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The relaxation of aligned deuterium spins
Volume 121, number 8,9
PHYSICS LET’FERSA
18 May 1987
TIlE RELAXATION OF ALIGNED DEUTERIUM SPINS
P.P.J. DELHEIJ, D.C. HEALEY and G.D. WAIT TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3
Received 28 November 1986; accepted forpublication 21 March 1987
The nuclearspin orientation ofdeuterated butanol was modified by high intensity radio-frequency irradiation ofthe residual polarized proton spin system. For the resultingdeuterium alignment a decay time of 32 h was obtained at low free proton concentration. This provides evidence for alignment relaxation through the proton spin—spin interaction reservoir.
The spin system of deuterium nuclei can be polarized dynamical’yby microwave irradiation [1]. Once a sizable polar~zationhas been obtained, cooling of the sample to a temperature below 0.1 K will slow down the dec4iy of this polarization drastically, resulting in rela*ation times of several hundred hours (frozen spin tedhnique). In this state the orientation of the deuterium spins can be altered by high intensity radio-frequency (if) irradiation near either the deuterium Larmor frequency of 16.7 MHz (if burning) or the proton Larmor frequency of 108 MHz (if pumping). These frequencies apply in a magnetic field of2.55 T. This technique was explored at CERN where decay tithes r( D, D) up to 6 h [2] were measured for the reorientation induced by if pumping. From the temperature dependence of this decay It was concluded that the relaxation proceeds through contact with the electron spin—spin interaction reservoir. In general the shape of the nuclear magnetic resonance spectra (nmr) was reproduced at TRIUMF [3]. However, ~hemeasured decay times rc~for the if burning effet~ts(55 and 80 h) were an order of magnitude larger than r(D, D) of the if pumped spectra. This difference was attributed to a reduced free proton conc~entrationin the rf burned sample. A consequence of this assumption is an increase of r(D, D) in that sample. The expenmental procedure consists of polarizing the protons and deuterons in the sample with microwave irradiation. Next, the deuterium polarization .
.
0375-9601/87/1 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
is burnt off in frozen spin mode and the alignment is induced by if pumping through the proton spin system. At the lower proton concentration the pumping speed in the deuterium quadrupole reservoir is reduced severely. Therefore, the if frequency was chosen 45 kHz (after an unsuccessful trial with 125 kHz that was applied to the highest proton concentration) above the proton Larmor frequency, where a high pumping speed was reported although with a reduced final alignment [4]. Then, the area of the deuterium nmr signal was measured and this was repeated after one hour and after three hours. An example ofour nmr spectra is given in fig. 1. The area of the spectrum was calculated by adding the channel contents after the low-frequency half of the spectrum was multiplied by —1. The value of the 3
I
—
—
Z
I
_~
6.40
6.65
16.90
FREQUENCY CM Hz) Fig. 1. A nmr spectrum of deuterium is shown after if pumping of the deuterium quadrupole interaction reservoir. The proton concentration was 4%.
461
Volume 121, number 8,9
PHYSICS LETTERS A
Table 1 Decay times of the ifinduced deuterium alignment. Temperature (mK) 90 97 105
r(D, D) (h)
;(D, D) (h)
H-concentration
3.7 ±0.6 32 ±6
3.7 ± 0.6 50 ±11
18 4
21
53 ±14
2
±5
(%)
relaxation time r(D, D) was obtained from an exponential fit to the decay ofthe spectrum area. The sensitivity of our nmr system resulted in an output of —47 mV at the low frequency peak for a polarization of —0.21. The difference in the absolute value of the peak heights is caused by a remnant of the initial deuterium polarization after the if burning. Although the signal to noise ratio isconsiderably reduced, it is clear from a comparison with fig. 9c of ref. [3] that the shape of the nmr spectra is very similar. In table 1 our results are listed for three samples. The first one containednon-deuterated butanol to a proton concentration of 18% which is described in ref. [31.The other two samples consisted of deuterated (enrichment 98%) l-butanol-Dl0 and 5% by volume D 20 (enrichment 99.8%). The second one was doped with conventional EHBA [5], resulting in a proton concentration of 4%. The third sample contained fully deuterated EHBA, giving a proton concentrationof 2%. It is obvious that the reduction ofthe proton concentration causes an increase of the relaxation time r(D, D) of the quadrupole orientation. This points to the role of the proton sprn—sprn interaction reservoir in the relaxation process. The effect is even more pronounced ifthe data are renormalized with respect to the temperature. The assumption is that the decay of the quadrupole Ori entation shows a temperature dependence similar to the relaxation of the proton polarization in butanol (as was observed for propanediol [2]), for which a
462
18 May 1987
6 law was measured [61. After scaling to 90 mK acc’o~dingto this temperature dependence, the values under ;(D, D) in table 1 are calculated. MeasT
uring the temperature with two carbon resistors gives an estimated error less than 2 mK for the scaled tern-
perature differences. This contributes about 12% to the error of , which is added in quadrature to the
error of r. It is clear that r(D, D) and r~assume comparable values for similar samples at the same temperature. The introduction of deuterated EHBA seems not to affect the alignment decay time, although this reduces the proton concentrationby a factor 2. This is in agreement with the view that the protons, located close to the electron magnetic dipole moments, should be considered as part of the electron non-Zeeman interaction reservoir [71. It was shown previously [2,3] that the production of alignment and polarization can be described in very similar terms. Here, evidence is presented that this applies also to the relaxation. The centers which are responsible for the orientation (the free protons and paramagnetic impurities, respectively) mediate also the relaxation process.
