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 MDDC - 953 
 
 UNITED STATES ATOMIC ENERGY COMMISSION 
 
 A SIMPLE DEMONSTRATION OF RESONANCE SCATTERING 
 
 by 
 
 Alexander Langsdorf, Jr. 
 Wayne Arnold 
 
 Argonne National Laboratory 
 
 This document consists of 2 pages. 
 Date of Manuscript: May 1, 1947 
 
 Date Declassified: May 7, 1947 
 
 This document is for official use. 
 
 Its issuance does not constitute authority 
 
 for declassification of classified copies 
 
 of the same or similar content and title 
 
 and by the same author(s). 
 
 Technical Information Division, Oak Ridge Directed Operations 
 Oak Ridge, Tennessee 
 
 I 
 
 Di 
 
 U.S. DEPOSlTORv 
 
A SIMPLE DEMONSTRATION OF RESONANCE SCATTERING* 
 
 By Alexander Langsdorf, Jr. and Wayne Arnold 
 
 The Breit-Wigner theory predicts that at a neutron absorption resonance there should also be a 
 large scattering cross section and that the ratio of the peak cross sections at resonance should be in 
 the ratio of the widths P ^ and Fy. For the well-known resonances, such as those in gold, silver, and 
 indium, the neutron width I^ ^is much smaller than the gamma width, F so that the scattering 
 intensity expected to be found is quite weak. However, we have demonstrated the existence of the 
 scattering resonance, both by activation of foils and by use of a BF3 filled ion chamber detector, using 
 neutrons scattered from a beam brought out of the Argonne heavy-water pile. The results shown in 
 the table of data were obtained in the fall of 1944. They demonstrate that in the three elements tested, 
 the scattering resonance has a magnitude of at least 100 times 10"^^ cm^/atom, or over 10 times the 
 normal scattering cross sections found for these elements. 
 
 Data on the values of r ^ and F ^ obtained by modulated cyclotron velocity spectrometers indi- 
 cate that r may be expected to be 10 to 100 times F ^, which gives expected peak values of " gcat' 
 even larger than those found here. 
 
 Unpublished work similar to that here reported has been carried out since this work was done. 
 In May 1945, R.B. Sawyer, J. Blair, and E.O. WoUan issued a report (CP-3498) on a measurement 
 and analysis of the scattering cross section at the 1.44-ev indium resonance, which indicated a peak 
 scattering cross section of about 1100 x 10"^'' cm^/atom, and in January 1947 E. Fermi and L. 
 Marshall found 1000 x 10"^* cm^/atom for this resonance and about the same for the 5.2-ev silver 
 resonance (CP-3750). 
 
 In spite of the strength of the beam from the piles, all the above work suffered from fairly high 
 background intensities from neutrons scattered by air or which leaked into the detector in spite of 
 attempts to shield it. This, combined with the great loss in intensity due to the small solid angle of 
 the scattered neutrons picked up by the detectors, leads to great inaccuracy in the results. 
 
 Further improvement in technique is necessary and valuable, because if sufficient accuracy can 
 be attained in determining the resonance energy, F ^ 3^"^ ^ y> then the spin of the compound nucleus 
 should be evaluable with certainty. 
 
 That is, given a knowledge of 
 
 c'so/o'ao = Fn/ ry= ^ ' ^^^ 
 
 ^ao = "^ 
 
 1± 1/(21+ 1) 
 
 P/(l + P ) 
 
 and the undetermined sign of the spin term can be evaluated, if tt-k] and cr^o are known well, along 
 with P determined from the scattering experiments. 
 
 ♦ Paper presented at meeting of American Physical Society, Washington D. C, May 1-3, 1947 
 
 MDDC - 953 [1 
 
2] 
 
 MDDC - 953 
 
 In the paper next to be prepared by Fred Seidl, we are reporting work recently resumed in this 
 field following the discovery by Goldhaber of the remarkably strong resonance scattering by Mn. The 
 same difficulties in obtaining accuracy enough to define the sign of the spin term persist in this other 
 type of resonance. 
 
 It is interesting to note that in the absorption resonances a^^ only approaches its maximum 
 possible value when rjj~ Fy, at T^ = ry,aao = Tf- [l i 1/(21 + 1)] .^, while the scattering 
 approaches closely its maximum value of tiK [l ±. 1/(21 + 1)] whenever T „ » ^y This circumstance 
 makes the probability of finding extremely strong absorption resonances less likely that that of find- 
 ing extremely strong scattering resonances. Now that one such scattering resonance is known in 
 manganese, it will be of interest to find how common such scattering resonances may be. They may 
 turn out to be of fairly frequent occurrence. 
 
 Table 1 . Transmission of absorbers for scattered radiation 
 detected by BFj counter. 
 
 Absorber 
 
 
 Scatterer 
 
 
 (g/cm=) 
 
 
 (g/cm^) 
 
 
 
 Ag 0.042 
 
 
 C 0.079 
 
 0.13 Ag 
 
 0.74 i 0.07 
 
 
 0.976+ 0.015 
 
 1.04 Ag 
 
 0.61i 0.07 
 
 
 0.84 ± 0.02 
 
 0.1 B 
 
 0.71+0.10 
 
 
 0.64 ±0.03 
 
 0.42 In 
 
 0.79 ± 0.09 
 
 Scatterer 
 
 (g/cm^) 
 
 0.72 + 0.03 
 
 
 Au 0.26 
 
 
 C 0.079 
 
 0.26 Au 
 
 0.82+0.04 
 
 Scatterer 
 
 (g/cm=) 
 
 0.95+0.03 
 
 
 In 0.023 
 
 
 C 0.079 
 
 0.42 In 
 
 0.45+ 0.14 
 
 
 0.72 + 0.03 
 
 Table 2. Relative counts from detectors irradiated by scattered 
 neutrons in right angle geometry. 
 
 Detector 
 
 (g/cm^) 
 
 
 Scatterer 
 
 (g/cm^) 
 
 
 Air and 
 "leakage" 
 
 
 In 0.014 
 
 
 Ag 0.031 
 
 C 0.12 
 
 
 0.097 In 
 0.024 Ag 
 
 320 
 158 
 
 
 86 
 830 
 
 (1000) 
 (1000) 
 
 320 
 360 
 
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UNIVERSITY OF FLORIDA 
 
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