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A polarized proton target for low energy scattering experiments down to less than 10 MeV
NUCLEAR INSTRUMENTS
AND METHODS
98 (1972)
I5I-I55;
© NORTH-HOLLAND
PUBLISHING
CO
A P O L A R I Z E D P R O T O N TARGET FOR L O W ENERGY SCATTERING E X P E R I M E N T S D O W N TO LESS T H A N 10 MeV H. KUIPER, J. DIEKAMP, K. F R A N K , D. KRONIGER, K. KUTSCHERA and B. SEIDLER
Physikalisehes hzstitut der Univers#iit Erlangen-Niirnberg, Erlangen, Germany Received 12 August 1971 A polarized proton target for low energy experiments has been
in a magnetic field to 18.5 kG using 71 GHz microwaves from a
constructed and used at the Erlangen tandem accelerator. The free protons of the waters of hydration in a 3 x 3 x 0.05 mm3LMN crystal are polarized by the solid effect at a temperature of 1.4 K
klystron. 60% proton polarization is obtained. Constructional and operational details are discussed.
1. Introduction The increasing interest in experiments with oriented nuclei has led to the construction of a considerable number of polarized proton targets 1) in recent years. These targets, including the one to be described, have used the well-known technique of dynamic nuclear polarization2). The majority of polarized proton targets has been designed for experiments at high and intermediate energies, yet only one report about a polarized target for use with low energy charged particle beams exists3). In fact, low energy polarized proton targets essentially differ in their design from targets constructed for the hundred MeV or the GeV energy range and present a number of additional difficulties. Many of these can be traced back to the obvious necessity to limit the energy loss of beam and scattered particles in the target to at most a few MeV. As a consequence the geometrical dimensions of the target sample have to be very small and the microwave cavity must have thin walls for charged particles to pass through. Other problems are connected with the necessarily indirect cooling of the target sample and with the measurements of target polarization, which, in contrast to high energy targets, cannot reliably be performed by the conventional N M R method. In the description to follow it is shown how these problems were solved with the polarized proton target constructed at Erlangen. This target was used first for an (e,p) elastic scattering experiment at the Erlangen tandem accelerator. 2. General experimental set-up Fig. 1 shows the principal features of the target configuration. The magnet, cryostat and scattering chamber assembly are located at the end of a beam
[IEMPERAIURE [ MICRQWA~Zi~iOL ARIZAIIQN ~ MEASUREMENI [ SYSTEM ] LMEASUREMEN;J
I -PL;HtPii-I
BEaM
~-DIFFI_S]SN~. PLJf4P ] / ....
}: [ ,t v
,
;
Fig. 1. Principal arrangement of the polarized proton target. a: LMN target crystal, b: Si surface barrier detectors, c: waveguide, d: stainless steel tube supplying liquid 4He to the target, e: liquid Nz cooled radiation shield, f: electromagnetically actuated beam shutter, g: vacuum wall of scattering chamber. tube of the tandem accelerator. The beam hits the L M N target parallel to the c-axis of the crystal. The target sample is placed in the centre of an electromagnet and cooled down to about 1.4 K by a conventional, pumped ¢He cryostat. Microwave power is supplied by a 71 GHz reflex klystron. The scattered particles are detected by two Si surface barrier detectors. 3. Crystal The sample containing the protons to be polarized consists of a very thin slab of monocrystalline La2Mg3(NO3)12-24H20 , (LMN for short), doped with 1% Nd 3+ in the solution. This material has been chosen, since it is capable of the highest degree of polarization and because it allows the preparation of 151
152
H. KUIPER et al.
He-6ewar
......
leflon gasket '
I I --V?A flange Soft soldered
V2A fube
Ag ,~avegude RG98 L . . . . . .
