- Email: [email protected]
The Bonn polarized target NMR-system
__ __ l!l!iB
cQ&
Nuclear Instruments and Methods in Physics Research A 356 (1995) 74-78
NUCLEAR INSTRUMENTS 8 METHODS IN PHYStCS RESEARCH Secbon A
ELSEVIER
The Bonn polarized target NMR-system ’ G. Reicherz
*,
C. Bradtke, H. Dutz, R.M. Gehring, S. Goertz, D. Kramer ‘, W. Meyer, M. Pliickthun, A. Thomas Physikalisches Institut der Vniversitiit Bonn, Nussallee 12, 53115 Bonn, Germany
Abstract The Bonn frozen spin target is used for meson photoproduction experiments in combination with the facility PHOENICS at ELSA. The maximum polarization field of 7 T is provided by a superconducting solenoid. For electron scattering experiments a target apparatus which uses a superconducting 4 T magnet in a Helmholtz configuration is employed. For dynamic nuclear polarization of protons or deuterons various microwaves sources with frequencies of 70 GHz, 98 GHz or 140 GHz are available. Ammonia, butanol and lithium hydrids are used as target materials. The main part of the polarization detection system is the so-called Liverpool nuclear magnetic resonance module, which ensures a high flexibility for the polarization measurements of different nuclear species, e.g. “N at 2.5 T (fL = 10.8 MHz) and protons at 50 T (fL = 213 MHz).
1. Introduction Intermediate energy physics experiments with polarized targets have reached a state where more precise measurements of polarization observables can be performed. The first target asymmetry measurements of the pion photoproduction have been done at the new continuous wave electron stretcher accelerator ELSA in Bonn [l]. At high energies new experiments with polarized beams and polarized targets have been performed to study the spin structure of the nucleons [2-51. Improvements in the polarized target technology over many years have led to highly polarized protons and deuteron targets. Using butanol or ammonia, proton polarization values of 90% to 100% and for deuterons more than 50% polarization have been measured. In particular, the latest results of the experiments mentioned above have indicated that the systematic error due to the present accuracy of the polarization measurement is a limiting factor. Currently most of the polarized target systems use the so-called Liverpool nuclear magnetic resonance (NMR) module. Nevertheless, each experiment has its own specific requirements, e.g. the size of the target volume, geometry of the inductance coil, magnetic field etc. which requires an appropriate design of all NMR parameters to
* Supported by the Bundesministerium fir Forschung und Technologie. Corresponding author. 1 Now at Universit‘gt Bielefeld, FakultHt fir Physik, 33501 Bielefeld, Germany. l
0168-9002/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(94)01447-7
minimize error sources. On the other hand a high flexibility of the NMR-system is desirable for the studies of the polarization behaviour of different target materials e.g. lithium hydrides or -deuterides. In this article first the Bonn NMR-system is described. Then its operation to detect thermal equilibrium (TE) signals is outlined. Finally the main error sources for the polarization measurements are discussed.
2. NMR-system The degree of the nuclear polarization is measured with the continuous wave nuclear magnetic resonance (NMR) technique. The polarized nucleons or nuclei give the target material a complex susceptibility x(o) where x(w)
=x’(w)
- ix”(w).
(1)
,Y(o) is a function of the applied angular frequency w. The polarization P is connected to x(w) by the relation P = KIb^x’{(
w)
dw,
(2)
where K is a constant involving the properties of the nucleus concerned. For polarized protons and neutrons x(w) is zero for all frequencies except for a small band close to the Larmor frequency oo. The system works with the Q-meter technique. A coil with an inductance Lo, which is in or around the target material, is a part of a series K-circuit. The material modifies the inductance of the coil by the relation L(w)
=&(I
+ 4’rrVX(W)),
(3)
G. Reichrrz
Table
et al. / Nucl. Inrtr.
