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The national bureau of standards 4πβ-γ coincidence-counting and γ-ray intercomparator automatic sample changers
NUCLEAR
INSTRUMENTS
AND
METHODS
112
(1973)
2t3-2t7;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E N A T I O N A L BUREAU O F S T A N D A R D S 4n if-? C O I N C I D E N C E - C O U N T I N G AND ?-RAY I N T E R C O M P A R A T O R A U T O M A T I C S A M P L E C H A N G E R S S. B. G A R F I N K E L * , W. B. M A N N and J. L. P A R A R A S
National Bureau o f Standards, Washington, D.C. 20284, U.S.A. Instruments o f very simple and somewhat novel design will be described. The 4 ~ / 3 - 7' changer holds from 1 to 30 carriers o f sources, each of which can be counted for any preset number, up to ten, o f counting periods. The 7'-ray changer accommo-
dates from one to 24 source carriers. In comparing ?'-ray source count rates, the detector-to-source distances for individual sources are carefully controlled.
1. Introduction
use in the efficiency-tracing method, and on ?-ray point-sources to determine their relative ?-ray-emission rates.
In order to cope with the exploding demand for radioactivity standards that is concomitant with the rapidly expanding use of radioactive materials and their potential impact upon the environment, it is desirable both to automate measurements wherever possible, and to interface the results of such measurements to the input of a time-sharing computer for reduction to the activity or ?-ray emission-rate values for each source. In this way large numbers of readings can be taken on sources of different efficiency, say for * Deceased 22 December 1972.
2. The 4~ if-? coincidence-counting automatic sample changer Several national laboratories have designed and constructed automatic sample changers, for use in conjunction with 4n [3-? coincidence counting equipment, and some have been rather complicated in design. The NBS design is very simple, and, aside from the source carriers (which all changers utilize), has but four moving parts: a rotating ring, which stores the
j¢
Fig. I. Automatic changer and associated 4n fl-7' coincidence-counting equipment.
213 IV. C O I N C I D E N C E
COUNTING
214
s . B . GARFINKEL et al.
source carriers, a pneumatically-operated piston which moves the source carrier, in and out of the 4n 13 proportional counter, an electric motor for rotating the ring, and a spring-loaded solenoid-operated plunger which serves to locate the ring position when sources are being changed. The speed of the pneumaticallyoperated piston can be adjusted over a wide range, and characteristically moves sources into and out of the 4n 13 counter at a little less than 1 cm s -~ in order to avoid damage to the thin-film source mounts. A photograph of the assembled 4~ 13-7 coincidencecounting sample changer is shown in fig. 1. The 4~ 13 windowless gas-flow proportional counter, which was built from drawings kindly supplied by J. G. V. Taylor of the Chalk River Nuclear Laboratories cf Atomic Energy of Canada Ltd., is contained in the lead shielding on the left, directly below the 3½" i.d. × 8"o.d. cylindrical lead shield for the upper 3"× 3" NaI(TI) crystal-phctotube assembly. A similar crystal-phototube assembly is hung under the table, directly below the 4~ fl counter. The anode leads and gas-flow tubes can be seen entering the 4~ 13-counter shield, and a 14
15
*,
• •
microswitch, that senses when the source carrier is in position, can also be seen, partially hidden by the gas-flow tubes. The storage ring of the changer can accommodate from l to 30 source carriers. For each source carrier there is a corresponding index hole that can be seen on the periphery of the storage ring into which the solenoid-operated plunger can enter, in order to stop rotation of the carrier ring, through use of a microswitch as explained later, and locate the exact position for moving the source carrier into the 47t fl counter. Associated with each source carrier position there is also one of a system o f 3 ~ " holes arranged in a binaryto-decimal coded pattern to indicate, in conjunction with photocells, the identifying number of the source being counted. The lay-out of the outer ring of ~%" index holes and three inner rings of 3 ,, binary-to-decimal coded holes can be seen in the schematic drawing of the carrier ring shown in fig. 2, where the binary-codeddecimal system is also shown. Different positions of these holes correspond to the numbers 1, 2, 4, 8, 10, and 20.
