establishing neutron calibrations at ssdl using iso 8529 ... lecture on...ü neutron hluence in the...

EstablishingneutroncalibrationsatSSDLusingISO8529radionuclidesources

Dr.RobertoBedogniIAEAexpert,dosimetryandcalibration

Lecturecontents

•  Referencepublications•  Quantities•  Metrologyframework•  Transferinstruments•  Irradiators•  Scatteringcorrection


Referencepublications



ICRU-InternationalCommissiononRadiationUnitsandMeasurements

•  ICRUReport85(2011):fundamentalquantitiesandunitsforionizingradiation

•  ICRUReport66(2001):DeterminationofOperationalDoseEquivalentQuantitiesforNeutrons



ISO-InternationalStandardisationOrganisation

ISO8529Referenceneutronradiations

•  Part1(2000):Characteristicsandmethodsofproduction

•  Part2(2000):CalibrationfundamentalsofradiationprotectiondevicesrelatedtothebasicquantitiescharacterizingtheradiationTield

•  Part3(1998).Calibrationofareaandpersonaldosimetersanddeterminationoftheirresponseasafunctionofneutronenergyandangleofincidence

ISO 8529 deals with reference neutron radiation Tields in the energy range fromthermal up to 20 MeV, used for calibrating neutron-measuring devices forradiationprotectionpurposes,andfordeterminingtheirresponseasafunctionofneutronenergy.

ThereferenceradiationTieldsareproducedwith- neutronsfromradionuclidesources,includingneutronsfromsourcesina

moderator;- neutronsproducedbynuclearreactionswithchargedparticlesfromaccelerators;- neutronsfromreactors.



ISO-InternationalStandardisationOrganisation

ISO8529Referenceneutronradiations

•  Part1(2000)•  Part2(2000)•  Part3(1998)

ThesestandardsdescribehowtheneutronHieldscanbeproduced,tracedtoPSDL,andusedforcalibrations.

ThesestandardsareproducedandupdatedbyISOTechnicalCommittee85(Nuclearenergy)/SC2(RadiationProtection)/WG2WorkingGroup2(ReferenceRadiationTields).

ISO8529-1iscurrentlyunderrevision.Parts2and3willberevisedincoming2-3years.



ISO-InternationalStandardisationOrganisationTheISO8529seriesdoesnotcoversimulatedworkplaceneutronHieldswherearangeofdifferentissuesneedtobeconsidered.FortheseTieldsanewapproachwasneededandanewseriesoftwostandard,ISO12789,waswritten.ISO12789ReferenceradiationHields—SimulatedworkplaceneutronHields

•  Part1(2008):Characteristicsandmethodsofproduction•  Part2(2008):CalibrationfundamentalsrelatedtothebasicquantitiesPassivepersonaldosemetersarecoveredby:ISO21909Passiveneutrondosimetrysystems

•  Part1(2015):Performanceandtestrequirementsforpersonaldosimetry•  Part2(2019):MethodologyandcriteriaforthequaliTicationofpersonaldosimetrysystemsinworkplaces



IEC-InternationalElectrotechnicalCommission

TheInternationalElectrotechnicalCommissionhaveproducedstandardsontestmethodsforradiationprotectiondevices,e.g.IEC61005(2014)coveringareasurveyinstrumentsIEC61526(2010)coveringactivepersonaldosemeters



IAEA-InternationalAtomicEnergyAgency•  IAEA (1988).GuidelinesonCalibrationofNeutronMeasuringDevices,Technical

ReportsSeriesNo.285.

TRS285includedthetechnicalbasesthatledtotheproductionoftheISO8529SeriesofStandards.

•  IAEA (2000). Calibration of radiation protectionmonitoring instruments. SafetyReportsSeriesNo.16.


Quantities


Basicphysicalquantities

Fluence

unit:m-2orcm-2

Φ is quotient between the number of particle dN incident on the elemental spherehavingcrosssectionalareada,andthecrosssectionalareada;unitcm-2.•  Allparticlesareequallyweighted,independentlyontheirdirection.•  Aninstrumentwithisotropicresponseisneededtoaccuratelymeasurethequantity.

InMonteCarlocodestheTluenceinacellhavingvolumeΔVisoftencalculatedas

dadN

=Φ

Vi

ΔΦ ∑=

ℓ


WhereliaretheparticlepathlengthsinthevolumeΔV.

Fluencerate m-2s-1 morefrequentlycm-2s-1

€

˙ Φ =dΦdt

ThedistributionΦEoftheTluencewithrespecttoenergy(spectrum)

dEdEEΦ

Φ =)( m-2J-1,morefrequentlycm-2MeV-1


•  PlottingdΦ/dEvs.Eisnotpracticable,asEvariesovermanyordersofmagnitude•  PlottingdΦ/dEvs.log(E)doesnotpreservetheproportions(wedesirethattoequalareascorrespondequalneutronHluencevalues)

•  IfthelethargydistributionoftheHluence,EdΦ/dE,isplottedvs.ln(E),theproportionsarepreserved,as

d(ln(E)) E dΦdE∫ =

dEEE dΦdE∫ =Φ

Fastpeak

Epithermalcontinuous

Thermalpeak

Particleradiance(orangle/directiondistributionoftheTluence)

m-2sr-1,morefrequentlycm-2sr-1

Thedirection isoftenspeciTied intermsofcosine(θ),whereθ is thepolaranglewithrespecttoareferencedirectioninspace.


