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A study of track etch anisotropy in apatite
Nuclear Tracks, Vol. 12, Nos I-6, pp. 927-930, 1986. Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
A STUDY OF TRACK ETCH ANISOTROPY IN APATITE
Surinder Singh, Dilbag Singh, A.S. Sandhu, Gurmukh SinEh and H.S. Virk Department of Physics, Guru Nanak Dev University, Amritsar, India
ABSTRACT The track etch investigations on samples cut from different planes of apatite crystal are carried out in order to study the anisotropic etching behaviour of the crystal. The variation of the track etch characteristics with crystallographic orientations is observed. Because of track etch anisotropy in the crystal the fission track age determined on different planes of the crystal is found to vary. The etching conditions are standardised for different planes using HNO 3 as the track etchant. KEY WORDS Track etching, anisotropic, crystallographic orientations, fission track age. INTRODUCTION The most important advancement in the history of solid state nuclear track detectors (S~TD) is the discovery of chemical etching for track revealation (Price and Walker, 1962). Since then the SSNTD technique has gained popularity in diverse fields (Fleischar et al., 1975). In geochronology the technique is generally used for absolute age determination of minerals and rocLcs. Though a lot of work has been reported on fission track (f.t.) dating of minerals and rocks, a very little attention has been paid to the fundamental track etching characteristics of minerals and how these may affect the dating technique in routine use. In an anisotropic mineral, the atomic spacing is variable along different crystallographic orientations, hence the bulk etch rate, VG, the track etch rate, VT, and hence the etching efficiency,~ , might be different for tracks in different orientations. This anisotropic etching behaviour can affect the f.t. age of the minersl. In track etch work there are very few studies of this type (Durrani et el., 1974, Gleadow, 1978) because in most of the cases simple isotropic detectors like glasses and plastics are generally used. In f.t. geochronology one has to use natural minerals most of which are anisotropic and can have complicated track etch characteristics. In the present work an attempt has been made to study the track etch ~n~sotropy in apatite and to observe its effect on fission track dating of this mineral. Apatite crystal procured from Guebic Lavel Museum and of Wakefield area in Canada is used for such studies. EXPERIMENTAL PROCEDURE (a) S am~le preparation: In order to cut the different planes of the a p a t i t e crys%-al, it was marked with X,Y,U and Z axis. X,Y and U are inclined at 120 u with each other and Z axis is perpendicular to all these axis. The planes (lOll), (Olrl), (1TOO), (lOgO), (OleO) and (O001) cut from the crystal were 927
928
SURINDER SINGH et aZ.
taken for study. (b) Measurement of VG, V T and ~ ,
on different planes:
Samples from different
faces of the crystal were heated at a temperature of 530°C for one and a half hour to remove fossil traci~. The polished samples were irradiated with fission fragment source ( C ~ 52) in 2~ geometry. These were then etched in 67. HNO 3 at room temperature for various time intervals. The results for the variation of track length and track diameter with etching time for different planes are shown in Figs. 1 and 2 respectively. The bulk etch rate, VG, was calculated
I T
S
,I • --(ioll~ • - (lOft) • --(I l.oJ
• - @l~O
• --(loll)
•
• i(o I
. __
0
Fig. 1
zO
40
EO
ETCHING
TIME
IO
I00
• - (1100)
ft.) ~oot)
40
~.o
ETCHrNO
(see)
Variation of track length with etching time.
Fig .2
e,O TrM¢
¢m~
mO
• -
OOTo)
--
(otto)
m -
(0000
mO
(l@c)
variation of track diameter with etching time.
from the slope of the curve etching time versus trac~ diameter. V T was determined from the slope of the linear part of the curve showing the variation of projected track length with etchi.ng time (Fleischer et al., 1975( Singh et.al, 1984). The etching efficiency ( ~ ) was calculated from the relation: VG --1 - V T The results for V G, V T and ~] for different planes are given in Table 1. TABLE 1 Face lOl--O 011--1 l~O0 O001 OIT0 lO 1--1
The values of V G, V T and ~ for different planes of apatite VT (~m/sec)
VG (~m/s ec)
0.225 0.120 0.165 0.230 0.135 O. 200
0.040 0.020 0.0 20 0,0 27 0.015 O. 019
~
(~')
82.230 83.330 87.879 88.0 44 88.880 90 •530
(c) ~tandardisation of e t c h i ~ t i o n s : In order to determine the optimum etching time for fossil tra-~Lcs, a sample containing fossil tracLcs from (Ol~l) plane was etched in 6~. HN0 3 at room temperature for different time intervals and the tracL~ density was recorded corresponding to each time interval. For determining the optimum etching time for induced fission tracks, the annealed apatite samples from different planes irradiated with thermal neutrons (From CIRUS Reactor at BARC, Trombay with a dose of 5xl014 nvt) were etched in 6~. HNO 3 at room temperature for different time intervals.
TRACK ETCH ANISOTROPY IN APATITE
929
The induced track density was recorded at each etching interval for different planes. The results are shown in Figs. 3 and 4. (d) Fission _~ack a~e determination on different planes of apatite cr~stal. Fission track age on different planes wm~ estimated using the age equation (Singh et al., 1984) : T = 6.57x109 ln(l+9.25x10 "18 ~ x ~) where ~s and ~i are the fossil and induced track densities and ~ is the total thermal neutron dose. The results are given in Table II. TABLE II
Fission track age on different faces of apatite crystal = 4.54 x l014 (nvt).
Face
Sample No.
lOll
AC-I
7.38
3.95
50.63+2.53
AC-II
7.53
3.98
51.12+2.53
AC-I AC-II AC-I AC-II
8.38 8.43 4.83 4.83
3 •48 3.32 3.13 3.26
64.29+_3.32
64. 63+_3.31
64.97+--3.29 41.84+2.42 40.16+ 2.30
41.00÷-2.36
I0~0
AC-I AC-II
4.28 4.32
2.50 2.50
46.45+-2.95 46.73+_2.96
oIYO
AC-I
3.87
2.38
41.91+2.78
AC-II
3.73
2.38
42.63+_2.82
Ol~l ii--00
~s x lO -5 (Tracks/cm 2)
~i x 10 -5 (Tracks/cm 2)
T (m.y.)
Mean age (m.y)
50.87+2.33
46.39+2.96
42.27.2.32
RE~JLTS AND DISCUSSION The fossil track density in case of apatite increases with etching time becomes maximum and then decreases. The optimum etching time corresponding to maximum track density for fossil tracks is found to be 44 sec for (OIT1) plane of apatite. The induced track density on various c~ystsl faces shows similar behaviour of its variation with etching time. It has been found that the o p t i m ~ etching time in case of samples containing induced fission tracks is less than that for fossil tracks. However it has been observed that the optimum etching time for induced tracks on different faces of apatite crystal is the same, though a slight variation of induced tracL~ density is found. Thus we conclude that spontaneous tracks etch more slowly than the induced tracks. Our results corroborate the findings of Reimer (1974) and Poupeau et al. (1980) for apatite. In both the fossil and induced s~mples the decrease in track density with prolonged etching may be due to overlapping of tracks or due to progressive removal of surface zone containing externally implanted tracks (Gleadow, 1978). From Table (1), it is evident that VG, V T anti,vary with the crystallographic orientations. This shows that the track etching behaviour_cf apatite is anisotropic. The value of V T varies from 0 .i~0 ~mTsec for oI~I plane to a maximum of 0.230 ~um/sec for a plane perpendicular to Z-axis of the crystal. ¥G varies from 0.015 jam/sec for 01~O plane to 0.040 jum/sec, for i0~0 plane. ~ varies from 82.23 to 90.63~. The variation of V G and V T with the crystallographic
930
SURINDER SINGH et aZ.
