ACE News #1: 9/24/97

First Data from the Cosmic Ray Isotope Spectrometer (CRIS)



CRIS was the second instrument aboard ACE to be turned on, on August 27, 1997. Its purpose is to study the isotopic composition of galactic cosmic rays with excellent mass resolution and unprecedented collecting power, over an element interval of 2 <= Z <= 30 (He to Zn) with energies from ~100 to ~500 MeV/nuc. All 60 of the silicon solid state detectors and their associated electronics in the four ``telescopes'' of CRIS are operating well. The Scintillating Optical Fiber Trajectory (SOFT) hodoscope, which is used to measure particle trajectories, is achieving excellent position resolution of ~100 µm. We are presently using flight data to examine SOFT and silicon detector threshold settings in order to optimize trajectory identification over the entire charge range and to properly sort events into prioritized event buffers on the basis of their charge and energy. Shown in the figure is a nuclear charge histogram from CRIS, including events of boron through magnesium collected between 8/27 and 9/11/97. The C and O peaks extend off the top of the figure, to levels of 3062 and 3560 counts per bin, respectively. More than 20,250 O events are included in this figure, which were required to satisfy various consistency cuts and stop in the instrument between the third and the eighth detectors. This corresponds to an energy interval of about 100 to 250 MeV/nuc for O. The selection criteria used include less than half of the available data. Much of the remaining data will be useful once analysis procedures have been refined. Because the energy signals deposited in the solid state detectors depend on the atomic mass as well as the nuclear charge of the particle, element peaks in the ``charge'' histogram show subpeaks due to multiple isotopes. Two isotopes of B, C, and N and three of O, Ne, and Mg are clearly seen. Once trajectory calculating algorithms and energy calibrations are further refined, the resolution should improve further, allowing the smaller, lower abundance peaks to be more cleanly separated from their higher abundance neighbors. These data will then provide precise measurements of the relative abundances of the different isotopes in galactic cosmic rays. The statistical accuracy of the data already approaches that of the best measurements from previous instruments. The CRIS instrument involves investigators from Caltech, the Jet Propulsion Laboratory, Washington University, and the Goddard Space Flight Center.

.....contributed by Dr. Richard Leske, Caltech.


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