Information Notice No. 80-33: Determination of Teletherapy Timer Accuracy

                                                       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.