- Email: [email protected]
The new neutron powder diffractometer at “Demokritos” research reactor
ARTICLE IN PRESS
Physica B 350 (2004) 162–165
The new neutron powder diffractometer at ‘‘Demokritos’’ research reactor K. Mergia*, A. Salevris, S. Messoloras National Centre for Scientific Research ‘‘Demokritos’’, 15310 Aghia Paraskevi Attikis, Greece
Abstract The new neutron powder diffractometer, NEDI, recently installed at GGR-1 research reactor at the National Research Center of Scientific Research ‘‘Demokritos’’, Greece, is described. Instrumental aspects are discussed and examples of neutron diffraction patterns from standard samples are presented. r 2004 Elsevier B.V. All rights reserved. PACS: 83.85.Hf; 29.40.Cs; 61.12.Ld Keywords: Powder diffractometer; Position-sensitive detector; Focusing monochromator
1. Introduction
2. Description of the instrument
A new two-axis neutron powder diffractometer, NEDI, was recently installed at GRR-1 research reactor at the National Centre for Scientific Research ‘‘Demokritos’’ which has a power of 5 MW and light water is used as a moderator and coolant. The diffractometer was designed so as to enhance the diffraction data quality which can be obtained from a reactor of modest thermal power. Its applications are mainly focused on the study of crystalline and magnetic structures, phase transitions induced, e.g. by temperature change or by pressure, residual stress measurements and kinetics of chemical reactions.
The diffractometer was installed at a beam port which is at 30 angle to the reactor core. An in-pile main beam collimator of 1.87 m length is installed in the beam tube ending at a rotating drum which enables opening and closing of the beam. A sapphire filter of 75 mm thickness is used in the primary beam to reduce the fast neutron flux [1]. Fig. 1 shows the schematic layout of the instrument. The neutron-diffractometer radiation shielding was optimised by Monte Carlo neutron and photon transport code calculations (MCNP) [2]. The main shielding materials used were boron-doped polyethylene as neutron thermalising and absorbing material or lead–boron-doped polyethylene as neutron thermalising–absorbing and gamma absorbing material.
*Corresponding author. Fax: +30-210-6533431. E-mail address: [email protected] (K. Mergia).
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.04.019
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
Position sensitive detectors
sample aperture Collimators
neutron beam
monochromator
Reactor face
Fig. 1. Schematic layout of ‘‘Demokritos’’ neutron diffractometer.
2.1. Collimation and monochromatisation Two alternative multi-crystal monochromators are available: a germanium (Ge/hkk) and a pyrolytic graphite (PG/002). The Ge monochromator is made from plastically deformed germanium (5 1 1) wafers fabricated at Riso. National Laboratory. By applying uniaxial pressure at elevated temperatures to the Ge wafers a mosaic spread of around 150 is obtained [3–5]. In this way, a higher neutron flux is achieved by utilising a broader neutron wavelength band without deterioration of the resolution. The Ge monochroma( tor provides wavelengths in the range 0.41–2.95 A. The monochromator consists of five composite strips cut from composite germanium slabs composed of 20 wafers each. The strips are 16 mm in height, 50 mm in length and 8 mm in thickness. The pyrolytic graphite uses the (0 0 2) reflection ( and can provide wavelengths from 1.28 to 5.81 A and is used in combination with a graphite filter for the suppression of high order harmonics. The PG has a mosaicity of 300 . Both monochromators are set up in reflection geometry in order to obtain the best efficiency [6]. The neutron beam impinging on the monochromator has dimensions 90 40 mm2 (height width) and its natural divergence taking into
163
account the in-pile collimator and the beam tubes is 1.2 in the vertical and 0.6 in the horizontal direction. Two Soller collimators placed before the monochromator and made from mylar-foil covered with gadolinium oxide offer a collimation of 150 and 300 with transmission of 93% and 96%, respectively. Appropriate mechanical devices permit the exchange of the collimators and their alignment, the exchange of the monochromators, the Bragg angle setting of the monochromator, the alignment of the monochromator with the neutron beam and the focusing of the monochromator from 50 cm to infinity. Optimisation of the focusing conditions has been carried out by a position-sensitive detector and is discussed elsewhere [7]. The settings of the collimators and monochromators are controlled by a PC and software developed locally. The collimator and monochromator units are housed within the main instrument shield, which is constructed as to allow easy access to both units. The main monochromator shield provides 2yM take-off angles at 22 , 45 , 60 , 90 and 120 . 