Search

Our recent work in the application of gas microstrip detector (GMD) technology to the fields of digital X-ray imaging and radiation dosimetry is described. The GMD can measure the position and the energy of individual photons at the high counting rates encountered in X-ray imaging. GMD based imaging systems have high detective quantum efficiency and permit improvement of image quality and contrast using display windowing and measured energy information. Results are presented on the performance of a prototype GMD imaging system operated with a xenon/methane 90/10 gas mixture at 1 atmosphere. Results are also presented on the performance of a GMD filled with tissue equivalent gases for applications in the field of radiation dosimetry in mixed neutron and /spl gamma/ fields. The results show that the GMD can be used for dosimetric discrimination between different types of radiation in mixed field environments.

The performance of the ATLAS electromagnetic liquid argon/brass forward calorimeter with its new readout geometry consisting of tube/rod electrodes with cylindrical shell gaps, has been evaluated with a full depth prototype in a testbeam experiment with electrons in 1995. The results for signal linearity of better than 1% and a constant term in the relative energy resolution of 3% meet and even exceed the original performance requirements very well. Space resolution in the order of 0.5 mm in the high energy limit, and an insignificant signal dependency on the electron impact angle have been found in addition.

MicroStrip Gas Counters (MSGCs) have been fabricated with gold electrodes on ion-implanted Upilex (polyimide) using both wet-etch and lift-off processes. The quality of lithography from the wet-etch process has now improved to the point where there is less than one defect per (5 cm × 2 cm) print. Recently, four of these detectors were operated as a beam telescope at TRIUMF. They were read out through fast low-noise amplifiers into 100 MHz flash-ADCs and data are presented here on their efficiency, timing, and spatial resolution.

Muons, which are produced naturally in the upper atmosphere, can be used to scan cargo for special nuclear materials (SNM). Preliminary simulated results show that detecting the presence of these materials can be accomplished by measuring the scattering of cosmic ray muons. Machine learning tools have been used on these data to classify it as SNM or not. The muon exists long enough, and is penetrating enough, that it can be used to passively scan cargo to detect SNM. By measuring the deflection angles of muons after they exit a container, one can determine whether or not SNM are present. Different detector approaches have been evaluated by considering the performance, cost, and robustness of several technologies. Simulations have been performed to help design the detectors and to determine the effectiveness of the proposed techniques. Realistic cargo containers have been simulated. Two types of techniques can be used to determine whether the cargo containers contain SNM. More traditional methods use an expert system which uses knowledge of physics to compute physical information about the cargo. The other approach is to use Machine Learning classifiers, which can be used to determine if the cargo contains SNM. These techniques include the following algorithms: decision trees, neural networks, special vector machines, and k nearest neighbours. Preliminary results from the two approaches to classification have been obtained and will be discussed in the paper.

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.

Plastics are desirable as substrates for gas microstrip detectors (GMDs) because of their flexibility, low density and long radiation length. GMDs have been fabricated on white Tedlar which has bulk electrical conductivity and on ion-implanted Upilex which has a thin electrically conductive layer on the surface of an insulator. The shielding effect of the thin conductive surface layer on the GMD gains has been investigated. High X-ray rate measurements show no charging or loss of gain for ion-implanted Upilex GMDs. Three such GMDs were recently tested in a high intensity beam at CERN. The anode signals were read out using fast, low noise, high gain amplifiers. Preliminary results of the test are presented.

The CRIPT project brings together several Canadian institutions, concerned with the detection of Special Nuclear Materials (SNM), addressing a requirement at ports-of-entry for detection of SNM in the many thousands of containers that flow into and out of Canada every day, while at the same time providing a way of monitoring the contents of nuclear waste storage containers, for waste management and non-proliferation reasons. The scale of these applications is large — typically tens of cubic metres, requiring detectors that can cover a large area at a minimum cost. The CRIPT project considered drift chambers adapted from high energy physics experience, as one of its technology choices. We report here on the performance of a full scale prototype, which covers an area of 2.4m × 1.2m with only 6 readout channels, and provides a 2D data point with accuracies (in x and z) of around 2 mm

Due to the severe radiation environment, the ATLAS experiment has chosen a compact tungsten/liquid argon forward hadronic calorimeter. The electrode design is unique and consists of hexagonally packed, tubular, thin gap electrodes running parallel to the beam direction. We describe the design criteria, the novel construction methods based on sintered tungsten components, and initial high energy beam tests at CERN.

Gas microstrip detectors (GMDs) could be ideal for use in digital X-ray imaging because of their ability to measure accurately, with virtually no noise, the energy and the position of X-ray photons at radiological counting rates. Thus a GMD based imaging system will have high detective quantum efficiency and will also be capable of dual energy radiography using a single X-ray exposure. As proof of principle, a prototype atmospheric pressure GMD imaging system has been built and its performance characterized by imaging standard phantoms and biological specimens. Using 8 keV X-rays from a copper target a line spread function with a FWHM of 230 /spl mu/m (determined by the photoelectron range of 190 /spl mu/m) was obtained for a gas mixture of 90% Ar and 10% isobutane.

The CRIPT Cosmic Ray Imaging and Passive Tomography system began data taking in September 2012. CRIPT is a “proof of principle” muon tomography system originally proposed to inspect cargo in shipping containers and to determine the presence of special nuclear materials. CRIPT uses 4 layers of 2 m x 2 m scintillation counter trackers, each layer measuring two coordinates. Two layers are used to track the incoming muon and two for the outgoing muon allowing the trajectories of the muon to be determined. The target volume is divided into voxels, and a Point of Closest Approach algorithm is used to determine the number of scattering events in each voxel, producing a 3D image. The system has been tested with various targets of depleted uranium, lead bricks, and tungsten rods. Data on the positional resolution has been taken and the intrinsic resolution is unfolded with the help of a simulation using GEANT4. The next steps include incorporation of data from the spectrometer section, which will assist in determining the muon's momentum and improve the determination of the density of the target.

A novel method of determining electron drift velocity at high electric fields uses plastic MicroStrip Gas Counters (MSGCs) operating in a test beam. The data from angled tracks through a MSGC provided a distance versus time relationship from which a drift velocity of 36.5 ± 2 μm/ns was obtained for an 90 10 argon/isobutane gas mix and an electric field of 4 kV/cm.