Response of radiation protection dosemeters in mixed high-energy photon and electron radiation fields

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.     

Determination of personal dose equivalents in accelerator radiation fields

Values for the dose equivalent are required for radiation protection purposes, but determination of such values can be quite difficult for high energy radiations. The accurate determination of personal dose equivalents in accelerator radiation fields requires the propel use of appropriate radiological quantities and units, knowledge of the dose equivalent response of the personal dosemeters used, measurement or calculation of the fluence spectrum in the workplace and the fluence spectrum of the reference radiation used to calibrate the dosemeters, in addition to knowledge of the appropriate fluence-to-dose equivalent conversion coefficients. This information can then be used to select the appropriate dosemeters, set up the optimum calibration conditions, or to establish correction factors that account for differences in the calibration and workplace fluence spectra. High energy neutrons account for a significant fraction of the dose equivalent received by workers at accelerator facilities, and this work discusses the procedures and methods needed to determine dose equivalent produced by neutrons in the vicinity of high energy particle accclerators.     

Individual neutron monitoring in workplaces with mixed neutron/photon radiation

EVIDOS ('evaluation of individual dosimetry in mixed neutron and photon radiation fields') is an European Commission (EC)-sponsored project that aims at a significant improvement of radiation protection dosimetry in mixed neutron/photon fields via spectrometric and dosimetric investigations in representative workplaces of the nuclear industry. In particular, new spectrometry methods are developed that provide the energy and direction distribution of the neutron fluence from which the reference dosimetric quantities are derived and compared to the readings of dosemeters. The final results of the project will be a comprehensive set of spectrometric and dosimetric data for the workplaces and an analysis of the performance of dosemeters, including novel electronic dosemeters. This paper gives an overview of the project and focuses on the results from measurements performed in calibration fields with broad energy distributions (simulated workplace fields) and on the first results from workplaces in the nuclear industry, inside a boiling water reactor and around a spent fuel transport cask.     

Response of alanine and radio-photo-luminescence dosemeters to mixed high-energy radiation fields

Alanine and Radio-Photo-Luminescence (RPL) dosemeters are passive dosemeters used to monitor absorbed dose in all kind of radiation fields. However, up to now both dosemeter types are calibrated to photon sources only. In order to study the response of RPL and alanine dosemeters to mixed high-energy particle fields like those occurring at CERN's accelerators, an irradiation campaign at the CERN-EC High-Energy Reference field Facility (CERF-field) was performed. In this facility a copper target is irradiated by hadrons with a momentum of 120 GeV/c. Dosemeters were exposed to various mixed radiation fields by placing them at various positions on the surface of the target. In addition to the experiment FLUKA Monte Carlo simulations were carried out, which provide information concerning the energy deposition at the dosemeter locations. This paper compares the measurements with the simulation results and discusses the radiation field compositions present at the various dosemeter positions on the target.     

Novel reference radiation fields for pulsed photon radiation installed at PTB

Currently, ～70 % of the occupationally exposed persons in Germany are working in pulsed radiation fields, mainly in the medical sector. It has been known for a few years that active electronic dosemeters exhibit considerable deficits or can even fail completely in pulsed fields. Type test requirements for dosemeters exist only for continuous radiation. Owing to the need of a reference field for pulsed photon radiation and accordingly to the upcoming type test requirements for dosemeters in pulsed radiation, the Physikalisch-Technische Bundesanstalt has developed a novel X-ray reference field for pulsed photon radiation in cooperation with a manufacturer. This reference field, geared to the main applications in the field of medicine, has been well characterised and is now available for research and type testing of dosemeters in pulsed photon radiation.     

The right choice: extremity dosemeter for different radiation fields

Measurements of weakly penetrating radiation in personal dosimetry present problems in the design of suitable detectors and in the interpretation of their readings. For the measurement of the individual beta radiation dose, personal dosemeters for the fingers/tips are required. LiF:Mg,Cu,P is a promising thermoluminescent (TL) material which allows the production of thin detectors with sufficient sensitivity. Dosimetric properties of two different types of extremity dosemeters, designed to measure the personal dose equivalent Hp(0.07), have been compared: LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD700H). A type test for energy response for photon and beta radiation according to ISO 4037-3 and ISO-6980 was carried out and the results for both dosemeters were compared. Simultaneous measurements with both types of dosemeters were performed at workplaces, where radiopharmaceuticals containing different radioisotopes are prepared and applied. Practices in these fields are characterized by handling of high activities at very small distances between source and skin. The results from the comparison of the two-dosemeter types are presented and analysed with respect to different radiation fields. Experiments showed a satisfactory sensitivity for the thinner dosemeter (TLD 700H) for detecting beta radiation at protection levels and a good energy response.     

