development of optical monitor of alpha radiations based on cr-39

Applied Radiation and Isotopes 81 (2013) 184ndash189

Contents lists available at ScienceDirect

Applied Radiation and Isotopes

0969-80httpd

n CorrE-m

journal homepage wwwelseviercomlocateapradiso

Development of optical monitor of alpha radiations based on CR-39

Pranav M Joshirao a Jae Won Shin b Chirag K Vyas a Atul D Kulkarni c Hojoong Kim cTaesung Kim cd Seung-Woo Hong ab Vijay K Manchanda an

a Department of Energy Science Sungkyunkwan University Suwon 440-746 Republic of Koreab Department of Physics Sungkyunkwan University Suwon 440-746 Republic of Koreac Department of Mechanical Engineering Sungkyunkwan University Suwon 440-746 Republic of Koread SAINT Sungkyunkwan University Suwon 440-746 Republic of Korea

H I G H L I G H T S

Fiber Optics Reflectance Sensor based on CR-39 was employed to monitor online low level alpha radiations emitted from 241Am

Change in reflectance was found to be inversely related to roughness of the film The sensitivity of the detector far exceeds the sensitivity obtained by measuring the track density Experimentally measured track density matched with Monte Carlo GEANT4 simulation results

a r t i c l e i n f o a b s t r a c t

Available online 24 June 2013

KeywordsReflectance sensorAlpha radiationsCR-39Track densityGEANT4 simulation

43$ - see front matter amp 2013 Elsevier Ltd Axdoiorg101016japradiso201306012

esponding author Tel +82 312996272 faxail addresses vkm25749gmailcom vkm49

Fukushima accident has highlighted the need to intensify efforts to develop sensitive detectors tomonitor the release of alpha emitting radionuclides in the environment caused by the meltdown of thedischarged spent fuel Conventionally proportional counting scintillation counting and alpha spectro-metry are employed to assay the alpha emitting radionuclides but these techniques are difficult to beconfigured for online operations Solid State Nuclear Track Detectors (SSNTDs) offer an alternative off linesensitive technique to measure alpha emitters as well as fissile radionuclides at ultra-trace level in theenvironment

Recently our group has reported the first ever attempt to use reflectance based fiber optic sensor(FOS) to quantify the alpha radiations emitted from 232Th In the present work an effort has been madeto develop an online FOS to monitor alpha radiations emitted from 241Am source employing CR-39 asdetector Here we report the optical response of CR-39 (on exposure to alpha radiations) employingtechniques such as Atomic Force Microscopy (AFM) and Reflectance Spectroscopy

In the present work GEANT4 simulation of transport of alpha particles in the detector has also beencarried out Simulation includes validation test wherein the projected ranges of alpha particles in the airpolystyrene and CR-39 were calculated and were found to agree with the literature values An attempthas been further made to compute the fluence as a function of the incidence angle and incidence energyof alphas There was an excellent correlation in experimentally observed track density with the simulatedfluence The present work offers a novel approach to design an online CR-39 based fiber optic sensor(CRFOS) to measure the release of nanogram quantity of 241Am in the environment

amp 2013 Elsevier Ltd All rights reserved

1 Introduction

Future of nuclear power globally depends on the innovations tostrengthen reactor safety and strategies to deal with spent fueldischarged from nuclear reactors For countries following closedfuel cycle management of high level liquid waste emanatingfrom the fuel reprocessing plants is another major issue With

ll rights reserved

+82 312994279skkuedu (VK Manchanda)

increasing inventory of the spent fuel and delay in the commis-sioning of geological repositories for its final destination risk ofradiation exposure to workersgeneral public has increased mani-folds Apart from intensifying efforts to enhance safety features inthe design of future reactors and addressing to the issue of safestorage of spent fuel Fukushima accident has also brought intofocus the need to monitor low level radiations in the environmentDue to the high linear energy transfer value alpha radiationsparticularly cause major damage to the living beings if inhaled oringested internally as aerosols or as dispersed particulate matter inthe environment Apart from this high levels of naturally-

Fig 1 Schematic diagram of the experimental setup

PM Joshirao et al Applied Radiation and Isotopes 81 (2013) 184ndash189 185

occurring radioactive aerosols particularly from decay products ofradon or thoron gas can pose a health risk at indoor locations suchas mines and dwellings However measurements of low levelalpha radiations presents a challenge due to their limited range inthe media through which they travel

