Context :

Aims : The purpose of this work is to search the Yangbajing neutron monitor data obtained between 1998 October and 2000 June for solar neutrons associated with solar flares .

Methods : Using the onset times of 166 BATSE-detected flares with the GOES peak flux ( 1 – 8 Å ) higher than 1.0 \times 10 ^ { -5 } \mathrm { Wm ^ { -2 } } , we prepare for each flare a light curve of the Yangbajing neutron monitor , spanning \pm 1.5 hours from the BATSE onset time .
Based on the light curves , a systematic search for solar neutrons in energies above 100 MeV from the 166 flares was performed .

Results : No statistically significant signals due to solar neutrons were found in the present work .
Therefore , we put upper limits on the > 100 MeV solar-neutron flux for 18 events consisting of 2 X and 16 M class flares .
The calculation assumed a power-law shaped neutron energy spectrum and three types of neutron emission profiles at the Sun .
Compared with the other positive neutron detections associated with X-class flares , typical 95 % confidence level upper limits for the two X-class flares are found to be comparable to the lowest and second lowest neutron fluxes at the top of the atmosphere .
In addition , the upper limits for M-class flares scatter in the range of 10 ^ { -2 } to 1 neutrons \mathrm { cm ^ { -2 } s ^ { -1 } } .
This provides the first upper limits on the solar-neutron flux from M-class solar flares , using space observatories as well as ground-based neutron monitors .

Conclusions :