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NMR system for imaging3He-4He mixtures on the melting curve
PHYSICA[
Physica B 194-196 (1994) 147-148 North-Holland
NMR System for Imaging 3He -4He Mixtures on the Melting Curve ¥ o r a m Swirski', Israel Shuster, Ehud J. Schmidt t , Emil Polturak, Stephen G. Lipson a nDepartment of Physics, Technion, Israel Institute of Technology, Haifa 32000, Israel A pulsed 10 MHz NMI% system for imaging 3He - 4 H e mixtures on the melting curve between 0.4K to 1.bK is
described. The spatial distribution of 3He , Tt and T~ relaxation times were mapped along the vertical direction, perpendicular to the liquid-solid interface. Multi-exponential fits for the relaxation times were made. These are useful to follow the relaxation of structural inhomogeneities in the crystal.
In our previous experiments, optical observations showed that the 3He distribution in 3He 4He crystals is highly inhomogeneons [1]. To learn more about these crystals, we decided to use NMI~ imaging, which provides a direct method for plotting the 3He spatial distribution as well as for mapping physical properties reflected ha the relaxation times T I and Tz. A 1D mapping was performed in the direction perpendicular to the liquid-solid interface in the sample cell. The location of each phase was determined using their different Tz and T2 • Here, we describe the NMP~ system. Experimental results are presented in [2]. The sample cell is a 1 cm diameter and height upright cylinder (see fig 1). It is made of 1266 Stycast and connected thermally to the 3He refrigerator using a gold wire. Cell temperature was measured on this wire. Using a heated fill line: we could grow crystals at a any fixed temperature between 0.45K and 1.2K, including temperatures lower than the minimum of the melting
Thermallink
Heated
to3He fddge
fill line
A J' li::i::i Iil F:i ; k- - a ~ : . ~ / Sphe.c= ~ transmitting / ~
Sadd
reC:ol~llng - " ' ~ " ~ ' -
coil
/ High voltage
Figure 1. Schematic view of the sample cell.
curve.
The N M K system consists of spherical transmitting and saddle receiving crossed coils, as well as gradient and shim coils. The Larmor frequency is 10 MHz. The spherical coil provides the best transmission H1 field homogeneity [3, 4] under the restrictions of limited cryostat space and transmitted pulse power. The field uniformity of the saddle receiving coil is slightly inferior to the spherical coil, but it provides the best J~lling factor in our geometry. The Q of the P~F coils is low (about 10) to enable transmission and reception "Present axid.rcss: P,.n/acl, P.O.B. tpresent addresl: Elsclnt, P.O.B.
2250,Halfa, Inrael 550,Haifa, I~rael
in. the bandwidth required for imaging. The home built receiver was designed for phase sensitive detection to enable extraction of the data in the frequency domain using a minimal spectra/ bandwidth. It's gain is 100 dB. The NMK pulses, gradient field and the receiver protection switch are programmed from a 386 PC using commercial waveform synthesizer boards. Exact timing and lengths of the pulses (including phase determination relative to the external timhag oscillator) was done by a home built board containi,~g T T L digital delay lines and switches.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0652-W
148
i :i:J~Calibration!curve ~I, •.,,.!!,.
46810
Frequency [kHz]
Figure 2. Signal profiles of a solid filled cell.
The signal from the receiver is digitized by a commercial digitizer (A/D) board located in the same computer. The controlling computer program was responsible for the timing of the pulses and the A/D. It also provides on-line view of the time domain signal and the frequency profiles ( F F T ) of selected echoes. A 40 echo C P M G sequence (with r ~ 1 msec) in the presence of a Z gradient field was used to evaluate the 3He concentration and T2 spatial distribution. Saturation recovery sequences with different pulse spacing were used to determine T1 distribution. The F F T transformed signal was fitted at each frequency to a multi-exponential decay model [5]. This model allows for the coexistence of spins with different relaxation times in the same region. The growth process of the crystals was monitored by imaging the relaxation times during growth and annealing, using the large differences in T1 between solid and liquid. In certain conditions T2 also provided good contrast [2]. Calibration measurements were conducted on uniform solid mixture samples that were allowed to anneal for many hours. The calibrations compensate for magnetic field inhomogeneities
(H0 and received H1 ).The raw data used for the calibration file is shown in 2. The calibration measurements were used later for the absolute determination of spatial spin ( 3 H e ) distribution in a cell filled with solid or with a solid-liquid mixture. One such profile for an all-solid sample is shown in 2. The gradient field homogeneity and the frequency - location relationship in the cell were determined by measuring the signal profile with the cell partially filled with liquid to various heights. The S/N ratio is limited by external noise, not by the receiver or by thermal noise in the coils. The relevant factors affecting the S/N are the number of spins, field gradient strength and temperature [6]. In a typical situation (1K on the melting curve with 3He concentration of 2%, gradient of 3 Gauss/cm) the cell could be divided into 30 slices with S/N of about 10 for each slice (2). This corresponds to a lower detection limit of 2 x 10 is spins at 1K. We do not signal average, in order to be able to follow temporal changes. This proved important in the study of the relaxation of He inhomogeneities as evidenced from the multiexponential decay of the spin echoes [2]. We thank B. Cowan and R.C. Richardson for useful discussions and advice. This work was supported by the US - Israel Binational Science Foundation and by the Israel Academy of Science. REFERENCES
1. Carmi, Y. et al., Phys. Rev. Lett. 62 (1989) 1364. 2. Shuster, I. et al., these proceedings, LT20 (1993). 3. Everett, :I.E. and Osemeikhian, J.E., J. Sci. Instrum. 43 (1966) 470. 4. Mansfield, P. and Morris, P.G., NMR Imaging in Biomedicine. Ed.: Waugh, J.S. (Academic Press, 1982). 5. Schmidt, E.J. et al., J. Appl. Phys. 59 (1986) No. 8, p. 2788. 6. Hoult, D.I. and Richards, R.E., J. of Mag. Res. 24 (1976) 71..
Physica B 194-196 (1994) 147-148 North-Holland
NMR System for Imaging 3He -4He Mixtures on the Melting Curve ¥ o r a m Swirski', Israel Shuster, Ehud J. Schmidt t , Emil Polturak, Stephen G. Lipson a nDepartment of Physics, Technion, Israel Institute of Technology, Haifa 32000, Israel A pulsed 10 MHz NMI% system for imaging 3He - 4 H e mixtures on the melting curve between 0.4K to 1.bK is
described. The spatial distribution of 3He , Tt and T~ relaxation times were mapped along the vertical direction, perpendicular to the liquid-solid interface. Multi-exponential fits for the relaxation times were made. These are useful to follow the relaxation of structural inhomogeneities in the crystal.
In our previous experiments, optical observations showed that the 3He distribution in 3He 4He crystals is highly inhomogeneons [1]. To learn more about these crystals, we decided to use NMI~ imaging, which provides a direct method for plotting the 3He spatial distribution as well as for mapping physical properties reflected ha the relaxation times T I and Tz. A 1D mapping was performed in the direction perpendicular to the liquid-solid interface in the sample cell. The location of each phase was determined using their different Tz and T2 • Here, we describe the NMP~ system. Experimental results are presented in [2]. The sample cell is a 1 cm diameter and height upright cylinder (see fig 1). It is made of 1266 Stycast and connected thermally to the 3He refrigerator using a gold wire. Cell temperature was measured on this wire. Using a heated fill line: we could grow crystals at a any fixed temperature between 0.45K and 1.2K, including temperatures lower than the minimum of the melting
Thermallink
Heated
to3He fddge
fill line
A J' li::i::i Iil F:i ; k- - a ~ : . ~ / Sphe.c= ~ transmitting / ~
Sadd
reC:ol~llng - " ' ~ " ~ ' -
coil
/ High voltage
Figure 1. Schematic view of the sample cell.
curve.
The N M K system consists of spherical transmitting and saddle receiving crossed coils, as well as gradient and shim coils. The Larmor frequency is 10 MHz. The spherical coil provides the best transmission H1 field homogeneity [3, 4] under the restrictions of limited cryostat space and transmitted pulse power. The field uniformity of the saddle receiving coil is slightly inferior to the spherical coil, but it provides the best J~lling factor in our geometry. The Q of the P~F coils is low (about 10) to enable transmission and reception "Present axid.rcss: P,.n/acl, P.O.B. tpresent addresl: Elsclnt, P.O.B.
2250,Halfa, Inrael 550,Haifa, I~rael
in. the bandwidth required for imaging. The home built receiver was designed for phase sensitive detection to enable extraction of the data in the frequency domain using a minimal spectra/ bandwidth. It's gain is 100 dB. The NMK pulses, gradient field and the receiver protection switch are programmed from a 386 PC using commercial waveform synthesizer boards. Exact timing and lengths of the pulses (including phase determination relative to the external timhag oscillator) was done by a home built board containi,~g T T L digital delay lines and switches.
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0652-W
148
i :i:J~Calibration!curve ~I, •.,,.!!,.
46810
Frequency [kHz]
Figure 2. Signal profiles of a solid filled cell.
The signal from the receiver is digitized by a commercial digitizer (A/D) board located in the same computer. The controlling computer program was responsible for the timing of the pulses and the A/D. It also provides on-line view of the time domain signal and the frequency profiles ( F F T ) of selected echoes. A 40 echo C P M G sequence (with r ~ 1 msec) in the presence of a Z gradient field was used to evaluate the 3He concentration and T2 spatial distribution. Saturation recovery sequences with different pulse spacing were used to determine T1 distribution. The F F T transformed signal was fitted at each frequency to a multi-exponential decay model [5]. This model allows for the coexistence of spins with different relaxation times in the same region. The growth process of the crystals was monitored by imaging the relaxation times during growth and annealing, using the large differences in T1 between solid and liquid. In certain conditions T2 also provided good contrast [2]. Calibration measurements were conducted on uniform solid mixture samples that were allowed to anneal for many hours. The calibrations compensate for magnetic field inhomogeneities
(H0 and received H1 ).The raw data used for the calibration file is shown in 2. The calibration measurements were used later for the absolute determination of spatial spin ( 3 H e ) distribution in a cell filled with solid or with a solid-liquid mixture. One such profile for an all-solid sample is shown in 2. The gradient field homogeneity and the frequency - location relationship in the cell were determined by measuring the signal profile with the cell partially filled with liquid to various heights. The S/N ratio is limited by external noise, not by the receiver or by thermal noise in the coils. The relevant factors affecting the S/N are the number of spins, field gradient strength and temperature [6]. In a typical situation (1K on the melting curve with 3He concentration of 2%, gradient of 3 Gauss/cm) the cell could be divided into 30 slices with S/N of about 10 for each slice (2). This corresponds to a lower detection limit of 2 x 10 is spins at 1K. We do not signal average, in order to be able to follow temporal changes. This proved important in the study of the relaxation of He inhomogeneities as evidenced from the multiexponential decay of the spin echoes [2]. We thank B. Cowan and R.C. Richardson for useful discussions and advice. This work was supported by the US - Israel Binational Science Foundation and by the Israel Academy of Science. REFERENCES
1. Carmi, Y. et al., Phys. Rev. Lett. 62 (1989) 1364. 2. Shuster, I. et al., these proceedings, LT20 (1993). 3. Everett, :I.E. and Osemeikhian, J.E., J. Sci. Instrum. 43 (1966) 470. 4. Mansfield, P. and Morris, P.G., NMR Imaging in Biomedicine. Ed.: Waugh, J.S. (Academic Press, 1982). 5. Schmidt, E.J. et al., J. Appl. Phys. 59 (1986) No. 8, p. 2788. 6. Hoult, D.I. and Richards, R.E., J. of Mag. Res. 24 (1976) 71..