P r a m g n a - J . Phys., Vol. 25, No. 3, September 1985, pp. 259-265. © Printed in India.
Annealing of heavy ion tracks in cellulose nitrate plastic track detectors
S M F A R I D
Department of Physics, Rajshahi University, Rajshahi, Bangladesh MS received 4 January 1985; revised 21 June 1985
Abstract. The effect of heat treatment on the latent tracks in cellulose nitrate plastic track detectors has been studied. The bulk etch rate increases with annealing temperature while the track diameters of different ions in cellulose nitrate decrease with increase in annealing time and temperature. Experimental results show that for heavier ions higher temperatures are needed for their complete erasure. The track length and track etch rate are decreased by the application of heat. Experiments reveal that annealing reduces track density. The vertical tracks are more stable than the oblique tracks and require higher temperature for their complete erasure.
Keywords. Solid state nuclear track detectors; annealing; bulk and track etch rates; track length; track density; vertical and oblique tracks.
PACS No. 29.40
1. Introduction
In solid state nuclear track detectors (ssrrro) paths of individual heavily ionizing charged particles are revealed by selective chemical etching of the radiation-damaged material along the particle's trajectory. Heating produces an apparent reduction of particle track length as well as its diameter (Fleischer et al 1975; Khan and Durrani 1972, 1973; Somogyi 1972). Hence, if the detectors are to be operated at a temperature higher than the normal room temperature, proper temperature-dependent corrections to the efficiency factor must be applied. In the present paper the experimental data are presented for the influence of different annealing conditions on bulk etch rate, track etch rate, etch pit diameters, track density and etchable ranges of different ions in cellulose nitrate plastic detectors.
2. Experimental procedure
Studies were performed on two types of cellulose nitrate plastics, simultaneously under identical experimental conditions. The detectors used were cellulose nitrate (Daicel, Japan) and cellulose nitrate (Russian) abbreviated henceforth as cNJ and CNR respectively. The samples were exposed to different ions of different energies (table 1) from cyclotron beams at JINR, Dubna (USSR). The angles of exposure were 90 ° and 30 ° with respect to the detector surface. The exposed samples were annealed in air in an 259
260 S M Farid
Table 1. Exposure and etching conditions of the detectors.
Angle of exposure
Detector Ion with energy w.r.t, detector Etching
material in MeV/N surface conditions
Cellulose nitrate 160, E = 8.75 30 ° 6 N NaOH
CN (Russian) ~o°Ne, E = 10-00 90 °, 30 ° 60°C Cellulose nitrate ~He, E = 1.75 90 ° 6 N NaOH
CN (Japan) ~°Ne, E = 8-5 90 °, 30 ° 60°C
oven. T h e oven t e m p e r a t u r e was controlled to within + 3°C. T h e samples were etched in stirred 6N N a O H solution at 60°C. The m e a s u r e m e n t s were taken with an
" O l y m p u s " microscope with an eyepiece m i c r o m e t e r o f least c o u n t L.C. = 0.215/tm at a magnification o f 900 x . T h e absence o f error bars in all the figures represents an accuracy better than the size o f the symbols used in the figures.
3. Results and discussion
3.1 Effect of annealing on bulk etch rate and on track diameter
Unexposed samples o f cNa and cNJ were annealed f o r 1 0 m i n u t e s at different temperatures. T h e annealed samples were then etched simultaneously in 6 N N a O H at 60°C. The data o f the removed layers o f the sheets were o b t a i n e d t h r o u g h direct thickness m e a s u r e m e n t s with microthickness gauge having least c o u n t = 0"5 #m. The bulk etch rate is determined by V b = Ad/2t, where Ad is the dissolved thickness o f the material in a k n o w n etching time t.
In cNJ sheets a b o v e 120°C, lib keeps increasing as a result o f the ever growing degree o f thermal degradation. In CrqR V b increases above the annealing temperatures o f 100°C.
A b o v e 140°C (in b o t h cases) the mechanical properties strongly deteriorate, the sheets b e c o m e glassy a n d brittle and the measurements o f the r e m o v e d layers b e c o m e unreliable.
cNJ and CNR detector samples exposed vertically to different ions (table 1) were etched in 6 N N a O H solution at a particular temperature. T h e track diameters were measured for different etching times. A linear relationship exists between track diameter and etching time for different etching temperatures. Different authors (Benton 1968; S o m o g y i 1972, Luck 1974; M a u r y a et al 1979) also observed linear relationship between track diameter and etching time in cellulose nitrate ( K o d a k and Daicel) detector for different etching temperatures.
