358 Th. Krist et al. can be seen that are nearly parallel to the surface. We characterized the crys- tal orientation by the ratio of the area under the respective peaks. From the change in the position of the peaks related to the bulk value, we determined the lattice distortion. As far as we know, no publication in the literature has dealt so far with XRD measurements on Ni/Ti supermirrors. In all samples, both Ni and Ti show polycrystalline structure. Generally four maxima occur in the spectra, which can be identified as face-centered cubic (fcc) Ni lattice (Nifcc(200), Nifcc(111)) and hexagonal close-packed (hcp) Ti lattice (Tihcp(011), Tihcp(002)). For the best quality mirror – prepared on a smooth substrate – the XRD spectra only show one Ni and one Ti peak. This means that both materials crystallize with a preferred orientation with the Ni(200) planes and Ti(011) planes parallel to the substrate surface. The medium quality mirror was prepared on a similarly smooth substrate; however, the quality is not the same. Again a similar orientation is preferred, but not to the same extent, even though Ni(111) and Ti(002) reflections appear. The worst mirror was prepared on a quite rough substrate. We can see that all four peaks appear with no preferred orientation in this case. Summarizing we can say that there are two ways of orientation: Ni(200) is connected to Ti(011), and Ni(111) to Ti(002) [5]. On a rough substrate both orientations are present and the reflectivity is low (R ∼ 50%). In this case σ ∼ 1.2nm, where σ is the rms microroughness measured by X-ray reflectivity. On a smooth substrate (σ ∼ 0.4 nm) the Ni(200)/Ti(011) orientation is preferred. The more this orientation is preferred, the better is the quality of the mirror (reflectivity R>80%), as can be seen in Fig. 22.2. In Fig. 22.3 one can see the extent of orientation depending on the reflec- tivity of supermirrors with m = 3. Circles represent the ratio of the area under the maxima Ni(200) and Ni(111). Triangles are for the ratio of the area under the maxima Ti(011) and Ti(002). Where no triangles are shown only the Ti(011) peak was present. Thus, we can find a relation between the quality of the mirror and the crystal orientation. To explain this correspondence TEM pictures were taken (Fig. 22.4). In the good quality mirror there are smooth parallel layers. In the second, less good mirror the layers are not parallel at some parts. The size of the deflection is about 1,000 A. The crystalline orientation is likely to be the same in both cases, but due to the deflection of the layers, the layers are not parallel to the surface and the reflections from the other lattice planes appear as well in the XRD spectra. The deflections start from the substrate, and grow almost straight upward. Their origin might be some inhomogeneity of the substrate surface on a mesoscale, which does not change the observed roughness. The Ni(200) lattice spacing is in all cases larger by 0.001–0.003nm than in the bulk. In accordance with the literature, during sputtering in a reactive atmosphere, nickel crystals grow such that Ni(200) planes are parallel to the surface, because gas atoms can be incorporated easily into the lattice in that 22 Neutron Supermirror Development 359 35 40 45 50 55 R = 50 % substrate σ = 1.2 nm 2 theta R = 67% substrate σ = 0.35 nm R = 86% substrate σ = 0.30 nm Ti (011) Ti (002) Ni (200) Ni (111) Fig. 22.2. XRD spectra of Ni/Ti supermirrors 60 65 70 75 80 85 90 10 20 30 40 Ratio of areas under the peaks Ni(200)/Ni(111) Ti(011)/Ti(002) R (%) @ m=3 Fig. 22.3. Extent of crystalline orientation in relation to the reflectivity of the supermirrrors with m =3 orientation. The increased lattice spacing also shows this incorporation. More- over, the dilatation of 0.001–0.0015nm in the Ti(011) direction indicates the diffusion of gas atoms through the Ni/Ti interface. The Ni(111) peak position is the same as in the bulk. It is related to the presence of small pure Ni phases. 