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Dynamic proton polarization in ammonia
NUCLEAR
INSTRUMENTS
AND
METHODS
82
(i97o)
205--207;
©
NORTH-HOLLAND
PUBLISHING
CO.
DYNAMIC PROTON POLARIZATION IN AMMONIA K. SCHEFFLER CERN, Geneva, Switzerland
Received 29 January 1970 By means of dynamic nuclear orientation, we have polarized the protons of ammonia (NHs) reproducibly up to 40~ at 1 K and 25 kG.
1. Introduction Because of its good ratio of hydrogen density to density of bound protons, ammonia might have several advantages over the polarized proton targets currently used in high-energy physics (lanthanum magnesium nitrate, butanol, ethylene glycol). After trying a number of paramagnetic centres such as sodium, porphyrexide, porphyrindine, diphenylpicryl-hydrazil, Ziegler's radical, di-tert.-butyl-nitroxide, per-chlorotriphenylmethyl and violanthrone, all of which gave low polarization or none at all, we doped the liquefied ammonia with Crv glycerol complexes.
centrifuged to get a solution that was free of unsolved particles; this was stored at 77 K. The mixtures themselves showed a dynamic polarization of about 45% at 1 K and 25 kG, with a proton relaxation time of T n = 5 m i n for t .... t ~ 3 0 m i n , similar to the results obtained in glycolZ). The ammonia (density at - 7 9 ° C = 0.817 g/cm3; melting point = - 77.7 ° C; boiling point = - 33.35 ° C) was doped with these mixtures in the following way: a certain amount of Cr-glycerol is transferred at r o o m temperature into the upper part of a scaled glass vessel C (see fig. 1), which can be connected either to a
2. Preparation of the target The Crv complexes were made by the reaction of glycerol with K2CrzO 7 (20:8) stirring the mixture at, say, 60°C for some time t .... tl). The mixture then was
50
I 4C
N2
I lr Vacuum
.--T-
NH 3
3O o_c
2o
-70°C-bath
~1
1
30
Fig. 1. Experimental apparatus for liquifying and doping the ammonia.
I
60 t react.[mir~
90
I
1
120
150
1-180-
Fig. 2. Polarization of ammonia versus reaction time of K2Cr2OTglycerol. Different signs correspond to different series of complex preparation.
205
206
K. S C H E F F L E R
vacuum line, an NHa-reservoir or an N2-bottle. After evacuating, the container is cooled down by a bath of methanol dry-ice to about - 70 ° C. By closing the vacuum line and opening the NH3-1ine, the ammonia is allowed to condense into the container and, while running down the walls, to dissolve all of the Crglycerol mixture. The volume of liquid ammonia can be measured by the scale on the glass container so that the weight of the solution is known. When the desired amount of ammonia is condensed, the NHa-line is closed and the N2-1ine is opened in order to get normal pressure, so that the container can easily be opened. After shaking to get an homogeneous solution, its contents are poured into a dewar filled with liquid nitrogen. The large spheres of the green-coloured doped ammonia obtained by using this procedure are crushed in liquid nitrogen to small pieces of about 10 mm 3 and smaller*. This target material, now ready for use, can be stored at 77 K. The crushed ammonia sample is transferred into a 5 cm a cavity in the same way as is done in this laboratory with butanol targets3), i.e. the pre-cooled cryostat as well as the pumping line is filled with cold He gas until normal pressure is achieved. The cryostat * It is also possible to prepare solid blocks o f t h e size of the cavity by slowly freezing t h e a m m o n i a within a n appropriate container. T h e s e blocks gave polarizations lower by a factor o f 1.6 c o m p a r e d to the c r u s h e d material.
50
40
3o
L_a
o_~
20
I
I
I
[
r
[
I
i
~
1
2
3
4
5
6
7
8
9
Time[h] Fig. 4. Polarization of a m m o n i a versus time o f polarization.
is then opened inside a plastic bag. Under He atmosphere the target material is now tranferred into the cavity through a small hole in the bag. The time needed for this operation is about 1 min, so that the temperature of the cavity remains well below the melting point of the ammonia. Also, there is negligible condensation of water onto the cryostat. The microwave power used to polarize the ammonia was about 80 mW/cm a.
150
rz 100 i_J
3. Results
o
h
50
I
,0
•
2'o
3'0
"
Cr-glycerot concentrQtionP/o]
Fig. 3. Time needed to achieve 0.7 of the maximum polarization versus Cr-glycerol concentration (weight-%) of the ammonia target.
