By forcing the solution through a special thin plastic filter having tiny holes, the particles of barium sulfate together with the radium that had been in the solution were caught on the surface of the filter. This was mounted on a solid disc so that it too could be placed in the alpha-particle spectrometer for the measurement of radioactivity from the radium.
Two weeks later the results were ready. Dad, the boys, and one of the experts from the museum met with the chemist to discuss them. For one of the two paintings, the polonium-210 radioactivity was about ten times that of the radium activity. The boys were disappointed because this meant that the painting could not have been 300 or 400 years old as it first appeared to be.
An alpha-particle spectrometer is used to measure the radioactivity of the radium and polonium prepared from the lead white.
A plastic disc on which is cemented a filter containing a nearly invisible deposit of barium sulfate (BaSO₄) that “carried” the radium.
But in the second painting the radioactivity from the polonium-210 and from the radium-226 were just about equal. That meant that this painting was at least 100 years old and, from its appearance, probably more. The boys were excited.
“We have a really valuable painting!” said Martin.
“Not so fast, boys,” cautioned Dad. “We don’t know who painted it and we don’t know exactly how old it is.”
The Gallery’s expert was happy too. He believed that the second picture was a genuine Dutch painting from the 17th century. It was a landscape and the artist might have been Aelbert Cuyp.