This article was written by Stephen Adams for The Telegraph
Previous research had discovered an outline of the peasant's head behind the Dutch painter's later work, Patch of Grass.
But this latest technique, which has never been used before, has unveiled the pigments van Gogh used in the original painting.
Over two days the scientists bombarded the painting with a powerful pencil-thin beam of X-rays, which caused the atoms in the picture to release "fluorescent" X-rays of their own which the scientists measured.
As the different chemicals that van Gogh used to paint the image release differing amounts of fluorescence, they were able to map the picture in great detail.
Elements from specific paint pigments allowed the team - led by Dr Joris Dik, from Delft University of Technology in the Netherlands, and Professor Koen Janssens from the University of Antwerp in Belgium, - to build up a "colour photo" of the picture.
They said the distribution of two chemicals, mercury and antimony, were particularly useful in reconstructing the image.
Vermillion, the red pigment important for pink flesh tones, contains the former while antimony is a component of Naples yellow, which van Gogh also used to paint the woman's face.
Writing in the journal Analytical Chemistry, they said: "We present the first-time use of synchrotron radiation based X-ray fluorescence mapping, applied to visualise a woman's head hidden under the work Patch of Grass by van Gogh.
"We recorded decimetre-scale X-ray fluorescence intensity maps, reflecting the distribution of specific elements in the paint layers. In doing so, we succeeded in visualising the hidden face with unprecedented detail."
Experts in van Gogh believe he often over-painted his work, doing so on about a third of his early works.
Art historians think van Gogh painted the peasant's image while staying in the Dutch village of Nuenen in 1884 or 1885.