Common method of measuring the concentration of atmo-spheric radioactive aerosol particles is to collect them on a filter bydrawing a known volume of air through it The activity depositedon the filter is then measured using scintillation detector SolidState Nuclear Track Detectors (SSNTDs) offer an alternativeapproach to monitor alpha radiations in the environment SSNTDsare insulating solids both naturally occurring and man-made Thepotential applications of SSNTDs are growing rapidly They havebeen employed in various areas related to nuclear scienceand technology viz nuclear physics nuclear imaging nucleartechnology space physics microanalysis mine safety environ-mental research uranium prospecting biomedical sciences mate-rial sciences and geological sciences (Fleischer et al 1975) Thereare several types of these detectors including inorganic crystalsglasses and plastics When a heavily ionizing charged particlepasses through such insulating solids it leaves a narrow trail ofdamage of about 50 Aring in diameter along its path This is calledlsquoLatent Trackprime as it cannot be seen with the naked eye It is possibleto view this latent track with an electron microscope The exactnature of the physical and chemical changes occurring at thedamage site depends on the charge(Z) and velocity term (βfrac14νcwhere ν is the particle velocity and c is the velocity of light) of theparticle on the chemical structure and storage conditions of thedetector material These latent tracks can be enlarged and can beviewed under an optical microscope by etching with somechemicals such as sodium hydroxide or hydrofluoric acid How-ever neither of these methods is satisfactory for real-time radia-tion detection due to their inherent off line character Thus there isa need to develop sensor for real-time detection of alpha particlesor heavy ions in the environment

A radiation sensor using an optical fiber consists of a scintilla-tor an optical fiber and a photo-detector The scintillation photonsinduced by the exposed radiations are detected by the photo-detector system through an optical fiber which is not influencedby temperature pressure or electromagnetic waves A fiber opticradiation sensor (FORS) ensures real-time measurement excellentspatial resolution and high flexibility Recently our group hasreported the first ever attempt to use CR-39 based reflectancesensor to quantify the alpha radiations emitted from 232Th(Kulkarni et al 2012)

An attempt has been made in the present work to develop fiberoptic radiation sensor to monitor alpha radiations emitted from241Am source employing CR-39 as a detector The fiber optic probeis configured in reflectance mode The proposed FORS utilizes afast photo detector instead of a photomultiplier tube (PMT) usedin scintillation photon optical sensors and it has the maximumsensitivity in the short wavelength region of visible light Theexposed SSNTDs are also analyzed by conventional track densitymeasurements and the data is correlated with the reflectance dataAs the reflectance of the exposed CR-39 film depends on itsroughness an attempt is also made to evaluate its surface topologyusing Atomic Force Microscopy (AFM)

In the case of a mono energetic flux of incident radiationdetermination of the real number of projectiles impinging ondetector per unit area per unit time can be obtained from thesource strength and geometry of source and detector However ina poly energetic system all particles incident on CR-39 may notresult in etchable tracks (Membrey et al 1993) Only those latentdamages registered on the SSNTD at an angle greater than thecritical angle of etching will be visible as tracks Thus thededuction of critical angle values for given etching conditions

can be useful to quantify the real particle flux under givenexposure (Stafford et al 1988 Dorschel et al 1999)

An attempt has also been made to correlate the number oftracks per unit area (track density) and critical angle obtainedexperimentally with the values obtained using Monte Carlosimulations (GEANT4) for a given source-detector geometry

2 Experimental

21 Schematic of set up

Fig 1 shows a schematic diagram of our experimental setup forexposure of detector film It consists of a CR-39 film pasted insidea petri-dish an 241Am source and a plastic cylindrical ring TheCR-39 (Allyl diglycol carbonate) film of 1 cm1 cm007 cm wasprocured from PPG Coatings Co Ltd (Hong Kong)

Two different sources were used in the present work In thefirst part for Fiber Optic Reflection and AFM studies 241Amcalibration source S1 was used Specifications of S1 are as followsBerthold Technologies Germany activity 729 Bq surface emissionrate 331 sminus1 in 2π sr relative uncertainty of the alpha surfaceemission rate 3 diameter of active surface 58 mm 241Am isincorporated into the surface of an anodized foil of 03 mmthickness thickness of the activated layer is appox 6 μm

In the second part for correlating track density with simulationstudies 241Am source S2 was used Specifications of S2 are asfollows Eckert and Zeigler USA EAB-241-PL activity 3585 Bqsurface emission rate 1775 sminus1 in 2π sr The total uncertainty ofthe alpha surface emission rate at 99 confidence level 3diameter of active surface 45 mm 241Am is electrodeposited onstainless steel backing

