LHAASO observed GRB 221009A with more than 5000 VHE photons up to around 18 TeV

We report the observation of GRB 221009A, which was detected by Swift (Kennea et al. GCN #32635), Fermi-GBM (Veres et al. GCN #32636, Lesage et al. GCN #32642), Fermi-LAT (Bissaldi et al. GCN #32637), IPN (Svinkin et al. GCN #32641) and so on. GRB 221009A is detected by LHAASO-WCDA at energy above 500 GeV, centered at RA = 288.3, Dec = 19.7 within 2000 seconds after T0, with the significance above 100 s.d., and is observed as well by LHAASO-KM2A with the significance about 10 s.d., where the energy of the highest photon reaches 18 TeV. This represents the first detection of photons above 10 TeV from GRBs. The LHAASO is a multi-purpose experiment for gamma-ray astronomy (in the energy band between 10^11 and 10^15 eV) and cosmic ray measurements.