Non-radioactive ion source using high energy electrons

A method for producing high energy electrons for ionization, comprising:

• exciting an electron emitter disposed within an evacuated tube behind an optically transparent portion of the evacuated tube, wherein the electron emitter includes a photocathode element that emits electrons when excited by photons received at the photocathode element through the optically transparent portion of the evacuated tube;
• controlling a flow of the electrons along the evacuated tube using electron optics, wherein the electron optics include a plurality of electrodes disposed along the evacuated tube, wherein the plurality of electrodes are biased to control a final electron energy of the electrons arriving at the membrane;
• and
• passing the electrons through a membrane disposed at an end of the evacuated tube downstream from the flow of electrons caused by the electron optics, wherein the electrons that pass through the membrane are high energy electrons, wherein the membrane has at least one property specified permeate through the membrane the high energy electrons having energies down to a cutoff value such that a range of electron energies of the high energy electrons emitted from the membrane is adjustable using:
• (i) control of the voltages of the plurality of electrodes of the electron optics that determines an upper limit of the range of electron energies and (ii) the at least one property of the membrane that determines the lower limit of the range of electron energies according to the cutoff value.