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Recoil-ion momentum spectroscopy
Nuclear Instruments and Methods in Physics Research 361 ff991) 415-422
415
~~rt~-~~l~an~
Recoil-ion momentum spectroscopy
U, Buck APIftir Striimungsforschung,
*
Gijttingen, Germany
Received 19 February 1991
Within recent years the spectroscopy of the target recoil momen~m in distant ion-atom collisions has been established as a third, independent and vafuabIe method of target spectroscopy besides the classica measurement of ejected electrons and X-rays. For a iarge set of cotfision systems, e.g. for fast and heavy projectiles impinging on gaseous targets, the determination of the recoil-ion transverse momentum provides the only possibility to obtain differential excitation or ionisatian cross sections. Moreover, since the requirements on the ion beam (emittance, focussingf are comparably low even for high precision measurements, experiments using exotic beams like antiprotons can be envisaged. The present stage of development of recoil-ion momentum spectroscopy (RIMS) is illustrated, its special ability to obtain information on many-body interactions in an ion-atom collision is demonstrated and the future potential of the method as to the momentum resolution, the solid angle and possible applications in many-particle coincidence measurements is outlined.
1. Zntroduetion The i~~esti~~t~o~ of atomic reactions like ionisation or excitation in energetic ion-atom collisions turned out to be of particutar value for comparison with theoretical approaches if differential cross sections, in dependence on the projectile laboratory scattering angle G, were determined by the experimentalists. Pioneering experiments by Everhart and Kessel 111 supporting the development of the molecular orbital picture in slow collisions [2] were followed by numerous angular differential measurements of X-ray emission yielding impact parameter dependent inner-shell ionisation probabilities [3]. The determination of the projectile trajectory allowed the observation of interference patterns in quasimolecular X-ray spectra and a
* Supported by GSI, Darmstadt, ~~ndesminjster~um t%r Forschung und Technologie (BMPT:), and Deutsche Forschungsgemeinschaft (DFG), Germany. First presented at 11th Int. Conference on the Applications of Accelerators in Research and Industry, Nov. 5-8, 1990, Denton, TX, USA. 0168483X/91/$03.50
precise spectroscopy of the quasimolecular orbitals formed during the colhsion [4]. Very recently, few angular differential data have also been reported for electron ejection and recoil-ion production [5,6]. However, since projectile scattering angle measurements are limited to a regime of 6) 2 5 x 10-’ xad (0.5 mm deflection on a distance of 10 ml, a large set of collision systems or excitation processes are beyond the scope of this technique: For heavy projectiles at C&V/u energies, delivered for instance by the new accelerator complex SIS/ESR at GSI, Darmstadt, typical 9 are below measurable values for nearly all collision-induced atomic reactions. Even at intermediate velocities of several MeV/u, for heavy ion impact the major part of single and also multiple target ionisation, i.e. more than 90% of the total electron ejection cross section and a main contribution to the stopping power is produced at large impact parameters leading to pxojectile deflections as small as microradians. This is illustrated in fig. 1 where the projectile energy is plotted versus the projectile scattering angle regime for all combinations of collision systems (U on We until Ne on Xe, available at SIS/ESR) at a representative recoil-ion transverse energy of 100 meV (shaded area). First
0 1991 - Eisevier Science Publishers B.V. All rights reserved
416
J. Ullrick / Recoil-ion momentum spectroscopy
measurements at 1.4 and 5.9 MeV/u (full dots in fig. 1) as well as nCTMC calculations indicate that more than 95% of the ionisation events are expected at even smaller scattering angles than indicated by the shaded area. The lower limit for a direct 6 measurement of 9 2 5 X lo-’ given in the figure presumes a transverse momentum spread of the ion-beam bp/p _< IO-’ usually obtained by collimating the beam. In most practical cases this limit is more on the order of 2 X 10m4 rad. Moreover, in storage rings for ions like CRYRING, TSR, or ESR the measurement of projectile scattering angles cannot be envisaged without disturbing the beam in such a way that further storage and cooling is impossible. In order to enable differential measurements in this regime which has not been accessible before, we have developed a novel technique, measuring the recoil-ion momentum transverse to the beam direction PRI instead of that of the projectile P, i . The investigation of single and multiple ionisation of He, Ne, and Ar targets in collisions with MeV/u, highly charged uranium (4 = 32, 75) or even protons illustrated that the momenta transferred to the recoil-ion (the projectile) are in the order of 3 a.u. or below for a major part of the ionisation events. For the recoiling target ion this means that one has to deal with transverse energies E s I in the subthermal regime E, _Ls 20 meV for a target at room temperature. Therefore, the first generation of recoil-ion momentum spectrometers which worked with the target gas at a temperature of 300 K (a detailed description can be found in refs. [7,8]) was followed by a new development, where the target is at 30 K [9]. Fig. 2 illustrates the improvement of the
-?10 2 E 2 w 51
_____._
____ r___._ Temperature/K
Fig. 2. Centroid (dots) and full width half maximum (bars) of the measured recoil-ion energy distributions for a representative energy transfer of 15 meV at different target temperatures.
transverse recoil-ion energy determination (full dots) and of the energy-resolution (bar: FWHM) for a representative transverse energy transfer in the collision of 15 meV with decreasing target temperature. In this paper we will describe some conceptual details and the main features of the cooled spectrometer. In addition to its ability to provide differential cross sections for a large set of collision systems which are not achievable with other techniques. the target recoil-momentum spectroscopy will be shown to be a third, valuable and independent type of spectroscopy besides the classical X-ray and electron spectroscopy. Main emphasis will be given to the future scope and potential of this technique.
