bNuclear Science Center, Aruna Asaf Ali Marg, New Delhi 110 067, India. The energy loss of 25 MeV protons in ferroelectric Barium Titanate [BT] single crystals.
Ferroelectrics, 323:65–70, 2005 Copyright © Taylor & Francis Inc. ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150190500308694
Temperature Dependent Energy Loss of Protons in Barium Titanate Single Crystals KHADKE UDAY KUMAR,a,∗ B. R. KERUR,a S. M. HANAGODIMATH,a M. T. LAGARE,a D. K. AVASTHI,b AND A. MANDALb a
Department of Physics, Gulbarga University, Gulbarga 585 106, Karnataka, India b Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi 110 067, India The energy loss of 25 MeV protons in ferroelectric Barium Titanate [BT] single crystals was measured as a function of the temperature of BT with an accuracy of 0.06%. When the protons traversed the crystal along the ferroelectric axis energy loss showed broad fluctuations with a peak-to-peak variation of 5% from the value at room temperature and the energy losses in ferroelectric and paraelectric phases of BT appeared to be nearly the same. When protons traversed the crystal in a direction perpendicular to the ferroelectric axis energy loss in the crystal remained independent of the crystal temperature in the ferroelectric phase. However, the energy loss in paraelectric phase appeared to be about 1.5% lower than that in the ferroelectric state. It is suggested that the observed results are indicative of a new mode of energy loss and can be related to the fluctuations in polarization of cluster of unit cells and the dynamics of their short-range order. Keywords Ferroelectrics; energy loss of charged particles; stopping power
Introduction When an energetic charged particle penetrates a material medium it loses energy mainly by two nearly independent processes: (i) elastic collisions with the nuclei known as nuclear energy loss, which dominates at an energy of about 1 keV/amu; and (ii) inelastic collisions with the atomic electrons of the matter known as electronic energy loss which dominates at an energy of about 1 MeV/amu or more. In this process, the target atom is not displaced but only excited or ionized. A new mode of energy loss in the form of a critical phenomenon was recognised by one of the present authors [1, 2], which involved the role of electric dipole moment of unit cells and their short-range order in a ferroelectric medium. It was observed that the energy loss of 620 keV and 942 keV electrons in polycrystalline barium titanate (BT) as well as that of 942 keV electrons in crystalline triglycine sulphate (TGS) was anomalously high by about 4% around the ferroelectric Curie temperature (Tc ). It was suggested that in the neighborhood of Tc an incident electron interacts with a cluster of unit Paper originally presented at AMF-4, Bangalore, India, December 12–15, 2003; received in final form April 10, 2005. ∗ Corresponding author.
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cells and loses its energy to the cluster by inducing greater disorder in the dipole moments in the cluster. Lagare and Umakantha also suggested an immediate application of the new phenomenon in radiation shielding [3]. In similar experiments Ayyub et al. [4] observed an increase in energy loss by about of 12% around Tc . They clearly established that the critical enhancement of the energy loss of fast electrons in different types of ferroelectric media is correlated to the magnitude and temperature occurrence of the dielectric response. There have been no further reports of experimental results on the energy loss of charged particles in ferroelectrics. There have been some attempts to explain these new experimental results on energy loss of electrons in ferroelectrics [5, 6]. However, the physical origin of this phenomenon has not been thoroughly understood. It is felt that more experimental data is necessary to pin point the mechanism responsible for the critical energy loss of charged particles in ferroelectrics [1, 4, 5]. It is therefore interesting to extend the experimental investigations to the energy loss of high-energy protons in ferroelectrics. In the present paper we report our findings on the measurements of the energy loss of 25 MeV protons in BT single crystals.
Experimental Details Ferroelectric Crystals Single crystals of ferroelectric Barium Titanate with two crystallographic orientations and a size of 10 mm × 10 mm were procured from MTI Corporation, USA. The crystals with larger (10 mm × 10 mm) faces parallel to the ferroelectric c-axis had a thickness of 0.796 mm and we will refer to them as BT-0. The crystals with larger faces perpendicular to the ferroelectric c-axis had a thickness of 0.849 mm and we will refer to them as BT-1. Both the crystals had a density of 6.03 gm/cm3 . Proton Spectrometer The proton beam from pelletron accelerator at Nuclear Science Center, New Delhi was used. The experimental chamber and spectrometer for recording the energy spectrum of the protons is explained in detail elsewhere [7] and is briefly described here. The proton beam of about 10 mm in diameter entered the scattering chamber of diameter 356 mm, maintained at 2.5 × 10−6 torr. A gold foil of 300 nm thickness with tantalum collimator of 4 mm dia was mounted normal to the beam direction at the center of the scattering chamber. The protons scattered by the gold foil at an angle of 30◦ were incident on the crystal and the protons transmitted through the crystal were detected by a 5 mm thick silicon surface barrier detector operated at room temperature. The scattered beam was used to reduce the proton intensity on to the crystal to about 1000 particles/sec so that there were no radiation induced defects in the crystal and that the detector did not suffer from radiation damage. The basic principle used for measuring the energy loss of protons in BT crystals was same as that used for electrons [1]. The crystal was held in an aluminium sample holder (SH) and its temperature was controlled by controlling the temperature of the SH by a PC based system with an accuracy of 0.01◦ C as explained earlier [7]. A 4 mm × 8 mm square hole through the SH served to collimate the beam incident on the sample and the transmitted beam reaching the detector. The size of the BT crystal and its positioning inside the SH was such that it covered 4 mm × 6 mm area on the lower part of the hole. Thus 80% of the beam was allowed to pass through the sample while the 20% of the beam, going through the upper open area of the hole, reaches the detector directly. Thus the detector receives both
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Figure 1. Typical proton energy spectrum showing the peaks corresponding to the incident beam and the transmitted beam. Inset shows the sample holder with crystal.
