Photoneutrons from a beryllium reflector: a potential source of problems with Zr-Au flux monitors in k0 standardization based neutron activation analysis.

The assumption that the shape of the epithermal neutron spectrum can be described, in any research reactor, by the 1/E1+α function is a fundamental starting point of the k0 standardization. This assumption may be questioned from a reactor physics viewpoint. The type of moderator, the existence of neutron reflectors, the additional production of (γ, n) neutrons and resonance capture by construction materials may be different for each reactor, with consequences for the shape of the neutron spectrum. This dependency may explain that various practitioners reported contradicting experiences with the use of Zr-Au flux monitors for the determination of the α-parameter. An objective view on the influence of the design of the reactor and irradiation facility on the shape of the neutron spectrum can be obtained by modeling. This has been applied in the Reactor Institute Delft for reactor configurations in which the irradiation facilities face the fuel elements with the presence of beryllium reflector elements. The Monte Carlo calculations indicate a distortion of the 1/E1+α relationship at the higher energy edge of the epithermal neutron spectrum. This distortion is attributed to the formation and thermalisation of both photoneutrons and (n, 2n) produced fast neutrons in the beryllium, and has a direct impact on the resonance activation of 95Zr, other than represented by the 1/E1+α function. The obtained relationship between neutron flux and neutron energy was also used for estimating the f-value and compared with the value obtained by the Delft Cr-Mo-Au flux monitor.

A TL/OSL emission spectrometer extension of the Risø reader.

The integration of a low cost, compact sized spectrometer with the Risø reader is described. The luminescence light emitted by the sample is transmitted by an optical fibre onto a fixed entrance slit of a spectrograph. The light is measured with a high sensitivity 2048-element CCD-linear array detector (Avantes PC2000) sensitive in the UV-VIS region. The CCD array has a low readout noise and a photon sensitivity of 86 photons per count. Examples are given of both thermally and optically stimulated 3-D spectra showing the luminescence spectra as a function of temperature and time respectively. Spectra from CaF2:Tm (TLD-300) chips could be distinguished from the background (2 SD) at a 0.7 Gy dose level.