Fluence-to-doseequivalentconversioncoefHicients

Thetypeofdose-equivalentmustbespeciTied.Forneutronsthefollowingarerelevant:

ü  H*(10) forambientmonitoringü  Hp(10,α) forindividualmonitoringThedoseequivalentisderivedbyenergy-integratingtheproductofthespectrumandthe energy- (and angle-) dependent conversion coefTicient tabulated in ICRP74 /ICRU57.

Aconvenientconcept is theunitspectrum,deTinedasthespectrumperunit Tluence.Theunitspectrumhasunitintegralandonlycontainsthe“shape”ofthespectrum.

IfanISOstandardTieldisused, ϕEandhΦaretabulatedinISO8529-1.


Neutrons

Free-HieldFluenceResponse RΦ ISO8529-2(2000)

Φ  istheconventionalquantityvalueofthe“free-Tield”Tluencefromthesource:

NOTincludingtheneutronsscatteredbytheroom(walls,Tloor,ceiling),surrounding structuresandmaterials,andtheair.

Gcorristhe“correctedindication”.

ü  ScatteredneutronsareaninTluencequantityoftypeS(ISO29661)ü  ThenaturalbackgroundisalsoaninTluencequantityoftypeSü  NonlinearityisaninTluencequantitytypeF

Doseequivalentresponse RH ISO8529-2(2000)


Metrologyframework


PrimaryandSecondaryStandards

Inneutronmetrologythefollowingareprimarystandards,intendedasestablishedbyprimaryreferencemeasurementprocedure(i.e.usingmeasurementstandards formagnitudesofdifferenttype):•  Activationtechniquesbasedonelementswithwell-knownradiativecapturecross

sections

ü  emission rate of radionuclide sources can be determined accuratelythroughthethermalneutronactivationof55Mn(sulphateinaqueoussolution)inthesocalled“manganesemoderatingbath”

ü  thermal neutron Hluence rates are measured through thermal neutronactivationof197Au(goldactivationfoils)

•  Nuclearreactionswithverywell-knownXs,suchasn–pscatteringinHydrogenatedmaterials

ü  Neutron Hluence in the 0.5 MeV - 10 MeV region can be measured withrecoilProtonTelescopesorRecoilprotonproportionalcounters(countingtherecoilprotonsfromaknownhydrogenatedvolume)


ThermalHluenceratemeasurements

MeasuringthermalHluenceratewithAufoilsandCdsubtractiontechnique197Au(n,γ)198AuradiativecapturecrosssectioninthermaldomainiswellknownandcanbeusedasprimarystandardforThermalneutronTluencerate.

198Aubetadecays(2.694d)andyields411.8keVγ(95.6%)

InAu–Au/Cdtechnique:

4πβcounterultralowbackgroundviaanticoincidencering(NPL)

DE<0.5eV =1Gt

Dbare −FDCd( )

𝐺𝑡 thermalneutronself-shieldingfactor(typ.1.02for10umfoils)

𝐹 CorrectionfactorforattenuationofepithermalneutronsinCd(typ.1.01)

𝜎0 conventionalthermalXs(at25meV)=98.69b

𝑔 Westcottfactor(forAu)=1.0046(departurefrom1/v)

Φwisthe“conventionalthermalneutronTluencerate”or“sub-CdcutoffTluencerateintheWestcottconvention”:the25meVKluenceratethatwouldproducetheobservedactivation.


MonoenergeticreferenceHieldsbetween1keVand20MeV

ISO8529-1:2001recommendsasetofenergiesforstandardsofmonoenergeticTluence,basedonanumberofdifferentneutron-producingreactions,fromacceleratororreactors.

MonoenergeticneutroninstallationatNPL–UK.

•  Small acceleratorsprovidingprotonsanddeuteronsup toanenergyof3,5MeVareenough togenerateneutronsofallrecommendedenergies.

•  For2,8MeVand14,8MeV,however,asmallacceleratorwithapotentialofupto fewhundredkilovolts,issufTicient

•  Parameters tobeknown:chargedparticleenergy,angle, Tluencemeasurementandmonitoring,neutronspectrum,sourcesofscatteredandcontaminantneutrons,targetageandthickness


Themanganesesulphatebath

•  Absolutedeterminationofthe4πsrneutronemissionratefromradionuclideneutronsources.

•  Asphericalvessel(50to150cmdiameter)isTilledwithaqueoussolutionofpureMnSO4(about20g/l).

•  Theneutronsourceisplacedatthecentreofthebath.

•  Neutronsarethermalizedinwaterandinduceradiativecapturein55Mn(100%oftheMnatoms).

•  CaptureinOxygenisnegligiblysmallandneutroncapturebyhydrogenandsulphurproducesstableisotopes.

