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Optical radiation from the sputtered species under gas excitation by the products of the 6Li(n,α)3H nuclear reaction
Journal Pre-proof Optical radiation from the sputtered species under gas excitation by the products of 6 3 the Li(n,α) H nuclear reaction Erlan Batyrbekov, Mendykhan Khasenov, Yuriy Gordienko, Kuanysh Samarkhanov, Yuriy Ponkratov PII:
S0022-2313(19)31792-2
DOI:
https://doi.org/10.1016/j.jlumin.2019.116973
Reference:
LUMIN 116973
To appear in:
Journal of Luminescence
Received Date: 11 September 2019 Revised Date:
2 December 2019
Accepted Date: 15 December 2019
Please cite this article as: E. Batyrbekov, M. Khasenov, Y. Gordienko, K. Samarkhanov, Y. Ponkratov, 6 3 Optical radiation from the sputtered species under gas excitation by the products of the Li(n,α) H nuclear reaction, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116973. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Optical radiation from the sputtered species under gas excitation by the products of the 6Li(n,α)3H nuclear reaction Erlan Batyrbekova, Mendykhan Khasenovb*, Yuriy Gordienkoс, Kuanysh Samarkhanovс, Yuriy Ponkratovс a National Nuclear Center of the Republic of Kazakhstan, 2 Beybit Atom St., 071100, Kurchatov, Republic of Kazakhstan b National Laboratory Astana, 53 Kabanbay Batyr Ave., 010000, Nur-Sultan, Republic of Kazakhstan c Institute of Atomic Energy Branch of the National Nuclear Center of the Republic of Kazakhstan, 10 Beybit Atom St., 071100, Kurchatov, Republic of Kazakhstan *Corresponding author. E-mail address: [email protected] Abstract The emission spectra during alkali metal sputtering into noble gases excited by the products of Li(n,α)3H nuclear reaction were studied. Lithium in the form of a capillary-porous structure (CPS) was heated up to a temperature of 730 K. At a temperature in the 520-620 K range, depending on a type of gas medium, intense radiation was observed for lines of lithium and lines of sodium and potassium found as impurities in lithium. The process activation energy (155-161 kJ/mol) obtained from the dependence of the optical radiation intensity on the lithium layer temperature is in good agreement with the lithium evaporation energy values. The vapour density that is significantly higher than that saturated lithium vapour under normal thermal heating was created by the αparticles and tritium nuclei released from the lithium layer and when the opposite wall was bombarded. Excitation of the sputtered lithium atoms occurs as the result of the plasma-chemical reactions in the gas. The Penning process for the lithium atoms is suggested to be the main channel for the excitation transmission from the noble gases to the lithium atoms. 6
Keywords: Nuclear reaction; Sputtering; Emission; Alkali metal; Mechanism of excitation 1.
Introduction
Investigations of the optical radiation of gas mixture plasma formed in the nuclear reactor core is of interest for designing ionizing radiation detectors and the devices that directly convert nuclear reaction energy into light. Uranium fission fragments, as well the products of 3He(n,p)3H and 10 B(n,α)7Li nuclear reactions were used in the nuclear reactor for gas ionization and excitation [1]. Products of the 6Li(n,α)3H nuclear reaction were used in [2, 3] for gas excitation in the study of the luminescence spectra at ~300 K. The quite long path length of tritium nuclei in lithium and gas medium allows the excitation of a large volume of gases and provides a large amount of power contained by the gas compared to the 10B(n,α)7Li reaction products. The temperature increases of the gas medium up to ~500 K leads to the appearance of alkali metal lines in the emission spectra [4]. Luminescence spectra of lithium sputtering of the target irradiated with Xe+ ions in vacuum at room temperature were studied in [5]. It was found that the processes leading to light emission during metal sputtering into a gas medium [4, 6] differ from the sputtering processes in the vacuum [7].
Cadmium and zinc sputtering in the gas medium during irradiation by the products of nuclear reactions and electron beam were studied in [6, 8, 9]. The intensity of cadmium and zinc lines during the sputtering increased sharply at temperatures above 430 and 460 K, respectively. The mechanism of the excited atoms release from the metal was studied in [6], and the temperature dependence of the luminescence intensity was explained by the droplet model. A brief description of the droplet model and its discussion will be presented in Section 4. The present paper examines the emission spectra of noble gases and alkali metals sputtering from the lithium layer in the nuclear reactor core at temperatures up to 730 K. 2.
Experimental Setup
The study was carried out at the IVG.1M nuclear reactor [10] at a thermal neutron flux density of up to 1.4·1014 n/cm2s. Ionization and excitation of the gas medium was generated via nuclear reaction products: 6
Li + n → 4He (2.05 MeV) + 3H (2.73 MeV)
(1)
The scheme of the in-pile experimental device is shown in Figure 1. The experimental cell is made of a stainless-steel pipe with a length of 900 mm and an inner diameter of 20 mm. A flange with an optical vacuum input (1) plugs the top part of the pipe. In [4], lithium was applied to the walls of the experimental cell by heating up to 570 K, and liquid lithium in a vacuum spread evenly over the surface and was retained due to the surface tension force. In the present work, the lithium stabilized in a matrix of capillary-porous structure [11] was used. The woven stainless-steel mesh was placed on the inner side of the experimental cell; the dimensions of the porous cell were 0.3×0.3 mm with a thickness of 0.1 mm. Lithium (1.55 g, LE-1 grade) was purified by vacuum distillation and gettering. Further, the sample of lithium CPS was prepared as follows: the cell with CPS was heated under conditions of oil-free pumping (up to a temperature of 570 K) and in an atmosphere of purified argon (up to a temperature of 1030 K). As a result, liquid lithium was evenly distributed over the porous structure of the mesh. Lithium with a natural composition (7.5% of 6Li) and an impurity content of less than 0.1% was used. The height of the applied lithium layer (2) was 200 mm. The experimental device was loaded into the dry physical experimental channel of the IVG.1M reactor core with a height of 800 mm. In a different circle of the IVG.1M reactor, the middle of the lithium layer was located in the centre of reactor core, either 100 or 150 mm above the centre, corresponding to the average height of the layer, at a thermal neutron flux density of 1.4·1014, 1014, and 5·1013 n/cm2s. The light radiation output was observed via a collimator with a quartz lens (3) located inside the experimental device and connected to a fibre optic cable (4) through an optical vacuum input. The light radiation from the experimental cell through a 10 m-long fibre was directed to the input of the QE65Pro (Ocean Optics) spectrometer located in the experimental hall of the IVG.1M reactor. Luminescence spectra in the wavelength range of 300-970 nm were recorded with the integration time from 0.01 up to 10 s and an optical spectrometer resolution of 1.5 nm. A USB2000+ spectrometer (Ocean Optics) was connected in parallel to record 980 nm xenon line.
