Few-nucleon physics with stored, cooled beams

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A631 (1998) 122c-136c

Few-Nucleon Physics with Stored, Cooled Beams H.O. Meyer Department of Physics, Indiana University, Bloomington, Indiana 47405 ([email protected])

Storage rings with internal targets offer a number of important advantages for experiments in few-nucleon physics. Most significant are the new possibilities concerning polarized beam and targets. The results of some recent experiments are discussed. Storage ring experiments involving polarized deuterons are now in the planning stage. Some aspects of polarized deuterons and early measurements are presented.

1. FEW-NUCLEON EXPERIMENTS: BENEFITS FROM STORAGE RINGS 1.1. Stored beams, internal targets In a storage ring, an ion beam, delivered from a suitable injector, can be accumulated. For the IUCF Cooler, the orbiting current is typically several hundred micro-Amperes, a limit mainly given by space-charge related effects. The beam can be either coasting (DC), or it can have a time structure (arbitrary number of bunches per circumference). In the latter case, the beam energy is accurately determined by the orbit frequency. The bunching rf device is also used to increase or decrease the beam energy. The stored beam is phase-space cooled (e.g., by means of a co-moving electron beam). This produces a beam of high quality with an emittance of typically 10-8n m and a relative energy spread AT/T on the order of 10 -4. The thickness of an internal target is limited by the strength of the cooling force and by the beam lifetime (which itself depends on beam cooling). A short lifetime means a low average luminosity because of the time spent filling the ring rather than taking data. The effect of the target on the emittance growth, and on the beam lifetime depends on energy. At energies below a few GeV the dominating mechanism that governs both is Rutherford scattering, which scales with the atomic number Z of the target like I/Z 2 [1]. As a rule of thumb, one finds that the average luminosity also drops with I/Z 2 as the target Z is increased. This is an important reason why low-Z targets are preferred, and why storage rings are attractive tools in particular in few-body nuclear physics. However, the main payoff of the storage ring technology clearly comes from the novel possibilities that are available for physics with polarized collision partners. These possibilities are briefly described in the next two sections. 0375-9474/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0375-9474(98)00019-0

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1.2. Polarized, internal targets Several methods to produce 1 polarized atoms are available. For instance, polarized hydrogen (or 0.5 deuterium) atoms can be produced by Qx 0 dissociating molecules, forming a -0.5 beam and then selecting a single -1 hyperfine state by a combination of 1 inhomogeneous magnetic fields and 0.5 I if-induced transitions between • • J Oy 0 magnetic substates. In a recent example, a beam of about 1 cm -o.5~"'~"~'~'="" ~ . . . . ~ , diameter with a flux of about 3.1016 -1 "' ' ¶ 0 2 4 6 8 10 12 polarized H atoms/s in a pure spin ~ime ( s a c ) state with polarization P=0.75 has been produced [2]. When this beam is directed from the side into a Figure 1. Horizontal and vertical target narrow, open-ended channel through polarization components Qx and Qy as a which the stored beam passes (a sofunction of time over a 12-s cycle. The called storage cell), a target thickness measurement uses up-down and left-right count of 1013 - 1014 atoms/cm2 can be rate ratios ofp+p elastic scattering. From t=8 s achieved. The polarization direction to t=12 s the target is polarized in the z is defined by a magnetic guide field direction (longitudinal). in the target region. Such polarized targets are pure, not susceptible to radiation damage, and offer complete freedom in choosing the polarization direction as well as the possibility of rapid reversal of its sign. As an example, Figure 1 shows the time dependence of the measured horizontal and vertical hydrogen target polarizations as the polarization is made to point along or opposite three orthogonal directions. For more information, the reader is refered to Ref. [3] and references mentioned therein.

