New measurement of the altitude dependence of the atmospheric muon intensity

We present a new measurement of atmospheric muons made during an ascent of the High Energy Antimatter Telescope balloon experiment. The muon charge ratio ${\mu }^{+}/{\mu }^{-}$ as a function of atmospheric depth in the momentum interval 0.3–0.9 GeV/c is presented. The differential ${\mu }^{-}$ intensities in the 0.3–50 GeV/c range and for atmospheric depths between $4–960\mathrm{g}/{\mathrm{c}\mathrm{m}}^2$ are also presented. We compare these results with other measurements and model predictions. We find that our charge ratio is $\sim 1.1$ for all atmospheric depths and is consistent, within errors, with other measurements and the model predictions. We find that our measured ${\mu }^{-}$ intensities are also consistent with other measurements, and with the model predictions, except at shallow atmospheric depths.

Figure 1

Schematic view of the HEAT-pbar Instrument. The solid black line labeled “S” is a piece of scintillator and is part of the instrument trigger.

Figure 2

Distribution of dE/dx as a function of rigidity for ascent data for positive rigidities. Superimposed are the Bethe-Bloch energy loss curves for various particle species.

Figure 3

Restricted mean energy loss distributions for ascent data with Gaussian fits for $-0.9\mathrm{G}\mathrm{V}<R<-0.3\mathrm{G}\mathrm{V}$.

Figure 4

Restricted mean energy loss as a function of rigidity for ascent data for (top) positive rigidities and (bottom) negative rigidities. The event selections (solid lines) are shown on each plot as well as the Bethe-Bloch energy loss prediction curves (dashed line). Note that the apparent higher density of negative muons compared to positive muons is an artifact of the plotting routine used for the figure; there are approximately equal numbers of points shown on each panel.

Figure 5

Atmospheric muon charge ratio as a function of atmospheric depth for a geomagnetic cutoff of 4.5 GV (Fort Sumner, NM, USA). Also shown are the ratios for the HEAT 1994 flight as well as other measurements. The curves are calculations with the three-dimensional TARGET algorithm. Indicated are the solar modulation parameters for each data set.

Figure 6

Atmospheric muon charge ratio as a function of atmospheric depth for a geomagnetic cutoff of $\ll 1\mathrm{G}\mathrm{V}$ (Lynn Lake, Manitoba, Canada) for two data sets. The curves are calculations with the three-dimensional TARGET algorithm. Indicated are the solar modulation parameters for each data set.

Figure 7

Ground-level differential ${\mu }^{-}$ intensities as a function of rigidity. Also shown are results from various other instruments. The plots have been scaled by a factor of $R^2$. The curves are calculations with the three-dimensional TARGET algorithm for solar minimum (solid line) and solar maximum (dashed line).

Figure 8

Differential ${\mu }^{-}$ intensities as a function of rigidity for various atmospheric depth ranges. Also shown are results from various other instruments. (Note: only subsets of measurements by other groups are shown here, where the choice of atmospheric depth bins matches with ours.) The plots have been scaled by a factor of $R^2$. The curves are calculations with the three-dimensional TARGET algorithm for solar minimum (solid lines) and solar maximum (dashed lines). See Figs. 5, 6 for each measurement’s solar and geomagnetic conditions.

Figure 9

${\mu }^{-}$ growth curves for various rigidity intervals. (Note: other groups have used different rigidity bins in their analyses, so that we only report HEAT measurements here.) The plots have been scaled by a factor of $R^2$. The curves are calculations with the three-dimensional TARGET algorithm for solar minimum (solid lines) and solar maximum (dashed lines). See Fig. 7 for each measurement’s solar and geomagnetic conditions.