16 17
,2
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O0
~ 18
•• •
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\\\
19
"-\
".'. /
21~" DIA,
Ii"
• a
I
~
/
* KADON BEARIN~ B~6031-t- 6L2
!
21"01~.
J/
~'/
•0
000
":.7" 26
~24"DIA,
3 I" DIA. _ _
e•
3
•
"~'~0
••
~o
27
SECTION ~ . ~ FULL SGALE
2 I
30
29
Fig. 2. Lay-out of rotating storage ring showing disposition of solenoid-operated-plunger index holes (larger diameter) and binary-todecimal coded holes for source identification.
THE NBS
4n/~-~ C O I N C I D E N C E
COUNTING
215
Fig. 3. View o f a u t o m a t i c 4nil- 7 c h a n g e r with lead shielding for 3" x 3" NaI(TI) crystal r e m o v e d to s h o w 4~fl c o u n t e r with source carrier in c o u n t i n g position. T h e bakelite box, straddling the storage ring to the right, houses the six photocells for source identification.
In fig. 3 positions 1-8 can be clearly seen in the nearest part of the carrier ring. Contained in the bakelite housing straddling the carrier ring on the right are six ¼" diameter semi-conductor photocells. These can be illuminated in different combinations, corresponding to the binary-to-decimal coded holes which, in that position, transmit light from a light located below the ring. The photocells are connected to binarycoded-decimal decoders which then permit of the sample number being recorded, together with the measurements data through suitable electronic interfacing, on to puched paper tape. These data are subsequently fed into a time-shared computer for reduction to activity values for the sources. Referring again to fig. 3, which also shows the 47t fl counter assembly, it can be seen that the selonoidoperated plunger is in the index hole of the sector containing the binary code for the number 4. Because of the necessary offsets of the plunger and photocell positions from the position of the source carrier in the counter this does not mean that source number 4 is
being counted. The offsets are such that source carrier number 27 is actually in the 4n/~ counter. These offsets are quite arbitrary and a matter of convenience in locating the plunger and photocell assembly. For the moment it is, however, also convenient to refer to the plunger being in the index hole in sector 4, as distinct from the actual carrier number. The index holes also provide for those cases where no source is present in the source carrier. The holes have shoulders and are threaded for the purpose of inserting special screws that fill the holes to the bottom surface of the source-carrier ring. Such an inserted screw can be seen in fig. 3 in sector 7. When the ring is rotating the plunger does not sense a hole where such a blocking screw is inserted and it therefore ignores this sourcecarrier position, which presumably has no source, and the ring continues rotating until an empty hole is presented for the plunger to enter. Let us now consider a complete counting cycle. We will suppose that the ring is rotating and the first open hole arrives above the spring-loaded solenoid-operated IV. C O I N C I D E N C E
COUNTING
216
s.B. GARFINKEL et al.
plunger, so that the plunger can enter the hole. The ring-drive motor is topped by a microswitch operated by the plunger as it is entering the hole (the plunger itself does n o t stop the ring), and the pneumatic piston pushes the source carrier into the 4n fl counter. This last operation is initiated by a circuit that senses that the motor has stopped. When the source carrier has been fully inserted into the counter, the microswitch shown to the right of the 4 n fl counter in fig. 3 is closed, which resets the counting electronics and puts a 40-s time-delay circuit into operation which provides sufficient time for the counting gas to pre-flush the counter, after which the counting period starts. A cycle consists of a preset number (1-10) of counting periods, after which the source-changing cycle is repeated. The counting periods themselves are pre-set either by a time period, by the number of counts in the//channel, or number of counts
in the coincidence channel, whichever of the three is achieved first. At the end of each counting period, the following data are automatically recorded: source number, calendar time (in seconds), duration of count (in hundredths of seconds), and counts in the fl, ? and coincidence channels. After the data have been recorded, all the scalers are reset, except for the calendar one, and a new count is started. The recording of a calendar time with each set of data allows all data to be computer-corrected to a fixed reference time. Both the calendar and count-duration scalers are fed from standard frequency signals, which are available in all buildings at the National Bureau of Standards. When the source has been counted for the predetermined number of counting periods, a relay causes the pneumatic piston to withdraw the carrier from the counter back into the carrier ring. At its furthermost point of withdrawal the spring-loaded pin (which can
Fig. 4. Automatic changer for intercomparison of 7-ray point sources. The source-identification photocells are located under the ring below the light reflector seen in the far corner.
