The fast neutron flux spectrum has been measured by the activation of the threshold detection foils at the inner and outer irradiation sites of the Miniature Neutron Source Reactor in China Institute of Atomic Energy Five threshold detection foils

i e Al

Fe

In

Ni and Mn were adopted in the experiment The ratios of fast-to-thermal neutron flux for inner and outer irradiation sites were calculated by the method of average cross section

to be 0 198±0 005 and 0 071±0 003

respectively The method of effective threshold energy was used to calculate the neutron flux in different energy ranges The axial and radial inhomogeneities of fast neutron flux for four irradiation sites and the inhomogeneity for different inner irradiation sites were also measured