The CR-39 films were exposed in air at 2945 K with relativehumidity of 25 The irradiation of the CR-39 films as a function ofdistance (dfrac141ndash5 cm) was done using the experimental setupdepicted in Fig 1 The CR-39 films were exposed for duration of10 min 20 min 40 min and 60 min The etching of the exposedsamples was done with 6 M NaOH for duration of 4 h at 60 1C Theoptical microscope Olympus BX51MndashN35MF with attached CCDcamera and image analysis software Tomoroscope was used fortaking the images of the exposed and etched CR-39 samples Thesoftware Image Pro plus 60 was used for analysis of images todeduce the track density of the samples

Track density for non-exposed CR-39 is 200750 cmminus2 whichcorresponds to sim1 for the maximum and sim5 for the minimumtrack density obtained with S1 source To keep the blank valuelow care is taken to protect the CR-39 from naturalartificial

1 X 2 Optical Fiber

To Photo Detector Spectrometer

To Light Source

Incident reflected Light

Film under test

Opt

ical

Fib

er

Inci

dent

ligh

t

Ref

lect

ed li

ght

CR-39Air

Air

Base plate241Am

d

Alpha particle

CR-39 Structure

241Am

Fig 2 Experimental set up for FORS (a) and a scheme of FORS for detection ofalpha particle radiations (b)

Fig 3 Comparison of track density with reflected intensity of CR-39 film

PM Joshirao et al Applied Radiation and Isotopes 81 (2013) 184ndash189186

radiations by storing it along with the film (provided by themanufacturer) in properly shielded environment

22 Fiber optics radiation sensor studies

A bifurcated polymer optical fiber with a 12 fiber coupler(50 50 Industrial Fiber Optics Inc IF-562) shown in Fig 2(a) wasused for the detection of the reflected signal The blue light source(λfrac14458 nm Newport USA) was used to launch the light into thefiber The reflected light was measured using a Si photodiodedetector (Thorlab PDA36A) or a spectrometer (2048 Avaspec) Thescheme of the FORS is depicted in Fig 2(b) along with the raydiagram and chemical structure of CR-39 and alpha particles TheFORS is configured in the reflectance mode When the incidentlight interacts with the CR-39 film a fraction of light is reflectedfrom the surface (both top and bottom) another fraction isabsorbed and the remainder is transmitted through The reflectedlight from the surface as well as through the bulk of CR-39 filmis measured by photo diode detector and spectrometer Theirradiated CR-39 film was found to have the absorbance in nearUV region Therefore the present work was based on blue incidentlight (λfrac14458 nm) A digital multimeter (Keithley 2700) was usedto retrieve the sensor output and the data was recorded on a PCwhich was used to analyze the sensor performance

Changes in the surface topology of CR-39 post irradiation weremeasured by AFM The AFM images were taken using lsquoVeecodi Innovarsquo AFM machine The roughness was monitored undertapping mode the area of each field being 5 μm5μm

The instruments which are used in the reflectance roughnessand track density experiments are all computer software con-trolled and have high repeatability and reproducibility (high

precision) In case of reflected intensity measurements the varia-tion in reflected intensity of CRFOS was found to be less than705 Hence considering all other experimental variations thepresented CRFOS measurement setup has repeatability and repro-ducibility of better than 710

23 Simulation

GEANT4 (Agostinelli et al 2003) simulation was performedfor the propagation of low energy alphas from 241Am source It isan open source program created by the GEANT4 collaborationand it helps modeling problems in nuclear and high energyphysics (Ivanchenko 2002 Rhee et al 2006) medical physics(Shin et al 2011a) and other applications (Shin et al 2011b) Inthis work GEANT4 version 94p02 was used A detailed descrip-tion and validation of the code can be found on the GEANT4website (GEANT4 2012) The alpha particles emitted from 241Amhave a continuous energy spectrum with three principal peaks at5485 MeV 5443 MeV and 5388 MeV with corresponding yieldsper decay of 0845 013 and 0016 respectively (Neti et al 2004)For practical simulations the energy spectra of the alpha particlesemitted from 241Am source were modeled from the data availablefrom the table of radionuclides (DDEP 2012)

3 Results and discussion

31 Fiber optics reflection studies

The reflected intensity of CR-39 film was recorded onlineduring exposure of alpha particles emanated from 241Am source(S1) as a function of time and distance Fig 3 shows the valuesobtained on exposure for 1 h

