- Email: [email protected]
Automatical spark counting of neutron-induced recoil tracks in polycarbonate foils
NUCLEAR
INSTRUMENTS
AUTOMATICAL
AND
METHODS
I28
SPARK COUNTING
0975) 447-453;
©
NORTH-HOLLAND
OF NEUTRON-INDUCED
IN POLYCARBONATE
PUBLISHING
CO.
RECOIL TRACKS
FOILS
J. J A S I A K * and E. PIESCH
Health Physics Division, Karlsruhe Nuclear Research Center, Karlsruhe, Germany
Received 7 July 1975 The new approach in automatical track counting of neutroninduced recoils in 12t~m thin Makrofol KG foils was realized by varying the spark counter voltage and using the voltage at the starting point of the electrical breakdown as a parameter for the counting sensitivity. Based on an optimized etching technique small changes in foil quality, in foil thickness and etching con-
1. Introduction The a u t o m a t i c counting o f track etching detectors 1) by using an electrical discharge in the p o l y m e r detector foil is a c o m m o n l y used technique which is mainly applied to low t r a c k density registration o f fission fragments generated in a 237Np o r 2 3 2 T h r a d i a t o r foil2). In personnel neutron d o s i m e t r y the practical a p p l i c a t i o n o f these detectors is limited to some extent by the restriction o f not using fissile material with a relatively high g a m m a b a c k g r o u n d in a personnel dosimeter badge. C o n s e q u e n t l y , these detectors have been used mainly for investigations in accident dosimerry or in routine m o n i t o r i n g as a neutron finger dosimeter 3. 4). The detection of n e u t r o n - i n d u c e d recoil particle tracks created directly in p o l y m e r s itself shows a similar energy d e p e n d e n c e 5) but a sensitivity which is a p p r o x i m a t e l y ten times higher than that for the fission fragment detection. Due to elastic collisions o f neutrons and heavy charged nuclei such as c a r b o n and oxygen the track length o f recoils as well as the energy deposited are smaller, so that a longer etching time is required which yields in an i n h o m o g e n e o u s p o r o u s surface o f the foil with a higher n u m b e r o f b a c k g r o u n d tracks. C o m p a r e d with other polymers like cellulose nitrate and cellulose acetate, namely p o l y c a r b o n a t e foils ( M a k r o f o l , Kimfol) with the best resistance due to e n v i r o n m e n t a l and fading influences 6) will preferably be used. It has been shown previously 7) that an a u t o m a t i c track counting o f recoil etch pits in M a k r o f o l foils has n o t yet been realized by using both -optical
systems in c o m b i n a t i o n
with
the
inter-
* IAEA fellow on leave from the Central Laboratory of Radiological Protection, Warsaw, Poland. 447
ditions as well as in neutron energy yield in a sensitivity of (0.3-2.0) × 10-~ sparks per neutron for '2s~Cf fission and Am--Be neutrons. The reproducibility for consecutive counting, the lower detection limit and the energy dependence have been investigated and compared with microscopical track counting.
ference c o n t r a s t m e t h o d due to different etch pit-size d i s t r i b u t i o n a n d u n f a v o u r a b l e c o n d i t i o n s o f trackt o - b a c k g r o u n d contrast, 500
MAKROFO L
KG COUNTING
ETCHED 16 HOURS 25% KOH , &0*C 400
,~ 0/$/*
3 rd 2 nd
f ~ I st 1 300
,/ [
,.
! J/
IRRAOIATED 10 rod FISSION NEUTRONS
~00
/ 0 300
~
S0~
7C0
900
SPARK COUNTING VOLTAGE IN V
Fig. I. Spark counts in irradiated polycarbonate foil as a function of spark voltage during first, second and third sparking.
448
J. J A S I A K A N D E. P I E S C H
103
MAKROFOL
KG
ETCHED
t6
BACKGROUND HOURS ,
25 % KOH
RATE 40 =C
UNIRRADIATED COUNTING
I0 2
3 rd ~
A,,---.~
•
A--"t""
A
2 nd
I01
./" 101
,
300
--
400
t
e
t
I
500
600
SPARK COUNTING
i
I
i
700
B00
VOLTAGE: IN V
Fig. 2. Spark counts in unirradiated polycarbonate foil as a function of spark voltage during first, second and third sparking.
- spark counting due to the lack of reproducibility and a high background rate of unirradiated foils. Investigations recently performed at O R N L have shownS), that a reproducibility of _+ 10% was found for an automatic spark counting in 8 l~m thick cellulose nitrate foils. As the polycarbonate foil seems to have a widespread application in fast neutron dosimetry mainly due to the low fading behaviour, investigations were performed with 12pm thick Makrofol KG foils using the conventional etching conditions and several sparking runs during which the electrical breakdown of the foil was reached. With regard to a practical application, the spark counting - high voltage characteristic, the background track density of unirradiated foils as well as the reproducibility and the energy response were investigated.