55Mn(n,γ)56Mn

56Mnbetadecays(2.58h)andyields847keVγ(100%)•  The speciTic Mn saturated activity is measured (4πβ-γ or NaI(Tl) counting) and the sourceemissionrateQisobtained(1-2%standardunc.)by:

ü  𝐴𝑚 56MnspeciTicsaturatedactivity(reachedafteroneirradiationday)ü  𝑀 massofthesolutionü  f captureprobabilityinMn/totalcaptureprobabilityü  𝛿 removalprobabilityby(n,α)and(n,p)inS,(n,α)inO,re-capturefromsource,escapefrom

vessel.

MnbathatLNHBCEAParisFrance


RecommendedRadionuclideneutronsourcesISO8529:2001

Sourcetypesü  ThepreferredoneisthespontaneousTission252Cfsource,asitisverysmall,haswellknownspectrum,lowphotonTield,andisavailablewithanyemissionrate.Thetime-dependentemissionratedependsondecayofalltheconstituents,including250Cfand248Cm.Ifmorethan5%oftheemissionisdueto250Cf+248Cm,frequentrecalibrationshouldtakeplace.Theshorthalflife(2.65a)requiresfrequentreplacements.

ü  (α,n)Am-BesourcesareconsiderablybiggerthanCf.Asthespectrumdependsoncapsulesizeandamountofactivematerial,theyareaffectedbyformat-to-formatspectralvariationsthatshouldbetakenintoaccount.


RecommendedRadionuclideneutronsourcesISO8529:2001

Anisotropy

ü  Neutronsourcesgenerallyshowanisotropicneutronemissionü  TheangularemissionratedB/dΩ=BΩ(sr-1)isspeciTiedintermsofthedirectionΩ thatisspeciTiedbytheanglesαandθ.

ü  TheanisotropyfactorFΩinagivendirectionΩ isdeTinedas:consideredthatB/4πistheangularemissionrateforanisotropicsource.

ü  For cylindrical sources the symmetryimplies that BΩ mainly depends uponangleθ.

ü  Forpracticalreasonstheθ=90°directionis used for calibrations and the sourceshould be put in slow rotation aboutthe cylindrical axis to eliminate theresidualanisotropyaroundangleα.



252Cfsourcespectrum


252Cfsourceanisotropy(X1capsule)

takenfrom:NPLReportCIRM24(1998)

D2Omoderated252Cfsource252Cfatthecentreof15cmradiusD2Ospherewith1mmthickexternalCdshell11,5%neutronscapturedinthesphere

Spectrumderivedfor3.0E+6n/s,4cm3cavityvolume25.2(height)x25.2(diam),3.7mm(cylinderwall),3.2mm(endwalls)


241Am-Besourcespectrum


241Am-Besourceanisotropy(X3capsule)

takenfrom:NPLReportCIRM24(1998)

EstablishingsecondarystandardsatSSDL

Primarystandardsinneutronmetrology:

-  Sourceemissionrate(Mnbath)-  ThermalneutronTluence(goldfoils)-  Neutron Tluence in the 0.5MeV - 10MeV region: Recoil Proton Telescope or Recoil proton

proportionalcounters(countingtherecoilprotonsfromaknownhydrogenatedvolume)

SecondarystandardsatSSDLshouldbeestablishedusingneutronsourceswithideally:

•  EmissionratedeterminedataPSDLusingtheMnbath•  AnisotropydeterminedataPSDLusingalong-counter•  Knownspectrum

Ifthesourcehas•  Unknownemissionrate,or•  Unknownanisotropy,or•  SpectrumdifferentthanthosereportedinISO8529:1(2001)

Thenthesecondarystandardshouldbeestablishedbymeansofatransferinstrument


Transferinstruments


Themoderatedambientdoseequivalentmeter(remcounter)•  Itshows”Tlat”energydependenceoftheH*(10)response(inprinciple)

•  Ifthemeteriswell-designed:

ü  M=q�H*(10)

ü  TheFluenceresponseisproportionaltotheenergy-dependentTluence-to-ambient-doseequivalentconversioncoefTicient(ICRP74/ICRU57)


•  Hankins(LA-2717(1962))realizedthatthe Tluence response of a 25.4 cmsphere with a thermal counter in itscentre was curiously similar to “doseequivalent” conversion coefTicient, inthe energy range from 100 keV to fewMeV.


Themoderatedambientdoseequivalentmeter

Leakecounter Anderson-BraunNM2B Studsvik2202D BertholdLB64113He200kPa BF3 BF3 3.5bar3He+1barCH421cmdiam. 24hx21cmdiam. 24hx21cmdiam. 25cmdiam.

PRACTICAL IMPLICATIONS OF NEUTRON SURVEY INSTRUMENT PERFORMANCE

2

and monoenergetic measurements. The calculated data also indicated a strong angle dependence of response for the instruments.

e Some limited folding of the new response functions with workplace fields was performed to ascertain the effect of the new response characteristic data. These showed some differences, particularly for the NM2.

FIGURE 1 The three designs of instrument modelled in the earlier work and in this project: the Leake (0949) on the left, the NM2 in the centre and the Studsvik 2202D on the right

In particular, the earlier study highlighted the variability in the measured data, and the sensitivity of the response to the angle of incidence of the neutrons. Few of the experimental measurements were very recent, which is a cause for concern given the number of model changes that each of the instruments has seen over the last 30-40 years. There was hence seen to be a need for the sensitivity of the response to be determined for natural manufacturing variability and for model-to-model differences.