The change in the reflected intensity was significant at 1 cmand it decreased rapidly with increasing distance as the fluence ofalpha radiations interacting with CR-39 decreased sharply from1 cm to 4 cm Reflected intensity at 5 cm distance was similar tothat of blank CR-39 which confirms that the range of alphaparticles in air is o5 cm Fig 3 shows that the track densitymeasured by optical microscope after chemical etching andreflected intensity follow the same pattern with increasing dis-tance of detector from the radiation source The interaction ofradiations with the CR-39 film leads to chemical changes like chainscissioncross linking leading to permanent structural damage andalteration in its physico-chemical properties like uvvis absor-bance topology and thermal behavior (Zaki 2008) The reflectanceis strongly dependent on film surface topology To characterizesurface topology of CR-39 film the surface roughness of irradiatedfilms was measured using AFM in terms of the root mean square

Table 1Variation of physical properties of CR-39 as a function of distance from the 241Amsource for irradiation time of 1 h

Sample distance AFM RMS roughness (nm) Reflected intensity (a u)

Blank 04089700017 53853727050 cm 04037700209 54829725040 cm 03921700012 55029728030 cm 03864700023 55853728020 cm 03823700010 56492727010 cm 03825700150 56604728505 cm 03824700007 566677285

Fig 4 Open GL picture showing the propagation of alpha particles emitted from241Am source located at the bottom CR-39 film is attached to the petri dish


(RMS) roughness AFM gives 3D image track profile unlike the 2Dimage of optical microscope It can be seen from Table 1 that thesurface roughness decreases with the alpha particle irradiation

Excellent inverse relation between reflectance and RMS rough-ness and direct relation between reflectance and track densitysuggest distinct possibility of employing reflectance change as amonitor of alpha radiation flux CRFOS is much more sensitive ascompared to conventional track density measurement methodThe 241Am source (S1) which we used during the present work hasthe activity 729 Bq which corresponds to sim6 ng It is to be noticedfrom Table 1 that from blank to 05 cm distance the reflectedintensity change is 2814 counts for change in roughness of00265 nm Since the precision of measurements in reflectancevalues is 710 when exposed to sim6 ng of 241Am at 05 cmdistance it implies that the change in reflectance for 06 ng241Am at 05 cm distance will be sim280 counts which can bemeasured with a precision of 7100 Thus the present methodcan be considered suitable to detect nanogram quantity of 241Am ifpresent in a dispersion and exposes CRFOS for a period of fewminutes The present CRFOS has repeatability and reproducibilitybetter than 710 However the conventional track densitymeasurement has poorer reproducibility varying between 75and 720 depending on the distance or the absolute value

It is observed that change in reflectance is a more sensitive aswell as more precise parameter as compared to the track densityto monitor the fluence of alpha particles It is possibly due to thefact that every incident alpha particle may contribute towardsreflectance change but may not contribute towards track densityAs mentioned earlier the latter depends in a poly energetic systemon the fraction of incident particles possessing the thresholdenergy and critical angle An attempt has been made in thepresent work to arrive at this fraction by simulation usingGEANT4 code

32 GEANT4 simulations

Fig 4 shows a snap shot of the propagation of alpha particlesfrom the 241Am source (S2) 241Am emits alpha particles in randomdirections Though alpha particles appear to propagate in straightlines in Fig 4 they actually go through thousands of collisionswith air molecules ionize them and eventually only a fraction ofthe emitted alphas reach the CR-39 film and stop inside the filmThe fraction of alphas reaching the CR-39 film depends on thefraction of surface area of the film to the total exposed surface areaby the source as well as its distance from the source

The projected range of alpha particles was calculated in the airand also within CR-39 There was excellent correlation of theprojected range obtained from GEANT4 simulations with theliterature values (NIST 2011 Lounis et al 2001) GEANT4 simula-tion results plotted by the circles in Fig 5(a) describe well thealpha particle projected ranges in dry air However our experi-ments are done at 215 1C and at relative humidity of 25 To dothe simulation for this situation we have modeled the humid air

by GEANT4 and re-calculated the projected range of alpha parti-cles The results are plotted by the squares in Fig 5(a) Althoughthe dry air and the humid air have different chemical composi-tions the projected ranges calculated for the dry air and the humidair are indistinguishable The projected range of the alpha particleswith energy of 5485 MeV is 418 cm and this is consistent withthe small values of track densities for exposure distance of 4 cmshown in Fig 3 The projected range of the alpha particles insidethe CR-39 material is calculated and plotted in Fig 5(b) It can beseen that the calculated projected range in CR-39 from ourGEANT4 simulation agrees with experimental data available(Lounis et al 2001)