The polarization of the ammonia doped with these paramagnetic centres varied with the t .... t of the Cr vglycerol mixture, as shown in fig. 2. The polarization showed no significant changes with Cr-glycerol concentration in the ammonia, which was varied between 10% and 30% by weight. The proton relaxation times were quite long and were not unique. The times needed in order to achieve 0.7 of the maximum of the polarization, increased with decreasing Cr-glycerol concentration in the ammonia (see fig. 3). No marked dependence on t .... t of the Cr-glycerol mixture used has been
D Y N A M I C P R O T O N P O L A R I Z A T I O N IN A M M O N I A
found. The maximum of the polarization is reached very slowly (5-9 h) (see fig. 4). In one of the samples, a polarization of 45% has been obtained reproducibly. The reason why this high value was not reached in other samples of similar composition is not known so far, and further investigations are in progress. The quoted polarizations have been calculated with the aid of the enhancement factor and using 0.25% polarization for the thermal equilibrium proton NMRsignal at 1 K and 25 kG, corresponding to free proton spins in ammonia. To confirm this, a measurement of the well-known asymmetry of the pp scattering was done with an 8 cm 3 ammonia target. This target, not optimized with respect to the polarization, showed values of p+ = (36.6 + 1.8)% and p - --- (33.6_+ 1.7)% or ~ = ( 3 5 . 1 _+1.8)% measured by NMR. An independent pp scattering experiment at 1.22 GeV/e led to ~ = (37.2 + 1.5)%, so that within the limit of error there is satisfactory agreement. I am grateful to Dr. O. Runolfsson for his valuable
207
assistance and for many stimulating discussions during this investigation. I want to thank Dr. J. C. Sens and his group for performing the pp scattering experiment with the ammonia target, and Dr. M. Borghini for his interest and support of this work. Note added in proof By dropping the doped liquid ammonia into liquid N2, we also prepared spheres of N H 3 with a diameter of ca. 2 mm. For 10%-samples the time of polarization (see fig. 3) is then reduced by a factor of about 2. References 1) This procedure is similar to the work done on glycol-K2CrsO7 by N. S. Garif'yanov, B. M. Kozyrev and V. N. Fedotov,
Dokl. Acad. Nauk SSSR, 178 (1968) 808 [Engl. transl.: Soviet Phys. Dokl. 13 (1968) 107]; see also ref. 2). 2) H. Glgttli, M. Odehnal, J. Ezratty, A. Malinowski and A. Abragam, Phys. Letters 29A (1969) 250. 8) S. Mango, 13. Run61fsson and M. Borghini, Nucl. Instr. and Meth. 72 (1969) 456.
INSTRUMENTS
AND
METHODS
82
(i97o)
205--207;
©
NORTH-HOLLAND
PUBLISHING
CO.
DYNAMIC PROTON POLARIZATION IN AMMONIA K. SCHEFFLER CERN, Geneva, Switzerland
Received 29 January 1970 By means of dynamic nuclear orientation, we have polarized the protons of ammonia (NHs) reproducibly up to 40~ at 1 K and 25 kG.
1. Introduction Because of its good ratio of hydrogen density to density of bound protons, ammonia might have several advantages over the polarized proton targets currently used in high-energy physics (lanthanum magnesium nitrate, butanol, ethylene glycol). After trying a number of paramagnetic centres such as sodium, porphyrexide, porphyrindine, diphenylpicryl-hydrazil, Ziegler's radical, di-tert.-butyl-nitroxide, per-chlorotriphenylmethyl and violanthrone, all of which gave low polarization or none at all, we doped the liquefied ammonia with Crv glycerol complexes.
centrifuged to get a solution that was free of unsolved particles; this was stored at 77 K. The mixtures themselves showed a dynamic polarization of about 45% at 1 K and 25 kG, with a proton relaxation time of T n = 5 m i n for t .... t ~ 3 0 m i n , similar to the results obtained in glycolZ). The ammonia (density at - 7 9 ° C = 0.817 g/cm3; melting point = - 77.7 ° C; boiling point = - 33.35 ° C) was doped with these mixtures in the following way: a certain amount of Cr-glycerol is transferred at r o o m temperature into the upper part of a scaled glass vessel C (see fig. 1), which can be connected either to a
2. Preparation of the target The Crv complexes were made by the reaction of glycerol with K2CrzO 7 (20:8) stirring the mixture at, say, 60°C for some time t .... tl). The mixture then was
50
I 4C
N2
I lr Vacuum
.--T-
NH 3
3O o_c
2o
-70°C-bath
~1
1
30
Fig. 1. Experimental apparatus for liquifying and doping the ammonia.
I
60 t react.[mir~
90
I
1
120
150
1-180-
Fig. 2. Polarization of ammonia versus reaction time of K2Cr2OTglycerol. Different signs correspond to different series of complex preparation.
