Ramola, R C

Sandhu, A S

Singh, Surinder

Virk, H S

IACS Institutional Repository

Indian J .  Phys. 6 4 A  (5 ) , 35 2 -3 5 7  (1 9 9 0 )Fission track dating of zircon crystalA  S  Sandhu, R C  Ramola, Surinder S ingh and H S  Virk Department of P h y s ic s , G uru  N anak Dev U n ivers ity , A m ritSar-143 0 0 5 , IndiaA b s t r a c t : The track  etch ing  and fiss io n  track  (f.t) age stu d ies are carried  out on various natural p lanes of z ircon crysta l co llected  from Kham mam D istrict of Andhra Pradesh. The etching stu d ies are carried  out using  tw o d ifferent se ts of etchants v iz . (a) 1 ; 1 m ixture of H jS O «  ; H P and (b) 4 : 1 5 : 6  m ixture of NaOH : KOH : L iO H  .H .,0 . It is  observed that the track etch ing  anisotropy is  more in the b a sic  m edium  (b ) than that in a c id ic  m edium  (a ). T h is  leads to  the  different va lues of track  etching e ffic ie n cy  on vario us p lanes of z irco n . B ecau se  of th is  an iso tro p ic etching behaviour of the crysta l, the fo ss il track density  is  found to vary, from plane to plane. H ow ever, the concordant f.t. ages are obtained on vario us p lanes of z irco n , by app lying  the appropriate co rrectio n s for anisotrop ic etching.Keywords : F iss io n  track dating, track  etch ing , z irco n  c rysta l, fo ss il track  d ensity , an isotrop ic etch ing .P A C S  N o s  ; 9 1 .6 0 . - x, 9 1 .6 5 . + pI . IntroductionChemical etching makes the fission tracks visible because the rate of etching along the damaged region of the track (V^) is greater than the general etching rate of the surface (Vo). The relationship between V^ and Vg controls the etching efficiency, y (Fleischer et al 1975). In minerals, where Vj. is much higher than Vq, tracks have the form of cylindrical holes, and a high etching efficiency (-^ 100X) is implied. In other materials, such as glasses, etched tracks have the appearance of broad cones because is only a little greater Vo (Gleadow 1978). In these materials, tracks at shallow angles to the surface are not revealed by etching when the vertical component of Vj. is less than or equal to Vg. The classic description of Vj., Vg and y  has proved extremely useful in understanding track etching in isotropic materials, such as glasses and plastics, but the etching behaviour of natural minerals is more complicated (Gleadow 1981, Sandhu et of 1987). Most of the minerals occur in the form of crystals w ith variable atomic spacing along different crystallographic orienta­tions. Hence their etching and annealing characteristics for radiation damage vary with the crystal orientation (Sandhu et al 1987, 1988).352Fission track dating of zircon crystal 353In the present investigations, the track etching and fission track age studies are carried out on various natural planes of zircon crystal collected from Khammam district of Andhra Pradesh. Fission track dating has proved to be quite suitable for zircon due to its relatively high uranium content and low  annealing rate of radiation damage compared to other minerals. Three planes of zircon crystal viz. (100), (101) and (001), are taken for study.2. Experimental procedureThe polished samples from different planes of zircon were irradiated with ®®“Cf fission fragment source in 2n- geometry. Prior to irradiation the samples were heated at 700"’C  for 6  hrs to erase all fossil tracks. The etching of the samples is performed using two different etchants namely :(i) NaOH : KOH : LiOH.HaO  (Zaun and Wagner 1985)(ii) HF : H g SO * (Krishnaswami et al 1974)A  new track etchant for zircon is reported earlier by Zaun and Wagner (1985). This new etchant (NaOH : KOH : L i0 H ,H30=6  : 1 4 : 1 )  reduces the etching time for track revelation compared with the eutectic melt etchant, proposed by Gleadow et of (1976). After considerable experimentation, a slight modification w as done in this new etchant for getting good results. For this 4  gms of NaOH, 15 gm s of KOH and 6 gm s L iO H .H jO  were mixed with 2 ml HgO in a teflon lined steel capsule. The irradiated samples from each plane of zircon crystal were immersed in this improved etchant and the capsule was closed air tight and placed in the muffle furnace pre-heated to 200 '’C. The