Another area of concern raised was the “mode of use”. This is important, because the instruments are designed to have an isotropic response, and are intended to measure an isotropic dose quantity. The calculations of the response show that the response is not isotropic, and observations of the manner in which the instruments are used in the workplace indicate that the user is commonly holding the device close to the body or places it on the floor. This influence of the user, and placement of the instrument on the floor, need to be investigated since they may have significant impacts on the response of the instruments.

1.1 Background

Survey meters are used to determine dose rates in the workplace for general health physics purposes. In particular they are used in the designation of controlled areas so their accuracy is of great importance in the workplace. Consequently, significant biases

ü  Ifcomparedwiththeh*(10)conversioncoefTicients,theresponseofthe25cmsphereshowslargeoverestimationintheepithermalregion

ü  Improvements to this concepts lead todifferentdesigns, all of themaimedatdepressing theepithermalresponsewhilekeepingtheMeVresponse.

ü  Interruptingthemoderatorwithaneutronabsorber(aperforatedcadmiumfoiloraboratedrubbershell)provedtohavesomeeffect..

TakenfromReportHPA(UK)HPA-RPD-016(2006)

Thelongcounter

•  Alongcounterisadirectionalmeterwith“Hlat”Hluenceresponsefrom1keVto15MeV(<10%dependence).Itwasdesignedinthe1950-60sbyHanson,DePangher,McTaggart.

•  Usuallyformedbyathermalneutrontubecounter(usuallyBF3)inacylindricalmoderatorwithtypicalsize44cmx∅38cm

•  TheTluenceresponse(≈10countscm2)isdeterminedwithinabout1.5%-2%usingsourceswithknownemissionrate(viaMn-bath)


•  Designprinciples-Lateralshield(PE+boron) -Holesforlow-energystreaming

•  Effective centre depends on energy and shiftsdeeperwithincreasingenergy.Itcanbeknownwithunc.≈0.6cm.

Bonnerspheres

•  Bramblettetal.,“Anewtypeofneutronspectrometer”NIMA9(1960)1-12.

•  HDPEspheresofmultiplediameters(5-30cm),sequentiallyexposedwiththesamethermalneutrondetectorintheircentre.

•  ThetracksinTigurecorrespondto: (a) high-energyneutronescapingfromtheassembly, (b) neutronreachingthedetectorwiththermalenergy,producingacount, (c) low-energyneutronabsorbedinthepolyethylene.

•  Theprobability foraneutron tobemoderatedandgivea “count” in thedetector isuniquelyrelatedtoitsenergy.

•  The energy thatmaximizes such probability is uniquely related to the sphere diameter: theresponse of a spherehas a peak at an energy value that is uniquely related to the spherediameter.


Bonnerspheres

•  Responsematrix=countsperunitTluenceasafunctionoftheenergyandthespherediameter,underuniformirradiationcondition.DerivedbyMonteCarlowithatleast5binsperdecade

•  Φ istheneutronTluenceincm-2;•  ϕ(E) istheenergydistributionoftheneutronTluencenormalizedto1cm-2anditsunitisMeV-1

(alsotermed“unitspectrum”)•  Ri(E) istheresponsefunctionofthesphere(incm2).•  Ci arethecountsinthei-thspheres


∫=max

min

)()(E

Eii dEEERC ϕΦ

Bonnerspheres

•  ThecountsofdifferentspheresexposedtothesameTluenceofagivenspectrumformaplotasafunctionofthediameter(countproTile).Thesearesmoothcurves,Tittedby5-6points.

•  TheproTilescontainthetotalityofthespectrometricinformation.

•  5-6wellchosenspheresareenoughtodescribeallpossiblevariationintheproTile.

•  Thespectrumisinferredvia“unfolding”startingfromtheresponsematrix,thecountproTile,theuncertainties,andagivenamountofpre-information(aMCsimulationforex.)neededtomakeupforthelackofinformation(theproblemisunderdetermined,becausewithtenmeasurementsorlessweinferacontinuousspectrumovermaybe100bins).


Bonnerspheres–centraldetectors


R. Bedogni et al. Nuclear Inst. and Methods in Physics Research, A 897 (2018) 18–21

Table 1Sensitivity of different BS central detectors, represented by the response of the 200 mm diameter sphere (or 8 in.)at 1 MeV.

Central detector model Sensitive volume and shape R (1 MeV, 200 mm) cm2 References

05NH1 8 kPa, 3HeCylindrical, 10 mm ù 9 mm

0.4 [7]

SP9 200 kPa, 3HeSpherical, 32 mm diameter

2.5 [8,9]

6LiI(Eu) ScintillatorCylindrical, 4 mm ù 4 mm

0.2 [6,10]

6LiI(Eu) ScintillatorCylindrical, 11 mm ù 3 mm

0.6 This work

Fig. 1. Structure of the large 6LiI(Eu) scintillator, 11 mm diameter ù 3 mmheight. All dimensions in mm. Courtesy of Scionix Holland BV.

operative scenarios. The BS based on this detector is here called LL-BSS(Large LiI(Eu) Bonner Sphere Spectrometer). Although its sensitivity islower than that of the SP9-based BSS, it responds 50% more than the05NH1 one, and is still very small compared with the size of the smallerspheres.

