Lab Detectors Measure Cosmic Rays Aboard NASA's ACE Spacecraft
October 31, 1997
By Paul Preuss, paul_preuss@lbl.gov
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.
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
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.
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.
