J. Armitage

Development of gas microstrip detectors for digital X-ray imaging and radiation dosimetry

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.

Electron signals in the Forward Calorimeter prototype for ATLAS

A pre-production prototype of the Forward Calorimeter (FCal) for the ATLAS detector presently under construction at the Large Hadron Collider (LHC) at CERN, Geneva, Switzerland, was exposed to electrons in the momentum range from 20 to 200 GeV/c in a test beam experiment at CERN in 1998. The measured performance, including a signal linearity within about ±1% and a high energy limit in the relative energy resolution of about 4%, meets the expectations for this kind of calorimeter, and exceeds the physics requirements for successful application in ATLAS.

Construction, commissioning and first data from the CRIPT muon tomography project

The Cosmic Ray Inspection and Passive Tomography (CRIPT) project is investigating muon scattering tomography (MST) for applications in border security, nuclear non-proliferation, and nuclear waste characterization. The construction of the full-scale prototype MST system began in the Summer of 2011 and was completed in September 2012. The CRIPT detector employs 12 layers of scintillator to track atmospheric muons before and after passage through a volume of interest, and to estimate each muons momentum. The total height of the system is 5.5 m and its weight is 20 tonnes. Details of its construction are presented. After the integration of the custom data acquisition electronics, the commissioning of the CRIPT detector began. The first tomographic images were obtained in October 2012 and are presented here.

Results for electrons from the 1995 ATLAS forward calorimeter prototype testbeam

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.

Digital X-ray imaging using gas microstrip detectors

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.

High rate and ageing properties of MSGCs on plastic substrates

Abstract We describe the high-rate performance of MSGCs fabricated on thin plastic (Upilex) sheets. The surface of each device is treated by one of three methods to bring the resistivity within the appropriate range: ion-implantation, nickel oxide coating and Inconel oxide coating. We also describe preliminary ageing results and their local effect on the MSGC.

FIRST IMAGES FROM THE CRIPT MUON TOMOGRAPHY SYSTEM

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 muons momentum and improve the determination of the density of the target.

Machine learning for the cosmic ray inspection and passive tomography project (CRIPT)

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.

Construction and initial beam tests of the ATLAS tungsten forward calorimeter

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.

Medical x-ray imaging with scattered photons

All medical x-ray imaging today is done using the transmitted photons, i.e., those x-ray quanta which do not suffer any interaction within the patient. An alternative is to use the more plentiful scattered photons. Backscatter is almost entirely Compton (incoherent) scatter, which is principally sensitive to the number of electrons per unit volume. Forward scatter is dominated by coherent scatter, which is the basis of x-ray diffraction. Its cross section varies with angle and photon energy in a material-specific manner, even for amorphous materials. The dependence on Z and chemical structure allows it to be very useful in distinguishing tissues within the patient. Many workers have demonstrated utilization of both types of scatter in the lab, but it has been difficult to compare the performance of these systems with conventional transmission imaging. Therefore, we devised a semi-analytic model of scatter imaging. Our calculations predict that for some imaging tasks the contrast and signal-to-noise ratio achieved by collecting a portion of the scatter (in an annular cone) will be superior to that achieved by conventional transmission imaging, for the same number of photons incident on the patient. Our analysis is reliant on the limited published data for coherent scattering for biological materials.