By Tom Mead
Secret recipe for straightening a large, thick, brass
1 12-ton concrete block
Assorted supports and packing
1 very good crane crew
1 extremely good engineer
It is rare amidst the extraordinary precision of a high
energy physics laboratory to find an instance where a precise,
close-tolerance problem is solved by repeated bashing with a massive
12-ton concrete radiation shielding block. This is a story of that rare
detector was built to study the millions of B mesons produced by the
PEP-II storage ring. One of the many layers within the 1200-ton detector
is the Instrumented Flux Return (IFR).
The brass-bashing all stars, from left: Scott
Jansson (RD), Jason Krebs (EFD), James Krebs (RD), George Bradford (EFD),
Robert Moore (EFD) (photo by Gerard Putallaz)
The forward IFR consists of 18 alternating layers of
Resistive Plate Chambers (RPCs) and steel plates. The steel layers absorb
pions, ensuring that only muons pass through to detection.
However, some pions were getting through the IFR and
contaminating the data. More effective pion-absorbing material was needed.
It was determined that brass plates installed as banks of
10 parallel plates in three vertical ranks would do the trick. Thirty
2000-pound brass plates, measuring 9’ x 6’ x 1" each, were rolled to a
flatness tolerance of .060 of an inch.
James Krebs (RD), BABAR
Chief Mechanical Engineer, and his swing shift team — George Bradford,
Verne Coughran, Scott Jansson, Jason Krebs and Robert Moore — removed five
layers of RPCs from the BABAR
detector to open up slots for the banks of brass plates. Most of the
one-inch-wide brass plates installed relatively easily.
Most, however, was not all. One plate was slightly too
thick and bowed 3/8 inches in two dimensions, making it concave. After
hours of failed attempts to install the plate, the team began searching
for another answer.
The team decided to investigate correcting the plate
in-house at IR-2. The force needed to bend the brass plate was 24,000 lb.
It was not obvious how to apply such a force to the plate without much
investment in additional tooling. An unusual answer was needed. It was
spotted in the hulking form of a 12-ton block of concrete that was being
used as a massive table inside the detector hall.
The team placed eight-inch box beams 48 inches apart,
built shim stacks between them, laid the plate down across the beams,
convex side up, and used the 50-ton overhead crane to lower the concrete
block down on the plate. This cycle was repeated 12 times, removing a
lower shim each time to allow the plate to bow further down, lifting the
block and letting the plate spring back. "It was so crude it was great,"
said Krebs. "It was the most fun I had during this shutdown." At the end
of the 12th cycle the plate was declared flat.
Still, inserting the plate into the slot did require some
encouragement. So the team rounded the plates’ edges, liberally slathered
the plates with pink, liquid soap from the nearby bathroom dispensers, and
eased it into the slot.
The entire ad hoc flattening and installation process took
four hours and resulted in an immediate, on-site, low-cost solution, a
plate of the specified flatness, and adherence to a vital laboratory
"I really admired the way Krebs went about it," said
physicist John Fry (BABAR).
"The morale of that crane team was tremendous. Quite apart from all the
skill and hard work, it was also just tremendous fun."
Krebs echoed that thought, saying, "These guys are the
best crew I have ever had. It is a privilege to work with them. I don’t
think I would have attempted this solution without them."
Thanks to teamwork and ingenuity, the brass plates are now
in their assigned slots and the pion infiltration problem has been solved.