Figure 1. Expected results for the B/C ratio from CALET
Of major scientific importance is CALET’s ability to help resolve one of the outstanding questions about cosmic ray propagation in the galaxy — what is the energy dependence of the process? Here it is secondary to primary ratios (e.g. B/C) that hold the key. In the simplest model of propagation, the energy dependence of the diffusion process is parameterized as E-d, and Kolomogorov theory predicts that d should approach 1/3 at very high energy. Current measurements do not extend to high enough energy, with sufficient precision, to infer the value of, or any energy dependence for d. With the combination of good charge resolution and high energy resolution, CALET is expected to provide important new data as illustrated in Figure 1. With a full 5 year mission CALET will extend the measurements well into the region where the separation between various values of d is measurable. Shown are the anticipated statistics for a result in accord with Iroshnikov-Kraichnan diffusion plus a modest amount of re-acceleration during transport in the interstellar medium, a model that is believed to best represent current experimental data. CALET should be able to verify such predictions and to observe a flattening, moving toward the d = 0.3 curve, if such exists. Without any atmospheric background to subtract, a space experiment such as CALET can produce unprecedented results.