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Highest polarizations in deuterated compounds
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 526 (2004) 43–52
Highest polarizations in deuterated compounds$ S.T. Goertz*, J. Harmsen, J. Heckmann, Ch. HeX, W. Meyer, E. Radtke, G. Reicherz Institut fur 150, Bochum 44780, Germany . Experimentalphysik I, Ruhr-Universitat . Bochum, Universitatsstr. .
Abstract Unlike the proton, which can be almost completely polarized even at temperatures as high as 1 K; achieving a high polarization in a deuteron system is a much more complicated task. The reason for that is the much lower magnetic moment of the deuteron compared to that of the proton. Since the nuclear polarization is determined by the ratio of the magnetic to the thermal energy, the only way to further enhance the deuteron polarization consists of reducing the effective temperature. Speaking in terms of the spin temperature theory, the nuclear Zeeman temperature has to be minimized. This can be done most effectively by using paramagnetic centers, which experience only a weak non-Zeeman interaction. In deuterated alcohols or diols such centers can be provided by either an irradiation with ionizing particles or by chemical doping with radicals of the trityl family. Both methods led to a substantial improvement of the maximum polarization in these substances. Values of about 80% have been measured in trityl-doped D-butanol and D-propanediol. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.P Keywords: DNP; Polarized targets; Spin temperature; Paramagnetic centers; EPR; NMR
1. Introduction
*
In order to provide a polarized solid target for a scattering experiment on the proton or on the neutron, one has to rely on hydrogen-rich substances or on materials containing polarizable and neutron-rich nuclei (preferably deuterons), respectively. The choice of the most suitable material depends on several factors:
*
*
The relative content of polarizable nucleons (dilution factor).
$ Supported by the ‘Bundesministerium fur . Bildung und Forschung’ and the ‘Deutsche Forschungsgemeinschaft’. *Corresponding author. E-mail address: [email protected] (S.T. Goertz).
*
The stiffness of the polarization against radiation damage (radiation resistance). The maximum available polarization. The presence of polarizable nuclei apart from protons and deuterons (polarized background).
The target materials used so far divide into two classes, one containing inorganic substances like ammonia (NH3 ; ND3 ) and lithium deuteride (6 LiD). They are highly polarizable and exhibit a good radiation resistance together with a highdilution factor. In particular 6 LiD provides the highest available content of polarizable nucleons compared to all nucleons present in the material. The reason is that the 6 Li nucleus may be
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.03.148
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considered as an a particle plus two loosely bound nucleons, a proton and a neutron. Just like in the case of the deuteron, the spins of these two valence nucleons are essentially parallel and thus responsible for the polarization of the 6 Li nucleus. While for experiments at high energies, one may profit from the highly polarized 6 Li nucleus in this way, for experiments at intermediate energies the problem raises, to which extent the experimental data are contaminated by nuclear binding effects and how to correct for them. That is why for these experiments organic materials like alcohols or diols, which make up the class of organic materials, are still preferred. Apart from the lower radiation hardness of these substances they suffer from another problem, namely the limited deuteron polarization available from the deuterated versions of these materials. Whereas for instance normal (protonated) butanol can be polarized up to 90% using a dilution refrigerator and a magnetic field of 2:5 T; in deuterated butanol the maximum polarization do not exceed 30–40% under the same conditions. From this point of view, a further development of the deuterated organic target materials concerning their maximum polarization as well as their radiation hardness is of high priority. Both properties are supposed to depend on the physics nature of the paramagnetic centers used for the DNP process. This contribution reports about the first and very successful polarization tests of alternatively doped D-butanol and D-propanediol.
2. Theoretical background The theoretical background underlying the invention of the new doping methods is described at length in the article ‘The dynamic nuclear polarization process’, which can be found in these proceedings. Thus only a brief summary of the basic idea is given in the present section. The nuclear (vector) polarization can be expressed using the Brillouin function: ! gmB 1 PI ¼ BI o a ð1Þ ¼ B I I I : 2 2kTIZ
Since BI is a monotonously increasing function of its argument, i.e. of the ratio of the magnetic energy to the thermal energy, an optimization of the polarization is equivalent to a minimization of the temperature TIZ of the nuclear Zeeman reservoir. In contrast to a proton spin system, which—depending on the actual width of the EPR line—may or may not be in thermal contact to the electron non-Zeeman (dipolar) reservoir, for a deuteron spin system with its low Larmor frequency this is always the case. Thus the inverse spin temperature aI of the deuteron Zeeman reservoir is given by the equilibrium value of the non-Zeeman inverse spin temperature of the electron system: beq ¼
WTD ðD2 =D2 Þ aL : 1 þ W ðTZS þ ðD2 =D2 ÞTDS Þ
ð2Þ
For the case of complete saturation W b1=TZS ; 1=TDS the above expression simplifies to beq ¼
CSZ
CSZ aL : þ ðTZS =TDS ÞCSD
ð3Þ
Although these relations are quantitatively valid only in the high-temperature limit, i.e. if the magnetic energy of the system is small compared to its thermal energy, they correctly describe the qualitative dependence of the maximum inverse spin temperature on the respective parameters for all temperatures.1 A further simplification, which has been done in the theoretical considerations of the contribution cited above, concerns the origin of the finite size of the EPR line. In the Provotorov theory it is assumed that the broadening of the electron Zeeman levels entirely stems from a local magnetic field Bl ; which is produced by the neighboring unpaired electrons (homogeneous broadening). In nature however inhomogeneous interactions like the hyperfine interaction with adjacent nuclei and a broadening due to a possible anisotropy of the g-factor are always present. Their respective strengths may be much larger than that of the dipole–dipole interaction. Nevertheless, it can be shown experimentally [2,3] that 1
Special solutions for the general case of arbitrary temperatures have been given by M. Borghini. They can be found in Ref. [1].
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also in this case the EPR line behaves in a similar way as it would do under an entirely homogeneous interaction. The only assumption is that the dipole–dipole interaction has to be strong enough to equalize the energy difference between energetically neighboring spin packets. Then Eqs. (2) and (3) are still valid provided the local frequency D ¼ gBl is extended by an inhomogeneous contribution Bl;inhom to the local field D2 ¼ gðB2l;hom þ B2l;inhom Þ
ð4Þ
which enters into the expression for the heat capacity of the non-Zeeman reservoir CSD ¼ NAD2 : Therefore relation (3) in combination with Eq. (1) may be taken as a guideline showing which parameters eventually determine the achievable polarization. Apart from the ratio of the two time constants of the electron spin system, its Zeeman (TZS ) and its dipolar relaxation time (TDS ), the most crucial parameter is the size of the heat capacity CSD : Since the inhomogeneous contribution to the local field is usually at least one order of magnitude larger than the homogeneous one, the best way to minimize CSD is to search for a doping method, which provides paramagnetic centers with a particularly weak inhomogeneous interaction.
