Fast detectors for Mössbauer spectroscopy )

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1 Fast detectors for Mössbauer spectroscopy ) A.L. Kholmetskii Department of Physics, Belarus State University, Minsk, Belarus M. Mashlan Palacký University, Olomouc, Czech Republic K. Nomura School of Engineering, The University of Tokyo, Japan O.V. Misevich, A.R. Lopatik Institute of Nuclear Problems, Minsk, Belarus Received 17 May 2001 The methods to increase the productivity (statistical quality) of Mössbauer measurements have been considered.some fast detectors for gamma- and secondary radiation have been described.these detectors allow in many cases to essentially reduce the time for the Mössbauer spectra accumulation with a given productivity. 1 Productivity of Mössbauer measurements It is well known (see, e.g. [1]) that the productivity (statistical quality) of Mössbauer measurements is determined by the expression Q = ε2 N( ), (1) ε +2 where ε is the value of resonant effect, N( ) N(0) ε = ; N( ) N(0) and N( ) are the numbers of counts in resonance and far from resonance, respectively. In the case of transmission measurements ε is always much smaller than unity, and we can use an approximate relation Q 1 2 ε2 N( ). (2) In order to understand how to increase the productivity Q in practice, it is necessary to express it via the parameters of the Mössbauer spectrometer, in particular, the Mössbauer source and the spectrometric section. As a rule, emission ) Presented at the International Colloquium Mössbauer Spectroscopy in Materials Science, Velké Losiny, Czech Republic, 3 8 September Czechoslovak Journal of Physics, Vol. 51 (2001), No

2 A.L. Kholmetskii et al. spectra of Mössbauer sources contain several gamma-lines. It is clear that ability of a detector to select Mössbauer radiation from background influences the ratio of the effect/background in Mössbauer spectrum and, therefore, together with a fast operation of a detector influences the productivity. We will show below that an analytical expression connecting the productivity of Mössbauer measurements with parameters of the spectrometric section can be obtained when introducing the notions of the spectrometric and absolute selectivities of detectors. Spectrometric selectivity can be defined as a ratio of count rates of useful events to background events detected in a selected energy window [2, 3]: S s = I out I b I b, (3) where I out is the total registered intensity and I b is the registered intensity of the background. For resonance detector one can additionally define the absolute selectivity as a ratio of counting efficiency in resonant condition η 0 and outside the resonance η [4, 5]: S a = η 0. (4) η It is important that both these values can be directly determined experimentally. The spectrometric selectivity is measured by means of method of filters, while the absolute selectivity is determined from the emission Mössbauer spectrum of radioactive source by means of Eq. (5) [4]: S a = ε em χf +1, (5) where ε em is the value of resonant effect in the emission Mössbauer spectrum, χ = I γ I γ + I b, I γ, I b are the intensities of Mössbauer and background radiation, and f is the Debay Waller factor for Mössbauer source. Further, in transmission geometry the intensity of Mössbauer radiation passing through an absorber under resonant condition and outside the resonance is defined by the known expression I(0) = I 0 e µρ [1 ff(c a )] ; I( ) =I 0 e µρ, (6) where I 0 is the intensity of the Mössbauer radiation in the solid angle of registration, µ is the electronic mass absorption coefficient for investigated sample, ρ the surface density of the absorber, C a the effective thickness of the resonant absorber, and F =e Ca/2 J 0 ( 1 2 C a). 764 Czech. J. Phys. 51 (2001)

3 Fast detectors for Mössbauer spectroscopy In the general analysis let us additionally introduce the intensity of background radiation and different registration efficiencies for resonant and non-resonant radiation. In this case Eq. (6) transforms into I out (0) = η 0 I 0 e µρ f [1 F (C a )] + η (1 f)i 0 e µρ + η b I b, (7) I out ( ) =η 0 I 0 fe µρ + η (1 f)i 0 e µρ + η b I b, where η b is the registration efficiency for background radiation I b. The experimentally measured magnitude of resonant effect is written as ε = I out( ) I out (0) (8) I out ( ) and the productivity of measurement is determined by the relation Q = ε 2 I out ( ) = [I out( ) I out (0)] 2. (9) I out ( ) In further discussion let us separately consider two cases: 1. Case of low count rate, far from limiting values of a detector. 2. Case of maximum count-rate of a detector. In the first case we can directly substitute the Eqs. (7) obtained above into Eq. (9). As a result we get the productivity of measurements Q = η 0I 0 e µρ S a S s [ff(c a )] 2 fs a S s +(1 f)s s +1. (10) This equation shows that the productivity of measurements depends on the parameters of the source, detector and absorber. However, comparing two different detectors we may use the ratio of productivities which depends on the parameters of the source and detectors in the case of resonant detectors: Q 1 = η 01 S a1 S s1 [fs a2 S s2 +(1 f)s s2 +1] Q 2 η 02 S a2 S s2 [fs a1 S s1 +(1 f)s s1 +1], (11) and solely on the parameters of the detectors in the case of non-resonant detectors: Q 1 = η 01 S s1 [S s2 +1] Q 2 η 02 S s2 [S s1 +1]. (12) That is why the latter expression can be applied in the search of optimal Mössbauer detector in transmission geometry. In order to simplify further analysis, one can introduce into consideration a conditional ideal detector with the counting efficiency equal to unity and spectrometric selectivity equal to infinity. Then the productivity ratio for used and ideal detectors acquires a very simple form: Q = ηs s Q id S s +1. (13) Equation (13) allows to make two important conclusions: Czech. J. Phys. 51 (2001) 765

4 A.L. Kholmetskii et al. 1. When the spectrometric selectivity reaches several units, its further increase does not essentially influence the productivity. 2. Productivity of measurements is a linear function of registration efficiency. Now let us consider the case of maximum count-rate of the detectors. This case is realised in practice either by the application of Mössbauer source with high activity, or compressed measuring geometry with increased solid angle of registration. In this case we ought to put into Eq. (9) I out ( ) =I L,whereI L is a limited count-rate. Then we get for productivity ratio Q 1 = I L1 (S a1 S s1 ) 2 [fs a2 S s2 +(1 f)s s2 +1] 2 Q 2 I L2 (S a2 S s2 ) 2 [fs a1 S s1 +(1 f)s s1 +1] 2. (14) One can see that in this case the influence of spectrometric selectivity of a detector is more essential than in the case of low count-rate due to contribution of background radiation to the total limited count-rate. Thus, our analysis shows that the productivity of Mössbauer measurements can be expressed in analytical form through the introduced parameters of the detectors: spectrometric selectivity, absolute selectivity, counting efficiency and limited countrate. Proceeding from the obtained expressions let us compare well-known detectors of Mössbauer radiation. Table 1 shows the characteristics of different detectors, obtained from the amplitude and emission spectra. The results of calculated values of productivity of measurements for the same detectors are presented in Table 2. These results allow to conclude that among widely distributed Mössbauer detectors the highest productivity of measurements is provided: in case of low count-rate by semiconductor detector (90 % from conditional ideal detector ), in case of limited count-rate by resonant scintillation detector (by several times in comparison with other detectors presented in Table 2). The extremely high admissible count-rate of resonant scintillation detector practically excludes any restrictions on the upper limit of activity A of the source. This limit is about 1500 GBk. In this hypothetical case the productivity of measurements can be increased several hundred times. With the real activity of 2 GBk the resonant scintillation detector continues to be the best. In order to realise the advantages of this detector in practice, we have developed a very fast preamplifier, working with resonant scintillation detectors, and providing quasi-gauss formation Table 1.Parameters of detectors. Detector type η S s, S a I L (c 1 ) NaI(Tl) 0.9 S s Semiconductor detector (SD) 0.9 S s = 10 4 Proportional Ar-Xe counter (PC) 0.5 S s Resonant scintillation detector for 57 Fe (RSD) 0.15 S s 2, S a Czech. J. Phys. 51 (2001)