~ferences [1] A. Abragam, The principles of nuclear magnetism (Oxford Univ. Press, Oxford, 1961). [2J W. de Boer, M. Borghini, K. Morimoto, TO. Niinikoski and F. Udo, J. Low Temp. Phys. 15 (1974) 249. [3] P.PJ. Delheij, D.C. Healey and G.D. Wait, Nucl. Instrum. MethodsA 251 (1986) 498. [4] W. de Beer, M. Borghini, K. Morimoto, T.O. Niinikoski and F. Udo, Phys. Len. A 46 (1973) 143. [5] M. Krumpolc and J. Rocek, J. Am. Chem. Soc. 101 (1979)
3206.
[6] P.P.J. Detheij, D.C. Healey and (ID. Wait, Proc. 6th mt. Symp. on Polarization phenomena in nuclear physics, Osaka 1985, supplement to J. Phys. Soc. Japan 55 (1986)1090. [7] M. Goldman, S.F,J. Cox and V. Bouffard, J. Phys. C 7 (1974) 2940.
PHYSICS LET’FERSA
18 May 1987
TIlE RELAXATION OF ALIGNED DEUTERIUM SPINS
P.P.J. DELHEIJ, D.C. HEALEY and G.D. WAIT TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3
Received 28 November 1986; accepted forpublication 21 March 1987
The nuclearspin orientation ofdeuterated butanol was modified by high intensity radio-frequency irradiation ofthe residual polarized proton spin system. For the resultingdeuterium alignment a decay time of 32 h was obtained at low free proton concentration. This provides evidence for alignment relaxation through the proton spin—spin interaction reservoir.
The spin system of deuterium nuclei can be polarized dynamical’yby microwave irradiation [1]. Once a sizable polar~zationhas been obtained, cooling of the sample to a temperature below 0.1 K will slow down the dec4iy of this polarization drastically, resulting in rela*ation times of several hundred hours (frozen spin tedhnique). In this state the orientation of the deuterium spins can be altered by high intensity radio-frequency (if) irradiation near either the deuterium Larmor frequency of 16.7 MHz (if burning) or the proton Larmor frequency of 108 MHz (if pumping). These frequencies apply in a magnetic field of2.55 T. This technique was explored at CERN where decay tithes r( D, D) up to 6 h [2] were measured for the reorientation induced by if pumping. From the temperature dependence of this decay It was concluded that the relaxation proceeds through contact with the electron spin—spin interaction reservoir. In general the shape of the nuclear magnetic resonance spectra (nmr) was reproduced at TRIUMF [3]. However, ~hemeasured decay times rc~for the if burning effet~ts(55 and 80 h) were an order of magnitude larger than r(D, D) of the if pumped spectra. This difference was attributed to a reduced free proton conc~entrationin the rf burned sample. A consequence of this assumption is an increase of r(D, D) in that sample. The expenmental procedure consists of polarizing the protons and deuterons in the sample with microwave irradiation. Next, the deuterium polarization .
.
