Nuclear Instruments and Methods in Physics Research BlO/ll (1985) 39-44

North-Holland, Amsterdam

39

A REVIEW OF RECENT HIGH ENERGY ION-ATOM COLLISION PHYSICS

Patrick RICHARD and Martin ST&KLI

James R. Macdonald Laboratory, Kansas State University, Manhattan, Kansas 66506, USA

Over the last two years there have been several experiments with high velocity ion beams directed towards the precision measurement of K X-ray transition energies in one- and two-electron ions. One of the purposes of these measurements has been to determine with some precision the 1s lamb shift in one-electron ions. To date measurements have been performed in the Z range from

17 up to 36. All of the experiments have used the same technique of measuring the total absolute energy of the 2p to Is transition of the one-electron ion. Over this range of Z, the Zpr,s-ls,,, transition energy varies from - 2958.5 eV to - 13429 eV, and the

predicted 1s Lamb shift varies from 0.9383(6) eV to 11.858(11) eV. The main parameter that has been varied in these experiments has

been the means of excitation used to produce the one-electron ion. These methods so far have included beam-foil excitation of high velocity beams, gas phase excitation of an acce-decel beam, direct ionization of a gas target atom (recoil ion) by a highly charged

high-velocity beam and excitation in a hot tokamak plasma. The results of these measurements will be discussed and compared with QED calculations. The limitations of the experiments and possible future improvements will be discussed. The potential of producing

a beam of low energy, bare and one-electron high Z recoil ions with high velocity heavy-ion beams will also be discussed.

1. Introduction

We have been asked to review some of the interest- ing physics that has been addressed in the area of high energy atomic collisions over the last two years since the last accelerator conference here in Denton. Fortunately, due to the excellent work in organizing this conference, most of these interesting subjects are being reported in this and other sessions. We will therefore summarize briefly some of the developments which we find most interesting, and discuss them as time and space permit. There are a total of eight sessions at this meeting dealing with atomic collisions and spectroscopy and a total of fifty papers, which testifies to the vitality of the study of atomic physics with accelerators.

The term high-energy ion-atom collisions is suffi- ciently ambiguous so as to leave much latitude in sub- ject matter. By subject. matter, however, collisions in which the velocity of the projectile equals or exceeds the orbital velocity of the electrons to be captured or ex- cited in the collision are most clearly classified as high- energy collisions. Following somewhat this guideline, but also noting the use particularly of very heavy ions in the most recent years, by using some of the new very high energy accelerators, we will discuss both the result- ing interesting collision studies and spectroscopy. Datz [l] gave a very good review of this field at the last meeting, and. for some of the subjects he mentioned as new endeavors, we are happy to report that there has been substantial progress.

If there were time and space, we would like to summarize and discuss briefly five areas of high velocity

0168-583X/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

collision studies which we find particularly interesting: 1) dielectronic recombination with high velocity ions; 2) resonant-transfer and excitation with He targets; 3) 1s Lamb shift in high Z one-electron ions; 4) production of slow bare ions,by the recoil method; 5) atomic processes with relativistic heavy ions.

2. Dieledronic recombination with high velocity ions

One of the most interesting developments using high velocity ions has been the study of dielectronic recombi- nation by merged ion-electron beam methods as we have learned from the previous paper by Dittner [2]. This ion-electron process of dielectronic recombination is a traditional low-energy resonance phenomena in which an electron recombines with an ion to form a doubly excited state which can be stabilized by the emission of a photon. This process has been calculated theoretically for several decades, but only recently has been observed by low energy electron-ion collisions [3]. There seem to be large differences between experiment and theory in these low energy collision systems, but the experiments are very difficult and not many data points are available. High-energy collisions have come to the aid of this field recently in the following manner. A high velocity ion beam from an accelerator can be merged with a correctly collimated monoenergetic electron beam with nearly matched velocity. By varying the ion beam energy the Oak Ridge group [4] has shown that the DR resonance signal can be observed above the background in the yield of single-charge captured beam vs. beam

I. ATOMIC PHYSICS/RELATED PHENOMENA

40 P. Richard M. Stiickli / A review of recent collision physics

energy. In addition this technique, now under control, does generate a reasonable bulk of data to compare with theory. This is of course significant since it is sometimes misleading to compare experiment and theory for a single case without the aid of systematics. An additional point to be made here is that we feel that there are many other very interesting similar experiments that will be pursued in the future using this technique. For example, DR to discrete-bound states of heavier sys- tems (Zelectron doubly excited states of high Z ions) can be observed with the availability of sufficiently high energy beams. I presume that such experiments will be pursued at the Holifield Laboratory at Oak Ridge. We at Kansas State University plan to use an on-line Crye- bis with a tandem-linac accelerator to study e-ion colli- sions [5].

