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A CCD That Can Stand up to Cosmic Rays
Jul 2002
BERKELEY, Calif., July 24 -- A single instrument aboard SNAP, the proposed SuperNova/Acceleration Probe, will make it unique among satellites: Its billion-pixel astronomical camera, the GigaCAM. Built from an array of revolutionary Berkeley Lab CCDs developed by Stephen Holland and his colleagues in the Lab's Engineering and Physics Divisions, the GigaCAM will be the largest and most sensitive astronomical CCD imager ever constructed.

Standard astronomical CCDs are fragile affairs, and their ability to obtain high-quality images degrades quickly in the hostile radiation environment of space -- one reason why astronauts have already replaced all of the Hubble Space Telescope's original imaging instruments.

"The Hubble and many other satellites were designed to be maintenance-friendly, but SNAP is going to be placed in an unreachable orbit," said Engineering's William Kolbe. He added that once the GigaCAM is carried aloft on its five-year-plus mission, it can't afford to fail.

At 300 microns (millionths of a meter) thick, the Berkeley Lab "high-resistivity, p-channel" CCDs are much more rugged than conventional astronomical CCDs measuring only a few tens of microns thick. In recent months Kolbe and Armin Karcher have been conducting tests at the 88-Inch Cyclotron to see just how well the new CCD can stand up to radiation damage.

Outer space in the lab

In space the culprits are cosmic rays, high-velocity particles packing a tremendous energetic punch that destroys pixels, increases "dark current" (a source of background noise) and, worst of all, degrades the efficiency of charge transfer from the pixels to the amplifiers at the edges of the chip. A few cosmic rays are massive atomic nuclei like iron, nickel or zinc, but the majority are protons and electrons.

The 88-Inch Cyclotron simulates the cosmic ray environment with both heavy ions and protons. to test spacecraft components ranging from memory chips to transistors to entire computer systems, and to calibrate detectors used in compiling space "weather" reports. CCD chips and solar cells are particularly prone to degradation from large numbers of protons generated during high solar activity.

The Light Ion Irradiation Station located in the 88-Inch Cyclotron's Cave 3, originally developed for radiation biology, is often used to test spacecraft components.

"At the Cyclotron, protons are delivered to target components at the Light Ion Irradiation Station located in Cave 3," says Peggy McMahan, research coordinator for the 88-Inch Cyclotron. "We owe this station to a group from the Life Sciences and Engineering Divisions, who worked with our operations staff to develop it in order to maintain a small radiation biology program here after the closure of the Bevalac in 1993."

In the station the dose is measured and the beam is "blown up" to four inches in diameter to uniformly irradiate silicon wafers (and the Petrie dishes used in life sciences experiments, too). The Materials Sciences, Chemical Sciences, and Advanced Light Source Divisions have also used the irradiation facility when the cyclotron is not busy with nuclear science experiments, its primary mission. Companies who have tested components for space applications on a fee basis include Eastman Kodak, Aerospace Corp., Lucent Technologies and Mitsubishi Electronics.

Test CCDs for the SNAP project are bombarded with beams of protons ranging in energy from 10 to 55 MeV (million electron volts); by testing several wafers, each at a different dose -- from a few billion to a hundred billion protons per square centimeter of surface -- dosage can be scaled to equal what CCDs would receive after several years in orbit

Performance check

Before irradiation, Kolbe and Karcher assess the test wafers in their laboratory for dark current, charge transfer efficiency, and "cosmetic" defects. Computer processing and other electronic tricks can compensate for cosmetic flaws like a few damaged ("hot") pixels, endemic to all CCDs, but dark current and charge transfer efficiency pose more serious challenges.

Dark current is electronic noise caused by thermal motion of the atoms that make up the chip; the colder the chip, the less the dark current. The Berkeley Lab CCD is much thicker than ordinary astronomical chips so there is more material in which dark current can be generated, but its high purity, negative "doping" and low operating temperature work to suppress dark current. In space, SNAP's GigaCAM will operate at about 140 degrees Kelvin (by comparison, nitrogen under normal atmospheric pressure liquefies at 77 degrees K).

Typically, the most serious radiation damage to CCDs is a steady reduction in charge transfer efficiency. Photons from distant objects like stars are focused on pixels in the CCD, and the brighter the object, the more photons are converted to charge. Negative electrons or positive "holes" are collected and transferred to the edge of the chip along specific channels, like buckets of water in a bucket brigade. The chip's electronics reconstruct the image of a star by associating the precise amount of charge and the precise location of the pixels that generated it.

"When you irradiate a CCD with protons, silicon nuclei are knocked out of their lattice position, and what was fairly perfect material develops defects," Kolbe says. "These form electron or hole 'traps' that can grab charge that's being transferred, hold onto it for a time then let it go later."

Armin Karcher notes that these inefficiencies in charge transfer "can affect the apparent brightness of objects in the sky and the interpretation of their spectra." The success of the SNAP satellite will greatly depend on its ability to measure supernova spectra with extreme accuracy.

To characterize the test CCDs, Kolbe and Karcher create star fields by exposing them to a small x-ray source. "Each x-ray photon deposits an artificial 'star' every 50 to 70 pixels, generating a cloud of charge," says Karcher. "We know how many electrons it takes to represent each of these, so when we read out the data we know what the reading should be."

And the winner is . . .

After irradiation at the 88-Inch Cyclotron, Kolbe and Karcher take the test wafers back into the laboratory to measure radiation effects from different doses. In the three batches tested so far they found that, while radiation dosage increased dark current, the effect was important only at high temperatures -- and the impact of radiation on charge transfer efficiency was remarkably small. Available studies indicate that the charge transfer efficiency of conventional CCDs falls off rapidly with increased radiation, while the Berkeley Lab CCDs are little affected even at very high doses.

The charge transfer efficiency of conventional CCDs rapidly decreases as radiation dose increases, but Berkeley Lab CCDs suffer little reduction in efficiency.

These results were not unexpected; after all, the Berkeley Lab CCD descends from a long line of detectors designed to withstand radiation from colliding beams of particles in giant research accelerators -- "much more hostile than outer space," Kolbe says.

"The high-resistivity p-channel CCDs exhibit extremely low dark current at the operating temperature," the researchers concluded in their latest report. "Radiation damage proved to be much less detrimental than in conventional CCDs. . . . Their potential lifetime in space is measured in decades, not years."

Kolbe and Karcher have devised new instruments to test larger CCD wafers, to measure the efficiency of their response at all wavelengths, and to investigate what effect pixel size, different levels of doping, and other manufacturing variables may have on their performance after radiation exposure.

"We're expanding our capabilities constantly," Karcher says. "We plan to keep going till SNAP flies."

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