An aerospace inspection facility needs a well-collimated thermal-neutron beam of at…
PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Radiation interactions, detection and dosimetry · Nuclear reactions
The Nucleus Resolved, Ch Neutrons: the reagent with no barrier
Problem
An aerospace inspection facility needs a well-collimated thermal-neutron beam of at least 1.0×107cm−2s−1 at an image plane 2.0m from the source, to radiograph turbine blades continuously. Four options are considered: a man-portable 241Am-Be source of the largest practical activity, 100Ci, yield 2.2×106ns−1Ci−1; a 252Cf source of the largest practical mass before self-heating becomes unmanageable, 1.0mg, yield 2.3×1012ns−1g−1; a modern sealed-tube d+t generator yielding up to 1.0×1010ns−1; and a research reactor thermal beam port.
((a)) Treating each of the first three as an isotropic point emitter (an idealisation generous to them, since any moderator needed to thermalise their fast neutrons would only reduce the flux delivered further), estimate the fast-neutron fluence rate each could deliver at 2.0m, and state which source type, if any, can meet the requirement, explaining why.
((b)) With the chosen facility running, a radiographer must occasionally approach the shielded beam port with the shutter closed. An area monitor there reads a leakage thermal-neutron fluence rate of ϕ=1.0×104cm−2s−1. Using the tissue kerma coefficient for thermal neutrons, Kth=2.0×10−12Gycm2, and the radiation weighting factor wR=2.5 appropriate below about 1keV, find the equivalent dose rate at that position in μSvh−1, and the time to accumulate a 20μSv per-visit administrative control level.