THE NBS
4re fl-7
COINCIDENCE
be seen in fig. 3 just short of entering the 47~fl counter) disengages from the source-carrier hole (similar holes can be seen in the other source carriers in fig. 3), and the microswitch, to the left of the plunger in fig. 3, is closed. The closing of this microswitch causes the solenoid-operated plunger to withdraw from the index hole in the carrier ring and to actuate the solenoidplunger microswitch that starts the ring-drive motor. The ring then rotates until the next open hole is sensed by the plunger and the whole counting cycle is repeated.
3. ~-ray intercomparator automatic sample changer An improved design for an automatic changer for the comparison of 7-ray sources has also been introduced to replace that of our older 7-ray intercomparatorl). The new version is shown in fig. 4. It incorporates the same "loading-platform" feature of the older version, in order to maintain high precision in the reproducibility of source-to-detector distance. A major innovation is the introduction of circular symmetry of the carrier ring around the detector axis, so that the effect on background due to the sources not being counted will always be the same whatever their position. Provided that all of the sources of the same radionuclide in the set of N being counted are not too different in activity from each other, the background due to the ( N - 1) not being counted at any given time, . which is also attenuated through a 4 zi t t i.d. × 12 tto.d. lead shield, should not vary within the desired limits of accuracy. The detector is a 3"× 3" NaI(T1) crystal-phototube assembly and the same ring-carrier design, with plunger positioning of the sources, used in the 4~ fl-7 coincidence counting sample changer, has been
COUNTING
217
adopted. The ring carrier is, however, designed only to store from 1 to 24 sources. The index hole (with threaded inserts) with spring-loaded solenoid-operated plunger and the binary-coded source number indicators are also similar. The cycling is also controlled in the same manner, using terminal microswitches at each end of the source carrier and plunger travel from the carrier ring into the central counting position and back again. The electronics equipment of the 4 ~ fl- ~ changer is used to effect the choice of number of counting periods, preset times or preset counts, and the recording of measurements data (but now just for the y-ray channel). Calendar time and source number are also recorded. The sources shown stored in the carrier ring in fig. 4 are 7-ray point-source standards of 85Kr implanted in aluminum foil and sandwiched between two plate-glass discs sealed together with optically epoxy resin.
Reference 1) S. B. Garfinkel, W. B. M a n n a n d W. J. Y o u d e n , J. Research NBS, 70C (1966) 53.
Discussion Reher: D r M a n n , y o u said that the identification o f the source to be m e a s u r e d is its position in the sample changer. I could imagine there is a certain possibility that the source is placed in the w r o n g position. D o y o u therefore k n o w a b o u t a system to identify the source itself? Mann: If this h a p p e n e d I t h i n k the discrepancies in the comp u t e d activities per unit m a s s w o u l d s t a n d o u t a n d the mistake could be corrected. It is m o r e i m p o r t a n t to k n o w if the changer m a l f u n c t i o n s in the middle o f the night! Taylor: A t C h a l k River we keep a log b o o k recording the identification a n d position o f each source. There have been n o problems.
IV. C O I N C I D E N C E
COUNTING
INSTRUMENTS
AND
METHODS
112
(1973)
2t3-2t7;
©
NORTH-HOLLAND
PUBLISHING
CO.
T H E N A T I O N A L BUREAU O F S T A N D A R D S 4n if-? C O I N C I D E N C E - C O U N T I N G AND ?-RAY I N T E R C O M P A R A T O R A U T O M A T I C S A M P L E C H A N G E R S S. B. G A R F I N K E L * , W. B. M A N N and J. L. P A R A R A S
National Bureau o f Standards, Washington, D.C. 20284, U.S.A. Instruments o f very simple and somewhat novel design will be described. The 4 ~ / 3 - 7' changer holds from 1 to 30 carriers o f sources, each of which can be counted for any preset number, up to ten, o f counting periods. The 7'-ray changer accommo-
dates from one to 24 source carriers. In comparing ?'-ray source count rates, the detector-to-source distances for individual sources are carefully controlled.
1. Introduction
use in the efficiency-tracing method, and on ?-ray point-sources to determine their relative ?-ray-emission rates.
In order to cope with the exploding demand for radioactivity standards that is concomitant with the rapidly expanding use of radioactive materials and their potential impact upon the environment, it is desirable both to automate measurements wherever possible, and to interface the results of such measurements to the input of a time-sharing computer for reduction to the activity or ?-ray emission-rate values for each source. In this way large numbers of readings can be taken on sources of different efficiency, say for * Deceased 22 December 1972.
2. The 4~ if-? coincidence-counting automatic sample changer Several national laboratories have designed and constructed automatic sample changers, for use in conjunction with 4n [3-? coincidence counting equipment, and some have been rather complicated in design. The NBS design is very simple, and, aside from the source carriers (which all changers utilize), has but four moving parts: a rotating ring, which stores the
j¢
Fig. I. Automatic changer and associated 4n fl-7' coincidence-counting equipment.
213 IV. C O I N C I D E N C E
COUNTING
214
s . B . GARFINKEL et al.
source carriers, a pneumatically-operated piston which moves the source carrier, in and out of the 4n 13 proportional counter, an electric motor for rotating the ring, and a spring-loaded solenoid-operated plunger which serves to locate the ring position when sources are being changed. The speed of the pneumaticallyoperated piston can be adjusted over a wide range, and characteristically moves sources into and out of the 4n 13 counter at a little less than 1 cm s -~ in order to avoid damage to the thin-film source mounts. A photograph of the assembled 4~ 13-7 coincidencecounting sample changer is shown in fig. 1. The 4~ 13 windowless gas-flow proportional counter, which was built from drawings kindly supplied by J. G. V. Taylor of the Chalk River Nuclear Laboratories cf Atomic Energy of Canada Ltd., is contained in the lead shielding on the left, directly below the 3½" i.d. × 8"o.d. cylindrical lead shield for the upper 3"× 3" NaI(TI) crystal-phctotube assembly. A similar crystal-phototube assembly is hung under the table, directly below the 4~ fl counter. The anode leads and gas-flow tubes can be seen entering the 4~ 13-counter shield, and a 14
15
*,
• •
microswitch, that senses when the source carrier is in position, can also be seen, partially hidden by the gas-flow tubes. The storage ring of the changer can accommodate from l to 30 source carriers. For each source carrier there is a corresponding index hole that can be seen on the periphery of the storage ring into which the solenoid-operated plunger can enter, in order to stop rotation of the carrier ring, through use of a microswitch as explained later, and locate the exact position for moving the source carrier into the 47t fl counter. Associated with each source carrier position there is also one of a system o f 3 ~ " holes arranged in a binaryto-decimal coded pattern to indicate, in conjunction with photocells, the identifying number of the source being counted. The lay-out of the outer ring of ~%" index holes and three inner rings of 3 ,, binary-to-decimal coded holes can be seen in the schematic drawing of the carrier ring shown in fig. 2, where the binary-codeddecimal system is also shown. Different positions of these holes correspond to the numbers 1, 2, 4, 8, 10, and 20.
