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Spectrophotometric measurement of track density in CR39 for high neutron dose determinations
0735-245X/91 $3.00 + .00 Pergamon Pre~ plc
NucL Tracks Radiat. Meas., Vol. 19, Nos I-4, pp. 241-244, 1991 Int. J. Radiat. AppL Inatrum., Part D Printed in Great Britain
SPECTROPHOTOHETRIC MERSUREMENT OF TRRCK DENSITY IN CR39 FOR NIGH NEUTRON DOSE DETERMINATIONS
~.M. BORDY, R. MEDIONI and G. PORTRL tEA, OPTISIDR, Centre d ' E t u d e s N u c l e a i r e s , B.P. n°6, 92285 F o n t e n a y - a u x - R o s e s , C~dex, FRANCE
RBSTRRCT
Due to t r a c k o v e r l a p p i n g , v i s u a l c o u n t i n g and a u t o m a t i c c o u n t i n g are L i m i t e d to low t r a c k densities. C o n v e n t i o n a l r e a d i n g t e c h n i q u e s cannot be used f o r high dose measurement. An a t t e m p t has been made t o d e v e l o p a t r a c k d e n s i t y d e t e r m i n a t i o n method s u i t a b l e f o r high n e u t r o n dose i r r a d i a t i o n (ex : c r i t i c a l i t y a c c i d e n t d o s i m e t r y ) . We have found t h a t the measurement of the o p t i c a l t r a n s m i s s i o n of a monochromatic l i g h t through CR39 d e t e c t o r i s a p o s s i b l e m e t h o d . D i f f e r e n t k i n d s of CR39 (produced i n B r i s t o l , I t a l y , or by Rmerican RCryl i c e ) were s t u d i e d • They were i r r a d i a t e d i n the f i e l d of the SILENE r e a c t o r and then c h e m i c a l l y e t c h e d . The r e s u l t s o b t a i n e d show t h a t : i t i s p o s s i b l e to p e r f o r m measurements from about 0.2 Gy to 100 6y, a linear relation between the o p t i c a l d e n s i t y and n e u t r o n karma can be o b t a i n e d by s u i t a b l y a d j u s t i n g chemical e t c h i n g c o n d i t i o n s .
KEYWORDS CR39 ; Chemical e t c h i n g
; S p e c t r o p h o t o m e t e r r e a d i n g ; High doses ; Neutron d o s i m e t r y .
INTRODUCTION S o l i d S t a t e Nuclear Track D e t e c t o r s have been w i d e l y used f o r many y e a r s . E x t e n s i v e s t u d i e s have been performed to f i n d a s o l u t i o n to r o u t i n e p e r s o n a l d o s i m e t r y . The main problems were background, d e t e c t i o n t h r e s h o l d and r e p r o d u c i b i l i t y . These d i f f i c u l t i e s are s t i l l not resolved satisfactorily f o r low dose measurements• Track d e n s i t y measurements are L i m i t e d to a maximum dose of about 0.1 Sv. Due to t r a r k o v e r l a p p i n g , a u t o m a t i c or v i s u a l c o u n t i n y cannot be used f o r high n e u t r o n doses• To f i n d a s o l u t i o n to t h i s problem, a study of CR39 o b t a i n e d from v a r i o u s m a n u f a c t u r e r s has been c a r r i e d o u t . A simple method of e v a l u a t i o n has been a p p l i e d : measurement of L i g h t t r a n s m i s s i o n i n c h e m i c a l l y etched CR39 samples.
EXPERIMENTAL PROCEDURE Nature of CR39 used and storage conditions Four different
CR3g products were investigated
Bristol 1000 um • manufactured in 1989 • s t o r e d in refrigerator •
t h i c k n e s s
:
@merican R c r y t i c s • t h i c k n e s s : 700 pm • r e c e i v e d i n Sept• 1983 • s t o r e d i n open a i r u n t i l 1987 then in r e f r i g e r a t o r
:
I n t e r c a s t Europe 5P@ ( I t a l y ) • t h i c k n e s s : 700 pm . r e c e i v e d i n June 1989 • s t o r e d in r e f r i g e r a t o r Non . . •
identified ; called t h i c k n e s s : 460 pm r e c e i v e d i n 1983 s t o r e d i n open a i r
All p r o d u c t s were p r o t e c t e d from d a y l i g h t
during storage•
241
" o l d " in t h i s
paper
242
J . M . B O R D Y et al.
