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Magnetic–electronic pressure studies of natural iron-bearing minerals and materials using 57Fe Mössbauer spectroscopy in a diamond anvil cell
Nuclear Instruments and Methods in Physics Research B 183 (2001) 413±418
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Magnetic±electronic pressure studies of natural iron-bearing ssbauer spectroscopy minerals and materials using 57Fe Mo in a diamond anvil cell S. Takele *, G.R. Hearne Department of Physics, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg-Gauteng, South Africa Received 23 January 2001; received in revised form 24 April 2001
Abstract The possibility of measuring 57 Fe M ossbauer spectra of natural isotopic abundance materials at variable high pressures and cryogenic temperatures in a diamond anvil cell (DAC) on a timescale comparable to conventional M ossbauer spectroscopy (MS) is described. This is exempli®ed by satisfactory spectra obtained for an ilmenite FeTiO3 sample (absorber) in 12 h at temperatures where the sample is paramagnetic and in 20±30 h below spinordering temperature of 60 K where resonance intensity is reduced due to magnetically split spectral components. A commercially available 57 Co Rh point source of 14.4 keV resonant radiation and a Kr±CO2 proportional counter have both been used. Suciently high count-rates are obtained by using both the 14.4 keV resonant c-ray and associated 1.8 keV escape peak events. To optimise the resonance eect, careful attention has been paid to minimize non-resonant radiation within the discriminator window set to select the 14.4 keV resonant radiation. This has been achieved by setting an appropriate source±DAC±detector geometry (i.e., solid angle) to reduce scattering events o components of the DAC to a minimum. By using conventional commercially available M ossbauer apparatus, magnetic±electronic properties of iron-bearing minerals and materials with an iron content greater than 20% may be investigated to pressures in excess of 10 GPa encompassing many minerals of the earth's interior. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 76.80.+y Keywords: High pressure; M ossbauer spectroscopy; Materials; Proportional counter
1. Introduction *
Corresponding author. Present address: Department of Physics, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth 6000, South Africa. Tel.: +27-41-5042255; fax: +2741-5042573. E-mail address: [email protected] (S. Takele).
Diamond anvil cells (DACs) have evolved from being bulky cumbersome devices to versatile miniature multipurpose instruments. Speci®cally for the case of M ossbauer spectroscopy (MS) measurements, miniature Merrill-Bassett [1] and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 9 5 - 4
414
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
piston±cylinder [2] DACs have been designed to permit pressure studies up to 35 GPa and also beyond 100 GPa, respectively, depending on the culet dimensions. The compact features ensure that the DAC is easily loaded into a cryostat for variable cryogenic temperature measurements. This permits an investigation of the magnetic properties of materials under pressure at cryogenic temperatures where many minerals order magnetically. The main constraint to MS applications in a DAC is the small sample area which is less than 10 3 cm2 (typical sample dimensions are 300 lm diameter 50 lm thickness) as compared to the area 1 cm2 of sample or source used in conventional MS. The additional constraint of 75% attenuation of the incident 14.4 keV c-ray radiation by the diamond anvils has precluded the application of high-pressure 57 Fe MS to natural Fe-bearing materials and minerals pressurized in a DAC at cryogenic temperatures. This is due to the protracted measuring times required to achieve satisfactory signal-to-noise ratios, especially in the case of magnetically ordered spectra [3±5]. To circumvent these constraints Hearne et al. [3] re®ned and extended the performance of the miniature Merrill-Bassett DAC 57 Fe MS studies at cryogenic temperatures. Their re®nements to the already existing high-pressure MS methodology included using 1. a point source with high speci®c activity to concentrate the 14.4 keV c-ray radiation ¯ux on the minute sample cavity, 2. 57 Fe isotopically enriched absorbers to enhance the resonance eect, and 3. an argon±methane gas-®lled proportional counter to achieve a high signal-to-background ratio. Enrichment of the absorber, however, has two disadvantages, namely that the isotope is expensive and only synthetic materials may be investigated. On the other hand, if materials of natural isotopic abundance are to be investigated using MS in a DAC at high pressure, measurements have to proceed for several days at room temperature to obtain a satisfactory signal-to-noise ratio. Although 57 Fe MS in a DAC at room temperature has been applied to natural abundance materials in recent years [4,6], almost no experimental
details have been published. Under the constraints typical of 57 Fe MS in a DAC, acquisition times greater than 48 h are required. It is of interest to improve the existing high-pressure 57 Fe MS methodology to permit a convenient investigation of magnetic±electronic properties of materials of natural isotopic abundance pressurized in a DAC. Data accumulation times should be minimized so that pressure measurements at extreme high or low temperatures are not prohibitive.
