ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2007, Vol. 71, No. 9, pp. 1302–1304. © Allerton Press, Inc., 2007. Original Russian Text © M.G. Kozin, I.L. Romashkina, S.A. Sergeev, L.V. Nefedov, V.P. Koshelets, L.V. Filippenko, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1336–1338.
On the Possibility of Application of Superconducting Tunnel-Junction Detectors in Mössbauer Spectroscopy M. G. Kozina, I. L. Romashkinaa, S. A. Sergeeva, L. V. Nefedova, V. P. Kosheletsb, and L. V. Filippenkob a
Skobel’tsyn Research Institute of Nuclear Physics, Moscow State University, Moscow, 119992 Russia b Institute of Radio Engineering and Electronics, Russian Academy of Sciences, ul. Mokhovaya 11, Moscow, 125009 Russia e-mail:
[email protected]
Abstract—It is shown that a Nb-based superconducting tunnel-junction detector can be used to record γ radiation in the energy range from 1 to 15 keV. The possibility of application of such detectors in Mössbauer spectroscopy is discussed. Prospects of application of tantalum absorbers are considered, in particular, the possibility of developing a cryogenic resonant detector for 181Ta. DOI: 10.3103/S1062873807090249
INTRODUCTION The superconducting tunnel-junction (STJ) detectors that are developed abroad exceed modern semiconductor detectors in energy resolution and some other parameters. This fact conditioned, despite not all details of the operation of such detectors are fully understood, their practical application in astronomy [1], X-ray fluorescence analysis [2], mass spectrometry of heavy biomolecules [3], and other fields. The state of the art of the investigations and practical applications of STJ and other cryogenic detectors, as well as the related problems of physics, technology, signal processing, obtainment of ultralow temperatures, etc. was considered in [4]. Recently, we have obtained an energy resolution of 78 eV in the 5.9-keV line [5, 6], using a detector area of 6400 µm2; this resolution exceeds that of the best silicon detectors by a factor of about 1.5. For a detector of similar design but with a larger area, we performed energy calibration using a 57Co Mössbauer source [7]. In this paper, these data are analyzed in view of the possibility of applying such detectors in Mössbauer spectroscopy. A possibility of developing a resonant STJ detector with a tantalum absorber is also considered.
Al2O3 buffer layer by photolithography and magnetron sputtering. The amplitude spectrum of the pulses recorded by the detector is shown in Fig. 1 (on the logarithmic scale). The 14.41-keV γ line (hereinafter, the Mössbauer isotope parameters are given according to the data of [8] and studies cited there) and the related FeKα (6.40 keV) and FeKβ (7.06 keV) X-ray lines can be seen well. In addition, fluorescence lines from the materials of the source, detector, and their environment, as well as the peaks of escaping niobium X rays, are observed. The origin of these lines was also considered in [7]. N
Fe Kα
Nb escape peaks
Fe Kβ
10000
1000
Al Kα Ag Lα
14.4 keV
Si Kα
In Lα
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RANGE OF MEASURED ENERGIES In this study, we used a detector based on the Ti/Nb/Al, AlOx/Al/Nb/NbN structure (the corresponding layer thicknesses are 30/100/8/2/13/150/30 nm) with an area of 20 000 µm2 and a 57Co(Rh) Mössbauer source with an activity of about 20 mCi. The functions of the layers in the detector electrodes, the experiment geometry, and other details were described in [7]. The entire structure is formed on a silicon substrate with an 1302
100 300
400 500 E, channel number
Fig. 1. Amplitude spectrum of pulses from a 57Co(Rh) Mössbauer source, recorded by a Ti/Nb/Al, AlOx/Al/Nb/NbN STJ detector (with an area of 20 000 µm2) at the temperature T = 1.4 K. E is the energy (channel number) and N is the number of pulses.
ON THE POSSIBILITY OF APPLICATION
It follows directly from Fig. 1 that this detector can record photons in the energy range approximately from 1 to 15 keV. This range includes (along with 57Fe) the energies of γ transitions of such Mössbauer isotopes as 181Ta (6.22 keV), 169Tm (8.41 keV), 83Kr (9.41 keV), and 73Ge (13.26 keV). Unfortunately, our detector, being a thin-film device, has a low detection efficiency (see Fig. 2): a 150-nm-thick niobium film absorbs 3.4% FeKα radiation and 0.37% 14.41-keV radiation. However, we should note that the experimental fact of detection of radiation with relatively high energy (in comparison with the standard value of 5.89 keV for the MnKα line) deserves attention. The point is that with an increase in the radiation energy the mean free path of photoelectrons increases, as well as the probability of emergence of the tracks of the electron collision cascade caused by the primary photoelectron beyond the detector electrode. As a result, the probability of forming the total absorption peak decreases, and differences arise in the shape of the backgrounds against which the 6.4- and 14.41-keV lines are observed and which can be described within the limits of thick and thin films, respectively [9]. The probability of forming the total absorption peak was estimated by Ukibe et al. [10]. They also detected synchrotron radiation with an energy up to 18 keV using a detector with a 200-nm-thick electrode. The STJ technology of current use does not seem to make it possible to significantly increase the absorber thickness (above 150 nm) because of the problem of a reliable current lead to the upper electrode of the detector. However, a heavier absorber can be used, in particular, a tantalum one. The background phonon signal, which arises upon radiation absorption in a silicon substrate, can be decreased, for example, by introducing an additional SiO2 buffer layer [11] in order to isolate the tunnel junction from the substrate. The background due to the rare events of absorption of higher energy (122 and 136 keV) radiation and the fluorescence induced by this radiation can be neglected. POSSIBILITY OF INCREASING THE DETECTOR EFFICIENCY WITH THE USE OF A TANTALUM ABSORBER Figure 2 shows the energy dependences of X-ray absorption in Nb and Ta films (the film thickness is 150 nm in both cases), plotted according to the data taken from the database [12]. The main process in this energy range is photoelectric absorption. It can be seen in Fig. 2 that the efficiency of a Ta absorber exceeds that of a Nb absorber by a factor of 2 for energies of about 6 keV and by almost an order of magnitude for 14-keV radiation. The technology of Ta-based STJ detectors, as well as that of Nb-based detectors, has been fairly well developed. Ta-based detectors are also stable to thermal cycling and their properties do not change with time;
1303
A, % 60 10
Maximum nuclear absorption
7.3
Ta 3.1
3.6 1
Nb 0.37 0.1
0
5
10
15
20 E, keV
Fig. 2. Dependences of the photoabsorptance A in Nb and Ta films with the thickness t = 150 nm on the radiation energy E. The absorptances for the 181Ta and 57Fe lines are indicated by numbers at the points of intersection of these dependences with the corresponding vertical lines (6.22 and 14.41 keV). For 181Ta, an individual point (60%) shows also the hypothetical absorptance due to the nuclear resonance (the recoilless absorption probability is assumed to be unity).
however, they require lower operating temperatures. Using a Ta-based detector, van Vechten and her team detected radiation of a 109Cd source [13]. Undoubtedly, such a detector can resolve the 119Sn Mössbauer line (23.88 keV) from the interfering SnKα X-ray radiation (25.27 keV). Application of such a detector would make it possible to measure 119Sn Mössbauer spectra without a Pd filter. RESONANT Ta-BASED DETECTOR Application of Ta also allows development of a cryogenic resonant detector based on the 181Ta isotope, which records the Mössbauer effect from conversion electrons. The content of this isotope in natural tantalum is 99.99%, and the maximum nuclear resonance cross section [14] for the 6.22-keV Mössbauer transition, determined by the square of the radiation wavelength, is 11 × 10–19 cm2, i.e., larger than the photoelectric cross section by a factor of more than 12 (the corresponding hypothetical absorption is shown by an individual point in Fig. 2). To estimate actual absorption, one should take into account that the best experimental linewidth observed until now is about 15Γ0 [15] (Γ0 = 7.5 × 10–11 eV corresponds to a Doppler shift of 3.6 µm s–1). The excited state formed as a result of the absorption will be deexcited mainly through emission of a conversion electron (the conversion coefficient is 70.5). Conversion occurs at the Ta M shell; therefore, electrons have a relatively low energy (about 4 keV) and, correspondingly, short mean free path. This situation is illustrated in Fig. 3, where the mean free path of conversion electrons is characterized by the radius R,
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KOZIN et al. t
R
Fig. 3. Illustration of the relationship between the absorber thickness t (150 nm) and the mean free path R (50 nm) of conversion electrons in tantalum. The radial lines originating from point of absorption of a γ photon characterize the volume in which the total absorption peak is formed.
which is smaller than the absorber thickness t. This situation differs from the case of resonant gas detectors, in which the resonant absorber should be thin for conversion electrons to allow their escape from it. To estimate the mean free path of conversion electrons in Ta, we used the Kanaya–Okayama formula [16] and the empirical relation reported by van Vechten [17]. These estimates give close values of R ≈ 30–50 nm. It can be seen in Fig. 3 that specifically the events occurring on the path of length of ~50–90 nm in the central part of the 150-nm-thick absorber contribute to the total absorption peak. Along with low temperature, another necessary condition for operation of an STJ detector is application of a weak (about 100 Oe) magnetic field in the junction plane in order to suppress the Josephson current. This field is oriented perpendicularly to the propagation direction of γ photons; therefore, 8 from 24 allowed transitions between the sublevels of the ground (Ig = 7/2, µg = 2.35µN) and excited (Ie = 9/2, µe = 5.2µN) states of 181Ta will be observed. The distance between the outer lines of the hyperfine structure is about 267 × 10−11 eV (36 Γ0 or about 13 µm s–1); this value exceeds the best experimental width by a factor of only little more than 2. The London penetration depth of magnetic field is comparable with the electrode thickness; therefore, the instrumental profile of the detector line will be determined, along with the factors related to the imperfections of the absorber crystal structure, by the applied field and the absorber thickness. Despite the above-mentioned difficulties, the use of a Ta-based detector should make it possible to tune off from the interfering tantalum X rays (TaLα with 8.1 keV) and obtain Mössbauer spectra of higher qual-
ity in comparison with scintillation or proportional counters. CONCLUSIONS It is well known that high energy resolution of a detector makes it possible to improve the quality of Mössbauer spectra. Obviously, future wide application of STJ detectors in Mössbauer spectroscopy is out of the question: even semiconductor detectors with high energy resolution, which were developed long ago, are rarely used for these purposes. However, in the cases where the signal-to-noise ratio in Mössbauer spectra must be high and the object of study and detector should be at a sufficiently low temperature, application of STJ detectors can be justified. ACKNOWLEDGMENTS We are grateful to S.K. Godovikov for the concept of a resonant detector based on the 181Ta isotope and for the helpful discussions. REFERENCES 1. Perryman, M.A.C. et al., Mon. Not. R. Astron. Soc., 2001, vol. 324, p. 899. 2. Frank, M. et al., Rev. Sci. Instrum., 1998, vol. 69, p. 25. 3. Frank, M. et al., Mass Spectrom. Rev., 1999, vol. 18, p. 155. 4. Proc. 11th Int. Workshop on Low-Temp. Detectors (LTD11), Tokyo, Japan, 2005, Nucl. Instrum. Methods Phys. Res., Sect. A, 2006, vol. 559. 5. Kozin, M.G. et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 2004, vol. 520, p. 250. 6. Kozin, M.G. et al., Izv. Akad. Nauk, Ser. Fiz., 2005, vol. 69, no. 1, p. 36. 7. Kozin, M.G. et al., Prib. Tekh. Eksp., 2006, no. 6, p. 135. 8. Leupold, O. et al., Hyperfine Interact., 1999, vol. 123/124, p. 612. 9. Van Vechten, D. et al., IEEE Trans. Appl. Supercond., 1995, vol. 5, no. 2, pt. 3, p. 3030. 10. Ukibe, M. et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 1998, vol. 402, p. 95. 11. Poelaert, A. et al., J. Appl. Phys., 1996, vol. 79, p. 2574. 12. http://www.nist.gov. 13. Porter, F.S. et al., IEEE Trans. Appl. Supercond., 1995, vol. 5, no. 2, pt. 3, p. 3026. 14. Shpinel’, V.S., Rezonans gamma-luchei v kristallakh (γRay Resonance in Crystals), Moscow: Nauka, 1969. 15. Dornow, V.A. et al., Nucl. Instrum. Methods Phys. Res., 1979, vol. 163, p. 491. 16. Kanaya, K. and Okayama, S., J. Phys. D: Appl. Phys., 1972, vol. 5, p. 43. 17. Van Vechten, D. and Wood, K.S., Phys. Rev. B: Condens. Matter Mater. Phys., 1991, vol. 43, p. 12 852.
BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES: PHYSICS
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2007