Fig. 1. Scheme of the experimental device. 1 – flange with vacuum optical input, 2 – CPS lithium layer, 3 – optical collimator, 4 – heaters, 5 – thermocouples, 6 – cadmium, 7 – flange for gas outlet and inlet.
The measurements were conducted at a constant IVG.1M reactor thermal power of 6 MW. Heating of the experimental device was implemented due to radiation heating of the steel pipe and energy release in the lithium layer, as well as ohmic heating. The temperature of the experimental device was controlled by changing the heater power and, if it was necessary to reach a certain temperature, by changing the nitrogen gas flux in the external blowing of a tube. During the experiment, the outer surface temperature was measured for 4 points at the experimental device height. In measurements with krypton, the temperature of the experimental cell and casing was maintained at the same height up to 620 K, and with further temperature increases of the lithium layer, the temperature in the experimental device casing remained equal to 620 K in order to avoid overheating of the collector with a quartz lens. In the subsequent reactor cycles, because the experimental cell must be located above the reactor core, the temperature in the top part of the experimental device casing was 50 K lower for the measurements with xenon and 150 K lower for
the measurements with neon and argon, compared with an experimental cell. The volume of the experimental device was degassed in the reactor core at 410 K temperature and then "flushed" several times with the tested gas prior to filling the cell, and gases with an impurities content of <0.001% were used. The “Liana” test-bench vacuum system was used for gas pumping and supplies [10]. A piece of cadmium (1.42 g) was placed on the bottom of the cell. It was assumed that it would be possible to compare two mechanisms of the formation of optical radiation during thermal evaporation of cadmium and during sputtering of a layer of lithium, respectively. However, the radiation intensity at the brightest lines of the cadmium atom (508.6 nm) and the cadmium ion (441.6 nm) did not exceed 200 rel. units. Possibly, the low intensity of cadmium lines is associated with the formation of a compound of cadmium with lithium. 3. Results Figure 2 shows the emission spectra during krypton excitation. Lithium lines dominate in the emission spectra from an experimental cell filled with krypton. In addition to krypton lines, sodium and potassium lines and a weak cadmium line are observed. No alkali metal lines are observed in the absence of the gas in the experimental device.
Fig. 2. Emission spectra of krypton at a pressure of 45 kPa (1) and irradiation from an experimental cell (2) at the residual pressure of ~10 Pa. The thermal neutron flux density is F=1.4·1014 n/сm2s, and the lithium layer temperature is 623 К. The integration time is 0.01 s (1) and 10 s (2). Line (2) shifted 2000 counts down.
In the lithium spectra, lines are also observed at 413.3 nm (2p-5d transition), 391.5 nm (2p6d), and 379.5 nm (2p-7d) in addition to the lines shown in Figure 2. Emission in the resonance lines of lithium at 670.8 nm was “trapped”. Radiation trapping consists in multiple reemission and reabsorption of resonance photons in an optically dense medium. In resulting photon flux, extending
beyond the system, not the most numerous (but also the most strongly absorbed photons) photons from center of the line predominate, but relatively few photons from distant line wings, which free path is comparable to the size of the system. In our experiments, a broadening of lithium resonance line was observed; in helium at a temperature of 620–670 K, the self-reversion of the line at 670 nm also was observed. Emission in the resonance lines of sodium at 589 and 589.6 nm (3s-3p transition) was also trapped, and sodium lines are also observed at 568.3 (3p-4d) and 819.5 nm (3p-3d) in the spectra. The 766.5 and 769.9 nm (4s-4p) and 404.4 and 404.7 nm (4s-5p) resonance lines are also observed in the potassium spectra. Sodium and potassium are found in lithium as impurities with chemical compositions of 0.04 and 0.005%, respectively. The amount of sodium and potassium may change during eight reactor cycles measurements of 3 hours each. The cadmium atom spectra are represented by the lines of the cadmium triplet (467.8, 480, and 508.6 nm) and by the lines at 326.1, 361 and 643.8 nm. A 441.6 nm cadmium ion line was observed only in helium and neon, and the 533.7 and 537.8 nm weak lines are found only in helium. The lines of the np-ns transitions (where n=3, 4, 5, and 6, for neon, argon, krypton, and xenon, respectively) dominate in the noble gases’ spectra, and there are also a number of 3p-3d transition lines in neon. The weak continuum in the region from 570–770 nm with neon to 570–950 nm with xenon corresponds to the A–X transitions of the Li–R molecules, where R is a noble gas atom [12]. Figure 3 shows the dependence of the elements’ characteristic line intensity on the lithium layer temperature. The intensities are given in relative units, without correction for the spectral sensitivity of the experimental facility. The peak intensity values, not corrected for line broadening related to the self-absorption are shown for the lithium and sodium resonance lines.
a)
b)
c)
d) Fig. 3. Dependence of the intensity of the lines of atoms of alkali metals and noble gases in neon (a), argon (b), krypton (c) and xenon (d) on the lithium layer temperature. The initial gas pressure at 420 K: Ne – 100, Ar – 88, Kr – 41, Xe – 37 kPa. The thermal neutron flux density: Ne, Ar – 5·1013, Xe – 1014, Kr – 1.4·1014 n/cm2s. The intensity of 763.5, 826.5, and 912.3 nm (b); 766.5 nm (c); and 590 and 766.5 nm (d) lines was increased 5 times; the intensity of 837.8 nm (3p-3d transition) (a) line was increased 10 times.