Detector samples exposed vertically to different ions were annealed for 10 rain utes at various temperatures. T h e choice o f the annealing time o f 10 rain was accounted for by o u r experience with plastic annealed for a longer period. After etching the samples simultaneously in 6 N N a O H at 60~C the etchpit diameters were measured as a function o f the r e m o v e d layer. The results are presented in figure 1 for 2°Ne-ion in CNR.
In the left hand corner o f figure 1 (inset) the track d i a m e t e r is plotted after removing a
Annealing o f heavy ion tracks 261
4 0
~L 3O
.~ E 2 0 c- O
1 0
3O
° i 0 0
iemoved~
Ioylr =10 (prnI I I 1 40 80 120
Temp (°C)
25 ° 100 °
120 °
Ann. temp.
(°C) CN(R)
2°Ne-ion, E = IOMeV/N 10
Ann.time = lOmin, 6N NQOH, 6 0 °C
~1 -~0)o
0 5 10 15 20
Removed Ioyer from single surface ( p r o )
Figure !. Relationship between track diameter and removed layer for 12o ° Ne-ion track s in CNR annealed for lO min at different temperatures.
10 pm thick surface layer as a function of annealing temperature. Thus to eradicate
~2o°Ne-ion tracks in CNR completely, an annealing period of 10 min must be applied at 130°C. Similar plots (not shown here) were also obtained for 12° Ne and z4He-ions in CNJ.
For complete thermal eradication of ~2°Ne and ~He-ion tracks, 10 min of annealing must be applied at 140°C and 135°C respectively in CNJ. The figures show that for heavier ions higher temperature is needed for complete erasure. The heavier ions will deposit more energy and hence there is greater damage. Thus a higher temperature (i.e.
higher thermal energy) is needed to cure the latent damage in the detector. Somogyi (1972) also observed the reduction in diameter after annealing tHe-ions tracks in CNJ.
3.2 Effect of annealing time on track diameter
Detector samples o f CNJ and CNR exposed vertically to the ions were annealed at 120°C for different lengths o f time. After etching the track diameters were measured. Figure 2 shows the effects o f annealing time on etch pit diameters of Z°Ne and ~He-ions entering at right angles, after removing a 15 pm thick surface layer of the individual detector samples. The same curves for ~0°Ne-ion in cNJ are shown in figure 3 for annealing temperatures of 120°C and 130°C. It is observed that each curve consists of a steeply and a slowly falling component. It is apparent that a part of the radiation-damaged region produced by the ions is already eradicated at the given temperature by short annealing; however in the more stable region presistent after the treatment, no considerable change is brought about even by a significant extension of annealing time.
At higher temperature of annealing (130°C in figure 3), the slowly falling component of
262 S M Farm
38,
32
(~] 'ZONe O C N (R) Io
a 4He A A] CN(J)
2
Ann. temp. = 120°C Ann. time = lOmin
24 ~ Removed layer = 15 pm
L 6N NaOH, 60°C
O I I I
1 2 5
Anneoling time (hr)
Figure 2. Relationship between track diameter and annealing time for x2°Ne-ion in CNR and for ~o°Ne and ~He-ions in cNJ detector annealed at 120°C.
3 2
2 4 E :3-
~ 1 6
E
0 c- O ID
~ 8
O
C N (J)
: : N e - i o n , 8 5 MeV/N
~N. 6 N NQOH, 60°C
_ ~ k ' ~ Removed Ioyer = 15pm
~ Ann. temp.
1 2 3 4
Anneoling time (hr)
Figure 3. Relationship between track diameter and annealing time for ~°Ne-ion in CN~
detector annealed at 120°C and 130°C.
Annealin O o f heavy ion tracks 263 the curve starts early. Since the measurements at other annealing temperatures for 2o°Ne and ~He-ions in CNR and CNJ respectively display a similar tendency in diameter decrease they are not presented here.
3.3 Effect of annealing temperature on V,
Experiments were conducted to show the decrease in V, of 2°Ne and ~He-ions. The ratio of track and bulk etch rates, V = V,/V b was evaluated for different temperatures from the initial slopes, S of the curves shown in figure 1 and similar figures using the relation (Somogyi 1972; Somogyi and Szalay 1972; Luck 1974, Schlenk et al 1975),
V = (I +0.25 S 2 ) / ( 1 -0.25 $2).
This etch rate ratio is plotted against annealing temperature as shown in figure 4. It is observed that V, decreases with annealing temperature.
3.4 Effect of annealing on diameter distribution
Samples of cNJ exposed to 2°Ne-ions were annealed for 10 min at 120°C. The areas of annealed and unannealed samples were measured precisely to obtain the track density.
