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Comments on the mechanism of the self-quenching streamer transition
580
Nuclear Instruments and Methods in Physics Research A307 (1991) 580 North-Holland
Letter to the Editor
Comments on the mechanism of the self-quenching streamer transition Tang Xiaowei Institute of High Energy Physics, Beijing, China Received 11 February 1991
Recently Koori et al. reported the evidence of the self-quenching streamer (SQS) transition in Kr-mixtures [Nucl. Instr. and Meth. A281 (1989) 243]. They suggested the existence of another SQS formation mechanism than those proposed hitherto. In this Letter we propose the electron collision mechanism to explain the reported experimental facts.
The problem of the mechanism of the SQS transition is a subject of heated discussion. Atac et al. [1] have proposed the recombination mechanism of the SQS transition, i.e. the energetic photons emitted from the excited molecular states of Ar which are produced through the recombination of Ar ions and electrons in an electron avalanche induce the SQS transition through the direct photoionization process. Zhang [2] proposed the deexcitation of metastable molecular states of Ar as the mechanism of the SQS transition. Recently Taylor [3] proposed a SQS transition mechanism based on the Townsend gas discharge theory in which the effects of photon emission and photoionization are totally neglected. We notice that the mechanism proposed by Zhang cannot account for the SQS transition of only quenching gas without rare gas [4] and the mechanism proposed by Taylor cannot account for the discontinuous SQS transition [5]. Recently Koori et al. have studied the properties of the SQS transition for Xe and Kr mixtures with quenching gases CH4, C2H6, CO 2 etc. [6,7]. It is argued that the maximum energy of photons emitted from the recombination of Xe and Kr is 10.1 eV for Kr and 8.5 eV for Xe respectively; they are insufficient to ionize the quenching gas molecules directly (e.g. the ionization potentials of quenching gases are 13.12 eV for CH4, 11.60 eV for C2H6, and 13.78 eV for CO2). Since the SQS transition is observed in the Xe and Kr mixtures, Koori et al. concluded that the SQS transitions would not be induced through the direct photoionization process. We would like to point out that in the gas discharges the energetic photons are emitted not only through the recombination mechanism but also through the electron collision mechanism. The electric field near the anode wire of the streamer tubes reaches a value higher than 5 x 10 4 V / c m . In such a high electric field the electron
collision in the electron avalanche development is the source of energetic photons. The energy of the photons emitted by electron collisions with the inner atomic shell is much higher than that of photons emitted in the recombination process, for example, the lowest energy of photons emitted in the M - N transitions is about 60 eV for Kr. We agree with the statement in ref. [6] that the recombination mechanism proposed by Atac et al. is invalid since the maximum energy of recombination photons in Xe and Kr is insufficient to ionize the quenching gas molecules. But, in our opinion, the argument in refs. [6,7] that in the Kr and Xe mixture the SQS transition would not be induced through the direct photoionization process is not convincing since the energy of photons emitted in the electron collision mechanism is sufficient to produce the photoionization process. We conclude that the experimental facts in refs. [6,7] can be easily explained by the electron collision mechanism since the energy of the photons emitted in the inner atomic shell electron collisions is higher than the ionization potentials of the quenching gases.
Acknowledgement I wish to thank Prof. Z.Z. Xu for discussions.
References [1] [2] [3] [4] [5] [6] [7]
M. Atac et al., Nucl. Instr. and Meth. 200 (1982) 345. L.S. Zhang, Nucl. Instr. and Meth. A247 (1986) 343. F.E. Taylor, Nucl. Instr. and Meth. A289 (1990) 283. N. Koori et al., IEEE Trans. Nucl. Sci. NS-33 (1986) 395. X.W. Tang, Nucl. Instr. and Meth., to be published. N. Koori et al., Nucl. Instr. and Meth. A281 (1989) 243. N. Koori et al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 223.
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
Nuclear Instruments and Methods in Physics Research A307 (1991) 580 North-Holland
Letter to the Editor
Comments on the mechanism of the self-quenching streamer transition Tang Xiaowei Institute of High Energy Physics, Beijing, China Received 11 February 1991
Recently Koori et al. reported the evidence of the self-quenching streamer (SQS) transition in Kr-mixtures [Nucl. Instr. and Meth. A281 (1989) 243]. They suggested the existence of another SQS formation mechanism than those proposed hitherto. In this Letter we propose the electron collision mechanism to explain the reported experimental facts.
The problem of the mechanism of the SQS transition is a subject of heated discussion. Atac et al. [1] have proposed the recombination mechanism of the SQS transition, i.e. the energetic photons emitted from the excited molecular states of Ar which are produced through the recombination of Ar ions and electrons in an electron avalanche induce the SQS transition through the direct photoionization process. Zhang [2] proposed the deexcitation of metastable molecular states of Ar as the mechanism of the SQS transition. Recently Taylor [3] proposed a SQS transition mechanism based on the Townsend gas discharge theory in which the effects of photon emission and photoionization are totally neglected. We notice that the mechanism proposed by Zhang cannot account for the SQS transition of only quenching gas without rare gas [4] and the mechanism proposed by Taylor cannot account for the discontinuous SQS transition [5]. Recently Koori et al. have studied the properties of the SQS transition for Xe and Kr mixtures with quenching gases CH4, C2H6, CO 2 etc. [6,7]. It is argued that the maximum energy of photons emitted from the recombination of Xe and Kr is 10.1 eV for Kr and 8.5 eV for Xe respectively; they are insufficient to ionize the quenching gas molecules directly (e.g. the ionization potentials of quenching gases are 13.12 eV for CH4, 11.60 eV for C2H6, and 13.78 eV for CO2). Since the SQS transition is observed in the Xe and Kr mixtures, Koori et al. concluded that the SQS transitions would not be induced through the direct photoionization process. We would like to point out that in the gas discharges the energetic photons are emitted not only through the recombination mechanism but also through the electron collision mechanism. The electric field near the anode wire of the streamer tubes reaches a value higher than 5 x 10 4 V / c m . In such a high electric field the electron
collision in the electron avalanche development is the source of energetic photons. The energy of the photons emitted by electron collisions with the inner atomic shell is much higher than that of photons emitted in the recombination process, for example, the lowest energy of photons emitted in the M - N transitions is about 60 eV for Kr. We agree with the statement in ref. [6] that the recombination mechanism proposed by Atac et al. is invalid since the maximum energy of recombination photons in Xe and Kr is insufficient to ionize the quenching gas molecules. But, in our opinion, the argument in refs. [6,7] that in the Kr and Xe mixture the SQS transition would not be induced through the direct photoionization process is not convincing since the energy of photons emitted in the electron collision mechanism is sufficient to produce the photoionization process. We conclude that the experimental facts in refs. [6,7] can be easily explained by the electron collision mechanism since the energy of the photons emitted in the inner atomic shell electron collisions is higher than the ionization potentials of the quenching gases.
Acknowledgement I wish to thank Prof. Z.Z. Xu for discussions.
References [1] [2] [3] [4] [5] [6] [7]
M. Atac et al., Nucl. Instr. and Meth. 200 (1982) 345. L.S. Zhang, Nucl. Instr. and Meth. A247 (1986) 343. F.E. Taylor, Nucl. Instr. and Meth. A289 (1990) 283. N. Koori et al., IEEE Trans. Nucl. Sci. NS-33 (1986) 395. X.W. Tang, Nucl. Instr. and Meth., to be published. N. Koori et al., Nucl. Instr. and Meth. A281 (1989) 243. N. Koori et al.. IEEE Trans. Nucl. Sci. NS-36 (1989) 223.
0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved