Thomas W. Raudorf

Radiation Damage Resistance of Reverse Electrode GE Coaxial Detectors

Two high-purity germanium coaxial detectors, having opposite electrode configurations from one another, but fabricated from the same germanium crystal, were irradiated simultaneously with fast neutrons from an unmoderated 252Cf source. Both detectors were 42 mm diam. The detector having the conventional electrode configuration was about 28 times more sensitive to radiation damage than was the detector having the p+ contact on the coaxial periphery. These results prove that germanium coaxial detectors having the conventional electrode configuration should not be used in any situation subject to significant radiation damage. This conclusion was anticipated because the defects produced by neutron and proton irradiation of germanium act predominantly as hole traps.

Effect of charge carrier trapping on germanium coaxial detector line shapes

Abstract A theoretical method to predict and quantify the effects of charge carrier trapping on germanium coaxial detector line shapes has been developed. This model was used to calculate line shapes which closely matched the measured line shapes of both conventional and reverse electrode high-purity germanium coaxial detectors that had suffered fast neutron damage. In accordance with experimentally established behavior, the theory predicts that reverse electrode detectors are vastly more resistent to the effects of neutron damage than conventional electrode detectors. The theory also predicts that the energy resolution and line shape of neutron-damaged conventional electrode detectors are far more dependent on detector diameter than are detectors of the reverse electrode configuration. The generally observed variation of the energy resolution and line shape as a function of bias voltage is shown to arise from an ¢E −1 dependence of the trap cross section on electric field.

Mechanism for fast neutron damage of Ge(HP) detectors

Abstract The effect on high purity germanium detectors of the disordered regions (r ∼ 100 A ) created by fast neutrons is theoretically and experimentally addressed. The hole trapping cross section of these defects is a function of their net negative charge and the applied electric field. We estimate σ ≈ 10−9 – 10−10 cm2 immediately after biasing an n-type detector and σ ≈ 10−10 – 10−12 cm2 as hole trapping and detrapping reach a steady state in the depleted detector. Resolution transients observed immediately after biasing n-type and p-type Ge(HP) coaxial detectors are reported and are shown to be consistent with the neutralization (n-type) or the charging (p-type) of the thermal equilibrium state. However on the basis of these transients we cannot exclude the possibility that point defects play a decisive role in the steady state resolution degradation. The ionization or activation of traps after cycling the p-type detector off/on can be consistently interpreted as due to an acceptor level near Ev + 0.27 eV. The duration of the transient observed in n-type germanium was reduced to less than 1 h by placing a 6.5 μCi 60Co source on the end cap. Thus this transient does not significantly diminish the advantage of the contracting polarity employed on n-type coaxial detectors in reducing the effect of the hole trapping on resolution.

Pulse Shape and Risetime Distribution Calculations for HPGe Coaxial Detectors

Studies of the timing properties of HPGe coaxial detectors have been complicated by the lack of precise information about the shapes of photon induced charge pulses with respect to collection time. Previous calculations of pulse shapes occurring in HPGe coaxial detectors (1234) have involved great simplifications concerning the electric field dependence of the carrier drift velocity. Furthermore these calculations have used rather incompletely described numerical techniques. Also no attempt has as yet been made to use the mathematically determined pulse shapes to generate a risetime distribution curve even though such a distribution is accessible to measurement. In this work it is demonstrated how accurate pulse shape calculations may be performed for the case of the coaxial HPGe detector geometry using a programmable calculator. Pulse shapes due to single photon interactions are treated first, then the procedures for calculating the pulse shapes resulting from multiple interactions are demonstrated. The distribution of risetimes (number of events versus 10-90% risetime) may be calculated from the pulse shapes. Risetime distributions so determined are especially interesting because such distributions can be directly measured.2 A comparison between calculated and measured risetime distributions is made later in this work.

Advances in germanium detector technology

This paper summarizes some of the recent advances in high-purity germanium (HPGe) detector technologies and their applications. These important advances were driven by the necessity to support the recent increase in applications, uses and requirements for germanium detectors. Performances of single and multi-element closely packed HPGe detector arrays are discussed for various applications. Some of the developmental work to further improve the packing density of both large and small germanium detectors by using monolithic segmentation technology is presented along with the results. Monolithically segmented large germanium detectors are described which have high efficiency and high-energy resolution. Such detectors supply both interaction position and energy information of incident high-energy photons thereby providing powerful tools for gamma-ray tracking, polarimetry studies, low-energy filtration, and low-energy background rejection, etc. Monolithically segmented small germanium detectors provide improved solid angle ratio in X-ray detection systems without sacrificing energy resolution or throughput. Advancements in this detector technology are required for the fourth-generation Synchrotron Light Sources that are at the planning stage. This paper also reports on a revolutionary monolithic structure that is believed to be the first ever fabricated on large HPGe crystal.

Charge trapping correction in Ge spectrometers

A charge-trapping model is developed which does not require the assumption of shallow-level detrapping. The model shows the charge-trapping deficit to be proportional to S/sub 0/t/sup N/, where S/sub 0/ is the peak amplitude of the shaping amplifier pulse, t is the charge collection time for the carrier being trapped, and 1.5 >

Performance of Reverse Electrode HPGE Coaxial Detectors after Light Damage by Fast Neutrons

Several reverse electrode HPGe coaxial detectors fast neutron-damaged to various fluences were annealed at room temperature for varying periods of time. For comparison, in one case a dry ice temperature anneal was made. The result of these annealings on the energy resolution at 1.33 MeV was measured. The effect of source intensity and energy on the energy resolution and line shape are discussed.

Neutron Damage in Ge(Hp) Coaxial Detectors

We report energy resolution, capacitance, and depletion voltage transients in neutron damaged coaxial Ge(HP) detectors fabricated from n-type and p-type germanium. The disordered region model for fast neutron degradation of germanium detectors was supported by 1) the large trapping cross sections observed (~ 10-11 cm2), 2) changes in the steady state charge on the traps over four decades of induced current density, and 3) the inability of the isolated defect hypothesis to self-consistently account for the ratio of thermal emission of holes in liquid nitrogen to liquid argon cooled detectors. The results of in-cryostat annealing of these detectors are reported.

High-rate and high-energy gamma-ray spectroscopy using charge trapping and ballistic deficit correction circuits

A study of the resolution of large, coaxial, reverse electrode, HPGe detectors was performed over the energy range from 100 keV to 10 MeV and triangular amplifier shaping times from 0.5 mu s to 6 mu s. Resolutions were calculated using an approach based on the Trammell-Walter equation. The effect of ballistic deficit was included in the calculations by the introduction of a term, the ballistic efficiency, to the Trammell-Water equation. Experimental data were collected over an energy range from 122 keV to 2.6 MeV on three detectors with relative efficiencies of 76%, 56%, and 29%. For these three detectors, the data indicated that the triangular shaping amplifier with charge trapping and ballistic deficit correction offered better resolution than a gated integrator for shaping times >or=2 mu s, while the gated integrator produced better resolution for shaping times >

Comparative Timing Performance of Large Volume Hpge Germanium Detectors

The performance characteristics of a timing spectrometer suitable for use with large volume germanium detectors was studied. Timing resolution was measured as a function of constant fraction shaping delay, timing filter amplifier shaping time constants, discriminator threshold setting, and detector bias. Timing resolution data was collected for 14 detector-preamplifier systems including 9, coaxial HPGe p-type detectors, and 5 coaxial HPGe n-type detectors. Timing resolutions at FWHM and FWTM as a function of energy are presented for the energy range 150 + 5OkeV to 1330 + 50keV.