Cryogenic Beam Loss Monitoring for the LHC

A Beam Loss Monitoring (BLM) system was installed on the outside surface of the LHC magnet cryostats to protect the accelerator equipment from beam losses. The protection is achieved by extracting the beam from the ring in case thresholds imposed on measured radiation levels are exceeded. Close to the interaction regions of the LHC, the present BLM system is sensitive to particle showers generated in the interaction region of the two beams. In the future, with beams of higher energy and brightness resulting in higher luminosity, distinguishing between these interaction products and possible quench-provoking beam losses from the primary proton beams will be challenging. The particle showers measured by the present BLM configuration are partly shielded by the cryostat and the iron yoke of the magnets. The system can hence be optimised by locating beam loss monitors as close as possible to the protected element, i. e. the superconducting coils, inside the cold mass of the magnets in superfluid helium at 1.9 K. The advantage is that the dose measured by the Cryogenic Beam Loss Monitor (CryoBLM) would more precisely correspond to the dose deposited in the superconducting coil. The main challenges of this placement are the low temperature of 1.9 K and the integrated dose of 2 MGy in 20 years. Furthermore the CryoBLM should work in a magnetic field of 2 T and at a pressure of 1.1 bar, withstanding a fast pressure rise up to 20 bar in case of a magnet quench. The detector response should be linear between 0.1 and 10 mGy/s and faster than 1 ms. Once the detectors are installed in the LHC magnets, no access will be possible. Hence the detectors need to be available, reliable and stable for 20 years. Following intense research it became clear that no existing technology was proven to work in such conditions. The candidates under investigation in this work are diamond and silicon detectors and an ionisation chamber, using the liquid helium itself as particle detection medium. All the selected detector technologies are based on ionisation and subsequent charge carrier transport within the detector bulk. Therefore laboratory measurements were performed to measure the charge carrier characteristics in the detector material in the temperature range from 1.6 to 300 K. In the silicon detector, charges were generated using laser light and - particles. For diamond detectors the measurements were done with -particles only. The temperature dependence of the drift velocity and of the mobility of the charge carriers was measured. To measure the detector’s characteristics with respect to particle detection at liquid helium temperatures, low intensity beam tests with minimum ionising protons were carried out. They allowed to prove that all tested detectors work at 1.9 K. The silicon detector Full Width Half Maximum (FWHM) of the signal from a MIP is 2.5 ± 0.7 ns at liquid helium temperatures. For the diamond detector the FWHM is 3.6 ± 0.8 ns. The signal width decrease from room temperature to liquid helium temperatures is of 54 % for silicon material and 28 % for diamond material. This allows bunch by bunch resolution of the LHC losses, as already demonstrated at room temperature. The radiation hardness of the solid-state detectors over 20 years of LHC operation was addressed during high intensity beam tests carried out at CERN in a liquid helium environment. A complete cryogenic system was installed in the irradiation area of the CERN East Hall. Data from the continuous monitoring of the signal development during irradiation and measurements from test cycles enabled the advantages and disadvantages of each detector technology to be identified. The expected reduction in detector sensitivity over 20 years (2 MGy) of LHC operation is of a factor of 14 ± 3 for the diamond detector. For the silicon detector the expected signal reduction is of a factor of 25 ± 5. Using liquid helium as particle detection medium has the advantage of no radiation hard- ness issues. The downside is the low electron and ion mobility in superfluid helium, which leads to a slower detector response. With the current design of the liquid helium chamber a successful protection from losses with a time constant above 180 s is ensured. These results show that the diamond and silicon detectors satisfy the criteria for use in a fast protection and feedback system, while the simultaneous use of the liquid helium chamber enables the calibration of the solid-state detectors and the reliable protection from steady state losses.