U-Junction Ge(Li) Drift Detectors

A U-junction drifting configuration has been used which permits the fabrication of large volume, low-capacitance detectors in much less time than previously required. For example, it is now possible, with good material, to fabricate in approximately one week an 18-cm3 diode with 3-pF capacitance. The U-junction structure is partially wrapped around three sides of a parallelepiped of the desired dimensions. The drifting then proceeds toward the remaining side of the detector. At the completion of the drift, only a narrow strip of P-type material remains along one edge. The portion of the lithium junction used for the rapid drift is now lapped off, leaving the low-capacitance junction structure for the collection of the charge. No further processing is required, and the diode is ready for mounting. Initial tests on two 6-cm3 U-junction detectors indicate that the diodes have the expected capacitance of about 2 pF and a relatively thin window. A resolution of 705 eV for the 60-keV gamma ray of Am241 has thus far been obtained, and the measured Fano factor of 0.157 is consistent with the currently accepted value.