3. Resonant-transfer and excitation with He targets

In our opinion the most exciting recent development in the field of ion-atom collisions has been the observa- tion of a large resonance signal in the energy depen- dence of coincidences between K X-ray and electron capture of projectiles for three-electron ions on He gas targets. This effort has been led by Tanis et al. [6]. RTE is the formation of doubly excited states ala dielectromc recombination mode using ion-atom collisions. The quasi-free target electrons play the role that the free electrons play in DR. RTE is thus observed as a reso- nance phenomena in the X-ray yield from the doubly excited state versus the projectile energy. The Compton profile of the initial bound electron broadens the reso- nance as observed in the laboratory rest frame. Tanis et al. [6] first observed a somewhat weak resonance in Sq++ Ar collisions by an X-ray coincidence with the net one-electron charge capture beam. Very recently RTE was observed as a very large effect in asymmetric colli- sions of S + He (71, Ca + He [8], V + He [8], S + He (81 and Si + He [9]. For symmetric systems and for light Z systems (e.g., F@+ He) [lo] the non-resonant two-step process has been shown to dominate.

Properties of resonant-transfer excitation: 1) Ratio of resonant to non-resonant cross section in-

creases as the target Z decreases. 2) The ratio of resonance to non-resonance cross sec-

tion increases with projectile Z. 3) Resonant cross section can be calculated within the

impulse approximation for all but the tightly bound electrons of the target.

4) Non-resonant cross section can be calculated most accurately for low Z targets and not at all reliably for symmetric collision systems.

5) The amplitudes for the two processes should be considered coherently.

6) Questions still remain concerning the impact param-

Energy (MeVV)

Fig. 1. X-ray production yield for K, and KS X-rays in coincidence with an electron capture (St3+ + He + S”+ + . . ) vs the energy of the S”+ projectile (from ref. [7]).

eter dependence of RTE vs. NTE and on the selec- tion rules.

The S + He data from Heidelberg is shown in fig. 1 [7]. Fig. 2 shows the calculated projectile Z-dependence of the resonant cross sections for He targets [8]. Pepmiller et al. [lo] pointed out that two electron systems could be investigated in high resolution X-ray studies by look- ing directly at the decay of the doubly excited states. In this conference Itoh [ll] will report on an extention of this technique to the Auger decay channel by scanning over the resonance energy in He+-He collisions and observing the 0’ electrons in high resolution. These latter two techniques offer promise for future experi- ments with asymmetric high Z collision partners. Still

Littim-like Projectiles

- 2cQo-

j

Tr- u-l

a tooo-

3

500-

,

0 200 400 600 600

EfMeW

Fig. 2. RTE cross sections vs projectile energy for various three-electron ions incident on helium (from ref. [8]).

P. Richor M. Stiickli / A review of recent collision physics

another version of this experiment is being tested at Heidelberg [12]. This method involves the measurement of the coincident K X-rays from the decay of the doubly excited states of a two-electron high 2 ion. No results are out on this additional method which uses two high efficiency Si(Li) X-ray detectors. Many questions con- cerning RTE still remain, however it is now firmly established as a process very closely related to if not exactly analogous to DR.

4. ‘Ihe 1s Lamb shift in high 2 ions

The 1s Lamb shift is a very well defined quantity for one-electron ions, which represents the ideal two-body problem. It is the difference between the real energy of the 1s level and the exactly calculated Dirac energy of the 1s level. The deviations in these two quantities is due to all corrections to the Dirac energy. These are the reduced mass correction, finite size of the nucleus, higher order and relativistic effects, and most important, quantum electrodynamic effects (QED). The reduced mass correction is often included in the Dirac energy rather than in the 1s Lamb shift. The calculation of these various corrections has reached a high level of sophistication [13,14], and it remains as a challenge to the experimentalist to achieve a similar level of sophisti- cation. Fundamental knowledge of atomic physics can be obtained from the measurement of the 1s Lamb shift. Since all the corrections included in the Lamb shift have a strong Z dependence, the Lamb shift becomes a larger fraction of the binding energy with increasing Z as shown in fig. 3.

