Neutron Activation Analysis and Autoradiography

Almost all of neutron activation analysis depends on the relatively high probability that a slow-moving neutron can be captured by an atomic nucleus. This reaction forms a new isotope with a mass number one unit larger than the target atom:

In this reaction the atomic number Z of the nucleus is unchanged.

When the atoms of an element are bombarded by neutrons in a reactor, some of them will absorb a neutron and become radioactive. For example, in the case of sodium,

A sodium nucleus with 12 protons and only 11 neutrons absorbs a neutron to become a sodium nucleus with 12 neutrons. This sodium-24 is radioactive and eventually decays:

Each sodium-24 atom that decays to a magnesium atom (with 13 protons and 11 neutrons) emits a beta particle (fi-, an energetic electron) and a gamma ray. Energy conservation is provided by the gamma rays. The number and energy of the gamma rays are unique for the decay of sodium-24 and thus constitute a kind of signature. Furthermore, the halflife of the sodium-24 (15 hours) is also a means of identification.

Thus, when a material containing some sodium is irradiated by neutrons in a nuclear reactor, one may (a) show that sodium is present by later observing the energies of gamma rays from sodium-24, and (b) determine how much is present by measuring how many such gamma rays are emitted.

Autoradiography is performed by placing a photographic film in direct contact with a painting after neutron irradiation. In contrast to neutron activation analysis, where gamma rays are detected, in autora-diography, it is the beta-rays (the energetic electrons) that are detected in the film. The gamma rays have such great penetration depths that they hardly interact with the film, whereas the electrons, with their shorter penetration, readily expose the film. In a sense this is a form of beta-radiography as used in watermark analysis, but since the radioactive elements in the painting itself are the source of beta rays, the technique is called "autoradiography."

TABLE G.2

Elements easily detected by neutron activation autoradiographic

TABLE G.2

Elements easily detected by neutron activation autoradiographic

procedures

Name

z

Symbol

Beta Half-Life

Sodium

11

Na

15 hours

Aluminum

13

Al

2.3 min

Manganese

25

Mn

2.6 hours

Cobalt

27

Co

5.3 years

Copper

29

Cu*

5.1 min & 12.8 hours

Arsenic

33

As

26.5 hours

Antimony

51

Sb

3 days

Gold

79

Au

3 days

Mercury

80

Hg

For neutron autoradiography we require that there be a high probability for the formation of a radioactive species that decays by emission of beta particles with a half-life that is not too short or too long. It is clear that quantitative analysis of the amount of an element present depends on when the measurement is made after the irradiation.

There are limitations to the method. The most obvious of these are the cases where neutron capture produces a stable isotope, since in such cases no activation occurs. This applies to most of the important light elements such as H, C, N, O, Mg, Si, S, Ti, and Fe. Similarly, if the most abundant isotope yielded a very long-lived radioactive product, the activity produced during an irradiation of some hours or days might still be insufficient for sensitive analysis. Such, for example, is the case for Ca and Ni. Also, when the contrary is true and the product has a very short half-life, decay of the activity may be inconveniently fast and cause problems of measurement as with Li, B, and F. All of these elements except Li, O, and F are readily identified with prompt gamma analysis.

In terms of elements found in pigments, for autoradiography where 3 emission is required for photographic films, the elements most easily

Neutron Activatione Analysis

Fig. G.4. Relative rates of energetic electron (beta ray) emission during the radioactive decay of neutron-activated pigments with the painting Saint Rosalie Interceding for the Plague-Stricken of Palermo (from Ainsworth et al., Art and Autoradiography (The Metropolitan Museum of Art, New York, 1982)).

Fig. G.4. Relative rates of energetic electron (beta ray) emission during the radioactive decay of neutron-activated pigments with the painting Saint Rosalie Interceding for the Plague-Stricken of Palermo (from Ainsworth et al., Art and Autoradiography (The Metropolitan Museum of Art, New York, 1982)).

detected are, in alphabetical order, aluminum, antimony, arsenic, cobalt, copper, gold, manganese, mercury, and sodium. These elements are listed by atomic number Z in Table G.2, which gives ^-emission half-life.

