Test of a crystalline undulator with positrons at BTF *University of Ferrara and INFN - Italy E. Bagli, L. Bandiera*, G. Cavoto, S. Dabagov, G. Germogli, V. Guidi*, A. Mazzolari First BTF Users Workshop –INFN LNF – May 6 th, 2014

Outlook Motivation Channeling in bent crystals Periodically bent crystal as CU for positrons and electrons CU fabrication methods at the SSL of Ferrara Conclusion

A CU can play the same role as a magnetic undulator in a conventional FEL. The motion of a projectile and the process of photon emission in a CU are very similar to that in a magnetic undulator. However, it can be built with submillimetric period (1-100 µm), which is two to five orders of magnitude smaller than the period of a conventional undulator, increasing the energy of radiated photons of some orders of magnitude more energetic than in a FEL with the same beam energy. An operating CU would provide highly monochromatic X-ray beam, with energies up to 1 MeV or higher. [1] A.V. Korol, A.V. Solov'yov, W. Greiner, Channeling and Radiation in Periodically Bent Crystals, Berlin, Heidelberg Springer 2013 Motivation

Crystal undulators can be realized with low cost materials, such as silicon or germanium and they have no functional costs, using the strong crystalline elastic force (~10 GeV/cm) to steer the particles.

Why BTF? Energy of the positron beam at BTF ( ~500 MeV) is ideal for the realization of a crystalline undulator for positrons (CUP) as expected from theory [1]. In parallel with the test of crystalline undulator at BTF, an investigation on the radiation emitted by positrons interacting with straight and bent crystals can be accomplished. This study could increase the knowledge regarding key parameters for realization of a working CU, e.g., the dechanneling length for positrons as a function of crystal curvature. [1] A.V. Korol, A.V. Solov'yov, W. Greiner, Channeling and Radiation in Periodically Bent Crystals, Berlin, Heidelberg Springer 2013

Channeling is the confinement of charged particles travelling through a crystal, by atomic planes or axes. LINDHARD (1964) Channeling occurs as the trajectory of particles forms an angle lower than the critical angle Positively-charged particles like protons and positrons are repulsed from the nuclei of the axis (plane) Negatively-charged particles like antiprotons and electrons are attracted towards the positively-charged nuclei of the axis (plane) Channeling

λ A channeled particle deviates from its initial direction by an angle equal to the bending angle of the crystal. Bent crystals can be used for extraction or collimation of particles from the circulating particle beam in an accelerator (UA9 experiment) Tsyganov (1976) Channeling in bent crystals

Periodically bent crystal (PBCr): channeling and undulator radiation One can distinguish 2 oscillatory motions, characterized by two different periods: The first one due to channeling. The channeling radiation is thereby different and more intense than bremsstrahlung in amorphous media; the second one can be ascribed to the PBCr. [1] A.V. Korol, A.V. Solov'yov, W. Greiner, Channeling and Radiation in Periodically Bent Crystals, Berlin, Heidelberg Springer Berlin Heidelberg 2013 The characteristic frequency of ChR and CUR are*: ω ch ~ 2γ 2 Ω ch and ω u ~ 2γ 2 Ω u If λu >>λch (or Ω u << Ω ch ), the characteristic frequencies are well separated. *if K<<1, undulator (dipole) regime of radiation and forward direction

Periodically bent crystal (PBCr): contribution of dechanneling Channeled negative paritcles are dechanneled faster than positrons due to higher probability to suffer nucler incoherent scattering; At beam energy ~ 500 MeV L d is some hundreds of µm for e + L d is about 10 µm for e - ; Length of the crystal should be of the order of L d to avoid a too big contribution of dechanneling, while contaning a large number of λ to have intense CUR production; A positron-based CU is easier to be accomplished. Particle dechanneling can be roughly represented by a exponential decay: n ch,0 is the fraction of channeled particles for z=0, L d, is called dechanneling length [1] A.V. Korol, A.V. Solov'yov, W. Greiner, Channeling and Radiation in Periodically Bent Crystals, Berlin, Heidelberg Springer 2013

