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Sensors and Actuators B 161 (2012) 697– 701

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

nline optical monitor of alpha radiations using a polymeric solid stateuclear track detector CR-39

tul Kulkarnia, Chirag K. Vyasb, Hojoong Kimc, Puran C. Kalsid, Taesung Kima,c,∗,ijay Manchandab,d,∗∗

Nanoparticle Technology Lab, School of Mechanical Engineering, Sungkyunkwan University, Suwon 440746, South KoreaEmerging Nuclear Technology Lab, Department of Energy Science, Sungkyunkwan University, Suwon 440746, South KoreaSungkyunkwan Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440746, South KoreaRadio Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

r t i c l e i n f o

rticle history:eceived 2 August 2011eceived in revised form 29 October 2011ccepted 6 November 2011vailable online 16 November 2011

a b s t r a c t

In the wake of Fukushima accident, there is a need to intensify efforts to efficiently monitor the release ofalpha emitting radionuclides caused by the core melt down or/and failure of first barrier containment ofthe nuclear fuel. As a consequence of the exposure of diethylene-glycol bis(allycarbonate) (CR-39) to alpharadiations (emitted from 232Th), radiation damage causes modification of its physico-chemical propertieslike absorbance, surface roughness and reflectance which could be correlated with the chemically etched

eywords:adiation sensorolid state nuclear track detectorsptical fiber sensoreflectance

track density, conventionally followed for the quantification of �-radiations offline. CR-39 based FiberOptic Sensor (CRFOS) based on the reflectance measurement has been developed with an aim to monitoronline the presence of �-emitting radionuclides (in/around a nuclear facility).

© 2011 Elsevier B.V. All rights reserved.

uclear radiations

. Introduction

Fukushima accident has raised questions about the fueleltdown and overheating of the discharged spent fuel lead-

ng to the possible release of alpha emitting radionuclides aserosols/dispersed particulate matter in the environment [1–3].lpha and other heavy particles do not pose the hazard of exter-al exposure as long as they are in a sealed container. However iniew of their high linear energy transfer values, effective biologicalose of these radiations if inhaled or ingested internally is 20 times

arger than the corresponding dose for beta/gamma radiations [4].t is mandatory to monitor alpha emitting radionuclides presentn the dispersed particulate matter within as well as in the sur-oundings of Nuclear Fuel Cycle facilities, Nuclear Power Reactors

s well as Accelerator facilities to ensure that the operational work-rs/general public are not subjected to over exposure. Such regularonitoring exercise is immensely beneficial particularly during

∗ Corresponding author at: Sungkyunkwan Advanced Institute of NanotechnologySAINT), Sungkyunkwan University, Suwon 440746, South Korea.el.: +82 31 299 6272; fax: +82 31 299 4279.∗∗ Corresponding author at: Emerging Nuclear technology Lab, Department ofnergy Science, Sungkyunkwan University, Suwon 440746, South Korea.el.: +82 31 299 6272; fax: +82 31 299 4279.

E-mail addresses: tkim@skku.edu (T. Kim), vkm49@skku.edu (V. Manchanda).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.11.016

a nuclear emergency as prompt redressal mechanism againstexcessive exposure can be activated.

Contrary to popular belief, alpha radiation is not always blockedby the outermost skin layer (the epidermis). Several alpha emittingisotopes, including naturally occurring 232Th, 238U and 235U, havea particle energy great enough to penetrate the epidermis whereit is thinnest on the body. In addition, alpha emitters can fasten inhair follicles and sweat pores and stick to any part of the skin. Forthese reasons, external dose to the skin from alpha emitters must betaken into account in determining the health effects of alpha radi-ation. International Commission on Radiological Protection (ICRP)recommendation for alpha emitters is specified in terms of totalbody burden. It varies with the biochemical as well as radiologi-cal properties of alpha emitter. For example, it is 100 nCi for 226Ra(bivalent metal ion with T1/2 = 1600 years) and 40 nCi for 239Pu(tetravalent metal ion with T1/2 = 24,500 years). Whereas 40 nCiof 239Pu corresponds to ∼250 ng, 100 nCi of 226Ra corresponds to100 ng. For members of the general public, who are not monitored,permissible body burden is set much lower (4 nCi for 239Pu). Themaximum deposit in lung for occupational worker is 16 nCi and forgeneral public it is 1.6 nCi. Considering this is the maximum burden

permitted in the whole life of a person, it is imperative that radio-analytical methods should be able to quantify these alpha emittersat a level several orders lower viz. sub picocurie (corresponding tosubpicograms or submilliBq of alpha emitters). Measurements of

