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An apparatus for electrochemical etching of recoil particle tracks in plastic
NUCLEAR INSTRUMENTS AND METHODS I38 (I976) 561--563;
© NORTH-HOLLAND PUBLISHING CO.
LETTERS TO T H E E D I T O R AN A P P A R A T U S FOR E L E C T R O C H E M I C A L E T C H I N G OF RECOIL PARTICLE TRACKS IN P L A S T I C FOILS* J. H. THORNGATE, D. J. CHRISTIAN and C. P. LITTLETON Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A.
Received 17 August 1976 A h~lgh voltage, ac power supply is described that was designed for use in studies of the electrochemical etching of recoil particle tracks in plastic foils. It was designed to operate over the frequency range from 40 to 4000 Hz with an output of at least 1000 V (rms) at load currents of at least 4 mA. Improvements in the etching cell to increase the safety of the system are also described. Certain plastic foils produce observable tracks when they are irradiated with charged particles and suitably etched1). This property has found important applications in personnel neutron dosimetry, usually where a radiator foil is used to provide the charged particles 2' 3). It is also possible to record the heavy recoil particles produced directly in the plastic foil by the neutrons4). The etching techniques used play a significant part in the ultimate sensitivity with which the recoil ions can be detected. Recently, an electrochemical etching process has been developed in which an alternating electric field is used to amplify the size of the etched tracksS-7). Successful measurements of as little as 7 mRad of fission neutrons have been reported using this technique with polycarbonate foils. This success has prompted further work at several laboratoriesg). This paper describes the apparatus developed for use in electrochemical etching studies that are a part of a program to develop personnel neutron dosimeters at this Laboratory. The complete apparatus is shown in fig. 1. The electronic system consisted of an external audio oscillator, a special power supply and a vacuum tube voltmeter. The frequency range used in these studies required the use of a vacuum tube voltmeter rather than a simple meter. The etching chamber is essentially the same as that described by Sohrabi 8' 9) with some minor modifications to the electrode system to provide additional safety and convenience. None of the important parameters of the etching procedure depend upon the size and geometry of the chamber so that it can be any size convenient for the dimensions and * Research sponsored by the Energy Research and Development Administration under contract with Union Carbide Corporation.
number of foils to be etched. The electrodes must be made of a metal that will not be damaged by the caustic etching solution. Those shown were made of platinum although they could also be made of palladium or stainless steel. The electrodes were suspended in the chamber from modified kovar vacuum feed-throughs. For safety reasons, the external high voltage connections were made via safe-high-voltage (SHV) coaxial connectors held in place by several layers of heatshrinkable polyethylene tubing. Use of high voltage coaxial cable increases the safety of the system but the capacity of the cable does load the supply somewhat. In addition, the cell had a special interlock that prevented removing live electrodes from the etching solution. This was done by mounting a disconnect switch in the lucite bar holding the electrodes. The
Fig. 1. Photograph of electrochemical etching apparatus.
562
J.H.
THORNGATE
lucite bar also helped to stabilize the position of the electrodes. An alternating current (ac) power supply that can be varied over a wide range of frequencies and voltages is required to optimize all of the parameters of the electrochemical etching process. The supply must be capable of producing output currents up to several milliamps. These requirements were met by the unit shown schematically in fig. 2. A separate audio oscillator was connected to point A for this work, in place of the oscillator and voltage amplifier shown. In this case the variable resistor labeled "voltage control", was set to limit the maximum input voltage so that the power amplifier could not be overloaded. An external oscillator was used primarily to obviate the need of calibrating the frequency scale. The oscillator and voltage amplifier shown in the diagram were built specially and worked well for this application. The audio oscillator is a standard Wien bridge circuit. On the basis only of ready availability, four no. 1859 lamps (10 V, 10 mA) are used as the nonlinear element required to stabilize the output voltage. The level adjust control is set so that the output of the oscillator is not distorted. Fixed capacitors and a dual potentiometer are used to adjust the oscillator IreOSCILLATOR
et al.