0375-9601/87/1 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
is burnt off in frozen spin mode and the alignment is induced by if pumping through the proton spin system. At the lower proton concentration the pumping speed in the deuterium quadrupole reservoir is reduced severely. Therefore, the if frequency was chosen 45 kHz (after an unsuccessful trial with 125 kHz that was applied to the highest proton concentration) above the proton Larmor frequency, where a high pumping speed was reported although with a reduced final alignment [4]. Then, the area of the deuterium nmr signal was measured and this was repeated after one hour and after three hours. An example ofour nmr spectra is given in fig. 1. The area of the spectrum was calculated by adding the channel contents after the low-frequency half of the spectrum was multiplied by —1. The value of the 3
I
—
—
Z
I
_~
6.40
6.65
16.90
FREQUENCY CM Hz) Fig. 1. A nmr spectrum of deuterium is shown after if pumping of the deuterium quadrupole interaction reservoir. The proton concentration was 4%.
461
Volume 121, number 8,9
PHYSICS LETTERS A
Table 1 Decay times of the ifinduced deuterium alignment. Temperature (mK) 90 97 105
r(D, D) (h)
;(D, D) (h)
H-concentration
3.7 ±0.6 32 ±6
3.7 ± 0.6 50 ±11
18 4
21
53 ±14
2
±5
(%)
relaxation time r(D, D) was obtained from an exponential fit to the decay ofthe spectrum area. The sensitivity of our nmr system resulted in an output of —47 mV at the low frequency peak for a polarization of —0.21. The difference in the absolute value of the peak heights is caused by a remnant of the initial deuterium polarization after the if burning. Although the signal to noise ratio isconsiderably reduced, it is clear from a comparison with fig. 9c of ref. [3] that the shape of the nmr spectra is very similar. In table 1 our results are listed for three samples. The first one containednon-deuterated butanol to a proton concentration of 18% which is described in ref. [31.The other two samples consisted of deuterated (enrichment 98%) l-butanol-Dl0 and 5% by volume D 20 (enrichment 99.8%). The second one was doped with conventional EHBA [5], resulting in a proton concentration of 4%. The third sample contained fully deuterated EHBA, giving a proton concentrationof 2%. It is obvious that the reduction ofthe proton concentration causes an increase of the relaxation time r(D, D) of the quadrupole orientation. This points to the role of the proton sprn—sprn interaction reservoir in the relaxation process. The effect is even more pronounced ifthe data are renormalized with respect to the temperature. The assumption is that the decay of the quadrupole Ori entation shows a temperature dependence similar to the relaxation of the proton polarization in butanol (as was observed for propanediol [2]), for which a
462
18 May 1987
6 law was measured [61. After scaling to 90 mK acc’o~dingto this temperature dependence, the values under ;(D, D) in table 1 are calculated. MeasT
uring the temperature with two carbon resistors gives an estimated error less than 2 mK for the scaled tern-
perature differences. This contributes about 12% to the error of , which is added in quadrature to the
error of r. It is clear that r(D, D) and r~assume comparable values for similar samples at the same temperature. The introduction of deuterated EHBA seems not to affect the alignment decay time, although this reduces the proton concentrationby a factor 2. This is in agreement with the view that the protons, located close to the electron magnetic dipole moments, should be considered as part of the electron non-Zeeman interaction reservoir [71. It was shown previously [2,3] that the production of alignment and polarization can be described in very similar terms. Here, evidence is presented that this applies also to the relaxation. The centers which are responsible for the orientation (the free protons and paramagnetic impurities, respectively) mediate also the relaxation process.
~ferences [1] A. Abragam, The principles of nuclear magnetism (Oxford Univ. Press, Oxford, 1961). [2J W. de Boer, M. Borghini, K. Morimoto, TO. Niinikoski and F. Udo, J. Low Temp. Phys. 15 (1974) 249. [3] P.PJ. Delheij, D.C. Healey and G.D. Wait, Nucl. Instrum. MethodsA 251 (1986) 498. [4] W. de Beer, M. Borghini, K. Morimoto, T.O. Niinikoski and F. Udo, Phys. Len. A 46 (1973) 143. [5] M. Krumpolc and J. Rocek, J. Am. Chem. Soc. 101 (1979)
3206.
[6] P.P.J. Detheij, D.C. Healey and (ID. Wait, Proc. 6th mt. Symp. on Polarization phenomena in nuclear physics, Osaka 1985, supplement to J. Phys. Soc. Japan 55 (1986)1090. [7] M. Goldman, S.F,J. Cox and V. Bouffard, J. Phys. C 7 (1974) 2940.