1. “BELOW THE KNEE” Working Group (Binns, Cherry, Hörandel, LeBohec, Mitchell, Müller, Moskalenko, Streitmatter, Vacchi, Yodh, et al) Report 4/28/05 Major Theme: How can direct cosmic ray measurements complement the indirect air shower observations? One obvious answer: Provide accurate measurements of the composition of cosmic rays up to an energies which overlap with the range of air shower installations. This was one of the major objectives for the proposed ACCESS space mission.

2. Recall ACCESS category A objectives: (From NASA ACCESS Report 2000) Measure spectra of H and He up to 10 15 eV. Measure spectra of Li to Ni to as high an energy as their flux allows; i.e., to at least 10 15 eV for the more abundant elements C, O, and Fe. Required exposure factors are then: For H and He: 1 m 2 sr year (or 300 m 2 sr days) For the heavier elements: 20 m 2 sr y (or 6000 m 2 sr d)

3. ACCESS was supposed to be implemented through a space mission which combined two detectors, calorimeter (O (0.5 m 2 sr)), and a TRD (O(5 m 2 sr)). These would meet objectives in space mission of a few years duration, and would provide some intercalibration between the two techniques. Questions considered by the Working Group: (1) Is there really an advantage in combining these two techniques in one instrument? (2) Can the ACCESS objectives be achieved by alternate, means; i.e., not requiring space flight.

4. We consider the current balloon instruments: Calorimeters (ATIC, CREAM) have typically 0.25 m 2 ster. They are most suitable for protons and helium. The TRD of TRACER has 5 m 2 sr. It can measure the heavier elements (boron and above). A typical Long Duration Balloon flight lasts 20 days, and therefore, would provide exposures of: 5 m 2 sr days for the calorimeters, 100 m 2 sr days for the TRD. It would require 60 LDB flights for each instrument to meet the requirements of ACCESS!

5. Conclusion: If we use balloon observations we need larger instruments than currently exist. (We also may have to be concerned about nuclear interactions in the residual atmosphere). For calorimeters, a significant increase is not be possible because of weight constraints. For TRD’s, an increase to a detector area of about 5x5 m 2 (as opposed to the current 2x2 m 2 ) may be possible. This would reduce the number of required TRD flights from 60 to 10.  For protons and helium, balloon measurements cannot reach the ACCESS goal. For the heavier nuclei, the gap between balloon flights and ACCESS is considerably smaller.

6. A POSSIBLE (?) ALTERNATIVE TO MEASURE THE ENERGY SPECTRUM OF PROTONS 10 11 TO 10 16 eV: Hadron Calorimeter (such as the one of Kascade), at high mountain altitude; detect surviving single protons. Some numbers: assume residual atmosphere to have 5 proton interaction lengths. Then 0.67% of protons will survive (factor 400 more than at sea level). If the hadron calorimeter has the same sensitivity as that of Kascade (320 m 2 sr) its effective geometric factor would be 2.14 m 2 sr. The ACCESS goal for protons would be achieved within 0.5 years of observation!

7. Comments on hadron calorimeter: Energy resolution is much better than for ballon/space calorimeter because of greater depth. (Kascade: around 20% for 11 interaction lengths). Interaction cross section in air must be precisely known. 3% error in cross section corresponds to 15% error in proton intensity. This would not affect the measured shape of the spectrum. Background due to He and other interacting nuclei must be studied.

8. THE BOTTOM LINE, FOR NOW: Long-duration balloon flights of a TRD based detector for the heavier nuclei, combined with measurements with a high altitude ground-based calorimeter for penetrating single protons seem to be a viable alternative for some of the major objectives of the space flight of an ACCESS payload. Another possibility to consider: direct observation from the ground of Cherenkov light of nuclei before they interact. NOTE: These considerations are devoid of any political or fiscal constraints!