is a measure of the deviation of the theoretical spectrum and the experimental spectrum, giving a best fit when it is at a minimum. The analysis programs vary the parameters of the theoretical spectrum to find a minimum in according to the following formula:

(4.1) |

where is the number of channels (576 in this case) and is the number of parameters that are free to vary. and are the number of counts in the th channel of the experimental and theoretical spectra respectively.

Dr John Bland, 15/03/2003