quency because it is usually easier to obtain a dual potentiometer than a dual variable capacitor of the right range. The combination of resistance and capacitance shown allows the frequency to be varied from 40 to 430 Hz on the low range and from 370 to 4000 Hz on the high range. The voltage amplifier serves three functions: it isolates the oscillator output from the load ; it increases the output voltage to the required range (about 6 V rms in this case); and it provides a means of setting the maximum output voltage. The power amplifier uses complimentary transistors and dual power supplies in a bridge configuration so that the quiescent output is zero volts. An integratedcircuit operational amplifier is used in a unity-gain configuration to provide stability. Finding a suitable output transformer was the most difficult problem encountered in building the supply. The transformer shown in the diagram is a special oscilloscope power transformer with two 6.3 V filament windings and a high voltage winding for 2700 V at 5 mA. The filament windings were connected in series because it was found that the transformer and power amplifier performed better at higher frequencies when a higher voltage, lower current signal was used. A lower input voltage could be used with a transformer POWER AMPLIFIER
VOLTAGE AMPLIFIER +V
ir ~ ~
0.7t 3 w
[MAX
,
VOLTAGE~ET) IOk
2.2/z5
2~" . ) U I ' ~ ~1
.... '\
L./
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iN I r-.~-.~
)F-14°°6~- 6 ~4~4100~
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~-v
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I
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I
I
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2700V
-V
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l
POWER SUPPLY
IA
~
FULL WAVE BRIDGE
+
* MATCHEDPAIR ** MATCHEDPAIR ONHEATSINK
0.45 H / 0 . 9 A
V
oJo
1t0 V
,;oo ov
0¢
FULL WAVE BRD I GE Fig. 2. Variable frequency, high voltage ac supply-circuit d i a g r a m .
0.,45H/0.9A
~TPUT
5mA
APPARATUS FOR ELECTROCHEMICAL ETCHING 1700
,
LI
,
,
ii
i
563
I
1600
4500
• -
.4"
t400
t300
1.0 rnA .5
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.
.
.
mA
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2.0
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0.5 mA
1200
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jo
~.0 mA Q
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• Q
--~
3.5 mA 4.0
1000 t0
mA
I' °5oA ~ i 40
80 t00
l 200
n 400
I 1--'I' 800
1000
r 2000
4000
f (Hz)
Fig. 3. maximum power supply output voltage as a function of frequency and load current. that requires it but, in every case, the turns ratio m u s t be sufficient to p r o v i d e the desired o u t p u t voltage. There are no s t a n d a r d a u d i o t r a n s f o r m e r s t h a t m a y be used because their windings w o u l d have to have impedance ratios o f a b o u t 4 x 10 4 to 1 in o r d e r to transform 6 V to 1200V. P o w e r limitations generally prevent a c o m b i n a t i o n o f higher voltages a n d lower turns from being used. C o m m e r c i a l l y available oscilloscope p o w e r t r a n s f o r m e r s are available that will w o r k over a reduced frequency range. The low frequency limit o f the supply (40 Hz) is set by a n o n l i n e a r i t y that occurs in the o u t p u t o f the p o w e r amplifier. A n u p p e r frequency o f 4000 H z is all that is practical when a 60 H z t r a n s f o r m e r is used in the o u t p u t because the core is t o o massive to be driven effectively at higher frequencies. Even so, this provides two decades o f v a r i a t i o n in the current due to the change in the capacitive reactance o f the etching apparatus. The m a x i m u m voltage that can be o b t a i n e d from this supply as a function o f frequency and current is
shown in fig. 3. M e a s u r e m e n t s were limited to voltages greater t h a n 1 kV to prevent d a m a g e to the supply. D u r i n g n o r m a l etching, a current limiting resistor could be put in the o u t p u t circuit to prevent d a m a g e to the circuit if the etch tracks penetrate the foil a n d the o u t p u t o f the p o w e r supply is short circuited. C u r r e n t limiting in the dc p o w e r supply might also be used.
References 1) L. Medveczky and G. Somagi, Atomki Koslem 8 (1966) 226. 2) W. G. Cross and L. Tommasino, Rad. Eft. 5 (1970) 85. 3) M. Sohrabi and K. Becker, Nucl. Instr. and Meth. 104 (1972) 409. 4) K. Becker and M. Abd-E1 Razel, Nucl. Instr. and Meth. 1t24 (1975) 557. 5) L. Tommasino, CNEN Report RT/PROT(71), vol. 1 (1970). 6) L. Tommasino and G. Armellini, Rad. Eft. 20 (1973) 253. 7) M. Sohrabi and K. Becker, ORNL-TM-3605 (1971). 8) M. Sohrabi, Health Phys. 27 (1974) 598. 9) M. Sohrabi and K. Z. Morgan, Operational health physics, Proc. 9th Midyear Topical Syrup. of the Health Physics Society, Denver (1976) p. 853.
© NORTH-HOLLAND PUBLISHING CO.
LETTERS TO T H E E D I T O R AN A P P A R A T U S FOR E L E C T R O C H E M I C A L E T C H I N G OF RECOIL PARTICLE TRACKS IN P L A S T I C FOILS* J. H. THORNGATE, D. J. CHRISTIAN and C. P. LITTLETON Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A.
Received 17 August 1976 A h~lgh voltage, ac power supply is described that was designed for use in studies of the electrochemical etching of recoil particle tracks in plastic foils. It was designed to operate over the frequency range from 40 to 4000 Hz with an output of at least 1000 V (rms) at load currents of at least 4 mA. Improvements in the etching cell to increase the safety of the system are also described. Certain plastic foils produce observable tracks when they are irradiated with charged particles and suitably etched1). This property has found important applications in personnel neutron dosimetry, usually where a radiator foil is used to provide the charged particles 2' 3). It is also possible to record the heavy recoil particles produced directly in the plastic foil by the neutrons4). The etching techniques used play a significant part in the ultimate sensitivity with which the recoil ions can be detected. Recently, an electrochemical etching process has been developed in which an alternating electric field is used to amplify the size of the etched tracksS-7). Successful measurements of as little as 7 mRad of fission neutrons have been reported using this technique with polycarbonate foils. This success has prompted further work at several laboratoriesg). This paper describes the apparatus developed for use in electrochemical etching studies that are a part of a program to develop personnel neutron dosimeters at this Laboratory. The complete apparatus is shown in fig. 1. The electronic system consisted of an external audio oscillator, a special power supply and a vacuum tube voltmeter. The frequency range used in these studies required the use of a vacuum tube voltmeter rather than a simple meter. The etching chamber is essentially the same as that described by Sohrabi 8' 9) with some minor modifications to the electrode system to provide additional safety and convenience. None of the important parameters of the etching procedure depend upon the size and geometry of the chamber so that it can be any size convenient for the dimensions and * Research sponsored by the Energy Research and Development Administration under contract with Union Carbide Corporation.
number of foils to be etched. The electrodes must be made of a metal that will not be damaged by the caustic etching solution. Those shown were made of platinum although they could also be made of palladium or stainless steel. The electrodes were suspended in the chamber from modified kovar vacuum feed-throughs. For safety reasons, the external high voltage connections were made via safe-high-voltage (SHV) coaxial connectors held in place by several layers of heatshrinkable polyethylene tubing. Use of high voltage coaxial cable increases the safety of the system but the capacity of the cable does load the supply somewhat. In addition, the cell had a special interlock that prevented removing live electrodes from the etching solution. This was done by mounting a disconnect switch in the lucite bar holding the electrodes. The
Fig. 1. Photograph of electrochemical etching apparatus.
562
J.H.
THORNGATE
lucite bar also helped to stabilize the position of the electrodes. An alternating current (ac) power supply that can be varied over a wide range of frequencies and voltages is required to optimize all of the parameters of the electrochemical etching process. The supply must be capable of producing output currents up to several milliamps. These requirements were met by the unit shown schematically in fig. 2. A separate audio oscillator was connected to point A for this work, in place of the oscillator and voltage amplifier shown. In this case the variable resistor labeled "voltage control", was set to limit the maximum input voltage so that the power amplifier could not be overloaded. An external oscillator was used primarily to obviate the need of calibrating the frequency scale. The oscillator and voltage amplifier shown in the diagram were built specially and worked well for this application. The audio oscillator is a standard Wien bridge circuit. On the basis only of ready availability, four no. 1859 lamps (10 V, 10 mA) are used as the nonlinear element required to stabilize the output voltage. The level adjust control is set so that the output of the oscillator is not distorted. Fixed capacitors and a dual potentiometer are used to adjust the oscillator IreOSCILLATOR
et al.
