A new neutron monitor and extended conversion coefficients for HP(10).

A new personal dose equivalent monitor for neutrons, the 'HpSLAB', is introduced. The device consists of a 30x30x15 cm3 polymethyl-methacrylate slab hosting a superheated drop detector embedded at a depth of 10 mm. The personal dose equivalent monitor was characterised experimentally with fast neutron calibrations in the 0.144-14.8 MeV range and numerically with Monte Carlo simulations. In order to evaluate the performance of the device, its response was compared to the fluence-to-directional dose equivalent conversion coefficients, hp(10;alpha,E). Since published coefficients only cover neutron angles of incidence up to 75 degrees, a new extended set of coefficients was computed for angles of incidence up to 180 degrees. The method used in these calculations was the very same used in the generation of the dose equivalent coefficients recommended by International Commission on Radiological Protection publication 74. The response of the HpSLAB follows with good approximation the trend of the conversion coefficients for monoenergetic neutrons above approximately 0.5 MeV. The device was extensively tested in broad-spectrum workplace-fields encountered at nuclear installations and its response was on average within a factor 1.4 of the reference personal dose equivalent values, regardless of angle and energy distribution of the neutron fluence.

Uncertainty in radiation dosimetry: basic concepts and methods.

Principally, uncertainty is associated with measured and computed values. The basic concepts of uncertainty are outlined, an overview on methods for its determination is given and the application of uncertainties in quality assurance is discussed. Based on the Guide to the expression of uncertainty in measurement and a forthcoming supplement to it, the paper identifies modelling of measurement or computation and the use of a probability distribution functions (PDFs) for expressing the degree of belief in possible values as basic concepts. All presently used proper methods for determining uncertainty can be derived from the Markov formula. Optimisation of measurements and computations as well as the acceptance of results by others are identified as the two major tasks of quality assurance based on analysing and stating uncertainty.

Pitfalls and modelling inconsistencies in computational radiation dosimetry: lessons learnt from the QUADOS intercomparison. Part II: Photons, electrons and protons.

'QUADOS', a concerted action of the European Commission, has promoted an intercomparison aimed at evaluating the use of computational codes for dosimetry in radiation protection and medical physics. This intercomparison was open to all users of radiation transport codes. Eight problems were selected for their relevance to the radiation dosimetry community, five of which involved photon and proton transport. This paper focuses on a discussion of lessons learned from the participation in solving the photon and charged particle problems. The lessons learned from the participation in solving the neutron problems are presented in a companion paper (in this issue).

Pitfalls and modelling inconsistencies in computational radiation dosimetry: lessons learnt from the QUADOS intercomparison. Part I: Neutrons and uncertainties.

The QUADOS EU cost shared action conducted an intercomparison on the usage of numerical methods in radiation protection and dosimetry. The eight problems proposed were intended to test the usage of Monte Carlo and deterministic methods by assessing the accuracy with which the codes are applied and also the methods used to evaluate uncertainty in the answer gained through these methods. The overall objective was to spread good practice through the community and give users information on how to assess the uncertainties associated with their calculated results.

QUADOS intercomparison: a summary of photon and charged particle problems.

QUADOS, a Concerted Action of the European Commission, has promoted an intercomparison aimed at evaluating the use of computational codes for dosimetry in radiation protection and medical physics. This intercomparison was open to all users of radiation transport codes. Eight problems were selected for their relevance to the radiation dosimetry community, five of which involved photon and proton transport. This paper focuses on the analysis of the photon and charged particle problems. The neutron problems were presented in a paper at the NEUDOS9 conference.

Photon and neutron dose contributions and mean quality factors phantoms of different size irradiated by monoenergetic neutrons.

