In connection with ideas to produce undulator-like radiation in the hundreds of keV up to the MeV region by means of positron and electron channeling, there is renewed interest to study various channeling phenomena also experimentally. With electrons experiments have been performed at the Mainz Microtron MAMI to explore channeling-radiation emission by a 4-period epitaxially grown strained

layer Si1−xGex undulator with a period length of λu = 9.9 μm. Unfortunately, high-quality positron beams of sufficient intensity are not easily accessible. The only serious candidate in Europe seems to be the Beam Test Facility (BTF) at INFN/LNF, Frascati, Italy. Some requirements to extent BTF in a facility which is also well suited for positron channeling-radiation experiments will be outlined

Azadegan, B.

Backe, H.

Buonomo, B.

Dabagov, S. B.

Dizdar, A.

Krambrich, D.

Lauth, W.

Lundsgaard Hansen, J.

Mazzitelli, G.

Quintieri, L.

Uggerhøj, U. I.

Wagner, W.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10983-9Colloquia: Channeling 2010IL NUOVO CIMENTO Vol. 34 C, N. 4 Luglio-Agosto 2011Future aspects of X-ray emission from crystal undulatorsat channeling of positronsH. Backe(1)(∗), D. Krambrich(1), W. Lauth(1), B. Buonomo(2),S. B. Dabagov(2)(3), G. Mazzitelli(2), L. Quintieri(2),J. Lundsgaard Hansen(4), U. I. Uggerhøj(4), B. Azadegan(5),A. Dizdar(6) and W. Wagner(7)(1) Institute of Nuclear Physics, J. Gutenberg-University Mainz - D-55099 Mainz, Germany(2) INFN - Laboratori Nazionali di Frascati - Via E. Fermi 40, F000U Frascati (RM), Italy(3) RAS - P.N. Lebedev Physical Institute - 119991 Moscow, Russia(4) Department of Physics and Astronomy, University of AarhusDK-8000 Aarhus C, Denmark(5) Sabzevar Tarbiat Moallem University - Sabzevar, Iran(6) Department of Physics, Istanbul University - Istanbul, Turkey(7) Helmholtz-Zentrum Dresden-Rossendorf - Postfach 51 01 19, D-01314 Dresden, Germany(ricevuto il 22 Dicembre 2010; pubblicato online il 22 Settembre 2011)Summary. — In connection with ideas to produce undulator-like radiation in thehundreds of keV up to the MeV region by means of positron and electron channeling,there is renewed interest to study various channeling phenomena also experimentally.With electrons experiments have been performed at the Mainz Microtron MAMIto explore channeling-radiation emission by a 4-period epitaxially grown strainedlayer Si1−xGex undulator with a period length of λu = 9.9μm. Unfortunately,high-quality positron beams of sufficient intensity are not easily accessible. Theonly serious candidate in Europe seems to be the Beam Test Facility (BTF) atINFN/LNF, Frascati, Italy. Some requirements to extent BTF in a facility which isalso well suited for positron channeling-radiation experiments will be outlined.PACS 41.60.-m – Radiation by moving charges.PACS 61.85.+p – Channeling phenomena (blocking, energy loss, etc.).PACS 87.56.bd – Accelerators.1. – IntroductionUndulator-like radiation in the hundreds of keV up to the MeV region can be pro-duced by means of positron channeling as discussed in a number of papers, see e.g. [1-4].However, one obstacle for the success of radiation production by this means lies in the(∗) E-mail: backe@kph.uni-mainz.dec© Societa` Italiana di Fisica 175176 H. BACKE, D. KRAMBRICH, W. LAUTH, ETC.fact that the fabrication of suitable crystalline undulators is an experimental challenge.Our approach to realize them is based on the method of epitaxially growing graded com-position Si1−xGex strained layer superlattices [5]. In a preceding paper of this issue [6]promising experiments with such an undulator crystal have been described employing270 MeV and 855 MeV electrons from the Mainz Microtron MAMI. The results encour-aged us to propose experimental investigations with 500 MeV positron beam of goodemittance as well to be developed at the Beam Test Facility BTF of INFN/LNF Fras-cati, Italy. In the following some basic considerations of such an experiment are outlined.2. – Advantages of positronsIt has been shown in ref. [6] that an undulator crystal has been prepared at MBEAarhus which exhibits some of the expected features in experiments with electrons. How-ever, the absence of the expected photon distribution in combination with the suppressionof the channeling radiation for K parameters in the order of 0.4 for the experiment at855MeV raises the question whether electrons are as well suited as positrons to produceundulator radiation [7, 8]. To discuss this question we refer to fig. 1(a) where the sumof the centrifugal potential Uc(x, z′) = pvk2uA cos(kux) · z′ at maximum and the crystalpotential U(z′) are depicted, for details see ref. [6]. The coordinate x coincides