The DAΦNE–BTF is able to deliver positron beams in the energy range of interest for the CUP. For example: ε = 500 MeV, a/d =10 (PBC amplitude) λu =23.35 µm << λch = 2µm L= 15 λu = 350 µm ~ Ld Si (110) planes ω u ~ 90 keV for CUR ω ch ~ MeV for ChR ChR CUR [1] A.V. Korol, A.V. Solov'yov, W. Greiner, Channeling and Radiation in Periodically Bent Crystals, Berlin, Heidelberg Springer 2013 CU for positrons (CUP)

CUP at DAΦNE-BTF Until now, the experiments with positron beams have not presented an evidence of CUR signal, mainly due to low quality of the beam and of the periodic crystalline structure. Recently, it was proposed to use DAΦNE BTF for CUP experiments [4]. Some limitations in the experimental setup were highlighted by the authors: 1.Too high angular divergence: 1 mrad vs. θc =0.2 mrad; 2.High background: not able to resolve channeling radiation peaks. If limitations 1 and 2 are overcome, BTF will be a perfect facility for the test of feasibility of a CU IHEP: [1] A.G. Afonin, V.T. Baranov, S. Bellucci, NIMB 234, (2005) [2] V.T. Baranov, S. Bellucci, V.M. Biryukov et al, JETP Lett. 82, (2005) [3] V.T. Baranov, S. Bellucci, V.M. Biryukov et al., NIM 252, (2006) [4] L. Quinteri et al., Positron channeling at the Da ΦNE BTF facility: the CUP experiment, Proceedings of the Channeling 2008, pp World Scientific, Singapore/Hackensack (2010).

Investigation of CU for electrons: dechanneling length A CU for electrons is much more desirable because there are many facility with electrons. The experimental and theoretical difficulties for design of a CU for electrons are in the lack of knowledge about the dechanneling length. A serius investigation about L d for electrons has been started only few years ago [1]. Measurements were done with a sub-GeV electron beam at MAMI accelerator. The L d was extrapolated from the emitted radiation spectrum resulting to be about 35 µm for (110) Si planes. Recently the group of INFN-IceRad experiment demonstrated the possibility to deflect a sub-GeV (855 MeV) electron beam with the usage of a bent crystal at MAMI [2]. The value of L d was measured directly from the deflected beam distribution, resulting to be about 20 µm for (111) Si bent (R=30.5mm) planes. [1] W. Lauth et al, Int. J. Mod. Phys. A 25, 136 (2010) [2] A. Mazzolari, E. Bagli, L. Bandiera, V. Guidi et al., Phys. Rev. Lett. 112, (2014) Further studies on coherent interactions of electrons with straight, bent and PB crystals would strengthen the scientific and strategic role of BTF

CU fabrication Some methods of fabrication*: Acustic Wave; Graded Ge x Si 1−x ; Laser ablation; Superficial grooving**; Deposition of amorphous or crystalline layers**; Ion implantation**. *A proposal for study and realization of a CU was submitted to the PRIN MIUR call ** All the presented methods are studied, used and improved by INFN-LOGOS experiment for realization of curved crystals for X-ray focusing through Laue lenses

Micromachining of silicon Hard x-ray optics Concentrated photovoltaics Gas sensing via chemoresistive metal oxides Crystal Fabrication at Sensor & Semiconductors lab of Ferrara

Grooves Crystalline material: silicon or germanium CU fabrication: superficial grooving method SGM

Amorphous material Groove crystalline material compression CU fabrication: SGM Surface grooving produces permanent plastic deformation in the neighborhood of the grooves. Such plasticized layer transfers coactive forces to the crystal bulk, thus producing an elastic strain field within the crystal.

CU fabrication: SGM Plasticization of the surface of the plates proved to induce a net and uniform curvature within a crystal which resulted in a wavy profile that propagates deep into the bulk [1]. [1] Bellucci, V., Camattari, R., Guidi, V., Neri, I. Exp. Astron. 31 (2011), [2] V. Guidi et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 40–46 [3] S. Bellucci et al., Phys. Rev. Lett. 90 (2003) An alternated pattern of parallel indentations performed on both surfaces may be of some interest for the realization of a millimetric or even a submillimetric undulator [2,3].

