antiproton beams

NUCLEAR PHYSICS A

NuclearPhysicsA558 (1993) 655c-664~ North-Holland,Amsterdam

Atomic collision experiments using keV-MeV proton/antiproton beams E. Uggerhej* CERN, CH - 1211 Geneva 23, Switzerland

Abstract Experimental results (PS-194) are discussed for keV-MeV protons and antiprotons penetrating gases and solids. Large differences are found in ionization and energy loss. These experimental results have led to new theoretical developments but the agreement with some of the antiproton measurements is not overwhelming. Future experiments with well-defined low energy antiproton beams are discussed.

1. INTRODUCTION Since the beginning of this century the penetration of charged particles through matter has occupied a large number of physicists beginning with Thompson, Rutherford, and Bohr. The theoretical and experimental investigations of such problems have led to many new discoveries and have played a decisive role in the development of modern physics. The successful interplay between development of new particle beams and detection techniques have resulted in a large variety of very detailed experimental results. Today, however, the understanding of collision processes is still not complete. Theory is now developed to a degree where contributions from second/higher order Born terms are important. Very strong, higher order effects are expected for projectile velocities comparable to orbital velocities of the electrons. Processes, where two (or more) electrons are involved, are of special interest because of the electronelectron correlations. Here the theoretical descriptions are very complicated because of the many-body problems. For a very recent review, see Ref. 1. In perturbation treatments even and odd Born terms have opposite parity under charge conjugation of projectiles. For experimental investigations of these higher order terms, particle/antiparticle beams (protons, pions, muons, electrons) in the keV-MeV energy region are unique. Out of the available systems pro-

* On leave from Institute for Synchrotron Aarhus C, Denmark.

03759474/93/$06.00

Radiation, University

of Aarhus, 8000

0 1993 - Elsevier Science Publishers B.V. All rights reserved.

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ton/antiproton beams are superior because: 1) they are heavy, stable particles with classical paths, 2) intense antiproton (p) beams are available from LEAR, 3) proton (p) beams are easily available for reference and test experiments. In the previous years PS-194 has studied deviations from exact invariance under charge conjugation of several atomic processes. These measurements consist of single and double ionization of noble gases, inner-shell ionization and energy loss with antiproton projectiles. Large differences between antiproton and proton impact are found in the energy range 40 keV to 4 MeV. These findings have inspired new theoretical developments and changed the interpretation of several phenomena hitherto studied with positive ions. During the last two years PS-194 has had only a few days of LEAR running, so most of the experimental results were already presented in group reports [2,3]. Here, a brief review of the situation is given together with a discussion of future experiments.

2.

EXPERIMENTAL

SETUP

The experimental setup consists of a small vacuum chamber which surrounds the exit (100pm) Be-window of the LEAR beamline, such that the focus is in the target area. The inner of the chamber has two versions, as shown in Fig. 1, i.e., one for measurements of ionization in gases (Fig. la), and one for slowing-down processes in solid targets (Fig. lb). In all, cases the experiment uses 100 MeV/c 6 from LEAR, which are slowed down in the Be-window and degrader foils. The energy of the 6 are measured by TOF between a 100 pm thick start scintillator and a stop scintillator placed downstream. For energy loss measurements in foils (Fig. lb) TOF on both incident and exit sides of the target are detected. The timing signals from the target foils are obtained through counting the emitted secondary electrons. Calibrations of TOF are performed by standard delay and by changing the length of the flight paths. With different degrader foils at the entrance, the energy region for 5 has been covered down to around 20 keV. The large energy spread from degradation means that quasi-monoenergetic beams can be selected in the off-line analysis using the TOF signals. It should be pointed out, however, that through degradation only faint low-energy beams are obtained and the multiple scattering is severe. In ionization experiments (Fig. la) the produced ions (single/multiple) are accelerated to a channeltron detector. From difference in arrival time, the different charge states are measured. In this way it is very simple to obtain relative cross sections just by taking ratios like in the first He-experiments [4].

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110 - J)s

0

1 2

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b)

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Figure 1. Schematic for experimental setup for a) gas targets. The numbers refer to: 1) Beamline vacuum chamber; 2) Ion flight tube; 3) Channeltron; 4) Stop scintillator; 5) Start scintillator; and 6) Degrader. b) Solid targets. The numbers refer to: 1) Beamline vacuum chamber; 2) Start scintillator; 3) Target foil; 4) and 5) Electrical mirror; 6) Channel-plate detector, and 7) Stop detector. 3.

