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Characteristic and quasimolecular X-rays from collision systems with Z1 + Z2 = 133, 134 under single and multiple collision conditions
Volume 62A, number 6
PHYSICS LETTERS
19 September 1977
CHARACTERISTIC AND QUASIMOLECULAR X-RAYS FROM COLLISION SYSTEMS WITH Z1 + = 133, 134 UNDER SINGLE AND MULTIPLE COLLISION CONDITIONS G. KRAFT, P. ARMBRUSTER, W. ENDERS, F. FOLKMANN, S. HAGMANN, Ch. HEITZ
*
and P.H. MOKLER
GSI, Postfach 541, D-6100 Darmstadt, F.R. Germany Received 16 June 1977 Measurements with Hg-vapor targets bombarded with 18 MeV to 150 MeV xenon ions show that M-MO X-rays are excited in a single collision, and target L-lines are suppressed in gas targets compared to solids, furthermore, projectile L-lines show a pressure dependent energy shift of 50 eV between iO~ and 1 Torr.
In the first experiments showing the existence of molecular X-rays in the Ar-Ar system, Saris et al. [1] studied the excitation mechanism by investigating gaseous solid targets. the densitya dependence of the and molecular X-rays From they concluded double collision process for creating inner shell vacancies: In a second collision the surviving fraction of excited projectile atoms produced in a first collision can transfer vacancies via molecular orbitals into deeper shells. De-excitation during the collision causes the MO Xrays, dc-excitation after the collision contributes to the characteristic lines. In heavy collision systems like I Au a broad line of M MO X-rays has been observed and interpreted [2] in the framework of double collision processes. As previous measurements [3] with diluted gold targets allowed no unique conclusions, to test this assumption we have measured the X-ray spectra in gaseous targets. Here, the density is about 10—6 smaller than in solid targets suppressing efficiently contributions from double collision processes. In the experiment Hg vapor was bombarded with 18 to 150 MeV Xe ions from the UNILAC in Darmstadt. The target was a closed cell with an 1 pm Feentrance window for the Xe beam and a 31 pm thick Hostaphan window perpendicular to the beam direction for the 80 mm2 Si(Li) X-ray detector which has a resolution better than 200 eV at 6 keV. The vapor pressure could be varied by heating from 20°to 120° centigrade corresponding to l0~ to 1 torr. To avoid electronic pile up, the counting rate was held at about 200 counts/per second at a duty cycle of 10% by using —+
*
Permanent address: CRN Strasbourg, France.
low beam currents and inserting an 18 pm Al absorber between gas cell and the Si(Li) detector. The spectra of the gas targets are compared with the 2corresponding Au target. spectra measured within a solid pg/cm The spectra shown fig. 1 100 are the raw data without any absorption correction. To evaluate intensity ratios the different characteristic lines are fitted [4] with gaussian distributions and the absorption corrections are done for the center of the lines. To compare the different MO continua, which change their shape and position with impact energy and target Z, the spectra are corrected channel by channel for absorption, integrated over a region where no characteristic line gives a significant contribution to the, counting rate, and divided by the integration width of about 2 keV giving an average number of counts per keV over the MO region. Although it was not possible to get an absolute normalisation due to galvanic currents arising from heating the target, several conclusions can be drawn comparing the different spectra and intensity ratios of solid and gaseous targets. 1) At low energy the MO X-rays are found for solid and gaseous targets (fig. la, Ib) with comparable intensity relative to the Xe L-lines. This fact is mdcpendent of pressure. Even at the very low Hg pressure of l0—~torr, corresponding to a density ratio between solid and gas of i0~ the yield of the MO X-rays is not suppressed. That means, the MO X-rays are strongly excited in a single collision. With higher energies the yield of the MO X-rays relative to the Xe L-yield decreases rapidly in the gas target (fig. Ic) and in the solid target the MO region is then covered by X-rays coming from other effects as e.g. radiative electron 409
Volume 62A, number 6 3
4
~
5 0
PHYSICS LETTERS
6
7
8
I
9
10
I
11
12
I
13
19 September 1977
14
I
~ e ,..Au solid target) ~ Hg vapor target)
10
iü~ 1
](L,M~)J(L
-.
