- Email: [email protected]
Real-time radiography at the NECTAR facility
Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Real-time radiography at the NECTAR facility n ¨ T. Bucherl , Ch. Lierse von Gostomski
Lehrstuhl f¨ ur Radiochemie (RCM), Technische Universit¨ at M¨ unchen (TUM), Walther-Meissner-Str. 3, 85748 Garching, Germany
a r t i c l e i n f o
abstract
Available online 25 January 2011
A feasibility study has shown that real-time radiography using fission neutrons is possible at the ¨ NECTAR facility, when using an improved detection system for fast variations (Bucherl et al., 2009 [1]). Continuing this study, real-time measurements of slowly varying processes like the water uptake in medium sized trunks (diameter about 12 cm) and of slow periodic processes (e.g. a slowly rotating iron disk) are investigated successfully using the existing detection system. & 2011 Elsevier B.V. All rights reserved.
Keywords: Neutron Radiography Real-time NECTAR Fission
1. Introduction The non-destructive characterization of dynamic processes by means of fission neutron radiography is a challenging task as the available neutron fluxes and the efficiencies of the position sensitive detection (PSD) systems are much lower than those at facilities using X-rays, gamma-rays or thermal and cold neutrons. However, the unique properties of fission neutrons, i.e. their high sensitivity for hydrogen and less sensitivity for dense materials (e.g. shielding materials like lead) seem reasonable for the investigation of dynamic processes. For the investigation of dynamic processes real-time measurements are required. The time resolution for a specific PSD system is defined by the minimum time period between two succeeding measurements being made up by the shutter time ts, the integration time tm and the data transfer and/or storage time td. Thus, the minimum cycle time Dt is given by
Dt ¼ 2ts þtm þ td
ð1Þ
with ts and td being fixed values and only tm being user defined. Fig. 1 shows the typical time response for two succeeding measurements starting at time i and (i +1). The integration time ti must be adjusted such that the image quality is of significant statistics. On the other hand, the longer the integration time the more blurred the image will be due to in-motion unsharpness. If periodic processes have to be investigated, triggered realtime measurements may be applied to improve the statistics even for very short integration times by summing up several images measured for the same time interval. This is illustrated in Fig. 2, where i and (i +1) denote the moments of two succeeding trigger signals, i.e. the time interval of one period of the process. n
Corresponding author. Tel.: +49 89 289 14328; fax: + 49 89 289 14347. ¨ E-mail address: [email protected] (T. Bucherl).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.061
The trigger signal at point i starts one measurement sequence, i.e. after an offset (or delay) time to the data acquisition for one image is performed. This is repeated identically N times for succeeding trigger signals. Then, after some pre-processing like filtering, these N images are summed up and normalized. This procedure is repeated for increasing offset times (i.e. starting at point 0 in discrete time steps until t0 is equal to a complete period). The normalized images can then be merged to visualize
Fig. 1. Time response of two succeeding measurements starting at i and (i +1).
Fig. 2. Time response of a triggered real-time measurement.
176
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
the periodic process. To optimize the overall measuring time required, additional measurements within a period can be started at point i0 , if the time for a single measurement (Dt + to) is less than the time between two succeeding triggered measurements at point i and (i+ 1). The positions i0 must be selected to fit the positions of i identically. In 2008 first investigations took place demonstrating the principle suitability of the NECTAR facility for real-time radiography [1]. This was verified by the first imaging of a slowly
Fig. 3. Photography of the experimental set-up for the measurement of water uptake in a trunk. The trunk is loaded by an iron cylinder. In the background the converter plate of the detector system is visible.
varying process and was presented in 2009 [2]. Based on these results the investigations of other slowly varying and of periodic processes are continued.
Fig. 5. Time dependence of the water uptake in the trunk.
2 min
200 min
1000 min
2000 min
Fig. 4. Difference images of the water uptake in a trunk after about 2, 200, 1000 and 2000 min after filling water in the bowl (from upper left to lower right).