2. Experiment For the spark counting investigation a commercially available Makrofol KG foil 12/~m thick was used instead of 0.3 mm thick Makrofol E for optical counting. The foils were glued with chloroform onto thick Makrofol E frames, washed in ultrasonic bath and dried at 40°C before etching. The etching was performed with the foils fixed in a holder in a thin-welled polyethylene container of 3 1 volume positioned in a water bath at (40+0.1)°C by
using magnet stirring of etchant and water bath. Etchant concentrations of 25%, 30% and 35% KOH in water solution and etching times of 4-20 h were investigated with regard to optimum etching conditions at the electric breakdown in non-irradiated foils which has been found for 25-30% K O H (specific density 1.285) after 16 h etching at 40~C. After etching the foils were rinsed at first in destilled, then for one hour in running water, followed finally by an ultrasonic bath in destilled water and than dried one hour at 40°C. A reproducible spark counting was realized by means TABLE 1
Consecutive sparking at 460 V. N u m b e r of consecutive spark counting
I 2 3 4 5 6 7
N u m b e r of spark counts Irradiated foil Unirradiated foil
382 392 383 377 389 366 396
82 93 106 112 I10 118 135
AUTOMATICAL SPARK COUNTING
MAKROFOL
Consecutive sparking at the same voltage results in a constant count only for irradiated foils. This is presented in table ! for sparking at 460 V, which shows a deviation of < 5% from the mean value. Unirradiated foils, however, show an increase of spark counts up to 50%. As the foils are etched and spark-counted up to the point where the electrical breakdown occurs, also unirradiated foils show an increase of counts as a function of voltage (see fig. 2). The plateau observed at the second and third counting of an unirradiated foil is not as regular as in irradiated foils. For the optimum etching conditions at 40°C and 16 h etching time voltages above 500 V yield an increase in spark counts due to an electrical breakdown and/or a multiple counting on the same hole. The reproducibility of spark counting was investigated by irradiation of five foils in the same neutron
KG
ETCHED 16 HOURS 25% KOH, 40=C 80C
FOIL No &
600
/
5
40(1
449
~
1000
FOIL No o
MAKROFOL KG
2OO
IRRADIATED 10 rod FISSION NEUTRONS
.
•
i
I 500
i
,
i
800
i
I
i
700
.
,,f
i 900
SPARK COUNTING VOLTAGE IN V
4
/
t'
COUNTING
2nd
0 , 300
/ I j r
ETO'IED 16 HOURS 25"/, KOH,60 QC
600
Fig. 3. Spark counts in five polycarbonate foils irradiated with 10 Rad as a function of spark voltage during second sparking.
of a c o m m o n l y used counter for fission fragment detection with a constant pressure of 0.63 kg during counting and an area of 1.96 cm 2 and 0.5 cm z, respectively. For each foil the spark counts vs high voltage characteristic was investigated in the range 300-720 V by using steps of 20 V without changing the position of the aluminized mylar foil. The foils were washed in ultrasonic bath before the consecutive counting run.
400
200
I
3. Results In fig. 1 the number o f spark counts has been presented vs spark voltage for three times o f consecutive counting o f a foil irradiated with 10 Rad o f 252Cf fission neutrons. During the first c o u n t i n g there is no characteristic plateau which is found for the spark counting o f fission fragments. After the second and third counting a clearly marked plateau is observed.
0
.
30O
COUNTING
3rd
.
.
.
i
500
i
i
,
.
i
7OO
,
i
.
i
900
SPARK COUNTINGVOLTAGE IN V
Fig. 4. Spark counts in five polycarbonate foils irradiated with 10 Rad as a function of spark voltage during third sparking.
450
J. J A S I A K A N D E. P I E S C H
DETECTOR
1
NUMBER 237 OF SPARKS
2
3
Z,
5
382
389
721
624
Fig. 5. Replicas o f spark counts in aluminized mylar foil o f five polycarbonate foils irradiated with 10 Rad counted at 460 V, after third sparking.
replicas of sparks in the aluminized mylar foil after spark counting at 460 V. The distribution of holes in the replica is not uniform for most of the foils. One explanation is that the foil quality is not homogeneous enough, which yields a different foil thickness after the etching process. Therefore the hole density is lower and zero, respectively, in one part of the foil area or too high due
fluence. As can be seen in figs. 3 and 4 there is a large discrepancy in foil response and also the plateau is not well-defined during the third counting. The relatively large increase of counts as a function of voltage may be explained by multiple sparkings, the spread in response by the inhomogeneity of the spark distribution over the foil area. These effects are demonstrated in fig. 5 by the i0/'
DOSE
~o~o......o..._a~o,,_.,_~~o"~-'o 10 3
I0 rad
~ o~ o..~.,o......o.._,._o.~...o.,u..._e~ n~.~L.~.~a.~..~.D ..,..,.-0 5 rad
I
-
13 rod
,~.-,,~.o
0.6 rod
102
o
~
:
~
i
UNIRRADIATED
tt)
m
101 MAKROFOL
KG
ETCHED AT HOURS 30% KOH, ~.0°C IRRADIATED WITH FISSION NEUTRONS 2 nd COUNTING
z
10 0
300
,
i
t.O0
l
I
i
,5O0
I
600
SPARK COUNTING VOLTAGE IN V Fig. 6. Spark counts in polycarbonate foils exposed to different doses and in unirradiated foil during second sparking.
