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Calculation of spectra and operational radiation protection quantities for the D2O-moderated Cf-252 source at PTB

23.12.2021

For the calibration of neutron measuring instruments, PTB uses a neutron reference field produced by a Cf‑252 source which is moderated by means of heavy water. Having exact knowledge of the neutron spectrum is of decisive importance for determining the values of the operational radiation protection quantities in this reference field. A detailed simulation model of the Cf‑252 source moderator assembly has been prepared and investigated using new spectral data and two different Monte Carlo transport codes, MCNP6.1 and PHITS3.22, with the evaluated nuclear data libraries ENDF/B‑VIII.0, ENDF/B‑VII.1, ENDF/B‑V, ENDL85 and JENDL 4.0. This new model improves and replaces earlier calculations for the D2O-moderated Cf‑252 neutron source, which were carried out more than 20 years ago.

The PTB D2O‑Cf‑252 assembly consists of a Cf‑252 source placed in a cylindrical, stainless steel capsule with a height of 11.3 mm and an external diameter of 10.8 mm. The source capsule is guided to the center of the assembly by a stainless steel tube with a wall thickness of 1 mm. The heavy water is enclosed in a spherical shell made of 1 mm thick stainless steel surrounded by a half‑shell made of cadmium (Cd). The geometrical model is shown in Figure 1.

 source assembly model

Figure 1: Model of the PTB D2O‑Cf‑252 source assembly.

The calculations were carried out using the High‑Performance Computing Cluster at PTB Berlin. The use of the computing cluster reduced the computing time and allowed the uncertainty to be significantly reduced.

The spectral emission rates for the two codes and different cross‑section data are shown in Figure 2. In the so‑called lethargy format selected here, the product of the spectral emission rate and energy is plotted against the energy in a semilogarithmic scale. In this representation, equal areas below the curves for given energy intervals correspond to the same neutron intensity.

Figure 2: Comparison of the emission rates at a distance of 100 cm in the lethargy format for the D2O‑Cf‑252 source assembly in vacuum using codes MCNP6.1 and PHITS3.22 with different cross‑section data.

The results show that the evaluated nuclear data libraries influence the calculated operational radiation protection quantities in the 3‑5 % range. The dosimetric quantities calculated by the PHITS code and data library ENDF/B‑VII.1 agree well with the MCNP6 results.

Contact:

Opens local program for sending emailDr. A. Al-Qaaod, Department 6.4, Working Group 6.43