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Conversion electron Mossbauer spectroscopy with the use of channel electron multipliers operating at low temperatures
Nuclear Instruments and Methods 190 (1981) 433-435 North-Holland Publishing Company
433
LETTERS TO THE EDITOR
CONVERSION ELECTRON MOSSBAUER SPECTROSCOPY WITH THE USE OF CHANNEL ELECTRON MULTIPLIERS OPERATING AT LOW TEMPERATURES J.A. SAWlCKI, T. TYLISZCZAK 1 and O. GZOWSKI 2 Institute of Physics, Jagiellonian University, Cracow, Poland 1 Institute o f Physics, Technical University, Cracow, Poland 2 Institute of Physics, Technical University, Gdahsk, Poland Received 9 June 1981 and in revised form 6 July 1981 A simple method of measuring the conversion electron M~hssbauerspectra at low temperatures down to 4.2 K is presented. The method utilizes channel electron multipliers as high performance detectors of low-energy electrons at cryogenic conditions. A versatile low-cost insert unit for M~hssbauereffect measurements with the source, resonant scatterer and channel electron detector cooled down to liquid helium temperature is also described. The applications o f conversion electron M6ssbauer spectroscopy (CEMS) has in recent years been a subject of considerable interest. The high sensitivity of this technique enables ~the study o f solid layers as thin as 10 nm or less. In the most favorable case o f 14.4 keV gamma ray transition in STFe the CEMS spectra of even 1014-10 is resonant atoms per cm 2 ( 1 0 - 1 0 0 ng s 7Fe/cm 2) are observed relatively easily. Therefore, the technique has been most successfully applied in investigations of ion implantation, surface deposition, corrosion, adsorbtion, surface magnetisation and other close-to-surface phenomena, see e.g. refs. 1 - 3 . However, the applications of CEMS method are seriously limited by the lack of a suitable efficient tecfinique o f measurements at low temperatures. Helium proportional or avalanche counters, which are widely employed in CEMS measurements at room temperature were shown to be practically inapplicable at temperatures much below 77 K because o f the dramatic drop in the gas gain [4,5]. The main drawback o f magnetostatic or electrostatic electron analyzers, which so far have been used for low temperature CEMS measurements, e.g. refs. 6 - 8 , is, besides the relative complexity o f the equipment, their usually small count rate associated with poor transmission. The m e t h o d reported in this work employs a single channel electron multiplier which, as it appeared during experiments, can be safely cooled down to liquid helium temperature without losing its excellent features as a detector o f low-energy electrons. The properties of the detector were also not adversely 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1981 North-Holland
affected either by repetitive cycling between atmospheric pressure and vacuum or by repetitive cooli n g - w a r m i n g cycles. This is contrary to the data in the literature [9] suggesting that the performance o f channel electron multipliers can deteriorate at low temperatures largely due to changed electric characteristics. The scheme of one of the experimental arrangements used by us is shown in fig. 1. As is seen the resonant scattering sample and a charmeltron are m o u n t e d at very close distance to each other inside a small vacuum chamber, evacuated via a supporting stainless steel tube. The movable M6ssbauer source is located above the chamber and gamma rays pass through an indium-sealed beryllium window. The
TO PUMP
I
i- 0.3kv
1era Fig. 1. The tail of the insert unit which combines the source, scatterer and channeltron detector used for CEMS measurements at low temperatures. The velocity transducer, pumping head and the electric leads-through are installed at the top
and are not shown here. During measurements at low temperatures the tail is immersed in liquid helium or nitrogen in a suitable storage vessel.