18MeV Xe on Hg
~
~ Z
S
101
~ It i ((It
:~~ 102
18MeV Xe on Au
1M3) J (L3M1,5)
lOo
~~
~~~ j r”
10
::
~
Xe
~
~
Au
)(L2M±J(L.M~
Hg
1 L~ M~~I (LiM J(L3M~)
1tXIMeV XeonHg
(
•‘
counts
~normalized to lOl
101
the Xe L-Lnes 18
I
3
4
5
I
I
6
7
8
9
ENERGY
11
13
~°
14
IkeVi
rig. 1. X-ray spectra of Hg and Au targets. a) 18 MeV Xe ions on Hg 2vapor of —1 c) Torr. 18 MeV Xe on ionsHgonvapor an 100 Au target. 100b) MeV Xe ions of ~O.2 Torr. Mg/cmAll the spectra are taken with an 18 ~.smAl absorber and are not corrected for absorption in chamber and detector windows,
capture [5] (fig. ic). These points in fig. 2 are in brackets, 2) At low gas pressure and small collision energy (fig. la) the target L-intensity, normalized to the Xe L-lines, is much smaller than at higher energy or in the solid target (fig. lb). As shown in fig. 2 the yield of the Hg L-radiation at low energy is about 20% of the corresponding solid target value. This indicates that in the solid target the target L-lines are mainly a consequence of excitation by multiple collisions. It is sufficient that only a small fraction of the Xe projectiles are carrying vacancies from one collision to the next, during which these holes are transferred to the target L-shell. With increasing energy the number of projectile holes as well as the chance to survive will be greater 410
54 105154
MeV Projectile Energy
Fig. 2. Intensities of the different characteristic lines and the MO continuum normalized to the Xe L-lmes at —1 Torr pressure in the Hg target. .
.
.
.
.
which2).will a higher intensity of target L-radiation (fig. Onyield the other hand, direct ionisation in gases and solids increases with higher collision energies. 3) For the Xenon L-lines the intensity ratio of LN to LM transitions is a factor of 2 greater with solid than with gas targets (fig. 2). Additionally, the L N transitions (L 3N5 at about 5 keV; L2N4 and L1N23 at about 5.5 key) are well separated lines (fig. la, c) while in the solid target (fig. lb) the lines are broader and poorly resolved. All the data are normalized to the projectile L-lines and therefore the comparison between solid and gaseous target may be influenced by a difference in the fluorescence yield of the L-shell. This fluorescence yield depends mainly on the number of M-holes [6] and it should not vary too much if only outer electrons are excited or stripped off. The following analysis of the Xe L-line positions gives evidence that the M-shell configuration of Xe in Hg vapor at higher pressure and in solids is the same, supporting our normalisation procedure.
Volume 62A, number 6
PHYSICS LETTERS
154 MeV Xe
1eV]
—
Hg (vapor)
40 20
~-
~4
4600
19 September 1977
-~
/
~
AeIL
2M4~LlM23
T4~
SolId Target Value
leVi
~_jJ_-
1,0
Xe
1L3 M45)
2
4200 20
30
40
ifi
50
1 60
70
80
90
100
110
120
130
Torr 140
°C temperature
Fig. 3. Energy-shift of the xenon L M transitions as a function of target pressure.
After the Fe entrance-window the xenon projectiles have a charge distribution with the maximum at l2~ and 18 MeV and 29~’at 150 MeV. Hence, the N shell should be empty at 150 MeV and no L N transition should occur if only a single L vacancy were to be produced in the collision. But it has been shown [6] that after violent collisions creating L shell vacancies the charge state is much higher than the equilibrium value. In collisions with the excitation of one Lelectron, some M-electrons must be simultaneously ionized or transferred to higher shells allowing the observed L N transitions. Moreover, M electrons can be transferred to the N shell by more probable soft collisions leading to no Lvacancies. At low pressure the M holes have time to deexcite between two collisions. At higher pressure the time between the soft collisions shortens and more M holes have a chance to survive until the next collision. The more violent collisions producing an Lhole will start from a different state of the M shell. This change of the occupation of the M shell is reflected by an energy shift of the L M transition. The experiment shows (fig. 3) that the main variation of the energy of the projectile lines occurs at a pressure of about 10—1 Torr corresponding to 3.5 X 1015 atoms/cm3. Above this pressure the number of M holes has reached its solid target value. At all measured impact energies from 18 MeV to 150 MeV the solid target value of ti~eL M transition energy was reached above 100°centigrade or 3 X 10—1 Torr. The time t between two collisions changing the M-shell configu‘
ration is given by t = (n v a)~,where n is the target density, v is the velocity and a the cross section for M hole production, which is about 10—16 cm2 [7]. For the projectile energies used one obtains times between 10—10 and I 0~ sec as life times of the Mshells configurations produced in the soft collisions. Long lived isomeric states or outer shell transitions govern the L-shell transition energies. With higher density an equilibrium state of collision induced transfer of M electrons and holes is established, but more and more configurations of outer electrons are p05sible as spectator states. These will not vary the main transition energy of the L M lines and the fluorescence yield very much but will broaden the observed L-lines as can be seen by comparing solid target and gas measurement (fig. la, lb). In conclusion the measurements show that the M-MO continuum radiation emitted during heavy ion collisions results predominantly from single collision processes. However, these results also show that multiple collision processes are influencing the outer shell occupation, as well as the production of L-shell vacancies of the heavier partner. One of us (GK.), would like to acknowledge the tolerance of his wife and the patience of his twins for waiting to be born until this experiment was working successfully. We would also like to thank J.R. Macdonald, D. Liesen and A. Warczak for many helpful discussions.
411
Volume 62A, number 6
PHYSICS LETTERS
References 11] F.W. Saris, W.F. van der Weg, H. Tawara and R. Lambert, Phys. Rev. Lett. 28 (1972) 717. 12] P.H. Mokler, H.J. Stein and P. Armbruster, Phys. Rev. Lett. 29 (1972) 827. 131 P. Armbrustcr, in Proc. 2nd Intern. Conf. on Inner Shell Ionization Phenomena, eds. W. Mehlhorn and R. Brenn (Freiburg, 1976) p. 21.
412
14] W.
19 September 1977
Schuh, Institut für Kernphysik, KOin has written the computer program. [5] AR. Sohval, J.P. Delvaile, K. Kalata and H.W. Schnopper, J. Phys. B9 (1976) L 47. [6] S. Hagmann, Thesis K’ôln, 1977 and GSI report Pa-1-77. [7] HO. Lutz, H.J. Stein and C.D. Moak, Phys. Rev. Lett. 28 (1972) 8.
PHYSICS LETTERS
19 September 1977
CHARACTERISTIC AND QUASIMOLECULAR X-RAYS FROM COLLISION SYSTEMS WITH Z1 + = 133, 134 UNDER SINGLE AND MULTIPLE COLLISION CONDITIONS G. KRAFT, P. ARMBRUSTER, W. ENDERS, F. FOLKMANN, S. HAGMANN, Ch. HEITZ
*
and P.H. MOKLER
GSI, Postfach 541, D-6100 Darmstadt, F.R. Germany Received 16 June 1977 Measurements with Hg-vapor targets bombarded with 18 MeV to 150 MeV xenon ions show that M-MO X-rays are excited in a single collision, and target L-lines are suppressed in gas targets compared to solids, furthermore, projectile L-lines show a pressure dependent energy shift of 50 eV between iO~ and 1 Torr.
In the first experiments showing the existence of molecular X-rays in the Ar-Ar system, Saris et al. [1] studied the excitation mechanism by investigating gaseous solid targets. the densitya dependence of the and molecular X-rays From they concluded double collision process for creating inner shell vacancies: In a second collision the surviving fraction of excited projectile atoms produced in a first collision can transfer vacancies via molecular orbitals into deeper shells. De-excitation during the collision causes the MO Xrays, dc-excitation after the collision contributes to the characteristic lines. In heavy collision systems like I Au a broad line of M MO X-rays has been observed and interpreted [2] in the framework of double collision processes. As previous measurements [3] with diluted gold targets allowed no unique conclusions, to test this assumption we have measured the X-ray spectra in gaseous targets. Here, the density is about 10—6 smaller than in solid targets suppressing efficiently contributions from double collision processes. In the experiment Hg vapor was bombarded with 18 to 150 MeV Xe ions from the UNILAC in Darmstadt. The target was a closed cell with an 1 pm Feentrance window for the Xe beam and a 31 pm thick Hostaphan window perpendicular to the beam direction for the 80 mm2 Si(Li) X-ray detector which has a resolution better than 200 eV at 6 keV. The vapor pressure could be varied by heating from 20°to 120° centigrade corresponding to l0~ to 1 torr. To avoid electronic pile up, the counting rate was held at about 200 counts/per second at a duty cycle of 10% by using —+
*
Permanent address: CRN Strasbourg, France.