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
2. Experiments and results All measurements are performed at the NECTAR facility. A collimated fission neutron beam (f ¼5.4 105 cm 2 s 1; L/D ¼ 233 716) is used for the investigation of all slowly varying processes, while for periodic processes a non-collimated beam (f ¼1.2 107 cm 2 s 1; L/D ¼132712) is used. A detailed description of the facility and the parameters is given in Ref. [4].
177
For quantitative evaluation, i.e. the determination of the water content as a function of time, the pixel values pi within a predefined area in the difference images are determined and
2.1. Real-time measurements As examples of slowly varying processes, the absorption of water in trunks is investigated by placing them in a water quench and by applying drops of water from the top. In a first experiment a tree trunk (diameter about 12 cm) is placed in a plastic bowl. An iron cylinder on top of the trunk acts as additional mass to avoid floating when water is added to the bowl (Fig. 3). A reference image is registered before adding the water to the bowl. Then, without changing the alignment of the experimental set-up, the bowl is carefully filled with about 250 ml of water and a series of 1000 radiographs back-to-back is started immediately, thus covering a time period of about 52 h. The integration time is 60 s, the shutter time 0.1 s and the data storage time 2 s. Thus, the time between two succeeding images is 62.2 s. All images are filtered [3], dark image corrected, normalized to the flat field image and then subtracted by the adequately treated reference image. The resulting difference images, at about 2, 200, 1000 and 2000 min, after filling the bowl are shown in Fig. 4. Within the first 200 min, the water is mainly soaring within the bark until it reaches a maximum height at about 5 cm from the bottom. Then water uptake in the inner part of the trunk takes place, too, while the water content in the bark declines. Correspondingly the water level within the bowl decreases continuously.
Fig. 7. Sample consisting of 4 slices of birch trunks. The topmost trunk has a small pitch into which the water is dropping.
Fig. 6. Difference image of the radiographs for t ¼ 2000 min and t ¼750 min showing the water transfer from the bark into the inner part of the trunk and through its lower cross-section. White colored pixels indicate a loss of water at 2000 min compared to 750 min.
Fig. 8. Experimental set-up placed in an aluminum container.
178
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
corrected for the background. Then, the volume of water V within the trunk is quantified using the equation V ¼
1
mHs O
ln
Y
! pi wh
ð2Þ
i ¼ area
with m being the linear attenuation coefficient of water, pi the value of the i-th pixel and w and h the width and height of a pixel, respectively. The linear attenuation coefficient was determined experimentally by another experiment to be 0.86 70.06 cm 1. The resulting time dependence of the water uptake in the trunk is shown quantitatively in Fig. 5. As already identified qualitatively by the radiographs (Fig. 4), the water uptake within the investigated time interval can be described by two different mechanisms. For the first 750 min the soaring within the bark is the main mechanism, well described by an exponential rise. Subsequently the water uptake within the inner part of the trunk becomes dominant at least for the remaining 1250 min. Fig. 6 shows the difference of the radiographs measured at 2000 and 750 min. As the areas in the bark show negative values (white color), this clearly indicates that there is a water transfer from the bark into the inner part of the trunk in addition to the water uptake through its lower crosssection. These effects can be approximated by a linear relation. Of course, the water uptake must converge to a maximum for longer times as the feeding of water is finite. In another experiment the absorption of water in trunks was studied, when water is admitted from the top. The sample consists out of 4 slices of birch trunks of about 10 cm in diameter and different heights put on top of each other (Fig. 7). A dropping funnel with a flexible hose attached to the Teflon plug valve is used as the resource for the water. The dropping speed was
adjusted via a crimping unit to an experimentally determined speed of 0.25570.005 ml/min, thus being sufficient for 1960 min of water feeding. The topmost trunk had a small pit into which the water dropped. The complete experimental set-up was placed in a sealed aluminum container (Fig. 8). The measurements are performed with the same parameters as used in the previous experiment, i.e. the period is 62.2 s, and the same data processing. Some results are shown in Fig. 9. Within the first 50 min of water feeding there is an accumulation of the water on the upper cross-section of the topmost trunk.
Fig. 10. Experimental set-up for the triggered real-time measurement. The neutron beam is coming from the left.
Fig. 9. Time series of difference images of the water uptake in the trunks.
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
179
Fig. 11. Some results of the triggered real-time measurement. The time interval between two neighboring images is 10 s, each.
Then, water transfer through the bark downwards to the subjacent trunk takes place and results in the formation of a water layer on its upper cross-section after about 90 min. In parallel water infiltration into the inner region of the trunk from all sides takes place. About 110 min later, the same behavior of the water transfer takes place for this subjacent trunk and – at later times – for the other trunks. After the water feeding was finished (at about 1960 min), the diffusion into the inner parts of the trunks continued until the end of the experiment after 6000 min.
angularity, the dynamic process can be visualized. Some of the results are shown in Fig. 11. Evaluation of individual images results in an unsharpness of the images of about 31. This is much more than defined by the pure in-motion unsharpness that is calculated to about 0.211 and is caused by the beam divergence as a non-collimated beam is used for maximum fission neutron flux.
3. Conclusions 2.2. Triggered real-time measurements In a series of experiments the limits of triggered real-time measurements using the actually existing detection system [4] is investigated. For this purpose an iron disk (diameter 300 mm; thickness 4 mm) is attached to a stepping motor with a reduction gear in between. An inductive switch is activated by a small iron cylinder fixed on the disk, thus triggering the measurements. The disk has several holes of 2 mm diameter at different positions. In a first experiment the detectability of these holes for integration times of less than 20 s per image is investigated. Due to the low absorption of fission neutrons in iron, the contrast achieved was too low to detect these holes. Therefore some polyethylene (PE) objects of different sizes (up to 20 mm thickness) and shapes are glued on the disk (Fig. 10) showing sufficient contrast even for very short integration times. Based on these results and the given limits of the detection system (minimum shutter time 0.1 s), an integration time of 0.5 s and a speed of rotation of the disk of 0.4 turns per min are selected for the final experiment. In total, at 304 equally distributed angular positions for the continuously rotating disk 630 images are measured. The parameters selected enable the registration of 22 images per rotation at different angular positions, thus optimizing the overall measuring time. After filtering [3], all images corresponding to a specific angular position are summed up. Arranging these images according to the
The investigations have shown the suitability of the NECTAR facility for the quantitative investigation of slowly varying processes, especially of water in large sized wooden objects with a time resolution of about 1 min. Triggered real-time measurements with integration times down to 0.5 s are possible but show image unsharpness mainly caused by the beam divergence. The efficiency of these measurements will be improved in the near future using a pco.1600 CCD-camera with 4 GByte on-board memory. This will reduce the read out and data storage time by more than a factor of 6. As it is working without a mechanical shutter, time resolutions of 0.1 s and less will also become possible. References ¨ [1] T. Bucherl, F.M. Wagner, Ch. Lierse v. Gostomski, Nucl. Instr. and Meth. A 605 (2009) 47. [2] T.J. Buecherl, C. Lierse J. Guo, Radiography and tomography using fission neutrons, in: Proceedings of the IEEE Nuclear Science Symposium Conference Record on The NECTAR Facility, 2009, CN1-3, ISBN: 978-1-4244-3962-1. ¨ [3] K. Osterloh, T. Bucherl, Ch. Lierse von Gostomski, U. Zscherpel, U. Ewert, S. Bock, Filtering algorithm for dotted interferences, this proceedings. ¨ [4] T. Bucherl, Ch. Lierse v. Gostomski, H. Breitkreuz, M. Jungwirth, F.M. Wagner, NECTAR—a fission neutron radiography and tomography facility, this proceedings.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Real-time radiography at the NECTAR facility n ¨ T. Bucherl , Ch. Lierse von Gostomski
Lehrstuhl f¨ ur Radiochemie (RCM), Technische Universit¨ at M¨ unchen (TUM), Walther-Meissner-Str. 3, 85748 Garching, Germany
a r t i c l e i n f o
abstract
Available online 25 January 2011
A feasibility study has shown that real-time radiography using fission neutrons is possible at the ¨ NECTAR facility, when using an improved detection system for fast variations (Bucherl et al., 2009 [1]). Continuing this study, real-time measurements of slowly varying processes like the water uptake in medium sized trunks (diameter about 12 cm) and of slow periodic processes (e.g. a slowly rotating iron disk) are investigated successfully using the existing detection system. & 2011 Elsevier B.V. All rights reserved.
