Estimation of neutron-equivalent dose in organs of patients undergoing radiotherapy by the use of

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Estimation of neutron-equivalent dose in organs of patients undergoing radiotherapy by the
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2012 Phys. Med. Biol. 57 6167
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IOPPUBLISHING PHYSICSINMEDICINEANDBIOLOGY
Phys.Med.Biol.57(2012) 6167–6191 doi:10.1088/0031-9155/57/19/6167
Estimation of neutron-equivalent dose in organs of
patients undergoing radiotherapy by the use of a novel
online digital detector
FSa´nchez-Doblado1,2,CDomingo3,FGo´mez4,BSa´nchez-Nieto5,
JLMun˜iz6,MJGarc´ıa-Fuste´3,MRExpo´sito2,RBarquero7,
GHartmann8,JATerro´n1,JPena4,RMe´ndez6,FGutie´rrez9,
FXGuerre10,JRosello´11,LNu´n˜ez12,LBrualla-Gonza´lez11,
FManchado2,ALorente13,EGallego13,RCapote14,DPlanes11,
JILagares6,XGonza´lez-Soto4,FSansaloni6,RColmenares15,
KAmgarou3,EMorales3,RBedogni16,JPCano2andFFerna´ndez17
1ServiciodeRadiof´ısica,HospitalUniversitarioVirgenMacarena,Sevilla,Spain
2DepartamentodeFisiolog´ıaMe´dicayBiof´ısica,UniversidaddeSevilla,Sevilla,Spain
3DepartamentodeF´ısica,UniversitatAuto`nomadeBarcelona,Barcelona,Spain
4DepartamentodeF´ısicadePart´ıculas,UniversidaddeSantiagodeCompostela,Santiagode
Compostela,Spain
5DepartamentodeF´ısica,PontificiaUniversidadCato´licadeChile,Santiago,Chile
6CentrodeInvestigacionesEnerge´ticas,MedioambientalesyTecnolo´gicas(CIEMAT),Madrid,
Spain
7ServiciodeProteccio´nRadiolo´gica,HospitalUniversitarioR´ıoHortega,Valladolid,Spain
8DepartmentofMedicalPhysicsinRadiotherapy,DeutschesKrebsforschungszentrum(DKFZ),
Heidelberg,Germany
9Optoelectronics&NewTechnologiesLaboratory,AlterTechnology,Sevilla,Spain
10HirexEngineering,RamonvilleS,Agne,France
11UnidaddeRadioterapiaHospitalGeneralUniversitariodeValencia,ERESA,Valencia,Spain
12ServiciodeRadiof´ısica,HospitalUniversitarioPuertadeHierro,Majadahonda,Spain
13DepartamentodeIngenier´ıaNuclear,UniversidadPolite´cnicaMadrid,Madrid,Spain
14InternationalAtomicEnergyAgency(IAEA),Viena,Austria
15UnidaddeRadiof´ısicaCl´ınica,HospitalUniversitarioRamo´nyCajal,Madrid,Spain
16LaboratoriNazionalidiFrascati,IstitutoNazionalediFisicaNucleare(INFN),Italy
17ConsejodeSeguridadNuclear(CSN),Madrid,Spain
E-mail:[email protected]
Received7May2012,infinalform18July2012
Published13September2012
Onlineatstacks.iop.org/PMB/57/6167
Abstract
Neutron peripheral contamination in patients undergoing high-energy photon
radiotherapy is considered as a risk factor for secondary cancer induction.
Organ-specific neutron-equivalent dose estimation is therefore essential for a
reasonableassessmentoftheseassociatedrisks.Thisworkaimedtodevelopa
methodtoestimateneutron-equivalentdosesinmultipleorgansofradiotherapy
patients. The method involved the convolution, at 16 reference points in an
anthropomorphic phantom, of the normalized Monte Carlo neutron fluence
energyspectrawiththekermaandenergy-dependentradiationweightingfactor.
