The form of photography we spoke about in the last blog is called autoradiography.  You might ask whether this is really a form of photography and whether it truly belongs in a blog about photography.  The thing is that light comes in many flavors.  Light is an electromagnetic wave, much like the waves at a beach.  The distance between one wave and the next is referred to as the wavelength.  Visible light has wavelengths between about .350 microns and .750 microns.  Remember that a micron is a thousandth of a millimeter. But the spectrum of electromagnetic radiation extends beyond visible light.  Increasing wavelength we have infrared light, microwaves, sub mm waves, and radio waves.  In the other direction we have ultraviolet light, X-rays, and gamma rays.  So really we are taking a pretty narrow view, if we confine ourselves to visible light simply because that is what the human eye can see.

So suppose we had a painting that contained a pigment that contained arsenite, ⁷⁶As.  ⁷⁶ As has a half life of just over a day.  This means that if you start off with a gram of ⁷⁶As, it will give off gamma rays so fast that a day later you will only have ½ a gram, two days later ¼ gram etc. On the other hand if you have a gram of copper ⁶⁴Cu, it decays twice as fast; so a 1/4 gram after one day and 1/8 gram after two days.  So suppose you have a green pigment in a painting and want to determine if it contains arsenite, an arsenic ore, or malachite, a  copper ore, you simple place a sheet of photographic film under the painting for an an hour or so develop it and repeat the process a day and then two days later.  After 24 hours the density of the film below a green region decreases in half if it is made of arsenic or to 1/4 if it is made of copper.  Then to a 1/4 for arsenic or a 1/16 for copper after 48 hours.

It’s as simple as that, except for one thing.  There is very little ⁷⁶As in arsenite.  It is almost completely ⁷⁵As.  There’s a similar story for copper, which likes to be either ⁶³Cu or ⁶⁵Cu.  Atoms consists largely of two types of particles: protons and neutrons.  All arsenic atoms have 33 protons.  The so called isotopes differ by the number of neutrons.  In ⁷⁵As there are 22 neutrons.  In ⁷⁶As there are 23.  Now here’s the problem, natural samples of arsenite have precious little ⁷⁶As, certainly not enough to expose a photographic film in an hour.

To turn ⁷⁵As into ⁷⁶As you’ve got to shoot neutrons into your painting.  To accomplish this you have to take your painting to your local nuclear reactor and transmutate the metals in your pigments..  Most art museums don’t have nuclear reactors in their basements.  Fortunately, many physics departments do.  These are the basics of neutron activated autoradiography.  Remember this next time you want to catch a Spanish forger.

* For those who want to dig into this a bit more deeply check out Neutron Activation of Paintings.