Optical Gain Measurements for a Portable Plastic-Scintillator-Based Muon Tomography System

Transcription

1 Optical Gain Measurements for a Portable Plastic-Scintillator-Based Muon Tomography System Prepared By: Kenneth Moats Zernam Enterprises Inc. 101 Woodward Dr, Suite 110 Ottawa, ON K2C 0R4 PWGSC Contract Number: W Technical Authority: David Waller, Defence Scientist, DRDC Ottawa Research Centre Contractor s Publication Date: March 201 Disclaimer: The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada. This document was reviewed for Controlled Goods by DRDC using the Guide to Canada s Export Controls. Contract Report DRDC-RDDC-201-C102 May 201

2 Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 201 Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 201

3 Optical Gain Measurements for a Portable Plastic-Scintillator-Based Muon Tomography System Kenneth Moats Contract Report DRDC Ottawa March 201

4 Abstract A portable plastic-scintillator-based muon tomography system could be used by the Canadian Armed Forces (CAF) to image suspect Special Nuclear Material (SNM) objects that are approximately 1 m 3. The main challenge for image reconstruction in a portable muon tomography system is achieving a precise angular resolution. In order to do so, the optical yield of the scintillator bars must be improved. The work in this report focuses on determining the optical gain that could be achieved in such a system by inserting optical couplers between the plastic triangular scintillating bars and the wavelength shifting fibres. This report considers four optical couplers, namely water, BC 30, EJ 00, and NOA 3, as well as de-gassed BC 30, EJ 00, and NOA 3. Test data for this system using cosmic-ray muon sources was collected at DRDC Ottawa Research Centre and was analyzed by writing code in C++ with the ROOT analysis package. It was found that the measured optical gain values are quite similar for each optical coupler. It was also found that de-gassing the optical couplers had no noticeable effect on the optical gain. Either of the optical couplers considered in this study, whether de-gassed or not, are capable of improving the optical yield of a portable plastic-scintillator-based muon tomography prototype by approximately 40%. Further study is needed to determine how much these optical couplers will improve the tracking and position resolution of a portable muon tomography system, which will allow smaller muon trackers without sacrificing angular resolution. A Geant4 optical simulation of a portable muon tomography prototype, dubbed MuPIC (Muon Portable Imager for Counter-terrorism) is underway at DRDC Ottawa Research Centre. These simulations and laboratory studies should be continued to optimize the design of this prototype for future use by the CAF to detect SNM. ii

5 Table of Contents Abstract ii Table of Contents iii List of Tables iv List of Figures v 1 Introduction Muon Tomography CRIPT: Cosmic Ray Inspection and Passive Tomography A Portable Muon Tomography Experiment Data Analysis and Optical Gain 2.1 Data Collection Data Analysis Control Bars Temperature Dependence Optical Gain Summary 2 References 2 iii

6 List of Tables 2.1 List of optical couplers, with the corresponding bar numbers, run numbers for the empty and filled bars, and the fibre diameter used. Bars listed with a filled run of N/A have not yet been filled with an optical coupler List of control bar run numbers used in this study Measured optical gain for each optical coupler, calculated using the ratio of normalized mean charges of filled and empty runs. These values and their statistical uncertainties correspond to the yellow regions in Figure 2.9, and were calculated using the results from channels 3 and 4 (the red points in Figure 2.9), not using the normalized MPV (the cyan points in Figure 2.9), which have larger statistical uncertainties iv

7 List of Figures 1.1 Dimensions of the triangular cross section of the scintillator bars, with a hole in the centre to hold the Kuraray Y-11 multi-clad WLS fibre. All dimensions are in mm Configuration of the Mega Triangle Bars, with our numbering scheme for the bars and channels Mean charge in each channel vs. the mean charge in channel 1 for each 1-hour time interval of all good runs, using a charge threshold of Q min = 2 pc in the outer bars Mean charge in each channel vs. the mean temperature for each 1-hour time interval of all good runs, using a charge threshold of Q min = 2 pc in the outer bars Mean charge in each channel vs. the mean temperature for each 1-hour time interval of all control bar runs, using a charge threshold of Q min = 2 pc in the outer bars Normalized mean charge in each channel vs. the mean temperature for each 1- hour time interval of all control bar runs, using a charge threshold of Q min = 2 pc in the outer bars Temperature-corrected mean charge for each 1-minute interval of run Mean temperature for each 1-minute interval of run v

