Louis-Andre Hamel

CHARGE TRANSPORT AND SIGNAL GENERATION IN CDTE PIXEL DETECTORS

Abstract The current and charge signals on individual pixels of CdTe γ-ray cameras are calculated. The time and position dependent free carrier densities following the absorption of a γ-ray in the detector are first calculated by solving the continuity equations. These density distributions are then combined with the physical electric field and the pixel weighting field to provide the current induced through an individual pixel from which the charge signal is also obtained. Simulations are made for various pixel sizes. These simulations confirm that the energy resolution of electron collecting pixel detectors must improve when smaller pixels are used since small pixels are less sensitive to holes.

A superheated droplet detector for dark matter search

Abstract We discuss the operation principle of a detector based on superheated droplets of Freon-12 and its feasibility for the search of weakly interacting cold dark matter particles. In particular we are interested in a neutralino search experiment in the mass range from 10 to 10 4 GeV/c 2 and with a sensitivity of better than 10 −2 events/kg/d. We show that our new proposed detector can be operated at ambient pressure and room temperature in a mode where it is exclusively sensitive to nuclear recoils like those following neutralino interaction, which allows a powerful background discrimination. An additional advantage of this technique is due to the fact that the detection material, Freon-12, is cheap and readily available in large quantities. Moreover we were able to show that piezoelectric transducers allow efficient event localization in large volumes.

Signal generation in CdZnTe strip detectors

The energy resolution of CdTe and CdZnTe detectors is usually limited by the poor transport properties of holes. Devices segmented into small pixels have been observed to exhibit improved energy resolutions. Simulations have shown that this small pixel effect is due to the fact that small pixels are mostly sensitive to carriers moving close to the pixel, within a distance of the order of the pixel size. In this paper, signals are calculated for CdZnTe strip detectors in order to determine to what extent a similar small electrode effect is produced by strips. The free carrier density distributions following the absorption of a /spl gamma/-ray are calculated by solving the continuity equations. Combined with the strip weighting field, this provides the signal induced in the strip. Simulations are made for various detector geometries and for both the anode and cathode strips. Simulated signals are compared with actual signals measured on CdZnTe detectors.

Response of amorphous silicon p-i-n detectors to ionizing particles

Abstract Prototypes of thin film p-i-n amorphous silicon detectors, having intrinsic layer thicknesses ranging from 3 to 18 μm, were designed and fabricated, and their response to protons of 1.3 to 11.6 MeV, and to α particles of 1.6 to 14.6 MeV was studied. The collection efficiency of the charges generated by a particle is found to decrease sharply with increasing dE dx , for dE dx values greater than 10 keV μm . However, there exists a domain of oblique angles of incidence, relatively high applied biases and low dE dx values over which the signal amplitude varies linearly with energy loss. Pulse shape studies show the signal to consist of a steep leading edge, attributed to prompt electron collection, and a slow component showing evidence of multiple-trapping transport of holes, which is a possible cause for incomplete collection of the generated charge at the maximum attainable field of 60 V μm . Finally, all detectors give signal amplitudes compatible with a mean electron-hole pair creation energy of 3.4 to 4.4 eV in a-Si: H.

Monte Carlo simulations of Meyer-Neldel effect on carrier time-of-flight in a-Si:H

Abstract We present Monte Carlo simulations of multiple-trapping transport with Meyer-Neldel effect in a-Si:H assuming exponential band tails. The transit time tT, and the dispersion parameters, α1 and α2, before and after the transit time, are extracted from the simulated currents. The simulations show that including the Meyer-Neldel effect improves the agreement of α1 and α2 with experimental data, both at low and high temperatures and fits the time-of-flight drift mobility measurements. Best fits to the data yield T0 = 263 K, TMN = 464 K, v00 = 5 × 109 s−1 and μ0 = 4 cm2 V−1 s−1 for electrons and T0 = 409 K, TMN = 809 K, v00 = 6.5 × 1010 s−1 and μ0 = 0.5 cm2 V−1 for holes.

Performance and simulation of CdZnTe strip detectors as sub-millimeter resolution imaging gamma radiation spectrometers

We report /spl gamma/-ray detection performance measurements and computer simulations of a sub-millimeter pitch CdZnTe strip detector. The detector is a prototype for /spl gamma/-ray astronomy measurements in the range of 20-200 keV. The prototype is a 1.5 mm thick, 64/spl times/64 orthogonal stripe CdZnTe detector of 0.375 mm pitch in both dimensions, with approximately one square inch of sensitive area. Using discrete laboratory electronics to process signals from an 8/spl times/8 strip region of the prototype we measured good spectroscopic uniformity and sub-pitch (/spl sim/0.2 mm) spatial resolution in both x and y dimensions. We present below measurements of the spatial uniformity, relative timing and pulse height of the anode and cathode signals, and the photon detection efficiency. We simulated the photon interactions and signal generation in the strip detector and the test electronics and we compare these results to the data. The data indicate that the cathode signal-as well as the anode signal-arises primarily from the conduction electrons rather than the holes. This presents a detection efficiency limitation which must be considered in the design of strip detector systems.

Passivation of gas microstrip detectors and stability of long-term operation

Abstract We have studied the long-term operation of gas microstrip detectors which have been passivated with a layer of nickel oxide. We have used as the active gas CF4/isobutane (80:20) and three different types of substrates: Tedlar, glass and Upilex. In all three cases we found that the detectors are stable after passivation and can operate for a month without changes in gain at rates of MHz. The total accumulated charge was approximately 100 mC.

Signal formation in a-Si:H particle detectors

The response of a hydrogenated amorphous silicon thin film p-i-n diode to protons is presented as a function of the applied bias. A typical pulse shape is also presented. These data are explained on the basis of a model whose main features are fast ( >

Transport properties and performance of CdZnTe strip detectors

We report on recent tests and computer simulations of a sub-millimeter pitch CdZnTe strip detector under study as a prototype imaging spectrometer. This paper presents new measurements and new analyses of previous measurements of the CdZnTe transport properties for this prototype and resolves previously reported quantitative discrepancies. Measurements of charge signals produced by /spl alpha/-particles are used to determine the transport properties. These are then used in the model to predict detection efficiencies and spectroscopic performance for /spl gamma/-rays that are compared with the data.

Single-sided CZT strip detectors

We report progress in the study of thick CZT strip detectors for 3-d imaging and spectroscopy and discuss two approaches to device design. Unlike double-sided strip detectors, these devices feature both row and column contacts implemented on the anode surface. This electron-only approach circumvents problems associated with poor hole transport in CZT that normally limit the thickness and energy range of double-sided strip detectors. The work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma radiation measurements while minimizing the number and complexity of the electronic readout channels. These devices can achieve similar performance to pixel detectors for both 3-d imaging and spectroscopy. The low channel count approach can significantly reduce the complexity and power requirements of the readout electronics. This is particularly important in applications requiring large area detector arrays. We show two single-sided strip detector concepts. One, previously reported, features rows established with collecting contacts and columns with non-collecting contacts. Another, introduced here, operates on a charge sharing principle and establishes both rows and columns with collecting contacts on the anode surface. In previous work using the earlier strip detector concept we reported simulations and measurements of energy and spatial resolution for prototype 5 and 10 mm thick CZT detectors. We now present the results of detection efficiency and uniformity measurements conducted on a 5 mm thick detector using a specific configuration of the front-end electronics and event trigger. We discuss limitations associated with implementation of this approach.