4. Discussion Figure 2 shows that the intensity of lithium and sodium lines while sputtering into a vacuum is more than 104 times smaller than the intensity of the line while sputtering into a gas medium. The specificity of sputtering into the gas medium shows a sharp increase in the alkali metal line intensity at temperatures higher than 520, 570 and 620 K in krypton, xenon, and argon or neon medium, respectively. Figure 4 shows the temperature dependence of the luminescence intensity of lithium in the (1/kT, lnI) coordinates, where k is the Boltzmann constant. The rapid increase in the luminescence intensity is well approximated by:
~exp
(2)
where A is the activation energy of this process. The A values obtained for 610.4 nm lithium lines are 1.67 eV in argon, 1.63 eV in neon, and 1.61 eV in xenon. These values are in good agreement with the evaporation energy of lithium, which is equal to 1.63 eV (156.9 kJ/mol [13]), and are very different from the activation energy during lithium self-diffusion (55.2 kJ/mol [14]). Since the saturation vapour pressure of lithium is small (4·10-6 Pa at 520 K and 0.08 Pa at 720 K [15]), the radiation process cannot be associated with the usual thermal evaporation of lithium [4].
Fig. 4. The dependence of the lithium emission intensity at 610.4 nm in argon and xenon on the lithium layer inverse temperature.
The mechanism of the excited cadmium atoms release under metal cadmium irradiation by alpha particles was studied in [6]. Based on the proximity of the obtained values for the activation energies of the metal atoms and ion emissions to the values of the activation energy for selfdiffusion (Q) it is concluded that the line intensity temperature dependence is related to the metal atoms’ self-diffusion [6, 8]. The values of activation energy were A=75-85 kJ/mol [6] at the tabulated value of Q=76-80 kJ/mol for cadmium [14] and A~70 kJ/mol [8] at Q=91.6 kJ/mol [14] for zinc. The coefficients of sputtering of metal cadmium by α-particles with the energy of 5.3 MeV increased from 4·10-18 g/particle obtained at 293 K (sputtering in a vacuum) to ~3·10-14 g/particle during the sputtering into helium at the cadmium temperature of 473 K [6]. A high sputtering coefficient corresponds to the emission of approximately 108 metal atoms per alpha particle. The temperature dependence of the luminescence intensity was explained by the droplet model [6]. When a particle is moderated in Cd, it interacts with atoms of the crystal lattice, knocks out some of them to interstitial sites, and forms the so-called temperature wedge within which the entire kinetic energy of this particle is contained in the form of vibrations of the lattice and displaced atoms. The temperature within this wedge increases considerably and may exceed the melting temperature of the metal. In this case, the metal structure differs from an amorphous structure of a real liquid and it is actually an overheated solid. A microdroplet of the heated metal that contains Cd atoms displaced from lattice sites is ejected due to thermal stresses in the foil. The displaced Cd atoms diffuse inside the droplet in search for vacancies. The self-diffusion coefficient for Cd at T~430-530 K increases by many orders of magnitude and promotes an accelerated emission of the displaced atoms out of the metal. It should be noted that the metal temperature in the drop will be approximately equal to or above the melting point and is loosely related to the initial temperature. One difference in the sputtering in lithium is that for irradiation with α-particles [6] or 3 He(n,p)3H nuclear reaction products [8], the charged particle flies into the metal (cadmium or zinc).
For the 6Li(n,α)3H nuclear reaction, the ions are emitted directly from the metal layer and only a fraction of particles that reach the opposite wall fly into the metal. Another difference is that lithium is in the liquid phase, while cadmium and zinc are irradiated in the solid state. Consequently, the lithium vapour density, which is different from the density of the saturated steam, is generated in the gas excitation area. For ion energies in the MeV range electronic sputtering processes contribute to surface sputtering [16, 17]. A major part of the energy of heavy ions is transferred to electrons along the ion track. A coupling between the electrons and phonons causes large local heating in cylindrical volume. Surface atoms may be removed by evaporation in a jet from the heated volume [17]. The sputtered particles are emitted predominantly as neutral atoms and generally less than 5% are ions, a certain fraction can be emitted as atom clusters [16]. The direct contribution of emitted ions and excited atoms to the radiation is insignificant. This is evidenced by the absence of appreciable optical radiation in a vacuum at the lithium layer temperature of 623 K (Figure 2). Additionally, no band in the region of 450–550 nm is observed in the emission spectra of lithium with noble gases, while this band is observed upon thermal evaporation of lithium and corresponds to the B–X transition of the lithium dimer Li2 [18]. Thus, the excitation of lithium atoms or other alkali metal in a noble gas is described as follows: R + (α/T) → R+ + (α/T) + e
(3)
R + (α/T) → R* + (α/T)
(4)
R+ + 2R → R2+ + R
(5)
R2+ + M → M+ + 2R
(6)
R* + M → M+ + R + e
(7)
M+ + 2R → MR+ + R
(8)
MR+ + M + R → M2+ + 2R
(9)
M2+ + e → M* + M
(10)
R2+ + e → R* + R
(11)