The alpha particles reach the detector at varying energies andangles and thus follow different path and energy profile withinthe detector The calculated energy and angular distributions ofalpha particles at the surface of the CR-39 film are shown in Fig 6for different values of d from the alpha source to the surface of CR-39 The energy spectra of alphas for dfrac141 cm 2 cm and 3 cm areshown in Fig 6(a) When the CR-39 film is closer to the source(dfrac141 cm) more alpha particles can reach the scoring region withhigher energies due to lesser collisions of alphas with air mole-cules As expected the energy distribution of alphas for dfrac141 cm ispeaked at 45 MeV which is about 1 MeV lower than the energy ofthe alphas emitted from the source The energy distributions fordfrac142 cm and 3 cm are peaked at sim35 MeV and sim2 MeV respec-tively Also the number of alphas for dfrac143 cm are reduced by afactor of 5 as compared to those for dfrac141 cm Fig 6(b) shows thatfor a smaller distance such as dfrac141 cm more alpha particles reachthe detector at smaller angles For larger distance from the sourcethe alpha particles reach the detector at larger angles For eachdistance d there is a characteristic threshold angle θprime as can beseen in Fig 1 This threshold angle θprime is obviously determined bythe geometry of the source with respect to the CR-39 film Theangle θ at which the alphas really impinge on the CR-39 is unlikelyto be smaller than this threshold angle θacute for the present source-detector schematic shown in Fig 1 In fact our simulation resultsshows that only less than about 05 of alphas reach the CR-39film with angles smaller than the threshold angle θprime The values ofθprimecalculated from Fig 1 are 2001 3601 and 4751 for dfrac141 cm 2 cmand 3 cm respectively and agree well with our simulation resultsshown in Fig 6(b) Peak maxima for simulated critical angleshowever were observed at 301 (1 cm) 501 (2 cm) and 601 (3 cm)

As noted in the introduction only those latent damagesregistered on the SSNTD at an angle greater than the critical angle

Fig 5 Projected ranges of the alpha particles in the air (a) and in CR-39 material (b)

Fig 6 Energy (a) and angular distribution (b) of alphas scored at the CR-39 surface

Table 2The critical angles (deg) obtained experimen-tally (θc) and the threshold angles (θprime) deter-mined by the detector system geometry

d (cm) θc (deg) θprime(deg)

1 154 2002 320 3603 383 475

Table 3Track densities (cmminus2) observed in the CR-39 film with 241Am source of 3585 Bq

Distance(cm)

Duration of exposure (min)

10 20 40 60

1 3426877076 7661775351 132421711835 1747857107332 2522773796 5333374564 8983374408 133522776873 1336073648 2400073087 4645074074 6300075125


of incidence will be visible as tracks after etching process Thecritical angle values can be obtained from the expressionθcfrac14sinminus1 (VbVt) where Vb is the bulk etch rate and Vt is the tracketch rate The critical angles obtained experimentally from theabove relation (assuming constant Vt) are listed in Table 2 alongwith the threshold angles determined by the geometry of thesystem Table 2 shows that for all the values of d (1 cm 2 cm and3 cm) the threshold angles are larger than the critical angles forour detector system The critical angles θc are the characteristics ofthe material CR-39 whereas the threshold angles θprime are purelygeometrical Whereas the bulk etch rate was measured withsufficient accuracy by measuring the track diameters the tracketch rate was determined indirectly by measuring the trackdiameters on the detector surface and assuming a constant tracketch rate along the particle trajectories In deeper detector layersthis assumption is however not valid because the energy losssharply increases at the end of the particle trajectory therebyincreasing the track etch rate

Since θc (calculated assuming constant Vt) is less than θprime mostalphas reaching the CR-39 film are likely to leave a track on thefilm in the present configuration provided they have the thresh-oldoptimum energy to cause the damage which can be observedas a track under optimized etching conditions Thus we cancompare the calculated number of alphas incident on the surfaceof CR-39 film with the experimentally measured values of thetracks The latter represents the number of alphas which cause

measurable damage in CR-39 film Table 3 summarizes the trackdensities (cmminus2) observed in the CR-39 film with 241Am source ofactivity 3585 Bq (S2) as a function of distance and exposure time

