SummaryThe cross section of the reaction 6Li(n,3H)'He has been measured, with good energy resolution, over the neutron range 2·0--2·65 MeV. This corresponds to an excitation of 7Li from 9· 0 to 9· 55 MeV, a region in which photodisintegration experiments indicate the presence of one or more levels. No resonance structure was observed in this experiment and possible reasons for this are discussed. Kern and Kreger 1958). These results show that, with the exception of the broad p-wave resonance at 255 keV, no resonance structure is apparent over the whole of the measured energy range. This is surprising as several fairly well-established levels are known to be present in the compound system, 7Li*. " In the experiments reported here, the cross section of 6Li(n,3H)4He was measured, with good energy resolution, over a neutron energy range 2 ·0-2 . 65 MeV; this corresponds to an excitation of the compound 7Li system from 9·0 to 9 ·55 MeV where a fairly well-established level of 7Li is known to be present (Ajzenberg and Lauritsen 1955). From a knowledge of the neutron and triton partial widths which have been established for this level by y-ray absorption experiments it would appear that, to an order of magnitude, the resonance cross section to this level by neutron absorption in 6Li should be several hundred millibarns. The experimental techniques used here were Sufficiently sensitive so that a 10 per cent. change in the cross-section curve having a 40 keV half-width should be readily detected.

The energy spectrum of the protons which are emitted when nickel is bombarded with 14 ·1 MeV neutrons has been studied recently by several workers-Allan (1957), Colli, Facchini, et al. (1958), and Colli, Pignanelli, et al. (1958). The results of these experiments do not allow an accurate estimate of the relative importance of contributions from the (n,p) reaction and the (n,np) reaction. Such information is necessary when any reaction mechanism is postulated to account for the experimental results. In these experiments, therefore, activation techniques have been used to measure these two cross sections separately for the isotope 58Ni at 14·1 MeV; in addition, the 68Ni(n,2n)57Ni cross section has been remeasured.Nickel foils were irradiated with monoenergetic neutrons produced by the 8H(d,n)4He reaction, the neutron flux being measured by counting the associated oc.-particles emitted into a known solid angle. During irradiation, the nickel foils were placed 3·5 cm from the centre of the target. However, because the deuteron beam wanders slightly over the surface of the tritium target during an irradiation, an estimate of the flux through the sample based on the distance from the centre of the beam is unreliable; for this reason, copper disks were placed on either side of the irradiated nickel foil and the induced 65Cu(n,2n)64Cu activity compared with the same activity in disks which were placed 15 cm from the neutron source. This technique allowed the integrated flux at the specimen to be measured with an accuracy better than ±7 per cent.58CO formed by the (n,p) reaction during irradiation decays to an excited state of 58Fe by positron emission and by the electron capture process. The k/~+ branching ratio has been measured accurately by Good, Peaslee, and Deutsch (1946), and their value of 5 ·89 was used here. The number of 58CO atoms produced during the irradiation was estimated by counting the positron annihilation quanta with a scintillation spectrometer, allowance being made for self-absorption in the nickel foils. The geometry of the counter was eliminated by calibrating the spectrometer with a standardized 22N a source, and the counting errors were estimated to be less than 12 per cent. Three independent measurements of the cross section were made, and the weighted mean of these gave 58Ni(n,p)58Co (14·1 MeV) =(560 ±110) X10-27 cm 2 .* Manuscript