J. Chem. Chem. Eng. 8 (2014) 870-875
doi: 10.17265/1934-7375/2014.09.005
L3-Edge Jump and Shift on White-Line of Pd Interlayer
for Trilaminar Neutron Production Target under H2+ Irradiation
Shintaro Ishiyama1*, Ryo Fujii2, Masaru Nakamura2 and Yoshio Imahori2
1. Quantum Beam Science Center, JAEA (Japan Atomic Energy Agency), Ibaraki 319-1195, Japan
2. Cancer Intelligence Care System, Incorporated, Tokyo 135-0063, Japan
Abstract: Interlayer Pd for the Li/Pd/Cu neutron target for BNCT (boron neutron capture therapy) was characterized after 0.1-5 keV H2+ irradiation by XAFS (X-ray absorption fine structure) technique, and following conclusions were derived: (1) from
the XAFS obrvation of white line of Pd, remarkable Pd L3edge jump was found in 1.1-3 times higher than before irradiation in low irradiation fluence; (2) this fact indicates increa of hole density in Pd 4d-band, whereas, no change was obrved for XASF spectra
of Ag sample under the same irradiation conditions; (3) remarkable Pd L3edge shift of 0.12-0.66 eV was also found with increa of
H2+ irradiation energy in low fluence, and drastically decread after peak in high irradiation energy and fluence; (4) implanted protons deposited in Pd as negative under the balance of electron population enhanced by proton irradiation and charge transfer.
Key words: Boron neutron capture therapy, lithium target, palladium, X-ray absorption fine structure, H2+.
1. Introduction
Many types of pilot innovative accelerator-bad neutron sources for neutron capture therapy were propod [1-6] and the Li/Pd/Cu trilaminar target model was propod[3-7] to avoid the irradiation blistering of the lithium target and adopted by 25 kW BNCT (boron neutron capture therapy) implement deployed in National Cancer Centre, Japan in 2013.
Previously, white-line edge jump and shift of Pd interlayer was reported [7] by the follow-up studies concerning detailed characterization of this type of target model under 3 keV H2+ irradiation (Fig. 1).
There is no report about white-line jump and shift of the materials under irradiation, thus, prent paper characterized Pd interlayer for the Li/Pd/Cu target model after H2+ irradiation by XAFS (X-ray absorption fine structure) using X-rays from synchrotron light source.
*Corresponding author: Shintaro Ishiyama, Dr., rearch field:materialscience.E-mail:*************************.jp.2. Experiments
2.1 Specimens
Metallic palladium foil (10 mm × 10 mm × 0.1 mm) and Ag foil (10 mm × 10 mm × 0.1 mm) with 99.9% was ud in prent experiment.
2.2 Apparatus
Experiments were performed at the BL (beam line)-27A station of the KEK-PF (Photon Factory in the High Energy Accelerator Rearch Organization) [3-4]. The X-rays were emitted from the bending magnet, and the photon energy was tuned by an InSb (indium-stibium)(111) double crystal monochromator. The photon energy of the beam line covers from 1.8 keV to 4.2 keV, and the typical photon flux was ~ 1010 photons·cm-2·s-1. The energy resolution of the monochrometer was 0.9 eV at 3,000 eV.
The analysis chamber consisted of a manipulator, an electron energy analyzer, and a cold cathode ion gun. The ba pressure of the analysis chamber was
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L 3-Edge Jump and Shift on White-Line of Pd Interlayer for Trilaminar Neutron
Production Target under H 2+
Irradiation
871
E (photon energy) (eV) E (photon energy) (eV)
(a) (b)
Fig. 1 Pd L 3-edge XAFS spectra for the sample (a’) before and (b’) after H 2+ irradiation: (a) edge jump; (b) edge shift.