2. Present apparatus troscopy
total i~i~~n
5 to-5 2 10-B2 s to-7 2 5 1o*2 Projectile Scattering Angle 6/
5 to-4 2 rad
Fig. I. Projectile laboratory scattering angle versns projectile kinetic energy. The shaded area indicates the projectile scattering angle regime coincident to recoil-ion energies of 100 meV for various collision systems from Ne on Xe (right-hand sotid line) up to U on He Cleft-hand solid line) collisions. Full dots: investigated collision systems [8,17].
i
for transverse
momentum
spec-
The principle behind the recoil-ion momentum spectrometer is to measure the transverse momentum of the recoiling target-ion after the collision, instead of that of the projectile (for a detailed description see ref. [7]). As shown in fig. 3, recoil ions are produced in a static gas cclt (a cylinder of 10 mm diameter coaxial to the beam) and drift to the wall of this cylinder in a time interval At inversely proportional to the transverse velocity (momentum) which they gained within the collision. They leave the reaction zone through an aperture of 1 mm x 20 mm, positioned along the beam axis, are accelerated, charge state analyzed and detected. Compared to the first RIMS version, where the aperture was a hole of 1 mm diameter, the recoil-ion detection efficiency is enlarged by a factor of 20. The ion-optics has been adopted in such a way that the
J. Uliri~~ / Rec#i~-ion momentum spectroscopy
a.) along
the
beam
b) perpendicular
417
to the beam
*-----
Fig. 3. Recoil-ion momentum spectrometer with cold target. a) Cut along the beam axis; b) Cut perpendicular to the beam axis.
separation and simultaneous detection of 10 Ne charge states is possible. Due to the limited extension of the
gas target along the beam only recoil-ions scattered into a certain polar angular regime can be observed. The calculated angular acceptance, shown in fig. 4, shows a constant behaviour over a wide angular range between 20 O and 150 o with a sharp fall off to zero for the other angles. The recoil-ion time-of-flight (TOF) is obtained by a coincidence with the scattered projectile. The time resolution is below 10 ns and small compared to typical recoil-ion - TOF in the order of a few microseconds inside the cylinder. The target cylinder is drilled in a
3
S 0.6. ‘ij Z % 0.4.
polar emission angle Fig. 4. Calculated polar angular acceptance of the recoil-ion momentum spectrometer.
copper block and mounted on the top of the cold finger of a cryopump. The target gas is precooled on its way to the cylinder and reaches the target area through 8 channels (of 1 mm diameter) along the beam axis in order to ensure a constant target pressure along the beam axis. The temperature which can be as low as 30 K is measured by a Pt resistor on top of the copper block. In the present version, the copper block is gold-plated, the inner surface of the cylinder is completely coated by a copper grid (mesh width: 0.12 mm x 0.12 mm) to form a “perfect” Faraday cup and to minimise fringe fields at the recoil-ion aperture. As shown in fig. 5 the exact potential curves at the aperture were calculated showing that the full potential of 500 V drops to a value of 5 mV within 0.4 mm. The small variation in the recoil-ion drift path has to be considered for the exact determination of the recoil-ion momentum. Before each experiment, the spectrometer has to be carefully cleaned in an ultrasonic bath. Typicalfy, after about five hours of beamtime, the recoii-ion momentum spectra change slowly, probably due to a freezing out of rest gas components on the surface of the spectrometer. Warming up above the nitrogen freezing point and cooling down again yielded the original spectra. Different material combinations (grid, cylinder) and recoil-ion drift lengths (2.5 mm, 3.5 mm, 5 mm1 were used to check possible influences of fringe fields. Within statistical error bars identical results were ob-
J. Ullrich / Recoil-ion momentum spectroscopy
418
drift
0.8
region
0.6
0.4
0.2
Fig. 5. Electric
potential
curves at the recoil-ion
For the different experiments performed, we derive that further systematical errors should be below + 5 meV. The thermal motion of the target atoms at 30 K essentially shifts the measured energy distribution by +4 meV. For the comparison with theory the momentum distribution has to be unfolded with respect to the remaining thermal motion momentum distribution. tained.
3. PRI_PPI
0.2
0.0 beam
direction
0.L
0.8
0.6
Z/mm
aperture
which is covered
by a copper
grid
(straight line): At scattering angles around 0.7 mrad the coincident recoil-ion energy starts to fall below the two-body value with a maximum deviation at 6 = 0.5 mrad and a saturation in energy is observed at a value of 8 f 5 meV (thermal motion unfolded). The overall feature of the experimental data is in good agreement with theoretical results of classical many-body calculations (nCTMC) [lo]. Deviations from the two-body behaviour were found theoretically to be
-coincidences
To test the spectrometer, the transverse recoil-ion momentum PRI (or energy E, i) was measured in coincidence with the final projectile transverse momenturn Pp, (or scattering angle: -9 = P, I/P,, ; P,, is the incoming projectile momentum) for the 0.5 MeV p on He collision system at the 2 MV Van de Graaff accelerator of the Institut fir Kernphysik, Universitat Frankfurt. For single ionisation where only three free particles emerge after the encounter the transverse momentum balance is fully determined by the simultaneous determination of P, I and P, I For negligibly small momentum of the emitted electron P,, or isotropic electron emission the mean recoil-ion transverse momentum (P, I > equals Pp, which would enable a direct test of the spectrometer. The experimental results (open circles in fig. 6), however, displayed distinct deviations from this two-body behaviour
3 E \ > too P e
0.5
MeV p -> He
W a u
10
u
5 f
1 0.1
Projectile Scattering
1
Angle 6/
mad
Fig. 6. Projectile scattering angle 19 versus transverse recoil-ion energy E, I. Open circles: Experimental data corrected for the thermal motion influence at 30 K. Bars: Systematical experimental error. Straight line: Two-body kinematics. Full line: nCTMC results.
J. Vllrich / Recoil-ion momentum spectroscopy
due to three-body interactions between the heavy particles, the proton, and the He+ ion and the emitted electron for single ionisation. (For a detailed discussion see refs. [9,11].) Similar theoretical results have been obtained applying a semiclassical quantum statistical model [12]. The observed deviation between theory and experiment at scattering angles below 0.5 mrad are still unexplained: Sensitive tests of the spectrometer at the 1 meV level are extremely delicate but are in progress in order to substantially reduce possible systematical experimental errors. The experimental results, however, demonstrate that the simultaneous spectroscopy of the transverse recoilion momentum and charge state in dependence of the transverse momentum transfer to the projectile provides new insight into the dynamics of ionising collisions, which can hardly be achieved by other techniques.
4. Spectroscopy momentum
of the parallel and transverse recoil-ion
Theoretical calculations predict strong deviations from the expected 90 o polar angular scattering of the recoil ions c-9,) at large impact parameters due to the interaction with the ionised electron(s) [13]. Since the exact position along the beamline within the target cylinder of the present apparatus is not specified, recoil-ion momentum components along the beam direction Pa,, cannot be measured and the theoretical predictions cannot be proven. Therefore, we have developed a modified version of the apparatus described above, were the observable collision region is defined within the target cylinder by implementation of another aperture of 2 mm diameter. First results on recoil-ion polar emission angles in dependence of the recoil-ion energy with the cold target have been obtained but careful checks of the ion-optics (to observe these angles) as well as calculations of the solid angle for each 6, are presently in progress. Since the efficiency of this spectrometer is a factor of 20 smaller such experithan the one for the P, I measurement, ments are extremely time consuming.
