Focalizing thermal neutron beams at and below micron scales: discussion on BNCT

INTRODUCTION. Hollow neutron guides (with transverse dimension the order of cms.) are routinely used to conduct slow (cold, thermal, epithermal) neutron beams along several tens of m. At later stages, actual guides could be... [ view full abstract ]

INTRODUCTION. Hollow neutron guides (with transverse dimension the order of cms.) are routinely used to conduct slow (cold, thermal, epithermal) neutron beams along several tens of m. At later stages, actual guides could be employed to focalize the beams onto smaller transverse scales ( say, down to mm) for various applications: for instance, BNCT. Due to the growing research and technologically relevant areas in which slow neutrons are employed, one may suppose that, besides supermirrors, there will be further increasing improvements on neutron focusing in the future (for instance, for non-destructive analysis of materials and in the framework of accelerator-based and spallation sources). Motivated by that, and concentrating on beam focalization onto shorter scales, we remind the following experiments establishing confined propagation of slow neutrons in: a) thin films (Y. P. Feng et al., Phys. Rev. B49 (1994), 10814), after previous proposals, b) thin polycapillary glass fibres or PGF (H. Chen et al., Nature, 357 (1992), 390), leaving aside previous independent proposals based upon thin circular waveguides (say, Ti fibres).

METHODS. One important limitation of both a) and b) is that, typically, the transmitted confined slow (say, thermal) neutron flux is smaller than the incoming (unconfined) one by about 10^-3 to 10^-4. In a), a thin Titanium (Ti) planar waveguide (specifically, a TiO₂/Ti/Si layer) with thickness 120 nm along about 20 mm was considered. We outline a proposal for confined thermal neutron propagation with transverse sizes down to about 0.1 μm along longer distances, about tens of cm, by modifying adequately a) (upon coupling the Ti layer to a suitable thin planar film). 

RESULTS. One can estimate roughly that the proposed device could transmit (along the longer distances indicated above) about 10⁵ to 10⁶ neutrons/cm²s coming from, say, a suitable accelerator-based source (yielding 10⁹ neutrons/cm²s). 

DISCUSSION. We also entertain proposals by combining a) and b), namely, by suitable coupling of thin Ti films to a polycapillary glass fibre.Possible relationships (consistent with the above flux limitation) of those proposed arrangements to accelerator-based BNCT for small tumours (say, those requiring smaller neutron fluxes), below the mm scale, are discussed.