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measured when the Sun is active. Since Hayabusa hovers at the Home Posi-
tion (HP) several kilometers sunward (or earthward) the asteroid, apparent
size of Itokawa becomes just in the fields of view of the XRS. With long-term
observation, the XRS will map the surface in the longitudinal direction by
asteroid rotation. Composition at the rotation phase especially when the
enhanced solar activity occurs will be accurately determined for larger num-
bers of elements. That of local areas can be measured from the lower altitude
during the descent for sample collection. For its small solar phase angle less
than 10 , the HP is favored for spectrometry but not suitable for geomor-
phology or detailed shape modeling. For this reason, Hayabusa takes a few
weeks' trip to the zones at larger solar phase angle (XO). Dependency of the
XRF intensities on the solar phase angle can be compared, which informs
the surface microscopic roughness by particle size effect 9 .
3.2. Design concept of the XRS
The configuration [or schematic diagram] of the XRS is shown in Fig. 1. The
specifications of the XRS are summarized and tabulated in Table 1. Much
improvement is achieved for the precision of the XRS observation, relative
to the conventional proportional counters. As the X-ray detector we adopt
Hamamatu's Deep-1 type two-dimensional CCD 10 of 1-inch square because
energy resolution (better than 150 eV at 5.9 keV) is required high enough to
discriminate each XRF peak: moreover, a relatively large detection area of
25 cm 2 is available with four CCDs arrayed. A mesh-supported ultra-thin
beryllium window of 5 µ m thick has been developed to achieve complete
light shield and high transparency of soft X-rays. With its large detection
area, the XRS can obtain statistically significant counts in several minutes,
especially at the energy of 1-3 keV. A thin latticed collimator made of
phosphor blonde is developed to limit the fields of view of 3 . 5 ×
3 . 5 ,just-
in size of the asteroid when observed from the HP, to achieve high signal
to background ratio. In addition, a standard sample plate is mounted so
that elemental analysis becomes more accurate with conducting in situ
calibration of XRF with which to compare the observation data.
There remains severe constraints in mass and power supply. The suit-
able operation temperature of CCD is less than minus 50 C. Since the use
of coolant or mechanical cooler is not favored for this small mission, the
thermal design of the instrument is such that can keep the detector cool
enough with radiation cooling through a small radiator. The compact elec-
tronics is realized by using the onboard computer SH-OBC that has three
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