Cosentino_-_Neutron_backscattering_for_the_search_of_the_Battle_of_Anghiari-1

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Neutron back scattering for the search of the Battle of Anghiari

V.R. Bom a,, A. Cosentino b, M. Seracini b, R. Rosa c

a Delft University of Technology, Department of Applied Physics, Mekelweg 15, 2629 JB Delft, The Netherlandsb Center of Interdisciplinary Science for Art, Architecture and Archaeology, University of California San Diego, 9500 Gilman Dr. La Jolla, CA 92093, USAc ENEA, Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy

a r t i c l e i n f o

Article history:

Received 17 June 2009

Received in revised form30 July 2009

Accepted 31 July 2009

Keywords:

Neutron back scattering

Leonardo Da Vinci

Battle of Anghiari

Non-destructive testing

a b s t r a c t

The Battle of Anghiari is a wall painting made by Leonardo Da Vinci around 1505. Its present day

location is unknown but some indications suggest that the mural might be concealed behind a brick

wall. Test measurements are presented demonstrating that neutron back scattering (NBS) can be used tosearch through the wall for the painting. NBS is a non-destructive technique to establish the presence of

the hydrogen contained in the painting materials that were probably used by Da Vinci.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In 1503 Leonardo Da Vinci accepted a commission from the

chief magistrate of the Republic of Florence to paint a large muralon a wall of the Hall of 500 in the Palazzo Vecchio. This painting

was to commemorate the historic Battle of Anghiari (BoA) in

which the Florentine army defeated the Milanese one in 1440 and

would be Leonardo Da Vincis largest and most substantial work,

about three times the size of the famous Last Supper mural he

painted in Milano in 1495. Work began presumably in June 1505

but it would never be completed. Contemporaries describe the

painting as Da Vincis most magnificent work ever and it is of

great interest because the artist used a new experimental painting

technique based on oil. The unfinished mural remained in the Hall

of 500 until 1563 when the architect and painter Giorgio Vasari

undertook renovation of the space. The ceiling and the walls were

raised and Vasari himself covered the new walls with frescoes and

all traces of the Battle of Anghiari were lost. Some scholars believethat Vasari was a far too great admirer of Leonardo Da Vinci to

have destroyed the artwork and that it would have been a simple

enough matter to have built a new wall over the work of Leonardo,

as was sometimes done in those days.

A first investigation to determine if the BoA was behind one of

the walls was carried out in 1975 by Seracini, Newton and Asmus,

using a variety of introspection methods. The research was

unsuccessful due to the lack of appropriate technologies. Further

research was done by Seracini in 2000 using laser scanning,

thermography and ground penetrating radar (Pieraccini et al.,

2005). The thermographic investigation showed that Vasari had

built brick walls to support six new murals and the radarmeasurements provided insight in the internal structure of east

and west walls. The west wall appeared to be homogeneous as

radar images showed no traces of an internal interface. However,

inside the east wall a discontinuity clearly appeared at 15 cm

depth, which could correspond to an air gap between the masonry

built by Vasari and the original wall, maybe to preserve the BoA

mural.

The presence of the BoA behind the masonry front wall might

be confirmed by using non-destructive and non-invasive neutron

techniques such as neutron nanosecond analysis (NNA) and

neutron back scattering (NBS). NNA is based on n;g reactions

and can be used to detect the elemental composition of materials

inside the wall. Neutron back scattering is based on the slowing

down of fast neutrons by hydrogen nuclei. NBS can only detecthydrogenous materials but has a high speed of operation

compared to the NNA technique. The hydrogen in the case of

the BoA search would be present in the materials used by

Leonardo Da Vinci such as Greek pitch, linseed oil, walnut oil and

gesso, which have been used according to original documents.