where R is a noble gas atom, and M is an alkali metal atom. The population of the lithium levels occurs during the process of recombination (10) of molecular ions with electrons. Figure 3 shows that the intensity of the np-ns–transitions of the noble gas lines decreases monotonically with increasing temperature or is essentially unchanged as observed, for example, for intensities of transitions from four lower 6p-levels of xenon (see 881.9 nm line in Figure 3d). With a sharp increase in the intensity of alkali metal lines, there is no bend in curves of the line intensity dependence of np-ns-transitions of noble gas atoms on temperature of lithium layer. The population of the lithium 2p–levels practically do not affect the population of the np–levels of noble gas atoms, including cascading transitions from higher levels. Cascading transitions from d–levels [1, 19, 20] are considered to be the main channels for the populating the noble gases’ 2p–levels. D–levels are populated in the processes of dissociative recombination of molecular ions with electrons [1] and during the direct excitation of atoms by secondary electrons [20]. The Penning process (7) with buffer gas atoms in a ns-states (two resonance, from which the radiation is trapped, and two
metastable) appears to be the main channel leading to the population of the lithium 2p levels. Process (6) competes with the dissociative recombination of noble gases’ molecular ions with electrons (10), leading to the population of noble gas atom levels. The dominance of process (6) will lead to a decrease in the intensity of the np-ns transitions of noble gases. Apparently, the excitation transfer from the noble gas atoms to lithium atoms in the Penning process (7) or recharging (6) occurs more efficiently in krypton than in argon and neon. In krypton, a sharp increase in the lithium lines intensity starts at a temperature that is 100 K lower than that in argon and neon. The intensity observed for the line at 610.4 nm decreases in the krypton medium at the lithium layer temperature above 670 K and apparently is also related to the trapped emission on this transition. 5. Conclusions Alkali metal sputtering into a gas medium excited by the products of the 6Li(n,α)3H nuclear reaction was studied. Intense radiation in the lines of lithium, sodium and potassium atoms appears at temperatures in the 520-620 K range, depending on the gas medium. The good agreement between the activation energy of the emission process for the alkali metals lines with the energy of lithium evaporation indicates that the emission process is related to the evaporation under nuclear particles passage through a lithium layer. Luminescence for the alkali metal lines does not affect the radiation for the lines of the np-ns transitions of noble gas atoms. The Penning process with buffer gas atoms in a ns-states is assumed to be the main channel of the excitation transmission from the noble gases to the lithium atoms. The obtained results are of interest for developing the methods for generating energy in the form of optical radiation from a nuclear reactor. Acknowledgements This work was supported by the Ministry of Energy of the Republic of Kazakhstan (Program “Development of nuclear energy in the Republic of Kazakhstan for the 2018-2020 years”), Nazarbayev University (grant ORAU). References [1] S.P. Melnikov, A.N. Sizov, A.A. Sinyanskii, G. H. Miley, Lasers with Nuclear Pumping. Springer, Heidelberg (2015). [2] E.G. Batyrbekov, Yu.N. Gordienko, Yu.V. Ponkratov, M.U. Khasenov, I.L. Tazhibayeva, N.I. Barsukov, T.V. Kulsartov, Zh.A. Zaurbekova, Ye.Yu. Tulubayev, M.K. Skakov, Fusion Engineering and Design 117 (2017) 204-207. [3] K.K. Samarkhanov, E.G. Batyrbekov, M.U. Khasenov, Yu.N. Gordienko, Zh.A. Zaurbekova, V.S. Bochkov, Eurasian Chemico-Technological Journal 21 (2019) 115–123. [4] Yu. N. Gordienko, M.U. Khasenov, E.G. Batyrbekov, K.K. Samarkhanov, Yu.V. Ponkratov, A.K. Amrenov, Laser and Particle Beams 37 (1) (2019) 18-24. [5] K. Jensen, E. Veje, Z. Phys. 269 (1) (1974) 293-300. [6] A.I. Mis’kevich, Quantum Electronics 32 (9) (2002) 803-808. [7] R. Kelly, Phys. Rev. B 25 (2) (1982) 700-712. [8] A.I. Mis’kevich, Lui Tao, Optics and Spectroscopy 105 (5) (2008) 691–698. [9] A.I. Mis’kevich, Liu Tao, Technical Physics 55 (2) (2010) 264-269.
[10] A.O. Sadvakassova, I.E. Tazhibayeva, E.A. Kenzhin, Zh.A. Zaurbekova, T.V. Kulsartov, Yu.N. Gordiyenko, Ye.V. Chikhray, Fusion Science and Technology 60 (1T) (2011) 9-15. [11] I. Lyublinski, A. Vertkov, V. Lazarev, J. Nucl. Mater. 463 (2015) 1156–1159. [12] A. Scheps, Ch. Ottinger, G. York, A. Gallagher, J. Chem. Phys. 63 (6) (1975) 2581-2590. [13] D. Henriquesa, V. Motalova, L. Benczea, T. Markus, ECS Transactions 46 (1) (2013) 303-312. [14] I.S. Grigoryev, E.Z. Meilikhov (Eds). (1991). Handbook of Physical Quantities, Energoatomizdat, Moscow. [15] D. R. Lide (Ed.). CRC Handbook of Chemistry and Physics, the 84th Edition, CRC Press (2003-2004). [16] R. Behrisch, W. Eckstein, in: R. Behrisch, W. Eckstein (Eds.), “Sputtering by Particle Bombardment, Experiments and Computer Calculations from Threshold to MeV Energies”, Springer-Verlag, Berlin, (2007) 1-19. [17] W. Assmann, M. Toulemonde, Ch. Trautmann, in: Behrisch, W. Eckstein (Eds.), “Sputtering by Particle Bombardment, Experiments and Computer Calculations from Threshold to MeV Energies”, Springer-Verlag, Berlin, (2007) 401-450. [18] Sh. Mochizuki, M. Sasaki, R. Ruppin, J. Phys.: Condens. Matter 9 (27) (1997) 5801–5814. [19] X. K. Hu, J. B. A. Mitchell, R. H. Lipson, Phys. Rev. A 62(5) (2000) 052712. [20] M.U. Khasenov, Laser and Particle Beams 32 (3) (2014) 501-508.
