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The PSI 100 cm3 frozen spin target
s __ __ @
Nuclear Instruments and Methods in Physics Research A 356 (1995) 53-55
NUCLEAR INSTFIUMENTS aMEmooS IN PHYStCS RESEARCH
ELSEXIER
The PSI 100 cm3 frozen spin target B. van den Brandt
*, P.
Hautle, J.A. Konter, S. Mango
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Abstract For measurements of the 2-spin and 3-spin transfer parameters in the n-p system in the 300-600 MeV range, a large (100 cm3> frozen spin polarized target has been built and put into operation at PSI. The holding coil system allows a virtually arbitrary polarization direction, quick polarization reversal and large opening angles.
1. The magnet system For dynamically polarized targets a magnetic field in the range of 2.5-5 T with a homogeneity of low4 over the target volume is required to achieve sizeable polarizations. At the same time large opening angles from the target to the detectors are necessary. These requirements are difficult to fulfill for a large target. For this reason the so-called frozen spin target concept has been developed [l]. We have built a system in which a 100 cm3 target is dynamically polarized in a high homogeneity superconducting solenoid, housed in a room temperature bore cryostat, surrounding the target dilution refrigerator. (see Fig. 1). In a second step the target polarization is frozen in by lowering the temperature to Tmin= 50 mK and subsequently the magnetic field B to 0.8 T. Then the magnetic field is taken over by a holding coil system, integrated in the target refrigerator cryostat, and the polarization solenoid is removed from its polarizing i.e. beam obstructing position (see Fig. 2). The holding coil system consists of a split pair magnet and a saddle coil magnet, providing resp. the vertical and horizontal holding fields (see Fig. 4). A linear combination of these fields together with the rotatability of the cryostat around the vertical axis, in dilution mode, allows virtually any quantization direction in space, only limited by the two pillars of the magnet support (see Fig. 3). Moreover, the twofold coil system allows a quick polarization reversal by magnetic field rotation. Initial tests have shown a reversal time of 12 min. The characteristics of the magnet system are: - Polarizing coil: 5 T coil in room temperature bore cryostat; B )I z-axis (vertical); AB/B = 1.7 x 10e4over 100 cm3 cube; cryostat vertically moveable.
* Corresponding author.
Vertical holding coil: 1.2 T superconducting split pair coil in target cryostat; B 11z-axis; AB/B = 10% over the target volume. Horizontal holding coil: 1.1 T superconducting saddle coil in target cryostat; B I z-axis; AB/B = 7%. Opening angles of holding coil system (see Fig. 3): 2 (Y= 145”; 2/3 = 100”; 4 = 165”. Dimensions of holding coil system: ri = 73 mm; r0 = 110 mm.
_. The dilution refrigerator The vertical dilution refrigerator (Fig. l), constructed at PSI, has been incorporated in a cryostat in which also the twofold superconducting holding coil system has been integrated. The main He reservoir, suppleting the magnet system, serves at the same time as a buffer volume for the two precooling loops of the dilution refrigerator insert. Precooling of the incoming 3He is accomplished in a triple exchanger between two streams of 4He from the 4.2 K and sub-A bath, with maximal recovery of the enthalpy of the outcoming 3He-gas from the inside pumping tube (Fig. 4). Condensation of the mixture takes place in a 4He pot at = 1.2 K. The target loading is performed with an up-scaled version of a former PSI design [2], in which the target cell is an integral part of a central insert. This insert rod essentially consists of a cylindrical waveguide plus baffle system and a Vespel block with a 5” conical section, mating the central acces hole in the supporting structure of the heat exchanger. This loading system enables us to load a rectangular target cell into an also rectangular mixing chamber, thus minimizing the amount of background material in the target region. At present, we have implemented a cylindrical mixing chamber and target cell. A dummy target, mounted under-
0168-9002/95/%09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(94)01443-4
POLARIZED TARGET MATERLUS
54
B. uan den Brandt et al. / Nucl. Instr. and Meth. in Phys. Res. A 356 (1995) 53-55
Fig. 4. Schematic drawing of the holding coils arrangement, showing the split pair for the vertical field and the saddle coil for the horizontal field.
Fig. 1. Target in polarization
mode.
Split pair ‘Saddle coil
neath the target cell in the mixing chamber, can be advanced on beam for background measurements, without interrupting the refrigerator operation (see Fig. 5). The present version of the dilution refrigerator is equipped with a single concentric heat exchanger between still and mixing chamber. The minimum temperature which can be achieved is around 50 mK. In frozen spin mode relaxation times of about 1100 h have been reached in 0.8 T. At high circulation rates, the cooling power is lagging behind the expectations because of a too high 4He-3He ratio of the circulating mixture, contrary to measurements in the initial phases of the development. This puzzling behaviour has to be investigated further.
