Design and preliminary testing of the Multi-spectral...

Design and preliminary testing of the Multi-spectral Imaging System for the Detection of Radiological Contamination Elizabeth Inrig, Ian Watson, Vern Koslowsky, Michael Dick and Patrick Forget

Defence R&D Canada – Ottawa

Technical Memorandum DRDC Ottawa TM 2011-195

December 2011

Design and preliminary testing of the Multi-spectral Imaging System for the Detection of Radiological Contamination

Elizabeth Inrig Ian Watson DRDC Ottawa Vern Koslowsky Michael Dick Patrick Forget Bubble Technology Industries

Defence R&D Canada – Ottawa Technical Memorandum DRDC Ottawa TM 2011-195 December 2011

Principal Author

Original signed by Elizabeth Inrig

Elizabeth Inrig

Defence Scientist/CARDS

Approved by

Original signed by Julie Tremblay-Lutter

Julie Tremblay-Lutter

Section Head/CARDS

Approved for release by

Original signed by Chris McMillan

Chris McMillan

DRP Chair/DRDC Ottawa

This project was funded in part by the CBRNE Research & Technology Initiative (CRTI) under project CRTI-07-0104TD.

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2011

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2011

DRDC Ottawa TM 2011-195 i

Abstract ……..

Field detection and measurement of the activity and extent of alpha ( ) contamination is a particularly challenging task for radiation detection personnel. With funding from the CBRNE Research & Technology Initiative (CRTI-07-0104TD), the Multi-spectral Imaging System for the Detection of Radiological Contamination (MISDORC), is being developed with the aim of providing a new tool for in-field alpha detection. This detector uses the air fluorescence in the vicinity of the radioactive source to create an image of the contamination. The optical and mechanical design of the detector are briefly described here, along with the concept of use. The detector is currently at the laboratory prototype stage, and the results of preliminary tests of the prototype, using both sealed and unsealed sources, are presented. The detector is now in the final stage of development and is expected to be completed in early 2012.

Résumé ….....

La détection et la mesure pratique de la contamination alpha ( ) représente un défi particulièrement difficile pour le personnel de détection des radiations. Grâce à un financement fourni dans le cadre de l’Initiative de recherche et de technologie CBRNE (IRTC-07-0104TD), on a entrepris la conception d’un nouvel outil de détection de la contamination radiologique alpha en terrain d’opération, soit le Système d'imagerie multispectrale de détection de contamination radiologique (Multi-spectral Imaging System for the Detection of Radiological Contamination – MISDORC). Ce détecteur produit une image de la contamination d’après la fluorescence de l’air aux environs d’une source radioactive. La conception optique et mécanique du détecteur, ainsi que son concept d’utilisation, est brièvement décrite dans le présent document. La mise au point du détecteur est actuellement rendue à l’étape du prototype de laboratoire, dont les résultats des essais préliminaires exécutés au moyen de sources de rayonnement alpha scellées et non scellées sont aussi présentés. La conception du détecteur tire à sa fin et devrait se terminer au début de 2012.

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DRDC Ottawa TM 2011-195 iii

Executive summary


Elizabeth Inrig; Ian Watson; Vern Koslowsky; Michael Dick; Patrick Forget; DRDC Ottawa TM 2011-195; Defence R&D Canada – Ottawa; December 2011.

Introduction or background: This project, funded by the CBRNE Research & Technology Initiative (CRTI), aims to provide a new tool for in-field alpha ( ) detection. Due to their short range in air and other materials, particles are particularly difficult to detect, and conventional detection requires a painstaking examination of potentially contaminated surfaces using a fragile probe held within centimetres of the surface. This detector, by contrast, uses a novel approach by imaging the air fluorescence in the vicinity of the radioactive source, greatly increasing the range at which the contamination can be detected. Due to the low intensity and spectral characteristics of the fluorescence signal, the detector must operate under extremely low light conditions.

Results: The optical and mechanical components of the detector are now complete, but the system is currently operating with some temporary components, including the data acquisition electronics and focal plane detector. The detector in its current form is considered to be a laboratory prototype. Preliminary tests of the prototype have been carried out using both sealed and unsealed sources. Under darkroom conditions, the detector has proven capable of imaging a 22 kBq (0.6 Ci) 241Am smoke detector source at 1.5 m, and deliberate reduction in the device sensitivity achieved by reducing the optical aperture indicates that sources as small as 7 kBq (0.2

Ci) would be detectable. Testing was carried out at DRDC Ottawa in March, June, and July 2011 using both sealed 241Am sources and coupons prepared with unsealed 225Ac, a short-lived -emitting radioisotope. Due to problems involving light leakage between the filter and photomultiplier tube, as well as difficulties eliminating all sources of interfering light, the measurement results were somewhat inconsistent and the limit of detection was higher than expected. Further tests in July 2011 showed that increasing the UG-11 filter thickness resulted in an improved signal-to-noise ratio when interfering light from artificial sources was present, but provided no improvement when natural sunlight was the source of the interference.

