Investigation of electronic conductivity of liquid argon-nitrogen mixtures

Nuclear Instruments and Methods in Physics Research A234 (1985) 451-454 North-Holland, Amsterdam

451

INVESTIGATION OF ELECTRONIC CONDUCTIVITY OF LIQUID ARGON-NITROGEN MIXTURES A.S. B A R A B A S H , A.A. G O L U B E V , O.V. K A Z A C H E N K O a n d B.M. O V C H I N N I K O V Institute of Nuclear Research, Academy of Sciences of the USSR, Moscow 117312, USSR Received 26 June 1984

Electron drift velocitiesand the electron attachment coefficientsfor electronegativeimpurities are measured in liquid argon-nitrogen mixtures. It is shown that nitrogen molecules have no electronegativity.

1. Introduction

The purpose of the present work is to study properties of liquid argon-nitrogen mixtures as media for pulse ionization chambers. Nitrogen admixtures to argon may be used, say, for shifting the spectrum of the argon scintillation radiation from the far ultra-violet region to that of visible light in order to get in this way additional scintillation information in the liquid ionization chamber. The presence of nitrogen in large-mass detectors makes it possible, in principle, to use them for investigation of neutral currents in processes of neutrino scattering on nitrogen nuclei [1,2]. The necessary conditions for the use of a liquid as a filling for a pulse ionization chamber are non-electronegativity of its molecules and a sufficiently high electron drift velocity in this medium under the action of an external electric field. The information on the affinity of nitrogen molecules for electrons which was available before this work, was rather controversial. A number of theoretical and experimental works on the electron mobility in gaseous nitrogen [3-7] indicated that the nitrogen molecules are not electronegative. On the other hand, it was reported [8,9] that in liquid argon the coefficient of the electron attachment to nitrogen molecules is less than that for oxygen molecules only by a factor of 200-300. A recent work [10] showed that even a microscopical admixture of nitrogen to liquid argon is sufficient to destroy the electron conductivity in argon. The mobility of negative charges in liquid nitrogen was found to be close to that of negative ions (10-2-10 -3 cm2. V -1 • s -1) [11,12], a fact that can be considered as more evidence of the electronegativity of nitrogen molecules. The results of the present work show that nitrogen molecules are not electronegative. The electron drift velocities and the attachment coefficients to electronegative admixtures in liquid argon-nitrogen mixtures 0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

were also measured. Preliminary measurements of the electron drift velocity in argon-nitrogen mixtures were performed in a previous work [13].

2. The apparatus 2.1. Ionization chamber The measurements have been performed by means of a planar pulse ionization chamber with a screening grid (fig. 1). The diameters of the chamber cathode and grid are 55 mm, the anode diameter is 35 mm; the cathode and anode gaps in the chamber are dgc = 20 mm, dga = 3 mm. The screening grid was wound with wire of beryllium bronze 0.1 mm in diameter, with a pitch of 1.0 mm. A 239pu source emitting about 10 2 a-particles per second is put on the cathode. The signals from the anode come to a charge-sensitive preamplifier BUS-2-96, further they are put via the amplifier BUS-2-97 to an oscillograph and the amplitude analyzer AI-256. The front duration of the signal and the signal amplitude were measured. 2.2. Preparation of the chamber To maintain a high purity of the filling media during the measurements, the internal surfaces of the chamber were carefully degassed. The chamber was evacuated for 24 h under a temperature of 100°C by means of an cathode

Fig. 1. Scheme of the experimental set-up.

452

A.S. Barabash et al. / Electronic conductivity of liquid argon - nitrogen mixtures

oil-vapour pump with a liquid nitrogen trap; then the chamber was repeatedly washed out with liquid argon in which the contents of electronegative admixtures did not exceed 10 -9 of the equivalent oxygen contents.

20

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% 2.3. Purification of the argon-nitrogen mixture The argon-nitrogen mixture was purified by means of the nickel-on-kieselguhr ( N i / S i O 2) adsorbent at room temperature. Previously we have shown that it is possible to use the N i / S i O 2 adsorbent for deep purification of noble gases, hydrogen and methane from electronegative impurities up to their residual concentration C ~< 10 -9 of the equivalent oxygen contents [14]. It is mainly oxygen that is removed from the gases in this process. Other possible impurities which are not removed by the adsorbent, e.g. H20, CO 2, CO, H 2, N H 3, HC1, saturated and some unsaturated hydrocarbons, do not capture low-energy electrons, so their microscopical admixtures produce no effect in the operation of the pulse liquid ionization chamber. One can suppose that nitrogen is purified from oxygen by N i / S i O 2 up to a residual concentration of - 1 0 -9, as for noble gases, hydrogen and methane. To get an accurate quantitative test of the purification of nitrogen from oxygen one has to perform additional measurements with calibration mixtures.

