A VERSATILE COUNTER FOR CONVERSION MÖSSBAUER SPECTROSCOPY

Transcription

1 A VERSATILE COUNTER FOR CONVERSION MÖSSBAUER SPECTROSCOPY I. BIBICU 1, G. NICOLESCU 2, L. CIOLACU 2, L. SERBINA 2 1 National Institute for Materials Physics, Bucharest 77125, Romania, bibicu@infim.ro 2 National Institute of Physics and Nuclear Engineering, IFIN-HH, Bucharest 77125, Romania Received June 26, 29 A multi-purpose gas-flow proportional counter for surface Mössbauer spectroscopy is described. The main improvements obtained are: the height of the detection volume can be changed in large limits from 1 to 38 mm, the detection volume can be symmetrical or not in respect with anode plan, the anode changing is easily and different anode configuration can be used. It is suitable for studies with 57 Fe, 119 Sn and 151 Eu isotopes. The performances of the detector are presented. Key words: gas-flow detector, backscatering experiment, surface studies, Fe-57, Sn-119, Eu INTRODUCTION Mössbauer spectroscopy [1] is based on the resonant recoil free absorption and emission of low energy γ-rays. The effect is significant when the nuclei are imbedded in a rigid matrix. The nuclear resonance absorption is successively followed by nuclear and atomic processes resulting in the emission of various radiations: γ-ray, conversion electrons, X-rays and Auger electrons. This secondary radiation is employed to obtain an emission-type Mössbauer spectrum by the backscattering method, in which the counts of secondary radiations increase at the velocity of nuclear resonance. The information obtained by the backscattering method is restricted to the layer to which the secondary radiation employed in the measurement can escape from the surface of sample. The detection of the three backscattered particles permits surface studies to be performed at various depths. The smallest depths can be investigated by electron detection. The conversion electron signal is quite high for the 57 Fe and 119 Sn isotopes. The purpose of this paper is to describe a new version of a gas-flow proportional detector for detection of the conversion electrons or characteristic X-ray. The improved performances of the multi-purpose detector are demonstrated by measurements on samples containing the isotopes Fe-57, Sn-119 and Eu-151. The energy of the backscattered particles is in a large range [2]. Rom. Journ. Phys., Vol. 56, Nos. 1 2, P , Bucharest, 211

2 2 Counter for conversion Mössbauer spectroscopy DESCRIPTION The sample to be studied is mounted inside the detector and the sample itself is a part of the cathode of the detector. It was designed in a cylindrical geometry. The cross section of the detector is shown in Fig. 1 and the dimensions are in mm. Fig. 1. Cross section of the detector 1 and 2, main parts of the detector body; 3 input piece; 4 sample holder; 5 teflon insulator; 6 stainless steel ring; 7 sample; 8 collimator; 9 gas connection; 1 high voltage connector; 11 mylar windows; 6, 12 tightness piece; S Mössbauer source. The body of the detector, made of aluminium, consists of only two main parts (1 and 2), in order to lower the leak rate. The background due to photoelectrons is minimised by using low-z materials as much as possible. The anode, gold-coated tungsten wire, 25 µm diameter, is supported in the center of the detector by a stainless steel ring 6. The ring can be changed easily with another, with the same dimensions. On the ring are mounted parallel anode wires. The number of wires and the distance between them can be changed. Thus for o specific geometry and measurement type can be find an optimum anode configuration. The ring diameter is higher than twice of sample (7) diameter. The investigated part of the sample has a diameter higher than 2 mm. The electric field at the sample is thus practically constant. The casing of the stainless steel ring is made of teflon insulator (5). The Mössbauer sample is mounted inside the counter by means of a sample holder 4. The holder allows an easy manipulation of the sample outside the detector and it can always be repositioned in a reproducible manner with respect to the detector body. The holder also allows performing simultaneously surface and transmission measurements for samples with suitable thickness for transmission measurements.

