Page 168 - Instrumentation Reference Book 3E

152 Measurement of vacuum
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I; n &G 11

Figure 10.14 Hot-cathode ionization gauge.
involved: at the filament one takes place in which
molecules of active gases combine to form stable
solid compounds; in the other, at the ion-collector,
the positive ions embed themselves beneath its sur-
face. The speed of pumping can be reduced by Figure 10.15 The Bayard-Alpert ionization gauge.
lowering the filament temperature and by reducing
the rate of collection of ions. 10.3.2.4 The Bayard-Alpert ionization gauge
(1950)
Relationship between ion current and pressure If The soft X-ray eSfect The conventional ioniza-
the pressure is p, the ion current i+, and the tion gauge described in Section 10.3.2.3 is not
electron current i, it is found for a given gas, say able to measure pressures below lop6 Pa, due to
nitrogen, that the presence of a spurious current in the ion-
collector circuit produced by processes occurring
z+ Ocpz- (10.1) in the gauge, and independent of the presence of
Therefore gas. This current is produced by soft X-rays gen-
erated when electrons strike the grid. The phe-
it = KpiL (10.2) nomenon is called the soft X-ray effect. The
wavelength X of this radiation is given by
where K is a constant called the gauge factor for
nitrogen. The SI unit is Pa-', and its value for an X = 1200/Vnm (10.4)
average gauge is 0.1 Pa-'. The value for a par-
ticular gauge is given by the maker. For other where V is the grid potential. If this is 120 volts
gases then X = 10nm. Radiation of this wavelength
cannot escape through the gauge envelope, but
i+ = CKpiL (10.3) when absorbed by the ion-collector causes the
emission of photoelectrons which are collected
where C is the relative sensitivity, with respect to by the grid. In the collector circuit the loss of an
nitrogen. Its approximate value for various gases electron cannot be distinguished from the gain of
is as follows: a positive ion, so that the process results in a
Gas He H2 N2 Air Ar Xe Organic steady spurious ion current superimposed on the
vapors true ion current. The spurious current is about
C 0.16 0.25 1.00 1.02 1.10 3.50 >4.0 lo-'' A and is of the same order of magnitude as
true ion current at IOp6 Pa. It is therefore difficult
Equation (10.3) shows that for a given gas and to measure pressures below this value with this
value oft type of gauge. A modified design due to Bayard
and Alpert shown in Figure 10.15 enables the
i+ KP area of the ion-collector, and hence the spurious
current, to be reduced by a factor of lo3, thereby
This valuable property of the gauge is obtained extending the range to lo-' Pa.
by stabilizing i- by means of an electronic servo
system which controls the filament temperature,
and can switch the filament off if the gauge pres- Construction The filament F is mounted outside
sure becomes excessive. a cylindrical grid G, having a fine wire collector C