Significance: The air fluorescence imager will provide a new tool for rapid imaging of contamination over a large area. This represents a much-needed stand-off radiation detection capability that may be applied to criminal investigations, radiation safety and hazard avoidance, and nuclear decommissioning activities.

Future plans: The detector is now in the final stage of development and is expected to be completed in early 2012. Initial testing of the final prototype will be carried out at BTI, followed by further testing at DRDC Ottawa using sealed and unsealed sources. Finally, the performance of the detector in real-world conditions will be evaluated through testing at nuclear facilities with existing contaminated areas.

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Sommaire .....


Elizabeth Inrig; Ian Watson; Vern Koslowsky; Michael Dick; Patrick Forget ; DRDC Ottawa TM 2011-195 ; R & D pour la défense Canada – Ottawa; décembre 2011.

Introduction : Le projet CRTI-07-0104TD, qui est financé dans le cadre de l’Initiative de recherche et de technologie CBRNE (IRTC), vise la conception d’un nouvel outil de détection de la contamination alpha ( ) en terrain d’opération. En raison de leur courte portée dans l’air et d’autres matériaux, les particules sont particulièrement difficiles à détecter et exigent, lorsque l’on emploie des techniques de détection classiques, un processus laborieux exécuté au moyen d’une sonde manuelle fragile qu’il faut tenir à quelques centimètres des surfaces potentiellement contaminées. Par contraste, le MISDORC repose sur une nouvelle approche qui implique l’imagerie de la fluorescence de l’air aux environs de la source radioactive, ce qui accroît considérablement la portée de détection de la contamination. Par contre, les caractéristiques spectrales et la faible intensité du signal de fluorescence exige l’utilisation de ce détecteur dans des conditions de luminosité extrêmement faible.

Résultats : Les composants optiques et mécaniques du détecteur ont tous été mis au point, mais le système fonctionne actuellement grâce à certains composants temporaires, y compris des éléments électroniques de collecte de données et un détecteur à plan focal, alors le system est considéré comme prototype de laboratoire. Les essais préliminaires du prototype ont été exécutés au moyen de sources de rayonnement scellées et non scellées. Dans une chambre noire, le détecteur a pu produire l’image d’une source de 22 kBq (0,6 Ci), soit un détecteur de fumée à 241Am, à 1,5 m. De plus, après que la sensibilité de l’appareil eut été abaissée en réduisant la taille de son ouverture optique, on a constaté qu’il serait possible de détecter des sources d’un rayonnement aussi faible que 7 kBq (0,2 Ci). Les essais ont été exécutés dans les installations de RDDC Ottawa, en mars, en juin et en juillet 2011, au moyen de sources scellées de 241Am et petite surfaces préparées avec une source non scellée de 225Ac, soit un isotope radioactif de vie courte qui émet un rayonnement . En raison de problèmes liés à une fuite de lumière entre le filtre et le tube photomultiplicateur, ainsi qu’à l’élimination de toutes les sources de parasites lumineux, les mesures prises se sont avérées quelque peu incohérentes et la limite de détection plus élevée que prévue. D’autres essais effectués en juillet 2011 ont montré qu’un accroissement de l’épaisseur du filtre UG-11 a permis d’améliorer le rapport « signal-bruit » en présence de parasites lumineux issus de sources artificielles, mais pas en présence de parasites lumineux émis par la lumière naturelle du soleil.

Importance : L’imageur de fluorescence de l’air constituera un nouvelle outil permettant de produire rapidement des d’images de la contamination sur de vastes étendues. Il répondra ainsi au besoin important de détecter à distance la radioactivité, notamment lors d’enquêtes criminelles et d’activités de déclassement nucléaire, de même qu’à des fins de sécurité radiologique et de protection contre les dangers d’irradiation.

DRDC Ottawa TM 2011-195 v

Perspectives : La conception du détecteur tire à sa fin et devrait se terminer au début de 2012. Des essais initiaux du prototype final seront réalisés dans les installations de BTI, puis d’autres essais seront exécutés dans celles de RDDC Ottawa au moyen de sources de rayonnement scellées et non scellées. Enfin, le rendement du détecteur en campagne sera évalué lors d’essais menés dans des installations nucléaires comportant des zones contaminées.

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DRDC Ottawa TM 2011-195 vii

Table of contents

Abstract …….. ................................................................................................................................. iRésumé …..... ................................................................................................................................... iExecutive summary ....................................................................................................................... iiiSommaire ....................................................................................................................................... ivTable of contents ........................................................................................................................... viiList of figures .............................................................................................................................. viiiList of tables .................................................................................................................................. ix1 Introduction............................................................................................................................... 1

1.1 The detection challenge................................................................................................. 11.2 Detector concept............................................................................................................ 11.3 Project status.................................................................................................................. 1

2 Detector design overview ......................................................................................................... 32.1 Physical subcomponents and measurement concept ..................................................... 32.2 Optical design................................................................................................................ 42.3 Mechanical design ......................................................................................................... 6

3 Laboratory prototype testing..................................................................................................... 73.1 March 2011 – Am-241 sources in 5B............................................................................ 73.2 June 2011 – Ac-225 coupons and Am-241 sources in T-32.......................................... 9

