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Lightweight aerial vehicles for monitoring, assessment and mapping of radiation anomalies
Journal of Environmental Radioactivity 136 (2014) 127e130
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Lightweight aerial vehicles for monitoring, assessment and mapping of radiation anomalies J.W. MacFarlane a, *, O.D. Payton a, A.C. Keatley a, G.P.T. Scott b, H. Pullin a, R.A. Crane c, M. Smilion d, I. Popescu d, V. Curlea e, T.B. Scott a a
Interface Analysis Centre, HH Wills Physics University of Bristol, UK Imperial College London, UK Water Research Laboratory, University of New South Wales, Australia d National Institute For Metals and Radioactive Resources Bucharest, Romania e National Company of Uranium, Romania b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2014 Received in revised form 8 May 2014 Accepted 12 May 2014 Available online
The Fukushima Daiichi nuclear power plant (FDNPP) incident released a significant mass of radioactive material into the atmosphere. An estimated 22% of this material fell out over land following the incident. Immediately following the disaster, there was a severe lack of information not only pertaining to the identity of the radioactive material released, but also its distribution as fallout in the surrounding regions. Indeed, emergency aid groups including the UN did not have sufficient location specific radiation data to accurately assign exclusion and evacuation zones surrounding the plant in the days and weeks following the incident. A newly developed instrument to provide rapid and high spatial resolution assessment of radionuclide contamination in the environment is presented. The device consists of a low cost, lightweight, unmanned aerial platform with a microcontroller and integrated gamma spectrometer, GPS and LIDAR. We demonstrate that with this instrument it is possible to rapidly and remotely detect ground-based radiation anomalies with a high spatial resolution (<1 m). Critically, as the device is remotely operated, the user is removed from any unnecessary or unforeseen exposure to elevated levels of radiation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Radiation monitoring Unmanned aerial system Characterisation Gamma spectrometry
1. Introduction Following the large release of radio-active material at the FDNPP incident (Stohl et al., 2012; Chino et al., 2011; Hirao et al., 2013; Terada et al., 2012; Schoeppner et al., 2013; Morino et al., 2011) only a limited amount of data relating to the quantity and geographical distribution of the released radiation was available to decision makers of evacuation and disaster response (Omoto, 2013; Nuclear Accident Independent Investigation Commission, 2012; Povinec et al., 2013). Static monitoring infrastructure surrounding the site, which had traditionally been used for routine plant monitoring, was compromised following the tsunami, with 23 of the 24 monitoring points rendered inoperable (Omoto, 2013). This resulted in an effective data blackout for radiological information
* Corresponding author. Interface Analysis Centre, HH Wills Physics University of Bristol, BS8 1TL, UK. Tel.: þ44 117 331 1171. E-mail address: [email protected] (J.W. MacFarlane). http://dx.doi.org/10.1016/j.jenvrad.2014.05.008 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
pertaining to the incident, in the hours and days following. It was not until four days following the event, that the first data revealing the extent of radiological contamination in the surrounding area was recorded. This data was produced by a fleet of 15 cars equipped with GM tubes reporting measurements 20 km away from the site (Povinec et al., 2013). Data gathered in this manner presented a potentially significant dose hazard to the operators in return for a spatially limited data set (Nuclear Accident Independent Investigation Commission, 2012). Traditional airborne measurements to identify highly contaminated areas were conducted (Yoshida and Kanda, 2012; Lyons and Colton, 2012) but the results were not made available until 11 days after the incident. These flights used extremely expensive equipment, carrying sensitive and heavy, volume-style radiation detectors, operating at a relatively high altitude (150e300 m) above the surface (Povinec et al., 2013). Resultantly all the recorded data was of low (km scale) spatial resolution and limited to within 30 km of the site (Povinec et al., 2013). The available data, which were obtained through the use of weather prediction models, and assessments of likely released
