There are two ways to get X-rays, :
1) acceleration and deceleration of charges, electrons/ions giving synchrotron radiation in one case and brehmstralung radiation in the second.
2) de-excitation of electrons falling from a higher energy level/band to a lower energy level/band with an X-ray magnitude energy difference
In 1953, Soviet scientists showed that triboluminescence caused by peeling a roll of an unidentified Scotch brand tape in a vacuum can produce X-rays.6 In 2008, American scientists performed an experiment that showed the rays can be strong enough to leave an X-ray image of a finger on photographic paper
and a recent research ;
Putterman’s group recorded X-ray emission in the form of intense bursts some billionth of a second long (with the width of the X-ray pulses calibrated using the well characterized radiowaves), and found that these bursts are correlated with very slight slippages (i.e. reductions in force) in the otherwise smooth removal of the tape from its reel.
The wiki entry attributes the phenomenon of light to triboluminescence
riboluminescence is an optical phenomenon in which light is generated through the breaking of chemical bonds in a material when it is pulled apart, ripped, scratched, crushed, or rubbed (see tribology). The phenomenon is not fully understood, but appears to be caused by the separation and reunification of electrical charges.
The more recent article speculates:
The group believes that as the tape peels the acrylic adhesive on the exposed tape becomes positively charged and the outer surface of the remaining polyethylene roll acquires a negative charge. This causes electric fields to build up to values that trigger discharges.
Breaking a bond gives kinetic energy to the constituents, and electrons and ions will be separated and left in excited states. The reorganization of charges happens with the emission of photons. For X-rays it means those bonds are strong enough to be left in breaking at energy levels high enough to release X-rays in the de-excitations.
The newer link explains it as :
The researchers say that at the reduced pressure in the experiment — about one millionth of an atmosphere — the discharges accelerate the electrons to energies that generate X-rays when they suddenly decelerate in the positive side of the tape. However, the researchers remain stumped as to how the diffuse mechanical energy needed to peel the tape is focused to the extent that it can produce X-rays, and even more strangely how it can do so in the form of nanosecond pulses. This, says Putterman, gives us a “new inspiring mystery to dig into”.
Searching on triboluminescence one finds that the exact mechanism is a research subject, explanations centering on excitations and de-excitations of the atoms in the material. See a spectrum of triboluminescence here, though not of x-ray energies.