Nuclear Instruments and Methods in Physics Research A

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1 Nuclear Instruments and Methods in Physics Research A 624 (2010) Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: Expanding the detection efficiency of silicon drift detectors D.M. Schlosser a,, P. Lechner a, G. Lutz a, A. Niculae b, H. Soltau a, L. Strüder c, R. Eckhardt a, K. Hermenau a, G. Schaller c, F. Schopper c, O. Jaritschin b, A. Liebel b, A. Simsek b, C. Fiorini d, A. Longoni d a PNSensor GmbH, Römerstr. 28, München, Germany b PNDetector GmbH, Otto-Hahn-Ring 6, München, Germany c MPI Halbleiterlabor, Otto-Hahn-Ring 6, München, Germany d Politecnico di Milano and INFN Sezione di Milano, Milano, Italy article info Available online 20 April 2010 Keywords: Silicon drift detector (SDD) X-ray detector Low X-ray energy Light elements Gamma ray detection Quantum efficiency Scintillator Hard X-ray abstract To expand the detection efficiency Silicon Drift Detectors (SDDs) with various customized radiation entrance windows, optimized detector areas and geometries have been developed. Optimum values for energy resolution, peak to background ratio (P/B) and high count rate capability support the development. Detailed results on sensors optimized for light element detection down to Boron or even lower will be reported. New developments for detecting medium and high X-ray energies by increasing the effective detector thickness will be presented. Gamma-ray detectors consisting of a SDD coupled to scintillators like CsI(Tl) and LaBr 3 (Ce) have been examined. Results of the energy resolution for the 137 Cs 662 kev line and the light yield (LY) of such detector systems will be reported. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Silicon Drift Detectors (SDDs), introduced in 1984 [1], are nowadays being used in a rising number of different applications. As a result of the continuous improvements in the detector technology, the SDDs with integrated FET [2] fabricated by the Semiconductor Laboratory of the Max-Planck-Institute together with the company PNSensor have established themselves as stateof-the-art detectors for Energy Dispersive X-ray (EDX) spectroscopy. The working principle of a SDD is based on sideward depletion [3]. The SDD geometry enables a low anode capacitance and the monolitical integration of the first amplification step, a junction gate field effect transistor (JFET), in the silicon. This geometry reduces stray capacitances and avoids pick-up noise or microphony. The reduction of the detector capacitance leads to a low electronic noise, hence to an improved energy resolution, even at high count rates. The detector efficiency is an important feature of an EDX system. It influences on the one hand the ultimate quality of the measurement process and on the other hand the overall measurement time which is of great importance in industrial applications. There are three factors determining the detection efficiency: detector key performance (energy resolution, P/B ratio), detector quantum efficiency, detector area and geometry. The influence of all three factors will be discussed in the following. Detector systems consisting of SDDs in combination with a scintillator have also been developed and investigated as gamma ray detectors with superior spatial and energy resolution for gamma energies above 100 kev in comparison to photodiodes and photomultipliers (PMTs) [4 7]. The advantages of SDDs as photodetectors are the high quantum efficiency (QE), the low electronic noise at moderate cooling temperatures ( 20 1C) and their compactness. In the next chapters the detector noise and the P/B ratio, the light element, medium and hard X-ray detection capability and the detection of gamma-rays will be discussed. 2. Improvement in detector noise and P/B ratio For energy dispersive solid state detectors the main contributions to the energy resolution, expressed as full width at half maximum (FWHM) (Eq. (1)), is the noise due to the statistical fluctuation in the number of electron hole (N eh ) pairs created by an incident X-ray, the so-called Fano noise (Eq. (2)), and the electronics of detector and first amplification stage (Eq. (3)): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi FWHMðEÞ¼2:35 e ENC Fano ðeþ 2 þenc 2 el: ð1þ The Fano noise (Eq. (2)) depends on detector material properties including F¼Fano factor and e ¼ mean energy to create an electron hole pair and the E¼X-ray energy. It sets the lowest limit for the detector energy resolution: Corresponding author. address: dieter.schlosser@pnsensor.de (D.M. Schlosser). ENC 2 Fano ðeþ¼ F e E: ð2þ /$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi: /j.nima

2 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) The three terms of the electronic noise (Eq. (3)) are describing the contributions of the serial white noise, the 1/f noise of the integrated JFET and the shot noise associated to the leakage current I l of the detector [8]. Additional noise sources from other electronic components are neglected in the formula, respectively, hidden in the factors A 1 A 3. ENC 2 el ¼ 4kT Ctot 2 3g A 1 1 m t þ2pa f Ctot 2 A 2 þqi l A 3 t ð3þ with k is the Bolzmann constant, T the temperature, g m the transconductance of the JFET, a f the constant parameterizing the JFET 1/f noise, q the elementary charge, t ¼ shaping time, and A 1, A 2, A 3 the constants depending on the filter functions of the shaper. The serial white and 1/f noise strongly depend on the total capacitance C tot seen by the detector anode, which is a sum of the anode and the gate-drain capacitance of the integrated JFET and the depletion capacitances between the anode and the neighboring regions [9] and other parasitic contributions if existent. The parallel noise is mainly determined by the value of the leakage current I l. Fig. 1 shows the FWHM, calculated according to Eq. (1), in dependence of the incident X-ray energy for two values of the electronic noise, given in equivalent noise charge (ENC). The lower limit of the FWHM is determined by the Fano noise (Eq. (2)), if ENC el ¼0 electrons. An increase of ENC el to 4, 10 electrons leads to a higher relative increase of the FWHM especially at lower X-ray energies. In the low energy regime the benefit of a low electronic noise value on the energy resolution is the greatest. Possibilities to improve the energy resolution are the reduction of the total detector capacitance and the leakage current. A reduction of C tot from 150 to 80 ff is achieved by moving the anode and the integrated JFET from the center (standard SDD) to the border of the SDD (droplet SDD¼SD 3 ) [10,11]. The JFET located at the SDD border has a second positive effect. Irradiation of the integrated JFET can be avoided by mounting an appropriate collimator. Undesired background events can be reduced. This circumstance leads to a higher P/B ratio [10]. Furthermore the leakage current could be decreased, through a new fabrication technology, poly-silicon, to a stable level down to 200 pa/cm 2 at room temperature, reducing the energy resolution further. The dependence of the energy resolution of a 10 mm 2 SD 3 on the count rate is illustrated in Fig. 2. Energy resolution values down to a full width at half maximum (FWHM) of 123 ev at the Mn2K a line have been currently measured at moderate temperatures of T¼ 20 1C. The charge sensitive amplifier (CSA) Fig. 2. Count rate dependent energy resolution of the Mn K a line measured with a10mm 2 SD 3. The energy resolution is about 123 ev at count rates of some kcps and changes to about 125 ev for count rates up to 130 kcps. Fig. 3. Superposition of spectra of Boron, Carbon and Oxygen measured with a 10 mm 2 SD 3, with pn-window at T¼ 20 1C. The FWHM of the B2K a,c2k a and O2K a lines are down to 38, 42 and 48 ev. readout configuration in combination with the pulsed reset operation mode ensures a nearly constant energy resolution up to a few hundred kcps [9]. 3. Optimizing the detector for light element performance Fig. 1. Energy resolution against X-ray energy for different contributions of the electronic noise. Apart from the detector noise, the detector entrance window is important at low X-ray energies. A modified new detector entrance window, pn-window, has been developed for optimum light element detection. The pn-window reduces the loss of generated electrons in the SDD p+ layer of the entrance window after an interaction of a X-ray photon in this region, so that a lower number of events with partial charge collection are detected. This leads to an improved energy resolution measured at lower X-ray energies. The spectra of boron and carbon measured with a 10 mm 2 SD 3 detector are plotted in Fig. 3. Energy resolution values of 38 ev for Boron line (138 ev) or 42 for Carbon line (277 ev) have been determined.

3 272 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) Fig. 4. Spectra of two 10 mm 2 SD 3 with a standard entrance window (EW) (gray) and a optimized EW (black), while irradiating with a 55 Fe source. The optimization of the P/B and P/V ratios by the pn-window is obvious. Fig. 6. FWHM at Mn K a of a circular 30 mm 2 SDD vs. shaping time at 20 and 30 1C. At optimal shaping times of 2 or 3 ms FWHM of 129 ev have been measured. Fig. 7. FWHM at Mn2K a of a square 100 mm 2 SDD vs. shaping time at 20, 25, 30 and 351C. Fig. 5. Spectrum of a 20 mm 2 SD 3 with optimized pn-window, while irradiating with a 55 Fe source. The FWHM of the Mn2K a line is 125 ev and a P/B ratio of up to has been measured. The P/B and P/V (peak to valley) ratio of the SDDs with pn-window is improved by the same physical mechanism also for higher X-ray energies e.g. Mn K a. This effect is shown in Fig Optimization of area and geometry to increase the detection efficiency for the fluorescent X-rays The optimization of the geometry of the detection area is important for applications requiring maximum collection efficiency of the incoming photons. This can be achieved by maximizing the detection area, by matching the detector geometry to the geometry of the system, as well as by maximizing the detection solid angle for the incident radiation. Fig. 5 shows the spectrum of 55 Fe measured with a droplet shaped, SD 3, detector, of which the detection area has been increased to 20 mm 2. The energy resolution at Mn2K a line is 125 ev at T¼ 20 1C with a P/B ratio of Prototypes of 30 mm 2 SD 3 have also been developed. They are still under investigation. Fig. 6 shows the FWHM at Mn2K a of a 30 mm 2 circular SDD against shaping time at two different temperatures. In Fig. 7 the Fig. 8. Detector consisting of mm 2 cells with an on chip collimator. FWHM at Mn2K a of a 100 mm 2 circular SDD for four temperatures is plotted against the shaping time. Increasing the detector area furthermore to several cm 2 by maintaining the energy resolution and avoiding dead area is possible with monolitical SDD arrays. Detectors with large areas have been produced in that way [11]. The one presented here has an area of mm 2, consisting of six cells. The energy resolution of this detector is down to 140 ev at the Mn2K a line and moderate temperatures of 201 for each channel. Operation is possible for input count rates of up to 3 Mcps. By means of an adapted radiation entrance window, these detectors can also be used in combination with scintillators for gamma ray detection (Figs. 8 and 9).

4 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) Fig Fe spectra of each channel of the six element SDD. Fig. 12. SDD without a housing, electrically coupled by a flex lead, as a detector to be integrated arbitrarily into analysis instruments e.g. electron microscopes, where the space for the detector is limited. cooling plate peltier element Fig. 10. Rococo 2 consists of four SDDs, with an area of 15 mm 2 each, a central hole and a on chip collimator. ceramic SDD chip Fig. 13. Example of a construction for the flexlead SDD shown in Fig. 12, which is cooled in this case by a cooling plate and a peltier element. Fig. 11. Fluorescent X-rays from the sample detected by Rococo 2. The sample is irradiated by an X-ray beam through the central hole of the SDD. A maximum solid angle covered by Rococo 2 can be achieved by placing the sample as near as possible to the detector. Specific devices with a central hole for close arrangement of the detector to the sample to ensure high collection efficiency of X-ray fluorescence photons have been developed and tested (Fig. 10) [12]. In such a geometry the detector covers a bigger solid angle from the excitation point of the fluorescent X-rays on the sample, so that the detected fraction of fluorescent X-rays is increased (Fig. 11). Such detector consists of four SDDs, with an active area of 15 mm 2 each, on a single chip and is suitable in various systems with X-rays and electrons as exciting beam (Fig. 11). Energy resolution of the Mn K a line down to 129 ev has been measured. Besides the design of the sensors themselves the SDD mounting is very flexible and accounts for the small amount of space available in an electron microscope. The fact that for SDDs with integrated FET an external bulky cooling mechanism is not necessary to reach optimum performance becomes very important. As demonstrated in Fig. 12 the detector can be built up in a very slim version connected electrically by a small flex lead and cooled by a peltier element, which is connected to the ceramic by a cooling plate (Fig. 13). This detector architecture allows various integration possibilities into electron microscopes.

5 274 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) Optimizing the detection efficiency for medium and hard X-rays To improve the detection efficiency for medium and high X-ray energies the thickness of SDDs ð450 mmþ has to be increased. This can be done by using a thicker silicon substrate. A disadvantage of using SDDs with thicker substrates is besides the difficulty in starting a new silicon production line the increase of leakage current. To reach the same electronic noise level and energy variance the operation temperature has to be decreased. A new detector arrangement is avoiding these difficulties and offering new opportunities: It consists of a stack of two SDD detector chips (Fig. 14) leading to an overall thickness of 900 mm. The thermal coupling of the SDD chips to the peltier cooler is good, giving rise to a negligible temperature difference between the two chips and a typical operation temperature of 20 1C only. The spectra of 109 Cd + 55 Fetakenwiththe2-layerSDDareshown in Fig. 15. Opportunities of the 2-layer SDD are obvious: on the one side the good performance for X-ray energies Eo10 kev remains unchanged compared with 450 mm thick SDDs, on the other side trace contaminations of heavy elements can be easily distinguished. The calculated QE of the 2-layer SDD compared to the SDD with the standard thickness is plotted in Fig. 16. For energies beyond the Ag K a line it is increased by almost a factor of two. The ratio of the theoretical QEs of a standard SDD and a 2-layer SDD is QE ð450 mmþ ¼ 0:56 ðfig:16þ: QE ð900 mmþ ð4þ Fig. 16. Calculated quantum efficiency (QE) of a 450 and 900 mm thick SDD. The ratio of the measured counts with the first SDD layer to the total measured counts at the Ag K a line is counts layer 1 counts layer 1þcounts layer 2 ¼ 0:57 which is in good agreement with the theoretical value. 6. Extension of the energy range of SDDs into the gamma range ð5þ ceramic 1 ceramic 2 Peltier SDD chip 1 SDD chip 2 Silicon of thicknesses up to several millimeters is transparent for g-rays with energies E4100 kev. Gamma rays of such energies can be detected with a detector consisting of a SDD coupled to a scintillator, where the SDD is used as a photodetector for the scintillation light (Fig. 17). Fig. 14. Drawing of the two layer SDD, with a total thickness of 900 mm. Fig. 17. CsI(Tl) wrapped into a reflector and coupled to a SDD. The formula of the relative energy resolution RðE g Þ ðfwhmðe g Þ=E g Þ of such a detector system is given in Eq. (6). ENC el is the equivalent noise charge of the SDD and the electronics, N eh are the generated electron hole pairs in the SDD, R intr is the intrinsic resolution of the gamma detector and E g the energy of the g-photons. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ENC 2 el RðE g Þ¼2:35 N 2 eh ðe gþ þ 1 N eh ðe g Þ þr2 intr ðe gþ: ð6þ Fig Cd + 55 Fe spectra of the two channels of the 2-layer SDD shown in Fig. 14. For a good energy resolution as many of the following criteria as possible have to be fulfilled. The SDD entrance window should be optimized to maximize scintillation photon detection, thus maximizing the number of generated e h pairs. The electronic noise, ENC el, should be as low as possible. The scintillation decay time (Table 1) should be faster than the optimal shaping time for the SDD and the drift time of the electrons in the SDD to the anode to ensure the use of the optimum shaping time for minimum ENC el and a high count rate capability. A high light yield (LY) of

6 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) the scintillator (Table 1) is desirable and a low intrinsic resolution, R intr, of the gamma detector improves the energy resolution in addition. For compact scintillators, crystals with high densities are needed to minimize the volume by keeping the stopping power for gamma rays. Two scintillators with high LYs have been investigated, CsI(Tl) and LaBr 3 (Ce), manufactured by Scionix Netherlands and Saint Gobain Crystals. The LY of CsI(Tl) is about 54 photons/kev and of LaBr 3 (Ce) 63 photons/kev with a maximum of the scintillation light spectrum at 550 nm for CsI(Tl) and 380 nm for LaBr 3 (Ce). The scintillation decay times of CsI(Tl) are 600 and 3400 ns and of LaBr 3 (Ce) 16 ns. In Table 1 the values of the refraction index and density of the scintillators are also listed. Fig. 18 shows the calculated quantum efficiencies (QEs) of two SDD entrance windows for photons with a propagation direction perpendicular to the entrance window. One entrance window has been optimized for photons with wavelengths around 400 nm and the other for photons around 550 nm to fit the scintillation photon spectra. In Fig. 19 the N eh pairs per kev, which are generated in the SDD, are plotted against the shaping time. The LY has been determined from the pulse hight of the 662 kev 137 Cs line. Because of the slow scintillation decay times of CsI(Tl) (Table 1), long shaping times of some ms ðt shaping 410 msþ are needed to minimize ballistic deficit, which leads to a higher contribution of the leakage current to the electronic noise. In contrast the optimal shaping time of a SDD with minimal noise is around 0.5 and 1 ms. This corresponds better to LaBr 3 (Ce). Fig. 19 shows that already at 1 ms all charges are collected. The lower number of eh-pairs when measuring with LaBr 3 (Ce) compared to CsI(Tl) is mainly caused by the loss of photons on their way out of the vacuum sealed LaBr 3 (Ce) housing into the SDD. This loss of photons can be Fig. 19. Dependence of the LY of the gamma detector, SDD + LaBr 3 (Ce) or CsI(Tl), on the shaping time. Table 1 Properties of CsI(Tl) and LaBr 3 (Ce) given by Scionix Netherlands and S. Gobain Crystals: LY scintillator light yield, t decay decay time of scintillation light, l max maximum of scintillation spectrum, n refraction index at l max, r density. Scintillator LY (photons/kev) t decay (ns) l max (nm) n r (g/cm 3 ) CsI(Tl) ; LaBr 3 (Ce) Fig. 20. Spectra of gamma detectors consisting of SDD coupled to LaBr 3 (Ce) or to CsI(Tl) scintillators, while irradiating with a 137 Cs source. Fig. 18. The QEs of two SDD EWs for photons, which propagate perpendicular towards the EW coming from the vacuum, are plotted against the photon wavelength (continuous lines). The EWs have been optimized for photons with wavelengths around 400 or 550 nm. These wavelength ranges fit to the scintillation spectra of CsI(Tl) and LaBr 3 (Ce) (dashed lines). reduced by coupling the scintillator LaBr 3 (Ce) directly onto the SDD. Commercially available LaBr 3 (Ce) scintillators are normally packed into vacuum sealed housings, because LaBr 3 (Ce) is hygroscopic. Unfortunately it has been not possible for us to receive an unpacked LaBr 3 (Ce) scintillator yet. The spectra in Fig. 20 have been measured with a detector consisting of a SDD, coupled to a cylindrical shaped CsI(Tl) (gray curve) or LaBr 3 (Ce) (black curve) scintillator, while irradiating with a 137 Cs source. Both scintillators are wrapped into reflectors. The energy resolution of the 662 kev 137 Cs line down to 2.8% has been measured with LaBr 3 (Ce) or 4.3% with CsI(Tl). In spite of a lower number of optical photons entering the SDD, if the SDD is coupled to LaBr 3 (Ce) (Fig. 19) the energy resolution of the 662 kev 137 Cs line is better compared to a detector consisting of SDD + CsI(Tl), because of a better intrinsic resolution of LaBr 3 (Ce) at E g ¼ 662 kev and a lower electronic noise contribution, ENC el, due to a shorter shaping time of 0:5 ms (LaBr 3 (Ce)) compared to 10 ms (CsI(Tl)). The energy resolution strongly depends on the scintillator quality. For a gamma detector consisting of LaBr 3 (Ce) coupled to a photomultiplier (PMT) or a large area avalanche photo diode (LAAPD) energy resolution values of 2.7% or 3.1% have been

7 276 D.M. Schlosser et al. / Nuclear Instruments and Methods in Physics Research A 624 (2010) measured for the 662 kev 137 Cs line [13]. The readout of the light pulses generated in LaBr 3 (Ce) with SDDs lead to a better energy resolution above gamma energies of 100 kev compared to the readout of the same LaBr 3 (Ce) scintillator with PMTs, LAAPDs or photo diodes (PDs) [6]. In case that CsI(Tl) is coupled to a PMT values of 5.2% have been measured for the 662 kev 137 Cs line [14]. Gamma detectors consisting of CsI(Tl) or LaBr 3 (Ce) coupled to SDDs with an adapted entrance window for scintillation photons achieve similar or superior energy resolutions for the 662 kev 137 Cs line compared to gamma detectors consisting of CsI(Tl) or LaBr 3 (Ce) coupled to PMTs, LAAPDs or PDs. 7. Summary and conclusions Various possibilities for the enhancement of the detection efficiency of SDDs have been presented. SDDs with optimum energy resolution and high count rate capability build up the baseline. Improvement of the radiation entrance window leads to a reduction of partial events, resulting in excellent light element performance and shifting the detection efficiency to low energies. Large SDDs with active areas up to 600 mm 2 improve the detection efficiency for specific applications as TXRF or synchrotron needs. New fascinating SDD geometries as the SDD with the hole in the middle for the exciting beam or as the distributed element systems arranging SDDs via flex lead architecture almost arbitrarily into the analytical instruments allow much higher detection efficiency as in the past and will have a strong impact on the analysis methods especially in micro analysis. A double stage 1 mm thick SDD shows enhanced detection efficiency for medium X-rays as Ag with undisturbed energy resolution. Detectors consisting of a SDD, with optimized entrance window for scintillation light, coupled to scintillators, LaBr 3 (Ce) and CsI(Tl), for gamma ray detection have been investigated for gamma spectroscopy and results of the energy resolution for the 137 Cs line, which belong to the best resolution measured yet, have been tabled. References [1] E. Gatti, P. Rehak, J.T. Walton, Nucl. Instr. and Meth. 226 (1984) 129. [2] P. Lechner, S. Eckbauer, R. Hartmann, S. Krisch, D. Hauff, R. Richter, H. Soltau, L. Strüder, C. Fiorini, E. Gatti, A. Longoni, M. Sampietro, Nucl. Instr. and Meth. A 377 (1996) 346. [3] E. Gatti, P. Rehak, Nucl. Instr. and Meth. A 225 (1984) 524. [4] P. Lechner, R. Eckhard, C. Fiorini, A. Gola, A. Longoni, A. Niculae, R. Peloso, H. Soltau, L. Struder, Hard X-ray and gamma-ray imaging and spectroscopy using scintillators coupled to silicon drift detectors, Proc. SPIE, vol. 7021, 2008, p , doi: / [5] C. Fiorini, A. Gola, M. Zanchi, A. Longoni, P. Lechner, H. Soltau, L. Strüder, IEEE Trans. Nucl. Sci. NS-53 (4) (2006) [6] M. Moszyński, C. Plettner, A. Nassalski, T. Szcześniak, L. Świderski, A. Syntfeld-Kaz uch, W. Czarnacki, G. Pausch, J. Stein, A. Niculae, H. Soltau, IEEE Trans. Nucl. Sci. NS-56 (3) (2009) [7] C. Fiorini, A. Gola, R. Peloso, A. Longoni, P. Lechner, H. Soltau, L. Strüder, Nucl. Instr. and Meth. A 604 (2009) 101. [8] B. Beckhoff, B. KanngieBer, N. Langhoff, R. Wedell, H. Wolff (Eds.), Handbook of Practical X-Ray Fluorescence Analysis, Springer Verlag, Berlin/Heidelberg, [9] A. Niculae, P. Lechner, H. Soltau, G. Lutz, L. Strüder, C. Fiorini, A. Longoni, Nucl. Instr. and Meth. A 568 (2006) 336. [10] P. Lechner, A. Pahlke, H. Soltau, X-Ray Spectrom. 33 (2004) 256. [11] P. Lechner, C. Fiorini, A. Longoni, G. Lutz, A. Pahlke, H. Soltau, L. Strüder, Adv. X-ray Anal. 47 (2004) 53. [12] C. Fiorini, A. Longoni, Nucl. Instr. and Meth. B 266 (2008) [13] M. Moszyński, L. Świderski, T. Szcześniak, A. Nassalski, A. Syntfeld-Kaz uch, W. Czarnacki, G. Pausch, J. Stein, P. Lavoute, F. Lherbert, F. Kniest, IEEE Trans. Nucl. Sci. NS-55 (2008) [14] A. Syntfeld-Kaz uch, M. Moszyński, L. Świderski, W. Klamra, A. Nassalski, IEEE Trans. Nucl. Sci. NS-55 (3) (2008) 1246.