Optical radiation from the sputtered species under gas excitation by the products of the 6 Li(n,α)3H nuclear reaction Erlan Batyrbekov, Mendykhan Khasenov, Yuriy Gordienko, Kuanysh Samarkhanov, Yuriy Ponkratov
• • •
With increasing temperature of the lithium, intense radiation arises at lithium lines Emission on lithium lines occurs as a result of plasma-chemical processes in a gas The main process of excitation of lithium atoms is assumed to be the Penning process
CRediT author statement Erlan Batyrbekov: Project administration, Supervision Mendykhan Khasenov: Conceptualization, Investigation, Writing - Original Draft Yuriy Gordienko: Investigation, Resources Kuanysh Samarkhanov: Investigation, Writing - Review & Editing Yuriy Ponkratov: Formal analysis, Resources
To the Editorial Board of the J. of Luminescence The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. “Optical radiation from the sputtered species under gas excitation by the products of the 6Li(n,α)3H nuclear reaction”. Corresponding author: Mendykhan Khasenov
S0022-2313(19)31792-2
DOI:
https://doi.org/10.1016/j.jlumin.2019.116973
Reference:
LUMIN 116973
To appear in:
Journal of Luminescence
Received Date: 11 September 2019 Revised Date:
2 December 2019
Accepted Date: 15 December 2019
Please cite this article as: E. Batyrbekov, M. Khasenov, Y. Gordienko, K. Samarkhanov, Y. Ponkratov, 6 3 Optical radiation from the sputtered species under gas excitation by the products of the Li(n,α) H nuclear reaction, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116973. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Optical radiation from the sputtered species under gas excitation by the products of the 6Li(n,α)3H nuclear reaction Erlan Batyrbekova, Mendykhan Khasenovb*, Yuriy Gordienkoс, Kuanysh Samarkhanovс, Yuriy Ponkratovс a National Nuclear Center of the Republic of Kazakhstan, 2 Beybit Atom St., 071100, Kurchatov, Republic of Kazakhstan b National Laboratory Astana, 53 Kabanbay Batyr Ave., 010000, Nur-Sultan, Republic of Kazakhstan c Institute of Atomic Energy Branch of the National Nuclear Center of the Republic of Kazakhstan, 10 Beybit Atom St., 071100, Kurchatov, Republic of Kazakhstan *Corresponding author. E-mail address: [email protected] Abstract The emission spectra during alkali metal sputtering into noble gases excited by the products of Li(n,α)3H nuclear reaction were studied. Lithium in the form of a capillary-porous structure (CPS) was heated up to a temperature of 730 K. At a temperature in the 520-620 K range, depending on a type of gas medium, intense radiation was observed for lines of lithium and lines of sodium and potassium found as impurities in lithium. The process activation energy (155-161 kJ/mol) obtained from the dependence of the optical radiation intensity on the lithium layer temperature is in good agreement with the lithium evaporation energy values. The vapour density that is significantly higher than that saturated lithium vapour under normal thermal heating was created by the αparticles and tritium nuclei released from the lithium layer and when the opposite wall was bombarded. Excitation of the sputtered lithium atoms occurs as the result of the plasma-chemical reactions in the gas. The Penning process for the lithium atoms is suggested to be the main channel for the excitation transmission from the noble gases to the lithium atoms. 6
Keywords: Nuclear reaction; Sputtering; Emission; Alkali metal; Mechanism of excitation 1.
Introduction
Investigations of the optical radiation of gas mixture plasma formed in the nuclear reactor core is of interest for designing ionizing radiation detectors and the devices that directly convert nuclear reaction energy into light. Uranium fission fragments, as well the products of 3He(n,p)3H and 10 B(n,α)7Li nuclear reactions were used in the nuclear reactor for gas ionization and excitation [1]. Products of the 6Li(n,α)3H nuclear reaction were used in [2, 3] for gas excitation in the study of the luminescence spectra at ~300 K. The quite long path length of tritium nuclei in lithium and gas medium allows the excitation of a large volume of gases and provides a large amount of power contained by the gas compared to the 10B(n,α)7Li reaction products. The temperature increases of the gas medium up to ~500 K leads to the appearance of alkali metal lines in the emission spectra [4]. Luminescence spectra of lithium sputtering of the target irradiated with Xe+ ions in vacuum at room temperature were studied in [5]. It was found that the processes leading to light emission during metal sputtering into a gas medium [4, 6] differ from the sputtering processes in the vacuum [7].
Cadmium and zinc sputtering in the gas medium during irradiation by the products of nuclear reactions and electron beam were studied in [6, 8, 9]. The intensity of cadmium and zinc lines during the sputtering increased sharply at temperatures above 430 and 460 K, respectively. The mechanism of the excited atoms release from the metal was studied in [6], and the temperature dependence of the luminescence intensity was explained by the droplet model. A brief description of the droplet model and its discussion will be presented in Section 4. The present paper examines the emission spectra of noble gases and alkali metals sputtering from the lithium layer in the nuclear reactor core at temperatures up to 730 K. 2.
Experimental Setup
The study was carried out at the IVG.1M nuclear reactor [10] at a thermal neutron flux density of up to 1.4·1014 n/cm2s. Ionization and excitation of the gas medium was generated via nuclear reaction products: 6
Li + n → 4He (2.05 MeV) + 3H (2.73 MeV)
(1)
The scheme of the in-pile experimental device is shown in Figure 1. The experimental cell is made of a stainless-steel pipe with a length of 900 mm and an inner diameter of 20 mm. A flange with an optical vacuum input (1) plugs the top part of the pipe. In [4], lithium was applied to the walls of the experimental cell by heating up to 570 K, and liquid lithium in a vacuum spread evenly over the surface and was retained due to the surface tension force. In the present work, the lithium stabilized in a matrix of capillary-porous structure [11] was used. The woven stainless-steel mesh was placed on the inner side of the experimental cell; the dimensions of the porous cell were 0.3×0.3 mm with a thickness of 0.1 mm. Lithium (1.55 g, LE-1 grade) was purified by vacuum distillation and gettering. Further, the sample of lithium CPS was prepared as follows: the cell with CPS was heated under conditions of oil-free pumping (up to a temperature of 570 K) and in an atmosphere of purified argon (up to a temperature of 1030 K). As a result, liquid lithium was evenly distributed over the porous structure of the mesh. Lithium with a natural composition (7.5% of 6Li) and an impurity content of less than 0.1% was used. The height of the applied lithium layer (2) was 200 mm. The experimental device was loaded into the dry physical experimental channel of the IVG.1M reactor core with a height of 800 mm. In a different circle of the IVG.1M reactor, the middle of the lithium layer was located in the centre of reactor core, either 100 or 150 mm above the centre, corresponding to the average height of the layer, at a thermal neutron flux density of 1.4·1014, 1014, and 5·1013 n/cm2s. The light radiation output was observed via a collimator with a quartz lens (3) located inside the experimental device and connected to a fibre optic cable (4) through an optical vacuum input. The light radiation from the experimental cell through a 10 m-long fibre was directed to the input of the QE65Pro (Ocean Optics) spectrometer located in the experimental hall of the IVG.1M reactor. Luminescence spectra in the wavelength range of 300-970 nm were recorded with the integration time from 0.01 up to 10 s and an optical spectrometer resolution of 1.5 nm. A USB2000+ spectrometer (Ocean Optics) was connected in parallel to record 980 nm xenon line.