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radionuclides, were used by advisers to inform the evacuation plan for the surrounding population and the designation of exclusion zones where data readings were not available (Terada et al., 2012; Morino et al., 2011; Povinec et al., 2013). Worryingly, if a similar disaster were to occur today, we still lack the necessary tools to rapidly provide a time resolved, high spatial resolution and accurate plot of radiation released into the environment. Furthermore, traditional radiological assessment techniques would typically subject the operator, whether a driver or pilot, to an unknown and potentially significant radiation dose. The use of unmanned aerial systems can offer an interesting solution into the detection of radiation in the environment. There have been several previous studies examining aspects of this topic, these include examples which have explored the use of fixed wing aerial vehicles, (Kurvinen et al., 2005; Pllnen et al., 2009) or indoor UAV system (Boudergui et al., 2011), or even very large aircraft (Barnes and Austin, 2009). On these systems a variety of sensor arrays have been tested including air sampling sensors for the detection of airborne radiation particles (Pllnen et al., 2009; Pllnenb et al., 2009) and scintillating devices (Kurvinen et al., 2005). Fixed wing aircraft (Kurvinen et al., 2005; Pllnen et al., 2009) typically operate at higher altitude (up to 4500 m), and move at a minimum ground speed of 90 km h1. Here we present a new instrument for detection and assessment of radionuclide contamination in the environment, using small multi-rotor unmanned aerial systems. The instrument incorporates a microcontroller operated, lightweight, low volume, semi-conductor gamma ray spectrometer integrated with a small aerial platform. The instrument securely transmits the location, identity and intensity of radionuclide contamination to a remote operator or base station. Implementing small multi-rotor unmanned aerial systems, and the benefit these systems bring including reduced operation speed and greater manoeuvrability, allows for the device to produce high spatial resolution maps of radiological contamination in the environment. 2. Experimental The aerial platform consists of a modified multi-rotor (six propeller) aerial vehicle (Hexa XL, Mikrokopter) which records the GPS location of the instrument at a high frequency (10 Hz). At each recorded location a spectrum of the energy of incoming incident radiation is recorded (GR1, Kromek). The height of the device above the surface (±10 mm at <100 m) is simultaneously measured and recorded via the use of LIDAR (AR2500, Acuity). An Arduino mega ADK microcontroller unit is used to combine the data streams (GPS position, LIDAR height, radiation spectra). The data is stored locally on the instrument and concurrently transmitted to the user in real time (500 ms delay) as a 128 bit secured encrypted data stream, to a remote base station which can be up to 7 km from the instrument used to control the device. The payload, consisting of the LIDAR and
gamma spectrometer and associated microcontroller, is mounted on a gimbaled stage such that the spectrometer and LIDAR remain directed normal to the surface, regardless of pitch of the aerial platform (see Fig. 1). The system is powered by two 7.4 V lithium polymer batteries giving, currently, a total survey flight time of upto 12 min, with a maximum aerial speed of approximately 25 m s1. The presented system has a theoretical ability to survey 18 km of flight path in one flight, however, realistically the survey is typically operated at a slower speed to increase the sensitivity of the instrument. Typically areas of several tens to a hundred meters squared can be surveyed in one continuous flight before the batteries of the system need to be replaced. The batteries take typically 30 min to recharge, requiring 6 batteries in rotation to provide continuous survey coverage. Complementary sensors can be added to the device including thermal and visual cameras. The system can be operated manually, using traditional radio-controls or semi autonomously via programmed GPS way-points. Utilizing simple interface software, these way-points may be pre-selected or transmitted to the instrument in flight. Way-point matrices can generate survey routes that provide detailed geographical coverage of a designated area. The way-points can include automated landing and take-off, such that the device can gather long exposure gamma radiation spectra at a the region of interest. In this case the payload has a relatively small power requirement such that the system may install itself as a static ground-level monitoring point, by landing and reducing flight energy expenditure, for an extended period (weeks). Where landing is not possible, or appropriate, the instrument can hover and autonomously maintain its position over a set location even in severe weather conditions due to its low aerodynamic cross-section. Where possible, typically the instrument is operated at low altitude (<3 m) in order to maximise the radiation sensitivity, reduce the effects of background radiation shine on the instrument and to increase the spatial resolution of the data. Operating at <3 m altitude is only possible depending on the environment the instrument is functioning in. At the lower altitude care must be taken that the instrument does not collide with an obstructing obstacle, which currently is determined by the operator.
