analysis methods for airborne radioactivity

204 Pages·2008·2.03 MB·English

TKK Dissertations 129
Espoo 2008
ANALYSIS METHODS FOR AIRBORNE RADIOACTIVITY
Doctoral Dissertation
Jarmo J. Ala-Heikkilä
Helsinki University of Technology
Faculty of Information and Natural Sciences
Department of Engineering Physics
TKK Dissertations 129
Espoo 2008
ANALYSIS METHODS FOR AIRBORNE RADIOACTIVITY
Doctoral Dissertation
Jarmo J. Ala-Heikkilä
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission
of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium
K216 at Helsinki University of Technology (Espoo, Finland) on the 20th of August, 2008, at 12
noon.
Helsinki University of Technology
Faculty of Information and Natural Sciences
Department of Engineering Physics
Teknillinen korkeakoulu
Informaatio- ja luonnontieteiden tiedekunta
Teknillisen fysiikan laitos
Distribution:
Helsinki University of Technology
Faculty of Information and Natural Sciences
Department of Engineering Physics
P.O. Box 4100
FI - 02015 TKK
FINLAND
URL: http://www.tkk.fi/Units/AES/
Tel. [phone]
Fax [phone]
E-mail: [email protected]
© 2008 Jarmo J. Ala-Heikkilä
ISBN 978-951-22-9439-8
ISBN 978-951-22-9440-4 (PDF)
ISSN 1795-2239
ISSN 1795-4584 (PDF)
URL: http://lib.tkk.fi/Diss/2008/isbn9789512294404/
TKK-DISS-2491
Yliopistopaino
Helsinki 2008
AB
ABSTRACT OFDOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY
P.O.BOX1000,FI-02015TKK
http://www.tkk.fi
Author JarmoJohannesAla-Heikkilä
Nameofthedissertation
AnalysisMethodsforAirborneRadioactivity
Manuscriptsubmitted 11.02.2008 Manuscriptrevised 23.05.2008
Dateofthedefence 20.08.2008
X Monograph Articledissertation(summary+originalarticles)
Faculty FacultyofInformationandNaturalSciences
Department DepartmentofEngineeringPhysics
Fieldofresearch Radiationphysics
Opponent(s) Prof. StanleyG.Prussin
Supervisor Prof. RainerSalomaa
Instructor D.Sc. PerttiAarnio
Abstract
High-resolutiongamma-rayspectrometryisananalysismethodwellsuitableformonitoringairborneradioactivity.
Manyofthenaturalradionuclidesandamajorityofanthropogenicnuclidesareprominentgamma-rayemitters. With
gamma-rayspectrometrydifferentradionuclidesarereadilyobservedatminuteconcentrationsthatarefarfromhealth
hazards.Thegamma-rayspectrometricanalysesappliedinairmonitoringprogrammescanbedividedintoparticulate
measurementsandgasmeasurements.Inthiswork,methodsapplicableforparticulatesampleanalysishavebeen
presented,implementedinanalysissoftware,andevaluatedwithawidevarietyofcases. Ourgoalhasbeentodevelop
acollectionoftoolsthatenablesacompletequantitativeexplanationofallcomponentsofameasuredgamma-ray
spectrumwithaminimumofuserintervention.
Inahigh-resolutionspectrum,allessentialinformationiscontainedinthepeaks.Mostofthemarefull-energypeaks
thatcanbeexplainedwithgamma-raylinesinareferencelibrary.Amongthefull-energypeaks,however,thereare
annihilationescapepeaks,X-rayescapepeaks,coincidencesumpeaks,andrandomsumpeaksthatarenottabulated.
Calculationmethodsforthesespecialpeaksarepresented,implementedinSHAMAN,andevaluatedinthiswork.
ExpertsystemSHAMANcombinesacomprehensiveENSDF-basedreferencelibrarywithaninferenceenginethat
appliespruningrulestoselecttheacceptablecandidatenuclides,utilizingacollectionofcalculationalmethods.Its
performancewithairfilterspectraisevaluatedinthisworkwithfourdifferentspectrumsets. Theperformanceof
SHAMANisalsobenchmarkedagainstanotheridentificationsoftwareincompletelyautomatedanalysis.
Insummary,expertsystemSHAMANiswellapplicableatanyorganizationwhereenvironmentalradioactivityis
monitored.Thesystemcanalsobeutilizedinotherapplications,butthecurrentrulebasehasbeentailoredmost
comprehensivelyforairfilterspectra.Theanalysiscapabilitiesofthesystemcanbeimprovedinotherapplicationsof
gamma-rayspectrometrybytuningtheanalysisparametersofthesystem. Naturally,thereisstillroomfor
improvementinthemethodologyofSHAMAN,butalreadywiththecurrentlyimplementedfeaturesitisareliable
expertsystemfornuclideidentification.