1.3. Polarized, stored beam The magnetic moments of stored particles precess around the magnetic fields in the ring. Depolarizing resonances occur when this precession is consonant with the particle motion. Moving away from such a resonance, the polarization lifetime quickly increases [4], and the stored polarization is usually remarkably stable. Accelerating and decelerating the beam hardly affects its polarization, provided no resonances are crossed in the process. This can be used to export a given polarization standard to arbitrary energies [5]. The spin closed orbit is the eigenvector of the rotation of a magnetic moment in a single revolution. Polarization components normal to the spin closed orbit rotate around it and average to zero over time, thus, only the component along this vector is preserved. The spin closed orbit is given by the fields in the ring; it is independent of the history of

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the beam prior to injection. Normally, the spin closed orbit is vertical, but non-vertical fields (e.g., solenoids along the beam axis) can be used to manipulate it, and to produce non-vertical beam polarization at the target. Crossing a depolarizing resonance at slow speed (adiabatic passage) causes the spin closed orbit to reverse direction. A resonance can be produced by a magnetic rf field, and the resonance can be crossed by varying its frequency. It has been demonstrated that such a device is able to flip the sign of the polarization of protons with better than 98% efficiency [6]. This is an important tool in eliminating systematic errors in polarization experiments. Most of our experience to date has been with polarized spin-V2 particles (protons). Experimentation with (spin-l) deuterons is starting now, and several open questions still need to be answered for this more complicated case. Some of these topics are addressed in Section 5 below.

2. PROTON-PROTON SCATTERING BELOW THE PION THRESHOLD Much of our understanding of the nucleon-nucleon (NN) force has been gained through the study of free nucleon-nucleon scattering. Observed quantities are parametrized in terms of empirical phase shifts. The phase shifts, in turn, form the basis for tests of NN interaction models, such as those that try to explain the NN force based on the exchange of mesons. It is generally believed that the pp scattering phase shifts below 1 GeV are known well but there are differences between the analyses from different groups. Considering the sheer size of the pp data basis, it is clear that any new contribution can only come from a body of measurements with a large weight (small experimental uncertainties) of observables that are not yet well represented in the data base. These requirements are met by a new generation of spin correlation measurements carried out at storage rings. 2.1. Proton elastic scattering with the IUCF Cooler: 200 MeV A polarized hydrogen target facility has been installed in the IUCF Cooler in the spring of 1993. The target is placed in a magnetic guide field which can be changed in direction (up, down, left, right, along and opposite the beam axis) within a few milliseconds. Downstream from the target is a stack of scintillators and wire chambers to detect direction and energy deposited by charged reaction products, emitted between 5 ° and 17 °. To commission the apparatus and to study the many, new experimental questions, we decided to investigate first p+p elastic scattering. For this purpose, we introduced additional detectors alongside the target cell to observe the low-energy recoil protons from this reaction in coincidence with the forward protons, resulting in a very clean definition of the events of interest (the background was well below 1%). Yields were then observed in four azimuthal directions, for the six target polarization directions, and vertically up and down polarized beam. A new analysis method has been developed [7] to deduce from these 48 yields the analyzing power Ay, and the spin correlation coefficients A~x,Ayy,A~, as well as a host of information on systematic effects that need

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to be studied for a high-precision experiment. The achieved statistical uncertainty is approximately _+0.01, while systematic errors are less than that, and the error of the absolute overall normalization is 2.4%.

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Figure 2. Analyzing power and spin correlation coefficients in p+p elastic scattering at 197.8 MeV. The points are the data of Ref. [9] minus the corresponding values calculated with the SAID solution SM94 [10]. The solid line is the difference between the solutions WI96 and SM94, shows the response of the analysis to the inclusion of the new data.

The practical considerations that led to the choice of beam energy (197.8 MeV) were the following. On one hand this is the energy at which the beam was injected, thus the technical problem of accelerating the polarized beam did not arise. On the other hand, an accurate p+p analyzing power datum for calibration of the measurement is available at this energy [8]. The latter allows one to determine the absolute target and beam polarizations from the data. The results of this first phase of the experiment, including many details concerning the technique, have recently been published [9].

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The effect of the measurement on partial wave analyses is demonstrated in Figure 2 in which the data are compared to two solutions of the Virginia analysis [10]. The first (SM94) has been published prior to this experiment, while for the second (WI96) the new data are included in the data base. The points in Figure 2 are our data [9] minus the corresponding values calculated with SM94. The solid line is the difference between WI96 and SM94, and thus shows the response of the analysis to the new data.