Gesso is the Italian word for Board chalk (akin to the Greek

word gypsum), and is a powdered form of the mineral calcium

carbonate, CaSO4 2H2O. Each calcium carbonate molecule nor-

mally contains two water molecules bound into the crystal. Gesso

was traditionally mixed with animal glue, usually rabbit-skin

glue, to be used as a primer coat preventing paint from soaking

into the support layer. Leonardo Da Vinci plastered his wall with

ARTICLE IN PRESS

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/apradiso

Applied Radiation and Isotopes

0969-8043/$- see front matter & 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.apradiso.2009.07.025

Corresponding author.

E-mail address: [email protected] (V.R. Bom).

Applied Radiation and Isotopes 68 (2010) 6670
http://-/?-http://www.elsevier.com/locate/apradisohttp://dx.doi.org/10.1016/j.apradiso.2009.07.025mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.apradiso.2009.07.025http://www.elsevier.com/locate/apradisohttp://-/?-


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gesso because he tried out an experimental technique using the

wall as it were a big panel painting, and used oil based paints.

Gesso was not applied by Vasari. He used mortar instead, made

from sand and lime, to build the wall and as preparation for his

frescoes. The Vasari paintings do not interfere with the NBS

method since dried mortar does not contain hydrogen.

The feasibility of NBS for the search for the BoA has been tested

using mock walls at the Delft University of Technology in the

Netherlands and at the ENEA laboratories in Casaccia near Rome,Italy. The results of both tests are presented in this paper, leading

to the conclusion that the NBS technique may be applied

successfully in the search for the BoA mural and that the method

does not constitute any risks neither to the existing Vasari

paintings nor to the public in the Hall.

2. Neutron back scattering

Neutron back scattering is a well established method to show

the presence of hydrogen. It is used, among others, for land mine

detection (Brooks et al., 1999; Datema et al., 2001, 2002; Bom

et al., 2005).

A NBS detector operates by irradiating the surface under

investigation with high energy (MeV) neutrons. The neutrons lose

energy by scattering from atomic nuclei beneath the surface and

become thermal after a number of collisions. The thermalization

process takes far fewer collisions when scattering from hydrogen

as compared to other elements. The concentration of thermal

neutrons in regions containing hydrogen-rich materials will

therefore be relatively high. A thermal neutron detector that

monitors the neutron flux coming back from the irradiated surface

will show an increased count rate above hydrogenous regions. An

important advantage of the NBS method is the high speed of

operation (Bom et al., 2006). The main limitation of the NBS

method lies in the sensitivity to moisture. The masonry in case of

the BoA search, however, is very dry because the east wall in the

Hall of 500 is an inner wall and there is no observed leakage from

the roof. The hydrogen containing materials inside the east wall

can therefore only be the oils, the resin and the gesso used by

Leonardo Da Vinci.

The NBS system used is the Delft University neutron back-

scatter imaging detector (DUNBID). It consists of 16 position

sensitive thermal neutron proportional counter tubes that are

placed in a tray covering an area of 50 50cm2. The position of a

neutron hit along a tube is determined by charge division, the

tube number provides the position in the direction perpendicular

to the tubes. Both positions are fed into a process computer and a

2D image of the back scattered thermal neutron intensity is

formed, reflecting the hydrogen distribution beneath the surface.

There is extensive blurring in the image due to the heavy

scattering of the neutrons. The advantage of using a 2D approach

is the enhanced sensitivity for the detection of discontinuities incomparison to only monitoring the overall count rate. The spatial

resolution is approximately 3 3 cm2. The detection efficiency is

almost 1 for neutrons with thermal energies and decreases to

around 104 for 2.5 MeV neutrons. The neutrons are provided by a

radioisotope 252Cf point source that is positioned in the center of

the detector. The sensitivity for hydrogen detection varies over the

detector area due to the dependence of the neutron flux on the

distance from the source.

3. Test structures

The east wall can be regarded as composed of three main

structural elements: a front wall, the mural and a back wall. It is

impossible to build an exact reproduction of the east wall cross

section because the pictorial technique used by Leonardo Da Vinci

and the composition of the wall on which the mural was painted

are unknown. DUNBID was evaluated on two different test

structures to cope with this limitation: the Delft structure and

the ENEA one. They were designed to cover a range of likely

compositions of the east wall. The real east wall, regarding to

neutron back scattering, is expected to behave in a way that will

be in between the two structures. Both structures are composed ofthe three main parts.

