- Email: [email protected]
PiBrED—A novel bragg edge detector for neutrons
Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
PiBrED—A novel bragg edge detector for neutrons V. Finocchiaro a,b, F. Aliotta b, D. Tresoldi b, R.C. Ponterio b, C.S. Vasi b, G. Salvato b,n a b
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita di Messina, Salita, Sperone 31, I-98166 Messina, Italy CNR-IPCF, Istituto per i Processi Chimico-Fisici, Viale F. Stagno d’Alcontres 37, I-98158 Messina, Italy
a r t i c l e i n f o
abstract
Article history: Received 6 May 2011 Received in revised form 27 July 2011 Accepted 9 August 2011 Available online 22 August 2011
We present a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. The optical signal coming from a scintillator is collected by an optical fiber and is detected by an avalanche photodiode. A fast, Field Programmable Gate Array based, readout allows to obtain transmission spectra within reasonable acquisition times. The performances of the instrument have been tested by measuring the transmission spectra of iron powder samples with two different scintillators. The instrument accuracy in detecting the Bragg edges positions is comparable with the state of the art for similar devices. & 2011 Elsevier B.V. All rights reserved.
Keywords: Neutron detector Bragg edge detector Energy selective neutron imaging
1. Introduction The need of investigating the matter properties and its microscopic structure leads to the development of several sophisticated analysis methods. Many different radiation probes are today employed in a vast realm of experimental techniques; among them, an important role is played by the spectroscopic and diffraction techniques that use neutrons as radiation probe. Neutrons directly interact with atomic nuclei and only very weakly with their electronic shells. Then using neutrons as a probe allows to investigate the internal structure of many materials obtaining information, which are complementary to those from electrons and X-rays. In particular, the use of cold and thermal neutrons, whose wavelengths are of the inter-atomic distance order, allows to establish the lattice structure of a sample and to get many information about its elemental composition. The Bragg edge neutron transmission technique [1] consists in measuring the flux of the neutrons transmitted through a polycrystalline sample as a function of the incident neutron energy. The analysis of the measured spectrum allows to extract useful information about both the investigated material nature and its physical status. The Bragg edge pattern allows to identify the microscopic structure of a given sample; moreover, a spatially resolved analysis of the Bragg edge energies can highlight changes in the d-spacing of the sample lattice revealing material textures and/or internal strains [2]. The Time-Of-Flight (TOF) technique at pulsed spallation sources provides a straightforward way for obtaining energy
n
Corresponding author. Tel.: þ39 090 39762251; fax: þ39 090 3974130. E-mail address: [email protected] (G. Salvato).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.025
resolved spectra. Spatially resolved Bragg edge measurements require a detector that is able to record the neutron flux intensity as a function of both position and energy. Despite of the conceptual simplicity, only few detector prototypes seem promising to record neutron transmission spectra with spatial resolution in the order of 10 mm and temporal resolution better than 10 ms. Some of these prototypes are described in Refs. [3–6]. The fundamental features of a spatially resolved neutron transmission detector for Bragg edge analysis are: spatial and temporal resolution, detector area, time required to obtain a given signal to noise ratio, dynamic range and readout time. It is very difficult to optimize the whole set of parameters e.g. high spatial resolution and large sensible area require a high number of small pixels, which implies a long exposure time to acquire sufficient statistics. In this paper we present a single pixel prototype of a new kind of pixelated transmission detector called PiBrED. Once extended to a greater number of pixels, this detector can allow to perform spatially resolved measurements and residual stress analysis. Furthermore it can be a useful tool for energy selective neutron imaging. As an example of the PiBrED capabilities, we present two iron powder transmission spectra collected with the single pixel prototype at the ROTAX beamline (ISIS, UK).