3 152 I.Bibicu et al. 3 In this case it is necessary another counter placed behind the versatile conuter. The front side of the detector is closed with an input piece 3, identically with sample holder. The parts 3 and 4 are equipped with thin aluminized mylar windows 11. In the input piece 3 may be inserted a collimator 8. The components 3 and 4 can be moved independently in respect with detector body and the space between them represents the detection volume. The height of the detection volume can be fixed, mechanically, in large limits from 1 to 38 mm, moving the two components. The detection volume can be settled symmetrical or not in respect with anode plan. By changing the volume detection and flow gas it is possible to make measurements by electron, X-ray or gamma ray detection. For conversion electron detection a 94% He + 6% CH 4 or 99% He + 1% C 4 H 1 mixtures was used and for X-ray a 9% Ar + 1% CH 4 mixture. The gas flow rate can be set in the range of 5 1 cm 3 /hour by a flowmeter working at a pressure of about.1 Mpa. It is possible to set relatively low flow rates with sufficiently small deviations, so that no fluctuations in the amplifying factor appear. The access of the gas was designed such to avoid vibration of the anode, due to the flow rate and any gas leak. We used an economical radiation shielding for counter, consisting of a combination of lead, copper and steel disks. To destroy the characteristic radiation, alternate mounting of lead, copper and steel disks was used. In order to absorb unfavourable X-rays from the sources, a plexiglas filter was placed in front of the shielding. 3. RESULTS The measurements were made, at room temperature, with a new constructed, versatile flow-gas proportional counter, suitable for studies with all mentioned isotopes. We used the following Mössbauer sources: 57 Co diffused in Rh matrix, 119m Sn diffused in CaSnO 3 matrix and 151 Sm diffused in samarium oxide. Measurements were carried out by inserting the proportional counter into an Elscint AME-5 Mössbauer spectrometer using a compatible CMCA-55 Wissel unit for data acquisition. We used as Mössbauer samples stainless steel, β-sn metallic foil and Eu 2 O 3. The thickness of the detection space was adjusted to minimum necessary required by signal/noise ratio [3], [4]. The detector shows the best performance for electrons at a voltage level of 9 11 V and for X-rays at a level voltage of V. Gas flow rate was in the range 5 1 cm 3 /hour. The parameters of the Mössbauer spectra were calculated using a computer fitting program, which assumes a Lorentzian line shape. The isomer shifts were referred to α-fe. The electron amplitude spectra of the detector with 57 Co and 119m Sn sources and stainless steel, respectively β-sn samples inside are presented in figure 2 and 3. Eu did not emit electrons by the de-excitation of the nuclear excited states [5].

4 4 Counter for conversion Mössbauer spectroscopy Fig. 2. Pulse height spectrum of electrons scattered from stainless steel sample, recorded with 57 Co source without filter; 3 mm thickness of the detecting volume; 3 minutes acquisition time Fig. 3. Pulse height spectrum of electrons scattered from β-sn sample, recorded with 119m Sn source without filter; 5 mm thickness of the detecting volume; 3 minutes acquisition time. In the case of Fe we obtained a spectrum better than the previously reported ones [6]. It has a visible structure and it is similar with that one obtained using the rhodium foil 3% enriched in 57 Fe. [7]. A good setting of the SCA window for the resonant part of the pulse-height spectrum now it is possible for iron studies. The amplitude spectrum of the detector with 119m Sn source and β-sn sample inside displays a large peak, with a good resolution corresponding to electron energies: kev [2]. The amplitude spectra of the detector for X-ray, with the mentioned sources, without filter, are shown in figure 4, 5 and 6 respectively. The amplitude spectra have a good resolution for low energy X-ray, especially for Sn low energy X-ray ( kev) and low effectiveness for Mössbauer radiation. These facts prove the performances of the detector.

5 154 I.Bibicu et al kev 12 8 Escape peak KeV 1 2 Fig. 4. Pulse height spectrum of photons recorded with 57 Co source without filter and with stainless steel sample: 17 mm thickness of the detecting volume; 6 minutes acquisition time , 25.27keV kev Fig. 5. Pulse height spectrum of photons recorded with 119m Sn source without filter and with β-sn sample; 2 mm thickness of the detecting volume; 3 minutes acquisition time kev kev Fig. 6. Pulse height spectrum of photons recorded with 151 Sm source without filter and with Eu 2 O 3 sample; 14 mm thickness of the detecting volume; 3 minutes acquisition time.

