- Email: [email protected]
Characterization of mica crystals as reflectors for an ultracold neutron doppler converter
ELSEVIER
Physica B241 243 (1998) l l0 112
Characterization of mica crystals as reflectors for an ultracold neutron doppler converter M.L. Crow* Department (?['Physics. Unicersity q/Rhode lshmd, East Hall, Kingston, RI 02881. USA
Abstract Natural mica crystals have been characterized for use as reflectors for an ultracold neutron source using Bragg scattering of cold (400 m/s) neutrons from a moving mica reflector, recently installed at the Los Alamos neutron science center (LANSCE). In particular, the trioctahedral Mg-rich mica phlogopite has been obtained in large sheets. The structure factor for the first-order Bragg reflection is a factor of four larger than for muscovite, the first variety of mica examined. The observed peak reflectivity for a phlogopite of 0.25 mm thickness is about 8.2% at 1.9 ,A, with a mosaic spread of about 0.3' FWHM, in good agreement with the ideally imperfect crystal model. This can be extrapolated to a reflectivity of about 63% at the 17.4 A operating wavelength of the ultracold neutron source. Improvements are possible by obtaining Fe-rich mica, which should be almost as effective as the alternative approach of using synthetic perfluorinated mica. 1998 Elsevier Science B.V. All rights reserved.
Keywords: Ultracold neutrons; Mica; Muscovite: Phlogopite
1. Introduction Micas are aluminosilicate minerals with a sheet structure with two layers of silicate tetrahedra arranged about an intervening layer of hydrated metal-oxide octahedra [1]. The interlayer space is occupied by alkali (usually K) cations. The interlayer lattice spacing ( ~ 10 ,A) is the largest among minerals readily available in large crystals, and is therefore of interest as a cold-neutron monochromator or analyzer.
*Corresponding author. Tel. + 1 401 874 6558: + 1 401 782-4201: e-mail: [email protected].
fax:
The conversion of cold neutrons to ultracold neutrons (UCN) by reflection from a moving crystal [2,3] motivates this study. The large lattice spacing of mica allows a much lower reflector velocity ( ~ 200 m/s) than, e.g., graphite ( ~ 600 m/s). This U C N production method is well suited to pulsed spallation sources; the crystal movement can be synchronized with the neutron pulses to produce an efficient converter. Natural muscovite mica is readily available in large, flat, optically transparent sheets. The lowstructure factor and high-extinction lead to a low reflectivity in muscovite, however, and the incoherent attenuation by hydrogen limits the usable thickness.
0921-4526/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved Pll S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 ( ) 5 2 5 5
M.L. Crow / Physica B 241-243 (1998) 110- 112
In some natural mica, fuorine partially substitutes for the hydroxyl ions. Hydrogen-free perfluorinated mica has been synthesized [1]; this mica can be stacked to a thickness of a centimeter or more to obtain a high neutron reflectivity, since incoherent scattering by hydrogen is usually the only strong neutron attenuation in mica. Perfluorinated mica crystals have been examined as neutron reflectors, and used on the prototype UCN converter at Argonne [3]. Unfortunately, large perfluorinated mica crystals are not consistently produced in quantity. A search for natural mica of other compositions, with better neutron reflection characteristics, has yielded specimens of natural phlogopite, The three octahedral sites in each formula unit of this mica are predominantly occupied by Mg, with small impurities of Fe and other transition metals. The phlogopite samples described here have Mg in over 90% of the octahedral sites. "Thick" pieces of natural phlogopite ( > 0.05 cm), typically have ripples in the flat surface with spacing ranging from a few mm to several cm, a geological artifact of their high formation temperature. Nevertheless, samples have been found with large, nearly flat areas.
