Radiant energy – Invisible radiant energy responsive electric signalling – Including ionization means
Reexamination Certificate
1999-12-06
2001-08-21
Berman, Jack (Department: 2881)
Radiant energy
Invisible radiant energy responsive electric signalling
Including ionization means
Reexamination Certificate
active
06278121
ABSTRACT:
The invention relates to an ionization chamber for radiometric measuring instruments, particularly for traversing systems, which measure the mass per unit area and consist of a housing, which is filled with a gas and has at least one radiation inlet window and a number of collector electrodes in the housing with insulated electrical connections, which are taken to the outside, there being an electrical potential difference (voltage) between the housing and the collector electrodes.
In industrial equipment for the radiometric measuring of sheets of materials, ionization chambers usually are used as detectors during their production or processing. The ionization chambers consist of a housing, a collector electrode and a filler gas. The radiation, entering through the radiation inlet window, produces free charge carriers (ions and electrons) in the filler gas. A voltage, applied between the electrode and the housing, produces in the chamber an electrical field, which the charge carriers follow. The thus resulting current between the electrode and the housing (in the &mgr;A up to the pA range) is measured and converted, for example, into voltage signals. The measurement signal, highly insulated from the housing, is passed from the interior of the chamber over a gas-tight feed-through with a connection to a signal lead to the outside. A ring electrode, which acts as a guard ring, is introduced around the connection in the insulating layer of the feed-through. This guard ring prevents the voltage dropping off directly over a continuous insulated segment between the housing and the electrode, as shown in
FIG. 1
as state of the art for an axially symmetrical ionization chamber. The guard ring thus prevents the occurrence of interfering leakage currents, so that the basic current of an ionization chamber without radiation is minimal at its operating voltage (generally a few hundred volts), and is typically less than 0.1 pA.
The radiometric system of measuring in a production or processing plant consists of a source of ionizing radiation, the detector, that is, the ionization chamber and the material to be measured. The degree of interaction between the radiation and the material being measured (such as absorption, back scatter, fluorescence) is a measure of the amount of material to be determined, mostly given as weight per unit area or density. The system of radiation source and detector can in most places be moved transversely to the sheet of material.
With the transition from a single detector to a detector with several independent measuring sites, new possibilities have opened up in the technique of measuring the weight per unit area for solving previously unsurmounted measurement problems. The additional information, so developed, offers a base for a more effective and accurate control of production processes.
On the one hand, the local resolution of the transverse profile can be refined.
The local resolution of a measurement of a transverse profile with the conventional, axially symmetrical ionization chamber is limited naturally by the diameter of the chamber. For example, the resolvable structure is stated in the technical literature to be twice the extent of the detector. A finer local resolution must be achieved with detectors of a lesser extent in the direction at right angles to the sheet of material. These detectors can be disposed, so that a transverse profile section can be measured with a resolution higher than in the case of a corresponding single detector.
The use of an array of semiconductor detectors (silicon pin diodes), which are operated in the current mode, is known. However, the semiconductor detectors are highly susceptible to temperature changes of the type that occur constantly in industrial production plants. As a result, the measurement signals are distorted.
On the other hand, the energy spectrum can be used as a consequence of the interaction of X-rays with the material being measured.
For certain coating processes, the atomic number of the substrate differs only slightly from the atomic number of the layer that is to be applied (for example, zinc on steel). In this case, the known beta back scattering method is unsuitable. However, a possible X-ray fluorescence radiation of the two-component system provides information about the thickness of the layer applied. The energy of the fluorescence radiation is specific for each element, while the intensity depends on the amount of material, which is being investigated and, with that, on the thickness of the layer. Due to the K edge effect, selective filters strongly absorb, for example, the X-rays emanating from the layer and largely transmit the radiation emanating from the substrate. Two detector sections with different filters can, after being calibrated, be used to measure one component of the two-component system.
These detector sections can be disposed in a multi-chamber, as described. Beyond the two-component system, n−1 components of an n-component system can be determined with a chamber with n measurement sections in principle in certain cases.
The technique described is published as U.S. Pat. No. 3,514,602. A chamber is divided here into two sections, the signals of which are subtracted from one another as analog signals, in order to obtain an output signal as a measure of the desired quantity to be measured. This corresponded to the state of the art at that time. The present state of the art corresponds to a converted (low impedance) chamber section output signal, which can be supplied to appropriate, further, computer-based or processor-based processing.
These measurement problems, listed by way of examples, therefore led to the object of developing an ionization chamber for radiometric measuring instruments, especially for traversing systems, which measure the mass per unit area, have an adequate sensitivity and, at the same time, make available the customary good values of ionization chambers with respect to vacuum tightness, basic current and temperature dependence.
This objective, on which the invention is based, is accomplished by an ionization chamber of the type named above owing to the fact that the interior of the housing is divided into a plurality of adjacent and mutually delineated measurement sections with the respective collector electrodes, that the collector electrodes are connected with electrical connections, which are taken through the insulator of a gas-tight multiple lead-through to the outside and that the insulator is provided with an electrically conductive region, which surrounds the electrical connections jointly and is insulated electrically from the housing as well as from the connections, yet lies in a currentless state at the electrode potential.
Due to this inventive solution, it becomes possible to measure the weight per unit area by a method which, when compared to a method using conventional ionization chambers, achieves, for example, a finer local resolution or also an energy selection of the radiation emanating from the site of the measurement.
Further developments of the invention arise out of the dependent claims.
In this regard, the region, surrounding the connections, is constructed as a guard ring in the form of a metal ring, which surrounds several collector electrode connections simultaneously. The insulator, between the housing and the guard ring, can be part of an insulating pipe, the one metal contact of which is connected gas-tight with the housing. A multiple feed-through can be connected gas-tight with the opposite metal contact.
In a variation of the invention, the guard ring can also be constructed as ajunction electrode, which surrounds the connections on at least one side of the insulator. Preferably, however, the junction electrode is disposed on the inside as well as on the outside of the housing. The two junction electrodes are connected electrically with one another and jointly with a connector pin and accordingly are at the potential of the guard ring.
In a continuation of the invention, the measurement sections are separated from o
Hildebrandt Steffen
Laube Siegfried
Urban Franz-Josef
Berman Jack
Jordan and Hamburg LLP
Smith II Johnnie L
Vacutec Messtechnik GmbH
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