This was then scaled with the total neutron fluence measured with passive
0031-9155/12/196167+25$33.00 ©2012InstituteofPhysicsandEngineeringinMedicine PrintedintheUK&theUSA 6167
6168 FSa´nchez-Dobladoetal
detectors,atthesamereferencepoints,inordertoobtaintheequivalentdoses
in organs. The latter were correlated with the readings of a neutron digital
detector located inside the treatment room during phantom irradiation. This
digital detector, designed and developed by our group, integrates the thermal
neutronfluence.Thecorrelationmodel,appliedtothedigitaldetectorreadings
duringpatientirradiation,enablestheonlineestimationofneutron-equivalent
dosesinorgans.Themodeltakesintoaccountthespecificirradiationsite,the
field parameters (energy, field size, angle incidence, etc) and the installation
(linacandbunkergeometry).Thismethod,whichissuitableforroutineclinical
use, will help to systematically generate the dosimetric data essential for the
improvementofcurrentrisk-estimationmodels.
1.Introduction
There is a growing concern about the risk of secondary radiation-induced tumours as a
consequence of the radiotherapy treatments. The comprehensive review article by Xu et al
(2008)detailstheprinciples,methodsandunresolvedissuesrelatedtothissubject.
Both scattered and leakage photon radiations, as well as secondary neutrons, are
responsiblefortheperipheralunwanteddose.Anout-of-fieldphoton-absorbeddoseistypically
largerthananeutron-absorbeddose(Bednarzetal2009),butbecauseoftheradiationweighting
factor(w ),neutronsmaybethedominantsourceoftissuedamagefromsecondaryradiation
R
atsomelocations(Kryetal2005).
Epidemiological risk assessments should evaluate the secondary cancer rate incidences
in a cohort of patients in relation to their organ-specific equivalent doses. In this sense,
dosimetricdata,properlygenerated,areessentialtoestablishareasonableassessmentofthese
associatedrisks.Therearepreviousdosimetricstudiesdevotedtoassessneutronandphoton
contamination.Xuetal(2008)coveredsomeofthesemajordosimetricstudies.
Treatment planning procedures provide detailed dosimetric information on the volume
coveredbytheCTscan.However,toourknowledge,nosystematicdosimetrymethodology
forthequantificationofneutron-equivalentdosesinorgansoutsidetheplanningvolumehas
beenproposedyet.
The aim of this work was to develop a methodology for the systematic estimation of
neutron-equivalent doses in organs for patients undergoing high-energy (above 10 MV)
external photon beam radiotherapy. A measurement- and Monte Carlo (MC)-based model
is presented here as a general applicable method for the online estimation of equivalent
dose in specific organs of radiotherapy patients. First, an appropriate active digital neutron
detector, based on the relation existing between upsets in digital memories and thermal
neutronfluence,wasdesignedanddeveloped(Go´mezetal2010).Thisdetectorovercomesthe
problemsofcurrentactive(mainlysaturationandphotonsensitivity)andpassive(withtime-
consuming and cumbersome readout procedures) detectors. Whole-body anthropomorphic
phantom measurements, carried out with passive detectors, combined with a detailed MC
model are correlated with the digital detector readings under a variety of conditions which
potentiallycouldaffectsecondaryneutronradiation.Thesamemethodologyisapplicableto
anyotherneutron-sensitivedetectorafteraspecificcalibrationprocedure.
Beingabletosystematicallyestimateandreporttheneutron-equivalentdoseinperipheral
organs,similarlytowhattreatmentplanningsystemsperforminthecaseofin-fieldorgans,will
constituteasteptowardsprovidingsomeoftheessentialdataneededforapropersecondary
Estimationofneutron-equivalentdose 6169
Figure 1. (Left) Scheme of the anthropomorphic phantom designed and built for this work
(NORMA). It shows the points where measurements and simulations were performed. Points
HandArepresenttheisocentresforheadandabdomentreatments,respectively.(Topright)The
phantomonthetreatmentcouchandthedigitaldetectorlocatedinthemeasurementposition(circle)
areshown.(Bottomright)DedicatedcavitiesinsideNORMAtoallocatethepassivedetectorsare
shown.
cancerriskanalysis.Thelatterwillbeusefulforabettertreatmentstrategyselection.Itwill
also be a valuable tool for comparing the relative level of secondary radiation for different
acceleratorsandtreatmentprocedures.
2.Materialandmethods
Thepurposeofthisworkentailsestablishingamethodologytocorrelatethereadingsofthe
activedetectorandtheneutron-equivalentdosesinpre-establishedspecificorgansofpatients
undergoingexternalphotonbeamradiotherapy.
