We will review the status of the Laue lens development of space astrophysics and the importance of the curved crystals for optimizing the lens performance

Frontera, F.

Virgilli, E.

English

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DOI 10.1393/ncc/i2011-10955-1Colloquia: Channeling 2010IL NUOVO CIMENTO Vol. 34 C, N. 4 Luglio-Agosto 2011Crystals for X/gamma-ray space telescopesF. Frontera(∗) and E. VirgilliDipartimento di Fisica, Universita` di Ferrara, - Via Saragat 1, 44100 Ferrara, Italy(ricevuto il 22 Dicembre 2010; pubblicato online il 9 Settembre 2011)Summary. — We will review the status of the Laue lens development of spaceastrophysics and the importance of the curved crystals for optimizing the lens per-formance.PACS 95.55.Ka – X- and γ-ray telescopes and instrumentation.PACS 95.85.Pw – γ-ray.1. – IntroductionFor a breakthrough in the study of the high-energy physical processes occurring inGalactic and extragalactic sources, it is crucial to have an instrumentation that coversa broad energy band (from fraction of keV to several hundreds of keV) and that itshows a high flux sensitivity even on short time scales (possibly on time scales shorterthan source time variability). To meet both requirements it is crucial to use focusingtelescopes. The current experimental scenario is the following: low-energy X-ray (0.1–10 keV) telescopes are available and are well tested in space; medium-energy X-ray (upto 70–80 keV) telescopes, based on multilayer mirrors, will be tested in space in the nextfuture (American “NUSTAR” mission, Japanese “ASTRO–H” mission); high-energy X-ray (> 70/100 keV) telescopes are still under development.But, why to extend the band up to high-energy X-rays (also called soft gamma-rays)(> 70/100 keV)? For a review of the science goals in this energy band, see, e.g., ref. [1].In short, some scientific reasons are the following:– to study the spectral formation physics in the presence of strong magnetic fields;– to study soft and very soft states of Galactic Black Hole Binaries (BHBs);– to investigate the physical origin of high-energy cut-offs in AGNs spectra;(∗) E-mail: frontera@fe.infn.itc© Societa` Italiana di Fisica 485486 F. FRONTERA and E. VIRGILLIFig. 1. – Sensitivity to continuum emission of the IBIS-ISGRI telescope aboard the INTEGRALsatellite now in flight. Reprinted from ref. [4].– to establish the origin of the high energy (> 60 keV) Cosmic X-ray Diffuse Back-ground;– to determine the production sites the e+/e− annihilation line, and its origin.As an example, we consider the last science goal. Positron production can occur ina variety of cosmic explosions and acceleration sites. It is expected to be the resultof the presence of antimatter but it is also expected in the case of dark matter. Thusthe observation of the characteristic 511 keV annihilation line provides a powerful toolto probe plasma composition, temperature, density and ionization degree. A claim foran annihilation line from a compact source (Nova Muscae) was reported but never con-firmed [2]. Instead evidence of diffuse annihilation line emission from the Galactic Centerregion was found with the INTEGRAL satellite [3]. The origin of this emission is stillnot established, even if the authors of the paper [3] suggest as possible origin the hardX-ray compact sources located in the region. The test of this assumption and thus estab-lishing the sources of the 511 keV line in compact sources requires much more sensitivity,achievable only with focusing gamma-ray telescopes.The requirements for future hard X/γ-ray focusing telescopes can be summarized asfollows:– broad band (80/100–600 keV and beyond);– continuum sensitivity 2 orders of magnitude better than current non-focusing tele-scopes (design goal: 10−8 ph/(cm2 s keV) in an observation time T = 105 s);– good imaging capability (angular resolution ≤ 1 arcmin).As an example of limit sensitivity of the current instrumentation, we show in fig. 1 thecontinuum sensitivity of the ISGRI instrument aboard the INTEGRAL satellite.2. – Laue lens principlesWe propose as focusing instrument in the soft gamma-ray band a Laue