698 A. Kulkarni et al. / Sensors and Actuators B 161 (2012) 697– 701

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ig. 1. Proposed scheme of CRFOS for detection of alpha particle radiations from 232

hoto-detector and reflected-absorbance is measured by spectrometer. CR39 3D crom 10 to 50 mm to evaluate the effect of irradiation distance. The distance betwe

uch low quantities of alpha radiations/alpha emitters are a greathallenge.

Conventionally offline scintillation counting/alpha spectrome-ry are employed to track the alpha emitting radionuclides like U,h and Pu in the dispersed particulate matter in the environmentf a nuclear facility [5]. Whereas minimum detection limit for theormer is about 2–3 mBq, one can detect even 0.5 mBq using sili-on surface barrier detector (alpha spectrometry). It is necessary tonsure that apart from the radiation background, electronic back-round is low enough to allow the measurement of such low levelf alpha radiations. Solid State Nuclear Track Detectors (SSNTD’s)ffer an alternative technique to measure U, Th and Pu at ultra-traceevel in the environment [6–8]. As energetic and heavy chargedarticles travel through matter, they produce along their pathsylindrical shaped regions which have been atomically and elec-ronically disturbed. The lengths of these zones of radiation damageepend critically on the charge and mass of the traversing particlend on the chemical composition of matter traversed. The tracksroduced in the various insulating materials could be made observ-ble by optical microscopy. The chemical reagent reacted muchore rapidly along the sensitized tracks than with the undam-

ged areas of the insulating materials. The preferential chemicaltching converted the tracks to much larger, very nearly cylindri-al channels, which scattered light and thus appeared as dark linesn the field of an optical microscope. These detectors can be usedn two modes viz. Fission Track Analysis (FTA) mode and Alpharack Analysis (ATA) mode. Employing Fission Track Analysis (FTA),t is possible to assay picogram level of natural uranium [9]. Theetection limit for 239Pu is much lower (around 10 �Bq or ∼1 fg).

hese detection limits are governed by the isotopic compositionith respect to fissile isotopes, fission cross section of the fissile

toms and the energy/fluence of the projectile neutrons. However,n case of ATA, depending on the concentration and half-life of alpha

urce. Reflected light form CR-39, as indicated in the ray diagram, is measured usingal structure as well as alpha particle structure is shown. The distance “d” is variedical fiber and CR 39 film is kept constant @ 10 mm.

emitting radionuclide, the detection limits is in the same range asalpha spectrometry viz. 0.5 mBq though there is an advantage ofavoiding any sensitive electronics which is accompanied with highbackground. Whereas diethylene-glycol bis (allylcarbonate) (CR-39) is commonly used for monitoring alpha radiations, bisphenol-Apolycarbonate (Lexan) is used for monitoring heavy ions generatedin the neutron induced fission of U, Th and Pu [10–13]. Dependingon the isotopic composition and concentration level of these ele-ments, it is required to expose the CR-39 detectors for few minutesto few weeks followed by chemical etching and quantification oftrack density using optical microscope [14,15]. Though this tech-nique is simple but it is off line, involves time consuming chemicaltreatment and thus is not effective during any emergency situationrequiring prompt response.

In the present work, an effort has been made to develop anonline fiber optic sensor (FOS) to monitor CR-39 detector exposedto alpha radiations emitted (3.93 kBq/g of 232Th) present in 1 g ofTh(NO3)4·5H2O. This is the first ever attempt to use reflectancebased FOS to quantify the alpha radiations flux using CR-39detector. The proposed FOS utilises fast photo-detector insteadof photomultiplier tube (PMT) used in scintillation photon opticalsensors for radiation measurements [16–20]. The FOS with CR-39henceforth will be referred in the text as CRFOS.