First,asetofirradiationconditionswhichincludedsingle-beamincidencesandcomplete
radiotherapy treatments (see section 2.2) was defined. Second, a detailed model of the
photoneutronfieldinsidearadiotherapytreatmentroomandatspecificpointsinsideawhole-
body anthropomorphic phantom was generated for the different irradiation conditions and
treatmentfacilitieschosen.Thispartofthestudywasperformedbycombiningexperimental
neutronspectrometrywithMCsimulations.Third,fluencemeasurementswerecarriedoutfor
the defined irradiation setups. More specifically, simultaneous readings carried out with the
digitaldetectorinsidethetreatmentroomandwithtwotypesofpassivedetectors(poly-allyl-
diglycol-carbonate (PADC) and 6Li/7Li thermoluminiscence dosimeters (TLD)) at specific
pointsinsidetheanthropomorphicphantomwereperformed.
2.1.NORMA:theanthropomorphicphantom
A whole-body anthropomorphic female phantom called NORMA (figure 1) was designed
and built—in polyethylene and low-density wood for lungs—with 16 measurement points
distributed along it and covering different depths. The selection of the points in NORMA
representingeachorganwasmadeusingtheorgansizesandpositionsfromtherevisedversion
6170 FSa´nchez-Dobladoetal
Table1.NORMApointstoorganscorrelation.
Organsandtissues NORMApoints
Thyroid 4
Oesophagus 4,9,16
Lung 7,8
Breast 5,6,15
Stomach 9,11,16
Liver 9,10,11,16
Colon 11,12
Urinarybladder 10
Ovary 11,12
Skin 15
Bone 1,3,9,12,13,14,15
Bonemarrow 9,12,15
Remainder Allexcept7,8,15
of the Cristy anthropomorphic phantom (available at http://ordose.ornl.gov/resources/
phantom).Table1showsthecorrespondencebetweenorganandNORMApoints.Theauthors
areawareofthelimitationsofthismethodology.However,dosesinsidethephantomdonot
show extremely sharp changes and the selected points could be accepted given our level of
uncertainty. Nevertheless, extra measurements points were added in some organs to better
simulatethedifferentcontributionstothedose(e.g.,askinpointforbreast).Thus,neutron-
equivalentdosesinorganswereestimatedthroughanaverageofthecorrespondingNORMA
points.
2.2.Irradiationconditions
Sevendifferentirradiationconditionsweresetupasfollows:
(i) head and neck complete treatment, eight beam incidences (at 0◦, 45◦, 90◦, 135◦, 180◦,
225◦,270◦,315◦),10 × 10cm2fieldsize,isocentreatpointH(seefigure1),
(ii) abdomencompletetreatment,thesamebeamincidencesandfieldsizeasini,isocentre
atpointA(seefigure1),
(iii) fixed2 × 2cm2irradiationfield,gantryat0◦,isocentreatpointA,
(iv) fixed5 × 5cm2irradiationfield,gantryat0◦,isocentreatpointA,
(v) fixed20 × 20cm2irradiationfield,gantryat0◦,isocentreatpointA,
(vi) fixed10 × 10cm2irradiationfield,gantryat0◦,isocentreatpointA,
(vii) fixed10 × 10cm2irradiationfield,gantryat270◦,isocentreatpointA.
Alltheseirradiationsweresetto1000monitorunits(MU).18Thedoseraterangedfrom
300to600MUmin–1.Theusedenergydependsonthetreatmentfacility.
For treatments (i) and (ii), dosimeters were placed at all 16 points in NORMA (see the
insetoffigure1).Theinfluenceoftheirradiationfieldsizewasstudiedfromcases(iii)to(vi),
withtwomeasurementpoints(2and8)insideNORMA,whiletheinfluenceofthebeamentry
anglewasstudiedthroughcases(vi)and(vii),withsixmeasurementspoints(2,7,8,11,13
and14).Additionally,somerealintensity-modulatedradiotherapytherapy(IMRT)treatments
werecompared,fromthepointofviewofneutronproduction,withcases(i)and(ii)usingthe
samenumberofmonitorunitsandgantryangles.
18OneMUwascalibratedtocorrespondto1cGyphotondosemeasuredatthedepthofmaximumdoseinwater
(source-to-surfacedistanceequalto100cm).
