- Email: [email protected]
Performance of TOF powder diffractometers on reactor sources
ARTICLE IN PRESS
Physica B 385–386 (2006) 1019–1021 www.elsevier.com/locate/physb
Performance of TOF powder diffractometers on reactor sources Judith Petersa,, Hans-Ju¨rgen Bleif a, Gyo¨rgy Kalib, Laszlo Rostac, Ferenc Mezeia,d a
Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany Budapest Research Reactor, 1121 Budapest, Konkoly Thege u´t 29-33, Hungary c Research Institute for Solid State Physics and Optics, 1525 Budapest, P.O.B. 49, Hungary d LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87544, USA b
Abstract In 1998, a prototype of a time-of-flight (TOF) powder diffractometer was built at KFKI in Budapest in collaboration with the HahnMeitner-Institut (HMI) in Berlin. At a reactor source the neutron pulses are produced by a chopper system, which allows for shorter pulses than those obtained at pulsed spallation sources in the wavelength range most relevant for diffraction work, i.e. lX0.7 A˚. Furthermore, the chopper system provides an ideal symmetric line shape. First results proved the high potential of the approach, namely an excellent resolution of 1–5 103 for Dd/d was achieved. The prototype is presently rebuilt as a user instrument at the Budapest Neutron Centre. At HMI Berlin a new much more complex TOF powder diffractometer (EXED ¼ extreme environment diffractometer) with higher resolution is under construction. It will benefit from variable resolution to achieve either ultrahigh resolution or very high intensities at conventional resolutions. EXED is devoted to studies under extreme sample conditions, for instance the TOF technique permits the access of a broad range of Q-values or d-spacing domains under scattering angle access strongly restricted by the use of highest field magnets. The whole instrument was simulated by Monte Carlo (MC) technique, and the simulations yield promising results. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 28.41.Rc; 02.70.Uu Keywords: Time of flight; Diffractometer; Reactor source
1. Introduction In 1963, Buras and Leciejewicz [1] first demonstrated the great potential of the Time of flight (TOF) technique applied to neutron diffractometry on a continuous source. Since then several instruments have used this technique for neutron diffraction, e.g., TOF at KFKI in Budapest/ Hungary [2] and POLDI at PSI near Zu¨rich/Switzerland [3], and the results are very good. The main advantage is the possibility to either gain ultrahigh or variable resolution, where the latter option enables us to achieve very high intensity at conventional resolutions. Another advantage is the capability to use scattering angle domains strongly restricted by extreme sample environment equipment. The Budapest instrument and the new instrument EXED under construction at the Corresponding author. Tel.: +49 30 8062 3068; fax: +49 30 8062 3094.
E-mail address: [email protected] (J. Peters). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.325
HMI in Berlin are presented here as prototypes which use these opportunities to the full capacity. 2. The TOF diffractometer at KFKI Budapest The TOF diffractometer project started in 1996 in collaboration with the HMI Berlin and the instrument was installed at the Budapest Research Reactor. After a longer reactor shutdown period, the instrument has been rebuilt at a thermal beam and will be ready in September 2005 (see Fig. 1). The length of a disk chopper pulse depends on the chopper speed and on the convolution between the opening window of the disc chopper and the cross section of the guide. Thus, to achieve shortest neutron pulses this instrument uses the so-called eye-of-the-needle technique, i.e. the width of the guide is reduced from 25 to 10 mm at the position of the first three choppers with a 4.5 m long compressor section in the neutron guide and opens again to
ARTICLE IN PRESS J. Peters et al. / Physica B 385–386 (2006) 1019–1021
1020
25 mm within another antitrumpet section. The double disk chopper has two windows: a 1.51 opening for short pulses (10 ms) and a 151 window for long pulses (20–200 ms). First experimental results (Fig. 2a) show the best resolution that has ever been achieved on a reactor for an 18 m instrument. The resolution is wavelength dependent, e.g. near backscattering, where TOF instruments have the highest resolution, Dd/d ¼ 0.001 for d ¼ 1 A˚ and pulses of 10 ms. These measurements were carried out on a sintered alumina sample. Furthermore, with a TOF powder diffractometer lattice constants can be determined very precisely by using single crystals, since there is no broadening of the diffraction line caused by small particle size (or strain). Fig. 2b shows the c-lattice constant distribution of pyrolytic graphite, measured at a resolution o4 104 using the single reflection 002. The total flight time was about 35 650 ms with a time uncertainty of about 14 ms. Another very useful feature of a chopper system is the possibility to adjust the instrumental resolution to the instrinsic width of the measured signal by adjusting the
Fig. 1. Schematic representation of the Budapest TOF Diffractometer including the first three choppers. The last chopper and the sample position are not shown here.
