Materials Transactions, Vol. 54, No. 12 (2013) pp. 2233 to 2237 ©2013 The Japan Institute of Metals and Materials Low Temperature and Pressure Synthesis of Lithium­Nitride Compound with H2O Addition on Lithium Target for BNCT Shintaro Ishiyama1,+, Yuji Baba1, Ryo Fujii2, Masaru Nakamura2 and Yoshio Imahori2 1Quantum Beam Science Directorate, Japan Atomic Energy Agency, Naka-gun, Ibaraki 319-1195, Japan 2Cancer Intelligence Care Systems, Inc., Tokyo 135-0063, Japan Low temperature synthesis of lithium­nitride compound was conducted on the lithium target for BNCT by N2/H2O mixing gas squirt in the ultra high vacuum chamber, and the following results were derived. (1) Lithium­nitride compound was synthesized on the lithium target ¹8 under 101.3 Pa N2 gas squirt at room temperature and in the ultra high vacuum chamber under the pressure of 1 © 10 Pa. (2) Remarkable contamination by O and C was observed on the lithium­nitride compound synthesized under the squirt pressure of 13.3­80 Pa/1.33­4.7 Pa N2/ H2O mixing gas. (3) No contamination and synthesis of Li­N compound was observed under the squirt pressure of 0.013­0.027 Pa/0­0.005 Pa N2/H2O mixing gas. (4) Contamination by O and C was enhanced with excessive addition of H2O at the pressure of over 1.33 Pa. [doi:10.2320/matertrans.M2013242] (Received June 26, 2013; Accepted September 25, 2013; Published November 9, 2013) Keywords: boron neutron capture therapy, neutron source, lithium target, lithium nitride, nitrogen gas, contamination, H2O addition 1. Introduction Implemented deployment of accelerator-driven neutron source for Boron Neutron Capture Therapy (BNCT) is scheduled in 2013 in National Cancer Center, Japan. This BNCT system was designed with the production of neutrons via threshold 7Li (p, n) 7Be reaction at 25 kW proton beam with energy of 2.5 MeV and starts its installation at middle of 2013. Many types of pilot innovative accelerator-based neutron source for neutron capture therapy with lithium target were designed1­4) and these designs face serious problems such as evaporation of lithium with the progressive power run-up. In the previous paper, we have proposed that the evaporation can be reduced by synthesis of Li3N on the surface of Li target exposed to proton beam, because lithium nitride is thermally very stable up to 1086 K and exhibited Li3N synthesis on lithium target by in-situ Li deposition and 5) ion implantation technique. Fig. 1 Procedure of nitridation of Li target surface on Cu; (a) Li deposition The conceptual lithium target model for BNCT is process on Cu target, (b) N2 gas squirt with H2O and (c) Li­N compound illustrated in Fig. 1(c). Heat load receiving area of the target formation on Li surface. is consisted of Li target (³100 µ mt) with Li3N thin layer and copper substrate. photoelectron spectroscopy (XPS) using X-rays from syn- There are many reports4­9) about nitridation techniques of chrotron light source. lithium, and direct synthesis of Li3N in low temperature and pressure N2 gas with the presence of H2O and O2 is also one 2. Experimental Method of very attractive nitridation techniques6­8) in practical use for BNCT target production. 2.1 Specimens However, very high level of oxygen and carbon contam- High-purity copper (Cu) plates (5 mm © 5mm© 1 mmt) inations on the lithium­nitride compound layer surface was were used as a substrate. As a source material for deposition, reported in previous low temperature direct synthesis study metallic lithium rod (5 mm¤ © 8 mm) purchased from under the ultra-high vacuum condition.8,9) Kojundo Chemical Laboratory Co. Ltd. was used. Purity Therefore, present paper primarily intends to ascertain the of the lithium was higher than 99.98% and Na(0.004%), cause of these contaminations observed in direct synthesis Ca(0.006%), K(0.001%), Fe(0.001%), Si(0.001%), of the Li­N compounds on lithium in nitrogen gaseous N(0.006%) and Cl(0.001%) were contained in this pellet. atmosphere with H2O addition. The surface condition of the lithium­nitride compounds was characterized by X-ray 2.2 Apparatus Experiments4,5,8,9) were performed at the BL-27A station +Corresponding author, E-mail: [email protected] of the Photon Factory in the High Energy Accelerator 2234 S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori Research Organization (KEK-PF). The X-rays were emitted 2.4 Nitridation procedure with H2O at room temper- from the bending magnet, and the photon energy was tuned ature by an InSb (111) double crystal monochromator. The energy Nitridation procedure of Li/Cu target was illustrated in resolution of the monochromator was 0.9 eV at 2000 eV. Figs. 1 and 3 shows the gas mixing apparatus connected in The analysis chamber consisted of a manipulator, an the main chamber, in which Li/Cu specimen was installed electron energy analyzer, and a cold cathode ion gun. The and N2 gas and H2O was supplied from a bottle of base pressure of the analysis chamber was 1 © 10¹8 Pa. The compressed nitrogen and a glass test tube, respectively. preparation chamber consisted of a vacuum evaporator and a Partial pressure of N2 gas and H2O was measured by Pirani sample transfer system. The base pressure of the preparation gage and N2/H2O mixing gas was squirted out of the nozzle chamber was 1 © 10¹6 Pa. The sample can be transferred to the Li/Cu target under the pressure combination of 0.01­ between two chambers without exposing the sample to air. 101.3 Pa/0­4 Pa. Exposing time of the Li/Cu was controlled XPS spectra were measured with hemispherical electron within 5­60 min. energy analyzer (VSW Co. Class-100). The X-rays were irradiated at 55 degree from surface normal and a take-off 3. Results and Discussion direction of photoelectrons was surface normal. Typical photon energy used was 2000 eV. An X-ray tube with yttrium 3.1 Chemical conditions of lithium deposition surface anode (Y M¦ line, h¯ = 132.3 eV) was also used to measure Figures 4(a) and 4(b) show XPS scan spectra for the Li 1s lines. The binding energy was normalized by C 1s of copper surface before and after Li deposition, respectively. adventitious organic carbons adsorbed on the samples at The pressure during the deposition was 3 © 10¹4 Pa and the 284.8 eV. deposition time was 20 min. Narrow scan in Li 1s region after Li-deposition is also shown as small inset in Fig. 4(b). After 2.3 Lithium deposition the deposition, the intensity of the Cu 2p peak from the The evaporator consisted of a tantalum crucible sur- copper substrate decreased, and O 1s, C 1s and Li 1s peaks rounded by the spiral type tungsten filament. The crucible were observed. Here, this spectra pattern (Fig. 4(b)) with low was floated at +1.5 kV, while the filament was grounded. Therefore, the crucible was heated by the bombardment of 1.5 keV electrons. The distance between the crucible and the substrate was 50 mm. A shutter that is electrically isolated from the ground was equipped between the crucible and the substrate in order to precisely control the evaporation rate of the source material. Since a part of the evaporated lithium atoms is ionized due to the surface ionization, a positive current was observed at the shutter. The thickness of the film was precisely determined by the product of the shutter current and the evaporation time that was calibrated by XPS measurements. The vacuum pressure during the lithium deposition was 1.3 © 10¹4 Pa, and the deposition time of lithium was 50 min. Figure 2 shows experimental situation of lithium deposition on Cu specimen in the main chamber. Fig. 2 Lithium deposition situation in preparation chamber. Fig. 3 Main chamber with N2/H2O gas mixing apparatus. Low Temperature and Pressure Synthesis of Lithium­Nitride Compound with H2O Addition on Lithium Target for BNCT 2235 (a) (b) Fig. 5 XPS semi wide-scan spectra for the Li/Cu target exposed to N2 gas; (a) 0, (b) 5 and (c) 60 min and was categorized as pattern B. Table 1 Testing conditions of nitridation synthesis on lithium target. H O Exposing N pressure 2 Categorized 2 pressure time I /I (Pa) pattern N1s O1s (Pa) (min) 101.3 0 5, 60 B 0.3­3 101.2 0.1 5 B 2 80 1.33 5 C 0.03 27 4.7 5 D ³0 13.3 4.0 5 D ³0 Fig. 4 (a) XPS wide-scan spectra for copper surface, (b) XPS wide-scan 0.027 0.005 5 A 0 spectra for copper surface after the Li deposition and was categorized as 0.013 0.004 5 A 0 pattern A. 0.013 0 5 A 0 level of O and C contamination without N1s peak is categorized as pattern A. The higher intensities of the O 1s Figure 5 shows the XPS semi-wide scan spectra of the Li/ and C 1s peaks compared with that of the Li 1s peak is due to Cu target after 101.3 Pa N2 gas squirt for 5 and 60 min. O 1s, the extremely low photoionization cross sections of Li 1s by N 1s and Li 1s peaks were observed and the intensity of the 2000 eV photons. In the previous work,4,5) we have shown O1s peak decreased with nitridation time, whereas N1s peak that O 1s and C 1s peaks for the Li-deposited sample come increased.