Fig. 1. Scheme of the experimental device. 1 – flange with vacuum optical input, 2 – CPS lithium layer, 3 – optical collimator, 4 – heaters, 5 – thermocouples, 6 – cadmium, 7 – flange for gas outlet and inlet.
The measurements were conducted at a constant IVG.1M reactor thermal power of 6 MW. Heating of the experimental device was implemented due to radiation heating of the steel pipe and energy release in the lithium layer, as well as ohmic heating. The temperature of the experimental device was controlled by changing the heater power and, if it was necessary to reach a certain temperature, by changing the nitrogen gas flux in the external blowing of a tube. During the experiment, the outer surface temperature was measured for 4 points at the experimental device height. In measurements with krypton, the temperature of the experimental cell and casing was maintained at the same height up to 620 K, and with further temperature increases of the lithium layer, the temperature in the experimental device casing remained equal to 620 K in order to avoid overheating of the collector with a quartz lens. In the subsequent reactor cycles, because the experimental cell must be located above the reactor core, the temperature in the top part of the experimental device casing was 50 K lower for the measurements with xenon and 150 K lower for
the measurements with neon and argon, compared with an experimental cell. The volume of the experimental device was degassed in the reactor core at 410 K temperature and then "flushed" several times with the tested gas prior to filling the cell, and gases with an impurities content of <0.001% were used. The “Liana” test-bench vacuum system was used for gas pumping and supplies [10]. A piece of cadmium (1.42 g) was placed on the bottom of the cell. It was assumed that it would be possible to compare two mechanisms of the formation of optical radiation during thermal evaporation of cadmium and during sputtering of a layer of lithium, respectively. However, the radiation intensity at the brightest lines of the cadmium atom (508.6 nm) and the cadmium ion (441.6 nm) did not exceed 200 rel. units. Possibly, the low intensity of cadmium lines is associated with the formation of a compound of cadmium with lithium. 3. Results Figure 2 shows the emission spectra during krypton excitation. Lithium lines dominate in the emission spectra from an experimental cell filled with krypton. In addition to krypton lines, sodium and potassium lines and a weak cadmium line are observed. No alkali metal lines are observed in the absence of the gas in the experimental device.
Fig. 2. Emission spectra of krypton at a pressure of 45 kPa (1) and irradiation from an experimental cell (2) at the residual pressure of ~10 Pa. The thermal neutron flux density is F=1.4·1014 n/сm2s, and the lithium layer temperature is 623 К. The integration time is 0.01 s (1) and 10 s (2). Line (2) shifted 2000 counts down.
In the lithium spectra, lines are also observed at 413.3 nm (2p-5d transition), 391.5 nm (2p6d), and 379.5 nm (2p-7d) in addition to the lines shown in Figure 2. Emission in the resonance lines of lithium at 670.8 nm was “trapped”. Radiation trapping consists in multiple reemission and reabsorption of resonance photons in an optically dense medium. In resulting photon flux, extending
beyond the system, not the most numerous (but also the most strongly absorbed photons) photons from center of the line predominate, but relatively few photons from distant line wings, which free path is comparable to the size of the system. In our experiments, a broadening of lithium resonance line was observed; in helium at a temperature of 620–670 K, the self-reversion of the line at 670 nm also was observed. Emission in the resonance lines of sodium at 589 and 589.6 nm (3s-3p transition) was also trapped, and sodium lines are also observed at 568.3 (3p-4d) and 819.5 nm (3p-3d) in the spectra. The 766.5 and 769.9 nm (4s-4p) and 404.4 and 404.7 nm (4s-5p) resonance lines are also observed in the potassium spectra. Sodium and potassium are found in lithium as impurities with chemical compositions of 0.04 and 0.005%, respectively. The amount of sodium and potassium may change during eight reactor cycles measurements of 3 hours each. The cadmium atom spectra are represented by the lines of the cadmium triplet (467.8, 480, and 508.6 nm) and by the lines at 326.1, 361 and 643.8 nm. A 441.6 nm cadmium ion line was observed only in helium and neon, and the 533.7 and 537.8 nm weak lines are found only in helium. The lines of the np-ns transitions (where n=3, 4, 5, and 6, for neon, argon, krypton, and xenon, respectively) dominate in the noble gases’ spectra, and there are also a number of 3p-3d transition lines in neon. The weak continuum in the region from 570–770 nm with neon to 570–950 nm with xenon corresponds to the A–X transitions of the Li–R molecules, where R is a noble gas atom [12]. Figure 3 shows the dependence of the elements’ characteristic line intensity on the lithium layer temperature. The intensities are given in relative units, without correction for the spectral sensitivity of the experimental facility. The peak intensity values, not corrected for line broadening related to the self-absorption are shown for the lithium and sodium resonance lines.
a)
b)
c)
d) Fig. 3. Dependence of the intensity of the lines of atoms of alkali metals and noble gases in neon (a), argon (b), krypton (c) and xenon (d) on the lithium layer temperature. The initial gas pressure at 420 K: Ne – 100, Ar – 88, Kr – 41, Xe – 37 kPa. The thermal neutron flux density: Ne, Ar – 5·1013, Xe – 1014, Kr – 1.4·1014 n/cm2s. The intensity of 763.5, 826.5, and 912.3 nm (b); 766.5 nm (c); and 590 and 766.5 nm (d) lines was increased 5 times; the intensity of 837.8 nm (3p-3d transition) (a) line was increased 10 times.
