Direction distributions of neutrons and reference values of the personal dose equivalent in workplace fields

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Supported by the European Commission, the EVIDOS project started in November 2001 with the broad goal of evaluating state of the art dosimetry techniques in representative workplaces of the nuclear industry. Seven European institutes joined efforts with end users at nuclear power plants, at fuel processing and reprocessing plants, and at transport and storage facilities. A comprehensive programme was devised to evaluate capabilities and limitations of standard and innovative personal dosemeters in relation to the mixed neutron-photon fields of concern to the nuclear industry. This paper describes the criteria behind the selection of dosimetry techniques and workplaces that were analysed, as well as the organisation of the measurement campaigns. Particular emphasis was placed on the evaluation of a variety of electronic personal dosemeters, either commercially available or previously developed by the partners. The estimates provided by these personal dosemeters were compared to reference values of dose equivalent quantities derived from spectrometry and fluence-to-dose equivalent conversion coefficients. Spectrometry was performed both with conventional multisphere and with some original instrumentation providing energy and direction resolution, based on silicon detectors and superheated drop detectors mounted on or in spherical moderators. The results were collected in a large, searchable database and are intended to be used in the harmonisation of dosimetric procedures for mixed radiation fields and for the approval of dosimetry services in Europe.

Section: Direction Spectrometry and Reference Values For H P (10) And Ementioning

confidence: 99%

Evaluation of individual dosimetry in mixed neutron and photon radiation fields (EVIDOS). Part I: scope and methods of the project

D’Errico

Bartlett²,

Bolognese-Milsztajn

et al. 2006

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“…surface oriented to the direction 180 (back). Table 2 shows the measured personal neutron dose equivalent rate H p (10) DISN , measured with the dosemeter types PE 4% LiNO 3 (bare and with 1 mm B 4 C) and A-150 1.25% BN (bare and with 2 mm B 4 C), respectively, and the values H Ã (10) BSS (6) and H p (10) DS (7) for all locations and positions. The uncertainties for the values H Ã (10) BSS and H p (10) DS are in the order of 10% and up to 30%, respectively.…”

Section: Field Measurementsmentioning

confidence: 99%

Performance of a personal neutron dosemeter based on direct ion storage at workplace fields in the nuclear industry

Boschung¹,

Fiechtner²,

Wernli³

2006

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In the framework of the EVIDOS project, funded by the EC, measurements were carried out using dosemeters, based on ionisation chambers with direct ion storage (DIS-N), at several workplace fields, namely, at a fuel processing plant, a boiling and a pressurised water reactor, and near transport and storage casks. The measurements and results obtained with the DIS-N in these workplaces, which are representative for the nuclear industry, are described in this study. Different dosemeter configurations of converter and shielding materials were considered. The results are compared with values for personal dose equivalent which were assessed within the EVIDOS project by other partners. The advantages and limitations of the DIS-N dosemeter are discussed. DOSEMETER DESIGN AND DOSE DETERMINATIONThe personal neutron dosemeter DIS-N, developed by RADOS, is based on ionisation chambers with direct ion storage and a double-chamber system which allows for differential readings to separate the neutron from the photon dose equivalent (1) . However, ionisation chambers are sensitive to both neutrons and photons. Therefore, application of the double-chamber principle requires that the photon energy sensitivities of the two chambers are almost equal and the neutron energy sensitivities of the two chambers are significantly different.Four different types of dosemeter were investigated within the EVIDOS project. The chamber with a high sensitivity for fast neutrons is build with tissue-equivalent wall material (A-150) or polyethylene (PE) for the detection of recoil protons. The response to thermal neutrons can be modified by adding boron nitride (BN) to A-150 or LiNO 3 to PE and by surrounding the dosemeter with additional 'boron-covers' (containing 40% B 4 C). Thermal neutrons can then be detected by the secondary charged particles of the (n,a) reaction with 10 B and 6 Li, respectively, while the boron-covers will reduce the thermal neutron flux.The chamber with a low sensitivity to neutrons was in all cases made up of Teflon (polytetraflouroethylene) containing 60% graphite. Owing to a 1-mm-thick tin shielding around the chambers, photon energies <100 keV may be neglected.The following combinations of wall materials and boron-covers of the chamber with a high-neutron response to fast neutrons were used: PE with 4% LiNO 3 . PE with 4% LiNO 3 and 1 mm B 4 C.A-150 with 1.25% BN. A-150 with 1.25% BN and 2 mm B 4 C.The measured quantities N 1 and N 2 (arbitrary units, a.u.) in both chambers 1 (high-neutron sensitivity) and 2 (low-neutron sensitivity) in a mixed neutron/ photon field are due to the sum of neutron-induced and photon-induced effects and can be written as follows:where H n and H g are the neutron and photon dose equivalent; S 1n and S 2n are the neutron sensitivities of chambers 1 and 2; and S 1g and S 2g are the photon sensitivities of chambers 1 and 2. The sensitivities S i are average sensitivities dependent on the angular and energy distribution of the neutron and photon fields. The neutron dose equivalent can then be ...

“…The H Ã (10) rate measured using each of the instruments, divided by that calculated directly from the Bonner sphere determinations of the fluence-energy distribution (6,7) , gives the bias in the reading of the instrument. A theoretical value for the bias can also be obtained by folding the instrument response with the energy distribution and dividing this value by the reference H Ã (10).…”

Section: Response In Evidos Fieldsmentioning

confidence: 99%

“…Ratio of the measured ambient dose equivalent rate, H Ã (10) M , to the reference value (6,7) , H Ã (10) R . Ratio of the calculated response of the instrument, H Ã (10) C , to H Ã (10) R .…”

Section: Response In Evidos Fieldsmentioning

confidence: 99%

Neutron area survey instrument measurements in the EVIDOS project

Tanner¹,

Bolognese-Milsztajn²,

Boschung³

et al. 2006

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Neutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These results have an important impact in the interpretation of the results from the project, since operationally the survey instrument will be used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, since the response of these instruments is in some cases very well characterised, their systematic deviations from the reference quantities provide an important verification of the determination of those quantities.