- Email: [email protected]
Ion-X-ray coincidence measurements in heavy ion collisions
Nuclear Instruments and Methods North-Holland, Amsterdam
ION-X-RAY
in Physics
COINCIDENCE
Research
B24/25
(1987) 69-73
MEASUREMENTS
69
IN HEAVY ION COLLISIONS
*
W.G. GRAHAM Physics Department,
Universi(y of Ulster, Coleraine BT52 ISA,
E.M. BERNSTEIN, Department
of Physics,
K.H. BERKNER, Lawrence
Western Michigan
and
Berkeley L.aborato~,
and
MT 49008, USA
J.W. STEARNS
and
lJniversi(v of Culifornia,
K.W. JONES
National Laboratory,
J.A. TANIS
Universit.v, Kalamuzoo,
A.S. SCHLACHTER
B.M. JOHNSON, Brookhuven
M.W. CLARK
N. Irelund
Berkelqv, CA 94720, USA
M. MERON
Upton, NY I 1973, USA
R.H. MCFARLAND Department
of Physics, University of Missouri, Rolla, MI 65401, USA
T.J. MORGAN Physics Department,
Wesleyun University, Middletown,
CT 06457, USA
M.P. STOCKLI Department
of Physics, Kunsas Stute Universi(v, Munhuttan,
KS 66506, USA
X-ray emission associated with projectile charge-changing events in fast ion-atom collisions can be used to isolate and investigate excitation, ionization and charge transfer, as well as combinations of these processes. The major emphasis to date has been on the study of two-electron processes. Several such processes have been studied in detail. Resonant and nonresonant transfer and excitation (RTE and NTE) occur when electron capture and projectile excitation, with stabilization by X-ray emission, take place together in a single collision. Loss and excitation (LE) occurs when a projectile electron is removed and the ion is excited in a single collision. The cross sections for loss and excitation have been measured for a wide range of highly stripped projectiles over a broad energy range. The results shed light on projectile K X-ray production processes in heavy-ion collisions. The use of ion-X-ray coincidence techniques in exploring these and other collision processes will be discussed.
1. Introduction X-ray emission associated with projectile chargechanging events in fast ion-atom collisions can be used to isolate and investigate excitation, ionization and charge transfer, as well as combinations of these processes with an overall goal of understanding funda-
* This work is supported in part by the Science and Engineering Research Council, Great Britain and the US DOE, Division of Chemical Sciences and Applied Plasma Physics. 0168-583X/87/$03.50 0 Hlsevier (North-Holland Physics Publishing
Science Publishers Division)
B.V
mental atomic interactions. The major emphasis to date has been on the study of two-electron processes, with the projectile ions and projectile X-rays being detected in coincidence. In particular this has led to the discovery of resonant transfer and excitation [1,2]. The techniques are now being extended by including measurements of the target ions and target X-rays with capabilities for X-ray/X-ray coincidences and target recoil ion/target or projectile X-ray coincidences. In this paper some of our current measurements will be reviewed and the extension of these techniques to other coincidence partners discussed. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
70
W.G. Graham et al. / Ion - X-rq
2. Experimental procedure
+ A(q+P)*+ + [B]
The measurements reported here were obtained using the SuperHILAC accelerator at the Lawrence Berkeley Laboratory and the tandem Van de Graaff facility at the Brookhaven National Laboratory. The apparatus and experimental techniques have been previously described [1,2]. Briefly, a well collimated beam of ions of the desired energy and charge state is incident on the differentially pumped target gas cell, where the gas pressure is measured using a capacitance manometer. X-rays produced in the target region from the projectile ions or target atoms are detected using one of two Si(Li) detectors. The beam leaving the target gas cell is charge state analyzed using either magnetic (LBL) or electrostatic (BNL) deflection. The charge-changed components of the beam are detected using an array of solidstate detectors, although there are plans to replace these detectors with position sensitive wire counters. At present the intense non-charge-changed component of the emerging beam is collected in a Faraday cup. The cross sections are obtained from the dependence on target gas thickness of the fractional yield of ions in a given charge state, X-rays, or coincident events as compared to the total incident beam. The linear dependence of the fractional yields on gas pressure is used to ensure that single collision conditions prevail. The X-ray detectors are generally able to resolve the K, (n=2+l)andKp (n=3+1)componentsof the projectile K X-ray spectra so that the relative contributions of K, and K, to capture and excitation, loss and excitation, and excitation alone can be determined. If sufficiently heavy projectiles or targets are used X-rays from transitions to the L shell can also be observed. The relative uncertainties in the measurements can be determined from one standard deviation of the least squares fit to the data and are generally of the order of - 5% for single-electron capture and loss u4,1 and total projectile and target X-ray production o,,~; and of the order of *15% for projectile ion X-ray coincidence measurements oft:;. Systematic uncertainties mainly arise in the determination of the target thickness and the X-ray detection efficiency and solid angle leading to overall uncertainties in the absolute cross sections of about + 12% for u,,l, *20’% for uKaP and k258 for a$:;.