4. Discussion Figure 2 shows that the intensity of lithium and sodium lines while sputtering into a vacuum is more than 104 times smaller than the intensity of the line while sputtering into a gas medium. The specificity of sputtering into the gas medium shows a sharp increase in the alkali metal line intensity at temperatures higher than 520, 570 and 620 K in krypton, xenon, and argon or neon medium, respectively. Figure 4 shows the temperature dependence of the luminescence intensity of lithium in the (1/kT, lnI) coordinates, where k is the Boltzmann constant. The rapid increase in the luminescence intensity is well approximated by:
~exp
(2)
where A is the activation energy of this process. The A values obtained for 610.4 nm lithium lines are 1.67 eV in argon, 1.63 eV in neon, and 1.61 eV in xenon. These values are in good agreement with the evaporation energy of lithium, which is equal to 1.63 eV (156.9 kJ/mol [13]), and are very different from the activation energy during lithium self-diffusion (55.2 kJ/mol [14]). Since the saturation vapour pressure of lithium is small (4·10-6 Pa at 520 K and 0.08 Pa at 720 K [15]), the radiation process cannot be associated with the usual thermal evaporation of lithium [4].
Fig. 4. The dependence of the lithium emission intensity at 610.4 nm in argon and xenon on the lithium layer inverse temperature.
The mechanism of the excited cadmium atoms release under metal cadmium irradiation by alpha particles was studied in [6]. Based on the proximity of the obtained values for the activation energies of the metal atoms and ion emissions to the values of the activation energy for selfdiffusion (Q) it is concluded that the line intensity temperature dependence is related to the metal atoms’ self-diffusion [6, 8]. The values of activation energy were A=75-85 kJ/mol [6] at the tabulated value of Q=76-80 kJ/mol for cadmium [14] and A~70 kJ/mol [8] at Q=91.6 kJ/mol [14] for zinc. The coefficients of sputtering of metal cadmium by α-particles with the energy of 5.3 MeV increased from 4·10-18 g/particle obtained at 293 K (sputtering in a vacuum) to ~3·10-14 g/particle during the sputtering into helium at the cadmium temperature of 473 K [6]. A high sputtering coefficient corresponds to the emission of approximately 108 metal atoms per alpha particle. The temperature dependence of the luminescence intensity was explained by the droplet model [6]. When a particle is moderated in Cd, it interacts with atoms of the crystal lattice, knocks out some of them to interstitial sites, and forms the so-called temperature wedge within which the entire kinetic energy of this particle is contained in the form of vibrations of the lattice and displaced atoms. The temperature within this wedge increases considerably and may exceed the melting temperature of the metal. In this case, the metal structure differs from an amorphous structure of a real liquid and it is actually an overheated solid. A microdroplet of the heated metal that contains Cd atoms displaced from lattice sites is ejected due to thermal stresses in the foil. The displaced Cd atoms diffuse inside the droplet in search for vacancies. The self-diffusion coefficient for Cd at T~430-530 K increases by many orders of magnitude and promotes an accelerated emission of the displaced atoms out of the metal. It should be noted that the metal temperature in the drop will be approximately equal to or above the melting point and is loosely related to the initial temperature. One difference in the sputtering in lithium is that for irradiation with α-particles [6] or 3 He(n,p)3H nuclear reaction products [8], the charged particle flies into the metal (cadmium or zinc).
For the 6Li(n,α)3H nuclear reaction, the ions are emitted directly from the metal layer and only a fraction of particles that reach the opposite wall fly into the metal. Another difference is that lithium is in the liquid phase, while cadmium and zinc are irradiated in the solid state. Consequently, the lithium vapour density, which is different from the density of the saturated steam, is generated in the gas excitation area. For ion energies in the MeV range electronic sputtering processes contribute to surface sputtering [16, 17]. A major part of the energy of heavy ions is transferred to electrons along the ion track. A coupling between the electrons and phonons causes large local heating in cylindrical volume. Surface atoms may be removed by evaporation in a jet from the heated volume [17]. The sputtered particles are emitted predominantly as neutral atoms and generally less than 5% are ions, a certain fraction can be emitted as atom clusters [16]. The direct contribution of emitted ions and excited atoms to the radiation is insignificant. This is evidenced by the absence of appreciable optical radiation in a vacuum at the lithium layer temperature of 623 K (Figure 2). Additionally, no band in the region of 450–550 nm is observed in the emission spectra of lithium with noble gases, while this band is observed upon thermal evaporation of lithium and corresponds to the B–X transition of the lithium dimer Li2 [18]. Thus, the excitation of lithium atoms or other alkali metal in a noble gas is described as follows: R + (α/T) → R+ + (α/T) + e
(3)
R + (α/T) → R* + (α/T)
(4)
R+ + 2R → R2+ + R
(5)
R2+ + M → M+ + 2R
(6)
R* + M → M+ + R + e
(7)
M+ + 2R → MR+ + R
(8)
MR+ + M + R → M2+ + 2R
(9)
M2+ + e → M* + M
(10)
R2+ + e → R* + R
(11)