and Meth. in Phvs. Rex A 3556 119951 74-78
I
9 RF-excitation
Variable parameters in the RF-circuit for the proton and deuteron NMR-system Proton
Deuteron
600
600
R, /12
10
s
URF
95 mV
YO mV
“2 .iT nS
106.5 MHz
16.35 MHz
6.96 m
6.48 m
VL Sweeps
100
< 10000
200
8000
Coil windings
3
10
R,, /fJ
ADC diode
x(w) or phase
sensitive
detection
x”(w)
QhdB NMR-coil
where 77 is the filling factor of the coil. The coil and the capacitor C are connected by a coax cable with a length of nh/2. Changes in x(w) cause changes in the Q of this resonant system which may be measured as a change in the impedance. The main part of the NMR-system is the Liverpool NMR module [6]. This module consists of RF amplifiers and a phase sensitive detector (BRM ‘1. The frequency bandwidth is variable between 5 and 250 MHz which covers the Larmor frequencies of all used materials at magnetic fields between 2.5 and 5.0 T. A disadvantage is the built-in, wide banded amplifiers, which cause a smaller signal to noise ratio compared to the narrow banded amplifiers. Two different RF-synthesizer (U,,) are used. One is digitally coupled via a GPIB 488 board and the second one is DC-coupled. The frequency of the DC-coupled generator is scanned by a sawtooth voltage output by a DAC. The DC-coupled can be modulated for the “lock in” technique [7]. A comparison of the variable parameters in the RF-circuit for the proton and deuteron NMR-system operating at 2.5 T is listed in Table 1. These variable parameters of the NMR-circuit are shown schematically in Fig. 1. The output voltage of the Liverpool NMR-box is a parabola with a height I 100 mV and has an offset of about - 3 V. The first LF (low frequency)-card is a DC-compensation board which subtracts the measured DC-offset after every sweep, and is triggered by a TTL signal. The amplification factor V,, is I, IO or 100. After the DC-compensation there is a following second LF-card for the parabola compensation. This board has a further amplification factor Vpz of 1, IO or 100. One sweep is divided into a maximum of 500 channels and the sweep duration is 50-60 ms. The first step before the parabola compensation is the detection of the parabola voltage outside the NMR-resonance and the storage of the voltage array in the memory. During the sweep procedure the appropriate parabola voltage will be output by a DAC. This voltage is subtracted from the signal. After the parabola compensation a second LF-amplifica-
Fig. 1. The variable parameters of the NMR-circuit:
amplitude of
the RF-excitation,
capacitor
constant current
dumping resistor R,.
A/2-cable
resistor R,,,
c‘.
and the inductance L of the
NMR-coil.
This causes a better utilization of the ADC resolution. The h/2-cable outside the refrigerator is an UT85 type with a propagation delay of 4.72 X lo-’ s/m. The A/Z-cable and the RF-part is temperature stabilized by water, to minimize electronic shifts. The result of this procedure explained above is demonstrated in Fig. 2, where the thermal equilibrium (TE) signal for deuterons in ‘LiD is plotted. Two different control systems are available. One is based on a VME-bus system with MC68000 processor and the GEM shell. The second system is based on a PC with National Instrument boards and the Turbo Vision shell from Borland C+ +. Both systems are connected via MIDI or RS232 with the target control system. The target control system is a second VME bus system with different l/O boards for control of the target periphery like pumption is possible.
-
300
signal
signal 250
,*.‘i
baseline
- -- - signal
’ :, baselind ’ ! :
B=ZT
with baseline with
withoutsubtraction
at-----J 0
Lip TE-signal T=42K;
100
L----J 200
300
400
channel Fig. 2. The deuteron TE-signal
2
before “subtraction”
Balanced ring modulator.
(upper),
(T = 4.2 K, B = 2.5 T) of ‘LiD after “subtraction”
and inclusive
baseline fit (middle) and subtracted baseline (lower) is presented.
POLARIZED TARGET MATERIALS
G. Reicherz et al. /Nucl. Instr. and Meth. in Phys. Res. A 356 (1995) 74-78
76
r
FWHM=34,7kHz
P=-90%
FWHM=36.2kHz
TE-signal butanol B=5.OT, T=l K;
FWHM=46.6kHz
50
Fig. 3. Overview of the NMR system.
ing system, microwaves, refrigerator, (resistance bridge AVS), magnet power supplies and others. The experiments slow control software is the same for both VME systems. By switching the menu between “NMR” and “DAQ” only the systems are exchangeable. An overview of the Bonn NMR-system is shown in Fig. 3.