2. Experimental

The scheme of the CRFOS is depicted in Fig. 1 along with theray diagram and 3D chemical structure of CR-39 and alpha par-ticle. The CRFOS is configured in the reflectance mode. When the

incident light interacts with the CR-39 film, a fraction of light isreflected from the surface (both top and bottom), another fractionis absorbed and the remainder is transmitted through. The reflectedlight from the surface as well as through the bulk of CR-39 film is

A. Kulkarni et al. / Sensors and Actu

Fig. 2. CRFOS response to alpha particle radiation from 232Th source measured byphoto detector after curve fitting (where R2 = 0.99) for various alpha particle irradi-aui

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tion distances of 10, 20 and 50 mm. The CRFOS reflectance–absorbance measuredsing spectrometer after every 15 min up to 60 min by spectrometer is shown in

nset. The reflectance–absorbance increased with irradiation time.

easured by photo detector and spectrometer. From the spectro-copic evaluation of the irradiated CR-39 film, it was found to havehe absorbance in near UV region. Therefore the present work wasased on blue incident light (� = 458 nm).

The commercially available CR 39 films were used for this studyKodak, USA). These films were cut in equal size of 15 mm × 15 mmo fit in the fabricated CRFOS radiation sensor. CR-39 samplesf thickness 500 �m (composition C12H18O7, molecular weight

ig. 3. AFM 3D image of CR 39 film before (a) and after alpha particle irradiation at var0 min of irradiation time. The surface topology for irradiation distance of 10 and 20 mm

o the original surface topology. The RMS roughness for 10, 20 and 50 mm irradiation distwithout alpha particle irradiation) is 1.3 nm.

ators B 161 (2012) 697– 701 699

274 a.m.u., density 1.32 g cm−3) were irradiated with 232Th powder(Th(NO3)4·5H2O).

A bifurcated polymer optical fiber POF with a 1 × 2 fiber coupler(50:50, Industrial Fiber Optics Inc., IF-562) is used for the detec-tion of the reflected signal. The light source (Newport, USA) was tolaunch the light into the fiber, and the reflected light was measuredusing a Si photodiode detector (Thorlab, PDA36A) and a Spectrome-ter (2048, Avaspec). A digital multimeter (Keithley, 2700) was usedto retrieve the sensor output and the data was recorded on a PC,which were used to analyze the sensor performance.

The thorium powder was kept below the film at varying dis-tance from the film viz. 5, 10, 20, 30, 40 and 50 mm. The reflectancemeasurement was carried out by recording the optical signal imme-diately after placing the thorium under the CRFOS. Reflectanceintensities were registered after every 1 min up to 60 min. To char-acterize the surface topology of the unirradiated and irradiatedCR-39 films, the surface roughness was measured using atomicforce microscope (AFM) (diInnova, Veeco, USA) in terms of theroot-mean-square (rms) roughness. Tapping mode operation of theAFM was used with Al coated silicon cantilever (NCHR, NanoWorldAG, Swiss) and 2825 kHz tapping frequency. The tip has pyramidshape and curvature radius of 8 nm in its end. For each CR-39 filmthree different areas with a size of 2 �m × 2 �m were scanned witha resolution of 512 × 512 scan lines to determine the rms rough-ness. An initial setpoint of 6 V was applied on the tip and tappingamplitude of 0.134 V was used with 0.5 Hz scan rate during mea-surements. The surfaces of the detectors were imaged directly inair at room temperature. The measured surfaces were converted

into 3-dimensional image and analyzed by SPIP software (ImageMetrology A/S, Denmark). Height scale (Z-axis) was identical forthe 3-D images between 0 and 20 nm. Bright color indicates higherZ value.

ious distances of 10 mm (b), 20 mm (c) and 50 mm (d) from the 232Th source forshowed distinct changes where as that for 50 mm shows little change with respectance is 0.91, 0.90 and 1.2 nm respectively whereas the RMS roughness of blank film

