Volume 41A, number 3 PHYSICS LETTERS 25 September 1972

DYNAMIC POLARIZATION OF PROTONS IN METHANOL - ETHYLENEGLYCOL MIXTURES

J. SVOBODA and M. ODEHNAL Institute of Nuclear Physics, Czechoslovak Academy of Sciences,

I?e2, Czechoslovakia

Received 21 July 1972

Results of a measurement of the polarization of protons in mixtures of ethyleneglycol and methanol at 4.2 K and 1.3 K in a field of 9 kG are presented. Higher polarizations were obtained than in pure ethyleneglycol and also a slightly better free-to-bound proton ratio.

The aim of recent developments of polarized

proton targets has been to increase the proton polarization with simultaneous improvement of the

free-to-bound proton ratio. Currently used target materials like butanol [l] and ethyleneglycol (EC) [2,3] represent a considerable amelioration in the latter aspect. We present here a preliminary report on the dynamic polarization of protons in EC - methanol mixtures at 4.2 K and 1.3 K and in a magnetic field

of 8.9 kG. The samples of EG doped with paramag- metic Cr” were prepared in the usual way [2] and then mixed at room temperature with methanol

whose concentrations ranged from 10% to 75% (by weight). Samples thus obtained were rapidly frozen in liquid nitrogen.

The studies of proton polarization and nuclear spin-lattice relaxation times TI, were performed as a function of the Crv - concentration (represented by the reaction time tR, for which a mixture of EC and of K2Cr207 were heated at 70°C), and of the methanol concentration in the mixtures.

The proton signal enhancements were measured by comparing the amplitudes of the enhanced NMR absorption signal with thermal equilibrium signal and the proton relaxation times were determined by ob- serving a decay of the enhanced signal after switching off the microwave power. The results of the measure- ments at 1.3 K are shown in figs. 1 and 2. It can be seen that addition of methanol increases the proton polarization enehancements compared to the ones in pure EG, starting with EG a minimum reaction time of which is 15 min. With increasing tR the maximum polarization moves towards the higher methanol con-

Fig. 1. The proton polarization enhancement Eat 1.3 K in the EG-methanol mixtures as a function of the methanol concen- tration (in weight %), for EG prepared under various reaction times tR (e.g. EG 20 corresponds to tR = 20 min). The plotted values of E are the mean values of negative and positive en- hencement factors, after taking into account the corrections for Q-meter. The error bars are not drawn and the lines are

meant to guide the eye.

centrations. Similarly the relaxation times r, are lenghtened in these samples by adding methanol. On the other hand, the “slow” EG’s (tR < 15 min) have exhibited an opposite behaviour. With increasing amount of methanol added the polarization enhance- ment decreases which corresponds to a decrease in Tin, caused by a more rapid relaxation of protons in pure methanol than of protons these EG samples. The

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Volume 41A. number 3 PHYSICS LETTERS 25 September 1972

FIG. 2

Fig. 2. The proton spin-lattice relaxation times Th, at 1.3 K as depending on the methanol concentration for various fR. EG 3; EG 5 and EG 10 - right-hand scale, the others left-hand scale.

patterns similar to those shown in figs. 1 and 2 were observed at 4.2 K, too. The maximum enhancement at4.2KwasE= 100forEG 15(tR= 15min)kith 10% of methanol, while for pure EC we obtained a maximum E = 75 for EC 10. The maximum enhance- ment at 1.3 K amounted to about 140 for several samples, although the enhancements for “fast” EG’s (tR > 15 min) were limited by insufficient micro- wave power (the maximum available microwave power in the cavity did not exceed 15 mW). This value can be compared with a maximum enhancement factor of 95 acquired in pure EG 15 (see fig. 1). Besides, we have observed a permanent discrepancy between the magnitudes of negative and positive polar- izations at 1.3 K, the negative being greater (about 1%). It is questionable whether this discrepancy occurs through some systematic error (corrections for nonlinearity of Q-meter were taken into account) or because of the physical reasons. A similar question was raised by Shapiro [3] for the case of butanol where, however, the sign of discrepancy was opposite.

A distance between peaks of positive and negative polarizations ranged from 22 G to 32 G at 4.2 K. When going down to 1.3 K this distance spreads more

apart, the difference being greatest for “slow” EG’s,

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where the distance approaches the values of 54 G while the theoretical distance of pure “solid effect” is 26 G. With increasing fR the difference of distances at 1.3 K and 4.2 K decreases and ultimately for EG 30 and EG 50 the distance reaches a value of about 33 G. A peak-to-peak distance of EPR absorp- tion curve was measured at 4.2 K to be around 32 G, but a concentration depedence of EPR line-width was not studied.

In view of the improvement in the absolute proton polarization it is suggested that the EG-alcohol mix- ture studied here might prove to be useful alternatives to EG as polarized target materials. These initial results are promising and it would be desirable to perform the similar experiments at higher magnetic fields and at lower temperatures. A detailed account of the physical phenomena accompanying this study will be given elsewhere.

References Vi

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S. Mango, 6. Runolfson and M. Borghini, Nucl Inst. Meth

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H. Glzttili, M. Odehnal, J. Ezratty, A. Malinovski and

A. Abragam, Phys. Letters 29A (1969) 250.

Proc. of the IInd Intern. Conf. on Polar. targets, University of California, Berkeley (197 1).