3.2.1 Source and coupon preparation............................................................................ 93.2.2 Air fluorescence measurements ......................................................................... 113.2.3 Activity measurements and comparison with air fluorescence signals.............. 12

3.3 July 2011 – Am-241 in T-32 and BTI dark room........................................................ 134 Conclusions and future work .................................................................................................. 16References ..... ............................................................................................................................... 17Annex A Calculations for 225Ac solution preparation ................................................................. 19

viii DRDC Ottawa TM 2011-195

List of figures

Figure 1: The Multi-spectral Imaging System for the Detection of Radiological Contamination ............................................................................................................... 2

Figure 2: MISDORC conceptual design.......................................................................................... 3

Figure 3: MISDORC optical design ................................................................................................ 4

Figure 4: Temporary (left) and final (right) detection systems. ...................................................... 5

Figure 5: Fluorescence images of 241Am sources: a 21 kBq (0.58 Ci) point source (left) and a 190 kBq (5.1 Ci) 10 cm x 10 cm area source (right). The area source is pictured at centre [4]. .................................................................................................................. 7

Figure 6: A 1.85 MBq (50 Ci) 241Am source masked by dim room light (100 s exposure). Filtering, using a Fourier transform (centre) of the raw signal (left) reveals the fluorescence signal from the source (right) with interference from the room light eliminated [4]. ............................................................................................................... 8

Figure 7: Images of a 21 kBq (0.58 Ci) 241Am source concealed by books. Photo (left) and air fluorescence image (right) [4].................................................................................. 8

Figure 8: Ac-225 decay chain.......................................................................................................... 9

Figure 9: T-32 layout for coupon preparation and measurement .................................................. 10

Figure 10: Ac-225 coupon configurations, 10 cm x 10 cm (left), 5 cm x 5 cm (top centre), point source (bottom centre), and multiple source areas (right).................................. 11

Figure 11: Setup for testing in T-32, June 2011 ............................................................................ 11

Figure 12: Air fluorescence images of 5 cm x 5 cm (left) 10 cm x 10 cm (right) 225Ac coupons (nominal activities as indicated).................................................................................. 12

Figure 13: Variation of fluorescence signals from 241Am sources with UG-11 filter thickness.... 14

Figure 14: Effect of varying UG-11 filter thickness on attenuation of LED interference (top) and sunlight interference (bottom). ............................................................................. 15

DRDC Ottawa TM 2011-195 ix

List of tables

Table 1: MISDORC physical subcomponents................................................................................. 4

Table 2: Optical design parameters ................................................................................................. 5

Table 3: Am-241 sources used in testing......................................................................................... 7

Table 4: Ac-225 coupons prepared and nominal alpha activities.................................................. 10

Table 5: HPGe measurements of 225Ac coupons ........................................................................... 13

Table 6: Comparison of relative activities and relative fluorescence intensities of coupons ........ 13

x DRDC Ottawa TM 2011-195

Acknowledgements

Funding for this research was provided by the CBRNE Research and Technology Initiative (CRTI) under project CRTI-07-0104TD. Thanks go out to the following members of the project team for their contributions to the experimental plan and participation in the testing of the laboratory prototype: Patrick Forget, Vern Koslowsky, Michael Dick, and Mike Robins (BTI); Larry Wong (CNSC); and Roger Hugron (DGNS).

DRDC Ottawa TM 2011-195 1

1 Introduction

1.1 The detection challenge

The detection and delineation of radiologically contaminated areas poses a significant challenge, particularly when the radioisotopes involved emit short-range alpha or beta radiation with little or no gamma signature. Identifying contaminated areas is important in a number of applications, including criminal and national security investigations and nuclear energy decommissioning activities. The investigation following the 2006 poisoning of Russian national Alexander Litvinenko in the United Kingdom provided a clear example of the deficiencies of currently available alpha detectors.

The radioisotope used to poison Litvinenko, 210Po, emits alpha radiation with only an extremely weak gamma signature. In order to detect the presence of 210Po in the field, a suspect area must be surveyed inch-by-inch at close proximity to the surface, an extremely time-consuming process. A painstaking investigation, requiring the efforts of large numbers of radiation detection personnel over several weeks, revealed multiple trails of 210Po leading in and out of London, with contamination discovered in locations such as restaurants, hotel rooms, offices, cars, and even airplanes.

1.2 Detector concept

A previous project (CRTI-01-0203RD), funded under the CBRNE Research and Technology Initiative (CRTI), developed the Simultaneous Multi-Spectral Imager (SMSI). This detector employs a novel technique: instead of detecting the radiation directly, it measures an optical signal that originates as a result of the excitation of the air surrounding a radioactive source, resulting in detection at a range greater than that of the radiation itself. In field tests, SMSI proved capable of detecting unsealed alpha emitters from over one kilometre, much further than the range of alpha particles in air (a few centimetres).

