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First steps towards real-time radiography at the NECTAR facility
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 605 (2009) 47–49
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First steps towards real-time radiography at the NECTAR facility T. Bu¨cherl a,, F.M. Wagner b, Ch. Lierse v. Gostomski a a b
¨t Mu ¨ r Radiochemie (RCM), Technische Universita ¨ nchen (TUM), Germany Lehrstuhl fu ¨t Mu ¨ nchen, Germany Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universita
a r t i c l e in fo
abstract
Available online 4 February 2009
The beam tube SR10 at Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) provides an intense beam of fission neutrons for medical application (MEDAPP) and for radiography and tomography of technical and other objects (NECTAR). The high neutron flux of up to 9.8E+07 cm2 s1 (depending on filters and collimation) with a mean energy of about 1.9 MeV at the sample position at the NECTAR facility prompted an experimental feasibility study to investigate the potential for real-time (RT) radiography. & 2009 Elsevier B.V. All rights reserved.
Keywords: Fission neutron Radiography Real time
1. Introduction Real-time (RT) radiography is one of the high end applications in non-destructive inspection. It opens up deep insight in the functionality of timely varying systems, like in biology, engineering, etc. A test measurement at the radiography and tomography system NECTAR [1] at the beam tube SR10 at the Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) using the uncollimated neutron beam was performed using the sample shown in Fig. 1. The original purpose of this measurement was to answer a specific question in medical application. The resulting radiography (Fig. 2 left), that was achieved in a fraction of measurement time usually required when using the collimated neutron beam (Fig. 2 right) immediately raises the question of the possibility of RT radiography at the NECTAR facility.
2. Basics All investigations were performed at the NECTAR [1] facility without any special modifications. The main components considered in these investigations were the ‘‘filterbench’’ and the CCD-based detector system. The components installed on the ‘‘filterbench’’ (Fig. 3) allow manipulation of the neutron beam geometry or of the neutron spectrum. For the first a collimator usually used for radiography measurements is available, for the latter some lead and polyethylene slabs of different thicknesses. For the investigations reported in this paper, only the collimator and lead filters were used. respectively. The corresponding neutron fluxes for eight different experimental set ups are shown in Table 1. Corresponding author. Tel.: +49 8928914328; fax: +49 8928914347.
E-mail address: [email protected] (T. Bu¨cherl). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.126
The detector system is based on a peltier cooled CCD camera (ANDOR DV434-BV, 1024 pixel 1024 pixel, working temperature 50 1C). The main limitations for real-time application using this detector system are the speed of the shutter of the camera, which was declared by the supplier to be about 50 ms, and the read out time of the CCD (about 1 s minimum). This leads to the conclusion that using this detector system no RT radiography is possible, but the basic ideas can be investigated anyhow. pp-Converters of 2.0 mm (number 1) and 2.4 mm (number 2) thicknesses (characteristic parameters are described in [2]), respectively, convert the fast neutrons to visible light for detection. The data of Table 1 imply that omitting the collimator, that is used for routine radiographic and tomographic investigations, will result in an increase of the available fission neutron flux at the measurement position by a factor of about 9, thus giving a total neutron flux of about 9.8E+07 cm2 s1. This is associated with an increase of the gamma flux as a lead absorbing layer of about 100 mm in thickness contained in the collimator is omitted. This means that an excellent neutron to gamma discrimination (of the converter) is of great importance (see [2]), too.
3. Measurements In a first series, open beam images for varying integration times, ranging from 0.01 to 1 s were measured for all configurations listed in Table 1. The corresponding dark image (i.e. camera shutter closed) was measured, too. For neutron conversion, the pp-converter number 2 was used as it showed to have the highest detection efficiency of all large area converters actually available at the NECTAR facility [2]. For the determination of the L/D value, the method described by Yoshii et al. [3] was used for the uncollimated beam arrangement without any lead filter. An iron cylinder of 5 cm
ARTICLE IN PRESS 48
¨ cherl et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 47–49 T. Bu
Table 1 Available neutron fluxes for the investigations. ]
Filtering
Neutron flux at measurement position (cm2 s1)
1
Collimated Uncollimated with lead filter 0 mm 25 mm 50 mm 75 mm 100 mm 125 mm 150 mm
1.10E+07
2 3 4 5 6 7 8
Fig. 1. Photography of the test sample consisting of a plastic can containing a thighbone and a polyethylene rod, both partly covered by distilled water in the lower part.
mean counts/pixel
1200
9.8E+07 5.2E+07 3.1E+07 1.9E+07 8.4E+06 7.1E+06 4.5E+06
0 mm Pb 25 mm Pb 50 mm Pb 75 mm Pb 100 mm Pb 125 mm Pb 150 mm Pb
1000 800 600 400 200 0 0
2x10
7
4x107 6x107 8x107 1x108
fluence [cm-2s-1] Fig. 4. Results of the open beam measurements. The mean counts per pixel for different thicknesses of the lead filters are plotted versus the neutron fluence.
