ABSTRACT
Figure 12.1 Excitation function for the neutron capture reaction 197Au(n,g)198Au. The oscillations of the reaction cross section in the energy range between 5 and 5000 eV are referred to as resonances (Data from JEFF 3.1.1 (OECD-NEA 2009)).Since neutron capture occurs preferentially at low enough neutron energies, En, there is little momentum transferred to the target nucleus, and it can generally be assumed that the activated nucleus remains very close to its original lattice position [4, 8]. This implies that the range of the recoiling activation product is much shorter than it is in the case of proton or deuteron activation. Therefore, NPs may be directly labelled by nuclear transmutation that occurs in the volume of the radiolabelled NP itself, rather than by recoil implantation of an activation product in a second NP, which is typical for proton or deuteron activation [4]. This may allow radiolabelling of NPs in liquid suspension, which may be impossible in the case of irradiation with light ions as the recoiling radiolabels would be lost in the liquid medium rather than becoming implanted in a nearby NP [4]. Radiolabelling of NP suspensions exposed to a neutron flux facilitates cooling, which is hampered in dry proton or deuteron irradiated powders by their poor thermal conductivity [11]. On the other hand the damaging effect of the high neutron and g-ray radiation dose prevailing in a nuclear reactor needs to be considered very carefully in the planning of a neutron activation of a NP batch. For example neutron activation of poly-lactic acid Ho-core microparticles (diameter 37 μm) in a reactor indicated the need of irradiating the particles in dry conditions and for limited time and neutron flux
to avoid significant alteration of the particle characteristics [12]. Nevertheless, whenever NPs may be activated in liquid suspension, this may provide a significant advantage over labelling techniques using exposure of NPs to light ion beams. 12.2 Activation Methods and Practical
Activation in research reactors offers high neutron fluxes and can lead to high activity concentrations in the NPs of the order of 1 MBq/mg rather easily [1-3]. The activity can be adjusted by modifying exposure time, i.e. the time the material stays in its position in the reactor. Usually, research reactors offer different irradiation positions within the reactor core, that differ by the neutron spectrum and the total flux density that can be obtained locally. A look at Fig. 12.1 emphasises that the most important contribution to the activation is made by thermal neutrons (in the range between 0.01 and 0.1 eV). It is therefore also obvious that the effect of a neutron irradiation can to a certain extent be adjusted by the irradiation position in the reactor, which affects the total neutron flux and the energy spectrum of the neutrons. An example is given in Fig. 12.2. It is shown that the position of highest total flux in the centre of the reactor core has approximately the same neutron fluxes for “thermal” (En ≤ 0.55 eV), “epithermal” (0.55 eV ≤ En ≤ 0.1 MeV) and fast (En ≥ 0.1 MeV) neutrons. At the expense of the total neutron flux available, the contribution of ‘epithermal’ neutrons can be minimised in the position of the so-called fast transfer thimble, and in the position of the so-called rotary specimen rack the contribution of fast neutrons can be minimised.In any case there is a high dose rate from neutron and g-radiation, which may destroy some NP types (e.g. organic or organic coated). Therefore, this aspect has to be considered thoroughly when planning an irradiation. It may limit the achievable activity concentration as it limits exposure time for organic components, and may even completely exclude the option of neutron activation in a reactor in some cases.