Keywords: Neutron Radiography Real-time NECTAR Fission
1. Introduction The non-destructive characterization of dynamic processes by means of fission neutron radiography is a challenging task as the available neutron fluxes and the efficiencies of the position sensitive detection (PSD) systems are much lower than those at facilities using X-rays, gamma-rays or thermal and cold neutrons. However, the unique properties of fission neutrons, i.e. their high sensitivity for hydrogen and less sensitivity for dense materials (e.g. shielding materials like lead) seem reasonable for the investigation of dynamic processes. For the investigation of dynamic processes real-time measurements are required. The time resolution for a specific PSD system is defined by the minimum time period between two succeeding measurements being made up by the shutter time ts, the integration time tm and the data transfer and/or storage time td. Thus, the minimum cycle time Dt is given by
Dt ¼ 2ts þtm þ td
ð1Þ
with ts and td being fixed values and only tm being user defined. Fig. 1 shows the typical time response for two succeeding measurements starting at time i and (i +1). The integration time ti must be adjusted such that the image quality is of significant statistics. On the other hand, the longer the integration time the more blurred the image will be due to in-motion unsharpness. If periodic processes have to be investigated, triggered realtime measurements may be applied to improve the statistics even for very short integration times by summing up several images measured for the same time interval. This is illustrated in Fig. 2, where i and (i +1) denote the moments of two succeeding trigger signals, i.e. the time interval of one period of the process. n
Corresponding author. Tel.: +49 89 289 14328; fax: + 49 89 289 14347. ¨ E-mail address: [email protected] (T. Bucherl).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.061
The trigger signal at point i starts one measurement sequence, i.e. after an offset (or delay) time to the data acquisition for one image is performed. This is repeated identically N times for succeeding trigger signals. Then, after some pre-processing like filtering, these N images are summed up and normalized. This procedure is repeated for increasing offset times (i.e. starting at point 0 in discrete time steps until t0 is equal to a complete period). The normalized images can then be merged to visualize
Fig. 1. Time response of two succeeding measurements starting at i and (i +1).
Fig. 2. Time response of a triggered real-time measurement.
176
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
the periodic process. To optimize the overall measuring time required, additional measurements within a period can be started at point i0 , if the time for a single measurement (Dt + to) is less than the time between two succeeding triggered measurements at point i and (i+ 1). The positions i0 must be selected to fit the positions of i identically. In 2008 first investigations took place demonstrating the principle suitability of the NECTAR facility for real-time radiography [1]. This was verified by the first imaging of a slowly
Fig. 3. Photography of the experimental set-up for the measurement of water uptake in a trunk. The trunk is loaded by an iron cylinder. In the background the converter plate of the detector system is visible.
varying process and was presented in 2009 [2]. Based on these results the investigations of other slowly varying and of periodic processes are continued.
Fig. 5. Time dependence of the water uptake in the trunk.
2 min
200 min
1000 min
2000 min
Fig. 4. Difference images of the water uptake in a trunk after about 2, 200, 1000 and 2000 min after filling water in the bowl (from upper left to lower right).