8 2.8 Normalized mean charge for each bar, using a charge threshold of Q min = 2 pc in the outer bars. Refer to Table 2.1 for a list of the optical couplers used in each bar. The empty runs for each bar are shown in the bottom panel and the corresponding optical coupler filled runs are shown in the top panel. Note that empty run 41 for bar 1 was found to have bad data due to a shifted fibre, and that there is not yet any optical coupler data for bars 3 44 or for bars, which are yet to be filled Optical gain for each middle bar, using a charge threshold of Q min = 2 pc in the outer bars. Refer to Table 2.1 for details on the optical couplers used in each bar. The average optical gain values for each optical coupler, along with their statistical uncertainties, were calculated using the red points from channels 3 and 4 and are shown as the yellow shaded regions. The corresponding values are given in Table vi

9 1. Introduction The objective of the Special Nuclear Material (SNM) detection work being conducted at Defence Research and Development Canada (DRDC) is to develop technologies that improve the ability of the Canadian Armed Forces (CAF) to detect SNM. Muon tomography work began at DRDC Ottawa Research Centre (ORC) in 2009 with project CRTI RD [1], for which a large muon tomography prototype, named CRIPT (Cosmic Ray Inspection and Passive Tomography), was developed. 1.1 Muon Tomography Muon tomography relies on two or more planes of position-sensitive muon detectors arranged above and below a volume to be imaged, potentially containing SNM or dense shielding material [2]. The upper detector layer measures the position and direction of the incoming muon tracks. As the muons pass through the volume, they are deflected from multiple Coulomb scattering much more by SNM or dense shielding material than by low-density, low-z materials. As the lower detector layer measures the position and direction of the outgoing muon tracks, measuring muon scattering through cargo containers, as was done by CRIPT, allows one to reconstruct images of objects inside, and to identify potential threats involving SNM. The multiple Coulomb scattering angles in muon tomography are sampled from a distribution that is assumed to be Gaussian. Although the actual distribution has heavier tails than a Gaussian, this is a good approximation for the central 98% of scattering angles. 1

10 2 This distribution has a mean scattering angle of zero degrees, and a width (in mrad) of θ MS = 13. [ ( )] L L ln βpc X 0 X 0 mrad (1.1) where β is the velocity of the muon as a fraction of the speed of light, c, p is the muon momentum in MeV/c, L is the path length of the muon through the material, and X 0 is the radiation length of the material [3]. The radiation length (in cm) is related to the density ρ (in g/cm 3 ), and atomic properties of the material by X 0 = 1.4 A ρz(z + 1) ln(28/ Z) cm (1.2) where Z is the number of protons per nucleus and A is the number of nucleons per nucleus [3]. It is evident that muons are deflected much more by high-z SNM or dense shielding material than by low-density, low-z materials. As an example, for muons with 1 GeV/c momentum traversing a 10 cm thickness of material, θ MS = 14 mrad for aluminum (Al), θ MS = 3 mrad for iron (Fe), θ MS = 4 mrad for lead (Pb), and θ MS = 8 mrad for uranium (U). Using sophisticated reconstruction techniques, the shape and composition of materials traversed by muons can be deduced by a high-statistics measurement of the deflection angles [4]. 1.2 CRIPT: Cosmic Ray Inspection and Passive Tomography The potential for military applications of muon tomography was identified during the execution of the CRIPT project, which uses four planes of extruded plastic, triangular scintillator bars manufactured at Fermilab [] for the MINERνA neutrino detector [, ]. Each panel has 121 triangular strips with a length of 2 m. The entire CRIPT detector is roughly metres tall and weighs 22 tonnes. The CRIPT experiment achieved position resolution on the

