Neutron Monitor Electronics for Spectral Information from a Single Neutron Monitor
D. Ruffolo, Mahidol University, Bangkok, Thailand
Neutron monitors have provided continuous tracking since about 1950 of the neutron count rate at various sites, with a precision as good as 0.1% for hourly rates, to monitor variations in the cosmic ray flux. Because different sites have different rigidity cutoffs (thresholds), ranging from about 1 GV to 17 GV, it is commonly thought that these sites also provide continuous monitoring of the cosmic ray spectrum over that rigidity range. However, in practice there are systematic uncertainties and local atmospheric effects that limit the accuracy of tracking spectral variations in that way, especially for time scales shorter than a year. Therefore, several techniques have been developed that can successfully measure spectral variations of galactic cosmic rays or solar energetic particles at a single site (or neighboring sites) or from a single neutron monitor. After a brief overview of such techniques, we will specifically describe techniques using special electronics to provide timing and multiplicity information. We describe technical problems with a common definition of multiplicity in terms of the number of events detected within a time window, because these are strongly affected by chance coincidences, which are sensitive to atmospheric pressure. A more successful technique is to statistically remove chance coincidences from the distribution of time delays between successive counts to determine the “leader fraction” of counts that did not follow a temporally associated count from the same cosmic ray shower. This technique has been validated using data from latitude surveys and comparison with data from the spaceborne AMS-02 detector. At high cutoff rigidity, e.g., above 17 GV, the leader fraction provides superior resolution of spectral variations in comparison with spacecraft detectors. Electronics have been further developed to collect statistics on cross-counter time delays and events with high counter multiplicity. Partially supported by grant RTA6280002 from Thailand Science Resesarch and Innovation.