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Possible variant of neutron therapy facility is performed at figure. Negative hydrogen ion beam is injected into electrostatic tandem accelerator with vacuum insulation. After charge-exchange of negative hydrogen ion into proton inside the charge-exchange tube in the center of high-voltage electrode, the proton beam is formed at the outlet of the tandem. It is accelerated up to double voltage of high-voltage electrode. Neutron generation is proposed to be carried out by dropping an intensive proton beam onto lithium target using ⁷Li(p,n)⁷Be threshold reaction.

In ordinary mode, at proton energy of 2.5 MeV, the neutron source produces neutron beam with maximum energy of 790 keV appropriate directly for fast neutron therapy and for neutron-capture therapy after moderation. Usually, a patient is placed in 0.5 m or farther from the target behind the shielding.

The most efficient operating mode of facility is at proton energy of 1.915 MeV that is 34 keV higher than the threshold of the ⁷Li(p,n)⁷Be reaction. In this mode, neutron beam is generated kinematically collimated in forward direction and its average energy of 30 keV, is directly applicable for boron neutron-capture therapy. Due to very fast increase in cross-section, which is the feature of this reaction, and pronounced neutron flux in forward direction, the forward yield of 30 keV (mean energy) neutrons is only one order lower than the full neutron flux in forward direction generated by 2.5 MeV protons and possessing wide energy spectrum. In this case, patient may be placed in 10 cm distance from the target, that increase considerably the neutron flux density or decrease the current requirements.

World experience available in the field of use of different types of accelerators as neutron sources for remote fast neutron radio-therapy was analyzed. Since neutrons with 0.5 - 1.5 MeV energy are optimal for fast neutron therapy, the optimal neutron source is accepted to be realized by drop of 1 mA 2 MeV deuterium beam onto beryllium target. Construction of such source is supposed to be simpler than one for neutron capture therapy.

The whole installation is placed in two-floor building in four separate rooms. The high-voltage source (HVS) and main powerful power supply sources are mounted in one of the rooms (I) of the first floor. The accelerator-tandem is mounted through the hole in ceiling above the HVS, so that its main part - vacuum tank with inputting insulator, potential electrodes and charge-exchange target is situated at the second floor (II). The axis of accelerated and injected beams is on about 1 m distance above the floor of the second floor. There are a source of negative hydrogen ions with differential vacuum pumping system and the optical system of beam transport for injection into accelerator placed from the one side of accelerator. The beam after charge-exchange process accelerated up to 2.5 MeV (doubled energy) comes from the other side of tandem and then the parallel shift system displaces the beam to the transport channel. This system separates the high intensity proton beam and low current beam of neutrals which can be used both to control the efficiency of charge-exchange process and for precise measurements (after additional stripping) of beam energy by means of special bending magnets.

Proton beam is directed by transport channel in two medical rooms where therapeutic neutron beams are generated by dropping the proton beam onto neutron generating target.

In the following sections, main elements of the source are described in details:

Facility
Negative hydrogen ion source
Ion-optical channel
Vacuum insulation tandem accelerator
High-voltage power supply
Charge-exchange target
Proton channel
Neutron producing target
Radiation fields and absorbed doze
Vacuum technique
Shielding