The results of experimental investigation of parametric radiation generated by 5.7 MeV electrons in a multilayer structure consisting of 100 Cr/Sc bi-layers deposited on a Si3N4 membrane are presented. The multilayer structure was specially created for generation of parametric radiation with photon energy in "water-window" spectral rang

Andre, J. M.

Kaplin, V. V.

Kubankin, A.

Le Guen K.

Uglov, S. R.

Journal of Physics : Conference Series

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RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/012017Cr/Sc multilayer radiator for parametric EUV radiation in “water-window” spectral rangeS R  Uglov1, V V K aplin1, A S K ubankin 2, J-M  A ndre3, K  Le Guen3, Ph Jonnard4, S de Rossi4, E M eltchakov4 and F Delmotte41 Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia2 Belgorod State University, Belgorod, Russia3 CNRS UMR 7614, Laboratoire de Chimie Physique - Matiere et Rayonnement Sorbonne Universites, UPMC Paris 06, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France4 Laboratoire Charles Fabry, Institut d'Optique, Graduate School, 2 avenue Augustin Fresnel F-91127 Palaiseau, FranceE-mail: uglov@tpu.ruAbstract. The results of experimental investigation of parametric radiation generated by 5.7 MeV electrons in a multilayer structure consisting of 100 Cr/Sc bi-layers deposited on a Si3N4 membrane are presented. The multilayer structure was specially created for generation of parametric radiation with photon energy in “water-window” spectral range. First test measurements of angular distributions of radiation have been done and discussed.1. In troductionIn a number o f papers [1 -  6] it was shown that artificial periodic structures, as well as crystals, are suitable to generate tunable quasi-monochromatic X-rays by relativistic electrons. The energies Eph of the photons emitted from the periodic structure lie in a narrow spectral range which is determined by the Bragg diffraction law. The mechanism of radiation generation in multilayer X-ray mirrors (MXM) is similar to the mechanisms of Parametric X-ray Radiation (PXR) and Diffracted Transition Radiation (DTR) generations in periodic crystal structures. Experiments [4 -  7] have been carried out for generation in MXM of X-rays with photon energies Eph = 6 -  15 keV.Recently in [10], it was experimentally shown that a multilayer mirror may be used for generation o f quasimonochromatic radiation in the range o f extreme ultraviolet (Eph around 70 eV). So, by using structure with shorter period, one can get the quasimochromatic tunable source o f photons in “water- window” spectral range. The “water-window” spectral range lies between the carbon and oxygen K absorption edges (284 and 543 eV, respectively). Radiation in this domain is required for many scientific and industrial applications, in particular, for the soft X-ray microscopy of biological objects.This paper presents the construction of a multilayer radiator for generation o f tunable radiation with photon energy in “water-window” spectral range and first results concerning the radiator test using the 5.7 MeV electron beam of the microtron M-5 o f the Tomsk Polytechnic University. The multilayer radiator was specially prepared for generation of the parametric radiation (PR) in the "water window" spectral range with very thin substrate in order to decrease the contribution of background radiation.Content from  this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/0120172. Cr/Sc rad ia to rIn the experiment, the radiator consists o f a pile o f 100 Cr/Sc bi-layers with a period d  = 2.34 nm caped by a 2.5 nm thick B4C layer; the whole stack is deposited onto a 0.5 ^m thick Si3N4 membrane. The thickness of Cr and Sc layers, a and b, respectively, are the same. Scheme of the Cr/Sc multilayer radiator is shown in figure 1.Figure 1. Scheme of Cr/Sc multilayer radiator. The thickness o f B4C cap layer is 2.5nm; Si frame is 500 ^m thick.The multilayer structure was prepared using the facility o f the Laboratoire Charles Fabry by magnetron sputtering technique under constant argon pressure using plasma discharges by radio frequency power [11]; it was characterized by grazing incidence X-ray reflectivity GIXR and X-ray emission spectroscopy XES. Interfacial imperfection o f Cr/Sc systems are mainly due to roughness with a typical rms roughness height o f about 0.5 nm at each interface as indicated by GIXR; the analysis o f Cr 3d valence states by XES does not evidence the presence o f interfacial compounds resulting from the interdiffusion o f the Cr and Sc layers [12].0.010 0.005 0.000i^-0.005- 0.010a>аз 0.005in 0.000-0.005- 0.010-0.015-" V .......1................1................1........: / /  4  ^.Х/7СГ IЗГ S c  j P ^ " - __ _ (a )  :: у/ J V  -------: l' S c^ ,4  // ( b ): " " - - - j: Cr/Sc У ____________ —■ Ту/200 300 400 500 600 700 800Photon energy, eVFigure 2. Dielectric susceptibilities: (a) Cr and Sc layers; (b) Cr/Sc bi-layer.The global multilayer radiator (multilayer + substrate) was also tested using the X-ray diffractometer “Shimadzu XRD 6000”. The width o f rocking-curve measured using the Cu K X-rays2RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/012017was about 0.2o that is in about 10 times more than that of theoretical one. All these characterizations indicate that the optical quality o f this radiator is yet very far from perfection which has likely affected its PR emissive properties as shown hereafter. The mechanical properties of the Si3N4 membrane acting as substrate have not yet been studied in details; nevertheless it is likely that a large part o f the relatively poor optical quality o f the radiator can be attributed to the substrate.Real y/ and imaginary X / parts o f the dielectric susceptibilities sa b (ю) = 1 + y/ (ю) + z'-xV ) o f Cr,Sc layers and average one sCr/Sc (ю) o f Cr/Sc bi-layer are shown in figure 2. The value o f sCr/Sc (ю) was calculated as (a-sCr + b-sSc) / (a + b). The reflectivity of our Cr/SC multilayer structure supposed to be ideal (no interfacial roughness and interdiffusion) was calculated using the Parratt-method [13] for a number o f Bragg angles and photon energies and shown in figure 3. The calculation evidences the reflectivity enhancements just below the L edge o f Sc (398 eV) at the Bragg angle 0O = 42o and around L edge o f Cr (574 eV) at 0O = 27o as a result o f the anomalous scattering [14]Figure 3. Reflectivity o f ideal Cr/Sc mirror as function o f Bragg angles and photon energies.3. Theoretical backgroundThe angular and spectral-angular densities o f radiation were calculated using different theories [3, 5]. The calculation shows that the maximum PR intensity is emitted near the direction 0Dy = 1/y (y is the Lorentz factor) with respect to the plane of diffraction. Therefore, the search for the PR effect should be carried out near the expected direction o f maximum intensity of the radiation by scanning the detector through the angular distribution in a plane that is at an angle 0Dy = 1/y with respect to the diffraction plane VN (N is the normal to the target surface). As it is known, the back transition radiation (BTR) formed at the interaction o f electrons with the input surface o f the target is emitted also in the same direction. To suppress the BTR contribution, a filter (0.2 цш LiF + 0.1 цш Palyren) was used.The angular distributions of 30 -  600 eV BTR and PR calculated for a number o f Bragg angles 00 by taking into account the photon absorption in the filter are shown in figures 4(a) and 5. Figure 4(b) illustrates the influence o f the filter on BTR intensity generated at 00 = 40° on upper Cr layer o f the multilayer structure. The Cr surface was taken for calculation o f the BTR contribution because the intensity o f BTR from Cr surface is higher than that from B4C, Si3N4 or Sc surfaces. The angular distributions o f BTR were calculated using the Pafomov’s theory [15].3RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/012017Figure 4. Angular distributions of 30 -  600 eV BTR: (a) 0O between 25° and 85°; (b) at 0O = 40°, with and without a filter.The angular distributions of PR were calculated using the theory [6]. The upper solid curve in figure 5 shows the dependence of photon energy versus 0D = 200 . As can be seen from a comparison of the data presented in figures 4(a) and 5, BTR contribution to overall radiation yield can be neglected at using the filter. Also is very important to note the difference of changing of the BTR and PR intensities via the angle 00 . The BTR intensity increases with decrease of the angle 00 , while PR intensity has a maximum around 00 « 40° .1.4о  1.2X’a) 1.0Ш(/) 0.8q . 0.6G 0.4■a 0.20.040°Kedge of oxygenBragg A ." energy \- . 35°\ 4 5 ° ^ --------- Kedge o f carbonUlО0оОсо\  5*  60°65°70° 75° 80° ОСО -; 00=25°k f  I- — ...... Y"\ \ / \ s ^  Л у х ..85—. ..—8000)600 ^2>шсa)400 с20020 40 60 80 100 120 140 160 180 2000Dx, degreeFigure 5. Angular distributions of PR for angles 00 between 25° and 85°.4. Experim ental procedureExperiment on the angular distribution of the radiation generated by the 5.7 MeV electrons into the Cr/Sc radiator was carried out using the electron beam delivered by the microtron M-5 at Tomsk. The radiation was detected by a channel electron multiplier (CEM-model VEU-6). A detailed description of the installation can be found in [8 -  10]. The geometry of the interaction between the electron beam and the target and of the generation of radiation is shown in figure 6.4RREPS2015 IOP PublishingJournal of Physics: Conference Series 732 (2016) 012017 doi:10.1088/1742-6596/732/1/012017Figure 6. Scheme of geometry of the interaction of electron beam with the radiator and generation of radiation at experimental setup based on microtron M-5. Electron energy -  5.7 MeV; repetition rate -  25 Hz, pulse duration -  0.4 ^s, electron current on target -  4 pC per pulse.A series of angular distributions was measured for several angles between the electron beam and the radiator surface in the range of 35o -  72o. The distributions were measured in the diffraction plane VN and at the angle 0Dy = 1/y = 5.14o (y = 11.15) with respect