111
2. Calculated reflectivities Reflection and attenuation characteristics of various micas can be calculated using nominal structures [1] and tabulated cross-section data [-4], if the crystals can be assumed to be ideally imperfect. Table 1 shows the first-order structure factors for some micas. The first-order structure factor in muscovite is very low. The structure factor is four times higher in phlogopite, and even higher in the Fe-rich trioctahedral mica biotite (more difficult to obtain in large, flat pieces). The attenuation factors are shown in Table 2 at 1.9 ~, and at 17.4 ~, clearly illustrate the advantage of synthetic perfluorinated mica crystals. Table 3 Table 1 The square of the first-order structure factor for some micas
Name
Composition
W2I (barns)
Muscovite Phlogopite Fluorophlogopite Biotite (50% Fe)
K 2AI4(Si3A1)zOzo(OH 14 K 2M g6(Si3Al)2020(OH)4 K2Mg6(SisA1)EOzo(F)4 K2Mg3Fe3(Si3AI)zOz0(OH)4
3.13 13.07 17.17 23.92
Table 2 Macroscopic attenuation factors due to incoherent scattering and absorption for some micas tabulated at 1.9 ~, and at 17.4 Name Muscovite Phlogopite Fluorophlogopite
1.9 ~, 17.4 A 1.9 ~, 17,4 ,~ 1.9 A 17.4 ,~
Incoherent(cm 1)
Absorption(cm 1)
Total(cm
0.688 0.688 0.656 0.656 0.002 0.002
0.018 0.186 0.016 0.164 0.013 0.116
0.706 0.874 0.672 0.820 0.015 0.118
1)
Table 3 Calculated first-order peak neutron reflectivities of some micas, assuming ideally imperfect crystals with the indicated wavelength 2, FWHM, and thickness t. Measurements on muscovite and phlogopite are discussed in this work Name
2 ( ~,t
FWHM ( )
t (cm)
Peak reflectivity
Muscovite
1.9 17.4 1.9 1.9 17.4 1.9 1.9 17.4
0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3
0.025 0.025 0.025 0.10 0.025 0.025 0.10 0.025
0.030 0.037 0.071 0.15 0.63 0.11 0.32 0.69
Pfilogopite
Fluorophlogopite
112
M.L. Crow / Physica B 241-243 (1998) 110 112
lists the expected reflectivities for micas with indicated thicknesses and mosaic spread, using the ideally imperfect model E5,6].
3. Measurements M u s c o v i t e : Rocking curve shapes of flat, window
quality muscovite crystals has been measured using diffractometers at the Rhode Island Nuclear Science Center (RINSC) reactor and at the Brookhaven High-Flux Beam Reactor, Since the rocking curve shapes of these crystals are not easily resolved using broad mosaic graphite monochromators, the shapes have been measured using pairs of muscovite crystals. The overall rocking curve widths of the samples, typically 6 cm 2 and 0.25 mm thick, are usually about 0.2 '~ FWH M, but the shape exhibits sharp sub-peaks, indicating large crystallites. Muscovite crystal reflectivities have been measured on the two-axis diffractometer at the RINSC reactor. A time-of-flight analysis has been used to select a single wavelength for each reflectivity measurement. The measured reflectivities, about 1.9% at 1.9 A,, are low compared to the predicted imperfect crystal values (Table 3). Elastic deflection has been tried to decrease the extinction; although the integrated reflectivity increases by up to 30%, it is still lower than expected in the imperfect limit. P h l o g o p i t e : Measurements have been made on phlogopite crystals of dimension 6 x 6 x 0.025 cm. The narrowest observed F W H M is about 0.3, and the highest peak reflectivity is ~ 8.2%. Phlogopite rocking curves lacked the subpeaks observed in the muscovite, suggesting smaller crystallites. Other phlogopite crystals ranging up to about 0.2 cm thick have reflectivities corresponding closely to that predicted using the imperfect crystal model.
4. Discussion This study has shown that natural phlogopite is a superior reflector to natural muscovite for two
reasons: (1) It has a significantly higher first-order structure factor, and (2) It is found in less perfect crystals and therefore has little extinction. The reflectivity of phlogopite, unlike that of muscovite, is consistently predictable using the ideally imperfect model. At 17.4 A, the predicted peak reflectivity of 0.025 cm thick phlogopite with 0.3 ° F W H M is about 63%, a usable reflection efficiency for a UCN converter. There are two likely methods of achieving better mica reflectivity. The possibility of thick perfluorinated mica stacks is limited by cost and availability of this material. If natural high Fe biotite can be obtained in large sheets, its use should be feasible in applications which require large analyzer areas, such as backscattering, as well as for the UCN conversion application.
Acknowledgements The author thanks T.J. Bowles, S.J. Seestrom, and the Los Alamos National Laboratory ultracold neutron group for suggesting and funding this work, and A. Steyerl and the URI Department of Physics for their support. Measurements were made at the Rhode Island Nuclear Science Center and the Brookhaven High Flux Beam Reactor. The author recognizes helpful discussions with A.C. Nunes, D.G. Johnson, O.D. Hermes, and U. Wildgruber.
References Ill S.W. Bailey,Structures of layer silicates,in: G.M. Brindley, G. Brown (Eds.), Crystal Structures of Clay Minerals and Their X-Ray Identification, Mineralogical Society, London, 1980, pp. 1 123. E2] T.J. Bowles, T.O. Brun, R.E. Hill, C.L. Morris, S.J. Seestrom, M.U Crow, Presented at joint APS/AAPTmeeting, April 1997. [3] T.W. Dombeck,J.W. Lynn, S.A. Werner, T. Brun, J. Carpenter, V. Krohn, R. Ringo, Nucl. Instr. and Meth. 165 (1979) 139. [4] V.V. Sears, Neutron News 3 (1992)26. [5] S.A. Werner, A. Arrot, Phys. Rev. 140 (1965)676 686. [6] G.E. Bacon, R.D. Lowde,Acta Crystallogr. 1 (1948) 303.
