But this is a serious scientific
pursuit. More than that, its a business, putting IUs powerful cyclotron
at the disposal of the worlds space and aeronautics industries. The Radiation
Effects Research Program (RERP) makes it possible for the likes of NASA
and Boeing to answer a pressing question: Can todays microelectronic circuitry
withstand the onslaught of radiation, especially in outer space?
Theres no escaping radiation.
Charged ions rain down on us from solar flares; electrons jostle us constantly
in the form of electromagnetic radiation. Most of the ions are absorbed by our
atmosphere before they reach the earth. But outside of Earths atmosphere,
the incidence of charged particles is vastly increased. Just one wayward ion
with an attitude can unseat the most ambitious space mission.
|Members of the Radation Effects Research Programfrom left, Ken Murray, Barbara von Przewoski, and Charles Fosterhelp scientists test a wide variety of devices for use in space, from circuit boards to supercomputers. Photo Tyagan Miller.|
Space is a nasty environment,
says Charles Foster, IUCF staff physicist
and director of RERP. Its a vacuum, for one thing. There are particles,
cosmic rays, heavy ions and fragments of them, which can cause problems, depending
on the conditions. The components on semiconductor devices are getting smaller
and smaller, andthat makes them individually
more sensitive and more susceptible to damage.
High-velocity ions can permeate
a semiconductor device, producing freed electrons and holes (when
an electron leaves an atom, the resulting ion has a positive electric charge,
referred to as a hole). These electrons and holes then carry currents
within the device. Because semiconductor devices work by carefully regulating
the currents flowing through them, any unexpected alterations can wreak havoc.
This is especially true of sensitive areas such as memory cells, where information
may be altered or erased, or false commands can be generated. In control systems,
the alterations can be catastrophic, producing failures in propulsion and navigation.
Circuits anywhere can suffer burnout, and systems can be switched irretrievably
into the wrong mode (a situation known as latchup). More generally,
exposure to large quantities of ions can degrade a semicon-ductors material,
shortening its life considerably.
This all makes Will this
thing work in space? an urgent question. Its bad enough when your
hard drive crashes at homea systems failure in outer space is another
thing altogether. The question is complicated by radical changes in the economics
of space exploration over the past couple of decades.
A quarter-century ago,
Foster explains, the military and space sectors drove the development
of electronic technology, so they could dictate their needs.
Now, the commercial market for
microelectronics is so huge that military and space consumers account for a
mere 1 percent. Consequently, many of the devices taken into space these days
are similar, if not identical, to the kinds of gadgets you buy for your own
home or office.
Foster cites an example. Everyone
knows about the problems NASA had with the optics on the Hubble spacecraft.
But there was also a problem with one of the Hubbles computers. This computer
latched up whenever the spacecraft passed through a band of high-velocity electrons
and ions called the South Atlantic Anomaly. The thrusters would fire and make
the flight path unstable. So the computer had to be turned off whenever the
Hubble passed through the anomaly. In the end, someone had to go up to replace
This, Foster concludes, is the
quandary facing the space and military sector: How do they know, when
they buy off-the-shelf systems, that these things will survive the mission?
Well, they bring them here and test them.
The radiation effects program
began in 1992, after a number of discussions and experiments over the preceding
years. Foster says he had argued for some time that at some point, funding
for basic nuclear physics was going to wind down, and we needed to find other
uses for the facility. In the summer of 1992, Foster and Alan Skees, a
student, built a new beam line from the main branch of the facility, complete
with an end station.
It was all done on a shoestring,
says Foster. The National Science Foundation doesnt directly fund
applied research, but the director of the cyclotron, John
Cameron, allowed us to use equipment that was already around.
Around the same time, Ken Murray,
a scientific consultant in California, contacted Foster. I had been working
at UC Davis, Murray says, and Id heard the IU machine was
more powerful and better suited to radiation-effects experiments.
