In Part I of this thesis, a new magnetic spectrometer experiment which measuredthe β spectrum of ^(35)S is described. New limits on heavy neutrinoemission in nuclear β decay were set, for a heavy neutrino mass range between12 and 22 keV. In particular, this measurement rejects the hypothesisthat a 17 keV neutrino is emitted, with sin^2 θ = 0.0085, at the 6δ statisticallevel. In addition, an auxiliary experiment was performed, in which anartificial kink was induced in the β spectrum by means of an absorber foilwhich masked a fraction of the source area. In this measurement, the sensitivityof the magnetic spectrometer to the spectral features of heavy neutrinoemission was demonstrated.
In Part II, a measurement of the neutron spallation yield and multiplicityby the Cosmic-ray Underground Background Experiment is described.The production of fast neutrons by muons was investigated at an undergrounddepth of 20 meters water equivalent, with a 200 liter detector filledwith 0.09% Gd-loaded liquid scintillator. We measured a neutron productionyield of (3.4 ± 0.7) x 10^(-5) neutrons per muon-g/cm^2, in agreement withother experiments. A single-to-double neutron multiplicity ratio of 4:1 wasobserved. In addition, stopped π^+ decays to µ^+ and then e^+ were observed aswas the associated production of pions and neutrons, by the muon spallationinteraction. It was seen that practically all of the π^+ produced by muons were also accompanied by at least one neutron. These measurements serve as the basis for neutron background estimates for the San Onofre neutrino detector.
