BATAN’s thermal neutron TAS: First results

Abstract

Revitalization project of the thermal neutron TAS – Triple Axis Spectrometer encoded as SN1 at the Experimental Hall of G.A. Siwabessy Research Reactor of BATAN Indonesia had been performed. For evaluating purpose of the newly developed hardware and software, inelastic scattering experiments to measure lattice dynamics on Cu and Ge single crystals had been carried out. The results had been compared and in a good agreement with references. Though still not optimal, these results show that the revitalization process has been successful in restoring the function of the SN1 as an inelastic scattering instrument. This can be considered as the first important step toward the utilization of the instrument for materials characterization on lattice dynamics and magnetic excitation.

Keywords

BATAN TAS inelastic neutron scattering

1. Background

Triple Axis Spectrometer, abbreviated as TAS, is a sophisticated neutron scattering instrument used for investigating structure as well as dynamics of material such as lattice dynamics (phonon) and magnetic spin excitation (magnon). Neutron scattering facilities around the world are competing in building this kind of instrument with good performance. For example, there is PANDA, a cold neutron based TAS located at FRM, Germany which recently commissioned its polarization option [6]. There is also TAIPAN, a thermal neutron based TAS located at ANSTO, Australia which successfully performed first experiment investigating soft phonon modes in superionic conductor ${Cu}_{2 - δ} Se$ , modulated antiferromagnetic/ferromagnetic structure in a quasi-single crystral FePt₃ thin film, and orbital peierls state in the vanadates [4]. Australia also has another TAS named SIKA, a cold-neutron based one which can be used in conventional and multiplexing mode [13]. NCNR America has just built its fifth TAS with the double focusing monochromator [9]. China also has designed a cold neutron based TAS to be installed at CARR in the near future [3].

Fig. 1.

BATAN’s thermal neutron TAS – Triple Axis Spectrometer.

Table 1

Technical data of BATAN SN1 Triple Axis Spectrometer

Beam tube	S4
Beam size at monochromator	35 mm × 55 mm (W × H)
Core–monochromator distance	2000 mm
Monochromator–sample distance	1780 mm
Sample–analyzer distance	850 mm
Analyzer–detector distance	630 mm
Monochromator take off angle	$15 < 2 T_{m} < 50$
Sample scattering angle	$- 5 < 2 T_{s} < 120$
Analyzer take off angle	$- 5 < 2 T_{a} < 90$
Incident energy range	$13 < E_{i} < 106 meV$
Energy transfer range	$1 < E < 50 meV$
Flux at the sample position	$1 \times 10^{5} n/ {cm}^{2} /s$

Fig. 2.

Constant Q scan with fix E_i/k_i (left) and fix E_f/k_f (right).

Researches on advanced materials, in particular eco-friendly materials, are actively performed and in progressively increase both in quality and quantity. One example that fall into the eco-friendly criterion is Lead-free piezoelectric material as replacement for PZT, the Lead-based ones [11]. Recent researches have shown the use of neutron scattering instruments in general and TAS in particular in searching candidates of Lead-free piezoelectric materials such as Bismuth Sodium Titanate [1,2,5,7,10]. This emphasizes that neutron scattering is indeed a powerful technique to investigate materials and a good chance to carry on the development of science and technology in the world.

Fig. 3.

Neutron count at different values of energy (E) and wave vector (q) of transverse acoustic branch 1, TA1 phonon of Cu single crystal in [ $ζ ζ 0$ ] direction.

Triple axis spectrometer is one of seven neutron instruments available at the National Nuclear Energy Agency of Indonesia – BATAN (Badan Tenaga Nuklir Nasional) and encoded as SN1. SN1 was installed at the Experimental Hall of G.A. Siwabessy Research Reactor. The reactor is located about 30 km in the south part of Jakarta, the capital city of Indonesia. The installation process was performed in 1993–1994. In 1995, a preliminary result of inelastic neutron scattering experiments was obtained from SN1. This data became the first and unfortunately the last data coming out from the instrument, since then. This was due to hardware and software problems of SN1 that has been in an idle state as an inelastic scattering instrument since 1996. A team was formed with the main goal to revitalize and optimize SN1 so it could be reutilized for inelastic scattering experiments. Based on SN1 manuals, previous works, and by using available references, the team had started to work under the BATAN’s Research Project for Development of Neutron Facilities and Spectrometry Technique 2011–2014. In 2011 the main program involved the designing and developing electronics hardware to control all the electro-mechanical and pneumatic systems. In 2012 the program was focused on designing and developing new software to control the hardware and for data acquisition. We had implemented four modes of inelastic scattering measurements, i.e. constant scattering vector Q with fixed initial energy $E_{i}$ and fixed final energy $E_{f}$ , and constant energy transfer E with fixed $E_{i}$ and fixed $E_{f}$ by applying UB matrix calculation [8]. In 2013 the main program was characterizing the newly developed hardware and software. The elastic scattering measurements using standard powder samples such as Ni, Si, Al₂O₃, and a Vanadium rod are routinely measured for instrument alignments and wavelength calibrations. Inelastic scattering experiments to measure phonons have been performed on available single crystals samples i.e. Cu and Ge. In 2014–2015 we are optimizing SN1 on how to increase the intensity by developing a focusing monochromator and analyzer systems and to decrease the background by building new shielding for unwanted thermal neutrons. It is expected that the neutron intensity at sample position will be increased by factor of 4 to 5, and simultaneously produce lower background counted by the main detector. The sample environment with a cryostat is also being installed for future temperature dependent measurements.

