16  Fission Track dating

 

‘Fission tracks’ are not strictly speaking radiogenic nuclides, but they are the damage tracks left by fission products, which represent a special kind of radiogenic nuclide. As such, the abundance of fission tracks in geological materials increases over time in the same way as a radiogenic isotope. Fission track dating was first developed simply as a dating tool for general application, but their susceptibility to thermal resetting, originally a disadvantage, as been put to very good use as a measure of cooling, uplift or burial processes.

 

 

16.1     Track formation

 

The spontaneous fission of 238U releases about 200 MeV of energy, much of which is transferred to the two product nuclides as kinetic energy. They travel about 7 :m in opposite directions, leaving a single trail of damage through the medium which is about 15 :m long. Fission fragment tracks were originally observed in cloud chambers and photographic emulsions. Subsequently, Silk and Barnes (1959) produced artificial tracks in muscovite by irradiating uranium-coated flakes in a reactor. The resulting fragment tracks were observed at high magnification under the electron microscope.

 

            ‘Fission tracks’ (Fleischer et al., 1964) are only found in insulating materials. Fleischer et al. (1965a) proposed that the passage of the charged fission fragment causes ionisation of atoms along its path by stripping away electrons (Fig. 16.1a). The positively charged ions then repel each other, creating a cylindrical zone of disordered structure (Fig. 16.1b). This, in turn, causes relaxation stress in the surrounding matrix. It is the resulting 100 D (10 nm)-wide zone of strain (Fig. 16.1c) which is actually seen under the electron microscope. Conductors do not display fission tracks because the free movement of electrons in their lattice structure neutralises the charged damage zone.

Fig. 16.1. Schematic illustration of the process of formation of a fission track in a crystalline insulating solid. After Fleischer et al. (1975).

 

            The ability to generate tracks depends on the mass of the ionising particle and the density of the medium. In muscovite, the lowest mass particle which can generate tracks by irradiation is about 30 atomic mass units (a.m.u.). Fission fragments, with masses of ca. 90 and 135 a.m.u. respectively, are well above this threshold, so that they always generate tracks. On the other hand, " particles, the major product of uranium decay, are so far below the critical mass that they cannot create tracks. Neither can they cause track erasure (Fleischer et al., 1965b).

 

            Price and Walker (1962a) showed that when irradiated material was abraded to expose fission tracks at the surface, the damage zone could be preferentially dissolved by mineral acids, leading initially to a very fine channel only 25 D wide. However, this could be enlarged by further chemical etching to yield a wide pit which was observable under the optical microscope. Price and Walker (1962b) first discovered ‘fossil’ fission tracks in minerals, created by the spontaneous fission of dispersed uranium atoms. They went on to suggest (Price and Walker, 1963) that their density could be used as a dating tool for geological materials up to a billion years old. This was verified by Fleischer et al. (1965a) who obtained dates on artificial and natural glasses and minerals which were in agreement with other methods (Fig. 16.2).

Fig. 16.2. A comparison of specimen ages determined by fission track analysis with those from historical or other radiometric sources. After Fleischer et al. (1965a).

 

            Price and Walker (1963) demonstrated that spontaneous fission of 238U was the only significant source of tracks in most natural materials. Induced fission of 235U by natural thermal neutrons can be ignored, as can cosmic-ray-induced fission of uranium. Spallation recoils induced by cosmic rays could, in principle, generate tracks in geological material exposed at the surface for very long time-periods. This is the principal source of tracks in meteorites (e.g. Lal et al., 1969), but atmospheric shielding reduces their abundance to negligible levels in terrestrial rocks (Fleischer et al., 1975). Therefore the total production of spontaneous fission tracks (Fs) per unit volume of rock can be derived from the general decay equation [1.9]:

 

                        8fission

            Fs  =    ))))  238U (e8" t ! 1)                                                 [16.1]

                        8"

 

The 238U fission decay constant is ca. 7 H 10!17 yr!1 (t1/2 = 9.9 H 1015 yr; Naeser et al., 1989). There is some disagreement as to its exact value, but it will be seen below that this uncertainty need not enter into geological age determinations. Fissiogenic decay is over a million times lower than the " decay constant of 238U, so it can be ignored in determining the isotopic abundance of uranium through time.

 

            After polishing and etching a surface of the material to be dated, a fraction q of the total tracks will be visible at the surface. Therefore the measured spontaneous fission track density, Ds, will be q H Fs:

 

                                    8fission

            Ds  =     q          ))))  238U (e8" t ! 1)                                     [16.2]

                                    8"

 

Price and Walker recognised that the most effective way of measuring the uranium concentration was to irradiate the sample with neutrons in a reactor, and thereby produce artificial tracks by the induced fission of 235U. Based on equation [16.2], the induced track density will be:

 

                Di  =   q  235U N F                                                                [16.3]

 

where N is the thermal neutron flux per unit volume and F is the cross section of 235U for induced fission by thermal neutrons. If the sample material, including uranium concentration and etching procedure, is identical for these two experiments then the ratio of track densities can be used to solve for t, and then q goes out of the equation and the uranium concentrations are replaced by the 238U/235U isotope ratio only:

    Ds           8fission         137.88

    ))  =     )))))  @  )))))  (e8" t !1)                                           [16.4]

    Di             8"            N  F

 

This can be rearranged to yield an equation in terms of t:

 

            1          |         Ds    8"        N F              |

   t   =  ))  ln   | 1  +  )) @ )))) @ ))))       |                                   [16.5]

            8"        |         Di    8fission   137.88         |

 

It is possible to determine N and F directly by using flux monitors such as iron wire or copper foil. However, these types of flux monitors may not react to reactor conditions in exactly the same way as geological material. Therefore, an alternative procedure is to do a fission track analysis of a standard material with known uranium concentration. Fleischer et al. (1965a) used fragments of glass microscope-slides to calibrate the Brookhaven graphite reactor in this way. However, this does not avoid the uncertainty of the 238U fission decay constant.

 

            To eliminate both the flux term and the decay constant term, many workers started to use minerals dated by K)Ar as internal standards for the irradiation. Fleischer and Hart (1972) formalised this system into the ‘zeta calibration’. A sample of known age is used to calculate . by rearranging equation [16.4] and dividing both sides by the track density Dd in a given glass dosimeter:

 

                 N F                         e8" t  ! 1

 .  =     )))))))))))     =    ))))))))                                     [16.6]

            137.88 8fission Dd           8" (Ds/Di) Dd

 

To date an unknown sample, the age equation [16.5] is now modified by substitution of .:

 

               1      |         . 8" Ds Dd           |

   t   =  ))  ln   | 1  +  ))))))           |                                               [16.7]

               8"     |             Di                   |

 

The failure to resolve the decay constant problem can perhaps be attributed to this method, which transfers the uncertainty into the age determination of the geological reference material. Use of such material was recommended for all fission track dating studies by a working group of the IUGS Subcommission on Geochronology (Hurford, 1990). One of the most well-known of these standards is the 28 Myr-old Fish Canyon Tuff, Colorado (Naeser et al., 1981; Hurford and Green, 1983).

 

 

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