2. Architecture of the PiBrED The single pixel PiBrED is a neutron detector based on a scintillator screen contained in a light-tight aluminum can, coupled
384
V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
via optical fiber with an Avalanche Photo-Diode (APD) detector (see Fig. 1). The light signal from the scintillator, collected by the optical fiber (core diameter of about 100 mm) in close contact with its surface, is detected by the single photon counting avalanche photodiode (EG&G SPCM-200 PQ-F500). Each detected photon produces a TTLlike pulse, whose time width is of 40 ns, which is sent to the signal processing and readout electronics to be processed. The processing unit has been implemented by means of a Field Programmable Gate Array (FPGA). The electronics allows to construct a histogram of the detected events as a function of the arrival time. The reference time is taken from the trigger signal, T0, synchronous with the pulse generation from the spallation source. To describe the FPGA behavior, let us consider the time interval between two consecutive source pulses. This interval is internally divided into a number of time windows tW. At the end of each time interval, the collected counts are summed to the previously obtained ones in the corresponding histogram bin. At present, the signal processing electronics allows to reach a minimum of 120 ns for tW. In Fig. 2 it is shown a block diagram of the synthesized electronics inside the FPGA. The signal produced by the Timer is used to start each single measurement cycle with a programmable delay from T0. This delay is provided in order to discard the signal produced by the fast neutrons and the gamma flashes coming from the neutron spallation source. The 32-bit counter is used to record the total number of the measurement source triggers. The whole measurement process is managed by the Control Unit while the Signal Processing Unit provides a temporally resolved count of the pulses produced by the detector array. The histogram is built into a fast, 8-bit depth, Random Access Memory (RAM) synthesized inside the FPGA. When one of the RAM locations reaches its maximum
Fig. 1. Experimental setup.
count value (255), all the collected data are added to the previously collected ones, contained in a Double Data Rate Synchronous Dynamic RAM (DDR SDRAM), 32-bit wide. The width of the SDRAM allows to avoid any data transfer to the controlling PC during the measurements, apart for the real time monitoring. For the real time monitoring purposes, the data download from the SDRAM to the PC is also performed by means of a thread with a priority lower than that of the measurement thread, so that there is almost no dead time on the measurement. A double buffer mechanism is also implemented and, when a data transfer to the external PC is triggered, the data contained in the SDRAM are copied in different locations before being transferred. This task is concurrent to the measurement accumulation one. With this architecture, the measurement dead time is only due to the data transfer from the RAM to the SDRAM module, which is a very fast process. This transfer time, tt, can be calculated as tt ¼120 10 9 number_of_pixel histogram_bins [s]; if the period of the neutron source is of 20 ms, a readout will be required every 5 s at worst. The interface between the FPGA and the PC is provided by the Universal Serial Bus (USB) Controller and a UM245R module [7]. Measurement parameters, commands to PiBrED and measurement data are exchanged via the USB.
3. Measurements and data analysis At the ROTAX beamline the flux of neutrons, thermalized by a 95 K liquid methane moderator, is higher than 106 neutron s 1 cm 2. We have performed transmission measurements on two iron powder samples contained in two thin cylindrical aluminum cans (35 mm and 70 mm diameters) using two different scintillators. The first scintillator, S1, is based on 6LiF/ZnS:Ag in a 4:1 ratio with a thickness of 450 mm while the second one, S2, is made of 6LiF/ ZnS:Cu,Al,Au in a 4:1 ratio with a thickness of 225 mm. The light emitted by S1 is peaked around 450 nm while S2 emits at about 520 nm [8]. To normalize the transmitted signal, the unperturbed beamline spectrum have been recorded using both scintillators. The neutron spectra at the ROTAX beamline, as measured by both scintillators, are reported, in counts per seconds, in Fig. 3. As shown in Fig. 3, it seems that the two scintillators have different detection efficiencies: scintillator S1, the blue emitting one, is more efficient in the high energy range while S2, the green emitting one, appears to be more efficient in the low energy region. Such small differences should be further investigated to clarify the role played by the different doping materials and the different thickness of the two scintillators. A rough estimate of the PiBrED neutron detection efficiency can be gained by integrating the above curves. The values
Fig. 2. Block diagram of the PiBrED electronics.
V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
385
Fig. 3. ROTAX beamline spectrum measured by the two scintillators, S1 (black line) and S2 (gray line). The background contributions have been subtracted and the spectra are presented both in TOF (a) and in wavelength (b). In (b) the subtracted background level is also reported (dashed line).
