HEAD Division Meeting 1999, April 1999
Session 21. Other
Poster, Tuesday, April 13, 1999, 8:30am-6:02pm, Gold Room

## [21.01] A Measurement of the Atmospheric Positron Annihilation Line at Low Geomagnetic Latitudes

I.N. Azc\'{a}rate (Instituto Argentino de Radioastronom\'{\i}a)

An experiment carried out with a collimated gamma-ray detector is described. The detector system was transported by a stratospheric balloon, that was launched from Paraná, province of Entre R\'{i}os, Argentina, (geomagnetic cut-off 11.1 GV). The presence of a peak was observed, as a characteristic feature of the energy-loss spectrum in the detector. That peak corresponds to the 511 keV line produced by positron annihilation in both the atmosphere and the lead collimator. The contribution to the counting rate below" the peak due to the 511 keV photons produced in the lead is computed. The resulting unidirectional flux for the atmospheric 511 keV line, at an atmospheric depth of 4.5 g.cm-2, is:\\ (6.5±0.6) \times 10-3 phot.cm-2.s-1.str-1.

A growth curve of this unidirectional line flux between 300gcm-2 and ceiling altitude (4.5 gcm-2) is also obtained from a good data set. The shape of the growth curve and the value of the atmospheric depth at which the Pfotzer maximun is present is in good agreement with other determinations.

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