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Aims. Lunar soil and rocks are not protected by a magnetic field or anatmosphere and are continuously irradiated by energetic particles that canproduce cosmogenic radioisotopes directly inside rocks at different depthsdepending on the particle's energy. This allows the mean fluxes of solar andgalactic cosmic rays to be assessed on the very long timescales of millions ofyears. Methods. Here we show that lunar rocks can serve as a very good particleintegral spectrometer in the energy range 20-80 MeV. We have developed a newmethod based on precise modeling, that is applied to measurements of $^{26}$Al(half-life ~0.7 megayears) in lunar samples from the Apollo mission, andpresent the first direct reconstruction (i.e., without any a prioriassumptions) of the mean energy spectrum of solar and galactic energeticparticles over a million of years. Results. We show that the reconstructedspectrum of solar energetic particles is totally consistent with that over thelast decades, despite the very different levels of solar modulation of galacticcosmic rays ($\phi=496\pm 40$ MV over a million years versus $\phi= 660\pm 20$MV for the modern epoch). We also estimated the occurrence probability ofextreme solar events and argue that no events with the F(>30 MeV) fluenceexceeding $5*10^{10}$ and $10^{11}$ cm$^2$ are expected on timescales of athousand and million years, respectively. Conclusions. We conclude that themean flux of solar energetic particles hardly depends on the level of solaractivity, in contrast to the solar modulation of galactic cosmic rays. Thisputs new observational constraints on solar physics and becomes important forassessing radiation hazards for the planned space missions.

Solar energetic particles and galactic cosmic rays over millions of years as inferred from data on cosmogenic $^{26}$Al in lunar samples