Estimationofneutron-equivalentdose 6171
Table2.Descriptionofirradiationfacilities.
Nominal Bunker
Facility Linac energy(MV) volume(m3)
HospitalUniversitarioVirgenMacarena SiemensPrimus 15 317(b1)
(HUVM),Seville(Spain)a 162(b2)
HospitalUniversitarioPuertadeHierro VarianClinac2100 15 205
(HUPH),Majadahonda(Spain)
HospitalUniversitarioRamo´nyCajal ElektaSynergy 15 172
(HURC),Madrid(Spain)
HospitalGeneralUniversitariodeValencia SiemensPrimus 18 150
(HGUV),Valencia(Spain)
Universita¨tsKlinikum(UK),Heidelberg SiemensMevatron 23 169
(Germany)
aAtHUVM,twodifferentbunkers(b1andb2)withthesamelinac.
Irradiation facilities are described in table 2. In particular, the two linacs in Seville are
of special interest in order to study bunker size dependence, as they are identical machines,
operatingunderidenticalconditions,withtheonlydifferencebeingthetreatmentroomvolume:
one of them considerably large (bunker 1: 317 m3) and the other one considerably smaller
(bunker2:162m3).
2.3.NeutronspectraMCsimulations
TheMCsimulationwascarriedoutusingtheMCNPX-2.6.0code(Hendricksetal2008)and
followingthedetailedmodelofaSiemensPRIMUSandcodelibrariesdescribedbyPenaetal
(2005).MoredetailsaboutthesesimulationsweregivenbyGo´nzalez-Sotoetal(2012).
In order to take into account the fact that the amount of neutrons produced per MU
dependsappreciablyontheenergyoftheprimaryelectronbeam,aMCcommissioningwas
performed. The primary electron source was chosen to yield (for a 10 cm × 10 cm field at
0◦),atapointlaterallydisplaced50cmfromtheisocentre,thesamefastneutronfluenceas
theonemeasuredbytheBonnerSphereSpectrometersystem(seesection2.4).Theprimary
electronsourcewasmodelledasaGaussiandistributioncentredatthechosenenergywitha
relativeFWHM=14%andaradialspatialdistributionwithFWHM=1.5mm.Additionally,
differentwaterdepth–dosecurvesandprofilesweregeneratedfromtheMCphasespaceand
comparedwiththemeasureddata(depthprofilemeandeviationwasbelow2%).
Theaimofthesessimulationswastoobtainthefollowing:
– Neutron fluence spectra at the isocentre produced by the Siemens linac operating at
differentnominalenergies(15,18,21,25MV)fordifferentsquarefields.
– Neutron fluence spectra of the same machine operating at 15 MV at the location of the
digitaldetector(seesection2.8)forninedifferentbunkerssizes(withthesameheightbut
different shapes and sizes covering the full range of typical treatment rooms, including
bunkers1and2inSeville).Inthiscase,eachbeamincidencewascarriedoutinasingle
simulation,includingthetreatmentroom,thephantomandthelinac.
– Neutron fluence spectra in all specific points in the phantom, for the head and neck
(case(i))andabdomen(case(ii))treatmentsforlinacinstalledinbunker1.Thesespectra
werecalculatedusinganf5tally(pointdetector).However,anadditionalsimulationwas
performedusingbothf5andf4(fluxaveragedovercell)talliestoverifythattheobtained
resultswereequivalent.
6172 FSa´nchez-Dobladoetal
All simulations were carried out using 48 double-processor computers, as a part of the
cluster belonging to the Medical Physics Group, Department of Medical Physiology and
Biophysics,UniversityofSeville(Lealetal2004).
2.4.Neutronspectrometryinthetreatmentroom
Amongthemanyavailableneutronspectrometrytechniques,themultisphereorBonnerSphere
Spectrometer isthemostusedforradiationprotectionpurposes (Thomas andAlevra 2002),
owing to advantageous characteristics such as a wide energy range (from thermal to GeV
neutrons),alargevarietyofactiveorpassivethermalsensorsthatallowadaptingthesensitivity
tothespecificworkplace,agoodphotondiscriminationandasimplesignalmanagement.The
passive Bonner Sphere Spectrometer used in this work was described by Ferna´ndez et al
(2007a,2007b),includingtheresponsematrixanalysis.
