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DETERMINATION AND REALISATION OF CALIBRATION FIELDS FOR NEUTRON PROTECTION DOSIMETRY AS DERIVED FROM SPECTRA ENCOUNTERED IN ROUTINE SURVEILLANCE

Objective

The objective of this collaborative project is to produce, in the laboratory, a few well characterized neutron fields that replicate typical spectral neutron fluence distributions encountered in radiation protection practice. These fields are needed for the calibration of neutron area and personnel dosemeters which generally do not have the energy response required to determine dose equivalent quantities.
A database of neutron spectra measured around nuclear installations has been compiled together with protection-level instrument response functions and conversion factors from neutron to dosimetric quantities. A computer program has been written to handle these data.
Based on this collection of workplace spectra it is planned to develop realistic calibration fields for performing improved instrument calibrations. This apprach will overcome the problem that exists at present with the calibration of neutron protection level instruments, arising from the poor dosimetric response of these instruments, which results in all measurements made with them having substantial uncertainties.

New data to be included in a European neutron spectra database were obtained from various French nuclear installations, using the Bonner sphere technique and proton recoil spectrometry.

A new unfolding code has been written for use with the NE213 spectrometer. It incorporates a determination of the efficiency including the irradiation geometry and a detection threshold, and the use of a trapeziodal shape for the response function to correct the measured proton distribution, and it replaces the differentiation technique by a convolution product. The results have been checked against calibrated sources.

The main characteristics (calibration curves, efficiency, energy resolution) of SP2 proportional counters have been redetermined. This reduces the systematic discrepancy between result obtained using Bonner spheres and proton recoil techniques.

Calculations have been made to determine a suitable replica for the most common type of neutron spectrum. A suitable set up is described.

An experimental set up for use as a radiation source for intercomparison of spectrometry techniques has been constructed and characterised. A second one is under development.

Published data on neutron spectra have been collected and catalogued. Nearly 90 neutron spectra from Russian publications were translated and relevant data extracted.

The response matrix of a Bonner sphere spectrometer set with a spherical helium-3 counter has been calculated, for use in the unfolding of Bonner sphere measurements obtained at working sites. These aspects were studied: the pressure dependence of the response when the gas pressure of the helium-3 counter in varied; the influence of the polyethylene density of the spheres on the response; the possibility of representing the whole matrix in the sphere diameter domain by a log normal distribution (Zarborowski technique).

To derive the pressure dependence of the response the calculations were performed for different counter gas pressures for several Bonner spheres and neutron energies. The response data were interpolated to generate the response matrix with 49 energy points in log equidistant intervals from 0.01 eV to 30 MeV for spheres from 2 to 15 inches in diameter steps of 0.5 inches. A first comparison with recent experimental data exhibited excellent agreement.
The project consists of four distinct parts:
the measurement of the spectral neutron fluence typically encountered in practice;
the preparation of a catalogue of all measured spectra in an agreed format including the calculation of relevant dose equivalent quantities and the response of commonly used neutron dosemeters;
the inspection of this catalogue in order to extract a few representative spectra and their expansion in terms of the calibration spectra already available or, if necessary, newly defined ones; .SP 0 the computational prediction of configurations consisting of the usual neutron sources (accelerator, reactor or radionuclide based) and appropriate moderators in order to produce these reference fields in the laboratory.

Four European laboratories, well experienced in the field of neutron metrology and dosimetry, will cooperate in this project. Spectrometric measurements will be performed independently by each laboratory, chiefly in their own country. Setting up and analysis of the catalogue, however, require a close cooperation.

Coordinator

PHYSIKALISCH-TECHNISCHE BUNDESANSTALT
Address
Bundesallee 100
Braunschweig
Germany

Participants (3)

Commissariat à l'Energie Atomique (CEA)
France
Address
Centre D'études De Fontenay-aux-roses
92265 Fontenay-aux-roses
GSF - FORSCHUNGSZENTRUM FUER UMWELT UND GESUNDHEIT GMBH
Germany
Address
Ingolstaedter Landstrasse 1
85764 Neuherberg
National Physical Laboratory (NPL)
United Kingdom
Address
Queen's Road
TW11 0LW Teddington