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SSINS No.: 6830 Accession No.: 8006190078 IN 80-33 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 September 15, 1980 Information Notice No. 80-33: DETERMINATION OF TELETHERAPY TIMER ACCURACY Description of Circumstances: New requirements for full calibration and spot-check measurements of teletherapy units in SSSS35.21 - 35.25 of 10 CFR Part 35 became effective on July 9, 1979. One of the determinations required is "timer accuracy" [SSSS35.21(b)(4) and 35.22(b)(1)]. Recent inspections conducted by the U.S. Nuclear Regulatory Commission (NRC) have determined that many licensees and consultant physicists have misunderstood this requirement. The purpose of this information notice is to clarify the meaning of "determination of timer accuracy (i.e., timer error)" as used in these regulations. There is a finite error in source output measurements due to the time required for the source shutter mechanism to travel from the full "off" to full "on" condition and return. This error may be positive or negative, depending on when the timer movement starts with respect to source travel. It is the determination of this "timer error" that is required in the regulations as part of full calibrations and monthly spot-check measurements. The timer error, once determined, may be corrected by adding its value (positive or negative) to the treatment time or exposure time during calibrations and spot-check measurements. Omission of this correction may introduce substantial error into short irradiations. Further, a significant change in timer error from month to month may indicate a timer malfunction or source movement mechanism failure or both. The timer error and acceptable methods for its determination are described in detail in SS4.14 of ANSI Standard N449.1-1978*. Comparison of the console timer with a stopwatch is not an acceptable substitute for this determination. ACTION TO BE TAKEN BY LICENSEES: Full calibration and spot check measurements performed on your teletherapy unit should include the determination of timer error as described above. This information notice should be brought to the immediate attention of the qualified expert (10 CFR Part 35.24) performing these calibrations and measurements. No response to this information notice is required. If you need additional information regarding this matter, contact the Fuel Facility and Material Safety, Branch of the appropriate NRC Regional Office. * Copies of ANSI Standard N449.1-1978 may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018.
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