The 1s Lamb shift has been studied rather vigorously for high Z ions using high resolution X-ray spec- troscopy of the 2p-1s Ly a transition coupled with a transfer calibration. At the time of the last Denton meeting, there were preliminary results on oneelectron Fe and Ar by Briand et al. [15] and on Cl by Richard et al. [16]. These measurements were at the 15%, 44% and 11% level for Fe, Ar and Cl, respectively. These three measurements used a high velocity beam to excite the 2p level of the projectile in a target foil. There are three limitations in accuracy in these initial experiments: the transfer calibration, spectator electrons and the Doppler correction due to the emission from a moving frame. Since the time of the last meeting, there have been three additional different techniques used to attempt to im- prove on the results. The techniques are (1) hot toka- mak-plasma excitation of the one-electron ions [17], (2) recoil-ion excitation [18], and (3) accel-deeel of bare ions coupled with one-electron capture in a He gas [19].

The first of these three methods as performed at MIT [17] mainly provides an independent excitation method and a much reduced Doppler effect and satellite effect, but encountered other problems which limited

I 3 IO 30 100 NUCLEAR CHARGE Z

Fig. 3. Different energies and corrections contributing to the 1s electron binding energy of a one electron ion vs nuclear charge as calculated by Johnson and Soff [13]. Absolute values are

plotted but negative values are indicated by dashed lines. The

dash-dotted line shows the Lamb shift which is a sum of many contributions and corrections as indicated in the figure (from

ref. [20].

this method to the 11% level. The recoil-ion method as performed at GSI [18] makes use of the ionizing power of a high velocity Uq+ (q - 66) ion on an Ar atom and has the large improvement of reducing substantially the Doppler correction (in fact none had to be applied in estimating the errors). The limiting feature of this ex- periment is however the large observed but unresolved satellite tails on the Ly a X-rays. Even with this limita- tion the measurement with Ar recoil ions has led to a determination of the Lamb shift at the 1.5% level of accuracy. The third method is the accel-decel of bare ions performed at Heidelberg [19]. The Ly a X-rays of Cl are studied in this effort by the one-electron capture of bare chlorine in a He gas. This method offers an improvement in terms of the reduced Doppler effect and the near elimination of spectator electrons. The analysis of this latter experiment is incomplete and therefore we anxiously await the results.

We have proposed an improved version of our Cl experiment at Brookhaven [20]. By using the accel-de- ccl method discussed above, excitation in various gases

and solids, and improved measurement of the emission angle, we estimate that an accuracy at the 0.5% level should be attained. Fig. 4 shows the geometry we plan to use.

There is one more effort we wish to discuss, which is

I. ATOMIC PHYSICS/RELATED PHENOMENA

42 P. Richard, M. StaCkli / A review of recent collision physics

Fig. 4. Schematic of the experimental setup for the proposed

Brookhaven experiment. The goal is to determine the 1s Lamb

shift to an accuracy of 0.58, which is more accurate than any

other Lamb shift measurements in hydrogen-like heavy ions

(from ref. [20]).

the attempt to measure the Lamb shift in even higher 2

one-electron ions. Briand et al. [21] have performed a measurement for Kr ions at GSI. The one-electron and two-electron spectra are shown in fig. 5. Due to the low

Fig. 5. The hydrogen-like and helium-like Kr spectrum ob-

tained with an 18.5 MeV/amu Kr beam passing through a C-foil (from ref. [21]).

13420 13450

Table 1

Measurements of the 1s Lamb shift in H-lie heavy ions. The reported Ly (I values were compared to the theoretical values from ref.

[13] using a conversion constant of 1239.8520 eV.nm. The measured relative deviation refers to the difference between the

experimental deduced Lamb shift value and the theoretical Lamb shift value divided by the theoretical Lamb shift value. The

experimental relative accuracy is the ratio between the experimental uncertainty and the theoretical Lamb shift value.

Year Author(s) Element Excitation method Reported Lamb shift Measured Exp.