7 d Actuators B 161 (2012) 697– 701

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Fig. 4. Comparison of CRFOS reflected intensity with the change in RMS roughness(a) and with measured track density (b). Beyond the irradiation distance of 20 mm

00 A. Kulkarni et al. / Sensors an

Further, the irradiated CR-39 films were etched under theptimised chemical etching conditions for the appearance of theegistered tracks under optical microscope. The irradiated CR-9 films were dipped in the etchant (6 N NaOH) at about 60 ◦Cor 2 h accompanied with mechanical stirring. After the chemi-al etching procedure, the films were carefully washed with theistilled water before taking their microscopic images. The trackensities of chemically etched CR-39 films after alpha particle

rradiation were measured using optical microscope and Image Joftware.

. Results and discussion

The reflected intensity of CR-39 film was recorded online dur-ng exposure to 232Th (spread as 1 g of Th(NO3)4·5H2O on an areaf 0.785 cm2) for 1 h, varying the distance of source from the CR-9 film from 5 to 50 mm. However, the representative data for0, 20 and 50 mm is discussed here. The change in the reflected

ntensity (Fig. 2) was identical (∼12 mV) for irradiation distancef 10 and 20 mm. However, for 50 mm irradiation distance, it wasnly 2 mV. It can be explained by the fact that the mean rangef alpha particle (of around 4 MeV) in air is only around 35 mm.herefore flux of alpha particles drops drastically at 50 mm irra-iation distance. This observation also confirms that the change

n the reflected intensity is due to exposure to alpha particle irra-iation and not because of alpha emitting 220Ra gas released as aecay product of 232Th. Obviously, the latter can travel much longeristance (of few meters) before it decays to 216Po with a half-lifef 55.6 s. The interaction of radiation with the CR-39 film leads tohemical changes like chain scission/cross linking leading to per-anent structural damage and alteration in its physico-chemical

roperties like UV/Vis absorbance, topology and thermal behaviour21]. The results for reflectance and absorbance were recordedvery minute for 1 h. Fig. 2 shows that the reflectance–absorbanceinset) increased continuously with time suggesting the change inhe reflectance intensity of CRFOS as an excellent means to monitorhe presence of alpha radiations.

The reflectance is strongly dependent on film surface topol-gy. To characterize surface topology of CR-39 film, the surfaceoughness of irradiated films was measured using atomic forceicroscopy (AFM) in terms of the root mean square (RMS) rough-

ess. AFM gives 3D image track profile unlike the 2D image ofptical microscope. It can be seen that the surface roughnessecreases with the alpha particle irradiation (Fig. 3).

The RMS roughness of CR-39 film decreases from 1.3 to 0.90 nmn irradiation thereby causing increase in reflectance as the alphaarticle exposure increases. As expected, the track density afterhemical etching (number of tracks per unit area) was found to belmost the same for irradiation distance of 10 mm and 20 mm. How-ver, track density decreased significantly at an irradiation distancef 50 mm. Qualitatively, the trend is similar to that observed witheflected intensity. Excellent inverse relation between reflectancend RMS roughness (Fig. 4a) and direct relation between reflectancend track density (Fig. 4b) suggest distinct possibility of employingeflectance change as a monitor of alpha radiation flux.

The precision (1�) of reflectance measurement in the presentork is around ±0.01 mV suggesting the possibility of measuring

he signal generated by milligram quantities of 232Th for exposureime of few minutes. Detection limit is expected to be significantlyetter for 239Pu (specific activity is 106 times of 232Th) and 241Amspecific activity is 5 × 107 times of 232Th) for identical configura-

ion of CRFOS set-up and similar exposure time. Thus it may beossible to achieve the sensitivity of CRFOS towards alpha emit-ers similar to FTA which as mentioned in the introduction isn order of magnitude better than ATA. Pico curie level of alpha

the reflected intensity drops down whereas the RMS roughness increases steeply.It is also seen that the track density measured after chemical etching of CR-39 hasdirect relationship with the reflected intensity.

