MoEDAL expands

The MoEDAL collaboration deployed a test array of 18 plastic Nuclear Track Etch Detector (NTD) stacks – covering an area of 1 m2 – in the MoEDAL/VELO cavern at Point 8 of the LHC ring in November 2009. This small array was supplemented by a further 110 stacks this past January. The MoEDAL test array, which now covers an area of 8 m2, will reveal its secrets early in 2013. The full MoEDAL detector will be installed in the next long shutdown of the LHC in 2013.


View of the MoEDAL detectors installed at Point 8 of the LHC ring in January 2011.

MoEDAL (Monopole and Exotics Detector At the LHC), the seventh LHC experiment, was approved by the CERN Research Board at the end of 2009. Its goal is to search for very specific exotics such as highly ionising massive stable (or pseudo-stable) particles with conventional electrical charge and magnetic monopoles. “The main LHC experiments are designed to detect conventionally charged particles, with conventional ionisation patterns, produced with a velocity near the speed of light. However, the MoEDAL detector is designed to search for highly ionising particles that are either highly charged and/or moving very slowly (less than half the speed of light),” says James Pinfold, spokesperson of the experiment. Thus, MoEDAL has a detection efficiency and physics range that is complementary to the conventional general-purpose LHC detectors.

In its final configuration, scheduled for 2013, MoEDAL will consist of 400 NTD stacks [see box] distributed over an area of 25 m2 on the walls of the cavern that houses the LHCb's VELO detector at Point 8 of the LHC ring. "In November 2009, we installed a test array of 18 NTD stacks covering a total area of one square metre," explains James. During the recent winter shutdown, we added a further test array of 110 NTD stacks. MoEDAL NTD stacks now occupy about 8 m2 of the MoEDAL/VELO cavern wall, which is a third of the total scheduled area."

NASA Space Radiation Laboratory where the two NTDs will be sent for calibration tests.

The MoEDAL team also removed two of the original NTDs during the shutdown. They are on their way to the NASA Space Radiation Laboratory at the Brookhaven National Laboratory in the United States, where they will undergo tests to determine the calibration needed to analyse the detectors currently in place. "The Laboratory produces heavy ion beams at sufficiently high energies to calibrate our NTDs," explains James. "These tests will allow us to see how our detectors respond to particles with ionisation similar to the ones we are hoping to detect."

The 128 NTDs currently installed will be removed in 2013. When they are analysed in the laboratory they will be examined for the tell-tale signs of direct production of magnetic monopoles and/or stable massive particles. Like photographic film, the MoEDAL detectors will not reveal their content until they are "developed". Let's hope that they have pleasant surprises in store for us...

Further information about MoEDAL can be found here

Nuclear Track Detectors (NTDs)

The NTD stacks of the MoEDAL experiment comprise ten layers of plastic and are particularly sensitive to highly ionising particles such as magnetic monopoles but are indifferent to the high-energy particles produced in the proton-proton collisions at the LHC. Their response to highly ionizing particles is well understood and their analysis provides accurate information on the detected particles’ charge, energy loss and trajectory.

NTDs are completely passive: they do not require a trigger, gas, high voltage or readout electronics. They are radiation-resistant, easily calibrated with heavy-ion beams, and their response to highly ionising particles is very well understood. Last but not least, they provide a full and cost-effective coverage of the experimental acceptance.


by Anaïs Schaeffer