2
LIqu,d Zke
U II
The waveguide supplying the microwave power is fed right through the liquid helium vessel, as seen from fig. 2. By lowering the pressure in the helium dewar with an Edwards Speedivac 1SC900 rotating vacuum p u m p (nominal pumping speed 15 1/sec), the target can be cooled down to about 1.4 K. The temperature of the cavity base can be measured via the 4He vapour pressure or, more conveniently, by means of Allen Bradley carbon resistors with an accuracy better than _+ 0.005 K. The useful helium inventory of the cryostat is about 3.5 1. Continuous operation is possible for about 28 h at 4.2 K or for about 10 h at 1.4 K between two helium transfers. The magnet is a standard iron core electromagnet with 250 m m diameter pole pieces tapered to 140 m m diameter and producing a field of 18.5 kG at the centre
//Hgrd soldered 109
7
6
51
2 3
0
1
8
11
"-To diaphragms
Fig. 2. Cooling system and cavity assembly (V2A= stainless
steel). small specimens of well-defined dimensions. The crystals are grown from the saturated aqueous solution by automatically lowering the temperature in a Peltier element controlled thermostat. In this way we obtain crystals suited for target application within a day or so. These are given their final dimensions of approximately 3 x 3 x 0.05 m m 3 by carefully polishing on a wet satin cloth. During this procedure the thickness and plane parallelity of the crystal are controlled with a microscope.
4. Cryostat and magnet The cryogenic system involves a conventional 4He cryostat. The bottom of the helium dewar is demountable and sealed with a thin teflon foil gasket (see fig. 2). This foil proved very useful since it endures 30 or more cool-down warm-up cycles, thus comparing favourably with the conventional indium seal. A thin stainless steel tube of 18 m m o.d. has been welded to the demountable bottom flange. The other end of the tube has been hard-soldered to the bore of a copper block, called cavity base, which is cooled by direct contact with liquid helium. On this base the microwave cavity containing the target crystal is mounted in a manner to be described below (sect. 7).
2cm
Fig. 3. Cavity and detector arrangement as mounted inside the liquid N2 cooled radiation shield. 1: cavity base, 2: waveguide, 3: cavity cover, 4: cavity core, 5, 7, 10: tantalum diaphragms along the incident beam path, 6: stainless steel tube, 8: tantalum diaphragms defining the scattering angles, 9: liquid N2 cooled copper radiation shield, 11 : aluminium housing for the detectors, 12: Si surface barrier detectors, 13 : beam shutter (closed position), 14: shutter driving coil.
POLARIZED PROTON TARGET
of a 42 mm gap. Inside a sphere of 1 cm diameter at the field centre the deviations of the field strength from the centre value are not greater than 0.4 G at 18.5 kG, corresponding to a homogeneity of about 2 x 10 -5 The time stability of the magnetic field strength has been checked by EPR measurements and turned out to be of the order of 5 x 10 .6 over a period of about ten minutes. 5. Particle trajectories and energy losses The strong magnetic field at the target causes curved trajectories for the beam as well as for the scattered particles. For a well-defined scattering geometry all relevant particle trajectories have to be known. For this purpose a computer program has been devised capable of calculating the particle trajectories on the basis of the measured field distribution. For the practical adjustment the incident beam is made visible by means of two ZnS coated beam viewers which can be,, flipped into the beam electromagnetically. Marks on the viewers applied under control of a theodolite, de,fining the precalculated direction of the beam, ensure that the incident particles hit the target at the correct angle. Each viewer is monitored by a TV camera. For the scattered particles another computer program yields energy and scattering angle (at the target) of those particles that pass through a diaphragm of given geometrical size and position and then enter a de,tector. The program takes also into account the energy losses of the charged particles in the scattering process as well as in traversing the target crystal and copper cavity walls. With the known values of the atomic stopping powers for Cu 4) and L M N s) we have calculated the positions of the diaphragms shown in fig. 3. These are chosen such that of the recoil protons from the L M N target only those emitted under (32__ 3) and ( - 3 2 + 3) degrees (lab.syst.) reach the de,tectors. By similar calculations the instrumental asymmetry could be reduced to about 2.5% (experimental value). This arrangement has been used in an (e,p) elastic scattering experiment designed to measure the target polarization and to compare the results with those of the PIF method 6) (cf. sect. 8). An electromagnetically operated shutter mechanism was constructed (see fig. 3), which exposes the target to the particle flux only during the well-defined counting periods. In this way all unnecessary bombardment of the target is avoided, which is important in view of the serious problem of radiation damage7). The shutter can be actuated either manually or under control of the electronic counter circuit.