Keywords gamma-rayspectrometry,airborneradioactivity,radionuclideidentification
ISBN(printed) 978-951-22-9439-8 ISSN(printed) 1795-2239
ISBN(pdf) 978-951-22-9440-4 ISSN(pdf) 1795-4584
Language English Numberofpages 188p.
Publisher TKK
Printdistribution TKK
X Thedissertationcanbereadathttp://lib.tkk.fi/Diss/2008/isbn9789512294404/

AB
VÄITÖSKIRJAN TIIVISTELMÄ TEKNILLINEN KORKEAKOULU
PL1000,02015TKK
http://www.tkk.fi
Tekijä JarmoJohannesAla-Heikkilä
Väitöskirjannimi
Analyysimenetelmiäilmanradioaktiivisuudenvalvontaan
Käsikirjoituksenpäivämäärä 11.02.2008 Korjatunkäsikirjoituksenpäivämäärä 23.05.2008
Väitöstilaisuudenajankohta 20.08.2008
X Monografia Yhdistelmäväitöskirja(yhteenveto+erillisartikkelit)
Tiedekunta Informaatio-jaluonnontieteidentiedekunta
Laitos Teknillisenfysiikanlaitos
Tutkimusala Säteilyfysiikka
Vastaväittäjä(t) Prof. StanleyG.Prussin
Työnvalvoja Prof. RainerSalomaa
Työnohjaaja TkTPerttiAarnio
Tiivistelmä
Korkeanresoluutiongammaspektrometriaonanalyysimenetelmä,jokasoveltuuhyvinilmanradioaktiivisuuden
valvontaan,silläuseimmatluonnollisetjaihmisentekemätradionukliditlähettävätgammasäteilyä.Käytännössä
gammaspektrometriallaonmahdollistahavaitaminimaalisiaaktiivisuuspitoisuuksia,jotkaeivätolelähelläkään
terveysriskiä.Gammaspektrometrisetanalyysitvoidaanjaotellahiukkas-jakaasunäytemittauksiin.Tässätyössä
esitelläänmenetelmiähiukkasnäytteidenanalyysiin,ohjelmoidaannetietokoneohjelmaksijaarvioidaanniiden
käyttökelpoisuussuurentestispektrijoukonavulla. Tavoitteenaonkehittääsellainentyökalukokoelma,jonkaavulla
kaikkigammaspektrinkomponentitvoidaanselittääkvantitatiivisestijajokatoimiimahdollisimmanautomaattisesti.
Korkeanresoluutiongammaspektrissäkaikkitietoonspektrinpiikeissä. Suurinosapiikeistäontäysenergiapiikkejä,
joitavastaavatgammaviivatlöytyvätreferenssikirjastosta.Täysenergiapiikkienseassaonkuitenkin
annihilaatiopakopiikkejä,röntgenpakopiikkejä,koinsidenssisummapiikkejäjasatunnaissummapiikkejä,joitaeiole
taulukoitu.Tässätyössäesitellään,ohjelmoidaanjaevaluoidaanlaskentamenetelmiänäilleerikoispiikeille.
TyössäonkehitettyasiantuntijajärjestelmäSHAMANia. SeyhdistääkattavanENSDF-pohjaisenreferenssikirjastonja
laskentamenetelmätpäättelykoneeseen,jokakäyttääkarsintasääntöjäanalysoitavannäytteenoikean
nuklidikoostumuksenpäättelemiseen. SHAMANinsuorituskykyilmafiltterienanalyysissaevaluoidaantässätyössä
neljänlaajanspektrikokoelmanavulla. Lisäksisuorituskykyäarvioidaanvertailemallatuloksiatoisen
tunnistusohjelmistonsaavuttamiin.
Yhteenvetonavoidaantodeta,ettäSHAMANsoveltuumainiostiluonnonradioaktiivisuudenvalvontatehtävään.Sitä
voidaankäyttäämyösmuissasovelluskohteissa,joissapyritääntäydelliseengammaspektrintulkintaan,mutta
nykyversiontietämyskantaonparhaitenräätälöityilmafiltterispektreille.Analyysikykyämuissasovelluksissavoidaan
parantaaohjelmistonparametrejasäätämällä. MyösSHAMANinmetodologiassaonkehittämisenvaraa,muttajo
nykyisillälaskentamenetelmilläseonluotettavaasiantuntijajärjestelmäradionuklidientunnistukseen.