3. Thermal equilibrium signals The correct way to calibrate the polarization value is to measure the proportionality factor K given in Eq. (2) during the thermal equilibrium. TE-signals for ammonia and butanol are taken at 1 K and for lithium hydrides at 2 and 4.2 K. For big TE-signals like protons we reduce the quality factor Q of the circuit to minimize the modulation and the non-linearities. The TE-signal and highly polarized signals for protons in butanol taken at 1 K and 5 T are shown in Fig. 4. An explanation for the signal shifts depending on the polarization values is given in Ref. [l]. In Fig. 5a TE deuteron signal from 6LiD and 14ND3 is plotted. The NMR-parameters for both measurements are nearly the same. As can be seen the line shape of these signals is completely different. The more complicated deuteron line shape in 14ND3 has its origin in the interaction of the deuteron quadrupole moment with the electric field gradient. Such an electric field gradient does not exist in the face-centered cubic lattice structure of the lithium hydrides. As a result of this the deuteron line shape in 6LiD is approximately Gaussian. Thus the TE-signal with a width of 3 kHz (FWHM) is easy to detect, even at a temperature of 4.2 K (see Fig. 5). This NMR-technique is not suitable for signals with a line width of greater than 1 MHz. In particular the signal
200
loo channel”
Fig. 4. Negative, positive and TE polarized The dashed lines are fitted by Gaussians.
250
proton signals at 5 T.
for the spin 1 particle 14N, where the peak distance is 2.4 MHz, is extremely difficult to detect. One method of obtaining such signals is the “lock in”-technique. The voltage Us which has measured will be modulated with a parameter p. This parameter p is periodically varied by a reference frequency o, around a value po. In the case of NMR there are two methods with which the signal voltage can be modulated, the RF-frequency and the magnetic field. In most cases, the magnetic field modulation is complicated, because there is no room for a modulation coil inside the refrigerator and the polarization magnet is normally not designed for a fast modulation (0, > 50 Hz). It is much easier to modulate the RF-excitation by using a DC-coupled synthesizer. By sweeping the
D TE-sig. from ‘LiD T=4.2K, B= 2.5T
g 2 “a 5
80
60 40
D TE-sig. from 14ND,
-
T=l .OK. B= 2.5T 20 0
..‘. .. I
16.20
*
1
16.25
*
’
16.30
*
’
16.35
*
‘a
16.40
1%
16,45
’
16.50
11 16.55
f[MHz] Fig. 5. For comparison a TE deuteron signal 14ND3 is plotted. The NMR circuit-parameters same. Note the temperature difference.
from ‘LiD and are nearly the
77
G. Reicherz et al. / Nucl. Instr. and Meth. in Phw. Rex A 356 (1995) 73- 78
Table 2 The errors in the measurement of the polarization for protons and deuterons in ammonia and butanol, as well as A P/ P of ’ Li and deuterons in ‘LiD
100
0
200
300
proton
deuteron. ‘Li
deuteron
channel
CT = 4.2 K. B = 2.5 T) of deuterons in “LID.
otained with the “normal” “lock
h LID
400
channel
Fig. 6. TE-signals
Ammonia. butanol
in”-technique
detection method (left) and with the
(right). The sweep times are approximarely
equal. The dashed lines are the Gaussian fits of the lineshapes.
4. Determination RF over the NMR resonance, one can detect only signals with a small bandwidth (Al’< 10 kHz) (6), because the resolution of the “lock in”-amplifiers is too small in the wings of the circuit parabola. However, if one sweeps the magnetic field over the resonance, the measurement of the signals with very large bandwidth (‘“N) is possible. By this method the RF-frequency can always be kept in the center of the parabola, where there is no unwanted contribution of the dispersive part of the susceptibility. It is obvious that during the sweeping of the magnetic field the DNP is not recommendable. It has also been taken into account that the TE polarization changes during the sweeping procedure of the magnet field. In addition, the differenmust be tial signal, taken by the “lock in”-technique integrated to evaluate the degree of the poalization.
