- Email: [email protected]
Neutron gamma fraction imaging: Detection, location and identification of neutron sources
Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron gamma fraction imaging: Detection, location and identification of neutron sources K.A.A. Gamage a,n, G.C. Taylor b a b
Department of Engineering, University of Lancaster , Lancaster LA1 4YR, UK National Physical Laboratory, Hampton Road,Teddington, Middlesex TW11 0LW, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 23 March 2015 Accepted 25 March 2015 Available online 2 April 2015
In this paper imaging of neutron sources and identification and separation of a neutron source from another neutron source is described. The system is based upon organic liquid scintillator detector, tungsten collimator, bespoke fast digitiser and adjustable equatorial mount. Three environments have been investigated with this setup corresponding to an AmBe neutron source, a 252Cf neutron source and both sources together separated in space. In each case, events are detected, digitised, discriminated and radiation images plotted corresponding to the area investigated. The visualised neutron count distributions clearly locate the neutron source and, relative gamma to neutron (or neutron to gamma) fraction images aid in discriminating AmBe sources from 252Cf source. The measurements were performed in the low scatter facility of the National Physical Laboratory, Teddington, UK. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.
Keywords: Neutron gamma fraction imaging Mixed-radiation field Organic liquid scintillator Bespoke fast digitiser
1. Introduction Fast and accurate detection, location and identification of radioactive sources (particularly neutron sources) are important tasks in nuclear decommissioning, international safeguards application and security applications. For example, the threat of the illicit transport of radioactive materials across international borders by terrorists demands that precise radiation screening techniques and instrumentation are in place [1]. Similarly, in nuclear decommissioning applications, it is important to characterise contamination of structures, site and contaminated land in a fast, accurate and efficient way in order to accelerate the clean-up programme, while reducing the net cost of decommissioning. The presence of neutrons and gamma rays is detected and characterised by various mechanisms. Radiation portal monitors and gamma radiation imaging systems are currently deployed to detect radioactive material where safety and security is vital. These systems are generally passive (not designed to generate or emit radiation with which to stimulate a response) and, often use plastic scintillators for gamma-ray detection and 3He-filled gas proportional counters for neutron detection. In nuclear decommissioning applications, detection and location of radioactive materials is typically based on gamma-ray detection. Such systems are primarily equipped with NaI(Tl) or CsI(Tl) scintillation detectors, where CARTOGAM, RadScan, RadCam, and the Gamma Visor n
Corresponding author. Tel.: þ 44 1524593873; fax.: þ44 1524381707. E-mail address: [email protected] (K.A.A. Gamage).
http://dx.doi.org/10.1016/j.nima.2015.03.072 0168-9002/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.
are a few example gamma-ray imaging systems currently used in decommissioning applications. Recent advances in pulse-shape discrimination (PSD) methods and digital pulse-processing capabilities enable the use of organic liquid scintillation detectors in real-time radiation imaging applications [3,4]. Organic liquid scintillation detectors are sensitive to both neutrons and gamma rays. Pulse-shape analysis determines whether the event was caused by either a neutron or a gamma-ray event, based on the decay characteristics of the pulse. This provides an added benefit, particularly in the detection of neutron sources, where it permits both gamma rays and neutrons to be used to detect and locate the source. In this paper, detection and location of neutron sources, and identification of one neutron source from another are described. The technique is based on a recently developed organic liquid scintillate detector based mixed-field imaging system, where neutron gamma fraction images are generated to identify the neutron source. The remainder of the paper includes the details of the experimental setup at the National Physical Laboratory (Section 2); details and the results of pulse shape discrimination and neutron gamma fraction images (Section 3); discussion of results and conclusions (Section 4).
2. Experimental method In this work a system shown in Fig. 1 was used to collect data under three mixed-field radiation environments at the low scatter
10
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Fig. 2. Schematic of the experimental setup.
on the fly, in order to provide maximum flexibility in postprocessing analysis.
Fig. 1. The mixed-field imaging system used in this research [2].