3. Projectile charge state/projectile
X-ray coincidence
In measurements of projectile charge state/projectile K X-ray coincidences the following processes can be studied: Aq+ +fB -+ A(qPJ’)*+ + [B] -+A4+*+[B]
coincidence meusurements
UgP,
(1)
UR,
(2)
u$+“,
(3)
where A is the projectile ion, B the target atom and q is the incident projectile charge state; p is normally one but measurements with p = 2 have been made though as yet unpublished. The square brackets indicate that no information on the final state of the target atom is available. The intermediate excited states formed in the above processes subsequently decay by either photon emission or Auger electron emission. In this paper cases where an X-ray is emitted are considered. If the atomic number of the projectile is sufficiently large, detectors allow the K and L X rays to be observed and the OLand p lines to be resolved. The total K X-ray production cross section uknp normally reported in the literature is the sum of processes (l), (2) and (3). A measurement of the X-ray emission in coincidence with the incident charge state q after the target cell allows the cross sections for projectile excitation alone to be determined. This is a more fundamental cross section and more useful in comparisons with theoretical calculations. Comparing the sum of the separately determined cross sections for processes (l), (2) and (3) with the total X-ray production cross section would provide a good experimental verification of the coincidence measurements. As yet X-ray coincidence measurements with the main beam have not been reported since detectors which can handle the high count rates required of the mainbeam are needed. With the use of the wire counter in atomic physics experiments, many new applications of these detectors will be possible. The collisions described in eqs. (1) and (3) are two-electron processes involving projectile excitation and either electron capture or electron loss. Coincidence techniques are particularly useful in studying these processes since they are then isolated from such singleelectron processes as “pure” capture or excitation. 3.1. Electron capture and excitation The measurement of electron capture in coincidence with both K and L X-ray production, process (l), has received considerable attention [l-5]. This represents a collision in which an electron is captured and the projectile excited in a single encounter. A typical energy dependence of u&-’ is shown in fig. 1, in this case for S13+ in He [3]. The maximum near 130 MeV is attributed to resonant-transfer and excitation, RTE, while the low energy maximum near 30 MeV is evidence of nonresonant-transfer and excitation, NTE. RTE is the simultaneous electron capture and excitation in the projectile ion in a single collision as a result of an electron-electron interaction. Since its recent discovery [1,2] this particular process has been studied in
W.G. Graham et al. / Ion - X-ray coincidence measurements
71
certain energies. RTE has also been observed to make a sizeable contribution to the total electron capture cross section [6]. 3.2. Radiative electron capture (REC)
.\ i \ \*. J..
60
I
60
I
I
I
Another form of electron capture is radiative electron capture (REC) [7]. While occurring to some extent in all collision systems, REC is found to be the dominant form of electron capture in very fast, highlystripped ion collisions with light targets [8]. REC is the inverse of the photoelectric effect and it occurs when an electron is captured to an inner shell with the simultaneous emission of a photon. The X-ray spectrum for this process has been observed in several experiments but because of the small relative strength of REC compared with total X-ray production higher density solid targets have been used [9] where multiple collision effects can complicate the interpretation. As shown in fig. 2, observing the spectra in the electron capture/X-ray coincidence channel substantially improves the REC signal in the spectra and allows measurements of REC cross sections to be made in gas targets.
. I
3.3. Electron loss and excitation
100 120 140 160 160 20C
E(MeV) Fig. 1. Projectile K X-ray cross sections for 15-200 MeV SL3+ + He collisions. cKoS is the cross section for the total sulphur K X-ray production and u&$ is the cross section for sulphur K X-rays coincident with single electron capture. The tnaxunum in u&,k near 130 MeV is attributed to RTE and that near 30 MeV is attributed to NTE. The dashed curve is a calculated RTE cross section (ref. [3]).