LL-BSS response matrix was derived with MCNPX [11] and experi-mentally validated with reference monoenergetic neutron fields at NPL.

2. The central detector

The LL-BSS central detector used in this work is produced by ScionixHolland BV. Its internal structure is shown in Fig. 1.

The optical window (glass) was coupled with a PM tube (10 mmdiameter active window) through optical grease. The PM operates atpositive 1100 V. The anode of the PM tube is directly connected tothe analog input on a commercial digitizer (NI USB 6366X, BNC con-nectivity). In house LabView-based software was developed to processthe signal from the detector. The anodic signal generated by thermalneutron events exhibits a steep rising front (<1 �s, corresponding to thetime sample of the digitizer) followed by a slower decay. The softwaredigitally processes the waveform from the PM anode and measures theheight of the rising fronts. The Pulse height distribution (Fig. 2) isderived on this basis.

The system was found to be immune to pile up and saturation effectsfor counting rates up to at least 105 s*1.

The main peak in the spectrum (Fig. 2) is due to the completecollection of both ↵ (2.05 MeV, range in LiI 8 �m) and Triton (2.73 MeV,range in LiI 48 �m) products from the neutron capture reaction in 6Li.The straggling component comes from the partial escape of the reactionproducts. As these partial escapes denote a genuine neutron captureevent, an asymmetric Region of Interest (ROI) for counting purposeswas extended from (centroid—5 FWHM) to (centroid +3 FWHM). Thelarge photon-neutron separation allowed such a large ROI.

The secondary electrons from photon interactions are found in thefalling tail located below 100 mV. A very large separation exists fromphoton and thermal neutron events, allowing for a high neutron-to-photon discrimination capability. Tests with 137Cs sources showed thatphoton kerma rates up to at least 10 mGy h*1do not influence the regionof the spectrum where the neutron peak grows.

Fig. 2. Pulse height spectrum of the large 6LiI(Eu) scintillator exposed tothermal neutrons.

Explanations for this large neutron-gamma separation, with respectto the traditional 4 mm (diameter) ù 4 mm (height) cylindrical counter,could be:

– With respect to the traditional analog chain (shaper amplifier,Multichannel Analyser), the one used in this work directly dig-itizes the anodic signal, thus reducing the sources of noise andsignal broadening;

– With respect to the old counter, the new one improves thelight collection efficiency because the ratio between the areain contact with the optical window and the detector volume ishigher.

3. The response matrix

Bonner Spheres were manufactured using HDPE and their nominaldiameters were (in mm) 60, 70, 80, 90, 100, 110, 125, 150, 170,200, 250 and 300. Their average diameters and the dimensions of allmechanical features (such as the cavity for the detector) were accuratelydetermined. The actual HDPE density was measured for every spherewith accuracy better than 1%.

The LL-BSS response matrix (Fig. 3) was determined with MCNPXby simulating an exposure with a uniform parallel neutron beam havingthe same diameter as the studied sphere, and impinging the sphere alongthe detector cylindrical axis. The ENDF/B-VII cross section library [12]below 20 MeV, and the room temperature S(↵, �) cross sections forthermal treatment in polyethylene, were used. The Bertini intra nuclearcascade model and the Dresdner evaporation model were used above 20MeV [13].

The simulated response (unit: cm2) is here defined as, the number of(n,↵) capture events in the central detector, per unit incident neutronfluence, as a function of the sphere size and of the monoenergeticneutron energy. Pedix ‘‘i’’, with i = 1,… , 12, denotes the sphere.

19

11x36LiI(Eu)NIMA897(2018)18–21

2.8cm33He10atm

TransferinstrumentsforestablishingsecondaryStandards

Usingasphericalsurveymeterofalongcounter

IfthesourceatSSDLisa252Cf:•  ThespectrumcanbeassumedtobeidenticaltotheISOtabulation•  UncertaintyintheTluence-to-H*(10)conversioncoefTicientisonly1%•  Thefree-Tieldquantityatthecalibrationdistanceintheuseddirectioncanbe

determinedprecisely(unc.3-4%)

IfthesourceatSSDLisa241Am-Be•  ThespectrumisprobablydifferentthantheISOtabulation•  The241Am-BesourceusedatthePSDLtocalibratethetransferinstrumentis

probablydifferentthanthesourceattheSSDL.•  Additionaluncertaintiesareneededtoaccountforthedifferences(howto

evaluatethem?)•  Additional4%uncertaintyintheTluence-to-H*(10)conversioncoefTicient

UsingBonnerspheres

Thefree-TieldTluenceorH*(10)atthecalibrationdistanceintheuseddirectioncanbedeterminedprecisely(3-4%)foranysourcetypeandcapsuletype.


Irradiators


Calibrationroom

Ingeneral,irradiationroomshavethickconcretewalls.Insidedimensionsshouldbeaslargeaspracticallypossible.

Large buildings with low scattering walls (aluminium) have been built to reducescattering.

