a ’ SPECTRUM OF NEUTRONS FROM EXPLOSIONS ANCLASSIFED a 6. solloway Z 29 January 1947 — vis whether the spectrum of I understand there is some question as t9 holes in it. neutrons is a smooth function or has peaks and I believe some light ct of some magnitude. can be shed on this point, but anything further is @ proje g a maximum at The fission neutrons spectrum is a smooth function of energy havin about 1.8 Mev and extending, with small ordinate, up to 11 Mev. Collisions of the neutrons with matter after the explosion will decrease the average energy of the neutrons. If there should be a strong resonance scattering or absorption in the air atoms, then the spectrum need not necessarily remain a smooth function. In oxygen there are several resonances in the scattering cross-section; at about 5.€ hiev (3 barns over a background of 1 barn), at 0.9 Mev (6 barns over a background of 2 barns) and a rapidly rising cross-section towards low energies (about 6 barns at 0.4 Mev). The vresencs of a scattering resonance which has a cross-section say twice that on either side of the resonance leads to the condition that there are fewer neutrons existing at the energy of the resonance, since neutrons are scattered out of that energy region twice as fast as tly are scattered into it. a “hole” in the spectrum at the energy of the resonance. Thus, one would expect ‘iowaver, one should not expect any sharp holes, since the resonances are fairly broad and the energy lost per collision is not a definite fraction but depends upon the angle of collision. In nitrogen there are absorption resonances at 1.45 Mev (0.085 barns over background of 0.01 barn), at C.7 Mew (0.12 barn over 0.01 barn) and at 0.55 Mev (0.06 barn over 0.005 barn. These absorption resonances ure not strong enough to appreciably change the spectrum, since only 1/10 of the neutrons see these resonances; the average scattering cross-section is about 10 times greater than the resonance cross-section. YNCLASSIFTED