Irradiation
Both faces of ER39 d e t e c t o r s were covered w i t h cardboard (2 mm t h i c k n e s s ) and exposed in the f i e l d of the SILENE source which i s an e x p e r i m e n t a l s o l u t i o n f u e l e d r e a c t o r . The experiments were performed with the source surrounded by a Lead shield. Different samples were irradiated in free air at different distances (0.5 to B m) from the r e a c t o r a x i s and received i n t e g r a t e d neutron doses v a r y i n g between 0.3 and 100 6y. For some irradiation locations in the irradiation hall, neutron spectra have been measured with activation detectors CSN@C2) ; kerma doses in tissue were then derived. For all other irradiation points, relative measurements were performed with silicon diodes and sulphur activation monitors.
I n t e g r a t e d gamma doses were determined w i t h thermoluminescent d e t e c t o r s . The highest value measured was 5.5 6y. I t i s w e l l known t h a t t h i s l e v e l of gamma dose does not a f f e c t the CR39 behaviour to neutrons (Portwood et aL., 1986, Zamani et aL., 198B). Etchinq and readinq c o n d i t i o n s A s p e c i a l study was not performed to f i n d the e t c h i n g parameters. Standard c o n d i t i o n s d e s c r i b e d in the l i t e r a t u r e were used : samples were c h e m i c a l l y etched in KOH s o l u t i o n (GN) at 50 C. Depending on the i r r a d i a t i o n l e v e l of each d e t e c t o r we v a r i e d the e t c h i n g time from 2 to 10 hours. A 5pectrophotometer has been used to measure l i g h t t r a n s m i s s i o n through the etched samples (Mourgues et aL., 1973, I L i c et a l . , 1986). RESULTS
Reading wavelength d e t e r m i n a t i o n For non irradiated samples from different suppliers, Fig. mission is constant for wavelength values higher than about For i r r a d i a t e d samples from @merican @ c r y l i c s , transmission ( F i g . 2 ) . The same v a r i a t i o n was observed f o r the o t h e r types
I and 2 show that the trans400 nm (Derndt et aL., 1984). decreases w i t h i n c r e a s i n g dose of CR39.
In the following experiments, measurements were performed at B50 nm in order to minimize the d i f f e r e n c e between the f o u r kinds of CR39. T
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+++++~
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xxxXXXxxXXX xxxXxx xxXXX xxXX xxXIxx _.A&iAIAAIIAAjAA Xx _&li&&A& AAIm"
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T r a n s m i s s i o n CT) as a f u n c t i o n of w a v e l e n g t h (W) f o r o l d Cm), B r i s t o l ( + ) , I t a Lian CO) and A m e r i c a n (~) b a c k g r o u n d CR39. (chemical etching KOH 6 N , 5 0 ° C , 2hq5min).
2.
i
i 400
i
; m
!
! IB
u
NO
xa/
Transmission (T) as a func Lion of Wavelength (W). Amer i c a n A c r y L i c s CR39. 32 Gy C o ) , 26 G y C + ) , 1 2 . 5 Gy CO), 7.25 Gy(~), 3.6 Gy(x), back ground(V). Chemical etching KOH 6N, 5 0 " C , 2 h l S m i n .
HIGH NEUTRON DOSE DETERMINATIONS
243
Linearity F i g . 3 shows the v a r i a t i o n of o p t i c a l d e n s i t y measured i n I t a l i a n CR39 versus n e u t r o n karma dose up to 12 Gy and f o r d i f f e r e n t chemical e t c h i n g t i m e s . This v a r i a t i o n i s l i n e a r , at L e a s t , in a c e r t a i n range of doses. I t i s t h e r e f o r e p o s s i b l e to a d j u s t the e t c h i n g time to the dose range to be measured. The same b e h a v i o u r was observed f o r the t h r e e o t h e r p r o d u c t s .
1.g 1,8
1.? 1.6 1.5
t
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//
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1.4 1.3 bl
1.2 1.1 1 0.g
3
8
0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
I
I
2
I
I
I
4
I
I
I
6
I
g
I
10
I
I
12
K t i s s u CGY)
Fig. 3
Optical densitv as a f u n c t i o n of K tissu for different chemical etching t i m e (KOH 6N, 5 0 ° C ) : 4 h 2 5 m i n ( o ) , 5hl0min(+), 7hl0min (~), 9hS0min (v). Intercast Europe SPA CR39.
