WASHINGTON (States News Service) NASAs brand-new Chandra X-ray telescope has suffered what scientists think is radiation damage caused by the telescopes reflection of Earths Van Allen belt. Partial degradation of eight of ten one-inch-square silicon chips has reduced the telescopes ability to distinguish between different X-ray energies, or "colors," scientists on the Chandra project said.
In early September, scientists noted that there was an "energy resolution" loss in one of the telescopes detectors. It was transmitting less accurate information, essentially more out-of-focus, about incoming X-rays. They determined that damage to the chips had occurred each time the telescope traveled through the Earths radiation belts, which contain high concentrations of high-energy protons.
Although the satellite was made to handle these belts, where it spends about one-sixth of its 64-hour orbit, "we think that the protons are able to reflect off the telescope mirror better than we would have expected," said Harvey Tananbaum, director of the Chandra X-ray Center in Cambridge, Mass. Since the mirror focuses all energy at the detectors, more protons were impacting the chips harder than the designers expected, exposing them to greater radiation.
The detector was part of the Advanced Charge-Coupled Device Imaging Spectrometer (ACIS), a device that records X-rays from distant sources focused onto it by Chandra's telescope. The eight partially deteriorated silicon chips are all of one type, called front-illuminated. The other two chips are back-illuminated and had no damage. The chips read incoming X-rays and transfer them into electric signals, which scientists use to find out where the X-ray came from and its energy level.
For the last thirty days, satellite controllers have prevented the damage from getting worse by using a motorized mechanism to move the ACIS camera so it is shielded from the Earths radiation belt. They move the detectors out of the focus point and move the high-resolution camera there instead. Since the camera has its own protective door, everything is protected, and there is a back-up motor just in case, said Chandra NASA Project Scientist Martin Weisskopf. "[The motorized door] was designed to do this in every orbit," Weisskopf said. "It was designed for thousands of motions."
Scientists hope they can repair the damage to Chandra. They are reproducing the effect in lab simulations to better understand what is happening, and they have already tried cooling the chips, which Weisskopf said "actually improves energy resolution."
With the deteriorated chips, the instrument now receives less information about cosmic X-rays. "The fact that we get a signal is still there, its just that we dont get its energy as precisely," Tananbaum said.
Chandra is still the most powerful X-ray telescope ever built, and scientists remain confident that the damage will do little to impair the telescopes science.
"Really, the impact is not very major, and its something that we can live with and perhaps we can even mitigate," said Weisskopf.
"The procedures that we have put into place to prevent further deterioration have been successful," Weisskopf added. "[Only] seven percent of our observations are in fact impacted at all."
Chandra can use the back-illuminated chips to get the same scientific results that would have been available from the now-damaged front-illuminated chips. The only drawback is that this process takes longer, Weisskopf said, because the back-illuminated chips have to be aimed at four sections of the sky to cover the same area as would be seen all at once with the front-side chips.
advertisement
