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I N measurements of total cross sections for highenergy neutrons, detectors accepting neutrons of relatively wide ranges of energy have been used. By utilizing the known spectrum of the neutron beam and the threshold acceptance energy values of the detectors an effective energy has been stated to about 2 percent. 1 A scintillation counter telescope allows the use of considerably narrower energy acceptance bands in the determination of total cross sections. 2 The increased resolution of such a telescope makes it a suitable instrument to analyze the variation of total cross sections as a function of energy as well as permitting a more precise determination of the effective energy.By means of such a telescope, a good-geometry attenuation experiment has been performed with neutrons produced from a J-in. beryllium target bombarded by the 110-Mev internal proton beam of the Harvard 95-in. synchrocyclotron. 3 The neutron beam, collimated by pipes of rectangular cross section (so chosen to give maximum counting rate for a given energy resolution), was first monitored by a triple coincidence scintillation counter telescope of neutron threshold energy approximately 35 Mev (Fig. 1). Then the neutron beam, attenuated by samples of various elements, passed through a second rectangular collimator and its intensity was determined by another scintillation counter telescope. The second telescope (which counted exchange protons following the conventional polyethylenecarbon subtraction method) consisted of 8 plastic scintillators mounted on 1P21 photomultipliers. Five, six, seven, and eightfold coincidences were used, with the resultant energy bands determined by range differences.The values of the cross sections are listed in Table I . 4 The cross section of hydrogen was determined from a polyethylene-carbon subtraction, of deuterium from a heavy water-ordinary water subtraction and of oxygen by an ordinary water-hydrogen subtraction. It is to be emphasized that the energies quoted and the errors given are the absolute and not the effective values. The FIG. 1. Geometry of the experiment.TABLE I. Total cross sections for neutrons. (10~2 7 cm 2 ). Neutron energy (Mev) Pb Fe Si Al 0 c D H Neutron energy (Mev) Pb C Neutron energy (Mev) Pb 94.8±1.