Coaxial Germanium Research Articles

Double injection in cylindrical germanium structures

Current-voltage characteristics and electric field distributions have been measured on large hollow-core coaxial germanium devices operating in the double-injection semiconductor regime. The I-V characteristics obey an I α V2 relationship and the magnitude of the current is predicted to within a factor of 2 by I = δqμpμnτ (p0−n0)V2, where the term δ is a constant scaling factor dependent upon the concentric cylindrical geometry. The shape of the electric field is obtained from potential probe measurements taken along the radius r of a p+ − π − n+ structure with the current density directed radially outward. In this case the field follows an E α [(r2/r)2 − 1]1/2 dependence over the central portion of the device and diffusion effects are present at the inner (r1) and outer (r2) injecting boundaries. A first-order correction for diffusion at the contacts is made by the application of an effective radii difference r2eff − r1eff = r2 − r1 − 2QLa, where La is the ambipolar diffusion length and Q is a constant with values ≈ 0.5. A small-signal equivalent circuit representation for double injection in the concentric cylindrical geometry is established from impedance measurements over the frequency range 103≲ω≲106. Field and charge carrier distributions for cylindrical double injection in the diffusion-dominated regime are also discussed.

The Efficiency Instability Problem of Coaxial Germanium Detectors

The generally accepted belief in constant fullenergy peak efficiency for coaxial germanium gamma ray detectors maintained at liquid nitrogen temperature has been found to be invalid in many cases. A majority of the available raw germanium ingots possess a shortcoming which causes detectors to decline in efficiency with time, if the standard techniques described in the literature have been employed for their production. A strong correlation of efficiency instability with ingot has been established. The time rate of change varies markedly from ingot to ingot. Over 60% of the ingots studied resulted in detectors whose efficiencies declined at rates greater than 8% per year. A substantial fraction of the cases studied displayed relative decreases of 10-20% in a period of six months. Declines of as much as 10-25% in several weeks were also observed.

Calculated figure of merit for planar and co-axial germanium detectors

Abstract Theoretical values of the figure of merit for germanium detectors have been calculated and comparisons between alternative planar and co-axial geometries are made.

Improvements in Large Volume Coaxial Germanium Spectrometers

Large volume (16-54 cm3) germanium lithium drift diodes coaxially drifted with one open end have shown excellent performance as ?-ray spectrometers. However limitations have been found in the coincidence resolving times possible with short coaxial diodes due to large variations in charge collection times which result from the non-uniformity of the electric field - especially in the closed end portion. Leading edge time distributions as a function of axial position were made on a longer (6.6 cm) coaxial diode with one open end using 511 keV ?-rays. In an attempt to improve the electric field uniformity, the closed end was cut off to produce a double open-ended detector 4.2 cm long, and a single open ended detector 2.1 cm long. Both diodes were excellent spectrometers. In the 4.2 cm detector (C = 55 pF) the full width at tenth maximum W0.1 of the leading edge time distributions varied from 34 nsec to 19 nsec due to a large variation in core size along the axis. In the 2.1 cm detector (C = 34 pF) W0.1 varied from 17 nsec in the coaxial region to 30 nsec at the closed end. A 6.1 cm long coaxial diode was made with two open ends, a circular crosssection, and a circular core cross-section nearly uniform along the length. W0.1 of the leading edge time distribution for this improved diode was 19 nsec for the whole detector, and varied by less than 2 nsec from end to end.