Another two-electron process is one in which one projectile electron is lost and the ion is excited in the single collision of a highly-stripped heavy ion with an atom, i.e. loss and excitation in a single collision (LE) [lo] (process (3)). The cross sections for electron loss and excitation, u$Lj have been measured with the same collision sys-
SINGLES
some detail and is the subject of another paper in this session. NTE [3,4] is a two-step process in which two independent interactions occur in a single encounter. The mechanisms involved in this case are electron-nucleus interactions (1) between the target nucleus and a projectile electron resulting in excitation and (2) between the projectile nucleus and a target electron resulting in electron capture. Such a combination of excitation and capture events does not depend resonantly on the incident projectile velocity, however NTE does exhibit a maximum in its energy dependence. Qualitatively this may be viewed as a result of the product of an increasing excitation cross section and a decreasing singleelectron-capture cross-section. Results of measurements of RTE and NTR indicate that these processes can account for a sizeable fraction of the K-shell excitation events at certain energies e.g. in the present case, of St3+ in He, NTE contributes approximately 15% to the total K,, cross section at
3
COINCIDENCE
$
X-RAY
ENERGY
(arb..units)
Fig. 2. Typical X-ray spectra for 260 MeV Ca19+ +He. Total X-rays, (b) X-rays in coincidence with Ca*‘+. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
(a)
W. G. Graham et al. / Ion - X-q
72
II
’
’
.
’
’
1 ’
““‘I
Li-like Cal’+ to He-like Cal*+. Note that a,,, decreases by approximately a factor of 2 in going from Ca16+ to Cal” and from Cal*+ to Cal’+, which can be accounted for by the reduction of the number of electrons remaining in the L and K shells, respectively, i.e., from 2 to 1. This effect is also evident in K,, in going from Cala+ to Ca19+. For charge states lower than Cal*+ , uKap decreases slowly with decreasing charge state due to the fuller L-shell for the lower charge states. The loss-and-excitation cross sections ugib exhibit a pronounced decrease between Ca16+ and Cal’+. This decrease may be associated with the decrease in uq+i, since uxaS remains relatively unchanged in going from Ca16+ to Ca “+ . The relative magnitude of the decrease in u&‘,fr is much larger than that in uq+i. This is particularly interesting since the increase in the fluorescence yield from the final charge states Cal’+ and Cala+ (where the fluorescence yield should be one) would be expected to reduce the change in magnitude of u$zi in going from the incident ion Ca16+ to Cal’+. This may indicate a correlation between the loss and excitation events in the collision process. An important feature of the data [lo] is that ua:k is significantly less than a,+ This suggests that, for the cases shown, projectile K X-ray production is through the excitation, rather than the removal, of a 1s electron, since if a 1s electron were lost it would be observed in the loss and excitation coincidence channel.
‘.....* Uq+l
250 MeV Cd+
I
10-2'
E
10-?~o’
coincidence measurements
’
i ’
PROJECTILE
’
” 15 CHARGE
\,A
’
. “‘IJ20
STATE
Fig. 3. Charge state dependence of cross sections for 250 MeV Ca4+ in H, and He. Solid symbols, H, target; Open symbols, He target. Lines are to guide the eye 3 a,+13 uKafl~
.-.-.,
a+$;;.
terns and at the same time as, the RTE measurements. Electron-capture cross sections in this energy range typically decrease rapidly with increasing projectile energy [11,12]. The electron-loss and electron-excitation cross sections have a much weaker energy dependence and generally they exhibit a broad maximum, as predicted by theory, at a velocity between v, and 2v,, where u, is the velocity of the electron most likely to be lost or excited in the projectile, and v is the projectile velocity. The relative magnitudes of electron-capture and electron-loss cross sections depend strongly on the charge state of the ion [13]. The cross section crgzh, generally exhibits a similar weak energy dependence [lo], with a broad maximum being found around an energy corresponding to v = v,. At lower charge states multiple ionization of the projectile must also be considered. Two-electron loss processes can be studied in the present apparatus and it has been found that u4+* is consistently approximately two orders of lower than a,+i [14]. Preliminary analysis of data for u$$ suggest that for Cal*+ this is typically 25 times smaller than OR”,,& The charge-state dependence of the cross sections can be illustrated by considering measurements at one energy. Cross sections for Caqi (q = 12 to 19) in H, and He at 250 MeV are presented in fig. 3. The main features are the changes in the magnitude of the cross sections at shell boundaries. For example, u,+i decreases by almost an order of magnitude in going from
4. Projectile ion/target
X-ray coincidences
If heavy gas targets are used, e.g. Ar, target K or L X-rays can be detected in coincidence with the projectile ion when a vacancy in the appropriate K or L shell of the target is created. This allows the cross section for the capture of a target K or L-shell electron to be measured. When target X-ray production is studied in coincidence with projectile loss the probability of vacancy production in both the target and projectile in a single collision can be obtained. Early work on the projectile ion-target/X-ray coincidence technique indicated that it could be important in studying K-vacancy sharing during a collision [15]. Work is presently underway to examine these processes in more detail.
