A Boron neutron cancer treatment system, comprising: a secondary moderator (112, 166, 170) having a central treatment chamber for receiving a subject; and six substantially identical neutron generators (68, 118), each comprising a pre- moderator block of moderating material having an upper surface, a lower surface, a first and a second end, opposite side surfaces angled inward by thirty degrees along at least a portion of the height, a first length, a first width substantially less than the first length, and a first thickness, a cylindrical acceleration chamber having a first diameter substantially equal to the first width of the pre-moderator block, sealed at one end to the upper surface of the pre-moderator block adjacent the first end of the pre-moderator block, with a vertical axis perpendicular to the upper surface, the acceleration chamber having a height and a top cover at a second end away from the pre-moderator block, a vacuum pump engaging the acceleration chamber at a right angle to the vertical axis and configured to evacuate the acceleration chamber to a moderately high vacuum, a plasma ion chamber opening into the acceleration chamber through an ion extraction iris through the top cover of the acceleration chamber on the vertical axis of the acceleration chamber, a gas source configured to provide deuterium gas to the plasma ion chamber, a microwave energy source configured to ionize the gas in the plasma ion chamber, a cylindrical primary isolation well extending a substantial distance into the pre-moderator block from the upper surface, centered on the vertical axis of the acceleration chamber, a secondary isolation well substantially in a shape of a hollow cylinder surrounding the primary isolation well, to a depth somewhat less than the substantial distance of the primary isolation well, within the first diameter of the acceleration chamber, a water-cooled titanium target disk having a target surface orthogonal to the axis of the acceleration chamber, the target disk having a diameter substantially smaller then a diameter of the primary isolation well and positioned at a lower extremity of the primary isolation well, the target disk being configured to be biased to a substantial negative DC voltage, and electrically grounded metal cladding covering all otherwise exposed surfaces of the pre-moderator block; wherein the six neutron generators (68, 118) are positioned around the secondary moderator (112, 166, 170) with the axis of each acceleration chamber passing through the center of the treatment chamber, and with the angled sides of the neutron generators fully adjacent. a secondary moderator (112, 166, 170) having a central treatment chamber for receiving a subject; and six substantially identical neutron generators (68, 118), each comprising a pre- moderator block of moderating material having an upper surface, a lower surface, a first and a second end, opposite side surfaces angled inward by thirty degrees along at least a portion of the height, a first length, a first width substantially less than the first length, and a first thickness, a cylindrical acceleration chamber having a first diameter substantially equal to the first width of the pre-moderator block, sealed at one end to the upper surface of the pre-moderator block adjacent the first end of the pre-moderator block, with a vertical axis perpendicular to the upper surface, the acceleration chamber having a height and a top cover at a second end away from the pre-moderator block, a vacuum pump engaging the acceleration chamber at a right angle to the vertical axis and configured to evacuate the acceleration chamber to a moderately high vacuum, a plasma ion chamber opening into the acceleration chamber through an ion extraction iris through the top cover of the acceleration chamber on the vertical axis of the acceleration chamber, a gas source configured to provide deuterium gas to the plasma ion chamber, a microwave energy source configured to ionize the gas in the plasma ion chamber, a cylindrical primary isolation well extending a substantial distance into the pre-moderator block from the upper surface, centered on the vertical axis of the acceleration chamber, a secondary isolation well substantially in a shape of a hollow cylinder surrounding the primary isolation well, to a depth somewhat less than the substantial distance of the primary isolation well, within the first diameter of the acceleration chamber, a water-cooled titanium target disk having a target surface orthogonal to the axis of the acceleration chamber, the target disk having a diameter substantially smaller then a diameter of the primary isolation well and positioned at a lower extremity of the primary isolation well, the target disk being configured to be biased to a substantial negative DC voltage, and electrically grounded metal cladding covering all otherwise exposed surfaces of the pre-moderator block; wherein the six neutron generators (68, 118) are positioned around the secondary moderator (112, 166, 170) with the axis of each acceleration chamber passing through the center of the treatment chamber, and with the angled sides of the neutron generators fully adjacent.