153
6. Particle detection In the following we describe a particle detection arrangement used in the above mentioned (c~,p) elastic scattering experiment. In order to obtain a welldefined scattering geometry the diaphragms defining the scattering angles are mounted directly on the cavity assembly (see fig. 3). Si barrier detectors, contained in an aluminium housing, are fixed to the inside of the liquid N2 cooled radiation shield, close to the diaphragms. Due to geometrical restrictions and a 10 #m thick copper foil in front of the detectors only two particle groups are detected: 1) recoil protons from the polarized target and 2) those ~ particles elastically scattered by the copper cavity walls that did not penetrate the crystal. These two particle groups are not well separated in energy. Since, however, we are only interested in particle counting rates rather than in energy measurement, we achieve particle discrimination using diodes with a very small depletion depth such, that the ~ particles are stopped in the sensitive volume while the protons are not. In this way spectra like that shown in fig. 4 are obtained, the left peak representing the recoil protons. 7. Microwaves 7.1. GENERAL DESCRIPTION
In our case, the microwave apparatus is used for two very different purposes. First it serves to saturate the so-called "forbidden" transitions, thus polarizing the target. Secondly it provides a very useful tool for measuring the nuclear polarization inside the target by PIF (cf. sect. 8). Microwave power from a 71 GHz klystron oscillator is fed into the target cavity resonator. Variations of the reflected power, due to magnetic resonance in the cavity can be observed by a 1N53 microwave diode connected with the main waveguide via a 10 dB directional coupler or alternatively by a circulator (see fig. 5). Standard lock-in technique is used to detect the EPR signal. A microwave spectrometer like the one described here is not able to distinguish between absorption and dispersion in the EPR signal, but this is not of any significance in our experiments. 7.2.
MICROWAVE GENERATOR AND FREQUENCY STABILIZATION
As a microwave source we use a Philips Y K 1010 reflex klystron with a maximum output power of 350 mW at 71 GHz. Exceptionally high frequency stability of this oscillator has been achieved by extending a phase locking technique, well-known at lower frequencies8), to the
154
H. KUIPER et al.
.\".
15qO ~
10L
•.
..
5105
.< r
100
I 200
: 300
JJO
500 CHANNEL~JMUER
Fig. 4. Typical spectrum showing the recoil proton peak (at left) and a peak representing ~ particles elastically scattered at the copper foil cavity walls (at right).
71 GHz region (see upper part of fig. 5). A quartz oscillator supplying a reference frequency of about 1.4 GHz serves as a frequency standard of a relative stability of better than 10 - 7 per day. In a mixer diode (1 N53) the 48th harmonic of this reference is generated and superheterodyned with the klystron "local oscillator" (LO) frequency. In the stabilized state an intermediate frequency IF of 30 MHz appears at the mixer output. This IF is compared in a phase discriminator circuit with another 30 MHz frequency derived from the quartz standard• The phase comparator dc output, which is proportional to the phase difference of the two 30 MHz frequencies, is used to control the klystron reflector voltage and thereby the LO frequency. It is the principal advantage of a phase control that the frequency stability of the klystron so stabilized is equal to that of the quartz standard. Another advantage is the accurate and convenient resettability of the klystron frequency which is a consequence of the quartz oscillator reference frequency being variable by 10 kHz steps. 7.3. CAVITYRESONATOR For the cavity resonator, excited in the H~o ~ mode by a coupling iris from the waveguide, we have chosen a "sandwich" construction consisting of three main parts which may be called base, core and cover (see fig. 3). The cavity volume is formed by the core and
two copper foils (thickness 10 pro), one on each side of the core. The rectangular cavity thus formed has the dimensions 10x 1 0 x 2 . 1 9 mm 3. One of the foils supports the target crystal glued on it with K e I - F , a non-hydrogenous grease. The core is a precision-milled copper frame, cut into two pieces, thus facilitating the access to the resonator volume and making the core easy to manufacture. Cover, core, base and foils are mounted together with four fastening screws. For better thermal and electrical contact it proved to be useful to introduce two flat lead gaskets between the foils and the cover or the base, respectively. For the mode chosen the homogeneity of the microwave magnetic field in the target region is sufficiently good. After taking apart and later re-assembling the resonator, the cavity resonance frequency can be set again to the previous value by more or less tightening one of the four screws mentioned above (possible variation about 300 MHz).
8. Nuclear polarization The maximum proton polarization obtained was 62%. Since the N M R method fails, the target polarization is measured by PIF, a method specially developed for nuclear polarization measurement in low energy
r i I
Klvstraq
i
I
Mcgnel
/ I
L Fig. 5. Block diagram showing the EPR signal detection system and the frequency stabilization scheme by phase-locking the klystron to a quartz standard.