16 17
,2
~0
O0
~ 18
•• •
mo/ ' "
\\\
19
"-\
".'. /
21~" DIA,
Ii"
• a
I
~
/
* KADON BEARIN~ B~6031-t- 6L2
!
21"01~.
J/
~'/
•0
000
":.7" 26
~24"DIA,
3 I" DIA. _ _
e•
3
•
"~'~0
••
~o
27
SECTION ~ . ~ FULL SGALE
2 I
30
29
Fig. 2. Lay-out of rotating storage ring showing disposition of solenoid-operated-plunger index holes (larger diameter) and binary-todecimal coded holes for source identification.
THE NBS
4n/~-~ C O I N C I D E N C E
COUNTING
215
Fig. 3. View o f a u t o m a t i c 4nil- 7 c h a n g e r with lead shielding for 3" x 3" NaI(TI) crystal r e m o v e d to s h o w 4~fl c o u n t e r with source carrier in c o u n t i n g position. T h e bakelite box, straddling the storage ring to the right, houses the six photocells for source identification.
In fig. 3 positions 1-8 can be clearly seen in the nearest part of the carrier ring. Contained in the bakelite housing straddling the carrier ring on the right are six ¼" diameter semi-conductor photocells. These can be illuminated in different combinations, corresponding to the binary-to-decimal coded holes which, in that position, transmit light from a light located below the ring. The photocells are connected to binarycoded-decimal decoders which then permit of the sample number being recorded, together with the measurements data through suitable electronic interfacing, on to puched paper tape. These data are subsequently fed into a time-shared computer for reduction to activity values for the sources. Referring again to fig. 3, which also shows the 47t fl counter assembly, it can be seen that the selonoidoperated plunger is in the index hole of the sector containing the binary code for the number 4. Because of the necessary offsets of the plunger and photocell positions from the position of the source carrier in the counter this does not mean that source number 4 is
being counted. The offsets are such that source carrier number 27 is actually in the 4n/~ counter. These offsets are quite arbitrary and a matter of convenience in locating the plunger and photocell assembly. For the moment it is, however, also convenient to refer to the plunger being in the index hole in sector 4, as distinct from the actual carrier number. The index holes also provide for those cases where no source is present in the source carrier. The holes have shoulders and are threaded for the purpose of inserting special screws that fill the holes to the bottom surface of the source-carrier ring. Such an inserted screw can be seen in fig. 3 in sector 7. When the ring is rotating the plunger does not sense a hole where such a blocking screw is inserted and it therefore ignores this sourcecarrier position, which presumably has no source, and the ring continues rotating until an empty hole is presented for the plunger to enter. Let us now consider a complete counting cycle. We will suppose that the ring is rotating and the first open hole arrives above the spring-loaded solenoid-operated IV. C O I N C I D E N C E
COUNTING
216
s.B. GARFINKEL et al.
plunger, so that the plunger can enter the hole. The ring-drive motor is topped by a microswitch operated by the plunger as it is entering the hole (the plunger itself does n o t stop the ring), and the pneumatic piston pushes the source carrier into the 4n fl counter. This last operation is initiated by a circuit that senses that the motor has stopped. When the source carrier has been fully inserted into the counter, the microswitch shown to the right of the 4 n fl counter in fig. 3 is closed, which resets the counting electronics and puts a 40-s time-delay circuit into operation which provides sufficient time for the counting gas to pre-flush the counter, after which the counting period starts. A cycle consists of a preset number (1-10) of counting periods, after which the source-changing cycle is repeated. The counting periods themselves are pre-set either by a time period, by the number of counts in the//channel, or number of counts
in the coincidence channel, whichever of the three is achieved first. At the end of each counting period, the following data are automatically recorded: source number, calendar time (in seconds), duration of count (in hundredths of seconds), and counts in the fl, ? and coincidence channels. After the data have been recorded, all the scalers are reset, except for the calendar one, and a new count is started. The recording of a calendar time with each set of data allows all data to be computer-corrected to a fixed reference time. Both the calendar and count-duration scalers are fed from standard frequency signals, which are available in all buildings at the National Bureau of Standards. When the source has been counted for the predetermined number of counting periods, a relay causes the pneumatic piston to withdraw the carrier from the counter back into the carrier ring. At its furthermost point of withdrawal the spring-loaded pin (which can
Fig. 4. Automatic changer for intercomparison of 7-ray point sources. The source-identification photocells are located under the ring below the light reflector seen in the far corner.