with thebeam direction, z′ with the normal of the (110) planes in which the electrons channel. Forthe situation shown, the barrier height is reduced from Um = 22.6 eV to about 12.6 eV,i.e. by nearly a factor of two. This fact already enhances the dechanneling probabilityconsiderably. But even worse, a still channeled electron resides rather close to an atomicplane, indicated by the arrow in fig. 1(a). Here it experiences a much stronger transverseenergy increase than a weakly bound electron in a flat crystal at E⊥/Um ≈ 1. As aconsequence, a dramatically enhanced dechanneling probability is expected. In otherwords, deeply bound electrons experience a rather strong transverse energy heating andrapidly boil off from the potential pocket at the curved parts of the undulator.A completely different scenario is observed for positrons. Of course, the barrier re-duction is similar as for electrons with the difference that the potential shown in fig. 1(a)must be reflected at the horizontal z′/dp axis, see fig. 1(b). Also in this case positrons arelost due to the reduction of the barrier height. In addition, the local potential minimumshifts to 0.3 dp and the oscillating positrons come at one of their turning points rather6080'020400,z'/#eV'40600,z'/#eV'(a) (b)4020/Uc+x 020020/Uc+x 01.51.00.5 0.0 0.5 1.0 1.58060U+z'/1.51.00.5 0.0 0.5 1.0 1.56040U+z'/z'sdpz'sdpFig. 1. – Sum of the maximum centrifugal potential Uc(x = 0, z′) = Ee ·A · k2u · dp · (z′/dp) andcrystal potential U(z′) at pv  Ee = 855MeV for the (110) plane; A = 4.0 A˚, λU = 9.9μm, anddp = 1.92 A˚, the distance between (110) planes, for electrons (a), and for positrons (b).FUTURE ASPECTS OF X-RAY EMISSION FROM CRYSTAL UNDULATORS ETC. 177close to the thermally vibrating atoms comprising the atomic planes. This effect as wellas the rather small barrier width close to the local maximum must be taken into ac-count in a more detailed investigation. Nevertheless, at least part of the bound positronsexperience a more than two orders of magnitude reduced transverse energy heating aselectrons. This fact can be concluded from fig. 2 of ref. [9]. Therefore, the dechannelinglength as obtained for planar crystals is expected to be maintained also in the undulatorcrystal for a certain fraction of channeled positrons.3. – Experiment proposal at the DAΦNE Beam Test Facility BTFIn the following we discuss a possible experiment with positrons at BTF at INFN/LNFFrascati, Italy. Positrons which are normally released for the DAΦNE damping ring fromthe end of a high-intensity LINAC can also be diverted into a 100m2 BTF hall. Accordingto refs. [10, 11], a positron beam with an energy of 500MeV, an emittance of 5–10mmmrad and up to 1010 positrons per pulse can be provided at a repetition rate of up to49Hz. The beam emittance can be reduced on the expense of the beam intensity bymeans of slit systems.For the calculations it has been assumed that a positron beam can be prepared inthe BTF hall, e.g. by collimation accepting much smaller pulse intensities than 1010positrons, with beam emittances εh = εv = 0.1mmmrad in the horizontal and verticaldirections, respectively. At beam spot sizes with standard deviations σh = σv = 1.5mmthe angular spreads are σ′h = σ′v = 0.067mrad which are, according to eq. (A.2) of ref. [6],small in comparison with the critical angle ψcrit = 0.30mrad. It is also worthwhile tocompare these numbers with the rms values for multiple scattering through small anglesin amorphous matter which amounts for a 40μm thick silicon layer to θ0 = 0.39mrad [12].This angle is somewhat larger than the critical angle ψcrit. However, due to the reducedinteraction of the positron moving in the channel with crystal lattice atoms in comparisonwith amorphous matter, no problems because of this fact are expected.4. – The undulator radiation spectrumFor the undulator described in ref. [6], fig. 1, and a positron beam energy of 500MeVthe undulator parameter is K = 0.246, and the undulator radiation which peaks at233 keV would have a width of 52 keV. If line broadening effects as quoted in the captionto fig. 2(a) are taken into account properly, the line broadens to a width of 70.5 keV. Infig. 