5. Conclusion Ion-X-ray coincidence techniques provide a powerful method of identifying and measuring the “well defined” collision cross sections required for a fundamental understanding of atomic interactions. The use of such techniques has led to the discovery of resonanttransfer and excitation and shows promise in investigating many other processes such as radiative electron
W.G. Graham et al. / Ion - X-ray coincidence measurements
capture, loss and excitation vacancy sharing.
in a single collision and
[91R. Anholt,
References [II J.A. Tanis, E.M. Bernstein, W.G. Graham, M. Clark, SM. Shafroth, B.M. Johnson, K.W. Jones and M. Meron. Phys. Rev. Lett. 49 (1982) 1325. 121 J.A. Tanis, E.M. Bernstein, W.G. Graham, M.P. Stockli, M. Clark, R.H. McFarland, T.J. Morgan, K.H. Berkner, AS. Schlachter and J.W. Steams, Phys. Rev. Lett. 53 (1984) 2551. [31 J.A. Tanis, E.M. Bernstein, M.W. Clark, W.G. Graham, R.H. McFarland, T.J. Morgan, B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. A31 (1985) 4040. 141 M. Clark, D. Brandt, J.K. Swenson and S.M. Shafroth, Phys. Rev. Lett. 54 (1985) 544. [51 E.M. Bernstein, M.W. Clark, J.A. Tanis, W.G. Graham, R.H. McFarland, J.R. Mowat, D.W. Mueller, M.P. Stockli, K.H. Berkner, R.J. McDonald, A.S. Schlachter and J.W. Steams, to be published in Nucl. Instr. and Meth. B. [61 W.G. Graham, E.M. Bernstein, M.W. Clark, J.A. Tanis, K.H. Berkner, P. Gohil, R.J. McDonald, AS. Schlachter, J.W. Steams, R.H. McFarland, T.J. Morgan and A. Muller, Phys. Rev. A33 (1986) 3591. H.D. Betz, J.P. Delvaille, K. Kalata, [71 H.W. Schnopper, A.K. Sohval, K.W. Jones and H.E. Wegner, Phys. Rev. Lett. 29 (1972) 898. R. Anholt, J. Eichler, H. Gould, Ch. 181 W.E. Meyerhof, Munger, J. Alonso, P. Thieberger and H.E. Wegner, Phys. Rev. A32 (1985) 3291.
13
[III
[I21
[131
[I41
[151
S.A. Andriamonje, E. Morenzoni, Ch. Stoller, J.D. Molitoris, W.E. Meyerhof, H. Bowman J.S. Xu, 2.2. Xu, J.O. Rasmussen and D.H.H. Hoffmann, Phys. Rev. Lett. 53 (1984) 234. W.G. Graham, E.M. Bernstein, M.W. Clark, J.A. Tanis, K.H. Berkner, R.J. McDonald, AS. Schlachter, J.W. Steams, R.H. McFarland, T.J. Morgan, A. Muller and M.P. Stockli, to be published in Nucl. Instr. and Meth. B. A.S. Schlachter, J.W. Steams, W.G. Graham, K.H. Berkner, R.V. Pyle and J.A. Tanis, Phys. Rev. A27 (1983) 3372. M.W. Clark, E.M. Bernstein, J.A. Tanis, W.G. Graham, R.H. McFarland, T.J. Morgan, B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. A33 (1986) 762. W.G. Graham, K.H. Berkner, E.M. Bernstein, M. Clark, R.H. McFarland, T.J. Morgan, AS. Schlachter, J.W. Steams, M.P. Stockli and J.A. Tanis, J. Phys. B18 (1985) 2503. J.A. Tanis, E.M. Bernstein, M.W. Clark, W.G. Graham, R.H. McFarland, T.J. Morgan, A. Muller, M.P. Stockli, K.H. Berkner, P. Gohil. A.S. Schlachter, J.W. Steams, B.M. Johnson, K.W. Jones, M. Meron and J. Nason, 2nd US-Mexico Symp. on Atomic and Molecular Physics: Two Electron Phenomena, Cocoyoc, Mexico (1986) to be published. J.A. Tanis, E.M. Bernstein, W.G. Graham, M. Clark, S.M. Shafroth, B.M. Johnson, K. Jones and M. Meron. XIII Int. Conf. on the Physics of Electronic and Atomic Collisions, Berlin (1983) Abstracts of Contributed Papers, p. 443.
I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
ION-X-RAY
in Physics
COINCIDENCE
Research
B24/25
(1987) 69-73
MEASUREMENTS
69
IN HEAVY ION COLLISIONS
*
W.G. GRAHAM Physics Department,
Universi(y of Ulster, Coleraine BT52 ISA,
E.M. BERNSTEIN, Department
of Physics,
K.H. BERKNER, Lawrence
Western Michigan
and
Berkeley L.aborato~,
and
MT 49008, USA
J.W. STEARNS
and
lJniversi(v of Culifornia,
K.W. JONES
National Laboratory,
J.A. TANIS
Universit.v, Kalamuzoo,
A.S. SCHLACHTER
B.M. JOHNSON, Brookhuven
M.W. CLARK
N. Irelund
Berkelqv, CA 94720, USA
M. MERON
Upton, NY I 1973, USA
R.H. MCFARLAND Department
of Physics, University of Missouri, Rolla, MI 65401, USA
T.J. MORGAN Physics Department,
Wesleyun University, Middletown,
CT 06457, USA
M.P. STOCKLI Department
of Physics, Kunsas Stute Universi(v, Munhuttan,
KS 66506, USA
X-ray emission associated with projectile charge-changing events in fast ion-atom collisions can be used to isolate and investigate excitation, ionization and charge transfer, as well as combinations of these processes. The major emphasis to date has been on the study of two-electron processes. Several such processes have been studied in detail. Resonant and nonresonant transfer and excitation (RTE and NTE) occur when electron capture and projectile excitation, with stabilization by X-ray emission, take place together in a single collision. Loss and excitation (LE) occurs when a projectile electron is removed and the ion is excited in a single collision. The cross sections for loss and excitation have been measured for a wide range of highly stripped projectiles over a broad energy range. The results shed light on projectile K X-ray production processes in heavy-ion collisions. The use of ion-X-ray coincidence techniques in exploring these and other collision processes will be discussed.
1. Introduction X-ray emission associated with projectile chargechanging events in fast ion-atom collisions can be used to isolate and investigate excitation, ionization and charge transfer, as well as combinations of these processes with an overall goal of understanding funda-
* This work is supported in part by the Science and Engineering Research Council, Great Britain and the US DOE, Division of Chemical Sciences and Applied Plasma Physics. 0168-583X/87/$03.50 0 Hlsevier (North-Holland Physics Publishing
Science Publishers Division)
B.V
mental atomic interactions. The major emphasis to date has been on the study of two-electron processes, with the projectile ions and projectile X-rays being detected in coincidence. In particular this has led to the discovery of resonant transfer and excitation [1,2]. The techniques are now being extended by including measurements of the target ions and target X-rays with capabilities for X-ray/X-ray coincidences and target recoil ion/target or projectile X-ray coincidences. In this paper some of our current measurements will be reviewed and the extension of these techniques to other coincidence partners discussed. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
70
W.G. Graham et al. / Ion - X-rq
2. Experimental procedure
+ A(q+P)*+ + [B]
The measurements reported here were obtained using the SuperHILAC accelerator at the Lawrence Berkeley Laboratory and the tandem Van de Graaff facility at the Brookhaven National Laboratory. The apparatus and experimental techniques have been previously described [1,2]. Briefly, a well collimated beam of ions of the desired energy and charge state is incident on the differentially pumped target gas cell, where the gas pressure is measured using a capacitance manometer. X-rays produced in the target region from the projectile ions or target atoms are detected using one of two Si(Li) detectors. The beam leaving the target gas cell is charge state analyzed using either magnetic (LBL) or electrostatic (BNL) deflection. The charge-changed components of the beam are detected using an array of solidstate detectors, although there are plans to replace these detectors with position sensitive wire counters. At present the intense non-charge-changed component of the emerging beam is collected in a Faraday cup. The cross sections are obtained from the dependence on target gas thickness of the fractional yield of ions in a given charge state, X-rays, or coincident events as compared to the total incident beam. The linear dependence of the fractional yields on gas pressure is used to ensure that single collision conditions prevail. The X-ray detectors are generally able to resolve the K, (n=2+l)andKp (n=3+1)componentsof the projectile K X-ray spectra so that the relative contributions of K, and K, to capture and excitation, loss and excitation, and excitation alone can be determined. If sufficiently heavy projectiles or targets are used X-rays from transitions to the L shell can also be observed. The relative uncertainties in the measurements can be determined from one standard deviation of the least squares fit to the data and are generally of the order of - 5% for single-electron capture and loss u4,1 and total projectile and target X-ray production o,,~; and of the order of *15% for projectile ion X-ray coincidence measurements oft:;. Systematic uncertainties mainly arise in the determination of the target thickness and the X-ray detection efficiency and solid angle leading to overall uncertainties in the absolute cross sections of about + 12% for u,,l, *20’% for uKaP and k258 for a$:;.
3. Projectile charge state/projectile
X-ray coincidence
In measurements of projectile charge state/projectile K X-ray coincidences the following processes can be studied: Aq+ +fB -+ A(qPJ’)*+ + [B] -+A4+*+[B]
coincidence meusurements
UgP,
(1)
UR,
(2)
u$+“,
(3)
where A is the projectile ion, B the target atom and q is the incident projectile charge state; p is normally one but measurements with p = 2 have been made though as yet unpublished. The square brackets indicate that no information on the final state of the target atom is available. The intermediate excited states formed in the above processes subsequently decay by either photon emission or Auger electron emission. In this paper cases where an X-ray is emitted are considered. If the atomic number of the projectile is sufficiently large, detectors allow the K and L X rays to be observed and the OLand p lines to be resolved. The total K X-ray production cross section uknp normally reported in the literature is the sum of processes (l), (2) and (3). A measurement of the X-ray emission in coincidence with the incident charge state q after the target cell allows the cross sections for projectile excitation alone to be determined. This is a more fundamental cross section and more useful in comparisons with theoretical calculations. Comparing the sum of the separately determined cross sections for processes (l), (2) and (3) with the total X-ray production cross section would provide a good experimental verification of the coincidence measurements. As yet X-ray coincidence measurements with the main beam have not been reported since detectors which can handle the high count rates required of the mainbeam are needed. With the use of the wire counter in atomic physics experiments, many new applications of these detectors will be possible. The collisions described in eqs. (1) and (3) are two-electron processes involving projectile excitation and either electron capture or electron loss. Coincidence techniques are particularly useful in studying these processes since they are then isolated from such singleelectron processes as “pure” capture or excitation. 3.1. Electron capture and excitation The measurement of electron capture in coincidence with both K and L X-ray production, process (l), has received considerable attention [l-5]. This represents a collision in which an electron is captured and the projectile excited in a single encounter. A typical energy dependence of u&-’ is shown in fig. 1, in this case for S13+ in He [3]. The maximum near 130 MeV is attributed to resonant-transfer and excitation, RTE, while the low energy maximum near 30 MeV is evidence of nonresonant-transfer and excitation, NTE. RTE is the simultaneous electron capture and excitation in the projectile ion in a single collision as a result of an electron-electron interaction. Since its recent discovery [1,2] this particular process has been studied in
W.G. Graham et al. / Ion - X-ray coincidence measurements
71
certain energies. RTE has also been observed to make a sizeable contribution to the total electron capture cross section [6]. 3.2. Radiative electron capture (REC)
.\ i \ \*. J..
60
I
60
I
I
I
Another form of electron capture is radiative electron capture (REC) [7]. While occurring to some extent in all collision systems, REC is found to be the dominant form of electron capture in very fast, highlystripped ion collisions with light targets [8]. REC is the inverse of the photoelectric effect and it occurs when an electron is captured to an inner shell with the simultaneous emission of a photon. The X-ray spectrum for this process has been observed in several experiments but because of the small relative strength of REC compared with total X-ray production higher density solid targets have been used [9] where multiple collision effects can complicate the interpretation. As shown in fig. 2, observing the spectra in the electron capture/X-ray coincidence channel substantially improves the REC signal in the spectra and allows measurements of REC cross sections to be made in gas targets.
. I
3.3. Electron loss and excitation
100 120 140 160 160 20C
E(MeV) Fig. 1. Projectile K X-ray cross sections for 15-200 MeV SL3+ + He collisions. cKoS is the cross section for the total sulphur K X-ray production and u&$ is the cross section for sulphur K X-rays coincident with single electron capture. The tnaxunum in u&,k near 130 MeV is attributed to RTE and that near 30 MeV is attributed to NTE. The dashed curve is a calculated RTE cross section (ref. [3]).
Another two-electron process is one in which one projectile electron is lost and the ion is excited in the single collision of a highly-stripped heavy ion with an atom, i.e. loss and excitation in a single collision (LE) [lo] (process (3)). The cross sections for electron loss and excitation, u$Lj have been measured with the same collision sys-
SINGLES
some detail and is the subject of another paper in this session. NTE [3,4] is a two-step process in which two independent interactions occur in a single encounter. The mechanisms involved in this case are electron-nucleus interactions (1) between the target nucleus and a projectile electron resulting in excitation and (2) between the projectile nucleus and a target electron resulting in electron capture. Such a combination of excitation and capture events does not depend resonantly on the incident projectile velocity, however NTE does exhibit a maximum in its energy dependence. Qualitatively this may be viewed as a result of the product of an increasing excitation cross section and a decreasing singleelectron-capture cross-section. Results of measurements of RTE and NTR indicate that these processes can account for a sizeable fraction of the K-shell excitation events at certain energies e.g. in the present case, of St3+ in He, NTE contributes approximately 15% to the total K,, cross section at
3
COINCIDENCE
$
X-RAY
ENERGY
(arb..units)
Fig. 2. Typical X-ray spectra for 260 MeV Ca19+ +He. Total X-rays, (b) X-rays in coincidence with Ca*‘+. I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
(a)
W. G. Graham et al. / Ion - X-q
72
II
’
’
.
’
’
1 ’
““‘I
Li-like Cal’+ to He-like Cal*+. Note that a,,, decreases by approximately a factor of 2 in going from Ca16+ to Cal” and from Cal*+ to Cal’+, which can be accounted for by the reduction of the number of electrons remaining in the L and K shells, respectively, i.e., from 2 to 1. This effect is also evident in K,, in going from Cala+ to Ca19+. For charge states lower than Cal*+ , uKap decreases slowly with decreasing charge state due to the fuller L-shell for the lower charge states. The loss-and-excitation cross sections ugib exhibit a pronounced decrease between Ca16+ and Cal’+. This decrease may be associated with the decrease in uq+i, since uxaS remains relatively unchanged in going from Ca16+ to Ca “+ . The relative magnitude of the decrease in u&‘,fr is much larger than that in uq+i. This is particularly interesting since the increase in the fluorescence yield from the final charge states Cal’+ and Cala+ (where the fluorescence yield should be one) would be expected to reduce the change in magnitude of u$zi in going from the incident ion Ca16+ to Cal’+. This may indicate a correlation between the loss and excitation events in the collision process. An important feature of the data [lo] is that ua:k is significantly less than a,+ This suggests that, for the cases shown, projectile K X-ray production is through the excitation, rather than the removal, of a 1s electron, since if a 1s electron were lost it would be observed in the loss and excitation coincidence channel.
‘.....* Uq+l
250 MeV Cd+
I
10-2'
E
10-?~o’
coincidence measurements
’
i ’
PROJECTILE
’
” 15 CHARGE
\,A
’
. “‘IJ20
STATE
Fig. 3. Charge state dependence of cross sections for 250 MeV Ca4+ in H, and He. Solid symbols, H, target; Open symbols, He target. Lines are to guide the eye 3 a,+13 uKafl~
.-.-.,
a+$;;.
terns and at the same time as, the RTE measurements. Electron-capture cross sections in this energy range typically decrease rapidly with increasing projectile energy [11,12]. The electron-loss and electron-excitation cross sections have a much weaker energy dependence and generally they exhibit a broad maximum, as predicted by theory, at a velocity between v, and 2v,, where u, is the velocity of the electron most likely to be lost or excited in the projectile, and v is the projectile velocity. The relative magnitudes of electron-capture and electron-loss cross sections depend strongly on the charge state of the ion [13]. The cross section crgzh, generally exhibits a similar weak energy dependence [lo], with a broad maximum being found around an energy corresponding to v = v,. At lower charge states multiple ionization of the projectile must also be considered. Two-electron loss processes can be studied in the present apparatus and it has been found that u4+* is consistently approximately two orders of lower than a,+i [14]. Preliminary analysis of data for u$$ suggest that for Cal*+ this is typically 25 times smaller than OR”,,& The charge-state dependence of the cross sections can be illustrated by considering measurements at one energy. Cross sections for Caqi (q = 12 to 19) in H, and He at 250 MeV are presented in fig. 3. The main features are the changes in the magnitude of the cross sections at shell boundaries. For example, u,+i decreases by almost an order of magnitude in going from
4. Projectile ion/target
X-ray coincidences
If heavy gas targets are used, e.g. Ar, target K or L X-rays can be detected in coincidence with the projectile ion when a vacancy in the appropriate K or L shell of the target is created. This allows the cross section for the capture of a target K or L-shell electron to be measured. When target X-ray production is studied in coincidence with projectile loss the probability of vacancy production in both the target and projectile in a single collision can be obtained. Early work on the projectile ion-target/X-ray coincidence technique indicated that it could be important in studying K-vacancy sharing during a collision [15]. Work is presently underway to examine these processes in more detail.
