The MEGA experiment, which searched for the muon- and electron-number violating decay μ-> e + γ, is described. The spectrometer system, the calibrations, the data taking procedures, the data analysis, and the sensitivity of the experiment are discussed. The most stringent upper limit on the branching ratio of μ-> e + γ) < 1.2 x 10^{-11} was obtained.

A new measurement of the Michel parameter ρ in normal muon decay has been performed using the MEGA positron spectrometer. Over 500 million triggers were recorded and the data are currently being analyzed. The previous result has a precision on the value of ρ±0.0026. The present experiment expects to improve the precision to ±0.0008 or better. The improved result will be a precise test of the standard model of electroweak interactions for a purely leptonic process. It also will provide a better constraint on the WR−WL mixing angle in the left‐right symmetric models.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation J. J. Szymanski, J. F. Amann, K. Baker, D. Barlow, K. Black, R. D. Bolton, M. Brooks, S. Carius, Y. Chen, M. D. Cooper, P. S. Cooper, J. Crocker, M. Dzemidzic, R. J. Fisk, J. Flick, W. Foreman, C. A. Gagliardi, D. Haim, A. Hallin, R. Harrison, G. Hart, C. M. Hoffman, G. E. Hogan, E. B. Hughes, E. V. Hungerford, K. Johnston, C. Jui, G. J. Kim, J. E. Knott, D. D. Koetke, M. A. Kroupa, T. Kozlowski, K. Lan, D. Lee, F. Liu, R. Manweiler, R. Marshall, B. W. Mayes, R. E. Mischke, F. J. Naivar, B. M. K. Nefkens, J. Novak, M. A. Oothoudt, J. N. Otis, R. Phelps, L. E. Piilonen, C. Pillai, L. Pinsky, J. Price, M. W. Ritter, S. Schilling, T. D. S. Stanislaus, K. M. Stantz, W. Stephens, J. Sturrock, L. Tang, B. Tippens, R. E. Tribble, X.‐L. Tu, L. A. Van Ausdeln, W. vonWitsch, D. Whitehouse, C. Wilkinson, B. Wright, S. C. Wright, Y. Zhang, W. Zhou, X.‐G. Zhou, K. O. H. Ziock; MEGA: A search for the decay μ→eγ. AIP Conf. Proc. 10 July 1995; 338 (1): 789–792. https://doi.org/10.1063/1.48450 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

The MEGA experiment, which is a search for the decay murarregamma with a branching ratio sensitivity of about 10-13, employs highly modular, fast detectors, state-of-the-art electronics, and a staged trigger with on-line filters. The detectors are contained in a 1.5-T solenoidal field produced by a superconducting magnet. Positrons are confined to the central region and are measured by a set of thin MWPCs. Photons are measured by one of four layers of pair spectrometers in the outer region. Most aspects of the design have been validated in engineering runs; data taking will begin in 1990 with much of the electron arm and one pair spectrometer layer installed.

Imaging of coronary arteries using a venous instead of an arterial injection of contrast agent could provide a much safer method to diagnose heart disease. The tunability, intensity, and collimation of synchrotron radiation x-ray beams makes possible imaging systems with greatly improved imaging sensitivity. A pair of fan x-ray beams, a movable patient chair, and a multielement x-ray detector are used to acquire a pair of x-ray images above and below the iodine K edge. The logarithmic subtraction of these two images produces an image with excellent sensitivity to contrast agent and minimal sensitivity to bone and tissue. High-quality images from a dog and preliminary images from five humans have been obtained. Improvements are being made to the system to increase the effective radiation flux and to measure the position of both x-ray beams.

Because coronary disease represents the principal health problem in the Western, industrialized world, and because of the risks and costs associated with conventional methods of visualizing the coronary arteries, an effort has been underway at the Stanford Synchrotron Radiation Laboratory to develop a less invasive coronary imaging procedure based on iodine K-edge dichromography. A pair of line images, recorded within a few milliseconds of each other, is taken with two monochromatic X-ray beams whose energy closely brackets the K-edge of iodine, 33.17 keV. The logarithmic subtraction of the images produced by these beams results in an image which greatly enhances signals arising from attenuation by iodine and almost totally suppresses signals arising from attenuation by soft tissue and bone. The high sensitivity to iodine allows the visualization of arterial structures after an intravenous injection of contrast agent and its subsequent 20-30 fold dilution.