Following the standard terminology, (cid:2)E(E) ≡ (d(cid:2)(E)/dE) is the ene(cid:2)rgy spectrum
of the absolute neutron fluence so that the total neutron fluence is (cid:2) = (cid:2) (E)dE.
E E
Similarly,ϕE(E)≡ (dϕE(E)/dE)istheenergy(cid:2)spectrumoftheunitneutronfluenceorenergy
distributionoftheneutronfluencesothatϕ = ϕ (E)dE =1.Itis,therefore,obviousthat
E E
(cid:2) (E)=(cid:2)ϕ (E)foranyvalueofE.Itisalsocommonpracticetorepresentneutronspectra
E E
intermsoflethargy,plottingtheproductE·(cid:2) (E)orE·ϕ (E)versusE(inthelogarithmic
E E
scale). Such a plot has the visual property that equal areas below the graph represent equal
neutronfluences.
Inthiswork,theFRUITcodehasbeenusedforunfolding(Bedognietal2007),inorder
toobtaintheenergydistributionoftheneutronfluenceϕ (E)andthetotalneutronfluence(cid:2)
E
fromthereadingsofthedetectorsatthecentreoftheBonnerspheresandfromtheevaluated
∗
response matrix. Ambient dose-equivalent values are subsequently evaluated from the h (E)
conversioncoefficients(ICRU1998)as
(cid:3) (cid:3)
H∗ = (cid:2) (E)·h∗(E)·dE =(cid:2) ϕ (E)·h∗(E)·dE. (1)
E E
E E
Spectrometric measurements were made in all six irradiation facilities, with a static
10 × 10 cm2 field at 0◦, at a reference point located at the same height as the isocentre
but laterally displaced 50 cm (without the NORMA phantom on the couch), as well as at
the place where the digital detector is located (see section 2.8) (with NORMA) (Domingo
et al 2009b, 2010a). This set of data allowed us to verify the MC simulation results for the
Seville’sfacilities.Measurementswereperformedwithstatisticaluncertaintiesoftheorderof
1%. The combination of these uncertainties with the response matrix uncertainties and with
the ones related to the unfolding process allows us to determine fluence values with overall
uncertaintiesintherangeof3–5%andambientdose-equivalentvalueswithuncertaintiesin
therangeof4–7%.
2.5.PADC-baseddosimeters
AminiaturizedversionoftheUniversitatAuto`nomadeBarcelonaetchedtrackneutron(PADC)
dosimeters(Garc´ıaetal2005,Domingoetal2009b)hasbeenusedinthisworktodetermine
neutron fluence at all 16 specified points in the NORMA phantom (figure 1). Two extra
dosimeters were placed on top of the digital detector for each irradiation condition. These
‘miniature’dosimeterswereobtainedbycuttingthestandard(6 × 6)cm2dosimeterintonine
equal (2 × 2) cm2 pieces, which fit into our etching cells and which can be placed in the
required spots (figure 1). The standard electrochemical etching procedure from Universitat
Auto`nomadeBarcelona,describedbyBouassouleetal(1999),hasbeenapplied.Trackdensity
Estimationofneutron-equivalentdose 6173
Figure2.Neutronlethargyspectraofneutronfieldssuppliedforcalibrationoftheetchedtrack
neutron PADC dosimeters. Only spectra of types II and III were actually used for calibration
purposes(seethetext).
in the etched plates was measured using a semi-automatic counting system (Domingo et al
2009b).
The PADC dosimeters are sensitive to the whole energy range and their response had
alreadybeenexperimentallyvalidatedwithISOsourcesandrealisticneutronfieldsasspecified
bytheISOstandards(ISO1998,2000)attheInstitutdeRadioprotectionetdeSuˆrete´Nucle´aire
facilities(Garc´ıaetal2005),aswellasinquasi-monoenergeticneutronbeamsattheInstitute
for Reference Materials and Measurements from the European Joint Research Centre JRC-
IRMM(Ge¨el,Belgium)VanderGraafaccelerator(Domingoetal2009a).