5 Low count-rate Fast detectors for Mössbauer spectroscopy Table 2.Productivity of measurements. Limited count-rate Q SD 0.9Q ideal Q PC 0.5Q ideal Q PC 0.9Q SD Q NaI 0.7Q ideal Q NaI 0.6Q SD Q RSD 0.4Q ideal Q RSD 270Q SD (with A = 1500 GBk) Q RSD 4Q SD (with A =2GBk) of a very short pulse with a high amplification. Its principal characteristics are the following: temperature drift of output signal < 20 µv/grad, total amplification 2000, duration of front of output pulse 5ns, duration of pulses corresponding to conversion 7.3 kev electrons 20ns, non-linearity of transmitting characteristic < 0.2%. At the same time, a disadvantage of the resonant scintillation detector is its low registration efficiency. 2 Fast scintillation detector with a crystal YAlO 3 :Ce For further increase of the productivity of transmission Mössbauer measurements it would be necessary to create a detector, which combines a high limited count-rate with a high registration efficiency. We have reached the optimal combination of these parameters in a scintillation detector with yttrium-aluminium perovskite, YAP [2, 3, 6]. Principal parameters of this scintillator in comparison with NaI(Tl) are presented in Table 3. One can see that light yield of YAP is about Table3.ThecharacteristicsofYAlO 3:Ce scintillator. YAlO 3:Ce NaI(Tl) Effective atomic number Density, g/cm Light yield, % Scintillation decay time, ns Refractive index Emission spectrum maximum, nm Light yield temperature coefficient, %/K Hardness, Mho Melting point, C Hygroscopic no yes Czech. J. Phys. 51 (2001) 767

6 A.L. Kholmetskii et al. Table 4.Calculated productivity Q YAP of transmission measurements for YAP. Low count-rate Limited count-rate 0.75 Q SD 5.6 Q SD 1.4 Q PC 6.8 Q PC 0.9 Q NaI 8.8 Q NaI 1.8 Q RSD 1.8 Q RSD (with A =2GBk) 40 % of the light yield of NaI(Tl). Therefore, the energy resolution of YAP is by 30 % worse than for NaI(Tl). This leads to some decrease of spectrometric selectivity. However, the above obtained equation (13) shows that this parameter does not essentially influence the productivity of measurements. At the same time, the decay time of YAP is almost one order of magnitude shorter than that of NaI(Tl). This defines a principal possibility to greatly increase the limited count-rate of YAP detector. Simultaneously one can provide a registration efficiency, close to unity, with appropriate choice of thickness of the scintillator. We have got the following parameters of spectrometric section with YAP detector (with the optimal thickness of 0.35 mm) and special fast spectrometric preamplifier: η =0.9, S s =3,I L =10 5 c 1. Table 4 represents the results of calculation of the productivity of measurements with YAP detector in comparison with other known detectors. These results allow to conclude that under limited count-rate the YAP detector increases the productivity of measurements up to (5 8) times. It is quite important that a limited count-rate of YAP corresponds to the activity of the Mössbauer source of about 2 GBk, i.e., the numerical estimations correspond to real measuring conditions. 3 Air scintillation detector for conversion electron Mössbauer spectroscopy All the detectors considered above are applicable for registration of gammaquanta. In the case of conversion electron Mössbauer spectroscopy (CEMS) we have developed the so called air scintillation detector which combines a fast operation with the possibility to investigate the samples with almost arbitrary form and size [7]. It is based on three ideas: 1) the registration of light flashes accompanying the discharge which is produced by electron in working gas between two flat electrodes; 2) the use of natural air as working gas in order to provide a big working volume of the detector, which allows to investigate the samples of almost arbitrary sizes; 3) the use of isolating film between two electrodes. In such a case any discharge in air creating by electron is quenched on isolating film, which prevents the formation of self-sustaining discharges in natural air. We conditionally call this phenomenon as microdischarge. An approximate scheme of air scintillation detector is shown in Fig. 1. The sample under investigation is placed near the input window of the photomultiplier. The sample is irradiated by a collimated tangential beam from a 768 Czech. J. Phys. 51 (2001)

7 Fast detectors for Mössbauer spectroscopy Photomultiplier Sample HV Output Collimator Mössbauer source Air Fig.1. Scheme of air scintillation detector. Hermetic chamber Mössbauer source. The sample, photomultiplier and source are placed in a hermetic chamber filled by natural rarefied air. The sign of high voltage on the sample is opposite to the sign of high voltage on the photocathode of the photomultiplier. The electrons leave the surface of the sample and cause the microdischarges in the gap between the source and the photomultiplier. The intensity of the electric field in the gap is determined by the difference of the electric potentials between the sample and the photocathode. Simplicity of the described construction of the detector is provided by the triple function of the photomultiplier: its photocathode is one of the electrodes; its glass bulb plays the role of the isolating film between the electrodes to prevent the formation of selfsustaining discharges in natural air; it properly detects the light pulses. This detector works without energy resolution and can be applied only for integral CEMS. Nevertheless, it has a comparably high spectrometric selectivity with respect to low-energy electrons. It is defined by the energy dependence of ionising losses, de dx 1 E, (15) that allows to realise such conditions where the detection efficiency for middle energy electrons, produced by high energy lines of Mössbauer source (122 kev and 136 kev in case of 57 Co) will be much less than that for Auger and conversion electrons. Under optimal conditions we get the spectrometric selectivity S s =2for 57 Fe Mössbauer spectroscopy. Substituting this value into Eq. (13) and taking into account that the registration efficiency of air scintillation detector is close to unity under optimal conditions, we obtain Q 0.7Q id. The high count-rate of the detector is provided by a short duration of the light pulses from microdischarges in gas, as well as by the glancing incidence of gammarays on the sample surface. In this case the count rate increases by approximately Czech. J. Phys. 51 (2001) 769

