A Total-Absorption Detector for 60-MeV Protons Using Lithium-Drifted Germanium

A lithium-drifted germanium diode has been used for total-absorption detection of 59-MeV protons from the Oak Ridge Isochronous Cyclotron. The detector is 1.9 cm in diameter, has a depletion depth of 6 mm, is cooled to less than 85° K, and is sealed in an aluminum can with a 0.0026-in.-thick window. The diode was oriented so that the protons entered in a direction parallel to the detector junction. The energy resolution attained for 59-MeV protons was 150 keV (FWHM), uncorrected for energy straggling in windows of 76 keV, for approximately 60 keV of beam resolution, and for electronic noise. The peak-to-total ratio, determined by using an anticoincidence collimator, was as high as 0.94, which is comparable to 0.96 observed elsewhere for NaI. When the collimator was moved along a line parallel to the junction and perpendicular to the beam, the energy resolution and peak-to-total ratios remained constant within the experimental accuracy over a 10-mm scanned distance. As the collimator was moved in a direction perpendicular to the junction and toward the depleted material, the peak-to-total ratio decreased, as was expected from multiple scattering calculations. When the diode was connected by a 125-ohm terminated coaxial cable to a fast amplifier, a rise time of 4-5 nsec was observed. Since the protons entered the detector parallel to the junction, pictures obtained show the superposition of nearly rectangular current pulses arising from hole and electron collection. The length of the pulses is correlated with the point of incidence of the collimated beam.