While SMSI was designed for long-range detection of radiological contamination, this project aims to provide a capability for rapid near-field detection and imaging through the development of the Multi-spectral Imaging System for the Detection of Radiological Contamination (MISDORC). The system is designed to be set up quickly and able to be run with minimal training. The user will initiate an automatic scan of an area of interest and the system will identify areas contaminated with alpha-emitting (and possibly beta-emitting) radioisotopes. The time required to scan a room for contamination could potentially be reduced from a number of hours (using conventional detectors) to tens of minutes. The fluorescence signal is relatively weak, so measurements must be carried out in the dark, although a certain amount of artificial light can be filtered from the signal.

1.3 Project status

CRTI-07-0104TD [1] began in 2009 and is scheduled to be completed in the summer of 2012. The optical and mechanical design and construction of the detector are complete, as pictured in

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Figure 1. Preliminary testing has been carried out using a temporary detection system and electronics; the system as tested is referred to as the laboratory prototype. Under darkroom conditions, the detector has proven capable of imaging a 22 kBq (0.6 Ci) 241Am smoke detector source at 1.5 m and deliberate reduction in the device sensitivity achieved by reducing the optical aperture indicates that sources as small as 7 kBq (0.2 Ci) would be detectable. This report describes the detector design in general terms and provides results from three tests carried out at DRDC Ottawa. Further details of the detector design, the concept of use, and the results of the final prototype testing will be presented in the project’s final report.

Figure 1: The Multi-spectral Imaging System for the Detection of Radiological Contamination


2 Detector design overview

2.1 Physical subcomponents and measurement concept

The conceptual design of the air fluorescence imager is shown in Figure 2. As pictured at left, the optical components are mounted on a motorized pan/tilt device. The assembly is shielded from dust and light by an outer enclosure that is not pictured here. The right panel of the figure gives a cross-sectional view of the mounted optical components and electronics.

Figure 2: MISDORC conceptual design

Table 1 lists the primary subcomponents of the MISDORC detector. A potentially contaminated surface is not imaged using a continuous scan, but rather by using a series of overlapping images, or scan segments, taken across the user-defined measurement area. The measurement process begins with the user selecting the boundary points of the scan. This is done using the alignment lasers, directed by the pan/tilt motor, which is controlled manually via a wireless tablet PC.

The control system records the pan and tilt angles at each vertex, and the user initiates the setup sequence using the tablet PC. The ultrasonic range-finder then measures the shortest distance to the surface. The position of and distance to each scan segment are calculated from the measured shortest distance, pan/tilt angles, and instrument field of view. An image of the surface is captured using the webcam; this image is used as a backdrop for the UV images from the surface scan when measurements are complete.

When the control system indicates that the system is ready, the user extinguishes the room lights and initiates the scan with the tablet PC. The over-exposure detection circuit verifies that the room is sufficiently dark for photomultiplier use before the scan begins. For each overlapping exposure taken during the measurement sequence, the focal-plane detector is adjusted for optimum focus. Background images are acquired with the PMT shutter closed and are interleaved with the UV images. The entire measurement sequence takes several minutes for a 2 m x 3 m surface. While all sub-components are fully integrated in the laboratory prototype, the

Tripod

Pan/tilt unit

Imager


control system as described will only be completed for the final prototype, so automated surface measurements were not carried out in the trials described here. Instead, single measurements were taken by manually directing the optics at the object of interest [2].

Table 1: MISDORC physical subcomponents

Physical Subcomponent FunctionAlignment Lasers Visible alignment aid.Ultrasonic Range-Finder Measures standoff distance for focusing purposes.Pan/ Tilt Unit Permits remote adjustment of the optical line of sight.Webcam Photographs the wall under normal room lighting conditions.

Focus Adjustment Adjusts position of focal plane detector, based on the distance provided by the range finder, to achieve optimal image focus.

Tablet PC Allows the user to control the pan/tilt unit locally and to control the measurement process.

Over-Exposure Detection Circuit

Prevents PMT overexposure and irreparable damage to the PMT.

PMT Data Acquisition System

Reads out image acquired by photomultiplier tubes.

Laptop Receives all measured images (UV and visible)

2.2 Optical design

The optics, pictured in Figure 3, consist of seven lenses, six spherical and one aspherical, made from fused silica in order to provide high light transmission in the UV. Anti-reflection coatings on all lenses help to minimize light loss at each lens surface. An optical filter in front of the image plane helps to attenuate unwanted stray light.

Figure 3: MISDORC optical design


Table 2 provides the specifications for a typical standoff distance of 1.5 m. The small F/# value is indicative of the high light-collection efficiency of the optics. The field of view is ±17º, resulting in a measurement area of 0.94 m along the diagonal at a 1.5 m standoff distance. The one aspherical component is a phase mask, a specially contoured lens that improves the depth-of-field through a controlled blurring of the image that is amenable to numerical correction. The phase mask can be removed and replaced with a blank as desired to provide a conventional imaging capability.

Table 2: Optical design parameters

Parameter Value Parameter Value

Magnification 0.15 Effective focal length 188.4 mm Entrance pupil 200 mm Maximum radial field (full

diagonal) 940 mm

F/# 1.23 Field of view (half angle) ~17 degrees Total track length 345 mm Weight of glass 9.2 kg Number of spherical / aspherical lenses

6 / 1 Operating wavelength 280-360 nm

In terms of resolution, the focal plane detector, rather than the optics, is the limiting factor. For the final prototype, each pixel will correspond to an object about 2 cm in diameter at 1.5 m, which corresponds well with the range of alpha particles in air. The laboratory prototype tested here employs a temporary detection system, shown on the left in Figure 4. It consists of one 8x8 PMT (Hamamatsu 8500-03) covering 5 cm x 5 cm with 64 pixels. The 8x8 PMT was mounted temporarily in the final mechanical assembly and image readout was accomplished through an off-the-shelf data acquisition system. The final detection system, shown in the right panel of Figure 4, will consist of four 16x16 PMTs (Hamamatsu H9500-03), covering 10 cm x 10 cm with 1024 pixels, and will use a custom on-board data acquisition system designed by BTI [3].

Figure 4: Temporary (left) and final (right) detection systems.


2.3 Mechanical design

As shown in Figure 2, the optical module is supported by a tripod and directed at the target by a pan/tilt unit. The optical module of the air fluorescence imager has two principal subassemblies: one for mounting the optics, previously pictured in Figure 3, and one for supporting the PM tubes and electronics. Three circuit boards stacked behind the optical assembly carry the data acquisition electronics, and the card nearest the optics supports the four pixelated PMTs. To focus, a stepping motor acting on a lead screw moves the stack of circuit cards forward and backward on four linear bearings. Other important components include the base plate, which supports the optics and attaches to the pan/tilt unit and tripod, rubber-covered feet for bench-top use, and robust handles for moving and installing the optical assembly. Light-tight covers, which slide over the lens assembly, are not pictured [2, 3].


3 Laboratory prototype testing

3.1 March 2011 – Am-241 sources in 5B

The first in-house tests were carried out in a windowless room in the basement of building 5B at DRDC Ottawa. A number of 241Am sources (point sources and area sources) of activities ranging from approximately 21–1850 kBq (0.58–50 Ci) were used, as listed in Table 3. The sources were placed vertically, approximately 1.5 m from the detector. As the system is not yet capable of automated scans, measurements consisted of a single image (generally a 10 second exposure) taken with room lights extinguished. Examples of fluorescence images of point and area 241Am sources are shown in Figure 5. The point source (left) and area source (right) are easily distinguished despite the limited resolution of the laboratory prototype’s focal plane detector.

Table 3: Am-241 sources used in testing

Source Description Activity

[MBq ( Ci)] 10x10 cm Analytics area source 0.19 (5.1) 0.013 (0.34) Point source (~1 cm diameter) from NRD Static Control 1.85 (50) 0.30 (8) Smoke detector source 0.022 (0.6) 9 cm diameter Petri dish with evaporated 241Am 0.37 (10) 0.74 (20)

Figure 5: Fluorescence images of 241Am sources: a 21 kBq (0.58 Ci) point source (left) and a 190 kBq (5.1 Ci) 10 cm x 10 cm area source (right). The area source is pictured at centre [4].

Figure 6 shows the effect of dim room light on the fluorescence signal from a 1.85 MBq (50 Ci) source (left). For artificial interfering light of constant frequency, a Fourier transform (centre) can be employed to separate the raw fluorescence data from the unwanted interfering light. In this case, the filtered image is shown at right in Figure 6 [4].


Figure 6: A 1.85 MBq (50 Ci) 241Am source masked by dim room light (100 s exposure).

Filtering, using a Fourier transform (centre) of the raw signal (left) reveals the fluorescence signal from the source (right) with interference from the room light eliminated [4].

For the final test, a 21 kBq (0.58 Ci) 241Am smoke detector source was placed in a recess between two books, as in Figure 7. A source (or loose contamination) in this configuration would be particularly difficult to detect using a conventional contamination meter, which must come within a few centimetres of the source in order to detect the emitted alpha radiation. As shown at right, the source is clearly visible despite the fact that the active area of the source is directed away from the detector. An ADM-300 survey meter equipped with an ABP-100 / probe failed to detect the source.

Figure 7: Images of a 21 kBq (0.58 Ci) 241Am source concealed by books. Photo (left) and air fluorescence image (right) [4].


3.2 June 2011 – Ac-225 coupons and Am-241 sources in T-32

3.2.1 Source and coupon preparation

In order to provide a more realistic simulation of distributed alpha contamination, trials were planned using a short-lived alpha emitter, actinium 225 (225Ac, t1/2 = 10.0 days), in addition to the sources listed in Table 3. Because 225Ac has no long-lived daughter isotopes, it is possible to prepare coupons for measurement with no risk of long-term contamination of the experimental area. The decay chain, shown in Figure 8, includes both alpha- and beta-emitting daughters.

Figure 8: Ac-225 decay chain

Due to the elevated risk involved in the use of unsealed alpha emitters, which have high ingestion and inhalation dose coefficients and therefore low Annual Limits on Intake (ALIs), a radiation practice management plan was prepared and approved by Director General Nuclear Safety (DGNS) for this experiment [5]. The layout of T-32 for the preparation of contaminated coupons and air fluorescence measurements is illustrated in Figure 9. The testing was carried out over the course of three days, with the coupons being prepared on the first day and allowed to dry overnight, measurements carried out with the laboratory prototype on the second day, and measurements taken with the Trans-SPEC high-purity germanium (HPGe) gamma spectrometer and / contamination meters on the third day.

Oak Ridge National Laboratory in the US supplied 18.5 MBq (0.5 mCi) of 225Ac, which was shipped as dried nitrate in a vial. Details of the calculations for the preparation of solutions are provided in Annex A. The stock solution was prepared using the entire contents of the vial. Distilled water was added to the vial to dissolve the dried nitrate, and the resulting solution was added to the primary solution (referred to as Solution A).

The nitrate proved more difficult to dissolve than was expected, so after rinsing five times with distilled water (with gentle agitation the last two times) a significant amount of 225Ac nitrate remained in the vial. Based on readings taking with the RadEye PRD, which were influenced by proximity to other radiation sources and therefore of limited accuracy, we estimate that 50-80% of the 225Ac was removed from the vial. In the future, an acidic solution should be used to dissolve the actinium nitrate prior to removal from the vial. The removal efficiency could have been measured more precisely if the vial had been assayed with the Trans-SPEC both before and after source dilution; however, the absolute activities of the coupons are not crucial to this experiment, as the relative activities of the solutions are known and the impact of varying the total activities and surface activity concentrations (Bq/cm2) on the observed air fluorescence can still be evaluated.


Figure 9: T-32 layout for coupon preparation and measurement

Four solutions were prepared with nominal specific activities of 8 Ci/ml (Solution A), 2 Ci/ml (Solution B), 0.5 Ci/ml (Solution C), and 0.125 Ci/ml (Solution D). The coupons were then contaminated using multi-tip and single-tip pipettes with various configurations and total activities. The geometries and activities prepared are listed in Table 4, below, while Figure 10 shows the layout of the various coupon types. The coupons were prepared on the first day and allowed to dry overnight. Because the point sources required a larger volume of solution to achieve the desired total activity, white glue was used to form a 1 cm diameter circle on the coupon surface to keep the solution from spreading while it dried.

Table 4: Ac-225 coupons prepared and nominal alpha activities

Coupon size (cm)

Coupons required Source area (cm)

Approx. nominal activities* (MBq [ Ci])

10 x 10 7 Point (~1 cm2) 0.04, 0.15, 0.52 [1, 4, 14] 5 x 5 0.04, 0.15, 0.52, 2.1 [1, 4, 14, 58] 30 x 30 9 10 x 10 0.15, 0.52, 2.1, 8.5 [4, 14, 58, 230] Multiple Various**

*Because 225Ac has three alpha-emitting daughters, the total alpha activity is four times the 225Ac activity. **The five 30 x 30 coupons with multiple source areas were prepared as follows:

1. Two 5x5 sources, 0.52 and 2.1 MBq (14 and 58 Ci) 2. Two 5x5 sources, 0.04 and 0.15 MBq (1 and 4 Ci) 3. Four 5x5 sources, 0.04, 0.15, 0.52, and 2.1 MBq (1, 4, 14, and 58 Ci) 4. One each of point, 5x5, 10x10 sources, all 0.52 MBq (14 Ci) 5. One each of point, 5x5, 10x10 sources, all 0.15 MBq (4 Ci)


Figure 10: Ac-225 coupon configurations, 10 cm x 10 cm (left), 5 cm x 5 cm (top centre), point source (bottom centre), and multiple source areas (right)

3.2.2 Air fluorescence measurements

T-32 was prepared for the air fluorescence measurements by blocking all light leaks with a combination of duct tape and black garbage bags, as visible in Figure 11. Once this was complete, no light was visible to the naked eye. Measurements were taken with sources and coupons mounted on a retort stand visible at the right of Figure 11. Despite the low-light conditions, the measurements were hampered by a highly variable background and light scatter, and the detector was unable see sources that were detected under darkroom conditions, where a 0.022 MBq (0.6 Ci) 241Am source was clearly visible.

Figure 11: Setup for testing in T-32, June 2011

The laboratory prototype was able to image several of the stronger 241Am sources and 225Ac coupons. Images of the two strongest 5 cm x 5 cm and 10 cm x 10 cm 225Ac source areas are


shown in Figure 12. Investigation following the tests revealed a poor light seal around the edge of the filter and PMT, which certainly had a significant impact on the quality of the measurements. This will be resolved in the final prototype by permanently bonding and sealing the filter to the PMT. In addition, small sunlight leaks were observed in T-32, particularly around the entry door. The amount of light entering around the door varied with the position of the sun, contributing to the changeable background and light scatter.

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Figure 12: Air fluorescence images of 5 cm x 5 cm (left) 10 cm x 10 cm (right) 225Ac coupons (nominal activities as indicated).

