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Radiation protection dosimetry on high-energy particle accelerators

26.07.2006

Radiation detectors that monitor the exposure of persons working at high-energy particle accelerators, i.e., at the Gesellschaft für Schwerionenforschung mbH (GSI) in Darmstadt or the European Organization for Nuclear Research (CERN) in Geneva, must meet special requirements, as the radiation in working areas is dominated by neutron radiation. This topic has increasingly gained in importance due to the large number of new particle accelerators for medical applications (e.g., for radiation therapy with protons and carbon ions) that are available, A difficult metrological challenge results from the fact that the neutron radiation has a very large energy range covering more than 12 powers of ten and that the interaction of neutrons shows a very strong energy dependence. In addition, neutrons of very high energies (larger than 20 MeV) [1] which cannot be measured correctly with conventional monitoring devices are present in the working areas of the accelerators, e.g., behind the massive concrete shielding. In the past few years, however, new instruments have been developed which are also sensitive to high energy neutrons.

The "European Radiation Dosimetry Group" (EURADOS) is responsible for the CONRAD (COordinated Network for RAdiation Dosimetry) project (2005-2007) funded by the European Commission. Within the scope of the work package "Complex mixed radiation fields at workplaces", a benchmark experiment was carried out at the GSI in July 2006 to check the suitability of the measuring devices and procedures used by European research institutes for radiation protection. Within the scope of this project, 12 institutes have measured - under realistic conditions and behind shielding - the neutron dose equivalent rate at seven different positions with more than 20 different instruments and procedures, Figure 1.

Figure 1 : Benchmark experiment carried out at the GSI in July 2006. Left and center: moderator spheres of the multiple-sphere spectrometers of GSF and PTB, respectively. Right: neutron survey meter (nüm) of a participating institute.

The dose equivalent can be calculated with lower uncertainty if the neutron fluence as a function of energy (i.e., the spectral neutron fluence or, briefly, the neutron spectrum) is determined with suitable spectrometers. PTB’s task within the scope of the CONRAD benchmark experiment was to measure the neutron spectra at four of the seven positions. A brief description of the multiple-sphere spectrometer NEMUS that was used, including additional literature, can be found under "Spectrometer NEMUS" on the PTB’s Internet page "Neutron Spectrometry". The neutron spectra at the remaining three positions and at a joint position were measured by the GSF - Forschungszentrum für Umwelt und Gesundheit in Neuherberg near Munich.

The evaluation of the spectrometric measurements has been carried out. Figure 2 shows that the neutron spectra that PTB measured at the four positions are very different from each other, even though these position are only a few meters apart. In a next step, these results as well as the dose equivalent rates calculated from them will be made available to the participants as reference values.

Figure 2 : Neutron spectra of four positions behind the shielding of "Cave A" of GSI. All spectra are normalized to the charge of the C ions impinging on the target.

By the end of the year, the results of Monte Carlo calculations, the results of the neutron spectrometry as well as the results of the comparison of the different detectors with the reference values will be submitted for publication as a comprehensive three-part report to the magazine “Radiation Measurements“.

Literature:

  1. G. Fehrenbacher, B. Wiegel, H. Iwase, T. Radon, D. Schardt, H. Schuhmacher, J. Wittstock:
    Spectrometry behind Concrete Shielding for Neutrons Produced by 400 MeV/u 12C Ions Impinging on a Thick Graphite Target,
    Proceedings of the 11th Intern. Congress of the International Radiation Protection Association (IRPA11); Madrid, Spain, Mai 2004, contrib. 5i4, www.irpa11.com