Harri Arvela

Composition and spectra of cosmic-ray hadrons at sea level

Energy spectra of neutrons, protons and pions were measured at sea level using a cosmic-ray hadron spectrometer consisting of a double neutron monitor and scintillation counters. All the hadron spectra were determined simultaneously. The analysis of the observed neutron multiplicity distributions was based on computer simulations of hadron cascades in the spectrometer. The neutron spectrum was determined in the range 0.05-1000 GeV. The proton-to-neutron ratio was estimated below 2 GeV, where the contribution of pions is of minor importance. Assuming a smooth rise of the p/n ratio above 2 GeV to a constant value at 5 GeV, the pion spectrum was determined between 2 and 50 GeV. At higher energies, where accompanied hadrons distort the measured spectra, upper bounds of the pion intensities were estimated. The composition of the hadron flux at sea level is discussed on the basis of the present and previous experimental and theoretical results.

A double neutron monitor applied to cosmic-ray multiplicity studies

Abstract A double neutron monitor consisting of two neutron monitors, one on top of the other, has been constructed. A thick layer of moderating material between the monitors enables two independent measurements of multiplicity. The total thickness of the target layer in the double monitor amounts to about 3 inelastic mean free paths of high-energy hadrons in lead. The construction and functional characteristics of the monitor are discussed. The data collection and recording is based on a microprocessor system. The average efficiency for detecting evaporation neutrons was found to be e = (12.5 ± 0.5)% in both monitors. The contribution of cosmic-ray muons to the counting rates of different multiplicities and the barometric coefficients were determined.

Interplanetary propagation of relativistic solar protons

Particle fluxes and pitch angle distributions of relativistic solar protons at Earths orbit have been determined by Monte Carlo calculations. The analysis covers two hours after the release of the particles from the Sun and total of 8 × 106 particle trajectories were simulated. The pitch angle scattering was assumed to be isotropic and the scattering mean free path was varied from 0.1 to 4 AU.The intensity-time profiles after a delta-like injection from the Sun show that the interplanetary propagation is clearly non-diffusive at scattering mean-free paths above 0.5 AU. All pitch angle distributions have a steady minimum at 90 °, and they become similar about 20 min after the arrival of first particles.As an application, the solar injection profile and the interplanetary scattering mean-free path of particles that gave rise to the GLE on 7 May, 1978 were determined. In contrast to the values of 3–5 AU published by other authors, the average scattering mean-free path was found to be about 1 AU.

Accompanied hadrons in sea-level detectors

The coincidences of cosmic-ray particles in sea-level detectors were investigated theoretically. The object of the work was to clarify the role of accompanied particles in the measurement of the spectrum of hadrons. The calculations were based on a Monte-Carlo model of atmospheric hadron cascades. The counting rates and energy spectra of hadrons with different numbers of accompanied charged hadrons were calculated by using the area of the detector as a parameter. The effect of accompanied hadrons on the measured spectra was found to be appreciable at total energies above 500 GeV.

Spectra of cosmic-ray hadrons in the range 0.1–1000 GeV

The energy spectra of cosmic-ray neutrons, protons, and pions were determined at sea level in the range 0.1 – 1000 GeV. The results are based on the measurements of the neutron-multiplicity distributions produced by cosmic hadrons in the Turku spectrometer. Below 0.5 GeV, the values γ = 1.47 ± 0.04 and 0.8 ± 0.1 were obtained for the slopes of the neutron and proton spectra. At 1000 GeV, the nucleon spectra are much steeper with γ = 2.79 ± 0.06. The pion spectrum was found to obey the power law with γ = 1.41 ± 0.05 in the low-energy region up to 30 GeV. Above 500 GeV, the spectrum of pions, containing an admixture of accompanied particles, was found to reach the high energy limit with γ = 2.92 ± 0.06.

Search of periodic scintillations in cosmic radiation in the range 2–50 mHz

Abstract Neutron monitor measurements using 10-second recording intervals were started at Oulu in September 1987. A special analysis method based on the power spectra was adopted to compensate for the relative low statistics associated with short recording intervals. The preliminary result based on the analysis of 6274 one-hour length data records strongly indicates the existence of periodic scintillations in the whole frequency range 2–50 mHz.

Cosmic-ray intensity oscillations immediately before and after the GLE on February 16, 1984

We have recently started a systematic study of short-period (periods less than 60 min) variations in the cosmic-ray intensity. By making use of the MEM, we have found intensity oscillations with periods varying from 16 to 27 minutes during the GLE on February 16, 1984.

Shape of energy spectrum of hadrons at sea level

The energy spectrum of cosmic-ray hadrons between 10 GeV and 10 TeV at sea level was calculated by using a Monte-Carlo method. The results show a strong steepening of the hadron spectrum at about 200 GeV. The change in the spectral slope is interpreted as a combined effect due to the energy-dependent production cross section of hadron-air-nucleus collisions and the saturation of the relative intensity of pions in the hadron family. The magnitude of the change is found to vary from 0.5 to 0.7 depending on the form of the production cross section. The results agree with the experimental spectra obtained by the Turku double neutron monitor.