The track densities observed experimentally and by GEANT4simulations are compared in Fig 7 for four different exposuredurations Results agree very well in most conditions but there aresome discrepancies seen at dfrac141 cm for exposure of 40 min and60 min Possible reasons for this discrepancy are (a) coalescing ofthe tracks for larger exposure time (b) uncertainty in the thresh-old energy (needed for registration of tracks) and (c) variation oftrack etch rate (Vt) particularly for short distance (dfrac141 cm)Overlap of tracks has been reported by several authors(Wertheim et al 2010a Zylstra et al 2012) Confocal laserscanning microscopy used by UK group to quantify the coalescingtracks has suggested their proportion as high as 30 (Wertheimet al 2010b) It is of interest to understand the conditions for theonset of the coalescence of the tracks Further studies are neededin this direction

4 Conclusions

Fiber Optics Reflectance Sensor based on CR-39 offers apromising approach to monitor low level alpha radiations emittingfrom 241Am source Change in reflectance was found to beinversely related to roughness of the film as seen by AFM and

Fig 7 Comparison of experimentally observed track density with the numberdensity of alpha particles incident on the CR-39 film


directly related to track density measured by optical microscopeThe present method can be considered suitable to detect nano-gram quantity of 241Am if present in a dispersion and exposesCRFOS for a period of few minutes The present CRFOS hasrepeatability and reproducibility better than 710 Howeverthe conventional track density measurement has poorer reprodu-cibility varying between 75 and 720 depending on theabsolute value of distance of detector from the source There wasgood agreement in the observed track density and that calculatedusing a Monte Carlo simulation exercise based on GEANT4 forvarying distance of detector and source of radiations

We will like to extend our work to design a devicesensorwhich can be installed in a nuclear facility like operating reactorspent fuel storage pond fuel reprocessing facility fuel fabricationplant waste management plant to respond to the alpha particlesemitted from accidental release of transuranics like microgramamount of 239Pu nano gram amount of 241Am picogram amountof 244Cm Such detector may also be useful in a facility where241Am is used for the production of smoke detectors or forindustrial gauging applications Many of these facilities handlegramskilogram quantities of alpha emitters There is a distinctpossibility that under accidental conditions milligram amounts ofthese transuranics can be released to the environment Theprinciple of the present work can be employed to detect nanogramquantities of 241Am if CRFOS can be designed to be exposed to theemitted alpha radiations through aerosolsdispersions in thefacility environment for a few minutes

Acknowledgments

This research was supported by the World Class University(WCU) program to VKM and SWH through the NationalResearch Foundation of Korea (NRF) funded by the Ministry ofEducation Science and Technology (R31-2008-10029)

References

Agostinelli S et al 2003 GEANT4-a simulation toolkit Nucl Instr Meth Phys ResA 506 250ndash303

DDEP 2012 Decay data evaluation project Available from langhttpwwwnucleideorgDDEP_WGDDEPdatahtmrang

Dorschel B Bretschneider R Hermsdorf D Kadner K Kuhne H 1999Measurement of track etch rates along proton and alpha particle trajectoriesin CR ndash 39 and calculation of the detection efficiency Radiat Meas 31 103ndash108

Fleischer RL Price PB Walker RM 1975 Nuclear Tracks in Solids Principlesand Applications University of California Press Berkeley

Geant4 2012 Geant4 a toolkit for the simulation of the passage of particlesthrough matter Available from langhttpgeant4webcernchgeant4rang

Ivanchenko VN 2002 Geant4 physics potential for HEP instrumentation NuclInstr Meth Phys Res A 494 514ndash519

Kulkarni A Vyas CK Kim H Kalsi PC Kim T Manchanda VK 2012 Onlineoptical monitor of alpha radiations using a polymeric solid state nuclear trackdetector CR-39 Sensors Actuators B 161 697ndash701

Lounis Z Djeffal S Morsli K Allab M 2001 Track etch parameters in CR-39detectors for proton and alpha particles of different energies Nucl Instr MethPhys Res B 179 543ndash550

Membrey F Fromm M El Rahamany A Chambaudet A 1993 Critical angle forlight ions registered in a CR ndash 39 SSNTD Variations with energy etchingconditions and etching time Nucl Tracks Radiat Meas 21 (3) 417ndash424

Neti PVSV et al 2004 A multi-port low-fluence alpha-particle irradiatorfabrication testing and benchmark radiobiological studies Radiat Res 161732ndash738