2.2. Proton elastic scattering with the IUCF Cooler: 250-450 MeV In a second phase of this experiment, the detector acceptance has been improved, covering the full angular range beyond 4 ° in the lab, and the technique was developed to accelerate and decelerate the beam without significant loss of the polarization. This allowed us to take data at 200 MeV and at some higher energy with the same beam. In this way, the measurement at the higher energy was calibrated by the data acquired at the injection energy [5]. Figure 3 shows the resulting polarization observables for bombarding energies between 200 and 450 MeV, obtained in less than one week of running time. In this energy range practically no spin correlation coefficients were previously known. The analysis of the data has been concluded and an account is presently being prepared for publication. 2.3. The spin correlation coefficient Az~ The recent addition of spin precessing solenoids to the IUCF Cooler makes it possible to produce a proton beam which, at the target position, is longitudinally polarized. Using this new capability, a measurement of the fourth correlation coefficient, A~z, over the full angular range has been carried out at 200 MeV [11].

3. PROTON-PROTON INTERACTION ABOVE THE PION THRESHOLD 3.1. Elastic scattering A ring is particularly well suited for measuring excitation functions, because data can be acquired while the beam energy is slowly changed. A study of pp elastic scattering which is making use of this feature is currently in progress at the COSY storage ring in Jtilich. The detector setup features a segmented, cylindrically symmetric arrangement of scintillator detectors, observing both outgoing protons, which allows the measurement of angular distributions from 15° to 75 ° in the lab. A polypropylene fiber is used as an internal target, and the luminosity is monitored by observing the current of secondary electrons as well as energetic electrons emitted from the fiber. In a first stage, differential cross sections between 500 and 2500 MeV have been measured by this so-called EDDA facility [12]. The development of accelerated polarized beam (overcoming depolarizing resonances) and the recent installation of a polarized target will soon make spin correlation measurements possible. Accurate data in this energy region are sparse, and much is needed in testing meson-exchange models which predict the increased importance of heavy-meson exchange [13].

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near threshold

Meson production near threshold is a class of experiments where storage ring technology clearly has had a profound impact, the most noteworthy case being the reaction pp-.ppx °. Within the first few hundred MeV above threshold, the angular momentum ~ in the final (NN) system, as well as the angular momentum Q~of the pion with respect to the NN pair, are both either 0 or 1. This greatly limits the number of

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contributing partial waves, and, as the bombarding energy is lowered towards threshold only a single partial wave survives (the one with the lowest Iron and Q~, or weakest energy dependence). Furthermore, large momentum transfer and small impact parameter (a small incident angular momentum is required by angular momentum conservation) filter out the short-range features of the NN interaction. Storage rings offer many advantages for this type of experiment. In particular, a pure and windowless target is crucial since it excludes an admixture of heavier target nuclei for which the production threshold would be much lower [3]. The most complete total cross section data to date have been obtained at IUCF in the channel pp-,ppn ° [14]. These data have recently been confirmed by an experiment at CELSIUS (Uppsala) [15]. From the energy dependence and the distribution of the protons in the exit channel it has been demonstrated that within the first 20 MeV above threshold the cross section is completely dominated by the 3Po-,'So(l~=0) partial wave. Before the internal-target data became available, it was believed that the ppoppn ° cross section is well explained by the impulse term (the pion is emitted by one of the nucleons), with negligible contribution from rescattering (where the pion is emitted by one of the nucleons and then rescattered by the other). This model, however, underestimates the data by about a factor of five (!) [16]. So far, two candidates have been suggested for the physics which is missing in the simple model. The first takes into account short-range effects due to the exchange of heavy mesons (HME) in a "z-graph" (with an intermediate negative-energy state) [17]. The important contributions arise from the exchange of o and 60 mesons. Using the same meson masses and meson-nucleon coupling constants that parametrize also the distorting potential, the missing factor of five could be explained in full [18]. This explanation is interesting, since it means that this reaction provides the first evidence of the exchange of mesons heavier than the pion, and it would prove that the axial current in a nuclear system may be enhanced. The second candidate is a rescattering term driven by the offshell isoscalar nN amplitude which is much larger than its on-shell equivalent. The current status of the theory is reviewed in Ref. [3]. The theoretical controversy calls for an extension of pion-production measurements to include polarization observables. The additional information would be used in isolating the contributions from individual partial waves other than the lowest one. At IUCF, for instance, an experiment to measure the spin-dependent total cross sections AOL and AoT is in progress. From these observables, together with the unpolarized total cross section otot,it is possible to deduce the incoherent contributions of the lowest three partial waves in a model-independent way [19] to further test the HME hypothesis or aspects of the nNN three-body system. The Cooler measurement of pion production with polarized beam and target uses the same facility as I mentioned earlier (Section 2.1.). Target performance and the necessary machine capabilities have been tested in the course of the p+p elastic scattering measurement, and appropriate modifications of the detector stack to trace and stop protons up to 200 MeV have been implemented. In addition, a neutron detector, supplied by Pittsburgh University, has been added. Charged nucleons in the exit channel are contained in the detector acceptance up to a bombarding energy of 375 MeV. It is