Table 1 g i v e s the range of l i n e a r i t y deduced from these e x p e r i m e n t s . The upper l i m i t s rep r e s e n t the mean v a l u e s o b t a i n e d f o r the d i f f e r e n t kinds of CR39. For e t c h i n g times g r e a t e r than 4h25, a L i m i t of 0.3 6y has been quoted, but the e f f e c t i v e t h r e s h o l d i s pro~abLy Lower. Some e x p e r i m e n t s w i l l be performed in the f u t u r e to determine t h i s t h r e s h o l d . The maximum e t c h i n g time i s L i m i t e d by the o p a c i f i c a t i o n of the non i r r a d i a t e d dosemeters ; t h i s L i m i t was not reached even a f t e r 9h50.
Table 1 . Etching time lh55 2h45 4h25 5h10 7h10 9h50
Linearity
f o r CR39.
Linearity
range
2.50 0.75 0.30 0.30 0.30 0.30
60 Gy 20 12 10 4 2
to
244
.I.M. BORDY et el.
Reproducibility between different products To compare the results obtained with the four studied CR38, we have calculated the slope of the Linear r e g r e s s i o n f o r each product (dose = slope x o p t i c a l density), the straight Line obtained passes through the o r i g i n . Table 2 . e t c h i n g time Italian Bristol American Old
Slope values ( 6 y ) . 4h25
5h10
7h10
5h50
13.30 12.83 12.10 10.54
9.35 9.26 8.50
4.35 4.22 3.59 3.54
2.21 2.22 2.12 I
7.57
Table 2 shows the slope values deduced from e x p e r i m e n t a l r e s u l t s corresponding to d i f f e r e n t etching times. The three identified products have similar results (~ 5~) ; however American @crylics (1983) has systematically Lower results than Italian and Bristol fabrication (19B5). The o l d CR39 presents slope values of about 20% lower than the three o t h e r p r o d u c t s . This i s p r o b a b l y due to the storage c o n d i t i o n s ( s e v e r a l years in the open a i r ) . I t was observed t h a t storage in open a i r at room temperature of the American g c r y l i c s CR35 has not significantly influenced its dosimetric properties. This result has to be confirmed in the future.
CONCLUSIONS The advantage of t h i s technique i s the easiness and r a p i d i t y of sample e v a l u a t i o n ; e t c h i n g times are Long for low values of karma, but they can be decreased by increasing the etching temperature. The differences of sensitivity and background values obtained for these four pro0ucts seem to be Lower than those normally encountered with electrochemical etching associated to conventional track counting techniques. These first results are very encouraging. Further invebl~gations will be made to confirm them and to obtain more information on the reproducibility, the threshold and the influence of Long time storage under Laboratory conditions.
REFERENCES Berndt, M., G. Siegmon, R. Beaujean and W. ENGE (1984). A new nuclear track filter of CR39. Nuclear tracks, 8, n ° I-4, 589-592. llic, R., 3. Rant, M. Humar, 6. Somogyi and I. Hunyadi (1955). Neutron radiographic characteristics of M@-ND type (aLlyl-diglycol-carbonate) nuclear track detectors. NuCLear tracks, I~2, n° I-5, S33-936. Mourgues, M. and ~.M. Renavot (1973). @pplication de [a technique ionographioue a l'anaLyse de la r ~ p a r t i t i o n des f i s s i o n s dans Les ~l~ments c o m b u s t i b l e s . I@E~-SM-160145. Portwood T. and g . L . Henshaw (1586). The e f f e c t of gamma dose on the alpha response of CR39. Nuclear t r a c k s , 12, n=1-6, 105-108. Zamani, M., E. Savvides, J. P e t r a k i s and 5. CharaLambous (1986). 6amma dose d i s c r i m i n a t i o n p r o p e r t i e s of SSNT d e t e c t o r s . Nuclear t r a c k ~ , 1 2 , r e 1 - 6 , 141-144.