lens. Lauelenses use the interference between the periodic nature of the electromagnetic radiationand a periodic structure such as the matter in a crystal. In a Laue lens, the photonspass through the full crystal, using its entire volume for interacting coherently. In orderCRYSTALS FOR X/GAMMA-RAY SPACE TELESCOPES 487Fig. 2. – Geometry of a Laue lens (see text).to be diffracted, an incoming gamma-ray must satisfy the Bragg condition:(1) 2dhkl sin θB = nhcE,where dhkl (in A˚) is the spacing of the lattice planes (hkl), n is the diffraction order,hc = 12.4 keV·A˚ and E is the energy (in keV) of the gamma-ray photon.A Laue lens for astrophysical applications is made of a large number of crystals,in transmission configuration (Laue geometry), that are disposed such that they willconcentrate the incident radiation onto a common focal spot. A convenient way tovisualize the geometry of a crystal lens is to consider it as a spherical cup covered withcrystal tiles having their diffracting planes perpendicular to the sphere (see fig. 2). Thefocal spot is on the symmetry axis at a distance f = R/2 from the cup, with R beingthe radius of the sphere of which the spherical cup is a part; f is called the focal length.The main requirement for the disposition of the crystals in the lens is to get a smoothdependence of the lens effective on photon energy. Two are the possible dispositions:ring-like for high focal lengths (> 20m) and spiral-like for lower focal lengths (see fig. 3).3. – Ongoing developments and main issuesThe development of Laue lenses is mainly performed in Europe within two institutes:CESR (Centre d’Etude Spatiale des Rayonnements) in Toulouse (France) and the PhysicsDepartment of the University of Ferrara, in Italy. In the former insitution, the main goalis the development of narrow-band (800–900 keV) Laue lenses for nuclear astrophysics,while in the latter institution the goal is the development of broad-band Laue lenses(70/100–600 keV).488 F. FRONTERA and E. VIRGILLIFig. 3. – The basic design of a crystal diffraction lens in Laue geometry. Flat crystal tiles areassumed. Left: concentric rings of a given radius r concentrating a constant energy E. Right:crystal tiles disposed along an Archimedes’ spiral result in a continuously varying energy E.Given the footprint of the crystals, the image in the focal plane has a minimum size equal tothat of the crystal size.The main issues to be faced for the Laue lens development are two:a) development of a proper technology for the massive production of suitable crystals;b) development of a technology for assembling thousands of crystals in a reasonabletime, compatible with a space project.4. – Crystals for Laue lensesBoth mosaic crystals and curved crystals are suitable to be used for a Laue lens. Seeref. [1] for an exhaustive review.4.1. Mosaic crystals. – Mosaic crystals are made of misaligned perfect microcrystals,with misalignment distribution well described by a Gaussian function:(2) W (Δ) =1√2πηexp[−Δ22η2],where Δ is the magnitude of the angular deviation from the mean, while βm = 2.35η isthe FWHM of the mosaic spread and it is called crystal mosaicity.The energy passband of a mosaic crystal is given by(3) ΔE =2dhkl · E2 · βmnhc,where E is the energy derived from eq. (1). In fig. 4 we clarify the concept of mosaicstructure.Mosaic crystals of Copper are currently produced by ILL, while mosaic crystals ofgermanium are developed by IKZ, Berlin. Instead mosaic crystals of GaAs are being de-veloped by CNR, IMEM, Parma, Italy. Also available are commercial crystals, producedCRYSTALS FOR X/GAMMA-RAY SPACE TELESCOPES 489Fig. 4. – Mosaic structure concept.by, e.g., Mateck Gmbh. In fig. 5, top panel, we show the expected peak reflectivity ofmosaic crystals made of different materials, for optimized values of the crystallite thick-ness and mosaic crystal thickness. Instead in fig. 5, right panel, we show the measureddiffraction efficiency (ratio between diffracted and transmitted beams) at 300 keV.4.2. Curved crystals. – Curved cystals have important advantages with respect tomosaic cystals:– No limit to their diffraction efficiency, while for mosaic crystals the maximumtheoretical value of the diffraction efficiency is 0.5.