The resonance of unpaired electrons in externally imposed magnetic fields of a few kilogauss gives rise to absorption frequencies in the microwave region analogous to nuclear magnetic resonance signals now widely observed in the lower-frequency radio region. Such microwave or radio signals can give much information about the immediate surroundings of the particle at resonance.' Unpaired electrons in biological materials exist normally in only very minute quantities on metallic ions or in organic free radicals. Nevertheless, Commoner, Townsend, and Pake2 have been able to detect the resonance of organic free radicals in lyophilized biological substances, and we at Duke University3 have been able to observe resonances of Mn++ and Cu+ ions and of natural free radicals in unlyophilized living plants.In the present work, X-irradiation is used to produce the unpaired electrons which are observed. This method appears to be applicable to almost any biological material. One can use it to study radiation damage to tissues as well as to obtain structural information. The results which are described here represent only the initial phases of the program of study of these and other biologically significant substances.Several amino acids have been examined. Some typical resonance curves are shown in Figure 1. These represent essentially first derivatives of the absorption curves. They were obtained with the usual automatic recording spectrometer, employing bolometer detection with low-frequency magnetic modulation of the resonance. Fifty-kilovolt X-rays were used for the irradiation. All measurements were made at room temperature.In all the biological substances examined, a g factor very close to that for the free electron spin was obtained. This indicates almost complete quenching of the orbital momentum, as would be expected if the unpaired electron is in a molecular orbital. In practically all the samples a complex structure was observed. All samples were examined both at 9 kmc. and at 23 kmc. This allows one to ascertain whether the structure arises from nuclear interactions or from internal crystalline field splitting, since the spacings of the latter but not those of the former are sensitive to the strength of the externally imposed field. Also, the nuclear hyperfine structure can often be recognized by its symmetrical pattern.In the strong fields employed in our experiments the nuclear-spin and electronspin vectors precess separately about the applied field, and only the components of their magnetic moments which lie along this field experience an uncanceled interaction. Except for s orbitals, the splitting of the spin resonance by a nucleus of spin I and magnetic moment ,, is given by
The sequence of experiments resulting in the development of steady-state dense hot-electron plasmas is briefly described. These plasmas are produced in magnetic-mirror machines by radiation at the electron-cyclotron frequency. The electron-cyclotron plasma with the greatest stored energy to date has a volume of ∼ 50 liters, an electron “temperature” of 120 keV, a density of 4 — 7 × 1011/cm3, an d an average beta of ∼ 0.4. This plasma is created in the EPA Facility by a 50-kW c.w. 10.6-gHz microwave power source. The construction and operation of the machine are briefly discussed. The analysis and interpretation of the bremsstrahlung and diamagnetic measurements employed to determine these parameters are given. Diamagnetic and particle decay measurements on a smaller machine, the Physics Test Facility, are also described.
Two types of instabilities have been studied in an electron-cyclotron plasma confined by magnetic mirrors and heated by continuous wave microwave power at 10.6 Gc. One type of instability results in the loss of energetic electrons (Ee > 100 keV) across magnetic field lines, accompanied by oscillations in the 3–30 Mc range. Suppression of this instability is accomplished by increased background pressure. The second type is characterized by loss of fast electrons along field lines through the magnetic mirrors and accompanied by radio-frequency oscillations at 5.3 Gc (=½ωce). The loss is also characterized by a threshold in magnetic field corresponding to locating the heating zones in the mirror throats. The first type of instability appears to be flute-like and driven by ▿B drifts. The second type may be initiated by the mirror instability in the plasma.
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