A high efficiency, low background detector for measuring pair-decay   branches in nuclear decay
  We describe a high efficiency detector for measuring electron-positron pair transitions in nuclei. The device was built to be insensitive to gamma rays and to accommodate high overall event rates. The design was optimized for total pair kinetic energies up to about 7 MeV. 
C. Tur, A. H. Wuosmaa, S. M. Austin, K. Starosta, J. Yurkon, A.   Estrade, N. Goodman, J. C. Lighthall, G. Lorusso, S. T. Ma...