3. The pumping system
Fig. 2. Target in frozen spin mode.
Fig. 3. Horizontal
The pumping and gas-handling system pertaining to the dilution refrigerator, are designed for a circulation rate of n, = 30 mmol/s. The pumping system comprises of a series combination of Roots blowers Balzers WKP 4000, Alcatel RSV 1000, RSV 300B and ADP 80, all equipped with canned motors and compressing completely oil-free.
cut through pillars of the holding coil system showing the opening angles.
B. van den Brandt et al. / Nucl. Insrr. and
Med
in Phvs. Res. A 356 (1995) 53-55
----
-I
2K-pumping tube 1 --
multiple HE microwave-tube
-
separator hlet 2K-pot inlet baffles
A -
--
2K-pot HE
-t
e D
1
--\
1
-
beam
Still HE Still heater plate
tubes-in-tube HE
microwave-horn
dummy target
--
Fig. 5. Dilution refrigerator, with the central access top loading rod in the down position (target on beam, dilution mode) (left drawing) and (right drawing) in the lifted-up position (dummy target in beam height).
4. Performance The system, installed on the NA2 polarized neutron in operation for several physics runs now. Proton polarizations of up to 80% could be achieved in a 100 cm3 target consisting of a 1-butanol/water mixture, doped with u 2 X 1019 EHBA-CrV/cm’ after _ 5 h of microwave irradiation in 2.5 ‘I’, a time-lirkation imposed by the cryogenic duty cycle of the magnet. Polarization decay times have been 2 1100 h at I” s 70 beam line, has been
mK in a holding field of 0.8 T.
Rotation of the holding field had no detectable influence on the polarization decay time. Polarization reversal by AFP on the proton system, with a HF coil geometry by far not optimized, helped in the last run to achieve the highest polarization reported. References [II T.O. Niinikoski and F. Udo, Nucl. Instr. and Meth. 134 (1976) 219. [2] B. van den Brandt, J.A. Konter and S. Mango, Nucl. Instr. and Meth. A 289 (1990) 526.
POLARIZED TARGET h4ATERLUS
Nuclear Instruments and Methods in Physics Research A 356 (1995) 53-55
NUCLEAR INSTFIUMENTS aMEmooS IN PHYStCS RESEARCH
ELSEXIER
The PSI 100 cm3 frozen spin target B. van den Brandt
*, P.
Hautle, J.A. Konter, S. Mango
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Abstract For measurements of the 2-spin and 3-spin transfer parameters in the n-p system in the 300-600 MeV range, a large (100 cm3> frozen spin polarized target has been built and put into operation at PSI. The holding coil system allows a virtually arbitrary polarization direction, quick polarization reversal and large opening angles.
1. The magnet system For dynamically polarized targets a magnetic field in the range of 2.5-5 T with a homogeneity of low4 over the target volume is required to achieve sizeable polarizations. At the same time large opening angles from the target to the detectors are necessary. These requirements are difficult to fulfill for a large target. For this reason the so-called frozen spin target concept has been developed [l]. We have built a system in which a 100 cm3 target is dynamically polarized in a high homogeneity superconducting solenoid, housed in a room temperature bore cryostat, surrounding the target dilution refrigerator. (see Fig. 1). In a second step the target polarization is frozen in by lowering the temperature to Tmin= 50 mK and subsequently the magnetic field B to 0.8 T. Then the magnetic field is taken over by a holding coil system, integrated in the target refrigerator cryostat, and the polarization solenoid is removed from its polarizing i.e. beam obstructing position (see Fig. 2). The holding coil system consists of a split pair magnet and a saddle coil magnet, providing resp. the vertical and horizontal holding fields (see Fig. 4). A linear combination of these fields together with the rotatability of the cryostat around the vertical axis, in dilution mode, allows virtually any quantization direction in space, only limited by the two pillars of the magnet support (see Fig. 3). Moreover, the twofold coil system allows a quick polarization reversal by magnetic field rotation. Initial tests have shown a reversal time of 12 min. The characteristics of the magnet system are: - Polarizing coil: 5 T coil in room temperature bore cryostat; B )I z-axis (vertical); AB/B = 1.7 x 10e4over 100 cm3 cube; cryostat vertically moveable.
* Corresponding author.
Vertical holding coil: 1.2 T superconducting split pair coil in target cryostat; B 11z-axis; AB/B = 10% over the target volume. Horizontal holding coil: 1.1 T superconducting saddle coil in target cryostat; B I z-axis; AB/B = 7%. Opening angles of holding coil system (see Fig. 3): 2 (Y= 145”; 2/3 = 100”; 4 = 165”. Dimensions of holding coil system: ri = 73 mm; r0 = 110 mm.
_. The dilution refrigerator The vertical dilution refrigerator (Fig. l), constructed at PSI, has been incorporated in a cryostat in which also the twofold superconducting holding coil system has been integrated. The main He reservoir, suppleting the magnet system, serves at the same time as a buffer volume for the two precooling loops of the dilution refrigerator insert. Precooling of the incoming 3He is accomplished in a triple exchanger between two streams of 4He from the 4.2 K and sub-A bath, with maximal recovery of the enthalpy of the outcoming 3He-gas from the inside pumping tube (Fig. 4). Condensation of the mixture takes place in a 4He pot at = 1.2 K. The target loading is performed with an up-scaled version of a former PSI design [2], in which the target cell is an integral part of a central insert. This insert rod essentially consists of a cylindrical waveguide plus baffle system and a Vespel block with a 5” conical section, mating the central acces hole in the supporting structure of the heat exchanger. This loading system enables us to load a rectangular target cell into an also rectangular mixing chamber, thus minimizing the amount of background material in the target region. At present, we have implemented a cylindrical mixing chamber and target cell. A dummy target, mounted under-
0168-9002/95/%09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(94)01443-4
POLARIZED TARGET MATERLUS
54
B. uan den Brandt et al. / Nucl. Instr. and Meth. in Phys. Res. A 356 (1995) 53-55
Fig. 4. Schematic drawing of the holding coils arrangement, showing the split pair for the vertical field and the saddle coil for the horizontal field.
Fig. 1. Target in polarization
mode.
Split pair ‘Saddle coil
neath the target cell in the mixing chamber, can be advanced on beam for background measurements, without interrupting the refrigerator operation (see Fig. 5). The present version of the dilution refrigerator is equipped with a single concentric heat exchanger between still and mixing chamber. The minimum temperature which can be achieved is around 50 mK. In frozen spin mode relaxation times of about 1100 h have been reached in 0.8 T. At high circulation rates, the cooling power is lagging behind the expectations because of a too high 4He-3He ratio of the circulating mixture, contrary to measurements in the initial phases of the development. This puzzling behaviour has to be investigated further.
3. The pumping system
Fig. 2. Target in frozen spin mode.
Fig. 3. Horizontal
The pumping and gas-handling system pertaining to the dilution refrigerator, are designed for a circulation rate of n, = 30 mmol/s. The pumping system comprises of a series combination of Roots blowers Balzers WKP 4000, Alcatel RSV 1000, RSV 300B and ADP 80, all equipped with canned motors and compressing completely oil-free.
cut through pillars of the holding coil system showing the opening angles.
B. van den Brandt et al. / Nucl. Insrr. and
Med
in Phvs. Res. A 356 (1995) 53-55
----
-I
2K-pumping tube 1 --
multiple HE microwave-tube
-
separator hlet 2K-pot inlet baffles
A -
--
2K-pot HE
-t
e D
1
--\
1
-
beam
Still HE Still heater plate
tubes-in-tube HE
microwave-horn
dummy target
--
Fig. 5. Dilution refrigerator, with the central access top loading rod in the down position (target on beam, dilution mode) (left drawing) and (right drawing) in the lifted-up position (dummy target in beam height).
4. Performance The system, installed on the NA2 polarized neutron in operation for several physics runs now. Proton polarizations of up to 80% could be achieved in a 100 cm3 target consisting of a 1-butanol/water mixture, doped with u 2 X 1019 EHBA-CrV/cm’ after _ 5 h of microwave irradiation in 2.5 ‘I’, a time-lirkation imposed by the cryogenic duty cycle of the magnet. Polarization decay times have been 2 1100 h at I” s 70 beam line, has been
mK in a holding field of 0.8 T.
Rotation of the holding field had no detectable influence on the polarization decay time. Polarization reversal by AFP on the proton system, with a HF coil geometry by far not optimized, helped in the last run to achieve the highest polarization reported. References [II T.O. Niinikoski and F. Udo, Nucl. Instr. and Meth. 134 (1976) 219. [2] B. van den Brandt, J.A. Konter and S. Mango, Nucl. Instr. and Meth. A 289 (1990) 526.
POLARIZED TARGET h4ATERLUS