NucL Tracks Radiat. Meas., Vol. 19, Nos I-4, pp. 241-244, 1991 Int. J. Radiat. AppL Inatrum., Part D Printed in Great Britain
SPECTROPHOTOHETRIC MERSUREMENT OF TRRCK DENSITY IN CR39 FOR NIGH NEUTRON DOSE DETERMINATIONS
~.M. BORDY, R. MEDIONI and G. PORTRL tEA, OPTISIDR, Centre d ' E t u d e s N u c l e a i r e s , B.P. n°6, 92285 F o n t e n a y - a u x - R o s e s , C~dex, FRANCE
RBSTRRCT
Due to t r a c k o v e r l a p p i n g , v i s u a l c o u n t i n g and a u t o m a t i c c o u n t i n g are L i m i t e d to low t r a c k densities. C o n v e n t i o n a l r e a d i n g t e c h n i q u e s cannot be used f o r high dose measurement. An a t t e m p t has been made t o d e v e l o p a t r a c k d e n s i t y d e t e r m i n a t i o n method s u i t a b l e f o r high n e u t r o n dose i r r a d i a t i o n (ex : c r i t i c a l i t y a c c i d e n t d o s i m e t r y ) . We have found t h a t the measurement of the o p t i c a l t r a n s m i s s i o n of a monochromatic l i g h t through CR39 d e t e c t o r i s a p o s s i b l e m e t h o d . D i f f e r e n t k i n d s of CR39 (produced i n B r i s t o l , I t a l y , or by Rmerican RCryl i c e ) were s t u d i e d • They were i r r a d i a t e d i n the f i e l d of the SILENE r e a c t o r and then c h e m i c a l l y e t c h e d . The r e s u l t s o b t a i n e d show t h a t : i t i s p o s s i b l e to p e r f o r m measurements from about 0.2 Gy to 100 6y, a linear relation between the o p t i c a l d e n s i t y and n e u t r o n karma can be o b t a i n e d by s u i t a b l y a d j u s t i n g chemical e t c h i n g c o n d i t i o n s .
KEYWORDS CR39 ; Chemical e t c h i n g
; S p e c t r o p h o t o m e t e r r e a d i n g ; High doses ; Neutron d o s i m e t r y .
INTRODUCTION S o l i d S t a t e Nuclear Track D e t e c t o r s have been w i d e l y used f o r many y e a r s . E x t e n s i v e s t u d i e s have been performed to f i n d a s o l u t i o n to r o u t i n e p e r s o n a l d o s i m e t r y . The main problems were background, d e t e c t i o n t h r e s h o l d and r e p r o d u c i b i l i t y . These d i f f i c u l t i e s are s t i l l not resolved satisfactorily f o r low dose measurements• Track d e n s i t y measurements are L i m i t e d to a maximum dose of about 0.1 Sv. Due to t r a r k o v e r l a p p i n g , a u t o m a t i c or v i s u a l c o u n t i n y cannot be used f o r high n e u t r o n doses• To f i n d a s o l u t i o n to t h i s problem, a study of CR39 o b t a i n e d from v a r i o u s m a n u f a c t u r e r s has been c a r r i e d o u t . A simple method of e v a l u a t i o n has been a p p l i e d : measurement of L i g h t t r a n s m i s s i o n i n c h e m i c a l l y etched CR39 samples.
EXPERIMENTAL PROCEDURE Nature of CR39 used and storage conditions Four different
CR3g products were investigated
Bristol 1000 um • manufactured in 1989 • s t o r e d in refrigerator •
t h i c k n e s s
:
@merican R c r y t i c s • t h i c k n e s s : 700 pm • r e c e i v e d i n Sept• 1983 • s t o r e d i n open a i r u n t i l 1987 then in r e f r i g e r a t o r
:
I n t e r c a s t Europe 5P@ ( I t a l y ) • t h i c k n e s s : 700 pm . r e c e i v e d i n June 1989 • s t o r e d in r e f r i g e r a t o r Non . . •
identified ; called t h i c k n e s s : 460 pm r e c e i v e d i n 1983 s t o r e d i n open a i r
All p r o d u c t s were p r o t e c t e d from d a y l i g h t
during storage•
241
" o l d " in t h i s
paper
242
J . M . B O R D Y et al.
Irradiation
Both faces of ER39 d e t e c t o r s were covered w i t h cardboard (2 mm t h i c k n e s s ) and exposed in the f i e l d of the SILENE source which i s an e x p e r i m e n t a l s o l u t i o n f u e l e d r e a c t o r . The experiments were performed with the source surrounded by a Lead shield. Different samples were irradiated in free air at different distances (0.5 to B m) from the r e a c t o r a x i s and received i n t e g r a t e d neutron doses v a r y i n g between 0.3 and 100 6y. For some irradiation locations in the irradiation hall, neutron spectra have been measured with activation detectors CSN@C2) ; kerma doses in tissue were then derived. For all other irradiation points, relative measurements were performed with silicon diodes and sulphur activation monitors.