activity has been measured earlier indirectly in Irish Sea water(where effluents of a fuel reprocessing plant were preconcentrated)using Cellulose Nitrate (LR-115) solid state nuclear track detector[22]. Thus it may be possible to employ such a device for moni-toring the aerosol/dispersed alpha emitting radionuclides not onlywithin but also around spent fuel or MOX fuel [(U, Pu) O2] han-dling facilities. It should be possible to set an alarm signal whichcan be activated remotely once the defined threshold reflectanceintensity change has occurred. In the present work, some exper-iments were also carried out with LR-115 detector and similarresults were obtained on exposure to alpha radiations. Choice ofsuitable detector material (like Lexan) can further extend the scopeof this approach to monitor heavy ions encountered in acceleratorsor during nuclear fission reactions.

4. Conclusions

To conclude, the present work offers a novel approach to mon-itor online alpha radiation flux employing the reflectance changesin the exposed organic polymer viz. CR-39 through optical fiberremotely. It can serve as an excellent tool to detect the release ofalpha emitting radionuclides either from the spent fuel or from fuelunder irradiation in a reactor, in an emergency. The sensitivity of

the developed technique appears far better than the usual trackdensity measurements due to alpha radiations and seems to matchthe sensitivity of fission track detectors for fissile nuclides. Thecumulative and irreversible nature of changes in exposed polymer

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acilitates the quantification of ultra trace levels of alpha emit-ers particularly those with high specific activity like 238Pu, 239Pu,41Am, 244Cm as and when these are dispersed within/around auclear facility.

cknowledgements

This research was supported by World Class University (WCU)rogram through the National Research Foundation of Koreaunded by the Ministry of Education, Science and Technology (R31-008-10029) to V.M. and is supported by the National Researchoundation of Korea (NRF) funded by the Ministry of Education,cience and Technology (2011-0006292) to T.K.

eferences

[1] D. Clery, Current designs address safety problems in fukushima reactors, Sci-ence 331 (2011), 1506–1506.

[2] G. Brumfiel, The meltdown that wasn’t, Nature 471 (2011) 417–418.[3] J. Kaiser, Radiation risks outlined by bombs, weapons work, and accidents,

Science 331 (2011) 1504–1505.[4] G. Friedlander, J.W. Kennedy, J.M. Miller, Nuclear and Radiochemistry, John

Wiley and Sons, USA, 1964, p. 126.[5] G.F. Knoll, Radiation Detection and Measurement, John Wiley and Sons, Inc.,

USA, 1989, pp. 231, 376.[6] R.L. Fleischer, P.B. Price, R.M. Walker, E.L. Hubbard, Track registration in various

solid-state nuclear track detectors, Phys. Rev. 133 (1964) A1443–A1449.[7] M.F. Zaki, Y.H. El-Shaer, Particularization of alpha contamination using CR-39

track detectors, Pramana-J. Phys. 69 (2007) 567–574.[8] A.R. Moorthy, C.J. Schopfer, S. Banerjee, Plubnium from atmospheric weapons

testing fission tmck analysis of urine samples, Anal. Chem. 60 (1988)857A–860A.

[9] S. Carpenter, C.H. Cheek, Anal. Chem. 78 (1970) 6955–6958.10] B. Center, F.H. Ruddy, Detection and characterization of aerosols containing

transuranic elements with the nuclear track technique, Anal. Chem. 48 (1976)2135–2138.

11] N. Dwaikat, M. El-hasan, M. Sueyasu, W. Kada, F. Sato, Y. Kato, G. Saffarini, T.Iida, A fast method for the determination of the efficiency coefficient of bareCR-39 detector, Nucl. Instrum. Methods B 268 (2010) 3351–3355.

12] S.A. Durrani, R.K. Bull, Solid State Nuclear Track Detection, Principles, Methodsand Applications, Pergamon Press, Great Britain, 1987, p. 49.

13] J.W. Mitchell, S. Carpenter, Parts per billion doping and characterization ofuranium distribution in an epoxy polymer matrix, Anal. Chem. 78 (2006)6955–6958.