5. Conclusion Ion-X-ray coincidence techniques provide a powerful method of identifying and measuring the “well defined” collision cross sections required for a fundamental understanding of atomic interactions. The use of such techniques has led to the discovery of resonanttransfer and excitation and shows promise in investigating many other processes such as radiative electron
W.G. Graham et al. / Ion - X-ray coincidence measurements
capture, loss and excitation vacancy sharing.
in a single collision and
[91R. Anholt,
References [II J.A. Tanis, E.M. Bernstein, W.G. Graham, M. Clark, SM. Shafroth, B.M. Johnson, K.W. Jones and M. Meron. Phys. Rev. Lett. 49 (1982) 1325. 121 J.A. Tanis, E.M. Bernstein, W.G. Graham, M.P. Stockli, M. Clark, R.H. McFarland, T.J. Morgan, K.H. Berkner, AS. Schlachter and J.W. Steams, Phys. Rev. Lett. 53 (1984) 2551. [31 J.A. Tanis, E.M. Bernstein, M.W. Clark, W.G. Graham, R.H. McFarland, T.J. Morgan, B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. A31 (1985) 4040. 141 M. Clark, D. Brandt, J.K. Swenson and S.M. Shafroth, Phys. Rev. Lett. 54 (1985) 544. [51 E.M. Bernstein, M.W. Clark, J.A. Tanis, W.G. Graham, R.H. McFarland, J.R. Mowat, D.W. Mueller, M.P. Stockli, K.H. Berkner, R.J. McDonald, A.S. Schlachter and J.W. Steams, to be published in Nucl. Instr. and Meth. B. [61 W.G. Graham, E.M. Bernstein, M.W. Clark, J.A. Tanis, K.H. Berkner, P. Gohil, R.J. McDonald, AS. Schlachter, J.W. Steams, R.H. McFarland, T.J. Morgan and A. Muller, Phys. Rev. A33 (1986) 3591. H.D. Betz, J.P. Delvaille, K. Kalata, [71 H.W. Schnopper, A.K. Sohval, K.W. Jones and H.E. Wegner, Phys. Rev. Lett. 29 (1972) 898. R. Anholt, J. Eichler, H. Gould, Ch. 181 W.E. Meyerhof, Munger, J. Alonso, P. Thieberger and H.E. Wegner, Phys. Rev. A32 (1985) 3291.
13
[III
[I21
[131
[I41
[151
S.A. Andriamonje, E. Morenzoni, Ch. Stoller, J.D. Molitoris, W.E. Meyerhof, H. Bowman J.S. Xu, 2.2. Xu, J.O. Rasmussen and D.H.H. Hoffmann, Phys. Rev. Lett. 53 (1984) 234. W.G. Graham, E.M. Bernstein, M.W. Clark, J.A. Tanis, K.H. Berkner, R.J. McDonald, AS. Schlachter, J.W. Steams, R.H. McFarland, T.J. Morgan, A. Muller and M.P. Stockli, to be published in Nucl. Instr. and Meth. B. A.S. Schlachter, J.W. Steams, W.G. Graham, K.H. Berkner, R.V. Pyle and J.A. Tanis, Phys. Rev. A27 (1983) 3372. M.W. Clark, E.M. Bernstein, J.A. Tanis, W.G. Graham, R.H. McFarland, T.J. Morgan, B.M. Johnson, K.W. Jones and M. Meron, Phys. Rev. A33 (1986) 762. W.G. Graham, K.H. Berkner, E.M. Bernstein, M. Clark, R.H. McFarland, T.J. Morgan, AS. Schlachter, J.W. Steams, M.P. Stockli and J.A. Tanis, J. Phys. B18 (1985) 2503. J.A. Tanis, E.M. Bernstein, M.W. Clark, W.G. Graham, R.H. McFarland, T.J. Morgan, A. Muller, M.P. Stockli, K.H. Berkner, P. Gohil. A.S. Schlachter, J.W. Steams, B.M. Johnson, K.W. Jones, M. Meron and J. Nason, 2nd US-Mexico Symp. on Atomic and Molecular Physics: Two Electron Phenomena, Cocoyoc, Mexico (1986) to be published. J.A. Tanis, E.M. Bernstein, W.G. Graham, M. Clark, S.M. Shafroth, B.M. Johnson, K. Jones and M. Meron. XIII Int. Conf. on the Physics of Electronic and Atomic Collisions, Berlin (1983) Abstracts of Contributed Papers, p. 443.
I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA