Depth resolved ion beam analysis of objects of art


Abstract 

The external proton beam progressively gains more attention, particularly in connection with recent new possibilities combining non-vacuum PIXE and RBS. To identify paint layer arrangements and their elemental composition external PIXE was applied varying the incident proton energy. This technique is checked on a set of test paint layers combining pigments typically applied in the Middle Ages as well as presently. Thin surface paint has been successfully characterized by means of simultaneous external RBS complementary to PIXE. Presented as an example are depth resolved measurements on a red colored detail of the painting "14 Nothelfer" from Lucas Cranach the Elder.

1. Equipment

Fig.1 The Rossendorf external proton beam facility at the 5 MV tandem accelerator: Exit pipe and detector arrangement

The figure gives a schematic representation of the Rossendorf external equipment. The main component is the exchangeable exit pipe completing the terminal vacuum tube after the 2 µm thick HAVARâ exit foil. A continuous helium gas stream (3 l/min), fed in from the bottom of the pipe, reduces proton stopping as well as X-ray absorption and prevents the emission of Ar X-radiation from air. The helium gas may escape only from the end of the exit pipe which is at a distance of 0.1-0.3 mm from the object during measurement. non-contact analysis is indispensable when, e.g., examining mechanically highly sensitive objects such as pastel drawings. Graphite linings inside the terminal vacuum tube and along the 11 mm proton path through the pipe prevent the beam halo from striking the stainless steel.

As shown in the figure, the detectors are arranged as close as possible to the beam spot. X-rays from elements Z>12 are measured at 135o backward angle by means of a conventional Si(Li) detector (12.5 mm2 active area, DE = 170 eV at 5.9 keV). The counting rates are limited by a selectable set of apertures and Mylar absorbers in front of the detector. Backscattered protons are also accumulated at 135 o using a light protected and cleanable silicon surface barrier detector (Canberra PIPS, 100 mm2 active area, 300 µm depletion depth, DEa = 30 keV at 5.4 MeV). Light element (Z<14) analysis takes place by PIGE using a large volume HPGe detector of 60% relative efficiency (DE = 1.9 keV at 1.33 MeV). The characteristic g-radiation can be detected from the rear of the object or, using a modified exit pipe, at 52o backward direction. The choice of geometry depends on the type of materials as well as the thickness and shape of the individual object. PIGE works only for a selection of light elements (B, Mg, Na, Al, Si) which are of special interest for distinguishing different types of modern or historical glasses.

At typical parameters of exposure no parasitic X-rays from components of the equipment can be detected. This has been checked by bombarding a silicon wafer. Besides the Si K-line only the pure secondary electron bremsstrahlung continuum is observed. Clean carbon glass substrates confirm this fact.

The proton fluency is monitored by internal RBS. For this, the collimated beam passes a rotating chopper consisting of three tantalum pins just before the HAVARâ exit window. Calibration of this unit takes place by means of an external Faraday cup.

Evaluation of the PIXE spectra is carried out using both the "thick target" and the "layered target" versions of the program GUPIX. The RBS spectra are simulated by means of RUMP.

GUPIX:

J.A. Maxwell, J.A. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B43 (1989) 218

RUMP:

L.R. Doolittle, Nucl. Instr. and Meth. B9 (1985) 344

2. Identification of layered objects: Paint layers

PIXE measurements at different proton energy Ep give relative concentrations of chromo-phoric elements Zc >at varied information depth. With increasing Ep the progressive increase/decrease of the concentration of Zc indicates the presence of this element within a deeper/upper layer of the pigment arrangement.

Fundamental information on this technique have been got by PIXE measurements at test paint layer arrangements, e.g. the combination "verdigris - white lead".

Test paint layer arrangements

 
verdigris
     
verdigris + white lead
white lead
white lead
verdigris
 
 
chalk
ground
   
 
presspahn
plate
   

Fig.2 Cross section of a test paint layer arrangement of verdigris Cu(CH3COO)2.H2O and white lead 2PbCO3.Pb(OH)2 (TALENSâ oil paints) on chalk ground CaCO3

PIXE results for the test paint layers as a function of the incident proton energy E

Tab.1 Relative concentrations (wt%) of chromophoric elements visible in the PIXE spectra from the test paint arrangement described above. GUPIX (thick target option) was used for evaluation of PIXE spectra. *Thicknesses measured by optical microscopy at cross sections prepared from tiny samples.