3. Materials and methods The source samples used within this study were specimens collected from the Cornubian batholith, Southwest UK. The batholith consists of six major and several smaller bodies of granite, the larger bodies of granite, from east to west are Dartmoor (600 km2), Bodmin moor (190 km2), St Austell (85 km2), Carnmenellis (130 km2), Lands End (190 km2), and the Isles of Scilly (area not defined). The St Austell granitic intrusion, from which the majority of the samples used in the study arise, is a small composite body
Fig. 1. The radiation detection system, left displays an overview of the system, right an exploded view of payload. a) hexacopter aerial platform, b) integrated payload c)gimbaled stage, d) LIDAR detector e) GPS board f) gamma ray spectrometer g) wireless transmitter h) payload housing.
J.W. MacFarlane et al. / Journal of Environmental Radioactivity 136 (2014) 127e130
129
(85 km2) that contains most of the Li-mica granite, with associated uranium bearing minerals, the Cornubian batholith. The gamma ray spectrometer (GR1 Kromek) used in this study was a small volume (100 mm3) CZT coplanar-grid detector. The energy range of the spectrometer is 30 keV to 3.0 MeV, with an energy resolution of 2.0 2.5% FWHM @ 662 keV. The electronic noise on the detector is <10 keV FWHM. 4. Results and discussion Fig. 2 demonstrates a simple experiment to demonstrate the capability of the instrument. Naturally radioactive rock samples containing uranium bearing minerals and associated daughter products were arranged within a 20 cm2 region of a marked 20 m2 geometric grid. The total activity of the test specimens was recorded as 260 Bq, representing a relatively weak radiation source. The associated gamma spectra, 800 s collection, of the minerals is shown in Fig. 2a, recorded decay energies of the radioisotopes: Bi
Fig. 2. Distribution of radiation mapped by the system. a) Spectra of radiation source, b) intensity map produced by instrument, c) hand survey data for comparison.
Fig. 3. Distribution of radiation mapped by the system. a) intensity map produced by instrument, b) hand survey data for comparison.
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Fig. 4. Diagrammatic representation of the reduction of ray path sampled as the detector plate is increased in distance from the source.
214, Pb 214, Ra 226, Th 230, Th 234, U 235. The grid test area was autonomously mapped using a pre-programmed GPS flight path at an altitude of 2.5 m. The radioactivity distribution map produced by the instrument is shown in Fig. 2b. The data are validated by comparison data with a coincident ground based survey area with a detector height of 1 m (Fig. 2c). The same detector was used for both the ground and aerial surveys. With the radiation sources located together towards the top left of the test grid, the arising data for both aerial and ground surveys is closely correlated (Fig. 2). The data records a clearly defined radioactive hot-spot in the same location in both surveys. A clear and rapid reduction in the recorded radiation intensity with distance from the test source is recorded, following the physically predicted z law (where z represents altitude/distance from the source). Fig. 3 illustrates a more complex radiological contamination analogy. The uranium bearing rock samples were arranged in two distinct 20 cm2 regions within the 20 m2 geometric grid. The two regions had differing activities, with location I having an activity of: 90 Bq and location II: 170 Bq. Again the aerial survey data (Fig. 3a) are in close positive agreement with the ground based survey (Fig. 3b). The two sources are clearly resolved within the data, with both surveys successfully resolving the differing relative intensities of the sources. Differences in the mapped shape of the hot-spot regions are apparent between the aerial and ground based surveys. Specifically the data from the ground based survey exhibits a greater overall signal strength and improved spatial resolution is metre scale. The operation at low altitude (closer proximity) provides an improved mapping sensitivity. The sensitivity of a fixed size detector is rapidly reduced with increasing distance from the source, as it effectively intercepts a decreasing proportion of the available