° TO: “ere Wee + ae _ pe ORR “pis Cocmmczt contahss wINTER-GFFICE MEMORANLAM “ego "IGE wes) pare January Slst, 1947 2. . _ | tr Wright Langham 2 B =” 2a. eeet, “yl ee fetAes ar, GT eo. yo t FROM: SUBJECT: . .G, A. Linenberger | Neutron. results fron Crossroads measurements ‘ UNCLASSIFIED | - (we I am returning herein your letter fror H. Scoville and will take up in order the questions raised by him. Conoerningthe neutrons in the physiologically interesting re- gion from between ca. 1/10 and 2 Mev., reference should be made to my Crossroads Tech-. nical Instrumentation Report--Project iio. Vi1-2 (Los Alamos reference notation is LANS- 447), kp which the number of neutrons having energies greater than 5 Mev. is given for 4$solid angle as a function of distance.. If this curve (taking proper account of dis- tance équared factors) is compared with the phosphorous activity curve sent you by Scoville, 1 think a few qualitative considerations will indicate that the slope of the lat- -ter curve may be considered the upper limit for the slope of the number-distance curve for any neutrons of intermediate energies. «also, the number of these neutrons will not The be less for a given distance than the number indicated by the phosphorous curve. reasoning briefly runs as follows. Assume for the moment no captnre by nitrogen and oxygen in the air. The scattering mean-free-path of those neutrons for which phosphorous is of any value as a detector (i.e., slaw to thermal region) is sufficiently short as to require that they be "born" of higher energy neutrons at or near the point of capture by phosphorous; ieee, they do not emerge from the bomb with these energies. The “parents” _ of the "phosphorous neutrons” may then in a sense be considered as all those neutrons which the sulphur doss not see; and, were it not for capture, they would be equal in number to the parent néutrons. That the slope of the distritution curve for neutrons of any intermediate energy will be less than that of the phosphorous curve, is based on two con-. Siderations; (a) the usually reliable assumption that the scattering erogs section will decrease with increasing energies; and (b) the fact that the slowing down distance in- ' Greases with increasing energy. Applying (a) and (b) in reverse will serve to make plausi- ble that at the same tire the slope of the sulphur curve will be a lower limit for the slope of the number-distansce curve for neutrons of an intermediate energy. Here, however, no comparison can be made concerning the actual numbers of neutrons. In fact, it is rea#onable to expect that due to general degradation more neutrons emerge with energies less than 3 Mev. than with energies greater than this. MEO aL yey 7 + Foaa 2 - =e It is by no means a simple matter to make any sort of quantitative analysis of the energy spectrum of neutrons from the bomb; however, 1 am appending a memorandum by Holloway which may shed some light on the matter. : ‘The cross section for absorption of slow neutrons by phosphorous used by Scoville agrees with the generally accepted value of that quantity for thermal neutrons, and hence known about the variation of this cross section with energy so that 1 am at a loss to know wpst to suggest as a better value. § (n,p)P® On the other hand, the cross section for the reaction may be regarded as a step function rising very steeply at 5 Mev. to. ® constant value of about 0.4 barns (See Klema, los Alamos report LA-515}. As for the absorption of slow neutrons by steel, the thermal absorption cross section of Fe is about 2.5 barns; so that the capture mean-free-path or e-folding distance comes to 2.8 cm. -Fe 3 enolos.. cCAL3b jd Form 25 UNCLASSIFIED @. a. LINEL BERGER