POLARIZED PROTON TARGET p o l a r i z e d targets. This m e t h o d is based on the k n o w n fact, t h a t the p o l a r i z e d nuclei establish an internal field 9) inside the target sample, which can be m e a s u r e d by E P R m e t h o d s a n d used to deduce the nuclear p o l a r i z a t i o n . C o m p l e t e discussions o f the theoretical basis 6) a n d the experimental p r o c e d u r e ' ° ) o f the P I F m e t h o d will soon be published.
W e wish to express our t h a n k s to Dr. A. Pfihler, Mr. H. D a n n h e i m and Mr. B. G a i s s m a i e r for their valuable c o n t r i b u t i o n s during the early stage o f this work. W e are very grateful to Profs. R. F l e i s c h m a n n a n d J. Christiansen for their e n c o u r a g i n g interest a n d effective support. The B u n d e s m i n i s t e r i u m ftir Bildung u n d Wissenschaft has given considerable financial s u p p o r t to this w o r k which is gratefully acknowledged.
155
References 1) Proc. Intern. Conf. Polarized targets and ion sources (Saclay, 1966). ~) C. D. Jeffries, Dynamic" nuclear orientation (New York, 1963). 3) p. Catillon, M. Chapellier and D. Garreta, Nuck Phys. B 2 (1967) 93. 4) W. Whaling, Handbuch der Physik 34 (Springer, Berlin, 1958) p. 198. ~) H. Dannheim, B. Gaissmaier, D. Kr6niger, H. Kuiper, K. H. Kutschera and B. Seidler, Z. Physik 241 (1971) 130. e) K Kutschera, H. Kuiper, D. Kr6niger and B. Seidler, Nucl. Phys., to be published. 7) T. W. P. Brogden, O. N. Jarvis, J. Orchard-Webb and M. R. Wigan, Proc. 2nd Intern. Syrup. Polarization phenomena of nucleons (Karlsruhe, 1965) p. 298. 8) M. Peter and M. W. P. Strandberg, Proc. Inst. Radio Engo 43 (1955) 86. ~) H. Kuiper, Z. Physik 232 (1970) 325. a0) K. Kutschera, D. Krbniger, H. Kuiper and B. Seidler, to be published.
AND METHODS
98 (1972)
I5I-I55;
© NORTH-HOLLAND
PUBLISHING
CO
A P O L A R I Z E D P R O T O N TARGET FOR L O W ENERGY SCATTERING E X P E R I M E N T S D O W N TO LESS T H A N 10 MeV H. KUIPER, J. DIEKAMP, K. F R A N K , D. KRONIGER, K. KUTSCHERA and B. SEIDLER
Physikalisehes hzstitut der Univers#iit Erlangen-Niirnberg, Erlangen, Germany Received 12 August 1971 A polarized proton target for low energy experiments has been
in a magnetic field to 18.5 kG using 71 GHz microwaves from a
constructed and used at the Erlangen tandem accelerator. The free protons of the waters of hydration in a 3 x 3 x 0.05 mm3LMN crystal are polarized by the solid effect at a temperature of 1.4 K
klystron. 60% proton polarization is obtained. Constructional and operational details are discussed.
1. Introduction The increasing interest in experiments with oriented nuclei has led to the construction of a considerable number of polarized proton targets 1) in recent years. These targets, including the one to be described, have used the well-known technique of dynamic nuclear polarization2). The majority of polarized proton targets has been designed for experiments at high and intermediate energies, yet only one report about a polarized target for use with low energy charged particle beams exists3). In fact, low energy polarized proton targets essentially differ in their design from targets constructed for the hundred MeV or the GeV energy range and present a number of additional difficulties. Many of these can be traced back to the obvious necessity to limit the energy loss of beam and scattered particles in the target to at most a few MeV. As a consequence the geometrical dimensions of the target sample have to be very small and the microwave cavity must have thin walls for charged particles to pass through. Other problems are connected with the necessarily indirect cooling of the target sample and with the measurements of target polarization, which, in contrast to high energy targets, cannot reliably be performed by the conventional N M R method. In the description to follow it is shown how these problems were solved with the polarized proton target constructed at Erlangen. This target was used first for an (e,p) elastic scattering experiment at the Erlangen tandem accelerator. 2. General experimental set-up Fig. 1 shows the principal features of the target configuration. The magnet, cryostat and scattering chamber assembly are located at the end of a beam
[IEMPERAIURE [ MICRQWA~Zi~iOL ARIZAIIQN ~ MEASUREMENI [ SYSTEM ] LMEASUREMEN;J
I -PL;HtPii-I
BEaM
~-DIFFI_S]SN~. PLJf4P ] / ....