PADC-based dosimeters are commonly calibrated for personal dosimetry in terms of
personaldoseequivalentH tocharacterizeexternalirradiationofanindividual.Inthiswork,
p
dosimetersareplacedinsidetheNORMAanthropomorphicphantomandintendtomeasure
the neutron field inside the phantom. Therefore, specific calibration irradiations of PADC
dosimeters were carried out at the Physikalisch-Technische Bundesanstalt primary neutron
facilitytotakeintoconsiderationthespecificcharacteristicsofneutronfieldsinsideNORMA
(previously estimated by the MC simulation, see the following text). Neutron irradiations
took place free in air with the purpose of characterizing the dosimeter response in terms of
fluencewhenplacedinalocation,nomatterwhere,inthepresenceofagivenneutronfield.
Threefluenceenergydistributions(seefigure2),andatdifferentincidenceanglestotakeinto
accounttheangulardependence,wereusedforcalibration(Bedognietal2012,Domingoetal
2012).Intable3,thefieldspectralcharacteristicsandcalibrationfactorsforthethreedifferent
neutroncalibrationspectraarereported.
Fastneutronsappearfromphotoneutron–productionreactionsintheacceleratorheadand
components.Whentheyscatterintheroomwallsandotherelementsinsidethetreatmentarea,
includingthepatientorthephantom,theybecomeepithermalandthermal.Therefore,while
thefastandtheepithermalneutroncomponentsarehighlydirectional,thethermalcomponent
can be approximated by an isotropic field at the point of measurement. This reasoning has
6174 FSa´nchez-Dobladoetal
Table3.CharacteristicsoftheneutronfieldsusedforcalibrationofthePADC-baseddosimeters
andtheresultingfluenceresponse.
SpectrumtypeI SpectrumtypeII SpectrumtypeIII
Description 252Cfsource 252Cf(D2O+Cd)+ 252Cfsource+
Shadowblock Shadowcone
Type Fast Epithermal+Thermal Fast+Epithermal+Thermal
p 0.052 0.312 0.211
t
p 0.103 0.592 0.38
e
p 0.845 0.097 0.409
f
F (Tracks/neutron) (1.118 ± 0.066) × 10−4 (4.65 ± 0.48) × 10−6 (1.762 ± 0.090) × 10−5
D
F (Tracks/neutron) (5.16 ± 0.90) × 10−5 (2.15 ± 0.24) × 10−6 (8.13 ± 0.58) × 10−6
iso
ptisthethermalfraction,peistheepithermalfraction,pfisthefastfractionandFDisthefluenceresponsefor
normalincidenceandFisoforisotropicincidence.
beenusedtocalculatethecalibrationfactorFwhichconvertstracksmeasuredinthedosimeter
(NPADC)intofluence((cid:2)PADC)inthefollowingway:
NPADC
(cid:2)PADC = , whereF = (p +p )·F +p ·F (2)
f e D t ISO
F
withp,p andp beingthefractionoffast,epithermalandthermal19neutronsinthespectrum
f e t
considered, respectively, and F and F being the corresponding calibration factors for
D ISO
directionalandisotropicincidences(seetable3).
Comparison of spectra in figure 2 with MC spectra at the phantom measurement
points (figure 8) illustrates that calibration spectrum type II is the most representative for
all measurement points, except for point 15, where spectrum type III is more adequate.
Consequently, the particular calibration factor for a PADC dosimeter placed at point i in a
givenirradiation jiscalculatedas
(cid:4) (cid:5)
Fi,j = pi,j+pi,j FII+pi,jFII fori(cid:5)=15
f e D t ISO
and (cid:4) (cid:5) (3)
Fi,j = pi,j+pi,j FIII+pi,jFIII fori=15
f e D t ISO
wherepi,j,pi,jandpi,jarethecorrespondingfractionsoffast,epithermalandthermalneutrons
f e t
obtainedfromtheMCsimulationforthepointandirradiationofinterest,andFII,FII ,FIII
D ISO D
and FIII are the F and F calibration factors for spectra of types II and III, respectively
ISO D ISO
(seetable3).Thetotalneutronfluenceatpointiduringirradiation jissubsequentlyevaluated
fromthereadingNPADCofthecorrespondingdosimeteras
i,j
NPADC
(cid:2)PADC = i,j . (4)
i,j Fi,j
2.6.Lithermoluminiscencedosimeters
Standard6LiF/7LiFpairsofdosimetersTLD-600/TLD-700(3 × 3 × 0.9mm3chips)have
been used as an independent system to assess the thermal neutron fluences in the selected
pointsinsideNORMA.