IONIZATION

3.1. Single ionization In two recent publications we described the first measurements of ionization of helium and molecular hydrogen by antiproton impact [5,6]. Figure 2 shows our measured cross sections for single ionization and 5 impact on He together with molecular hydrogen. Similar cross sections measured for proton impact are shown

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for comparison. Above 500 keV, there is no difference between the proton and antiproton results. At energies between 500 and 50 keV, the proton cross section exceed that of the antiprotons, owing to the polarization of the target atom during the collision; the same effect as responsible for the Barkas effect in stopping power (see below). At energies below 50 keV, the difference seems to be reversed. This indicates that at these energies “binding” and “deflection” effects become important in the collision dynamics. Binding can contribute to the proton/antiproton difference in close collisions, where the charge seen by the atomic electrons can be the nuclear charge plus the projectile charge. This effect results in a larger ionization cross section for antiprotons than for protons because of less/more binding for the two particles. Bending can also cause a difference in ionization with protonslantiprotons. This effect originates from the different trajectories followed by a positive and a negative projectile in the Coulomb field of the target nucleus. Bending is the dominating effect when light projectiles such as positrons and electrons are considered [ll, whereas the binding effect is most easily investigated by comparing results for heavy positive and heavy negative projectiles such as protons and antiprotons. As can be seen from Fig. 2, binding seems to be important for energies below 50 keV, but here is very little experimental information. Recent calculations [7,81 predict that

ANDERSEN et. al. (9Oa) SHAH et. al. (85)(89a)

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Figure 2. The cross section for ionization of (a) helium, and (b) H, (non-dissociative) by impact of antiprotons and protons.

at very low projectile energies the binding effects become dramatic. The calculations show that the electronic states for the antiproton-hydrogen and for the antiproton-helium systems evolve into the continuum at certain internuclear dis-

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tances. In other words, close to the “united atom limit”, where the proton and an~pro~n (in the former case) are close together, the atomic electron feels Coulomb attraction from the transient nucleus. This results in a cross section ionization which does not go to zero as the projectile velocity decreases (as proton impact), but instead remains of atomic dimensions.

the no for for

In a large perspective, we believe that investigations involving such exotic transient “atoms” with very low nuclear charge, can lead to new and interesting physical insight. Such experiments, however, require better defined low energy Ebeams than the degraded ones. 3.2. Double ionization The very first results around double ionization in He gases from ij/p impact are still under theoretical consideration. The best agreement until today is obtained by Reading and Ford [91, as shown in Fig. 3a. The calculated curve for proton impact agrees very well with the experimental results but for antiprotons the curve is below experimental points. In Fig. 3b is shown calculations by the same

b)

ENERGY (MeV/amu)

b in units of a,/2

Figure 3. a> Calculated (Ref. 9) and measured (Ref. 4) double and single cross sections for $p incident on He-gas. b) Single and double ionization probabilities P(b) for F/p as a function of impact parameters b (Ref. 10). authors [lOI of single and double cross parameters. Clearly, the large difference lisions. Experimentally, these phenomena measurements in which it will be possible lisions.

sections for $p as between c/p impact will be investigated to discern between

a function of impact belongs to closer colthrough differential close and distant col-

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4. ENERGY

LOSS - THE BARKAS

EFFECT

The Barkas effect is the difference in energy loss (range) of particles and antiparticles. For positive projectiles, target electrons are attracted but repelled from negative particles. This result in a reduced ionization energy loss for antiprotons, f 125 ,1 > 2 100 & 6

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Figure 4. Measured antiproton stopping powers of a) Si and b) gold foils compared to proton values. as compared to protons. Further on, negative projectiles have a longer range than the positive ones. In the plane-wave Born approximation the energy loss of charged particles is proportional to the square of the projectile charge (Z,e). This