samples were step etched and removed periodically from the capsule for track length and diameter measurements. The mean track length (corrected for dip) and diameter were measured at each etching event using an optical microscope at a magnification of 1 2 5 0 x (+ o i l  immersion). In order to study the etching behaviour of fission tracks in zircon using the acidic mixture (Krishnaswami et al 1974), 5  ml of 48;^ HF w as mixed with 5 ml of 9 8 ^  H gSO * in the teflon capsule. Etching of the samples was carried out under pressure at 175 C. The track length and diameter were measured at regular intervals. All length measurements were made only of horizontal tracks, characterized by low dip angle. These tracks were sharply focussed along their entire length and have a very bright reflection in the incident light.It w as observed during the experimental work that the tracks were not revealed on the basal plane (001) of zircon crystal using NaOH : KOH  : LiOH.HaO mixture. This may be due to very high bulk etch rate normal to 001 plane, which w as characterized by the quick removal of polished layer containing externally implanted354 A S Sandhu R C Ramola Surinder Singh and H  S Virkfission tracks. However, using acidic etchant, it was possible to observe the etched tracks on basal plane (001).Fission track age on different planes of zircon crystal w as determined by using the external detector method (Hurford and Green 1982). The measure­ments were repeated with the two different sets of etchants namely : with a 1 : 1 mixture of HF and HgSO^ and with the basic mixture viz. NaOH : KOH  ; LiOH.HgO. After counting the fossil tracks, the samples from different planes of zircon were irradiated with thermal neutrons (^ --4 .5 x  10^* n.cm"*) together with external muscovite detectors (which were subsequently etched for 50  min in 48‘V> HF to reveal the induced tracks).3. Results and discussionThe track etch rate, Vj., is calculated from the slope of the linear part of the curve, etching time versus track length. The bulk etch rate, V^, is calculated from the one half of the slope of the curve, etching time versus track diameter. The etching efficiency (■»?) and critical angle (0«) are calculated from the following relations (Fleischer et al 1975);i j = 1 - s in  0eandefl=sin-1^0( 1 )(2)The plane 100 show  low critical angle and higher etching efficiency (97.6%) compared to the other planes, using basic medium as the track etchant (Table 1).T a b le  I .  The va lues of etch ing  e ffic ie n cy  ( v )  and critica l angle (9«) for d ifferent p lanes o f z irco n .P laneN a O H : KOH ; L iO H .H .O H F ; H ,S 0 46c v {% ) 0c ’)(% )10 0 1.38" 9 7 .6 4 .5 “ 92.1101 22.9=' 61.1 5 .1 “ 91.1001 — — 6 .5 “ 9 0 .4Also, the etching efficiency Is found to be higher for 100 plane using HF : H ,S O *  as the track etchant. On comparing the results reported in Table 1, it appears that HF : H g S O i show s more isotropic etching behaviour. However, for the planes parallel to c-axis (100 plane), the NaOH ; KOH : L iOH  mixture seems to be the best etchant, as this etchant yields high etching efficiency (97.6%), as compared to that (92.1%) obtained by HF : H ^SO a etchant.Fission track doting of zircon c r y s t a l 355The fission track age of different planes of zircon crystal is determined using the follow ing relation (Price and Walker 1963):T = 6 .4 5 x 1 0 "  In ( 1 + 9 . 3 0 x 1 0 - ^ « x g x ^ x d > )Pi(3)where, P ,  and pt are the fossil and induced track density ; and g is the geometry factor.It is desirable in external detector method (ED M ) to measure the fossil track density on surfaces w ith identical etching efficiency as that of muscovite detector. It is observed (Table 1) that only the prismatic plane of zircon possess a high etchingT a b le  2. M easured va lues of geom etry factor for d ifferent p lanes of z irco n .C rysta l Pe.rt. H F : H •iSO^N a O H ; KO H  : L iO H .H .Oplane ( a I O " ) P in( x I O " ) PinPin( x I O " ) p in10 0 1 6 .3 8 2 8 .2 4 0 .5 8 3 2 .1 2 0.51101 1 6 .7 2 27 .87 0 .6 0 18.91 0 .8 8001 1 6 .4 2 2 5 .2 6 0 .6 5 — —efficiency (97.6%) closely resembling to that of the nnuscovite detector, whichapproaches 100% (Fleischer et a l  1975). Moreover, in order to get more reliable f.t.T a b le  3. F is s io n  track age for d ifferent p lanes of z irco n  using  NaOH ; KO H  :L iO H  as the track  etchant.Therm al neutron flu en ce  ( ^ ) = 4 . 