2(b) a calculation on the basis of the classical undulator theory [13] is presented inwhich line broadening effects due to dechanneling, coherence loss by phase fluctuations,and a damping of the amplitude have been omitted. However, in contrast to fig. 2(a),the angular spread of the beam, the beam spot size, and the finite detector aperture havebeen taken into account. The peak shifts to 210 keV and the line broadens from 53 keVfor a point-like source and a point-like detector to 72.4 keV. This broadening is about thesame as that for fig. 2(a). If all line broadening effects are taken into account, a properquadratic subtraction and summation results in a total line width of 87 keV which issmall enough for the envisaged experiment. Notice that a finite number of periods isinvoked in both distributions of fig. 2 and must not be taken into account twice.5. – Radiation backgroundTwo radiation background components will be discussed in the following. One is theunavoidable bremsstrahlung background from the undulator target itself, the other is the178 H. BACKE, D. KRAMBRICH, W. LAUTH, ETC.5MeV/'2530(a) (b)234103s+eM15201sMeV'0.0705 MeV 0.0724 MeV0 0 0 1 0 2 0 3 0 4 0 501dNsd³Z#10 0 0 1 0 2 0 3 0 4 0 50510S#. . . . . .³Z #MeV'd. . . . . .³Z #MeV'Fig. 2. – (a) Calculated radiation spectrum from a 4-period undulator with a period lengthof 9.9μm and amplitude of 4.0 A˚ at a positron beam energy of 500MeV. Assumed were adechanneling length dech = 126μm, a coherence length due to phase fluctuations caused bymultiple scattering phase = 106μm, and a relaxation of the undulation amplitude in crystalundulator relax = 40μm. (b) Expected spectral distribution as calculated on the basis of theclassical undulator theory [13]. Beam spot size and divergence at the target position: σh =σv = 1.5mm, σ′h = σ′v = 0.067mrad; beam emittance: εh = εv = 0.1mmmrad; distance targetto detector: 6m, diameter of aperture in front of detector: 4mm.background produced in the beam transport system and the beam dump.For the former background component the bremsstrahlung contribution has beencalculated under the same conditions as for the spectrum shown in fig. 2(b). The resultis that a peak-to-background ratio of about 20 can be expected. However, this numberwill be somehow reduced due to the fact that not all positrons channel. Assuming thatthe fraction of channeled positrons is 20%, the peak-to-background ratio reduces to 4which still is a sufficient large number for the proposed experiment.For the latter background component it has been assumed that the positron beam isdeflected with a dipole magnet, which has been positioned directly behind the undulatortarget, by 12.5◦ horizontally. Detector and beam dump are situated at a distance of6m from the target. Simulation calculations have been performed with the computercode GEANT4. Figure 3 shows that the bremsstrahlung background can be reduced bya large factor if the experiment is performed under vacuum conditions. The remainingcount rate mainly originates from bremsstrahlung production in the target itself. Thebackground from gamma rays and neutron production in the beam dump turns out tobe negligibly small.6. – Count rate estimateThe integrated count rate seen by a 2′′ × 2′′ NaI detector at 6m distance from the39.6μm thick silicon target, without any aperture in front of the detector, amounts to3.64 · 10−3/positron, see fig. 3 (inset). Production of channeling radiation may increasethis number by a factor of 2. With an aperture of 4mm diameter at 6m distance in frontof the detector the count rate will be reduced by a factor of about 7. To avoid pileup,the detector should not count more than 0.2 events in a single pulse, meaning that thenumber of positrons at the target must not exceed 200/pulse. Since the preparation ofthe beam emittance of εh = εv = 0.1mmmrad, causes a reduction of the beam intensityby a factor of about 103–104, a beam intensity of 2 ·105–2 ·106/pulse is needed which caneasily be provided. At 50Hz repetition rate of the LINAC the intensity in front of theFUTURE ASPECTS OF X-RAY EMISSION FROM CRYSTAL UNDULATORS ETC. 179 [keV]ωh0 100 200 300 400 500 600 700 800 900 1000 keV)+/(e-6   10ωhdN/d020406080100 [MeV]ωh0 20 40 60 80 100 120 140 160 180 200 MeV)+/(e-6   10ωhdN/d110210310410+/e-3Integrated count rate: 3.64x10Fig. 3. – (Colour on-line) Results of GEANT4 simulations. Silicon target thickness 39.6μm,distance target to a 2′′ × 2′′ NaI detector 6m, deflection angle of the positron beam 12.5◦. Theupper black histogram corresponds to an experiment in air, the lower ones to experiments undervacuum conditions under different assumptions: dedicated beam dump (blue), without radiator(red). The inset shows the spectra up to a photon energy of 200MeV in a semi-logarithmicrepresentation.slit and aperture system must be in the order of 107–108 positrons/s. With an integratedcount rate of about 3.5 · 10−4/positron for the undulator radiation peak, as estimatedfrom fig. 2(b), and an assumed channeling probability of 0.2, a total number of about1.5 · 108 positrons on target would be sufficient to collect 104 events in the undulatorpeak. With 200 positrons/pulse and a LINAC repetition rate of 49Hz this