3. Doping methods There are two methods of producing paramagnetic centers in a solid-state target material. One is the radiation doping method, which in principle works for all materials independently of their state at room temperature. Here defects are produced by a radiation-induced fission of the respective molecules, which may undergo several secondary reactions to eventually build up a paramagnetic center. The second method only works for substances possessing a liquid phase not to high above room temperature. In this case a certain amount of a stable chemical radical can be admixed, followed by a rapid freezing of the liquid to ensure a homogeneous distribution of the unpaired electrons, which are associated with the radical molecules. Each of these methods have their own specific problems. In the case of the radiation doping,
45
several things are a priori completely unknown and must be determined by experiment: *
*
*
The number of different radiation defects and their production efficiency. The nature of the defects including the questions, whether they are paramagnetic or not. The properties of the paramagnetic defects regarding their inhomogeneous interactions.
Unfortunately, from the experimental side there is only a limited influencing control on these points. Only the temperature during the irradiation as well as its duration and the intensity of the particle beam can be varied. But at least regarding the third point a rather general statement can be made: The radiation doping method recommends itself in particular for the doping of deuterated substances, or more precisely, for the doping of substances containing only those spin carrying nuclei, which possess a small magnetic momentum. The reason is that in this case the unpaired electron, which wave function extends more or less to the neighboring nuclei, will undergo only a relatively weak hyperfine interaction. Consequently, if the second inhomogeneous interaction, the g-factor anisotropy is not to strong, the width of the EPR line made up by these electrons will be rather narrow. These arguments led to the decision to test the radiation doping method also for the alcohols, substances, which are usually doped by the chemical method. Apart from the requirement that the substance has to be liquid in a temperature region, where chemical radicals are stable, the problematic nature of this method consists of the availability of suitable radical products. In the past only two different types of chemical radicals have been gained acceptance. These are Porphyrexide and TEMPO as members of the nitroxyl family as well as EHBA (EDBA) containing a Cr(V) complex.
4. Magnetic field dependence of conventionally doped D-butanol Since several years it has been tried to improve the deuteron polarization available from deuterated
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alcohols. In this connection the dependence of the polarization on the strength of the external magnetic field have been measured in hoping to create higher values at higher magnetic fields. At least a behavior like that is suggested by the theory, since the argument of the Brillouin function (Eq. (1)) exhibit a quadratic dependence on the field strength, one given by the magnetic energy and one by the minimum achievable spin temperature. Nevertheless the experimental results are exactly the opposite. Fig. 1 shows the maximum deuteron polarizations as measured in D-butanol doped with the two nitroxyl radicals and with EDBA [4]. These results are in agreement with earlier measurement made by the polarized target group of the PSI [5]. They show a maximum around 2.5–3:5 T followed by a rapid decrease at higher fields. Apart from the sample doped with Porphyrexide the maximum polarization do not exceed some 10% at a field of 5 T: The striking discrepancy between theory and experiment may be explained by a strong anisotropy of the gfactors in these radicals. For its measurement EPR lines were taken using a X-band spectrometer at liquid nitrogen temperature and compared with best fitting simulations. As an example Fig. 2 shows the EPR line of TEMPO doped butanol together with the respective line shape simulation.
The results for the average deviation Dg from gC2 are the following: Radical
Dg=g ½103
TEMPO Porphyrexide EHBA/EDBA
3:8570:2 3:070:3 6:070:2
In Fig. 3 these numbers are translated into the corresponding EPR line widths drawn versus the applied magnetic field. In addition to the g-factor anisotropy the two nitroxyl radicals also possess some hyperfine interaction due to a nitrogen nucleus nearby the unpaired electron. It is reflected by the prominent three line structure of the EPR line as shown in Fig. 2. Thus, in contrast to EDBA, the respective resonance lines have a finite width also at vanishing magnetic field. Nevertheless, for magnetic fields above a certain value the EPR line widths of all of the three radicals are linearly dependent on the magnetic field. In this, the explanation for the unusual behavior of the achievable polarization may be found, since the mechanism for the cooling of the deuteron Zeeman reservoir assumes that the inhomogeneous contribution to the electron non-Zeeman interactions must be weak enough to be balanced by the homogeneous (dipolar) contribution. This
Simulation Experiment
|Polarization| [%]
40
30
20
EDBA Porphyrexide TEMPO TEMPO + 5% D2O
10
Axx = 0.73 mT gxx = 2.0094 Ayy = 0.63mT gyy = 2.0065 Azz = 3.60 mT gzz = 2.0017
0 2
3 4 Magnetic Field [T]
5
Fig. 1. Dependence of the maximum polarization in fully deuterated butanol on the strength of the external magnetic field measured in a dilution refrigerator. The lines are only for eye guiding.
325
330
335 340 Magnetic Field [mT]
345
Fig. 2. EPR line of 0.5% (mass) TEMPO solved in butanol together with the best fitting simulation. The corresponding components of the g-tensor and the hyperfine tensor are given in the insert.
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dependence of chemically doped D-butanol is connected to the properties of the radicals used for the DNP process, since radiation doped samples of the same material behave completely different. This will be discussed in the next section.
5. Radiation-doped alcohols 5.1. The process of radiation doping
Fig. 3. Full-widths of the inhomogeneously broadened EPR versus the magnetic field for the different chemical radicals: (1) EHBA/EDBA, (2) TEMPO, (3) Porphyrexide. For comparison also the double values of the Larmor frequencies of protons (4) and deuterons (5) are included.
assumption may be violated above a certain magnetic field. Although this explanation is highly speculative, it is supported by several observations: *
*
The decrease of the polarization with increasing magnetic field is strongest in the case of EDBA, which itself exhibits the highest g-factor anisotropy of all of the investigated radicals. It is weakest in the case of Porhpyrexide having the smallest anisotropy.
A further hint comes from the behavior of normal (protonated) butanol doped with the same radicals. These substances show—instead of a decrease—an increase of the polarization at high fields. At first sight it may seem that this is in contradiction to what has been said before, but taking into account the highly different values of the magnetic moment for protons and deuterons, this observation may fit to the present picture as well: Due to the much higher Larmor frequency of the protons their Zeeman reservoir is in a much closer contact to the Zeeman reservoir of the electrons than it would be possible for a deuteron spin system. Thus an additional polarization channel may open up, which is the DNP via the differential solid effect. To sum up, at least one definitive statement can be made. The reason for the magnetic field
Deuterated as well as normal butanol have been irradiated at the Bonn injection Linac, which provides 20 MeV electrons with a time-averaged beam current of 10–20 mA: The samples prepared as frozen beads with diameters of about 2 mm were loaded into a special irradiation cryostat operating at the temperature of liquid argon (87 K). Compared to the inorganic materials the efficiency of defect creation in alcohols is almost two orders of magnitude higher. Thus an accumulated dose of 1015 e =cm2 ; corresponding to an irradiation time of only some minutes, is sufficient to create a radical density high enough for the DNP process to work. In this way two samples of fully deuterated butanol with different radiation doses were produced. By comparing the integrated EPR line intensities with those of chemically doped solutions the densities of the paramagnetic centers could be determined to be 1 1019 =g (sample No.1) and 2 1019 =g (sample No.2), respectively. 5.2. Polarization results In order to decide whether or not an irradiation is a suitable method for the doping of the alcohols at all, the samples were first tested in a 4 Heevaporation refrigerator at a magnetic field of 2:5 T: The results for the maximum polarization, the build-up and the relaxation time at about 1 K were the following:
No.
Pol. (%)
Build-up (s)
Relax. (s)
1 2
12.7 10.1
800 242
2924 1148
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For comparison in chemically doped D-butanol deuteron polarizations between 8% and 9% are usually obtained under the same conditions. With these encouraging results in mind the samples were polarized in a dilution refrigerator at magnetic fields of 2.5 and 5 T: Fig. 4 summarizes the measured values. In contrast to the behavior of chemically doped D-butanol, the polarization of the radiation-doped version of this substance increases with increasing magnetic field at least up to a field value of 5 T: At the time of the experiment (2001), a value of more than 70% was the highest deuteron polarization ever observed at a magnetic field of 5 T and the highest one in an organic solid-state target material at all. It was recently surpassed only by the results obtained from trityl-doped D-butanol and Dpropanediol, which will be presented in the next section. In order to get a better understanding of how the properties of the paramagnetic centers influence the efficiency of the DNP process as well as to draw a clean dividing line between the two doping methods, the structure of the radiation induced paramagnetic defect had to be determined. For this purpose alcohols of different chain lengths as well as of different degrees of deuterization have been irradiated and investigated in the X-band spectrometer.
70
-70.8 %
|Polarization| [%]
60
-55.1 % -50.3 %
50
+45.5 %
40 -34.8 %
30
Sample 1 Sample 2
20
5.3. The nature of the radiation-induced defects in the alcohols Beads of normal, partially and fully deuterated alcohols from methanol up to pentanol have been irradiated using the 2 MeV electrons from a 90 Sr radioactive source for periods of about 1 h: Classified by the major features of their EPR lines the samples may be divided into three groups: Fully protonated samples. The structure of the spectra is given by a pronounced hyperfine splitting due to the hydrogen atoms adjacent to the unpaired electron (e.g. top line in Fig. 6). Fully deuterated samples. The spectra essentially consist of a single line with a weak to moderate splitting due to hyperfine interaction with the adjacent deuterons (Fig. 5(a) and (b)). Partially deuterated samples. The hyperfine splitting with the residual protons dominates the spectra. The presence of the deuterons manifests itself only by a weak modulation of the proton HFS peaks (bottom line in Fig. 6). In contrast to the EPR line of D-butanol, which hyperfine peaks are not sufficiently resolved to allow an unambiguous assignment of the spectrum to a certain configuration, the origin of the Dethanol spectrum is rather obvious. Here a clear structure composed out of nine nearly equidistant lines is observed. They stem from an ethanol molecule, which lost one of its deuterium atoms. With the exception of the OD-group the residual unpaired electron spreads over the whole molecule with almost equal densities at the sites of the n ¼ 4 deuterium nuclei. Thus 2n þ 1 ¼ 9 hyperfine lines are expected in agreement with the observation. For the longer alcohols the interpretation of the EPR spectra is more complicated. Nevertheless it has been suggested by several authors [6–9] that the most probable electronic configuration of the radical molecule is of the type
10 0 0
1
2 3 Magnetic Field [T]
4
5
Fig. 4. Maximum deuteron polarizations of radiation doped Dbutanol measured in a dilution refrigerator. The lines are only for eye guiding.
A hydrogen or a deuterium atom is removed from the hydroxyl group and the wave function of the unpaired electron spreads over the atoms bound to the a- and b-carbons. In contrast to ethanol, in
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CH3(CH2)3OH
Signal Amplitude
CD3(CH2)3OH CD3CD2(CH2)2OH
CD3(CD2)2CH2OH
325
(a)
332
333
334
335
336
337
338
4 2 1 5 9 8
6 7
333 334 335 336 337 338 333
335 340 Magnetic Field [mT]
345
Fig. 6. EPR spectra of differently deuterated butanol. From top to bottom: n-butanol, n-butanol-4.4.4-D3, n-butanol4.4.4.3.3-D5, n-butanol-4.4.4.3.3.2.2-D7.
direct after irradiation after 5 min of bleaching
3
332
330
334 335 336 Magnetic Field [mT]
337
338
(b) Fig. 5. EPR lines of electron-irradiated solid D-butanol (a) and D-ethanol (b). In (b) the EPR line as measured directly after the irradiation (gray) is shown together with the one obtained after bleaching under normal light for 5 min (black). The residual line shown in the insert are due to solvated electrons.
which the methyl end of the molecule rotates freely under all circumstances, the rotation of the b with respect to the hydroxyl group (a-group) is hindered in the longer alcohols at low tempera-
tures. This leads to different hyperfine constants for the two b-hydrogen (deuterium) atoms. Consequently more than the n þ 1 ¼ 4 lines, which are expected for an interaction with n ¼ 3 equivalent hydrogen atoms, are visible. Values of 20 mT are found in the literature for the a- and one of the b-protons, whereas the second b-proton couples with a strength of 40 mT to the electron [10]. Our measurements are in agreement with these considerations. As an example Fig. 6 shows the EPR spectra of several butanol samples with increasing degrees of deuterization. Starting from the methyl towards the hydroxyl group more and more hydrogen atoms are replaced by deuterium atoms. The prominent five line structure, which is in agreement with the hyperfine constants given above, is essentially unchanged until the b-protons are finally replaced by deuterons, too. At that point the EPR line shape changes completely from the quintet to a doublet, for which the only explanation is that the unpaired electron stems in fact from the hydroxyl group. The interaction with the b-deuterons is reflected by the rather delicate fine structure of the line. These arguments, which are also supported by further EPR measurements made in normal, partially and fully deuterated methanol, propanol as well as in some higher alcohols, strongly suggest that also the paramagnetic center of radiation doped D-butanol is of the hydroxyl radical type.
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From the symmetry of the EPR lines it is apparent that the anisotropy of the g-factor of these centers is small, at least much smaller than that of the nitroxyl and Cr(V) radicals. The major broadening mechanism consists of the hyperfine interaction of the unpaired electron, which—in contrast to the g-factor anisotropy—gives a contribution to the EPR line width independently of the magnetic field. Thus the overall line width remains relatively narrow even at fields as high as 2.5 and 5 T: This is supported by a likewise narrow frequency curve of this material, i.e. by the deuteron polarization as a function of the applied microwave frequency as shown in Fig. 9. In this the reason for the high-polarization values in irradiated D-butanol may be found, since a narrow EPR line is synonymous to a small heat capacity of the electron non-Zeeman reservoir, which can thus be cooled very efficiently.
(a)
(b)
Fig. 7. Structures of the trityl radicals OX063 (a) and ‘Finland D36’ (b). The molecular weights are 1426.78 and 1036.14, respectively.
~ 0.22 mT
13C
6. Trityl doped D-butanol and D-propanediol The very good results obtained from irradiated D-butanol opened up the question, whether also stable chemical radicals of a similarly excellent suitability for the DNP process may be found. In fact these radicals exist. They belong to the socalled trityl family. Radicals of this well-known type have been further developed by researchers from Amersham Health R&D,2 in order to use them as dopants for DNP in medical imaging.3 The company kindly provided to us several hundred milligrams of the radical OX063 and of two types of their ‘Finland’ radical, the chemical structures of which are shown in Fig. 7. The EPR line of OX063 dissolved in frozen D-propanediol is presented in Fig. 8. Apart form the 1% natural abundance of the isotope 13 C causing the two very weak satellite lines there is no hyperfine structure present, since the nearest spin carrying nuclei are far away from the location of the unpaired electron. The width of the structureless main line is only given by a relatively weak g-factor anisotropy together with some dipolar 2 3
See the acknowledgements to this contribution. See the corresponding article in these proceedings.
13C
34.0
334.5
335.0 335.5 Magnetic Field [mT]
336.0
Fig. 8. X-band EPR spectrum of the trityl radical OX063 dissolved in D-propanediol at liquid nitrogen temperature.
broadening. From the peak-to-peak separation of the X-band signal as well as from bolometric measurements at high field (2:5 T) an upper limit for the g-factor anisotropy of Dg=gC3 104 can be given, which is one order of magnitude less than the corresponding values of the nitroxyl radicals. The width of the frequency curve of this material as shown in Fig. 9 fits well to this estimation. In addition to D-propanediol doped with OX063 also D-butanol doped with ‘Finland D36’ has been tested under lowest temperatures in a 3 He=4 Hedilution refrigerator at a magnetic field of 2:5 T: The reason for the different radicals used with the two substances is their different solubility. While
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80
10
70
5
55 MHz
0 -5
Polarization [%]
Deuteron Polarization [%]
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60 50 40 30 20
-10
10 -15
140 MHz
69.80 69.85 69.90 69.95 70.00 70.05 70.10 Microwave Frequency [GHz] Fig. 9. Deuteron polarization as a function of the applied microwave frequency in irradiated D-butanol (open circles) and in trityl doped D-propanediol (closed circles) at 1 K; 2:5 T:
OX063 is readily solved in highly polar substances like the diols, it is not solvable in the longer chained alcohols as it is the case for the ‘Finland D36’ radical. The optimum spin density for both materials was found to be 1:5 1019 =g; which is somewhat lower than the usual nitroxyl concentration of 2 1019 =g: The highest deuteron polarizations under the conditions mentioned above were achieved after about 1.5 days of continuous microwave irradiation. They correspond to 81% for D-propanediol and to and þ80% for Dbutanol, while values of 760% can be obtained within 10 h of polarizing time. As an example Fig. 10 shows the polarization build-up for Dbutanol. Two remarks should be made at that point: 1. The polarizations reported here are actually not sharp upper limits. They rather reflect those values, which can be obtained in a reasonable time, since at highest polarizations the build-up rate is (a) extremely sensitive to the correct frequency and (b) not higher than one percent per several hours anymore. 2. The reason that in each of the cases only one polarization direction has been driven to a final value is given by the limited running time of the experiment as well. Nevertheless it is expected that the maximum polarization in the respective opposite directions will turn out to be similarly high.
0
0
10
20 Time [h]
30
Fig. 10. Build-up curve of D-butanol doped with ‘Finland D36’ at TC150 mK; B ¼ 2:5 T:
(-1 -> 0) (-1 -> 0)
(0 -> +1)
(0 -> +1) 16.1 16.2 16.3 16.4 16.5 NMR frequency [MHz]
16.1 16.2 16.3 16.4 16.5 NMR frequency [MHz]
Fig. 11. NMR signals of D-propanediol (left) and D-butanol (right) with polarizations of 81% and +80%, respectively.
The polarization measurement have been performed using the asymmetry of the strengths of the two deuteron Zeeman transitions m ¼ þ1-m ¼ 0 and m ¼ 0-m ¼ 1; which are energetically separated by the so-called quadrupole splitting. A polarization of 80% corresponds to a ratio of about 6:1 for the intensities of the two transitions. This is demonstrated in Fig 11. Finally also the relaxation time of the D-butanol sample has been determined. At the reduced field of 0:42 T and the base temperature of the refrigerator of 65 mK a value of 133 h was measured, which is very, similar to what is known
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from nitroxyl-doped D-butanol. Thus also with the new materials experiments in the frozen spin mode are possible without any restrictions.
7. Summary After more than 40 years of research in polarizable solid-state target materials for the first time deuterated substances could be prepared, which can be polarized almost completely. The basis for these developments is still given by the spin temperature theory evolved already in the 1960s. It provides the necessary hints, in which way the process of dynamic nuclear polarization can be made more efficient. The basic idea of the theory consist in expressing the occupation distribution of energy levels belonging to a certain interaction by a temperature, which is in general independent of the physical temperature of the sample (lattice). The interactions of interest are the two Zeeman interaction, the one of the nuclear system and the one of the electron system, as well as the non-Zeeman interaction among the electrons. Each of these interactions is connected to a certain heat reservoir possessing its own heat capacity and its own temperature. The DNP process is then launched by means of the active (microwave induced) cooling of the reservoir of the electron Zeeman interaction. Depending on the kind of nuclei to be polarized as well as on the size of the electron inhomogeneous interaction, the nuclear Zeeman reservoir is cooled either via a direct contact to the electron Zeeman reservoir (solid-state effect) or indirectly via the electron non-Zeeman reservoir (EST or DONKEY effect). Due to the small magnetic moment of the deuteron and the finite EPR width of paramagnetic centers dissolved in a solid-state material, a deuteron spin system is always in thermal contact to the electron non-Zeeman reservoir. Thus it is the EST (DONKEY) effect, which provides the relevant polarization mechanism in this case. It works the more effectively the smaller the non-Zeeman heat capacity of the electron spin system is. Its size is determined almost completely by the strength of the inhomogeneous interactions (hyperfine structure and g-factor anisotropy), since compared to
them the dipole–dipole interaction is at least one order of magnitude weaker for the usual concentrations of some 1019 spins per gram. Two different doping methods—both providing weakly interacting paramagnetic centers—have been tested for deuterated butanol, the standard target material for experiments on polarized deuterons (neutrons) with low-to-moderate beam energies and intensities. Each of these methods, the irradiation and the chemical doping with radicals of the trityl family, provides a degree of polarization never seen in this substance before. With the help of the radiation doping values in excess of 70% at 5 T could be obtained, whereas the trityl doping even allowed a polarization of 80% at a magnetic field as low as 2:5 T: Furthermore, the materials prepared using the new doping methods are just as suitable for the use in frozen spin experiments as conventionally doped D-butanol. With these developments future scattering experiments on polarized neutrons or deuterons will achieve a precision similar to those on polarized protons nowadays.
Acknowledgements The trityl radicals were kindly provided by Amersham Health R&D. Furthermore the authors want to thank Jan Henrik Ardenkjaer-Larson and Jan Wolber for their incessant readiness for open and fruitful discussions.
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A. Abragam, Rep. Prog. Phys. 41 (1978) 396. V.A. Atsarkin, M.I. Rodak, Sov. Phys. Usp. 15 (1973) 251. St. Goertz, et al., Prog. Particle Nuc. Phys. 49 (2002) 403. J. Harmsen, Ph.D. Thesis, Bochum University, 2002. S. Trentalange, et al., in: W. Meyer, E. Steffens, W. Thiel (Eds.), Proceedings of the Ninth Symposium on High Energy Spin Physics, Bonn, 1990, p. 325. B. Smaller, M.S. Matheson, J. Chem. Phys. 28 (1958) 1169. H. Zeldes, R. Livingston, J. Chem. Phys. 30 (1959) 40. R.S. Alger, et al., J. Chem. Phys. 30 (1959) 695. C. Chachaty, E. Hayon, J. Chem. Phys. 61 (1964) 1115. V.A. Roginskii, V.I. Tupikow, EPR of Free Radicals in Radiation Chemistry, Wiley, New York, 1974.
Nuclear Instruments and Methods in Physics Research A 526 (2004) 43–52
Highest polarizations in deuterated compounds$ S.T. Goertz*, J. Harmsen, J. Heckmann, Ch. HeX, W. Meyer, E. Radtke, G. Reicherz Institut fur 150, Bochum 44780, Germany . Experimentalphysik I, Ruhr-Universitat . Bochum, Universitatsstr. .
Abstract Unlike the proton, which can be almost completely polarized even at temperatures as high as 1 K; achieving a high polarization in a deuteron system is a much more complicated task. The reason for that is the much lower magnetic moment of the deuteron compared to that of the proton. Since the nuclear polarization is determined by the ratio of the magnetic to the thermal energy, the only way to further enhance the deuteron polarization consists of reducing the effective temperature. Speaking in terms of the spin temperature theory, the nuclear Zeeman temperature has to be minimized. This can be done most effectively by using paramagnetic centers, which experience only a weak non-Zeeman interaction. In deuterated alcohols or diols such centers can be provided by either an irradiation with ionizing particles or by chemical doping with radicals of the trityl family. Both methods led to a substantial improvement of the maximum polarization in these substances. Values of about 80% have been measured in trityl-doped D-butanol and D-propanediol. r 2004 Elsevier B.V. All rights reserved. PACS: 29.25.P Keywords: DNP; Polarized targets; Spin temperature; Paramagnetic centers; EPR; NMR
1. Introduction
*
In order to provide a polarized solid target for a scattering experiment on the proton or on the neutron, one has to rely on hydrogen-rich substances or on materials containing polarizable and neutron-rich nuclei (preferably deuterons), respectively. The choice of the most suitable material depends on several factors:
*
*
The relative content of polarizable nucleons (dilution factor).
$ Supported by the ‘Bundesministerium fur . Bildung und Forschung’ and the ‘Deutsche Forschungsgemeinschaft’. *Corresponding author. E-mail address: [email protected] (S.T. Goertz).
*
The stiffness of the polarization against radiation damage (radiation resistance). The maximum available polarization. The presence of polarizable nuclei apart from protons and deuterons (polarized background).
The target materials used so far divide into two classes, one containing inorganic substances like ammonia (NH3 ; ND3 ) and lithium deuteride (6 LiD). They are highly polarizable and exhibit a good radiation resistance together with a highdilution factor. In particular 6 LiD provides the highest available content of polarizable nucleons compared to all nucleons present in the material. The reason is that the 6 Li nucleus may be
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.03.148
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considered as an a particle plus two loosely bound nucleons, a proton and a neutron. Just like in the case of the deuteron, the spins of these two valence nucleons are essentially parallel and thus responsible for the polarization of the 6 Li nucleus. While for experiments at high energies, one may profit from the highly polarized 6 Li nucleus in this way, for experiments at intermediate energies the problem raises, to which extent the experimental data are contaminated by nuclear binding effects and how to correct for them. That is why for these experiments organic materials like alcohols or diols, which make up the class of organic materials, are still preferred. Apart from the lower radiation hardness of these substances they suffer from another problem, namely the limited deuteron polarization available from the deuterated versions of these materials. Whereas for instance normal (protonated) butanol can be polarized up to 90% using a dilution refrigerator and a magnetic field of 2:5 T; in deuterated butanol the maximum polarization do not exceed 30–40% under the same conditions. From this point of view, a further development of the deuterated organic target materials concerning their maximum polarization as well as their radiation hardness is of high priority. Both properties are supposed to depend on the physics nature of the paramagnetic centers used for the DNP process. This contribution reports about the first and very successful polarization tests of alternatively doped D-butanol and D-propanediol.
2. Theoretical background The theoretical background underlying the invention of the new doping methods is described at length in the article ‘The dynamic nuclear polarization process’, which can be found in these proceedings. Thus only a brief summary of the basic idea is given in the present section. The nuclear (vector) polarization can be expressed using the Brillouin function: ! gmB 1 PI ¼ BI o a ð1Þ ¼ B I I I : 2 2kTIZ
Since BI is a monotonously increasing function of its argument, i.e. of the ratio of the magnetic energy to the thermal energy, an optimization of the polarization is equivalent to a minimization of the temperature TIZ of the nuclear Zeeman reservoir. In contrast to a proton spin system, which—depending on the actual width of the EPR line—may or may not be in thermal contact to the electron non-Zeeman (dipolar) reservoir, for a deuteron spin system with its low Larmor frequency this is always the case. Thus the inverse spin temperature aI of the deuteron Zeeman reservoir is given by the equilibrium value of the non-Zeeman inverse spin temperature of the electron system: beq ¼
WTD ðD2 =D2 Þ aL : 1 þ W ðTZS þ ðD2 =D2 ÞTDS Þ
ð2Þ
For the case of complete saturation W b1=TZS ; 1=TDS the above expression simplifies to beq ¼
CSZ
CSZ aL : þ ðTZS =TDS ÞCSD
ð3Þ
Although these relations are quantitatively valid only in the high-temperature limit, i.e. if the magnetic energy of the system is small compared to its thermal energy, they correctly describe the qualitative dependence of the maximum inverse spin temperature on the respective parameters for all temperatures.1 A further simplification, which has been done in the theoretical considerations of the contribution cited above, concerns the origin of the finite size of the EPR line. In the Provotorov theory it is assumed that the broadening of the electron Zeeman levels entirely stems from a local magnetic field Bl ; which is produced by the neighboring unpaired electrons (homogeneous broadening). In nature however inhomogeneous interactions like the hyperfine interaction with adjacent nuclei and a broadening due to a possible anisotropy of the g-factor are always present. Their respective strengths may be much larger than that of the dipole–dipole interaction. Nevertheless, it can be shown experimentally [2,3] that 1
Special solutions for the general case of arbitrary temperatures have been given by M. Borghini. They can be found in Ref. [1].
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also in this case the EPR line behaves in a similar way as it would do under an entirely homogeneous interaction. The only assumption is that the dipole–dipole interaction has to be strong enough to equalize the energy difference between energetically neighboring spin packets. Then Eqs. (2) and (3) are still valid provided the local frequency D ¼ gBl is extended by an inhomogeneous contribution Bl;inhom to the local field D2 ¼ gðB2l;hom þ B2l;inhom Þ
ð4Þ
which enters into the expression for the heat capacity of the non-Zeeman reservoir CSD ¼ NAD2 : Therefore relation (3) in combination with Eq. (1) may be taken as a guideline showing which parameters eventually determine the achievable polarization. Apart from the ratio of the two time constants of the electron spin system, its Zeeman (TZS ) and its dipolar relaxation time (TDS ), the most crucial parameter is the size of the heat capacity CSD : Since the inhomogeneous contribution to the local field is usually at least one order of magnitude larger than the homogeneous one, the best way to minimize CSD is to search for a doping method, which provides paramagnetic centers with a particularly weak inhomogeneous interaction.
3. Doping methods There are two methods of producing paramagnetic centers in a solid-state target material. One is the radiation doping method, which in principle works for all materials independently of their state at room temperature. Here defects are produced by a radiation-induced fission of the respective molecules, which may undergo several secondary reactions to eventually build up a paramagnetic center. The second method only works for substances possessing a liquid phase not to high above room temperature. In this case a certain amount of a stable chemical radical can be admixed, followed by a rapid freezing of the liquid to ensure a homogeneous distribution of the unpaired electrons, which are associated with the radical molecules. Each of these methods have their own specific problems. In the case of the radiation doping,
45
several things are a priori completely unknown and must be determined by experiment: *
*
*
The number of different radiation defects and their production efficiency. The nature of the defects including the questions, whether they are paramagnetic or not. The properties of the paramagnetic defects regarding their inhomogeneous interactions.
Unfortunately, from the experimental side there is only a limited influencing control on these points. Only the temperature during the irradiation as well as its duration and the intensity of the particle beam can be varied. But at least regarding the third point a rather general statement can be made: The radiation doping method recommends itself in particular for the doping of deuterated substances, or more precisely, for the doping of substances containing only those spin carrying nuclei, which possess a small magnetic momentum. The reason is that in this case the unpaired electron, which wave function extends more or less to the neighboring nuclei, will undergo only a relatively weak hyperfine interaction. Consequently, if the second inhomogeneous interaction, the g-factor anisotropy is not to strong, the width of the EPR line made up by these electrons will be rather narrow. These arguments led to the decision to test the radiation doping method also for the alcohols, substances, which are usually doped by the chemical method. Apart from the requirement that the substance has to be liquid in a temperature region, where chemical radicals are stable, the problematic nature of this method consists of the availability of suitable radical products. In the past only two different types of chemical radicals have been gained acceptance. These are Porphyrexide and TEMPO as members of the nitroxyl family as well as EHBA (EDBA) containing a Cr(V) complex.
4. Magnetic field dependence of conventionally doped D-butanol Since several years it has been tried to improve the deuteron polarization available from deuterated
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alcohols. In this connection the dependence of the polarization on the strength of the external magnetic field have been measured in hoping to create higher values at higher magnetic fields. At least a behavior like that is suggested by the theory, since the argument of the Brillouin function (Eq. (1)) exhibit a quadratic dependence on the field strength, one given by the magnetic energy and one by the minimum achievable spin temperature. Nevertheless the experimental results are exactly the opposite. Fig. 1 shows the maximum deuteron polarizations as measured in D-butanol doped with the two nitroxyl radicals and with EDBA [4]. These results are in agreement with earlier measurement made by the polarized target group of the PSI [5]. They show a maximum around 2.5–3:5 T followed by a rapid decrease at higher fields. Apart from the sample doped with Porphyrexide the maximum polarization do not exceed some 10% at a field of 5 T: The striking discrepancy between theory and experiment may be explained by a strong anisotropy of the gfactors in these radicals. For its measurement EPR lines were taken using a X-band spectrometer at liquid nitrogen temperature and compared with best fitting simulations. As an example Fig. 2 shows the EPR line of TEMPO doped butanol together with the respective line shape simulation.
The results for the average deviation Dg from gC2 are the following: Radical
Dg=g ½103
TEMPO Porphyrexide EHBA/EDBA
3:8570:2 3:070:3 6:070:2
In Fig. 3 these numbers are translated into the corresponding EPR line widths drawn versus the applied magnetic field. In addition to the g-factor anisotropy the two nitroxyl radicals also possess some hyperfine interaction due to a nitrogen nucleus nearby the unpaired electron. It is reflected by the prominent three line structure of the EPR line as shown in Fig. 2. Thus, in contrast to EDBA, the respective resonance lines have a finite width also at vanishing magnetic field. Nevertheless, for magnetic fields above a certain value the EPR line widths of all of the three radicals are linearly dependent on the magnetic field. In this, the explanation for the unusual behavior of the achievable polarization may be found, since the mechanism for the cooling of the deuteron Zeeman reservoir assumes that the inhomogeneous contribution to the electron non-Zeeman interactions must be weak enough to be balanced by the homogeneous (dipolar) contribution. This
Simulation Experiment
|Polarization| [%]
40
30
20
EDBA Porphyrexide TEMPO TEMPO + 5% D2O
10
Axx = 0.73 mT gxx = 2.0094 Ayy = 0.63mT gyy = 2.0065 Azz = 3.60 mT gzz = 2.0017
0 2
3 4 Magnetic Field [T]
5
Fig. 1. Dependence of the maximum polarization in fully deuterated butanol on the strength of the external magnetic field measured in a dilution refrigerator. The lines are only for eye guiding.
325
330
335 340 Magnetic Field [mT]
345
Fig. 2. EPR line of 0.5% (mass) TEMPO solved in butanol together with the best fitting simulation. The corresponding components of the g-tensor and the hyperfine tensor are given in the insert.
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dependence of chemically doped D-butanol is connected to the properties of the radicals used for the DNP process, since radiation doped samples of the same material behave completely different. This will be discussed in the next section.
5. Radiation-doped alcohols 5.1. The process of radiation doping
Fig. 3. Full-widths of the inhomogeneously broadened EPR versus the magnetic field for the different chemical radicals: (1) EHBA/EDBA, (2) TEMPO, (3) Porphyrexide. For comparison also the double values of the Larmor frequencies of protons (4) and deuterons (5) are included.
assumption may be violated above a certain magnetic field. Although this explanation is highly speculative, it is supported by several observations: *
*
The decrease of the polarization with increasing magnetic field is strongest in the case of EDBA, which itself exhibits the highest g-factor anisotropy of all of the investigated radicals. It is weakest in the case of Porhpyrexide having the smallest anisotropy.
A further hint comes from the behavior of normal (protonated) butanol doped with the same radicals. These substances show—instead of a decrease—an increase of the polarization at high fields. At first sight it may seem that this is in contradiction to what has been said before, but taking into account the highly different values of the magnetic moment for protons and deuterons, this observation may fit to the present picture as well: Due to the much higher Larmor frequency of the protons their Zeeman reservoir is in a much closer contact to the Zeeman reservoir of the electrons than it would be possible for a deuteron spin system. Thus an additional polarization channel may open up, which is the DNP via the differential solid effect. To sum up, at least one definitive statement can be made. The reason for the magnetic field
Deuterated as well as normal butanol have been irradiated at the Bonn injection Linac, which provides 20 MeV electrons with a time-averaged beam current of 10–20 mA: The samples prepared as frozen beads with diameters of about 2 mm were loaded into a special irradiation cryostat operating at the temperature of liquid argon (87 K). Compared to the inorganic materials the efficiency of defect creation in alcohols is almost two orders of magnitude higher. Thus an accumulated dose of 1015 e =cm2 ; corresponding to an irradiation time of only some minutes, is sufficient to create a radical density high enough for the DNP process to work. In this way two samples of fully deuterated butanol with different radiation doses were produced. By comparing the integrated EPR line intensities with those of chemically doped solutions the densities of the paramagnetic centers could be determined to be 1 1019 =g (sample No.1) and 2 1019 =g (sample No.2), respectively. 5.2. Polarization results In order to decide whether or not an irradiation is a suitable method for the doping of the alcohols at all, the samples were first tested in a 4 Heevaporation refrigerator at a magnetic field of 2:5 T: The results for the maximum polarization, the build-up and the relaxation time at about 1 K were the following:
No.
Pol. (%)
Build-up (s)
Relax. (s)
1 2
12.7 10.1
800 242
2924 1148
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For comparison in chemically doped D-butanol deuteron polarizations between 8% and 9% are usually obtained under the same conditions. With these encouraging results in mind the samples were polarized in a dilution refrigerator at magnetic fields of 2.5 and 5 T: Fig. 4 summarizes the measured values. In contrast to the behavior of chemically doped D-butanol, the polarization of the radiation-doped version of this substance increases with increasing magnetic field at least up to a field value of 5 T: At the time of the experiment (2001), a value of more than 70% was the highest deuteron polarization ever observed at a magnetic field of 5 T and the highest one in an organic solid-state target material at all. It was recently surpassed only by the results obtained from trityl-doped D-butanol and Dpropanediol, which will be presented in the next section. In order to get a better understanding of how the properties of the paramagnetic centers influence the efficiency of the DNP process as well as to draw a clean dividing line between the two doping methods, the structure of the radiation induced paramagnetic defect had to be determined. For this purpose alcohols of different chain lengths as well as of different degrees of deuterization have been irradiated and investigated in the X-band spectrometer.
70
-70.8 %
|Polarization| [%]
60
-55.1 % -50.3 %
50
+45.5 %
40 -34.8 %
30
Sample 1 Sample 2
20
5.3. The nature of the radiation-induced defects in the alcohols Beads of normal, partially and fully deuterated alcohols from methanol up to pentanol have been irradiated using the 2 MeV electrons from a 90 Sr radioactive source for periods of about 1 h: Classified by the major features of their EPR lines the samples may be divided into three groups: Fully protonated samples. The structure of the spectra is given by a pronounced hyperfine splitting due to the hydrogen atoms adjacent to the unpaired electron (e.g. top line in Fig. 6). Fully deuterated samples. The spectra essentially consist of a single line with a weak to moderate splitting due to hyperfine interaction with the adjacent deuterons (Fig. 5(a) and (b)). Partially deuterated samples. The hyperfine splitting with the residual protons dominates the spectra. The presence of the deuterons manifests itself only by a weak modulation of the proton HFS peaks (bottom line in Fig. 6). In contrast to the EPR line of D-butanol, which hyperfine peaks are not sufficiently resolved to allow an unambiguous assignment of the spectrum to a certain configuration, the origin of the Dethanol spectrum is rather obvious. Here a clear structure composed out of nine nearly equidistant lines is observed. They stem from an ethanol molecule, which lost one of its deuterium atoms. With the exception of the OD-group the residual unpaired electron spreads over the whole molecule with almost equal densities at the sites of the n ¼ 4 deuterium nuclei. Thus 2n þ 1 ¼ 9 hyperfine lines are expected in agreement with the observation. For the longer alcohols the interpretation of the EPR spectra is more complicated. Nevertheless it has been suggested by several authors [6–9] that the most probable electronic configuration of the radical molecule is of the type
10 0 0
1
2 3 Magnetic Field [T]
4
5
Fig. 4. Maximum deuteron polarizations of radiation doped Dbutanol measured in a dilution refrigerator. The lines are only for eye guiding.
A hydrogen or a deuterium atom is removed from the hydroxyl group and the wave function of the unpaired electron spreads over the atoms bound to the a- and b-carbons. In contrast to ethanol, in
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CH3(CH2)3OH
Signal Amplitude
CD3(CH2)3OH CD3CD2(CH2)2OH
CD3(CD2)2CH2OH
325
(a)
332
333
334
335
336
337
338
4 2 1 5 9 8
6 7
333 334 335 336 337 338 333
335 340 Magnetic Field [mT]
345
Fig. 6. EPR spectra of differently deuterated butanol. From top to bottom: n-butanol, n-butanol-4.4.4-D3, n-butanol4.4.4.3.3-D5, n-butanol-4.4.4.3.3.2.2-D7.
direct after irradiation after 5 min of bleaching
3
332
330
334 335 336 Magnetic Field [mT]
337
338
(b) Fig. 5. EPR lines of electron-irradiated solid D-butanol (a) and D-ethanol (b). In (b) the EPR line as measured directly after the irradiation (gray) is shown together with the one obtained after bleaching under normal light for 5 min (black). The residual line shown in the insert are due to solvated electrons.
which the methyl end of the molecule rotates freely under all circumstances, the rotation of the b with respect to the hydroxyl group (a-group) is hindered in the longer alcohols at low tempera-
tures. This leads to different hyperfine constants for the two b-hydrogen (deuterium) atoms. Consequently more than the n þ 1 ¼ 4 lines, which are expected for an interaction with n ¼ 3 equivalent hydrogen atoms, are visible. Values of 20 mT are found in the literature for the a- and one of the b-protons, whereas the second b-proton couples with a strength of 40 mT to the electron [10]. Our measurements are in agreement with these considerations. As an example Fig. 6 shows the EPR spectra of several butanol samples with increasing degrees of deuterization. Starting from the methyl towards the hydroxyl group more and more hydrogen atoms are replaced by deuterium atoms. The prominent five line structure, which is in agreement with the hyperfine constants given above, is essentially unchanged until the b-protons are finally replaced by deuterons, too. At that point the EPR line shape changes completely from the quintet to a doublet, for which the only explanation is that the unpaired electron stems in fact from the hydroxyl group. The interaction with the b-deuterons is reflected by the rather delicate fine structure of the line. These arguments, which are also supported by further EPR measurements made in normal, partially and fully deuterated methanol, propanol as well as in some higher alcohols, strongly suggest that also the paramagnetic center of radiation doped D-butanol is of the hydroxyl radical type.
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From the symmetry of the EPR lines it is apparent that the anisotropy of the g-factor of these centers is small, at least much smaller than that of the nitroxyl and Cr(V) radicals. The major broadening mechanism consists of the hyperfine interaction of the unpaired electron, which—in contrast to the g-factor anisotropy—gives a contribution to the EPR line width independently of the magnetic field. Thus the overall line width remains relatively narrow even at fields as high as 2.5 and 5 T: This is supported by a likewise narrow frequency curve of this material, i.e. by the deuteron polarization as a function of the applied microwave frequency as shown in Fig. 9. In this the reason for the high-polarization values in irradiated D-butanol may be found, since a narrow EPR line is synonymous to a small heat capacity of the electron non-Zeeman reservoir, which can thus be cooled very efficiently.
(a)
(b)
Fig. 7. Structures of the trityl radicals OX063 (a) and ‘Finland D36’ (b). The molecular weights are 1426.78 and 1036.14, respectively.