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planned to measure analyzing powers, spin-dependent total cross sections, and spin correlation coefficients of the reactions pp -. ppn°, pp -. dn ÷ and pp -, pnr~÷ between 300 and 400 MeV. At present, the first data are in hand and production running is expected in the near future.

4. INTERACTIONS BETWEEN THREE NUCLEONS 4.1. Pion production in p+d collisions Pion production in p+d collisions is experimentally very similar to the NN case and benefits from the storage ring environment in much the same way. Such measurements address the question whether the production mechanism is affected by the presence of other nucleons. Adding just one nucleon presumably results in a nuclear "system" of manageable complexity. Given the short range of the production process, one would expect that elementary production dominates (the pion is produced in an NN encounter and one nucleon acts as a spectator). Such a model [20] reproduces the nearthreshold total pd-.pdTz° total cross sections, obtained with the 1UCF Cooler [21], quite well. However, the interesting physics lies in a departure from the simple spectator picture, as seen for instance, when comparing the measured differential cross sections as a function of the angle at which the spectator proton is emitted with predictions by the spectator model. A follow-up experiment with a deuteron beam on a proton target is currently under way at CELSIUS (W. Scobel, Hamburg University, private communication). 4.2. Spin-dependent 3He wavefunctions The 3He nucleus is of interest because it is a relatively tightly bound system for which essentially exact Faddeev solutions exist that describe the ground-state wave function. These calculations predict that the ground-state is predominantly (about 90%) composed of a singlet proton pair and a neutron which is responsible for the 3He spin. Recent interest in the spin-dependent nucleon wave functions is motivated by the possible use of 3He as a polarizable neutron target. The spin-dependent momentum distributions, given by the difference of the singlenucleon momentum density distributions for a neutron (proton) with spin parallel or antiparallel to the 3He spin, have recently been studied at the IUCF Cooler. To this effect, quasi-elastic np or pp scattering was observed with a polarized, internal 3He target and a polarized, stored proton beam. The spin correlation data were analyzed assuming the validity of the plane-wave impulse approximation [22] and good agreement with theoretical models of 3He was found up to 300 MeV/c nucleon momentum. 4.3. Search for evidence of a three-body force Faddeev calculations that are restricted to NN forces have not been able to reproduce the binding energies of 3H and 3He, making it necessary to include the interaction between three nucleons. However, so far, direct evidence for three-body forces has not yet been established. The best place to look is the three-nucleon system,

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where empirical searches can be guided by Faddeev calculations. For instance, a state-ofthe-art Faddeev calculation has recently been used to survey the effect of the so-called Tucson-Melbourne force on spin observables in the nd-.nnp break-up reaction [23], and it was found that spin correlation coefficients, involving tensor-polarized deuterons can be very sensitive in certain parts of the phase space, in particular in the so-called "finalstate" configuration where two of the outgoing nucleons have the same momentum (see Figure 4). A qualitative understanding why this configuration is so sensitive to the threebody force is currently not available, but one suspects that the constraint is important that arises from the fact that the nn pair is predominantly in a ~S0 angular-momentum state.