– Better lens focusing for a proper crystal curvature.The better lens focusing is shown in fig. 6. It is found that for the same focal length,the angular resolution of a lens made of curved crystals significantly improves. Asan example, for 20m focal length, the angular resolution increases from 3.5 arcminto 20 arcsec, with the source image area that decreases by a factor about 100. As aconsequence of only this area decrease, the lens sensitivity increases by a factor atleast 10 with respect to flat mosaic crystals. As an example, in fig. 7 we comparethe sensitivity of a 20m focal length Laue lens made of curved mosaic crystals withthat made of flat mosaic crystals tiles of size 15× 15mm2 and 30 arcsec mosaicity.Fig. 5. – Left panel: expected maximum reflection efficiency of mosaic crystals of differentmaterials (reprinted from ref. [1]. Right panel: Measured diffraction efficiency of a Cu(111)mosaic crystal sample at 300 keV. Reprinted from ref. [5].490 F. FRONTERA and E. VIRGILLIFig. 6. – On axis response function (PSF) of a lens of 15m focal length made of mosaic crystalsof Cu(111) with 30 arcsec mosaicity, for an on-axis source. Left panel: Flat crystals with cross-section of 15× 15mm2. Right panel: Curved crystals with a curvature radius of 30m.There are several production methods to get curved crystals, among which the grow-ing of a two-component crystal whose composition varies along the crystal growth axis(see, e.g., ref. [6]), and the indentation of one face of a wafer. The last technique isbeing developed at the University of Ferrara with very satisfactory results for silicon andgermanium (see fig. 8). Also the deposition of a coating on a wafer is being tested at thesame University.In spite of these exciting results, further efforts are needed: a) to extend the bendingtechniques to other crystal materials, even mosaic crystals; b) to bend thicker crystals;c) massive production (thousands of pieces) of properly curved crystal tiles as requestedby space lenses.5. – Lens assembly technology developmentCurrently new lens assembly technologies are being developed at the University ofFerrara, for building broad passband (70/100–600 keV) Laue lenses, and at CESR, forbuilding high-energy narrow-passband (800–900 keV) Laue lenses. Results of these de-velopments are reviewed in ref. [1].Fig. 7. – 3σ Sensitivity of a 20m focal length Laue lens made of flat mosaic crystal tiles ofCu(111) with 30 arcsec mosaicity and 70–300 keV nominal energy passband, compared with thecorresponding lens in the case that the crystals are properly curved. The time measurementassumed is 105 s, while the energy bins have an amplitude ΔE = E/2.CRYSTALS FOR X/GAMMA-RAY SPACE TELESCOPES 491Fig. 8. – Measured rocking curve, in transmission geometry at 150 keV, of a Si(111) crystalcurved at the University of Ferrara. Δθ gives the difference between the incidence angle ofthe monochromatic photon beam and the Bragg angle. Open circles: ratio between measuredintensity of the diffracted beam and measured intensity of the transmitted beam (also calleddiffraction efficiency). Filled circles: difference between transmitted and diffracted intensities.Note that the angle Δθ, through the Bragg law, is related to the reflected photon energy. Thusthe figure also shows the energy bandwidth of the curved crystal. Reprinted from ref. [7], wherethe crystal sample was tested.The crystal tile positioning accuracy in the lens is the most critical issue. It dependson the mosaic spread and the focal length. Longer focal lengths require better positioningaccuracies, thus the development is more challenging for lenses working at the highestenergies.5.1. Activity at the University of Ferrara. – In Ferrara, the developed crystal assem-bling technology is found suitable for building lenses with moderately short (≤ 10−15m)focal lengths. It does not require any mechanism for a fine adjustment of the crystal ori-entation once the crystal is positioned in the lens frame. Using this technology, a firstlens prototype with 6m focal length has already