3. M e a s u r e m e n t s results

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of the e x p e r i m e n t a l

In the measurements the chamber was filled with the liquid mixture only to the grid level. The cathode with the a-source was in the gaseous phase of the argon-nitrogen mixture which was under a pressure of Pg = 7 k g f / c m 2. Under these conditions the ionization tracks of a-particles were completely in the gas, so it was possible to work in the region of low electric field strengths without the necessity to take into account the electron-ion pair recombination on the a-particle tracks. Besides, the electrons drifting from the a-particle tracks to the grid were not absorbed. The ratio of the electric field strengths in the anode (EA) and cathode ( E c ) gaps was E A / E c >/2, so that all the drifting electrons passed through the grid.

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Fig. 2. Electric field dependences of the electron drift velocity in liquid argon-nitrogen mixtures at different nitrogen concentrations: zx 7%, [] 16%, O 25%.

itself, d o , and the electron drift velocity was determined from the front duration of the ionization signal, z:

(])

v = d~a/~.

The obtained electric field dependences of the electron drift velocity in argon-nitrogen mixtures are presented in fig. 2. In fig. 3 we give the dependence of the electron drift velocity on the nitrogen concentration in the mixture for E A = 2.25 k V / c m . As can be seen, the obtained dependence is in agreement with both a high mobility of electrons in liquid argon [14], and with a low 106

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3.1. Electron drift velocity The magnitudes of the gas pressure and the electric field in the anode gap which were maintained during the measurements of the electron drift velocity were Pg = 7 k g f / c m 2, E A < 4 k V / c m . Under these conditions the dimensions of the electron cloud generated by the a-particle in the liquid-filled anode gap in the direction of the electric field E A were much less than the gap

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Fig. 3. Dependence of the electron drift velocity on the nitrogen concentration (7/) in liquid argon-nitrogen mixtures at an electric field strength E A = 2.25 kV/cm; +, the results for liquid argon [15]; I, the results for liquid nitrogen [11,12].

A.S. Barabash et aL / Electronic conductivity of liquid argon- nitrogen mixtures

mobility of electrons in liquid nitrogen [11,12]. The low electron mobility in liquid nitrogen can, evidently, be explained by the fact that microscopical vacuum cavities are formed around electrons [15], or in frameworks of other theories describing electrons localized in liquids [16,17]. In the liquid argon-nitrogen mixture we observe a transition from a high electron mobility in liquid argon to a low electron mobility in liquid nitrogen. A similar variation of the electron mobility was detected, for example, in liquid methane-ethane mixtures [16].

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3.2. Attachment of electrons to impurities in liquid argon-nitrogen mixtures

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0

The number of electrons passing a distance x without attachment to electronegative impurities in the medium, Q(x), is given by the formula

Q( x ) = Qo e x p ( - c k x ) ,

(2)

where Q0 is the initial number of electrons withdrawn from the a-particle track, c is the concentration of electronegative impurities, k is the coefficient of electron attachment to the impurities. The electron field dependence of the attachment coefficient is usually taken in the form [9,13]

(3)

k = aE -1.

In the present case the amplitude of the anode signal is given by the expression

Q= d~ foa*aQ(x ) dx.

(4)

Using expressions (2)-(4), we get Q

Qo

= 1 - exp( - acd~aE A')

acdgaEA 1

(5)

The experimentally obtained E A dependences of the ratio Q/Qo for various nitrogen concentrations in the mixture are presented in fig. 4. The quantities Q0 were measured in the ionization chamber under the same conditions (Pga~, tga~, Ec), as those for the signals Q, but without the liquid in the anode gap, i.e. without absorption of electrons. The coefficients a were calculated by means of eq. (5) for the experimental ratios Q/Qo at E A = 2.25 kV/cm. Two assumptions on the electronegative impurities in the argon-nitrogen mixtures were considered separately: (a) the impurity concentration does not depend on the mixture composition, (b) nitrogen is completely responsible for the presence of impurities, so the impurity concentration is proportional to the concentration of nitrogen. The obtained dependences of the coefficients on the nitrogen concentration in the mixture (fig. 5) indicate that the coefficients are increasing with the nitrogen concentration in both cases. The rise of the coefficients a with increase in the nitrogen concentration is due, mainly, to the increase of

