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Neutron monitor for pulsed radiation

29.09.2008

On high-energy accelerators such as, for example, DESY, GSI or CERN, radiation losses, which lead to a neutron dose from 10 to 20 µSv, occur at irregular intervals. This dose is measured with passive, integrating detectors (e.g. with TLDs [thermoluminescence detectors] or with bubble detectors). Due to the long dead time of the devices in the short pulse time, this dose is, however, not indicated by the active dosemeters which are presently in use for environmental monitoring [1].

The neutron monitor developed by us is based on the recording of activity products. It was the aim of the development to obtain a monitor which directly indicates the neutron dose - also in the case of a high background of photon radiation - both in pulsed and in continuous neutron fields, and which gives an alarm within a few seconds when the dose increases.

To furnish proof of neutrons, activity reactions on silver were selected. Natural silver contains approximately identical fractions of the isotopes 109Ag and 107Ag, which form - via a neutron capture reaction - the isotopes 110Ag and 108Ag which, in turn, pass into 110Cd and 108Cd via β- decay with half-lives of 25 s or 144 s, respectively:

109Ag + n → γ + 110Ag → 110Cd + β- (Emax = 2.9 MeV)

(48% in natAg, thermal cross section: 90 b, half-life 110Ag: 25 s)

107Ag + n → γ + 108Ag → 108Cd + β- (Emax = 1.7 MeV)

(52% in natAg, thermal cross section: 38 b, half-life 108Ag: 144 s).

The resulting β- radiation with maximum energies of 2.9 MeV or 1.7 MeV is measured with semiconductor detectors. The figure shows the structure of the detector capsule located in the centre of a moderating sphere of polyethylene, 30 cm in diameter.

4 silicon diodes are located in the centre of the detector capsule. Two symmetrically arranged diodes are covered on both sides with silver layers (0.25 mm in thickness) and two diodes are covered with tin layers (0.36 mm in thickness). The thickness of the silver layers has been optimized for the detection of the beta radiation to be measured, and the thickness of the tin layers has been selected in such a way that the absorption for photon radiation is comparable for all detectors.

The detectors covered with silver are sensitive to neutron and photon radiation, the detectors covered with tin are sensitive only to photon radiation. The neutron dose can be determined by a subtraction of the counting events and a calibration in the neutron field.

To reduce the uncertainties caused by the subtraction, a threshold was set in the pulse height spectrum at an energy of 662 keV. Hence, the monitor becomes insensitive to low-energy photon radiation with energies up to 662 keV (137Cs radiation). Setting of this pulse height threshold also allows the higher-energy β- radiation of 110Ag to be detected more efficiently. An evaluation of the decay curves shows that the reaction products with the shorter half-life (25 s) contribute more strongly to the indication (by a factor of 8) than those with the long half-life. This is considerably higher than the factor 2.3 to be expected from the ratio of the cross sections (90 b and 38 b, see above) and the isotope frequencies. Hence, an alarm can be given within a few seconds whenever the dose strongly increases. The mean neutron detection sensitivity amounts to (9.0 ± 0.4) counting events per µSv and is thus sensitive enough to record the radiation losses mentioned above.

The construction and the way of operation of the monitor, the results of the measurements performed with the monitor in neutron fields (radionuclide sources, monoenergetic neutron fields up to 15 MeV) and in photon fields (137Cs, 60Co, 6 MeV), the results of calculations in neutron fields with energies up to 1 GeV and uncertainty estimates in mixed neutron/photon fields were presented and published at the IRPA12 (12th International Congress of the International Radiation Protection Association) in Buenos Aires [2].

A few days before the meeting, PTB also presented the invention to the German Patent Office. The monitor has a good chance of being implemented in practice.

Figure : Construction of the detector capsule of the neutron monitor.

Literature

  1. Leuschner, A.:
    The 12B counter: an active dosemeter for high-energy neutrons,
     Radiat. Prot. Dosim. 116 (2005) 144.
  2. Luszik-Bhadra, M.; Hohmann, E.:
    A new neutron monitor for pulsed fields at high-energy accelerators,
    Proceedings of the IRPA 12 "Strengthening Radiation Protection Worldwide", Buenos Aires, Argentina, 19-24 October 2008, to be published at: irpa12.irpa.net.