2. Experimental The 57 Fe M ossbauer facility for high-pressure studies at cryogenic temperatures is similar to the one previously reported by Hearne et al. [3] except that a detachable extension tail exists at the bottom of the cryostat having Mylar windows on the bottom and sides of the extension tail. This is designed for both vertical and horizontal transmission MS experiments. A top-loading probe similar to the one described by Hearne et al. [3] has been designed to accommodate the miniature MerrillBassett-type DAC for variable high-pressure cryogenic-temperature 57 Fe MS measurements in a vertical transmission geometry. A custom-made 57 Co(Rh) point source with speci®c activity of 2000 mCi/cm2 has been used. It was produced 1 by thermally diusing 57 Co activity into a 12 lm thick rhodium foil and then ®xing an active area of 0:5 0:5 mm2 into the tip of a tapered aluminium holder (see Fig. 1). It was determined by MS using a single-line absorber (stainless steel) at room temperature and the point source at 4.2 K that the source is initially an unsplit single line, implying that the 57 Co activity is uniformly distributed in the rhodium matrix. The magnetic ordering temperature, Tm of the source increases by 4 K/at.% of iron in rhodium as a result of the magnetic interaction between nearest-neighbor cobalt and iron atoms. 2 We 1 The point source is supplied by Wissel GmbH (W urstrasse 8, D-82319 Starnberg, Germany). 2 Indicated in Amersham International products-catalogue on M ossbauer sources (see also [3]).
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
415
temperature to a saturation value of 30 K after a sucient number of 57 Co nuclei have decayed over an extended period of months [3]. A heating scheme similar to that applied by Hearne et al. [3] may be used to maintain the source temperature above Tm when measurements of the sample at T < Tm are required. The detector for MS measurements in a DAC must be able to discriminate between the resonant M ossbauer c-rays and non-resonant radiation from both the high-energy c-ray and X-ray transitions originating in the 57 Co(Rh) source as well as from scattering o the constituents of the cell. Proportional counters with high eciency and resolution are required. To achieve this, Hearne et al. [3] used a custom-developed argon±methane gas-®lled proportional counter with relatively high eciency of 65% for detecting the 14.4 keV resonant radiation. Their pulse-height (energy) spectrum in the example of an FeI2 sample at 20 GPa clearly shows the 14.4 keV peak dominating both background as well as all other X-ray peaks. An alternative is to use a commercially available Kr±CO2 gas-®lled proportional counter having an eciency of 12% for the 14.4 keV radiation and a resolution of 60%. In addition to the 14.4 keV resonant transition, the associated 1.8 keV escape peak [9] 3 may be used to register the M ossbauer eect. This escape peak typically contributes 40% of the total count-rate (700 counts/s) as established when an empty pressurized sample cavity at a DAC±detector (D±D) separation of 70 mm was used.
Fig. 1. Schematic diagram of the arrangement of the DAC for MS measurements. Label S refers to the point source, D denotes the diamonds, B denotes the backing plates, G is the gasket, A is the pressurized absorber, and C is the detector.
expect a progressive build-up of iron within the rhodium matrix as a consequence of the decay of 57 Co having a half-life of 270 days. Consequently Tm of the source will rise above liquid helium
3 The 1.8 keV escape peak is produced as a result of secondary processes within the detector. Incident radiation energy impinging upon a counter gas gives rise to a proportionate number of ionization events that constitute the detection signal. Ionization is a result of the photoelectric eect and accompanying X-ray events. In the case of a Kr±CO2 counter, the incident 14.4 keV resonant radiation exceeds the absorption edge of the counter gas Kab 14:32 keV. A K-shell photoelectron may be produced accompanied by a characteristic krypton Ka X-ray of 12.6 keV energy. These 12.6 keV X-rays may escape from the counter gas without producing any primary ionization. In this case the only ionization events registered are those corresponding to the energy dierence 14.4± 12.6 keV deposited in the counter, thus giving rise to a characteristic low-energy escape peak at 1.8 keV.