TLD readouts have been carried out using a Harshaw reader, model 4000, with linear
heatingfromroomtemperatureupto300◦Cataheatingrateof3◦Cs−1.Apre-irradiation
19Wewillusetheconventionalneutronclassificationaccordingtotheirkineticenergy,i.e.thermalneutrons:upto
0.4eV,theso-calledCadmiumcut;epithermalneutrons:from0.4eVto0.1MeV;andfastneutrons:from0.1MeV
to20MeV.
Estimationofneutron-equivalentdose 6175
thermal treatment of 1 h at 400 ◦C, followed by a reproducible cooling down to room
temperature, was always employed before reusing the detectors. Temperature and duration
oftheheatingandcoolingstageswereproperlycontrolled.
NeutroncalibrationwascarriedoutatPhysikalisch-TechnischeBundesanstaltinscattered
neutronreferenceradiationfieldsproducedbyabare252CfandaD O-moderated252Cfneutron
2
source (Kluge et al 1997), and for gamma, using a 137Cs calibration source at a secondary
standarddosimetrylaboratoryinCIEMAT.
The methodology employed to analyse all the experimental glow curves obtained was
basedonthenumericaltechniquesdevelopedbyDelgadoandGo´mez-Ros(1990,1992)and
Mun˜iz et al (1999, 2004). On the basis of the results of this glow curve analysis, thermal
neutronfluenceswereobtainedusingthedifferencesbetweenTLD-600andTLD-700results
foreachmeasuredpointinNORMA(R)andthecalibrationfactorsasfollows:
(cid:6) (cid:7) γ
f
(cid:2)TLD = fn Ri,j −kRi,j , k= 700, (5)
i,j 600/700 600 700 fγ
600
wherethecalibrationfactorsusedare fn =488ncm−2au(12%,k=2)forneutronsand
600/700
fγ =1.86×10−4mGyau–1(8%,k=2)and fγ =1.99×10−4mGyau−1(10%,k=2)for
700 600
gammas.
2.7.Evaluationofneutronfluence,doseequivalentandequivalentdose
BothPADCandTLDpassivedetectorswereusedtoevaluatetheneutronfluenceateachpoint
iinsideNORMAforeachirradiationcondition j,leadingtothevalues(cid:2)PADC(4)and(cid:2)TLD(5),
i,j i,j
respectively. These measurements were repeated in all irradiation facilities listed in table 2.
It is well known that 6Li/7Li TLD pairs allow a relatively precise estimation of the thermal
neutron component, but they are insensitive to the epithermal and fast components. On the
other hand, the PADC response is well characterized for fast neutrons, but relevant to large
uncertaintiesappearingintheepithermalandthermalranges.Takingallthatintoaccount,the
bestestimationoffluenceiscalculatedbycombiningthemeasurementsofbothdetectorsas
follows:
(cid:8) (cid:9)
(cid:2)i,j =(cid:2)Ti,LjD+ pie,j+pif,j (cid:2)Pi,AjDC. (6)
Uncertainties for the best estimation of the total fluence are obtained from standard
propagation of uncertainties of the TLD and PADC measurements and are expressed at the
standard(1SD)level.
Absorbeddoseisthemostusefulmeasurablequantity,asitallowsprotectionquantities
to be evaluated from tabulated weighting factors. Unfortunately, it is extremely difficult in
practice to measure the absorbed dose due to neutron irradiation because of the complexity
ofneutronsinteractionmechanismswithmatter.Allsecondarychargedparticlesoriginating
aroundthevolumeofinterestwouldhavetobeconsideredandtheirenergydepositioninside
theregionofinterestwouldhavetobeevaluated.
Alternatively, in the kerma approximation, the absorbed dose can be approximated by
kerma which, in turn, may be evaluated from fluence through the kerma factors. In this
situation, neutron dose-equivalent value H in a tissue-like point might be obtained from the
appropriatekermafactorsk(E)andtheradiationqualityfactorsQ(E)as
(cid:3)
Hi,j =(cid:2)i,j k(E)·Q(E)·ϕi,j(E)·dE, (7)
E
wherethemeasurablequantities,i.e.thetotalneutronfluence(cid:2) anditsenergydistribution
i,j
ϕ , have been determined in this work for all points of interest for different irradiation
i,j