Front brick wall: Samples of the bricks used by Vasari for the

front wall 15 4 30:5 cm are available and so the reproduction

of the front wall was not an issue.

Mural: The technique used by Da Vinci can be guessed from the

supplies he bought between August 1504 and April 1505, namely:

gesso, Greek pitch, and white soda. The back wall must first have

been covered with wall gesso. A ground layer of Volterrano gesso

mixed with Greek pitch, most likely made to react with the white

soda (sodium hydroxide), may have been applied next on the wall

gesso. Leonardo must have painted a ground color with Alexan-

drian white and linseed oil on this ground layer, finally followed

by the pigments, mixed with linseed and walnut oils. It cannot be

guessed how much gesso, Greek pitch and linseed oil were applied

on each square meter, therefore the mural of the Delft structure

was made with a high concentration of linseed oil and the ENEA

structure mural was made with three different smaller amounts.

Back wall: The east wall probably is made of mortar, stones and

likely clay elements and its average thickness is about 70 cm, but

the actual composition is not known. The Delft back wall consists

of 1 m dry sand, which depth is enough to be considered infinite,

and the ENEA test structure has a back wall made of bricks (less

silicon and more aluminum) with a natural moisture level and a

finite depth. The actual back wall in the Palazzo Vecchio is

supposed to be in between these two extreme cases.

3.1. Delft structure

The Delft structure was built at the Delft test lane facility,

which was especially constructed to test NBS demining systems.

The facility consists of a box of 3 8 1 m3 filled with sand. The

sand is heated from the bottom to keep it as dry as possible. This

box constitutes the back wall.

The mural was simulated by a plasterboard 200 60

0:9 cm3 laid down on the sand, building a horizontal wall

structure. The plasterboard is made of a gypsum layer and covered

with paper and is assumed to suitably simulate gesso for the

neutrons. A section 60 60cm2 of the plasterboard was painted

with linseed oil 1:38 l=m2 to simulate the mural on the gesso. An

aluminum plate 60 60 0:5 cm3 was also used, to evaluate the

sand box background. Three different front walls, made with

bricks 6:5 5 20cm3, were built on top of the plasterboard

and the aluminum plate and tested (see Fig. 1):

(A) Two layers of bricks 260 60 10cm3.

(B) Three layers of bricks 260 60 15cm3.

(C) Three layers of bricks 260 60 15cm3 plus a 5 c m

distance DUNBID-wall.

3.2. ENEA structure

The ENEA structure was built at the ENEA laboratory in

Casaccia, Rome. The front wall 180 120 15cm3 was built

with one layer of bricks. The mural was replicated using one of

four plasterboards 100 120 0:9 cm3 of which three were

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painted with different quantities of linseed oil: 0.41, 0.83 and

1:25 l=m2.

The back wall was built with two layers of bricks and was 5 cm

away from the front wall. Fig. 2 shows the ENEA structure cross

section.

4. Results

4.1. Delft structure

Measurements were done with each of the three front walls (A,

B and C, see Section 3.1 and for the following four experimental

configurations (see Fig. 1):

DELFT-1. Background: DUNBID above the aluminum plate.

DELFT-2. Plasterboard: DUNBID above the plasterboard with-

out linseed oil.

DELFT-3. Linseed oil plasterboard: DUNBID between the

plasterboard without and with linseed oil.

DELFT-4. Linseed oil 1:38 l=m2: DUNBID above the plasterboard

with linseed oil.

Counts were collected with an acquisition time of 500 s using a252Cf source of 1:5 105 n=s. The background count rate was

270c/s. Fig. 3 shows the count rate excess above the count rate

measured at the background position (configuration DELFT-1), as

a fraction of that background count rate, for the three front wall

configurations A, B and C.

