N㎜ 十
Bandpass 50 keV−5MeV
Effective Area 800 cm2 at 100 keV／400 cm2 at l MeV
Time Resolu60n 31．25 ms for GRB，1sfor All−Sky−Monitor
30 CHAPTER 3．1NSTRUMEN1五TION
o
， L r ． 1 雫 〉 「 「 L ． r． 「 w −一 ・
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、「
｝ 1．
1
0 6 IO l5
m㎝．図 「mm「o．噛【㎜ぬ1 ．
Fig．3．3：Left：XIS Effective area of one XRT＋XIS system． for the FI and BI CCDs．
no contamination． Right：The Encircled Energy Function（EEF）showing the fractiollal
ellergy within a given radius for one quadrant of the XRT−I telescopes on飢zαたμat 4．5 and 8．O keV（Serlemitsos et aL 2007）．The four XISs are true imagers， with a large field of view（〜18 ×18 ）， alld moderate spectral resolution
The HXD is a Ilon−imaging instrument with an ef允ctive area of〜260 cm2， featuring a
compound−eye con丘guration and an extremely low background． It extends the bandpass
of the mission with its nominal sensitivity over the 10−600 keV band（cf． Fig．3．4）．The HXD consists of two types of sensors：2mm thick silicon PIN diodes sensitive over
10−60keV， and GSO crystal scintillators placed behind the PIN diodes covering 30−
600keV． The HXD field of view is actively collimated to 4．5°×4．5°by the we1レshaped BGO scintillators， which， in combination with the GSO scintillators， are arranged in the
so−called phoswich con丘guration． At energies below〜100 keV， an additional passive
collimation further reduces the丘eld of view to 34 ×34ノ． The energy resolution is〜3．O k・V（FWHM）飴・th・PIN di・d・・，・nd 7．6／逗％（FWHM）f・・the sci・til1・t・rs（wh・・e E i、en・，gy i・M・V）． Th・HXD time res・1・ti・n f・・b・th・en・・rs i・61μ・・Whil・th・HXD is intended mainly to explore the faintest hard X−ray sources， it can also tolerate very b，ight・・urce・up t・〜10 C・・b． Th・HXD・1・・h品an・ll−sky m・・it・・（th・Wide−band All−sky Monitor（WAM）， which can detect GRB and other sources（3．1）． In this paper，we do not use HXD，
3．1．2 X−Ray Telescopes（XRTs）
5鵬αkμhas丘ve light−weight thin−foil X−・Ray T＞lescopes（XRTs）． The XRT5 have been
developed jointly by NASA／GSFC， Nagoya University， Tokyo Metropolitan University，
・・dISAS／JAXA． Th・・e a・e g・a・i・g−i・・id・・ce・e且ecti…ptics c・n・i・ti・g・f・・mp・・tly nested， thin collical elements， Because of the reflectors「small thickness， they permit high density nesting and thus provide large collecting ef五ciency with a moderate imaging capability in the energy range of O．2−12 keV， all accomplished in telescope units under 20 kg each．
Four XRTs onboard 3批αえμ（XRT−1）are used fbr the XIS， and the other XRT（XRT−S）
31．THE SσZ41αノSATELL∫TE 31
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三 ま
婁
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5
10
／
／
m l（｝鐘 εner噺（｝鳴v戊
Fig．3．4：Total effective area of the HXD detec七〇rs， PIN and GSO， as a function of energy
（Kokubun e七a1・2006）・
Sun Shade
X］匿r
： 脳 xm
XRr−S −【O o
Q XRI−12 ◎ o
・xm二13
EOB TOP PLArE
一
Fig 3．5：Layout of the XRTs on the S批α丸spacecraft（Serlemitsos et a1．2007）．
is for the XRS． The XRTs are arranged on the toP PIate of the Extensible Optical Bench
（EOB）in the manner shown in Figure 3．5． The ex七ernal dimensions of the 4 XRT−Is are the same（See Table 3．2， which also includes a comparison with the ASCA telescopes）．
The HPD of the XRTs range from 1．8ノ七〇2．3 ， which is the diame七er within which half of the focused X−ray is enclosed． The angular resolution does not signi丘cantly depend on the energy of the inciden尤X−ray in the energy range of S屹αえ叫0．2−12 keV The effective areas are typically 440 cm2 at 1．5 keV and 250 cm2 a七8keV． The focal lengths are 4．75 mfOr the XRT−1． lndividua1 XRT quadrants have their component focal lengths deviated 血om the design values by a few cm The optical axis of the quadrants of each XRT are aligned within Z丘・m the mechanical axis． The丘eld・f view（the diameter飴r a half・f
32 CHAPTER 3．1NS TRσMENTA質ON
Table 3．2：Telescope dimensions and design parameters of XRT−1， compared with ASCA XRT．
S批αえμXRT−I ASCA
Number of telescopes 4 4 Focal length 4．75 m 3．5 m Inner Diameter 118 mm 120 mm Outer Diameter 399 mm 345 mm Height 279 mm 220 mm Mass／Tblescope 19．5 kg 9．8 kg Number of nested shells 175 120、