been developed and tested. It makes useof mosaic crystals of Cu(111). The difference between the measured response functionof the first prototype and that expected in the case of a perfect mounting of the crystalsin the lens is shown in fig. 9. As can be seen, only the center part of the measuredimage (i.e., the black region) is subtracted by the expected image. The corona still visi-ble in the difference image is the result of the cumulative error (mainly that due to themechanical separation of the lens from the countermask) done during the lens assemblyprocess. A new prototype is being developed that takes into account the experiencegained constructing the first one [8].For long focal length lenses (up to 100m), new crystal assembly technologies arerequired. To this end, a new project, “LAUE”, supported by Italian space agency ASI,has been started with the contribution of the Italian industry (main contractor DTM,Modena). The main goals of the LAUE project are the following:– To develop a technology for building petals of long-focal-length lenses;– to assemble and test a lens petal (see fig. 10) with 20m focal length;– to develop a technology for a massive production of proper crystals;492 F. FRONTERA and E. VIRGILLIFig. 9. – Difference between the PSF measured and that obtained with a Monte Carlo code byassuming a perfect positioning of the crystal tiles in the lens. Reprinted from ref. [9].Fig. 10. – Sketch of the lens petal planned to be developed within the LAUE project.– to produce the crystals needed to assemble the lens petal above;– to perform a feasibility study of a space lens.Participants to the LAUE project include, in addition to DTM and the High-EnergyAstrophysics group of the University of Ferrara (UNIFE), the Sensor and SemiconductorLaboratory of UNIFE, the CNR Institute IMEM (Istituto dei Materiali per l’ Elettronicaed il Magnetismo) Parma; the Institute INAF–IASF (Istituto di Astrofisica Spazialee Fisica cosmica) Bologna; branches of Milan and Turin of the Thales-Alenia Spaceindustry in Italy.6. – ConclusionsThe hard X/γ-ray band covered by Laue lenses is crucial for high-energy astrophysics.Given their much higher expected sensitivity (two orders of magnitude better thanthe current instrumentation), Laue lenses can face key importance astrophysical issues.Curved Diffracting Crystals (CDC) appear the best crystals for Laue lenses. Technologiesalready developed (e.g., indentation) to produce CDC are very promising. A technologyCRYSTALS FOR X/GAMMA-RAY SPACE TELESCOPES 493developed for assembling Laue lenses with low focal length(up to 10–15 meters) are al-ready tested, with further results expected in a few months. A technology for buildinghigh-focal-length Laue lenses is under development (LAUE project) with the industryinvolvement.∗ ∗ ∗The lens devolopment is the result of a big effort of several people. We wish to thankthe significant efforts by V. Carassiti, F. Evangelisti, S. Squerzanti from INFNSection of Ferrara; L. Landi, G. Loffredo, F. Nobili from University of Ferrara;N. Auricchio, E. Caroli and J. Stephen from INAF, IASF Bologna. We acknowl-edge the support of ASI, that has permitted the past and present activity in this field.REFERENCES[1] Frontera F. and Von Ballmoos P., X-ray Opt. Instrum., 2010 (2010) 109740.[2] Goldwurm A., Ballet J., Cordier B. et al., Astrophys. J. Lett., 389 (1992) L79.[3] Weidenspointner G., Skinner G., Jean P. et al., Nature, 451 (2008) 159.[4] Lebrun F., Leray J. P., Lavocat P. et al., Astron. Astrophys., 411 (2003) L141.[5] Barriere N., Rousselle J., von Ballmoos P. et al., J. Appl. Crystallogr., 42 (2009)834.[6] Keitel S., Malgrange C., Niemoeller T. and Schneider J. R., Acta Crystallogr. A,55 (1999) 855.[7] Barrie`re N. M., Natalucci L., Abrosimov N. et al., Proc. SPIE, 7437 (2009).[8] Ferrari F., Frontera F., Loffredo G. et al., Proc. SPIE, 7437 (2009).[9] Frontera F., Loffredo G., Pisa A. et al., Proc. SPIE, 7011 (2008).

Crystals for X/gamma-ray space telescopes

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Crystals for X/gamma-ray space telescopes

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