The first one provides kinematically collimated neutrons via near-threshold ⁷Li(p,n)⁷Be reaction at proton energies of 1.883 – 1.9 MeV. Escaping angle of neutrons is within 45° with respect to the proton beam. The range of neutron energies is 2 to 87 keV. Reflectors, collimator, and moderators are not used in these open geometry conditions. Target will be created as a 2 – 3 m m thick lithium target on the surface of 5 cm tungsten disk cooled by liquid metal heat carrier or as drop-stream lithium target separated geometrically from the proton beam receiver.

In the second mode, two therapeutically useful orthogonal neutron beams are produced at proton energy of 2.5 MeV. Proton beam is focused in 1 cm diameter on 100 m m thick liquid lithium stream target placed in the center of a moderator-collimator. The beam losses 25 % of its power passing through the target and drops on the large square receiver.

INTRODUCTION: Nowadays ⁷Li(p,n)⁷Be reaction seems to be the most perspective neutron source for accelerator based BNCT facility. There are two conceptions of this source with acceptable for BNCT requirements. The first one operates at proton energy of 2.3 – 2.8 MeV and neutrons are generated from thick lithium target. Then neutron beam is moderated and shaped in the Beam Shaping Assembly [1, 2]. Another one (suggested by V. N. Kononov et al. [3]) is based on near reaction threshold kinematic collimation of neutrons. This type of source does not require such moderation and shaping of beam as the first one. Thickness of lithium layer for neutron production in these two sources is ~ 100 m m and ~ 3 m m, respectively.

In designing neutron source, accompanying 0.478 MeV gamma rays yield from ⁷Li(p,p`)⁷Li reaction and gamma rays from radioactive decay of ⁷Be should be considered [4]. In the first conception of source at proton energy of 2.5 MeV and beam current of 10 mA, yield of 0.478 MeV gamma ray is 2.2 ´  10¹² photon per second. In near threshold conception at proton energy of 1.885 MeV this yield is 1.4 ´  10¹⁰ photon per second. Radionuclide ⁷Be has half-decay time of 53.5 days and in 10 % has positron decay with accompanying 0.478 MeV gamma rays and in 90 % K-capture with 0.511 MeV gamma rays. After one hour 2.5 MeV 10 mA proton exposition thick ⁷Li target has the activity of about 2.3 ´  10⁹ Bk. Therefore, in designing such source not only the problem of cooling of lithium layer should be solved, but it is also very important to consider the whole nuclear process in the target.

RESULTS AND DISCUSSION: In the course of work on accelerator source of epithermal neutrons for the hospital-based boron neutron capture therapy [5], two ways of development of lithium neutron production targets are considered: a thin one for operation in open geometry near threshold of ⁷Li(p,n)⁷Be reaction, and a target with moderator and collimator for operation at proton energy of 2.5 MeV. The first variant of target is shown in Fig. 1. Steel disk (1) 50 mm in diameter, 3 mm thick, is a proton beam absorber. It is cooled by liquid metal flowing in opposite directions in neighbouring channels (2). A molybdenum plate (3) 0.2 mm thick is diffusely welded on the disk, a proton beam is directed on it. The plate is covered with a layer of lithium several micrometers thick. The target together with the tubes transporting cooling liquid metal (4) is placed in the end of a long cylinder (5) closed with stainless foil (6) separating the cylinder vacuum volume from atmosphere. The covering of molybdenum foil surface with lithium is fulfilled with special system directly in the target device. The moistening system consists of the ring (7) made of thin stainless tube with holes 0.1 mm diameter (8). Drops of liquid lithium are pressed out through the holes. At heating the rings over 400 – 600 °C, lithium is evapourated and the vapour molecules are reflected to the surface with heated to high temperature screen of special form (9) providing homogeneous covering of cool molybdenum surface with lithium. The covering mode (that is lithium layer thick via moistening time and temperature of moistening device) is determined during operation on special set-up. At operation near threshold for proton energy drop D Ĺ_Đ = 1900 – 1880 keV = 20 keV, the lithium target thick should be 2 – 3 m m. A gamma flow from ⁷Li(p,p:g )⁷Li reaction increases linearly, that may indicate target thickness. As proton beam power increases up to 100 kW (current of 50 mA), the density of the heat flow removed from the target increases up to 5 kW cm^–2. Lithium temperature on the target may be 300 – 400 °C, and lithium will evapourate intensively. Therefore, the moistening mode may be cyclic (in between the radiation sessions or more rarely) or continuous (moistening-evapourating in dynamic equilibrium). At evapouration of lithium from the target with proton beam, it will set on cool inner surfaces of the target device. In the upper part of the target cylinder, there is a diaphragming hole small in diameter (~ 1 mm) coinsiding with intermediate image of a proton beam providing condition of differential pumping out of beam line and accelerator with high vacuum from target device volume with lithium vapour.

The problem of intensive cooling of proton beam absorber at density of heat flow from the target surface of q = 1 – 5 kW cm^–2 is solved by using liquid metal heat carrier, gallium-indium alloy. The heat removal mode is characterized by the following temperature drop D T_Li = D T_Mo + D T_Mo-Ga + D T_Ga + T₀. At q = 1 kW cm^–2 and heat carrier velocity v_Ga = 5 m c^–1 D T_Mo = 20 °, D T_Mo-Ga = 46 °, D T_Ga = 60 °, so lithium layer temperature is T_Li = 150 °C. At q = 5 kW cm^–2 and v_Ga = 10 m c^–1, this value is T_Li = 450 °C.

The second variant of target development (Fig. 2) is using thin jet liquid lithium target placed in the center of moderator-reflector of neutrons and partial drop of energy by proton beam of 2.5 MeV up to the threshold energy of 1.881 MeV. To provide this energy drop of 0.6 MeV by protons, the target thickness should be 100 m m, so that less than 30 % of the beam full power will be released in the target.

The proton beam is focused into less than 1 cm on the target 70 – 100 m m thick and 1 cm wide, flowing from a narrow nozzle with the velocity of »  10 m c^–1. After passing through the target, a horizontal proton beam of 1.9 MeV will have an angle of multi-scattering 40 mrad and energy dispersion of ±  10 %. Short permanent magnet with the field of 1 T placed in the output of the moderator turns the beam 30° up and spreads it over the distant proton absorber surface of the large square with a simple water-cooling system. Two neutron beams come out of the moderator in the opposite directions perpendicularly to the proton beam and may be used independently in two different rooms.

Fig. 1. Neutron generating lithium target. 1 — steel disk with cooling channels, 2 — cooling channels, 3 — molybdenum foil with lithium layer, 4 — liquid metal heat carrier input tubes, 5 — cylindrical body of target device, 6 — vacuum separating foil, 7 — liquid lithium input tube, 8 —holes for liquid lithium flow out, 9 — reflecting screen.

Fig. 2. Liquid lithium jet target device.

REFERENCES:

1. D. A. Allen and T. D. Beynon. Med. Phys. 27 (2000) 1113.
2. D. L. Bleuel, R. G. Donahue et al. Med. Phys. 25 (1998) 1725.
3. V. N. Kononov et al. Proc. 1^st Int. Workshop on Accelerator Based Neutron Sources for BNCT. Jackson, USA, Sept. 11-14, 1994, Vol. 2, 447-483; CONF-94096.
4. V. Kononov et al. Preprint IPPE-2643, Obninsk, 1997.
5. B. Bayanov et al. Accelerator based neutron source for the neutron capture therapy at hospital. 9^th Int. Symp. on Neutron Capture Therapy for Cancer. Osaka, Japan, Oct. 2-6, 2000.