5. Present scope of RIMS Experiments using the cooled target recoil spectrometer demonstrate that an accuracy in the determination of the recoil-ion energy of k5 meV down to absolute energies of 10 meV is achievable. Since this accuracy is independent of the projectile velocity a precision in the relative momentum measurement corresponding to projectile scattering angles between lo-’ rad and 10m5 rad, depending on the energy and the
419
collision system, can be obtained. This enables a differential investigation of atomic reactions in a regime which has not been accessible before. It is this regime of very low momentum transfers which mainly contributes to single and multiple ionisation, to kinematic, radiative as well as resonant electron capture, to Felectron ejection and to the stopping power. measureFurthermore, the E, i - 19 coincidence ments have shown that the projectile and recoil-ion scattering contains different information. The theoretically observed saturation value of about 1 meV for the proton-He collision system reflects the mean energy of the emitted electron, or correspondingly, of the emitted electrons for many electron targets. Thus, recoil-ion momentum spectroscopy seems to be especially well suited to investigate collective processes which might occur due to the many-body Coulomb interaction during a collision [14]. A striking advantage of RIMS is the fact that the high accuracy can be obtained even with beams of a large transverse momentum spread of Ap/p = lo-‘. This is due to the feature that only the exact position of the projectile inside the spectrometer is needed to get the recoil-momentum information. This position can be obtained by a two-dimensional position sensitive detection of the projectiles directly behind the collision region. Therefore, the differential investigation of single and multiple target ionisation by antimatter beams, available at LEAR (CERN), can be envisaged. Due to the low intensities and large momentum spreads of such degraded beams a further collimation which would be necessary for the -9-differential measurement is not possible. Thus, even in a regime of transverse momentum transfers (lop4 to 10-s) which can usually be covered by the conventional technique, RIMS is able to give precise information if the beam quality rejects a 6 measurement. A further advantage of the method lies in the fact that all momentum transfers are measured simultaneously. However, the present stage of development still bears problems which should be solved in a third generation of RIMS. First, the energy resolution stated above can only be reached for a He target since all other gas targets form dimers or molecules if they are cooled down to the low temperatures necessary to reduce thermal motion influences. Since thermal motion is well known, it is possible to unfold the obtained spectra, however, structures in the spectra smaller than the thermal motion distribution (for example discrete lines in the recoil-momentum due to Auger decay of an excited target ion) cannot be resolved. Furthermore, due to the geometry of the present apparatus, certain recoil-ion polar emission angles cannot be detected (see fig. 4). This may influence the measured transverse recoil-ion momentum distributions since different atomic reactions might show com-
420
J. Ullrich / Recoil-ion momentum spectroscopy
pletely different angular emission patterns. This is not a severe constraint but has to be considered when comparing with theory. In principal RIMS is ideally suitable to perform coincidences with the emitted electrons or X-rays and, thus, providing detailed information on a specific ionisation or excitation process or, applying multi-coincidence techniques, on many-particle reactions occurring during the collision. However, the present setup strongly hinders such measurements, since first the solid angle for the recoil-ion detection is in the order of 10m3 only and, even more severe, since it is impossible to get a large solid angle for X-rays or electron detection, due to the geometric configuration of the spectrometer. Another feature, the ability of the apparatus to simultaneously detect all recoil-ion transverse momenta, i.e. all impact-parameters, which might be appealing at first glance turns out to be impeding if processes are to be investigated which mainly take place at smaller impact parameters or higher recoil-ion energies. The enormous number of recoil-ions created at large impact parameters produce random coincidences in the time-spectra and easily cover the true coincidences of interest. It would be very desirable if
only recoil ions in a certain adjustable momentum range would be detected. Due to the long recoil-ion drift path of 5 mm or at minimum 2.5 mm extended time windows are needed to obtain the full TOF-information, since typical drift times range between 1 us up to 20 us for Ne-ions in the meV regime. Compared to the conventional coincidence techniques (e-, X-ray, recoil-ion-projectilecoincidences) this is worse by a factor of 1000 requiring a 1000 times more efficient background suppression. Furthermore, since the shape of the TOF spectra reflect the momentum distribution the recoil-ions, the exponential decrease observed in the TOF-spectra for high particle rates should be below a factor of 2 over the time window, which limits the projectile yield to values below 3 x lo4 for a perfect dc ion-beam. Due to fluctuation in the ion-beam intensity on the microsecond scale realistic projectile yields very often were found to be limited below 104.
6. Third generation
RIMS
In order to overcome most of the difficulties stated above, a rigorously new concept of RIMS was devel-
PUMP
Fig. 7. New recoil-ion momentum spectrometer, presently machined. V,,ti: velocity transferred to the target during the collision. I$_,: Supersonic jet velocity. TAC: Time-to-amplitude converter. 4,: Projectile laboratory scattering angle. ‘pp: Azimuthal projectile scattering angle. PS PPAD: position sensitive parallel plate avalanche detector. PS MCP: position sensitive microchannel plate.