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
2. Experiments and results All measurements are performed at the NECTAR facility. A collimated fission neutron beam (f ¼5.4 105 cm 2 s 1; L/D ¼ 233 716) is used for the investigation of all slowly varying processes, while for periodic processes a non-collimated beam (f ¼1.2 107 cm 2 s 1; L/D ¼132712) is used. A detailed description of the facility and the parameters is given in Ref. [4].
177
For quantitative evaluation, i.e. the determination of the water content as a function of time, the pixel values pi within a predefined area in the difference images are determined and
2.1. Real-time measurements As examples of slowly varying processes, the absorption of water in trunks is investigated by placing them in a water quench and by applying drops of water from the top. In a first experiment a tree trunk (diameter about 12 cm) is placed in a plastic bowl. An iron cylinder on top of the trunk acts as additional mass to avoid floating when water is added to the bowl (Fig. 3). A reference image is registered before adding the water to the bowl. Then, without changing the alignment of the experimental set-up, the bowl is carefully filled with about 250 ml of water and a series of 1000 radiographs back-to-back is started immediately, thus covering a time period of about 52 h. The integration time is 60 s, the shutter time 0.1 s and the data storage time 2 s. Thus, the time between two succeeding images is 62.2 s. All images are filtered [3], dark image corrected, normalized to the flat field image and then subtracted by the adequately treated reference image. The resulting difference images, at about 2, 200, 1000 and 2000 min, after filling the bowl are shown in Fig. 4. Within the first 200 min, the water is mainly soaring within the bark until it reaches a maximum height at about 5 cm from the bottom. Then water uptake in the inner part of the trunk takes place, too, while the water content in the bark declines. Correspondingly the water level within the bowl decreases continuously.
Fig. 7. Sample consisting of 4 slices of birch trunks. The topmost trunk has a small pitch into which the water is dropping.
Fig. 6. Difference image of the radiographs for t ¼ 2000 min and t ¼750 min showing the water transfer from the bark into the inner part of the trunk and through its lower cross-section. White colored pixels indicate a loss of water at 2000 min compared to 750 min.
Fig. 8. Experimental set-up placed in an aluminum container.
178
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
corrected for the background. Then, the volume of water V within the trunk is quantified using the equation V ¼
1
mHs O
ln
Y
! pi wh
ð2Þ
i ¼ area
with m being the linear attenuation coefficient of water, pi the value of the i-th pixel and w and h the width and height of a pixel, respectively. The linear attenuation coefficient was determined experimentally by another experiment to be 0.86 70.06 cm 1. The resulting time dependence of the water uptake in the trunk is shown quantitatively in Fig. 5. As already identified qualitatively by the radiographs (Fig. 4), the water uptake within the investigated time interval can be described by two different mechanisms. For the first 750 min the soaring within the bark is the main mechanism, well described by an exponential rise. Subsequently the water uptake within the inner part of the trunk becomes dominant at least for the remaining 1250 min. Fig. 6 shows the difference of the radiographs measured at 2000 and 750 min. As the areas in the bark show negative values (white color), this clearly indicates that there is a water transfer from the bark into the inner part of the trunk in addition to the water uptake through its lower crosssection. These effects can be approximated by a linear relation. Of course, the water uptake must converge to a maximum for longer times as the feeding of water is finite. In another experiment the absorption of water in trunks was studied, when water is admitted from the top. The sample consists out of 4 slices of birch trunks of about 10 cm in diameter and different heights put on top of each other (Fig. 7). A dropping funnel with a flexible hose attached to the Teflon plug valve is used as the resource for the water. The dropping speed was
adjusted via a crimping unit to an experimentally determined speed of 0.25570.005 ml/min, thus being sufficient for 1960 min of water feeding. The topmost trunk had a small pit into which the water dropped. The complete experimental set-up was placed in a sealed aluminum container (Fig. 8). The measurements are performed with the same parameters as used in the previous experiment, i.e. the period is 62.2 s, and the same data processing. Some results are shown in Fig. 9. Within the first 50 min of water feeding there is an accumulation of the water on the upper cross-section of the topmost trunk.