The annealed and unannealed samples were etched simultaneously in 6 N NaOH at 60°C. Figure 5 shows the diameter distribution of 2°Ne-ion tracks in cNJ after removing 15 #m thick surface layer of the individual detectors. The experiments reveal that after annealing, not only the track density is reduced but the diameters of the etched 2°Ne-ion tracks are also diminished, as a result of lower etching velocity in the damaged trial. Since the annealing conducted for 4He-ion in cNJ and ~Zo°Ne-ion in CNR confirms the same conclusions, the histograms of annealed and unannealed ~He and
~°Ne-ion tracks in cNJ and CNR respectively are not presented here. Benton (1968)also observed a decrease in diameter and track density of heavy ions in CN plastic detectors prepared in the laboratory with different amount of additives.
[ (o
I CN(J)
>-- o e-lon
1 2 ~'~ \ \ 6 N NoOH
(2:: 0 L I
50 100
Anntemp (°C)
150
>~
24
~16
8
o P12
I ~ CN(R) (b)
f °Ne-'°n
0 I
5 0 100 150
Ann. temp. (°C)
Figure 4 Etch rate ratio, V = Vt/Vb as a function o f annealing temperature for (a) ~o°Ne and
~He-ions in CNJ and (b) for 2°Ne-ion in CNR detectors.
264 S M Farm
2 4 0
200
160
>. 120
tj c
C:T
Lt_
80
40
0
Figure 5.
detector.
19 C N ( J )
2O I 0 N e - I ° n ' 8 . 5 MeV/N
Ann. temp. = 120°C --Ann. time =tOrero 6N NoOH, 6 0 ° C Removed layer = 15pro
__J---~ Un onneoled
~- - -], A n n e a l e d
_ j I
r--I
I i
r - -
I I
i r--I I I
I [
22
"-1
I I
I
I
L-- I
I
I
1 [_ I
25 28
Diameter (pro)
L
\
31 34
Diameter distribution of unannealed and annealed ~°Ne-ion tracks in cNJ
3.5 Effect of temperature on maximum etchable range
Detector samples exposed to loNe and 160-ions at an angle of 30 ° with respect to 20 detector surface were annealed at different temperatures for 10 minutes. The samples were then etched simultaneously in 6 N NaOH at 60°C until the track ends become round. The maximum etched track lengths were determined following the procedure of Benton (1968), Dwivedi and Mukherji (1979) and Farid and Sharma (1983, 1984). The effect of annealing temperature on maximum etchable track length (i.e. range) of 2°Ne and 16 O-ions in CNR and 20°Ne-ions in cNJ is shown in figure 6. It is evident that the track length is decreased by the application of heat. It is also noted that 2o°Ne and 160-ion tracks is Cr~R are completely erased when annealed for l0 min at 125°C and 123°C respectively while 2o°Ne-ion tracks in CNJ are completely erased at 130°C. Comparing figure 6 with figure 1 and similar figures it can be concluded that the vertical tracks are more stable than oblique tracks and require higher temperature for their complete erasure.
Annealing of heavy ion tracks 265 2 4 0
o c . ( J )
E ~ " " ~ \ 6 N N o O H , 6 0 ° C
~ Omin
o ~
- 120
2
6O
laJ
o I I I ~ %
4 0 8 0 1 2 0 160
Annealing temp. (°C)
Figure 6. Variation of maximum etchable track length (i.e. range) with annealing tempera- ture for l~O and ~°Ne-ions in CNR and ~°Ne-ion in CNJ detector.
References
Benton E V 1968 Study o f charged particle tracks in cellulose nitrate, USNRDL-TR-68-14 Dwivedi K K and Mukherji S 1979 Nucl. lnstrum. Methods 159 433
Dwivedi K K and Mukherji S 1979 Nucl. Instrum. Methods 161 317 Farid S M and Sharma A P 1983 Int. J. Appl. Radiat. Isotopes 35 181 Farid S M and Sharma A P 1984 Radiat. Eft. 80 121
Fleischer R L, Price P B and Walker R M 1975 Nuclear tracks in solids (Berkeley: University of California Press)
Khan H A and Durrani S A 1972 Nucl. lnstrum. Methods 98 229 Khan H A and Durrani S A 1973 Nucl. lnstrum. Methods 113 51 Luck H B 1974 Nucl. Instrura. Methods 114 139
Maurya A L, Bose S K and Tuli S K 1979 Radiat. Eft. 40 115 Schlenk B, Somogyi G and Valek A 1975 Radiat. Eft. 24 147 Somogyi G 1972 Radiat. Eft. 16 245
Somogyi G and Szalay 1972 Atomki Kozl. 14 113
