[en] A solution containing 0.20 mM of ferrous ammonium sulphate, 5.0 mM benzoic acid and 0.20 mM xylenol orange in 0.05 N sulphuric acid was used for the calibration of ²⁵²Cf, ¹³⁷Cs, ²²⁶Ra and ⁶⁰Co needle sources. A known volume of this solution contained in a pyrex glass tube was irradiated with the source kept in a glass capillary at the centre of the solution. A few hours of irradiation were required for each needle and the absorbance of the solution was measured at 540 nm against the unirradiated solution. The accuracy of this method was found to be within a few percent and is particularly useful for ²⁵²Cf sources as the neutron component of the source contributes significantly to the absorbance produced. (author)