11 3 order of 3. mm with relatively short scanning times of 0 seconds that are required for efficient scanning at border crossings. Obtaining sufficient statistics to achieve good position resolution, and hence precise enough angular resolution to reconstruct a detailed image with these short scanning times is the most difficult challenge for muon tomography. The CRIPT experiment achieved angular resolutions of mrad, with a high true positive fraction of 90% and a low false positive fraction 10% [1, 8, 9]. 1.3 A Portable Muon Tomography Experiment A much smaller, portable system could be used to image suspect SNM objects that are approximately 1 m 3 (in principle it could be moved around to image larger objects, but this would take longer). The current DRDC effort focuses on simulation and laboratory studies of a smaller, portable muon tomography system for use by the CAF. The system will use two trackers, each with a detector area of 1 m 2 and a height of 40 cm. In order for a pair of soldiers to carry the system into the field relatively easily, the weight of each tracker must be a maximum of 100 kg. One advantage of this smaller detector is that the imaging times can be much longer than at a border crossing, on the order of a few hours, which avoids the most difficult challenge that the CRIPT experiment faced. On the other hand, the challenge for portable muon scattering tomography is achieving an adequate angular resolution. The new system s angular resolution for muon tracks should be similar to CRIPT s: σ θx z = mrad for a 2-dimensional scattering angle resolution in the x z plane (the y-direction is vertical). CRIPT s upper and lower tracking layers are 100 cm apart, so the position resolution for the portable system must be better to achieve the same angular resolution. The goal is to achieve a position resolution of σ x (40 cm/100 cm) 3. mm = 1.4 mm. The portable muon detection system will use the same triangular plastic scintillator bars as the CRIPT experiment, but with a length of 1 m or 0. m, instead of 2 m. However,

12 4 Figure 1.1: Dimensions of the triangular cross section of the scintillator bars, with a hole in the centre to hold the Kuraray Y-11 multi-clad WLS fibre. All dimensions are in mm. for the experiments performed for this study, 4-cm-long bars were used. The dimensions of the triangular cross section of the bars is shown in Figure 1.1. For more details on the properties of the bars and the physics of the scintillation process, please refer to [10, 11]. To further increase the attenuation length and channel the scintillated light, wavelength shifting (WLS) fibres are inserted along the length of the bars through their centres. Kuraray Y-11 multi-clad WLS fibres [12] of diameter 1.2 mm are used to shift the scintillation photons from blue and UV light to green light for detection by Hamamatsu S122-00C Silicon Photomultipliers (SiPMs) [13] mounted at each end of the fibres. To reduce blue and UV light losses from the scintillator-air-wls fibre interfaces via total internal reflection, an optical coupler was inserted into the gap between the fibre and the scintillator bars. The optical gain using various optical couplers was studied in a previous work [10] using four bars arranged horizontally. Four of these optical couplers (Water, BC 30 [14], EJ 00 [1], and NOA 3 [1]) were further studied in [11]. The triggering mechanism was improved in [11] by considering an alternate geometry, dubbed the Mega Triangle configuration, which is expected to give more reliable results as it allows one to use external triggers with

13 muon data. Test data for this system using cosmic-ray muon sources was collected at DRDC Ottawa Research Centre and was analyzed by writing code in C++ with the ROOT analysis package [1]. It was found in [10] and [11] that the measured optical gain values are quite similar for each optical coupler. It was suggested that the presence of air bubbles when inserting the optical coupler could account for this behaviour. To examine this effect, a series of trials was performed using de-gassed bars, in which the optical coupler was inserted using syringes and a vacuum to reduce the presence of air bubbles. Another possible explanation is that additional temperature corrections had not been taken into account. Therefore, the goal of this study was to examine the temperature correction in greater detail and determine an improved estimate of the optical gain using de-gassed optical couplers. The results of this data analysis are presented in Section 2. These optical gain results and future simulations and measurements of position and angular resolution will allow us to determine if a plastic-scintillator-based portable muon tomography system is feasible and whether it will meet user requirements for use in the field.

14 2. Data Analysis and Optical Gain 2.1 Data Collection Four 4-cm-long triangular plastic scintillator bars with Kuraray Y mm diameter multi-clad WLS fibres along their centres were arranged in the Mega Triangle configuration shown in Figure 2.1, and sealed in a light-tight box. To measure the light output at the ends of the fibres, Hamamatsu S122-00C SiPMs were connected to a Vertilon SIB-108 board and a Vertilon IQSP482 charge integrating data acquisition (DAQ) system [18]. The outer bars were used for triggering, so were not filled with an optical coupler. The bars in the middle were measured first empty, then filled with one of four optical couplers: Water, BC 30, EJ 00, or NOA 3. A list of the optical couplers used, along with the corresponding bar number, run numbers and fibre diameters are listed in Table 2.1. Up to four separate bars were used for each optical coupler, after which, data was taken with empty control bars inserted into the middle. Some control bar runs were found to have bad data mainly due to the fibres being shifted inside the bars, which was also a problem for run 41 in Table 2.1. The data for such control bar runs were discarded. Table 2.2 lists the control bar runs that did not have these issues, so their data was included in the analysis. The optical gain values were found to be quite similar for each of these optical couplers [11]. It was suggested that the presence of air bubbles when inserting the optical coupler may account for this behaviour. To examine this effect, a series of trials with de-gassed BC 30, NOA 3 and EJ 00 optical couplers was completed using bars 2, listed in Table 2.1. Note that for bars 3 44, the 1.-mm and 2.0-mm St. Gobain fibres were tested, but without optical couplers. Bars were measured empty and have not yet been filled