Fig. 2. Radiographies of a test sample similar to that shown in Fig. 1. Left: uncollimated neutron beam, 0 mm lead. Right: collimated neutron beam with 100 mm lead.
These measurements were performed using the pp-converter number 2. In the last measurement series, an iron slab of 5 cm 5 cm edge length was placed at the centre of the turntable. The detector (with pp-converter number 1) was at a distance being typically for routine measurements. One side of the slab was oriented perpendicular to the detector screen and parallel to the incoming neutron beam. Then radiographs with 10 s integration time for all configurations were measured. To ensure that the block was correctly oriented, each measurement was repeated by changing the angular position of the block from 11 to +11 in steps of 0.11. Additionally, the corresponding open beam images (i.e. same arrangement but without the iron slab) and the dark images were measured, too.
4. Evaluation and discussion
Fig. 3. Photography of the ‘‘filterbench’’ with the Pb and PE filters and the table for inserting the collimator into the neutron beam, respectively.
diameter was placed at different distances from the detector plane and the images as well as the corresponding open beam images and the dark images were measured for 10 s each.
For all dark image corrected open beam images the same area of 100 pixels 100 pixels was selected in the centre of the images and the mean values and its standard deviations were calculated. Fig. 4 shows the resulting mean count rates per pixel as a function of the neutron fluence (which is the neutron flux multiplied by the measurement time) for all uncollimated cases investigated (see Table 1). Fig. 5 shows the result when no lead filter was in the neutron beam. A linear relation between the mean count rate per pixel and the neutron fluence was measured for measurement times larger than 100 ms. For shorter times, the speed of the mechanical shutter was the limiting factor. Nevertheless, the linear relation affords to estimate a minimum measurement time for which the signal (i.e. the number of counts per pixel) is still sufficiently large to be significant, neglecting the shutter and the read out time of the CCD. Inserting the neutron flux in the linear relation (Fig. 5) results in c ¼ 1:8 þ 1135t
(1)
ARTICLE IN PRESS ¨ cherl et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 47–49 T. Bu
y = -1.8309 +1.1581e-05x R=0.99955
limit defined by shutter
1000 800 600 400 200
Filtering
L/D
Collimated Uncollimated, no lead filter
233716 132712
55000
0 0
2x10
7
4x107 6x107 8x107 1x108
fluence [cm-2s-1] Fig. 5. Results of the open beam measurements without lead filter (see Fig. 4).
1000 mean counts/pixel
Table 2 L/D values for the collimated and uncollimated arrangements.
50000 45000 40000 35000
y = 853.35 * e(-0.020936x) R = 0.99881
0
20
40
60 80 100 120 140 pixels
Fig. 7. Measured edge spread profile (circles) and best fit of the ESF according to [4].
100
10 20
intensity [arb. units]
mean counts/pixel
1200
49
40
60 80 100 120 140 160 thickness Pb filter [mm]
Fig. 6. Dependence of the mean count rate per pixel from the total thickness of the lead filters.