Arrangement
Ep [MeV]
Ca
Cu
Pb
Chalk (~ 360 µm)*

1.4
2.6
3.9
99.7
99.6
99.6
0
0
0
0
0
0>
Verdigris (~ 89 µm)
1.4
2.6
3.9
2.5
19.3
27.5
97.5
80.7
72.0
0
0
0
White lead (~ 30 µm)

1.4
2.6
3.9
0.2
0.4
0.2
0
0
0
92.2
91.5
93.8
Verdigris (~ 40-88 µm) on

white lead (~ 12-30 µm)

1.4
2.6
3.9
2.3
2.1
0.5
89
51
35
8
45
62
Verdigris mixed (1:1) with

white lead (~ 165 µm)

1.4
2.6
3.9
?
0.4
0.2
11
16
22
82
78
74

PIXE - elemental concentrations with increasing Ep,discussion of the trends (Tab. 1):

white lead on chalk Pb: no Ep dependency
verdigris on chalk Cu: decreasing

Ca: increasing

verdigris on white lead Cu: decreasing

Pb: increasing

Ca: low concentration

Verdigris + white lead Cu: slowly increasing (inhomogeneities)

Pb: slowly decreasing (inhomogeneities)

Ca: low concentration




RBS - the complement

 

 

Fig. 3:
RBS spectra (Ep = 3.9 MeV) of paint layer arrangements:



  • zinc white plus chromium oxide (mixed)
  • chromium oxide on zinc white (layered)
  • chromium oxide on chalk (single)

RBS - interpretation of the spectra (Fig. 3):

The location of the RBS surface edge (2) between the Zn edge and the Cr edge identifies Zn atoms to be covered with a coating, i.e. the chromium oxide on zinc white structure. Note that the presence of Mn - Cu is excluded by PIXE.

3. Example: The oil painting "14 Nothelfer" (Lucas Cranach the Elder)

Fig. 4 X-ray spectra from the red robe of the holy Christopherus ("14 Nothelfer", Lucas Cranach the Elder) taken at two proton energies

Tab. 2: Relative concentrations* (wt%) of main and secondary elements visible in the PIXE spectra of the red robe of the holy Christopherus (Easel painting "14 Nothelfer", L. Cranach the Elder).

Ep [MeV]
 
 
Hg
Pb
Ca
2.1
56.9
32.1
8.7
3.9
35.9
57.1
4.9

* the difference to 100 wt% includes the trace elements K, Fe, Cu, Zn

With increasing proton energy the data of Tab. 2 show clearly:

  • The concentration of Pb increases whereas that of Hg drops, hence
    1. the outermost visible layer, containing Hg, is interpreted as cinnabar (HgS)
    2. The assignment of Pb to an inner white lead 2PbCO3.Pb(OH)2 imprimatur or minium Pb3O4 cannot be determined from this measurement.

Ca X-rays originating from a chalk grounding must be excluded because of the strong absorption within the Hg and Pb containing paint material. Hence, Ca is ascribed as a secondary element of the used pigments and different in concentration for both the top and underlaying paint material.

4. Concluding remarks

By means of test paint layers it is demonstrated that external PIXE analysis with proton energy variation allows one to identify and to attribute pigments in layered arrangements. The thickness of extended paint layers can be estimated by PIXE.

If the layers have dimensions of only some microns, PIXE with proton energy variation fails. Complementary RBS makes it possible to completely characterize thin pigment layers near the surface. In particular, external RBS allows estimation of varnish film thicknesses which are of special importance for PIXE evaluations of the underlaying pigment layers.

Regarding an object of historic interest the depth arrangement of the paint materials possibly represents the "handwriting" of the artist or his workshop. This knowledge is of interest for art scientists and restorers.

5. Selected references

5.1 Reviews (IBA analysis of art objects)

  1. B.L. Doyle, D.S. Walsh, S.R. Lee; Nucl. Instr. and Meth. B54 (1991) 244
  2. M. Menu; Nucl. Instr. and Meth. B75 (1993) 469
  3. P.A. Mandò: Nucl. Instr. and Meth. B85 (1994) 815

5.2 Recent external proton beam studies

  1. 4. L. Giuntini, P.A. Mandò; Nucl. Instr. an Meth. B85 (1994) 744
  2. T. Kupila-Rantala, J. Räisänen; Nucl. Instr. and Meth. B84 (1994) 368
  3. G. Demortier, Y. Mociaux; Nucl. Instr. and Meth. B85 (1994) 112
  4. W. Wagner, C. Neelmeijer; Fresenius J. Anal. Chem. 353 (1995) 297

5.3 Historical painting materials and their identification

  1. H.P. Schramm, B. Hering; in: Historische Malmaterialien und ihre Identifizierung, ADVA, Berlin, 1989, ISBN 3-326-00202-5

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