facility of the National Physical Laboratory (Teddington, UK). The system comprised an organic liquid scintillator detector (4 ml cylindrical scintillator cell), a heavy tungsten collimator (outer diameter of 57 mm, 10 mm thickness and 250 mm length), adjustable equatorial mount and a bespoke fast digitiser (500 MHz, 12 bit resolution) [2]. The detector was placed inside the tungsten collimator which was then mounted on the equatorial mount. The detector output was connected to the digitiser and samples from the digitiser were streamed to a personal computer through an Ethernet connection. The mount and digitiser were synchronised and controlled through MATLAB. The sources (canister containing a small amount of radioactive material) used in this research were set up as shown in Fig. 2. The distance between the source plane and the detector front surface was 191 cm, where sources were approximately 45 cm apart from each other on the wall and 35 cm above the horizontal central axis of the collimator (where the height of the horizontal central axis of the collimator was 112 cm). Events were acquired under three environments: AmBe source only (NPL reference: 1000/1095; canister type X3; neutron emission rate of 2:149 106 s 1 ); 252 Cf source only (NPL reference: 4774; canister type X35; neutron emission rate of 2:24 107 s 1 ); both AmBe source and 252Cf source. In each case, the detector (controlled by the equatorial mount) collected data at a sequence of positions in azimuth and elevation constituting a full scan of the area on which the sources were set up i.e. data were recorded for a total of 35 positions (5 in elevation and 7 in azimuth) for each of the 3 source configurations. At each position data were taken for 1 min time period. The maximum count rate of the system was approximately around 50 kilo-pulses per second. The digitiser is designed to perform sampling at 500 MHz (i.e. each sample value taken at 2 ns intervals) with an amplitude resolution of up to 12 bits. The Field-Programmable Gate Array (FPGA) in the digitiser was set up to send raw digital pulse shapes, as opposed to discriminating
3. Results Organic liquid scintillator based detectors are sensitive to both neutrons and gamma rays. Experimentally collected digital samples consequently sent through a pulse shape discrimination algorithm (pulse gradient analysis was used in this research) in order to discriminate neutron events from gamma-ray events. Pulse gradient analysis is based on a comparison of the peak amplitude and the amplitude of a sample occurring at a defined time interval after the peak amplitude, generally known as the discrimination amplitude. Scatter plots (i.e. plots of discrimination amplitude versus peak amplitude) were obtained for the position giving the maximum number of counts for each of the 3 configurations to provide an indication of the mixed nature of each field investigated. Pulse shape discrimination scatter plots shown in Figs. 3(a), 4(a) and 5 are corresponding to AmBe source, 252Cf source and both sources respectively (scales of arbitrary units are common in each case). In each scatter plot the upper plume corresponds to neutron events and the lower to gamma-ray events. The discrimination line established on the basis of the previous experiments conducted for pure gamma and combined gamma and neutron sources [3]. The spatial distribution of neutron counts (after the pulse shape discrimination), for the case of the AmBe source and for the case of 252 Cf source, are shown in Fig. 3(a) and in Fig. 4(b) respectively. Fig. 6(a) represents the relative gamma to neutron fraction image (i.e. spatial distribution of gamma to neutron count ratio) and Fig. 6(b) represents the relative neutron to gamma fraction image (i.e. spatial distribution of neutron to gamma count ratio) for the case of both AmBe and 252Cf sources. In each case the count distribution transformed to the Cartesian xy-plane, relative to the co-ordinates of the detector mount system, giving a spatial dependence of the field in the actual plane on which the source is located. Also, the scale on the colour map corresponding to the intensity of the parameter considered in each case (for example number of
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Discrimination amplitude (a.u.)
70
11
data discrimination line
60 50 40
n
30 20
γ
10 0
0
50
100
150
200
250
300
Peak amplitude (a.u.) Fig. 3. For AmBe source, (a) pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) and (b) the spatial distribution of neutron counts. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Discrimination amplitude (a.u.)
50
data discrimination line
40 30
n
20 10 0
γ 0
50
100 150 200 Peak amplitude (a.u.)
250
300
Fig. 4. For 252Cf source, (a) pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) and (b) the spatial distribution of neutron counts. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Discrimination amplitude (a.u.)
60
data discrimination line
50 40 30
n
20 10 0
γ 0
50
100 150 200 Peak amplitude (a.u.)
250
300
Fig. 5. Pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) for both the 252Cf source and the AmBe source.
neutron counts as the parameter for the cases shown in Figs. 3(b) and 4(b)) as a function of position, where the colour map follows visible spectrum order with red corresponding to maximum and blue to minimum intensity.