1 × 10-8 Pa. The sample was horizontally located, and
it can be rotated around the vertical axis. The preparation chamber consisted of a vacuum evaporator and a sample transfer system. The ba pressure of the preparation chamber was 5 × 10-6 Pa. The sample can be transferred between two chambers without exposing the sample to air.
The X-rays were irradiated at 55 degree from surface normal and a take-off direction of photoelectrons was surface normal. Typical photon energy ud was 2,000 eV. The binding energy was normalized by Carbon 1 s of adventitious organic carbons adsorbed on the samples at 284.8 eV.
The XAFS spectra were measured by plotting a sample drain current as a function of photon energy. The sample current was normalized by the photon flux measured by the drain current of aluminum foil located in front of the sample.
2.3 H 2+
Irradiation
The targeted sample surface was bombarded with
hydrogen ions in the analysis chamber using a cold-cathode ion gun (OMEGATRON Co. OMI-0045CK) [7]. High-purity hydrogen gas (>
99.99%) was ud as an ion source. Most of the produced ions were molecular ions, H 2+. The energy
of the H 2+ ions was 0.1-5.0 keV. The typical ion flux was 1.4 × 1014 atoms·cm -2·s -1, and the pressure during the ion bombardment was 1.2 × 10-3 Pa. The direction of the ion beam was surface normal.
3. Results
3.1 White-Line for Pd Interlayer after H 2+ Irradiation
Fig. 2 shows the normalized Pd L 3-edge XAFS spectra for the sample before (①) and after irradiation (②, ③). A sharp peak is obrved at 3,171.0 eV (marked C) for metallic palladium before
H 2+ irradiation. This peak is so-called “white-line” corresponding to the dipole transition from the Pd 2p 3/2 to the valence unoccupied 4d orbital just above the Fermi level [8].
There are many peak points obrved in higher energy more than that of point C. This fact indicated that EXAFS (extended X-ray absorption fine structure) data provide information about the local structure, such as the nature and number of surrounding atoms and inter-atomic distance surrounding palladium atoms in the energy regions.
It is clear that the intensity of the absorption edge has incread with respect to that before H 2+
irradiation
(edge jump). This edge jump can be directly related to N o r m a l i z e d a b s o r p t i o n (a r b .u .)
N o r m a l i z e d a b s o r p t i o n (a r b .u .)
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L 3-Edge Jump and Shift on White-Line of Pd Interlayer for Trilaminar Neutron
Production Target under H 2+
Irradiation
872
E (photon energy) (eV)
Fig. 2 Pd L 3-edge XAFS spectra for the sample after H 2+ irradiation: ① 0 keV; ② 0.1 keV; ③ 3 keV and ④ 5 keV.
an increa in the number of holes in the d band [9].
Edge shift to higher energy is also clearly obrved in Fig. 2 and this shift implied that positively charged palladium atom and the energy shift is corresponding to that of metallic gadolinium 0.9 eV [10].
Previous study reported that many peak points obrved in higher energy regions gradually disappeared with increasing of irradiation time and this suggests the dominant prence of a low Z scatter (i.e., hydrogen) [10], which are interstitial solute atoms in fcc (face-centered cubic) palladium structure.
Normalized absorption and peak energy data obtained in prent study were provided with irradiation conditions in Table 1 where the normalized absorption was defined by the ratio of I p (peak count rate) and I 0 (grand level count rate) in XAFS spectra.
3.2 Ag Sample after H 2+ Irradiation
Fig. 3 shows Ag (next to Pd in the periodic table) L 3-edge XAFS spectra before and after H 2+ irradiation. There are many peaks with not clear sharp except d , s and p orbit edge peaks.