~ 0.22 mT
13C
6. Trityl doped D-butanol and D-propanediol The very good results obtained from irradiated D-butanol opened up the question, whether also stable chemical radicals of a similarly excellent suitability for the DNP process may be found. In fact these radicals exist. They belong to the socalled trityl family. Radicals of this well-known type have been further developed by researchers from Amersham Health R&D,2 in order to use them as dopants for DNP in medical imaging.3 The company kindly provided to us several hundred milligrams of the radical OX063 and of two types of their ‘Finland’ radical, the chemical structures of which are shown in Fig. 7. The EPR line of OX063 dissolved in frozen D-propanediol is presented in Fig. 8. Apart form the 1% natural abundance of the isotope 13 C causing the two very weak satellite lines there is no hyperfine structure present, since the nearest spin carrying nuclei are far away from the location of the unpaired electron. The width of the structureless main line is only given by a relatively weak g-factor anisotropy together with some dipolar 2 3
See the acknowledgements to this contribution. See the corresponding article in these proceedings.
13C
34.0
334.5
335.0 335.5 Magnetic Field [mT]
336.0
Fig. 8. X-band EPR spectrum of the trityl radical OX063 dissolved in D-propanediol at liquid nitrogen temperature.
broadening. From the peak-to-peak separation of the X-band signal as well as from bolometric measurements at high field (2:5 T) an upper limit for the g-factor anisotropy of Dg=gC3 104 can be given, which is one order of magnitude less than the corresponding values of the nitroxyl radicals. The width of the frequency curve of this material as shown in Fig. 9 fits well to this estimation. In addition to D-propanediol doped with OX063 also D-butanol doped with ‘Finland D36’ has been tested under lowest temperatures in a 3 He=4 Hedilution refrigerator at a magnetic field of 2:5 T: The reason for the different radicals used with the two substances is their different solubility. While
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80
10
70
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55 MHz
0 -5
Polarization [%]
Deuteron Polarization [%]
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60 50 40 30 20
-10
10 -15
140 MHz
69.80 69.85 69.90 69.95 70.00 70.05 70.10 Microwave Frequency [GHz] Fig. 9. Deuteron polarization as a function of the applied microwave frequency in irradiated D-butanol (open circles) and in trityl doped D-propanediol (closed circles) at 1 K; 2:5 T:
OX063 is readily solved in highly polar substances like the diols, it is not solvable in the longer chained alcohols as it is the case for the ‘Finland D36’ radical. The optimum spin density for both materials was found to be 1:5 1019 =g; which is somewhat lower than the usual nitroxyl concentration of 2 1019 =g: The highest deuteron polarizations under the conditions mentioned above were achieved after about 1.5 days of continuous microwave irradiation. They correspond to 81% for D-propanediol and to and þ80% for Dbutanol, while values of 760% can be obtained within 10 h of polarizing time. As an example Fig. 10 shows the polarization build-up for Dbutanol. Two remarks should be made at that point: 1. The polarizations reported here are actually not sharp upper limits. They rather reflect those values, which can be obtained in a reasonable time, since at highest polarizations the build-up rate is (a) extremely sensitive to the correct frequency and (b) not higher than one percent per several hours anymore. 2. The reason that in each of the cases only one polarization direction has been driven to a final value is given by the limited running time of the experiment as well. Nevertheless it is expected that the maximum polarization in the respective opposite directions will turn out to be similarly high.
0
0
10
20 Time [h]
30
Fig. 10. Build-up curve of D-butanol doped with ‘Finland D36’ at TC150 mK; B ¼ 2:5 T:
(-1 -> 0) (-1 -> 0)
(0 -> +1)
(0 -> +1) 16.1 16.2 16.3 16.4 16.5 NMR frequency [MHz]
16.1 16.2 16.3 16.4 16.5 NMR frequency [MHz]
Fig. 11. NMR signals of D-propanediol (left) and D-butanol (right) with polarizations of 81% and +80%, respectively.
The polarization measurement have been performed using the asymmetry of the strengths of the two deuteron Zeeman transitions m ¼ þ1-m ¼ 0 and m ¼ 0-m ¼ 1; which are energetically separated by the so-called quadrupole splitting. A polarization of 80% corresponds to a ratio of about 6:1 for the intensities of the two transitions. This is demonstrated in Fig 11. Finally also the relaxation time of the D-butanol sample has been determined. At the reduced field of 0:42 T and the base temperature of the refrigerator of 65 mK a value of 133 h was measured, which is very, similar to what is known
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from nitroxyl-doped D-butanol. Thus also with the new materials experiments in the frozen spin mode are possible without any restrictions.
7. Summary After more than 40 years of research in polarizable solid-state target materials for the first time deuterated substances could be prepared, which can be polarized almost completely. The basis for these developments is still given by the spin temperature theory evolved already in the 1960s. It provides the necessary hints, in which way the process of dynamic nuclear polarization can be made more efficient. The basic idea of the theory consist in expressing the occupation distribution of energy levels belonging to a certain interaction by a temperature, which is in general independent of the physical temperature of the sample (lattice). The interactions of interest are the two Zeeman interaction, the one of the nuclear system and the one of the electron system, as well as the non-Zeeman interaction among the electrons. Each of these interactions is connected to a certain heat reservoir possessing its own heat capacity and its own temperature. The DNP process is then launched by means of the active (microwave induced) cooling of the reservoir of the electron Zeeman interaction. Depending on the kind of nuclei to be polarized as well as on the size of the electron inhomogeneous interaction, the nuclear Zeeman reservoir is cooled either via a direct contact to the electron Zeeman reservoir (solid-state effect) or indirectly via the electron non-Zeeman reservoir (EST or DONKEY effect). Due to the small magnetic moment of the deuteron and the finite EPR width of paramagnetic centers dissolved in a solid-state material, a deuteron spin system is always in thermal contact to the electron non-Zeeman reservoir. Thus it is the EST (DONKEY) effect, which provides the relevant polarization mechanism in this case. It works the more effectively the smaller the non-Zeeman heat capacity of the electron spin system is. Its size is determined almost completely by the strength of the inhomogeneous interactions (hyperfine structure and g-factor anisotropy), since compared to
them the dipole–dipole interaction is at least one order of magnitude weaker for the usual concentrations of some 1019 spins per gram. Two different doping methods—both providing weakly interacting paramagnetic centers—have been tested for deuterated butanol, the standard target material for experiments on polarized deuterons (neutrons) with low-to-moderate beam energies and intensities. Each of these methods, the irradiation and the chemical doping with radicals of the trityl family, provides a degree of polarization never seen in this substance before. With the help of the radiation doping values in excess of 70% at 5 T could be obtained, whereas the trityl doping even allowed a polarization of 80% at a magnetic field as low as 2:5 T: Furthermore, the materials prepared using the new doping methods are just as suitable for the use in frozen spin experiments as conventionally doped D-butanol. With these developments future scattering experiments on polarized neutrons or deuterons will achieve a precision similar to those on polarized protons nowadays.
Acknowledgements The trityl radicals were kindly provided by Amersham Health R&D. Furthermore the authors want to thank Jan Henrik Ardenkjaer-Larson and Jan Wolber for their incessant readiness for open and fruitful discussions.
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