J. Ullrich
/ Recoil-ion
oped and presently is under construction. As shown in fig. 7 the gas target is no longer extended along the beam axis but is a supersonic jet of 1 mm diameter perpendicular to the ion beam. Recoil-momenta transferred to the jet-particles during a collision with a projectile will blow up the spatial extension of the jet perpendicular to its propagation, i.e. it will cause a change in the transverse velocity (u~,~ in the figure) as well as in the parallel velocity u, of the particles in the jet. A two-dimensional position-sensitive microchannel plate detector (active diameter 40 mm) monitors the spatial deflection of each recoil-ion perpendicular to the jet-direction (ux, L’~); a TOF measurement, achieved by a recoil-ion projectile (or electron) coincidence technique will provide the L’, information. In a second stage of development, a magnetic deflection of the recoil-ions will be implemented in order to simultaneously determine the recoil-ion charge state. The inherent temperature of such a gas jet is known to be below 1 K [15]. From this a recoil-ion energy resolution in the sub-meV regime can be expected. For a He-target this corresponds to an electron energy resolution of below one eV. Calculations, taking into account further sources of uncertainty in the recoil-ion energy determination like the jet divergence, the time resolution, or the definition of the exact interaction region show that an overall resolution corresponding to an electron energy resolution below 10 eV should be achievable. In this case discrete lines in the energy spectra due to Auger decays of metastable states produced in the collision may become observable. In contrast to the present setup even heavier target gases can be cooled and thus a similar energy resolution can be achieved [16]. The formation of clusters can be minimized under certain conditions of operation to a few percent, they can easily be further discriminated due to their large mass compared to the ions of interest. The new apparatus is well suited for any kind of (multiple) coincidence measurements: The recoil-ion solid angle of about 50% is extremely large due to the superimposed jet velocity, there is plenty of space available around the collision region to mount even several further electron or X-ray detectors. Since the full three-dimensional recoil-ion emission pattern is monitored any coincidence automatically provides polar- and azimuth angle dependent data as a function of the momentum transfer during the collision. By variation of the accelerating electric field after the drift region and of the jet velocity (changing the nozzle temperature) it is easily possible to be selective on a certain regime of momentum transfers predominantly contributing to the reaction of interest. (Recoil-ions containing transverse momenta larger than the desired value are outside the dimension of the channel plate, those with a momentum too low pass the detector through its middle hole like the undeflected jet.)
momentum
spectroscopy
421
Due to the superposed parallel jet-velocity up to corresponding energies of 1 eV the extension of the time-window can be reduced by roughly a factor of 10 for the very distant collisions which in turn allows to increase the incident projectile - and thus the coincidence yield by the same factor. Since the collision region is well defined by the jet position, the apparatus is even Iess sensitive on the beam divergence than the present setup. Also, disturbances by fringe fields and contact potentials should be easier to handle since no target cell of small size is necessary and no cold material is present where ingredients of the rest gas could freeze out.
7. Conclusion Recoil-ion momentum spectroscopy has been demonstrated to be a valuable method for the differential investigation of atomic reactions in a regime of very small momentum transfers (large impact parameter) that has not been accessible up to now. It should be kept in mind that due to the long range of the Coulomb potential it is this regime that mainly contributes to collision induced ionisation and excitation cross sections, to S-electron emission and to the total stopping power. Therefore, it is essential to understand these processes on the level of single collisions in order to describe more complex reactions like the interaction of fast heavy ions with dense matter like semiconductors or biological systems (radiation damage). The present apparatus has shown to provide detailed information on the dynamics of the ionisation process: It was observed that the momenta of the ejected electrons can substantially influence the heavy particle trajectories. In order to accurately describe the dynamics of single or multiple ionisation in a collision, theoretical models explicitely have to consider the many-body nature of the process and must implement the coupling between nuclear and electronic motion. The concept of a new, high resolution recoil-ion momentum spectrometer was introduced which should enable to determine recoil-energies in the sub-meV regime simultaneously in ail three spatial dimensions. This spectrometer is especially designed to perform multiple coincidences with the emitted electrons or X-rays, thus being an important tool for the exploration of the quantum-mechanical many-body Coulomb interaction.
Acknowledgement
The authors gratefully acknowledge many fruitful discussions with R.E. Olson, PH. Mokler, M. Horbatsch, CL. Cocke, and S. Hagmann.
422
J. Ullrich / Recoil-ion momentum spectroscopy
References [l] Q.C. Kessel and E. Everhart, Phys. Rev. 146 (1966) 16; E. Everhart and Q.C. Kessel, Phys. Rev. 146 (1966) 27. [2] U. Fano and W.L. Lichten, Phys. Rev. Lett. 14 (1965) 621. [3] P.H. Mokler and D. Liesen, in: Progress in Atomic Spectroscopy, Part C, eds. H.J. Beyer and H. Kleinpoppen (Plenum Press, New York 1984) pp. 321-359; R. Schuch, in: Electronic and Atomic Collisions, ed. S. Datz (North-Holland, Amsterdam, 1981) pp. 151-167. [4] H. Schmidt-B&king, R. Schuch, I. Tserruya, R. Hoffmann, B. Johnson, K.W. Jones, E. Justiniano, M. Meron, P.H. Mokler, H. Ingwersen, W. Schadt, M. Schulz and B. Fricke, Nucl. Instr. and Meth. BlO/ll (1985) 69. [5] A. Skutlartz, S. Hagmann, C. Kelbch and H. SchmidtB&king, Physica Scripta T22 (1988) 307. [6] R. Schuch, H. Schone, P.D. Miller, H.F. Krause, P.F. Dittner, S. Datz and R.E. Olson, Phys. Rev. Lett. 60 (19881 925. [7] J. Ullrich, H. Schmidt-B&king and C. Kelbch, Nucl. Instr. and Meth. A268 (1988) 216.
[8] J. Ullrich, M. Horbatsch, V. Dangendorf, S., Kelbch and H. Schmidt-Backing, J. Phys. B21 (1988) 611. 191 R. Darner, J. Ullrich, H. Schmidt-B&king and R.E. Olson, Phys. Rev. Lett. 63 (1989) 147. [lo] R.E. Olson, in: Electronic and Atomic Collisions, eds. H.B. Gilbody, W.R. Newell, F.H. Read and A.C.H. Smith (Elsevier, New York, 1988) pp. 271-285. [ll] R. Diirner et al., Proc. 5th Int. Conf. on the Physics of Highly-Charged Ions, Giessen 1990, to be published. [12] M. Horbatsch, J. Phys. B22 (1989) L639. [13] R.E. Olson, J. Ullrich and H. Schmidt-B&king, Phys. Rev. A39 (1989) 5572. [14] A.D. Gonzales, S. Hagmann, T.B. Quinteros, B. Krassig, H. Schmidt-B&king, R. Koch and A. Skutlartz, J. Phys. B23 (19901 L303. [15] U. Buck, M. Diiker, H. Pauly and D. Pust, Proc. of 4th Int. Symp. on Molecular Beams (1974) p. 70. [16] H. Haberland, U. Buck and M. Tolle, Rev. Sci. Instr. 56 (1985) 1712. [17] J. Ullrich, R.E. Olson, R. Darner, S. Kelbch, U. Berg and H. Schmidt-B&king, J. Phys. B22 (1989) 627.