5 x 1 0 ‘  ^ n cm~*Sam pleNo.10 0  p lane (g = 0 .5 1 ) 101 p lane (g==0.88)P i( x I O ' )pi  (ctJ.)( x I O ^ )T ±  l a(m .y .)Pt( x I O " )p1 (<'.(!•) ( x I O * )T ±  l a(m .y .)1 8.41 16.41 6 6 9 .5  ±  7 .8 4 .8 9 1 6 .5 0 6 6 8 .2  ±  8 .22 8 .7 2 1 6 .7 4 6 7 9 .9  ±  6 .3 4 .8 8 1 6 .6 0 663.1  ±  7 .83 8.61 1 6 .4 5 683.1 ±  8 .2 5 .1 3 16.91 6 8 3 .2  ±  5 .94 8 .6 2 1 6 .4 8 6 8 2 .6  ±  6 .8 4 .8 5 16 .1 5 6 7 6 .7  ±  8 .3M ean M ean6 7 8 .8  ±  3 .7 6 7 2 .8  ±  3 .8ages on different planes of zircon, geometry factor (g) for each plane is determined using both types of etchants. The value of g is  obtained (Table 2) by comparing the density of induced fission tracks on an internal surface (p,a, geometry) of each plane w ith that induced simultaneously on an adjacent muscovite detector (Pe*<i» 2rr geometry).356 A S Sondhu R C Ramolo Surinder Singh ond H S VirkO©oCOoCOXO)3COoI£JCO oa  cc i £ o  © ^3= X‘•5£  II§> s^  oy coS 0)o c  8 2 ii: giJ E *2 ® H iSto ^^ elo. oCL T-Xto£^  XSL w>«E•. <=>Q- «“X©aI  o cnzCD0041a>T—00CO''t<oo>CO(D4oCOcdc>iCDCDCMCD4iCDCOCDCOCDCOCOCDCM 00M?-fl 4C  CDcd’ S LOP^CO^  P*» ^  CDCOCDa>CDCM o> 00 P^CD 00 p^ 00«—•4 4 H 400 00CD cd CM »—CD p^ 00 00CD CD CD CD00 ID CM COcq IDCD cd cd cdT—CO 00 O iCO CO cqK p^r * CO LD00 oo' O) 004 4 4tp^ 00 m*00 <7> ID r*CO 00 P*«CO CO CD CDCM ID CDCD ID IDCD cd cd CDT—cto©400r'CDC©©HICOCDP^CDCDP^CM COFission t ra c k  d a tin g  o f  z irco n  c ry s ta l 3 5 7It is observed (Table 2) that only the plane parallel to c-axis (1(X)) of zircon shows the value of g very close to the ideal value (i.e. g=2 ir/^n=0.5). For other, planes the value of g deviates considerably from the ideal value using both types of the etchants. Since pi„ and Pe.a. are measured in different materials, the observed track density ratio (g=Pe-d*/ftn) is affected due to difference in their etching efficiency.It is observed that (Tables 3 and 4) the fossil track density varies significantly for different planes of zircon using both types of etchants. This is due to the anisotropic track etching along various crystallographic orientations in zircon. No remarkable variation of induced track density has been observed for various planes of zircon, since the same has been determined on the external muscovite detector. However, f.t. age obtained for each crystal plane is nearly the same (Tables 3, 4), irrespective of the etchant used. Thus the external detector method can successfully be applied for zircon, provided the appropriate geometry factor, has been evaluated. The f.t. age obtained in the present study is higher than that reported by Virk and Koul (1984) for zircon samples belonging to the same area. The lower value of f.t. age reported by Virk and Koul (1984) is due to the reason that the authors have measured the f.t. age by population method and have ignored the possible crystallo­graphic orientation effects.AcknowledgmentsFinancial assistance by Council of Scientific and Industrial Research (CSIR), New  Delhi and irradiation facilities provided by Bhabha Atomic Research Centre (BARC), Bombay are gratefully acknowledged.ReferencesFleischer R L, Price P B  and  W a lk e r  R  M  1 9 7 5  N uclear Tracks w  So lid s, Princip les and A pp lica­tions (Berkley, USA : University of California Press)Gleadow A J W, Hurford A J and Quaife R D 1976 Ea rth  P lanet. Sci. Lett. 33 273 Gleadow A J W 1978 Nucf. Tracks 2  105 ----1981 N u c l. T ra cks  5 3Hurford A J and Green P F 1982 Ea rth  P lanet. S c i . Le tt. 59 343Krishnaswami S, Lai D, Prabhu N and Macdougall D 1974 Ea rth  P lanet. S c i . L e tt. 22 51Price P B and Walker R M 1963 J. Ceophys. Res. 68 4847Sandhu A S, Singh Surinder and Virk H S 1987 Ind ian  J .  Pure Appl. Phys. 26 351“~1988 M in e r a l . ] .  141Virk H S and Koul S L 1984 J. Assoc. E x p l . Ceophys 519 Zaun P and Wagner G A 1985 N aturw issenschaften  72  143

Fission Track Dating of Zircon Crystal

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Fission Track Dating of Zircon Crystal

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