low beam currents and inserting an 18 pm Al absorber between gas cell and the Si(Li) detector. The spectra of the gas targets are compared with the 2corresponding Au target. spectra measured within a solid pg/cm The spectra shown fig. 1 100 are the raw data without any absorption correction. To evaluate intensity ratios the different characteristic lines are fitted [4] with gaussian distributions and the absorption corrections are done for the center of the lines. To compare the different MO continua, which change their shape and position with impact energy and target Z, the spectra are corrected channel by channel for absorption, integrated over a region where no characteristic line gives a significant contribution to the, counting rate, and divided by the integration width of about 2 keV giving an average number of counts per keV over the MO region. Although it was not possible to get an absolute normalisation due to galvanic currents arising from heating the target, several conclusions can be drawn comparing the different spectra and intensity ratios of solid and gaseous targets. 1) At low energy the MO X-rays are found for solid and gaseous targets (fig. la, Ib) with comparable intensity relative to the Xe L-lines. This fact is mdcpendent of pressure. Even at the very low Hg pressure of l0—~torr, corresponding to a density ratio between solid and gas of i0~ the yield of the MO X-rays is not suppressed. That means, the MO X-rays are strongly excited in a single collision. With higher energies the yield of the MO X-rays relative to the Xe L-yield decreases rapidly in the gas target (fig. Ic) and in the solid target the MO region is then covered by X-rays coming from other effects as e.g. radiative electron 409
Volume 62A, number 6 3
4
~
5 0
PHYSICS LETTERS
6
7
8
I
9
10
I
11
12
I
13
19 September 1977
14
I
~ e ,..Au solid target) ~ Hg vapor target)
10
iü~ 1
](L,M~)J(L
-.
18MeV Xe on Hg
~
~ Z
S
101
~ It i ((It
:~~ 102
18MeV Xe on Au
1M3) J (L3M1,5)
lOo
~~
~~~ j r”
10
::
~
Xe
~
~
Au
)(L2M±J(L.M~
Hg
1 L~ M~~I (LiM J(L3M~)
1tXIMeV XeonHg
(
•‘
counts
~normalized to lOl
101
the Xe L-Lnes 18
I
3
4
5
I
I
6
7
8
9
ENERGY
11
13
~°
14
IkeVi
rig. 1. X-ray spectra of Hg and Au targets. a) 18 MeV Xe ions on Hg 2vapor of —1 c) Torr. 18 MeV Xe on ionsHgonvapor an 100 Au target. 100b) MeV Xe ions of ~O.2 Torr. Mg/cmAll the spectra are taken with an 18 ~.smAl absorber and are not corrected for absorption in chamber and detector windows,
capture [5] (fig. ic). These points in fig. 2 are in brackets, 2) At low gas pressure and small collision energy (fig. la) the target L-intensity, normalized to the Xe L-lines, is much smaller than at higher energy or in the solid target (fig. lb). As shown in fig. 2 the yield of the Hg L-radiation at low energy is about 20% of the corresponding solid target value. This indicates that in the solid target the target L-lines are mainly a consequence of excitation by multiple collisions. It is sufficient that only a small fraction of the Xe projectiles are carrying vacancies from one collision to the next, during which these holes are transferred to the target L-shell. With increasing energy the number of projectile holes as well as the chance to survive will be greater 410
54 105154
MeV Projectile Energy
Fig. 2. Intensities of the different characteristic lines and the MO continuum normalized to the Xe L-lmes at —1 Torr pressure in the Hg target. .