with c as counts registered per pixel in time t. Accepting a statistical uncertainty of 10%, the time t is about 80 ms, for 25% it is about 14 ms and for 30% it is about 10 ms, respectively. If the detection efficiency can be improved (actually, it is less than 1%), this will decrease these minimum times directly proportionally! As the detection system (i.e. the converter) used is slightly sensitive to gamma rays it was tested whether (dense) objects in the beam change the detection efficiency drastically by preferring either the neutron or the gamma signal. By plotting the mean counts per pixel versus the total thicknesses of lead filters for measurement times of 60 s each (Fig. 6), in a first approximation the decrease in the measured combined neutron and gamma signal can be expressed by a single exponential showing the negligible influence in change of the detection efficiency ratio for neutron to gamma rays and/or the low sensitivity for gamma rays at all. Showing that the available neutron flux at the NECTAR facility makes RT radiography possible, the next questions was related to the spatial resolution. For the evaluation of the measurement of the L/D value, the measured images were corrected for the dark current of the CCD camera and then normalized with the likewise dark current corrected open beam images. From these data, horizontal line profiles through the image of the iron cylinder at its maximumwidth were extracted and the required parameters were derived. The results (Table 2) show the expected decrease in the L/D value compared to that of the collimated arrangement [1]. The degradation of the L/D value directly affects the spatial resolution. This was derived from the evaluation of the measurement data for the iron slab. After normalization, edge spread functions (ESF) were extracted for all data (Fig. 7). For the
determination of the modulation transfer function (MTF) value at 10%, a function proposed by Harms et al.[4] was used to fit the ESF (see also [2]). Apparently, this function does not fit completely the edges, i.e. the underlying assumptions in deriving this function might not be completely correct for fission neutrons and have to be improved in the future. The derivation of the ESF, the line spread function (LSF), was fitted to the analytical derivative of the function by Harms. The MTF value at 10% then resulted in about 0.3 linepairs/mm compared to about 0.8 linepairs/mm for the arrangement using the collimator. Qualitatively this difference in the MTF-values was expected when comparing Fig. 2 left and right.
5. Conclusion The investigations showed that RT radiography is possible at the NECTAR facility, but on the expense of reduced spatial resolution compared to the radiography measurements performed up to now. Nevertheless the spatial resolution seems to be sufficient for a large number of applications where other methods fail due to the opaqueness of the objects (e.g. high density of the cladding, etc.). One basic requirement for a future application of RT radiography at the NECTAR facility is the improvement of the detection system regarding efficiency, read out time of the CCD and minimization of shutter time. The development of a completely new detection system seems to be unavoidable. All existing components have to be charged on their suitability and might be replaced.
References [1] T. Bu¨cherl, Ch. Lierse v. Gostomski, in: Proceedings of Science, PoS(FNDA2006)033, 2006 /http://pos.sissa.itS. [2] J. Guo, et al., Nucl. Instr. and Meth. A (2009), these proceedings. [3] K. Yoshii, H. Kobayashi, Nucl. Instr. and Meth. A 377 (1996) 68. [4] A. Harms, et al., Mathematics and Physics of Neutron Radiography, D. Reidel Publishing Company, 1986.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First steps towards real-time radiography at the NECTAR facility T. Bu¨cherl a,, F.M. Wagner b, Ch. Lierse v. Gostomski a a b
¨t Mu ¨ r Radiochemie (RCM), Technische Universita ¨ nchen (TUM), Germany Lehrstuhl fu ¨t Mu ¨ nchen, Germany Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universita
a r t i c l e in fo
abstract
Available online 4 February 2009
The beam tube SR10 at Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) provides an intense beam of fission neutrons for medical application (MEDAPP) and for radiography and tomography of technical and other objects (NECTAR). The high neutron flux of up to 9.8E+07 cm2 s1 (depending on filters and collimation) with a mean energy of about 1.9 MeV at the sample position at the NECTAR facility prompted an experimental feasibility study to investigate the potential for real-time (RT) radiography. & 2009 Elsevier B.V. All rights reserved.
Keywords: Fission neutron Radiography Real time
1. Introduction Real-time (RT) radiography is one of the high end applications in non-destructive inspection. It opens up deep insight in the functionality of timely varying systems, like in biology, engineering, etc. A test measurement at the radiography and tomography system NECTAR [1] at the beam tube SR10 at the Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) using the uncollimated neutron beam was performed using the sample shown in Fig. 1. The original purpose of this measurement was to answer a specific question in medical application. The resulting radiography (Fig. 2 left), that was achieved in a fraction of measurement time usually required when using the collimated neutron beam (Fig. 2 right) immediately raises the question of the possibility of RT radiography at the NECTAR facility.
2. Basics All investigations were performed at the NECTAR [1] facility without any special modifications. The main components considered in these investigations were the ‘‘filterbench’’ and the CCD-based detector system. The components installed on the ‘‘filterbench’’ (Fig. 3) allow manipulation of the neutron beam geometry or of the neutron spectrum. For the first a collimator usually used for radiography measurements is available, for the latter some lead and polyethylene slabs of different thicknesses. For the investigations reported in this paper, only the collimator and lead filters were used. respectively. The corresponding neutron fluxes for eight different experimental set ups are shown in Table 1. Corresponding author. Tel.: +49 8928914328; fax: +49 8928914347.