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target nucleon), and Coulomb effects (which are not taken into account by the model) are reduced, while three-nucleon forces are expected to be more important than at lower energies.

5. FIRST STEPS TOWARDS EXPERIMENTS WITH POLARIZED DEUTERONS

5.1. Polarization of spin-1 particles For a spin-1 particle, the component ins of the spin along an arbitrarily chosen direction ("quantization axis") has one of three values, namely, m~=+l, 0, or - 1. For an unpolarized assembly of spin-1 particles, the magnetic substates rn~=l, 0, and -1 are equally populated, i.e., NI=N0=N_ 1=1/3, no matter in which direction the quantization axis is chosen. If the population of the rn~=l state is enriched at the expense of the n~=- 1 state, the system is said to have vector polarization, pz=Nl-N_l (analogous to s=l/2 polarization). If the number of particles in the rrq=0 state is enriched or depleted, the assembly acquires tensor polarization. The system can at the same time be vector- and tensor-polarized. For the special choice of quantization axis (also called spin-alignment axis) where the system has axial symmetry the degree of tensor alignment is measured by p==l-3N 0. In addition to the three vector components, five independent components of a second-rank tensor are required to describe the polarization of an assembly of spin- 1 particles. For a reaction with a two-body final state only three tensor quantifies matter; they are often expressed in terms of the spherical tensor moments t~0, t21, and t22 [24]. Their sizes depend on Pzz and the orientation of the spin alignment axis with respect to the reaction plane. In order to control individual contributions of different tensor moments to the spin-dependent reaction cross section, the experimenter can either rotate the reaction plane around the beam axis, or tilt the spin alignment axis (away from vertical). In a ring, the latter requires a spin rotator, i.e., precession about some non-vertical (e.g., solenoid) field. Such a rotator has been mentioned in Section 2.3.; for deuterons, it is discussed in the next paragraph.

5.2. Manipulation of the polarization of stored spin-1 particles The magnetic moment of deuterons is smaller than that of protons (the corresponding g-factors are gd--0.8574 and gp=2.7928). Thus, for equal precession, a field about three times stronger is required. Even though they are less effective, spin rotators for stored deuterons are by no means hopeless! For instance, with the two solenoids of about 1 Tm strength, currently operational in the Cooler, it is possible to tilt the spin alignment axis of a 90 MeV deuteron beam by about 45 o away from vertical (in the direction of the beam axis) at the location of the polarized target. In Section 1.3 1 briefly discussed the reversal of spin-½ polarization by crossing a depolarizing resonance. Such a resonance can be generated artificially by a radiofrequency solenoid. It is known and has been demonstrated experimentally [25] that the strength of the resonance has to correspond to the speed at which the resonance is

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crossed. The resonance strength 8 induced by a longitudinal rf field is defined to be the frequency with which the stored particles would precess around the beam axis if they would always encounter the solenoid at peak field. It can be easily verified that

8 _ 2~LRC Yg k fBpeakdl ,

where k=laN/~=0.1598 (Tin) ~, c is the speed of light, and L R is the ring circumference. The only ingredient that depends on the stored particles is the factor g/y, which for deuterons is about a factor of 3 smaller than for protons. This difference can easily be compensated by increasing the field strength and (or) decreasing the crossing speed. As the resonance is crossed, the spin closed orbit goes from vertical (up) via lying the ring plane to vertical down, or the angle a between the vertical direction and the spin closed orbit goes from 0 to 180 °. While doing so, the spin closed orbit rotates rapidly around the vertical direction, thus polarization components in the ring plane are averaged over time. Figure 5 shows vector and tensor polarization with respect to a vertical quantization axis as a function of a. For a complete reversal of the spin closed orbit, the vector polarization changes sign, but the tensor polarization is not affected. An interesting variant of this method might turn out to be when the resonance is made weak on purpose such that after crossing the spin closed orbit ends up in the ring plane. In this case, the vector polarization vanishes and the tensor polarization is decreased by a factor of two, but has changed sign.

