Design and dosimetry of a facility to study health effects following exposures to fission neutrons at low dose rates for long durations

Abstract

Purpose: During extended missions into deep space, astronauts will be exposed to a complex radiation field that includes high linear energy transfer (LET) radiation from high energy, heavy ions (HZE particles) at low dose rates of about 0.5 mGy/d for long durations. About 20% of the dose is delivered by ions with LET greater than 10 keV/µm. There are sparse empirical data in any species for carcinogenic effects from whole-body exposures to external sources of mixed or high LET radiation at this level of dose rates. For the induction of solid tumors, acute exposures to HZE ions have been shown to be substantially more effective per unit dose than low LET exposures associated with photons. To determine the health effects of high LET radiation at space-relevant dose rates on experimental animals, we developed a vivarium in which rodents could be irradiated with Californium (²⁵²Cf) neutrons for protracted periods of time.

Materials and methods: The neutron source is a panoramic irradiator containing ²⁵²Cf located in a concrete shielded vault with a footprint of 53 m². The vault can accommodate sufficient caging to simultaneously irradiate 900 mice and 60 rats for durations up to 400 d at a dose rate of 1 mGy/d and is approved for extended animal husbandry.

Results: The mixed field fluence is a combination of neutrons and photons emitted directly from the source and scattered particles from the concrete walls and floor. Mixed field dosimetry was performed using a miniature GM counter and CaF₂:Dy thermoluminescent dosimeters (TLD) for photons and tissue-equivalent proportional counters (TEPC) for neutrons. TEPC data provided macroscopic dose rates as well as measurements of radiation quality based on lineal energy, y, and LET. The instantaneous dose rate from the source decreases with a half-life of 2.6 years. The exposure time is adjusted weekly to yield a total dose 1 mGy/d. The photon contribution is 20% of the total dose. The uncertainty in the delivered dose is estimated to be ±20% taking into account spatial variations in the room and random position of mice in each cage. The dose averaged LET for the charged particle recoil nuclei is 68 keV/µ.

Conclusions: We have developed a facility to perform high LET studies in mice and rats at space relevant dose rates and career-relevant doses using neutrons emitted from the spontaneous fission of ²⁵²Cf.

Keywords: Neutron dosimetry; high LET; low dose rate; radiation.