orientation may be @ae to the variable atomic spacing along different orientations in the crystal. The detailed x-ray work is needed to establish this fact. The f.t. age (Table II) determined on different planes of apatits crystal is found to vary from 41.O0+2.36 to 64.@=~+3.31 m.y~ It has been observed that the samples from the sam~ face give almost the ~same age. The large variation in the observed age with crystallographic orientations is due to the anisotropy of track etching in the crystal. It is necessary to determine the f.t. age on different planes of the crystal in order to get an accurate age. CONCLUSIONS i. The track etching behaviour of apatite is anisotropic. 2. Fission trac~ age should be determined on different planes of the crystal to get an accurate age. ACENOWL EDG~IENTS The authors achnowledge the financial assistance of CSIR. They are than~Z~l to Mr. Jagmohan Singh for help in preparation of the samples. The irradiation facilities provided by BARC, Trombay are thankfully acknowledged. REFERENCES Durraui, S.A., H.A. Khan., R.E. Bull., G.W. Dorling and J.H. Fremlin (1974) Proc. 5th Lunar Sol. Conf., Geochim Cosmochlm. Acta. Suppl. ~, 2543256O • Fleischer, R.L., P.B. Price., and R.M. Walker (1975) Nuclear Tracks in Solids, Principles and Applications, University of California Press, Berkeley, U.S.A. Gleadow, ~ . (1978) Nucl. Track.Dot. 2_, 105-117. Poupeau, G.J.Csrpena., D. Maihle and V.K. Ceylan (1980) C.R. Acad. Sci. Paris. 290 (O), IIB9-i192. Price, P.B. and R.M. Walker (1962) J. Appl. Phys. 33, 3407-3412. Reimer, G.M. (1974) Trans. Amer. Nucl. Soc. 18, 87-9Q Singh, N.P., M. Singh., S. Singh and H.S. Virk (1984)"Nucl. Tracks and Rad. Measurements. 8., Nos. 1-4, 41-44. T
12~
w
T w
~2o i I
Fig.3
,
20 ~TCHING
a
,
,
,
T')0 IME
~4Q ec)
50
Optimum etching time curve for fossil trac~s,
Fig.4
Optimum etching time curve for induced fission tracks.
0191-278X/86 $3.00+.00 Pergamon Journals Ltd.
A STUDY OF TRACK ETCH ANISOTROPY IN APATITE
Surinder Singh, Dilbag Singh, A.S. Sandhu, Gurmukh SinEh and H.S. Virk Department of Physics, Guru Nanak Dev University, Amritsar, India
ABSTRACT The track etch investigations on samples cut from different planes of apatite crystal are carried out in order to study the anisotropic etching behaviour of the crystal. The variation of the track etch characteristics with crystallographic orientations is observed. Because of track etch anisotropy in the crystal the fission track age determined on different planes of the crystal is found to vary. The etching conditions are standardised for different planes using HNO 3 as the track etchant. KEY WORDS Track etching, anisotropic, crystallographic orientations, fission track age. INTRODUCTION The most important advancement in the history of solid state nuclear track detectors (S~TD) is the discovery of chemical etching for track revealation (Price and Walker, 1962). Since then the SSNTD technique has gained popularity in diverse fields (Fleischar et al., 1975). In geochronology the technique is generally used for absolute age determination of minerals and rocLcs. Though a lot of work has been reported on fission track (f.t.) dating of minerals and rocks, a very little attention has been paid to the fundamental track etching characteristics of minerals and how these may affect the dating technique in routine use. In an anisotropic mineral, the atomic spacing is variable along different crystallographic orientations, hence the bulk etch rate, VG, the track etch rate, VT, and hence the etching efficiency,~ , might be different for tracks in different orientations. This anisotropic etching behaviour can affect the f.t. age of the minersl. In track etch work there are very few studies of this type (Durrani et el., 1974, Gleadow, 1978) because in most of the cases simple isotropic detectors like glasses and plastics are generally used. In f.t. geochronology one has to use natural minerals most of which are anisotropic and can have complicated track etch characteristics. In the present work an attempt has been made to study the track etch ~n~sotropy in apatite and to observe its effect on fission track dating of this mineral. Apatite crystal procured from Guebic Lavel Museum and of Wakefield area in Canada is used for such studies. EXPERIMENTAL PROCEDURE (a) S am~le preparation: In order to cut the different planes of the a p a t i t e crys%-al, it was marked with X,Y,U and Z axis. X,Y and U are inclined at 120 u with each other and Z axis is perpendicular to all these axis. The planes (lOll), (Olrl), (1TOO), (lOgO), (OleO) and (O001) cut from the crystal were 927
928
SURINDER SINGH et aZ.
taken for study. (b) Measurement of VG, V T and ~ ,
on different planes:
Samples from different
faces of the crystal were heated at a temperature of 530°C for one and a half hour to remove fossil traci~. The polished samples were irradiated with fission fragment source ( C ~ 52) in 2~ geometry. These were then etched in 67. HNO 3 at room temperature for various time intervals. The results for the variation of track length and track diameter with etching time for different planes are shown in Figs. 1 and 2 respectively. The bulk etch rate, VG, was calculated
I T
S
,I • --(ioll~ • - (lOft) • --(I l.oJ
• - @l~O
• --(loll)
•
• i(o I
. __
0
Fig. 1
zO
40
EO
ETCHING
TIME
IO
I00
• - (1100)
ft.) ~oot)
40
~.o
ETCHrNO
(see)
Variation of track length with etching time.
Fig .2
e,O TrM¢
¢m~
mO
• -
OOTo)
--
(otto)
m -
(0000
mO
(l@c)
variation of track diameter with etching time.
from the slope of the curve etching time versus trac~ diameter. V T was determined from the slope of the linear part of the curve showing the variation of projected track length with etchi.ng time (Fleischer et al., 1975( Singh et.al, 1984). The etching efficiency ( ~ ) was calculated from the relation: VG --1 - V T The results for V G, V T and ~] for different planes are given in Table 1. TABLE 1 Face lOl--O 011--1 l~O0 O001 OIT0 lO 1--1
The values of V G, V T and ~ for different planes of apatite VT (~m/sec)
VG (~m/s ec)
0.225 0.120 0.165 0.230 0.135 O. 200
0.040 0.020 0.0 20 0,0 27 0.015 O. 019
~
(~')
82.230 83.330 87.879 88.0 44 88.880 90 •530
(c) ~tandardisation of e t c h i ~ t i o n s : In order to determine the optimum etching time for fossil tra-~Lcs, a sample containing fossil tracLcs from (Ol~l) plane was etched in 6~. HN0 3 at room temperature for different time intervals and the tracL~ density was recorded corresponding to each time interval. For determining the optimum etching time for induced fission tracks, the annealed apatite samples from different planes irradiated with thermal neutrons (From CIRUS Reactor at BARC, Trombay with a dose of 5xl014 nvt) were etched in 6~. HNO 3 at room temperature for different time intervals.
TRACK ETCH ANISOTROPY IN APATITE
929
The induced track density was recorded at each etching interval for different planes. The results are shown in Figs. 3 and 4. (d) Fission _~ack a~e determination on different planes of apatite cr~stal. Fission track age on different planes wm~ estimated using the age equation (Singh et al., 1984) : T = 6.57x109 ln(l+9.25x10 "18 ~ x ~) where ~s and ~i are the fossil and induced track densities and ~ is the total thermal neutron dose. The results are given in Table II. TABLE II
Fission track age on different faces of apatite crystal = 4.54 x l014 (nvt).
Face
Sample No.
lOll
AC-I
7.38
3.95
50.63+2.53
AC-II
7.53
3.98
51.12+2.53
AC-I AC-II AC-I AC-II
8.38 8.43 4.83 4.83
3 •48 3.32 3.13 3.26
64.29+_3.32
64. 63+_3.31
64.97+--3.29 41.84+2.42 40.16+ 2.30
41.00÷-2.36
I0~0
AC-I AC-II
4.28 4.32
2.50 2.50
46.45+-2.95 46.73+_2.96
oIYO
AC-I
3.87
2.38
41.91+2.78
AC-II
3.73
2.38
42.63+_2.82
Ol~l ii--00
~s x lO -5 (Tracks/cm 2)
~i x 10 -5 (Tracks/cm 2)
T (m.y.)
Mean age (m.y)
50.87+2.33
46.39+2.96
42.27.2.32
RE~JLTS AND DISCUSSION The fossil track density in case of apatite increases with etching time becomes maximum and then decreases. The optimum etching time corresponding to maximum track density for fossil tracks is found to be 44 sec for (OIT1) plane of apatite. The induced track density on various c~ystsl faces shows similar behaviour of its variation with etching time. It has been found that the o p t i m ~ etching time in case of samples containing induced fission tracks is less than that for fossil tracks. However it has been observed that the optimum etching time for induced tracks on different faces of apatite crystal is the same, though a slight variation of induced tracL~ density is found. Thus we conclude that spontaneous tracks etch more slowly than the induced tracks. Our results corroborate the findings of Reimer (1974) and Poupeau et al. (1980) for apatite. In both the fossil and induced s~mples the decrease in track density with prolonged etching may be due to overlapping of tracks or due to progressive removal of surface zone containing externally implanted tracks (Gleadow, 1978). From Table (1), it is evident that VG, V T anti,vary with the crystallographic orientations. This shows that the track etching behaviour_cf apatite is anisotropic. The value of V T varies from 0 .i~0 ~mTsec for oI~I plane to a maximum of 0.230 ~um/sec for a plane perpendicular to Z-axis of the crystal. ¥G varies from 0.015 jam/sec for 01~O plane to 0.040 jum/sec, for i0~0 plane. ~ varies from 82.23 to 90.63~. The variation of V G and V T with the crystallographic
930
SURINDER SINGH et aZ.
orientation may be @ae to the variable atomic spacing along different orientations in the crystal. The detailed x-ray work is needed to establish this fact. The f.t. age (Table II) determined on different planes of apatits crystal is found to vary from 41.O0+2.36 to 64.@=~+3.31 m.y~ It has been observed that the samples from the sam~ face give almost the ~same age. The large variation in the observed age with crystallographic orientations is due to the anisotropy of track etching in the crystal. It is necessary to determine the f.t. age on different planes of the crystal in order to get an accurate age. CONCLUSIONS i. The track etching behaviour of apatite is anisotropic. 2. Fission trac~ age should be determined on different planes of the crystal to get an accurate age. ACENOWL EDG~IENTS The authors achnowledge the financial assistance of CSIR. They are than~Z~l to Mr. Jagmohan Singh for help in preparation of the samples. The irradiation facilities provided by BARC, Trombay are thankfully acknowledged. REFERENCES Durraui, S.A., H.A. Khan., R.E. Bull., G.W. Dorling and J.H. Fremlin (1974) Proc. 5th Lunar Sol. Conf., Geochim Cosmochlm. Acta. Suppl. ~, 2543256O • Fleischer, R.L., P.B. Price., and R.M. Walker (1975) Nuclear Tracks in Solids, Principles and Applications, University of California Press, Berkeley, U.S.A. Gleadow, ~ . (1978) Nucl. Track.Dot. 2_, 105-117. Poupeau, G.J.Csrpena., D. Maihle and V.K. Ceylan (1980) C.R. Acad. Sci. Paris. 290 (O), IIB9-i192. Price, P.B. and R.M. Walker (1962) J. Appl. Phys. 33, 3407-3412. Reimer, G.M. (1974) Trans. Amer. Nucl. Soc. 18, 87-9Q Singh, N.P., M. Singh., S. Singh and H.S. Virk (1984)"Nucl. Tracks and Rad. Measurements. 8., Nos. 1-4, 41-44. T
12~
w
T w
~2o i I
Fig.3
,
20 ~TCHING
a
,
,
,
T')0 IME
~4Q ec)
50
Optimum etching time curve for fossil trac~s,
Fig.4
Optimum etching time curve for induced fission tracks.