Sample fabrication z x y θ Dicing machine Commercially available pure Si wafers are diced to form a strip (e.g. 0.2X5X45mm 3 ) by using a high-precision dicing saw (DISCO TM DAD3220), equipped with a rotating diamond blade.

Optical profilometry: interferometric microscope vertical resolution = 1 nm lateral resolution = 1 μm which provide morphological characterization of crystalline samples Sample Characterization

CU fabrication: Deposition of Si3N4 layers a) Starting material: silicon wafer b) LPCVD deposition of silicon nitride thin layer, coating on both sides of the wafer (800°) g) Si3N4 etching in hot H3PO4 using the SiO2 as mask h) the residual stress in silicon nitride causes a bending to the desired curvature the silicon substrate, realizing the undulator c) deposition of a SiO2 film on the silicon nitride layer to realize the selective mask d) deposition of a photoresist polymer e) development of photoresist (photolitography) f) etching in HF of SiO2 V. Guidi et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 40–46

CU fabrication: Deposition of crystalline layer Deposition by LEPECVD of a crystalline layer of Ge over a Si plate would ensure better adhesion of the patterned structure to the substrate and a stronger stress, leading to a stronger curvature and/or a lower pitch for the CU; As a further advantage, this method enables either the deposition of a patterned structure of Ge onto a Si substrate and viceversa. This latter option is highly challenging because a crystal with higher atomic number should improve the confinement of particles under channeling. Crystalline layer

CU fabrication: Ion implantation IONS Omogeneus flux of Ions (He, Ar) impinges on the free surface of the wafer creating damages and so an amorphization of the crystalline structure. From swelling studies, a density reduction of 3.1% for the pure amorphous Si with respect to crystalline Si was found [1]. The grow of volume of the amorphized region transfers coactive forces to the crystal bulk, thus producing an elastic strain field within the crystal, which results in a net curvature within the crystal bulk. [1] P. K. Giri at al., Phis. Rev. B 65, (2001)

Comparing methods SGM is simplier and cheaper than other methods, but can reach λ u not much smaller than 20 µm, which in any case is good for CUP at BTF; Deposition and Ion implantation can reach λ u of about 1-2 µm which may be good also for CU based on electrons at BTF.

Conclusion The possibility to test a CU for positron at BTF has been presented; Three different methods for design and realization of a functional CU were listed and discussed; The CU fabrication methods availabe at SSL can all be used to fabricate a CU suited for the BTF positron beam. The beam line should have a fraction of mrad divergence to test the undulator under ideal conditions and to study and verify the feasibility of this new radiation source.

Thank you for attention!

Feasibility of a CU based on a periodically bent crystal

Introducing the critical radius of curvature, Rc, above which canneling is still possible, Bent vs. straight crystals (110) Si planes interplanar potential energy for positive particles the critcal angle in a bent crystal is reduced as follow: centrifugal term [1] V.M. Biryukov, Y.A. Chesnokov Crystal Channeling and Its Application at High-Energy Accelerators, Springer-Verlag Berlin Heidelberg (1997).

Planar Channeling

BENT vs STRAIGHT CRYSTALS (a) Straight Crystal (b) Bent Crystal Volume reflection was predicted by Taratin and Vorobiov In 1988

1.Unperturbed beam 2.Channeling 3.Dechanneling 4.Volume reflection 5.Volume capture 6.Unperturbed beam  cry

Investigation of CU for electrons: dechanneling length Θinc ~ 0 µrad Deflection = 910±5 µrad Fraction of channeled particles is 20.1±1.2% (±3σ around channeling peak) Deflected beam distribution for channeling CH DCH L d ~ 19.5 μm Estimated dechanneling length

CU fabrication: Graded SiGE A graded Si1-xGex crystal was tested vs. 400 GeV/c protons at H8-SPS beamline at CERN to probe its capability to steer high-energy particles [2] The crystal had the shape of a parallelepiped though its (111) atomic planes were curved at a radius of 25.6 m because of the graded Ge content. Measured deflection efficiency was 62% under planar channeling and 96% under volume reflection. [2] E. Bagli et al., Phys. Rev. Lett. 110, (2013) Arhus

Ion implantation dose