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Hard X-ray Multilayer Optimisation for Astronomical Missions

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Presentation on theme: "Hard X-ray Multilayer Optimisation for Astronomical Missions"— Presentation transcript:

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

4 Resolving the XRB by focusing optics

5 Total reflection In X ray regions refractive index are close to and little less than 1 for grazing angles lower than a critical angle total reflection phenomenon takes place. Present day focalising telescopes are based on it 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

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 2d sin q = n l) Variable d-spacing  Is possible to obtain high reflectivity on a broader energy band Funzionamento multilayer spesssore variabile,costante formule e problema inverso.

7 Multilayer for broad band reflection 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 Mirror shell and optical module of SWIFT telescope
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 How to choose the best thickness values?
The reflectivity vs energy curve is determined by layers thicknesses sequence 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 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.


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

Crea i dati Legge un dato FINE SIMPLEX FUNZIONE PRINCIPALE no si USCITA Scrive il risultato The SIMPLEX ALGORITHM results 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” ( Iterated Simplex Optimisation for X-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

d-spacing sequence described by a power law: with: “a” ranging between 0 and ∞ “b” ranging between -∞ and 1 “c” ranging between 0 and ∞ the G 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

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

17 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.

18 Integrated effective area and parameters along shells (XEUS)

19 XEUS: a really fantastic telescope……
XMM J. Swift – Gullivers’s Travels PART I: A VOYAGE TO LILLIPUT

20 XEUS mission

21 XMM-Newton Mission Newton XEUS XEUS Number of modules 3 1 10.0 m
Mission  XMM - Newton XEUS Number of modules 3 1 10.0 m Max diameter 0.7 m Min diameter 0.3 m 1.3 m Geom. 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


23 Mirror Shell, Segments & Petals
Gli specchi di XEUS Mirror Shell, Segments & Petals 562 shell (296 XEUS I 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 Petali a forma di settori angolari CREDIT: ESA

24 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

25 XEUS I optimization results: XEUS I optimization:
Aeff = 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 XEUS I optimization: Shells 1-250: N=200, optimization with power law Parameters: a,b,c, Γ1, ΓN For shells  N=30 D=80 Å 1/1 - D=80 Å 1/4 2/4 1-118 Local minimum F.O.M. Shells

26 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

27 Comparison of the reflectivities of Gold, a W/Si multilayer (with and without a W-thick/C bilayer overcoating) at the incidence angle of 1 deg For XEUS II we assumed a constant d-spacing W/Si multilayer (30 bilayers, d = 80 Å) plus a n overcoating made by: W  80 Å + C 50 Å for all the XEUS II mirror shells

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

29 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).

30 What are the best material to survive in the very cold XEUS environment?
The XEUS optics module will have to operate for a long time in extreme thermal conditions, with the temperature cyclically fluctuating between –30 and –40 °C (depending on the orbit altitude)  to prevent fast aging effects, with the increase of the internal stresses and micro-roughness, a good agreement between the CTEs of the two materials of a multilayer could be an obvious advantage. Material Young’s Modulus (GPa) Thermal Expansion coefficient 25 oC Breaking strength (Mpa) Sputtering Yield Temperature at vapour pressure =10-4/10-6 ton Possible coupling Ni 200 13.0 Not available 1.3 1270/1072 C 6.5  9 (graphite) 7.1 8  15 (graphite) 0.36 2015/1872 Ni, Pt Pt 168 8.8 1.1 1750/1492 W 411 4.5 0.60 2800/2407 Si Mo 329 4.8 1930 0.70 2150/1822 47  131 2.6 120 0.5 1340/1147 Mo, W The Pt/C couple appears as a good candidate

31 Example of Pt/C Multilayer optimisation for future projects
Absorption edge at 78.4 KeV

32 Thickness variations with depth for different multilayer designs:
Bilayers thickness varies from ~20 to >120 Å Accurate control of deposition is needed to achieve high reflectivities.