Themagnitudeofthecorrectionforroom-andair-scatteredneutrons,andtheresultinguncertaintyinthefree-Tieldquantities,dependcriticallyonthesizeoftheroom.

In all cases, the effects of scattered neutrons shall be determined. Details of therecommendedcalibrationproceduresaredealtwithinISO8529-2

“The room should be such that scatter contributions are as low as possible, but in any case theyshouldnotcauseanincreaseininstrumentreadingofmorethan40%atthecalibrationpoint”


Panoramicirradiators

Apanoramicirradiatoris,ideally,aneutronsourcesuspendedinairinthecentreofaverylargeroom.

Foraroomwithsize10x10x3m3thescatteringcanproducea+40%(ISOcriterion)inaremballreadingat2m.Usefulcalibrationrange<2m.


ENEA-BolognaSSDL

Collimated-beamarecommerciallyavailablewithdifferentdegreeofautomation(shuttermotionisthemostimportant).Thesearenotrecommended,asthespectruminthecalibrationlinesigniTicantlydiffersfromthatofthebaresource.

Automatedirradiators

Automated irradiatorsnearly eliminate Individual exposure (except for loading andunloadingsources).

•  Sourceinshieldedbank(1m3)•  Multiplesourceselection•  Automatedsourceextractionandmotion•  Workpositionfarfrombanktoreducescattering

•  Irradiationbenchuptoabout3m•  Remotelycontrolledinstrumentholder•  Ancillaryequipment(cones,D2Osphere)canalsobeautomated


ENEA-BolognaSSDLManufacturersalsoprovidesafetysystems:

•  Signalizationlights(sourceinbank,sourceinmotion,sourceout)•  Emergencybuttons•  Searchbuttons•  Countingphotocells,interlockeddoors•  Motorizeddoors(typ.20cmHDPE+boronorCd+lead)•  Videomonitoringand/orintercom

Disadvantages:cost,troubleshooting,bulkystructures

Manualirradiators

Astheydonothaveautomations,manual irradiatorshavetheadvantageofbeingaslightaspracticallyachievable.

•  Sourceinashieldedbank•  Manuallytransferredtoworkposition•  Manipulator(1.5m)•  Irradiationbenchupto2-3meters•  Ancillaryequipment(cones,D2Osphere)manuallyoperated


ENEA-BolognaSSDL

Example: a light-structure manual irradiator in a 8 x 8 x 5 (h) m3 undergroundbunkerwith light aluminium roofmayprovide+10%scattering increase in a remballreadingplacedat1m.For radiation protection reasons, sources up to about 2E+6 n/s can behandled.

CalibrationequipmentCalibration room equipment includes rigid structures, designed to minimizescatteredradiation,withthepurposesof

•  suspendingthesourceataproperheight(minimumscatterfromgroundandroof)•  positioningtheinstrumentindistanceandheightfromground.

Itshouldbepossibletovarythesource-to-detectordistance.•  Suspendingtheshadowconeandregulatingitsposition•  AllocatingtheD2Omoderatingsphere


Laserlinersareusefulto:

ü  Markthesource-detector-lineonhorizontalandverticalplanes

ü  usefuldistancesfromthesource(1m,1.5m)

Typicalmanualirradiator

CalibrationequipmentExampleofminimalmanualirradiator


≈3kg/m

Sourceinslowrotationtoreducetheeffectofanisotropy

Simulatedroomandequipment


•  Thesimulatedroomhassize6x6x3(h)m3,20cmconcretewalls,Tloorandroof.•  TheirradiatorstructureisbuiltwithsteelproTileswithlinearmass2.7kg/m

•  Theholdersuspendsthesourceat1.8mfromground•  Thetypicalsphericalmeterisa25cmdiameterHDPEspherewithaspherical

3Heproportionalcounterinitscentre(3.2cmdiameter,5E+19atoms/cm3)•  Theinstrumentisleaningona30x30x1cm3aluminiumplate•  Thesourceisacylindricalvolumesource(3.1cm(h)x2.24cmdiameter)X3likewithemissionrate2E+6s-1andAm-Bespectrum

Top view. The source in in the roomcentreandthesphereistheremball

Lateralview.TheyellowlinesaretheStructure(horizontalline)andsourceholder(verticalline)

Scatteringcorrection


Neutrons

Free-1ieldFluenceResponse RΦ Doseequivalentresponse RH

Φ “conventionalquantityvalue”ofthe“free-Bield”Bluencefromthesource,i.e.NOTincluding

theneutronsscatteredbytheroom(walls,1loor,ceiling),surroundingstructuresandmaterials,andtheair.

Gcorr“correctedindication”fornon-linearity,background,andespeciallyscattering

Scatteredneutronsareduetotheroom,thesurroundingmaterialsandtheair.NotcorrectingwouldmakeRΦ and RHdependentonthecalibrationroom.

ISO8529-2providesmethodstodeterminethepartofindicationcomingfromscatteredneutrons,Gs


“free-1ield”1luencerate ΦΦ(oritsrate)isderivedbypropagatingthesourceemissionrateinthecalibrationdirection,takingairattenuationintoaccount.

