Radiant energy – Radiant energy generation and sources – With container for radioactive source and radiation...
Patent
1993-12-10
1995-07-11
Anderson, Bruce C.
Radiant energy
Radiant energy generation and sources
With container for radioactive source and radiation...
2504961, G21F 502
Patent
active
054323533
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nuclear gauges for making measurements of traveling webs in continuous sheet-making systems.
2. State of the Art
In continuous sheet-making systems, on-line measurements are highly desirable. The on-line measurements can provide, for instance, early indications of upsets in process conditions, thus allowing process controls to be effected before substantial quantities of substandard material are produced. In practice, however, accurate on-line measurements are difficult to make. The measurement difficulties arise, in part, because modern sheet-making machines are large and operate at high speeds. Some paper-making machines, for example, produce paper webs that are 100 to 400 inches (2.5 to 10.0 meters) wide at rates of up to 100 feet (30 meters) per second.
For making on-line measurements of properties of traveling webs in continuous sheet-making systems, it is common to employ sensors that periodically traverse, or scan, the webs. One type of scanning sensor is the nuclear gauge. In operation, nuclear gauges direct nuclear radiation (beta rays) against a surface of a traveling web while detecting the absorbed (or transmitted) radiation. (The quantity of nuclear radiation absorbed over a given area is a measure of the basis weight of the absorbing material.) Nuclear scanning gauges typically use radioactive krypton gas as the beta ray source.
When using nuclear gauges, safety is a major concern. For safety reasons, it is important that nuclear sources are appropriately shielded, especially when not in use, to prevent accidental exposure of personnel who might be working near the gauge. In conventional practice, shielding of a nuclear gauge is accomplished by mounting the nuclear source material to a protective housing that has a shuttered window. When the nuclear gauge is in use, the shuttered window is opened to allow radiation to be emitted onto a traveling web. When the gauge is not in use, the shuttered window is closed, thus blocking radiation and reducing the opportunities for accidental exposure.
Although shielding of nuclear gauges is necessary, the design of the shielding must not impair the accuracy of measurements that are made by the gauge.
SUMMARY OF THE INVENTION
The present invention, generally speaking, provides a nuclear gauge for making measurements of traveling webs in continuous sheet-making processes. In the preferred embodiment, the nuclear gauge comprises an enclosure means, an encapsulated nuclear source, and a rotatable means mounted in the enclosure means for carrying the encapsulated nuclear source between two angularly-displaced positions. In practice, the two angularly-displaced positions comprise a) a first position whereat the encapsulated nuclear source is positioned for making measurements of a web that travels past the gauge, and b) a second position whereat the encapsulated nuclear source is positioned to face a sidewall of the enclosure means at a location remote from the first position. Also, in practice, the nuclear gauge includes an aperture which is formed through the enclosure means to intersect the cylindrical cavity for providing a window through which the encapsuled nuclear source, when located in the first position, can emit radiation onto a web that travels past the window. Still further in practice, the outer diameter of the wheel-like member approximates the inside diameter of the cylindrical cavity so that as the wheel-like member is rotated, the spacing between the periphery of the wheel-like member and the interior wall of the cylindrical cavity remains essentially constant.
In the preferred embodiment, the nuclear gauge further includes a plug of material having a low atomic number, which plug is mounted in the sidewall of the cavity at the second position for absorbing beta rays that are emitted from the encapsulated nuclear source. In practice, the low atomic number material is selected from the group consisting of aluminum, beryllium, carbon and Delrin.
Further i
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Axelrod Steve
Boissevain Mathew
Goss John D.
Hegland Philip M.
Wiley Scott C.
Anderson Bruce C.
Measurex Corporation
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