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The deuterium-tritium neutron generator should be fully shielded for the safety of the operators participating in the experiments since the D-T neutron generator is commonly used in activation experiments. In this study, MCNP5 code was used to simulate the shielding effect of the neutron thermalization device previously designed by our group with Pb and boron-containing polyethylene as the shielding material. The neutron dose rate outside of the previous thermalization device can not meet the requirement, so a concrete wall is needed between the device and the operators. Two models are designed with concrete walls. One model is that the device and the experimental operators are not in the same room, another one is that the device and the experimental operators are in the same room, and there is an L-shaped concrete wall between them. In both models, the dose rate to the operators was less than 5 μSvh-1.

Diverse phospholipid motions are key to membrane function but can be quite difficult to untangle and quantify. High-resolution field cycling 31 P NMR spin-lattice relaxometry, where the sample is excited at high field, shuttled in the magnet bore for low-field relaxation, then shuttled back to high field for readout of the residual magnetization, provides data on phospholipid dynamics and structure. This information is encoded in the field dependence of the 31 P spin-lattice relaxation rate ( R 1 ). In the field range from 11.74 down to 0.003 T, three dipolar nuclear magnetic relaxation dispersions (NMRDs) and one due to 31 P chemical shift anisotropy contribute to R 1 of phospholipids. Extraction of correlation times and maximum relaxation amplitudes for these NMRDs provides (1) lateral diffusion constants for different phospholipids in the same bilayer, (2) estimates of how additives alter the motion of the phospholipid about its long axis, and (3) an average 31 P- 1 H angle with respect to the bilayer normal, which reveals that polar headgroup motion is not restricted on a microsecond timescale. Relative motions within a phospholipid are also provided by comparing 31 P NMRD profiles for specifically deuterated molecules as well as 13 C and 1 H field dependence profiles to that of 31 P. Although this work has dealt exclusively with phospholipids in small unilamellar vesicles, these same NMRDs can be measured for phospholipids in micelles and nanodisks, making this technique useful for monitoring lipid behavior in a variety of structures and assessing how additives alter specific lipid motions.

High resolution (13)C NMR field cycling (covering 11.7 down to 0.002 T) relaxation studies of the sn-2 carbonyl of phosphatidylcholines in vesicles provide a detailed look at the dynamics of this position of the phospholipid in vesicles. The spin-lattice relaxation rate, R(1), observed down to 0.05 T is the result of dipolar and CSA relaxation components characterized by a single correlation time tau(c), with a small contribution from a faster motion contributing to CSA relaxation. At lower fields, R(1) increases further with a correlation time consistent with vesicle tumbling. The tau(c) is particularly interesting since it is 2-3 times slower than what is observed for (31)P of the same phospholipid. However, cholesterol increases the tau(c) for both (31)P and (13)C sites to the same value, approximately 25 ns. These observations suggest faster local motion dominates the dipolar relaxation of the (31)P, while a slower rotation or wobble dominates the relaxation of the carbonyl carbon by the alpha-CH(2) group. The faster motion must be damped with the sterol present. As a general methodology, high resolution (13)C field cycling may be useful for quantifying dynamics in other complex systems as long as a (13)C label (without attached protons) can be introduced.