Physica B241 243 (1998) l l0 112
Characterization of mica crystals as reflectors for an ultracold neutron doppler converter M.L. Crow* Department (?['Physics. Unicersity q/Rhode lshmd, East Hall, Kingston, RI 02881. USA
Abstract Natural mica crystals have been characterized for use as reflectors for an ultracold neutron source using Bragg scattering of cold (400 m/s) neutrons from a moving mica reflector, recently installed at the Los Alamos neutron science center (LANSCE). In particular, the trioctahedral Mg-rich mica phlogopite has been obtained in large sheets. The structure factor for the first-order Bragg reflection is a factor of four larger than for muscovite, the first variety of mica examined. The observed peak reflectivity for a phlogopite of 0.25 mm thickness is about 8.2% at 1.9 ,A, with a mosaic spread of about 0.3' FWHM, in good agreement with the ideally imperfect crystal model. This can be extrapolated to a reflectivity of about 63% at the 17.4 A operating wavelength of the ultracold neutron source. Improvements are possible by obtaining Fe-rich mica, which should be almost as effective as the alternative approach of using synthetic perfluorinated mica. 1998 Elsevier Science B.V. All rights reserved.
Keywords: Ultracold neutrons; Mica; Muscovite: Phlogopite
1. Introduction Micas are aluminosilicate minerals with a sheet structure with two layers of silicate tetrahedra arranged about an intervening layer of hydrated metal-oxide octahedra [1]. The interlayer space is occupied by alkali (usually K) cations. The interlayer lattice spacing ( ~ 10 ,A) is the largest among minerals readily available in large crystals, and is therefore of interest as a cold-neutron monochromator or analyzer.
*Corresponding author. Tel. + 1 401 874 6558: + 1 401 782-4201: e-mail: [email protected].
fax:
The conversion of cold neutrons to ultracold neutrons (UCN) by reflection from a moving crystal [2,3] motivates this study. The large lattice spacing of mica allows a much lower reflector velocity ( ~ 200 m/s) than, e.g., graphite ( ~ 600 m/s). This U C N production method is well suited to pulsed spallation sources; the crystal movement can be synchronized with the neutron pulses to produce an efficient converter. Natural muscovite mica is readily available in large, flat, optically transparent sheets. The lowstructure factor and high-extinction lead to a low reflectivity in muscovite, however, and the incoherent attenuation by hydrogen limits the usable thickness.
0921-4526/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved Pll S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 ( ) 5 2 5 5
M.L. Crow / Physica B 241-243 (1998) 110- 112
In some natural mica, fuorine partially substitutes for the hydroxyl ions. Hydrogen-free perfluorinated mica has been synthesized [1]; this mica can be stacked to a thickness of a centimeter or more to obtain a high neutron reflectivity, since incoherent scattering by hydrogen is usually the only strong neutron attenuation in mica. Perfluorinated mica crystals have been examined as neutron reflectors, and used on the prototype UCN converter at Argonne [3]. Unfortunately, large perfluorinated mica crystals are not consistently produced in quantity. A search for natural mica of other compositions, with better neutron reflection characteristics, has yielded specimens of natural phlogopite, The three octahedral sites in each formula unit of this mica are predominantly occupied by Mg, with small impurities of Fe and other transition metals. The phlogopite samples described here have Mg in over 90% of the octahedral sites. "Thick" pieces of natural phlogopite ( > 0.05 cm), typically have ripples in the flat surface with spacing ranging from a few mm to several cm, a geological artifact of their high formation temperature. Nevertheless, samples have been found with large, nearly flat areas.
111
2. Calculated reflectivities Reflection and attenuation characteristics of various micas can be calculated using nominal structures [1] and tabulated cross-section data [-4], if the crystals can be assumed to be ideally imperfect. Table 1 shows the first-order structure factors for some micas. The first-order structure factor in muscovite is very low. The structure factor is four times higher in phlogopite, and even higher in the Fe-rich trioctahedral mica biotite (more difficult to obtain in large, flat pieces). The attenuation factors are shown in Table 2 at 1.9 ~, and at 17.4 ~, clearly illustrate the advantage of synthetic perfluorinated mica crystals. Table 3 Table 1 The square of the first-order structure factor for some micas
Name
Composition
W2I (barns)
Muscovite Phlogopite Fluorophlogopite Biotite (50% Fe)
K 2AI4(Si3A1)zOzo(OH 14 K 2M g6(Si3Al)2020(OH)4 K2Mg6(SisA1)EOzo(F)4 K2Mg3Fe3(Si3AI)zOz0(OH)4
3.13 13.07 17.17 23.92
Table 2 Macroscopic attenuation factors due to incoherent scattering and absorption for some micas tabulated at 1.9 ~, and at 17.4 Name Muscovite Phlogopite Fluorophlogopite
1.9 ~, 17.4 A 1.9 ~, 17,4 ,~ 1.9 A 17.4 ,~
Incoherent(cm 1)
Absorption(cm 1)
Total(cm
0.688 0.688 0.656 0.656 0.002 0.002
0.018 0.186 0.016 0.164 0.013 0.116
0.706 0.874 0.672 0.820 0.015 0.118
1)
Table 3 Calculated first-order peak neutron reflectivities of some micas, assuming ideally imperfect crystals with the indicated wavelength 2, FWHM, and thickness t. Measurements on muscovite and phlogopite are discussed in this work Name
2 ( ~,t
FWHM ( )
t (cm)
Peak reflectivity
Muscovite
1.9 17.4 1.9 1.9 17.4 1.9 1.9 17.4
0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3
0.025 0.025 0.025 0.10 0.025 0.025 0.10 0.025
0.030 0.037 0.071 0.15 0.63 0.11 0.32 0.69
Pfilogopite
Fluorophlogopite
112
M.L. Crow / Physica B 241-243 (1998) 110 112
lists the expected reflectivities for micas with indicated thicknesses and mosaic spread, using the ideally imperfect model E5,6].
3. Measurements M u s c o v i t e : Rocking curve shapes of flat, window
quality muscovite crystals has been measured using diffractometers at the Rhode Island Nuclear Science Center (RINSC) reactor and at the Brookhaven High-Flux Beam Reactor, Since the rocking curve shapes of these crystals are not easily resolved using broad mosaic graphite monochromators, the shapes have been measured using pairs of muscovite crystals. The overall rocking curve widths of the samples, typically 6 cm 2 and 0.25 mm thick, are usually about 0.2 '~ FWH M, but the shape exhibits sharp sub-peaks, indicating large crystallites. Muscovite crystal reflectivities have been measured on the two-axis diffractometer at the RINSC reactor. A time-of-flight analysis has been used to select a single wavelength for each reflectivity measurement. The measured reflectivities, about 1.9% at 1.9 A,, are low compared to the predicted imperfect crystal values (Table 3). Elastic deflection has been tried to decrease the extinction; although the integrated reflectivity increases by up to 30%, it is still lower than expected in the imperfect limit. P h l o g o p i t e : Measurements have been made on phlogopite crystals of dimension 6 x 6 x 0.025 cm. The narrowest observed F W H M is about 0.3, and the highest peak reflectivity is ~ 8.2%. Phlogopite rocking curves lacked the subpeaks observed in the muscovite, suggesting smaller crystallites. Other phlogopite crystals ranging up to about 0.2 cm thick have reflectivities corresponding closely to that predicted using the imperfect crystal model.
4. Discussion This study has shown that natural phlogopite is a superior reflector to natural muscovite for two
reasons: (1) It has a significantly higher first-order structure factor, and (2) It is found in less perfect crystals and therefore has little extinction. The reflectivity of phlogopite, unlike that of muscovite, is consistently predictable using the ideally imperfect model. At 17.4 A, the predicted peak reflectivity of 0.025 cm thick phlogopite with 0.3 ° F W H M is about 63%, a usable reflection efficiency for a UCN converter. There are two likely methods of achieving better mica reflectivity. The possibility of thick perfluorinated mica stacks is limited by cost and availability of this material. If natural high Fe biotite can be obtained in large sheets, its use should be feasible in applications which require large analyzer areas, such as backscattering, as well as for the UCN conversion application.
Acknowledgements The author thanks T.J. Bowles, S.J. Seestrom, and the Los Alamos National Laboratory ultracold neutron group for suggesting and funding this work, and A. Steyerl and the URI Department of Physics for their support. Measurements were made at the Rhode Island Nuclear Science Center and the Brookhaven High Flux Beam Reactor. The author recognizes helpful discussions with A.C. Nunes, D.G. Johnson, O.D. Hermes, and U. Wildgruber.
References Ill S.W. Bailey,Structures of layer silicates,in: G.M. Brindley, G. Brown (Eds.), Crystal Structures of Clay Minerals and Their X-Ray Identification, Mineralogical Society, London, 1980, pp. 1 123. E2] T.J. Bowles, T.O. Brun, R.E. Hill, C.L. Morris, S.J. Seestrom, M.U Crow, Presented at joint APS/AAPTmeeting, April 1997. [3] T.W. Dombeck,J.W. Lynn, S.A. Werner, T. Brun, J. Carpenter, V. Krohn, R. Ringo, Nucl. Instr. and Meth. 165 (1979) 139. [4] V.V. Sears, Neutron News 3 (1992)26. [5] S.A. Werner, A. Arrot, Phys. Rev. 140 (1965)676 686. [6] G.E. Bacon, R.D. Lowde,Acta Crystallogr. 1 (1948) 303.