360 Th. Krist et al. Fig. 22.4. TEM picture taken on a high and a low quality Ni/Ti supermirror 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 20 40 60 80 100 Reflectivity (%) m as prepared 4.5 years later (a) Fig. 22.5. (a) Reflectivity curves of a Ni/Ti supermirror before and after extended storage. (b) Adherence check 22.2.3 Stability of Supermirrors Extended Storage We have performed tests on unused supermirrors 4.5 years after their pro- duction. The reflectivity was found to be the same as at the time they were produced. The adherence checked by strong tesa tapes also meets the requirements (Fig. 22.5). Stability Under Heat Load We studied the structural changes during heating by X-ray diffraction with a heatable vacuum chamber as sample holder, to be able to perform in situ measurements during heating under low pressure. We simulated the same 22 Neutron Supermirror Development 361 30 Ti(012) Ti(011) Ti(002) Ti(010) 2 theta (degree) Ni(200) Ni(111) 140 C 120 C 100 C RT 35 40 45 50 55 60 (a) 30 35 40 45 50 55 60 Ti(012) Ti(011) Ti(002) Ti(010) 2 theta (degree) Ni(200) Ni(111) 170 C 230 C 300 C 350 C (b) Fig. 22.6. (a, b) XRD spectra of a Ni/Ti supermirror taken in situ during heating over two temperature ranges 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 20 40 60 80 100 SM-1 (m=3) at RT after treatment at 135 C Reflectivity (%) scattering vector Q (A −1 ) 0.01 0.02 0.03 0.04 0.05 0.06 0.07 SM-2 (m=3) at RT after treatment at 135 C scattering vector Q (A −1 ) Fig. 22.7. Neutron reflectivity curves of two heated Ni/Ti supermirrors circumstances as in the real neutron guides, where there is a pressure of about 10 −4 bar. Figure 22.6a shows the change of the XRD spectrum up to 140 ◦ C. The only change is the sharpening and slight shifting of the Ni(200) maximum. Figure 22.6b shows the further changes up to 350 ◦ C. In this temperature region fundamental structural transformations occur. Based on these results we expect that up to about 140 ◦ C the supermirror structure will be stable, and its reflectivity does not change. Experiments on the changes of reflectivity have been made by heat treatment of supermir- rors at 100, 120, and 135 ◦ C for 50 min in a vacuum chamber The reflectivity does not change after the treatment at 100 and 120 ◦ C. However, after the 135 ◦ C treatment the critical scattering vector may increase and the reflectivity decreases with 2–3% above m =1.21 (Fig. 22.7). 362 Th. Krist et al. The detailed process of structural changing during heating is as follows: As described above in good mirrors at room temperature the polycrys- talline Ni and Ti layers show a preferred orientation of Ni fcc(200)/Tihcp(011) with some dilatation. When heating to 100 ◦ C, peak broadening and further dilatation can be observed, the extent of which is higher than can be explained by simple thermal expansion. This process is likely to be due to gas atoms bound in the Ni layers during the sputtering in reactive atmosphere. They start to diffuse at the interfaces into the very good getter Ti. Up to 230 ◦ C, Ti gradually becomes amorphous and/or forms an amorphous compound. At the same time at 100 ◦ C the Ni(200) peak sharpens and shifts unex- pectedly toward higher angles. That is, the lattice spacing decreases even below the bulk value and the crystallite size seems to increase. The reason for this change is not yet clear: it may be ascribed to a rearrangement of the Ni lattice due to the diffusion of the gas atoms. By further heating, a slight thermal expansion occurs. At 140 ◦ C the Ni(111) peak arises. Above 170 ◦ C, Ni gradually transforms to a yet unidentified compound characterized by a broad maximum at d ∼ 0.180nm, and partly to NiO. Stability Under Irradiation One of the currently used substrates for supermirrors in neutron guides is Borofloat 33 (13 wt% B 2 O 3 ) glass produced by the company Schott. This material has the advantages of low surface roughness due to the float technol- ogy and the absorption of neutrons coming through the multilayers, providing shielding for the guide system via the following reaction n+ 10 B ⇒ α + 7 Li + γ (1.47 MeV). However the effect of this process on the glass and the coating is not yet clear. It is possible that this process or the energy released may cause damage if the neutron dose is large enough. The question is how large is the onset of neutron dose damage that deter- mines the lifetime of a borofloat guide piece? Recently at the ILL, Grenoble it was found that at the first part of the out-of-pile guide the coating from a borofloat substrate pealed off after three years