Posted on: 6th May 2013

Measuring the number of neutrons produced is one of the most basic yardsticks of success of a fusion experiment, yet it’s surprising tricky to ensure your neutron detectors are calibrated correctly, says the leader of the team that has just finished calibrating JET’s neutron diagnostic systems.

“The physics was only a part of the activity,” says Project Leader, Dr Brian Syme. “Obtaining the neutron source, the safety issues and all the engineering developments associated with source handling were three quarters of the project!”

The process sounds simple enough – simply hold a radioactive source that produces a known number of neutrons at a set of known locations in the torus. From the detector counts you have an answer. However to get to that moment took the team three years of careful planning, including intensive neutronics calculations by colleagues in the Slovenian and Swedish associations.

Because the source – Californium-252 – would give a yearly dose to any personnel at 1 metre in about an hour, everything was done using the remote handling system.

“The remote handling system carried the source in a special baton, which was designed and manufactured here,” says Dr Syme. After manufacture the hollow baton was taken to the suppliers of the source, the National Physical Laboratory, who inserted the centimetre-scale Californium source into it, using their own remote handling system. It was then returned to JET inside an industry-standard one tonne polythene ‘transport flask’. This flask was then opened by the remote handling mascot, the baton removed and taken into the torus, to obtain the aforementioned answers.

In addition to the components needed for holding and transferring the source, contingencies needed to be covered, says Dr Syme: “In case something went wrong with the source handling system, a portable safety shield was built, to be carried by the other boom arm. The source would be picked up by that portable shield and transferred out of the vessel to a safe shielded location, so normal repairs could proceed.  But we didn’t need to use it.”

“Yes, everything went really smoothly, experimentally,” says joint project leader Dr Sergey Popovichev. “It was busy running two shifts a day for two weeks but we missed only one of the 375 planned measurements, due to a timing consideration.”

The measurements were made with the source in a range of positions evenly distributed throughout the torus, to simulate neutrons coming from all parts of the plasma. Two systems were calibrated, a time-resolved fission chamber detector system, and an activation system which measures the total number of neutrons produced in a plasma pulse. As a cross-calibration, an independent counting system run by EFDA’s Polish Associate IPPLM was also employed.

“We did it at the end of the shutdown to so that JET was as close to real running conditions as possible, with all the tiles in place. We also made sure there was water and Galden (coolant) in the machine, which normally would not be there during maintenance.”

Now the detailed task of making sense of the measurements begins, and could take several months. “I won’t make any brave statements about the absolute calibrations at this stage!” smiles Dr Syme. And as to whether there might be any adjustments to previous results of the last few years, the scientists would not be drawn. “The system is so different now, we can really only say that we have made a calibration for the ITER-Like Wall measurements,” says Dr Popovichev.

In addition to the actual data the team has acquired a lot of experience, says Dr Syme. “We have learnt a lot that will be vital for ITER going forwards. Operating in modern times with a sealed JET and a strong emphasis on health and safety, you have to be much more disciplined. It’s not like when JET was first built and you could just about walk straight into the torus!