4. Discussion and conclusion The scatter plot for the AmBe source (shown in Fig. 3(a)) indicates a good separation of neutron events from gamma-ray events. The concentration of number of events gradually decreases with the peak amplitude, where neutron–gamma separation is much better with
the increasing peak amplitude. Similar variations in pulse shape discrimination can also be observed in the case of 252Cf (Fig. 4(a)) as well as in the case of the both sources (Fig. 5). Also, it is important to highlight the density of gamma-ray events in the lower quadrants of the scatter plots. Gamma to neutron count ratio is higher in 252Cf compared to AmBe [4], as can be seen in the pulse shape discrimination scatter plots of AmBe and 252Cf. The visualised neutron count distributions shown in Figs. 3(b) and 4(b) clearly locate the neutron sources (AmBe source and 252Cf respectively) in the experimental environment investigated. These images demonstrate the capability of the system to detect and locate neutron sources, where by using gamma to neutron (or neutron to gamma) count ratio this method can be further advanced to identify the neutron source. For example, Fig. 6(a) shows the spatial distribution of gamma to neutron count ratio and Fig. 6(b) shows the spatial distribution of neutron to gamma count ratio. As gamma to neutron count ratio is higher in 252Cf compared to AmBe, the highest intensity point of Fig. 6(a) corresponds to the location of the 252Cf source and similarly, highest intensity point of Fig. 6(b) corresponds to the location of the AmBe source as the neutron to gamma ratio is higher in AmBe. A method to detect, locate and identify neutron source with a single detector has been presented. It is expected to increase the speed of the operation significantly with an array of detectors. For example, using the system described in this research (as highlighted before, it contained a single organic liquid scintillator detector), scanning process was taken approximately 40 min for each case. The processing time of the system can be considerably reduced by optimising the number of detectors in an array with
12
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Fig. 6. The spatial distribution for both the fraction image.
252
Cf source and the AmBe source, (a) relative gamma to neutron fraction image and (b) relative neutron to gamma
the help of an artificial intelligence technique. The method has significant potential for the assay of mixed field environments in the nuclear industry, security applications and others. Acknowledgements The authors would like to acknowledge the financial support of Engineering Department and Faculty of Science and Technology, Lancaster University, UK (FST-RG-11/12-6). We also would like to acknowledge Dr. David Thomas and Dr. Nigel Hawkes for providing access to, and assistance with, the neutron metrology facility at the National Physical Laboratory, Teddington, UK.
References [1] R.T. Kouzes, E.R. Siciliano, J.H. Ely, P.E. Keller, R.J. McConn, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 584 (2008) 383. [2] K.A.A. Gamage, M.J. Joyce, G.C. Taylor, Applied Radiation and Isotopes 70 (2012) 1223. [3] K.A.A. Gamage, M.J. Joyce, J.C. Adams, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 635 (2011) 74. [4] K.A.A. Gamage, M.J. Joyce, N.P. Hawkes, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 642 (2011) 78.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron gamma fraction imaging: Detection, location and identification of neutron sources K.A.A. Gamage a,n, G.C. Taylor b a b
Department of Engineering, University of Lancaster , Lancaster LA1 4YR, UK National Physical Laboratory, Hampton Road,Teddington, Middlesex TW11 0LW, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 23 March 2015 Accepted 25 March 2015 Available online 2 April 2015
In this paper imaging of neutron sources and identification and separation of a neutron source from another neutron source is described. The system is based upon organic liquid scintillator detector, tungsten collimator, bespoke fast digitiser and adjustable equatorial mount. Three environments have been investigated with this setup corresponding to an AmBe neutron source, a 252Cf neutron source and both sources together separated in space. In each case, events are detected, digitised, discriminated and radiation images plotted corresponding to the area investigated. The visualised neutron count distributions clearly locate the neutron source and, relative gamma to neutron (or neutron to gamma) fraction images aid in discriminating AmBe sources from 252Cf source. The measurements were performed in the low scatter facility of the National Physical Laboratory, Teddington, UK. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.
Keywords: Neutron gamma fraction imaging Mixed-radiation field Organic liquid scintillator Bespoke fast digitiser
1. Introduction Fast and accurate detection, location and identification of radioactive sources (particularly neutron sources) are important tasks in nuclear decommissioning, international safeguards application and security applications. For example, the threat of the illicit transport of radioactive materials across international borders by terrorists demands that precise radiation screening techniques and instrumentation are in place [1]. Similarly, in nuclear decommissioning applications, it is important to characterise contamination of structures, site and contaminated land in a fast, accurate and efficient way in order to accelerate the clean-up programme, while reducing the net cost of decommissioning. The presence of neutrons and gamma rays is detected and characterised by various mechanisms. Radiation portal monitors and gamma radiation imaging systems are currently deployed to detect radioactive material where safety and security is vital. These systems are generally passive (not designed to generate or emit radiation with which to stimulate a response) and, often use plastic scintillators for gamma-ray detection and 3He-filled gas proportional counters for neutron detection. In nuclear decommissioning applications, detection and location of radioactive materials is typically based on gamma-ray detection. Such systems are primarily equipped with NaI(Tl) or CsI(Tl) scintillation detectors, where CARTOGAM, RadScan, RadCam, and the Gamma Visor n
Corresponding author. Tel.: þ 44 1524593873; fax.: þ44 1524381707. E-mail address: [email protected] (K.A.A. Gamage).