irradiation and at the same time the glass surface was destroyed. During this period the total incoming dose is estimated to be 3 × 10 16 ncm −2 . The other mirrors on normal float (no B content) or polished Borkron glass (prepared without float-technology) were found to be stable at the ILL. In Gatchina, and in Budapest, however, they found no damage at in-pile borofloat guides exposed to similar doses. This indicates an emerging need for the detailed examination of radiation damage of guide substrates and coatings. In the framework of COST action we have performed several irradiation tests in the reactor water in various neutron channels at the 10MW BNC reactor. The samples were packed in an Al capsule, and wrapped in Al foil to transmit the cooling effect of the surrounding water. 22 Neutron Supermirror Development 363 Fig. 22.8. Destruction of the Ni/Ti supermirror coating and the glass surface of borofloat glass after various irradiation doses larger than 1.5×10 17 ncm −2 in channel 11/2 Irradiation of uncoated glass substrates was performed in the 69/3 channel for 72 h. Borofloat and normal float glass alone were not damaged at a dose of 5×10 19 ncm −2 in reactor water. However, the color of borofloat glass changed to brownish. Irradiation of NiTi supermirror coatings on several glass substrates was performed in different channels of the reactor. The dose applied varied between 10 17 and 10 19 ncm −2 . In the channel 11/2 during the irradiation of borofloat glass with a supermirror coating, we found coating and the glass surface destruction and discoloration (Fig. 22.8). However, the noncoated side of the glass remained intact. Float glass with a supermirror coating remained intact in all cases. We have performed tests in another channel, channel 17, where a smaller fast neutron flux and less gamma radiation is expected. Here, as you can see in Fig. 22.9, even the coated side of the borofloat glass remained intact under a dose of 4 × 10 18 ncm −2 . Based on these results we can conclude that the borofloat glass without a metal layer is stable under irradiation. The destruction of coated borofloat glass under irradiation does not depend on the thermal flux of neutrons accord- ing our experience. The destroying factor is probably the thermal effect of high gamma radiation on the metal layer, which causes the glass surface destruction as well. By irradiating NiTi coatings on Si substrates, which absorb neutrons only to a small extent, we can investigate the stability of the supermirror coating itself. Moreover, in that case we have the opportunity to check the reflectivity as well. We have applied about 10 19 ncm −2 on coated Si wafers and measured the reflectivity curves before and after irradiation. We found that the reflec- tivity above m =1.8 is degraded by about 3%. The slope of the reflectivity curve above the maximum value, m =3.2, is somewhat less steep compared to the curve obtained before irradiation. 364 Th. Krist et al. Fig. 22.9. Ni/Ti supermirror coating on a borofloat glass surface after an irradiation dose larger than 4 × 10 18 ncm −2 in channel 17 22.2.4 Development of m = 4 Supermirror Technology Subsequent to the development of the m =3.65 Ni/Ti neutron supermirror, the task of realization of the considerably more difficult m = 4 multilayer system has been taken on. For this purpose the multilayer system to be sputtered has been extended, namely instead of the 900 layers for the m =3.65 mirror, a system of at least 1,600 layers is needed for reaching m = 4, with decreasing layer thickness. The layer system has been optimized by means of the code for reflectivity computation (REFLEX) [6]. To achieve high quality mirrors, appropriate substrate is needed. The sub- strate quality has been assessed by X-ray reflectometry. Finally the Schott Borofloat glass has been chosen (roughness <0.4 nm, lower density surface layer <1.5 nm), allowing excellent, reproducible m = 3 coating. At m = 4, a reflectivity of 72% has been obtained, the expected result after the 76% obtained for m =3.65 (see Fig. 22.1). Further experiments are planned using higher number of layers in order to improve the reflectivity. Today users (e.g., Spallation National Source, USA) require about 60% reflec- tivity for m = 4 supermirrors. Thus the quality of the produced supermirror exceeds the internationally expected quality level. 