When gold is struck by X rays it fluoresces. A characteristic double peak is produced in the spectral region between about 9 Kev and about 12.5 Kev. Many other metals of value also produce peaks profiles that allow then to be found, assessed in purity, and identified.
In general, XRF or X-ray fluorescence, causes the emission of secondary X-rays, called fluorescent X-rays by using high energy X-rays or gamma rays to expel tightly held electrons from the inner shells of the atom. When an electron in an inner orbital is kicked out by an X-ray, what happens is that another electron from an outer orbital (a shell further out), drops down to fill the hole left behind. As the outer shell electron falls toward the nucleus, it releases energy in the form of a photon, which has the exact amount of energy equal to the energy difference of the two orbitals. So the X-ray fluorescence of gold predictably occurs in two peaks at two specific energy levels around 9 to 12.5 Kev.
The wavelength for X ray fluorescence can be calculated using Planck’s law Lambda equals hc/E.
If a sample has many elements in it, a monochromator can help you see the X-ray fluorescence coming off the gold amidst all the other secondary X-rays peaks you get back.
The returning secondary radiation will always be much weaker than the primary X-ray beam you send out to test a sample. If it has to go any distance, the returning beam will be greatly attentuated because of low penetrating power. Ideally, you want a vacuum between the sample and the sensor of the returning secondary X-rays. But not always, if you are close enough.
As you may know, Paper Bullion Bank Bills are authenticated based on rare earth materials printed on them, and holograms that contain encrypted file numbers. However these bills contain fine particles of gold right in the paper of the bill, enmeshed between the fibers of the paper. This means that even without an official authentication machine, that gives you the 100% secure authentication of each individual bank bill, you could do a quick check of the gold content in the bill with a hand-held X-ray fluorescence examining machine that is readily available in the market for commercial testing equipment, that runs on battery power, and that is not costly to buy or to run.
Such a quick and dirty check will tell you the presence and the purity of the gold involved, but, of course, it will not tell you the precise amount — like 1/10 oz.
Paper Bullion Bank Bills are imprinted with the exact amount of gold contained in them which ranges from 1/10 oz to 1/2 oz.