3.2.3 Activity measurements and comparison with air fluorescence signals

On the third day of the experiment, the 225Ac coupons were measured using the Trans-SPEC-100 (HPGe), ADM-300 with ABP-100 ( / ) probe, and RadEye AB100 ( / ). Only those of simple geometry (point source, 5 cm x 5 cm, and 10 cm x 10 cm) were measured (i.e. not the multiple


source area coupons). The / measurements were intended to give an indication of the relative alpha activities of the coupons, but the higher-activity coupons overloaded the detectors, so no reliable results were obtained. Gamma spectra were collected using the Trans-SPEC high-purity germanium gamma spectrometer. As shown in Table 5, the measured relative activities of the coupons are in good agreement with the nominal relative activities.

Table 5: HPGe measurements of 225Ac coupons

Ratio (relative to highest-activity coupon)

Coupon

Nominal activity (MBq)

217At 218 keV (net counts)

213Bi 440 keV (net counts)

Nominal activity Net peak counts (avg. of 2 peaks)

1A 0.53 3195 3940 1.00 1.00 1B 0.13 743 905 0.25 0.23 1C 0.03 300 317 0.06 0.09 5A 2.13 14206 17229 1.00 1.00 5B 0.53 3816 4725 0.25 0.27 5C 0.13 956 1138 0.06 0.07 5D 0.03 749 857 0.02 0.05

10A 8.52 47480 58189 1.00 1.00 10B 2.13 13321 16128 0.25 0.28 10C 0.53 3365 4251 0.06 0.07 10D 0.13 1096 1386 0.02 0.02

The problems encountered when carrying out the air fluorescence measurements, including inconsistent background and light scatter, made it difficult to quantitatively compare the fluorescence signals from the lower-activity coupons. Clear fluorescence signals were, however, observed from several sources, including the two strongest 5 x 5 cm and 10 x 10 cm sources (as shown in Figure 12). The net fluorescence signal from these sources was integrated over the 4 x 4 pixel area of greatest signal intensity. For a given geometry, the intensity of the fluorescence signal is expected to be proportional to the total activity. For these pairs of 5 cm x 5 cm and 10 cm x 10 cm coupons, the relative activities and relative fluorescence signals were comparable, as shown in Table 6.

Table 6: Comparison of relative activities and relative fluorescence intensities of coupons

Coupon ratio Ratio of nominal

activities Ratio of measured

activities Ratio of air

fluorescence intensity 5B / 5A 0.25 0.23 0.25

10B / 10A 0.25 0.27 0.41

3.3 July 2011 – Am-241 in T-32 and BTI dark room

Following the testing in T-32 in June 2011, where interference from scattered light was an issue, further testing was carried out in July 2011 to explore the effect of varying the UG-11 filter thickness on the performance of the detector. Tests were carried out both under controlled conditions in the dark (windowless) room at BTI and under field conditions in T-32 at DRDC


Ottawa. A series of measurements was taken of the 241Am sources used in the previous tests with filter thicknesses from 1 mm to 3 mm. For the measurements taken in T-32, the detector was situated at 1.55 m from the source, and all LEDs and other artificial light sources were left uncovered. As in the June trials, external sunlight was blocked out to the extent possible. Selected fluorescence images from the T-32 and BTI testing are shown in Figure 13.

Filter 8 Ci – DRDC Ottawa T-32 0.6 Ci – BTI dark room

1 mm

2 mm

3 mm

0

5

10

15

20

25

30

35

40

45

Figure 13: Variation of fluorescence signals from 241Am sources with UG-11 filter thickness

A quantitative analysis of the data (see graphs in Figure 14) shows that increasing the filter thickness results in significant attenuation of artificial light (such as interference from LEDs), resulting in an improved signal-to-noise ratio (SNR). When the interfering light is natural sunlight, however, a thicker filter provides no improvement in the SNR. This is a result of the scattered sunlight having spectral features in the UV bandpass of the UG-11 filter. Increasing the filter thickness too much is detrimental to the SNR as the signal is also attenuated, bringing it closer to the baseline electronic noise level of the system. Based on these results, an additional


optical element (for the phase mask blank position) is being manufactured with a 2 mm UG-11 filter. This element will allow for additional attenuation of interfering artificial light when necessary.

Figure 14: Effect of varying UG-11 filter thickness on attenuation of LED interference (top) and

sunlight interference (bottom).


4 Conclusions and future work

The initial tests conducted in the basement of 5B in March 2011 provided a good demonstration of the capabilities of the laboratory prototype. In particular, the ability of the detector to detect an alpha source in a difficult geometry was demonstrated, as was the effectiveness of an algorithm to reduce interference from artificial room light using a Fourier transform. No particular technical challenges were encountered in this testing, and the detector performance was comparable to that observed in testing at BTI.

While the room in 5B where the March tests were carried out was completely devoid of sunlight, this was not the case for the June tests in T-32 in the RAD compound. Although efforts were made to eliminate all sources of stray light, it was evident that even though the room appeared completely dark to the naked eye, it was impossible to completely eliminate sunlight interference. This interference, combined with problems with the PMT filter seal, resulted in inconsistent measurement results and higher than expected limits of detection. The June tests did, however, provide an opportunity to work with an unsealed -emitter, 225Ac, for the first time. Procedures for preparing radioactive solutions and contaminated coupons were validated, and this experience will prove valuable in future experiments.