NIST 2011 Stopping-power and range tables for helium ions (ASTAR) Availablefrom langhttpphysicsnistgovPhysRefDataStarTextASTARhtmlrang

Rhee JT et al 2006 Study of the neutron sensitivity for the double gap RPC of theCMSLHC by using GEANT4 J Korean Phys Soc 48 33ndash39

Shin JW Hong SW Lee C-I Suh T-S 2011a Application of a GEANT4simulation to a 60Co therapy unit J Korean Phys Soc 59 12ndash19

Shin JW Park T-S Hong SW Park JK Kim JT Chai J-S 2011b Estimates ofSEU for semiconductors using MC50 cyclotron and GEANT4 simulationJ Korean Phys Soc 59 2022ndash2025

Stafford PM Horton JL Hogstrom KR Deluca Jr PM Holslin D 1988 Thecritical angle dependence of CR ndash 39 nuclear track detectors for 3ndash10 MeVprotons and 7ndash15 MeV alpha particles Nucl Tracks Radiat Meas 14 373ndash378

Wertheim D Gillmore G Brown L Petford N 2010a A new method for imagingparticle tracks in solid state nuclear track detectors J Microsc 237 1ndash6

Wertheim D Gillmore G Brown L Petford N 2010b 3-D imaging of particletracks in solid state nuclear track detectors Nat Hazards Earth Syst Sci 101033ndash1036

Zaki MF 2008 Gamma-induced modification on optical band gap of CR-39 SSNTDJ Phys D Appl Phys 41

Zylstra AB et al 2012 A new model to account for track overlap in CR-39 dataNucl Instr Meth Phys Res A 681 84ndash90

Fig 1 Schematic diagram of the experimental setup


occurring radioactive aerosols particularly from decay products ofradon or thoron gas can pose a health risk at indoor locations suchas mines and dwellings However measurements of low levelalpha radiations presents a challenge due to their limited range inthe media through which they travel

Common method of measuring the concentration of atmo-spheric radioactive aerosol particles is to collect them on a filter bydrawing a known volume of air through it The activity depositedon the filter is then measured using scintillation detector SolidState Nuclear Track Detectors (SSNTDs) offer an alternativeapproach to monitor alpha radiations in the environment SSNTDsare insulating solids both naturally occurring and man-made Thepotential applications of SSNTDs are growing rapidly They havebeen employed in various areas related to nuclear scienceand technology viz nuclear physics nuclear imaging nucleartechnology space physics microanalysis mine safety environ-mental research uranium prospecting biomedical sciences mate-rial sciences and geological sciences (Fleischer et al 1975) Thereare several types of these detectors including inorganic crystalsglasses and plastics When a heavily ionizing charged particlepasses through such insulating solids it leaves a narrow trail ofdamage of about 50 Aring in diameter along its path This is calledlsquoLatent Trackprime as it cannot be seen with the naked eye It is possibleto view this latent track with an electron microscope The exactnature of the physical and chemical changes occurring at thedamage site depends on the charge(Z) and velocity term (βfrac14νcwhere ν is the particle velocity and c is the velocity of light) of theparticle on the chemical structure and storage conditions of thedetector material These latent tracks can be enlarged and can beviewed under an optical microscope by etching with somechemicals such as sodium hydroxide or hydrofluoric acid How-ever neither of these methods is satisfactory for real-time radia-tion detection due to their inherent off line character Thus there isa need to develop sensor for real-time detection of alpha particlesor heavy ions in the environment

A radiation sensor using an optical fiber consists of a scintilla-tor an optical fiber and a photo-detector The scintillation photonsinduced by the exposed radiations are detected by the photo-detector system through an optical fiber which is not influencedby temperature pressure or electromagnetic waves A fiber opticradiation sensor (FORS) ensures real-time measurement excellentspatial resolution and high flexibility Recently our group hasreported the first ever attempt to use CR-39 based reflectancesensor to quantify the alpha radiations emitted from 232Th(Kulkarni et al 2012)

An attempt has been made in the present work to develop fiberoptic radiation sensor to monitor alpha radiations emitted from241Am source employing CR-39 as a detector The fiber optic probeis configured in reflectance mode The proposed FORS utilizes afast photo detector instead of a photomultiplier tube (PMT) usedin scintillation photon optical sensors and it has the maximumsensitivity in the short wavelength region of visible light Theexposed SSNTDs are also analyzed by conventional track densitymeasurements and the data is correlated with the reflectance dataAs the reflectance of the exposed CR-39 film depends on itsroughness an attempt is also made to evaluate its surface topologyusing Atomic Force Microscopy (AFM)