After 0.1-5 keV H 2+ irradiation, no remarkable edge jump and shift are found around higher energy regions. The results implied that there is no change in the number of holes in the d band and positively charged
河南中招考生服务平台Table 1 The normal absorption and peak energy before and after H 2+ irradiation. H 2+ energy (keV) Current (μA)
Time (min) I p /I 0
Peak energy
(eV) 0 0 0 1 3,171.0 0.1 19 10 1.02 3,171.12 0.1 18.1 95 1.085 3,171.40 0.1 17 145 1.085 3,171.42 3 27.5 2 2.8 3,171.15
3 27.5 10 2.86 3,171.3 3 27.5 60 2.89 3,171.55 3 27.5 180 2.90 3,171.6 5 25 2 1.572 3,171.66
5 25 10 0.784 3,171.22 5 54 20 0.11 3,171.26
E (photon energy) (eV)
Fig. 3 Ag L 3-edge XAFS spectra before and after H 2+ irradiation.
Ag atom. Within the single particle approximation, the oscillator strength of the white line of the L 3 edge of the d transition metals would be proportional to the hole population in the d -band [10]. Actually, in the ca of Ag, the d bands are esntially full and no white line is obrved at the L -edge.
4. Discussion
The Li/Pd/Cu trilaminar Li target consisted with Pd interlayer is irradiated by high energy and flux protons during neutron production for BNCT, thus, irradiation effects on proton accelerated energy are discusd here.
4.1 Edge Shift of Pd White-Line
Fig. 4 shows Pd L 3 edge shift after H 2+ irradiation.
N o r m a l i z e d a b s o r p t i o n (a r b .u .)
N o r m a l i z e d a b s o r p t i o n (a r b .u .)
Pd
Ag
① 0 keV
② 0.1 keV ③ 3 keV ④ 5 keV
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营销宣传L 3-Edge Jump and Shift on White-Line of Pd Interlayer for Trilaminar Neutron
Production Target under H 2+
Irradiation
873
The edge shift incread with irradiation fluence and
saturated for 0.1 keV and 3 keV irradiations [7],
whereas, the edge shift decread after peak in the
ca of 5 keV. Here, Region I was defined as the range of fluence less than 1017 n and Region II was higher fluence range. In Region II, remarkable decrea of the intensity change was obrved in the ca of 5 keV. This is due to the sputtering
enhanced by higher proton energy irradiation [3-5] and caus the new formation of irradiation surface, which disturbs edge jump and shift under irradiation.
The relationship between the peak energies and
irradiation energy is shown in Fig. 5. The edge shift
increas with irradiation energy in Region I.
The difference between the energy at half of maximum of the absorption edge spectra in Fig. 2 is found to be 0.12-0.66 eV in this region, whereas, the edge shift drastically decread in Region II after peak in the ca of 5 keV.
The edge shifts implied yield of positively
charged palladium atoms, and we assumed that the
three conduction electrons are transferred to hydrogen atoms implanted in Pd metallic [10]. This fact suggests the charge transfer from the palladium atom to the hydrogen atoms and
H-transformation/deposition in Pd.
4.2 Edge Jump of Pd White-Line
Fig. 6 shows the Pd L 3 edge jump in white-band of Pd after H 2+ irradiation as a function of fluence with irradiation energy. In the Region I, the L 3 edge peaks immediately incread up to about 1.1 and 3 times higher than before irradiation and then saturated in the ca of 0.1 eV and 3 eV irradiati
on. The edge jump drastically decread after peak in the ca of 5 keV. Fig. 7 shows the relationship between the edge jump and irradiation energy of Pb in the Regions I and II. There is a peak near-by 3 keV in the edge jump and the values drastically decrea with increasing irradiation energy.
面板开关1E 18 2E 18 4E 18 6E 18
H 2+ implantation fluence (n)
Fig. 4 The change of edge sift of Pd interlayer after H 2+
irradiation as a function of fluence with irradiation energy.
E 0 (H 2+ irradiation energy) (keV)
Fig. 5 The relationship between the edge sift and
irradiation energy in the Regions I and II.
楚天极目
1E 18 2E 18
4E 18 6E 18
凉夏H 2+ irradiation fluence (n)
测绘报告
Fig. 6 The change of edge jump of Pd interlayer after H 2+ irradiation as a function of fluence with irradiation energy.