6 6 Counter for conversion Mössbauer spectroscopy 155 The Mössbauer spectrum obtained by the detection of electrons from metallic Sn is presented in the Fig. 7. This spectrum evidences presence of two Mössbauer lines: one corresponding to β-sn and the other to SnO 2 oxide β Sn Relative emission SnO Velocity [mm/s] Fig. 7. Conversion electron Mössbauer spectrum of metallic β-sn foil at room temperature; mono-channel analyzer selected the peak presented in figure 3. The parameters obtained for Sn are: resonance effect (ε) = %, line width (w) =.88 mm/s. These are better than those obtained in the transmission geometry: ε = 7.92 %; w =.97 mm/s. The smaller line width is expected in the backscattering geometry due to lack of saturation broadening. The backscattering measurement evidenced presence of the SnO 2 on the sample surface; this fact was not possibly in transmission measurement. The line width is closed to theoretical value. The thickness of the SnO 2 film is estimated around 4 nm [8]. The isomer shift of SnO 2, slight different from SnO 2 reference sample, shows a change in its stoichiometry. 1.6 Relative emission Velocity [mm/s] Fig. 8. Conversion X-ray Mössbauer spectrum of metallic β-sn foil at room temperature; monochannel analyzer selected the energy range kev.

7 156 I.Bibicu et al. 7 The Mössbauer spectrum obtained by the detection of X-ray from Sn is presented in the figure 8. It shows the presence of a single line, corresponding to β-sn. The resonance effect ε = 5.98 % is lower than the value obtained in the transmission geometry (7.9%). Also the line width is lower:.92 mm/s instead.98 mm/s obtained in the transmission geometry. After our knowledge the Mössbauer spectrum obtained by detection of low energy X-ray for 119 Sn is realized for the first time in the world. Fig. 9 shows the Mössbauer spectra of the Eu 2 O 3 sample obtained by detection of backscattered X-rays. The detection of the conversion X-rays compensate for smaller resonance effect by its significant smaller line width: 2.32 mm/s instead 3.24 mm/s in transmission for 1mg/cm 2 Eu 2 O 3 sample Relative emission Velocity [mm/s] Fig. 9. Conversion X-ray Mössbauer spectrum of Eu 2 O 3 at room temperature. 4. CONCLUSIONS A versatile counter for conversion Mössbauer spectroscopy at room temperature has been described and its performances were presented. The detector design is better than previously reported [9]-[11]. The device can be applied for both surface and bulk studies. It is suitable for studies with 57 Fe, 119 Sn and 151 Eu Mössbauer isotopes. Acknowledgements. The authors thank for the financial support of the National University Research Council Grant/ competition and National Authority for Scientific Research (ANCS, under Core Research Programme PN9/29-PN45/29). REFERENCES 1. R.L. Mössbauer, Nuclear Resonance Fluorescence of gamma Radiation in 191 Ir. Z. Physik, 151, p , E. Browne, R.B. Firestone, Table of radioactive Isotopes, Ed. John Wiley & Sons, 1986.

8 8 Counter for conversion Mössbauer spectroscopy Zs. Kajcsos, Ch. Sauer, W. Zinn, Criteria for optimizing DCEMS, Hyperfine Interactions, 57, p , Zs. Kajcsos, Ch. Sauer, W. Zinn, W. Meisel, H. Spiering, M. Alflen, P.Gütlich, High-performance Mössbauer spectroscopy: criteria, possibilities, limitations, Hyperfine Interactions, 71, , I. Bibicu, M.S. Rogalski, G. Nicolescu, A detector assembly for simultaneous conversion electron conversion X-ray and transmission Mössbauer spectroscopy, Meas. Sci. Technol 7, p , I. Bibicu, M.S. Rogalski, G. Nicolescu, An improved proportional counter for conversion and transmission Mössbauer spectroscopy, Rom. J. Phys. 45, no. 1 2, p , G.P. Huffman, Applications of Mössbauer Spectroscopy, Academic Press, Inc, New-York, 198, p Y. Isozumi, D.I. Lee, I. Kadar, A new detector assembly for conversion and X-rays from Mössbauer effect, Nucl. Instrum. Methods, 12, p , A.S. Kamzin, L.A. Grigoriev, Properties of surface layers and the interior of a crystal studied by Mössbauer spectroscopy, Sov. Tech.-Phys. Lett., 16, p , I. Bibicu, M.S. Rogalski, G. Nicolescu, Toroidal proportional detector for conversion X-ray and transmission Mössbauer spectroscopy, Nucl. Instr. and Meth. in Phys. Res., B 94, p , 1994.