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
• low dose rate
• radiation
• high LET

Acknowledgments

The authors thank Dr. Robert L. Ullrich of the Radiation Effects Research Foundation, Hiroshima, Japan, for helpful discussions on low dose rate neutron exposures. We thank Dr. Adam Rusek of the NASA Space Radiation Laboratory and Drs, John Norbury, Tony Slaba, Steven Blattnig of the NASA Langley Research Center, for reviewing the dosimetry protocol. We acknowledge the support of James Abraham and the CSU Radiation Control Office in providing assistance relating to licensing and regulatory compliance.

Disclosure statement

The authors report no conflicts of interest.

Additional information

Funding

This work was supported with federal funds from the National Aeronautics and Space Administration under grant NNX15AK13G. Addition funds were provided by the Department of Environmental and Radiological Health Sciences, College of Veterinary Medicine and Biosciences, and Office of the Vice-President for Research, Colorado State University.

Notes on contributors

Thomas B. Borak

Thomas B. Borak, is a Professor Emeritus in the Department of Environmental and Radiological Health Sciences at Colorado State University.

Laurence H. Heilbronn

Laurence H. Heilbronn, is a Professor in the Department of Nuclear Engineering at the University of Tennessee Knoxville.

Nathan Krumland

Nathan Krumland, is an undergraduate student in the Department of Chemical and Biological Engineering at Colorado State University.

Michael M. Weil

Michael M. Weil, is a Professor in the Department of Environmental and Radiological Health Sciences at Colorado State University and Director of the NASA Specialized Center of Research on Carcinogenesis.