40 -
_g
The most accurate determination of the deuteron polarization involves measuring the factor K in Eq. (2) by the TE-method (“area method”). To minimize the errors a temperature stabilization of the h/2-cable and the RF-part as well as a very stable and reproduceable magnetic field is required. A polarization measurement accuracy of better than 4.5% has been obtained (see Section 5). It is not recommended to determine the polarization degree of the deuterons in D-butanol or ND, by the method” (P = (R’ - I)/(R’ + R so-called “asymmetry + 1)) [8]. For NMR-signals detected with a series circuit Q-meter, the dispersive part of the susceptibility can result in errors. It is extremely important to center the deuteron signal exactly on the NMR-circuit parabola, where wrong contributions to the signal area i.e. parts at the edges of the pedestals are negligible (see Fig. 7). The signals, outlined by dotted and dashed lines, were off-centered by shifting the magnetic field slightly. As can be seen clearly, the asymmetric signal shape varies strongly. Consequently, the deuteron polarization measured by the “asymmetry method”, also varies e.g. 6% and 60% for the dotted and dashed signals, respectively.
30-
3 f
_
0.
-
5
of the deuteron polarization
20 -
5. Polarization
errors
10 -
-lO-
’
0
’
’
100
’
’
200
’
’ 300
.
’ 400
channel Fig. 7. Three dynamic deuteron signals with equal polarization, but with different
positions on the circuit parabola (1000
scan). For explanation see text.
kHz
The principal contributions to the error in the evaluation of the polarization are the uncertainty in the temperature AT of the material while taking the TE-signal, the determination of the TE-signal area AFE,,, the non-linearities of RF-amplifiers AV,, and the LF-amplifiers AV,, and the changes in the electronic length of the h/2-cable due to the shifts in the temperature Al,,,. The errors are listed in Table 2.
POLARIZED
TARGET
MATERIALS
78
G. Reicherz et al. /Nucl.
Instr. and Meth. in Phys. Res. A 356 (1995) 74-78
6. Summary The Bonn polarized target NMR-System is based on the Liverpool NMR-module. Presently there are two control systems. One is based on the VME bus with a MC68000 processor and the second is based on PC with National Instrument boards. The NMR-system consists of three NMR modules, which can be controlled in parallel with different frequencies for different nuclear spin species. The LF-part exists of two compensation cards, one for the DC-offset and an other for the parabola compensation. The system has been used in several particle experiments: a measurement of the target asymmetry of the single pion photoproduction [I], a measurement of the polarization behaviour of ammonia in an intense electron beam [9] and the measurement of the polarization behaviour of LiH(D) at low temperature irradiation [IO]. The polarization measurement error for protons and deuterons in ammonia and butanol is 2.2% and 4.5%, respectively. Proton polariza-
tion in 7LiH is determined to a precision of 2.2%, whereas the polarization measurement error of 6Li and deuterons in ‘LiD is 2.8%.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
[lo]
H. Dutz et al., Nucl. Instr. and Meth. A 340 (1993) 272. B. Adeva et al., Phys. Lett. B 302 (1993) 533. D. Adams et al., Phys. Lett. B 329 (1994) 399. P.L. Anthony et al., Phys. Rev. Lett. 71 (1993) 959. R. Arnold et al., Conf. on the Intersections of Particle and Nuclear Physics St. Petersburg, Florida, 1994. G. Court et al., Nucl. Instr. and Meth. A 324 (1993) 433. G. Reicherz, Dissertation BONN-IR-94-16. G. Guckelsberger and F. Udo, Nucl. Instr. and Meth. 137 (1976) 415. A. Thomas et al., these Proceedings (7th Workshop on Polarized Target Materials and Techniques, Bad Honnef, Germany, 1994) Nucl. Instr. and Meth. A 356 (1995) 5. S. Goertz et al., ibid, p. 20.
cQ&
Nuclear Instruments and Methods in Physics Research A 356 (1995) 74-78
NUCLEAR INSTRUMENTS 8 METHODS IN PHYStCS RESEARCH Secbon A
ELSEVIER
The Bonn polarized target NMR-system ’ G. Reicherz
*,
C. Bradtke, H. Dutz, R.M. Gehring, S. Goertz, D. Kramer ‘, W. Meyer, M. Pliickthun, A. Thomas Physikalisches Institut der Vniversitiit Bonn, Nussallee 12, 53115 Bonn, Germany
Abstract The Bonn frozen spin target is used for meson photoproduction experiments in combination with the facility PHOENICS at ELSA. The maximum polarization field of 7 T is provided by a superconducting solenoid. For electron scattering experiments a target apparatus which uses a superconducting 4 T magnet in a Helmholtz configuration is employed. For dynamic nuclear polarization of protons or deuterons various microwaves sources with frequencies of 70 GHz, 98 GHz or 140 GHz are available. Ammonia, butanol and lithium hydrids are used as target materials. The main part of the polarization detection system is the so-called Liverpool nuclear magnetic resonance module, which ensures a high flexibility for the polarization measurements of different nuclear species, e.g. “N at 2.5 T (fL = 10.8 MHz) and protons at 50 T (fL = 213 MHz).
1. Introduction Intermediate energy physics experiments with polarized targets have reached a state where more precise measurements of polarization observables can be performed. The first target asymmetry measurements of the pion photoproduction have been done at the new continuous wave electron stretcher accelerator ELSA in Bonn [l]. At high energies new experiments with polarized beams and polarized targets have been performed to study the spin structure of the nucleons [2-51. Improvements in the polarized target technology over many years have led to highly polarized protons and deuteron targets. Using butanol or ammonia, proton polarization values of 90% to 100% and for deuterons more than 50% polarization have been measured. In particular, the latest results of the experiments mentioned above have indicated that the systematic error due to the present accuracy of the polarization measurement is a limiting factor. Currently most of the polarized target systems use the so-called Liverpool nuclear magnetic resonance (NMR) module. Nevertheless, each experiment has its own specific requirements, e.g. the size of the target volume, geometry of the inductance coil, magnetic field etc. which requires an appropriate design of all NMR parameters to
* Supported by the Bundesministerium fir Forschung und Technologie. Corresponding author. 1 Now at Universit‘gt Bielefeld, FakultHt fir Physik, 33501 Bielefeld, Germany. l
0168-9002/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(94)01447-7
minimize error sources. On the other hand a high flexibility of the NMR-system is desirable for the studies of the polarization behaviour of different target materials e.g. lithium hydrides or -deuterides. In this article first the Bonn NMR-system is described. Then its operation to detect thermal equilibrium (TE) signals is outlined. Finally the main error sources for the polarization measurements are discussed.
2. NMR-system The degree of the nuclear polarization is measured with the continuous wave nuclear magnetic resonance (NMR) technique. The polarized nucleons or nuclei give the target material a complex susceptibility x(o) where x(w)
=x’(w)
- ix”(w).
(1)
,Y(o) is a function of the applied angular frequency w. The polarization P is connected to x(w) by the relation P = KIb^x’{(
w)
dw,
(2)
where K is a constant involving the properties of the nucleus concerned. For polarized protons and neutrons x(w) is zero for all frequencies except for a small band close to the Larmor frequency oo. The system works with the Q-meter technique. A coil with an inductance Lo, which is in or around the target material, is a part of a series K-circuit. The material modifies the inductance of the coil by the relation L(w)
=&(I
+ 4’rrVX(W)),
(3)
G. Reichrrz
Table
et al. / Nucl. Inrtr.
and Meth. in Phvs. Rex A 3556 119951 74-78
I
9 RF-excitation
Variable parameters in the RF-circuit for the proton and deuteron NMR-system Proton
Deuteron
600
600
R, /12
10
s
URF
95 mV
YO mV
“2 .iT nS
106.5 MHz
16.35 MHz
6.96 m
6.48 m
VL Sweeps
100
< 10000
200
8000
Coil windings
3
10
R,, /fJ
ADC diode
x(w) or phase
sensitive
detection
x”(w)
QhdB NMR-coil
where 77 is the filling factor of the coil. The coil and the capacitor C are connected by a coax cable with a length of nh/2. Changes in x(w) cause changes in the Q of this resonant system which may be measured as a change in the impedance. The main part of the NMR-system is the Liverpool NMR module [6]. This module consists of RF amplifiers and a phase sensitive detector (BRM ‘1. The frequency bandwidth is variable between 5 and 250 MHz which covers the Larmor frequencies of all used materials at magnetic fields between 2.5 and 5.0 T. A disadvantage is the built-in, wide banded amplifiers, which cause a smaller signal to noise ratio compared to the narrow banded amplifiers. Two different RF-synthesizer (U,,) are used. One is digitally coupled via a GPIB 488 board and the second one is DC-coupled. The frequency of the DC-coupled generator is scanned by a sawtooth voltage output by a DAC. The DC-coupled can be modulated for the “lock in” technique [7]. A comparison of the variable parameters in the RF-circuit for the proton and deuteron NMR-system operating at 2.5 T is listed in Table 1. These variable parameters of the NMR-circuit are shown schematically in Fig. 1. The output voltage of the Liverpool NMR-box is a parabola with a height I 100 mV and has an offset of about - 3 V. The first LF (low frequency)-card is a DC-compensation board which subtracts the measured DC-offset after every sweep, and is triggered by a TTL signal. The amplification factor V,, is I, IO or 100. After the DC-compensation there is a following second LF-card for the parabola compensation. This board has a further amplification factor Vpz of 1, IO or 100. One sweep is divided into a maximum of 500 channels and the sweep duration is 50-60 ms. The first step before the parabola compensation is the detection of the parabola voltage outside the NMR-resonance and the storage of the voltage array in the memory. During the sweep procedure the appropriate parabola voltage will be output by a DAC. This voltage is subtracted from the signal. After the parabola compensation a second LF-amplifica-
Fig. 1. The variable parameters of the NMR-circuit:
amplitude of
the RF-excitation,
capacitor
constant current
dumping resistor R,.
A/2-cable
resistor R,,,
c‘.
and the inductance L of the
NMR-coil.
This causes a better utilization of the ADC resolution. The h/2-cable outside the refrigerator is an UT85 type with a propagation delay of 4.72 X lo-’ s/m. The A/Z-cable and the RF-part is temperature stabilized by water, to minimize electronic shifts. The result of this procedure explained above is demonstrated in Fig. 2, where the thermal equilibrium (TE) signal for deuterons in ‘LiD is plotted. Two different control systems are available. One is based on a VME-bus system with MC68000 processor and the GEM shell. The second system is based on a PC with National Instrument boards and the Turbo Vision shell from Borland C+ +. Both systems are connected via MIDI or RS232 with the target control system. The target control system is a second VME bus system with different l/O boards for control of the target periphery like pumption is possible.
-
300
signal
signal 250
,*.‘i
baseline
- -- - signal
’ :, baselind ’ ! :
B=ZT
with baseline with
withoutsubtraction
at-----J 0
Lip TE-signal T=42K;
100
L----J 200
300
400
channel Fig. 2. The deuteron TE-signal
2
before “subtraction”
Balanced ring modulator.
(upper),
(T = 4.2 K, B = 2.5 T) of ‘LiD after “subtraction”
and inclusive
baseline fit (middle) and subtracted baseline (lower) is presented.
POLARIZED TARGET MATERIALS
G. Reicherz et al. /Nucl. Instr. and Meth. in Phys. Res. A 356 (1995) 74-78
76
r
FWHM=34,7kHz
P=-90%
FWHM=36.2kHz
TE-signal butanol B=5.OT, T=l K;
FWHM=46.6kHz
50
Fig. 3. Overview of the NMR system.
ing system, microwaves, refrigerator, (resistance bridge AVS), magnet power supplies and others. The experiments slow control software is the same for both VME systems. By switching the menu between “NMR” and “DAQ” only the systems are exchangeable. An overview of the Bonn NMR-system is shown in Fig. 3.
3. Thermal equilibrium signals The correct way to calibrate the polarization value is to measure the proportionality factor K given in Eq. (2) during the thermal equilibrium. TE-signals for ammonia and butanol are taken at 1 K and for lithium hydrides at 2 and 4.2 K. For big TE-signals like protons we reduce the quality factor Q of the circuit to minimize the modulation and the non-linearities. The TE-signal and highly polarized signals for protons in butanol taken at 1 K and 5 T are shown in Fig. 4. An explanation for the signal shifts depending on the polarization values is given in Ref. [l]. In Fig. 5a TE deuteron signal from 6LiD and 14ND3 is plotted. The NMR-parameters for both measurements are nearly the same. As can be seen the line shape of these signals is completely different. The more complicated deuteron line shape in 14ND3 has its origin in the interaction of the deuteron quadrupole moment with the electric field gradient. Such an electric field gradient does not exist in the face-centered cubic lattice structure of the lithium hydrides. As a result of this the deuteron line shape in 6LiD is approximately Gaussian. Thus the TE-signal with a width of 3 kHz (FWHM) is easy to detect, even at a temperature of 4.2 K (see Fig. 5). This NMR-technique is not suitable for signals with a line width of greater than 1 MHz. In particular the signal
200
loo channel”
Fig. 4. Negative, positive and TE polarized The dashed lines are fitted by Gaussians.
250
proton signals at 5 T.
for the spin 1 particle 14N, where the peak distance is 2.4 MHz, is extremely difficult to detect. One method of obtaining such signals is the “lock in”-technique. The voltage Us which has measured will be modulated with a parameter p. This parameter p is periodically varied by a reference frequency o, around a value po. In the case of NMR there are two methods with which the signal voltage can be modulated, the RF-frequency and the magnetic field. In most cases, the magnetic field modulation is complicated, because there is no room for a modulation coil inside the refrigerator and the polarization magnet is normally not designed for a fast modulation (0, > 50 Hz). It is much easier to modulate the RF-excitation by using a DC-coupled synthesizer. By sweeping the
D TE-sig. from ‘LiD T=4.2K, B= 2.5T
g 2 “a 5
80
60 40
D TE-sig. from 14ND,
-
T=l .OK. B= 2.5T 20 0
..‘. .. I
16.20
*
1
16.25
*
’
16.30
*
’
16.35
*
‘a
16.40
1%
16,45
’
16.50
11 16.55
f[MHz] Fig. 5. For comparison a TE deuteron signal 14ND3 is plotted. The NMR circuit-parameters same. Note the temperature difference.
from ‘LiD and are nearly the
77
G. Reicherz et al. / Nucl. Instr. and Meth. in Phw. Rex A 356 (1995) 73- 78
Table 2 The errors in the measurement of the polarization for protons and deuterons in ammonia and butanol, as well as A P/ P of ’ Li and deuterons in ‘LiD
100
0
200
300
proton
deuteron. ‘Li
deuteron
channel
CT = 4.2 K. B = 2.5 T) of deuterons in “LID.
otained with the “normal” “lock
h LID
400
channel
Fig. 6. TE-signals
Ammonia. butanol
in”-technique
detection method (left) and with the
(right). The sweep times are approximarely
equal. The dashed lines are the Gaussian fits of the lineshapes.
4. Determination RF over the NMR resonance, one can detect only signals with a small bandwidth (Al’< 10 kHz) (6), because the resolution of the “lock in”-amplifiers is too small in the wings of the circuit parabola. However, if one sweeps the magnetic field over the resonance, the measurement of the signals with very large bandwidth (‘“N) is possible. By this method the RF-frequency can always be kept in the center of the parabola, where there is no unwanted contribution of the dispersive part of the susceptibility. It is obvious that during the sweeping of the magnetic field the DNP is not recommendable. It has also been taken into account that the TE polarization changes during the sweeping procedure of the magnet field. In addition, the differenmust be tial signal, taken by the “lock in”-technique integrated to evaluate the degree of the poalization.
40 -
_g
The most accurate determination of the deuteron polarization involves measuring the factor K in Eq. (2) by the TE-method (“area method”). To minimize the errors a temperature stabilization of the h/2-cable and the RF-part as well as a very stable and reproduceable magnetic field is required. A polarization measurement accuracy of better than 4.5% has been obtained (see Section 5). It is not recommended to determine the polarization degree of the deuterons in D-butanol or ND, by the method” (P = (R’ - I)/(R’ + R so-called “asymmetry + 1)) [8]. For NMR-signals detected with a series circuit Q-meter, the dispersive part of the susceptibility can result in errors. It is extremely important to center the deuteron signal exactly on the NMR-circuit parabola, where wrong contributions to the signal area i.e. parts at the edges of the pedestals are negligible (see Fig. 7). The signals, outlined by dotted and dashed lines, were off-centered by shifting the magnetic field slightly. As can be seen clearly, the asymmetric signal shape varies strongly. Consequently, the deuteron polarization measured by the “asymmetry method”, also varies e.g. 6% and 60% for the dotted and dashed signals, respectively.
30-
3 f
_
0.
-
5
of the deuteron polarization
20 -
5. Polarization
errors
10 -
-lO-
’
0
’
’
100
’
’
200
’
’ 300
.
’ 400
channel Fig. 7. Three dynamic deuteron signals with equal polarization, but with different
positions on the circuit parabola (1000
scan). For explanation see text.
kHz
The principal contributions to the error in the evaluation of the polarization are the uncertainty in the temperature AT of the material while taking the TE-signal, the determination of the TE-signal area AFE,,, the non-linearities of RF-amplifiers AV,, and the LF-amplifiers AV,, and the changes in the electronic length of the h/2-cable due to the shifts in the temperature Al,,,. The errors are listed in Table 2.
POLARIZED
TARGET
MATERIALS
78
G. Reicherz et al. /Nucl.
Instr. and Meth. in Phys. Res. A 356 (1995) 74-78
6. Summary The Bonn polarized target NMR-System is based on the Liverpool NMR-module. Presently there are two control systems. One is based on the VME bus with a MC68000 processor and the second is based on PC with National Instrument boards. The NMR-system consists of three NMR modules, which can be controlled in parallel with different frequencies for different nuclear spin species. The LF-part exists of two compensation cards, one for the DC-offset and an other for the parabola compensation. The system has been used in several particle experiments: a measurement of the target asymmetry of the single pion photoproduction [I], a measurement of the polarization behaviour of ammonia in an intense electron beam [9] and the measurement of the polarization behaviour of LiH(D) at low temperature irradiation [IO]. The polarization measurement error for protons and deuterons in ammonia and butanol is 2.2% and 4.5%, respectively. Proton polariza-
tion in 7LiH is determined to a precision of 2.2%, whereas the polarization measurement error of 6Li and deuterons in ‘LiD is 2.8%.
References [l] [2] [3] [4] [5] [6] [7] [8] [9]
[lo]
H. Dutz et al., Nucl. Instr. and Meth. A 340 (1993) 272. B. Adeva et al., Phys. Lett. B 302 (1993) 533. D. Adams et al., Phys. Lett. B 329 (1994) 399. P.L. Anthony et al., Phys. Rev. Lett. 71 (1993) 959. R. Arnold et al., Conf. on the Intersections of Particle and Nuclear Physics St. Petersburg, Florida, 1994. G. Court et al., Nucl. Instr. and Meth. A 324 (1993) 433. G. Reicherz, Dissertation BONN-IR-94-16. G. Guckelsberger and F. Udo, Nucl. Instr. and Meth. 137 (1976) 415. A. Thomas et al., these Proceedings (7th Workshop on Polarized Target Materials and Techniques, Bad Honnef, Germany, 1994) Nucl. Instr. and Meth. A 356 (1995) 5. S. Goertz et al., ibid, p. 20.