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approximation

cannot account for differences in energy loss for particlefantipar-

title systems. The next term in the Born series is proportional

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the required characteristic for describing charge-dependent phenomena. Firstly, the Barkas effect was measured by PS-194 in thin Si-detectors [11,12]. Latest, the effect was measured in gold foils [13]. Both results are shown in Fig. 4. Here it is seen that the antiproton measurements merge with the proton stopping powers at high energy, where it is know that the Barkas effect is small [141. At lower energies, however, the antiproton stopping power is more than a factor of two lower (Au) than for protons. Furthermore, the stopping power is constant within the experimental uncertainty and no characteristic maximum is observed for antiprotons as for protons. The measurements described above will be extended to other targets, to study the 2, dependence of the Barkas-term. In particular, foils of aluminium, silicon, copper, silver, and gold will be used. Silicon is included to complement the previous indirect measurements with semiconductor detectors. Also measurements at lower energies seem possible, simply by using thinner foils and degradation to lower energies. Finally, the method allows the energy-loss straggling to be determined at low energy, where the energy resolution is good. The relative contribution from the close collisions to the straggling is larger than to the stopping power itself, and hence straggling results together with stopping results will probe the contributions from the close and distant collisions. Also measurements of stopping power for p at very low energies, where nuclear stopping sets in, are of great interest.

5.

K-SHELL

IONIZATION

The investigation of inner-shell vacancy production by excitation and ionization during ion-atom collisions is a powerful tool for the study of electronic states of atoms. It also provides important insight into collision processes involving a manybody problem, in a case where the basic force is known. Experimental investigations of inner-shell excitation are plentiful with protons and positive heavy ions. Accurate data are rather well described by different models over a wide range of energies and target materials 1151. However, all of these models contain adjustable parameters, some of which should allow to describe e.g. the binding effect (the inner-shell electrons are more tightly bound by the presence of an additional positive charge near the nucleus), the polarization of the wave function of inner-shell electrons due to the projectile, Coulomb deflection of the projectile trajectory, etc. Experimental studies of 6 K-shell excitation were first suggested by Brandt and Basbas [16]. Later, also Mehler et al. [17] have shown that large enhancements of c induced cross sections may be expected, when compared to proton results. Also calculations on this matter using the semiclassical approximation @CA) method have been presented [18]. The first PS-194-results on K-shell excita-

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tions by p beams were presented in Ref. 3. Unfortunately, the background from 5 annihilation in degraders ruin x-ray detection at lower 6 energies, where large E/p differences are expected. If a direct p beam (without Be window and degraders) with energies down to -0.5 MeV were available, these very interesting investigations could be carried through. A direct beam would reduce considerably the background in the x-ray detectors.

6.

CHANNELING

WITH MeV ANTIPROTONS

For many years, channeling effects have been investigated both theoretically and experimentally for positive particles in the MeV region. Today, the understanding is so profound that it is possible to describe nearly all aspects of channeling in good agreement with experimental results. Consequently, channeling is now used as a tool in many applications in the energy region from keV to multihundred GeV. Around the CERN SPS several crystals are today installed for: 1) beam splitting, 2) beam extraction, 3) strong field effects, etc. (for a review see Ref. 19).

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Figure 5. Angular dependence of emission yields of es (150-250 keV) and e+ (200300 keV from CUDSembedded in a Cu crystal (Ref. 20). Investigations of channeling for negative particles are much more scarce. MeV electrons were used for such channeling experiments which gave a qualitative picture of the effect. Many experimental problems arise using electron beams, because of the strong multiple scattering which requires very thin crystals. The influence of channeling of the energy loss and X-ray excitation were not measured due to experimental difficulties. Moreover, the small electron mass leads to the well known diffraction effects associated with coherent scattering from many lattice atoms. In fact, these problems were one of the motivations for GeV-channeling

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at CERN. For GeV negative particles, strong channeling effects have been found for many processes but the influence of channeling on close encounter processes for MeV heavy negative particles have not been investigated. Measurements with antiproton beams of energies in the MeV range will offer a very important extension of the knowledge and include for the first time nonrelativistic, heavy, strongly interacting negative projectiles. The observation of antiproton and proton channeling can serve to elucidate the discriminatory manner in which the crystal interacts with negative and positive particles, as shown in Fig. 5. The investigations can be performed in a collimated MeV p beam by measuring the variations of close encounters (annihilation, K- X-rays) yields around crystal directions. Calculations predict enhancements of such processes by a factor of -6 [191.

7.

