Context :

Aims : Lunar soil and rocks are not protected by a magnetic field or an atmosphere and are continuously irradiated by energetic particles that can produce cosmogenic radioisotopes directly inside rocks at different depths depending on the particle ’ s energy .
This allows the mean fluxes of solar and galactic cosmic rays to be assessed on the very long timescales of millions of years .

Methods : Here we show that lunar rocks can serve as a very good particle integral spectrometer in the energy range 20 – 80 MeV .
We have developed a new method based on precise modeling , that is applied to measurements of ^ { 26 } Al ( half-life \approx 0.7 megayears ) in lunar samples from the Apollo mission , and present the first direct reconstruction ( i.e. , without any a priori assumptions ) of the mean energy spectrum of solar and galactic energetic particles over a million of years .

Results : We show that the reconstructed spectrum of solar energetic particles is totally consistent with that over the last decades , despite the very different levels of solar modulation of galactic cosmic 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 of extreme solar events and argue that no events with the F ( > 30 MeV ) fluence exceeding 5 \cdot 10 ^ { 10 } and 10 ^ { 11 } cm ^ { -2 } are expected on timescales of a thousand and million years , respectively .

Conclusions : We conclude that the mean flux of solar energetic particles hardly depends on the level of solar activity , in contrast to the solar modulation of galactic cosmic rays .
This puts new observational constraints on solar physics and becomes important for assessing radiation hazards for the planned space missions .