205
206
K. S C H E F F L E R
vacuum line, an NHa-reservoir or an N2-bottle. After evacuating, the container is cooled down by a bath of methanol dry-ice to about - 70 ° C. By closing the vacuum line and opening the NH3-1ine, the ammonia is allowed to condense into the container and, while running down the walls, to dissolve all of the Crglycerol mixture. The volume of liquid ammonia can be measured by the scale on the glass container so that the weight of the solution is known. When the desired amount of ammonia is condensed, the NHa-line is closed and the N2-1ine is opened in order to get normal pressure, so that the container can easily be opened. After shaking to get an homogeneous solution, its contents are poured into a dewar filled with liquid nitrogen. The large spheres of the green-coloured doped ammonia obtained by using this procedure are crushed in liquid nitrogen to small pieces of about 10 mm 3 and smaller*. This target material, now ready for use, can be stored at 77 K. The crushed ammonia sample is transferred into a 5 cm a cavity in the same way as is done in this laboratory with butanol targets3), i.e. the pre-cooled cryostat as well as the pumping line is filled with cold He gas until normal pressure is achieved. The cryostat * It is also possible to prepare solid blocks o f t h e size of the cavity by slowly freezing t h e a m m o n i a within a n appropriate container. T h e s e blocks gave polarizations lower by a factor o f 1.6 c o m p a r e d to the c r u s h e d material.
50
40
3o
L_a
o_~
20
I
I
I
[
r
[
I
i
~
1
2
3
4
5
6
7
8
9
Time[h] Fig. 4. Polarization of a m m o n i a versus time o f polarization.
is then opened inside a plastic bag. Under He atmosphere the target material is now tranferred into the cavity through a small hole in the bag. The time needed for this operation is about 1 min, so that the temperature of the cavity remains well below the melting point of the ammonia. Also, there is negligible condensation of water onto the cryostat. The microwave power used to polarize the ammonia was about 80 mW/cm a.
150
rz 100 i_J
3. Results
o
h
50
I
,0
•
2'o
3'0
"
Cr-glycerot concentrQtionP/o]
Fig. 3. Time needed to achieve 0.7 of the maximum polarization versus Cr-glycerol concentration (weight-%) of the ammonia target.
The polarization of the ammonia doped with these paramagnetic centres varied with the t .... t of the Cr vglycerol mixture, as shown in fig. 2. The polarization showed no significant changes with Cr-glycerol concentration in the ammonia, which was varied between 10% and 30% by weight. The proton relaxation times were quite long and were not unique. The times needed in order to achieve 0.7 of the maximum of the polarization, increased with decreasing Cr-glycerol concentration in the ammonia (see fig. 3). No marked dependence on t .... t of the Cr-glycerol mixture used has been
D Y N A M I C P R O T O N P O L A R I Z A T I O N IN A M M O N I A
found. The maximum of the polarization is reached very slowly (5-9 h) (see fig. 4). In one of the samples, a polarization of 45% has been obtained reproducibly. The reason why this high value was not reached in other samples of similar composition is not known so far, and further investigations are in progress. The quoted polarizations have been calculated with the aid of the enhancement factor and using 0.25% polarization for the thermal equilibrium proton NMRsignal at 1 K and 25 kG, corresponding to free proton spins in ammonia. To confirm this, a measurement of the well-known asymmetry of the pp scattering was done with an 8 cm 3 ammonia target. This target, not optimized with respect to the polarization, showed values of p+ = (36.6 + 1.8)% and p - --- (33.6_+ 1.7)% or ~ = ( 3 5 . 1 _+1.8)% measured by NMR. An independent pp scattering experiment at 1.22 GeV/e led to ~ = (37.2 + 1.5)%, so that within the limit of error there is satisfactory agreement. I am grateful to Dr. O. Runolfsson for his valuable
207
assistance and for many stimulating discussions during this investigation. I want to thank Dr. J. C. Sens and his group for performing the pp scattering experiment with the ammonia target, and Dr. M. Borghini for his interest and support of this work. Note added in proof By dropping the doped liquid ammonia into liquid N2, we also prepared spheres of N H 3 with a diameter of ca. 2 mm. For 10%-samples the time of polarization (see fig. 3) is then reduced by a factor of about 2. References 1) This procedure is similar to the work done on glycol-K2CrsO7 by N. S. Garif'yanov, B. M. Kozyrev and V. N. Fedotov,
Dokl. Acad. Nauk SSSR, 178 (1968) 808 [Engl. transl.: Soviet Phys. Dokl. 13 (1968) 107]; see also ref. 2). 2) H. Glgttli, M. Odehnal, J. Ezratty, A. Malinowski and A. Abragam, Phys. Letters 29A (1969) 250. 8) S. Mango, 13. Run61fsson and M. Borghini, Nucl. Instr. and Meth. 72 (1969) 456.