where R is a noble gas atom, and M is an alkali metal atom. The population of the lithium levels occurs during the process of recombination (10) of molecular ions with electrons. Figure 3 shows that the intensity of the np-ns–transitions of the noble gas lines decreases monotonically with increasing temperature or is essentially unchanged as observed, for example, for intensities of transitions from four lower 6p-levels of xenon (see 881.9 nm line in Figure 3d). With a sharp increase in the intensity of alkali metal lines, there is no bend in curves of the line intensity dependence of np-ns-transitions of noble gas atoms on temperature of lithium layer. The population of the lithium 2p–levels practically do not affect the population of the np–levels of noble gas atoms, including cascading transitions from higher levels. Cascading transitions from d–levels [1, 19, 20] are considered to be the main channels for the populating the noble gases’ 2p–levels. D–levels are populated in the processes of dissociative recombination of molecular ions with electrons [1] and during the direct excitation of atoms by secondary electrons [20]. The Penning process (7) with buffer gas atoms in a ns-states (two resonance, from which the radiation is trapped, and two
metastable) appears to be the main channel leading to the population of the lithium 2p levels. Process (6) competes with the dissociative recombination of noble gases’ molecular ions with electrons (10), leading to the population of noble gas atom levels. The dominance of process (6) will lead to a decrease in the intensity of the np-ns transitions of noble gases. Apparently, the excitation transfer from the noble gas atoms to lithium atoms in the Penning process (7) or recharging (6) occurs more efficiently in krypton than in argon and neon. In krypton, a sharp increase in the lithium lines intensity starts at a temperature that is 100 K lower than that in argon and neon. The intensity observed for the line at 610.4 nm decreases in the krypton medium at the lithium layer temperature above 670 K and apparently is also related to the trapped emission on this transition. 5. Conclusions Alkali metal sputtering into a gas medium excited by the products of the 6Li(n,α)3H nuclear reaction was studied. Intense radiation in the lines of lithium, sodium and potassium atoms appears at temperatures in the 520-620 K range, depending on the gas medium. The good agreement between the activation energy of the emission process for the alkali metals lines with the energy of lithium evaporation indicates that the emission process is related to the evaporation under nuclear particles passage through a lithium layer. Luminescence for the alkali metal lines does not affect the radiation for the lines of the np-ns transitions of noble gas atoms. The Penning process with buffer gas atoms in a ns-states is assumed to be the main channel of the excitation transmission from the noble gases to the lithium atoms. The obtained results are of interest for developing the methods for generating energy in the form of optical radiation from a nuclear reactor. Acknowledgements This work was supported by the Ministry of Energy of the Republic of Kazakhstan (Program “Development of nuclear energy in the Republic of Kazakhstan for the 2018-2020 years”), Nazarbayev University (grant ORAU). References [1] S.P. Melnikov, A.N. Sizov, A.A. Sinyanskii, G. H. Miley, Lasers with Nuclear Pumping. Springer, Heidelberg (2015). [2] E.G. Batyrbekov, Yu.N. Gordienko, Yu.V. Ponkratov, M.U. Khasenov, I.L. Tazhibayeva, N.I. Barsukov, T.V. Kulsartov, Zh.A. Zaurbekova, Ye.Yu. Tulubayev, M.K. Skakov, Fusion Engineering and Design 117 (2017) 204-207. [3] K.K. Samarkhanov, E.G. Batyrbekov, M.U. Khasenov, Yu.N. Gordienko, Zh.A. Zaurbekova, V.S. Bochkov, Eurasian Chemico-Technological Journal 21 (2019) 115–123. [4] Yu. N. Gordienko, M.U. Khasenov, E.G. Batyrbekov, K.K. Samarkhanov, Yu.V. Ponkratov, A.K. Amrenov, Laser and Particle Beams 37 (1) (2019) 18-24. [5] K. Jensen, E. Veje, Z. Phys. 269 (1) (1974) 293-300. [6] A.I. Mis’kevich, Quantum Electronics 32 (9) (2002) 803-808. [7] R. Kelly, Phys. Rev. B 25 (2) (1982) 700-712. [8] A.I. Mis’kevich, Lui Tao, Optics and Spectroscopy 105 (5) (2008) 691–698. [9] A.I. Mis’kevich, Liu Tao, Technical Physics 55 (2) (2010) 264-269.
[10] A.O. Sadvakassova, I.E. Tazhibayeva, E.A. Kenzhin, Zh.A. Zaurbekova, T.V. Kulsartov, Yu.N. Gordiyenko, Ye.V. Chikhray, Fusion Science and Technology 60 (1T) (2011) 9-15. [11] I. Lyublinski, A. Vertkov, V. Lazarev, J. Nucl. Mater. 463 (2015) 1156–1159. [12] A. Scheps, Ch. Ottinger, G. York, A. Gallagher, J. Chem. Phys. 63 (6) (1975) 2581-2590. [13] D. Henriquesa, V. Motalova, L. Benczea, T. Markus, ECS Transactions 46 (1) (2013) 303-312. [14] I.S. Grigoryev, E.Z. Meilikhov (Eds). (1991). Handbook of Physical Quantities, Energoatomizdat, Moscow. [15] D. R. Lide (Ed.). CRC Handbook of Chemistry and Physics, the 84th Edition, CRC Press (2003-2004). [16] R. Behrisch, W. Eckstein, in: R. Behrisch, W. Eckstein (Eds.), “Sputtering by Particle Bombardment, Experiments and Computer Calculations from Threshold to MeV Energies”, Springer-Verlag, Berlin, (2007) 1-19. [17] W. Assmann, M. Toulemonde, Ch. Trautmann, in: Behrisch, W. Eckstein (Eds.), “Sputtering by Particle Bombardment, Experiments and Computer Calculations from Threshold to MeV Energies”, Springer-Verlag, Berlin, (2007) 401-450. [18] Sh. Mochizuki, M. Sasaki, R. Ruppin, J. Phys.: Condens. Matter 9 (27) (1997) 5801–5814. [19] X. K. Hu, J. B. A. Mitchell, R. H. Lipson, Phys. Rev. A 62(5) (2000) 052712. [20] M.U. Khasenov, Laser and Particle Beams 32 (3) (2014) 501-508.
Optical radiation from the sputtered species under gas excitation by the products of the 6 Li(n,α)3H nuclear reaction Erlan Batyrbekov, Mendykhan Khasenov, Yuriy Gordienko, Kuanysh Samarkhanov, Yuriy Ponkratov
• • •
With increasing temperature of the lithium, intense radiation arises at lithium lines Emission on lithium lines occurs as a result of plasma-chemical processes in a gas The main process of excitation of lithium atoms is assumed to be the Penning process
CRediT author statement Erlan Batyrbekov: Project administration, Supervision Mendykhan Khasenov: Conceptualization, Investigation, Writing - Original Draft Yuriy Gordienko: Investigation, Resources Kuanysh Samarkhanov: Investigation, Writing - Review & Editing Yuriy Ponkratov: Formal analysis, Resources
To the Editorial Board of the J. of Luminescence The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. “Optical radiation from the sputtered species under gas excitation by the products of the 6Li(n,α)3H nuclear reaction”. Corresponding author: Mendykhan Khasenov