34 The Platinum choice as a reflecting coating
Platinum can act, like Gold, as a release agent in the replication process

35 the “Formation Flight” architecture opens the opportunity to realize hard X-ray (E > 10 keV) telescopes based on low grazing angles and large focal lengths Wolter I optics the design of the SIMBOL-X baseline relies on Pt single-layer mirrors with a 30 m focal length, with shell diameters similar to those assumed for XMM (but with much smaller reflection angles) possible improvements of the design can be achieved by increasing the external diameter and using multilayer reflecting coatings for more external shells

36 HEXIT-SAT & SIMBOL-X: due missioni per lo studio dell’Universo in raggi X duri tramite ottiche focalizzanti Giovanni Pareschi INAF – Osservatorio Astronomico di Brera (on behalf of large collaborations) HEXIT-SAT Simbol-X

37 Risoluzione del background cosmico X nella regione del picco (~30 keV)
De Luca e Molendi 2003

38 Progetto ASI “Progetto preliminare Payload per Astronomia delle alte energie”
Uno studio finanziato da ASI (Unità Paylod Elettrottici) è appena iniziato per produrre un prototipo completo (1 mirror shell “multilayer” sottile + 1 rivelatore CdZnTe) di telescopio per raggi X duri. Scopo  dimostrare che in Italia le tecnologie per la realizzazione di un payload a focalizzazione per astronomia in raggi X duri sono ormai mature per una missione satellitare

39 SIMBOL–X & HEXIT-SAT Emissione al di sotto di 10 keV : meccanismi termici & non termici La componente non termica è fondamentale per studiare i fenomeni di accrescimento ed accelerazione Gli strumenti realizzati finora presentano una sensibilità peggiore di circa 2 ordini di grandezza in corripondenza della separazione tra fenomeni termici e non termici SIMBOL–X ed HEXIT-SAT sono due missioni al cui studio e proposizione sta partecipando parte della comunità italiana « Alte Energie » per risolvere questo problema  introduzione di ottiche focalizzanti per raggi X duri Sensibilità: > 100 volte meglio di INTEGRAL-IBIS (E < 50 keV) Risoluzione angolare: arcsec HEW Banda energetica di lavoro: 0.5 – 70 keV

40 SIMBOL–X P.I.: P. Ferrando
Decisione per selezione Studio di Fase A entro Luglio 2004 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

41 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

42 SIMBOL-X mission concept
Formation fligth with 30 m focal length Can serve as XEUS pathfinder Main features Operative band: –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 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

43 Architettura ‘Formation flight’ per SIMBOL-X
’’Pointed Telescope’’ Orbita Circolare a Km di altitudine  possibilità di più di 100 ks di osservazioni ininterrotte Posizionamento relativo: ± 1 cm lungo l’asse del telescopio ± 1 cm perpendicolare Ricostruzione di assetto: conoscenza della posizione dell’asse entro 3 arcsec Orbita a basso bachrground di particelle Almeno due anni di osservazioni reali Missione di classe ‘’medium-size’’

44 SIMBOL-X: area efficace in asse

45 80 cm diam + ML 70 cm diam 60 cm diam (baseline)


47 HEXIT-SAT mission

48 (High Energy X-ray Imaging Telescope - SATellite)
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

49 Multilayer a larga banda (supermirrors)
se il d-spacing viene variato in modo continuo (supermirror) e l’assorbimento non è eccessivo (raggi X duri, > 10 keV) si possono creare bande di riflessione 3-4 volte maggiori di quelle in riflessione totale per specchi a singolo strato ad es. in Au, Pt, Ir. La distribuzione dei d-spacing segue in questo caso una legge di potenza: d(i) = a / (b+i)c i = indice del bistrato a  l/(2 sin qc) c  b> -1


51 HEXIT-SAT Main features
Number of modules 4 Number of nested mirror shells 50 Reflecting coating 200 bilayers W/Si Geometrical profile Wolter I (lin. approx) Focal Length 8000 mm Total Shell Height 800 mm Plate scale 26 arcsec/mm Material of the mirror walls electroformed Ni Min-MaxTop Diameter mm Min - Max angle of incidence deg Min-Max wall thickness mm Total Mirror Weight (1 module) 65 Field-of-View (diameter FWHM 15 arcmin Single module effective area 75 keV

52 Principali caratteristiche della missione HEXIT-SAT
Lifetime  years Orbital Altitude  600 Km Mass  1000 Kg (TBC) Orbit Duration 95 min Scientific Data Center: ASI SDC (Frascati) Ground Stations: #1  Malindi Lat: o, Long: 40.05, Alt.: 0 Km 3.5o orbit: 150min/day – 650 sec/orbit 50o orbit: min/day – 500 sec/orbit (with large spread min/max) #2  Fucino Lat: o, Long: , Alt.: 0 Km 3.5o orbit: No Tracking 50o orbit: min/day – 550 sec/orbit

53 4 modules

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

55 Summary and conclusions
The software ISOXM ( Iterated Simplex Optimisation for X-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.

56 The End

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