}: [ ,t v
,
;
Fig. 1. Principal arrangement of the polarized proton target. a: LMN target crystal, b: Si surface barrier detectors, c: waveguide, d: stainless steel tube supplying liquid 4He to the target, e: liquid Nz cooled radiation shield, f: electromagnetically actuated beam shutter, g: vacuum wall of scattering chamber. tube of the tandem accelerator. The beam hits the L M N target parallel to the c-axis of the crystal. The target sample is placed in the centre of an electromagnet and cooled down to about 1.4 K by a conventional, pumped ¢He cryostat. Microwave power is supplied by a 71 GHz reflex klystron. The scattered particles are detected by two Si surface barrier detectors. 3. Crystal The sample containing the protons to be polarized consists of a very thin slab of monocrystalline La2Mg3(NO3)12-24H20 , (LMN for short), doped with 1% Nd 3+ in the solution. This material has been chosen, since it is capable of the highest degree of polarization and because it allows the preparation of 151
152
H. KUIPER et al.
He-6ewar
......
leflon gasket '
I I --V?A flange Soft soldered
V2A fube
Ag ,~avegude RG98 L . . . . . .
2
LIqu,d Zke
U II
The waveguide supplying the microwave power is fed right through the liquid helium vessel, as seen from fig. 2. By lowering the pressure in the helium dewar with an Edwards Speedivac 1SC900 rotating vacuum p u m p (nominal pumping speed 15 1/sec), the target can be cooled down to about 1.4 K. The temperature of the cavity base can be measured via the 4He vapour pressure or, more conveniently, by means of Allen Bradley carbon resistors with an accuracy better than _+ 0.005 K. The useful helium inventory of the cryostat is about 3.5 1. Continuous operation is possible for about 28 h at 4.2 K or for about 10 h at 1.4 K between two helium transfers. The magnet is a standard iron core electromagnet with 250 m m diameter pole pieces tapered to 140 m m diameter and producing a field of 18.5 kG at the centre
//Hgrd soldered 109
7
6
51
2 3
0
1
8
11
"-To diaphragms
Fig. 2. Cooling system and cavity assembly (V2A= stainless
steel). small specimens of well-defined dimensions. The crystals are grown from the saturated aqueous solution by automatically lowering the temperature in a Peltier element controlled thermostat. In this way we obtain crystals suited for target application within a day or so. These are given their final dimensions of approximately 3 x 3 x 0.05 m m 3 by carefully polishing on a wet satin cloth. During this procedure the thickness and plane parallelity of the crystal are controlled with a microscope.
4. Cryostat and magnet The cryogenic system involves a conventional 4He cryostat. The bottom of the helium dewar is demountable and sealed with a thin teflon foil gasket (see fig. 2). This foil proved very useful since it endures 30 or more cool-down warm-up cycles, thus comparing favourably with the conventional indium seal. A thin stainless steel tube of 18 m m o.d. has been welded to the demountable bottom flange. The other end of the tube has been hard-soldered to the bore of a copper block, called cavity base, which is cooled by direct contact with liquid helium. On this base the microwave cavity containing the target crystal is mounted in a manner to be described below (sect. 7).
2cm
Fig. 3. Cavity and detector arrangement as mounted inside the liquid N2 cooled radiation shield. 1: cavity base, 2: waveguide, 3: cavity cover, 4: cavity core, 5, 7, 10: tantalum diaphragms along the incident beam path, 6: stainless steel tube, 8: tantalum diaphragms defining the scattering angles, 9: liquid N2 cooled copper radiation shield, 11 : aluminium housing for the detectors, 12: Si surface barrier detectors, 13 : beam shutter (closed position), 14: shutter driving coil.