The absolute count rate for the background measurements

varies for cases A, B and C as: 1.00, 0.93 and 0.82, respectively,

showing a decreasing rate as the distance of the detector to the

back wall (the sand in this case) increases. The plasterboard

(configuration DELFT-2) could be detected in all the setups with

an excess count rate between 12% and 15%. The plasterboard

painted with linseed oil (configuration DELFT-4) could be detected

with a 2528% excess count rate. DUNBID could also report a

lesser linseed oil amount (configuration DELFT-3) giving an excess

fraction between 19% and 22%. Fig. 3 shows two trend lines for the

two front wall configurations A and C. The slope of the line is an

indication for the sensitivity of the method to the presence of

Fig. 1. Layout of the experimental configurations of the Delft test structure.

Fig. 2. Layout of the experimental configuration of the ENEA test structure.

Fig. 3. The count rate excesses above background as fraction of the background

count rate for the three front wall configurations A, B and C mentioned in the text

and the three configurations DELFT-4, DELFT-3 and DELFT-2 (from left to right).

The drawn and dashed lines reflect the trend for the 15-cm bricks with standoff

and the 10-cm bricks front walls, respectively (walls A and C).

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linseed oil. The sensitivity is higher the closer the detector is to

the oil-containing layer as could be expected.

4.2. ENEA structure

The experimental configurations were:

ENEA-1. Background: The gap between the front and back wall

is empty.

ENEA-2. Plasterboard: The plasterboard without oil is insertedbetween the two walls.

ENEA-3. Oil0:41 l=m2: The plasterboard with 0:41 l=m2 lin-

seed oil is placed between the two walls.

ENEA-4. Oil0:83 l=m2: The plasterboard with 0:83 l=m2 is

linseed oil placed between the two walls.

ENEA-5. Oil1:25 l=m2: The plasterboard with 1:25 l=m2 lin-

seed oil is used.

ENEA-6. Empty=oil1:25 l=m2: The plasterboard with linseed

oil 1:25 l=m2 covers half DUNBID while the other half is left empty.

ENEA-7. Plasterboard=oil1:25 l=m2: DUNBID covers half the

bare plasterboard and half the plasterboard with 1:25 l=m2 board.

Fig. 4 shows the relative excess count rates for measurements

made with the DUNBID detector placed at a standoff distance

from the front wall of 4 and 5 cm. All the data were collected withan acquisition time of 60 s and a 252Cf source of 2:1 106 n=s. The

background count rate was 3150 c/s.

The figure shows that with the NBS method slight variations in

oil concentration can be detected. The count rate is strongly

dependent on the distance between the detector and the wall or

oil. Measurements done without the back wall hardly showed any

effect of plasterboard or oil. A substantial back wall proved to be

necessary to observe the effect. The large difference of the

observed effect between the Delft and the ENEA tests (30% and

15%, respectively) may be explained by the difference between the

back walls used in both setups.

4.3. NBS images

The intensity of the back scattered neutrons can be determined

as a function of position using the position sensitivity of DUNBID.

The resulting images will be blurry because of the scattering of

the neutrons in the wall structures but may still give important

information about the distribution of the hydrogenous materials.

The sensitivity over the image varies strongly as the edges/cornersFig. 4. Excess count rates relative to the background for distances DUNBID-front-

wall of 4 and 5cm.

Fig. 5. Images of the backscattered neutron flux obtained with DUNBID. Corrections for background and sensitivity variations have been applied as mentioned in the text.

The intensity in the images is an indication for the oil content on the boards. Intensity 1.0 corresponds to a bare plasterboard ENEA-2. The two bottom images represent the

results for the configurations ENEA-6 (left) and ENEA-7 (right). All images have the same intensity scale.

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get less primary neutrons compared to the center because of the

central position of the source. Corrections have been applied by

subtraction of the background image to remove the scattering

from the bricks followed by a division by the background-

subtracted plasterboard image to correct for the sensitivity

variations. Fig. 5 shows DUNBID images obtained in this way

with the ENEA tests. The images have been compressed into 5 5

pixels to improve statistics, giving a resolution of 10 10cm2.

