The use of focusing telescopes in hard X-ray (E > 10 keV) astronomy will provide better flux sensitivity and imaging perfomances with respect to the directviewing detectors, utilized until now. We present recent results obtained from our Group regarding the possible use of Bragg-diffraction technique to design hard X-ray focusing telescopes

Frontera, F.

Pareschi, G.

Pasqualini, G.

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IL NUOVO CIMENTO VOL. 20 C, N. 5 Settembre-Ottobre 1997Bragg concentrators for hard (> 10 keV) X-ray astronomy:Status report()G. PARESCHI(1)(), F. FRONTERA(1)(2) and G. PASQUALINI(1)(1) Dipartimento di Fisica, Universita` di Ferrara - 44100 Ferrara, Italy(2) Istituto di TESRE/CNR - 40129 Bologna, Italy(ricevuto il 4 Aprile 1996; approvato il 24 Maggio 1996)Summary. — The use of focusing telescopes in hard X-ray (E > 10 keV) astronomywill provide better flux sensitivity and imaging perfomances with respect to the direct-viewing detectors, utilized until now. We present recent results obtained from our groupregarding the possible use of Bragg-diffraction technique to design hard X-ray focusingtelescopes.PACS 96.40 – Cosmic rays.PACS 95.55 – Astronomical and space-research instrumentation.PACS 95.85 – Astronomical observations.PACS 98.85.Nv – X-ray.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe flux sensitivity of direct-viewing detectors in X-ray astronomy is strongly limitedby the Poissonian noise, that increases with the detector surface. The only pratical wayto overcome this problem is to focus the X-rays collected from a large passive area, onto asmall-area detector. Focusing optics will also provide better imaging performances thandetectors with masks.For this reason, low-energy telescopes with focusing optics based on the phenomenonof total external reflection, has allowed to achieve much higher flux and angular resolutionsensitivities in the classical X-ray band (E < 10 keV) [1].Unfortunately, in the hard-X-ray band (E > 10 keV), where the critical grazing reflec-tion angles are extremely small and, consequently, the allowed collecting areas are verylimited, this technique becomes inefficient.() Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.() Current address: Danish Space Research Institute, Juliane Maries Vej 30, DK-2100Copenhagen, Denmark.G Societa` Italiana di Fisica 843844 G. PARESCHI, F. FRONTERA and G. PASQUALINITABLE I. – HAXTEL Configuration.Number of concentrator mirrors 28Mirror height h (cm) 60Top radius of the outer mirror (cm) 134Top radius of the inner mirror (cm) 10Focal length FL (cm) 380Mirror mechanical support material nickelMirror support thickness (mm) 1Nominal operation band (keV) 10–140Effective area at 15 keV (cm2) 1000Effective area at 60 keV (cm2) 100Effective area at 100 keV (cm2) 35Sensitivity at 15 keV (10 6 Crab and Tint= 105s) 8Sensitivity at 60 keV (10 6 Crab and Tint= 105s) 190Sensitivity at 100 keV (10 6 Crab and Tint= 105s) 670Several alternative concentration techniques have been recently proposed aiming atthe development of focusing optics in the high-energy X-ray band [2]; among them, the useof multiple small-angle reflection in the interior of glass microcapillars and multichannelplates, the reflection from continuously graded multilayers and the Bragg diffraction frommosaic crystals are particularly promising. In what follows we report on more recentresults regarding the latter technique currently under study in our group.2. – HAXTEL: a telescope concept based on mosaic crystal diffractionMosaic crystals can be described as a set of a large number of crystallites of micro-scopical or submicroscopical size, which are oriented almost, but not exactly, parallel toeach other. While the diffraction from each crystallite is coherent, there is no coherencebetween photons scattered from different blocks. The misalignement distribution of mi-croblocks lattice planes with respect to the mean orientation (mosaic spread) is, with goodapproximation, a Gaussian function.With respect to a perfect crystal, which achieves high reflectivity in a narrow angu-lar range centered in the Bragg peak, a mosaic crystal maintains a significant reflectionpower over a much larger angular range, depending on the mosaic spread.The idea to utilize Bragg diffraction technique (in reflection geometry) from mosaiccrystals to focus celestial hard X-rays with continuous photon spectra is described inseveral previous publications [3-6]. The proposed telescope consists of a set of confocalparaboloidal mirrors. The mirror positions satisfy the following criterion: top radius ofa given mirror is equal to the bottom radius of the adjacent outer mirror. Each mirrorsupport is covered by many pieces of mosaic crystals with average reflecting planes par-allel to the crystal surfaces. All the mirrors have the same height. With this geometricalconstruction it is possible to concentrate polychromatic photons into a small focal plaindetector. An important feature of Bragg telescopes is the strong dependence on the mini-mum threshold