15 Figure 2.1: Configuration of the Mega Triangle Bars, with our numbering scheme for the bars and channels. with an optical coupler. Data were collected using a cosmic-ray muon source, and the length of each run varied widely from run to run. The early muon runs were taken overnight ( 8 hours), as this had been done for the previous muon runs [10]. However, it was later realized that shorter runs ( 1 hour) could achieve statistical errors on the order of a few percent, which is sufficient for our purposes. This had the advantage of significantly speeding up the data collection, so beginning with run number 48, the run lengths were as short as 1 hour. 2.2 Data Analysis After performing measurements, the data were analyzed in several steps with code written in C++ and using ROOT version [1]. First, a temperature correction for the SiPM gain, given by the manufacturer [13] as Temperature Coefficient of Gain Gain = / C = 0.021/ C (2.1)

16 8 Table 2.1: List of optical couplers, with the corresponding bar numbers, run numbers for the empty and filled bars, and the fibre diameter used. Bars listed with a filled run of N/A have not yet been filled with an optical coupler. Bars Empty Runs Filled Runs Optical Coupler Fibre Diameter (mm) , 48, 48, , 1, 2, 0 Water , 493, 494, 49 N/A N/A , 49, 498, 499 N/A N/A , 10, 11, 12 28, 29, 30, 31 BC , 14, 41 a, 1 4,,, 8 EJ , 3, 3, 34 3, 4,, NOA , 0, 1, 2 N/A N/A ,,, 8 N/A N/A N/A N/A BC 30 Degassed , 84 9, 9 NOA 3 Degassed , 88, 89, 91 9, 98, 99, 01 EJ 00 Degassed 1.2 a Run 41 was later found to have bad data due to a shifted fibre, so the data for Bar 1 were excluded from this analysis. Table 2.2: List of control bar run numbers used in this study. Control Bar Run Numbers C1 490, 00, 1, 32, 2, 0,, 3, 9, 8, 92, 02 C2 491, 33, 3, 9, 8, 4, 80, 8, 93

17 9 was applied to the measured charge. The SiPMs at each end of the bars were labelled with a channel number as shown in Figure 2.1. Once the data were obtained and the temperature correction was applied, we imposed the trigger requirement that a muon must hit any two outside bars. This guarantees that the muon passed through the middle bar as well. We also expect a correlation between the light output at one end of a given bar and the light output at the other end. Therefore, our trigger condition can be written logically as [(Q 1 Q min AND Q 2 Q min ) AND (Q Q min AND Q Q min )] OR [(Q 1 Q min AND Q 2 Q min ) AND (Q Q min AND Q 8 Q min )] OR [(Q Q min AND Q Q min ) AND (Q Q min AND Q 8 Q min )] (2.2) where Q i refers to the charge in channel i and Q min is the charge threshold. In this study we present results using a charge threshold of Q min = 2 pc only (other thresholds as high as 20 pc were considered in [11]). If this trigger requirement was not satisfied, the event was rejected during the data analysis. The temperature-corrected charge distributions for all runs were plotted for empty bars, and also for the corresponding runs in which an optical coupler was inserted between the bar and the WLS fibre. As an example, see Figures 2.2 and 2.3 in [11]. We used these charge distributions to estimate the optical gain achieved by using each optical coupler. The challenge in doing so in our previous study [10] using four horizontal bars was that adding an optical coupler pushes a larger fraction of the total number of collected events above the charge threshold to pass the trigger condition. For this reason one could not simply use an observable such as the mean charge of the distribution to reliably determine the optical gain. However, by placing one bar on top of the other three in the Mega Triangle configuration shown in Figure 2.1, and using the trigger requirement given in Equation 2.2, these problems

18 10 are alleviated as the charge trigger is not applied to the middle bar whose optical coupler is being studied. In this case, one can then simply use the ratio of the mean charge of the distributions to estimate the optical gain as Optical Gain = Q filled Q empty (2.3) with a statistical uncertainty given by σ Optical Gain = (Optical Gain) (σ Qfilled Q filled ) 2 + ( σ Qempty Q empty ) 2. (2.4) In our previous study [11], it was shown that using the mean charge as an observable to estimate the optical gain includes some low-charge events that presumably are not due to muons alone. It was argued that the effects of these low-charge background events could be excluded by fitting a Landau distribution to the muon peak and using the Most Probable Value (MPV), which determines the charge at the location of the peak. Using the ratio of the Most Probable Value (MPV), between filled and empty runs allows a determination of the optical gain that is independent of these low-charge background events. However, it was found that the statistical errors were larger using this observable. 2.3 Control Bars The purpose of using control bars is to ensure that the charges in each bar do not significantly change from one empty run to another. However, in our previous study [11], the charge in the middle control bar was found to vary significantly from run to run. Furthermore, the mean charge in each of the outer bars appeared to vary in the same manner. This behaviour was observed for all runs, regardless of whether the middle bar was a control bar, an empty bar, or a filled bar. The correlation coefficient of the charge in any two channels was found

19 11 to be quite high ( ) in each case [11], except when examining the correlation with the charge in channels 3 and 4 (this should not be surprising as the middle bar in some of these runs is filled with an optical coupler or uses different fibre diameters). Since the charges in each channel are highly correlated, we may use the outer bars to normalize the mean charges of each channel (i = 1,..., 8) as follows: Q norm i = Q i Q outer (2.) where Avg( Q, Q, Q, Q 8 ) for i = 1, 2 (Bar 1) Q outer = Avg( Q 1, Q 2, Q, Q, Q, Q 8 ) for i = 3, 4 (Bar 2) Avg( Q 1, Q 2, Q, Q 8 ) for i =, (Bar 3) Avg( Q 1, Q 2, Q, Q ) for i =, 8 (Bar 4) (2.) and the average of the mean charges on the outer bars was computed using a weighted mean, given by with an uncertainty given by Avg( Q ) = σ Avg( Q ) = i Q i /σ 2 i i 1/σ2 i 1 i 1/σ2 i (2.). (2.8) More simply stated, we divided the mean charge in the middle bar by the average of the mean charges in the outer bars, whereas for the outer bars, we divide their mean charge by the average of the mean charges in the other outer bars. The same was done for the MPV in the middle bar. We found in [11] that there is much less variation in the normalized mean charge than in the uncorrected mean charge, with the standard deviation in the MPV

20 12 reduced by a factor of, and the standard deviation in the mean charge reduced by a factor of 12. The reason for this may be due to the temperature correction being applied to the charge in each channel. In our previous study [11] we found that the magnitude of the correlation coefficient between the mean charge in each channel and the initial temperature of each run is This was not as high as the correlation coefficient between the mean charge of any two channels, which was found to be This indicated that an additional temperature correction may have been needed. However, it was realized that using the initial temperature for each run may not be sufficient for the longer runs, as the temperature may vary significantly over the course of an overnight run. 2.4 Temperature Dependence To more rigorously examine the temperature dependence of our results, we split each run into 1-hour-long intervals and computed the mean charge and mean temperature during each of those time intervals. Similar to our analysis in [11], we then plot the mean charge in each channel vs. the mean charge in channel 1 for each 1-hour time interval of all good runs in Figure 2.2. We also plot the mean charge in each channel vs. the mean temperature for each 1-hour time interval of all good runs in Figure 2.3. Comparing these two plots, we see that the magnitude of the correlation coefficients for each channel are similar between the charge vs. charge plots and charge vs. temperature plots. This demonstrates that the temperature correction we have used behaves as expected. In channels 3 and 4, there are distinct clusters of data points at higher or lower charge, which is not surprising since these points correspond to runs in which the middle bar was filled with an optical coupler or used a larger fibre, respectively. However, there also appear to be distinct clusters of points in the other channels. These appear to be systematic shifts in the measured charges over the course of consecutive runs,