22.2.5 Increase of Homogeneity Over Large Substrate Sizes In some new neutron sources there is a need for using large cross-section guides. For that one has to produce supermirrors on substrates with a width 22 Neutron Supermirror Development 365 larger than the typical 50–100 mm, namely 200 or even 300 mm. We per- formed tests to determine whether the coating prepared on substrates with various widths is of the same quality. The length was in all cases the usual 500 mm. Substrates were coated in the same sputtering machine, under the same conditions and the neutron reflectivity curves were measured. We have found a larger critical angle for supermirrors deposited on larger substrates. On the basis of this change we can conclude that the integral thickness of the deposited layers is 3–4% smaller for substrates of 200 × 500 mm and 7–8% smaller for substrates of 300 × 500 mm, with respect to substrates of 50 × 500 mm size. The cause of this difference in the deposition process is not yet fully understood. Some electrical charging can be supposed which can be dependent on the substrate size. It was also concluded that this thickness variation of the layers does not influence the mirror quality (interface rough- ness, adherence) because after compensating for the difference in deposition rate we obtained the same reflectivity for each substrate size. 22.3 Polarizing Supermirrors 22.3.1 Neutron Polarization In a magnetic field the neutron energy has an additional energy term, the Zeeman term ±μB [7, 8]. The magnetic moment of the neutron, μ,hasthe value 61 neV T −1 and B is the magnetic field, which in Fe, for example, has a value of 2.2 T. The sign refers to the orientation of the neutron spin, which is either parallel or antiparallel to the magnetic field direction. The spin of a neutron is antiparallel to its magnetic moment. In ferromagnetic materials the Zeeman term has the same order of magnitude as the nuclear interaction. The refractive index, n, of a magnetic material for neutrons including nuclear and magnetic interactions is given by: n =1−λ 2 N(b ± p)/2π (22.2) with λ, the neutron wavelength and N the atomic density. The magnetic scattering length, p,isgivenby p =2μm n M/¯h 2 N (22.3) with m n , the neutron mass, M , the magnetization in the material and ¯h Planck’s constant divided by 2π. This refractive index gives rise to two critical angles for the total reflection for the two different spin components: sin Θ ± = λ √ (N(b ± p)/π (22.4) The product N(b ± p) is called scattering length density (SLD). Two quantities are used to characterize how well the spin components of a neutron beam have been separated. In terms of the number of neutrons in 366 Th. Krist et al. the two spin states, n + and n − ,thepolarization,P , and the flip ratio, f r ,are defined as: P =(n + − n − )/(n + + n − ) (22.5) f r = n + /n − . (22.6) The polarization of a sample is determined by using a neutron beam and a spin analyzer of known polarization. Methods of calibrating spin analyzers are discussed in [7]. 22.3.2 Neutron Polarizers Nowadays mainly three methods are used to polarize neutrons: by the use of Heusler alloys, by 3 He spin filters and by polarizing supermirrors. Heusler alloys like Cu 2 MnAl can simultaneously monochromatize and polarize a neu- tron beam [9]. The cross section for Bragg reflection for a magnetic field perpendicular to the scattering plane is given by the square of the sum of the nuclear and the magnetic atomic structure factor. If both have the same value, a high polarization can be achieved. In practice polarization values of 95% for reflected intensities of 90% can be achieved. Heusler alloys are expensive and not easily available on the market. They are mostly used for neutrons with wavelengths below 0.2 nm. 3 He spin filters exploit the spin-dependent absorption cross section of 3 He atoms for neutrons [10]. The cross section amounts at a neutron wavelength of 0.18 nm to 5,333 barn for antiparallel and to 5 barn for parallel spins. The 3 He atoms are kept in a cell with specially prepared walls to reduce polarization losses during wall reflections and are polarized either by spin exchange or by metastable optical pumping. The