T. J. Paulus

Timing Electronics and Fast Timing Methods with Scintillation Detectors

Time spectroscopy involves the measurement of the time relationship between two events. This paper reviews time pick-off techniques, practival time-pickoff circuits, and timing with scintillation detectors. A detailed comparison is made between leading-edge timing and constant-fraction timing. Typical timing resolution results are given for 60Co.

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.

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 >

A New Constant Fraction Timing System with Improved Time Derivation Characteristics

A constant fraction timing system having excellent time derivation properties over a wide dynamic range of signals is described. Leading edge walk is significantly reduced by arming the constant fraction zero-crossing circuit with a discriminator that is separate from the energy discriminator. Timing resolution with Ge(Li) detectors can be improved by the optional use of a slow risetime reject circuit, but only at the expense of reduced counting efficiency. This performance trade-off is described in detail. Extensive data are presented on the timing performance in ¿-¿ coincidence experiments.

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.

Comparative Timing Performance of Large Volume Ge(Li) and HPGE Coaxial Detectors

The timing performance of several large (8% to 12% relative efficiency) Ge(Li) and HpGe coaxial detectors has been measured and compared. The Ge(Li) detectors are capable of timing resolution which is generally 10% to 30% better than that of the HpGe devices. This resolution capability can be attributed to the higher bias voltages which can be applied to the Ge(Li) detectors. However, in some applications that involve a wide dynamic range of energies, the HpGe devices can provide better timing resolution than their Ge(Li) counterparts. Advancements in the state of the art are expected to result in HpGe detectors that can provide timing performance equivalent to the performance of the best Ge(Li) devices.

Characteristics of High-Rate Energy Spectroscopy Systems Using HPGe Coaxial Detectors and Time-Variant Filters

A high-rate, high-resolution gamma spectrometer system is described. The system consists of a reverse electrode HPGe coaxial detector, a transistor reset preamplifier, an active, semi-Gaussian prefilter, a gated integrator, and a unique data acquisition system consisting of a 10 ¿s, 13 bit ADC, fast FIFO memory, 8k by 23 bit data memory, and computer interface circuitry under the control of a Z-80A ¿P. The effects of the various components on the throughput are described and throughput data is presented. The resolution and peak shift for various shaping times are presented for count rates up to 1 Mcps input rate using a mixed 22Na and 60Co source. The low rate resolutions of 57Co and 60Co for various shaping times using either the semi-Gaussian or gated integrator output are discussed as well as the low energy resolution and peak shifts in the presence of high energy events.

Enhancement of peak-to-total ratio in gamma-ray spectroscopy

Abstract High resolution gamma-ray measurements require the use of germanium detectors. Gamma rays with low emission probabilities are often hot detectable above the background in which source-induced effects such as Compton-scattered events play a significant role. A Compton suppression system is required to reduce the background in the low energy range, and several such techniques are described for enhancing the peak-to-total ratio in a gamma-ray spectrometer.

A Versatile Constant Fraction 100 Mhz Discriminator

A new constant fraction discriminator having excellent timing characteristics over a wide dynamic range of signals has been designed and tested. To achieve maximum timing performance, the most critical block of circuitry has been implemented using hybrid microcircuit technology. This block includes the leading edge arming discriminator, the constant fraction shaping circuitry and the zero-crossing pickoff. The basic constant fraction building element indicated above operates to a minimum threshold of -30 mV and can accept input signals to -10 V without overload. Amplitude-dependent time walk is less than ±150 ps for a 200:1 dynamic range of input signals (tr = 1 ns). The count rate capability of this constant fraction hybrid exceeds 100 MHz.