statistics iscollected in 1.5 · 104 s or about 4 hours beam on target.7. – DetectorsFor the observation of the undulator radiation peak at 210 keV, a NaI detector seemsto be appropriate. Its energy resolution of about 8%, corresponding to about 17 keV,is sufficient. At such a resolution a broadening of the expected peak with a width of87 keV can be neglected. The time resolution should be as good as possible, i.e., of theorder of 1 ns, in order to suppress background which is expected to originate from theslit and collimator system in which more than 99.9% of the positrons must be discarded.The suppression can hopefully be maintained by means of the time-of-flight differencesbetween undulator-target events and the background events. In addition, a good detectorshielding is required.8. – DiscussionIt has been shown that the Beam Test Facility BTF, despite the low repetition rateof the LINAC of 50Hz, is well suited for undulator-radiation experiments at channelingof positrons. However, a number of modifications must be performed. The main onesare 1) removal of the conversion target, 2) replacement of the 500μm thick berylliumfoil, which separates the ultrahigh-vacuum system of DAΦNE and the LINAC from thehigh-vacuum system of BTF, by a thinner foil to avoid deterioration of the beam emit-tance, 3) transportation of the positron beam under high vacuum to avoid bremsstrahlung180 H. BACKE, D. KRAMBRICH, W. LAUTH, ETC.background from air, 4) construction and installation of a goniometer vacuum chamber,and 5) installation of several in-vacuum beam monitor detectors.It should be mentioned that a similar facility was in operation about 25 years agoat the Lawrence Livermore National Laboratory Electron-Positron Linear Accelerator,USA [14].9. – ConclusionsAs shown in ref. [6], undulator-like radiation has been observed employing an epitaxi-ally grown Si1−xGex 4-period λU = 9.9μm undulator and an electron beam with energiesof 270MeV and 855MeV. A corresponding experiment with positrons can presently notbe performed because a suitable machine in Europe is missing. Nevertheless, detailedexperimental studies also with high-quality positron beams are absolutely imperativeto judge wether radiation production in the hard-X-ray region by means of crystal un-dulators is a viable concept, or not. It has been shown in this paper that the BeamTest Facility BTF at INFN Frascati, Italy, would be a good candidate if a number ofmodifications are performed.∗ ∗ ∗This work has been supported by the Transnational Access to the Research Infrastruc-ture (TARI) at LNF: Contract Number 227431, Project N. 16 “Crystals Undulator forElectrons and Positrons”, the Istanbul University Scientific Research Projects (IU BAP9890), and the Deutsche Forschungsgemeinschaft DFG under contract BA 1336/2-1.REFERENCES[1] Dedkov G. V., Phys. Status Solidi B, 184 (1994) 535.[2] Korol A. V., Solov’yov A. V. and Greiner W., Int. J. Mod. Phys. E, 8 (1999) 48.[3] Korol A. V., Solov’yov A. V. and Greiner W., Int. J. Mod. Phys. E, 13 (2004) 867.[4] Korol A. V., Solov’yov A. V. and Greiner W., Proc. SPIE-Int. Soc. Opt. Eng., 5974(2005) 597405.[5] Mikkelsen U. and Uggerhøj E., Nucl. Instrum. Methods Phys. Res. A, 483 (2002) 455.[6] Backe H., Krambrich D., Lauth W., Hansen J. Lundsgaard and Uggerhøj UlrikI., X-Ray Emission from a Crystal Undulator - Experimental Results at Channeling ofElectrons, this Proceedings.[7] Tabrizi M., Korol A. V., Solov’yov A. V. and Greiner W., Phys. Rev. Lett., 98(2007) 164801.[8] Tabrizi M., Korol A. V., Solov’yov A. V. and Greiner W., Phys. G: Nucl. Part.Phys., 34 (2007) 1581.[9] Beloshitsky V. V. and Trikalinos Ch. G., Radiat. Eff., 56 (1981) 71.[10] Mazzitelli G., Bedogni R., Buonomo B., Esposito A., Gentile A., De Giorgi M.,Quintieri L., Borla O., Giannini G. and Go´mez-Ros Jose´ M., Neutron Source at theDAΦNE Beam Test Facility, in Proceedings of IPAC’10, Kyoto, Japan, 2010, pp. 415-417(MOPEB063).[11] Buonomo B., Mazzitelli G., Murtas F., Quintieri L. and Valente P., A wide rangeelectrons, photons, neutrons beam facility, in Proceedings of EPAC’08, Genoa, Italy, 2008,pp. 3321-3323 (THPC143).[12] Amsler C. et al., Phys. Lett. B, 667 (2008) 1.[13] Dattoli G., Giannessi L., Mezi L., Richetta M. and Torre A., Phys. Rev. A, 45(1992) 4023.[14] Klein R. K., Kephart J. O., Pantell R. H., Park H., Berman B. L., Swent R. L.,Datz S. and Fearick R. W., Phys. Rev. B, 31 (1985) 68.

Future aspects of X-ray emission from crystal undulators at channeling of positrons

http://eprints.bice.rm.cnr.it/17330/1/ncc10006.pdf

Future aspects of X-ray emission from crystal undulators at channeling of positrons

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository