affected.{{citation needed|date=December 2015}}

One experiment measured the soft error rate at the sea level to be 5,950&nbsp;[[failures in time]] (FIT = failures per billion hours) per DRAM chip.  When the same test setup was moved to an underground vault, shielded by over {{Convert|50|feet|m}} of rock that effectively eliminated all cosmic rays, zero soft errors were recorded.<ref>{{cite web|author-last=Dell|author-first=Timothy J.|date=1997|title=A White Paper on the Benefits of Chipkill-Correct ECC for PC Server Main Memory|url=https://asset-pdf.scinapse.io/prod/48011110/48011110.pdf|access-date=2021-11-03|website=ece.umd.edu|page=13}}</ref>  In this test, all other causes of soft errors are too small to be measured, compared to the error rate caused by cosmic rays.
 
Energetic neutrons produced by cosmic rays may lose most of their kinetic energy and reach thermal equilibrium with their surroundings as they are scattered by materials. The resulting neutrons are simply referred to as [[thermal neutrons]] and have an average kinetic energy of about 25 millielectron-volts at 25&nbsp;°C. Thermal neutrons are also produced by environmental radiation sources, including the decay of naturally occurring radioactive elements such as [[uranium]] and [[thorium]]. The thermal neutron flux from sources other than cosmic-ray showers may still be noticeable in an underground location and an important contributor to soft errors for some circuits.