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 GOAL!
5 Total reflectionIn X ray regions refractive index are close to and little less than 1for grazing angles lower than a critical angle total reflection phenomenon takes place. Present day focalising telescopes are based on itAt 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 bandFunzionamento 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 opticsMirror shell and optical module of SWIFT telescopeGrazing incidence optics employ nested shells to improve collecting areaEvery 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 bilayersDec 2003E-beam depositionby OAB/Media Lario
11 How to choose the best thickness values? The reflectivity vs energy curve is determined by layers thicknesses sequenceIt 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 responseIt can be useful to define a function (function of merit or FOM) whose value indicates “how good” is the chosen solutionEmploying numerical techniques the highest value of the merit function (best design) can be find.
13 SIMPLEX ALGORITHM (amoeba) Applicazione alla funzione di prova:Dato inizialeRCCminEFINE?sinoUSCITAIt is a quite atypical optimisation technique:It does not require derivative informationsThe method is LOCAL
14 ITERATED SIMPLEX METHOD Crea i datiLegge un datoFINESIMPLEXFUNZIONEPRINCIPALEnosiUSCITAScrive il risultatoThe 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 visualizationISOXM program functional scheme for IS optimisation
15 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 ∞the G parameter linearly changes along the stackthe 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 slopeiterated simplex optimization performed on a small number of selected shells distributed along the sequence of the diameters by using different FOMssequential 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 oneit 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…… XMMJ. Swift – Gullivers’s TravelsPART I: A VOYAGE TO LILLIPUT
23 Mirror Shell, Segments & Petals Gli specchi di XEUSMirror Shell, Segments & Petals562 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 mØ max. XEUS I = 4.04 mØ max. XEUS II = 9.9 mPetali a forma di settori angolariCREDIT: 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 rangestudy 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-spacingThe supermirror solution is currently being considered by the XEUS Telescope Working GroupOgasaka et al., 2003XEUS I
25 XEUS I optimization results: XEUS I optimization: Aeff = keVThe 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 increaseXEUS I optimization:Shells 1-250: N=200, optimization with power lawParameters: a,b,c, Γ1, ΓNFor shells N=30 D=80 Å1/1-D=80 Å1/42/41-118Local minimumF.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 energythe use of “broad-band” multilayer supermirrors made of many bi-layers is not viable even below 10 keV, due to the strong photoelectric absorptionHOWEVERthe 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 degFor 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 overcoatingmultilayer Bragg peak
29 Low-energy (0.93 keV) reflectivity enhancement of a Ni mirror by a 50 Å Carbon overcoating: experimental resultTest 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.MaterialYoung’s Modulus (GPa)Thermal Expansion coefficient 25 oCBreaking strength (Mpa)SputteringYieldTemperature at vapour pressure =10-4/10-6 tonPossible couplingNi20013.0Not available1.31270/1072C6.5 9 (graphite)7.18 15 (graphite)0.362015/1872Ni, PtPt1688.81.11750/1492W4114.50.602800/2407SiMo3294.819300.702150/182247 1312.61200.51340/1147Mo, WThe 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 ÅAccurate 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 opticsthe 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 focalizzantiGiovanni PareschiINAF – Osservatorio Astronomico di Brera(on behalf of large collaborations)HEXIT-SATSimbol-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-SATEmissione al di sotto di 10 keV : meccanismi termici & non termiciLa componente non termica è fondamentale per studiare i fenomeni di accrescimento ed accelerazioneGli strumenti realizzati finora presentano una sensibilità peggiore di circa 2 ordini di grandezza in corripondenza della separazione tra fenomeni termici e non termiciSIMBOL–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 duriSensibilità: > 100 volte meglio diINTEGRAL-IBIS (E < 50 keV)Risoluzione angolare: arcsec HEWBanda energetica di lavoro: 0.5 – 70 keV
40 SIMBOL–X P.I.: P. Ferrando Decisione per selezione Studio di Fase A entro Luglio 2004P.I.: P. FerrandoService d’Astrophysique CEA & Fédération de Recherche APCProgetto proposto al CNES (Bando “formation flight” 2004) da:Francia: Service d’Astrophysique CEA Saclay / CESR ToulouseLAOG Grenoble / LUTH MeudonItalia: 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 TübingenUK: Dept of Astronomy and Astrophysics, Leicester
41 SIMBOL–X P.I.: P. Ferrando Service d’Astrophysique CEA & Fédération de Recherche APCProgetto proposto al CNES (Bando “formation flight” 2004) da:Francia: Service d’Astrophysique CEA Saclay / CESR ToulouseLAOG Grenoble / LUTH MeudonItalia: 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 TübingenUK: Dept of Astronomy and Astrophysics, Leicester
43 Architettura ‘Formation flight’ per SIMBOL-X ’’Pointed Telescope’’Orbita Circolare a Km di altitudine possibilità di più di 100 ks di osservazioni ininterrottePosizionamento relativo:± 1 cm lungo l’asse del telescopio± 1 cm perpendicolareRicostruzione di assetto: conoscenza della posizione dell’asse entro 3 arcsecOrbita a basso bachrground di particelleAlmeno due anni di osservazioni realiMissione di classe ‘’medium-size’’
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 lengthExtensable optical bench to reduce the costsOrbita 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)ci = indice del bistrato a l/(2 sin qc) c b> -1
51 HEXIT-SAT Main features Number of modules4Number of nested mirror shells50Reflecting coating200 bilayers W/SiGeometrical profileWolter I (lin. approx)Focal Length8000 mmTotal Shell Height800 mmPlate scale26 arcsec/mmMaterial of the mirror wallselectroformed NiMin-MaxTop DiametermmMin - Max angle of incidencedegMin-Max wall thicknessmmTotal Mirror Weight (1 module)65Field-of-View (diameter FWHM15 arcminSingle module effective area75 keV
52 Principali caratteristiche della missione HEXIT-SAT Lifetime yearsOrbital Altitude 600 KmMass 1000 Kg (TBC)Orbit Duration 95 minScientific Data Center: ASI SDC (Frascati)Ground Stations:#1 Malindi Lat: o, Long: 40.05, Alt.: 0 Km3.5o orbit: 150min/day – 650 sec/orbit50o orbit: min/day – 500 sec/orbit(with large spread min/max)#2 Fucino Lat: o, Long: , Alt.: 0 Km3.5o orbit: No Tracking50o orbit: min/day – 550 sec/orbit
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.