Alcatel EA10 Description (short form)
This instrument is designed to detect the presence of helium (or hydrogen or helium 3) in installations with their own pumping system. It is usually used to detect leaks.
The assembly has two sub-assemblies:
- The mass spectrometer cell and its magnet, and an Alcatel PIl Pirani gauge to prevent the filament from lighting when the pressure is too high.
- A control module connected to the cell by a ten-foot cable.
The assembly is connected to the system under test by using a Pneurop NW25 flange.
A metal seal or a Viton seal should be used.
Analysis Cell and Pirani Gauge - Principle
This this a mass spectrometric analysis cell, adjusted to the mass of helium (m/e = 4) or another
"light mass", with a mass of 2 or 3.
- m/e: atomic mass of the particle divided by the number of electrons lost during ionization.
The principle of the magnetic deflection spectrometer is as follows :
the neutral molecules of the gas being analyzed enter an ionization chamber (or ion source) where they are bombarded by a beam of electrons from a heated tungsten filament. Many of the molecules are transformed into ions, which are then accelerated by an electric field.
The entire analysis cell is located within a magnetic field which curves the trajectories of the ions along different radii, as a function of their masses (more precisely, as a function of m/e). Thus, the ion beam, containing ions of different masses, breaks up into a number of beams, each containing only ions with the same m/e number. Helium ions (m/e = 4) are separated from lighter ions (H2 + H1, with smaller radii) or heavier ions ( N2+, 02+, with larger radii).
A constant magnetic field provided by a permanent magnet allows the accelerating electric field to be adjusted so that the helium ions (m/e = 4) describe a fixed trajectory (passing through diaphragms) and strike a target connected to the input of a DC amplifier.
The helium ion current is proportional to the partial pressure of helium in the unit and is a function of the flow through the leak. The total analysis cell pressure must be less than 2x10-4 mbar in order to prevent stray molecules from disturbing the trajectories of the electrons and ions ; however, there is no risk of damaging the heated filament until approximately 10-3 mbar.
Note 1 : in order to separate the helium ions reliably from the backround noise produced by the scattered ions, an electrode mounted in front of the target eliminates the low-energy secondary ions. This electrode is called the repeIler.
Note 2 : an auxiliary plate-shaped electrode is mounted at the top of the cell and collects ions with masses greater than that of helium. This electrode also permits measurement of the total analyzer pressure without using a Penning gauge. The Penning gauge is a noise source and should therefore not be used when measuring very weak signals (1x10-11 atm x cc/s). This electrode acts as the plate of a triode gauge, and is therefore referred to as a triode electrode.