Where

B sourceemissionrateFθ anisotropyfactorfortheuseddirectionl distancefromsourcecentreandinstrumentcentreΣ  AirlinearattenuationcoefBicientforthespeciBicspectrum


Scatteringmodel

MT(l)(totalindication)≈k/l2+A/l+SRoom:scatteringfromwalls,Bloor,ceiling.Roomscatteristypicallythemostimportantsourceofscatteredneutrons.SupportstructuresSupport structures should be as light as possible, with little or no hydrogenatedmaterials. Special care should be taken to minimize the mass of support structurenearestthesourceordetector.

AirAirout-scatterNeutronsemittedbythesourceareabsorbedanddeBlectedintheair.Airin-scatterTheinstrumentdetectsneutronsthatareoriginallyemittedinotherdirections,butaredeBlectedbytheairtowardstheinstrument.



Scattering

InstrumentindicationVsdistanceintheidealizedMonteCarlomodel

•  Free-Bield•  Aironly–note:Aironly>freeBield(in-scatterprevailsoverout-scatter)•  Airandroom

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Scattering

Scatter-relatedIncreaseinindicationVsdistance-idealizedMonteCarlomodel

•  Maximumusefulcalibrationdistance1.25m

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Scattering

Fourmethodsareproposedtodeterminethescatteredcontributiontotheinstrumentindication:

Shadow-conemethod:requiresaminimumoftwomeasurementsatthesamedistance

Generalized1it,semi-empirical,Reduced1ittingmethods

-Requireaninitialsetofcarefulmeasurementsasafunctionofthedistance.-AlotofpointsnearthesourcearefundamentalintheGen-Bitmethod,togetlowuncertainties

Asdifferentmethodsmaygiveslightlydifferentresults(withinfew%),thechoiceofthemethoddependsonroomcharacteristics.

Usuallytheshadow-conemethodisthequickestandmoreaccuratemethodforcalibrations


Geometrycorrection

F1(l)takesintoaccountthenonuniformilluminationoftheinstrumentatshortdistancesFormulasinISO8529-2weredeterminedusinganalyticapproximations.IfevenaverysimpliBiedMonteCarlomodelofthedeviceisavailable,itwouldprobablyprovidebettercorrection.


PointsourceandsphericaldetectorF1(l)dependsonl sourcetodetectordistancerD detectorradiusδ constant

Generalised1itRequires a lot of datapoints, less spaced (1-2 cm)near the source,more spaced atlargerdistances(5-10cmat1mandbeyond).PointsnearthesourceareCRUCIALforgettinglowuncertainties.

a) roomsize,shapeandsource/detectorsize:nolimitation;b) source-detectordistance:minimumdistance1cmbetweenthesurfacesofthe sourceandthedetector,maximumdistanceissetbytherequirementthatthe increasedreadingfromroomscattershouldbelessthan40%;

Advantages:MaybeusedwithanyoftheISOsources;

Disadvantages: A complete set of measurements needed for each instrument. Non-linearordriftingreadingsshouldbecarefullycorrected,sincetheycanbemaskedbytheBittingprocedure.Goodpositioningandcountingstatisticsrequired.


k free-Bieldindicationatunitdistancel centretocentredistanceF1(l) geometriccorrectionFA(l) airout-scattercorrection=eΣlAin Airin-scattercoefBicients RoomscattercoefBicient

Generalised1it

(1) FitF1/FAwitha6thdegreepolynomialandBindα0

(2)  FitMT(l)*l2witha6thdegreepolynomialandBindk*α0àgetk(Gcorr@1m)alotofpointsnearthesourceareneededtogetlowuncertainty!

(3) Fit1/l*(MT(l)/k*l2-F1/FA)withalineandBindAinandS

Advantage:youcanusethesameAinandSfornextinstrumentofthesametype



Generalised1it

Whatifyoudonothavealotofpointsnearthesource?–idealizedroommodel

FittingMT(l)*l2witha6thdegreepolynomialprovidesk*α0with20%uncertainty!!

k=32±6cps�m2

Truevalue(simulationwithoutroomandair) ktrue=(33.67±0.12)cps�m2

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Semi-empiricalmethod

k free-Bieldindicationatunitdistancel centretocentredistanceF1(l) geometriccorrectionA AirscattercoefBicient(in–out)S RoomscattercoefBicient

Roomsize: nolimitation;Roomshape: cubicalorclosetocubical;Source/detectorsize: nolimit;Distance: fromsourceplusdetectordiametersto40%limitAdvantages: SameRoom-scattercorrectioncanbeusedforallinstrumentsofthe

sametype

Disadvantages: 1. Canbeusedifthemainsourceofneutronscatteristheroom 2. Forcinga“physicalmodel”forthescattermayleadtoslightly biastheresults

Semi-empiricalmethod

(1) FitMT(l)*l2/F1witha3thdegreepolynomialandBindk=Gcorr@1m

(2) DetermineAandS

Applicationtotheidealizedroommodelk =(34.45±0.74) cps�m2

ktrue=(33.67±0.12) cps�m2S=0.28±0.06



Reduced1ittingmethod

k free-Bieldindicationatunitdistanced distancefromsourcetosurfaceofmetera “effective”positionofthedetectorcentre

