1938, the neutron generator at 600 kV acceleration potential in the Philips NatLab in Eindhoven. From 1936 Daniel den Hoed carries out tests with the neutron beam. Image Philips Company Archives.
The vertical panel in the centre, on which the vacuum pump is mounted, is earthed. Left of this lies horizontally the first acceleration tube to 300 kV (this was insulated in glass here), right the second section (with "Philite", the Philips version of Bakelite insulation). The actual neutron source at the right hand end is shielded by paraffin blocks for protection of the researcher from the neutrons. On the far left can be seen the source current generator at 50 kV, and behind the machine stand the two cascade type high voltage generators, each generating 300 kV. In the cascade of 2 acceleration tubes, ionized heavy hydrogen nuclei (deuterons) are accelerated to 600 kV and fired onto a plate of lithium, by which a reaction takes place: Li + d ~ 2He + n. The production of the machine for 600 kV is 6·109 neutrons per second .
Through the collaboration with Philips NatLab, Daniel den Hoed is able to carry out experiments with the neutron generator which was built in 1936 by researcher F.A. Heijn . The expectations are that neutrons will be especially important in medical applications; in the irradiation of tumours and for the production of short-life isotopes for research and treatment. From the experiments of Den Hoed in the Philips NatLab in Eindhoven, it appears that the neutrons have a similar effect on organic tissues as X-ray radiation*. However, there is some difference. While the X-rays work through the ionization which they cause within the tissue through their exchange with the electrons already present in the tissue, neutrons cause ionization as they collide and transfer energy into the hydrogen-atom nuclei present in the tissue which in turn cause ionization in the tissue. In the latter case, the ionization is much more local, while the ionization by X-rays is more homogeneous. It can also be expected that with neutrons deeper situated tissues can be irradiated .
Den Hoed apologises for the fact that he limits his work to some exploratory measurements. A busy work schedule does not permit extensive experiments. He arrives at the qualitative observation that neutrons will kill Drosophila eggs depending on the dosage and this effect appears to be stronger than that seen with X-ray radiation at the same ionizing strength . Based on other published experiments, Den Hoed assumes that neutron irradiation, at equal ionizing effect, damages tumour tissue more than the surrounding healthy tissue at a ratio of 4:3.
*This was presented at the 5th International Congress of Radiology, Chicago, 13-17 September, 1937