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/E^1+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/E^1+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/E^1+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.

Neutron activation of holmium poly(L-lactic acid) microspheres for hepatic arterial radio-embolization: a validation study

Poly(L-lactic acid) microspheres loaded with holmium-166 acetylacetonate (¹⁶⁶Ho-PLLA-MS) are a novel microdevice for intra-arterial radioembolization in patients with unresectable liver malignancies. The neutron activation in a nuclear reactor, in particular the gamma heating, damages the ¹⁶⁶Ho-PLLA-MS. The degree of damage is dependent on the irradiation characteristics and irradiation time in a particular reactor facility. The aim of this study was to standardize and objectively validate the activation procedure in a particular reactor. The methods included light- and scanning electron microscopy (SEM), particle size analysis, differential scanning calorimetry, viscometry, thermal neutron flux measurements and energy deposition calculations. Seven hours-neutron irradiation results in sufficient specific activity of the ¹⁶⁶Ho-PLLA-MS while structural integrity is preserved. Neutron flux measurements and energy deposition calculations are required in the screening of other nuclear reactors. For the evaluation of microsphere quality, light microscopy, SEM and particle size analysis are appropriate techniques.

A new monitor for routine thermal and epithermal neutron fluence rate monitoring in k0 INAA

The Zr-Au set for monitoring the thermal and epithermal neutron fluence rate and the epithermal spectrum parameter alpha is not always practicable for routine application of INAA in well-thermalized facilities. An alternative set consisting of Cr, Au and Mo provides values for the thermal neutron fluence rate, f and alpha that are not significantly different from those found via the Zr-Au method and the Cd-covered Zr-method. The IRMM standard SMELS-II was analyzed using the (Au-Cr-Mo) monitor and a good agreement was obtained.