High precision measurements of the ground state hyperfine structure interval of muonium and of the muon magnetic moment

Spinal fractures are common in the thoracolumbar region. Assessment of fracture instability is often made from fracture pattern seen on plain radiographs or CT scans. The purpose of this in vitro study was to correlate three-dimensional flexibility to each fracture type, i.e., endplate, wedge, and burst. Ten fresh cadaveric human spine specimens (T11-L1) were incrementally impacted in a high-speed trauma apparatus until a fracture occurred. All fractures were produced by the same mechanism (axial compression/flexion load). The occurrence of a fracture was monitored by lateral radiographs of the specimen, whose canal was lined with 1.6-mm steel balls. After each impact, the specimen was studied for its flexibility in flexion, extension, left and right lateral bindings, and left and right axial rotations. The flexibility was determined in response to the application of maximum pure moments of 7.5 Nm. Each moment was applied individually and in three load cycles. Parameters of neutral zone (NZ) and range of motion (ROM) were computed. Average flexion-extension ROM (and NZ) for intact, endplate, wedge, and burst fracture were respectively, 12.7 degrees (1.3 degrees), 13.9 degrees (1.7 degrees), 19.2 degrees (3.2 degrees), 22.0 degrees (6.0 degrees). The average lateral bending ROM (NZ) were 12.6 degrees (1.2 degrees), 13.6 degrees (1.9 degrees), 19.1 degrees (3.7 degrees), 27.2 degrees (9.8 degrees). The average axial rotation ROM (NZ) were 4.7 degrees (0.4 degree), 6.1 degrees (0.7 degrees), 7.1 degrees (1.0 degree), 12.9 degrees (3.1 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)

IntroductionInjuries of the thoracolumbar spine are serious disabling injuries. The treatment choice and clinical outcome are significantly dependent upon the determination of the stable/unstable status of the injured spine of the patient. There is controversy concerning the understanding and even the definition of clinical instability. However, mechanical derangement, in addition to clinical aspects such as pain and neurological deficit, is considered as one of the major determinants [21]. The mechanical injury can vary from a purely ligamentous injury to fractures of the endplate, vertebral body, and/or posterior elements. From the functional point of view, changes in the mechanical characteristics of the spine, documented in a three-dimenAbstract Injuries of the thoracolumbar spine are serious, disabling, and costly to society. These injuries vary from mild ligament tears to severe bony fractures. Increased range of motion (ROM) and neutral zone (NZ) have been suggested as indicators of the resulting clinical instability. The purpose of the present study was to investigate the relative sensitivities and merits of the ROM and NZ in relation to spinal injuries of the thoracolumbar junction. A graded spinal trauma experiment was designed, in which the threshold of injury and injury progression were examined. Ten thoracolumbar human spine specimens (T11-L1) were traumatized using a high-speed incremental trauma model. The ROM and NZ, which indicate altered mechanical properties, were determined for three physiological motions: flexion/extension (FE), lateral bending (LB), and axial rotation (AR). The injury threshold was found to be 84 J (or 84 Nm) by examining both ROM and NZ for all motion types (P < 0.05), but the NZ was more sensitive. At the injury threshold, the NZ showed an overall average increase of 566% above that of the intact, while the equivalent increase in the ROM was only 94%. The NZ was also a more sensitive parameter documenting the progression of the injury beyond the injury threshold. After the maximum trauma of 137 J, the NZs for the three motions (FE, LB, and AR) increased by 700%, 1700%, and 3000% above their respective intact values. Corresponding increases in the ROM were much smaller: 115%, 184%, and 425% respectively. Direct extrapolation of the in vitro experimental findings to the clinical situation, as always, should be done with care. Our findings, however, suggest that the ROM, as measured from functional radiographs of a traumatized patient, may underestimate the true injury to the spinal column.

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An experiment is in progress at the Los Alamos Meson Physics Facility (LAMPF) to determine with high precision the hyperfine structure interval AV (to -10 ppb) and the muon to proton magnetic moment ratio &,/pp (to -60 ppb) in the ground state of muonium @+e-). These precision gods correspond to increases in precision of AV and of &/pp by about a factor of 5 compared to present knowledge. The ExperimentThe general method of the experiment [1,2] is microwave magnetic resonance spectroscopy as applied to muonium, which relies on parity nonconservation in the n: + 4 pLf v decay to produce polarized p+ and in the p+ -+ e+V,,v, decay to indicate the spin direction of p ' . An earlier LAMPF experiment provides the present experimental values [3]: AveXp = 4 463 302.88(16) kHz (36 ppb) pp I pp = 3.183 346 1(11)(360 ppb) and The theoretical expression for AV can be Av(theory) = Av(binding,rad) + Av(recoi1) + Av(rad-recoil) + Av(weak) written as: Except for the small term Av(weak) coming from the weak interaction, this expression arises solely from the electromagnetic interaction of two pointlike leptons of different masses in their bound state. The present theoretical value is [4]: Avth=4,463,3O2.63( 1.34)(0.06)(0.17) kHz (0.3 ppm) The principal error of 1.34 kHz arises from the uticertainty in &,/pp. The second uncertainty arises from that in a based on the electron 8-2 experiment, and the third is the estimate of the error in the latest QED calculation [4]. Weak neutral current effects associated with Z exchange in the e-p interaction contribute 0.065 kHz or 15 ppb and are included in Avth. The experimental and theoretical values for AV agree well: Avth -AveXp = -0.23 1.4) l c H~ "th -= -(0.06 k 0.3)ppm. A V exp In the experiment in progress microwave induced Zeeman transitions are measured in a static magnetic field of 1.7 T. Resonance lines have been obtained by sweeping the magnetic field with a fixed microwave frequency. The new experiment improves on the earlier one [3] in several ways.First, the p+ beam intensity is now 1 x lo' p+/s, with a duty factor of 6 to 9% and with greater purity, achieved principally by use of an E x g separator to reduce the e+ background in the beam. Second, the magnetic field from a commercial Magnetic Resonance Imaging superconducting magnet system operating in persistent mode provides a field with a homogeneity of better than 1 ppm over the active region of the microwave cavity and with a stability of 0.01 ppm to 0.1 ppm/hr. Third, an electrostatic chopper in the muon beam line provides a muon beam with an on-period of 4 ps and an off-period of 10 ps, which allowed the observation of a resonance line from muonium atoms which had lived longer than the 2.2 vs mean lifetime T~ of muons [5]. Such resonance lines can be narrower than the natural linewidth determined by T~. Indeed linewidths narrower than the natural linewidth by factors of up to 3 were obtained.During runs in 1994 and 1995 with a total beam time of about 3 mos., data were taken with stopping Kr gas pressures of 0.5, 1.0 an...

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