Reflectors／Tblescope 1400 960 Geometric area／Telescope 873 cm2 558 cm2 Reflecting surface Gold Gold Substrate ma七erial Aluminum Aluminum Substrate thickness 155μm 127μm
Re且ector slant height 101．6 mm 101．6 mm
the effective area）for XRT−Is is about 17 a七1．5 keV and 13／at 8 keV．
Basic Components of XRT 一
The 8〔脇αえμXRTs consis七〇f closely nested thin−foil reflectors， reflecting X−ray at small grazing angles． An XRT is a cylindrical structure， having the following layered compo−
nents：1． a thermal shield at the entrance aperture to help maintain a uniform tempera−
ture；2． a pre−collima七〇r mounted on metal rings for stray light elimination；3． a primary stage for the first X−ray re且ection；4． a secondary stage for the second X−ray reflection；
5．abase ring for structural integrity and interface with the EOB． A11七hese components，
except七he base rings， are construc七ed in 90°segments． Four of these quadran七s are cou−
pled together by interconnect−couplers and also by the top and base rings（Figure 3．6）．
The telescope housings are made of aluminum for an optimal strength to mass ratio．
Including the alignment bars， collimating pieces， screws and washers， couplers， re−
taining plates， housing panels and rings， each XRT−I consists of over 4112 mechanically separated parts． In total， nearly 7000 qualified re且ectors were used and over l million cm20f gold surface was coated．
Reflectors Each reflector consists of a substrate also made of aluminum and an epoxy layer that couples the reflecting gold surface to the subs七rate． The reflectors are nominally 178μmin thickness． In shape， each reflector is a 90°segment of a section of a cone． The cone angle is designed to be the angle of on−axis incidence for the primary stage and 3 七imes that for七he secondary stage． They are 101．6 mm in slant length and with radii extending approximately from 60 mm at the inner part to 200 mm at the outer par七．
3．1， THE 8σZAκσ8ATELL∫TE 33
Fig．3．6：AS鵬α★μX−Ray Telescope（Serlemitsos et a1，2007）．
All re且ectors are positioned with grooved alignmellt bars， which hold the fbils at their circular edges． There are 13 alignment bars at each face of each quadrant， separated at apProximately 6．4°apart．
In the 5酩α処XRTs、 the conical approximation of the Wolter−I type geometrY is
used． This approximation f皿damentally limits the angle resolution achievable． More
signi6calltly、 the combination of the且gure error in the replication mandrels and the imper拒ction in the thermo−fbrming process（to about 4 micrometers in the］ow f士equency components of the figure error in the a」dal direction）1inlitsthe angular resolution to about lminute of arc（Misaki et al．2004）．Pre−collimator The pre−collimator， which blocks o任stray light that otherwise would
ellter the detector at a larger angle than intended、 consists of concentrically nested alu−minum Ibils similar to that of the re且ector substrates（Mori et al．2005）． They are shorter，
22mm in length， and thinner，120μm in thickness． They are positioned in a fashion siln−
ilar to that of the reHectors， by l3 grooved alunli皿m plates at each circular edge of the pieces． They are installed on top of their respecti、・e primary reflectors along the aコdal directiolL Due to their smaller thickness， they do not signi6cantly reduce the entrance aperture in that direction more than the reflectors already do． Pre−collimator foils（10 not have reHective surfaces（neither front nor back）． The relevant dimensions are listed血
Table 3．3．
Thermal Shields The 5脇畝u XRTs are designed to血nction in a thermal enviro皿ment
of 20±7．5 c C（figure 3．7）． The thermal shield is mechanically sustained by a frame皿ade of alumi皿ln， with a thickness of 4 mm． The f士ame has thirteen spokes which are along the alignlnent bars of the XRT． A stainless steel mesh with a wire pitch， width and34 CHAPTER 3．1NS TRσMEN工ATτON
Table 3．3：Design parame七ers for pre−collima七〇r