1 Introduction

The high energy component of gamma-ray bursts can be measured by means of extensive air shower (EAS) arrays located at mountain altitudes. The ${\rm e}^{\pm}$ generated in the atmosphere by gamma-rays with energy E > 10 GeV can reach the detector level; although the number of secondary particles is too small to reconstruct the primary direction, GRBs could be detected as a short time increase in the single particle counting rate. The background consists of ${\rm e}^{\pm}$ and $\mu^{\pm}$generated by low energy cosmic rays. All sources of background modulation have time scales (hours) much larger than the typical GRB duration (seconds), hence they do not affect the GRBs search.

  

Figure 1: Distribution of the excesses ${\rm C}-{\rm B}$, in unit of standard deviations, observed in coincidence with 292 BATSE GRBs

The EASTOP experiment is running since 1991 at Campo Imperatore in Italy, at 2000 m a.s.l. The array is devoted to the measurement of the various components of extensive air showers. In the GRBs search only the detector of the electromagnetic component is used [(Aglietta 1996)]. It consists of 35 scintillators of 10 m² each, spread over an area of $\sim 10^5$ m², operating at an energy threshold $E_{\rm th} \sim 3$ MeV. At the EASTOP altitude the single particle background is mostly due to muons and its measured rate is $\sim 500$ m^-2 s^-1.