The effects of ionisation density on the thermoluminescence response (efficiency) of LiF:Mg,Ti and LiF:Mg,Cu,P

In this paper, the various models dealing with the effects of ionisation density on the thermoluminescence (TL) response (efficiency) of TL LiF dosemeters are discussed. These include (i) the Unified Interaction Model (UNIM), which models photon/electron linear/supralinear dose response; (ii) the Extended Track Interaction Model (ETIM), which models heavy charged particle (HCP) TL fluence response; (iii) Modified Track Structure Theory (MTST), which models relative HCP TL efficiencies; and (iv) Microdosimetric Target Theory (MTT), which models both relative HCP efficiencies and photon energy response.     

Intercomparison measurements of extremity dosemeters in beta and/or photon radiation fields

Beta dosimetry, especially at the extremities, is gaining in importance due to the increasing use of beta particle sources, e.g. in brachytherapy. The dosimetric properties of personal dosemeters to be worn on the extremities and capable of measuring the personal dose equivalent, Hp(0.07), in beta and/or photon radiation fields were investigated within the scope of intercomparison measurements organised by the PTB in two steps. The results were evaluated on the basis of recommendations from the German Commission on Radiological Protection (SSK). In the first step 10 types of dosemeter were investigated in beta particle fields in a range of mean energies from 0.06 MeV to 0.8 MeV. In the second step, five selected beta dosemeter types were exposed to beta particles and, in addition, to photons and to mixtures of both. Three dosemeters fulfill the requirements for the whole range of mean beta energy used for the intercomparison and meet the requirements for photon radiation from 8 keV to 662 keV.     

Calibration of personal dosemeters in mixed neutron-photon fields: some problems and their solution

In neutron reference radiation fields, the conventional true value of the personal dose equivalent, H(p)(10), is derived from the spectral neutron fluence and recommended conversion coefficients. This procedure requires the phantom on which the personal dosemeter is mounted to be irradiated with a broad and parallel beam. In many practical situations, the change of the neutron fluence and/or the energy distribution over the surface of the phantom may not be neglected. For a selection of typical irradiation conditions in neutron reference radiation fields, the influence of this effect has been analysed using numerical methods. A further problem, which is of relevance for the calibration of dosemeters measuring both the neutron and the photon component of mixed fields, is the 'double counting' of the dose equivalent due to neutron-induced photons. The relevance of this conceptual problem for calibrations in mixed-field dosimetry was analysed.     

Bonner sphere neutron spectrometry at spent fuel casks

For transport and interim storage of spent fuel elements from power reactors and vitrified highly active waste (HAW) from reprocessing, various types of casks are used. The radiation exposure of the personnel during transportation and storage of these casks is caused by mixed photon–neutron fields and, frequently, the neutron dose is predominant. In operational radiation protection, survey meters and even personal dosemeters with imperfect energy dependence of the dose-equivalent response are used, i.e. the fluence response of the devices does not match the fluence-to-dose equivalent conversion function. In order to achieve more accurate dosimetric information and to investigate the performance of dosemeters, spectrometric investigations of the neutron fields are necessary. Therefore, fluence spectra and dose rates were measured by means of a simple portable Bonner multisphere spectrometer (BSS). The paper describes briefly the experimental set-up and evaluation procedure. Measured spectra for different locations, types of casks and inventory are discussed. The spectra provide a basis to determine dose rates and other integral quantities with higher accuracy and for choosing suitable area monitors, respectively, to establish correction factors applied to the dosemeter reading.     

