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Computer memories recall radiation dose.

Several years ago, it was noticed that certain high-density computer-memory chips were generating inexplicable errors. Research eventually showed the cause to be alpha particles, ionizing radiation emitted by trace levels of uranium and thorium in the chips' packaging material. Now a Navy physicist reports on efforts to harness the alpha-sensitivity of these chips in designing digital monitors to measure human exposures to neutrons--an especially potent form of radiation.

As an uncharged form of radiation, neutrons interact weakly with most materials, including those meant to detect them. Yet accurate monitoring of human exposure to them is important since they can exert up to 10 or more times the biological damage of X-rays or gamma rays.

John L. Davis, now at the Naval Research Laboratory in Washington, D.C., worked on the new computer-memory-based neutron dosimeter while at the Naval Surface Weapons Center in Silver Spring, Md. In the August HEALTH PHYSICS he describes how it works.

Because fast neutrons, the type emitted by nuclear reactions, are particularly difficult to trap and measure, dosimeters first use a "moderator"--something containing hydrogen--to interact with them and "slow" them down. For most dosimeters, the hydrogen source is the body of the human wearing the dosimeter. Interactions cause neutrons to shed some energy; when they have slowed enough to become thermal, or low-energy, neutrons they can be captured by a converter element such as boron or lithium. Capture by the converter promptly causes the neutron to emit an alpha particule--which is where the computer chip comes into play.

In a dynamic random-access memory (D-RAM) computer chip, "an individual memory cell consists of, among other things, small capacitors which are either charged or uncharged," Davis exmplains. This is how the memory stores information--as zeros and ones, represented in the chip by the presence or absence of charge.

But an alpha particle passing through silicon can discharge a capacitor. Davis says the trick is to see that each memory cell starts out filled with just enough charge so that an alpha-gnerated discharge even will essentially empty it. Then, when the chip's circuitry monitors its capacitors, any cell reading empty represents an alpha interaction. How many occur will correspond to the neutron dose received by the wearer.

In the Navy experiments, some chips proved to be more immune to alpha discharge, suggesting that oxide coatings made these chips more radiation resistant. Any commercial dosimeter would therefore require chips specially designed without these coatings.

Under Navy contract, Radiation Monitoring Devices of Watertown, Mass., is investigating the commercial potential for these devices. Frank Sinclair, a physicist with the firm, says that unlike existing neutron dosimeters, this one would allow instant and digital readout of dose.
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Author:Raloff, Janet
Publication:Science News
Date:Aug 31, 1985
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