http://dx.doi.org/10.1016/j.nima.2015.03.072 0168-9002/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved.
are a few example gamma-ray imaging systems currently used in decommissioning applications. Recent advances in pulse-shape discrimination (PSD) methods and digital pulse-processing capabilities enable the use of organic liquid scintillation detectors in real-time radiation imaging applications [3,4]. Organic liquid scintillation detectors are sensitive to both neutrons and gamma rays. Pulse-shape analysis determines whether the event was caused by either a neutron or a gamma-ray event, based on the decay characteristics of the pulse. This provides an added benefit, particularly in the detection of neutron sources, where it permits both gamma rays and neutrons to be used to detect and locate the source. In this paper, detection and location of neutron sources, and identification of one neutron source from another are described. The technique is based on a recently developed organic liquid scintillate detector based mixed-field imaging system, where neutron gamma fraction images are generated to identify the neutron source. The remainder of the paper includes the details of the experimental setup at the National Physical Laboratory (Section 2); details and the results of pulse shape discrimination and neutron gamma fraction images (Section 3); discussion of results and conclusions (Section 4).
2. Experimental method In this work a system shown in Fig. 1 was used to collect data under three mixed-field radiation environments at the low scatter
10
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Fig. 2. Schematic of the experimental setup.
on the fly, in order to provide maximum flexibility in postprocessing analysis.
Fig. 1. The mixed-field imaging system used in this research [2].
facility of the National Physical Laboratory (Teddington, UK). The system comprised an organic liquid scintillator detector (4 ml cylindrical scintillator cell), a heavy tungsten collimator (outer diameter of 57 mm, 10 mm thickness and 250 mm length), adjustable equatorial mount and a bespoke fast digitiser (500 MHz, 12 bit resolution) [2]. The detector was placed inside the tungsten collimator which was then mounted on the equatorial mount. The detector output was connected to the digitiser and samples from the digitiser were streamed to a personal computer through an Ethernet connection. The mount and digitiser were synchronised and controlled through MATLAB. The sources (canister containing a small amount of radioactive material) used in this research were set up as shown in Fig. 2. The distance between the source plane and the detector front surface was 191 cm, where sources were approximately 45 cm apart from each other on the wall and 35 cm above the horizontal central axis of the collimator (where the height of the horizontal central axis of the collimator was 112 cm). Events were acquired under three environments: AmBe source only (NPL reference: 1000/1095; canister type X3; neutron emission rate of 2:149 106 s 1 ); 252 Cf source only (NPL reference: 4774; canister type X35; neutron emission rate of 2:24 107 s 1 ); both AmBe source and 252Cf source. In each case, the detector (controlled by the equatorial mount) collected data at a sequence of positions in azimuth and elevation constituting a full scan of the area on which the sources were set up i.e. data were recorded for a total of 35 positions (5 in elevation and 7 in azimuth) for each of the 3 source configurations. At each position data were taken for 1 min time period. The maximum count rate of the system was approximately around 50 kilo-pulses per second. The digitiser is designed to perform sampling at 500 MHz (i.e. each sample value taken at 2 ns intervals) with an amplitude resolution of up to 12 bits. The Field-Programmable Gate Array (FPGA) in the digitiser was set up to send raw digital pulse shapes, as opposed to discriminating
3. Results Organic liquid scintillator based detectors are sensitive to both neutrons and gamma rays. Experimentally collected digital samples consequently sent through a pulse shape discrimination algorithm (pulse gradient analysis was used in this research) in order to discriminate neutron events from gamma-ray events. Pulse gradient analysis is based on a comparison of the peak amplitude and the amplitude of a sample occurring at a defined time interval after the peak amplitude, generally known as the discrimination amplitude. Scatter plots (i.e. plots of discrimination amplitude versus peak amplitude) were obtained for the position giving the maximum number of counts for each of the 3 configurations to provide an indication of the mixed nature of each field investigated. Pulse shape discrimination scatter plots shown in Figs. 3(a), 4(a) and 5 are corresponding to AmBe source, 252Cf source and both sources respectively (scales of arbitrary units are common in each case). In each scatter plot the upper plume corresponds to neutron events and the lower to gamma-ray events. The discrimination line established on the basis of the previous experiments conducted for pure gamma and combined gamma and neutron sources [3]. The spatial distribution of neutron counts (after the pulse shape discrimination), for the case of the AmBe source and for the case of 252 Cf source, are shown in Fig. 3(a) and in Fig. 4(b) respectively. Fig. 6(a) represents the relative gamma to neutron fraction image (i.e. spatial distribution of gamma to neutron count ratio) and Fig. 6(b) represents the relative neutron to gamma fraction image (i.e. spatial distribution of neutron to gamma count ratio) for the case of both AmBe and 252Cf sources. In each case the count distribution transformed to the Cartesian xy-plane, relative to the co-ordinates of the detector mount system, giving a spatial dependence of the field in the actual plane on which the source is located. Also, the scale on the colour map corresponding to the intensity of the parameter considered in each case (for example number of
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Discrimination amplitude (a.u.)