451
AUTOMATICAL SPARK COUNTING
to the electrical breakdown. In addition there are also multiple sparkings, for instance in foil nos. 3 and 4, giving a high increase of counts above 500 V. For further investigations therefore a half-logarithmic scale for the plateau curve was used. In fig. 6 the response of foils exposed to different doses are presented after the second counting compared with an unirradiated foil. As can be seen from this figure the number of spark counts is not proportional to the neutron dose. To improve the reproducibility of spark counting a high voltage at the starting point of the electrical breakdown was used as reference instead of a voltage within the plateau at 460 V. This is not a constant value and depends on the foil thickness and quality, respectively. This can be seen in fig. 7, in which the third spark counting is presented for four foils irradiated with the same dose. The voltage of interest shifts from 610 V to 530 V if the foil becomes thinner. It is possible to use the voltage of interest as a parameter to correct for the change in foil sensitivity, which is based on the spark counts at 460 V after the third counting. Different etching runs and irradiations with 252Cf and A m - B e neutrons were performed to check this assumption. The sensitivity of irradiated foils at 460 V is presented in fig. 8 as a function of voltage at the electrical breakdown point related to the sensitivity at 460 V. Excluding the foils, where no well-
defined plateau exists even for voltages below 460 V mainly due to multiple sparking, the inhomogeneity of foil sensitivity can be corrected within + 2 0 % by using a correction factor presented in fig. 8. It can be seen from this figure that a correction factor which is based on such an approximation yields the same sensitivity for fission neutrons and A m - B e neutrons. As the energy threshold for a spark counting of neutron-induced recoils is in the order of 2 MeV compared to 1 MeV for optical counting s) the diameter and depth of the etch pits are of importance for the track detection. To explain fig. 8 we have to consider the overlapping of three different effects: - fluctuation in f o i l thickness and etching conditions: electrical breakdown at lower voltages due to thinner foils or higher etching rates, - energy dependence o f etch pit size: electrical breakdown at lower voltages due to larger etch pits and multiple sparking, - energy dependence o f track registration: the elastic cross section and the sensitivity increase with neutron energy.
it should be pointed out that both the influence of small fluctuations in the foil thickness and in the etching conditions as well as the energy dependence of the foil sensitivity and of the etch pit diameter are in the order of a factor 7.
104
FOIL No ,,,,,,a. 4 ,--..3 ,,,,~ 2
103
MAKROFOL 102
KG
ETCHED 16 HOURS 30% KOH.40eC FISSION NEUTRONS 10 rad 3rd
101 300
400
500
COUNTING
600
700
800
SPARK COUNTING VOLTAGE IN V
Fig. 7. Spark counts in four polycarbonate foils exposed to the same dose, as a function of spark voltage during thrid sparking.
452
J. J A S I A K A N D E. P I E S C H
3.0
SENSITIVITY
OF SPARK COUNTING 2.0
2
%
~
THIRD SPARKINGj AT /,60 V 1.0
.,~
~ 1,11~G
~
+ '
~
Am-Be 252^, 05
~.
s z
N ...i
0.2
~:
0J
400
500
600
700
VOLTAGE IN V Fig. 8. Sensitivity of irradiated foils checked at 460 V third s p a r k i n g as a function of spark v o h a g e at the electrical b r e a k d o w n point.
Additional checks have shown that small changes of the sensitivity due to the etching conditions can be corrected. After irradiation with Am-Be neutrons the replica of spark counts show more multiple sparks and therefore a spark count vs voltage characteristic without a well-defined plateau. By using the special spark counting technique with the voltage at the electrical breakdown as a parameter of interest for the interpretation of the counts vs voltage characteristic, it is possible to measure fast neutrons approximately independent of energy for fission neutrons and Am--Be neutrons. Optical counting of neutron-induced recoils in cellulose nitrate LR-I 15 films with a dark red background, however, shows a high change of sensitivity with the neutron energy in the order of a factor of 12 between fission and 4.5 MeV neutrons '~) compared to non-coloured cellulose nitrate, the energy dependence of which is similar to that of polycarbonate foils (see table 2). It is of interest to point out, that only a small fraction of etch pits are spark-counted or registered by the red-dyed LR-115 film.