to this plane. To suppress the soft component of ordinary BTR, the measurements were carried out with the “LiF + Parylen” filter considered in section 3.5. F irst experim ental results and discussionAngular distributions of radiation generated in the Cr/Sc structure are shown in figure 7 and 8. The black points in figure 7 show the angular distribution of density of radiation generated in the Cr/Sc structure at 00 = 41o.0Dx, degreeFigure 7. The black points are for radiation emitted from Cr/Sc structure at 00 = 41o . The light points are for the Si3N 4 substrate without Cr/Sc structure. Lower curve shows background level.The light points are for radiation generated by the nude substrate (Si3N4  membrane) at the same angle 00  = 41o . The black and light points in figure 8 correspond to the angular distribution of radiation generated in the Cr/Sc structure at 00 = 55o and 00 = 35o , respectively. The lower dashed curves, shown in figures 7 and 8, present background level measured when the electron beam did not hit the target.5RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/012017Comparison of the results of measurements, presented in figure 7 and 8, with the calculations presented in figures 4(a) and 5, shows that the experimentally observed trend of change of the intensity of the maxima of the angular distributions, namely the existence of the global maximum of radiation intensity near 90 = 41o, is in qualitative agreement with the PR calculations.The FWHM of the PR angular distribution is about 6o that is about in 2 times less than typical value of FWHM of BTR one. As can be seen from figure 7, the Cr/Sc structure generates radiation with intensity more 2 times larger than the Si3N4 membrane, but this value is almost 10 times less than that derived from model calculations for an ideal periodic structure.40 50 60 70 80 90 100 110 120 130 140 150 1609dx, degreeFigure 8. The black and light points illustrate the emission from Cr/Sc structure for the two angles 90 = 55o and 90 = 35o, respectively. Lower curve shows background level.In addition, due to the small thickness of the substrate, the electron beam is scattered weakly, even at high angles of inclination of the target, and completely falls into the aperture of the Faraday cup, that eliminates the need of correction of the value of electron current measured, that associated with a change in the thickness of the target when the target is rotated.6. ConclusionThe presented results of the test experiment show the low yield of generated radiation. Probably, the reason of the essential difference between experimental and theoretical values of PXR yield results from interfacial roughness, mutual diffusion of layer materials and lack of flatness of the substrate because these factors can severely deteriorate efficiency of PXR generation, as it takes place concerning the decrease of reflectivity of real photons in X-ray optics due to imperfection of X-ray mirror. An improvement of the multilayer fabrication technique is necessary to minimize these radiator defects in order to create an effective source of radiation. Due to the thin (thickness -  0.5 цш) Si3N4 substrate of the multilayer structure, the contribution of background radiation generated in the target was less in about 3 times than that in experiments [9, 10] when a multilayer structures had the thick Si substrates. Additionally, the use of target with very thin substrates might be promising to increase the efficiency of this type of radiator when using cyclic accelerators because of possible multiple passes of electrons through them [16].It should be noted that the structure composed of Cr and Sc layers is capable to generate the Cherenkov radiation because the values of %cr/sc(®LCr) and/or %cr/sc(®LSc) may exceed 0 near the L absorption edges of Cr and Sc (see figure 2b). By means of optimizing of the ratio of the thicknesses6RREPS2015Journal of Physics: Conference Series 732 (2016) 012017IOP Publishingdoi:10.1088/1742-6596/732/1/012017of Cr and Sc layers, the effect o f diffracted Cherenkov radiation can be observed at the appropriate orientation of the structure with respect to an electron beam [17, 18].AcknowledgementsThe present research was partially supported by the Ministry o f Education and Science (state contract No. NIR MON 3761), and by the Russian Foundation for Basic Research (project No. 14-02-01032). The French team was supported by the Agence Nationale pour la Recherche, project No. ANR-10- BLAN-0924.References[1] Bazylev V A, Zhevago N K 1987 Radiation o f  fa st particles in matter and in external fields(Moscow: Nauka)[2] Andre J-M, Barchewitz R, Bonnelle C and Pardo B 1993 J. Opt. Paris 24 31[3] Andre J-M, Pardo В and Bonnellel C 1999 Phys. Rev. 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Cr/Sc multilayer radiator for parametric EUV radiation in "water-window" spectral range

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Cr/Sc multilayer radiator for parametric EUV radiation in "water-window" spectral range

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