415
~~rt~-~~l~an~
Recoil-ion momentum spectroscopy
U, Buck APIftir Striimungsforschung,
*
Gijttingen, Germany
Received 19 February 1991
Within recent years the spectroscopy of the target recoil momen~m in distant ion-atom collisions has been established as a third, independent and vafuabIe method of target spectroscopy besides the classica measurement of ejected electrons and X-rays. For a iarge set of cotfision systems, e.g. for fast and heavy projectiles impinging on gaseous targets, the determination of the recoil-ion transverse momentum provides the only possibility to obtain differential excitation or ionisatian cross sections. Moreover, since the requirements on the ion beam (emittance, focussingf are comparably low even for high precision measurements, experiments using exotic beams like antiprotons can be envisaged. The present stage of development of recoil-ion momentum spectroscopy (RIMS) is illustrated, its special ability to obtain information on many-body interactions in an ion-atom collision is demonstrated and the future potential of the method as to the momentum resolution, the solid angle and possible applications in many-particle coincidence measurements is outlined.
1. Zntroduetion The i~~esti~~t~o~ of atomic reactions like ionisation or excitation in energetic ion-atom collisions turned out to be of particutar value for comparison with theoretical approaches if differential cross sections, in dependence on the projectile laboratory scattering angle G, were determined by the experimentalists. Pioneering experiments by Everhart and Kessel 111 supporting the development of the molecular orbital picture in slow collisions [2] were followed by numerous angular differential measurements of X-ray emission yielding impact parameter dependent inner-shell ionisation probabilities [3]. The determination of the projectile trajectory allowed the observation of interference patterns in quasimolecular X-ray spectra and a
* Supported by GSI, Darmstadt, ~~ndesminjster~um t%r Forschung und Technologie (BMPT:), and Deutsche Forschungsgemeinschaft (DFG), Germany. First presented at 11th Int. Conference on the Applications of Accelerators in Research and Industry, Nov. 5-8, 1990, Denton, TX, USA. 0168483X/91/$03.50
precise spectroscopy of the quasimolecular orbitals formed during the colhsion [4]. Very recently, few angular differential data have also been reported for electron ejection and recoil-ion production [5,6]. However, since projectile scattering angle measurements are limited to a regime of 6) 2 5 x 10-’ xad (0.5 mm deflection on a distance of 10 ml, a large set of collision systems or excitation processes are beyond the scope of this technique: For heavy projectiles at C&V/u energies, delivered for instance by the new accelerator complex SIS/ESR at GSI, Darmstadt, typical 9 are below measurable values for nearly all collision-induced atomic reactions. Even at intermediate velocities of several MeV/u, for heavy ion impact the major part of single and also multiple target ionisation, i.e. more than 90% of the total electron ejection cross section and a main contribution to the stopping power is produced at large impact parameters leading to pxojectile deflections as small as microradians. This is illustrated in fig. 1 where the projectile energy is plotted versus the projectile scattering angle regime for all combinations of collision systems (U on We until Ne on Xe, available at SIS/ESR) at a representative recoil-ion transverse energy of 100 meV (shaded area). First
0 1991 - Eisevier Science Publishers B.V. All rights reserved
416
J. Ullrick / Recoil-ion momentum spectroscopy
measurements at 1.4 and 5.9 MeV/u (full dots in fig. 1) as well as nCTMC calculations indicate that more than 95% of the ionisation events are expected at even smaller scattering angles than indicated by the shaded area. The lower limit for a direct 6 measurement of 9 2 5 X lo-’ given in the figure presumes a transverse momentum spread of the ion-beam bp/p _< IO-’ usually obtained by collimating the beam. In most practical cases this limit is more on the order of 2 X 10m4 rad. Moreover, in storage rings for ions like CRYRING, TSR, or ESR the measurement of projectile scattering angles cannot be envisaged without disturbing the beam in such a way that further storage and cooling is impossible. In order to enable differential measurements in this regime which has not been accessible before, we have developed a novel technique, measuring the recoil-ion momentum transverse to the beam direction PRI instead of that of the projectile P, i . The investigation of single and multiple ionisation of He, Ne, and Ar targets in collisions with MeV/u, highly charged uranium (4 = 32, 75) or even protons illustrated that the momenta transferred to the recoil-ion (the projectile) are in the order of 3 a.u. or below for a major part of the ionisation events. For the recoiling target ion this means that one has to deal with transverse energies E s I in the subthermal regime E, _Ls 20 meV for a target at room temperature. Therefore, the first generation of recoil-ion momentum spectrometers which worked with the target gas at a temperature of 300 K (a detailed description can be found in refs. [7,8]) was followed by a new development, where the target is at 30 K [9]. Fig. 2 illustrates the improvement of the
-?10 2 E 2 w 51
_____._
____ r___._ Temperature/K
Fig. 2. Centroid (dots) and full width half maximum (bars) of the measured recoil-ion energy distributions for a representative energy transfer of 15 meV at different target temperatures.
transverse recoil-ion energy determination (full dots) and of the energy-resolution (bar: FWHM) for a representative transverse energy transfer in the collision of 15 meV with decreasing target temperature. In this paper we will describe some conceptual details and the main features of the cooled spectrometer. In addition to its ability to provide differential cross sections for a large set of collision systems which are not achievable with other techniques. the target recoil-momentum spectroscopy will be shown to be a third, valuable and independent type of spectroscopy besides the classical X-ray and electron spectroscopy. Main emphasis will be given to the future scope and potential of this technique.