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S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
3. Geometric considerations In conventional MS an attempt is made to maximize the count-rate by maximizing the solid angle of radiation ¯ux subtended by the combination of the sensitive area (window) of the detector, diameter of the sample and diameter of the source. This corresponds to reducing detector± sample±source distances to the smallest practical value but maintaining a minimum working distance to ensure that line-shape distortions due to cosine smearing eects [10±12] are negligible. Cosine smearing eects lead to line-shape distortions resulting from a range of incident resonant c-ray energies within the solid angle subtended by the source±detector con®guration for a given relative velocity between source and absorber. The cosine smearing eect may be rendered negligible by setting a minimum source±detector distance such that the angle a tan 1 s=2d is less than unity (e.g., see [12]), where s is the source diameter and d is the source±detector distance. In the case of MS DAC applications, cosine smearing eects are negligible at the minimum practical source±DAC±detector distances used, because of the comparatively small solid angle determined by the minute sample cavity. The inclination therefore is to minimize source-to-cavity and DAC-to-detector distances to achieve maximum count-rates. However, an inappropriate working geometry enhances the nonresonant background radiation within the discriminator window that has been set for the 14.4 keV resonant radiation. The non-resonant radiation arises from Compton scattering of highenergy c-rays originating in the source, and from photoelectric scattering of the c-rays and the resultant X-rays, which in turn are scattered. These scattering events may have ®nal non-resonant energies coincident with that of the 14.4 keV resonant radiation. The scattering is predominantly from components of the DAC. Fig. 1 is a schematic of the geometric arrangement of essential parts of the DAC for high-pressure MS. For the source of high speci®c activity (2000 mCi/cm2 and dimensions 0:5 0:5 mm2 the sample cavity is seen to act almost as an ideal point source of radiation. A minimum distance d may be set for which most of the source radiation
¯ux reaches the detector without ®rst intersecting components of the DAC. This reduces scattering from components of the DAC, and hence nonresonant radiation, to a minimum while maximizing the source±DAC±detector solid angle. An estimate of this optimal distance may be calculated from the geometry of Fig. 1. It is trivial to show that the optimum distance is d 2rh=t, where r is the radius of the detector window, h is the thickness of the diamond measured from the centre of the sample cavity and t is the diameter of the cone of the backing plate at the base of the diamond. Using typical values of r 13 mm, h 2 mm and t 0:8 mm, the optimum distance is estimated to be d 65 mm. 4. Results Fig. 2(a) depicts normalized pulse height spectra recorded for selected D±D distances with a 10 mCi point source. The DAC is at ambient pressure with an empty cavity of dimensions 300 lm 50 lm drilled in a Ta90 W10 gasket. These are typical conditions of MS in a DAC at high pressure. At the high D±D distance (65 mm in Fig. 2(a)), the 14.4 keV dominates both the non-resonant (X-ray) radiation peaks and the background when compared to the smaller D±D distance (10 mm in Fig. 2(a)). This is also clearly observed in Figs. 2(b) and (c) where the ratio of the 14.4 keV c-ray to that of the 6 keV X-ray is a maximum for D±D distances between 50 and 75 mm in agreement with the geometric considerations of the previous section. At D±D values beyond 70 mm the count rate of the 14.4 keV c-ray radiation also starts to decrease as some of the radiation ¯ux falls outside the sensitive area of the detector, see Fig. 2(c). 57 Fe MS measurements have been made on a natural specimen of ilmenite FeTiO3 pressurized in the DAC. A pair of 0.3 carat 16-sided diamonds each 2 mm thick with a truncated culet of 0.7 mm diameter have been used. A hole drilled in a preindented Ta90 W10 gasket constitutes the sample cavity of 0.3 mm diameter by 0.05 mm thick dimensions at the initial stage of the measurement. Shifts in the wavelength of ruby ¯uorescence peaks have been used for pressure calibration at room
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
(a)
(b)
(c)
Fig. 2. (a) Normalized pulse height spectra for a point source at selected D±D distances of 10 and 65 mm, (b) peak ratio of the 14.4 keV c-ray to the 6 keV X-ray as a function of D±D and (c) counts per second in a discriminator window set for the resonant 14.4 keV c-ray as a function of D±D. Experimental conditions in all cases involve a point source with an empty sample cavity (300 lm 50 lm) drilled in a Ta90 W10 gasket of a DAC.
temperature. Further details of the procedures used to measure the pressure have been published previously [13] (see also [1]). The variation of pressure with temperature for such a DAC in the range 20±300 K is less than 5%. Liquid argon is
417
used as a hydrostatic pressure-transmitting medium. All the spectra are recorded in a vertical transmission mode. Two independent singlechannel analyzers have been used to select the energy windows for the 1.8 keV escape peak and the 14.4 keV resonant transition as input to a fast linear ampli®er. 4 Data were collected in 1024 channels. In the analyzing program each spectrum and its mirror image were folded into 512 channels and adjacent channels were added to further reduce statistical scatter. Spectra recorded at room temperature on an ilmenite sample pressurized to 14 GPa indicated an appreciable change in maximum resonance intensity as the D±D distance decreases, being 6% at 70 mm and 4% at 5 mm. Hence, in all measurements a D±D distance of 70 mm has been maintained. With this D±D setting and the absorber at ambient conditions in the DAC, count rates of 210 and 350 counts/s have been obtained respectively in the discriminator windows set for the 1.8 keV escape peak and the 14 keV radiation ¯ux of a 5±10 mCi point source. There is 10% increase in the count rates in each window when the sample is pressurized to 14 GPa. Fig. 3 shows spectra recorded at 14 GPa in the range 300±20 K. Theoretical ®ts to the room temperature spectrum indicate that the sample consists of three distinct sub-spectra of which one is a magnetic component of 11% abundance and two paramagnetic doublets ascribed to ferric and ferrous contributions of 34% and 55% abundances, respectively, as obtained from the areas under each spectral component. The sub-spectra of the two formerly paramagnetic components start to display a magnetic splitting below 64 K. It is evident from Fig. 3 that at an onset temperature near 60 K there are line-broadening effects, a change in the relative intensity near zero velocity and also an apparent increase in intensities in the wings of the spectra. This is an indicative of the onset of magnetic ordering temperature which for this sample at 14 GPa appears to be in the range 54±64 K, similar to that of the material
4 The linear ampli®er is Canberra model 2020 manufactured by Canberra Industries, 45 Gracey Ave., Meriden, CT.
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S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
times. This has been achieved with conventional commercially available M ossbauer equipment with due regard to reducing non-resonant background contributions. This oers the possibility for magnetic±electronic studies of many natural iron-bearing minerals and materials under extreme P±T conditions of interest in the geosciences and in condensed matter physics. Acknowledgements Financial support from NRF-ORP (Pretoria) and University of the Witwatersrand is acknowledged with gratitude. Technical assistance from the Science Faculty workshop is gratefully appreciated. The natural sample of ilmenite used in these experiments has been obtained from Dr. J. Nell of the Mineral and Process Chemistry Division at Mintek (Randburg), South Africa. References
Fig. 3. Spectra of natural ilmenite FeTiO3 recorded at 14 GPa and variable cryogenic temperatures with a 5±10 mCi point source at a D±D distance of 70 mm. Solid lines in the spectrum at 300 K are from the theoretical ®ts to the data.
at ambient pressure [14]. The spectral data accumulation time at room temperature is 12 h. At lower temperatures where there is a magnetic splitting, data accumulation is extended to 20±30 h to obtain a satisfactory spectrum for a reliable analysis to be made. It may be noted that FeTiO3 represents a relatively dicult case scenario for high-pressure MS studies. The resonance intensity is distributed amongst three components and a good signal-tonoise ratio needs to be obtained for a reliable deconvolution of the spectrum. Nevertheless, this work demonstrates, that even under the severe constraints for 57 Fe MS in a DAC, adequately good spectra may be obtained on natural abundance specimens in reasonable data accumulation
[1] E. Sterer, M. Pasternak, R.D. Taylor, Rev. Sci. Instrum. 61 (1990) 1117. [2] G.Yu. Machavariani, M.P. Pasternak, G.R. Hearne, G.Kh. Rozenberg, Rev. Sci. Instrum. 69 (1998) 1423. [3] G.R. Hearne, M.P. Pasternak, R.D. Taylor, Rev. Sci. Instrum. 65 (1994) 3787. [4] G.Kh. Rozenberg, M.P. Pasternak, G.R. Hearne, C.A. McCammon, Phys. Chem. Miner. 24 (1997) 569 (High pressure 57 Fe M ossbauer spectra of natural has been measured at CuFe2 S3 room temperature for days). [5] M. Pasternak, R.D. Taylor, Hyp. Int. 47 (1989) 415. [6] H. Kobayashi, M. Sato, T. Kamimura, M. Sakai, H. Onodera, N. Kuroda, Y. Yamaguchi, J. Phys: Condens. Matter 9 (1997) 515 (Satisfactory spectra on Troilite (FeS) synthesized from natural materials were recorded at room temperature in 30±50 h using a clamp-type DAC). [9] H.R. Lemmer, O.J.A. Segaert, M. Grace, Proc. Phys. Soc. 68 (1955) 701. [10] J.J. Bara, B.F. Bogacz, M oss. E. Ref. Data J. 3 (1980) 154. [11] P. G utlich, R. Link, A. Trautwein, in: M ossbauer Spectroscopy and Transition Metal Chemistry, Springer, Berlin, 1978, p. 50. [12] C.A. McCammon, V. Chaskar, G.G. Richards, Mass. Sci. Technol. 2 (1991) 657. [13] R.D. Taylor, M.P. Pasternak, Hyp. Int. 53 (1990) 159. [14] Y. Ishikawa, N. Saito, M. Arai, Y. Watanabe, H. Takei, J. Phys. Soc. Jpn. 54 (1985) 312.