polarization efficiency, P , for neutrons depends on the polarization, P He ,ofthe 3 He atoms and the so-called opacity, O, of the gas: P =tan h(P He O), (22.7) with O = p bar −1 l cm −1 λ ˚ A −1 , (22.8) with p, the helium pressure in the cell, and l, the flight path of the neutron in the 3 He gas. The transmission of neutrons through the gas is given by T =cos h(P He O)T 0 exp(−O), (22.9) with T 0 , the absorption of the cell. Thus, the degree of neutron polarization can be chosen at the expense of the transmitted intensity. A polarization of 90% at a transmission of 30% of the incoming unpolarized beam is presently a reasonable compro- mise between maximum transmission and maximum polarization and can be reliably reached. 22 Neutron Supermirror Development 367 3 He spin filters need an expensive infrastructure and permanent mainte- nance and the technology is still strongly improving. Magnetic field gradients larger than 10 −4 reduce their polarizing efficiency. Their advantages are the absence of any small angle scattering and any sensitivity to the angles under which neutrons pass the filter. They enable a very high degree of polarization to be achieved if a corresponding reduction of the transmitted intensity is acceptable. Supermirrors in general were introduced above. Polarizing supermirrors exploit the fact that ferromagnetic materials have two strongly different scat- tering length densities (SLD) for the two spin components. After choosing two materials which exhibit the same SLD for one spin component, the supermir- ror sequence is calculated from the contrast of the two materials for the other spin component. Such a system reflects only the second spin component and transmits the first one. Polarizations up to 98% can be reached for intensities of 30–40% of the nonpolarized beam. Historically the first mirrors were made from the material pairs Fe−Ag [2] and Co−Ti [11]. Nowadays two groups of combinations are used: Fe-SiN x [12] and Fe 89 Co 11 −Si [6] or Co−Ti, FeCo−TiZr [13], and Fe 50 Co 48 V 2 −TiN x [14]. The materials in the first group have an SLD of the spin-down component close to the SLD of Si. They are used for solid-state devices where the neutrons travel inside thin Si wafers and one spin state is reflected from the supermirror coating at the walls of the wafers. The other spin component is not reflected by the supermirror since there is no or only a very small contrast to Si. The materials in the second group have an SLD close to or slightly below zero for the spin-down component. In this case no reflection of the spin-down neutrons occurs from the supermirror if the neutrons hit the supermirror in air. However, there are only two kinds of substrates which have the required small surface roughness and are available at reasonable prices for areas in the m 2 range: glass and Si wafers. From these substrates the spin-down compo- nent is reflected up to their critical angle. This amounts to m =0.5 for Si and m =0.6 for glass. To maximize the angular and wavelength range where neu- trons are reflected with good polarization, an antireflecting layer is introduced between the supermirror and the substrate, this antireflecting layer absorbing the neutrons before they reach the substrate. Such layers are made from Gd or Gd alloys or multilayers of Gd and Ti [15]. Polarizing supermirrors are not sensitive to magnetic fields and the tech- nology is quite mature. However, they show some small angle scattering, in some cases only if used in small magnetic fields, and they work only in an angular range on the order of 1 ◦ . They are most useful to polarize neutrons for wavelengths above 0.2 nm with a small angular divergence. 22.3.3 Increase of the Critical Angle In order to increase the available angular range of polarizing neutron supermir- rors and to facilitate the construction of polarizing devices, the critical angle [...]