ray-path. This is represented diagrammaticality in Fig. 4. The intercepted radiation can be modelled as 1/z2 functional decay, where z is height, and hence any aerial mapping is highly sensitive to the altitude at which the measurements are taken. By recording data at a low altitude the sensitivity of any radiation detection measurement is significantly increased, thereby allowing for the use of small, low weight detectors, which can be adequately carried by small aerial platforms. This approach is unlike most traditional aerial methods (AGENCY, 1999) and is only possible with hover-style aerial platform which have sufficiently dynamic stability and controllable air speeds for mapping. Multirotor vehicles are inherently stable in adverse weather conditions, with the system presented here tested to operate in relatively adverse weather conditions including rain, snow and measured
winds speeds of 20 m s1 For accuracy of mapping, the height of the detection unit above the ground surface needs to be precisely determined, as uncertainty in this measurement represents the greatest potential of error in the detection system. Within our system this measurement is performed via the use of a LIDAR detection sensor, with the arising height above surface measurement applied as a correction to the data, is effectively normalised for height. Here we have demonstrated that the system is capable of producing high resolution maps of small areas (20 m2) or regions which may be inaccessible to traditional techniques. In the real world environment the system can produce tens to hundreds of meter squared area. It is envisaged that this type of device can give an initial identification of regions which contain radiological contamination. The nature of this contamination may be highly complex, and may have been remobilised by weather events from the time of release to the time of detection. This must be taken into account when interpretation the data, in particular the absolute degree of contamination from the recorded data. The results of the two experiments presented demonstrate that the instrument can accurately record radiation identity and intensity data, linking this with accurately determined geospatial position to produce high spatial resolution maps of radiation anomalies. The high sensitivity of the instrument, when mapping at heights below 3 m, provides a means of accurately mapping radiation intensity with metre resolution and accuracy. This technology provides a much needed and low cost means of rapidly assessing environmental radiological contamination in the event of future nuclear incidents, and may equally be utilised for routine site monitoring. In order to deploy such technology at the time of a nuclear incident it is imperative that the operators and the equipment has be robustly and thoroughly tested, in both ability to perform the task and familiarity of operating the systems in the environments which they are likely to be deployed.
References Barnes, J., Austin, T., 2009. Aircraft Having an Integrated Radiation Detection System and Method Therefore. US Patent App. 12/121,956. Boudergui, K., Carrel, F., Domenech, T., Guenard, N., Poli, J.P., Ravet, A., Schoepff, V., Woo, R., 2011. Advancements in nuclear instrumentation measurement methods and their applications (ANIMMA). In: 2nd International Conference on, pp. 1e9. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G., Yamazawa, H., 2011. J. Nucl. Sci. Technol. 48, 1129e1134. Hirao, S., Yamazawa, H., Nagae, T., 2013. J. Nucl. Sci. Technol. 50, 139e147. I. A. E. Agency, 1999. Generic Procedures Formonitoring in a Nuclear or Radiological Emergency. IEIA, Vienna. Kurvinen, K., Smolander, P., Pllnen, R., Kuukankorpi, S., Kettunen, M., Lyytinen, J., 2005. J. Environ. Radioact. 81, 1e10. Lyons, C., Colton, D., 2012. Health Phys. 102, 509e515. Morino, Y., Ohara, T., Nishizawa, M., 2011. Geophys. Res. Lett. 38. Nuclear Accident Independent Investigation Commission, 2012. The Official Report of the Fukushima Nuclear Accident Independent Investigation Commission. NAIIC, Tokyo. Omoto, A., 2013. Nuclear Instruments and Methods in Physics Research Section a: Accelerators, Spectrometers, Detectors and Associated Equipment. Pllnen, R., Toivonen, H., Perjrvi, K., Karhunen, T., Ilander, T., Lehtinen, J., Rintala, K., Katajainen, T., Niemel, J., Juusela, M., 2009. Appl. Radiat. Isotop. 