B. Bayanov1, Yu. Belchenko1, V. Belov1, E. Bender1, M. Bokhovko2, G. Dimov1, V. Kononov2, O. Kononov2, N. Kuksanov1, V. Palchikov1, P. Petrov3, R. Salimov1, G. Silvestrov1, V. Shirokov1, A. Skrinsky1, N. Soloviov2, G. Smirnov3, A. Sysoev4, and S. Taskaev1

1Budker Inst. Nucl. Phys., Novosibirsk, Russia,
2Inst. Phys. and Power Engineering, Obninsk, Russia,

3Inst. Techn. Phys., Snezhinsk, Russia,

4Med. Radiological Res. Center, Obninsk, Russia

ABSTRACT: Accelerator source of epithermal neutrons for the hospital-based boron neutron capture therapy is proposed and discussed. Kinematically collimated neutrons are produced via near-threshold 7Li(p,n)7Be reaction at proton energies of 1.883 – 1.9 MeV. Steady-state accelerator current of 40 mA allows to provide therapeutically useful beams with treatment times of tens of minutes. The basic components of the facility are a hydrogen negative ion source, an electrostatic tandem accelerator with vacuum insulation, a sectioned rectifier, and a thin lithium neutron generating target on the surface of tungsten disk cooled by liquid metal heat carrier. Design features of facility components are discussed. The possibility of stabilization of proton energy is considered. At proton energy of 2.5 MeV the neutron beam production for NCT usage after moderation is also considered.

KEY WORDS: Accelerator, Tandem, Neutrons, Kinematical collimation

INTRODUCTION: The neutron therapy is presently realized in two versions: a neutron-capture therapy (NCT) and fast neutron therapy (FNT). At present, the most promising is the version of boron neutron capture therapy (BNCT) [1,2]. Boron containing compound enriched in the isotope 10B is synthesized. This compound introduced into the patient blood produces in the tumor cell the 10B isotope concentration of 30 m g/g while in surrounding normal tissue cell it is ~ 10 m g/g. In 10B(n,a )7Li neutron reaction, the charged particles are produced with the total kinetic energy of ~ 2.4 MeV and with a range in a tissue ~ 10 m m, i.e. of the order of a man's tumor cell size. Because of higher concentration of 10B-isotope in the tumor cells, mainly the cancer cells are destroyed.

The nuclear reactor is the most powerful stationary source of neutrons. At present, there are few active operating therapeutical beams on the nuclear reactors of various kinds and power in the world [3]. Serious disadvantages of reactor therapeutical facilities resulted in the problems of the development of a neutron source for NCT based on the compact and inexpensive accelerator which can be used for every cancer clinic being the subject of intense discussions. At present, various versions of neutron sources for NCT using cheap accelerators of direct action are conceptually developed [1, 2, 4].

For obtaining neutrons the nuclear reactions on light nuclei are supposed to be used (the proton energy is to be up to 2 – 2.5 MeV). The reaction 7Li(p,n)7Be is widely used for obtaining monoenergetic neutrons in nuclear physics experiments and it is quite well studied [6]. This reaction is a threshold one. Because of broad resonance near threshold, the reaction cross-section increases sharply over the threshold value and it has footstep form. Fig.1 shows the doubly differentiated yield of neutrons from a thick metal lithium target for various laboratory escape angles (step is 5° ) and an angular distribution of escaping neutrons in polar coordinates at an initial proton energy of 1886 keV. With an increase in proton energy the neutron escape angle is turned by 4 p  rad at Ep »  1920 keV and the total neutron yield increases up to 1 ´  1013 s–1 at an energy of protons 2.5 MeV and a beam current of 10 mA. At such intensity it turns to be possible to produce a source of appropriate to NCT epithermal neutrons by formation of the required spatial-energy distribution of neutrons with the compact moderator-collimator unit.

Fig. 1.

RESULTS AND DISCUSSION: Selecting a variant of accelerator for neutron source, it is desirable to provide the possibility of operation in two regimes both in the near threshold region [5] allowing to use the source for irradiating in open geometry without external collimator and for production of epithermal and fast neutrons at proton energy of 2.5 MeV with moderators.

In this project we offer to create a neutron source based on construction of vacuum insulation tandem accelerator (VITA) developed at BINP using the sectioned rectifier from electron accelerator of ELV type as a powerful source of high voltage. Advantages of tandem in comparison with accelerator on full energy from the point of providing maximum reliability in operation with high current continuous beam are obvious: the ion source is placed under ground potential and the operating voltage is only the half of full proton energy. The design of vacuum insulation tandem provides reliability significantly exceeding the one for tandem based on accelerating columns with ceramic insulators. The reliability of high voltage ELV rectifier was confirmed by many years of operation in industry. The use of such rectifier as a high voltage source for tandem supplying is attractive also its high efficiency.

Fig. 2. Possible variant of neutron source.

Possible variant of neutron source arrangement is performed at Fig.2. The whole facility is placed in two-floor building in four separate rooms. The high voltage power supply is mounted in one of the rooms (I) of the first floor. The accelerator-tandem is mounted through the hole in ceiling above the power supply. From one side of accelerator the source of H - with differential vacuum pumping system and optical system of beam transport for injection in accelerator are placed. The 40 mA beam accelerated up to 2.5 MeV after charge-exchange process comes from the other side of tandem and then the parallel shift system displaces the beam to the transporting 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 of beam energy after additional stripping by means of special bending magnets.

Proton beam is directed by transport channel in two medical rooms. The horizontal beam enters the medical room III for works with vertical jet liquid lithium neutron producing target [7]. Two neutron beams come out in the opposite directions perpendicularly to the proton beam and may be used directly for fast neutron therapy and for neutron capture therapy after moderation independently in two different rooms. The removal of heat released by proton beam is realized by pumping of liquid lithium through the heat exchanger. The transport channel has 90° bending magnet which directs the beam to another neutron-producing target situated in the irradiation room IV. This target is a steel disk cooled intensively by water or liquid metal coolant and covered by thin layer of lithium on which the proton beam is thrown. Such target can operate both with solid and liquid lithium, therefore it can be used only with vertically directed proton beam.

To obtain an uniform distribution of beam on the surface approximately 5 cm in diameter a method of recirculating scanning by means of wobbler magnet is used. This magnet is a rotating dipole of cobalt-samarium magnets. Possibility of applying of two variants of target is foreseen — the open target for operation in the region near threshold and the target with moderator and reflector.

There is a pool of the experience [8]. This work is supported by International Science and Technology Center, project ą 1484. Proposed accelerator source of epithermal neutrons for the hospital-based boron neutron capture therapy can be created and transferred for use in clinic.


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