[en] The AN/UDM-1A calibrator consists of a 4.44 TBq ¹³⁷Cs radioisotopic source in a cylindrical lead shield. The most typical application of the unit is radiac meter calibration where predictable dose rate distributions are desired. However, vertically skewed dose rate distributions vertically from the exit window centre line have been measured, and an investigation was undertaken to determine the cause. It is demonstrated that MCNP models of a well-defined problem can be developed to predict the effect of inaccuracies in the as-built configuration when compared to blueprints. The causes of the skewness are presented as well as the mitigation methodology adopted. (author)