2.2. Sample area The sample area consists of a table on air cushion pads allowing easy movements to the different beam exit tubes. Different sample environments can be easily assembled allowing convenient sample mounting. The incident to the sample neutron beam is defined by an adjustable in height and width aperture. The incident beam may be focused on a minimum sample height of 2 cm giving an intensity gain of a factor of about three. 2.3. Detector characteristics The detection system consists of seven linear position sensitive 3He detectors placed horizontally one on top of the other. The detectors have a diameter of 25.4 mm and length of 676 mm of which 610 mm is active. The active length of the detector corresponds to about 800 active electronic channels with a dimension of 0.67 mm each. The electronics of the position-sensitive detectors have been developed at Studsvik Neutron Research Laboratory and were constructed in collaboration
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
with them. The whole detector assembly can be moved on rails in and out towards the sample position and the acceptance angle of the detectors system can vary from 18 to 56 in Bragg angle 2y; which correspond to sample to centre of detector distances of 1665 and 500 mm, respectively. This gives high flexibility to the experiments, i.e. high resolution or high counting rate needed for a dynamic experiment. The whole detection system moves on air-pads around the sample axis by stepping motors in order to vary the 2y range of measurement from 5 to 120 . As a result of the optimisation of the main shield construction, a low background was achieved that permitted the use of very light shielding for the detectors. Boron carbide of 5 mm was adequate for the shielding of the detectors system. The shielding of the detectors has a conical-like shape so that only the sample is seen. Different size apertures and windows allow for the measurement of different diameter samples and at different sample to detector distances. This construction in combination with the incident beam defining aperture eliminates any scattering from the beam path or air sample environment, i.e. cryostat windows. For the determination of the position of the neutron capture event the charge division method is used. The assignment of electronic channels to positions and then finally to angles is performed using a cadmium mask having a set of holes in front of the detectors and a strong scatterer, i.e. perspex. The transformation of channels to angles is performed for each detector individually, the data are corrected for unequal amplifications [8] and other geometric effects and the binning of the data is performed when the data from all detectors are transformed to angles. In that way, a good agreement from the seven detectors is achieved not only of the Bragg peaks but also of the Bragg tails. In Fig. 2 are presented two Bragg reflections from Al2O3 from a bank of seven detectors where one observes that the overlay of the same Bragg peak is better than 3.60 , much better than the overall resolution of the instrument. The resolution of the detection system is mainly defined by the sample width and the physical detection width. The physical detection width was
300
400
3.6' 200
Intensity (c/900 s)
164
300
100
200 43.00 43.05 43.10 43.15 43.20 43.25
100
41
42
43
44
2θ (degrees) Fig. 2. Bragg reflections from Al2O3 received from the bank of seven detectors.
determined by the mask measurements of about 3.5 mm. A point sample at a sample–detector distance of 1600 mm gives a half-angle detection resolution of about 40 and at 500 mm of about 120 . A sample of diameter of 5 mm at 1600 mm will give a half-angle detection resolution of about 90 and at distance 500 mm of about 290 . Thus, the longest distance may be used for high-resolution experiments in connection with the Ge monochromator and the short distance for lower resolution but high counting rate experiments. Intermediate conditions are on choice. It should be noted that detection resolution is independent on detector angle or detection position on the detector stack.
3. NEDI neutron diffraction spectra As a final test of the diffractometer, powder diffraction pattern of standard samples were measured. In Fig. 3 the diffraction pattern from Y2O3 powder is presented together with its Rietveld refinement using FullProf software. The
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
sample diameter was 9 mm and the wavelength ( from Ge (5 1 1) at 90 . For the used was 1.54 A description of the peak shapes the Thompson– Cox–Hastings, Pseudo-Voigt function convoluted with axial divergence asymmetry function has been used [9]. Good agreement was obtained between refined and expected parameters.
6700 r1226.prf: Yobs Ycalc Yobs-Ycalc Bragg_position
5900 5100 4300
Intensity (a.u.)
165
3500
References 2700 1900 1100 300 -500 -1300 0
20
40
60
80
100
120
2 theta Fig. 3. Rietveld refinement of diffraction pattern of Y2O3 powder measured at ‘‘Demokritos’’ neutron diffractometer.
[1] I.E. Stamatelatos, S. Messoloras, Rev. Sci. Instrum. 71 (2000) 70. [2] I.E. Stamatelatos, A. Salevris, S. Messoloras, Demo Reports, DEMO 98/7 (1998). [3] M. Popovici, W.B. Yelon, J. Neutron Res. 3 (1995) 1. [4] B. Lebech, K. Theodor, B. Breiting, P.G. Kealey, B. Hauback, J. Lebech, S.Aa. Sørensen, K.N. Clausen, Physica B 241–243 (1998) 204. [5] A. Magerl, Physica B 213&214 (1995) 917. [6] A.K. Freund, Nucl. Instrum. Methods 238 (1985) 570. [7] K. Mergia, S. Messoloras, Rev. Sci. Instrum. 74 (2003) 931. [8] K. Mergia, A. Salevris, S. Messoloras, Appl. Phys. A 74 (2002) S145. [9] L.W. Finger, D.E. Cox, A.P. Jephcoat, J. Appl. Crystallogr. 27 (1994) 892.
Physica B 350 (2004) 162–165
The new neutron powder diffractometer at ‘‘Demokritos’’ research reactor K. Mergia*, A. Salevris, S. Messoloras National Centre for Scientific Research ‘‘Demokritos’’, 15310 Aghia Paraskevi Attikis, Greece
Abstract The new neutron powder diffractometer, NEDI, recently installed at GGR-1 research reactor at the National Research Center of Scientific Research ‘‘Demokritos’’, Greece, is described. Instrumental aspects are discussed and examples of neutron diffraction patterns from standard samples are presented. r 2004 Elsevier B.V. All rights reserved. PACS: 83.85.Hf; 29.40.Cs; 61.12.Ld Keywords: Powder diffractometer; Position-sensitive detector; Focusing monochromator
1. Introduction
2. Description of the instrument
A new two-axis neutron powder diffractometer, NEDI, was recently installed at GRR-1 research reactor at the National Centre for Scientific Research ‘‘Demokritos’’ which has a power of 5 MW and light water is used as a moderator and coolant. The diffractometer was designed so as to enhance the diffraction data quality which can be obtained from a reactor of modest thermal power. Its applications are mainly focused on the study of crystalline and magnetic structures, phase transitions induced, e.g. by temperature change or by pressure, residual stress measurements and kinetics of chemical reactions.
The diffractometer was installed at a beam port which is at 30 angle to the reactor core. An in-pile main beam collimator of 1.87 m length is installed in the beam tube ending at a rotating drum which enables opening and closing of the beam. A sapphire filter of 75 mm thickness is used in the primary beam to reduce the fast neutron flux [1]. Fig. 1 shows the schematic layout of the instrument. The neutron-diffractometer radiation shielding was optimised by Monte Carlo neutron and photon transport code calculations (MCNP) [2]. The main shielding materials used were boron-doped polyethylene as neutron thermalising and absorbing material or lead–boron-doped polyethylene as neutron thermalising–absorbing and gamma absorbing material.