67, 340e344. Pllnen-b, R., Toivonen, H., Perjrvi, K., Karhunen, T., Smolander, P., Ilander, T., Rintala, K., Katajainen, T., Niemel, J., Juusela, M., Palos, T., 2009. J. Radioanal. Nucl. Chem. 282, 433e437. Povinec, P.P., Hirose, K., Aoyama, M., 2013. Fukushima Accident. Elsevier, Boston. Schoeppner, M., Plastino, W., Povinec, P., Nikkinen, M., Ruggieri, F., Bella, F., 2013. Appl. Radiat. Isotop. 81, 358e361. Stohl, A., Seibert, P., Wotawa, G., Arnold, D., Burkhart, J.F., Eckhardt, S., Tapia, C., Vargas, A., Yasunari, T.J., 2012. Atmos. Chem. Phys. 12, 2313e2343. Terada, H., Katata, G., Chino, M., Nagai, H., 2012. J. Environ. Radioact. 112, 141e154. Yoshida, N., Kanda, J., 2012. Science 336, 1115e1116.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Lightweight aerial vehicles for monitoring, assessment and mapping of radiation anomalies J.W. MacFarlane a, *, O.D. Payton a, A.C. Keatley a, G.P.T. Scott b, H. Pullin a, R.A. Crane c, M. Smilion d, I. Popescu d, V. Curlea e, T.B. Scott a a
Interface Analysis Centre, HH Wills Physics University of Bristol, UK Imperial College London, UK Water Research Laboratory, University of New South Wales, Australia d National Institute For Metals and Radioactive Resources Bucharest, Romania e National Company of Uranium, Romania b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2014 Received in revised form 8 May 2014 Accepted 12 May 2014 Available online
The Fukushima Daiichi nuclear power plant (FDNPP) incident released a significant mass of radioactive material into the atmosphere. An estimated 22% of this material fell out over land following the incident. Immediately following the disaster, there was a severe lack of information not only pertaining to the identity of the radioactive material released, but also its distribution as fallout in the surrounding regions. Indeed, emergency aid groups including the UN did not have sufficient location specific radiation data to accurately assign exclusion and evacuation zones surrounding the plant in the days and weeks following the incident. A newly developed instrument to provide rapid and high spatial resolution assessment of radionuclide contamination in the environment is presented. The device consists of a low cost, lightweight, unmanned aerial platform with a microcontroller and integrated gamma spectrometer, GPS and LIDAR. We demonstrate that with this instrument it is possible to rapidly and remotely detect ground-based radiation anomalies with a high spatial resolution (<1 m). Critically, as the device is remotely operated, the user is removed from any unnecessary or unforeseen exposure to elevated levels of radiation. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Radiation monitoring Unmanned aerial system Characterisation Gamma spectrometry
1. Introduction Following the large release of radio-active material at the FDNPP incident (Stohl et al., 2012; Chino et al., 2011; Hirao et al., 2013; Terada et al., 2012; Schoeppner et al., 2013; Morino et al., 2011) only a limited amount of data relating to the quantity and geographical distribution of the released radiation was available to decision makers of evacuation and disaster response (Omoto, 2013; Nuclear Accident Independent Investigation Commission, 2012; Povinec et al., 2013). Static monitoring infrastructure surrounding the site, which had traditionally been used for routine plant monitoring, was compromised following the tsunami, with 23 of the 24 monitoring points rendered inoperable (Omoto, 2013). This resulted in an effective data blackout for radiological information
* Corresponding author. Interface Analysis Centre, HH Wills Physics University of Bristol, BS8 1TL, UK. Tel.: þ44 117 331 1171. E-mail address: [email protected] (J.W. MacFarlane). http://dx.doi.org/10.1016/j.jenvrad.2014.05.008 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
pertaining to the incident, in the hours and days following. It was not until four days following the event, that the first data revealing the extent of radiological contamination in the surrounding area was recorded. This data was produced by a fleet of 15 cars equipped with GM tubes reporting measurements 20 km away from the site (Povinec et al., 2013). Data gathered in this manner presented a potentially significant dose hazard to the operators in return for a spatially limited data set (Nuclear Accident Independent Investigation Commission, 2012). Traditional airborne measurements to identify highly contaminated areas were conducted (Yoshida and Kanda, 2012; Lyons and Colton, 2012) but the results were not made available until 11 days after the incident. These flights used extremely expensive equipment, carrying sensitive and heavy, volume-style radiation detectors, operating at a relatively high altitude (150e300 m) above the surface (Povinec et al., 2013). Resultantly all the recorded data was of low (km scale) spatial resolution and limited to within 30 km of the site (Povinec et al., 2013). The available data, which were obtained through the use of weather prediction models, and assessments of likely released
128
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radionuclides, were used by advisers to inform the evacuation plan for the surrounding population and the designation of exclusion zones where data readings were not available (Terada et al., 2012; Morino et al., 2011; Povinec et al., 2013). Worryingly, if a similar disaster were to occur today, we still lack the necessary tools to rapidly provide a time resolved, high spatial resolution and accurate plot of radiation released into the environment. Furthermore, traditional radiological assessment techniques would typically subject the operator, whether a driver or pilot, to an unknown and potentially significant radiation dose. The use of unmanned aerial systems can offer an interesting solution into the detection of radiation in the environment. There have been several previous studies examining aspects of this topic, these include examples which have explored the use of fixed wing aerial vehicles, (Kurvinen et al., 2005; Pllnen et al., 2009) or indoor UAV system (Boudergui et al., 2011), or even very large aircraft (Barnes and Austin, 2009). On these systems a variety of sensor arrays have been tested including air sampling sensors for the detection of airborne radiation particles (Pllnen et al., 2009; Pllnenb et al., 2009) and scintillating devices (Kurvinen et al., 2005). Fixed wing aircraft (Kurvinen et al., 2005; Pllnen et al., 2009) typically operate at higher altitude (up to 4500 m), and move at a minimum ground speed of 90 km h1. Here we present a new instrument for detection and assessment of radionuclide contamination in the environment, using small multi-rotor unmanned aerial systems. The instrument incorporates a microcontroller operated, lightweight, low volume, semi-conductor gamma ray spectrometer integrated with a small aerial platform. The instrument securely transmits the location, identity and intensity of radionuclide contamination to a remote operator or base station. Implementing small multi-rotor unmanned aerial systems, and the benefit these systems bring including reduced operation speed and greater manoeuvrability, allows for the device to produce high spatial resolution maps of radiological contamination in the environment. 2. Experimental The aerial platform consists of a modified multi-rotor (six propeller) aerial vehicle (Hexa XL, Mikrokopter) which records the GPS location of the instrument at a high frequency (10 Hz). At each recorded location a spectrum of the energy of incoming incident radiation is recorded (GR1, Kromek). The height of the device above the surface (±10 mm at <100 m) is simultaneously measured and recorded via the use of LIDAR (AR2500, Acuity). An Arduino mega ADK microcontroller unit is used to combine the data streams (GPS position, LIDAR height, radiation spectra). The data is stored locally on the instrument and concurrently transmitted to the user in real time (500 ms delay) as a 128 bit secured encrypted data stream, to a remote base station which can be up to 7 km from the instrument used to control the device. The payload, consisting of the LIDAR and
gamma spectrometer and associated microcontroller, is mounted on a gimbaled stage such that the spectrometer and LIDAR remain directed normal to the surface, regardless of pitch of the aerial platform (see Fig. 1). The system is powered by two 7.4 V lithium polymer batteries giving, currently, a total survey flight time of upto 12 min, with a maximum aerial speed of approximately 25 m s1. The presented system has a theoretical ability to survey 18 km of flight path in one flight, however, realistically the survey is typically operated at a slower speed to increase the sensitivity of the instrument. Typically areas of several tens to a hundred meters squared can be surveyed in one continuous flight before the batteries of the system need to be replaced. The batteries take typically 30 min to recharge, requiring 6 batteries in rotation to provide continuous survey coverage. Complementary sensors can be added to the device including thermal and visual cameras. The system can be operated manually, using traditional radio-controls or semi autonomously via programmed GPS way-points. Utilizing simple interface software, these way-points may be pre-selected or transmitted to the instrument in flight. Way-point matrices can generate survey routes that provide detailed geographical coverage of a designated area. The way-points can include automated landing and take-off, such that the device can gather long exposure gamma radiation spectra at a the region of interest. In this case the payload has a relatively small power requirement such that the system may install itself as a static ground-level monitoring point, by landing and reducing flight energy expenditure, for an extended period (weeks). Where landing is not possible, or appropriate, the instrument can hover and autonomously maintain its position over a set location even in severe weather conditions due to its low aerodynamic cross-section. Where possible, typically the instrument is operated at low altitude (<3 m) in order to maximise the radiation sensitivity, reduce the effects of background radiation shine on the instrument and to increase the spatial resolution of the data. Operating at <3 m altitude is only possible depending on the environment the instrument is functioning in. At the lower altitude care must be taken that the instrument does not collide with an obstructing obstacle, which currently is determined by the operator.
3. Materials and methods The source samples used within this study were specimens collected from the Cornubian batholith, Southwest UK. The batholith consists of six major and several smaller bodies of granite, the larger bodies of granite, from east to west are Dartmoor (600 km2), Bodmin moor (190 km2), St Austell (85 km2), Carnmenellis (130 km2), Lands End (190 km2), and the Isles of Scilly (area not defined). The St Austell granitic intrusion, from which the majority of the samples used in the study arise, is a small composite body
Fig. 1. The radiation detection system, left displays an overview of the system, right an exploded view of payload. a) hexacopter aerial platform, b) integrated payload c)gimbaled stage, d) LIDAR detector e) GPS board f) gamma ray spectrometer g) wireless transmitter h) payload housing.
J.W. MacFarlane et al. / Journal of Environmental Radioactivity 136 (2014) 127e130
129
(85 km2) that contains most of the Li-mica granite, with associated uranium bearing minerals, the Cornubian batholith. The gamma ray spectrometer (GR1 Kromek) used in this study was a small volume (100 mm3) CZT coplanar-grid detector. The energy range of the spectrometer is 30 keV to 3.0 MeV, with an energy resolution of 2.0 2.5% FWHM @ 662 keV. The electronic noise on the detector is <10 keV FWHM. 4. Results and discussion Fig. 2 demonstrates a simple experiment to demonstrate the capability of the instrument. Naturally radioactive rock samples containing uranium bearing minerals and associated daughter products were arranged within a 20 cm2 region of a marked 20 m2 geometric grid. The total activity of the test specimens was recorded as 260 Bq, representing a relatively weak radiation source. The associated gamma spectra, 800 s collection, of the minerals is shown in Fig. 2a, recorded decay energies of the radioisotopes: Bi
Fig. 2. Distribution of radiation mapped by the system. a) Spectra of radiation source, b) intensity map produced by instrument, c) hand survey data for comparison.
Fig. 3. Distribution of radiation mapped by the system. a) intensity map produced by instrument, b) hand survey data for comparison.