Fig. 10. Experimental set-up for the triggered real-time measurement. The neutron beam is coming from the left.
Fig. 9. Time series of difference images of the water uptake in the trunks.
T. B¨ ucherl, Ch. Lierse von Gostomski / Nuclear Instruments and Methods in Physics Research A 651 (2011) 175–179
179
Fig. 11. Some results of the triggered real-time measurement. The time interval between two neighboring images is 10 s, each.
Then, water transfer through the bark downwards to the subjacent trunk takes place and results in the formation of a water layer on its upper cross-section after about 90 min. In parallel water infiltration into the inner region of the trunk from all sides takes place. About 110 min later, the same behavior of the water transfer takes place for this subjacent trunk and – at later times – for the other trunks. After the water feeding was finished (at about 1960 min), the diffusion into the inner parts of the trunks continued until the end of the experiment after 6000 min.
angularity, the dynamic process can be visualized. Some of the results are shown in Fig. 11. Evaluation of individual images results in an unsharpness of the images of about 31. This is much more than defined by the pure in-motion unsharpness that is calculated to about 0.211 and is caused by the beam divergence as a non-collimated beam is used for maximum fission neutron flux.
3. Conclusions 2.2. Triggered real-time measurements In a series of experiments the limits of triggered real-time measurements using the actually existing detection system [4] is investigated. For this purpose an iron disk (diameter 300 mm; thickness 4 mm) is attached to a stepping motor with a reduction gear in between. An inductive switch is activated by a small iron cylinder fixed on the disk, thus triggering the measurements. The disk has several holes of 2 mm diameter at different positions. In a first experiment the detectability of these holes for integration times of less than 20 s per image is investigated. Due to the low absorption of fission neutrons in iron, the contrast achieved was too low to detect these holes. Therefore some polyethylene (PE) objects of different sizes (up to 20 mm thickness) and shapes are glued on the disk (Fig. 10) showing sufficient contrast even for very short integration times. Based on these results and the given limits of the detection system (minimum shutter time 0.1 s), an integration time of 0.5 s and a speed of rotation of the disk of 0.4 turns per min are selected for the final experiment. In total, at 304 equally distributed angular positions for the continuously rotating disk 630 images are measured. The parameters selected enable the registration of 22 images per rotation at different angular positions, thus optimizing the overall measuring time. After filtering [3], all images corresponding to a specific angular position are summed up. Arranging these images according to the
The investigations have shown the suitability of the NECTAR facility for the quantitative investigation of slowly varying processes, especially of water in large sized wooden objects with a time resolution of about 1 min. Triggered real-time measurements with integration times down to 0.5 s are possible but show image unsharpness mainly caused by the beam divergence. The efficiency of these measurements will be improved in the near future using a pco.1600 CCD-camera with 4 GByte on-board memory. This will reduce the read out and data storage time by more than a factor of 6. As it is working without a mechanical shutter, time resolutions of 0.1 s and less will also become possible. References ¨ [1] T. Bucherl, F.M. Wagner, Ch. Lierse v. Gostomski, Nucl. Instr. and Meth. A 605 (2009) 47. [2] T.J. Buecherl, C. Lierse J. Guo, Radiography and tomography using fission neutrons, in: Proceedings of the IEEE Nuclear Science Symposium Conference Record on The NECTAR Facility, 2009, CN1-3, ISBN: 978-1-4244-3962-1. ¨ [3] K. Osterloh, T. Bucherl, Ch. Lierse von Gostomski, U. Zscherpel, U. Ewert, S. Bock, Filtering algorithm for dotted interferences, this proceedings. ¨ [4] T. Bucherl, Ch. Lierse v. Gostomski, H. Breitkreuz, M. Jungwirth, F.M. Wagner, NECTAR—a fission neutron radiography and tomography facility, this proceedings.