This article dates back to August 2008, but I didn't pick up on it then. Thanks to Kat for sending me the link.
MUON DETECTORS HAVE a rich history of revealing the unusual and the unseen. Sixty years ago, scientists in Australia used them to measure layers of mountain snow. Today, Japanese scientists are testing them as a way to track magma rising in volcanoes. Border patrol agencies in the U.S. see the technology as a potential way to uncover radioactive materials hidden inside cargo containers and trucks.
It was about 40 years ago that archaeologists tapped muons for the first time. Luis Alvarez, a Nobel Prize-winning physicist at the University of California, Berkeley, wondered if muons might reveal chambers in the Second Pyramid of Chephren, one of the three great pyramids of Egypt that had somehow escaped the notice of archaeologists and looters for 4,500 years.
Alvarez and his team put a detector in the Belzoni Chamber, near the centre of the pyramid's base, and left it to collect muons for two years. They concluded that no additional chambers were hidden in the limestone above, although the scan could distinguish the four edges of the pyramid and what remained of its smooth limestone facing. While it would have been more exciting to discover a new chamber, this information still proved valuable for Egyptologists and archaeologists studying the pyramid – and it showed that the technique worked.
See the above page for the full story, which is not exclusively about Egyptian monuments. the above extract comes from the second page of the three page article.