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. 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.

12. ITERATED SIMPLEX ALGORITHM

13. SIMPLEX ALGORITHM (amoeba)
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

14. ITERATED SIMPLEX METHOD
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

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 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

22. LA MISSIONE XEUS

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