Philips Res. Repts 14, 98-100, 1959
R 363
SHOWING THE PRESENCE
OF VERY SMALL PERCENTAGES OF NEON IN HELIUM
by H. J. OSKAM *)
545.821 :546.29
Summary
Measurements of the afterglow in helium and in helium with 1.7.10-6
and 10-5 per cent neon are presented. The observed influence of the
admixed neon atoms on the afterglow curve is consistent with the value
of the cross-section of He+2 ions for a charge-transfer reaction with
neon atoms found recently.
Résumé
eet article traite des mesures de la disparition des électrons d'un
plasma de decharge gazeuse dans l'hélium pur et dans l'hélium avec
1,7.10-0 et 10-5 pour cent de néon, L'influence observée des atomes de
néon mélangés sur la courbe dormant la densité des électrons en fonction du temps, est conforme à la valeur de la section efficace des ions
He+2 pour une réaction à transfert de charge avec des atomes de néon,
qui a été trouvée récemment.
Zusammenfassung
Messungen der Elektronendichte in einem abklingenden Gasentladungsplasma in Helium und in Helium mit 1,7.10-6 und 10-5 Prozent Neon
werden mitgeteilt. Der beobachtete EinfluB der=beigemischten Neonatome auf die Abklingkurve steht im Einklang mit dem kürzlich gefundenen Wertdes Querschnitts von He+s-Ionenfür eine durch ein Neonatom ermittelte Ladungsübergangsreaktion.
Microwave investigation 1) of the properties of the afterglow in helium
(pressure ~ S mm Hg) with a small concentration of neon atoms indicated the
production of atomic neon ions by the reaction **)
Het
+ Ne -+ Ne+ + 2He.
(1)
The cross-section (at room temperature) of molecular helium ions for this
charge-transfer process with neon atoms was found to be Qce R:::l I·S.IO-I? cmê,
The conversion of Het ions into Ne+ ions results in an increasing rate of
electron loss from the plasma since th~ ambipolar diffusion coefficient of Ne+
ions in helium was found to be appreciably larger than that of the He+ and Het
ions in helium 1). Moreover, the N+ ions are found to be converted by threebody collisions with two normal atoms into molecular ions having a large
*) Present address: Department of Electrical Engineering, University of Minnesota, Minneapolis 14, Minn., U.S.A.
*.) Independently, Pahl and Weimer observed an indication ofthe occurrence ofthis reaction
when studying the positive column in "helium-neon mixtures with the aid of a mass
spectrometer (private communication).
,
PRESENCE OF NEON ,IN HELIUM
99
recombination
coefficient 1). The large value of Qce therefore indicates the
possibility to reveal the presence of very small concentrations
of neon atoms in
helium by measuring its afterglow properties.
.In this paper measurements of the influence of 1'7.10-6 and 10-5 per cent
neon on the afterglow of helium at a pressure of 22·6 mm Hg (reduced to 0 "C)
are presented. The helium was enclosed in a cylindrical quartz bottle of height
38 mm and diameter 21·5 mm. Calculations show that the loss of electrons from
the plasma in helium is determined, by ambipolar diffusion of Het ions during
the afterglow in this case, whereas the slope of the curve representing the
logarithm of the electron density versus time .should increase about 1 and 7 per
cent when adding 1,7.10-6 and 10-5 per cent neon respectively 1).
The electron density during the afterglow is measured with the aid of the
microwave method developed by Brown and Biondi 1-4). The frequency of the
microwave field was 9500 Mc/s and its power a ,few microwatts. The field inside
the cavity (enclosing the plasma) was resonating in the TMoIO mode. The construction and preparation of the gas container employed is described in detail
elsewhere 1). The container consisted of three small volumes (0·6 cmê) separated'
from the main volume (112 cmê) by thin glass walls. One' of the smallvolumes
was empty, whereas the two others were filled with neon at pressures such-as to
obtain the proper neon percentages when the thin glass walls were shattered
by means of a small iron ball. The main volume was filled with helium at a
pressure of 22·6 mm Hg (reduced at 0 "C). Part of the walls of the main volume .
was covered with Ba getter in order to absorb non-rare 'gases ,given off by the
glass during the sealing-off process and (or) during measurement.
The results are shown in fig. 1, in which the measured frequency shift of the
cavity is plotted on a logarithmic scale versus time during the afterglow period.
The first part of the curves is influenced by the initial density distribution of the
electrons. As soon as the contribution of the higher-order diffusion modes can
be neglected against that of the first mode the curves of fig. 1 become straight
lines. In this range the electron density responsible for a frequency shift of
1 Mc/s is found to be about 5.8.108 electrons/erne at the centre of the container.
The value of the ambipolar-diffusion
coefficient of Het ions inhelium can be
computed from the slope of the upper curve of fig. 1; the mobility p.o (Het)
of Het ions in helium at standard density (760 mm Hg at 0 "C) calculated from
this value' is found to be p.o(Het) = 17·8 cm/sec per volt/cm and is in agreement with the value recently found 1).
'
Shattering the thi~ glass wall of the empty small volume did not affect the
measurements. Taking into account the-influence of the smallvolumes
on the
total gas pressure, the calculated slopes ofthe curves corresponding to 1,7.10'-6
and 10-5 per cent neon are 179 and 191 sec-1 respectively. The relevant experimental values obtained from fig. 1 (180 and 193 sec-1 respectively) are in agreement with these values within the experimental error.
H. J. OSKAM
100
r-,
100
HeliJm-NeJ
1'o=22.6mm Hg
=0.176 cm2
~
~~
50
.x.
~
~
~20
....
.......
0%
~
~~~
'~
"<l
i
I
~
la
p~
~
g-
10-5%
~~ r"-.
~ 5
~ s-,
'<:::"'::: i'-lI....
Ol
~
1.7.10-6%
2
1
o
2
4
6
8
IQ
--
12
14
.... Time
16
18
20m sec.
95602
The afterglow in helium and in helium-neon mixtures for a helium pressure of 22·6 mm Hg
(reduced to 0 0C) and neon concentrations of 1'7.10-6 and 10-5 per cent; I/A2 = {',,/H)2 +
(2'405/1z)2 in which Hand R are the height and the diameter ofthe quartz cylinderrespectively.
Room temperature 24°C.
It can therefore be concluded that the observed influence of the presence of
very small concentrations of neon atoms in helium on the properties of the
afterglow in helium-neon mixtures is consistent with the occurrence of reaction
(1) and the value estimated for Qce.
Eindhoven, June 1958
REFERENCES
1) H. J. Oskam,
thesis, Utrecht 1957; Philips Res. Repts 13, 335-400, 1958.
2) D. J. Rose and S. C. Brown, J. appl. Phys, 23, 1028-1032, 1952.
3) M. A. Biondi
and S. C. Brown, Phys. Rev. 75,1700-1795,1949.
4) M. A. Biondi,
Rev. sci. Instrum. 22, 500-502, 1951.