TABLE I. Results for (y, n) reactions in O 18 and N 14 . QU N" Peak (Mev) 24.2 24.2

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S OME progress has been made toward the production of an approximately monochromatic meson beam with the 340-Mev protons from the Berkeley 184-inch synchro-cyclotron.Owing to the large peak at about 70 Mev in the spectrum of T + mesons produced at 0° by protons on protons, 1 hydrogen is peculiarly suited for use in a meson production target. Many more mesons are produced per nucleon in an 8-or 10-Mev interval from hydrogen than from any other material that has been studied. 2 A convenient target, rich in hydrogen and easy to handle, is polyethylene (CH 2 )n. Because of the hydrogen, the production from CH2 for a thin target is strongly peaked 3 at about 69 Mev. (See curve I, Fig. 1.) A 5-cm thick piece of CH2 was used as a meson production target. One would expect that such a thick target would broaden the peak considerably. This, however, is not the case. With a 5-cm CH 2 target, a 69-Mev meson created at the front of the target by a 340-Mev proton loses 15 Mev in the target and emerges with 54 Mev, whereas at the back of the target the protons have an energy of 323 Mev and produce mesons with a peak energy of 54 Mev.The experimental arrangement was designed with a meson scattering experiment in mind. For this purpose it was desirable to use the external proton beam of the 184-inch cyclotron. This beam emerges from a 2-inch diameter collimator and has at present a maximum intensity of about 6X10 -10 amp. 55 60 65 MES0M ENERGY (MEV) FIG. 1. The effective differential cross section per Mev per CH2 molecule per incident proton for the production of mesons in the forward direction by 340-Mev protons. Curve I: thin (&-in.) CH2 target. Curve II: 5-cm CH2 target. MAGNET . POLE PIECE 7 POLYETHYLENE FIG. 2. Arrangement of the apparatus in the magnetic field.Since the mesons come off from the CH 2 target predominantly in the forward direction, they must be separated from the proton beam. This is achieved with a magnetic field of 14,300 gauss across a gap of 3.4 inches, which gives a 54-Mev meson and a 340-Mev proton radii of curvature of 12.4 inches and 80 inches, respectively.In order to collimate the meson beam, and also to eliminate all particles other than mesons in the desired energy interval, a channel for the mesons was cut in the brass shielding (Fig. 2). The channel accepts mesons with energies between 48 and 60 Mev. A 54-Mev meson produced at 0° in the center of the target is turned through 85° before emerging from the center of the channel. The exit area of the channel is 2 inches wide by 2.5 inches high, subtending a solid angle of about 2.2X10 -2 steradian at the CH 2 target. With this arrangement the meson beam collimation was such that 0^0 2^9°.The meson energy spectrum was measured by means of nuclear emulsions embedded in absorbers. 2 Curve II of Fig. 1 shows the spectrum of the mesons leaving the 5-cm CH2 target in the forward direction. To obtain this curve, the meson density in the emulsions was corrected for (a) the loss of about 9 percent of the mesons due to decay in flight and (b) the loss of abo...

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