Quartz micro-balance results of pulse-resolved erosion/deposition in the JET-ILW divertor

•Sensitivity of quartz crystal microbalances (QMB) used at JET has been improved by a factor of 10 up to 1.4ng/cm2 (about 0.05 Be monolayer).•The stable deposition with average rate between 1.2 and 3ngcm−2s−1 was observed in inner divertor QMB when the strike point position is close to the bottom edge of the vertical target.•Net erosion was found on both the inner and outer divertor QMBs for strike points on the horizontal targets by pulse based measurements.•Observed net erosion signals can be explained by desorption-adsorption of the vacuum background gases on the deposited layer.