4. Application Contrary to the spark counting of fission fragments there is a need of repeated counting over the voltage range, checking the spark replicas with respect to homogeneity and multiple sparking and of taking
into account the background count. The spark counting ofunirradiated foils resulted in spark counts in the order of 10-150 sparks per cmz. For a practical application, however, the highest value must be considered which significantly spoils the lower detection limit. Compared to an optical counting, however, the background rate is only one quarter of the value. In table 2 the properties of optical and spark TAI]I.E 2 Neutron-induced recoil track registration in p o l y c a r b o n a t e foils. Recoil track registration a Optical c ount i ng Spark c ount i ng
Response "-"~'-'Cf Am-Be B a c kground rate
9 x 10-6 tracks,"n 15x10 6tracks,n 660 tracks.,'cm ~
(0.3-0.8) x 10- ~ sparks;'n (0.3 -2.0) × 10 ~ sparks/n 150 s p a r k s / c m 2
7.3 2.6 4.4 1.5
(2--5) × 108 n/cm e (7-18) Rem (0.75-5) × 10 ~ n,;cm 2 (2.6.-17) Rein
Lower detection tim# '-':"'-'C f Am-Be
× 107 n/ c m "~ Rcm x 107 n.:cm ~ Rem
a In Makrot'ol E. 300Hm thick, for optical counting, M a k r o t b l KG, 12Hm thick, for spark c o u n t i n g at 4 6 0 V during third s pa rki ng run.
AUTOMATICAL
counting of neutron-induced recoils in Makrofol polycarbonate foil are presented. The sensitivity of the neutron detection for fission and A m - B e neutrons is very low and was found to be in the order of 10% compared to the optical counting. Taking into account the background count and the sensitivity also for the unfavourable cases before a correction based on fig. 8 was used, the lower detection limit for the spark c o u n t i n g - t h e spark counts which double the background count - is higher by a factor 3-6 compared to optical counting. Consequently there are practically no advantages for the spark counting method in routine dosimetry because of the lowest detectable dose of 7-18 Rein. Taking into account the results found for cellulose nitrate at ORNL8), a similar sensitivity of approximately 0.6 x 10 -6 holes per neutron can be estimated. As the spark counts of unirradiated polycarbonate and cellulose nitrate foils are also of the same order the lower detection limit of the spark counting technique is in both cases not sufficient enough to be applied in routine monitoring. The reproducibility of neutron fluence measurements is limited in both polymer foils by small fluctuations in foil thickness and sensitivity. In this investigation only a value of + 2 0 % was found compared to + 1 0 % for cellulose nitrate 8) or to + 5 % for the optical counting technique applied to 300/~m thick Makrofol E foils. Therefore the optical counting of recoils seems to be a more accurate method for all other applications in the measurements of high fluence: in accident dosimetry as a threshold detector, as a dosimeter belt to estimate the direction of the radiation incidence 7) and in
SPARK COUNTING
453
micro-dosimetry to measure the depth dose distribution in tissue in a thin tissue layer ~o). Instead of the spark counting technique a much more promising electrochemical technique seems to lend itself better for a practical application in neutron monitoring. This method, originally proposed by Tommasino I ~), was recently improved by Sohrabi I z) resulting in a sensitivity of the same order as spark counting but with a lower detection limit of 10toRero and a reproducibility of approximately + 10%.
References 1) W. G. Cross and L. Tommasino, Proc. 1st Euratom Syrup. on Neutron dosimeto, in biology and medicine, Neuherberg (1972) p. 283. ") R. V. Griffith, UCRL-51362, Lawrence Livermore Laboratory Report (1973). :~) S. Pretre, Proc. IAEA Syrup. on Neutron monitoring /or radiation protection purposes, vol. 2, Vienna (1973) p. 99. 4) K. Buijs et al., Proc. IAEA Syrup. oll Neutron monitoring for radiation protection purposes, vol. 2, Vienna (1973) p. 159. ,5) K. J6zefowicz, Proc. IAEA Syrup. on Neutron monitoring Jor radiation protection purposes, vol. 2, Vienna (1973) p. 183. ~) A. M. Sayed and E Piesch, KFK-2032, Kernforschungszentrum Karlsruhe Rcport (1974). 7) E. Piesch, Proc. IAEA Syrup. on Advances in t, hysical and biological radiation detectors, Vienna (1970) p. 399. 8) K. Becker and M. Abd-el Razek, Nucl. Instr. and Meth. 12.4 (1975) 557. !~) G. M. Hassib and 1.. Medveczky, Proc. 2nd Symp. on Neutron dosimetry in bioh)gy attd medicine, Neuherbeg (1974). 10) E. Piesch and A. M. Sased, Intern. IAEA Syrup. on Adt,:ances in biomedical dosimetry, Vienna (1975). 11) L. Tommasino, C N E N Report R T / P R O T (71) I (1970). ze) M. Sohrabi, Health Phys. 27 (1974) 598; M. Sohrabi and K. Z. Morgan, 3rd Europ. IRPA Congr. on Criteria for raditition protection, Amsterdam (1975), paper 14.
