An electron therapy unit for delivering therapeutic electrons to a patient during an operation that is made up of a movable and stowable beam head that may be connected permanently or temporarily to either a base cabinet or a fixed structure using one or more optionally pivotable arms is provided. In an exemplary embodiment, the inventive electron therapy unit is a mobile unit suitable for in-hospital use or for shared use between hospitals or clinics. The unit is self-contained, small, light and easy to use. It has a very reliable, compact design, allowing for easy stowing to a small rugged configuration for transport. In another exemplary embodiment, the inventive electron therapy unit is a stationary unit. An ion chamber for use with such an optionally transportable electron therapy unit is also provided. The ion chamber employs two or more collector plates and associated bias plates, each having a centrally located hole that extends through the plate. Further provided is a method for reducing or eliminating the possibility of significantly higher current caused by electrical arcs and power excursions during operation of the inventive electron therapy unit.