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Magnetic±electronic pressure studies of natural iron-bearing ssbauer spectroscopy minerals and materials using 57Fe Mo in a diamond anvil cell S. Takele *, G.R. Hearne Department of Physics, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg-Gauteng, South Africa Received 23 January 2001; received in revised form 24 April 2001
Abstract The possibility of measuring 57 Fe M ossbauer spectra of natural isotopic abundance materials at variable high pressures and cryogenic temperatures in a diamond anvil cell (DAC) on a timescale comparable to conventional M ossbauer spectroscopy (MS) is described. This is exempli®ed by satisfactory spectra obtained for an ilmenite FeTiO3 sample (absorber) in 12 h at temperatures where the sample is paramagnetic and in 20±30 h below spinordering temperature of 60 K where resonance intensity is reduced due to magnetically split spectral components. A commercially available 57 Co Rh point source of 14.4 keV resonant radiation and a Kr±CO2 proportional counter have both been used. Suciently high count-rates are obtained by using both the 14.4 keV resonant c-ray and associated 1.8 keV escape peak events. To optimise the resonance eect, careful attention has been paid to minimize non-resonant radiation within the discriminator window set to select the 14.4 keV resonant radiation. This has been achieved by setting an appropriate source±DAC±detector geometry (i.e., solid angle) to reduce scattering events o components of the DAC to a minimum. By using conventional commercially available M ossbauer apparatus, magnetic±electronic properties of iron-bearing minerals and materials with an iron content greater than 20% may be investigated to pressures in excess of 10 GPa encompassing many minerals of the earth's interior. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 76.80.+y Keywords: High pressure; M ossbauer spectroscopy; Materials; Proportional counter
1. Introduction *
Corresponding author. Present address: Department of Physics, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth 6000, South Africa. Tel.: +27-41-5042255; fax: +2741-5042573. E-mail address: [email protected] (S. Takele).
Diamond anvil cells (DACs) have evolved from being bulky cumbersome devices to versatile miniature multipurpose instruments. Speci®cally for the case of M ossbauer spectroscopy (MS) measurements, miniature Merrill-Bassett [1] and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 9 5 - 4
414
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
piston±cylinder [2] DACs have been designed to permit pressure studies up to 35 GPa and also beyond 100 GPa, respectively, depending on the culet dimensions. The compact features ensure that the DAC is easily loaded into a cryostat for variable cryogenic temperature measurements. This permits an investigation of the magnetic properties of materials under pressure at cryogenic temperatures where many minerals order magnetically. The main constraint to MS applications in a DAC is the small sample area which is less than 10 3 cm2 (typical sample dimensions are 300 lm diameter 50 lm thickness) as compared to the area 1 cm2 of sample or source used in conventional MS. The additional constraint of 75% attenuation of the incident 14.4 keV c-ray radiation by the diamond anvils has precluded the application of high-pressure 57 Fe MS to natural Fe-bearing materials and minerals pressurized in a DAC at cryogenic temperatures. This is due to the protracted measuring times required to achieve satisfactory signal-to-noise ratios, especially in the case of magnetically ordered spectra [3±5]. To circumvent these constraints Hearne et al. [3] re®ned and extended the performance of the miniature Merrill-Bassett DAC 57 Fe MS studies at cryogenic temperatures. Their re®nements to the already existing high-pressure MS methodology included using 1. a point source with high speci®c activity to concentrate the 14.4 keV c-ray radiation ¯ux on the minute sample cavity, 2. 57 Fe isotopically enriched absorbers to enhance the resonance eect, and 3. an argon±methane gas-®lled proportional counter to achieve a high signal-to-background ratio. Enrichment of the absorber, however, has two disadvantages, namely that the isotope is expensive and only synthetic materials may be investigated. On the other hand, if materials of natural isotopic abundance are to be investigated using MS in a DAC at high pressure, measurements have to proceed for several days at room temperature to obtain a satisfactory signal-to-noise ratio. Although 57 Fe MS in a DAC at room temperature has been applied to natural abundance materials in recent years [4,6], almost no experimental
details have been published. Under the constraints typical of 57 Fe MS in a DAC, acquisition times greater than 48 h are required. It is of interest to improve the existing high-pressure 57 Fe MS methodology to permit a convenient investigation of magnetic±electronic properties of materials of natural isotopic abundance pressurized in a DAC. Data accumulation times should be minimized so that pressure measurements at extreme high or low temperatures are not prohibitive.
