Patent Number: 052218422
Section: summary

BACKGROUND OF THE INVENTION This invention pertains to a self-contained calibrator/irradiator and method for calibrating and irradiating large numbers of several types of personnel dosimetry equipment. Operators of nuclear facilities employing large numbers of personnel have had a long standing requirement for calibrating (exposing to precisely known and reproducible fields of photon radiation, typically using 137 Cs as a source of radiation) film badges, thermoluminescent dosimeters, pocket dosimeters and electronic dosimeters. Typically these calibration procedures have been accomplished by exposing a radiation source, either manually or remotely using a calibrator in the center of a ring of dosimeters which may be 100 cm. to 120 cm. in diameter. The ring diameter will vary dependent on the dose rate to be delivered to the dosimeters Because dosimetric equipment has widely varying ranges, multiple dose rates are required to cover the full range of dosimeters regularly used. The use of a radiation source, whether manually or remotely controlled, to calibrate dosimeters has required a dedicated room with typical minimum dimensions of 12'.times.12' which is essentially clear of other materials to reduce scattered radiation effect, from which all personnel must be evacuated while radiation (calibrations) is in progress and which typically requires shielded floor, walls and ceiling to reduce radiation in surrounding areas to levels in which personnel can be present during the irradiation. Later versions have required huge lead shielding structures and, although they did not require an entire dedicated room, they were over 1.5 meters in diameter, weighed well over 1400 kg and certainly could not be considered portable in that, once set up in a given location, they were seldom or never moved. As a result of the foregoing, it became clear to the applicant that there was a need for a much smaller calibrator/irradiator which is truly portable. BRIEF DESCRIPTION OF THE INVENTION Applicant has developed a completely self contained (shielded) calibrator for calibration of large numbers all types of personnel dosimetry equipment as listed above, with accuracy and reproducibility of greater than plus or minus 2%, scattered radiation component of less than 3%, which is portable (mounted on casters) in that it can be moved through standard doorways, and which has external radiation levels of less than 2 mr/hr which means that it can be used anywhere in a nuclear facility (laboratory or office) without personnel exclusion or any external shielding. Applicant's device provides automatic exposure control by means of preset time control of the radiation source exposure, multiple dose rate capability by means of automatically controlled attenuators and multiple source configuration and complete safety of operation because the cover (lid) of the exposure chamber and source exposure mechanism are completely interlocked. Multiple removable dosimeter holder racks are provided so that all types of pocket dosimeters, TLD's, film badges and electronic dosimeters may be calibrated in the same calibrator. The device includes a shielded cylindrical chamber including a shielded lid or cover which is interlocked with the radiation source to prevent inadvertent exposure of personnel to significant levels of radiation. A shielded rod is vertically movable along the axis of the chamber to place one or more radiation sources in position to irradiate the dosimeters. Additional attenuation means may be employed to provide reduced levels of radiation. The dosimeters to be radiated are placed in any of several ring-shaped racks which are positioned on a turntable in the chamber concentrically of the source rod such that they are rotatable around the source for a time sufficient to provide essentially uniform radiation of all the dosimeters in the rack, the time being controlled manually or by a timer connected to control the drive motor.