The International Commission on Radiological Protection (ICRP) in its Publication 60 introduced important changes in the concept of risk-related quantities. For external neutron radiation in particular the introduction of the equivalent dose with the radiation weighting factor wR instead of the dose equivalent concept with the quality factor Q(L) has many consequences. The value of wR is defined by the external neutron radiation field, while the radiation quality in the phantom depends on the radiation field at the position of interest and hence on the size of and the position in the phantom. It has been investigated to what extent the size of the phantom influences the mean radiation quality in the phantoms. For incident monoenergetic neutrons, mean photon dose contributions and mean quality factors have been calculated. Results are presented for various phantoms which characterize the conditions for a mouse, a rat, the ICRU sphere and a human body.

A theoretical concept for measuring doses from external radiation sources in radiation protection.

A physically consistent concept is presented for monitoring individual exposures from external radiation, giving quantitative relations between 'fitted' dosemeters satisfying this relation, and 'non-fitted' dosemeters which do not. The calibration of both types is discussed in detail. For non-fitted dosemeters, procedures to determine the error limits are given. The application to accident dosimetry is briefly discussed.

Depth dose-equivalent and effective energies of photoneutrons generated by 6-18 MV X-ray beams for radiotherapy.

Photoneutron production was investigated on Siemens KD 2 and Varian Clinac accelerators operating in the 6-18 MV range. Neutron dose equivalent rates were measured on the surface of a water phantom at the isocenter of the accelerators and also inside the phantom at depths of 1, 5, and 10 cm and off-axis distances of 0, 20, and 50 cm. Superheated drop detectors based on dichlorofluoromethane and etched-track detectors with boronated converters were employed in this study. The energy response of these detectors permits a direct measurement of dose equivalent without prior knowledge of the neutron energy spectra. Dose equivalent rates were assessed using the Q(L) relationship from ICRP publication 60, as well as using earlier data from ICRP publication 21. This permitted both a comparison with previously published data and an assessment of the impact of the recent ICRP recommendations--which were found to increase the dose equivalent levels by about 30%. In addition, the depth corresponding to 50% of maximum dose equivalent, dH50, was determined along the central axis of the beams and at 50 cm off-axis. Monte Carlo neutron transport calculations were performed to determine the depth-dose equivalent distributions in a phantom irradiated with monoenergetic neutrons. Effective energies of the photoneutron spectra were then estimated by comparing our measured dH50 values to those calculated for monoenergetic neutrons. It was found that the effective photoneutron energy is 1.8-2.1 MeV within the 10-18 MV x-ray beams, and it is 0.5-0.8 MeV for photoneutrons transmitted through the accelerator head. Data from this work cover most of the x-ray beam energies in clinical use and permit an assessment of integral dose values as well as specific organ doses to a radiotherapy patient.

A directional dose equivalent monitor for neutrons.

A directional dose equivalent monitor is introduced which consists of a 30 cm diameter spherical phantom hosting a superheated drop detector embedded at a depth of 10 mm. The device relies on the similarity between the fluence response of neutron superheated drop detectors based on halocarbon-12 and the quality-factor-weighted kerma factor. This implies that these detectors can be used for in-phantom dosimetry and provide a direct reading of dose equivalent at depth. The directional dose equivalent monitor was characterised experimentally with fast neutron calibrations and numerically with Monte Carlo simulations. The fluence response was determined at angles of 0, 45, 90, 135 and 180 degrees for thermal to 20 MeV neutrons. The response of the device is closely proportional to the fluence-to-directional dose equivalent conversion coefficient, h'phi (10; alpha, E). Therefore, our monitor is suitable for a direct measurement of neutron directional dose equivalent, H'(10), regardless of angle and energy distribution of the neutron fluence.

Intercomparison on the usage of computational codes in radiation dosimetry.

'QUADOS', a Concerted Action of the European Commission, has run an intercomparison aimed at evaluating the use of computational codes for dosimetry in radiation protection and medical physics. This intercomparison was open to all users of Monte Carlo, analytic and semi-analytic codes or deterministic methods. Its main aim was to provide a snapshot of the methods and codes currently in use. It also intended to furnish information on the methods used to assess the reliability of computational results and disseminate 'good practice' throughout the radiation dosimetry community. Eight problems were selected for their relevance to the radiation dosimetry community, three of which involve neutron transport. This paper focuses on the analysis of the neutron problems.