21 13 Channel 2 vs. Channel 1.. Entries = x =.18 ± y =.18 ± Cor(x, y) = Channel 1 Mean Charge (pc) x =.18 ± y = ± Cor(x, y) = Channel 1 Mean Charge (pc) x =.18 ± Cor(x, y) = Channel 1 Mean Charge (pc).. Channel 1 Mean Charge (pc) Channel vs. Channel 1.. Entries = 104 x =.18 ± y =.4 ± Cor(x, y) = Channel 1 Mean Charge (pc) Run , 11/30/1-02/2/1 Run 48, 03/11/1 Run 48-00, 03/14/1-04/29/1 13 Run 0-29, 0/09/1-0/1/1 Run 30-38, 0/1/1-0/29/1 12 Run 48-3, 0/20/1-09/01/1 Run 4-9, 09/01/1-11/04/ y = 8.1 ± Channel 1 Mean Charge (pc) 14 Entries = Channel 8 vs. Channel 1 Channel 8 Mean Charge (pc) Channel vs. Channel y = 1.88 ± y = ± Cor(x, y) = x =.18 ± Cor(x, y) = Channel 1 Mean Charge (pc) Entries = 104 x =.18 ± Channel Mean Charge (pc). Entries = Channel Mean Charge (pc) 14 Entries = 104 Channel vs. Channel 1 Channel Mean Charge (pc) Channel 4 Mean Charge (pc) 1 20 Cor(x, y) = 0.80 Channel 4 vs. Channel y =.08 ± Entries = x =.18 ± Channel 3 vs. Channel 1 Channel 3 Mean Charge (pc) Channel 2 Mean Charge (pc) Channel 1 Mean Charge (pc) Channel 1 vs. Channel 1 9 Entries = 104 Run 9-02, 01/0/1-01/12/1 x =.18 ± Empty Bars y = 11.1 ± 0.02 Filled Bars Cor(x, y) = & 2.0 mm Fibres. Channel 1 Mean Charge (pc) Figure 2.2: Mean charge in each channel vs. the mean charge in channel 1 for each 1-hour time interval of all good runs, using a charge threshold of Qmin = 2 pc in the outer bars.

22 14 Channel 2 Entries = 104 y =.18 ± Cor(x, y) = y =.08 ± Cor(x, y) = Entries = 104 x = ± y = 1.88 ± Cor(x, y) = Channel Mean Temperature ( C) Channel Entries = 104 y = ± 0.09 Cor(x, y) = Channel Entries = 104 x = ± 0.03 Mean Charge (pc) x = ± Mean Temperature ( C) y = ± Cor(x, y) = Mean Temperature ( C) Entries = 104 x = ± 0.03 Mean Charge (pc) 1 Mean Charge (pc) Channel 3 Entries = 104 x = ± 0.03 Mean Charge (pc) Mean Charge (pc) x = ± 0.03 Mean Charge (pc) Channel 1 y =.4 ± Cor(x, y) = Channel Mean Temperature ( C) Channel 8 Entries = 104 y = 8.1 ± Cor(x, y) = Mean Temperature ( C) Entries = 104 x = ± 0.03 Mean Charge (pc) Mean Charge (pc) x = ± Mean Temperature ( C) y = 11.1 ± Cor(x, y) = Run , 11/30/1-02/2/1 Run 48, 03/11/1 Run 48-00, 03/14/1-04/29/1 13 Run 0-29, 0/09/1-0/1/1 Run 30-38, 0/1/1-0/29/1 12 Run 48-3, 0/20/1-09/01/1 8. Run 4-9, 09/01/1-11/04/ Run 9-02, 01/0/1-01/12/ Empty Bars Filled Bars Mean Temperature ( C) 1. & 2.0 mm Fibres Mean Temperature ( C) Figure 2.3: Mean charge in each channel vs. the mean temperature for each 1-hour time interval of all good runs, using a charge threshold of Qmin = 2 pc in the outer bars.

23 1 and have been colour-coded based on run number. This is particularly evident in channels 2, and, with some clusters of points at higher or lower charge than expected. Since the bars in the outer channels are all empty, all points should fall along the same curve if the temperature correction has been done properly. This should especially be true for the control bar runs, so we also plot the mean charge in each channel vs. the mean temperature for each 1-hour time interval of all control bar runs from Table 2.2 in Figure 2.4. In this plot, we see the same clusters of points in channels 2, and. The cause of these systematic shifts in the measured charge was not fully determined. We fit the data to a straight line for each channel and computed the χ 2 of this fit for N degrees of freedom. We obtained a very poor fit for channels 2, and, with a reduced chi-square of χ 2 /N = 280/21, 22/21 and 01/21, respectively. We obtained a better fit for channels 1, and 8, with a reduced chi-square of χ 2 /N =.8/21, 91.3/21 and 1220/21, respectively. While the reduced chi-square for all channels corresponds to a small p-value, and thus a low probability that the data can be fit by a straight line, this is likely due to the fact that only statistical errors have been taken into account. If systematic errors could be estimated, the chi-square for these linear fits would be much smaller, resulting in a larger p-value. In any case, it is clear that the data in channels 2, and are less reliable than the data in channels 1, and 8. Based on these results, channels 2,, should not be used in the analysis and channels 1,, 8 should only be used when normalizing the mean charges using the charges in the outer bars. We then calculated the normalized mean charge using Equation 2., but instead of using Equation 2. to compute Q outer, the average of the mean charges on the outer bars, we use

