Effects of neutron radiation as cosmic radiation on food resources

Abstract

One of the major challenges in deep space exploration are effects of cosmic rays on human body and soft error, but equally important are their effects on food resources. In this study, we focused on neutrons as the secondary particle in the deep space environment and investigated how they affect food resources. Fast neutron was irradiated by the RIKEN Accelerator-driven compact Neutron Source (RANS) on meat sample. Biochemical analysis was conducted to present concrete effects of cosmic rays on food in deep space environment. Oxidative and nitrative modifications of proteins were detected by the electrophoresis and Western blotting. The result shows that nitrative modification of tryptophan, which is an essential amino acid in protein, tended to increase with increasing neutron dose.

Keywords

Neutron irradiation cosmic ray deep space environment food oxidative and nitrative modification of proteins

1. Introduction

In recent years, it has become clear that the effects of secondary radiation produced by interactions with the shielding walls in deep spacecraft are significant [3–5]. In deep space, such as Moon and Mars, with no Earth’s magnetic field nor atmosphere the shielding walls of spacecraft must be thicker to protect the human body from cosmic rays of high energy ions with wide range of atomic numbers. Inside the spacecraft with aluminum shielding walls thicker than 20 g/cm², for cosmic rays with energies below 100 MeV the neutron flux is larger than fluxes of proton and other particles. The flux of Z<11 nuclei with energies less than MeV increases rapidly and the flux of neutrons with energies below 100 MeV is outstandingly large [3–5]. In our study, fast neutrons were irradiated by RANS where the maximum energy of neutron is around 5 MeV corresponding to the main secondary particle flux inside the spacecraft in deep space environment.

The effects of cosmic rays on humans and electronic devices have been studied for human space exploration, but food resources needed to sustain life have not yet been examined. Human body cannot produce the essential amino acids so human must obtain proteins containing essential amino acids from food. It is known that the oxidative modification of proteins can sometimes cause protein dysfunction. Tryptophan, which is an essential amino acid, is modified with reactive nitrogen species to become 6-nitrotryptophan (6NO₂Trp) [6]. 6NO₂Trp has been detected in mice and human with atopic dermatitis [2]. Therefore, we adopt the oxidative and nitrative modification of protein as a specific indicator of qualitative and quantitative cosmic ray effects. Irradiation of gamma rays and electrons on foods is used for sterilization and preventing germination, but neutron irradiation on food is rarely examined. In this study, we perform biochemical analysis to detect the oxidative modification of proteins in a food sample irradiated by neutrons. The pork shoulder meat was chosen as a food sample since it contains high protein content composed of abundant muscle tissue.

2. Method

The study has two components: a neutron irradiation by RANS and a biochemical analysis to evaluate concrete effects of cosmic rays on food in deep space. Pork shoulder meat was obtained fresh about 4 days after slaughter. After irradiation with neutrons, the meat was immediately flash frozen at −80°C with liquid nitrogen and stored on dry ice (−79°C) or −80°C freezer.

2.1. Neutron irradiation by RANS

The neutron beam was produced by the RIKEN Accelerator-driven compact Neutron Source (RANS) (Fig. 1). Beryllium (Be) with 300 μm thickness is the target material for neutron production via the ⁹Be(p,n)⁹B reaction. The accelerator of RANS has adopted a radio-frequency quadrupole (RFQ) and drift tube linacs (DTL). With incident protons of 7 MeV energy, the produced neutrons have a maximum energy of about 5 MeV. For this study, we built a sample container (Fig. 2) so that we can irradiate the food samples near the Be target (without moderator) with high energy neutrons (∼5 MeV) corresponding to cosmic radiation inside the spacecraft in deep space.

Fig. 1.

RANS: target station (TS), drift tube linacs (DTL), radio-frequency quadrupole (RFQ).

Fig. 2.

Aluminum sample container: meat sample (red), indium foil (silver) and breast milk pack (clear bag).

The Particle and Heavy Ion Transport Code System (PHITS) was used for the Monte Carlo simulation to design the experiments. The conditions of the Monte Carlo calculations are as follows (Fig. 3). The 7 MeV proton beam bombards a 300 μm thickness Be target. The 10 × 10 ×12 cm³ sample is placed 3 cm away from the target. The sample is set to be a soft tissue standardized by ICRU with number composition of H $6.08 \times 10^{- 2}$ , C $5.57 \times 10^{- 2}$ , N $1.12 \times 10^{- 2}$ and O $2.87 \times 10^{- 2}$ . The Monte Carlo simulation of the neutron and photon irradiations are presented in Fig. 4. The obtained neutron dose is $10^{- 17}$ Gy per proton which, multiplied by the proton current 50 μA, yields 0.0031 Gy/s. The photon contribution is $10^{- 1}$ less than the neutron contribution.

