Richard H. Pehl

A new particle identifier technique for Z = 1 and Z = 2 particles in the energy range > 10 MeV

Abstract Protons, deuterons, tritons, helium-3 and α particles produced in nuclear reactions have previously been identified by use of ΔE and E counters to determine dE/dx and E. Multiplying these together produces an output that is dependent on the type of particle. This technique is based on the theoretical relationship between dE/dx, E and the mass and charge of the particle. Unfortunately there is an obvious restriction on the technique, since dE/dx changes as the particle passes through the ΔE counter. For the E(dE/dx) product to have any real meaning, the ΔE counter must be thin and absorb only a small part of the total energy. This limits the use of this technique in a given experiment to small energy ranges and to selected types of particle. The new identifier also uses a ΔE (thickness T) and E counter, but employs the empirical relationship: Range = aE1.73, where a depends on the type of particle. Using the relationship, one can show T/a = (E+ΔE) 1.73 −E 1.73 . The identifier employs logarithmic elements to calculate this quantity and to produce an output that has a fixed value for each type of particle. The thickness of the ΔE counter is not limited to a very small value, and the identifier can cope with mixtures of all five types of particle, each covering a fairly wide energy range. Experimental results using this identifier and lithium drifted silicon detectors are presented and illustrate the clear separation between He3 and He4 particles (difficult to achieve with the multiplier type of identifier).

Accurate determination of the ionization energy in semiconductor detectors

Abstract The average energy g3 expended for electron-hole pair generation in silicon and germanium lithium-drifted detectors by gamma rays, electrons, and alpha particles has been measured as a function of temperature. These data indicate that the difference between eα and ee− in silicon is considerably less than previously reported, and in germanium eα ∼ ee−.

Pulsed Feedback Tecniques for Semicondctor Detector Radiation Spectrometers

Methods of applying pulsed-charge feedback to the charge-sensitive preamplifiers used with semiconductor detectors are discussed. All have in commn the accumulation of radiation-induced charge pulses on a feedback capacitor to produce a voltage ramp at the output of the feedback stage, which is reset at an appropriate point by pulsing a charge feedback path. Advantages of pulsed feedback over the conventional dc feedback techniques are discussed, together with the precautions required to reduce the effect of the large reset pulse on the later electronics. The application of pulsed-light feedback to low energy X-ray spectrometers is discussed and results are presented. We also discuss sane aspects of this system that tend to limit its high-rate performance. A brief account of the use of a transistor current-switch feedback system to reduce overload problems in high-energy ?-ray spectrometers is also given.

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.

Low capacitance large volume shaped-field germanium detector

A large-volume (150 cm/sup 3/) germanium detector with a full-depletion capacitance of only approximately=1 pf has been fabricated. The effect of impurity space charge was utilized to obtain an appropriate electric field distribution in the detector so that carriers are collected on a small-area electrode. Detectors based on this principle are capable of very-low-noise operation and have immediate applications in experiments for the direct detection of dark matter. Detector pulse shapes and carrier-trapping effects were also examined for possible applications involving higher energy radiations. >

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.

Fast Neutron Radiation Damage of High-Purity Germanium Detectors

Seven high resolution, high-purity planar germanium and two lithium-drifted germanium detectors have been exposed to fluences of monoenergetic fast neutrons of 1.4, 5.5 and 16.4 MeV to study radiation damage effects. Seven of the exposures were made at 5.5 MeV using detectors made from both LBL and General Electric Company material. Initial degradation of 60Co energy resolution was generally observed after fluences of 3 × 10+9 n/cm2. After fluences of 1010 n/cm2, the detector resolutions were all affected, and replacement would be required in most gamma ray spectrometry; these results are consistent with previous damage studies on germanium detectors. Considerable variability in neutron damage threshold between detectors was observed within this fluence range which must be attributable to a material parameter that is not yet fully determined. This is the major finding in this study. After irradiation, a significant increase in material resistivity was observed as a series resistance in the diodes undepleted region at low biases. The observations were made by capacitance effects and lengthened pulse rise time. Annealing of damage was observed during storage at LN2 temperature after irradiation; resulting, in some cases, in improvement of resolution and in others, further degradation. Drastic resolution degradation was observed on cycling detectors to dry ice temperatures, (200° K) with the loss of the high series resistance and an increase in acceptor concentration. Further cycling to room temperature for periods of hours resulted in improvement of the energy resolution compared with the 200° K value.

High-Purity Germanium: Detector Fabrication and Performance

The availability of germanium with less than 1010 acceptors/cm3 has enabled us to fabricate and study the performance of germanium detectors of substantially larger volume than has hitherto been possible except by using lithium-drifting techniques. Detectors having a sensitive volume of up to 25 cm3 have been made. These detectors have a lithium diffused n+ contact and a metal barrier non-injecting back contact, and are totally depleted. The energy resolution of these detectors is similar to that of equivalent sized lithium-drifted detectors, but the high-purity detectors are easier to fabricate, and present considerably fewer handling problems.

Levels involving A (d52)52 state in light nuclei

The two-nucleon ( alpha ,d) stripping reaction is studied for targets of C/sup 12/, N/sup 14/, N/sup 15/, and O/sup 16/. In each case, strong levels believed to correspond to insertion of the captured nucleons into a J = 5 state of a (d/sub 5/2/)sup 2/ configuration are observed. In N/sup 14/ and F/sup 18/, single levels are observed at 9.0 and 1.1 Mev, respectively; in O/sup 16/ and O/ sup 17/ the level is split into a triplet (O/sup 16/) and a doublet (O/sup 17/) by coupling with the spin of the target core. In O/sup 16/ the levels iie at 14.7, 18.2, and 17.2 Mev; in O/sup 17/, they are at 7.6 and 9.0 Mev. By inelastic helium ion scattering the following parities are measured for levels in N/sup 14/: 3.95 and 7.03 Mev, positive; 4.91, 5.10, 5.89, and 5.83 Mev, negative. (auth)The two-nucleon α, d stripping reaction has been studied for targets of C12, N14, N15 and O16. In each case, strong levels believed to correspond to insertion of the captured nucleons into J = 5 state of a (d52)2 configuration are observed. In N14 and F18, single levels are observed at 9.0 and 1.1 MeV, respectively: in O16 and O17 the level is split into a triplet (O16) and a doublet (O17) by coupling with the spin of the target core. In O16 the levels lie at 14.7, 16.2 and 17.2 MeV; in O17 they are at 7.6 and 9.0 MeV. By inelastic helium ion scattering the following parities were measured for level in N14: 3.95 and 7.03 MeV positive; 4.91, 5.10, 5.69 and 5.83 MeV negative.