E-mail address: [email protected] (T. Bu¨cherl). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.126
The detector system is based on a peltier cooled CCD camera (ANDOR DV434-BV, 1024 pixel 1024 pixel, working temperature 50 1C). The main limitations for real-time application using this detector system are the speed of the shutter of the camera, which was declared by the supplier to be about 50 ms, and the read out time of the CCD (about 1 s minimum). This leads to the conclusion that using this detector system no RT radiography is possible, but the basic ideas can be investigated anyhow. pp-Converters of 2.0 mm (number 1) and 2.4 mm (number 2) thicknesses (characteristic parameters are described in [2]), respectively, convert the fast neutrons to visible light for detection. The data of Table 1 imply that omitting the collimator, that is used for routine radiographic and tomographic investigations, will result in an increase of the available fission neutron flux at the measurement position by a factor of about 9, thus giving a total neutron flux of about 9.8E+07 cm2 s1. This is associated with an increase of the gamma flux as a lead absorbing layer of about 100 mm in thickness contained in the collimator is omitted. This means that an excellent neutron to gamma discrimination (of the converter) is of great importance (see [2]), too.
3. Measurements In a first series, open beam images for varying integration times, ranging from 0.01 to 1 s were measured for all configurations listed in Table 1. The corresponding dark image (i.e. camera shutter closed) was measured, too. For neutron conversion, the pp-converter number 2 was used as it showed to have the highest detection efficiency of all large area converters actually available at the NECTAR facility [2]. For the determination of the L/D value, the method described by Yoshii et al. [3] was used for the uncollimated beam arrangement without any lead filter. An iron cylinder of 5 cm
ARTICLE IN PRESS 48
¨ cherl et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 47–49 T. Bu
Table 1 Available neutron fluxes for the investigations. ]
Filtering
Neutron flux at measurement position (cm2 s1)
1
Collimated Uncollimated with lead filter 0 mm 25 mm 50 mm 75 mm 100 mm 125 mm 150 mm
1.10E+07
2 3 4 5 6 7 8
Fig. 1. Photography of the test sample consisting of a plastic can containing a thighbone and a polyethylene rod, both partly covered by distilled water in the lower part.
mean counts/pixel
1200
9.8E+07 5.2E+07 3.1E+07 1.9E+07 8.4E+06 7.1E+06 4.5E+06
0 mm Pb 25 mm Pb 50 mm Pb 75 mm Pb 100 mm Pb 125 mm Pb 150 mm Pb
1000 800 600 400 200 0 0
2x10
7
4x107 6x107 8x107 1x108
fluence [cm-2s-1] Fig. 4. Results of the open beam measurements. The mean counts per pixel for different thicknesses of the lead filters are plotted versus the neutron fluence.
Fig. 2. Radiographies of a test sample similar to that shown in Fig. 1. Left: uncollimated neutron beam, 0 mm lead. Right: collimated neutron beam with 100 mm lead.
These measurements were performed using the pp-converter number 2. In the last measurement series, an iron slab of 5 cm 5 cm edge length was placed at the centre of the turntable. The detector (with pp-converter number 1) was at a distance being typically for routine measurements. One side of the slab was oriented perpendicular to the detector screen and parallel to the incoming neutron beam. Then radiographs with 10 s integration time for all configurations were measured. To ensure that the block was correctly oriented, each measurement was repeated by changing the angular position of the block from 11 to +11 in steps of 0.11. Additionally, the corresponding open beam images (i.e. same arrangement but without the iron slab) and the dark images were measured, too.
4. Evaluation and discussion
Fig. 3. Photography of the ‘‘filterbench’’ with the Pb and PE filters and the table for inserting the collimator into the neutron beam, respectively.
diameter was placed at different distances from the detector plane and the images as well as the corresponding open beam images and the dark images were measured for 10 s each.