Forsymmetricdevicesusel=d+aS ScattercoefBicient

Roomsize: nolimitation;Roomshape: nolimitation;Source/detectorsize: nolimitation;Distance: from1.5xsizeofinstrumentto40%limitAdvantages: 1. doesnotrequirelongseriesofdatapoints

2. Allowsestimating“effectivecentre”

Disadvantages: 1. Neglectgeometrycorrection 2. Doesnotworkinlow-scatterroomsasitassumesconstant scattering 3. Forcinga“physicalmodel”forthescattermayleadtoslightly biastheresults

Reduced1ittingmethod

(1) LinearlyFitMT(l)asafunctionof1/l2andget Sandk(k=Gcorr@1m)

Applicationtotheidealizedroommodelk = (35.36±0.32)cps�m2ktrue= (33.67±0.12)cps�m2S=7.57±0.42


Shadow-cone

Theshadow-coneisashieldingmadeoftwoparts:afrontend,20cmlongmadeentirelyofiron;andarearend,30cmlongmadeofpolyethylene,with5%ormoreboronloading.

Thechoiceofthefront-enddiameterhastobebasedonthesizeoftheavailableneutronsources

Theshadowconeshould:-  haveanegligibletransmissionforthedirectneutrons-  Coverthesolidangleofthedevice.

Pairsofmeasurementsatthesamedistance:Mc(l) (shadow-cone)deviceexposedtothein-scatteredradiationonly(room,air).MT(l) (totalBield) deviceexposedtothetotalBield(free-Bield,in-scatter,airout-scatter)FA(l) airout-scattercorrection=eΣl


Shadow-cone

Accurateestimationofthein-scatteredcontribution(withcone):

-  SCDtobeexperimentallyoptimized-  CDD>50cm(SDD>1m+SCD);-  MaxSDDlimitedtothe40%scatteringcriterion-  Theconeapertureshouldbesuchtoovershadowthedevicebymax.afactorof2-  Severalshadow-conesneededtocoverdifferentSDDanddevicesizes

Generalcriteria-  Roomsize:largeroompreferred-  Roomshape:nolimitation-  Sourcesize:preferablysmall.D2O-252CfrequiresalargeconeAdvantage: directmeasurementofeffectofin-scatteredneutrons;Disadvantage: asetofshadowconesisrequired.


Shadow-cone

Idealizedroommodel(MCsimulations)

Parametricstudytoinvestigatetheaccuracyofthemeasurementwhenchanging:-  SCD-  Coneminimumdiameter-  Coneshadowingratio-  SDDis1ixedto1.25m

Gcorrvalueisdeterminedbyrunningasimulationwithoutroomandwithoutair

Gcorr=(21.91±0.08)cps


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Shadow-cone

EffectofchangingSCDGreenlinesdenote±1%boundary dmin=5cm,dmax=15cm


SCD=20cmdmin=1dmax=15

SCD=20cmdmin=9dmax=18

Shadow-cone

EffectofchangingconedminSCD=20cm.Shadowingratioalwaysbetween1.1and2Greenlinesdenote±1%boundary

Shadow-cone

Effectofchangingtheshadowingratio(SR),byvaryingdmax.SCD=20cm,dmin=5cmGreenlinesdenote±1%boundary


dmax=9cmSR=0.3

dmax=21SR=2.4

Methodscomparison

Idealizedroommodel(MCsimulations)

GcorrvalueisdeterminedbyrunningasimulationwithoutroomandwithoutairTruevalue(simulationwithoutroomandair) 33.67±0.12

Generalized1it 32±7Semi-empirical 34.45±0.74Reduced1it 35.36±0.32Shadow-cone (SCD20cm,dmin=5cm,dmax=15cm) 34.0±0.5


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Uncertainties

ü  Sourceemissionrate(<2%inMnbath)

ü  Anisotropyfactor(typ.<1%formeasuredvalues)ü  calibrationdistance

ü  geometryfactorF1(l)

ü  uncertainty of the conversion coefBicient; by convention taken to be zero formonoenergeticneutrons,forbroadspectraseealsoISO8529-2(1%forCfand4%forotherspectra)

ü  instrumentreading

ü  scatteringcorrection(elaboratedfromBit/conesprocedures)


Considerationsforpersonaldosemeters

ü  Thequantitytobemeasuredforindividualmonitoringisthepersonaldoseequivalent,Hp(10).ü  ConversioncoefBicientsfromBluencetoHp(10)intheICRUtissueslabphantom(ICRU47)

ü  ISOrecommendsdistancefromphantomfacetosourcecentre75cm.ü  Scattercontributionneedtobedetermined(onceperdosemetertype)

ü  SimpliBiedprocedure(nophantom)isallowed(phantombackscatteronceperdosemetertype)

ü  Simultaneous calibration of several dosemeters: consider the effective distance for everydosemeter(donotexceed15cmfromcentreofphantomface)

Uncertainties

ü  Sourceemissionrateandanisotropyü  positioningü  Unc.onconversioncoefBicient(1%forCfand4%forotherspectra)ü  uncertaintiesduetosimpliBiedproceduresorscattercorrectionü  uncertaintyduetosimultaneousirradiationofseveraldosimeters;


establishing neutron calibrations at ssdl using iso 8529 ... lecture on...ü neutron hluence in the...

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