Large enhancement of deuteron polarization with frequency modulated microwaves

We report a large enhancement of 1.7 in deuteron polarization up to values of 0.6 due to frequency modulation of the polarizing microwaves in a two liter polarized target using the method of dynamic nuclear polarization. This target was used during a deep inelastic polarized muon-deuteron scattering experiment at CERN. Measurements of the electron paramagnetic resonance absorption spectra show that frequency modulation gives rise to additional microwave absorption in the spectral wings. Although these results are not understood theoretically, they may provide a useful testing ground for the deeper understanding of dynamic nuclear polarization. ” Supported by Ishida Foundation. Mitsubishi Foundation and Monbusho International Science Research Program. ” Supported by the National Science Foundation of the Netherlands. I4 Supported by Comision Interministerial de Ciencia y Tecnologia. ” Supported by the US-Israel Binational Science Foundation and The Israeli Academy of Sciences. ‘* Supported by KBN. ” Now at Technische Universitgt. Dresden. D-01062. Measurements of deep inelastic scattering of polarized muons from polarized protons and deuterons determine the spin dependent structure functions of the nucleon which allow fundamental tests of quantum chromodynamics and of models of nucleon structure [I]. The precision of these experiments is strongly related to the polarization of the target nucleons. Therefore, the large enhancement of our deuteron target polarization which we discovered during the data-taking for deep inelastic muon scattering [2] had a significant impact on our experiment at CERN. The discovery was associated with a faulty regulator of the high voltage power supply of a microwave source. After B. Adevn et al. I Nucl. lnstr. and Mrth. in Phys. Res. A 372 (1996) 339-343 341 controllable frequency modulation (FM) of the microwave tube was implemented, a gain by a factor of 1.7 in the maximum deuteron vector polarization and of 2.0 in the polarization growth rate were achieved. These increases have been of crucial importance because data-taking extends over many months and the statistical error is proportional to I IPN”‘, in which P = (Iz)lI is the target vector polarization and N is the number of scattered events. The magnitude of the enhancement has been reported earlier [3]. The purpose of this paper is to detail the full characteristics of this effect, to present new data on the electron paramagnetic resonance absorption (EPR) spectrum and to discuss briefly processes which may contribute to the FM phenomenon. The polarized target [4,5] consists of two cells each 40 cm long and 5 cm in diameter located in a large cylindrical multimode microwave cavity. The two target halves are polarized in opposite directions by dynamic nuclear polarization (DNP). The target material is glassy, perdeuterated I-butanol. C,D,OD with 5% by weight of deuterium oxide, doped with the paramagnetic EDBA-Cr( V) complex ]6] to a concentration of 7 X IO” cm-’ [7]. It is located in a magnetic field of 2.5 T with a uniformity of IO-” over the volume and is cooled by a dilution refrigerator. The DNP is obtained by applying microwave power near the EPR frequency of the paramagnetic complex. The deuteron vector polarization is measured with nuclear magnetic resonance (NMR) probes, each of which is part of a series tuned Q-meter circuit [8]. The material is sampled by Iive probes in each target cell. The polarization is determined from the integrated NMR signals, calibrated in thermal equilibrium at I K. The relative accuracy of the measurement SPIP is 5% [3]. The microwave power for DNP is produced by two extended interaction oscillators (EIO) with an emission bandwidth of about 0.1 MHz. The rate of polarization is optimized by controlling the microwave power and frequency. The frequency is controlled by the EIO cathode voltage with a sensitivity of about 0.4 MHz/V or by tuning the EIO cavity. The power is controlled by non-reflective attenuators. For materials in which the solid effect [9,10] dominates as a mechanism for DNP, it has been found that microwave FM can improve the rate of DNP. This appears to result from the fact that FM counteracts the effect of “hole burning” due to EPR absorption at a fixed frequency [I I]. In the glassy alcohol materials with Cr(V) complexes, where the dynamic nuclear cooling [ 121 is the dominant mechanism for DNP. hole burning is not expected. However a polarization enhancement of 10% to 20% was observed in a fluorinated alcohol leading to polarizations of ~0.80 for protons and 19F [13]. References to enhancements of a few percent at polarizations around 0.70 or of about 15% for a material with only a few percent polarization can also be found in Ref. [ 131. To our knowledge no studies have been reported for the effect of FM on deuteron polarization except in one case where FM was used to compensate for magnetic field inhomogeneity and improve the final polarization by 5% to ~0.30 [ 141. The large enhancement of deuteron polarization in our target due to FM came therefore as a surprise. Fig. I shows the typical time evolution of the deuteron polarization P,, without and with FM. For this figure the cathode voltages were modulated at I kHz with a -50 V peak-to-peak amplitude leading to a FM amplitude Af20 MHz for the 69 GHz microwave source. The maximum deuteron vector polarizations under these conditions were 0.43 and -0.49. The EPR spectrum was measured in our target at a constant frequency by scanning the magnetic field. Such a spectrum, shown in Fig. 2 without FM, was obtained using a 220 R Speer composite carbon resistor as a bolometer, located in the dilute phase of the mixing chamber outside the target material [ 151. The input power to the microwave cavity a,, is the sum of e,,,. the power absorbed by the material in the EPR process, and &,, the non-resonant power absorbed into the cavity. The power absorbed by the bolometer & is a constant fraction r of e,,. It can be expressed as &, = c(T”,, T&) where T,, is the temperature of the bolometer. T,, is the temperature of the dilute phase and c is a constant [l6]. During the EPR measurement the input power Q,, remains constant and we can neglect the variations of Z&. Consequently the relation Q,, = Q,,, + (j,, = Q,,, + (clr)(T?,, ~ T1,,) shows that &,, is a linear function of T‘&. The broad absorption band seen in Fig. 2 is due to the anisotropy of the g-factor of the EDBA-Cr(V) electron spin. The highest positive and negative polarizations without FM were obtained at frequencies ,f,: = 69.090 GHz and f,; = 69.520 GHz, respectively. The EPR spectra with better resolution at the edges of the absorption band are shown in Figs. 3a and 3b, both with and without FM. The data points with FM were 10 loo loo0 Time(min) Fig. I. Deuteron polarization as a function of time without FM (dark circles) and with FM (open circles). Positive and negative polarizations are shown.