INSTRUMENTS
AUTOMATICAL
AND
METHODS
I28
SPARK COUNTING
0975) 447-453;
©
NORTH-HOLLAND
OF NEUTRON-INDUCED
IN POLYCARBONATE
PUBLISHING
CO.
RECOIL TRACKS
FOILS
J. J A S I A K * and E. PIESCH
Health Physics Division, Karlsruhe Nuclear Research Center, Karlsruhe, Germany
Received 7 July 1975 The new approach in automatical track counting of neutroninduced recoils in 12t~m thin Makrofol KG foils was realized by varying the spark counter voltage and using the voltage at the starting point of the electrical breakdown as a parameter for the counting sensitivity. Based on an optimized etching technique small changes in foil quality, in foil thickness and etching con-
1. Introduction The a u t o m a t i c counting o f track etching detectors 1) by using an electrical discharge in the p o l y m e r detector foil is a c o m m o n l y used technique which is mainly applied to low t r a c k density registration o f fission fragments generated in a 237Np o r 2 3 2 T h r a d i a t o r foil2). In personnel neutron d o s i m e t r y the practical a p p l i c a t i o n o f these detectors is limited to some extent by the restriction o f not using fissile material with a relatively high g a m m a b a c k g r o u n d in a personnel dosimeter badge. C o n s e q u e n t l y , these detectors have been used mainly for investigations in accident dosimerry or in routine m o n i t o r i n g as a neutron finger dosimeter 3. 4). The detection of n e u t r o n - i n d u c e d recoil particle tracks created directly in p o l y m e r s itself shows a similar energy d e p e n d e n c e 5) but a sensitivity which is a p p r o x i m a t e l y ten times higher than that for the fission fragment detection. Due to elastic collisions o f neutrons and heavy charged nuclei such as c a r b o n and oxygen the track length o f recoils as well as the energy deposited are smaller, so that a longer etching time is required which yields in an i n h o m o g e n e o u s p o r o u s surface o f the foil with a higher n u m b e r o f b a c k g r o u n d tracks. C o m p a r e d with other polymers like cellulose nitrate and cellulose acetate, namely p o l y c a r b o n a t e foils ( M a k r o f o l , Kimfol) with the best resistance due to e n v i r o n m e n t a l and fading influences 6) will preferably be used. It has been shown previously 7) that an a u t o m a t i c track counting o f recoil etch pits in M a k r o f o l foils has n o t yet been realized by using both -optical
systems in c o m b i n a t i o n
with
the
inter-
* IAEA fellow on leave from the Central Laboratory of Radiological Protection, Warsaw, Poland. 447
ditions as well as in neutron energy yield in a sensitivity of (0.3-2.0) × 10-~ sparks per neutron for '2s~Cf fission and Am--Be neutrons. The reproducibility for consecutive counting, the lower detection limit and the energy dependence have been investigated and compared with microscopical track counting.
ference c o n t r a s t m e t h o d due to different etch pit-size d i s t r i b u t i o n a n d u n f a v o u r a b l e c o n d i t i o n s o f trackt o - b a c k g r o u n d contrast, 500
MAKROFO L
KG COUNTING
ETCHED 16 HOURS 25% KOH , &0*C 400
,~ 0/$/*
3 rd 2 nd
f ~ I st 1 300
,/ [
,.
! J/
IRRAOIATED 10 rod FISSION NEUTRONS
~00
/ 0 300
~
S0~
7C0
900
SPARK COUNTING VOLTAGE IN V
Fig. I. Spark counts in irradiated polycarbonate foil as a function of spark voltage during first, second and third sparking.
448
J. J A S I A K A N D E. P I E S C H
103
MAKROFOL
KG
ETCHED
t6
BACKGROUND HOURS ,
25 % KOH
RATE 40 =C
UNIRRADIATED COUNTING
I0 2
3 rd ~
A,,---.~
•
A--"t""
A
2 nd
I01
./" 101
,
300
--
400
t
e
t
I
500
600
SPARK COUNTING
i
I
i
700
B00
VOLTAGE: IN V
Fig. 2. Spark counts in unirradiated polycarbonate foil as a function of spark voltage during first, second and third sparking.
- spark counting due to the lack of reproducibility and a high background rate of unirradiated foils. Investigations recently performed at O R N L have shownS), that a reproducibility of _+ 10% was found for an automatic spark counting in 8 l~m thick cellulose nitrate foils. As the polycarbonate foil seems to have a widespread application in fast neutron dosimetry mainly due to the low fading behaviour, investigations were performed with 12pm thick Makrofol KG foils using the conventional etching conditions and several sparking runs during which the electrical breakdown of the foil was reached. With regard to a practical application, the spark counting - high voltage characteristic, the background track density of unirradiated foils as well as the reproducibility and the energy response were investigated.