2. Present apparatus troscopy
total i~i~~n
5 to-5 2 10-B2 s to-7 2 5 1o*2 Projectile Scattering Angle 6/
5 to-4 2 rad
Fig. I. Projectile laboratory scattering angle versns projectile kinetic energy. The shaded area indicates the projectile scattering angle regime coincident to recoil-ion energies of 100 meV for various collision systems from Ne on Xe (right-hand sotid line) up to U on He Cleft-hand solid line) collisions. Full dots: investigated collision systems [8,17].
i
for transverse
momentum
spec-
The principle behind the recoil-ion momentum spectrometer is to measure the transverse momentum of the recoiling target-ion after the collision, instead of that of the projectile (for a detailed description see ref. [7]). As shown in fig. 3, recoil ions are produced in a static gas cclt (a cylinder of 10 mm diameter coaxial to the beam) and drift to the wall of this cylinder in a time interval At inversely proportional to the transverse velocity (momentum) which they gained within the collision. They leave the reaction zone through an aperture of 1 mm x 20 mm, positioned along the beam axis, are accelerated, charge state analyzed and detected. Compared to the first RIMS version, where the aperture was a hole of 1 mm diameter, the recoil-ion detection efficiency is enlarged by a factor of 20. The ion-optics has been adopted in such a way that the
J. Uliri~~ / Rec#i~-ion momentum spectroscopy
a.) along
the
beam
b) perpendicular
417
to the beam
*-----
Fig. 3. Recoil-ion momentum spectrometer with cold target. a) Cut along the beam axis; b) Cut perpendicular to the beam axis.
separation and simultaneous detection of 10 Ne charge states is possible. Due to the limited extension of the
gas target along the beam only recoil-ions scattered into a certain polar angular regime can be observed. The calculated angular acceptance, shown in fig. 4, shows a constant behaviour over a wide angular range between 20 O and 150 o with a sharp fall off to zero for the other angles. The recoil-ion time-of-flight (TOF) is obtained by a coincidence with the scattered projectile. The time resolution is below 10 ns and small compared to typical recoil-ion - TOF in the order of a few microseconds inside the cylinder. The target cylinder is drilled in a
3
S 0.6. ‘ij Z % 0.4.
polar emission angle Fig. 4. Calculated polar angular acceptance of the recoil-ion momentum spectrometer.
copper block and mounted on the top of the cold finger of a cryopump. The target gas is precooled on its way to the cylinder and reaches the target area through 8 channels (of 1 mm diameter) along the beam axis in order to ensure a constant target pressure along the beam axis. The temperature which can be as low as 30 K is measured by a Pt resistor on top of the copper block. In the present version, the copper block is gold-plated, the inner surface of the cylinder is completely coated by a copper grid (mesh width: 0.12 mm x 0.12 mm) to form a “perfect” Faraday cup and to minimise fringe fields at the recoil-ion aperture. As shown in fig. 5 the exact potential curves at the aperture were calculated showing that the full potential of 500 V drops to a value of 5 mV within 0.4 mm. The small variation in the recoil-ion drift path has to be considered for the exact determination of the recoil-ion momentum. Before each experiment, the spectrometer has to be carefully cleaned in an ultrasonic bath. Typicalfy, after about five hours of beamtime, the recoii-ion momentum spectra change slowly, probably due to a freezing out of rest gas components on the surface of the spectrometer. Warming up above the nitrogen freezing point and cooling down again yielded the original spectra. Different material combinations (grid, cylinder) and recoil-ion drift lengths (2.5 mm, 3.5 mm, 5 mm1 were used to check possible influences of fringe fields. Within statistical error bars identical results were ob-
J. Ullrich / Recoil-ion momentum spectroscopy
418
drift
0.8
region
0.6
0.4
0.2
Fig. 5. Electric
potential
curves at the recoil-ion
For the different experiments performed, we derive that further systematical errors should be below + 5 meV. The thermal motion of the target atoms at 30 K essentially shifts the measured energy distribution by +4 meV. For the comparison with theory the momentum distribution has to be unfolded with respect to the remaining thermal motion momentum distribution. tained.
3. PRI_PPI
0.2
0.0 beam
direction
0.L
0.8
0.6
Z/mm
aperture
which is covered
by a copper
grid
(straight line): At scattering angles around 0.7 mrad the coincident recoil-ion energy starts to fall below the two-body value with a maximum deviation at 6 = 0.5 mrad and a saturation in energy is observed at a value of 8 f 5 meV (thermal motion unfolded). The overall feature of the experimental data is in good agreement with theoretical results of classical many-body calculations (nCTMC) [lo]. Deviations from the two-body behaviour were found theoretically to be
-coincidences
To test the spectrometer, the transverse recoil-ion momentum PRI (or energy E, i) was measured in coincidence with the final projectile transverse momenturn Pp, (or scattering angle: -9 = P, I/P,, ; P,, is the incoming projectile momentum) for the 0.5 MeV p on He collision system at the 2 MV Van de Graaff accelerator of the Institut fir Kernphysik, Universitat Frankfurt. For single ionisation where only three free particles emerge after the encounter the transverse momentum balance is fully determined by the simultaneous determination of P, I and P, I For negligibly small momentum of the emitted electron P,, or isotropic electron emission the mean recoil-ion transverse momentum (P, I > equals Pp, which would enable a direct test of the spectrometer. The experimental results (open circles in fig. 6), however, displayed distinct deviations from this two-body behaviour
3 E \ > too P e
0.5
MeV p -> He
W a u
10
u
5 f
1 0.1
Projectile Scattering
1
Angle 6/
mad
Fig. 6. Projectile scattering angle 19 versus transverse recoil-ion energy E, I. Open circles: Experimental data corrected for the thermal motion influence at 30 K. Bars: Systematical experimental error. Straight line: Two-body kinematics. Full line: nCTMC results.
J. Vllrich / Recoil-ion momentum spectroscopy
due to three-body interactions between the heavy particles, the proton, and the He+ ion and the emitted electron for single ionisation. (For a detailed discussion see refs. [9,11].) Similar theoretical results have been obtained applying a semiclassical quantum statistical model [12]. The observed deviation between theory and experiment at scattering angles below 0.5 mrad are still unexplained: Sensitive tests of the spectrometer at the 1 meV level are extremely delicate but are in progress in order to substantially reduce possible systematical experimental errors. The experimental results, however, demonstrate that the simultaneous spectroscopy of the transverse recoilion momentum and charge state in dependence of the transverse momentum transfer to the projectile provides new insight into the dynamics of ionising collisions, which can hardly be achieved by other techniques.
4. Spectroscopy momentum
of the parallel and transverse recoil-ion
Theoretical calculations predict strong deviations from the expected 90 o polar angular scattering of the recoil ions c-9,) at large impact parameters due to the interaction with the ionised electron(s) [13]. Since the exact position along the beamline within the target cylinder of the present apparatus is not specified, recoil-ion momentum components along the beam direction Pa,, cannot be measured and the theoretical predictions cannot be proven. Therefore, we have developed a modified version of the apparatus described above, were the observable collision region is defined within the target cylinder by implementation of another aperture of 2 mm diameter. First results on recoil-ion polar emission angles in dependence of the recoil-ion energy with the cold target have been obtained but careful checks of the ion-optics (to observe these angles) as well as calculations of the solid angle for each 6, are presently in progress. Since the efficiency of this spectrometer is a factor of 20 smaller such experithan the one for the P, I measurement, ments are extremely time consuming.