*Corresponding author. Fax: +30-210-6533431. E-mail address: [email protected] (K. Mergia).
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.04.019
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
Position sensitive detectors
sample aperture Collimators
neutron beam
monochromator
Reactor face
Fig. 1. Schematic layout of ‘‘Demokritos’’ neutron diffractometer.
2.1. Collimation and monochromatisation Two alternative multi-crystal monochromators are available: a germanium (Ge/hkk) and a pyrolytic graphite (PG/002). The Ge monochromator is made from plastically deformed germanium (5 1 1) wafers fabricated at Riso. National Laboratory. By applying uniaxial pressure at elevated temperatures to the Ge wafers a mosaic spread of around 150 is obtained [3–5]. In this way, a higher neutron flux is achieved by utilising a broader neutron wavelength band without deterioration of the resolution. The Ge monochroma( tor provides wavelengths in the range 0.41–2.95 A. The monochromator consists of five composite strips cut from composite germanium slabs composed of 20 wafers each. The strips are 16 mm in height, 50 mm in length and 8 mm in thickness. The pyrolytic graphite uses the (0 0 2) reflection ( and can provide wavelengths from 1.28 to 5.81 A and is used in combination with a graphite filter for the suppression of high order harmonics. The PG has a mosaicity of 300 . Both monochromators are set up in reflection geometry in order to obtain the best efficiency [6]. The neutron beam impinging on the monochromator has dimensions 90 40 mm2 (height width) and its natural divergence taking into
163
account the in-pile collimator and the beam tubes is 1.2 in the vertical and 0.6 in the horizontal direction. Two Soller collimators placed before the monochromator and made from mylar-foil covered with gadolinium oxide offer a collimation of 150 and 300 with transmission of 93% and 96%, respectively. Appropriate mechanical devices permit the exchange of the collimators and their alignment, the exchange of the monochromators, the Bragg angle setting of the monochromator, the alignment of the monochromator with the neutron beam and the focusing of the monochromator from 50 cm to infinity. Optimisation of the focusing conditions has been carried out by a position-sensitive detector and is discussed elsewhere [7]. The settings of the collimators and monochromators are controlled by a PC and software developed locally. The collimator and monochromator units are housed within the main instrument shield, which is constructed as to allow easy access to both units. The main monochromator shield provides 2yM take-off angles at 22 , 45 , 60 , 90 and 120 . 2.2. Sample area The sample area consists of a table on air cushion pads allowing easy movements to the different beam exit tubes. Different sample environments can be easily assembled allowing convenient sample mounting. The incident to the sample neutron beam is defined by an adjustable in height and width aperture. The incident beam may be focused on a minimum sample height of 2 cm giving an intensity gain of a factor of about three. 2.3. Detector characteristics The detection system consists of seven linear position sensitive 3He detectors placed horizontally one on top of the other. The detectors have a diameter of 25.4 mm and length of 676 mm of which 610 mm is active. The active length of the detector corresponds to about 800 active electronic channels with a dimension of 0.67 mm each. The electronics of the position-sensitive detectors have been developed at Studsvik Neutron Research Laboratory and were constructed in collaboration
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
