Monte Carlo modelling of an x-ray chamber for providing inactivation exposures to viruses.

The Monte Carlo radiation transport code MCNP6 has been used to model dosimetry for biological pathogen samples placed within a MultiRad 225 irradiation chamber, in order to inform virus deactivation protocols. Full characterisations of the photon spectra generated by the chamber's x-ray tube were achieved for both 190 and 220 kV potentials, with and without aluminium and copper beam filters of different thicknesses. Dose rate maps to air and water within the chamber were then derived, along with corresponding conversion coefficient data. The maps were determined for samples located both on a shelf and on a dry ice refrigeration chamber, at different distances from the source. The potential depth-dose profiles through samples were also investigated. The optimum choice of filter for use in virus inactivation procedures will rely on a compromise between dose homogeneity and dose rate.

The 2019-2020 EURADOS WG10 and RENEB Field Test of Retrospective Dosimetry Methods in a Small-Scale Incident Involving Ionizing Radiation.

With the use of ionizing radiation comes the risk of accidents and malevolent misuse. When unplanned exposures occur, there are several methods which can be used to retrospectively reconstruct individual radiation exposures; biological methods include analysis of aberrations and damage of chromosomes and DNA, while physical methods rely on luminescence (TL/OSL) or EPR signals. To ensure the quality and dependability of these methods, they should be evaluated under realistic exposure conditions. In 2019, EURADOS Working Group 10 and RENEB organized a field test with the purpose of evaluating retrospective dosimetry methods as carried out in potential real-life exposure scenarios. A 1.36 TBq 192Ir source was used to irradiate anthropomorphic phantoms in different geometries at doses of several Gy in an outdoor open-air geometry. Materials intended for accident dosimetry (including mobile phones and blood) were placed on the phantoms together with reference dosimeters (LiF, NaCl, glass). The objective was to estimate radiation exposures received by individuals as measured using blood and fortuitous materials, and to evaluate these methods by comparing the estimated doses to reference measurements and Monte Carlo simulations. Herein we describe the overall planning, goals, execution and preliminary outcomes of the 2019 field test. Such field tests are essential for the development of new and existing methods. The outputs from this field test include useful experience in terms of planning and execution of future exercises, with respect to time management, radiation protection, and reference dosimetry to be considered to obtain relevant data for analysis.

The effects of revised operational dose quantities on the response characteristics of a beta/gamma personal dosemeter.

The International Commission on Radiation Units and Measurements is considering revising the definitions of the operational dose quantities used for personal monitoring. This paper investigates the impacts of the proposed changes on the Public Health England two-element ß/γ personal thermoluminescence dosemeter (TLD), in terms of its energy and angle dependences of responses for both skin and whole-body dose assessments. In general, the photon response of the skin element would be unaffected by the proposal, though technical issues may arise during calibration. For body photon doses, the current TLD design still produces acceptable response characteristics in some circumstances, but in general it will need to be redesigned to better match the requirements of the new operational quantity; to that end, a simple adaption is demonstrated that might provide a partial solution. For electron/beta exposures, matching the combined responses of both the body and skin elements to the dose quantities may be more challenging. The performance criteria against which dosemeters are judged may also need to be revised to reflect the proposed change.

The effects of a revised operational dose quantity on the response characteristics of neutron survey instruments.

The ICRU is considering revising the definition of ambient dose equivalent. This paper investigates the impacts of the proposed change on four designs of neutron survey instrument, the GNU, HSREM, LB6411 and Studsvik 2202D, in terms of their respective energy dependences of response and their performances in realistic workplace fields. In some circumstances the current designs of instrument still produce acceptable characteristics, but in general they may need to be re-optimized to better match the requirements of the new operational quantity; to that end, a simple retrofit solution for the GNU is demonstrated. The performance criteria against which instruments are judged may also need to be revised to reflect the proposed change.

THE RESPONSE OF THE PHE NEUTRON PERSONAL DOSEMETER IN TERMS OF THE PROPOSED ICRU PERSONAL DOSE EQUIVALENT.

The ICRU is considering amending the definition of the operational quantity for personnel monitoring. The present work investigates the impact of the proposed change on the PHE neutron personal dosemeter, which utilizes electrochemically etched PADC and is currently optimized in terms of Hp(10). The energy-dependent dose response characteristics of the dosemeter, and its performance in realistic workplace neutron fields, are calculated and compared for both the current and proposed dose quantities, for both frontal and rotationally isotropic fields. Adoption of the proposed quantity would make the dosemeter more sensitive to normally incident neutrons, but it would require some modification to ensure that it is able to meet the recommendations of the current ISO standard at all energies and its directional dependence of response would be poorer. The implications of this are discussed.

