The interaction of neutron beams with3He gas is of interest for nuclear pumped lasers. The effects of spectral dependence of the neutron beam, neutron attenuation in the gas‐filled laser tube, and transport of the charged‐particle3He(n,p)3H reaction products are treated in detail. An expression for the energy density as a function of position within the tube, tube radius, operating pressure, and neutron fluence is given. The maximum energy density within the optical cavity is achieved when the tube radiusa(cm) is given bya=3.26/PwhereP(atm) is the operating pressure. The variation of radius by 50&percent; above and below optimum will change the energy density at most by 10&percent;, although performance degrades quickly for radii outside this range. If the optimum tube radius is used for each operating pressure, then the power density on the centerline (kW/cm3) is given as &xgr;CL=9.3×10−18Pf0in a thermal neutron environment off0(n/cm2 sec).