Lab Detectors Measure Cosmic Rays Aboard NASA's ACE Spacecraft

October 31, 1997

By Paul Preuss, [email protected]

Berkeley Lab's Engineering Division has provided the heart of one of nine experiments aboard NASA's Advanced Composition Explorer (ACE), a satellite launched last August to study the "composition" of outer space: the solar corona, the interplanetary and local-interstellar media, and the makeup of the

Cosmic ray nuclear-isotope detectors fashioned by the Measurement Science Group of the Lab's Engineering Department from lithium-ion-compensated silicon, shown packaged for delivery.
galaxy itself. Cosmic rays offer one of the best ways to sample the Milky Way at high energies, and that's what CRIS -- the Cosmic Ray Isotope Spectrometer -- was built to do.

Designed by a consortium led by Caltech astrophysicists, CRIS consists of 60 lithium-ion- compensated silicon detectors, each three millimeters thick and 100 millimeters in diameter. The Si(Li) detectors measure the total energy of incoming cosmic ray heavy nuclei and the rate of energy loss as the nuclei come to a stop. The data is combined with input from a trajectory detector of fiber-optic planes, which sits on top of the stacks of silicon detectors, to determine the mass and atomic number of each cosmic ray nucleus. CRIS is designed to observe all the stable or long-lived nuclei up through Z=30 (zinc) in the periodic table.

"Starting with ISEE-3, the Third International Sun-Earth Explorer launched in 1978, we've built similar experiments for six of the last seven satellites of this kind," says Jack Walton of the Measurement Science Group in the division's Engineering Science Department.

Each new cosmic-ray "telescope" has presented the Measurement Science Group with greater challenges in the fabrication of instruments that have to be exquisitely crafted and at the same time rugged enough to withstand the G-forces, vibration, and temperature extremes of a launch into space. The silicon detectors of CRIS are the biggest and most sensitive yet for this kind of instrument.

"Only one company in the world, Topsil, provides the kind of crystals we need," Walton says. "It's located in the village of Frederikssund on the west coast of Denmark -- a place so small that when five of us went to visit the plant, the only taxi in town had to make two trips to carry us all from the railroad station."

Yu Wong, along with Julie Lee, headed the team that fabricated the detectors; he explains that the silicon used in CRIS is doped with boron, which provides negative acceptor sites in the crystal lattice. After the wafers are sliced from a single crystal of silicon by a diamond saw, they are coated with lithium on one side; with ultrasound cutters and chemical etching, grooves are cut around the top edge of the wafer and a shallow "well" is created on the bottom.

Lithium is an "interstitial donor" to silicon; by heating the wafers to 110 degrees Celsius and applying a positive 500-volt bias, the positive lithium ions are made to drift into the bulk of the crystal, positioning themselves among the silicon atoms instead of replacing them. They pair with the negative boron acceptor sites and turn the entire wafer into what Walton calls "a big silicon diode," with a net electrical impurity concentration of merely about one part in ten trillion.

The completed detectors are measured for flatness to an accuracy of better than one micrometer, calibrated using accelerated beams of argon and rigorously tested for stability under high-vacuum conditions. The 60 finished wafers in the CRIS experiment are the survivors of more than 160 candidates virtually hand-made at Berkeley Lab.

The ACE satellite is headed for a unique orbit around a gravitationally stable libration point between the Earth and the sun that will keep it operational for at least five years. As an example of the scope and sensitivity of the CRIS experiment, it is anticipated that CRIS will collect some 100,000 iron nuclei (along with dozens of other species) during its first two years in space, enough to precisely determine the abundance of even the rarest iron isotope, iron 58, which probably accounts for only three tenths of one percent of all the iron in ACE's outer-space environment.

Berkeley Lab's Engineering Science Department staff, under the leadership of Joe Jaklevic, have become specialists in supplying detectors capable of just that kind of precision and endurance.

Progress of the ACE satellite as it races towards its planned orbit can be followed on the ACE website.

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