2. Experimental The 57 Fe M ossbauer facility for high-pressure studies at cryogenic temperatures is similar to the one previously reported by Hearne et al. [3] except that a detachable extension tail exists at the bottom of the cryostat having Mylar windows on the bottom and sides of the extension tail. This is designed for both vertical and horizontal transmission MS experiments. A top-loading probe similar to the one described by Hearne et al. [3] has been designed to accommodate the miniature MerrillBassett-type DAC for variable high-pressure cryogenic-temperature 57 Fe MS measurements in a vertical transmission geometry. A custom-made 57 Co(Rh) point source with speci®c activity of 2000 mCi/cm2 has been used. It was produced 1 by thermally diusing 57 Co activity into a 12 lm thick rhodium foil and then ®xing an active area of 0:5 0:5 mm2 into the tip of a tapered aluminium holder (see Fig. 1). It was determined by MS using a single-line absorber (stainless steel) at room temperature and the point source at 4.2 K that the source is initially an unsplit single line, implying that the 57 Co activity is uniformly distributed in the rhodium matrix. The magnetic ordering temperature, Tm of the source increases by 4 K/at.% of iron in rhodium as a result of the magnetic interaction between nearest-neighbor cobalt and iron atoms. 2 We 1 The point source is supplied by Wissel GmbH (W urstrasse 8, D-82319 Starnberg, Germany). 2 Indicated in Amersham International products-catalogue on M ossbauer sources (see also [3]).
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
415
temperature to a saturation value of 30 K after a sucient number of 57 Co nuclei have decayed over an extended period of months [3]. A heating scheme similar to that applied by Hearne et al. [3] may be used to maintain the source temperature above Tm when measurements of the sample at T < Tm are required. The detector for MS measurements in a DAC must be able to discriminate between the resonant M ossbauer c-rays and non-resonant radiation from both the high-energy c-ray and X-ray transitions originating in the 57 Co(Rh) source as well as from scattering o the constituents of the cell. Proportional counters with high eciency and resolution are required. To achieve this, Hearne et al. [3] used a custom-developed argon±methane gas-®lled proportional counter with relatively high eciency of 65% for detecting the 14.4 keV resonant radiation. Their pulse-height (energy) spectrum in the example of an FeI2 sample at 20 GPa clearly shows the 14.4 keV peak dominating both background as well as all other X-ray peaks. An alternative is to use a commercially available Kr±CO2 gas-®lled proportional counter having an eciency of 12% for the 14.4 keV radiation and a resolution of 60%. In addition to the 14.4 keV resonant transition, the associated 1.8 keV escape peak [9] 3 may be used to register the M ossbauer eect. This escape peak typically contributes 40% of the total count-rate (700 counts/s) as established when an empty pressurized sample cavity at a DAC±detector (D±D) separation of 70 mm was used.
Fig. 1. Schematic diagram of the arrangement of the DAC for MS measurements. Label S refers to the point source, D denotes the diamonds, B denotes the backing plates, G is the gasket, A is the pressurized absorber, and C is the detector.
expect a progressive build-up of iron within the rhodium matrix as a consequence of the decay of 57 Co having a half-life of 270 days. Consequently Tm of the source will rise above liquid helium
3 The 1.8 keV escape peak is produced as a result of secondary processes within the detector. Incident radiation energy impinging upon a counter gas gives rise to a proportionate number of ionization events that constitute the detection signal. Ionization is a result of the photoelectric eect and accompanying X-ray events. In the case of a Kr±CO2 counter, the incident 14.4 keV resonant radiation exceeds the absorption edge of the counter gas Kab 14:32 keV. A K-shell photoelectron may be produced accompanied by a characteristic krypton Ka X-ray of 12.6 keV energy. These 12.6 keV X-rays may escape from the counter gas without producing any primary ionization. In this case the only ionization events registered are those corresponding to the energy dierence 14.4± 12.6 keV deposited in the counter, thus giving rise to a characteristic low-energy escape peak at 1.8 keV.
