Measurements with silicon detectors at extreme neutron fluences

Thin pad detectors made from 75 $mu$m thick epitaxial silicon on low resistivity substrate were irradiated with reactor neutrons to fluences from 2.5$times 10^{16}$ n/cm$^2$ to 1$times 10^{17}$ n/cm$^2$. Edge-TCT measurements showed that the active detector thickness is limited to the epitaxial layer and does not extend into the low resistivity substrate even after the highest fluence. Detector current was measured under reverse and forward bias. The forward current was higher than the reverse at the same voltage but the difference gets smaller with increasing fluence. Rapid increase of current (breakdown) above ~ 700 V under reverse bias was observed. An annealing study at 60$^circ$C was made to 1200 minutes of accumulated annealing time. It showed that the reverse current anneals with similar time constants as measured at lower fluences. A small increase of forward current due to annealing was seen. Collected charge was measured with electrons from $^{90}$Sr source in forward and reverse bias configurations. Under reverse bias the collected charge increased linearly with bias voltage up to 6000 electrons at 2.5$times 10^{16}$ n/cm$^2$ and 3000 electrons at 1$times 10^{17}$ n/cm$^2$. Rapid increase of noise was measured above $sim$ 700 V reverse bias due to breakdown resulting in worse S/N ratio. At low bias voltages slightly more charge is measured under forward bias compared to reverse. However better S/N is achieved under reverse bias. Effective trapping times were estimated from charge collection measurements under forward bias showing that at high fluences they are much longer than values extrapolated from low fluence measurements - at 1$times 10^{17}$ n/cm$^2$ a factor of 6 larger value was measured.