Fundamentals of Vacuum Technology (1248463), страница 36
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This effect is called thepositive X-ray effect, and it depends on the anode voltage as well as on thesize of the surface of the ion collector.Under certain circumstances, however, there is also a negative X-ray effect.Photons which impinge on the wall surrounding the gauge head releasephotoelectrons there, which again flow towards the anode, and since theanode is a grid structure, they also flow into the space within the anode.
Ifthe surrounding wall has the same potential as the ion collector, e.g. groundpotential, a portion of the electrons released at the wall can reach the ioncollector. This results in the flow of an electron current to the ion collector,i.e. a negative current flows which can compensate the positive ion current.This negative X-ray effect depends on the potential of the outer wall of thegauge head.The ion desorption effectAdsorbed gases can be desorbed from a surface by electron impact. For anionization gauge this means that, if there is a layer of adsorbed gas on theanode, these gases are partly desorbed as ions by the impinging electrons.The ions reach the ion collector and lead to a pressure indication that isinitially independent of the pressure but rises as the electron currentincreases.
If such a small electron current is used so that the number ofelectrons incident at the surface is small compared to the number ofadsorbed gas particles, every electron will be able to desorb positive ions. Ifthe electron current is then increased, desorption will initially increasebecause more electrons impinge on the surface. This finally leads to areduction in adsorbed gas particles at the surface. The reading falls againand generally reaches values that may be considerably lower than thepressure reading observed with a small electron current.
As a consequenceof this effect in practice, one must ascertain whether the pressure readinghas been influenced by a desorption current. This can be done most simplyby temporarily altering the electron current by a factor of 10 or 100. Thereading for the larger electron current is the more precise pressure value.In addition to the conventional ionization gauge, whose electrode structureresembles that of a common triode, there are various ionization vacuumgauge systems (Bayard-Alpert system, Bayard-Alpert system withmodulator, extractor system) which more or less suppress the two effects,depending on the design, and are therefore used for measurement in thehigh and ultrahigh vacuum range.
Today the Bayard-Alpert system isusually the standard system.a) The conventional ionization vacuum gaugeA triode of conventional design (see Fig. 3.16 a) is used as the gauge head,but it is slightly modified so that the outer electrode serves as the iona)Conventionalionizationvacuum gaugesystemb)ionization vacuumgauge system forhigher pressures (upto 1 mbar)c)Bayard-Alpertionizationvacuum gaugesystemd)Bayard-Alpertionization vacuumgauge system withmodulatore)extractor ionizationvacuum gauge systemC CathodeA AnodeI Ion collectorFig.
3.15 Explanation of the X-ray effect in a conventional ionization gauge. The electrons eemitted by the cathode C collide with anode A and trigger a soft X-ray radiation(photons) there. This radiation strikes, in part, the ion collector and generatesI ion collectorSc screenM modulatorA anodeC cathodeR reflectorFig. 3.16 Schematic drawing of the electrode arrangement of various ionization vacuum gaugemeasuring systems85HomeVacuum measurementcollector and the grid within it as the anode. With this arrangement theelectrons are forced to take very long paths (oscillating around the gridwires of the anode) so that the probability of ionizing collisions and thus thesensitivity of the gauge are relatively high. Because the triode system cangenerally only be used in high vacuum on account of its strong X-ray effect,the gas sorption (pumping) effect and the gas content of the electrodesystem have only a slight effect on the pressure measurement.b) The high-pressure ionization vacuum gauge (up to 1 mbar)A triode is again used as the electrode system (see Fig.
3.16 b), but thistime with an unmodified conventional design. Since the gauge is designedto allow pressure measurements up to 1 mbar, the cathode must beresistant to relatively high oxygen pressure. Therefore, it is designed as aso-called non-burnout cathode, consisting of an yttria-coated iridium ribbon.To obtain a rectilinear characteristic (ion current as a linear function of thepressure) up to a pressure of 1 mbar, a high-ohmic resistor is installed inthe anode circuit.c) Bayard-Alpert ionization vacuum gauge (the standard measuringsystem used today)To ensure linearity between the gas pressure and the ion current over aslarge a pressure range as possible, the X-ray effect must be suppressed asfar as possible.
In the electrode arrangement developed by Bayard andAlpert, this is achieved by virtue of the fact that the hot cathode is locatedoutside the anode and the ion collector is a thin wire forming the axis of theelectrode system (see Fig. 3.16 c). The X-ray effect is reduced by two tothree orders of magnitude due to the great reduction in the surface area ofthe ion collector. When pressures in the ultrahigh vacuum range aremeasured, the inner surfaces of the gauge head and the connections to thevessel affect the pressure reading. The various effects of adsorption,desorption, dissociation and flow phenomena cannot be dealt with in thiscontext. By using Bayard-Alpert systems as nude gauge systems that areplaced directly in the vessel, errors in measurement can be extensivelyavoided because of the above mentioned effects.d) Bayard-Alpert ionization vacuum gauge with modulatorThe Bayard-Alpert system with modulator (see Fig.
3.16 d), introduced byRedhead, offers pressure measurement in which errors due to X-ray andion desorption effects can be quantitatively taken into account. In thisarrangement there is a second thin wire, the modulator, near the anode inaddition to the ion collector inside the anode. If this modulator is set at theanode potential, it does not influence the measurement. If, on the otherhand, the same potential is applied to the modulator as that on the ioncollector, part of the ion current formed flows to the modulator and thecurrent that flows to the ion collector becomes smaller.
The indicatedpressure pI of the ionization gauge with modulator set to the anode potentialconsists of the portion due to the gas pressure pg and that due to the X-rayeffect pg:pA = pg + pγα < 1.The pg share of the X-ray effect is the same in both cases. Afterdetermining the difference between (3.4) and (3.5), we obtain the equationfor the gas pressure pg:pg =p −pAM1− α(3.6)α can immediately be determined by experiment at a higher pressure(around 10-6 mbar) at which the X-ray effect and thus pγ are negligible.
Thepressure corresponding to the two modulator potentials are read off andtheir ratio is formed. This modulation method has the additional advantagethat the ion desorption effect is determined in this way. It permits pressuremeasurements up to the 10-11 mbar range with relatively little effort.e) Extractor ionization vacuum gaugeDisruptive effects that influence pressure measurement can also beextensively eliminated by means of an ion-optical system first suggested byRedhead. With this extractor system (see Fig. 3.16 e) the ions from theanode cylinder are focused on a very thin and short ion collector.
The ioncollector is set up in a space, the rear wall of which is formed by a cupshaped electrode that is maintained at the anode potential so that it cannotbe reached by ions emanating from the gas space. Due to the geometry ofthe system as well as the potential of the of individual electrodes, thedisruptive influences through X-ray effects and ion desorption are almostcompletely excluded without the need of a modulator. The extractor systemmeasures pressures between 10-4 and 10-12 mbar.
Another advantage isthat the measuring system is designed as a nude gauge with a diameter ofonly 35 mm so that it can be installed in small apparatus.3.4Adjustment and calibration;DKD, PTB national standardsDefinition of terms: Since these terms are often confused in daily usage, aclear definition of them will first be provided:Adjustment or tuning refers to the correct setting of an instrument.
Forexample, setting 0 and 100 % in THERMOVACs or setting the massspectrometer to mass 4 in the helium leak detector.Calibration inspection refers to comparison with a standard in accordancewith certain statutory regulations by specially authorized personnel (Bureauof Standards). If the outcome of this regular inspection is positive, anoperating permit for the next operation period (e.g. three years) is madevisible for outsiders by means of a sticker or lead seal. If the outcome isnegative, the instrument is withdrawn from operation.(3.4)After switching the modulator from the anode potential over to the ioncollector potential, the modulated pressure reading pM is lower than the pIreading because a portion of the ions now reaches the modulator.
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