In the case of a mono energetic flux of incident radiationdetermination of the real number of projectiles impinging ondetector per unit area per unit time can be obtained from thesource strength and geometry of source and detector However ina poly energetic system all particles incident on CR-39 may notresult in etchable tracks (Membrey et al 1993) Only those latentdamages registered on the SSNTD at an angle greater than thecritical angle of etching will be visible as tracks Thus thededuction of critical angle values for given etching conditions

can be useful to quantify the real particle flux under givenexposure (Stafford et al 1988 Dorschel et al 1999)

An attempt has also been made to correlate the number oftracks per unit area (track density) and critical angle obtainedexperimentally with the values obtained using Monte Carlosimulations (GEANT4) for a given source-detector geometry

2 Experimental

21 Schematic of set up

Fig 1 shows a schematic diagram of our experimental setup forexposure of detector film It consists of a CR-39 film pasted insidea petri-dish an 241Am source and a plastic cylindrical ring TheCR-39 (Allyl diglycol carbonate) film of 1 cm1 cm007 cm wasprocured from PPG Coatings Co Ltd (Hong Kong)

Two different sources were used in the present work In thefirst part for Fiber Optic Reflection and AFM studies 241Amcalibration source S1 was used Specifications of S1 are as followsBerthold Technologies Germany activity 729 Bq surface emissionrate 331 sminus1 in 2π sr relative uncertainty of the alpha surfaceemission rate 3 diameter of active surface 58 mm 241Am isincorporated into the surface of an anodized foil of 03 mmthickness thickness of the activated layer is appox 6 μm

In the second part for correlating track density with simulationstudies 241Am source S2 was used Specifications of S2 are asfollows Eckert and Zeigler USA EAB-241-PL activity 3585 Bqsurface emission rate 1775 sminus1 in 2π sr The total uncertainty ofthe alpha surface emission rate at 99 confidence level 3diameter of active surface 45 mm 241Am is electrodeposited onstainless steel backing

The CR-39 films were exposed in air at 2945 K with relativehumidity of 25 The irradiation of the CR-39 films as a function ofdistance (dfrac141ndash5 cm) was done using the experimental setupdepicted in Fig 1 The CR-39 films were exposed for duration of10 min 20 min 40 min and 60 min The etching of the exposedsamples was done with 6 M NaOH for duration of 4 h at 60 1C Theoptical microscope Olympus BX51MndashN35MF with attached CCDcamera and image analysis software Tomoroscope was used fortaking the images of the exposed and etched CR-39 samples Thesoftware Image Pro plus 60 was used for analysis of images todeduce the track density of the samples

Track density for non-exposed CR-39 is 200750 cmminus2 whichcorresponds to sim1 for the maximum and sim5 for the minimumtrack density obtained with S1 source To keep the blank valuelow care is taken to protect the CR-39 from naturalartificial

1 X 2 Optical Fiber


To Light Source


Film under test

Opt

ical

Fib

er

Inci

dent

ligh

t

Ref

lect

ed li

ght

CR-39Air

Air

Base plate241Am

d

Alpha particle

CR-39 Structure

241Am










23 Simulation
























1 154 2002 320 3603 383 475


Distance(cm)


10 20 40 60

1 3426877076 7661775351 132421711835 1747857107332 2522773796 5333374564 8983374408 133522776873 1336073648 2400073087 4645074074 6300075125






4 Conclusions






Acknowledgments


References




















1 X 2 Optical Fiber


To Light Source


Film under test

Opt

ical

Fib

er

Inci

dent

ligh

t

Ref

lect

ed li

ght

CR-39Air

Air

Base plate241Am

d

Alpha particle

CR-39 Structure

241Am










23 Simulation
























1 154 2002 320 3603 383 475


Distance(cm)


10 20 40 60

1 3426877076 7661775351 132421711835 1747857107332 2522773796 5333374564 8983374408 133522776873 1336073648 2400073087 4645074074 6300075125






4 Conclusions






Acknowledgments


References






































1 154 2002 320 3603 383 475


Distance(cm)


10 20 40 60

1 3426877076 7661775351 132421711835 1747857107332 2522773796 5333374564 8983374408 133522776873 1336073648 2400073087 4645074074 6300075125






4 Conclusions






Acknowledgments


References
























Acknowledgments


References




















development of optical monitor of alpha radiations based on cr-39

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