2. Materials and methods

2.1. Instrument specification

SN1 (Fig. 1) was positioned at the beam port number 4 (S4). The effective beam size cross section that deliver the neutron to the monochromator is relatively small i.e. 35 mm × 55 mm (W × H). The monochromator is a single flat PG(002) with dimension of 40 mm × 74 mm × 1.3 mm (W × H × T), while for the analyzer is also a single flat PG(002) with dimension of 49 mm × 74 mm × 1 mm (W × H × T). There is no filter set to block high energy neutron before the monochromator. The neutron sense is in W configuration. Table 1 gives summary of SN1 technical data.

2.2. Methods of measurement

First inelastic scattering experiments were performed using available Cu and Ge single crystals with the dimension of 10 mm × 10 mm × 10 mm for Cu and 10 mm × 50 mm × 100 mm for Ge respectively. Scattering planes were constructed by respectively aligning Cu[111] and Cu[2-20] and Ge[220] and Ge[004] scattering vectors. Configuration of neutron collimation was 40′–40′–40′ – Open (between source–monochromator, monochromator–sample, sample–analyzer, and analyzer–detector). The area of incident slit was 16 mm (horizontal) × 30 mm (vertical) while for scattered slit was 20 mm (horizontal) × 40 mm (vertical). The experiments were performed with 15 MW un-polarized thermal neutron and at room temperature. The wavelength of monochromatic neutron was calibrated using standard sample Silicon powder. The instrument was optimally aligned to find the optimal position of monochromator take off angle $2 θ_{m}$ and scattering angle $θ_{m}$ , and zero position of the sample goniometer $2 θ_{s}$ to achieve the maximum intensity. Neutron energy was calibrated using a Vanadium rod with the diameter of 5 mm and 30 mm in height. The optimal position of takeoff angle and scattering angle of the crystal analyzer $2 θ_{a}$ and $θ_{a}$ , respectively were obtained from the energy calibration. Two types of inelastic neutron scattering experiments were performed by scanning the energy transfer E at constant-Q (momentum transfer). First type, constant-Q with fixed neutron incoming energy $E_{i} = 30.5 meV$ and second type, constant-Q with fixed neutron final energy $E_{f} = 14.7 meV$ as illustrated in Fig. 2. The experiments by scanning momentum transfer Q at a constant energy transfer E with fixed $E_{i} = 30.5 meV$ were also performed to measure the signal of phonon of longitudinal acoustic (LA) and transverse acoustic (TA) modes. During whole measurements none of wavelength harmonic filters were installed before and after the sample.

Fig. 4.

Neutron count at different values of energy (E) and wave vector (q) of acoustic transversal mode branch-2, TA2 phonon of Cu single crystal in [ $ζ ζ 0$ ] direction.

Fig. 5.

Dispersion relation of acoustic mode phonon of Cu single crystal in [ $ζ ζ 0$ ] direction measured by SN1 and by Svensson et al. [12].

Data of inelastic scattering measurements were fitted with Gaussian fit + constant background method by applying following equation: $\begin{matrix} S (E) = BG (E) + I_{0} / β * sqrt (log (2.0) / π) * exp (- 4.0 * log (2.0) * {(E - E_{0})}^{2} / β^{2}), \end{matrix}$ where $\begin{matrix} BG (E) = {bg}_{0} + {bg}_{1} \cdot E + {bg}_{2} \cdot E^{2} + {bg}_{3} \cdot E^{3} . \end{matrix}$ Here S is fitting function, $I_{0}$ is the maximum intensity of scattered beam, β is a parameter, E and $E_{0}$ are neutron energy transfer, ${bg}_{0}$ , ${bg}_{1}$ , ${bg}_{2}$ , and ${bg}_{3}$ are background fitting parameters.

