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. This spectra pattern is here categorized as from the water and carbonates adsorbed on Li surface after pattern B. the Li-deposition, and the main chemical states of lithium is The binding energy of the peak of N 1s and Li 1s is not Li2O but metallic lithium. identified as 391.3 and 54.5 eV in the figure. From the results The considerable decrease of the Cu 2p peak after the of previous work, the binding energy of the Li 1s peak is Li-deposition suggests that copper surface was covered with 52.8 eV due to the metallic Li and 2.3 eV chemical shift was fairly thick film of lithium. The photon energy used for XPS observed after nitridation. So, we assigned that the Li 1s peak measurements was 2000 eV, so the kinetic energy of the Cu observed in Fig. 5 originated from lithium nitride compound. 2p photoelectrons was about 1070 eV. Considering that the Figure 6 shows XPS wide scan spectra of Li/Cu target inelastic mean free path (IMFP) of 1070 eV electrons in solid after 80 Pa/1.3 Pa N2/H2O mixing gas squirt for 5 min and lithium is about 1.3 nm,4,5) it is suggested that the thickness of very higher intensity of the O 1s and C 1s peaks were the lithium layer was fairly larger than this value. observed with very small peak of Li 1s. This spectra pattern is categorized as pattern C. Remarkable contamination by O 3.2 Contamination on the lithium­nitride compound and C was observed in this case, we discuss the contami- surface nation level on the nitridation conditions and use the intensity 6­8) According to the results, the presence of H2OinN2 gas of the photoelectrons, I and intensity ratio, IN1s/IO1s as is significant to promote nitridation of Li and believed to be contamination parameter, and are listed in Table 1. From assistant agent in nitridation chemical reaction between Li the table, it is found that the ratio IN1s/IO1s decreases with and N2 gas. Therefore, to establish a role of H2O addition increase of H2O pressure above 13.3 Pa N2. These results to these reactions, N2/H2O mixing gas was squirted to the mean that there is a progression of contamination by O and C Li/Cu target surface in present study. on Li­N compound surface with H2O addition. 2236 S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori

Fig. 6 XPS spectra of Li specimens after 80 Pa/1.3 Pa N2/H2O squirt for 5 min. N1s peak was clearly observed after nitridation with O and C contamination and was categorized as pattern C.

Fig. 8 XPS spectra of Li specimens before nitridation. Remarkable contamination by O and C was observed with Li1s peak and was categorized as pattern D. Fig. 7 Li/Cu targets after N2/H2O squirt; (a) after Li deposition, (b) 80 Pa/ 1.3 Pa N2/H2O mixing gas squirt and (c) 1­3 min after squirt.

A few 10 s after 80 Pa/1.3 Pa N2/H2O mixing gas squirt, the colour of Li/Cu surface was immediately changed from silver to black, then black colour was faded into transparent colour with white-tinged within few minutes, as shown in Fig. 7. In the case of 27 Pa/4.7 Pa N2/H2O mixing gas exposure for 5 min, the same spectra pattern as pattern C was obtained except N1s peak shown in Fig. 8 and this pattern with high level of contamination by O and C without N1s peak is categorized as pattern D. These results indicated that there are two types of reaction processes between lithium, H2OandN2. At first, N2 gas causes a chemical reaction with lithium as;

6Li þ 3N2 ) 2Li3N ð1Þ

After then, Li3N was decomposed in the presence of Fig. 9 Contamination process of the lithium­nitride compound surface; carbonates above-mentioned in Section 3.1. observed by XPS spectra of (a) pattern A, (b) pattern C and (c) pattern D.

2Li3N þ 3CO2 þ 3H2O ) 3Li2CO3 þ 2NH3 ð2Þ So, we tentatively assign that the O1s, C1s and Li1s peaks lithium target (pattern B), the surface of the Li­N compound observed in pattern B, C and D originate from Li2CO3. is partially over-layered by Li2CO3 with H2O addition These contamination processes are illustrated in Fig. 9. (Pattern C) and these over-layers expand on entire surface After formation of lithium­nitride compound on the surface of of Li­N compound by excessive addition of H2O (pattern D). Low Temperature and Pressure Synthesis of Lithium­Nitride Compound with H2O Addition on Lithium Target for BNCT 2237

On the contrary, the same spectra pattern as pattern A with to be assistant agent in nitridation chemical reaction between lower intensity of O 1s and C 1s peaks was obtained in the Li and N2 gas at room temperature. case of 0.013­0.027 Pa/0­0.05 Pa N2/H2O. These results mean that there is no contamination and nitridation reaction Acknowledgement expressed in eqs. (1) and (2) due to inadequate supply of N2 and H2O. The authors would like to thank the staff of the KEK-PF for their assistance throughout the experiments. They also 4. Conclusions thank the members of the Surface Reaction Dynamics Research Group, Quantum Beam Science Directorate, Japan To prevent vaporization damage of BNCT (Boron Neutron Atomic Energy Agency for their helpful discussion and Capture Therapy) lithium target during operation, synthesis experimental supports. The work has been conducted under of lithium­nitride compound was conducted on the lithium the approval of Photon Factory Program Advisory Commit- target by N2/H2O mixing gas squirt in the ultra high vacuum tee (PF-PAC 2012G175). chamber and the structures, chemical states of nitridated zone formed on the lithium surface were characterized by XPS, and the following results were derived; REFERENCES (1) Lithium­nitride compound was synthesized on lithium target under 101.3 Pa N2 gas squirt at room temperature and 1) B. Bayanov, V. Belov, V. Kindyuk, E. Oparin and S. Taskaev: Appl. 1 © 10¹8 Pa in the ultra high vacuum chamber. Radiat. Isot. 61 (2004) 817­821. (2) Remarkable contamination by O and C was observed 2) C. Willis, J. Lenz and D. Swenson: Proc. LINAC08, Victoria, BC, ­ Canada, MOP063, (2008) pp. 223­225. on the lithium nitride compound over-layer synthesized by 3) S. Halfon, M. Paul, A. Arenshtam, D. Berkovits, M. Bisyakoev, I. squirt with N2/H2O mixing gas under the pressure combi- Eliyahu, G. Feinberg, N. Hazenshprung, D. Kijel, A. Nagler and I. nation of 13.3­80 Pa/1.33­4.7 Pa. Silverman: Appl. Radiat. Isot. 69 (2011) 1654­1656. (3) No contamination and synthesis of Li­N compound 4) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Nucl. Instrum. Meth. Phys. Res. B 288 (2012) 18­22. was observed under 0.013­0.027 Pa/0­0.005 Pa N2/H2O 5) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Nucl. squirt. Instrum. Meth. Phys. Res. B 293 (2012) 42­47. (4) Contamination of O and C was enhanced with 6) T. Yamamoto, S. Yoshikawa and M. Koizumi: Yogyo-Kyokai-Shi 93 excessive addition of H2O over the pressure of 1.33 Pa. [11] (1985) 68­71 (in Japanese). (5) XPS observation suggested that Li2CO3 over-layered 7) M. Hasegawa, T. Sekine and N. Nakayama: Report of National Industrial ­ on the surface of Li­N compound. Research Institute of Nagoya, Vol. 28, No. 12 (1979) pp. 388 393. 8) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Mater. From these results, it is concluded that lithium is very Trans. 54 (2013) 1765­1769. sensitive to the existence of H2O in nitrogen gas, however no 9) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Mater. evidence was found to support the fact that H2O is believed Trans. 54 (2013) 1760­1764.