The intensity change of the white line feature is bad on the change of the d -occupancy of Pd sites and the L 3 absorption spectra in Fig. 2 indicates a
Pd
Pb Pb
E (p e a k e n e r g y ) (e V )
E (p e a k e n e r g y ) (e V ) 0.1 keV
3 keV 5 keV 5 keV
0.1 keV
3 keV
I p /I 0 (n o r m a l i z e d j u m p ) (a r b .u .)
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L 3-Edge Jump and Shift on White-Line of Pd Interlayer for Trilaminar Neutron
贴息是什么意思Production Target under H 2+
Irradiation
874
E 0 (H 2+
irradiation energy) (keV)
Fig. 7 The relationship between the edge jump and irradiation energy in the Regions I and II.
Fig. 8 Illustration of hydrogen and proton diffusivity in Pd: (a) gaous hydrogen; (b) 0.1 keV; (c) 3 keV and 5 keV proton in Region I and (d) 5 keV proton in Region II.
increa in the number of unoccupied states of Pd d -character upon fluence and irradiation energy. Also, the d -character is determined with nine and one stray electrons in d -occupancy and it is considered that irradiation proton enhances dissociation of the stray electrons from d -band and enhances population of H - by the charge transfer above-mentioned in Section 4.1. Fig. 8 illustrates the diffusion process of gaous hydrogen and irradiation proton into Pd. Hydrogen diffus from the surface of Pb with charge transfer and recombines on the opposite Pd surface (Fig. 8a). The irradiation protons implanted under the Pd surface accept charge transfer from the dissociated stray electrons in Pd and changed to H -, whereas, other protons behave as hydrogen in the ca of 0.1 keV
(Fig. 8b). The deposition of H - in Pd enhances with increa of irradiation energy (Fig. 8c), but the conditions are disturbed by sputtering at high fluence and energy irradiation (Fig. 8d).
5. Conclusions
For the purpo of avoiding the radiation blistering of the lithium target in BNCT device, palladium thin layer was inrted between copper substrate of the trilaminar lithium target.
In prent study, Pb interlayer was characterized after 0.1-5 keV H 2+ irradiation by XAFS technique,
which provides structural/electronic properties of solids, and information about the local structure, such as the nature and number of surrounding atoms and inter-atomic distances, and the following results were derived:
(1) From the XAFS obrvation of white line of Pd, remarkable Pd L 3 edge jump was found with increa of H 2+ irradiation energy less than 3 keV, and this fact indicates increa of hole density in Pd 4d -band and increa of free electron population in Pd, whereas no change was obrved from XASF spectra of Ag sample under the same irradiation conditions;
(2) Remarkable Pd L 3 edge shift to higher energy for Pd sample was also found with increa of H 2+ irradiation energy, and this fact indicates charge transfer from Pd to hydrogen atoms;
(3) Implanted protons deposited in Pd as negative under the balance of electron population enhanced by proton irradiation and charge transfer;
(4) Sputtering disturbance was obrved and critical over 3 keV H 2+ irradiation.
References
[1] Bayanov, B.; Belov, V.; Kindyuk, V.; Oparin, E.;
Taskaev, S. Appl. Radiat. Isot. 2004, 61, 817-821.
[2] Bayanov, B.; Burdakov, A.; Chudaev, V.; Ivanov, A.;
Konstantinov, S.; Kuznesov, A. Appl. Radiat. Isot. 2009, 67, 285-287.
[3] Ishiyama, S.; Baba, Y.; Fujii, R.; Nakamura, M.; Imahori,
Y. Nucle. Inst. and Method in Physics Rearch 2012, B288, 18-22.
I p /I 0 (n o r m a l i z e d j u m p ) (a r b .u .)
Hole population in d -band and e - charge transfer to H
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