Elements that do not give good, distinct images are lead and iron along with carbon and calcium, as indicated above. Thus pigments that do not cause distinct images are chalk (CaCo3), lead red and white, lead-tin yellow, and the iron in ochre. As pointed out by Sayre and Lecht-man (Reference E, number 6), "Of these the one which might seem most regrettably absent from the autoradiographic palette is lead white, one of the most important pigments in the history of painting."

An indication of the influence of the half-life on electron emission rates is shown in Figure G.4. In autoradiograph film, darkening is caused by electron emission, i.e., beta particles emitted in radioactive decay. All beta particles from elements that were studied were roughly equally effective in producing film darkening. As a result, it appears that for each element its contribution to film darkening at any given time is roughly

Autoradiography

Fig. G.5. The technique of autoradiography. A painting is exposed to a beam of neutrons and is then placed in contact with film at a given time after the neutron activation of the pigments. During the radioactive decay process, beta rays from a radioactive atom in a pigment pass out from the painting and penetrate into the film. Each beta ray exposes the film emulsion along its track. After development of the film, the amount of darkening in a given area indicates the number of beta rays striking the film and hence the amount of radioactive pigment in the painting in the area in contact with the painting.

Fig. G.5. The technique of autoradiography. A painting is exposed to a beam of neutrons and is then placed in contact with film at a given time after the neutron activation of the pigments. During the radioactive decay process, beta rays from a radioactive atom in a pigment pass out from the painting and penetrate into the film. Each beta ray exposes the film emulsion along its track. After development of the film, the amount of darkening in a given area indicates the number of beta rays striking the film and hence the amount of radioactive pigment in the painting in the area in contact with the painting.

proportional to the electron emission rate for that element at that time. For every painting a graph such as shown in Figure G.4 is constructed that shows for each element the calculated electron emission rates as a function of the time elapsed after activation. These graphs are valuable in determining which elements contribute to the film darkening in any specific autoradiograph. They facilitate the identification of the pigment or painting material whose image appeared in that autoradiograph.

Autoradiography is a nondestructive technique used here for the examination of underlying paint layers. The painting is placed in a beam of thermal neutrons for a short period of time, and then a series of photographic films is placed in contact with the surface of the painting (Figure G.5). Beta particles (electrons) emitted in the decay of the radioactive elements present in the painting sensitize the film.

Upon development and fixing, the film will show the distribution (location and density) of those pigments and other painting materials that contain radioactive elements at the time of film exposure. The localized nature of the image is due to the short range of the beta rays emitted in radioactive decay. As shown in Figure G.6, the beta rays penetrate the equivalent of only a few paint layers (about 500 microns, or 0.5 mm) and hence blur only the periphery of the image. The deeper-penetrating gamma rays have only a small interaction with the film emulsion. A series of such full-scale photographic images, called autoradiographs, consists of several consecutive exposures: The first ones are short exposures obtained only minutes after neutron activation; later exposures are started hours, days, and even weeks after activation. Film exposure times vary accordingly from several minutes for the first autoradiograph to several weeks for the last one. Because of the differences in the decay times of the radioactive elements, different images are generally observed among the series of autoradiographs.

Fig. G.6. A schematic illustration of energetic electrons (beta rays) emitted in radioactive decay of neutron activated pigment particles. The range of the beta rays (shaded area) is about 0.5 mm, depending on electron energy, so that the exposed image in the emulsion corresponds to the location of the radioactive pigment particles.

Fig. G.6. A schematic illustration of energetic electrons (beta rays) emitted in radioactive decay of neutron activated pigment particles. The range of the beta rays (shaded area) is about 0.5 mm, depending on electron energy, so that the exposed image in the emulsion corresponds to the location of the radioactive pigment particles.

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    How to draw autoradiography process?
    2 years ago

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