Proceedings of the 15th workshop on the mass of the electron neutrino

This booklet carries eight talks and three appendices presented at the 15th workshop on the mass of the electron neutrino held on January 10, 1991, at National Laboratory for High Energy Physics (or KEK), Japan. The workshop focuses on recent studies on mνe, studies and comments on 5p-3s line of Dy atom, measurement of the number of incident photons in mνe-PF experiments, measurement of absorption cross section of Dy atom for soft X-ray, evaluation of σiα of Dy atom, λN/λM experiments (synthesis of Ho(fod)3, measurement with heated proportional counter), and monochromator for soft X-ray in the range of 1.0-1.6 keV. In particular, discussions for mνe-PF experiments are made focusing on the development of a high-performance monochromator for soft X-ray in the range of 1.0-1.6 keV for measuring the fluorescent X-ray spectra of SEd, SEe and SEf. It is still nuclear whether the deterioration in a beryl crystal monochromator is associated with temperature changes or radiation damage, but cooling equipment for tests under decreasing temperature is now being constructed at KEK. Performance tests of a heated proportional counter for non-active Ho(fod)3 and the production of active Ho(fod)3 are also planned to be conducted. (N.K.)