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5.3. Depolarizing resonances and beam depolarization In the vertical bending field in a ring, the magnetic moment of the beam particles precesses about the field direction. The precession frequency in the particle frame is given by Gy, where y is the Lorentz factor, and G=g- 1 is given by the g-factor of the particles. The "- 1" arises from the fact that the beam is bent, and the particle frame also rotates around the vertical direction. So-called imperfection resonances occur at energies for which Gy has integer value. Since for deuterons Gd=-0.1426, while for protons Gp=1.7928, there are much fewer depolarizing resonances for deuterons than for protons. The lowest imperfection resonance occurs at 11.3 GeV. Intrinsic resonances are caused by the focusing fields and depend on the vertical and horizontal machine tunes vv and VhThey occur when Gy=n+mvh+lV~ (n,m,l=integer). There are also fewer intrinsic resonances for deuterons but they can also occur at low energy since Vv,h can be a small number. A polarized deuteron beam has recently been accelerated and decelerated through an intrisic resonance at KEK [26]. Depolarization of a stored beam, when it occurs, is caused by a random walk of the spin closed orbit [4]. In order to calculate its size one has to derive the change in polarization induced by a change of the quantization axis direction by some angle c~. For vector polarization this is simply cost~, or the Legendre polynomial Pl(cost~). It can be derived easily, that the corresponding change for tensor polarization equals P2(cosc0. From this, one concludes that the polarization lifetime of tensor polarization is exactly three times shorter than that of vector polarization.The same ratio between the vector and tensor polarization lifetimes follows if one assumes transitions to occur at some constant rate between any two neighboring substates ( [Arr~[= 1), ignoring transitions from ms--1 to nh=- 1 (F. Sperisen, IUCF, private communication). It will be interesting to test this simple prediction experimentally. 5.4. First measurements with the IUCF Cooler, involving polarized deuterons In anticipation of the p+d break up studies that are planned (see Section 4.3), short, exploratory measurements involving polarized deuterons have been staged at the IUCF Cooler. Very recently (summer 1997) a vector- and tensor-polarized beam has been stored at 90 MeV, and accelerated to 290 MeV. The beam polarization has been determined from p+d elastic scattering, and the effect of the Gdy =vv-5 intrinsic resonance has been encountered and avoided by adjusting the machine tune. A first test to show the feasibility of measurements with a polarized deuterium target (and a polarized proton beam) was carried out in July 1996. Even though only one day of beam time was invested into this test measurement, the quality of the data tumed out to be sufficient to extract some of the analyzing powers and spin correlation parameters in p--d elastic scattering. The setup was identical to the one used in the p+p elastic scattering experiment discussed in Section 2. The polarized deuterium target was produced by simply substituting deuterium for the hydrogen gas in the atomic beam source supplying the target cell. Since the source was not yet equipped with a second set of deuterium rf transitions, only a mixture of vector polarization pz= +1/3 and tensor polarization of p==- 1/3 was available (relative to the direction of the weak guide field of 3 G). The guide field direction could be chosen to be vertical, horizontal or

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longitudinal. Reversal of the vector polarization was accomplished by reversing the guide field. Coincidences between a forward scintillation counter and silicon micro-strip recoil detectors are recorded. Forward scattered protons are detected in the scintillation counter. Two wire chambers provide information about the azimuthal angle qb and the scattering angle 0 of the forward particle. The energy deposited in the silicon detectors was used to identify the recoil particles as deuterons, thus allowing identification of p+d elastic scattering events. The measurement used a 200 MeV vertically polarized stored proton beam. Data taking was divided into spin correlation measurements with the polarized deuterium target (60-90 min) and beam polarization measurements with an unpolarized hydrogen target (10 min) before and after the deuterium runs. The beam polarization was typically 0.65. It was reversed approximately every 90 minutes by changing the polarization of the injected beam at the ion source. The target polarization orientation was changed in 2 s intervals and cycled through the six available directions. Overall, approximately two million p+d elastic scattering events were recorded over a period of 20 hours. By combining yields taken with different combinations of beam and target polarization directions, it was possible to extract the vector analyzing powers of target, Ayd and beam, Ayp, three vector-vector spin correlation coefficients, Cx,,, Cy,~, Cz.~ two tensor-vector spin correlation coefficients, Cz~,y,Cxy,xand the deuteron tensor analyzing power A=. Figure 6 shows preliminary data for the three quantities Cx.x, Cy,y, Cz,x, together with a recent Faddeev calculation [27]. In future experiments, higher values of the deuteron target polarization could be achieved by means of additional deuterium rf transitions in the atomic beam source. These transitions would also allow a sign change of the tensor polarization, essential in reducing the systematic uncertainties of the measurement.

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Figure 6. Spin correlation coefficientsof the elastic scattering of 200 MeV protons from vector-polarized deuterons versus proton laboratory angle. The solid lines are predictions obtained with a modem Faddeev calculation [27].

H.O. Meyer~Nuclear Physics A631 (1998) 122c-136c

135c

6. FINAL REMARKS

This report should have made clear that the technology of storage rings with internal targets has had a pronounced impact on experimental nuclear physics, and continues to do so. At present, the energy frontier with such facilities is pushed by CELSIUS and COSY beyond the threshold for production of mesons with strangeness. A future machine in the 10 to 20 GeV range would allow us to study the role of quarks and gluons in the interactions between polarized hadrons. However, also at energies below 1 GeV, the potential of the new technology is by no means exhausted. In particular, experimentation with polarized, stored deuterons is just about to start. Contributions to this conference have clearly shown that there are a number of open questions in classical few-nucleon physics that depend, for an answer, on data of high quality and accuracy such as those provided by storage ring facilities. This report deals to a large extent with results obtained with the polarizedhydrogen/deuterium internal target facility at the IUCF Cooler. Construction of the facility and the nuclear physics research that resulted has been the work of the PINTEX group, a collaboration between IUCF, Indiana University, University of Wisconsin, and Western Michigan University, involving the following people: W.A. Dezam, J. Doskow, M. Dzemidzic, W. Haeberli, J.G. Hardie, B. Lorentz, H.O. Meyer, P.V. Pancella, R.E. Pollock, B. von Przewoski, F. Rathmann, T. Rinckel, A.D. Roberts, M.A. Ross, F. Sperisen, T. Wise, and M. Wolanski. More recently, a group from Pittsburgh University has also joined the collaboration.

REFERENCES .

2. 3. . .

6. 7. .

9. 10. 11.

R.E. Pollock et al., Nucl. Instr. Meth. A330 (1993) 380. T. Wise, A.D. Roberts and W. Haeberli, Nucl. Instr. Meth. A336 (1993) 410. Nuclear Physics with Light-Ion Storage Rings, H.O. Meyer, Annu. Rev. Nucl. Part. Sci., in print. Polarization Lifetime near an Intrinsic Depolarizing Resonance, H.O. Meyer et al., Phys. Rev. E (in print). R.E. Pollock et al., Phys. Rev. E55 (1997) 7606. B. v. Przewoski et al., Rev. Sci. Instr. 67 (1996) 165. Diagonal Scaling and the Analysis of Polarization Experiments in Nuclear Physics, H.O. Meyer, Phys. Rev. C (in print). B. v.Przewoski et al., Phys. Rev. C44 (1991) 44. W. Haeberli et al., Phys. Rev. C55 (1997) 597. R.A. Amdt, Phys. Rev. C50 (1994) 2731, and the interactive NN data base SAID, accessed by TELNET 128.173.176.61, login PHYSICS, password QUANTUM. B. Lorentz et al., Measurement of A= in pp Elastic Scattering with a Longitudinally Polarized Beam, in Proc. 12th Int. Symp. On High-Energy Spin Physics, Amsterdam (The Netherlands), 1996, eds. C.W. de Jager et al., World Sci., Singapore (1997), p.426.

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H.O. Meyer~Nuclear Physics A631 (1998) 122c-136c

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