434
J.A. Sawicki et al. / Conversion electron MOssbauer spectroscopy
insert unit can be easily and quickly cooled down by immersing into liquid nitrogen or into liquid helium inside ordinary storage vessels. The method presented above greatly reduces the overall size and cost of the equipment as well as facilitates long-lasting experimental runs. It also very much simplifies the experimental procedure, particularly in the case of M6ssbauer isotopes for which cooling of both the source and scatterer is necessary in view of low recoilless fractions. In addition, as the source is easily accessible by lifting up the insert unit one can perform experiments with short-lived parent nuclei. We also believe that the arrangement described here makes a new step in the process of miniaturization and integration of M6ssbauer spectroscopy. Channeltron detectors were also used by us in other cryogenic configurations. In one of them several channeltrons were attached close to the scatterer at the bottom of the liquid helium vessel, whereas the source was held at room temperature outside the cryostat. With this arrangement we were able to increase markedly the electron count rate because of the increased total solid angle. This equipment has been particularly useful in CEMS studies of iron-containing samples since most of the available STCo sources, e.g. in Cr or Pd hosts, except for STCo in Rh, exhibit undesirable line broadening of hyperfine splitting when cooled down to 77 K or below. Examples of experin3ental results obtained with the equipment presented here for s 7Fe implanted samples at the dose level of 10 is atoms/cm 2 as well as for S~Eu and 1195n M6ssbauer samples are presented elsewhere [ 10,11 ]. Experiments with other M6ssbauer isotopes are in progress. We shall postpone the discussion of technical details and specific M6ssbauer spectroscopic problems to subsequent publications and will only present some data concerning the performance of the channeltron detectors. The channeltron electron multipliers manufactured and used by us were made of Bi-doped and specially heat-treated lead glass. The surface of the glass is characterized by the small temperature dependence of the electric conductivity. The activation energy of the surface conductivity was of the order of 0.05 eV. Because of the weak dependence of the secondary electron yield o on temperature the gain G ~ - o n (n - n u m b e r of multiplication stages) of our multiplier does not change very much at low temperatures. The observed decrease in the electron pulse gain between 300 K and 4.2 K was around 20%. This could easily be compensated by
operation at somewhat higher voltages (about 2800 V at 300 K and 3300 at 4.2 K). The pulse shape and pulse distribution were practically unaffected by temperature changes. We also noticed that an unwanted capacity added by the transmission lines joining the multiplier to the amplifier held at room temperature was completely insignificant in view of the very high gain of the multiplier of ~108. The detection efficiency of channeltrons is usually very high for low-energy electrons, with the maximum being about 90% at about 300 eV and approximately 10% at 10 keV. We had noticed previously [12] that the resonant absorption of gamma rays is associated with, in addition to internal conversion and Auger electrons, intensive emission of secondary electrons at an energy of around 1-5 eV. We associate these electrons with the multiple secondary electron scattering processes in the sample. We have also shown earlier that the secondary electrons carry information on the resonant absorption events in the sample and therefore can be effectively used for the registration of M6ssbauer spectra. Typically, by the application of an accelerating voltage of 200-300 V between the channeltron cone and the scattering sample one can increase the count rate by a factor of about 2 to 3 and obtain a larger magnitude of the resonant effect by a factor of more than 2. The latter is explained by the increased contribution of resonant secondary electrons caused by increased efficiency of their detection after acceleration. The optimum accelerating voltage varies with the scatterer-channeltron geometry and is found experimentally. We noticed that careful collimation of the incident beam of gamma rays is necessary in order to obtain a good signal-to-noise ratio. In the case of STFe CEMS measurements we use STCo sources with an activity of about 100 mCi, which together with the metallic iron scatterer enriched in STFe result in count rates of around 2000/s. The magnitude of the resonant effect defined for the largest of the resonant lines in the iron spectrum approaches 400%. These parameters are comparable with the results achieved at room temperature by means of conventional helium/CH4 counters. Further improvements of the method, in particular by the use of multichannel plates and by an increase in the solid angle, may be feasible.
References [1] J. Stanek, J.A. Sawicki and B.D. Sawicka, Nucl. Instr. and Meth. 130 (1975) 613.
J.A. Sawicki et al. / Conversion electron MOssbauer spectroscopy
[2] B.D. Sawicka, J.A. Sawicki, Topics in Current Physics, vol. 25, ed., U. Gonser (Springer Verlag, Berlin, 1981). [3] M.J. Tricker, Surf. Defect Prop. Solids 6 (1977) 106; Proc. Conf. on Chemical applications of MtSssbauer spectroscopy, Houston (1980). [4] J.A. Sawicki, Nucl. Instr. and Meth. 152 (1978) 577. [5] J.A. Sawicki, J. Stanek, B.D. Sawicka and J. Kowalski, Nukleonika 24 (1979) 1161. [6] H. Bokemeyer, K. Wohlfahrt, E. Kankeleit and D. Eckhardt, Z. Phys. A274 (1975) 305.
435
[7] M. Carbuccichio, Nucl. Instr. and Meth. 144 (1977) 225. [8] O. Massenet, Nucl. Instr. and Meth. 153 (1978) 419. [9] F. Lecomte and V. Perez-Mendez, IEEE Trans. Nucl. Sci. NS-25 (1977) 964. [10] A. Perez, B.D. Sawicka, L. Fritsch, G. Marest, J.A. Sawicki and T. Tyliszczak, to be published. [11] T. Tyliszczak and J.A. Sawicki, to be published. [12] T. Tyliszczak, J.A. Sawicki, J. Stanek and B.D. Sawicka, J. Phys. (Paris) Colloq. 41 (1980) CI-117.
433
LETTERS TO THE EDITOR
CONVERSION ELECTRON MOSSBAUER SPECTROSCOPY WITH THE USE OF CHANNEL ELECTRON MULTIPLIERS OPERATING AT LOW TEMPERATURES J.A. SAWlCKI, T. TYLISZCZAK 1 and O. GZOWSKI 2 Institute of Physics, Jagiellonian University, Cracow, Poland 1 Institute o f Physics, Technical University, Cracow, Poland 2 Institute of Physics, Technical University, Gdahsk, Poland Received 9 June 1981 and in revised form 6 July 1981 A simple method of measuring the conversion electron M~hssbauerspectra at low temperatures down to 4.2 K is presented. The method utilizes channel electron multipliers as high performance detectors of low-energy electrons at cryogenic conditions. A versatile low-cost insert unit for M~hssbauereffect measurements with the source, resonant scatterer and channel electron detector cooled down to liquid helium temperature is also described. The applications o f conversion electron M6ssbauer spectroscopy (CEMS) has in recent years been a subject of considerable interest. The high sensitivity of this technique enables ~the study o f solid layers as thin as 10 nm or less. In the most favorable case o f 14.4 keV gamma ray transition in STFe the CEMS spectra of even 1014-10 is resonant atoms per cm 2 ( 1 0 - 1 0 0 ng s 7Fe/cm 2) are observed relatively easily. Therefore, the technique has been most successfully applied in investigations of ion implantation, surface deposition, corrosion, adsorbtion, surface magnetisation and other close-to-surface phenomena, see e.g. refs. 1 - 3 . However, the applications of CEMS method are seriously limited by the lack of a suitable efficient tecfinique o f measurements at low temperatures. Helium proportional or avalanche counters, which are widely employed in CEMS measurements at room temperature were shown to be practically inapplicable at temperatures much below 77 K because o f the dramatic drop in the gas gain [4,5]. The main drawback o f magnetostatic or electrostatic electron analyzers, which so far have been used for low temperature CEMS measurements, e.g. refs. 6 - 8 , is, besides the relative complexity o f the equipment, their usually small count rate associated with poor transmission. The m e t h o d reported in this work employs a single channel electron multiplier which, as it appeared during experiments, can be safely cooled down to liquid helium temperature without losing its excellent features as a detector o f low-energy electrons. The properties of the detector were also not adversely 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1981 North-Holland
affected either by repetitive cycling between atmospheric pressure and vacuum or by repetitive cooli n g - w a r m i n g cycles. This is contrary to the data in the literature [9] suggesting that the performance o f channel electron multipliers can deteriorate at low temperatures largely due to changed electric characteristics. The scheme of one of the experimental arrangements used by us is shown in fig. 1. As is seen the resonant scattering sample and a charmeltron are m o u n t e d at very close distance to each other inside a small vacuum chamber, evacuated via a supporting stainless steel tube. The movable M6ssbauer source is located above the chamber and gamma rays pass through an indium-sealed beryllium window. The
TO PUMP
I
i- 0.3kv
1era Fig. 1. The tail of the insert unit which combines the source, scatterer and channeltron detector used for CEMS measurements at low temperatures. The velocity transducer, pumping head and the electric leads-through are installed at the top
and are not shown here. During measurements at low temperatures the tail is immersed in liquid helium or nitrogen in a suitable storage vessel.
434
J.A. Sawicki et al. / Conversion electron MOssbauer spectroscopy
insert unit can be easily and quickly cooled down by immersing into liquid nitrogen or into liquid helium inside ordinary storage vessels. The method presented above greatly reduces the overall size and cost of the equipment as well as facilitates long-lasting experimental runs. It also very much simplifies the experimental procedure, particularly in the case of M6ssbauer isotopes for which cooling of both the source and scatterer is necessary in view of low recoilless fractions. In addition, as the source is easily accessible by lifting up the insert unit one can perform experiments with short-lived parent nuclei. We also believe that the arrangement described here makes a new step in the process of miniaturization and integration of M6ssbauer spectroscopy. Channeltron detectors were also used by us in other cryogenic configurations. In one of them several channeltrons were attached close to the scatterer at the bottom of the liquid helium vessel, whereas the source was held at room temperature outside the cryostat. With this arrangement we were able to increase markedly the electron count rate because of the increased total solid angle. This equipment has been particularly useful in CEMS studies of iron-containing samples since most of the available STCo sources, e.g. in Cr or Pd hosts, except for STCo in Rh, exhibit undesirable line broadening of hyperfine splitting when cooled down to 77 K or below. Examples of experin3ental results obtained with the equipment presented here for s 7Fe implanted samples at the dose level of 10 is atoms/cm 2 as well as for S~Eu and 1195n M6ssbauer samples are presented elsewhere [ 10,11 ]. Experiments with other M6ssbauer isotopes are in progress. We shall postpone the discussion of technical details and specific M6ssbauer spectroscopic problems to subsequent publications and will only present some data concerning the performance of the channeltron detectors. The channeltron electron multipliers manufactured and used by us were made of Bi-doped and specially heat-treated lead glass. The surface of the glass is characterized by the small temperature dependence of the electric conductivity. The activation energy of the surface conductivity was of the order of 0.05 eV. Because of the weak dependence of the secondary electron yield o on temperature the gain G ~ - o n (n - n u m b e r of multiplication stages) of our multiplier does not change very much at low temperatures. The observed decrease in the electron pulse gain between 300 K and 4.2 K was around 20%. This could easily be compensated by
operation at somewhat higher voltages (about 2800 V at 300 K and 3300 at 4.2 K). The pulse shape and pulse distribution were practically unaffected by temperature changes. We also noticed that an unwanted capacity added by the transmission lines joining the multiplier to the amplifier held at room temperature was completely insignificant in view of the very high gain of the multiplier of ~108. The detection efficiency of channeltrons is usually very high for low-energy electrons, with the maximum being about 90% at about 300 eV and approximately 10% at 10 keV. We had noticed previously [12] that the resonant absorption of gamma rays is associated with, in addition to internal conversion and Auger electrons, intensive emission of secondary electrons at an energy of around 1-5 eV. We associate these electrons with the multiple secondary electron scattering processes in the sample. We have also shown earlier that the secondary electrons carry information on the resonant absorption events in the sample and therefore can be effectively used for the registration of M6ssbauer spectra. Typically, by the application of an accelerating voltage of 200-300 V between the channeltron cone and the scattering sample one can increase the count rate by a factor of about 2 to 3 and obtain a larger magnitude of the resonant effect by a factor of more than 2. The latter is explained by the increased contribution of resonant secondary electrons caused by increased efficiency of their detection after acceleration. The optimum accelerating voltage varies with the scatterer-channeltron geometry and is found experimentally. We noticed that careful collimation of the incident beam of gamma rays is necessary in order to obtain a good signal-to-noise ratio. In the case of STFe CEMS measurements we use STCo sources with an activity of about 100 mCi, which together with the metallic iron scatterer enriched in STFe result in count rates of around 2000/s. The magnitude of the resonant effect defined for the largest of the resonant lines in the iron spectrum approaches 400%. These parameters are comparable with the results achieved at room temperature by means of conventional helium/CH4 counters. Further improvements of the method, in particular by the use of multichannel plates and by an increase in the solid angle, may be feasible.
References [1] J. Stanek, J.A. Sawicki and B.D. Sawicka, Nucl. Instr. and Meth. 130 (1975) 613.
J.A. Sawicki et al. / Conversion electron MOssbauer spectroscopy
[2] B.D. Sawicka, J.A. Sawicki, Topics in Current Physics, vol. 25, ed., U. Gonser (Springer Verlag, Berlin, 1981). [3] M.J. Tricker, Surf. Defect Prop. Solids 6 (1977) 106; Proc. Conf. on Chemical applications of MtSssbauer spectroscopy, Houston (1980). [4] J.A. Sawicki, Nucl. Instr. and Meth. 152 (1978) 577. [5] J.A. Sawicki, J. Stanek, B.D. Sawicka and J. Kowalski, Nukleonika 24 (1979) 1161. [6] H. Bokemeyer, K. Wohlfahrt, E. Kankeleit and D. Eckhardt, Z. Phys. A274 (1975) 305.
435
[7] M. Carbuccichio, Nucl. Instr. and Meth. 144 (1977) 225. [8] O. Massenet, Nucl. Instr. and Meth. 153 (1978) 419. [9] F. Lecomte and V. Perez-Mendez, IEEE Trans. Nucl. Sci. NS-25 (1977) 964. [10] A. Perez, B.D. Sawicka, L. Fritsch, G. Marest, J.A. Sawicki and T. Tyliszczak, to be published. [11] T. Tyliszczak and J.A. Sawicki, to be published. [12] T. Tyliszczak, J.A. Sawicki, J. Stanek and B.D. Sawicka, J. Phys. (Paris) Colloq. 41 (1980) CI-117.