The breakdown performance of CMS barrel module prototype detectors and test
devices with single and multi-guard structures were studied before and aft
er neutron irradiation up to 2.10(14) 1 MeV equivalent neutrons. Before irr
adiation avalanche breakdown occurred at the guard ring implant edges. We m
easured 100-300 V higher breakdown voltage values for the devices with mult
i-guard than for devices with single-guard ring. After irradiation and type
inversion the breakdown was smoother than before irradiation and the break
down voltage value increased to 500-600 V for most of the devices.