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Automatic dosimeter checker
NUCLEAR
I N S T R U M E N T S AND
METHODS
97 0971) 425-426; © N O R T H - H O L L A N D
PUBLISHING
CO.
A U T O M A T I C D O S I M E T E R CHECKER M. L. WALSH
Ontario Hydro, Pickering G.S., Pickering, Ontario, Canada Received 16 August 1971 An 'automatic' dosimeter checker is described which substantially reduces the time required to give a direct-reading gamma dosimeter and constancy check.
At Ontario H y d r o ' s Nuclear Power Stations, personal direct-reading g a m m a dosimeters are routinely issued to all employees performing radioactive work. A,; a safety precaution each man is required to perform a constancy check on his dosimeter every two weeks. Formerly, this test was performed with a checker consisting of a lead castle containing two 10 mCi 137Cs sources. The dosimeter was dropped into the castle and exposed for one minute. The timing was done manually with a stop clock. This method, even at a relatively small station, resulted in long lineups and at a multiunit station such as Pickering G.S., the working time lost would be unacceptable. Other methods were considered including use of a simple checker with a
SWIT^CH SNAP MICROSWITCH II LIGHT-r" /SNAPLIGHT
large source so that the time would be reduced, or a checker with holes for several dosimeters. The first of these methods was rejected because of the error involved in measuring very short time intervals using a stop clock, and the second because of possible access problems and confusion which would be created as several persons tried to place and time their dosimeters at the same time. Instead, an 'automatic' checker was designed and built. The automatic checker is illustrated in figs. 1 and 2. A 100 mCi 137Cs source is placed in a container which
, A C SUPPLY
q, WER I SWITCH
~ POLYNOID LINEAR CRAMER L
DOSIMETER
FUSE ~ w ' ¢ ;
//// ~
Q I L I (SNAPLIGHT )
c~ FAN
D
TIMER MOTOR
MICROSWITCH SEALIN(D CLUTCH
(LEAD) Fig. 1. Automatic dosimeter checker, retracted position.
c,
DOSIMETER No
-137
RETRACT
~
I
BOXSHIELD(LEAD)
FORWARD
Fig. 2. Automatic dosimeter checker, forward position, details not shown.
425
c/L2
®
SWITCH X X
)<
Fig. 3. Dosimeter checker, logic diagram.
426
M.L.
screws on to the central rod of a Polynoid linear actuator (model 01A0899-111-10). The source is normally in the recessed position as shown in fig. 1 and a dosimeter placed in the hole receives a negligible dose during the time it takes to place it in the checker and press the microswitch to start the exposure. In order to irradiate the dosimeter the actuator is energized in the forward position for a preset time using a Cramer interval time (model 3811) and then automatically resets itself. The dosimeter reading is then compared to the known exposure. The logic of the device is illustrated by the ladder diagram in fig. 3. The power switch energizes the actuator stator, a snap light I L l , a fan to cool the actuator, and the timer motor. Timer switch 3 is open and timer switch 2 is closed. The phase of the current across the capacitor C1 is such that the source is in the retracted position. When microswitch 1 is pressed the timer clutch is picked up, switch 3 closes for twenty
WALSH
seconds and switch 2 opens. The phase shift across C1 is such that the actuator places the source in the forward position. Snap light IL2 is energized to indicate that the D R D is being irradiated. After the preset time interval, switch 3 returns to the normally open position and switch 2 closes. The phase across C1 shifts so that the actuator returns the source to the retracted position. The present checker at Pickering has operated successfully to date for about 10 000 cycles requiring no maintenance. Also the working time lost checking D R D ' s has been noticeably reduced. The cost of the electrical components are inexpensive at approximately $100.00. An improved version of the checker incorporating a 500 mCi source should further reduce the time per check to about four seconds.
I wish to extend my appreciation to Mr. W. McComb fol this assistance with this project.
I N S T R U M E N T S AND
METHODS
97 0971) 425-426; © N O R T H - H O L L A N D
PUBLISHING
CO.
A U T O M A T I C D O S I M E T E R CHECKER M. L. WALSH
Ontario Hydro, Pickering G.S., Pickering, Ontario, Canada Received 16 August 1971 An 'automatic' dosimeter checker is described which substantially reduces the time required to give a direct-reading gamma dosimeter and constancy check.
At Ontario H y d r o ' s Nuclear Power Stations, personal direct-reading g a m m a dosimeters are routinely issued to all employees performing radioactive work. A,; a safety precaution each man is required to perform a constancy check on his dosimeter every two weeks. Formerly, this test was performed with a checker consisting of a lead castle containing two 10 mCi 137Cs sources. The dosimeter was dropped into the castle and exposed for one minute. The timing was done manually with a stop clock. This method, even at a relatively small station, resulted in long lineups and at a multiunit station such as Pickering G.S., the working time lost would be unacceptable. Other methods were considered including use of a simple checker with a
SWIT^CH SNAP MICROSWITCH II LIGHT-r" /SNAPLIGHT
large source so that the time would be reduced, or a checker with holes for several dosimeters. The first of these methods was rejected because of the error involved in measuring very short time intervals using a stop clock, and the second because of possible access problems and confusion which would be created as several persons tried to place and time their dosimeters at the same time. Instead, an 'automatic' checker was designed and built. The automatic checker is illustrated in figs. 1 and 2. A 100 mCi 137Cs source is placed in a container which
, A C SUPPLY
q, WER I SWITCH
~ POLYNOID LINEAR CRAMER L
DOSIMETER
FUSE ~ w ' ¢ ;
//// ~
Q I L I (SNAPLIGHT )
c~ FAN
D
TIMER MOTOR
MICROSWITCH SEALIN(D CLUTCH
(LEAD) Fig. 1. Automatic dosimeter checker, retracted position.
c,
DOSIMETER No
-137
RETRACT
~
I
BOXSHIELD(LEAD)
FORWARD
Fig. 2. Automatic dosimeter checker, forward position, details not shown.
425
c/L2
®
SWITCH X X
)<
Fig. 3. Dosimeter checker, logic diagram.
426
M.L.
screws on to the central rod of a Polynoid linear actuator (model 01A0899-111-10). The source is normally in the recessed position as shown in fig. 1 and a dosimeter placed in the hole receives a negligible dose during the time it takes to place it in the checker and press the microswitch to start the exposure. In order to irradiate the dosimeter the actuator is energized in the forward position for a preset time using a Cramer interval time (model 3811) and then automatically resets itself. The dosimeter reading is then compared to the known exposure. The logic of the device is illustrated by the ladder diagram in fig. 3. The power switch energizes the actuator stator, a snap light I L l , a fan to cool the actuator, and the timer motor. Timer switch 3 is open and timer switch 2 is closed. The phase of the current across the capacitor C1 is such that the source is in the retracted position. When microswitch 1 is pressed the timer clutch is picked up, switch 3 closes for twenty
WALSH
seconds and switch 2 opens. The phase shift across C1 is such that the actuator places the source in the forward position. Snap light IL2 is energized to indicate that the D R D is being irradiated. After the preset time interval, switch 3 returns to the normally open position and switch 2 closes. The phase across C1 shifts so that the actuator returns the source to the retracted position. The present checker at Pickering has operated successfully to date for about 10 000 cycles requiring no maintenance. Also the working time lost checking D R D ' s has been noticeably reduced. The cost of the electrical components are inexpensive at approximately $100.00. An improved version of the checker incorporating a 500 mCi source should further reduce the time per check to about four seconds.
I wish to extend my appreciation to Mr. W. McComb fol this assistance with this project.