Hard X-ray Multilayer Optimisation for Astronomical Missions

1. Hard X-ray Multilayer Optimisation for Astronomical Missions

2. X-ray Reflection and focalization techniques The problem of multilayer optimisation for hard X ray (E > 10 keV) reflection. Multilayer mirrors optimisation for future astronomical X-ray projects. Conclusions

3. Minimum detectable flux for past-present-future astronomical missions GOAL!

4. Resolving the XRB by focusing optics

5. At photon energies > 10 keV the cut-off angles for total reflection are very small also for heavy metals the attained geometrical areas are in general very small Total reflection • In X ray regions refractive index are close to and little less than 1 • for grazing angles lower than acritical angle total reflectionphenomenon takes place. Present day focalising telescopes are based on it

6. Multilayer mirrors reflection • For angles bigger than critical one, reflectivity is low, but not zero.. • A multilayer consists in a sequences of bilayers(everyone composed from a couple of light and heavy material), the waves reflected from every interface sum in phase. • Constant bilayer thickness (d-spacing)  Bragg (constructive interference @ 2d sin q = n l) • Variable d-spacing  Is possible to obtain high reflectivity on a broader energy band

7. Multilayer for broad bandreflection not only in tecnology, but also in nature Artificial multilayer (nm for X-ray reflection) Natural multilayers (µm, for visible light) Aspidomorpha Tecta; 1 mm

8. Geometry Wolter I for grazing incidence optics Mirror shell and optical module of SWIFT telescope • Grazing incidence optics employ nested shells to improve collecting area • Every shell is composed of a double profile (parabole + hyperbole in Wolter I design). This scheme gives reduced optical aberrations and a shorter focal length

9. Ni/C multilayer onto a Si wafer substrate Ni/C multilayer 20 bilayers Dec 2003 E-beam depositionby OAB/Media Lario

11. It is possible to calculate the multilayer reflectivity for a given layers thicknesses sequence but.. it is generally not possible to analitically design the thicknesses for a given Reflectivity vs Energy response How to choose the best thickness values? • The reflectivity vs energy curve is determined by layers thicknesses sequence • It can be useful to define a function (function of merit or FOM) whose value indicates “how good” is the chosen solution • Employing numerical techniques the highest value of the merit function (best design) can be find.

12. Applicazione alla funzione di prova: Dato iniziale R C E Cmin FINE? no si USCITA SIMPLEX ALGORITHM (amoeba) It is a quite atypical optimisation technique: • It does not require derivative informations • The method is LOCAL

13. PRINCIPALE Crea i dati FUNZIONE Legge un dato SIMPLEX Scrive il risultato FINE no si USCITA ITERATED SIMPLEX METHOD The SIMPLEXALGORITHMresults are strongly dependent from starting points. ITERATED SIMPLEX METHOD (IS)consists in repeated execution of simplex algorithm, starting every time from different simplexes in parameter space. The software package “ISOXM” (IteratedSimplexOptimisation forX-ray Multilayers ) has been developed following this approach. It is possible to obtain results for different functions of merit (FOM). The software comprises tools for results analysis and visualization ISOXM program functional scheme for IS optimisation

14. PARAMETERS FOR OPTIMISATION d-spacing sequence described by a power law: with: • “a” ranging between 0 and ∞ • “b” ranging between -∞and 1 • “c” ranging between 0 and ∞ • theG parameter linearly changes along the stack • the heavy-material top layer is of increased size + a Carbon overcoating is added  to allow a high response in the soft X-ray regime

15. Dispersion of parameters after an iterated simplex optimization. Since the starting parameters are generated in a closed region, they are left free to expand.

16. Optimization strategy • optimized parameters: a, b, c + the G slope • iterated simplex optimization performed on a small number of selected shells distributed along the sequence of the diameters by using different FOMs • sequential optimization of all the shells, based on the results of the immeditely previous optimization. Every shell is optimized with a single execution of the simplex algorithm starting from the best result of the previous one • it is possible to combine results obtained from different FOMs for each group of shells, obtaining at the end the “more performing” total effective area of the telescope.

17. Integrated effective area and parameters along shells (XEUS)

18. XEUS mission

19. Mission XMM - Newton XEUS XMM-Newton Number of modules 3 1 10.0 m Max diameter 0.7 m Min diameter 0.3 m 1.3 m Geom. area @ 1 keV 0.15 m2 (per mod.) 30 m2 Min. angle (I) Min. angle (II) 0.3 deg 0.18 deg 0.7 deg Max. angle (I) Max.angle (II) 0.67 deg 0.7 deg  1.4 deg Angular Resoltion (HEW) 15 arcsec 2 arcsec (goal level) Focal Length 7.5 m 50 m Credits: ESA XEUS Credits: ESA

20. Gli specchi di XEUS Mirror Shell, Segments & Petals • 562 shell (296 XEUS I + 266 XEUS II) • Because of the huge dimensions, Wolter shells must be realized assembling a big number of segments (0.5 m x 1 m x 1 mm). Segments (17500) are grouped in “petals” (128) that form 5 concentric rings (2 XEUS I + 3 XEUS II). • Ø min. XEUS I = 1.3 m • Ø max. XEUS I = 4.04 m • Ø max. XEUS II = 9.9 m CREDIT: ESA

21. Extension of the XEUS operative range to hard X-ray (E ≥ 80 keV) • Even if the XEUS focal length is very large, the f-number are relatively small also for XEUS I (34 -10)  only with the use of multilayer supermirrors it is possible the hard X-ray extension of the XEUS operative range • study performed in Japan (Nagooya Univ & ISAS) suggested the use of Pt/C supermirrors based on discrete blocks of constant bi-layers with different (constant)d-spacing The supermirror solution is currently being considered by the XEUS Telescope Working Group Ogasaka et al., 2003 XEUS I

22. Shells F.O.M. Local minimum XEUS I optimization: • Shells 1-250: N=200, optimization with power law Parameters: a,b,c, Γ1, ΓN For shells 251-296  N=30 D=80 Å 1-118 2/4 119-124 1/4 125-250 1/1 251-296 - D=80 Å XEUS I optimization results: • Aeff = 2000 cm2 @ 40 keV • The number of bi-layers could be further on reduced without a strong impact on the reflectivity  Reduction of the deposition time and of the roughness increase

23. Multilayer mirrors for XEUS II: a viable and suitable choice? • depth-graded multilayer supermirrors for the enhancement of the hard X-ray (E > 10 keV) response are not convenient, since with the XEUS II large angles (0.7 – 1.4 deg) we are far from the Bragg diffraction conditions (2 d sinq = n l) at high energy • the use of “broad-band” multilayer supermirrors made of many bi-layers is not viable even below 10 keV, due to the strong photoelectric absorption HOWEVER • the soft X-ray (0.5 – 4 keV) response of any high density material (Au, W, Ir, Ni, Pt…) can be increased with the introduction of a low density material overcoating, not sensitive to the photoelectric absorption effects in the total reflection region (and anyway transparent at higher photon energies…) • constant d-spacing multilayers (formed by a small number of bilayers) are able to provide narrow high-reflectivity Bragg peaks in the soft X-ray region

24. carbon overcoating multilayer Bragg peak Effect of the XEUS II Low-Energy Enhancement

25. Low-energy (0.93 keV) reflectivity enhancement of a Ni mirror by a 50 Å Carbon overcoating: experimental result Test performed at the PANTER-MPE facility (Credits: W. Burkert).

26. SIMBOL–X P.I.: P. Ferrando Service d’Astrophysique CEA & Fédération de Recherche APC Progetto proposto al CNES (Bando “formation flight” 2004) da: Francia: Service d’Astrophysique CEA Saclay / CESR Toulouse LAOG Grenoble / LUTH Meudon Italia: INAF - Observatorio Astronomico di Brera ( ma interesesse a questa missione già mostrato anche da ricercatori di altri enti in ambito INAF, IASF/CNR e Università) Germania: MPE Garching / PNSensor GmbH München / IAA Tübingen UK: Dept of Astronomy and Astrophysics, Leicester

27. SIMBOL-X mission concept • Formation fligth with 30 m focal length • Can serve as XEUS pathfinder Main features Operative band:0.5–70 keV Energetical resolution: < 130 eV @ 6 keV, 1 % @ 60 keV Angular resolution: < 30 arcsec (local. < 3 arcsec) Effective area:> 550 cm2 E < 35 keV 150 cm2 @ 50 keV Sensibility: 5 10-8 ph/cm2/s/keV (E < 40 keV) (5 s, 100 ks, DE = E/2) Si SDD (0.5 – 10 keV) detector+ CdZnTe (10 – 70 keV) detector

28. SIMBOL-X: area efficace in asse

29. 80 cm diam + ML 70 cm diam 60 cm diam (baseline)

31. HEXIT-SAT mission

32. HEXIT – SAT (High Energy X-ray Imaging Telescope - SATellite) Mission concept to be realized for main contribute at national level from a researchers of INAF, IASF e Universities. • It will be presented to the international community on the occasion of the next SPIE conference in Glasgow (Fiore et al., 2004) • It is based on on a multimodular telescope (4 units) with Wolter multilayer mirrors with 8 m of focal length • Extensable optical bench to reduce the costs • Orbita LEO equatoriale (“SAX like”)  optimal to have a low particles background

34. HEXIT-SAT Main features

35. 4 modules

36. Effects of using different FOMs The design can be chosen according to the mission target

37. Summary and conclusions • The software ISOXM(IteratedSimplexOptimisation forX-ray Multilayers ) for global Optimisation with different FOMs has been developed. The numerical optimization of depth-graded supermirrors described by power-laws for several missions has been executed with good results. • future work will be done to study a possible reduction of the number of bi-layers compared to the 200 units assumed for this study. • At larger incidence angles multilayer reflectors can be employed to enhance the reflectivity at low energies by mean of constant d-spacing multilayers with Carbon overcoating.The study showed a consistent increase of the XEUS effective area in the energy region between 0.5 and 5 keV. • the carbon overcoating could be useful, not only to enhance the reflectivity in the soft X-ray region, but also to prevent aging effects due to the exposure to Atomic Oxigen fluxes.

38. The End