3. Results and discussion

Figure 3 shows a result of energy scanning at several values of phonon wave vector q. The transverse acoustic phonons branch 1 were measured in Cu[ $ζ ζ 0$ ] direction at $q = 0.1 to 0.5$ . From Fig. 3, it can be seen that the neutron intensity decrease as the transfer energy E(q) increase. This indicates that the observed scattered neutron is really coming from the vibration of the atom (phonon) in the samples. The scanning energy of transverse acoustic branch 2 phonon of Cu in [ $ζ ζ 0$ ] direction at phonon wave vector $q = 0.1 to 0.4$ is shown in Fig. 4. As shown in previous results, the neutron intensity decrease with increasing energy transfer. Data of Figs 2 and 3 were plotted with a relationship between phonon energy and the phonon wave vector q i.e. phonon dispersion relation as shown in Fig. 5. Data measured by Svensson et al. [12] is also shown in Fig. 5 for comparison.

Figure 6 shows peaks of longitudinal acoustic phonon of Cu[ $ζ ζ ζ$ ] at $q = 0.05, 0.15, and 0.2$ . At larger q, it was difficult to observe the phonons due to the inverse relationship of neutron intensity and energy and also because of very low peak to background ratio. Neutron intensity of transverse acoustic phonon of Cu[ $ζ ζ ζ$ ] at $q = 0.05, 0.1, 0.15, 0.2, 0.3, 0.35, 0.4, 0.45, and 0.5$ is shown in Fig. 7. As previous measurement in different mode and direction, the inverse relationship between neutron count and energy transfer was observed. Figure 8 shows a phonon dispersion relation of Cu in [ $ζ ζ ζ$ ] direction from present measurement and data of reference [12]. It can be observed that SN1 has produced the same result as obtained by the reference.

Fig. 6.

Neutron count at different values of energy (E) and wave vector (q) of longitudinal acoustic LA mode of Cu single crystal in [ $ζ ζ ζ$ ] direction.

Fig. 7.

Neutron count at different values of energy (E) and wave vector (q) of acoustic transversal TA mode of Cu single crystal in [ $ζ ζ ζ$ ] direction.

Fig. 7.

(Continued.)

Fig. 8.

Dispersion relation of acoustic mode phonon of Cu single crystal in [ $ζ ζ ζ$ ] direction.

Constant Q with fixed $E_{f}$ mode scans were performed to measure the signal of transverse acoustic phonon of Ge[ $00 ζ$ ] with wave vector $q = 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0$ . These data are shown in Fig. 9. Measurements by constant Q with fixed $E_{f}$ mode show relatively lower intensity and narrower profile compare to the fixed $E_{i}$ mode.

Fig. 9.

Phonon groups for acoustic transverse mode of Ge single crystal in [00ζ] direction.

Experiments with constant energy transfer with fixed initial energy were also performed. An example of the measurements were shown in Fig. 10 which displays transverse acoustic phonon of Cu in the direction of [ $ζ ζ ζ$ ] for the value of $q = 0.05, 0.1, 0.15, and 0.3$ .

Fig. 10.

Phonon groups for acoustic transverse mode of Cu single crystal in [ $ζ ζ ζ$ ] direction.

4. Conclusion

BATAN’s thermal neutron Triple Axis Spectrometer SN1 has been successfully revived as an inelastic scattering instrument and is ready to perform the investigation of lattice dynamics and magnetic excitations of condensed matters. To enhance capability of the instrument, focusing system of monochromator and analyser are being developed to increase the intensity. New shielding is under development to decrease the background as well as installation of sample environment i.e. cryostat for temperature dependence measurements. During upgrading project SN1 is still open for users to perform inelastic scattering experiments.

Footnotes

Acknowledgements

This project is funded by Research Project of DIPA BATAN 2011–2015. Full support also received from the late Dr. Sutiarso, Head of Neutron Beam Technology Division and Drs. Gunawan M.Sc., the Director of Center for Sciences and Technology of Advanced Materials BATAN. The Ministry of Research and Technology of Republic of Indonesia has granted one of the authors (IS) a fellowship to attend the 6th Central European Training School on Neutron Technique (CETS 2012) in Budapest, Hungary with an additional stay in Budapest Neutron Centre – BNC for almost two months. We gratefully thank Dr. M.D. Lumsden of Oak Ridge National Lab. USA, Dr. Mark Koennecke of Paul Scherrer Institute Swiss, Dr. Gyula Torok, Dr. Alex Szakal, and Dr. Marko Marton of Budapest Neutron Center BNC Hungary, Dr. Taku J. Sato of Institute of Multidisciplinary Research for Advanced Materials Tohoku University Japan, for their significant assistances during the SN1 revitalization process, TAS data analysis using GnuPlot, calculation of TAS resolution function using fit3ax and ResLib, and the basic theory of resolution convolution to fit the raw data of TAS. The author (IS) is grateful to Prof. Dr. Evvy Kartini of the Center for Science and Technology of Advanced Materials – PSTBM BATAN for her valuable advice on neutron scattering and manuscript preparation.

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