phase angles of the choppers. This can result in a considerable intensity gain. 3. The EXED instrument under construction at HMI Berlin After the first successful tests of the Budapest TOF diffractometer, we started to build another TOF powder diffractometer at HMI Berlin in 2003. This instrument has a special focus on extreme environment conditions such as high magnetic fields up to 40 T. Such a magnet, which could e.g. be a horizontal closed solenoid exclusively made of superconducting material, including new high-Tc material, would clearly restrict the possible neutron scattering angles to opening cones of 301 in forward and backward direction. Thus an access to a broad domain of d-spacings or Q-values is only possible by a large incident spectrum and consequently by using the TOF technique. In order to obtain shortest pulses, instead of the eye-ofthe-needle configuration, EXED can alternatively run a high-speed-double disk chopper with up to 14 400 RPM in counter or parallel rotating mode or a Fermi chopper with two different slit packages (a straight package with a thickness of 1 cm and a curved package with a thickness of 2.5 cm) and with a maximum speed of 36 000 RPM. This allows us to choose the pulse length (FWHM) between 6 and 4000 ms. The FWHM of the wavelength band, transmitted by the Fermi chopper at highest speed, is 2.8 A˚ for the curved slit package and 10 A˚ for the straight slit package. The longest possible flight path L between the pulse shaping chopper and the detector is about 60 m with a secondary flight path sample detector of 6 m. These characteristics translate into a possible ultrahigh resolution of Dd/dE2 104 for wavelengths X4 A˚ in backward direction. Further details of the instrument performance have been described elsewhere [4]. Pulse shaping choppers, e.g. high-speed double-disk choppers or Fermi choppers, produce symmetric, closely Gaussian line shapes and peak widths almost independent
140
300 250
120
Al203 sintered
100 200
FWHM =58μs
150
Resolution Δd/d =4 x10-4
80 counts
counts
PG 002 Lorentzian
60 40
100
20
50
0 0 1.21
(a)
1.22
1.23 d-spacing [A]
1.24
35400
(b)
35500
35600 35700 35800 time-of-flight [µs]
35900
Fig. 2. Powder reflections of Al2O3 and a single crystal reflection of pyrolytic graphite measured at the Budapest TOF instrument.
ARTICLE IN PRESS J. Peters et al. / Physica B 385–386 (2006) 1019–1021
E9 EXED - with double disc chopper
20000
neutron counts [a.u.]
16000 12000 8000 4000 0 -4000 0.2
0.3
0.4
0.5
sin(θ)/ λ [A-1] Fig. 3. Rietveld refinement of MnO at 150 K. The EXED intensities lines have been shifted by 2000 for a better visibility of the peaks. Table 1 Comparison between peak widths of E9 and EXED intensity lines as function of [(sin y)/l] [(sin y)/l][A˚1]
FWHM(E9) [A˚1]
FWHM(EXED) [A˚1]
0.195 0.489
0.0014 0.00083
0.0004 0.002
1021
factors F2 were extracted by the Rietveld refinement program FULLPROF [7]. The structure factor as a function of the d-spacing was then used as input for the Monte Carlo (MC) simulations carried out by the software package VITESS [8]. The simulations included the complete EXED geometry (source, guides, chopper system, sample and detectors). The MnO crystal was chosen for its simple structure, because MC simulations are extremely time consuming. For the same reason, the double-disk chopper was chosen as the pulse shaping chopper for the simulations, although this set-up does not yield the best possible resolution. Fig. 3 shows a comparison between data collected at the E9 diffractometer of HMI and simulated data obtained with the EXED geometry, both recalculated as a function of [(sin y)/l] and divided by the corresponding Lorentz factors. This shows the good agreement between both, but the larger peak width at higher [(sin y)/l] for EXED data and at lower [(sin y)/l] for E9 data (see Table 1). With the curved slit package of the Fermi chopper, the resolution is one order of magnitude better for EXED in terms of Dd/d. Rietveld refinement of TOF data is also possible after a few modifications accounting for the very symmetric peak shape and for the dependence of the peak width from the TOF. Such calculations are presently in preparation. Acknowledgements
