Much recent biological research supports the view that close range interactions, of the order of nanometers, can be a dominant factor in the production of biological damage from ionizing radiation. Until now, energy distributions at this level could be calculated and inferred, but could not be measured experimentally. A new instrument, designed and developed in this Center displays the capability of achieving this goal. This newly designed wall-less proportional counter is in process of construction. The design is based on a previously produced conventional walled counter called the UMC (ultra-miniature counter) which has been shown to be successful in the measurement of single-event energy deposition distributions at simulated tissue equivalent sizes of about 10 nm, far lower than any previously achieved. The new wall-less design will provide much more accurate spectra for those high LET radiations whose spectra are most distorted by the so-called wall effect, such as neutrons. This counter, unlike the UMC, can be calibrated accurately. It will then be used to provide the first experimental determination of neutron ionization distributions in nanometer-size sites. With this counter it will be possible to do measurements on the full range of monoenergetic neutrons available at the RARAF facility, including the new low energy neutron facility, at site sizes 10 nm and larger. Measurement of photoneutron contamination from pulsed high energy radiotherapy beams is also possible. These measurements can in turn supply a solid experimental base for application of risk assessment in medical and health physics applications.
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