Detectors that don’t fear neutrons
High-intensity pulsed neutron fields are produced at particle accelerators such as CERN’s PS and LHC. The efficient detection of this stray pulsed radiation is technically difficult and standard detectors show strong limitations when measuring such fields. A new test performed at the HiRadMat facility has recently shed light on the performance of various neutron detectors exposed to extreme conditions.
In order to limit the required human intervention to the beginning and the end of the test, detectors were mounted on a dedicated wheel that CERN’s HiRadMat team built for the HRMT-15 experiment.
High-intensity pulsed neutron fields are among the toughest conditions a detector can be asked to face. Particle accelerators produce such stray radiation when primary beams are dumped or lost because of, for example, an orbit instability that can occur during ordinary operation. Accurately measuring the radiation levels is the first requirement in order for experts to be able to effectively protect personnel and the environment. “Due to the constraints of the electronics, building detectors that can stand high-intensity pulsed neutron fields is a technical challenge,” says Marco Silari, member of CERN’s Radiation Protection group and project leader of the HRMT-15 RPINST experiment. “Most detectors suffer from particle pile-up and the read-out becomes meaningless. In some cases, algorithms are used to compensate for losses due to high-intensity pulses but they usually do not give reliable results.”
The HRMT-15 experiment recently measured the performance of five neutron detectors exposed to the neutron flux produced when high-energy protons from the SPS are stopped in the HiRadMat dump installation. “HiRadMat is a unique facility that has allowed us to expose the detectors to extremely intense neutron fluxes that hit them in very short pulses,” explains Marco Silari. “The intensity of the primary proton beam from the SPS has been varied over four orders of magnitude up to about 1013 protons per bunch. These are extremely tough conditions for the detectors. Among the five we tested, only three of them passed the test while the others showed significant signs of failure.”
Among the detectors that passed the test with flying colours are two of the RAMSES detectors, which are installed all around the LHC tunnel, in service caverns of the LHC experiments and all along the LHC injector chain, and constantly monitor the radiation levels in the machine. “We also tested LUPIN, a new detector prototype developed by the Energy Department of Politecnico di Milano (Italy) in collaboration with CNAO, the Italian Centre for Hadrontherapy,” adds Giacomo Manessi, PhD student in the Radiation Protection group. “LUPIN’s performance was excellent, thus also making it a suitable detector for medical accelerators.”
The HRMT-15 Collaboration, whose work was partially supported by EuCARD HiRadMat transnational access, is about to publish the detailed results of the test in Nuclear Instruments and Methods in Physics Research Section A.