Monte Carlo simulation of the IRSN CANEL/T400 realistic mixed neutron-photon radiation field

The calibration of dosemeters and spectrometers in realistic neutron fields simulating those encountered at workplaces is of high necessity to provide true and reliable dosimetric information to the exposed nuclear workers. The CANEL assembly was set-up at IRSN to produce such neutron fields. It comprises a depleted uranium shell, to produce fission neutrons, then iron and water to moderate them and a polyethylene duct. The new presented CANEL facility is used with 3.3 MeV neutrons. Calculations were performed with the MCNP4C code to characterise this mixed neutron-photon expanded radiation field at the position where calibrations are usually performed. The neutron fluence energy and the direction distributions were calculated and the operational quantities were derived from these distributions. The photon fluence and corresponding ambient dose equivalent were also estimated. Comparison with experimental results showed an overall good agreement.     

Alanine blends for ESR measurements of thermal neutron fluence in a mixed radiation field

In this paper, the results of a study on the electron spin resonance (ESR) dosimetry to measure thermal neutron fluence in a mixed radiation field (neutron and photons) are presented. The ESR responses of alanine dosemeters with different additives are compared. In particular, the (10)B-acid boric and the Gd-oxide were chosen to enhance the sensitivity of alanine dosemeters to thermal neutrons. Irradiations were carried out inside the thermal column of the TAPIRO reactor of the ENEA center, Casaccia Rome. The main results are a greater neutron sensitivity and a smaller lowest detectable fluence for the dosemeters with gadolinium than for dosemeters of alanine with (10)B, which is well known to be much more sensitive to thermal neutrons than simple alanine.     

Alanine response to proton beams in the 1.6-6.1 MeV energy range

Alanine response to low-energy protons was studied with alanine dosemeters of 2 mm thickness, irradiated with proton beams of energy in the 1.6-6.1 MeV range. The detector's range-averaged relative effectiveness to 60Co radiation ranged from 0.61 to 0.65. For fluence values up to 5 x 10(10) protons x cm(-2), the alanine response was linear.     

Extremity ring dosimetry intercomparison in reference and workplace fields

An intercomparison of ring dosemeters has been organised with the aim of assessing the technical capabilities of available extremity dosemeters and focusing on their performance at clinical workplaces with potentially high extremity doses. Twenty-four services from 16 countries participated in the intercomparison. The dosemeters were exposed to reference photon ((137)Cs) and beta ((147)Pm, (85)Kr and (90)Sr/(90)Y) fields together with fields representing realistic exposure situations in interventional radiology (direct and scattered radiation) and nuclear medicine ((99 m)Tc and (18)F). It has been found that most dosemeters provided satisfactory measurements of H(p)(0.07) for photon radiation, both in reference and realistic fields. However, only four dosemeters fulfilled the established requirements for all radiation qualities. The main difficulties were found for the measurement of low-energy beta radiation. Finally, the results also showed a general under-response of detectors to (18)F, which was attributed to the difficulties of the dosimetric systems to measure the positron contribution to the dose.     

Conceptual design of spectrum changeable neutron calibration fields in JAERI/FRS

Spectrum changeable neutron calibration fields are planned to be established with an accelerator installed in Japan Atomic Energy Research Institute/Facility of Radiation Standards. The neutron fields are provided by bombarding a target surrounded by a moderator, with charged particles from the accelerator. In the fields, a wide variety of neutron spectra is provided with sufficient fluence rate for the calibration of dosemeters. In this study, necessity of the field was first discussed in view of relationship between readings of existing dosemeters and true dose equivalents where the dosemeters were used. Second, test simulation of neutron spectra was carried out with the Monte Carlo technique for some arrangements with a LiF target and quasi-cylindrical moderators with different materials. The simulated spectra were summarised in terms of fluence-average energy, fluence rate and calibration factor for the dosemeters.     

Evaluation of individual monitoring in mixed neutron/photon fields: mid-term results from the EVIDOS project

EVIDOS is an EC sponsored project that aims at an evaluation and improvement of radiation protection dosimetry in mixed neutron/photon fields. This is performed through spectrometric and dosimetric investigations during different measurement campaigns in representative workplaces of the nuclear industry. The performance of routine and, in particular, novel personal dosemeters and survey instruments is tested in selected workplace fields. Reference values for the dose equivalent quantities, H(*)(10) and H(p)(10) and the effective dose E, are determined using different spectrometers that provide the energy distribution of the neutron fluence and using newly developed devices that determine the energy and directional distribution of the neutron fluence. The EVIDOS project has passed the mid-term, and three measurement campaigns have been performed. This paper will give an overview and some new results from the third campaign that was held in Mol (Belgium), around the research reactor VENUS and in the MOX producing plant of Belgonucléaire.     