Fig. 4. Transmission spectra measured by the two scintillators, S1 (black line) and S2 (gray line). The background contributions have been subtracted.
obtained are 1.03 106 and 0.99 106 counts per second per cm2 in the hypothesis of a 104 mm2 detected area. These values are to be compared with the ROTAX beamline neutron flux that is claimed to be4106 neutrons per second per cm2. All the presented measurements have been collected with a tW ¼20 ms. The transmission spectra of iron powder have been accumulated for about 12 h and those of the unperturbed beam for about 3 h. The background is very small, has no dependence on TOF, and follows a Gaussian distribution with a mean value of 485 counts per second. The background contribution has been subtracted both from the transmission spectra and the unperturbed beam measurements. The transmission spectra obtained by means of the two scintillators are shown in Fig. 4. In principle, from the transmission spectra it is possible to extract the total cross-section for unit cell, s, as
s¼
lnðI01 ÞlnðI1 Þ lnðI02 ÞlnðI2 Þ ¼ N t1 Nt2
where I01 and I02 are the beam intensities (after background subtraction) measured, respectively, with the S1 and S2 scintillators, I1 and I2 the transmitted intensities (after background subtraction), N the number density of iron cells, t1 and t2 the sample thicknesses. Just for the sake of comparison, we report, in Fig. 5a and b, the experimental normalized results together with the theoretical outcomes from the BETMAn software [9]. To convert the measured data from TOF to wavelength, the flight path has been calibrated using the (2 1 1) reflection of the iron powder samples. Due to its sharp and well defined edge and its low sensitivity to deformations, the (2 1 1) reflection is generally used as the reference for strain measurements [10]. As shown in Fig. 4, the sample measured with the blue scintillator has a transmission that is lower than 1% at wave˚ furthermore, as shown in Fig. 3, the lengths greater than 3.2 A; beam intensity rapidly decreases at wavelengths greater than ˚ For these reasons the transmission data for the blue 3.5 A. scintillator are much noisier than that obtained with the green one. In order to estimate the PiBrED ability in accurately measuring the Bragg edge positions, we have calculated the value of the iron lattice constant using the wavelengths of the eight most intense measured edges [(1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (3 2 1), (3 3 0), (4 3 1)]. To fit the experimental data, we have used the edge model and the fitting procedure described in Ref. [1]. Fig. 6 shows the lattice constant values obtained by the fitting procedure. The average values and statistical uncertainties of the lattice constant are 2.866271.7 mA˚ for S1 and 2.865972.1 mA˚ for S2, which a represent a difference of only 100 me between both scintillators. This shows that the proposed system is robust enough to be used in strain determinations. On the other hand, the statistical uncertainty of the smaller edges is considerable higher. The largest differences for the lattice constant are found between the (3 3 0) and (3 1 0) reflections for both S1 and S2. Such differences correspond approximately to 1730 me and to 2500 me, respectively, and are too large for strain measurement. Comparing our results with those of Ref. [1], we can see that our measurements are affected by a noise level higher of a factor 2 or 3. This can be easily explained in terms of count statistics. In fact, even if the counting time of our measurements is 3 times longer than that in Ref. [1], the area of our detector is about 5000 times less ( 104 mm2 vs. 50 mm2). The results show that both scintillators have similar performances in spite of S1 is slightly faster than S2 [11].
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V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
Fig. 5. Theoretical cross-section of iron powder (gray) compared with those obtained with the scintillators S1 (a) and S2 (b).
directly scalable to a thousand pixels configuration. We would suggest an alternative architecture that uses a linear array of optical fibers coupled to an APD array. The 2D transmission pattern can be obtained mounting the linear array of optical fibers on top of a linear translation stage and performing a scan of the sample area in a way that mimic the optical scanners. Obviously, this solution will increase the time needed to carry out a measurement but, at the same time, will decrease the complexity and the cost of the instrument.
Acknowledgments
Fig. 6. Lattice constant of iron powder as measured using scintillators S1 (black, cross) and S2 (gray, square).
4. Conclusions In this paper we have presented a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. It is based on a scintillator screen, an optical fiber, an APD and a FPGA based readout. We have measured the TOF capability of the instrument using two different scintillators based on 6 Li/ZnS. Both configurations are able to measure the lattice constant of a sample with an accuracy better than 2.1 mA˚ in a 12 h long run with a neutron flux of 106 neutron s 1 cm 2. The results are comparable with the state of the art in the Bragg edge detection [1,2,12]. In view of a multipixel configuration, the instrument can achieve a spatial resolution matching the external diameter of the adopted optical fiber, provided a suitable choice of the spatial resolution of the scintillator. In order to produce a 2D transmission map of a given sample we are planning to realize a multipixel prototype of the PiBrED. The experimental setup proposed in this paper is not easily and
The Funding Agreement No. 06/20018 between CNR and STFC, concerning collaboration in mutual scientific research at the spallation neutron source ISIS (UK) is gratefully acknowledged. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beamtime allocation from the Science and Technology Facilities Council. We are indebted to Dr. W.A. Kockelmann for the stimulating discussions. References [1] J.R. Santisteban, L. Edwards, A. Steuwer, P.J. Withers, J. Appl. Cryst. 34 (2001) 289. [2] W. Kockelmann, G. Frei, E.H. Lehmann, P. Vontobel, J.R. Santisteban, Nucl. Instr. and Meth. A 578 (2007) 421. [3] A.S. Tremsin, et al., Nucl. Instr. and Meth. A (2010). doi:10.1016/j.nima. 2010.06.176. [4] T. Nakamura, et al., Nucl. Instr. and Meth. A 604 (2009) 158. [5] E.H. Lehmann, et al., in: Proceedings of the 12th International Workshop on Radiation Imaging Detectors, Robinson College, Cambridge UK, July 11–15th 2010. ¨ [6] E.H. Lehmann, G. Frei, G. Kuhne, P. Boillat, Nucl. Instr. and Meth. A 576 (2007) 389. [7] /http://www.ftdichip.com/S. [8] /http://www.appscintech.com/S. [9] S. Vogel, Ph.D. Thesis, Kiel University, Available online at:/http://eldiss. uni-kiel.de/S, 2000. [10] W. Reimers, A.R. Pyzalla, A. Schreyer, H. Clemens, Neutrons and Synchrotron Radiation in Engineering Materials Science, Wiley-VCH, 2008. [11] G. Salvato, et al., Nucl. Instr. and Meth. A 621 (2010) 489. [12] A.S. Tremsin, et al., IEEE Trans. Nucl. Sci. NS-56 (2009) 2931.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
PiBrED—A novel bragg edge detector for neutrons V. Finocchiaro a,b, F. Aliotta b, D. Tresoldi b, R.C. Ponterio b, C.S. Vasi b, G. Salvato b,n a b
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita di Messina, Salita, Sperone 31, I-98166 Messina, Italy CNR-IPCF, Istituto per i Processi Chimico-Fisici, Viale F. Stagno d’Alcontres 37, I-98158 Messina, Italy
a r t i c l e i n f o
abstract
Article history: Received 6 May 2011 Received in revised form 27 July 2011 Accepted 9 August 2011 Available online 22 August 2011
We present a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. The optical signal coming from a scintillator is collected by an optical fiber and is detected by an avalanche photodiode. A fast, Field Programmable Gate Array based, readout allows to obtain transmission spectra within reasonable acquisition times. The performances of the instrument have been tested by measuring the transmission spectra of iron powder samples with two different scintillators. The instrument accuracy in detecting the Bragg edges positions is comparable with the state of the art for similar devices. & 2011 Elsevier B.V. All rights reserved.
Keywords: Neutron detector Bragg edge detector Energy selective neutron imaging
1. Introduction The need of investigating the matter properties and its microscopic structure leads to the development of several sophisticated analysis methods. Many different radiation probes are today employed in a vast realm of experimental techniques; among them, an important role is played by the spectroscopic and diffraction techniques that use neutrons as radiation probe. Neutrons directly interact with atomic nuclei and only very weakly with their electronic shells. Then using neutrons as a probe allows to investigate the internal structure of many materials obtaining information, which are complementary to those from electrons and X-rays. In particular, the use of cold and thermal neutrons, whose wavelengths are of the inter-atomic distance order, allows to establish the lattice structure of a sample and to get many information about its elemental composition. The Bragg edge neutron transmission technique [1] consists in measuring the flux of the neutrons transmitted through a polycrystalline sample as a function of the incident neutron energy. The analysis of the measured spectrum allows to extract useful information about both the investigated material nature and its physical status. The Bragg edge pattern allows to identify the microscopic structure of a given sample; moreover, a spatially resolved analysis of the Bragg edge energies can highlight changes in the d-spacing of the sample lattice revealing material textures and/or internal strains [2]. The Time-Of-Flight (TOF) technique at pulsed spallation sources provides a straightforward way for obtaining energy
n
Corresponding author. Tel.: þ39 090 39762251; fax: þ39 090 3974130. E-mail address: [email protected] (G. Salvato).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.08.025
resolved spectra. Spatially resolved Bragg edge measurements require a detector that is able to record the neutron flux intensity as a function of both position and energy. Despite of the conceptual simplicity, only few detector prototypes seem promising to record neutron transmission spectra with spatial resolution in the order of 10 mm and temporal resolution better than 10 ms. Some of these prototypes are described in Refs. [3–6]. The fundamental features of a spatially resolved neutron transmission detector for Bragg edge analysis are: spatial and temporal resolution, detector area, time required to obtain a given signal to noise ratio, dynamic range and readout time. It is very difficult to optimize the whole set of parameters e.g. high spatial resolution and large sensible area require a high number of small pixels, which implies a long exposure time to acquire sufficient statistics. In this paper we present a single pixel prototype of a new kind of pixelated transmission detector called PiBrED. Once extended to a greater number of pixels, this detector can allow to perform spatially resolved measurements and residual stress analysis. Furthermore it can be a useful tool for energy selective neutron imaging. As an example of the PiBrED capabilities, we present two iron powder transmission spectra collected with the single pixel prototype at the ROTAX beamline (ISIS, UK).
2. Architecture of the PiBrED The single pixel PiBrED is a neutron detector based on a scintillator screen contained in a light-tight aluminum can, coupled
384
V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
via optical fiber with an Avalanche Photo-Diode (APD) detector (see Fig. 1). The light signal from the scintillator, collected by the optical fiber (core diameter of about 100 mm) in close contact with its surface, is detected by the single photon counting avalanche photodiode (EG&G SPCM-200 PQ-F500). Each detected photon produces a TTLlike pulse, whose time width is of 40 ns, which is sent to the signal processing and readout electronics to be processed. The processing unit has been implemented by means of a Field Programmable Gate Array (FPGA). The electronics allows to construct a histogram of the detected events as a function of the arrival time. The reference time is taken from the trigger signal, T0, synchronous with the pulse generation from the spallation source. To describe the FPGA behavior, let us consider the time interval between two consecutive source pulses. This interval is internally divided into a number of time windows tW. At the end of each time interval, the collected counts are summed to the previously obtained ones in the corresponding histogram bin. At present, the signal processing electronics allows to reach a minimum of 120 ns for tW. In Fig. 2 it is shown a block diagram of the synthesized electronics inside the FPGA. The signal produced by the Timer is used to start each single measurement cycle with a programmable delay from T0. This delay is provided in order to discard the signal produced by the fast neutrons and the gamma flashes coming from the neutron spallation source. The 32-bit counter is used to record the total number of the measurement source triggers. The whole measurement process is managed by the Control Unit while the Signal Processing Unit provides a temporally resolved count of the pulses produced by the detector array. The histogram is built into a fast, 8-bit depth, Random Access Memory (RAM) synthesized inside the FPGA. When one of the RAM locations reaches its maximum
Fig. 1. Experimental setup.
count value (255), all the collected data are added to the previously collected ones, contained in a Double Data Rate Synchronous Dynamic RAM (DDR SDRAM), 32-bit wide. The width of the SDRAM allows to avoid any data transfer to the controlling PC during the measurements, apart for the real time monitoring. For the real time monitoring purposes, the data download from the SDRAM to the PC is also performed by means of a thread with a priority lower than that of the measurement thread, so that there is almost no dead time on the measurement. A double buffer mechanism is also implemented and, when a data transfer to the external PC is triggered, the data contained in the SDRAM are copied in different locations before being transferred. This task is concurrent to the measurement accumulation one. With this architecture, the measurement dead time is only due to the data transfer from the RAM to the SDRAM module, which is a very fast process. This transfer time, tt, can be calculated as tt ¼120 10 9 number_of_pixel histogram_bins [s]; if the period of the neutron source is of 20 ms, a readout will be required every 5 s at worst. The interface between the FPGA and the PC is provided by the Universal Serial Bus (USB) Controller and a UM245R module [7]. Measurement parameters, commands to PiBrED and measurement data are exchanged via the USB.
3. Measurements and data analysis At the ROTAX beamline the flux of neutrons, thermalized by a 95 K liquid methane moderator, is higher than 106 neutron s 1 cm 2. We have performed transmission measurements on two iron powder samples contained in two thin cylindrical aluminum cans (35 mm and 70 mm diameters) using two different scintillators. The first scintillator, S1, is based on 6LiF/ZnS:Ag in a 4:1 ratio with a thickness of 450 mm while the second one, S2, is made of 6LiF/ ZnS:Cu,Al,Au in a 4:1 ratio with a thickness of 225 mm. The light emitted by S1 is peaked around 450 nm while S2 emits at about 520 nm [8]. To normalize the transmitted signal, the unperturbed beamline spectrum have been recorded using both scintillators. The neutron spectra at the ROTAX beamline, as measured by both scintillators, are reported, in counts per seconds, in Fig. 3. As shown in Fig. 3, it seems that the two scintillators have different detection efficiencies: scintillator S1, the blue emitting one, is more efficient in the high energy range while S2, the green emitting one, appears to be more efficient in the low energy region. Such small differences should be further investigated to clarify the role played by the different doping materials and the different thickness of the two scintillators. A rough estimate of the PiBrED neutron detection efficiency can be gained by integrating the above curves. The values
Fig. 2. Block diagram of the PiBrED electronics.
V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
385
Fig. 3. ROTAX beamline spectrum measured by the two scintillators, S1 (black line) and S2 (gray line). The background contributions have been subtracted and the spectra are presented both in TOF (a) and in wavelength (b). In (b) the subtracted background level is also reported (dashed line).
Fig. 4. Transmission spectra measured by the two scintillators, S1 (black line) and S2 (gray line). The background contributions have been subtracted.
obtained are 1.03 106 and 0.99 106 counts per second per cm2 in the hypothesis of a 104 mm2 detected area. These values are to be compared with the ROTAX beamline neutron flux that is claimed to be4106 neutrons per second per cm2. All the presented measurements have been collected with a tW ¼20 ms. The transmission spectra of iron powder have been accumulated for about 12 h and those of the unperturbed beam for about 3 h. The background is very small, has no dependence on TOF, and follows a Gaussian distribution with a mean value of 485 counts per second. The background contribution has been subtracted both from the transmission spectra and the unperturbed beam measurements. The transmission spectra obtained by means of the two scintillators are shown in Fig. 4. In principle, from the transmission spectra it is possible to extract the total cross-section for unit cell, s, as
s¼
lnðI01 ÞlnðI1 Þ lnðI02 ÞlnðI2 Þ ¼ N t1 Nt2
where I01 and I02 are the beam intensities (after background subtraction) measured, respectively, with the S1 and S2 scintillators, I1 and I2 the transmitted intensities (after background subtraction), N the number density of iron cells, t1 and t2 the sample thicknesses. Just for the sake of comparison, we report, in Fig. 5a and b, the experimental normalized results together with the theoretical outcomes from the BETMAn software [9]. To convert the measured data from TOF to wavelength, the flight path has been calibrated using the (2 1 1) reflection of the iron powder samples. Due to its sharp and well defined edge and its low sensitivity to deformations, the (2 1 1) reflection is generally used as the reference for strain measurements [10]. As shown in Fig. 4, the sample measured with the blue scintillator has a transmission that is lower than 1% at wave˚ furthermore, as shown in Fig. 3, the lengths greater than 3.2 A; beam intensity rapidly decreases at wavelengths greater than ˚ For these reasons the transmission data for the blue 3.5 A. scintillator are much noisier than that obtained with the green one. In order to estimate the PiBrED ability in accurately measuring the Bragg edge positions, we have calculated the value of the iron lattice constant using the wavelengths of the eight most intense measured edges [(1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (3 2 1), (3 3 0), (4 3 1)]. To fit the experimental data, we have used the edge model and the fitting procedure described in Ref. [1]. Fig. 6 shows the lattice constant values obtained by the fitting procedure. The average values and statistical uncertainties of the lattice constant are 2.866271.7 mA˚ for S1 and 2.865972.1 mA˚ for S2, which a represent a difference of only 100 me between both scintillators. This shows that the proposed system is robust enough to be used in strain determinations. On the other hand, the statistical uncertainty of the smaller edges is considerable higher. The largest differences for the lattice constant are found between the (3 3 0) and (3 1 0) reflections for both S1 and S2. Such differences correspond approximately to 1730 me and to 2500 me, respectively, and are too large for strain measurement. Comparing our results with those of Ref. [1], we can see that our measurements are affected by a noise level higher of a factor 2 or 3. This can be easily explained in terms of count statistics. In fact, even if the counting time of our measurements is 3 times longer than that in Ref. [1], the area of our detector is about 5000 times less ( 104 mm2 vs. 50 mm2). The results show that both scintillators have similar performances in spite of S1 is slightly faster than S2 [11].