Fig. 3.

Condition of PHITS calculation: Be target (orange at z=0 cm), vanadium (blue) and soft tissue (green) in the target station and void (gray).

Fig. 4.

PHITS calculation of neutron (upper) and photon (lower). Dose is indicated from $10^{- 23}$ Gy (blue) to $10^{- 17}$ Gy (red).

Fig. 5.

Ge semiconductor detector to count γ emitted by activation of In foil.

Fig. 6.

γ spectrum detected by Ge semiconductor detector.

The neutron flux in the experiment was measured from the indium (In) activation using a 10 × 10 × 0.14 mm³ indium foil activated as ¹¹⁵In(n,n’)^115mIn. The 336 keV gamma-rays emitted by ^115mIn $\to^{115}$ In $+ γ$ were detected by the germanium (Ge) semiconductor detector (Fig. 5) as shown in Fig. 6. The neutron flux was deduced from the number of gamma rays using the cross section of 1 MeV neutron for ¹¹⁵In $σ = 0.2$ barn (JENDL-4.0 nuclear data library).

2.2. Biochemical analysis

Fig. 7.

Electrophoresis CBB staining (left) and WB with anti-nitrotryptophan antibody (right). (Preliminary result.).

Fig. 8.

Nitrative modification of tryptophan by neutron irradiation. (Preliminary result.).

Electrophoresis and Western blotting (WB) were performed in order to detect oxidative and nitrative modifications of proteins in meat. Protein extracts from neutron-irradiated meat samples was prepared. Electrophoresis (sodium dodecyl sulphate-polyacrylamide gel electrophoresis, SDS-PAGE) with Coomassie-Brilliant Blue (CBB) staining was performed to visualize proteins in the gel. Proteins on SDS-PAGE gels (non-stained gels) were transferred to PVDF (Poly Vinylidene Di-Fluoride) membranes on a semi-dry system. The transferred membranes reacted with anti-6-nitrotryptophan monoclonal antibody and alkaline phosphatase-conjugated secondary antibody. The band signal on the membrane treated with chemiluminescent substrate was detected by CCD camera and the band signal intensity was analyzed using Image-J software [1].

3. Results

Neutrons were irradiated to pork meat in the range of 0.01 Gy to 3 Gy, corresponding to a proton beam current of RANS with 5.7 μA for 30 s to 34.4 μA for 1270 s. By adjusting the proton current and irradiation time, the radiation dose plan in the logarithmic scale (0.01, 0.03, 0.1, 1.0, 3.0) was achieved. The measurement of neutron flux by the indium activation analysis shows the linear correlation with the PHITS calculation values. This confirms the justification of the parameters for the neutron irradiation experiment by RANS.

The preliminary results of the biochemical analysis are shown in Fig. 7: The left figure shows proteins separated with SDS-PAGE visualized by CBB staining. No difference in the electrophoresis pattern of the proteins was observed upon neutron irradiation. This shows that neutron irradiation does not alter the composition of protein components. The right figure shows the results of WB using mouse antibody specific for nitrative modification of tryptophan residues in proteins. Some band signals appear to increase with increasing neutron fluence. Figure 8 shows the semi-quantification using Image-J of the band about 110 kDa in WB. The increase of this nitrative modification was observed with the increase of radiation dose up to 1 Gy. Interestingly, the amount of modification seems to saturate around 1 Gy irradiation dose.

4. Discussions and conclusions

Pork meat samples were irradiated with neutrons in the range of 0.01 Gy to 3 Gy while monitoring the neutron beam with the activation of indium. A correlation between neutron dose and modifications of proteins was observed. As a preliminary result, nitrative modification of tryptophan which is an essential amino acid in protein increased with increasing neutron dose. An interesting behavior is that the amount of modification seems to saturate around 1 Gy irradiation dose. Experiments to confirm reproducibility and verification are underway. The elementary process of oxidative and nitrative modification of proteins by neutron irradiation needs to be clarified.

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