Asiasanat gammasäteily,gammaspektrometria,radioaktiivisuusvalvonta,radionuklidientunnistus
ISBN(painettu) 978-951-22-9439-8 ISSN(painettu) 1795-2239
ISBN(pdf) 978-951-22-9440-4 ISSN(pdf) 1795-4584
Kieli englanti Sivumäärä 188s.
Julkaisija TKK
Painetunväitöskirjanjakelu TKK
X Luettavissaverkossaosoitteessahttp://lib.tkk.fi/Diss/2008/isbn9789512294404/

Abstract
High-resolution gamma-ray spectrometry is an analysis method well suitable for
monitoring airborne radioactivity. Many of the natural radionuclides as well as
a majority of anthropogenic nuclides are prominent gamma-ray emitters. With
gamma-ray spectrometry different radionuclides are readily observed at minute
concentrations that are far from health hazards. Thus, measurements during
normal conditions and minor anthropogenic releases can be utilized to increase
sensitivity of analysis methods and to learn to distinguish between source terms
from different release types.
Thegamma-rayspectrometricanalyses appliedinairmonitoringprogrammescan
bedividedintoparticulatemeasurements andgasmeasurements, ofwhichthefor-
mer category is technologically more mature. In this work, methods applicable
for particulatesample analysis have been presented, implemented in analysis soft-
ware, and evaluated with a wide variety of cases. Our goal has been to develop a
collection of tools that enables a complete quantitative explanation of all compo-
nents of a measured gamma-ray spectrum with a minimum of user intervention.
The importance of accurate and precise calibrations cannot be overestimated.
Energy and peak efficiency calibrations are fundamental for spectrum analysis,
since they are needed to convert the peak positions and areas to energies and
emission rates and further to nuclide identifications and quantifications. Use of
a shape calibration can also be recommended for its stabilizing effect on peak
search and area determination. If measurements are made in a close geometry, a
total efficiency calibration is also needed in order to enable corrections for true
coincidence summing effects.
In a high-resolution spectrum, all essential information is contained in the peaks.
Most of them are full-energy peaks that can be explained with gamma-raylines in
a reference library. Among the full-energy peaks, however, there are annihilation
escape peaks, X-ray escape peaks, coincidence sum peaks, and random sum peaks
that are not tabulated. They need to be explained quantitatively if a complete
interpretation of a measured spectrum is aimed at. Calculation methods for these
special peaks are presented, implemented, and evaluated in this work.
Expert system Shaman combines a comprehensive ENSDF-based reference li-
brary with an inference engine that applies pruning rules to select the acceptable
candidate nuclides, and with a collection of calculational methods including the
above-mentioned ones. Its performance with particulate air filter spectra has
been evaluated in this work with four different spectrum sets. Two of the sets
have been produced at different times by a global monitoring network that has
been set up for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), and the
two other sets, measured after two minor anthropogenic releases, by the Finnish
monitoring network run by STUK (S¨ateilyturvakeskus, Finnish Radiation and
Nuclear Safety Authority).
i
In the first CTBT-related set of 250 spectra the peak analysis was made with
tailor-made analysis software based on Genie. Its output was analyzed with
two Shaman versions and they both obtained a nuclide identification percentage
and a peak identification percentage of approximately 99% each. The spurious
nuclide percentage was 4–7%, translating to 0.4–0.7 spurious identifications per
spectrum. These results were obtained in a completely automated analysis mode,
so they can be considered excellent.
The second CTBT-related set included 6161spectra thatwere not analyzed man-
ually unlike the first spectrum set, only with an automated UniSampo–Shaman
pipeline system. Without a manual analysis we presented uncorrected peak iden-
tification percentages and uncorrected average numbers of spurious nuclides of
96.4% and 0.96, respectively. Both figures are less ideal than for the previous
spectrum set, but the explanation is that a smaller peak search threshold has
been utilized in the peak analysis phase. Therefore, the average number of peaks
is much larger than that in the first set and especially the share of small peaks
has increased. The performance level can still be considered very good, even if
there is some room for improvement.
Thespectrumsetsofthenationalnetworkhadbeenmeasuredafterminorreleases
fromNovayaZemlyainAugust1987andSosnovyyBorinMarch1992. Bothspec-
trumsets were analyzed with Sampoininteractive modeand thenidentified with
Shaman, so the results are not directly comparable to the automated analysis
results referred to above. However, these two spectrum sets showed that the iden-
tificationperformance of Shaman is not degraded in cases where air filter spectra
contain fission and activation products in addition to natural radioactivity.