... sputtering machines This was achieved by understanding the effects previously limiting the layer numbers to a few hundred, and finding ways to reduce them by improved sputtering processes In the near future, a further increase of the critical angle can be expected In the case of nonpolarizing supermirrors this will increase the divergence and hence the flux of neutrons transmitted through neutron guides and. .. stress in a multilayer results from the stress of each layer in the multilayer stack and from the interfacial stress within the structure Fig 23.1 Stress variation vs argon pressure in single layer W (dashed line), single layer Si (dotted line), and bilayer Si/W (solid line) [5] 374 Th Krist et al In the case of a periodic binary multilayer consisting of alternatively deposited materials, h and l,... 120 and 250 μm 23 Stress Reduction in Multilayers Used for X-Ray and Neutron Optics 377 After the sputtering the multilayers were characterized by X-ray reflection with a wavelength of 0.154 nm and neutron reflection with a wavelength of 0.47 nm In the case of X-rays, fits to the data using the program Parrat [16] enabled us to determine average values for the thickness and roughness of the two individual... Reduction in Multilayers Used for X-Ray and Neutron Optics 383 divided by spin down intensity, is about 30% higher for the supermirror grown with 35 V It can be concluded that, within the parameter range explored in this study, the variation in substrate bias in uences the growth conditions in a way which only changes the stress and not the reflectivity In conclusion, it can be said that an increase in layer... are studied and compared in terms of the interface quality, structure and thermal stability Specular/non-specular X-ray reflectometry, transmission electron microscopy and X-ray and electron diffraction were employed to obtain an insight into the interface phenomena UHV e-beam deposition with optimized in situ substrate heating was tested successfully as a simpler and cheaper alternative to in situ ion... substrate showing the reflectivity of both spin components of neutrons with a wavelength of 0.47 nm together with the polarization and the flip ratio second mirror 80 (polarization 97.5%) in the interval from m = 1 to 2 and 50 in the interval from m = 2 to 3.3 In conclusion it can be said that in the past 5 years the critical angle of polarizing and nonpolarizing supermirrors has been increased considerably,... are generally preferred in ML mirrors since polycrystalline grains may lead to rough interfaces Another adverse effect of polycrystallinity is grain boundary diffusion which is usually very fast and may result in discontinuous layers Grain boundary diffusion can be avoided and atomically flat interfaces achieved in epitaxial superlattices which were tested for mirror applications In particular, Al-based... was used to determine crystallinity and – by use of the Scherrer formula – the grain size of the FeCo layers Figure 23.7 shows that Fig 23.6 Stress values for FeCo−Si monochromators with varying thickness of the FeCo layer together with a linear fit Fig 23.7 Grain size in the FeCo layers depending on the layer thickness 23 Stress Reduction in Multilayers Used for X-Ray and Neutron Optics 379 Fig 23.8... linearly with increasing Γ The zero 23 Stress Reduction in Multilayers Used for X-Ray and Neutron Optics 385 Fig 23.13 Stress values of different Mo fractions (Γ ) for e-beam deposition (FOM) and magnetron sputtering by Windt et al [1] and Mirkarimi et al [2] The black circles have been produced with e-beam and ion treatment of the Si layers, the gray circle by e-beam and ion treatment of the Si and Mo layers,... optimization of fabrication conditions resulting in high-performance interference mirrors 392 M Jergel et al 24.2 Sample Choice and Preparation Because of lower absorption far above the K absorption edges, Cu and Ni are promising refractory metals to replace W or Pt in ultra-short period grazingincidence X-ray mirrors working close to 100 keV When combining them with a low-absorption spacer material, . sample is determined by using a neutron beam and a spin analyzer of known polarization. Methods of calibrating spin analyzers are discussed in [7]. 22.3.2 Neutron Polarizers Nowadays mainly three. layer in the multilayer stack and from the interfacial stress within the structure. Fig. 23.1. Stress variation vs. argon pressure in single layer W (dashed line), single layer Si (dotted line), and. varied between 10 17 and 10 19 ncm −2 . In the channel 11/2 during the irradiation of borofloat glass with a supermirror coating, we found coating and the glass surface destruction and discoloration