70
11
data discrimination line
60 50 40
n
30 20
γ
10 0
0
50
100
150
200
250
300
Peak amplitude (a.u.) Fig. 3. For AmBe source, (a) pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) and (b) the spatial distribution of neutron counts. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Discrimination amplitude (a.u.)
50
data discrimination line
40 30
n
20 10 0
γ 0
50
100 150 200 Peak amplitude (a.u.)
250
300
Fig. 4. For 252Cf source, (a) pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) and (b) the spatial distribution of neutron counts. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Discrimination amplitude (a.u.)
60
data discrimination line
50 40 30
n
20 10 0
γ 0
50
100 150 200 Peak amplitude (a.u.)
250
300
Fig. 5. Pulse shape discrimination using pulse gradient analysis (discrimination amplitude versus peak amplitude) for both the 252Cf source and the AmBe source.
neutron counts as the parameter for the cases shown in Figs. 3(b) and 4(b)) as a function of position, where the colour map follows visible spectrum order with red corresponding to maximum and blue to minimum intensity.
4. Discussion and conclusion The scatter plot for the AmBe source (shown in Fig. 3(a)) indicates a good separation of neutron events from gamma-ray events. The concentration of number of events gradually decreases with the peak amplitude, where neutron–gamma separation is much better with
the increasing peak amplitude. Similar variations in pulse shape discrimination can also be observed in the case of 252Cf (Fig. 4(a)) as well as in the case of the both sources (Fig. 5). Also, it is important to highlight the density of gamma-ray events in the lower quadrants of the scatter plots. Gamma to neutron count ratio is higher in 252Cf compared to AmBe [4], as can be seen in the pulse shape discrimination scatter plots of AmBe and 252Cf. The visualised neutron count distributions shown in Figs. 3(b) and 4(b) clearly locate the neutron sources (AmBe source and 252Cf respectively) in the experimental environment investigated. These images demonstrate the capability of the system to detect and locate neutron sources, where by using gamma to neutron (or neutron to gamma) count ratio this method can be further advanced to identify the neutron source. For example, Fig. 6(a) shows the spatial distribution of gamma to neutron count ratio and Fig. 6(b) shows the spatial distribution of neutron to gamma count ratio. As gamma to neutron count ratio is higher in 252Cf compared to AmBe, the highest intensity point of Fig. 6(a) corresponds to the location of the 252Cf source and similarly, highest intensity point of Fig. 6(b) corresponds to the location of the AmBe source as the neutron to gamma ratio is higher in AmBe. A method to detect, locate and identify neutron source with a single detector has been presented. It is expected to increase the speed of the operation significantly with an array of detectors. For example, using the system described in this research (as highlighted before, it contained a single organic liquid scintillator detector), scanning process was taken approximately 40 min for each case. The processing time of the system can be considerably reduced by optimising the number of detectors in an array with
12
K.A.A. Gamage, G.C. Taylor / Nuclear Instruments and Methods in Physics Research A 788 (2015) 9–12
Fig. 6. The spatial distribution for both the fraction image.
252
Cf source and the AmBe source, (a) relative gamma to neutron fraction image and (b) relative neutron to gamma
the help of an artificial intelligence technique. The method has significant potential for the assay of mixed field environments in the nuclear industry, security applications and others. Acknowledgements The authors would like to acknowledge the financial support of Engineering Department and Faculty of Science and Technology, Lancaster University, UK (FST-RG-11/12-6). We also would like to acknowledge Dr. David Thomas and Dr. Nigel Hawkes for providing access to, and assistance with, the neutron metrology facility at the National Physical Laboratory, Teddington, UK.
References [1] R.T. Kouzes, E.R. Siciliano, J.H. Ely, P.E. Keller, R.J. McConn, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 584 (2008) 383. [2] K.A.A. Gamage, M.J. Joyce, G.C. Taylor, Applied Radiation and Isotopes 70 (2012) 1223. [3] K.A.A. Gamage, M.J. Joyce, J.C. Adams, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 635 (2011) 74. [4] K.A.A. Gamage, M.J. Joyce, N.P. Hawkes, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 642 (2011) 78.