Active neutron dose meters for the application at high energy particle accelerators

The operation of a high energy particle accelerator facility is accompanied by neutron fields which are widely distributed in energy. As the neutron component dominates the dose outside the shielding neutron dose meters are needed for monitoring the radiation level at beam lines and experimental areas which can be accessed by personnel. For this purpose the so-called neutron rem-counter of the Andersson-Braun [1] type is used at many laboratories. The first design by Andersson and Braun was described to reproduce the dose equivalent for neutrons starting from thermal energy (1/40 eV) up to 10 MeV [1]. This energy range covers neutrons emitted by nuclear power installations. A standard AB-rem-counter consists of a proportional counter tube filled with a gas that has a high reaction cross section for neutrons such as boron-fluoride (BF3) or helium-3. The counting tube is surrounded by a moderator made of a hydrogen-rich material such as polyethylene eventually mixed with neutron capturing elements like boron or cadmium. The reaction cross section of B-10 or He-3 for neutron absorption is roughly inversely proportional to the neutron velocity. Consequently the reaction probability is the highest for thermal neutrons. On the contrary the dose equivalent resulting from neutron radiation is rising with energy reaching a transient maximum at about 20 MeV. The moderating and absorbing element in a rem-counter is required to transform the energy relation of the neutron detecting reaction into a detector response which resembles the conversion function for the ambient dose equivalent. In the vicinity of high energy particle