A pixel value of 1.0 would correspond to plasterboard. The two topimages show results obtained with the plasterboards with 0:41

and 1:25 l=m2 oil concentrations. The pixel values for the

0:41 l=m2 are around 1.0, indicating that such small oil

concentrations can hardly be distinguished from plasterboard

without oil.

The pixel values for the 1:25 l=m2 board are clearly above 1

showing the effect of the oil. The two bottom images in Fig. 5

show the two mixed compositions: ENEA-6 (to the left) and ENEA-

7 (to the right). The lack of counts is obvious in the left-hand part

of the image of the ENEA-6 configuration where there was no

board. In the image of the ENEA-7 combination the left-right

difference is still noticeable.

4.4. Uncertainties

The uncertainties in the results due to counting statistics are

small: 0.003 for the ENEA tests and 0.004 for the Delft tests. The

main source of errors is the variation in the distance of the NBS

detector to the wall since back scattering from the wall itself is the

main count rate source. A correction for distance variations can be

applied by measuring the standoff and careful calibration. The

effects of variations in the wall structures, possibly due to uneven

brick size or to irregular joints, are small because of the

considerable scattering and diffusion of the neutrons through

the structures that will average out small scale irregularities.

5. Safety

The time during which the wall is irradiated in one single

position is of the order of minutes thanks to the high speed of

operation of the NBS method. The radioactivity that is induced in

the wall structure is therefore only short living and is far below

the legal limits.

The radioactivity of a brick similar to the ones used for the

ENEA structure was measured before and after irradiation with

the source that was used for the ENEA tests. Gamma ray

measurements were performed of the natural radioactivity for

24 h. Then the brick was irradiated from a distance as used in the

tests for 7 days and measured again for 24 h. Two isotopes were

formed by this irradiation: 24Na T1=2 14:9 h and56Mn

T1=2 2:6 h. These two isotopes decay quickly and were detect-

able only after an irradiation time of many hours, long enough toreach the saturation activity. DUNBID requires a measurement

time that is more than two orders of magnitude smaller.

The 2:1 106 n=s Cf neutron source gave a dose rate at a

distance of 1 m of 5 mSv=h. This is below the allowed dose rate of

10mSv=h for radiological workers. DUNBID will be mounted on a

10m high scaffolding for the experiments in the Palazzo Vecchio.

The dose rate at the floor in that case will be only 0:05mSv=h,

which is far below the allowed rate for public.

6. Conclusion

It is certain that Leonardo Da Vinci made a painting on a wall in

the Palazzo Vecchio. What materials he used and in whatconcentrations remains uncertain, but an educated guess can be

made among others from still existing shopping lists. The setups

used in the tests described here were based on these guesses and

variations in the setups were applied to cover a wide range of

paint materials and concentrations.

The wall with the BoA mural may contain sections with and

without preparation layers and the prepared sections may be

painted or unpainted. It has been demonstrated that the NBS

device DUNBID may be applied successfully to search of the

prepared sections, painted or not, because the base layers give a

strong signal. The painted sections may also be found since the

concentration of painting materials used by Leonardo Da Vinci

likely is within the range of the concentrations used in these tests.

DUNBIDs imaging capabilities may prove useful in distinguishing

the various sections. The DUNBID NBS system may thus be

successfully applied in the search for the Battle of Anghiari

painting.

Acknowledgements

We thank Prof. DuVarney, Emory University Department of

Physics, for suggesting the use of neutron techniques to detect the

presence of pigments behind the masonry front wall in 2005. Prof.

DuVarney has been a consultant to the project since then.

The University of California starting from 2007 has supported

Prof. Seracinis efforts to pursue the search for the lost mural with

neutron techniques after appointing him Scientific Director of

CISA3: Center of interdisciplinary Science for Art, Architecture andArchaeology.

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