Testing in July aimed to explore the effect of varying filter thickness on attenuation of unwanted interfering light. While increased filter thickness did not result in any improvement of the signal-to-noise ratio when the interference was due to natural light, it did result in an improved signal when the interference came from artificial sources such as LEDs. A new filtered blank for the phase mask position is being manufactured, which will allow filter thickness to be varied as an additional parameter in future tests.

The final prototype is scheduled to be completed in early 2012. Following testing at BTI, the detector will be tested and characterized at DRDC Ottawa using 241Am and 225Ac sources similar to those used in the June 2011 tests. Additionally, more realistic trials are planned which will involve the use of 225Ac to contaminate surfaces and objects such as tables, door handles, and telephone receivers, effectively simulating a contaminated room such as might have been encountered during the Litvinenko investigation. Finally, planning is also in progress for testing at existing contaminated sites at nuclear facilities in Canada and the UK.


References .....

[1] E. Inrig, T. Cousins (2007), Project Charter: Multi-spectral Imaging System for the Detection of Radiological Contamination (CRTI-07-0104TD), Defence R&D Canada.

[2] E. Inrig, L. Erhardt, V. Koslowsky, B. Andrews, H. Ing, M. Dick and P. Forget (2011), “An air fluorescence imaging system for the detection of radiological contamination,” In Proceedings of SPIE 8018, 80180G, 2011, doi:10.1117/12.887352.

[3] Bubble Technology Industries Inc. (2011), Multispectral Imager for the Detection of Radiological Contamination: Fiscal Year Interim Report for DRDC Ottawa (Contract #W7714-081106/001/SQ), Bubble Technology Industries Inc.

[4] E. Inrig, V. Koslowsky, B. Andrews, M. Dick, P. Forget, H. Ing, R. Hugron, and L. Wong “Development and Testing of an Air Fluorescence Imaging System for the Detection

of Radiological Contamination”,

[5] E. . and Watson. (2011), Preliminary testing of MISDORC using sealed and unsealed sources (DRDC Ottawa TM 1548-20) Defence R&D Canada – Ottawa.

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DRDC Ottawa TM 2011-195


DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)

1. ORIGINATOR (The name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Centre sponsoring a contractor's report, or tasking agency, are entered in section 8.) Defence R&D Canada – Ottawa 3701 Carling Avenue Ottawa, Ontario K1A 0Z4

2. SECURITY CLASSIFICATION (Overall security classification of the document including special warning terms if applicable.)

UNCLASSIFIED

3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S, C or U) in parentheses after the title.) Design and preliminary testing of the Multi-spectral Imaging System for the Detection of Radiological Contamination

4. AUTHORS (last name, followed by initials – ranks, titles, etc. not to be used) Inrig, E.; Watson, I.; Koslowsky, V.; Dick, M.; Forget, P.

5. DATE OF PUBLICATION (Month and year of publication of document.) December 2011

6a. NO. OF PAGES (Total containing information, including Annexes, Appendices, etc.)

3

6b. NO. OF REFS (Total cited in document.)

5

7. DESCRIPTIVE NOTES (The category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered.) Technical Memorandum

8. SPONSORING ACTIVITY (The name of the department project office or laboratory sponsoring the research and development – include address.) Defence R&D Canada – Ottawa 3701 Carling Avenue Ottawa, Ontario K1A 0Z4

9a. PROJECT OR GRANT NO. (If appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant.)

CRTI-07-0104TD

9b. CONTRACT NO. (If appropriate, the applicable number under which the document was written.)

10a. ORIGINATOR'S DOCUMENT NUMBER (The official document number by which the document is identified by the originating activity. This number must be unique to this document.) DRDC Ottawa TM 2011-195

10b. OTHER DOCUMENT NO(s). (Any other numbers which may be assigned this document either by the originator or by the sponsor.)

11. DOCUMENT AVAILABILITY (Any limitations on further dissemination of the document, other than those imposed by security classification.)

Unlimited

12. DOCUMENT ANNOUNCEMENT (Any limitation to the bibliographic announcement of this document. This will normally correspond to the Document Availability (11). However, where further distribution (beyond the audience specified in (11) is possible, a wider announcement audience may be selected.)) Unlimited

13. ABSTRACT (A brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual.)

Field detection and measurement of the activity and extent of alpha ( ) contamination is a particularly challenging task for radiation detection personnel. With funding from the CBRNE Research & Technology Initiative (CRTI-07-0104TD), the Multi-spectral Imaging System for the Detection of Radiological Contamination (MISDORC), is being developed with the aim of providing a new tool for in-field alpha detection. This detector uses the air fluorescence in the vicinity of the radioactive source to create an image of the contamination. The optical and mechanical design of the detector are briefly described here, along with the concept of use. The detector is currently at the laboratory prototype stage, and the results of preliminary tests of the prototype, using both sealed and unsealed sources, are presented. The detector is now in the final stage of development and is expected to be completed in early 2012.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

radiation detection; alpha radiation; radiological contamination; air fluorescence

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