Lya values (theory) (cm-‘)

relative relative

deviation accuracy

(W) (56)

82 Schleinkofer et al. (231 S Beam foil 2619.35(36) eV

2622.59(27) eV

3.7365(g) A

3.7310(g) A

6951.9(7) eV

6973.8(6) eV

3318.1(5) eV

3323.2(5) eV

2.4966(3) A

2.4911(3) A

2958.62(10) eV

2962.46(10) eV

2958.59(10) eV 2962.47(12) eV

373.6522(19) pm

373.1105(19) pm

6146 +49 f47 +17 i35

-1 f62 -8 f62 +3 *I8

-14 *15 +9 *44

-16 It44 -2 *26 -9 f26 -8 *11 -9 *11 -4 ill

-10 f13 +0.4 il.5

+ 0.3 f1.5

82 Beyer et al. [24] Ar Recoils 9206

83 Briand et al. [15]

83 Briand et al. [15]

Fe Beam foil 32040

Ar Beam foil 9206

83 Dohmamt et al. [25] Ti Beam foil 18230

84

84

Ullne et al. [17]

Richard et al. [la]

Beyer et al. [18]

Cl

Cl

Tokamak plasma

Beam foil

Ar Recoils

Briand et al. [22] Rr Beam foil

Deslattes et al. [19] Cl e-capture from He

Marmar et al. [26] Ar Tokamak plasma

7568

7568

9206

95640 7568

9206

P. Richard hf. Stiickli / A review of recent collision physics 43

velocity (18.6 MeV/amu) of the Kr ions, very little Ly (Y intensity was obtained and therefore the slits of the spectrometer were opened up which eliminated the pos- sibility of making a precision measurement. In a second attempt Briand et al. [22] studied the Ly a radiation from Kr using a 36 MeV/amu Kr beam from the Ganil accelerators. The analysis of this data is still in progress, but the expected accuracy is of the order of 1 eV out of - 11.8 eV which is at the 8% level.

Table 1 summarizes the results to date on the 1s Lamb shift [15-19,22-261.

5. Production of slow bare ions by the recoil method

We have seen in the previous section that one-elec- tron Ar ions can be produced by high velocity Uq+ beams. In a recent study by Kelbch et al. [27] the cross sections for producing Arq+ recoils were measured using I_Jq+ beams in the energy range of 3.7 MeV/amu to 15.5 MeV/amu. The recoil-ion production cross sections were made by a coincidence between recoil-ion and the charge-changed beam. The recoil time-of-flight was used to determine the final Ar charge state and the U charge state was used to select one- and few-electron transfer to the projectile and direct Coulomb ionization with no electron transfer. Fig. 6 shows the time-of-flight spec- trum for Ar and the production of the very highest charge states Ar17+ and Ar’*+. Fig. 7 shows the mea- sured cross sections where we see that Ar”+ and Ar’*+ are produced with cross sections > 2 X 10” cm2.

In a subsequent study by Ulhich et al. [28], the feasibility of establishing a parasite ion source for bare heavy-ion production on a high-energy heavy ion accel- erator is discussed on the basis of the previous results. Fig. 8 shows the predictions for the secondary recoil ion RwrICLEs/sac IOK'

P 2

IOpj ,o & %

5 lo 15 I8 f- TIME OF FLIGHT cl

Fig. 6. Time of flight analysis of the charge state of Ar recoil ions produced with a highly charged high energy uranium beam (from ref. [27]).

Fig. 8. Particle intensities of Arq+ recoil ions vs their charge state q for a proposed parasite recoil ion source to be installed in a high energy heavy ion storage ring (from ref. 1281).

1 ’ 1155 M&‘/u) d5+ on Ar

d4 -

-15 10 -

“E Y

z? E IrP -

In

is

8

1617 -

IL? -

x

x x direct ionization . : total electron captwe

x

x x

x x

IllI 11 ‘I I’ ’ ““““A 5 10 l!i

RECOIL ION CHARGE STATE

Fig. 7. Cross sections for producmg Ar4+ recoil ions vs q for 15.5 MeV/amu U”+ incident on Ar (from ref. [27j).

I. ATOMIC PHYSICS/RELATED PHENOMENA

44 P. Richard hi. Stijckli / A review of recent collision physics

beam using these measured cross sections and assuming at 50 particle nA (- 3 X 10” part/s) primary uranium beam from the GSI Unitar, a gas cell of 1 cm length, and an operating pressure of 10m3 Torr. One obtains for this situation - lo* H-like Ar ions per second.

Plans are presently in progress to test the feasilibility of a parasite beam using much higher velocity ion beams with higher charge states at the Bevalac in Berke- ley. Such measurements at this time would allow us to estimate the feasibility of such a parasite ion source on the upgraded SIS-18 at GSI and the accompanying storage ring.

This work was supported by the US Department of Energy, Division of Chemical Sciences.

References

[l] S. Datx, IEEE Trans. Nucl. Sci. NS-30 (1983) 881. [2] P. Dittner, this Conference (AARI ‘84), not published. [3] D.S. Belie, G.H. Dunn, T.J. Morgan, D.W. Mueller and C.