2. Experiment For the spark counting investigation a commercially available Makrofol KG foil 12/~m thick was used instead of 0.3 mm thick Makrofol E for optical counting. The foils were glued with chloroform onto thick Makrofol E frames, washed in ultrasonic bath and dried at 40°C before etching. The etching was performed with the foils fixed in a holder in a thin-welled polyethylene container of 3 1 volume positioned in a water bath at (40+0.1)°C by
using magnet stirring of etchant and water bath. Etchant concentrations of 25%, 30% and 35% KOH in water solution and etching times of 4-20 h were investigated with regard to optimum etching conditions at the electric breakdown in non-irradiated foils which has been found for 25-30% K O H (specific density 1.285) after 16 h etching at 40~C. After etching the foils were rinsed at first in destilled, then for one hour in running water, followed finally by an ultrasonic bath in destilled water and than dried one hour at 40°C. A reproducible spark counting was realized by means TABLE 1
Consecutive sparking at 460 V. N u m b e r of consecutive spark counting
I 2 3 4 5 6 7
N u m b e r of spark counts Irradiated foil Unirradiated foil
382 392 383 377 389 366 396
82 93 106 112 I10 118 135
AUTOMATICAL SPARK COUNTING
MAKROFOL
Consecutive sparking at the same voltage results in a constant count only for irradiated foils. This is presented in table ! for sparking at 460 V, which shows a deviation of < 5% from the mean value. Unirradiated foils, however, show an increase of spark counts up to 50%. As the foils are etched and spark-counted up to the point where the electrical breakdown occurs, also unirradiated foils show an increase of counts as a function of voltage (see fig. 2). The plateau observed at the second and third counting of an unirradiated foil is not as regular as in irradiated foils. For the optimum etching conditions at 40°C and 16 h etching time voltages above 500 V yield an increase in spark counts due to an electrical breakdown and/or a multiple counting on the same hole. The reproducibility of spark counting was investigated by irradiation of five foils in the same neutron
KG
ETCHED 16 HOURS 25% KOH, 40=C 80C
FOIL No &
600
/
5
40(1
449
~
1000
FOIL No o
MAKROFOL KG
2OO
IRRADIATED 10 rod FISSION NEUTRONS
.
•
i
I 500
i
,
i
800
i
I
i
700
.
,,f
i 900
SPARK COUNTING VOLTAGE IN V
4
/
t'
COUNTING
2nd
0 , 300
/ I j r
ETO'IED 16 HOURS 25"/, KOH,60 QC
600
Fig. 3. Spark counts in five polycarbonate foils irradiated with 10 Rad as a function of spark voltage during second sparking.
of a c o m m o n l y used counter for fission fragment detection with a constant pressure of 0.63 kg during counting and an area of 1.96 cm 2 and 0.5 cm z, respectively. For each foil the spark counts vs high voltage characteristic was investigated in the range 300-720 V by using steps of 20 V without changing the position of the aluminized mylar foil. The foils were washed in ultrasonic bath before the consecutive counting run.
400
200
I
3. Results In fig. 1 the number o f spark counts has been presented vs spark voltage for three times o f consecutive counting o f a foil irradiated with 10 Rad o f 252Cf fission neutrons. During the first c o u n t i n g there is no characteristic plateau which is found for the spark counting o f fission fragments. After the second and third counting a clearly marked plateau is observed.
0
.
30O
COUNTING
3rd
.
.
.
i
500
i
i
,
.
i
7OO
,
i
.
i
900
SPARK COUNTINGVOLTAGE IN V
Fig. 4. Spark counts in five polycarbonate foils irradiated with 10 Rad as a function of spark voltage during third sparking.
450
J. J A S I A K A N D E. P I E S C H
DETECTOR
1
NUMBER 237 OF SPARKS
2
3
Z,
5
382
389
721
624
Fig. 5. Replicas o f spark counts in aluminized mylar foil o f five polycarbonate foils irradiated with 10 Rad counted at 460 V, after third sparking.
replicas of sparks in the aluminized mylar foil after spark counting at 460 V. The distribution of holes in the replica is not uniform for most of the foils. One explanation is that the foil quality is not homogeneous enough, which yields a different foil thickness after the etching process. Therefore the hole density is lower and zero, respectively, in one part of the foil area or too high due
fluence. As can be seen in figs. 3 and 4 there is a large discrepancy in foil response and also the plateau is not well-defined during the third counting. The relatively large increase of counts as a function of voltage may be explained by multiple sparkings, the spread in response by the inhomogeneity of the spark distribution over the foil area. These effects are demonstrated in fig. 5 by the i0/'
DOSE
~o~o......o..._a~o,,_.,_~~o"~-'o 10 3
I0 rad
~ o~ o..~.,o......o.._,._o.~...o.,u..._e~ n~.~L.~.~a.~..~.D ..,..,.-0 5 rad
I
-
13 rod
,~.-,,~.o
0.6 rod
102
o
~
:
~
i
UNIRRADIATED
tt)
m
101 MAKROFOL
KG
ETCHED AT HOURS 30% KOH, ~.0°C IRRADIATED WITH FISSION NEUTRONS 2 nd COUNTING
z
10 0
300
,
i
t.O0
l
I
i
,5O0
I
600
SPARK COUNTING VOLTAGE IN V Fig. 6. Spark counts in polycarbonate foils exposed to different doses and in unirradiated foil during second sparking.
451
AUTOMATICAL SPARK COUNTING
to the electrical breakdown. In addition there are also multiple sparkings, for instance in foil nos. 3 and 4, giving a high increase of counts above 500 V. For further investigations therefore a half-logarithmic scale for the plateau curve was used. In fig. 6 the response of foils exposed to different doses are presented after the second counting compared with an unirradiated foil. As can be seen from this figure the number of spark counts is not proportional to the neutron dose. To improve the reproducibility of spark counting a high voltage at the starting point of the electrical breakdown was used as reference instead of a voltage within the plateau at 460 V. This is not a constant value and depends on the foil thickness and quality, respectively. This can be seen in fig. 7, in which the third spark counting is presented for four foils irradiated with the same dose. The voltage of interest shifts from 610 V to 530 V if the foil becomes thinner. It is possible to use the voltage of interest as a parameter to correct for the change in foil sensitivity, which is based on the spark counts at 460 V after the third counting. Different etching runs and irradiations with 252Cf and A m - B e neutrons were performed to check this assumption. The sensitivity of irradiated foils at 460 V is presented in fig. 8 as a function of voltage at the electrical breakdown point related to the sensitivity at 460 V. Excluding the foils, where no well-
defined plateau exists even for voltages below 460 V mainly due to multiple sparking, the inhomogeneity of foil sensitivity can be corrected within + 2 0 % by using a correction factor presented in fig. 8. It can be seen from this figure that a correction factor which is based on such an approximation yields the same sensitivity for fission neutrons and A m - B e neutrons. As the energy threshold for a spark counting of neutron-induced recoils is in the order of 2 MeV compared to 1 MeV for optical counting s) the diameter and depth of the etch pits are of importance for the track detection. To explain fig. 8 we have to consider the overlapping of three different effects: - fluctuation in f o i l thickness and etching conditions: electrical breakdown at lower voltages due to thinner foils or higher etching rates, - energy dependence o f etch pit size: electrical breakdown at lower voltages due to larger etch pits and multiple sparking, - energy dependence o f track registration: the elastic cross section and the sensitivity increase with neutron energy.
it should be pointed out that both the influence of small fluctuations in the foil thickness and in the etching conditions as well as the energy dependence of the foil sensitivity and of the etch pit diameter are in the order of a factor 7.
104
FOIL No ,,,,,,a. 4 ,--..3 ,,,,~ 2
103
MAKROFOL 102
KG
ETCHED 16 HOURS 30% KOH.40eC FISSION NEUTRONS 10 rad 3rd
101 300
400
500
COUNTING
600
700
800
SPARK COUNTING VOLTAGE IN V
Fig. 7. Spark counts in four polycarbonate foils exposed to the same dose, as a function of spark voltage during thrid sparking.
452
J. J A S I A K A N D E. P I E S C H
3.0
SENSITIVITY
OF SPARK COUNTING 2.0
2
%
~
THIRD SPARKINGj AT /,60 V 1.0
.,~
~ 1,11~G
~
+ '
~
Am-Be 252^, 05
~.
s z
N ...i
0.2
~:
0J
400
500
600
700
VOLTAGE IN V Fig. 8. Sensitivity of irradiated foils checked at 460 V third s p a r k i n g as a function of spark v o h a g e at the electrical b r e a k d o w n point.
Additional checks have shown that small changes of the sensitivity due to the etching conditions can be corrected. After irradiation with Am-Be neutrons the replica of spark counts show more multiple sparks and therefore a spark count vs voltage characteristic without a well-defined plateau. By using the special spark counting technique with the voltage at the electrical breakdown as a parameter of interest for the interpretation of the counts vs voltage characteristic, it is possible to measure fast neutrons approximately independent of energy for fission neutrons and Am--Be neutrons. Optical counting of neutron-induced recoils in cellulose nitrate LR-I 15 films with a dark red background, however, shows a high change of sensitivity with the neutron energy in the order of a factor of 12 between fission and 4.5 MeV neutrons '~) compared to non-coloured cellulose nitrate, the energy dependence of which is similar to that of polycarbonate foils (see table 2). It is of interest to point out, that only a small fraction of etch pits are spark-counted or registered by the red-dyed LR-115 film.