I n t e g r a t e d gamma doses were determined w i t h thermoluminescent d e t e c t o r s . The highest value measured was 5.5 6y. I t i s w e l l known t h a t t h i s l e v e l of gamma dose does not a f f e c t the CR39 behaviour to neutrons (Portwood et aL., 1986, Zamani et aL., 198B). Etchinq and readinq c o n d i t i o n s A s p e c i a l study was not performed to f i n d the e t c h i n g parameters. Standard c o n d i t i o n s d e s c r i b e d in the l i t e r a t u r e were used : samples were c h e m i c a l l y etched in KOH s o l u t i o n (GN) at 50 C. Depending on the i r r a d i a t i o n l e v e l of each d e t e c t o r we v a r i e d the e t c h i n g time from 2 to 10 hours. A 5pectrophotometer has been used to measure l i g h t t r a n s m i s s i o n through the etched samples (Mourgues et aL., 1973, I L i c et a l . , 1986). RESULTS
Reading wavelength d e t e r m i n a t i o n For non irradiated samples from different suppliers, Fig. mission is constant for wavelength values higher than about For i r r a d i a t e d samples from @merican @ c r y l i c s , transmission ( F i g . 2 ) . The same v a r i a t i o n was observed f o r the o t h e r types
I and 2 show that the trans400 nm (Derndt et aL., 1984). decreases w i t h i n c r e a s i n g dose of CR39.
In the following experiments, measurements were performed at B50 nm in order to minimize the d i f f e r e n c e between the f o u r kinds of CR39. T
"r
4,
I
ViiVVVVvVvyvvWVVVvDgvvvvvvvvvfVg
0,9 -
,,A.o" ,.,"
o,!'
//
0,7,
/o- 61
,/
A
Vv 0.8 -
si
0.? -
~0/
0.6,
o,°':" ,........,...°.....:::::.,,.'"....,..
oil
03
O,4, /12/i !
0
-I ° 9 ~ f f + ~ # f f _00000 0.1 - I t + "O000oOu 0 IoooOOOOOOOO0000000ooooOODDU
0.1 / !
o
i
u
i
U m
|
Fig. I.
+++++~
0.2
0],
Fig.
1ltlt°
-
0.3,
-
xxxXXXxxXXX xxxXxx xxXXX xxXX xxXIxx _.A&iAIAAIIAAjAA Xx _&li&&A& AAIm"
xX &All&Aa X AA 1
8.1 " V
i 4, o i
-
V V
/
05
vv
T r a n s m i s s i o n CT) as a f u n c t i o n of w a v e l e n g t h (W) f o r o l d Cm), B r i s t o l ( + ) , I t a Lian CO) and A m e r i c a n (~) b a c k g r o u n d CR39. (chemical etching KOH 6 N , 5 0 ° C , 2hq5min).
2.
i
i 400
i
; m
!
! IB
u
NO
xa/
Transmission (T) as a func Lion of Wavelength (W). Amer i c a n A c r y L i c s CR39. 32 Gy C o ) , 26 G y C + ) , 1 2 . 5 Gy CO), 7.25 Gy(~), 3.6 Gy(x), back ground(V). Chemical etching KOH 6N, 5 0 " C , 2 h l S m i n .
HIGH NEUTRON DOSE DETERMINATIONS
243
Linearity F i g . 3 shows the v a r i a t i o n of o p t i c a l d e n s i t y measured i n I t a l i a n CR39 versus n e u t r o n karma dose up to 12 Gy and f o r d i f f e r e n t chemical e t c h i n g t i m e s . This v a r i a t i o n i s l i n e a r , at L e a s t , in a c e r t a i n range of doses. I t i s t h e r e f o r e p o s s i b l e to a d j u s t the e t c h i n g time to the dose range to be measured. The same b e h a v i o u r was observed f o r the t h r e e o t h e r p r o d u c t s .