quency because it is usually easier to obtain a dual potentiometer than a dual variable capacitor of the right range. The combination of resistance and capacitance shown allows the frequency to be varied from 40 to 430 Hz on the low range and from 370 to 4000 Hz on the high range. The voltage amplifier serves three functions: it isolates the oscillator output from the load ; it increases the output voltage to the required range (about 6 V rms in this case); and it provides a means of setting the maximum output voltage. The power amplifier uses complimentary transistors and dual power supplies in a bridge configuration so that the quiescent output is zero volts. An integratedcircuit operational amplifier is used in a unity-gain configuration to provide stability. Finding a suitable output transformer was the most difficult problem encountered in building the supply. The transformer shown in the diagram is a special oscilloscope power transformer with two 6.3 V filament windings and a high voltage winding for 2700 V at 5 mA. The filament windings were connected in series because it was found that the transformer and power amplifier performed better at higher frequencies when a higher voltage, lower current signal was used. A lower input voltage could be used with a transformer POWER AMPLIFIER
VOLTAGE AMPLIFIER +V
ir ~ ~
0.7t 3 w
[MAX
,
VOLTAGE~ET) IOk
2.2/z5
2~" . ) U I ' ~ ~1
.... '\
L./
O~!i~i::::
iN I r-.~-.~
)F-14°°6~- 6 ~4~4100~
,.v
I_v
~ok
v
~-v
~__
(VONTTA##L} - /
I
NpR, " I
I
I
[~
2700V
-V
o.Tv3w
l
POWER SUPPLY
IA
~
FULL WAVE BRIDGE
+
* MATCHEDPAIR ** MATCHEDPAIR ONHEATSINK
0.45 H / 0 . 9 A
V
oJo
1t0 V
,;oo ov
0¢
FULL WAVE BRD I GE Fig. 2. Variable frequency, high voltage ac supply-circuit d i a g r a m .
0.,45H/0.9A
~TPUT
5mA
APPARATUS FOR ELECTROCHEMICAL ETCHING 1700
,
LI
,
,
ii
i
563
I
1600
4500
• -
.4"
t400
t300
1.0 rnA .5
~ ~o'~..~
o
•
•/ / / ' / . ~ ' ~ . / . ? . . " - . ~ \X r •// / / //? / / . ~ - •' ~ . / . . ~ " , , X \1 -
"-
.
~
.
.
.
mA
- -
2.0
mA
2.5 mA
HO0
o~
0 mA - -
0.5 mA
1200
o-.-.~
- -
///
jo
~.0 mA Q
O
~
Q
04•
\
• Q
--~
3.5 mA 4.0
1000 t0
mA
I' °5oA ~ i 40
80 t00
l 200
n 400
I 1--'I' 800
1000
r 2000
4000
f (Hz)
Fig. 3. maximum power supply output voltage as a function of frequency and load current. that requires it but, in every case, the turns ratio m u s t be sufficient to p r o v i d e the desired o u t p u t voltage. There are no s t a n d a r d a u d i o t r a n s f o r m e r s t h a t m a y be used because their windings w o u l d have to have impedance ratios o f a b o u t 4 x 10 4 to 1 in o r d e r to transform 6 V to 1200V. P o w e r limitations generally prevent a c o m b i n a t i o n o f higher voltages a n d lower turns from being used. C o m m e r c i a l l y available oscilloscope p o w e r t r a n s f o r m e r s are available that will w o r k over a reduced frequency range. The low frequency limit o f the supply (40 Hz) is set by a n o n l i n e a r i t y that occurs in the o u t p u t o f the p o w e r amplifier. A n u p p e r frequency o f 4000 H z is all that is practical when a 60 H z t r a n s f o r m e r is used in the o u t p u t because the core is t o o massive to be driven effectively at higher frequencies. Even so, this provides two decades o f v a r i a t i o n in the current due to the change in the capacitive reactance o f the etching apparatus. The m a x i m u m voltage that can be o b t a i n e d from this supply as a function o f frequency and current is
shown in fig. 3. M e a s u r e m e n t s were limited to voltages greater t h a n 1 kV to prevent d a m a g e to the supply. D u r i n g n o r m a l etching, a current limiting resistor could be put in the o u t p u t circuit to prevent d a m a g e to the circuit if the etch tracks penetrate the foil a n d the o u t p u t o f the p o w e r supply is short circuited. C u r r e n t limiting in the dc p o w e r supply might also be used.
References 1) L. Medveczky and G. Somagi, Atomki Koslem 8 (1966) 226. 2) W. G. Cross and L. Tommasino, Rad. Eft. 5 (1970) 85. 3) M. Sohrabi and K. Becker, Nucl. Instr. and Meth. 104 (1972) 409. 4) K. Becker and M. Abd-E1 Razel, Nucl. Instr. and Meth. 1t24 (1975) 557. 5) L. Tommasino, CNEN Report RT/PROT(71), vol. 1 (1970). 6) L. Tommasino and G. Armellini, Rad. Eft. 20 (1973) 253. 7) M. Sohrabi and K. Becker, ORNL-TM-3605 (1971). 8) M. Sohrabi, Health Phys. 27 (1974) 598. 9) M. Sohrabi and K. Z. Morgan, Operational health physics, Proc. 9th Midyear Topical Syrup. of the Health Physics Society, Denver (1976) p. 853.