RP delves underground

The LHC’s winter technical stop is rapidly approaching. As in past years, technical staff in their thousands will be flocking to the underground areas of the LHC and the Linac2, Booster, PS and SPS injectors. To make sure they are protected from ionising radiation, members of the Radiation Protection Group will perform an assessment of the levels of radioactivity in the tunnels as soon as the beams have stopped.


Members of the Radiation Protection Group with their precision instruments that measure radioactivity.

At 7-00 a.m. on 8 December the LHC and all of the upstream accelerators will begin their technical stop. At 7-30 a.m., members of the Radiation Protection Group will enter the tunnel to perform a radiation mapping, necessary so that the numerous teams can do their work in complete safety. “Before we proceed underground, we always check first to make sure that the readings from the induced radioactivity monitors installed in the tunnels are all normal,” underlines Christophe Tromel, the engineer in charge of coordinating radiation protection operations for the SPS and LHC accelerators. “Normally, we are able to enter the tunnel 30 minutes after the beams stop circulating, the amount of time necessary to completely exchange the air in the tunnels. We are always the first ones on the scene.”

Walking, cycling or riding electric trucks, the members of the Radiation Protection Group make their rounds of the 45-odd kilometres of the LHC, SPS, and injection and transfer tunnels. With their meters they continually measure the ambient radiation level and record the readings for all machine components. It takes eight people working one-and-a-half days to survey the 27 kilometres of the LHC tunnel.

“We use two types of detector to measure the radiation: a scintillation detector and a Geiger counter. If the reading from the first detector is above 0.5 μSv per hour*, we change to the second, which is better suited to measuring higher levels of radiation,” explains Nadine Conan, the engineer in charge of coordinating radiation protection operations for the LHC and HiRadMat experiments. “And when we approach one of the more radioactive points (of the order of several millisieverts per hour), for example an injection, extraction or collimation region, we fit the Geiger counter with a special “Teletector” probe that allows us to remain at a safe distance from the radiation source.”

The upstream accelerators (Linac2, Booster and the PS) will be the first to be mapped, on 5 and 8 December. Most of the underground portion of the LHC will be open to the Radiation Protection Group members half an hour after the beams stop circulating, but a few areas will not be opened until several hours later. The SPS and its transfer tunnels will be mapped later, between 12 and 14 December.

Once all of the measurements have been taken, the Radiation Protection Group will draw up a “radiation map” for the underground facilities and put up signs marking the most highly radioactive points, so that a visual overview of the radiation levels in the area can be rapidly obtained. “For the LHC, the radiation maps change from one year to the next,”  says Nadine Conan. “They are affected by various factors, namely the increase in intensity, the operation mode, and technical modifications carried out on the accelerator.

In 2010, the number of trips to the underground facilities during the winter technical stop numbered in the tens of thousands. This year, the tunnels of the world’s biggest accelerator will once again witness a very large number of interventions.

* The level of radioactivity (from natural origins) measured around CERN is some 0.1 μSv per hour.

More information about radiation protection at CERN can be obtained here.

The radiation classification of the LHC tunnels can be consulted here.

by Anaïs Schaeffer