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3. Geometric considerations In conventional MS an attempt is made to maximize the count-rate by maximizing the solid angle of radiation ¯ux subtended by the combination of the sensitive area (window) of the detector, diameter of the sample and diameter of the source. This corresponds to reducing detector± sample±source distances to the smallest practical value but maintaining a minimum working distance to ensure that line-shape distortions due to cosine smearing eects [10±12] are negligible. Cosine smearing eects lead to line-shape distortions resulting from a range of incident resonant c-ray energies within the solid angle subtended by the source±detector con®guration for a given relative velocity between source and absorber. The cosine smearing eect may be rendered negligible by setting a minimum source±detector distance such that the angle a tan 1 s=2d is less than unity (e.g., see [12]), where s is the source diameter and d is the source±detector distance. In the case of MS DAC applications, cosine smearing eects are negligible at the minimum practical source±DAC±detector distances used, because of the comparatively small solid angle determined by the minute sample cavity. The inclination therefore is to minimize source-to-cavity and DAC-to-detector distances to achieve maximum count-rates. However, an inappropriate working geometry enhances the nonresonant background radiation within the discriminator window that has been set for the 14.4 keV resonant radiation. The non-resonant radiation arises from Compton scattering of highenergy c-rays originating in the source, and from photoelectric scattering of the c-rays and the resultant X-rays, which in turn are scattered. These scattering events may have ®nal non-resonant energies coincident with that of the 14.4 keV resonant radiation. The scattering is predominantly from components of the DAC. Fig. 1 is a schematic of the geometric arrangement of essential parts of the DAC for high-pressure MS. For the source of high speci®c activity (2000 mCi/cm2 and dimensions 0:5 0:5 mm2 the sample cavity is seen to act almost as an ideal point source of radiation. A minimum distance d may be set for which most of the source radiation
¯ux reaches the detector without ®rst intersecting components of the DAC. This reduces scattering from components of the DAC, and hence nonresonant radiation, to a minimum while maximizing the source±DAC±detector solid angle. An estimate of this optimal distance may be calculated from the geometry of Fig. 1. It is trivial to show that the optimum distance is d 2rh=t, where r is the radius of the detector window, h is the thickness of the diamond measured from the centre of the sample cavity and t is the diameter of the cone of the backing plate at the base of the diamond. Using typical values of r 13 mm, h 2 mm and t 0:8 mm, the optimum distance is estimated to be d 65 mm. 4. Results Fig. 2(a) depicts normalized pulse height spectra recorded for selected D±D distances with a 10 mCi point source. The DAC is at ambient pressure with an empty cavity of dimensions 300 lm 50 lm drilled in a Ta90 W10 gasket. These are typical conditions of MS in a DAC at high pressure. At the high D±D distance (65 mm in Fig. 2(a)), the 14.4 keV dominates both the non-resonant (X-ray) radiation peaks and the background when compared to the smaller D±D distance (10 mm in Fig. 2(a)). This is also clearly observed in Figs. 2(b) and (c) where the ratio of the 14.4 keV c-ray to that of the 6 keV X-ray is a maximum for D±D distances between 50 and 75 mm in agreement with the geometric considerations of the previous section. At D±D values beyond 70 mm the count rate of the 14.4 keV c-ray radiation also starts to decrease as some of the radiation ¯ux falls outside the sensitive area of the detector, see Fig. 2(c). 57 Fe MS measurements have been made on a natural specimen of ilmenite FeTiO3 pressurized in the DAC. A pair of 0.3 carat 16-sided diamonds each 2 mm thick with a truncated culet of 0.7 mm diameter have been used. A hole drilled in a preindented Ta90 W10 gasket constitutes the sample cavity of 0.3 mm diameter by 0.05 mm thick dimensions at the initial stage of the measurement. Shifts in the wavelength of ruby ¯uorescence peaks have been used for pressure calibration at room
S. Takele, G.R. Hearne / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 413±418
(a)
(b)
(c)
Fig. 2. (a) Normalized pulse height spectra for a point source at selected D±D distances of 10 and 65 mm, (b) peak ratio of the 14.4 keV c-ray to the 6 keV X-ray as a function of D±D and (c) counts per second in a discriminator window set for the resonant 14.4 keV c-ray as a function of D±D. Experimental conditions in all cases involve a point source with an empty sample cavity (300 lm 50 lm) drilled in a Ta90 W10 gasket of a DAC.
temperature. Further details of the procedures used to measure the pressure have been published previously [13] (see also [1]). The variation of pressure with temperature for such a DAC in the range 20±300 K is less than 5%. Liquid argon is
417
used as a hydrostatic pressure-transmitting medium. All the spectra are recorded in a vertical transmission mode. Two independent singlechannel analyzers have been used to select the energy windows for the 1.8 keV escape peak and the 14.4 keV resonant transition as input to a fast linear ampli®er. 4 Data were collected in 1024 channels. In the analyzing program each spectrum and its mirror image were folded into 512 channels and adjacent channels were added to further reduce statistical scatter. Spectra recorded at room temperature on an ilmenite sample pressurized to 14 GPa indicated an appreciable change in maximum resonance intensity as the D±D distance decreases, being 6% at 70 mm and 4% at 5 mm. Hence, in all measurements a D±D distance of 70 mm has been maintained. With this D±D setting and the absorber at ambient conditions in the DAC, count rates of 210 and 350 counts/s have been obtained respectively in the discriminator windows set for the 1.8 keV escape peak and the 14 keV radiation ¯ux of a 5±10 mCi point source. There is 10% increase in the count rates in each window when the sample is pressurized to 14 GPa. Fig. 3 shows spectra recorded at 14 GPa in the range 300±20 K. Theoretical ®ts to the room temperature spectrum indicate that the sample consists of three distinct sub-spectra of which one is a magnetic component of 11% abundance and two paramagnetic doublets ascribed to ferric and ferrous contributions of 34% and 55% abundances, respectively, as obtained from the areas under each spectral component. The sub-spectra of the two formerly paramagnetic components start to display a magnetic splitting below 64 K. It is evident from Fig. 3 that at an onset temperature near 60 K there are line-broadening effects, a change in the relative intensity near zero velocity and also an apparent increase in intensities in the wings of the spectra. This is an indicative of the onset of magnetic ordering temperature which for this sample at 14 GPa appears to be in the range 54±64 K, similar to that of the material
4 The linear ampli®er is Canberra model 2020 manufactured by Canberra Industries, 45 Gracey Ave., Meriden, CT.
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times. This has been achieved with conventional commercially available M ossbauer equipment with due regard to reducing non-resonant background contributions. This oers the possibility for magnetic±electronic studies of many natural iron-bearing minerals and materials under extreme P±T conditions of interest in the geosciences and in condensed matter physics. Acknowledgements Financial support from NRF-ORP (Pretoria) and University of the Witwatersrand is acknowledged with gratitude. Technical assistance from the Science Faculty workshop is gratefully appreciated. The natural sample of ilmenite used in these experiments has been obtained from Dr. J. Nell of the Mineral and Process Chemistry Division at Mintek (Randburg), South Africa. References
Fig. 3. Spectra of natural ilmenite FeTiO3 recorded at 14 GPa and variable cryogenic temperatures with a 5±10 mCi point source at a D±D distance of 70 mm. Solid lines in the spectrum at 300 K are from the theoretical ®ts to the data.
at ambient pressure [14]. The spectral data accumulation time at room temperature is 12 h. At lower temperatures where there is a magnetic splitting, data accumulation is extended to 20±30 h to obtain a satisfactory spectrum for a reliable analysis to be made. It may be noted that FeTiO3 represents a relatively dicult case scenario for high-pressure MS studies. The resonance intensity is distributed amongst three components and a good signal-tonoise ratio needs to be obtained for a reliable deconvolution of the spectrum. Nevertheless, this work demonstrates, that even under the severe constraints for 57 Fe MS in a DAC, adequately good spectra may be obtained on natural abundance specimens in reasonable data accumulation
[1] E. Sterer, M. Pasternak, R.D. Taylor, Rev. Sci. Instrum. 61 (1990) 1117. [2] G.Yu. Machavariani, M.P. Pasternak, G.R. Hearne, G.Kh. Rozenberg, Rev. Sci. Instrum. 69 (1998) 1423. [3] G.R. Hearne, M.P. Pasternak, R.D. Taylor, Rev. Sci. Instrum. 65 (1994) 3787. [4] G.Kh. Rozenberg, M.P. Pasternak, G.R. Hearne, C.A. McCammon, Phys. Chem. Miner. 24 (1997) 569 (High pressure 57 Fe M ossbauer spectra of natural has been measured at CuFe2 S3 room temperature for days). [5] M. Pasternak, R.D. Taylor, Hyp. Int. 47 (1989) 415. [6] H. Kobayashi, M. Sato, T. Kamimura, M. Sakai, H. Onodera, N. Kuroda, Y. Yamaguchi, J. Phys: Condens. Matter 9 (1997) 515 (Satisfactory spectra on Troilite (FeS) synthesized from natural materials were recorded at room temperature in 30±50 h using a clamp-type DAC). [9] H.R. Lemmer, O.J.A. Segaert, M. Grace, Proc. Phys. Soc. 68 (1955) 701. [10] J.J. Bara, B.F. Bogacz, M oss. E. Ref. Data J. 3 (1980) 154. [11] P. G utlich, R. Link, A. Trautwein, in: M ossbauer Spectroscopy and Transition Metal Chemistry, Springer, Berlin, 1978, p. 50. [12] C.A. McCammon, V. Chaskar, G.G. Richards, Mass. Sci. Technol. 2 (1991) 657. [13] R.D. Taylor, M.P. Pasternak, Hyp. Int. 53 (1990) 159. [14] Y. Ishikawa, N. Saito, M. Arai, Y. Watanabe, H. Takei, J. Phys. Soc. Jpn. 54 (1985) 312.