Early attempts at setting up a testing program had not been successful, but after Murray met with Foster and saw the programs potential, he decided to relocate to Bloomington. Murray brought with him the original version of Beam Monster, a software program he designed to regulate and measure the variables in a radiation test. After extensive modifications, Beam Monster became the control system for the new project. The high demand for use of the IU facility led to the building of a second beam line end station in March 1997, enabling two tests to be carried out simultaneously.
|RERP team member Ken Murray (in red shirt) is hidden by a radiation beam line as he adjusts a laptop computer for testing. Photo Charles Foster.|
The RERP end station is a cross
between a physics lab and a shooting gallery. The device to be tested is placed
in the path of a beam and subjected to a series of proton bursts. Murrays
Beam Monster software allows the user to control the beam and to measure the
outcome. A printout indicates whether or not the device being tested is rad
hardthat is, whether it is likely to survive the radiation levels
expected. But why use protons, which carry a lower charge, if its the
heavy ions that cause damage in space?
Generally, protons cause
no direct upsets in microelectronic devices, Foster acknowledges. But
when a proton interacts with a silicon nucleus, it produces a shower of fragments
that includes heavy ions, and these can do damage. Testing with protons is far
cheaper and more convenient. What we needed to know was whether the damage rate
for heavy ions could be estimated from the proton test results.
The answer was yes, as two NASA
scientists from the Johnson Space Center
determined. Patrick M. ONeill and William X. Culpepper, both regular users
of the IUCF beam, developed a proton-based model for testing that gives an approximate
idea of heavy ion failure rates and modes. NASA now uses this model extensively
and is one of the heaviest users of the RERP. The programs ever-growing
client list also includes Boeing, Lockheed Martin, McDonnell Douglas, Vanderbilt
and Princeton universities, and the Naval Research Laboratory.
A tremendous variety of electronic
guinea pigs have undergone the proton test, and so far, there have been no unpleasant
surprises. Laptops and PCs are frequent subjects. They stand up fairly well
to radiation, although their cache memories have a tendency to go on the fritz.
A leading brand of printer, intended for use on the space shuttle, fared less
well. Once we hit that with the beam, Foster remembers, it
spat paper all over the place. Other devices tested include communications
systems, an emergency rocket pack for spacewalks, and a global positioning system
designed for use on aircraft.
|The Cassini spacecraft and its attached probe were launched on a seven-year journey to Saturn on October 15, 1997. Scientists from NASA used Indiana University's Radiation Effects Research Program facility to do last-minute testing of devices intended for use on the flight. Photo courtesy NASA.|
Sometimes the tests have a very
quick turnaround. Such was the case with the Cassini
mission to Saturn, one of the most expensive and controversial ever mounted,
with a price tag of $3.4 billion and a propulsion system involving seventy-two
pounds of plutoniumthe most ever sent into space. The Cassini spacecraft
was due to launch on October 15, 1997. But on September 5, the IUCF got a call
from the NASA Jet Propulsion Laboratory requesting emergency access to the beam.
The Cassini team was concerned
about the viability of European optocouplers (connections that translate an
electrical signal into a light signal and vice versa) and needed to test them
with proton irradiation to qualify them for use on the mission. An engineer
from NASA JPL arrived on September 6 and began the test that night because Lockheed
Martin had already booked the facility during the day. By the early morning
of September 8, the tests were completed. The optocouplers were declared rad
hard and were installed in the Cassini spacecraft, which launched on schedule
and is due to arrive in orbit around Saturn in 2004.
The nature of the tests isnt
always so obvious. What these devices are and what their uses will be,
I dont always know, says Foster. If they want to tell me,
fine. If not, thats fine too. They might be guarding commercial or military
secrets. Once we had two competing companies who both wanted to test their own
versions of the same device, and we had to get one group out before the other
one came in.
The commercial bent of this work
doesnt prevent students from becoming involved. Undergraduates work ten
weeks in the summer on a project, which they conclude with a written report
and an oral presentation.
Some of them have made major
contributions to the program, Foster says. If nothing else, students
get a clearer idea of what basic or applied research is all about, and they
can make a more informed decision about the path they want to take.
The path of the program in the future seems assured. As Foster nears retirement, he is preparing IUCF staff physicist Barbara von Przewoski to take over the reins. And the need for radiation testing will only increase. In the 1960s, future Intel chairman Gordon Moore predicted that the number of transistors on a chip would double every year. He was proved right. Today, a microchip the size of a fingernail can accommodate more than nine million transistorsevery one a sitting target for pesky charged particles. As long as space missions continue and technology advances, business is not likely to slow down at IUs RERP facility.
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