Radiant energy – Calibration or standardization methods
Reexamination Certificate
1998-03-05
2001-08-07
Hannaher, Constantine (Department: 2878)
Radiant energy
Calibration or standardization methods
C250S231130, C313S093000
Reexamination Certificate
active
06271516
ABSTRACT:
DESCRIPTION
This invention relates to an environment radioactivity meter or briefly called radiameter, based upon a G.M. tube and provided with a device adapted to monitor the sensitivity degradation, in which a so weak control source is utilized as to be made by a nature occurring radioisotope, and further provided with a correction circuit adapted to automatically correct its response to the measured radioactivity. The concerned radiameter is also optionally provided with an alarm circuit, which furnishes a warning signal when the sensitivity degradation is higher than a pre-established threshold value.
The radiameter according to this invention is mainly intended for use in the environment monitoring networks for territorial control, comprising a large number of measurement stations and, compared with all radiameters presently in use, it has the advantageous feature that it provides an indication of the environment radioactivity with an effectively uniform response over the whole territory, regardless of the time elapsed in the individual stations after their latest revision. Furthermore, since it provides an indication of the sensitivity degradation in the measurement circuits at each station, which, in turn, is an indication of the residual possible operation time of the station before a further revision thereof becomes necessary, it enables a reasonable revision schedule to be established for each station.
BACKGROUND OF THE INVENTION
Aiming at avoiding too large differences in the responses to the environment radioactivity and at establishing a proper revision schedule, almost all radioactivity meters are presently subjected to periodic controls by using a radioactive source, to which the radiameter is manually exposed by an operator, or, in more sophisticated apparatuses, by adopting a radioactive source incorporated with the radiameter itself and by remotely or automatically controlling the apparatus by an operator. Since, during the time period in which the above control action is effected, the radiameter is not operative, said time period should be relatively short and, therefore, the source to be used should have such an activity as to cause the G.M. tube to generate pulses having a high intensity compared to the one generated by the standard environment radioactivity.
The sensitivity monitoring device included in a radiameter according to this invention, in contrast, uses a source that, when it is positioned adjacent to the G.M. tube, it causes a measurement having the same magnitude order as the one caused by the background environment radioactivity to be indicated. Such indication intensity is so low that it cannot be considered under strict legal terms as a radioactive source and, therefore, any biologic damage to the network maintenance personnel or to the station revision personnel is avoided.
The measurement of the activity of said source, which is contemporaneous with the measurement of the environment radioactivity does not jeopardize the response of the radiameter to said environment radioactivity and the measurement operation takes a very long time with respect to the radiameter integration time and, therefore, it is very accurate and enables even minimum sensitivity variations to be automatically corrected. This makes the measurement accuracy in the various stations very uniform and makes the radioactivity topographic maps produced by the environment monitoring network more reliable.
The indication of the sensitivity degradation improves the revision schedule of the stations, thereby making the management of the environment radiametric networks much more cost effective, with respect to all complex and very expensive maintenance operations up to now carried out.
The decay time of the nature occurring radioisotope which is used as the control source is assimilable to infinite and, therefore, it eliminates any need that its measurement values be corrected to compensate the source decay, thereby facilitating the sensitivity controls during the revision operations of the stations.
PRIOR ART
According to this invention, a G.M. tube digital intensity meter in which use is made, for instance, of a double information type device, such as the one disclosed in Italian Patent No. 1.073.902, granted on Apr. 17, 1985, in the name of ITAL ELETTRONICA S.p.A., comprises a sensitivity monitoring device adapted to automatically control the response to the measured radioactivity, including a weak radioactive source mounted upon an electrically operated movable support, which alternatively switches its location with respect to the G.M. tube, in a rapid and reproducible manner, into a close position, in which it remains during a time T′, or into a spaced position, in which it remains during a time T″. The type of the used radioisotope, the intensity of the source, the geometries of the support and of the G.M. tube are such that, in the close position, the intensity of the pulses generated by the source contained in the G.M. tube is of the same magnitude order as the one generated by the nature occurring background radioactivity, but, in the spaced position, it is negligible with respect thereto. A timer included within the device operates in order that T′ and T″ are both constant and, by way of exemplification, we shall assume hereinbelow that T′=T″. During the time T′ in which the source is in close position, a suitable circuit included in the device generates pulses having an intensity corresponding to the mean intensity generated within the G.M. tube by the source and electrically subtracts it from the pulse intensity measured by the concerned radiameter. In both positions of the source, the radiameter continuously operates and provides a correct measure, to which statistical oscillations are associated, the statistical oscillations occurring during the T′ times being greater than the statistical oscillations occurring during said T″ times, in view of the fact that, during the T′ times, the statistical oscillations of the environment radioactivity are combined with the oscillations of the statistical pulses generated by the source. The alternated time period in which the source is close to or spaced from the G.M. tube is of the same magnitude order as the integration time of the radiameter (in particular, they may be the same), in order to prevent the variations of the measured radioactivity from interfering with the measure furnished by the source.
The measurement of the mean intensity of pulses generated in the G.M. tube by the source is carried out by adding in two counters all N′ pulses arriving during M time periods T′ as well as all N pulses arriving during M time periods T″. Since, as it has been assumed, T′=T″, the mean intensity of the pulses generated by the source turns out to be (N′−N″)/MT′ and this is, therefore, the value subtracted by the circuit from the pulse intensity measured by this radiameter during the time periods T′, in order to measure the intensity caused by the environment radioactivity. When the value M is chosen sufficiently large, any statistical oscillations under which one can measure the pulse strength generated by the source can be reduced as desired. When the response of the radiameter to the environment radioactivity as measured is to be automatically corrected, the circuit according to this invention includes a non-volatile memory in which the value N°=(N′−N″), as measured during the calibration operation carried out at the starting test stage or during a complete overhaul stage, is stored. During the subsequent lifetime of the radiameter, should its sensitivity be altered, the measurement performed therein without any correction of the response will be altered according the (N′−N″)/N° ratio. Since the true value of the environment radioactivity is desired, the circuit for automatically correcting the radiameter response provides for multiplying the incorrect me
Casale Renato
Castellani Renato
Gabor Otilia
Hannaher Constantine
Ital Elettronica S.p.A.
Smith , Gambrell & Russell, LLP
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