8 A.L. Kholmetskii et al. one order of magnitude in comparison with normal incidence of gamma-beam due to a corresponding increase of the path length of the gamma-quanta in the surface layer referring to the maximum escape length of electrons. Hence, the count-rate of the air scintillation detector is several times larger compared to normal incidence used in standard CEMS detectors. The working parameters of air scintillation detector are the following: pressure of air is Pa, thickness of gap is 5 mm, the difference between the sample s and photocathode electric potentials is V, the angle between the sample plane and the gamma-beam axis is 5. Admissible dimensions of the sample are almost arbitrary in the range (0 0.5) m (this size is restricted by dimensions of the hermetic chamber). transparent electrode gaseous helium PM HV sample collected inverted lens Gamma-beam Fig.2. Scheme of light counter. At the same time, the air scintillation detector has some disadvantages. In particular, it does not allow to perform the measurements in a wide range of temperatures of the samples, and does not allow to apply strong magnetic fields to the sample. In order to overcome these restrictions, we propose the following modification of its construction, Fig. 2. Instead of air, the hermetic chamber is filled by gaseous helium, and the photomultiplier is removed from the chamber. In order to collect on the photomultiplier the light from microdischarges in the gap between the sample and the transparent electrode, we apply two collected inverted lenses to get an image of the gap on the input window of the photomultiplier. Application of helium will allow to work in a wide range of temperatures of the samples, and a distant photomultiplier will allow to apply a magnetic field to the sample. We call this detector light counter and currently we develop its design. 770 Czech. J. Phys. 51 (2001)

9 Fast detectors for Mössbauer spectroscopy 4 Conclusions In the case of transmission geometry the best value of the productivity in Mössbauer measurements is provided by YAP detector. The increase of the productivity in comparison with traditional Mössbauer detectors is several times with the activity of the source (1 2) GBk. In addition, YAP detector is very convenient for simultaneous iron and tin Mössbauer spectroscopy, because it has an optimal thickness simultaneously for 14.4 and 23.8 kev. Air scintillation detector is very perspective for integral CEMS, since it allows to investigate the samples of almost arbitrary forms and sizes. One may expect that its development light counter will allow to carry out the CEMS measurements in a wide temperature region with strong applied magnetic fields. References [1] V.I. Goldanskii et al.: Gamma-Resonance Instruments and Methods for phase analysis of Mineral Deposits, Atomizdat, Moscow, [2] A.A. Fyodorov, A.L. Kholmetskii, M.V. Korzhik, et al.: Nucl. Instrum. Meth. B 88 (1994) 462. [3] A.L.Kholmetskii, M.Mashlan, O.V.Misevich, et al.: Nucl.Instrum.Meth.B 124 (1997) 143. [4] A.L. Kholmetskii and O.V. Misevich: Mössbauer concentratometers.universitetskoe, Minsk, 1992, 96 p.(in Russian). [5] A.L. Kholmetskii, O.V. Misevich, N.M. Abramchuk, and S.M. Leshkov: Nucl. Instrum. Meth.B 94 (1994) 493. [6] A.L.Kholmetskii, M.Mashlan, and D.Janchik: in Mössbauer Spectroscopy in Material Science (Ed.by M.Miglierini and D.Petridis).Kluwer Academic Publisher, Dordrecht, 1999, p.391. [7] A.L.Kholmetskii, O.V.Misevich, M.Mashlan, et al.: Nucl.Instrum.Meth.B 129 (1997) 110. Czech. J. Phys. 51 (2001) 771