For all dark image corrected open beam images the same area of 100 pixels 100 pixels was selected in the centre of the images and the mean values and its standard deviations were calculated. Fig. 4 shows the resulting mean count rates per pixel as a function of the neutron fluence (which is the neutron flux multiplied by the measurement time) for all uncollimated cases investigated (see Table 1). Fig. 5 shows the result when no lead filter was in the neutron beam. A linear relation between the mean count rate per pixel and the neutron fluence was measured for measurement times larger than 100 ms. For shorter times, the speed of the mechanical shutter was the limiting factor. Nevertheless, the linear relation affords to estimate a minimum measurement time for which the signal (i.e. the number of counts per pixel) is still sufficiently large to be significant, neglecting the shutter and the read out time of the CCD. Inserting the neutron flux in the linear relation (Fig. 5) results in c ¼ 1:8 þ 1135t
(1)
ARTICLE IN PRESS ¨ cherl et al. / Nuclear Instruments and Methods in Physics Research A 605 (2009) 47–49 T. Bu
y = -1.8309 +1.1581e-05x R=0.99955
limit defined by shutter
1000 800 600 400 200
Filtering
L/D
Collimated Uncollimated, no lead filter
233716 132712
55000
0 0
2x10
7
4x107 6x107 8x107 1x108
fluence [cm-2s-1] Fig. 5. Results of the open beam measurements without lead filter (see Fig. 4).
1000 mean counts/pixel
Table 2 L/D values for the collimated and uncollimated arrangements.
50000 45000 40000 35000
y = 853.35 * e(-0.020936x) R = 0.99881
0
20
40
60 80 100 120 140 pixels
Fig. 7. Measured edge spread profile (circles) and best fit of the ESF according to [4].
100
10 20
intensity [arb. units]
mean counts/pixel
1200
49
40
60 80 100 120 140 160 thickness Pb filter [mm]
Fig. 6. Dependence of the mean count rate per pixel from the total thickness of the lead filters.
with c as counts registered per pixel in time t. Accepting a statistical uncertainty of 10%, the time t is about 80 ms, for 25% it is about 14 ms and for 30% it is about 10 ms, respectively. If the detection efficiency can be improved (actually, it is less than 1%), this will decrease these minimum times directly proportionally! As the detection system (i.e. the converter) used is slightly sensitive to gamma rays it was tested whether (dense) objects in the beam change the detection efficiency drastically by preferring either the neutron or the gamma signal. By plotting the mean counts per pixel versus the total thicknesses of lead filters for measurement times of 60 s each (Fig. 6), in a first approximation the decrease in the measured combined neutron and gamma signal can be expressed by a single exponential showing the negligible influence in change of the detection efficiency ratio for neutron to gamma rays and/or the low sensitivity for gamma rays at all. Showing that the available neutron flux at the NECTAR facility makes RT radiography possible, the next questions was related to the spatial resolution. For the evaluation of the measurement of the L/D value, the measured images were corrected for the dark current of the CCD camera and then normalized with the likewise dark current corrected open beam images. From these data, horizontal line profiles through the image of the iron cylinder at its maximumwidth were extracted and the required parameters were derived. The results (Table 2) show the expected decrease in the L/D value compared to that of the collimated arrangement [1]. The degradation of the L/D value directly affects the spatial resolution. This was derived from the evaluation of the measurement data for the iron slab. After normalization, edge spread functions (ESF) were extracted for all data (Fig. 7). For the
determination of the modulation transfer function (MTF) value at 10%, a function proposed by Harms et al.[4] was used to fit the ESF (see also [2]). Apparently, this function does not fit completely the edges, i.e. the underlying assumptions in deriving this function might not be completely correct for fission neutrons and have to be improved in the future. The derivation of the ESF, the line spread function (LSF), was fitted to the analytical derivative of the function by Harms. The MTF value at 10% then resulted in about 0.3 linepairs/mm compared to about 0.8 linepairs/mm for the arrangement using the collimator. Qualitatively this difference in the MTF-values was expected when comparing Fig. 2 left and right.
5. Conclusion The investigations showed that RT radiography is possible at the NECTAR facility, but on the expense of reduced spatial resolution compared to the radiography measurements performed up to now. Nevertheless the spatial resolution seems to be sufficient for a large number of applications where other methods fail due to the opaqueness of the objects (e.g. high density of the cladding, etc.). One basic requirement for a future application of RT radiography at the NECTAR facility is the improvement of the detection system regarding efficiency, read out time of the CCD and minimization of shutter time. The development of a completely new detection system seems to be unavoidable. All existing components have to be charged on their suitability and might be replaced.
References [1] T. Bu¨cherl, Ch. Lierse v. Gostomski, in: Proceedings of Science, PoS(FNDA2006)033, 2006 /http://pos.sissa.itS. [2] J. Guo, et al., Nucl. Instr. and Meth. A (2009), these proceedings. [3] K. Yoshii, H. Kobayashi, Nucl. Instr. and Meth. A 377 (1996) 68. [4] A. Harms, et al., Mathematics and Physics of Neutron Radiography, D. Reidel Publishing Company, 1986.