Timmer, Phys. Rev. Lett 50 (1983) 339. [4] P.F. Dittner, S. Datx, P.D. Miller, C.D. Moak, P.H. Stel-

son, C. Bottcher, W.B. Dress, G.D. Alton, N. Neskovic and CM. Fou, Phys. Rev. Lett. 51 (1983) 31.

[5] M. Stiickli, K. Cames, C.L. Cocke, B. Cumutte, J.S. Eck, T.J. Gray, J.C. Leg& P. Richard and J. Arianer, these Proceedings (AARI ‘84) Nucl. Instr. and Meth. BlO/ll (1985) 763.

[6] J.A. Tanis, EM. Bernstein, W.G. Graham, M. Clark, S.M. Shafroth, B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. Lett. 49 (1982) 1325.

(71 E. Justiniano, R. Schuch, R. Hoffmann, J. Konrad, M. Schulz and F. Ziegler, 5. Arbeitsbericht, Arbeitsgruppe Energiereiche Atomare Stosse, HMI Berlin, eds., B. Fricke et al (1984) p. 55.

[8] J.A. Tanis, these Proceedings (AARI ‘84) Nucl. Instr. and Meth. BlO/ll (1985) 128; C.S. Oglesby, E.M. Bernstein and J.A. Tanis, BuII. Am. Phys. Sot. 29 (1984) 743.

(91 M. Clark, these Proceedings (AARI ‘84) Nucl. Instr. and Meth. (1985) 124.

WI

WI

WI

[131 I141 I151

WI

1171

WI

1191 PO1

VI

WI

1231

~241

1251

WI

P.L. PepmiBer, P. Richard, J. Newcomb, R. Dillingham, J.M. Hail, T.J. Gray and M. Stockli, IEEE Trans. Nucl. Sci. NS-30 (1983) 1002. A. Itoh, these Proceedings (AARI ‘84) Nucl. Instr. and Meth. BlO/ll (1985) 97. E. Justiniano, R. Schuch, P. Richard, H. Schmidt-B&king and S. Kelbch, private communication. W.R. Johnson and G. Soff, to be published. P.J. Mohr, At. Data Nucl. Data Tables 29 (1983) 453. J.P. Briand, M. Tavemier, P. Indelicate, R. Marrus, and H. Gould, Phys. Rev. Lett. 50 (1983) 832; J.P. Briand, J.P. Masse, P. Indelicate, P. Chevallier, D. Girard-Vemhet, A. Chetioui, M.T. Ramos and J.P. Desclaux, Phys. Rev. A 28 (1983) 1413. P. Richard, M. Stock& R.D. Deslattes, P. Cowan, R.E. LaViBa, B. Johnson, K. Jones, M. Meron and R. Mann, Phys. Rev. A 29 (1984) 2939. E. Kane, J. mine, P. Richard and M. StaCkIi, J. Phys. B 17 (1984) L115. H.F. Beyer, R.D. Deslattes, F. FoIkmann and R.E. LaViIIa, J. Phys. B, to be published. R.D. Deslattes and R. Schuch, private communication. M. St&&Ii and P. Richard, Proc. Int. Conf. on Physics of Highly Ionized Atoms, Oxford (1984) to be published. J.P. Briand, P. IndeIicato, M. Tavemier, 0. Gorceix, D. Liesen, H.F. Beyer, B. Liu, A. Warcrak and J.P. Desclaux, 2. Physik A: Atoms and Nuclei 318 (1984) 1. J.P. Briand, P. IndeIicato, M. Tavemier, P. Richard and D. Liesen, unpublished. L. SchIeinkofer, F. Bell, H.D. Betz, G. Trollmann and J. Rothermel, Physica Scripta 25 (1982) 917. H.F. Beyer, R. Mann, F. Folkmam and P.H. Mokler, J. Phys. B15 (1982) 3853. H.D. Dohmann, D. Liesen and E. Pfeq, GSI Scientific Report 1982, GSI 83-1, Darmstadt (1983) p. 155. E.S. Marmar, J.E. Rice, E. KaIlne, J. K&Une, R.E. LaVilla and M. Stockh, Bull. Am. Phys. Sot. 29 (1984) 1319.

[27] S. Kelbch, J. UIIrich, R. Mann, P. Richard and H. Schmidt-B&king, to be published in J. Phys. B.

[28] J. UIlrich, C.L. Cocke, S. Kelbch, R. Mann, P. Richard and H. Schmidt-Backing, J. Phys. B 17 (1984) L785.