of the wavelength. Consequently, DT/T is no longer constant, where T is the time of flight. As early as 1975, Hewat [5] already stated that best high-resolution powder diffractometers ask for a compromise between resolution and intensity and showed that conventional monochromatic diffractometers can be optimized by matching the resolution required to resolve adjacent peaks for a cubic crystal at low and medium values of [(sin y)/l] (p0.6 A˚1). However, in the high [(sin y)/l] region TOF machines can more easily reach the required resolution. The region of best resolution of conventional diffractometers is determined by the design of the instrument and can be changed by a variation of the monochromator take-off angle, e.g. by rebuilding the instrument. The EXED design for ultra high-resolution described above will allow for an easy matching of the required resolution anywhere up to [(sin y)/ l]p1 A˚1 with the possibility to adjust it if higher intensity is more favourable. To illustrate these characteristics, we used a sample of MnO, which was measured at the E9 powder diffractometer [6] of HMI Berlin. The MnO crystal structure
We would like to thank Z. Sa´nta for providing us with Fig. 1, M. Reehuis for the MnO data collected at E9 and J. Rodriguez Carvajal for the support by using FULLPROOF. References [1] B. Buras, J. Leciejewicz, Nukleonika 8 (1963) 75. [2] H.J. Bleif, D. Wechsler, F. Mezei, Physica B 276–278 (2000) 181; J.A. Stride, D. Wechsler, F. Mezei, H.-J. Bleif, Nucl. Instrum Methods A 451 (2000) 480. [3] U. Stuhr, Nucl. Instrum Methods A 545 (2005) 319; U. Stuhr, et al., Nucl. Instrum Methods A 545 (2005) 330. [4] J. Peters, K. Lieutenant, D. Clemens, F. Mezei, in: Proceedings of the European Powder Diffraction Conference, Prag, 2–5 September 2004, Zeitschrift F. Krist. Suppl. 23 (2006) 189; J. Peters, F. Mezei, in: Proceedings of ICANS-XVII, Santa Fe, 25–29 April 2005, to be published. [5] A. Hewat, Nucl. Instrum Methods 127 (1975) 361. [6] W. Jauch, M. Reehuis, Phys. Rev. B 67 (2003) 184420. [7] J. Rodriguez Carvajal, Physica B 192 (1993) 55. [8] http://www.hmi.de/projects/ess/vitess; G. Zsigmond, K. Lieutenant, F. Mezei, Neutron News 13.4 (2002) 11.
Physica B 385–386 (2006) 1019–1021 www.elsevier.com/locate/physb
Performance of TOF powder diffractometers on reactor sources Judith Petersa,, Hans-Ju¨rgen Bleif a, Gyo¨rgy Kalib, Laszlo Rostac, Ferenc Mezeia,d a
Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany Budapest Research Reactor, 1121 Budapest, Konkoly Thege u´t 29-33, Hungary c Research Institute for Solid State Physics and Optics, 1525 Budapest, P.O.B. 49, Hungary d LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87544, USA b
Abstract In 1998, a prototype of a time-of-flight (TOF) powder diffractometer was built at KFKI in Budapest in collaboration with the HahnMeitner-Institut (HMI) in Berlin. At a reactor source the neutron pulses are produced by a chopper system, which allows for shorter pulses than those obtained at pulsed spallation sources in the wavelength range most relevant for diffraction work, i.e. lX0.7 A˚. Furthermore, the chopper system provides an ideal symmetric line shape. First results proved the high potential of the approach, namely an excellent resolution of 1–5 103 for Dd/d was achieved. The prototype is presently rebuilt as a user instrument at the Budapest Neutron Centre. At HMI Berlin a new much more complex TOF powder diffractometer (EXED ¼ extreme environment diffractometer) with higher resolution is under construction. It will benefit from variable resolution to achieve either ultrahigh resolution or very high intensities at conventional resolutions. EXED is devoted to studies under extreme sample conditions, for instance the TOF technique permits the access of a broad range of Q-values or d-spacing domains under scattering angle access strongly restricted by the use of highest field magnets. The whole instrument was simulated by Monte Carlo (MC) technique, and the simulations yield promising results. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 28.41.Rc; 02.70.Uu Keywords: Time of flight; Diffractometer; Reactor source
1. Introduction In 1963, Buras and Leciejewicz [1] first demonstrated the great potential of the Time of flight (TOF) technique applied to neutron diffractometry on a continuous source. Since then several instruments have used this technique for neutron diffraction, e.g., TOF at KFKI in Budapest/ Hungary [2] and POLDI at PSI near Zu¨rich/Switzerland [3], and the results are very good. The main advantage is the possibility to either gain ultrahigh or variable resolution, where the latter option enables us to achieve very high intensity at conventional resolutions. Another advantage is the capability to use scattering angle domains strongly restricted by extreme sample environment equipment. The Budapest instrument and the new instrument EXED under construction at the Corresponding author. Tel.: +49 30 8062 3068; fax: +49 30 8062 3094.
E-mail address: [email protected] (J. Peters). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.325
HMI in Berlin are presented here as prototypes which use these opportunities to the full capacity. 2. The TOF diffractometer at KFKI Budapest The TOF diffractometer project started in 1996 in collaboration with the HMI Berlin and the instrument was installed at the Budapest Research Reactor. After a longer reactor shutdown period, the instrument has been rebuilt at a thermal beam and will be ready in September 2005 (see Fig. 1). The length of a disk chopper pulse depends on the chopper speed and on the convolution between the opening window of the disc chopper and the cross section of the guide. Thus, to achieve shortest neutron pulses this instrument uses the so-called eye-of-the-needle technique, i.e. the width of the guide is reduced from 25 to 10 mm at the position of the first three choppers with a 4.5 m long compressor section in the neutron guide and opens again to
ARTICLE IN PRESS J. Peters et al. / Physica B 385–386 (2006) 1019–1021
1020
25 mm within another antitrumpet section. The double disk chopper has two windows: a 1.51 opening for short pulses (10 ms) and a 151 window for long pulses (20–200 ms). First experimental results (Fig. 2a) show the best resolution that has ever been achieved on a reactor for an 18 m instrument. The resolution is wavelength dependent, e.g. near backscattering, where TOF instruments have the highest resolution, Dd/d ¼ 0.001 for d ¼ 1 A˚ and pulses of 10 ms. These measurements were carried out on a sintered alumina sample. Furthermore, with a TOF powder diffractometer lattice constants can be determined very precisely by using single crystals, since there is no broadening of the diffraction line caused by small particle size (or strain). Fig. 2b shows the c-lattice constant distribution of pyrolytic graphite, measured at a resolution o4 104 using the single reflection 002. The total flight time was about 35 650 ms with a time uncertainty of about 14 ms. Another very useful feature of a chopper system is the possibility to adjust the instrumental resolution to the instrinsic width of the measured signal by adjusting the
Fig. 1. Schematic representation of the Budapest TOF Diffractometer including the first three choppers. The last chopper and the sample position are not shown here.
phase angles of the choppers. This can result in a considerable intensity gain. 3. The EXED instrument under construction at HMI Berlin After the first successful tests of the Budapest TOF diffractometer, we started to build another TOF powder diffractometer at HMI Berlin in 2003. This instrument has a special focus on extreme environment conditions such as high magnetic fields up to 40 T. Such a magnet, which could e.g. be a horizontal closed solenoid exclusively made of superconducting material, including new high-Tc material, would clearly restrict the possible neutron scattering angles to opening cones of 301 in forward and backward direction. Thus an access to a broad domain of d-spacings or Q-values is only possible by a large incident spectrum and consequently by using the TOF technique. In order to obtain shortest pulses, instead of the eye-ofthe-needle configuration, EXED can alternatively run a high-speed-double disk chopper with up to 14 400 RPM in counter or parallel rotating mode or a Fermi chopper with two different slit packages (a straight package with a thickness of 1 cm and a curved package with a thickness of 2.5 cm) and with a maximum speed of 36 000 RPM. This allows us to choose the pulse length (FWHM) between 6 and 4000 ms. The FWHM of the wavelength band, transmitted by the Fermi chopper at highest speed, is 2.8 A˚ for the curved slit package and 10 A˚ for the straight slit package. The longest possible flight path L between the pulse shaping chopper and the detector is about 60 m with a secondary flight path sample detector of 6 m. These characteristics translate into a possible ultrahigh resolution of Dd/dE2 104 for wavelengths X4 A˚ in backward direction. Further details of the instrument performance have been described elsewhere [4]. Pulse shaping choppers, e.g. high-speed double-disk choppers or Fermi choppers, produce symmetric, closely Gaussian line shapes and peak widths almost independent
140
300 250
120
Al203 sintered
100 200
FWHM =58μs
150
Resolution Δd/d =4 x10-4
80 counts
counts
PG 002 Lorentzian
60 40
100
20
50
0 0 1.21
(a)
1.22
1.23 d-spacing [A]
1.24
35400
(b)
35500
35600 35700 35800 time-of-flight [µs]
35900
Fig. 2. Powder reflections of Al2O3 and a single crystal reflection of pyrolytic graphite measured at the Budapest TOF instrument.
ARTICLE IN PRESS J. Peters et al. / Physica B 385–386 (2006) 1019–1021
E9 EXED - with double disc chopper
20000
neutron counts [a.u.]
16000 12000 8000 4000 0 -4000 0.2
0.3
0.4
0.5
sin(θ)/ λ [A-1] Fig. 3. Rietveld refinement of MnO at 150 K. The EXED intensities lines have been shifted by 2000 for a better visibility of the peaks. Table 1 Comparison between peak widths of E9 and EXED intensity lines as function of [(sin y)/l] [(sin y)/l][A˚1]
FWHM(E9) [A˚1]
FWHM(EXED) [A˚1]
0.195 0.489
0.0014 0.00083
0.0004 0.002
1021
factors F2 were extracted by the Rietveld refinement program FULLPROF [7]. The structure factor as a function of the d-spacing was then used as input for the Monte Carlo (MC) simulations carried out by the software package VITESS [8]. The simulations included the complete EXED geometry (source, guides, chopper system, sample and detectors). The MnO crystal was chosen for its simple structure, because MC simulations are extremely time consuming. For the same reason, the double-disk chopper was chosen as the pulse shaping chopper for the simulations, although this set-up does not yield the best possible resolution. Fig. 3 shows a comparison between data collected at the E9 diffractometer of HMI and simulated data obtained with the EXED geometry, both recalculated as a function of [(sin y)/l] and divided by the corresponding Lorentz factors. This shows the good agreement between both, but the larger peak width at higher [(sin y)/l] for EXED data and at lower [(sin y)/l] for E9 data (see Table 1). With the curved slit package of the Fermi chopper, the resolution is one order of magnitude better for EXED in terms of Dd/d. Rietveld refinement of TOF data is also possible after a few modifications accounting for the very symmetric peak shape and for the dependence of the peak width from the TOF. Such calculations are presently in preparation. Acknowledgements
of the wavelength. Consequently, DT/T is no longer constant, where T is the time of flight. As early as 1975, Hewat [5] already stated that best high-resolution powder diffractometers ask for a compromise between resolution and intensity and showed that conventional monochromatic diffractometers can be optimized by matching the resolution required to resolve adjacent peaks for a cubic crystal at low and medium values of [(sin y)/l] (p0.6 A˚1). However, in the high [(sin y)/l] region TOF machines can more easily reach the required resolution. The region of best resolution of conventional diffractometers is determined by the design of the instrument and can be changed by a variation of the monochromator take-off angle, e.g. by rebuilding the instrument. The EXED design for ultra high-resolution described above will allow for an easy matching of the required resolution anywhere up to [(sin y)/ l]p1 A˚1 with the possibility to adjust it if higher intensity is more favourable. To illustrate these characteristics, we used a sample of MnO, which was measured at the E9 powder diffractometer [6] of HMI Berlin. The MnO crystal structure
We would like to thank Z. Sa´nta for providing us with Fig. 1, M. Reehuis for the MnO data collected at E9 and J. Rodriguez Carvajal for the support by using FULLPROOF. References [1] B. Buras, J. Leciejewicz, Nukleonika 8 (1963) 75. [2] H.J. Bleif, D. Wechsler, F. Mezei, Physica B 276–278 (2000) 181; J.A. Stride, D. Wechsler, F. Mezei, H.-J. Bleif, Nucl. Instrum Methods A 451 (2000) 480. [3] U. Stuhr, Nucl. Instrum Methods A 545 (2005) 319; U. Stuhr, et al., Nucl. Instrum Methods A 545 (2005) 330. [4] J. Peters, K. Lieutenant, D. Clemens, F. Mezei, in: Proceedings of the European Powder Diffraction Conference, Prag, 2–5 September 2004, Zeitschrift F. Krist. Suppl. 23 (2006) 189; J. Peters, F. Mezei, in: Proceedings of ICANS-XVII, Santa Fe, 25–29 April 2005, to be published. [5] A. Hewat, Nucl. Instrum Methods 127 (1975) 361. [6] W. Jauch, M. Reehuis, Phys. Rev. B 67 (2003) 184420. [7] J. Rodriguez Carvajal, Physica B 192 (1993) 55. [8] http://www.hmi.de/projects/ess/vitess; G. Zsigmond, K. Lieutenant, F. Mezei, Neutron News 13.4 (2002) 11.