Neutron Monitor response functions

Alexander Mishev, University of Oulu, Finland

Neutron monitors (NMs) provide continuous ground-based recording of the hadronic component of secondary cosmic ray radiation. The introduction of an NM as a continuous recorder of cosmic ray (CR) intensity followed the design by Simpson in 1953, introduced during the International Geophysical Year (IGY) 1957-1958. The IGY neutron monitor was used worldwide as a detector to study CR variations. Lately, in the mid-sixties, the design of the IGY NM was optimized resulting in increased counting rates. This second generation of NM design is known as NM64 or supermonitor, in practice the standard device nowadays. At recent, mini-NM has been installed at several stations, showing good performance, specifically at low cut-off rigidity and high-altitude locations. Standard NM consists of sensitive to thermal neutrons proportional counters based on 3He or boron-trifluoride enriched to 10B, surrounded by a moderator, usually paraffin wax or polyethylene, a reflector made of the same material as the moderator and a lead producer. In order to use the NM as a ground-based detector to study CRs, it is necessary to establish a connection between the primary cosmic ray particles at the top of the Earth’s atmosphere and the count rate of the device. Since the secondary CRs, resulting from the primary cascade in the Earth’s atmosphere, can reach the ground level and eventually be registered by an NM, the corresponding NM response incorporates the full complexity of the atmospheric cascade development including secondary particle propagation in the atmosphere and the efficiency of the detector itself to register the secondaries. Here, different methods of determining the neutron monitor response function to primary CRs are reviewed and discussed including early and recent results as well as several applications.