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Lewis 1 has recently pointed out that a thicktarget yield curve taken at a narrow resonance with a charged particle beam of high-energy resolution should have a maximum just above the resonance energy followed by a shallow minimum, before it assumes a constant value. The qualitative explanation for this phenomenon, which the authors have chosen to call "the Lewis effect, " is as follows: In passing through target material a charged particle loses energy in discrete steps Q. If some of these steps are larger than the natural width of a narrow resonance, some of the particles incident on a target at an energy well above the resonance energy, E^, will jump over the resonance. If particles are incident at E^ then all will have for a finite time the correct energy to interact. The yield curve should therefore exhibit a peak near E^.Gamma-ray yield curves from the Al 27 (p,y)Si 28 resonance reaction at 992 kev were studied with a proton beam having an energy spread at half maximum of about 125 ev. The natural width of this resonance is 80 ±40 ev. 2 Yield curves were initially taken from aluminum films formed in the target chamber by evaporation of aluminum from a tungsten filament. A peak in the yield curves appeared but could not be reproduced. This was interpreted as being due to the accumulation of surface deposits on the target. A new method of target preparation was developed 3 in which aluminum is slowly and continuously deposited on the target backing while taking data. Figure 1 shows several yield curves exhibiting the Lewis effect. Curve A is a yield curve calculated as outlined below. The data labelled B and C were taken during continuous evaporation of aluminum. Curves D and E were initial runs from two different targets prepared by filament evaporation; subsequent runs in both cases failed to reveal the Lewis peak.The yield per proton at a mean beam energy E^ from an infinitely thick homogeneous target is given by ..<>*<***}where n^ is the number of aluminum atoms per unit volume, g(E b ,E i )dE i is the probability that a proton in the beam of mean energy E b has an incident energy between E^ and Ej +dE^

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