5. Present scope of RIMS Experiments using the cooled target recoil spectrometer demonstrate that an accuracy in the determination of the recoil-ion energy of k5 meV down to absolute energies of 10 meV is achievable. Since this accuracy is independent of the projectile velocity a precision in the relative momentum measurement corresponding to projectile scattering angles between lo-’ rad and 10m5 rad, depending on the energy and the
419
collision system, can be obtained. This enables a differential investigation of atomic reactions in a regime which has not been accessible before. It is this regime of very low momentum transfers which mainly contributes to single and multiple ionisation, to kinematic, radiative as well as resonant electron capture, to Felectron ejection and to the stopping power. measureFurthermore, the E, i - 19 coincidence ments have shown that the projectile and recoil-ion scattering contains different information. The theoretically observed saturation value of about 1 meV for the proton-He collision system reflects the mean energy of the emitted electron, or correspondingly, of the emitted electrons for many electron targets. Thus, recoil-ion momentum spectroscopy seems to be especially well suited to investigate collective processes which might occur due to the many-body Coulomb interaction during a collision [14]. A striking advantage of RIMS is the fact that the high accuracy can be obtained even with beams of a large transverse momentum spread of Ap/p = lo-‘. This is due to the feature that only the exact position of the projectile inside the spectrometer is needed to get the recoil-momentum information. This position can be obtained by a two-dimensional position sensitive detection of the projectiles directly behind the collision region. Therefore, the differential investigation of single and multiple target ionisation by antimatter beams, available at LEAR (CERN), can be envisaged. Due to the low intensities and large momentum spreads of such degraded beams a further collimation which would be necessary for the -9-differential measurement is not possible. Thus, even in a regime of transverse momentum transfers (lop4 to 10-s) which can usually be covered by the conventional technique, RIMS is able to give precise information if the beam quality rejects a 6 measurement. A further advantage of the method lies in the fact that all momentum transfers are measured simultaneously. However, the present stage of development still bears problems which should be solved in a third generation of RIMS. First, the energy resolution stated above can only be reached for a He target since all other gas targets form dimers or molecules if they are cooled down to the low temperatures necessary to reduce thermal motion influences. Since thermal motion is well known, it is possible to unfold the obtained spectra, however, structures in the spectra smaller than the thermal motion distribution (for example discrete lines in the recoil-momentum due to Auger decay of an excited target ion) cannot be resolved. Furthermore, due to the geometry of the present apparatus, certain recoil-ion polar emission angles cannot be detected (see fig. 4). This may influence the measured transverse recoil-ion momentum distributions since different atomic reactions might show com-
420
J. Ullrich / Recoil-ion momentum spectroscopy
pletely different angular emission patterns. This is not a severe constraint but has to be considered when comparing with theory. In principal RIMS is ideally suitable to perform coincidences with the emitted electrons or X-rays and, thus, providing detailed information on a specific ionisation or excitation process or, applying multi-coincidence techniques, on many-particle reactions occurring during the collision. However, the present setup strongly hinders such measurements, since first the solid angle for the recoil-ion detection is in the order of 10m3 only and, even more severe, since it is impossible to get a large solid angle for X-rays or electron detection, due to the geometric configuration of the spectrometer. Another feature, the ability of the apparatus to simultaneously detect all recoil-ion transverse momenta, i.e. all impact-parameters, which might be appealing at first glance turns out to be impeding if processes are to be investigated which mainly take place at smaller impact parameters or higher recoil-ion energies. The enormous number of recoil-ions created at large impact parameters produce random coincidences in the time-spectra and easily cover the true coincidences of interest. It would be very desirable if
only recoil ions in a certain adjustable momentum range would be detected. Due to the long recoil-ion drift path of 5 mm or at minimum 2.5 mm extended time windows are needed to obtain the full TOF-information, since typical drift times range between 1 us up to 20 us for Ne-ions in the meV regime. Compared to the conventional coincidence techniques (e-, X-ray, recoil-ion-projectilecoincidences) this is worse by a factor of 1000 requiring a 1000 times more efficient background suppression. Furthermore, since the shape of the TOF spectra reflect the momentum distribution the recoil-ions, the exponential decrease observed in the TOF-spectra for high particle rates should be below a factor of 2 over the time window, which limits the projectile yield to values below 3 x lo4 for a perfect dc ion-beam. Due to fluctuation in the ion-beam intensity on the microsecond scale realistic projectile yields very often were found to be limited below 104.
6. Third generation
RIMS
In order to overcome most of the difficulties stated above, a rigorously new concept of RIMS was devel-
PUMP
Fig. 7. New recoil-ion momentum spectrometer, presently machined. V,,ti: velocity transferred to the target during the collision. I$_,: Supersonic jet velocity. TAC: Time-to-amplitude converter. 4,: Projectile laboratory scattering angle. ‘pp: Azimuthal projectile scattering angle. PS PPAD: position sensitive parallel plate avalanche detector. PS MCP: position sensitive microchannel plate.