with them. The whole detector assembly can be moved on rails in and out towards the sample position and the acceptance angle of the detectors system can vary from 18 to 56 in Bragg angle 2y; which correspond to sample to centre of detector distances of 1665 and 500 mm, respectively. This gives high flexibility to the experiments, i.e. high resolution or high counting rate needed for a dynamic experiment. The whole detection system moves on air-pads around the sample axis by stepping motors in order to vary the 2y range of measurement from 5 to 120 . As a result of the optimisation of the main shield construction, a low background was achieved that permitted the use of very light shielding for the detectors. Boron carbide of 5 mm was adequate for the shielding of the detectors system. The shielding of the detectors has a conical-like shape so that only the sample is seen. Different size apertures and windows allow for the measurement of different diameter samples and at different sample to detector distances. This construction in combination with the incident beam defining aperture eliminates any scattering from the beam path or air sample environment, i.e. cryostat windows. For the determination of the position of the neutron capture event the charge division method is used. The assignment of electronic channels to positions and then finally to angles is performed using a cadmium mask having a set of holes in front of the detectors and a strong scatterer, i.e. perspex. The transformation of channels to angles is performed for each detector individually, the data are corrected for unequal amplifications [8] and other geometric effects and the binning of the data is performed when the data from all detectors are transformed to angles. In that way, a good agreement from the seven detectors is achieved not only of the Bragg peaks but also of the Bragg tails. In Fig. 2 are presented two Bragg reflections from Al2O3 from a bank of seven detectors where one observes that the overlay of the same Bragg peak is better than 3.60 , much better than the overall resolution of the instrument. The resolution of the detection system is mainly defined by the sample width and the physical detection width. The physical detection width was
300
400
3.6' 200
Intensity (c/900 s)
164
300
100
200 43.00 43.05 43.10 43.15 43.20 43.25
100
41
42
43
44
2θ (degrees) Fig. 2. Bragg reflections from Al2O3 received from the bank of seven detectors.
determined by the mask measurements of about 3.5 mm. A point sample at a sample–detector distance of 1600 mm gives a half-angle detection resolution of about 40 and at 500 mm of about 120 . A sample of diameter of 5 mm at 1600 mm will give a half-angle detection resolution of about 90 and at distance 500 mm of about 290 . Thus, the longest distance may be used for high-resolution experiments in connection with the Ge monochromator and the short distance for lower resolution but high counting rate experiments. Intermediate conditions are on choice. It should be noted that detection resolution is independent on detector angle or detection position on the detector stack.
3. NEDI neutron diffraction spectra As a final test of the diffractometer, powder diffraction pattern of standard samples were measured. In Fig. 3 the diffraction pattern from Y2O3 powder is presented together with its Rietveld refinement using FullProf software. The
ARTICLE IN PRESS K. Mergia et al. / Physica B 350 (2004) 162–165
sample diameter was 9 mm and the wavelength ( from Ge (5 1 1) at 90 . For the used was 1.54 A description of the peak shapes the Thompson– Cox–Hastings, Pseudo-Voigt function convoluted with axial divergence asymmetry function has been used [9]. Good agreement was obtained between refined and expected parameters.
6700 r1226.prf: Yobs Ycalc Yobs-Ycalc Bragg_position
5900 5100 4300
Intensity (a.u.)
165
3500
References 2700 1900 1100 300 -500 -1300 0
20
40
60
80
100
120
2 theta Fig. 3. Rietveld refinement of diffraction pattern of Y2O3 powder measured at ‘‘Demokritos’’ neutron diffractometer.
[1] I.E. Stamatelatos, S. Messoloras, Rev. Sci. Instrum. 71 (2000) 70. [2] I.E. Stamatelatos, A. Salevris, S. Messoloras, Demo Reports, DEMO 98/7 (1998). [3] M. Popovici, W.B. Yelon, J. Neutron Res. 3 (1995) 1. [4] B. Lebech, K. Theodor, B. Breiting, P.G. Kealey, B. Hauback, J. Lebech, S.Aa. Sørensen, K.N. Clausen, Physica B 241–243 (1998) 204. [5] A. Magerl, Physica B 213&214 (1995) 917. [6] A.K. Freund, Nucl. Instrum. Methods 238 (1985) 570. [7] K. Mergia, S. Messoloras, Rev. Sci. Instrum. 74 (2003) 931. [8] K. Mergia, A. Salevris, S. Messoloras, Appl. Phys. A 74 (2002) S145. [9] L.W. Finger, D.E. Cox, A.P. Jephcoat, J. Appl. Crystallogr. 27 (1994) 892.