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NEMUS: An extended Bonner sphere spectrometer (Neutron Multisphere Spectrometer)

The Bonner sphere spectrometer NEMUS can measure the neutron fluence as well as the energy distribution of neutrons. The PTB spectrometer of the PTB consists of a set of 12 Bonner spheres of polyethylene 7.62 cm (3") to 45.72 cm (18") in diameter. In the centre of each sphere there is a spherical proportional counter filled with 3He gas which is very sensitive to thermalized neutrons.

In addition to these polyethylene spheres, there are four spheres in which lead or copper shells are embedded in the polyethylene; as a result, the response of the spectrometer at high neutron energies (> 20 MeV) is considerably raised. NEMUS serves as a reference system for the spectrometry and dosimetry of unknown neutron fields in the wide energy range of thermal neutrons to about 400 MeV.

The total neutron fluence can be determined by suitable deconvolution methods with uncertainties of 5 %. With the aid of recommended conversion coefficients, dose equivalent quantities (e.g. the ambient dose equivalent, H*(10)) can be determined from the neutron spectrum with uncertainties which are normally of the order of 15 %.

 

The NEMUS spectrometer has been used recently for measurements of secondary neutrons at proton therapy facilities as well as for measurements of the neutron background at underground laboratories, which is highly relevant for fundamental research. Other previous areas of application include measurements at nuclear facilities and measurements of the neutron component of cosmic rays at ground level and on airplanes.

Further information

  • Measuring neutron radiation in Gorleben
    At the request of the Ministry of the Environment, Energy and Climate Protection of Lower Saxony, measurements were carried out in the centre of the town of Gorleben. Data from measurement hut No. 5, which is located approx. 2 km from the transport cask storage facility, have shown that exclusively neutrons from cosmic radiation contribute to the ambient dose equivalent.


  • Intrinsic background of the neutron detectors of the NEMUS spectrometer
    For measurements in low-intensity neutron fields, it is absolutely indispensable to know the intrinsic background of detectors. During a two-month measurement campaign, all 3He detectors of the NEMUS neutron spectrometer were investigated in the underground laboratory of PTB (UDO) and their lower detection limit was determined.
  • Bayesian analysis of spectrometric measurements in high-energy neutron fields
    The uncertainty of measurements made with an extended-range Bonner sphere spectrometer in high-energy neutron fields has been analysed by means of Bayesian methods. It was shown that the spectrum below approx. 1 MeV is well determined whereas the information from the measurements is not sufficient to determine the high-energy end of the spectrum with low uncertainty.
  • Radiation protection dosimetry at high-energy particle accelerators
    Radiation detectors that monitor the exposure of persons working at high-energy particle accelerators used for basic research, technology and medicine must meet special requirements, as the radiation in working areas is dominated by neutron radiation. In these environments, neutrons of high energies which cannot be measured correctly with conventional monitoring devices are present. A working group of the "European Radiation Dosimetry Group" (EURADOS), which is funded by the European Commission within the scope of the 6th framework program, has performed a benchmark experiment to check the suitability of the measuring devices and procedures used by European research institutes for radiation protection. Within the scope of this project, PTB has measured neutron spectra at the Gesellschaft für Schwerionenforschung (GSI) and determined reference values for the participants of this benchmark experiment.

Literatur

  • M. Reginatto, A. Kasper, H. Schuhmacher, B. Wiegel, A. Zimbal:
    Neutron fluence rate measurements at an underground laboratory: A Bayesian approach
    AIP Conf. Proc. 1553, 77, 2013
    doi: 10.1063/1.4819986

  • A. Zimbal, D. Degering, M. Reginatto, H. Schuhmacher, B. Wiegel, K. Zuber:
    Low-Level measuring techniques for neutrons: High accuracy neutron source strength determination and fluence rate measurement at an underground laboratory
    AIP Conf. Proc. 1549, 70, 2013
    doi: 10.1063/1.4818078

  • B. Wiegel and A. V. Alevra:
    NEMUS - The PTB Neutron Multisphere Spectrometer: Bonner Spheres and More
    Beitrag "International Workshop on Neutron Field Spectrometry in Science, Technology and Radiation Protection", 5.-8. Juni 2000; Nucl. Instrum. Meth. A 476, (2002) 36-41.
    doi: 10.1016/S0168-9002(01)01385-7
  • B. Wiegel, A. V. Alevra and B. R. L. Siebert:
    Calculations of the Response Functions of Bonner Spheres with a Spherical 3He Proportional Counter Using a Realistic Detector Model
    PTB-Bericht PTB-N-21, ISBN 3-89429-563-5, (1994).

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