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2.0

25

3.0

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[ , KV, cm"~ Fig. 4. Relative n u m b e r of electrons traveling in liquid A r + N 2 m i x t u r e s with no a t t a c h m e n t to i m p u r i t i e s for a distance of 3 m m : zx 7% N 2, [] 16% N 2, O 25% N2; - - - - - 100% A r (c < 10 - 9 of 0 2 equivalent).

the number of electron-impurity random collisions. These take place since the drift time of electrons increases with decreasing drift velocity: V(7% N 2)/V(25% N 2 ) = 7 . Another reason is, probably, an increasing contribution from many-particle processes involving the electron, impurity and nitrogen molecules participating in the electron capture by impurities, like those which take place in gaseous nitrogen under high pressures [3]. Thus, an increase in the nitrogen concentration in a liquid argon-nitrogen mixture results in a rise of the electron capture probability for molecules of electronegative impurities. No careful purification of nitrogen from its oxygen impurity was undertaken in refs. [8-10], and this was, evidently, the main reason why those authors concluded wrongly that nitrogen has a considerable electronegativity.

3.3. Pulse ionization chamber filled with fiquid argon-nitrogen mixture The ionization chamber which was described above, but without the a-source, was filled completely with the liquid 805[ Ar + 20% N 2 mixture, and was exposed to "y-quanta with an energy of 1836 keV. The electric field strengths in the chamber were E c = 1.5 k V / c m and E A = 3.0 kV/cm, and the ionization signals have been detected from the Compton electrons. The maximum amplitude of the signals amounts to about 30% of the amplitude which was obtained with pure liquid argon under the same conditions. Such a considerable diminishing of the ionization signals is due to the capture of the drifting electrons by electronegative impurity molecules, as well as, probably, by the fact that the recombination of the electron-ion pairs at the electron

454

A.S. Barabash et al. / Electronic conductivity of liquid argon nitrogen mixtures -

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Fig. 5. Dependence of coefficients a on the nitrogen concentration (T1) in liquid Ar + N 2 mixtures: (a) the impurity concentration is independent of the mixture composition, (b) impurities in the mixture are contributed by nitrogen only. tracks is higher in the argon-nitrogen mixture pure argon. In order to suppress the electron ment to impurities and the contribution from combination processes one has to increase the field strength in the chamber.

than in attachthe reelectric

4. Conclusions 1) Nitrogen molecules are not electronegative. 2) A few percent of nitrogen, purified from electronegative impurities, can be added to the liquid-argon ionization chamber with no appreciable change in its working characteristics. The authors are profoundly grateful to V.M. Lobashev who substantially supported the present work.

References

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[3] R.E. Goans and L.G. Christophorou, J. Chem. Phys. 60 (1974) 1036. [4] M.A. Morrison and L.A. Collins, Phys. Rev. A17 (1978) 918. [5] B. Crosswend and E. Weibel, Nucl. Instr. and Meth. 155 (1978) 145. [6] H. Massey, Negative ions (Cambridge, 1976). [7] K.M. Avotyan, G.V. Azizbekyan and P.S. Mnatsakanyan, Prib. Tekh. Eksper. 4 (1982) 60 (in Russian). [8] D.W. Swan, Proc. Phys. Soc. 82 (1963) 74. [9] W. Hofman et al. Nucl. Instr. and Meth. 135 (1976) 151. [10] J.C. Berset et al., Nucl. Instr. and Meth. 203 (1982) 133. [11] B. Halpern and R. Gomer, J. Chem. Phys. 51 (1969) 1031. [12] R.J. Loveland, P.G. Comber and W.F. Spear, Phys. Rev. B6 (1972) 319. [13] A.S. Barabash, A.A. Golubev, O.V. Kazachenko and B.M. Ovchinnikov, Preprint P-0181, INR Acad. Sci. USSR, Moscow (1980). [14] L.S. Muller, S. Howe and W.E. Spear, Phys. Rev. 166 (1968) 877. [15] A.G. Khrapak and I.T. Iakubov, Usp. Fiz. Nauk 129 (1979) 45 (in Russian). [16] B.N. Rao, R.L. Bush and K. Funabashi, Can. J. Chem. 55 (1977) 1052. [17] W.F. Schmidt, Can. J. Chem. 55 (1977) 2197.