4. Application Contrary to the spark counting of fission fragments there is a need of repeated counting over the voltage range, checking the spark replicas with respect to homogeneity and multiple sparking and of taking
into account the background count. The spark counting ofunirradiated foils resulted in spark counts in the order of 10-150 sparks per cmz. For a practical application, however, the highest value must be considered which significantly spoils the lower detection limit. Compared to an optical counting, however, the background rate is only one quarter of the value. In table 2 the properties of optical and spark TAI]I.E 2 Neutron-induced recoil track registration in p o l y c a r b o n a t e foils. Recoil track registration a Optical c ount i ng Spark c ount i ng
Response "-"~'-'Cf Am-Be B a c kground rate
9 x 10-6 tracks,"n 15x10 6tracks,n 660 tracks.,'cm ~
(0.3-0.8) x 10- ~ sparks;'n (0.3 -2.0) × 10 ~ sparks/n 150 s p a r k s / c m 2
7.3 2.6 4.4 1.5
(2--5) × 108 n/cm e (7-18) Rem (0.75-5) × 10 ~ n,;cm 2 (2.6.-17) Rein
Lower detection tim# '-':"'-'C f Am-Be
× 107 n/ c m "~ Rcm x 107 n.:cm ~ Rem
a In Makrot'ol E. 300Hm thick, for optical counting, M a k r o t b l KG, 12Hm thick, for spark c o u n t i n g at 4 6 0 V during third s pa rki ng run.
AUTOMATICAL
counting of neutron-induced recoils in Makrofol polycarbonate foil are presented. The sensitivity of the neutron detection for fission and A m - B e neutrons is very low and was found to be in the order of 10% compared to the optical counting. Taking into account the background count and the sensitivity also for the unfavourable cases before a correction based on fig. 8 was used, the lower detection limit for the spark c o u n t i n g - t h e spark counts which double the background count - is higher by a factor 3-6 compared to optical counting. Consequently there are practically no advantages for the spark counting method in routine dosimetry because of the lowest detectable dose of 7-18 Rein. Taking into account the results found for cellulose nitrate at ORNL8), a similar sensitivity of approximately 0.6 x 10 -6 holes per neutron can be estimated. As the spark counts of unirradiated polycarbonate and cellulose nitrate foils are also of the same order the lower detection limit of the spark counting technique is in both cases not sufficient enough to be applied in routine monitoring. The reproducibility of neutron fluence measurements is limited in both polymer foils by small fluctuations in foil thickness and sensitivity. In this investigation only a value of + 2 0 % was found compared to + 1 0 % for cellulose nitrate 8) or to + 5 % for the optical counting technique applied to 300/~m thick Makrofol E foils. Therefore the optical counting of recoils seems to be a more accurate method for all other applications in the measurements of high fluence: in accident dosimetry as a threshold detector, as a dosimeter belt to estimate the direction of the radiation incidence 7) and in
SPARK COUNTING
453
micro-dosimetry to measure the depth dose distribution in tissue in a thin tissue layer ~o). Instead of the spark counting technique a much more promising electrochemical technique seems to lend itself better for a practical application in neutron monitoring. This method, originally proposed by Tommasino I ~), was recently improved by Sohrabi I z) resulting in a sensitivity of the same order as spark counting but with a lower detection limit of 10toRero and a reproducibility of approximately + 10%.
References 1) W. G. Cross and L. Tommasino, Proc. 1st Euratom Syrup. on Neutron dosimeto, in biology and medicine, Neuherberg (1972) p. 283. ") R. V. Griffith, UCRL-51362, Lawrence Livermore Laboratory Report (1973). :~) S. Pretre, Proc. IAEA Syrup. on Neutron monitoring /or radiation protection purposes, vol. 2, Vienna (1973) p. 99. 4) K. Buijs et al., Proc. IAEA Syrup. oll Neutron monitoring for radiation protection purposes, vol. 2, Vienna (1973) p. 159. ,5) K. J6zefowicz, Proc. IAEA Syrup. on Neutron monitoring Jor radiation protection purposes, vol. 2, Vienna (1973) p. 183. ~) A. M. Sayed and E Piesch, KFK-2032, Kernforschungszentrum Karlsruhe Rcport (1974). 7) E. Piesch, Proc. IAEA Syrup. on Advances in t, hysical and biological radiation detectors, Vienna (1970) p. 399. 8) K. Becker and M. Abd-el Razek, Nucl. Instr. and Meth. 12.4 (1975) 557. !~) G. M. Hassib and 1.. Medveczky, Proc. 2nd Symp. on Neutron dosimetry in bioh)gy attd medicine, Neuherbeg (1974). 10) E. Piesch and A. M. Sased, Intern. IAEA Syrup. on Adt,:ances in biomedical dosimetry, Vienna (1975). 11) L. Tommasino, C N E N Report R T / P R O T (71) I (1970). ze) M. Sohrabi, Health Phys. 27 (1974) 598; M. Sohrabi and K. Z. Morgan, 3rd Europ. IRPA Congr. on Criteria for raditition protection, Amsterdam (1975), paper 14.