POLARIZED PROTON TARGET
of a 42 mm gap. Inside a sphere of 1 cm diameter at the field centre the deviations of the field strength from the centre value are not greater than 0.4 G at 18.5 kG, corresponding to a homogeneity of about 2 x 10 -5 The time stability of the magnetic field strength has been checked by EPR measurements and turned out to be of the order of 5 x 10 .6 over a period of about ten minutes. 5. Particle trajectories and energy losses The strong magnetic field at the target causes curved trajectories for the beam as well as for the scattered particles. For a well-defined scattering geometry all relevant particle trajectories have to be known. For this purpose a computer program has been devised capable of calculating the particle trajectories on the basis of the measured field distribution. For the practical adjustment the incident beam is made visible by means of two ZnS coated beam viewers which can be,, flipped into the beam electromagnetically. Marks on the viewers applied under control of a theodolite, de,fining the precalculated direction of the beam, ensure that the incident particles hit the target at the correct angle. Each viewer is monitored by a TV camera. For the scattered particles another computer program yields energy and scattering angle (at the target) of those particles that pass through a diaphragm of given geometrical size and position and then enter a de,tector. The program takes also into account the energy losses of the charged particles in the scattering process as well as in traversing the target crystal and copper cavity walls. With the known values of the atomic stopping powers for Cu 4) and L M N s) we have calculated the positions of the diaphragms shown in fig. 3. These are chosen such that of the recoil protons from the L M N target only those emitted under (32__ 3) and ( - 3 2 + 3) degrees (lab.syst.) reach the de,tectors. By similar calculations the instrumental asymmetry could be reduced to about 2.5% (experimental value). This arrangement has been used in an (e,p) elastic scattering experiment designed to measure the target polarization and to compare the results with those of the PIF method 6) (cf. sect. 8). An electromagnetically operated shutter mechanism was constructed (see fig. 3), which exposes the target to the particle flux only during the well-defined counting periods. In this way all unnecessary bombardment of the target is avoided, which is important in view of the serious problem of radiation damage7). The shutter can be actuated either manually or under control of the electronic counter circuit.
153
6. Particle detection In the following we describe a particle detection arrangement used in the above mentioned (c~,p) elastic scattering experiment. In order to obtain a welldefined scattering geometry the diaphragms defining the scattering angles are mounted directly on the cavity assembly (see fig. 3). Si barrier detectors, contained in an aluminium housing, are fixed to the inside of the liquid N2 cooled radiation shield, close to the diaphragms. Due to geometrical restrictions and a 10 #m thick copper foil in front of the detectors only two particle groups are detected: 1) recoil protons from the polarized target and 2) those ~ particles elastically scattered by the copper cavity walls that did not penetrate the crystal. These two particle groups are not well separated in energy. Since, however, we are only interested in particle counting rates rather than in energy measurement, we achieve particle discrimination using diodes with a very small depletion depth such, that the ~ particles are stopped in the sensitive volume while the protons are not. In this way spectra like that shown in fig. 4 are obtained, the left peak representing the recoil protons. 7. Microwaves 7.1. GENERAL DESCRIPTION
In our case, the microwave apparatus is used for two very different purposes. First it serves to saturate the so-called "forbidden" transitions, thus polarizing the target. Secondly it provides a very useful tool for measuring the nuclear polarization inside the target by PIF (cf. sect. 8). Microwave power from a 71 GHz klystron oscillator is fed into the target cavity resonator. Variations of the reflected power, due to magnetic resonance in the cavity can be observed by a 1N53 microwave diode connected with the main waveguide via a 10 dB directional coupler or alternatively by a circulator (see fig. 5). Standard lock-in technique is used to detect the EPR signal. A microwave spectrometer like the one described here is not able to distinguish between absorption and dispersion in the EPR signal, but this is not of any significance in our experiments. 7.2.
MICROWAVE GENERATOR AND FREQUENCY STABILIZATION
As a microwave source we use a Philips Y K 1010 reflex klystron with a maximum output power of 350 mW at 71 GHz. Exceptionally high frequency stability of this oscillator has been achieved by extending a phase locking technique, well-known at lower frequencies8), to the
154
H. KUIPER et al.
.\".
15qO ~
10L
•.
..
5105
.< r
100
I 200
: 300
JJO
500 CHANNEL~JMUER
Fig. 4. Typical spectrum showing the recoil proton peak (at left) and a peak representing ~ particles elastically scattered at the copper foil cavity walls (at right).