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Fig. 4. Diagrammatic representation of the reduction of ray path sampled as the detector plate is increased in distance from the source.
214, Pb 214, Ra 226, Th 230, Th 234, U 235. The grid test area was autonomously mapped using a pre-programmed GPS flight path at an altitude of 2.5 m. The radioactivity distribution map produced by the instrument is shown in Fig. 2b. The data are validated by comparison data with a coincident ground based survey area with a detector height of 1 m (Fig. 2c). The same detector was used for both the ground and aerial surveys. With the radiation sources located together towards the top left of the test grid, the arising data for both aerial and ground surveys is closely correlated (Fig. 2). The data records a clearly defined radioactive hot-spot in the same location in both surveys. A clear and rapid reduction in the recorded radiation intensity with distance from the test source is recorded, following the physically predicted z law (where z represents altitude/distance from the source). Fig. 3 illustrates a more complex radiological contamination analogy. The uranium bearing rock samples were arranged in two distinct 20 cm2 regions within the 20 m2 geometric grid. The two regions had differing activities, with location I having an activity of: 90 Bq and location II: 170 Bq. Again the aerial survey data (Fig. 3a) are in close positive agreement with the ground based survey (Fig. 3b). The two sources are clearly resolved within the data, with both surveys successfully resolving the differing relative intensities of the sources. Differences in the mapped shape of the hot-spot regions are apparent between the aerial and ground based surveys. Specifically the data from the ground based survey exhibits a greater overall signal strength and improved spatial resolution is metre scale. The operation at low altitude (closer proximity) provides an improved mapping sensitivity. The sensitivity of a fixed size detector is rapidly reduced with increasing distance from the source, as it effectively intercepts a decreasing proportion of the available ray-path. This is represented diagrammaticality in Fig. 4. The intercepted radiation can be modelled as 1/z2 functional decay, where z is height, and hence any aerial mapping is highly sensitive to the altitude at which the measurements are taken. By recording data at a low altitude the sensitivity of any radiation detection measurement is significantly increased, thereby allowing for the use of small, low weight detectors, which can be adequately carried by small aerial platforms. This approach is unlike most traditional aerial methods (AGENCY, 1999) and is only possible with hover-style aerial platform which have sufficiently dynamic stability and controllable air speeds for mapping. Multirotor vehicles are inherently stable in adverse weather conditions, with the system presented here tested to operate in relatively adverse weather conditions including rain, snow and measured
winds speeds of 20 m s1 For accuracy of mapping, the height of the detection unit above the ground surface needs to be precisely determined, as uncertainty in this measurement represents the greatest potential of error in the detection system. Within our system this measurement is performed via the use of a LIDAR detection sensor, with the arising height above surface measurement applied as a correction to the data, is effectively normalised for height. Here we have demonstrated that the system is capable of producing high resolution maps of small areas (20 m2) or regions which may be inaccessible to traditional techniques. In the real world environment the system can produce tens to hundreds of meter squared area. It is envisaged that this type of device can give an initial identification of regions which contain radiological contamination. The nature of this contamination may be highly complex, and may have been remobilised by weather events from the time of release to the time of detection. This must be taken into account when interpretation the data, in particular the absolute degree of contamination from the recorded data. The results of the two experiments presented demonstrate that the instrument can accurately record radiation identity and intensity data, linking this with accurately determined geospatial position to produce high spatial resolution maps of radiation anomalies. The high sensitivity of the instrument, when mapping at heights below 3 m, provides a means of accurately mapping radiation intensity with metre resolution and accuracy. This technology provides a much needed and low cost means of rapidly assessing environmental radiological contamination in the event of future nuclear incidents, and may equally be utilised for routine site monitoring. In order to deploy such technology at the time of a nuclear incident it is imperative that the operators and the equipment has be robustly and thoroughly tested, in both ability to perform the task and familiarity of operating the systems in the environments which they are likely to be deployed.
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