Making the super-LHC super safe
The LHC upgrade programme will deliver ten times as many collisions per hour as are possible at the moment, and that level of activity brings its own problems, says Thomas Otto at CERN.
“The events will splatter ten times as many ionising particles into the detectors,” he says. That’s good news for the physicists monitoring the collisions and those subatomic particles – they will have ten times as much data to analyse – but it’s bad news for the teams in charge of accelerator and detector maintenance.
“The extra collisions mean that the inner detectors will receive a ten times higher dose of ionising radiation in their lifetime,” Otto explains. As the subatomic particles produced in the collisions race through the detector they knock protons and neutrons, as well as electrons, off the atoms in their path. Those tiny changes in atomic structure can have a huge influence on the properties of the materials used to construct the detector.
At one level, that’s problematic because the radiation damage impairs the performance of the precision equipment. “Radiation damage is known to add noise to the data,” Otto says. “At some point there might be too much noise to do useful experiments and repairs will have to be made.”
But then a second, more serious issue emerges. The exposure to scores of high-energy particles can turn the harmless materials used to build the detector into a radioactive, more harmful form. Copper, for instance, is a common material in the detector. But if a copper atom loses just two of its 29 protons it turns into cobalt-60. “That’s an obnoxious radioactive isotope,” says Otto. “It has a long half life of five and a half years and a high gamma energy.”
In other words, cobalt-60 emits potentially dangerous gamma rays, and does so for a number of years. This is all occurring deep underground so there is no danger to the public, but the maintenance workers who will periodically descend to repair the accelerator and detectors must be made aware of the risks.
“Can we afford the higher collision rates when we see the potential effect on equipment and personnel? That’s what we want to find out,” says Otto. “The dose rates might allow for some work on some critical areas with a strict time limit to minimise exposure, but in other areas the dose rate might be so high that work will only be possible using manipulators commanded from a distance, which will obviously be quite costly.”
The critical areas obviously include the detectors dotted around the accelerator ring: it’s here that the LHC's two beams of protons are brought together to produce the collisions that give the physicists their data – and form the ionising radiation.
But high radiation levels might also be expected at the point that the protons are injected into the circular collider. “The particles, which begin with a straight path as they are first produced, are forced into a circular orbit at the injectors,” Otto says.
“Some particles will naturally be lost from the beam and crash into the surrounding material.” Those wayward protons lead to the creation of radioactive atoms in the material in the same way that radioactive atoms form in the detectors.
The safety team will identify all the critical areas of the LHC that are likely to produce high levels of radiation, and deliver those results in a report due in spring 2009. The next step is to work out how much radiation can be expected at each critical point. Working out those dose rates is more than simply a matter of running a few calculations, says Otto. “You can’t accept a computer prediction without validating it” when worker safety is at stake, he says. Instead, the safety team will need to check their calculations against real data.
To do that, the team will calculate the expected dose rates in the first-generation LHC when it is switched on again later in 2009. They will then be able to assess the accuracy of those calculations against real figures collected during the first months of operation. If the figures add up, the team can be confident that their estimates for the LHC upgrade project are also accurate.
“Maybe those results will show that we have overestimated the problem,” Otto says. “Or maybe we haven’t, and we will have to devise new methods to deal with higher activation and dose rates.”
The inner detector at CMS during construction. The detector will experience severe radiation levels after the upgrade work and will need to be redesigned to cope.