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{\plain \fs24 \b\f1 Abstract\par
}{\plain \fs24 \f2 Luminescence decay with time often shows a power-law dependence of the\par
}{\plain \fs24 \f2 form intensity }{\plain \fs24 \i\f3 I }{\plain \fs24 \f1 {\u8733\'20} }{\plain \fs24 \i\f3 t}{\plain \fs24 \super\f1 {\u8722\'20}}{\plain \fs24 \i\super\f3 k}{\plain \fs24 \f2 , where }{\plain \fs24 \i\f3 t }{\plain \fs24 \f2 is time and }{\plain \fs24 \i\f3 k }{\plain \fs24 \f2 is usually in the range 1{\u8211\'96}1.5.\par
}{\plain \fs24 \f2 It is shown here that this power law can result from the tunnelling of trapped\par
}{\plain \fs24 \f2 electrons to recombination centres that are randomly distributed, and that the\par
}{\plain \fs24 \f2 range of exponents matches that of the observations. The explanation accounts\par
}{\plain \fs24 \f2 for the most extreme case of an observed }{\plain \fs24 \i\f3 t}{\plain \fs24 \super\f1 {\u8722\'20}}{\plain \fs24 \super\f2 1}{\plain \fs24 \i\super\f4 .}{\plain \fs24 \super\f2 06}{\plain \fs24 \f2  }{\plain \fs24 \f2 dependence extending over nine\par
}{\plain \fs24 \f2 decades of time.}{\plain \fs20 \f1\fs20 \par
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