24 1 χ χ χ χ χ χ χ χ Figure 2.4: Mean charge in each channel vs. the mean temperature for each 1-hour time interval of all control bar runs, using a charge threshold of Q min = 2 pc in the outer bars.

25 1 the following: Avg( Q, Q 8 ) for i = 1, 2 (Bar 1) Q outer = Avg( Q 1, Q, Q 8 ) for i = 3, 4 (Bar 2) Avg( Q 1, Q 8 ) for i =, (Bar 3) Avg( Q 1, Q ) for i =, 8 (Bar 4) (2.9) where we have used a weighted mean given by Equations to calculate the average of these mean charges. We plot the normalized mean charge, computed using Equation 2.9, vs. temperature in Figure 2. for the control bar runs listed in Table 2.2. All points should fall along a horizontal line of constant charge. Although the same clusters of points in channels 2,, and are visible, we see that this normalized mean charge is much less sensitive to variations in temperature than the mean charge plotted in Figure 2.4. As a check of the effect of temperature on the charge, the temperature and charge are plotted for each minute-long interval of run 02 in Figures 2. and 2. respectively. We see that the temperature varies gradually, and only up to about %, whereas the mean charge varies randomly by about 10-20%. Therefore, this variation in charge does not seem to be due to temperature alone. These results suggest that, rather than the mean charge, we should use the normalized mean charge in our analysis, calculated using Equation 2.9. Based on Figure 2., this ensures that the charge in the control bars remains relatively constant with temperature. We will therefore use this normalized mean charge to compute the optical gain in the following section.

26 18 χ χ χ χ χ χ χ χ Figure 2.: Normalized mean charge in each channel vs. the mean temperature for each 1-hour time interval of all control bar runs, using a charge threshold of Q min = 2 pc in the outer bars.

27 19 Bar 1, Channel 1 Bar 1, Channel 2 Charge (pc) Time (min) Charge (pc) Time (min) Charge (pc) Bar 2, Channel Time (min) Charge (pc) Bar 2, Channel Time (min) Charge (pc) Bar 3, Channel Time (min) Charge (pc) Bar 3, Channel Time (min) Bar 4, Channel Bar 4, Channel 8 Charge (pc) Time (min) Charge (pc) Time (min) Figure 2.: Temperature-corrected mean charge for each 1-minute interval of run 02.

28 20 Temperature ( C) Temperature vs Time Time (min) Figure 2.: Mean temperature for each 1-minute interval of run 02.

29 2. Optical Gain 21 Figure 2.8 plots the normalized mean charge for each bar for which data were obtained, using only channels 1,, and 8 from the outer bars to normalize the charges of the inner bars (Equation 2.9). Refer to Table 2.1 for a list of the optical couplers used in each bar. The empty runs for each bar are shown in the bottom panel and the corresponding optical coupler filled runs are shown in the top panel. It is clear that the outer bars (channels 1, 2, 8) show very little variation from run to run when using this normalized mean charge. Note that the results from bar 1 are not shown here, as it was discovered that the fibres had been shifted by a few mm for empty run 41, resulting in an unusually small amount of charge in the channels on one end of the bars. By the time the data for this empty run were analyzed and this problem discovered, bar 1 had already been filled with the EJ 00 optical coupler. We therefore exclude bar 1 from the subsequent calculations of the EJ 00 optical gain. Note that optical coupler data for bars 3 44 and have not yet been obtained, as these bars are yet to be filled. Using the normalized mean charges shown in Figure 2.8, we plot the optical gain for each optical coupler in Figure 2.9, The optical gain values for channels 3 and 4 using the normalized mean charge are shown as the red points, whereas the optical gain values for channels 3 and 4 using the normalized MPV are shown as the cyan points. The statistical errors for the MPV are noticeably larger, so we do not use these cyan points when computing the average optical gain for each optical coupler. The average optical gain values for each optical coupler, along with their statistical uncertainties, are shown as the yellow shaded regions, and the corresponding values are given in Table 2.3. Interestingly, the measured optical gain values using degassed optical couplers do not show improvement over the corresponding values without de-gassing. The optical gain is also quite similar for each optical coupler, which is consistent with our findings in [10, 11]. We conclude that either of these optical couplers, whether de-gassed or not, are adequate