energy.Results of a study by De Chiara and Frontera [3], devoted to optimize the parame-ters of such a kind of optics, have shown that pyrolytic graphite (002) is much suitable asreflecting material with the following mosaic parameters: crystal thickness of 2 mm, mi-croblock thickness of about 100 times the lattice spacing d002of graphite (d002=3.354 A˚)BRAGG CONCENTRATORS FOR HARD (> 10 keV) X-RAY ASTRONOMY: STATUS REPORT 845and mosaic spread of 0.2, that is a good compromise between large integrated reflectivityand small defocusing of photons on the focal plane due to crystallites misalignement.In table I we summarize the parameters of HAXTEL (HArd X-ray TELescope), aconcept of a medium-size telescope configuration based on the geometrical constructiondescribed above. In table I we give also the values of the effective area and flux sensitivityat three different energies with an integration time of 105 s.3. – Recent resultsIn what follows we will review some properties of graphite Bragg concentrators wehave recently studied.3.1. Intrinsic polarimetric capability. – The reflectivity of a mosaic crystal (and conse-quently the telescope effective area Ae) depends on the polarization angle  between theelectric field of incoming X-ray photons and the plane of reflection. De Chiara and Fron-tera [3] reported the expression of the reflectivity R as a function of the photon energy E,glancing incidence angle  and polarisation angle .Exploiting this property of mosaic crystals, Bragg concentrators have an intrinsic ca-pability to detect polarization from astrophysical objects. Indeed the intensity of reflectedpolarized photons in the focal plane results to be azimuth modulated: different angularsectors of the photon distribution in the focal plane show different intensity [3, 7]. Fig-ure 1 shows the dependence of the percentage effective area on for the telescope config-uration described in table I in the case of polarized photons with energy 10.8 keV, 21 keVand 28 keV. For this purpose we used the reflectivity expression for mosaic crystals givenin [3] as a function of the polarization angle of incoming photons. We have obtained simi-lar results also with Monte Carlo simulations we have performed [7], in which we adoptedthe model of Sanchez del Rio and coworkers to describe the misalignement of normals tocristallites with respect the normal to the mean reflection plan [8].The expected performance of the telescope as a polarimeter was also evaluated. TheMinimum Detectable Polarization (MDP) with 99% of confidence level is shown in tableII for three different energy bands, three different source flux and an integration time of105 s [7]. For this evaluation we assumed celestial X-ray sources with a Crab-like spec-trum (I(E) = k E 2:1 photons/(cm2 s keV)).3.2. Imaging capability and field of view. – We recently reported the imaging capabil-ities of Bragg telescopes [6]. In order to investigate the angular resolving power, a MonteCarlo code that simulates beams of parallel X-ray photons incident on the telescope tophas been utilized. The code was previously described [5].TABLE II. – Minimum Detectable Polarization (%).Energy band (keV) Incident flux (mCrab)1000 100 1010–20 2.6 8.5 26.620–30 8 26 8330–40 31 98 –846 G. PARESCHI, F. FRONTERA and G. PASQUALINIFig. 1. – Percentage effective area as a function of polarization angle  at three different energiesfor HAXTEL; as can be seen, the modulation amplitude decreases with the photon energy (takenfrom [7]).Fig. 2. – Contour plot of the 15 keV light distribution on the detector plane due to two sources, oneon-axis and one 10 arcmin apart, for a 380 cm focal length Bragg telescope (taken from [6]).BRAGG CONCENTRATORS FOR HARD (> 10 keV) X-RAY ASTRONOMY: STATUS REPORT 847Fig. 3. – Effective area for a 380 cm focal length Bragg telescope with a minimum operative energyEm=10 keV and for a set of 9 modules withEm=35 keV allocated in a room of equivalent area (seealso fig. 4).As a result of such a study, we found that light distribution of photons in the focal planecannot be fitted with a Gaussian, even though it shows a sharp peak with a full width athalf-maximum (FWHM) that depends on photon energy. For the telescope configurationof table I the FWHM corresponds to an angular resolution of about 4 arcmin at 15 keVand 1.5 arcmin at 80 keV. The field of view results to be about 40 arcmin (FWHM) with off-axis images affected by aberration defects (coma). In fig. 2 we show the light distributionsof two sources separated by 10 arcmin at 15 keV; as one can observe, the two sources areclearly resolved.It is worthy to note that a position-sensitive detector, with good properties of spatialresolution (less of 1 mm) is needed to exploit the imaging properties of Bragg telescopes.Still better resolution (0.5 mm) is requested if we want to use the concentrators as po-larimeters, to allow the study of the azimutal distribution of reflected photons in the focalplane.For this reason a detector with the requested characteristics based on Germaniumstrips could be a good solution. Also Mercuric Iodide cameras, as recently proposed byDusi et al. [9], appear of great interest.3.3. Design optimization criteria. – A drawback of the telescope configuration de-scribed in table I is its large diameter (268 cm). For that, we have studied possible waysto decrease the dimensions of such kinds of telescopes [6,10]. In fact, as mentioned above,in a Bragg telescope the dimensions depends strongly on the minimum reflection energy848 G. PARESCHI, F. FRONTERA and G. PASQUALINIFig. 4. – A 380 cm focal length multimodular telescope configuration. The minimum operativeenergy Emof each module is 35 keV. The set of 9 modules is allocated in the same room needed bya same focal length telescope but with Em=10 keV.Em. In particular, the external diameter D decreases according to the following law:D(Em) = D10 keVEm10 keV 1;(1)whereD10 keVis the external diameter of a telescope atEm=10 keV. In the case of a 380 cmfocal length telescope (HAXTEL), the diameter diminishes from 268 cm when Em= 10keV (see table I) to 80 cm when, e.g., Em= 35 keV. Thus, if the telescope diameter mustbe kept within given values, the only solution is to rise the minimum operative energy ofthe telescope. Alternatively, the room needed by a telescope with a energy threshold of10 keV, can be used to allocate a set of 9 modules with a diameter of 80 cm andEm=35 keV,with a sensitive increase of the effective area at high energies (see fig. 3). A schematicview of this telescope configuration is shown in fig. 4.4. – Discussion and conclusionAs a result of the properties above studied, the Bragg diffraction from mosaic crystalsis a good candidate technique for the construction of a focusing telescope with an opera-tive range up to about 200 keV. We have shown the good imaging capability of the Braggconcentrators, in addition to a field of view that can be of particular interest for sky sur-veys. Another important characteristic of Bragg telescopes is the capability to detect thepolarization of incoming photons.A feature of these telescopes is their diameter that strongly decreases with energy:large diameter values are needed for a minimum operative energy of 10 keV. At this en-ergy, telescopes based on multilayers appear more compact [10]. Thus a possible payloadBRAGG CONCENTRATORS FOR HARD (> 10 keV) X-RAY ASTRONOMY: STATUS REPORT 849for a future wide-band X-ray astronomy mission could be based on a combination of mo-saic crystal concentrators devoted to higher-energy photons (e.g., Em>35 keV) with tele-scopes based on one of the other techniques described by Gorestein [2] (microcapillars,multichannel plate optics, multilayers) that appear more suitable to focus lower-energyphotons. For this aim, the use of Wolter I multilayer telescopes, recently proposed [11], isof particular interest.  This research is supported by the Italian Space Agency ASI, Ministero Universita` eRicerca Scientifica e Tecnologica and Consiglio Nazionale Ricerche of Italy.REFERENCES[1] BRADT H. V. D., OHASHI T. and POUNDS K. A., X-ray astronomy missions, Annu. Rev.Astron. Astrophys., 30 (1992) 391.[2] GORESTEIN P., Non-traditional grazing incidence optics for X-ray astronomy, Proc. SPIE,1546 (1991) 91.[3] DE CHIARA P. and FRONTERA F., Bragg diffraction technique for concentration of hardX-rays for space astronomy, Appl. Opt., 31 (1992) 1361.[4] FRONTERA F., DE CHIARA P., GAMBACCINI M., LANDINI G. and PASQUALINI G., HardX-ray imaging via crystal diffraction: the first results of reflectivity measurements, Proc.SPIE, 1549 (1991) 113.[5] PASQUALINI G., DE CHIARA P. and FRONTERA F., Development status of Bragg hard X-rayconcentrators for space astronomy, Nuovo Cimento C, 15 (1992) 879.[6] FRONTERA F. and PARESCHI G., Hard X-ray imaging via focusing optics with mosaiccrystals, Exp. Astron., 6 (1995) 25.[7] FRONTERA F., DE CHIARA P., PARESCHI G. and PASQUALINI G., Polarimetricperformance of a Bragg hard X-ray (> 10 keV) concentrator, Proc. SPIE, 2283 (1994) 85.[8] SANCHEZ DEL RIO M., BERNSTORFF S., SAVOIA A. and CERRINA F., A conceptual modelfor ray tracing calculations with mosaic crystals, Rev. Sci. Instrum., 63 (1992) 932.[9] DUSI W., CAROLI E., DI COCCO G., DONATI A., RAMUNNO G., GRASSI D. and PERILLOE., A mercuric iodide X-ray camera for high energy astronomy, Proceedings of the CapriWorkshop on Imaging in High Energy Astronomy, edited by L. BASSANI and G. DI COCCO(Kluwer Academic Publisher, Dordrecht) 1995, p. 351.[10] FRONTERA F. and PARESCHI G., Comparative study of hard (>10 keV) focusing techniquesfor space astronomy, Proc. SPIE, 2515 (1995) 2.[11] CITTERIO O., FRONTERA F., JOENSEN K., GORENSTEIN P. and PARESCHI G., Newconception multilayer hard X-ray telescope for X-ray astronomy, Proceedings of the CapriWorkshop on Imaging in High Energy Astronomy, edited by L. BASSANI and G. DI COCCO(Kluwer Academic Publisher, Dordrecht) 1995, p. 259.

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Bragg concentrators for hard (> 10 keV) X-ray astronomy: Status report

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