Geoscience Reference
In-Depth Information
4.2.
Grade analysis
When X-ray photons or high-energy charged particles enter into the CCD,
the derived electrons do not always stay in the single pixel (as single event)
but often split into neighboring pixels (as split event). The value of each
pixel is not proportional to the irradiated energy so that the value should
be summed over a split event. For each extracted X-ray event, data of
5 pixels stored in the FIFO memory are dealt with by the software analysis
according to whether the event is single or split. Grade-0 is attributed to the
single event, and grades 1-3 is to left-side split, right-side split, and both-
side split, respectively. Split threshold levels to judge whether the event is
single or split can be set by commands individually for each CCD.
After the classification of grades, energy histograms of 4096 energy chan-
nels with 16 bits depth are once produced in a given integration time for
each CCD chip and registered in the buffer area on the SH-OBC. Further-
more, those histograms are compressed typically into less than 1/10 size
and compiled in the CCSDS telemetry packet before delivery.
5. Laboratory Experiments
We have examined the XRS performance in the laboratory. Figure 3 shows
an example of results for powder specimen with simple composition such as
MgO, Al
2
O
3
,SiO
2
,CaCO
3
,TiO
2
,andFe
2
O
3
. A Cr-target X-ray generator
is used as the primary X-ray source. The temperature of CCD is cooled
at
50
◦
C. In this analysis, only the grade-0 data is used. The XRF line
spectra of major elements are clearly detected and discriminated each other.
The scattered Cr peaks are also found for all the spectra. The energy res-
olution ranging 1.25-7.1 keV is almost achieved to that we have expected.
Performance of the XRS
11
has been also proven for complex powder com-
pounds as shown in Fig. 4, whose composition is simulated to a typical
A12 (Apollo 12 site) basaltic soil.
12
Each XRF peak of component major
elements such as Mg, Al, Si, K, Ca, Ti, and Fe and scattered Cr peak are
clearly detected and discriminated each other. In addition to K-
α
peaks,
K-
β
peaks are detected for Si, Ca, Ti, Cr, and Fe, which indicates suciently
good energy resolution for quantitative elemental analysis as expected.
−
6. In-Flight Experiments
After Hayabusa was launched, the XRS have been operated for observa-
tion and in-flight calibration. Figure 5 shows an example of
in situ
XRF