the incident beam and the transmitted beam. Hence the recorded spectrum will have one peak at E1 corresponding to the incident beam and another peak at lower energy E2 corresponding to the beam transmitted through the BT crystal. A typical spectrum is shown in Fig. 1. The paths of incident and the transmitted beam through the SH are shown schematically in the inset to Fig. 1. The most probable values of E1 and E2 were obtained by fitting Gaussian curves to the two peaks and the most probable energy loss of the protons in the crystal was taken to be given by E = E1 − E2 . For the sake of brevity we will refer to E as energy loss of 25 MeV protons in the crystal. Dielectric Measurements Gold film electrodes of 20 micron thickness were deposited on the two surfaces of the crystals by vacuum evaporation method. Crystal was held in SH and its dielectric constant was measured as a function of temperature at 10 kHz using HP 4192 impedance analyzer. Dielectric measurement was done thrice: 1) at all the selected temperatures before starting the energy loss measurement to ensure that crystals were of high quality, 2) at each of the temperature just before energy loss measurement at that temperature and 3) at all the selected temperatures at the end of the energy loss measurement to verify whether the crystal has suffered any degradation in its ferroelectric properties.
Results and Discussions The online dielectric constant data for the BT1 crystal recorded manually just before energy loss measurements at each of the selected temperatures is shown in Fig. 2. The dielectric response data recorded before the starting of the energy loss measurement and after the energy loss measurements were similar showing that the crystals were of high quality and that its ferroelectric property was not affected due to protons passing through it. The results for BT0 were similar.
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Figure 2. Temperature dependent dielectric constant of BT1.
The energy of the incident protons, E1 , was 24.965 ± 0.005 MeV for both the crystals while energy of the transmitted protons, E2 , at room temperature was 16.735 ± 0.005 MeV for BT0 and 17.830 ± 0.01 MeV for BT1. Thus, the energy loss, E, was 8.230 ± 0.015 MeV in BT0 and 7.075 ± 0.015 MeV in BT1. The variation of E with crystal temperature is shown in Fig. 3 for BT0 and in Fig. 4 for BT1. The striking difference in the energy loss in the two crystals of different orientations can be clearly seen. In the case of BT0, E is almost constant at about 8.23 MeV over the ferroelectric phase and decreases as the Curie
Figure 3. Energy loss of protons in BT0.
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Figure 4. Energy loss of protons in BT1.
temperature is approached and reaches a lower value of about 8.10 MeV in paraelectric phase. In the case of BT1 E shows systematic broad fluctuations with a peak-to-peak variation of about 5%. We may now try to understand the difference in the E curves for the two crystals qualitatively in terms of the role played by the ferroelectric domains. In the case of passage of electrons in ferroelectrics, according to Ayyub et al. [4], the electrons lose additional energy when they move along ferroelectric axis as compared to when they are moving perpendicular to it. They associated this enhanced energy loss with the alignment of the domains along the c-axis by the high transverse electric field of the fast electrons moving through the domain. In the present case the protons would be moving at much smaller velocity as compared to the electrons and, hence, the transverse electric field would be weaker. In BT0 alignment of domains does not seem to be taking place. However in BT1 the electric field of protons may be enough to cause the alignment of some of the domains. This seems to be happening over a broad temperature range over which de-pinning of the domains is facilitated. The de-pinning of the domains and their subsequent alignment involve complex mechanisms and depend on the nature and history of the crystal. In the absence of quantitative study of the domain patterns the present results may be taken to show qualitatively correlation between the energy loss fluctuations and the polarization fluctuations in the crystals. Additional measurements with associated study of dynamics of the domains is necessary to establish the correlation between the short range order of the unit cell dipoles and the fluctuations in energy loss of charged particles in ferroelectrics.
Acknowledgments The authors would like to thank Prof. A. M. Umarji for the help in preparation and characterization of the ferroelectric samples. One of the authors (Khadke UdayKumar) is grateful to the CSIR for the financial assistance.
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