1.g 1,8
1.? 1.6 1.5
t
-"9~
//
,
/
~
~r-
"
1.4 1.3 bl
1.2 1.1 1 0.g
3
8
0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
I
I
2
I
I
I
4
I
I
I
6
I
g
I
10
I
I
12
K t i s s u CGY)
Fig. 3
Optical densitv as a f u n c t i o n of K tissu for different chemical etching t i m e (KOH 6N, 5 0 ° C ) : 4 h 2 5 m i n ( o ) , 5hl0min(+), 7hl0min (~), 9hS0min (v). Intercast Europe SPA CR39.
Table 1 g i v e s the range of l i n e a r i t y deduced from these e x p e r i m e n t s . The upper l i m i t s rep r e s e n t the mean v a l u e s o b t a i n e d f o r the d i f f e r e n t kinds of CR39. For e t c h i n g times g r e a t e r than 4h25, a L i m i t of 0.3 6y has been quoted, but the e f f e c t i v e t h r e s h o l d i s pro~abLy Lower. Some e x p e r i m e n t s w i l l be performed in the f u t u r e to determine t h i s t h r e s h o l d . The maximum e t c h i n g time i s L i m i t e d by the o p a c i f i c a t i o n of the non i r r a d i a t e d dosemeters ; t h i s L i m i t was not reached even a f t e r 9h50.
Table 1 . Etching time lh55 2h45 4h25 5h10 7h10 9h50
Linearity
f o r CR39.
Linearity
range
2.50 0.75 0.30 0.30 0.30 0.30
60 Gy 20 12 10 4 2
to
244
.I.M. BORDY et el.
Reproducibility between different products To compare the results obtained with the four studied CR38, we have calculated the slope of the Linear r e g r e s s i o n f o r each product (dose = slope x o p t i c a l density), the straight Line obtained passes through the o r i g i n . Table 2 . e t c h i n g time Italian Bristol American Old
Slope values ( 6 y ) . 4h25
5h10
7h10
5h50
13.30 12.83 12.10 10.54
9.35 9.26 8.50
4.35 4.22 3.59 3.54
2.21 2.22 2.12 I
7.57
Table 2 shows the slope values deduced from e x p e r i m e n t a l r e s u l t s corresponding to d i f f e r e n t etching times. The three identified products have similar results (~ 5~) ; however American @crylics (1983) has systematically Lower results than Italian and Bristol fabrication (19B5). The o l d CR39 presents slope values of about 20% lower than the three o t h e r p r o d u c t s . This i s p r o b a b l y due to the storage c o n d i t i o n s ( s e v e r a l years in the open a i r ) . I t was observed t h a t storage in open a i r at room temperature of the American g c r y l i c s CR35 has not significantly influenced its dosimetric properties. This result has to be confirmed in the future.
CONCLUSIONS The advantage of t h i s technique i s the easiness and r a p i d i t y of sample e v a l u a t i o n ; e t c h i n g times are Long for low values of karma, but they can be decreased by increasing the etching temperature. The differences of sensitivity and background values obtained for these four pro0ucts seem to be Lower than those normally encountered with electrochemical etching associated to conventional track counting techniques. These first results are very encouraging. Further invebl~gations will be made to confirm them and to obtain more information on the reproducibility, the threshold and the influence of Long time storage under Laboratory conditions.
REFERENCES Berndt, M., G. Siegmon, R. Beaujean and W. ENGE (1984). A new nuclear track filter of CR39. Nuclear tracks, 8, n ° I-4, 589-592. llic, R., 3. Rant, M. Humar, 6. Somogyi and I. Hunyadi (1955). Neutron radiographic characteristics of M@-ND type (aLlyl-diglycol-carbonate) nuclear track detectors. NuCLear tracks, I~2, n° I-5, S33-936. Mourgues, M. and ~.M. Renavot (1973). @pplication de [a technique ionographioue a l'anaLyse de la r ~ p a r t i t i o n des f i s s i o n s dans Les ~l~ments c o m b u s t i b l e s . I@E~-SM-160145. Portwood T. and g . L . Henshaw (1586). The e f f e c t of gamma dose on the alpha response of CR39. Nuclear t r a c k s , 12, n=1-6, 105-108. Zamani, M., E. Savvides, J. P e t r a k i s and 5. CharaLambous (1986). 6amma dose d i s c r i m i n a t i o n p r o p e r t i e s of SSNT d e t e c t o r s . Nuclear t r a c k ~ , 1 2 , r e 1 - 6 , 141-144.