14] V. Chavan, P.C. Kalsi, V.K. Manchanda, A novel room temperature-inducedchemical etching (RTCE) technique for the enlargement of fission tracks

in Lexan polycarbonate SSNTD, Nucl. Instrum. Methods A 629 (2011)145–148.

15] K.N. Yu, H.H.W. Lee, A.W.T. Wong, Y.L. Law, S.F.L. Cheung, D. Nikezic, F.M.F.Ng, Optical appearance of alpha particle tracks in CR-39 SSNTD, Nucl. Instrum.Methods B 263 (2007) 271–278.

ators B 161 (2012) 697– 701 701

16] B. Lee, K.W. Jang, D.H. Cho, W.J. Yoo, H.S. Kim, S.C. Chung, J.H. Yi, Development ofone-dimensional fiber-optic radiation sensor for measuring dose distributionsof high energy photon beams, Opt. Rev. 14 (2007) 351–354.

17] B. Lee, W.J. Yoo, D.H. Cho, K.W. Jang, S.C. Chung, G.R. Tack, Low-temperatureradiometric measurements using a silver halide optical fiber and infrared opti-cal devices, Opt. Rev. 14 (2007) 355–357.

18] E. Takada, D. Yamada, H. Kuroda, A new optical fiber sensor for multipointradiation measurement with sensing regions in its cladding, Meas. Sci. Technol.15 (2004) 1479–1483.

19] J.P.Y. Ho, C.W.Y. Yip, D. Nikezic, K.N. Yu, Differentiation between tracks anddamages in SSNTD under the atomic force microscope, Radiat. Meas. 36 (2003)155–159.

20] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 78(2006) 3859–3873.

21] M.F. Zaki, Gamma-induced modification on optical band gap of CR-39 SSNTD,J. Phys. D: Appl. Phys. 41 (2008).

22] S.A. Durrani, R.K. Bull, Solid State Nuclear Track Detection, Principles, Methodsand Applications, Pergamon Press, Great Britain, 1987, p. 29.

Biographies

Atul Kulkarni received the Ph.D. degree from the University of Pune, India in2005. His research topic involved multidisciplinary sensors. Since 2006, he hasbeen a Research Professor at Sungkyunkwan University, Nanoparticle TechnologyLab, School of Mechanical Engineering, South Korea. His research interests includedeveloping sensors using optical, nano-, and biotechnologies.

Chirag K. Vyas received his B.S. degree from Pune University, India in 2007. He iscurrently a graduate student in Department of Energy Science at SungkyunkwanUniversity, South Korea. His research interests include recovery and purification ofvaluables from irradiated nuclear fuels.

Hojoong Kim received the Bachelor’s degree from the Department of Mechani-cal Engineering, Sungkyunkwan University, Korea. He is a Ph.D. candidate at SKKUAdvanced Institute of Nanotechnology in same university. His research interests arechemical mechanical planarization (CMP) process and particle technology.

Puran C. Kalsi received his Ph.D. from J&K University, India in 1976. He worked asSenior Scientist at Radiochemistry Division of BARC, Mumbai, India since 1978. Hisresearch interests include Solid State Nuclear Track Detectors (SSNTDs), Destructiveand Nondestructive assay techniques for nuclear materials accounting the Thermalanalysis of solids and polymers.

Taesung Kim received the Ph.D. degree in 2003 from the Department of Mechani-cal Engineering, University of Minnesota, Twin Cities Campus, USA. He is currentlya Professor in the School of Mechanical Engineering, Sungkyunkwan Univer-sity, South Korea. His research interests are in nanoparticle instrumentationand manufacturing, nano/micro contamination and atmospheric aerosol control,nanogrinding/CMP, nanostructured material research, and sensors.

Vijay Manchanda received his Ph.D. from University of Bombay, India in 1975. He is

currently the visiting professor in Department of Energy Science at SungkyunkwanUniversity, South Korea. His research interests are Separation Chemistry ofActinides, Complex Chemistry of fission products with macrocyclic ligands, Radioan-alytical Chemistry, Chemical Quality control of nuclear fuels, Speciation of actinidesin aquatic environment.