The performance of Shaman was benchmarked against another identification
software based on Genie in two intercomparisons. In both cases, the results from
automated analysis were compared, thus neglecting the effect of a human analyst
that can compensate for errors in automated results. The first intercomparison
truly measured the difference in nuclide identification, since the same peak results
were fed into both identification implementations. In the second intercomparison
two automated analysis pipelines were compared where also the peak analysis
results are different due to different software. In both intercomparisons, Shaman
had a clear advantage in nuclide identification and peak explanation. The sta-
tistically inevitable disadvantage, a larger number of spurious nuclides, remained
under control.
In summary, the UniSampo–Shaman pipeline system is well applicable at any
organization where environmental radioactivity is monitored. The system can
also be utilized in other applications, but the current rule base of expert system
Shamanhasbeentailoredmost comprehensively forair filterspectra. The analy-
sis capabilities of the system can be improved in other applications of gamma-ray
spectrometry by tuning the analysis parameters of the system. This should not
require but a modest effort of an end-user who has the application-specific exper-
tiseandhassomescripting experience since amajorpartofShaman’s“expertise”
or “intelligence” is in its operating scripts and macros. Naturally, there is still
room for improvement in the methodology of Shaman, but with the currently
implemented features it is a reliable expert system for nuclide identification.
ii
Preface
This thesis presents the results from a long-lasting development project where
the methodology of expert system Shaman has been improved and its perfor-
mance has been evaluated at various stages of development. The project has
been financed primarily by Helsinki University of Technology (TKK), but over
the years financial support has also been obtained fromthe Ministry of Trade and
Industry and the Ministry for Foreign Affairs of Finland, as well as from the US
Department of Defense throughthe visiting scientist programme of the Prototype
International Data Centre (PIDC), Arlington, Virginia.
The author is most grateful for this long-term financial support. Especially
the extreme patience of my supervisor Prof. R. Salomaa is admirable. I also wish
to state my gratitude for the personal grants from the Jenny and Antti Wihuri
Foundation and the Swedish Academy of Technical Sciences in Finland (Wilhelm
Guerillot Fund) in mid-1990’s.
The assistance and co-operation of the radionuclide staff at the PIDC during
my two visits in 1996–98is highly appreciated. The following persons contributed
tothesuccessinonewayoranother: Dr.N.Duncan,Dr.R.Mason,Dr.W.Evans,
Ms. J. Bohlin, Mr. G. Novosel, Mr. E. Pratt, Mr. J. Bohner, Mr. A. Bennett,
Mr. J. Nicholson, Dr. K. Biegalski, Dr. S. Biegalski, Dr. L. Vladimirski, Mr. M.
Pickering, Mr. M. Wasim, and Mr. D. Williams.
Over the years, we have enjoyed a fruitful collaboration with the Security
Technology Laboratory (formerly Aerosol Laboratory) of STUK headed by Dr.
H. Toivonen, as well as the Finnish National Data Centre (FiNDC) hosted by
STUK and headed consecutively by Mr. M. Nikkinen, Mr. A. Isolankila, and Mr.
M. Moring.
A major part of the research has been done in the Radiation Physics Group of
theAdvancedEnergySystemslaboratoryatTKK.Thegroupisnotbigbutitsex-
pertise on gamma-ray spectrometry is state-of-the-art. The group leader and my
instructor Dr. P. Aarnio has introduced an extremely interesting research subject
to me and has led the work with scientific rigor through the years. Mr. T. Hakuli-
nen is the father of Shaman who has built and continues to build an exemplarily
robust computer code, a solid foundation that allows for further development.
Mr. M. Nikkinen has contributed to the Shaman project in his Master’s thesis
and later by developing the spectrum analysis software UniSampo, the preferred
front-end for Shaman. Prof. Emer. J. Routti, the founder of this group, has
shown continued interest in our work, inspiring us to carry on. Additionally, a
number of smart summer students have contributed to the research of our group
at various stages.
The working atmosphere in the Advanced Energy Systems laboratory has
been most pleasant and inspiring, but nevertheless, I would like to emphasize the
contribution of my family. Without its counterbalancing effect this work may
have been completed earlier, but my life would have been much less rewarding.
Therefore, I wish to dedicate this work to my dear wife Marika and our four
wonderful children, Ida, Fanny, Anton, and Sofia.
Jarmo Ala-Heikkil¨a
Espoo, May 23, 2008
iii