V. Radeka

Liquid-argon ionization chambers as total-absorption detectors☆

Abstract A new detector for the measurement of energy by total absorption, based on the use of multiple-plate ion chambers, is described. The use of liquid argon as the working medium and optimized readout results in an electronic noise contribution to the resolution of less than 0.1 GeV, in a large detector. The use of thin plates, 0.1 radiation length, ensures that sampling fluctuations are small The technique allows absolute calibration and very good gain stability. Tests on a detector large enough to absorb a high-energy electromagnetic shower are described, where the energy resolution is limited by the residual sampling fluctuations.

Implanted silicon JFET on completely depleted high-resistivity devices

To satisfy the increasing interest in the integration of electronics onto optical and ionizing particle fully depleted detectors, a nonconventional JFET (junction field-effect transistor), designed to operate on a completely depleted, 2-k Omega -cm resistivity silicon substrate, has been designed, fabricated, and tested at room temperature. The devices show very low gate leakage current, low output conductance, a transconductance per unit gate width of 3 mS/mm, and a pinch-off voltage of -1.5 V. The integration of the devices onto the detectors makes possible the matching of the input capacitance of the JFET to the detectors output capacitance, which is of the order of few hundreds of femtorads. The measured gate capacitance of 200 fF is shown to correspond to an expected resolution in charge measurements, at room temperature, of less than 40 electrons rms. The fabrication constraints, imposed by the limited number of production steps of the detectors, are reported.<<ETX>>

Position Sensing by Charge Division

A summary of a comprehensive analysis of theoretical and practical aspects of position sensing by charge division from resistive electrodes is presented. Properties of transformer decoupling of the resistive electrode from detection bias voltage are analyzed and compared to the usual capacitive decoupling methods. Optimization and limitation of signal shaping is discussed as a function of diffusion time constant, signal rise times, and noise.

Centroid finding method for position-sensitive detectors

Abstract A new centroid finding method for all detectors where the signal charge is collected or induced on strips or wires, or on subdivided resistive electrodes, is presented. The centroid of charge is determined by convolution of the sequentially switched outputs from these subdivisions or from the strips with a linear centroid finding filter . The position line width is inversely proportional to N 3 2 , where N is the number of subdivisions.

The pn-CCD on-chip electronics.

A new pn-CCD with an activa area of 3 × 1 cm2 was recently fabricated for ESAs X-ray Multi Mirror Mission (XMM). The front-end electronics has been integrated on the same chip as the detector, and its noise behaviour was investigated. X-rays from a 55Fe source have been used for the absolute calibration. The measured electronic Equivalent Noise Charge (ENC) of the on-chip amplifier was 8.8 e− at room temperature and 2.2 e− at the CCD operating temperature of 150 K. The improvements with respect to the last version with noise figures of 4.8 e− (at 150 K) are due to the reduction of the total input capacitance by a factor of 1.6, the improvement of the transistor transconductance by a factor of 2, and the reduction of 1ƒ noise because of the different p-well implant with a better thermal annealing.

Optimum Signal-Processing for Pulse-Amplitude Spectrometry in the Presence of High-Rate Effects and Noise

In this paper the idea of a general signal processing system which should satisfy various pulse rate and noise requirements is explored. Optimum processing functions (weighting functions) are considered for an ideal system, and for real conditions where effects like imperfect pole-zero cancellation are present. Time-variant filters of the gain-varying class are used to realize the required optimum weighting functions of finite width. It is shown how nonfinite-width weighting functions of some time-invariant filters can be modified into finite-width functions by switching. These switched-gain time-variant filters are somewhat limited in choice of weighting functions. A general processing system can be realized employing filters with continuously time-variant elements. In particular, a gain-varying element (i.e., an analog multiplier) can be used in conjunction with an integrator to realize arbitrary weighting functions, and therefore the theoretically maximum signal-to-noise ratio. The system is time-variant only for the noise and not for the signal, so that it does not require high precision of the time-variant element. The system output is independent of the gating interval, and does not require precise timing. A method for evaluation of such systems in terms of noise, ballistic deficit and sensitivity to parameter variations is given.

Speed and noise limits in ionization chamber calorimeters

Abstract Relations among the speed of response, the signal-to-noise ratio and the charge collection time in ionization chamber calorimeters are analyzed. A dominant limiting factor to the speed of response is the charge transfer time from the electrodes to the amplifier determined by the electrode capacitance CD and the inductance of connections Ls. The time parameter L s C D sol1 2 has to be at least an order of magnitude smaller than the required length of the overall calorimeter response. The charge collection time plays a secondary role in determining the speed of response, while together with the charge yield it affects the signal-to-noise ratio.

Proportional chambers for very high counting rates based on gas mixtures of CF4 with hydrocarbons

Very fast multiwire proportional chambers of low mass for very high counting rates were developed. The anode signals have an effective duration of 8 ns after pulse shaping and a time jitter of only 4 ns fwhm. This performance was achieved with gas mixtures of CF4 and hydrocarbons at atmospheric pressure in MWPCs with small interelectrode distances. Gas mixtures with high primary specific ionization density and low noise fast signal processing provide full efficiency at gas gains of a few times 104. At these conditions, the chambers can operate at rates of several times 107 s−1 cm−2. Various gas mixtures and chamber geometries were explored and test results are reported.

Trapezoidal Filtering of Signals from Large Germanium Detectors at High Rates

A filter with a trapezoidal weighting function is the optimum one when both noise and variations in the charge collection time are present, A trapezoidal filter was realized by a time-variant (gated filter) system. This paper presents the results obtained with large germanium detectors at high rates and higher energies, showing that trapezoidal filtering is essential for good resolution in this case.

Stress Dependence of Contact Potential: The ac Kelvin Method

Measurements of the influence of stress upon contact potential necessitate the observation of shifts in contact potential of the order of 100 μV or less. The ac Kelvin technique was selected to yield this sensitivity. The application of phase sensitive detection techniques to the ac Kelvin method is discussed, and the equations are solved. The success of the method depends upon the use of a guarded field effect transistor input stage located within the Kelvin probe. The effect of nonuniform contact potentials and nonuniform capacitor plate separation is analyzed, and conditions are given for the balance point to be independent of plate spacing. The stress dependent contact potential measurements may be continuously displayed using a new nonlinear second harmonic detection scheme. This method is discussed.