Pigment Response to Neutrons

A beam of neutrons incident on a painting will induce reactions within the elements present in the pigments. These reactions lead to the emission of beta rays and gamma rays. As is the case in x-ray emission, the energies of the emitted gamma rays identify the elements. Beta-ray emission allows one to examine the distribution of elements across a painting in a manner similar to x-radiography. An autoradiograph of Anthony Van Dyck's Saint Rosalie Interceding for the Plague-Stricken of Palermo (Figure 7.6a) reveals a self-portrait sketch painted in bone black oil paint residing beneath the image of Saint Rosalie (Figure 7.6b). Bone black pigment contains high concentrations of phosphorous, which when activated by neutrons generates through the emission of beta rays an image on film. Such exposures are timed to produce images of selected elements.

Neutrons are one of the constituents of nuclear matter and may be generated in a nuclear reactor. A flux of neutrons incident on the surface of a painting will penetrate the materials and be selectively absorbed by the elements in the pigments. The absorbed neutron causes a rearrangement of the nuclear structure of the element, leading to instability. The unstable nucleus moves toward a stable configuration by emitting a variety of nuclear particles such as beta rays and gamma rays. This process is referred to as radioactive decay. The rate of radioactive decay varies widely for different elements. The rate of decay is characterized by the "half-life," which is the time required for one half of the radioactive nuclei to decay.

The products of radioactive decay and their energies vary from element to element. In neutron-induced radioactive decay, the pigments are identified by the energies of the emitted gamma rays. Through the use of a gamma ray detector, one is able to document the elemental composition of a given pigment. The pigments also provide a picture of their location in a painting by the emission of beta rays, which expose film placed in contact with the surface of the painting. This process is called

Papin Sisters Crime Photos

Fig. 7.6a,b. (a) Anthony Van Dyck, Saint Rosalie Interceding for the Plague-Stricken of Palermo, c. 1624. Oil on canvas, 39 1/4 X 29 inches, The Metropolitan Museum of Art, New York. (b) A neutron-induced autoradiograph of Van Dyck's painting showing the self-portrait painted in bone black residing underneath the image of Saint Rosalie.

Fig. 7.6a,b. (a) Anthony Van Dyck, Saint Rosalie Interceding for the Plague-Stricken of Palermo, c. 1624. Oil on canvas, 39 1/4 X 29 inches, The Metropolitan Museum of Art, New York. (b) A neutron-induced autoradiograph of Van Dyck's painting showing the self-portrait painted in bone black residing underneath the image of Saint Rosalie.

autoradiography because emission from the pigments themselves provides the image rather than an external beam.

The oil painting shown in Figure 7.7a was made to take advantage of the unique image-producing qualities of neutron-induced autoradi-ography. It is composed of layers of images superimposed upon one another beginning with the head painted in raw umber. Over the image of the head lies a grid of blue lines followed by squares of the same cobalt blue color. Once the painting was completed, it was exposed to a flux of neutrons produced in a nuclear reactor designed for research purposes. The intervention of the neutron flux initiated the radioactive decay of a variety of elements present in the pigments. The emission of beta rays incident on photographic film in contact with the painting surface produced a series of images, three of which are shown in Figures 7.7b,c, and d. Film exposures are timed to correspond to the decay rate of specific elements. In the first exposure, (Figure 7.7b) made 15 minutes after activation and of 10 minutes duration, atoms of cobalt altered by the flux of neutrons (or isotopes, in this case cobalt 60m) provide a strong image of the blue lines and checkerboard pattern. The large shape of cadmium red, which in the painting overlie the blue color, blocks some of the beta radiation being emitted by the cobalt. A very light image of the head can also be seen, which in the next exposure is much stronger. In the second exposure (Figure 7.7c) a picture of the distribution of man-

Drawing Analysis Shape

Fig. 7.7a,b. (a) Painting of a head partially obscured by a grid and checkerboard in blue and a red shape. The head is painted in raw umber containing manganese, overlain by grid lines and squares painted in cobalt blue. The red shape is painted in cadmium red. (b) First autoradiograph of the painting produced by beta radiation emitted by the isotope cobalt 60m. Exposure is for 10 minutes, beginning 15 minutes after activation. (Continued on pg. 85.)

Fig. 7.7a,b. (a) Painting of a head partially obscured by a grid and checkerboard in blue and a red shape. The head is painted in raw umber containing manganese, overlain by grid lines and squares painted in cobalt blue. The red shape is painted in cadmium red. (b) First autoradiograph of the painting produced by beta radiation emitted by the isotope cobalt 60m. Exposure is for 10 minutes, beginning 15 minutes after activation. (Continued on pg. 85.)

Fig. 7.7c,d. (c) A 2-hour exposure beginning 2 hours after activation produced this autoradiograph generated by the isotope manganese 56. (d) Third autoradiograph with dominant image of Cadmium. The exposure was begun 4 days after activation, for a period of 2 days.

Fig. 7.7c,d. (c) A 2-hour exposure beginning 2 hours after activation produced this autoradiograph generated by the isotope manganese 56. (d) Third autoradiograph with dominant image of Cadmium. The exposure was begun 4 days after activation, for a period of 2 days.

ganese (present in the pigment raw umber used to paint the head) appears in an exposure taken during a period of 2 hours beginning 2 hours after activation. The cobalt isotope has by now degraded to such an extent that its beta radiation emission cannot be detected. For cadmium (the large shieldlike shape), the exposure was taken over a two-day period beginning four days after activation. It is shown in Figure 7.7d. The variations in value in the shape indicate differences in density of cadmium isotopes and therefore differences in thickness of the paint, even though in Figure 7.7a the red color appears as a uniform coating. Just beginning to be detected is a second isotope of cobalt (cobalt 60) with a much longer half-life than cobalt 60m.

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  • Ada
    Is cobalt oil paint radioactive?
    8 years ago
  • Adalgisa
    Is oil painting radioactive?
    1 year ago

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