CONCLUSION

As shown by the results from PS-194, beams of keV-MeV p/p are unique tools for investigations of atomic collisions in gases and solids. The many new results and surprises have led to new insight and have inspired a revival of theoretical developments in penetration phenomena. These studies now require more detailed investigations ~differential). New calculations in the fields of channeling and inner-shell excitation are awaiting experimental investigations [213. Of special interest are low energy 5; beams, where large differences between p and 5 collisions are expected.

8. AC~OWLE~GE~E~S The experiments of the Aarhus-CERN-PSI collaboration described in this paper have been carried out over the last years with L.H. Andersen, K. Elsener, P. Hvelplund, H. Knudsen, S.P. Moller, R. Medenwaldt, J.O.P. Pedersen, S. TangPetersen, E. Morenzoni, and T. Worm. The author is grateful for the collaboration and the stimulating discussions with his colleagues. Also the CERN staff around LEAR are gratefully acknowledged for their effort to create the best experimental conditions.

9.

REFERENCES

1 H. Knudsen and J.F. Reading, Phys.Report 212 (1992) 107. 2 K. Elsener, Atomic Physics with Low Energy Antiprotons. Proc. from LEAP 90, Stockholm (Singapore: World Scientific 1991) p. 361-372. 3 E. Morenzoni, Collisions ofAntiparticles with Atoms. Proc. from 4th Workshop on High-Energy Ion-Atom Collisions, September 1990, Debrecen, Hungary (Berlin, Sponger-Verlag 1991) p. 173-190.

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4 L.H. Andersen,

P. Hvelplund, H. Knudsen, S.P. Msller, A.H. Sorensen, K. Elsener, K-G. Rensfeldt, and E. Uggerhej, Phys.Rev. A 36 (1987) 3612. 5 L.H. Andersen, P. Hvelplund, H. Knudsen, S.P. Meller, J.O.P. Pedersen, S. Tang-Petersen, E. Uggerhej, K. Elsener, and E. Morenzoni, Phys.Rev. A 41

(1990)6536. 6 L.H. Andersen, P. Hvelplund, H. Knudsen, S.P. Meller, J.O.P. Pedersen, S.

7 8 9 10 11

12 13 14 15 16 17 18 19 20 21

Tang-Petersen, E. Uggerhsj, K. Elsener, and E. Morenzoni, J.Phys. B 23 (1990) L39.5. A. Muller-Groeling and G. Soff, Z.Phys. D 9 (1988) 223. M. Kimura and M. Inokuti, Phys.Rev. A 38 (1988) 3801. J.F. Reading and A.L. Ford, J.Phys. B 20 (1987) 374?. A.L. Ford and J.F. Reading, J.Phys. B 23 (1990) 2567. L.H. Andersen, P. Hvelplund, H. Knudsen, S.P. Moller, J.O.P. Pedersen, S. Tang-Petersen, E. Uggerhsj, K. Elsener, and E. Morenzoni, Phys.Rev.Lett. 62 (1989) 1731. R. Medenwaldt, S.P. Meller, E. Uggerhej, T. Worm, P. Hvelplund, H. Knudsen, and E. Morenzoni, Nucl.Instr. & Meth. B 58 (1991) 1. R. Medenwaldt, S.P. Meller, E. Uggerhej, T. Worm, P. Hvelplund, H. Knudsen, and E. Morenzoni, Phys.Lett. A 155 (1991) 155. H.H. Mikkelsen, H. Esbensen, and P. Sigmund, Nucl.Instr. & Meth. B 48 (19901 8. H. Paul and J. Muhr, Phys.Rep. 135 (1986)47. W. Brandt and G. Basbas, Phys.Rev. A 27 (1983) 578 and A 28 (1983) 3142. G. Mehler, B. Miiller, W. Greiner, and G. Soff, Phys.Rev. A 36 (1987) 1454. J.M. Hansteen, private communication. A.H. Serensen and E. Uggerhoj, Nucl.Sci.Appl., Vol. 3 (1989) 145. E. Uggerhej and J.U. Andersen, Can.J.Phys. 46 (1968) 543. L.L. Balashova, N.M. Kabachnik, V.I. Shulga, and Ch. Trikalinos, J.Phys. Condensed Matter 4 (1992) 4883 and references therein.