The NPL clinical linear accelerator (above) and secondary
standard ionisation chamber in water (inset) modelled
using EGSnrc: X-rays (yellow) and electrons (green).

The practical realisation of dosimetry standards at NPL is supported by theoretical and computational modelling. We use a variety of techniques to simulate the irradiation facilities and radiation detectors used at NPL, as part of a programme to establish improved dosimetry standards.

We have also carried out successful consultancies and collaborations with industry, academic and other government departments in computational modelling. We would welcome any enquiries regarding consultation or collaborations concerning problems where mathematical modelling may be able to provide a solution, such as in detector design, radiation shielding and irradiator design and validation.

Modelling based on the Monte Carlo method is one of the most important techniques that NPL uses in radiation dosimetry. In radiation transport applications, this is a realistic simulation of the physical scattering and absorption processes undergone by ionising radiation - photons, electrons, neutrons, light ions - as it passes through a set of predefined materials and geometries.

We use a wide variety of radiation transport codes, including EGS (and egspp), GEANT4, FLUKA, MCNPX and PTRAN, to model a whole range of situations across all dosimetry technical areas including:

A commercial gamma-ray irradiation facility simulated
using egspp for the comparison of dose-rate calculations
against the NPL real-time dosimeter.
Inset: Close-up of source rack showing active (green)
and blank source pencils.
• Accelerator beam lines and radiation sources
  Extensive simulations of the clinical and research linear accelerators, X-ray and gamma-ray facilities at NPL have been carried out. In addition, modelling of the ocular proton beam line at the Clatterbridge Centre for Oncology (CCO) and commercial electron beam and more complex gamma-ray irradiation facilities have been performed. A variety of LDR and HDR brachytherapy sources have also been modelled and characterised.
  
  
• Radiation detectors
  Many types of radiation detector used at NPL for the measurement and characterisation of ionising radiation, such as ionisation and well chambers, graphite calorimeters, germanium detectors have been modelled as well as investigations for the design of new detectors and standards.

Driven by a continuing demand for increased accuracy in the standards that we provide, NPL actively contributes to the development of improved transport algorithms and interaction data used in these codes. Many of the simulations make heavy use of the NPL distributed computing grid that utilises the spare computing capacity of hundreds of desktop computers across the site.

Analytical and numerical modelling also plays an essential role in radiation dosimetry. Finite Element analysis techniques are used extensively for modelling heat flow in a range of water and graphite calorimeters that are used for the realisation of absorbed dose.

A well ionisation chamber with an inserted 192I HDR brachytherapy source modelled using egspp. Inset: close up of the source.		The clinical 60MeV ocular proton beam line at the CCO: protons (blue), electrons (red), neutral particles (green) modelled using GEANT4. Inset: modulator wheel used to control the dose distribution in the patient.

NPL is also committed to the dissemination of knowledge and technical information in this research field. We have hosted many successful UK Monte Carlo User Group (MCNEG) meetings and international workshops on Monte Carlo codes and training courses on the EGS system.

For further information on modelling activities in Radiation Dosimetry, please contact Mark Bailey or David Shipley.

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