Secondary to Primary Ratio
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 18. 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.
Figure 2. Charge resolution for B-O obtained in the GSI
calibration run (preliminary data).
The anticipated results in Figure 1 are for the primary Silicon charge detector. As mentioned earlier a back-up CHD made from scintillator strips is now the primary. A prototype of this back-up detector was taken to the heavy ion accelerator at GSI. A beam of Ni was accelerated to 1.34 GeV/nucleon and hit an internal target in the Fragment Separator (FRS) beam line. This line allows tuning of beam fragments with fixed A/Z ratio, providing a sample of all ions down to B. (The width of the momentum distribution and few direct production channels make it difficult to obtain high yields of the lighter nuclei.) Figure 2 shows preliminary results for the B-O region. As is evident, there is good element separation with a (rms) charge resolution of ~0.14 charge units, and this is sufficient to perform the B/C measurement over most of the energy range shown in Figure 18. (It should be noted that a similar prototype was exposed at the HIMAC heavy ion accelerator in Japan at ~500 MeV/nucleon and gave comparable results.)
Of course, this is an accelerator test with a single energy and single angle of incidence but with some momentum spread in the beam. Determining the radiative strength of different effects contributing to the width of the charge peaks in Figure 2 is one of the tasks in the on-going analysis.
In summary, the revised CALET mission has been shown to be capable of meeting its science objectives and promises important new data -- and potential discoveries -- in the very high energy regime.