30 22 Mega Triangle Mean Charge (Normalized) vs. Bar Number - Filled Bars - Normalized by Good Channels, Threshold > 2.0 pc Normalized Mean Charge Channel 1 Channel 2 Channel 3 Channel 3 MPV Channel 4 Channel 4 MPV Channel Channel Channel Channel Bar Number Mega Triangle Mean Charge (Normalized) vs. Bar Number - Empty Bars - Normalized by Good Channels, Threshold > 2.0 pc Normalized Mean Charge Channel 1 Channel 2 Channel 3 Channel 3 MPV Channel 4 Channel 4 MPV Channel Channel Channel Channel Bar Number Figure 2.8: Normalized mean charge for each bar, using a charge threshold of Q min = 2 pc in the outer bars. Refer to Table 2.1 for a list of the optical couplers used in each bar. The empty runs for each bar are shown in the bottom panel and the corresponding optical coupler filled runs are shown in the top panel. Note that empty run 41 for bar 1 was found to have bad data due to a shifted fibre, and that there is not yet any optical coupler data for bars 3 44 or for bars, which are yet to be filled.

31 23 Mega Triangle Optical Gain vs. Bar Number using Normalized Mean Charge - Using Good Channels Only, Threshold > 2.0 pc Optical Gain Water BC30 EJ00 NOA3 Degassed NOA3 BC30 EJ00 Channel 1 Channel 2 Channel 3 Channel 3 MPV Channel 4 Channel 4 MPV Channel 1. Channel Channel 1. Channel Bar Number Figure 2.9: Optical gain for each middle bar, using a charge threshold of Q min = 2 pc in the outer bars. Refer to Table 2.1 for details on the optical couplers used in each bar. The average optical gain values for each optical coupler, along with their statistical uncertainties, were calculated using the red points from channels 3 and 4 and are shown as the yellow shaded regions. The corresponding values are given in Table 2.3. for improving the optical yield of a portable plastic-scintillator-based muon tomography prototype by approximately 40%.

32 24 Table 2.3: Measured optical gain for each optical coupler, calculated using the ratio of normalized mean charges of filled and empty runs. These values and their statistical uncertainties correspond to the yellow regions in Figure 2.9, and were calculated using the results from channels 3 and 4 (the red points in Figure 2.9), not using the normalized MPV (the cyan points in Figure 2.9), which have larger statistical uncertainties. Optical Coupler Average Optical Gain Water ± BC ± EJ ± NOA ± BC 30 De-Gassed ± NOA 3 De-Gassed ± EJ 00 De-Gassed ± 0.00

33 3. Summary A portable plastic-scintillator-based muon tomography system could be used by the CAF to image suspect SNM objects that are approximately 1 m 3. The main challenge for image reconstruction in a portable muon tomography system is achieving a precise angular resolution. In order to do so, the optical yield of the scintillator bars must be improved. The work in this report focuses on determining the optical gain that could be achieved in such a system by inserting optical couplers between the plastic triangular scintillating bars and the wavelength shifting fibres. This report considers four optical couplers, namely water, BC 30, EJ 00, and NOA 3, as well as de-gassed BC 30, EJ 00, and NOA 3. Test data for this system using cosmic-ray muon sources was collected at DRDC Ottawa Research Centre and was analyzed by writing code in C++ with the ROOT analysis package. It was found that the measured optical gain values are quite similar for each optical coupler. It was also found that de-gassing the optical couplers had no noticeable effect on the optical gain. Either of the optical couplers considered in this study, whether de-gassed or not, are capable of improving the optical yield of a portable plastic-scintillator-based muon tomography prototype by approximately 40%. Further study is needed to determine how much these optical couplers will improve the tracking and position resolution of a portable muon tomography system, which will allow smaller muon trackers without sacrificing angular resolution. A Geant4 [19] optical simulation of a portable muon tomography prototype, dubbed MuPIC (Muon Portable Imager for Counter-terrorism) is underway at DRDC Ottawa Research Centre. These simulations and laboratory studies should be continued to optimize the design of this prototype for future use by the CAF to detect SNM. 2

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