Neutron Activation Autoradiography
(a presentation by
Helmut Hörner)
M. Alfeld et. al, 2015:
„Neutron activation autoradiography
and scanning macro X-ray fluorescence analysis
of Rembrandt van Rijn’s Susanna and the Elders
(Gemäldegalerie Berlin):
A comparison of two methods
for imaging of historical paintings
with elemental contrast“
in: Applied Physics A · June 2015 · 119:795–805
Neutron Activation Autoradiography
“Susanna and the Elders”,
Rembrandt 1647, oil on mahogany,
Gemäldegalerie Berlin
Methods
• XRR: “X-Ray Radiography”
• X-raystransmittedthroughthepaintingarerecordedina
photographicmediumorareadetector
• MA-XRF: “Macro X-Ray Fluorescence Analysis”
• surfaceofthepaintingisscannedwithafocusedorcollimatedX
-raybeam
• NAAR: Neutron activation autoradiography
• Paintingisexposedtocoldorthermalneutrons;elementsform
radioactiveisotopeswithknownradioactivedecaypath
• Advantages / Disadvantages
X-Ray Radiography
Source:C.RichardJohnson,Jr.andWilliamA.Sethares.CountingVermeer:UsingWeaveMapstoStudyVermeer'sCanvases,modifiedbyH.Hörner
X-rays transmitted through the painting are recorded in a
photographic medium or area detector
Macro X-Ray Fluorescence Analysis
• ThesurfaceofthepaintingisscannedwithafocusedorcollimatedX
-raybeamwithadiameterofafewhundredmicrometres
• Someoftheatomsareionized,whichmeansthat
onemoreelectronsareejectedfromtheirorbits
• Electronsinhigherorbitals"fall"intothelower
orbitaltofillthehole.Aphotonisreleased,the
energyofwhichisequaltotheenergydifference
ofthetwoorbitalsinvolved.
• Theenergyofeachphotonisdetectedbyasilicondriftdetector,
whichmeasurestheenergyofeachincomingphotonbytheamount
ofionizationitproducesinthedetectormaterial.
ImgSources:OINA/SDDExplained/0112
physicsopenlab.org
Macro X-Ray Fluorescence Analysis
• scannerbuiltbyUniversityof
Antwerp
• oneX-raytube
• 0.8mmPbcollimator
• fourSDdetectors
• distance2-3cm
• elementsfromSu(Z=16)up
toSn(Z=50)detectedbyk-
levelradiation
• HeavierelementsbyL-and
M-levelradiation
• Hiddenlayers:elements
heavierthanCu(Z=29)are
detectable
• sensitivityof10,000
counts/s/masspercent
betweenZ=26andZ=42
• Step-size:0.6mm-1mm
NAAR: Neutron Activation Autoradiography
ImgSources:(1)DenkerAetal.NeutronActivationAutoradiography.In:Kardjilovetal(eds)NeutronMethodsforArchaeologyand
CulturalHeritage.Springer,Cham,2017.;(2)http://www.iki.kfki.hu/nuclear/research/index_en.shtml
NAAR: Neutron Activation Autoradiography
NAARatBERIIresearchreactorattheHelmholtz-ZentrumBerlin
coldneutronbeam,3.5cmx12.5cm,fluxof1.1x10
9
cm
-2
s
-1
,impinged3hrsinflatangle
XRR: X-Ray Radiography
• Advantages
• simple
• fast
• cheap
• Disadvantages:
• Dominatedbytheheaviestelements,typicallyPbfromlead
white(PbCO
3
and2PbCO
3
∙Pb(OH)
2
).
• Thestudyofhiddenpaintlayerscontaininglighterelements,
suchasFeandMninearthpigments,isseldompossible
• Alllayerscontributeequallytotherecordedradiograph.
• Noinformationaboutelementalcomposition.
Macro X-Ray Fluorescence Analysis
• Advantages
• Canstillbeperformed“in-house”(here:inthe
conservationstudioofthemuseum),
• andwithinreasonabletime-frame(here:38hours).
• Elementaldistributionimageseasytointerpret
• Disadvantages:
• DetectionofAsinthepresenceofHgandPbis
difficult(spectraloverlap);
• notsuitableforvisualizingthedistributionofP
(boneblack)insub-surfacelayers(e.g.sketches);
• considerablymoresurfacesensitivethanNAARand
XRR:e.g.morethan95%oftheradiationis
absorbedina170-μmleadwhitepaintlayer.
NAAR: Neutron Activation Autoradiography
• Advantages:
• Provideselementaldistributionimages(likeMA-XRF);
• providesahigherlateralresolutionthanMA-XRF(atleastinthe
implementationusedinthiscase);
• NAARislesshinderedbyabsorptionincoveringlayers;
• onlymethodcapableofvisualizingPinlowerpaintlayers
contributedbythepresenceof“BoneBlack”(Ca
3
(PO
4
)
2
.
• verysuitabletoolforalsodetecting,umber(Mn),copper-based
greensandblues(Cu)andvermilion(Hg)duetotheir
susceptibilitytoactivation
• Disadvantages:
• Requiresthestayofthepaintingataresearchfacilityfor
severalweeks;
• inter-elementinterferencescanbedifficulttoresolve;
• onlyalimitednumberofelements(mostnotablyMn,Cu,As,
Co,HgandP)contributetotheacquiredautoradiographs.
Where’s the Arm?
Conclusion
“Given the relatively short time and less effort required for MA-XRF
investigations, this technique is expected to be applied more frequently
in the future than NAAR.
However, due to the capability of NAAR to visualize the distribution of
certain elements through strongly absorbing covering layers, especially
P in the case of Rembrandt paintings, NAAR and MA-XRF ultimately
provide complimentary information.”