386
V. Finocchiaro et al. / Nuclear Instruments and Methods in Physics Research A 659 (2011) 383–386
Fig. 5. Theoretical cross-section of iron powder (gray) compared with those obtained with the scintillators S1 (a) and S2 (b).
directly scalable to a thousand pixels configuration. We would suggest an alternative architecture that uses a linear array of optical fibers coupled to an APD array. The 2D transmission pattern can be obtained mounting the linear array of optical fibers on top of a linear translation stage and performing a scan of the sample area in a way that mimic the optical scanners. Obviously, this solution will increase the time needed to carry out a measurement but, at the same time, will decrease the complexity and the cost of the instrument.
Acknowledgments
Fig. 6. Lattice constant of iron powder as measured using scintillators S1 (black, cross) and S2 (gray, square).
4. Conclusions In this paper we have presented a single pixel prototype of a pixelated Bragg edge detector for neutron transmission measurements. It is based on a scintillator screen, an optical fiber, an APD and a FPGA based readout. We have measured the TOF capability of the instrument using two different scintillators based on 6 Li/ZnS. Both configurations are able to measure the lattice constant of a sample with an accuracy better than 2.1 mA˚ in a 12 h long run with a neutron flux of 106 neutron s 1 cm 2. The results are comparable with the state of the art in the Bragg edge detection [1,2,12]. In view of a multipixel configuration, the instrument can achieve a spatial resolution matching the external diameter of the adopted optical fiber, provided a suitable choice of the spatial resolution of the scintillator. In order to produce a 2D transmission map of a given sample we are planning to realize a multipixel prototype of the PiBrED. The experimental setup proposed in this paper is not easily and
The Funding Agreement No. 06/20018 between CNR and STFC, concerning collaboration in mutual scientific research at the spallation neutron source ISIS (UK) is gratefully acknowledged. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beamtime allocation from the Science and Technology Facilities Council. We are indebted to Dr. W.A. Kockelmann for the stimulating discussions. References [1] J.R. Santisteban, L. Edwards, A. Steuwer, P.J. Withers, J. Appl. Cryst. 34 (2001) 289. [2] W. Kockelmann, G. Frei, E.H. Lehmann, P. Vontobel, J.R. Santisteban, Nucl. Instr. and Meth. A 578 (2007) 421. [3] A.S. Tremsin, et al., Nucl. Instr. and Meth. A (2010). doi:10.1016/j.nima. 2010.06.176. [4] T. Nakamura, et al., Nucl. Instr. and Meth. A 604 (2009) 158. [5] E.H. Lehmann, et al., in: Proceedings of the 12th International Workshop on Radiation Imaging Detectors, Robinson College, Cambridge UK, July 11–15th 2010. ¨ [6] E.H. Lehmann, G. Frei, G. Kuhne, P. Boillat, Nucl. Instr. and Meth. A 576 (2007) 389. [7] /http://www.ftdichip.com/S. [8] /http://www.appscintech.com/S. [9] S. Vogel, Ph.D. Thesis, Kiel University, Available online at:/http://eldiss. uni-kiel.de/S, 2000. [10] W. Reimers, A.R. Pyzalla, A. Schreyer, H. Clemens, Neutrons and Synchrotron Radiation in Engineering Materials Science, Wiley-VCH, 2008. [11] G. Salvato, et al., Nucl. Instr. and Meth. A 621 (2010) 489. [12] A.S. Tremsin, et al., IEEE Trans. Nucl. Sci. NS-56 (2009) 2931.