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Capability to compute neutron dose rate is introduced for the first time in the new version of the CASCADE.04 code. Two different methods, 'track length estimator' and 'collision estimator' are adapted for the estimation of neutron fluence rate needed to calculate the ambient dose rate. For the validation of the methods, neutron dose rates are experimentally measured at different locations of a 5Ci Am-Be source, shielded in Howitzer-type system and these results are compared with those estimated using (i) modified CASCADE.04.d and (ii) MCNP4A codes and it is found that the agreement is good. The paper presents details of modification and results of the comparative study.

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A guarded cylindrical graphite ionization chamber of nominal volume 1000 cm3 was designed and fabricated for use as a reference standard for low‐dose rate  137Cs brachytherapy sources. The air kerma calibration coefficient (normalNK) of this ionization chamber was estimated analytically using Burlin's general cavity theory, as well as by the Monte Carlo simulation and validated experimentally using Amersham CDCS‐J‐type  137Cs reference source. In the analytical method, the normalNnormalK was calculated for 662 keV gamma rays of  137Cs brachytherapy source. In the Monte Carlo method, the geometry of the measurement setup and physics‐related input data of the  137Cs source and the surrounding material were simulated using the Monte Carlo N‐Particle code. The photon energy fluence was used to arrive at the reference air kerma rate (RAKR) using mass energy absorption coefficient. The energy deposition rates were used to simulate the value of charge rate in the ionization chamber, and the normalNnormalK was determined. The analytical and Monte Carlo values of normalNnormalK of the cylindrical graphite ionization chamber for  137Cs brachytherapy source are in agreement within 1.07%. The deviation of analytical and Monte Carlo values from experimental values of normalNnormalK is 0.36% and 0.72%, respectively. This agreement validates the analytical value, and establishes this chamber as a reference standard for RAKR or AKS measurement of  137Cs brachytherapy sources.PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.bg, 87.55.Qr