J. Ullrich
/ Recoil-ion
oped and presently is under construction. As shown in fig. 7 the gas target is no longer extended along the beam axis but is a supersonic jet of 1 mm diameter perpendicular to the ion beam. Recoil-momenta transferred to the jet-particles during a collision with a projectile will blow up the spatial extension of the jet perpendicular to its propagation, i.e. it will cause a change in the transverse velocity (u~,~ in the figure) as well as in the parallel velocity u, of the particles in the jet. A two-dimensional position-sensitive microchannel plate detector (active diameter 40 mm) monitors the spatial deflection of each recoil-ion perpendicular to the jet-direction (ux, L’~); a TOF measurement, achieved by a recoil-ion projectile (or electron) coincidence technique will provide the L’, information. In a second stage of development, a magnetic deflection of the recoil-ions will be implemented in order to simultaneously determine the recoil-ion charge state. The inherent temperature of such a gas jet is known to be below 1 K [15]. From this a recoil-ion energy resolution in the sub-meV regime can be expected. For a He-target this corresponds to an electron energy resolution of below one eV. Calculations, taking into account further sources of uncertainty in the recoil-ion energy determination like the jet divergence, the time resolution, or the definition of the exact interaction region show that an overall resolution corresponding to an electron energy resolution below 10 eV should be achievable. In this case discrete lines in the energy spectra due to Auger decays of metastable states produced in the collision may become observable. In contrast to the present setup even heavier target gases can be cooled and thus a similar energy resolution can be achieved [16]. The formation of clusters can be minimized under certain conditions of operation to a few percent, they can easily be further discriminated due to their large mass compared to the ions of interest. The new apparatus is well suited for any kind of (multiple) coincidence measurements: The recoil-ion solid angle of about 50% is extremely large due to the superimposed jet velocity, there is plenty of space available around the collision region to mount even several further electron or X-ray detectors. Since the full three-dimensional recoil-ion emission pattern is monitored any coincidence automatically provides polar- and azimuth angle dependent data as a function of the momentum transfer during the collision. By variation of the accelerating electric field after the drift region and of the jet velocity (changing the nozzle temperature) it is easily possible to be selective on a certain regime of momentum transfers predominantly contributing to the reaction of interest. (Recoil-ions containing transverse momenta larger than the desired value are outside the dimension of the channel plate, those with a momentum too low pass the detector through its middle hole like the undeflected jet.)
momentum
spectroscopy
421
Due to the superposed parallel jet-velocity up to corresponding energies of 1 eV the extension of the time-window can be reduced by roughly a factor of 10 for the very distant collisions which in turn allows to increase the incident projectile - and thus the coincidence yield by the same factor. Since the collision region is well defined by the jet position, the apparatus is even Iess sensitive on the beam divergence than the present setup. Also, disturbances by fringe fields and contact potentials should be easier to handle since no target cell of small size is necessary and no cold material is present where ingredients of the rest gas could freeze out.
7. Conclusion Recoil-ion momentum spectroscopy has been demonstrated to be a valuable method for the differential investigation of atomic reactions in a regime of very small momentum transfers (large impact parameter) that has not been accessible up to now. It should be kept in mind that due to the long range of the Coulomb potential it is this regime that mainly contributes to collision induced ionisation and excitation cross sections, to S-electron emission and to the total stopping power. Therefore, it is essential to understand these processes on the level of single collisions in order to describe more complex reactions like the interaction of fast heavy ions with dense matter like semiconductors or biological systems (radiation damage). The present apparatus has shown to provide detailed information on the dynamics of the ionisation process: It was observed that the momenta of the ejected electrons can substantially influence the heavy particle trajectories. In order to accurately describe the dynamics of single or multiple ionisation in a collision, theoretical models explicitely have to consider the many-body nature of the process and must implement the coupling between nuclear and electronic motion. The concept of a new, high resolution recoil-ion momentum spectrometer was introduced which should enable to determine recoil-energies in the sub-meV regime simultaneously in ail three spatial dimensions. This spectrometer is especially designed to perform multiple coincidences with the emitted electrons or X-rays, thus being an important tool for the exploration of the quantum-mechanical many-body Coulomb interaction.
Acknowledgement
The authors gratefully acknowledge many fruitful discussions with R.E. Olson, PH. Mokler, M. Horbatsch, CL. Cocke, and S. Hagmann.
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J. Ullrich / Recoil-ion momentum spectroscopy
References [l] Q.C. Kessel and E. Everhart, Phys. Rev. 146 (1966) 16; E. Everhart and Q.C. Kessel, Phys. Rev. 146 (1966) 27. [2] U. Fano and W.L. Lichten, Phys. Rev. Lett. 14 (1965) 621. [3] P.H. Mokler and D. Liesen, in: Progress in Atomic Spectroscopy, Part C, eds. H.J. Beyer and H. Kleinpoppen (Plenum Press, New York 1984) pp. 321-359; R. Schuch, in: Electronic and Atomic Collisions, ed. S. Datz (North-Holland, Amsterdam, 1981) pp. 151-167. [4] H. Schmidt-B&king, R. Schuch, I. Tserruya, R. Hoffmann, B. Johnson, K.W. Jones, E. Justiniano, M. Meron, P.H. Mokler, H. Ingwersen, W. Schadt, M. Schulz and B. Fricke, Nucl. Instr. and Meth. BlO/ll (1985) 69. [5] A. Skutlartz, S. Hagmann, C. Kelbch and H. SchmidtB&king, Physica Scripta T22 (1988) 307. [6] R. Schuch, H. Schone, P.D. Miller, H.F. Krause, P.F. Dittner, S. Datz and R.E. Olson, Phys. Rev. Lett. 60 (19881 925. [7] J. Ullrich, H. Schmidt-B&king and C. Kelbch, Nucl. Instr. and Meth. A268 (1988) 216.
[8] J. Ullrich, M. Horbatsch, V. Dangendorf, S., Kelbch and H. Schmidt-Backing, J. Phys. B21 (1988) 611. 191 R. Darner, J. Ullrich, H. Schmidt-B&king and R.E. Olson, Phys. Rev. Lett. 63 (1989) 147. [lo] R.E. Olson, in: Electronic and Atomic Collisions, eds. H.B. Gilbody, W.R. Newell, F.H. Read and A.C.H. Smith (Elsevier, New York, 1988) pp. 271-285. [ll] R. Diirner et al., Proc. 5th Int. Conf. on the Physics of Highly-Charged Ions, Giessen 1990, to be published. [12] M. Horbatsch, J. Phys. B22 (1989) L639. [13] R.E. Olson, J. Ullrich and H. Schmidt-B&king, Phys. Rev. A39 (1989) 5572. [14] A.D. Gonzales, S. Hagmann, T.B. Quinteros, B. Krassig, H. Schmidt-B&king, R. Koch and A. Skutlartz, J. Phys. B23 (19901 L303. [15] U. Buck, M. Diiker, H. Pauly and D. Pust, Proc. of 4th Int. Symp. on Molecular Beams (1974) p. 70. [16] H. Haberland, U. Buck and M. Tolle, Rev. Sci. Instr. 56 (1985) 1712. [17] J. Ullrich, R.E. Olson, R. Darner, S. Kelbch, U. Berg and H. Schmidt-B&king, J. Phys. B22 (1989) 627.