An engineering run with a partial detector is scheduled for Fall of 1987. The run will test the response of Phillips TDCs and nonsparse latches, the performance of SEs and SIs, and the data path from Fastbus through the ACP system to the MicroVAX II. Debugging of the GPM and FBBC is currently in progress. Software development for the GPM, ACP nodes, and MicroVAX II host is in progress and will be ready for use by the Fall run. Production data taking will start in late 1988.

The application of coronary angiography is limited because it requires arterial invasion and the direct injection of contrast agent into the coronary arteries. A prototype system has been developed which achieves sufficient sensitivity to the iodinated contrast agent to allow the visualization of coronary arteries in dogs after its intravenous injection. The system uses two fan beams of x-rays from an electron storage ring and a 300 element linear silicon detector. Two interlaced images, spaced at 150 eV above and below the K absorption edge of iodine (33.2 keV), are acquired and the logarithmic subtraction of these two images produces an image which has maximal sensitivity to iodine and minimal sensitivity to soft tissue and bone. This approach appears suitable for studies on human subjects.

A program is underway at the Stanford Synchrotron Radiation Laboratory to evaluate at minimally invasive method of visualizing the coronary arteries in man using synchrotron radiation. The design of an X-ray imaging system to accomodate a newly-available X-ray beam 12.3 cm in width is described. The system performance as revealed with test images of phantoms and excised hearts is indicated, and the system components to be tested before clinical studies can begin are identified.

Two 30 cm diameter steel drums with four facets rotating at 1800 RPM have produced high-quality digital-subtraction radiographs by alternately blocking and transmitting dual-energy synchrotron X-ray beams. An eight times faster system with 16.5 cm diameter tenfaceted tin-plated aluminum drums rotating at 5760 RPM is presently under construction.

Coronary arteriograms are the clinical reference standard for assessing the status of coronary atherosclerosis. Because of the low sensitivity of conventional x-ray systems to contrast agents, the angiographic procedure requires the direct injection of the contrast agent into the coronary arteries. The hazards and expense associated with arterial invasion severely restrict the use of this method and have led to development of the presènt transvenous approach.1–3

Multichannel lithium-drifted silicon (Si(Li)) semiconductor detectors are well suited to line-scan medical imaging systems. They have excellent linearity, high efficiency, large dynamic range and good stability. A major limitation in many medical imaging systems using image intensifiers is the veiling-glare associated with the large differences in intensity between different parts of the image field. Images acquired with linear Si(Li) detectors should suffer less from veiling-glare, primarily because they are line-scan images, but also because the Si(Li) detector responds to X-rays directly rather than to the light produced in a scintillator. The effective veiling-glare has been measured in a linear, 64-channel Si(Li) detector of thickness 5 mm and center-to-center spacing between adjacent sensitive areas of 0.5 mm. Each elemental contact was 6 mm high and 0.4 mm wide. The measurement was made in two different ways, both employing a 33 keV beam of synchrotron X-rays at the Stanford Synchrotron Radiation Laboratory. In one method, a lead phantom in the shape of an isosceles triangle was illuminated across its width with a fan beam of 33 keV X-rays 0.5 mm high by 20 mm wide. The X-ray flux passing by the phantom was measured in each channel of the Si(Li) detector. In the second method, a highly collimated beam of 33 keV X-rays (0.5 mm high by 0.025 mm wide) was scanned across 15 channels of the detector in 0.01 mm steps. The measured intensity in each channel was recorded at each position of the beam and the photon flux fell to less than 0.1% at a distance of 3 mm from the edge of the beam. The results of both measurements were in mutual agreement. Both were compared also to calculated results based on the known total X-ray cross sections for Compton scattering and photoelectric absorption. To within the experimental error, the observed and calculated results were found to be generally in good agreement.

A safe and sensitive method of visualizing the coronary arteries is needed. The presently available method of visualization is highly invasive and carries too high a risk and cost. The intensity and tunability of synchrotron X-ray beams provides a new source of X-rays highly suited to the in vivo detection of iodinated contrast agents. The method offers the prospect of non-invasive visualization of the arteries by peripheral venous injection of contrast agent. The studies already conducted and those in preparation for human subjects are reviewed.