71 GHz region (see upper part of fig. 5). A quartz oscillator supplying a reference frequency of about 1.4 GHz serves as a frequency standard of a relative stability of better than 10 - 7 per day. In a mixer diode (1 N53) the 48th harmonic of this reference is generated and superheterodyned with the klystron "local oscillator" (LO) frequency. In the stabilized state an intermediate frequency IF of 30 MHz appears at the mixer output. This IF is compared in a phase discriminator circuit with another 30 MHz frequency derived from the quartz standard• The phase comparator dc output, which is proportional to the phase difference of the two 30 MHz frequencies, is used to control the klystron reflector voltage and thereby the LO frequency. It is the principal advantage of a phase control that the frequency stability of the klystron so stabilized is equal to that of the quartz standard. Another advantage is the accurate and convenient resettability of the klystron frequency which is a consequence of the quartz oscillator reference frequency being variable by 10 kHz steps. 7.3. CAVITYRESONATOR For the cavity resonator, excited in the H~o ~ mode by a coupling iris from the waveguide, we have chosen a "sandwich" construction consisting of three main parts which may be called base, core and cover (see fig. 3). The cavity volume is formed by the core and
two copper foils (thickness 10 pro), one on each side of the core. The rectangular cavity thus formed has the dimensions 10x 1 0 x 2 . 1 9 mm 3. One of the foils supports the target crystal glued on it with K e I - F , a non-hydrogenous grease. The core is a precision-milled copper frame, cut into two pieces, thus facilitating the access to the resonator volume and making the core easy to manufacture. Cover, core, base and foils are mounted together with four fastening screws. For better thermal and electrical contact it proved to be useful to introduce two flat lead gaskets between the foils and the cover or the base, respectively. For the mode chosen the homogeneity of the microwave magnetic field in the target region is sufficiently good. After taking apart and later re-assembling the resonator, the cavity resonance frequency can be set again to the previous value by more or less tightening one of the four screws mentioned above (possible variation about 300 MHz).
8. Nuclear polarization The maximum proton polarization obtained was 62%. Since the N M R method fails, the target polarization is measured by PIF, a method specially developed for nuclear polarization measurement in low energy
r i I
Klvstraq
i
I
Mcgnel
/ I
L Fig. 5. Block diagram showing the EPR signal detection system and the frequency stabilization scheme by phase-locking the klystron to a quartz standard.
POLARIZED PROTON TARGET p o l a r i z e d targets. This m e t h o d is based on the k n o w n fact, t h a t the p o l a r i z e d nuclei establish an internal field 9) inside the target sample, which can be m e a s u r e d by E P R m e t h o d s a n d used to deduce the nuclear p o l a r i z a t i o n . C o m p l e t e discussions o f the theoretical basis 6) a n d the experimental p r o c e d u r e ' ° ) o f the P I F m e t h o d will soon be published.
W e wish to express our t h a n k s to Dr. A. Pfihler, Mr. H. D a n n h e i m and Mr. B. G a i s s m a i e r for their valuable c o n t r i b u t i o n s during the early stage o f this work. W e are very grateful to Profs. R. F l e i s c h m a n n a n d J. Christiansen for their e n c o u r a g i n g interest a n d effective support. The B u n d e s m i n i s t e r i u m ftir Bildung u n d Wissenschaft has given considerable financial s u p p o r t to this w o r k which is gratefully acknowledged.
155
References 1) Proc. Intern. Conf. Polarized targets and ion sources (Saclay, 1966). ~) C. D. Jeffries, Dynamic" nuclear orientation (New York, 1963). 3) p. Catillon, M. Chapellier and D. Garreta, Nuck Phys. B 2 (1967) 93. 4) W. Whaling, Handbuch der Physik 34 (Springer, Berlin, 1958) p. 198. ~) H. Dannheim, B. Gaissmaier, D. Kr6niger, H. Kuiper, K. H. Kutschera and B. Seidler, Z. Physik 241 (1971) 130. e) K Kutschera, H. Kuiper, D. Kr6niger and B. Seidler, Nucl. Phys., to be published. 7) T. W. P. Brogden, O. N. Jarvis, J. Orchard-Webb and M. R. Wigan, Proc. 2nd Intern. Syrup. Polarization phenomena of nucleons (Karlsruhe, 1965) p. 298. 8) M. Peter and M. W. P. Strandberg, Proc. Inst. Radio Engo 43 (1955) 86. ~) H. Kuiper, Z. Physik 232 (1970) 325. a0) K. Kutschera, D. Krbniger, H. Kuiper and B. Seidler, to be published.