Figure 22 depicts an example of 14C measured in a lunar sample (Jull et al., 1998). The dotted line shows the expected production of radiocarbon by GCR. The production increases with depth due to the development of a nucleonic cascade in the matter, initiated by energetic GCR particles, similar to the atmospheric cascade. Less energetic particles of solar origin produce the isotope only in upper layers of the rock, since their low energy does not allow them to initiate a cascade. On the other hand, thanks to their high flux in the lower energy range, the production of 14C in the upper layers is much higher than that from GCR. Thus, by first measuring the isotope activity in deep layers one can evaluate the average GCR flux, and then the measured excess in the upper level yields an estimate for the SEP flux in both integral intensity and spectral shape. The result is based on model computations and therefore is slightly model dependent but makes it possible to give a robust estimate of the GCR and SEP in the past.
A disadvantage of this approach is that lunar samples are not stratified and do not allow for temporal separation. The measured isotope activity is a balance between production and decay and, therefore, represents the production (and the ensuing flux) integrated over the life-time of the isotope before the sample has been measured. However, using different isotopes with different life times, one can evaluate the cosmic-ray flux integrated over different timescales.
Estimates of the average SEP flux F30 on different timescales, as obtained from various isotopes measured in lunar samples, are collected in Table 3. The average F30 flux for the last five solar cycles (1954 – 2006) is consistent with the average flux estimated in the past for longer timescales from 103 to 107 years (cf. Reedy, 2002).
|Timescale||Method||Source||Reference||F30 (cm–2 s–1)|
|1954 – 2006||measurements||space-borne||Smart and Shea (2002)||35|
|500 yrs||nitrates||polar ice||McCracken et al. (2001b)||24|
|104 yrs||14C||lunar rock||Jull et al. (1998)||42|
|105 yrs||41Ca||lunar rock||Klein et al. (1990)||28|
|105 yrs||41Ca||lunar rock||Fink et al. (1998)||56|
|5 × 105 yrs||36Cl||lunar rock||Nishiizumi et al. (1995)||26|
|106 yrs||26Al||lunar rock||Kohl et al. (1978)||25|
|106 yrs||10Be, 26Al||lunar rock||Michel et al. (1996)||24|
|106 yrs||10Be, 26Al||lunar rock||Nishiizumi et al. (1995)||26|
|106 yrs||10Be, 26Al||lunar rock||Fink et al. (1998)||32|
|2 × 106 yrs||10Be, 26Al||lunar rock||Nishizumi et al. (1997)||35|
|5 × 106 yrs||53Mn||lunar rock||Kohl et al. (1978)||25|
|2 × 106 yrs||21Ne, 22Ne, 38Ar||lunar rock||Rao et al. (1994)||22|
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