Quality assurance of personal beta particle dosemeters used for individual monitoring of occupationally exposed persons

As a result of investigations and intercomparison measurements organised from 1996 to 1999 by PTB, several types of personal dosemeters, all based on TLD, were selected by the dosimetry services for the measurement of the personal dose equivalent H(p)(0.07) in beta and/or photon radiation fields. These dosemeters have now the status of legal personal beta partial-body dosemeters. Workplaces at which beta radiation might significantly contribute to the doses to the extremities are to be found today with increasing frequency in radiation therapy, radiation source production and nuclear power plants. Quality assurance for beta personal dosemeters is stipulated by guidelines for the official dosimetry service and is carried out by way of the intercomparison measurements organised periodically by the PTB. The results are evaluated based on the recommendations of the German Commission on Radiological Protection (SSK). The procedure of these intercomparison measurements will be explained in detail. The experience gained from three series of comparisons with seven types of fingerring dosemeters will be described and the results will be presented. The anonymity of the dosemeter types and of the participants in the intercomparison will be preserved.     

Photon energy response of luminescence dosemeters and its impact on assessment of Hp(10) and Hp(0.07) in mixed fields of varying energies of photons and beta radiation

X and gamma rays continue to remain the main contributors to the dose to humans. As these photons of varying energies are encountered in various applications, the study of photon energy response of a dosemeter is an important aspect to ensure the accuracy in dose measurement. Responses of dosemeters have to be experimentally established because for luminescence dosemeters, they depend not only on the effective atomic number (ratio of mass energy absorption coefficients of dosemeter and tissue) of the detector, but also considerably on the luminescence efficiency and the material surrounding the dosemeters. Metal filters are generally used for the compensation of energy dependence below 200 keV and/or to provide photon energy discrimination. It is noted that the contribution to Hp(0.07) could be measured more accurately than Hp(10). For the dosemeters exhibiting high photon energy-dependent response, estimation of the beta component of Hp(0.07) becomes very difficult in the mixed field of beta radiation and photons of energy less than 100 keV. Recent studies have shown that the thickness and the atomic number of metal filters not only affect the response below 200 keV but also cause a significant over-response for high energy (>6 MeV) photons often encountered in the environments of pressurised heavy water reactors and accelerators.     

Energy response of different types of RADOS personal dosemeters with MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P) TL detectors

The photon energy response of different RADOS (Mirion Technologies) personal dosemeters with MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P) thermoluminescence (TL) detectors was investigated. Three types of badges were applied. The irradiation with reference photon radiation qualities N (the narrow spectrum series), and S-Cs and S-Co nuclide radiation qualities, specified in ISO 4037 [International Organization for Standardization (ISO). X and gamma reference radiations for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy. ISO 4037. Part 1-4 (1999)], in the energy range of 16-1250 keV, were performed at the Dosimetry Laboratory Seibersdorf. The results demonstrated that a readout of a single MTS-N or MCP-N detector under the Al filter can be used to determine Hp(10) according to requirements of IEC 61066 [International Electrotechnical Commission (IEC). Thermoluminescence dosimetry systems for personal and environmental monitoring. International Standard IEC 61066 (2006)] for TL systems for personal dosimetry. The new RADOS badge with the experimental type of a holder (i.e. Cu/Al filters) is a very good tool for identifying the radiation quality (photon energy).     

Advantages of passive detectors for the determination of the cosmic ray induced neutron environment

Due to the pronounced energy dependence of the neutron quality factor, accurate assessment of the biologically relevant dose requires knowledge of the spectral neutron fluence rate. Bonner sphere spectrometers (BSSs) are the only instruments which provide a sufficient response over practically the whole energy range of the cosmic ray induced neutron component. Measurements in a 62 MeV proton beam at Paul Scherrer Institute, Switzerland, and in the CERN-EU high-energy reference field led to the assumption that conventional active devices for the detection of thermal neutrons inside the BSS, e.g. 6Lil(Eu) scintillators, also respond to charged particles when used in high-energy mixed radiation fields. The effects of these particles cannot be suppressed by amplitude discrimination and are subsequently misinterpreted as neutron radiation. In contrast, paired TLD-600 and TLD-700 thermoluminescence dosemeters allow the determination of a net thermal neutron signal.