INTERNATIONAL COMPARISON EXERCISE ON NEUTRON SPECTRA UNFOLDING IN BONNER SPHERES SPECTROMETRY: PROBLEM DESCRIPTION AND PRELIMINARY ANALYSIS.

This article describes the purpose, the proposed problems and the reference solutions of an international comparison on neutron spectra unfolding in Bonner spheres spectrometry, organised within the activities of EURADOS working group 6: computational dosimetry. The exercise considered four realistic situations: a medical accelerator, a workplace field, an irradiation room and a skyshine scenario. Although a detailed analysis of the submitted solutions is under preparation, the preliminary discussion of some physical aspects of the problem, e.g. the changes in the unfolding results due to the perturbation of the neutron field by the Bonner spheres, is presented.

USE OF A SIMPLE THERMALISED NEUTRON FIELD FOR QUALITY ACCEPTANCE OF WHOLE BODY TLDS.

The individual monitoring service of Public Health England (PHE) uses Harshaw™ whole-body and extremity thermoluminescent dosemeters (TLDs) with high-sensitivity lithium fluoride LiF:Mg,Cu,P, together with Harshaw 8800™ automated readers. The neutron-insensitive, (6)Li-depleted variety of TLD material is used by PHE because the service provides separate neutron and photon dosemeters. The neutron dosemeters are not sensitive to photons and vice versa Since insensitivity to neutrons is a supply requirement for TLDs, there is a need to test every new (annual) consignment for this. Because it is thermal neutrons that produce a response in (6)Li TLDs, a thermal field is needed. To this end, PHE has adopted the simple approach of sandwiching the TLDs between two ISO water-filled slab phantoms. In this arrangement, the fast neutrons from an Am-Be source are effectively thermalised. Details of the method are given, together with the results of supporting MCNP calculations and some typical results.

THE PHE FORTUITOUS DOSIMETRY CAPABILITY BASED ON OPTICALLY STIMULATED LUMINESCENCE OF MOBILE PHONES.

The Public Health England fortuitous dosimetry capability is reviewed, with particular attention focussed on the derivation of its energy and fading corrections, the Monte Carlo techniques used to generate the calibration factors between phone and body doses, and the procedures set in place to facilitate a reliable and effective service.

CALIBRATION OF THERMOLUMINESCENCE AND FILM DOSEMETERS FOR SKIN DOSES FROM HIGH-ACTIVITY MICROPARTICLES.

The use of EXT-RAD™ extremity TLDs and radiochromic film to measure doses from primarily beta-emitting microparticles is discussed. Specific calibration techniques have been developed, using both Monte Carlo modelling and experiments. Results for a (90)Sr/(90)Y microparticle are presented to illustrate the general techniques and to demonstrate reasonable agreement between the dosimetry methods.

Luminescence-based retrospective dosimetry using Al2O3 from mobile phones: a simulation approach to determine the effects of position.

Monte Carlo modelling has been performed in support of efforts to establish emergency dosimetry services based on optically or thermally stimulated luminescence (OSL/TL) of the Al(2)O(3) substrate present on the resistors found in mobile phones, which can act as fortuitous retrospective dosemeters for photon exposures. Specifically, a range of exposure conditions has been modelled to assess the dependence of the dosimetry on factors such as the position of resistors within a phone, the orientation of the phone relative to the source, and the location of the phone relative to its owner. Variations due to the resistors' positions and the phone's orientation were generally found to contribute just a few percent to the uncertainty on the dose assessments, though the electrical contacts surrounding the resistors could potentially enhance these by several 10s of percent. But, the location of the phone was found to impact dosimetry greatly. The largest discrepancies in the results were found for low-energy exposures: for (192)Ir, differences of up to an order-of-magnitude were found between resistor and whole body doses. The outcome of the work was to derive correction / calibration factors that can be applied to estimate whole body doses from OSL/TL readings, the accurate application of which would depend on the knowledge of the exposure geometry and the degree of conservatism acceptable for the dose assessment.

Development of a retrospective/fortuitous accident dosimetry service based on OSL of mobile phones.

Work is presented on the development of a retrospective/fortuitous accident dosimetry service using optically stimulated luminescence of resistors found in mobile phones to determine the doses of radiation to members of the public following a radiological accident or terrorist incident. The system is described and discussed in terms of its likely accuracy in a real incident.

Monte Carlo modelling of 90Sr/90Y and 85Kr beta fields for Hp(3) measurements.

In support of research aimed at developing a thermoluminescence dosemeter capable of accurately measuring ionising radiation doses to the lens of the eye, Monte Carlo modelling of a standard beta exposure set-up has been performed. It was found that electrons with an energy distribution corresponding to the beta emission spectrum from (85)Kr deposit negligible doses at a depth of 3 mm in tissue, but doses from (90)Sr/(90)Y are significant; free in air and fluence-to-Hp(3,θ°) and -Hp(0.07,θ°) conversion coefficient data were found for this field for exposures at 0°, 30° and 60° angles of incidence, and the response characteristics of the new eye dosemeter were evaluated. It was shown that the results were not affected greatly by the shape of the calibration phantom. However, it was demonstrated that the presence of intermediating air and beam flattening filters hardens the energy distribution of the field at the point of test, relative to a raw (90)Sr/(90)Y source, and this impacts dose depositions.

Measurements with the new PHE neutron survey instrument.

A novel design of survey instrument has been developed to accurately estimate ambient dose equivalent from neutrons with energies in the range from thermal to 20 MeV. The device features moderating and attenuating layers to ease measurement of fast and intermediate energy neutrons, combined with guides that channel low-energy neutrons to the single, central detector. A prototype of this device has been constructed and exposed to a set of calibration fields: the resulting measured responses are presented and discussed here, and compared against Monte Carlo data. A simple simulated workplace neutron field has also been developed to test the device.

Type testing of a head band dosemeter for measuring eye lens dose in terms of H(P)(3).

A new head band dosemeter, for the measurement of eye lens dose in terms of Hp(3), has been type tested by Public Health England's Centre for Radiation, Environmental and Chemical Hazards [formerly part of the UK Health Protection Agency (HPA)]. The type tests were based on the International Standard ISO 12794, drawing also upon earlier work at HPA. The results show that, unlike many existing dosemeters, the new head band dosemeter correctly measures Hp(3) for beta radiations as well as photons.

Doses and risks from uranium are not increased significantly by interactions with natural background photon radiation.

The impact of depleted uranium (DU) on human health has been the subject of much conjecture. Both the chemical and radiological aspects of its behaviour in the human body have previously been investigated in detail, with the radiological impact being assumed to be linked to the alpha decay of uranium. More recently, it has been proposed that the accumulation in tissue of high-Z materials, such as DU, may give rise to enhanced local energy deposition in the presence of natural background photon radiation due to the high photoelectric interaction cross sections of high-Z atoms. It is speculated that, in addition to producing short-range photoelectrons, these events will be followed by intense Auger and Coster-Kronig electron emission, thereby causing levels of cell damage that are unaccounted for in conventional models of radiological risk. In this study, the physical and biological bases of these claims are investigated. The potential magnitudes of any effect are evaluated and discussed, and compared with the risks from other radiological or chemical hazards. Monte Carlo calculations are performed to estimate likely energy depositions due to the presence of uranium in human tissues in photon fields: whole body doses, organ doses in anthropomorphic phantoms and nano-/micro-dosimetric scenarios are each considered. The proposal is shown generally to be based on sound physics, but overall the impact on human health is expected to be negligible.

A Monte Carlo analysis of possible cell dose enhancement effects by uranium microparticles in photon fields.

Uranium microparticles (radii: 50 nm-1.25 µm) were modelled surrounded by tissue and exposed to natural background radiation, in order to investigate potential dose enhancements from photon interactions. Generally, the results depended on the microparticle size. For a 0.5 µm radius microparticle in an isotropic field, it was found that the combined photon/electron doses deposited in 1 and 10 µm radii shells around it were raised by factors of ∼3.8 and ∼1.1, respectively; for a typical background photon fluence rate, these would correspond to increased energy depositions of a few 10s and a few 100s of eV y(-1), which are far less than the likely deposition rate resulting from the radioactive decay of a (238)U microparticle. The health hazard from uranium microparticle interactions with background photons was concluded to be negligible.

The MCNP-4C2 design of a two element photon/electron dosemeter that uses magnesium/copper/phosphorus doped lithium fluoride.

The Health Protection Agency is changing from using detectors made from 7LiF:Mg,Ti in its photon/electron personal dosemeters, to 7LiF:Mg,Cu,P. Specifically, the Harshaw TLD-700H card is to be adopted. As a consequence of this change, the dosemeter holder is also being modified not only to accommodate the shape of the new card, but also to optimize the photon and electron response characteristics of the device. This redesign process was achieved using MCNP-4C2 and the kerma approximation, electron range/energy tables with additional electron transport calculations, and experimental validation, with different potential filters compared; the optimum filter studied was a polytetrafluoroethylene disc of diameter 18 mm and thickness 4.3 mm. Calculated relative response characteristics at different angles of incidence and energies between 16 and 6174 keV are presented for this new dosemeter configuration and compared with measured type-test results. A new estimate for the energy-dependent relative light conversion efficiency appropriate to the 7LiF:Mg,Cu,P was also derived for determining the correct dosemeter response.