  We report on a measurement of thermal neutrons, generated by the hadronic
component of extensive air showers (EAS), by means of a small array of
EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement
Array), novel devices based on a compound alloy of ZnS(Ag) and $^{6}$LiF. This
array has been operated within the ARGO-YBJ experiment at the high altitude
Cosmic Ray Observatory in Yangbajing (Tibet, 4300 m a.s.l.). Due to the tight
correlation between the air shower hadrons and thermal neutrons, this technique
can be envisaged as a simple way to estimate the number of high energy hadrons
in EAS. Coincident events generated by primary cosmic rays of energies greater
than 100 TeV have been selected and analyzed. The EN-detectors have been used
to record simultaneously thermal neutrons and the air shower electromagnetic
component. The density distributions of both components and the total number of
thermal neutrons have been measured. The correlation of these data with the
measurements carried out by ARGO-YBJ confirms the excellent performance of the
EN-detector.
