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The Forefront of Space Science

When a Computer is Surprised
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Computers easily perform calculations that would be hardly possible for humans. When robots with computerized brains appear in the movies, they are often described as having a cold and cruel character. We may have such an image because the calculation is made by digital processing in which everything is divided into one (1) or zero (0). Would you be amazed to learn that such a cold computer could be surprised?

Computer Chips

The main parts of a computer are so-called computer chips such as memory and processor. In appearance, the chip is a shiny, thin metal plate about 1cm square. Strictly speaking it is not metal, but it appears metallic and very beautiful. The photo is a rare opportunity to see the interior of the chip, which is usually sealed.

A computer chip is also called a "large-scale integrated circuit," a large number of tiny switches or "transistors" are integrated in the chip. The switch has two states, ON and OFF, which is why it goes well with digital. Computer chips process information by switching ON and OFF rapidly in accordance with the precisely drawn up design diagram.

A spacecraft carries a number of computer chips that process information from onboard sensors and commands from the ground. The chips should operate according to their design diagram but sometimes the chips do not function correctly. The first anomaly occurred on a certain satellite in 1975. Memory data deviated from the values specified in the design. What happened in that computer chip?

Cosmic Rays

The universe is a vacuum, with no gravity and perhaps no other human life. Thus, the word suggests a place where nothing exists. In fact, however, something does exist and, above all, cosmic rays stand out.

Cosmic rays are in fact high-energy particles flying in the universe. If you look at their shiny trajectories, you may imagine lines. "Imagine" is a good word as they are too tiny to see. This is because they are atomic nuclei, the smallest constituent of matter. Cosmic rays or atomic nuclei are accelerated by the huge energy generated by celestial activity.

Draw a 1cm square on a piece of paper. If you were to go out into the universe carrying it, 100,000 cosmic rays would pass through the square within one second. Computer chips are exposed to such cosmic-ray showers.

Computer Chips are Amazed

What happens when a chip is struck by cosmic rays? Fig. 1 shows such a moment, and traces electrical signals from a computer chip like an electrocardiogram. When cosmic rays strike the chip it emits a large signal output. This is the moment the chip is amazed.

Figure 1
Figure 1. Event where computer switch is shocked
Krypton ions accelerated to 322MeV strike a transistor switch fabricated by FD-SOI technology (courtesy of Dr. Takahiro MAKINO, Japan Atomic Energy Agency).

The above record was obtained by selecting a single transistor switch aligned on the chip. The switch was kept in the OFF state --- current does not flow in this state ---, set in a measurement system so that current flows when the switch is changed to ON. Naturally, the value shown on the monitor before the cosmic rays struck was zero, but when a cosmic ray strikes, large current flows. The experiment was performed by the Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency. They produced artificial cosmic rays and bombarded the chip. The data were provided by the experimental team at the Institute.

Look carefully at the horizontal axis. The duration when current went through the switch (i.e., the period that the switch was amazed) was very short, one nano-second or one-billionth of a second. It is too short to measure with a stopwatch, so the team had to use an ultra-high speed digital oscilloscope. "Very fast" is, however, based on human criteria. It is not so fast for chips that operate at high-speed. Just like humans, shocked computer chips sometimes cannot move. There are cases where chips lost their memory because of a powerful shock. In fact, the chip malfunction onboard the satellite in 1975 was caused by this phenomenon.

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