THE NBS
4re fl-7
COINCIDENCE
be seen in fig. 3 just short of entering the 47~fl counter) disengages from the source-carrier hole (similar holes can be seen in the other source carriers in fig. 3), and the microswitch, to the left of the plunger in fig. 3, is closed. The closing of this microswitch causes the solenoid-operated plunger to withdraw from the index hole in the carrier ring and to actuate the solenoidplunger microswitch that starts the ring-drive motor. The ring then rotates until the next open hole is sensed by the plunger and the whole counting cycle is repeated.
3. ~-ray intercomparator automatic sample changer An improved design for an automatic changer for the comparison of 7-ray sources has also been introduced to replace that of our older 7-ray intercomparatorl). The new version is shown in fig. 4. It incorporates the same "loading-platform" feature of the older version, in order to maintain high precision in the reproducibility of source-to-detector distance. A major innovation is the introduction of circular symmetry of the carrier ring around the detector axis, so that the effect on background due to the sources not being counted will always be the same whatever their position. Provided that all of the sources of the same radionuclide in the set of N being counted are not too different in activity from each other, the background due to the ( N - 1) not being counted at any given time, . which is also attenuated through a 4 zi t t i.d. × 12 tto.d. lead shield, should not vary within the desired limits of accuracy. The detector is a 3"× 3" NaI(T1) crystal-phototube assembly and the same ring-carrier design, with plunger positioning of the sources, used in the 4~ fl-7 coincidence counting sample changer, has been
COUNTING
217
adopted. The ring carrier is, however, designed only to store from 1 to 24 sources. The index hole (with threaded inserts) with spring-loaded solenoid-operated plunger and the binary-coded source number indicators are also similar. The cycling is also controlled in the same manner, using terminal microswitches at each end of the source carrier and plunger travel from the carrier ring into the central counting position and back again. The electronics equipment of the 4 ~ fl- ~ changer is used to effect the choice of number of counting periods, preset times or preset counts, and the recording of measurements data (but now just for the y-ray channel). Calendar time and source number are also recorded. The sources shown stored in the carrier ring in fig. 4 are 7-ray point-source standards of 85Kr implanted in aluminum foil and sandwiched between two plate-glass discs sealed together with optically epoxy resin.
Reference 1) S. B. Garfinkel, W. B. M a n n a n d W. J. Y o u d e n , J. Research NBS, 70C (1966) 53.
Discussion Reher: D r M a n n , y o u said that the identification o f the source to be m e a s u r e d is its position in the sample changer. I could imagine there is a certain possibility that the source is placed in the w r o n g position. D o y o u therefore k n o w a b o u t a system to identify the source itself? Mann: If this h a p p e n e d I t h i n k the discrepancies in the comp u t e d activities per unit m a s s w o u l d s t a n d o u t a n d the mistake could be corrected. It is m o r e i m p o r t a n t to k n o w if the changer m a l f u n c t i o n s in the middle o f the night! Taylor: A t C h a l k River we keep a log b o o k recording the identification a n d position o f each source. There have been n o problems.
IV. C O I N C I D E N C E
COUNTING