.
.
.
.
which2).will a higher intensity of target L-radiation (fig. Onyield the other hand, direct ionisation in gases and solids increases with higher collision energies. 3) For the Xenon L-lines the intensity ratio of LN to LM transitions is a factor of 2 greater with solid than with gas targets (fig. 2). Additionally, the L N transitions (L 3N5 at about 5 keV; L2N4 and L1N23 at about 5.5 key) are well separated lines (fig. la, c) while in the solid target (fig. lb) the lines are broader and poorly resolved. All the data are normalized to the projectile L-lines and therefore the comparison between solid and gaseous target may be influenced by a difference in the fluorescence yield of the L-shell. This fluorescence yield depends mainly on the number of M-holes [6] and it should not vary too much if only outer electrons are excited or stripped off. The following analysis of the Xe L-line positions gives evidence that the M-shell configuration of Xe in Hg vapor at higher pressure and in solids is the same, supporting our normalisation procedure.
Volume 62A, number 6
PHYSICS LETTERS
154 MeV Xe
1eV]
—
Hg (vapor)
40 20
~-
~4
4600
19 September 1977
-~
/
~
AeIL
2M4~LlM23
T4~
SolId Target Value
leVi
~_jJ_-
1,0
Xe
1L3 M45)
2
4200 20
30
40
ifi
50
1 60
70
80
90
100
110
120
130
Torr 140
°C temperature
Fig. 3. Energy-shift of the xenon L M transitions as a function of target pressure.
After the Fe entrance-window the xenon projectiles have a charge distribution with the maximum at l2~ and 18 MeV and 29~’at 150 MeV. Hence, the N shell should be empty at 150 MeV and no L N transition should occur if only a single L vacancy were to be produced in the collision. But it has been shown [6] that after violent collisions creating L shell vacancies the charge state is much higher than the equilibrium value. In collisions with the excitation of one Lelectron, some M-electrons must be simultaneously ionized or transferred to higher shells allowing the observed L N transitions. Moreover, M electrons can be transferred to the N shell by more probable soft collisions leading to no Lvacancies. At low pressure the M holes have time to deexcite between two collisions. At higher pressure the time between the soft collisions shortens and more M holes have a chance to survive until the next collision. The more violent collisions producing an Lhole will start from a different state of the M shell. This change of the occupation of the M shell is reflected by an energy shift of the L M transition. The experiment shows (fig. 3) that the main variation of the energy of the projectile lines occurs at a pressure of about 10—1 Torr corresponding to 3.5 X 1015 atoms/cm3. Above this pressure the number of M holes has reached its solid target value. At all measured impact energies from 18 MeV to 150 MeV the solid target value of ti~eL M transition energy was reached above 100°centigrade or 3 X 10—1 Torr. The time t between two collisions changing the M-shell configu‘
ration is given by t = (n v a)~,where n is the target density, v is the velocity and a the cross section for M hole production, which is about 10—16 cm2 [7]. For the projectile energies used one obtains times between 10—10 and I 0~ sec as life times of the Mshells configurations produced in the soft collisions. Long lived isomeric states or outer shell transitions govern the L-shell transition energies. With higher density an equilibrium state of collision induced transfer of M electrons and holes is established, but more and more configurations of outer electrons are p05sible as spectator states. These will not vary the main transition energy of the L M lines and the fluorescence yield very much but will broaden the observed L-lines as can be seen by comparing solid target and gas measurement (fig. la, lb). In conclusion the measurements show that the M-MO continuum radiation emitted during heavy ion collisions results predominantly from single collision processes. However, these results also show that multiple collision processes are influencing the outer shell occupation, as well as the production of L-shell vacancies of the heavier partner. One of us (GK.), would like to acknowledge the tolerance of his wife and the patience of his twins for waiting to be born until this experiment was working successfully. We would also like to thank J.R. Macdonald, D. Liesen and A. Warczak for many helpful discussions.
411
Volume 62A, number 6
PHYSICS LETTERS
References 11] F.W. Saris, W.F. van der Weg, H. Tawara and R. Lambert, Phys. Rev. Lett. 28 (1972) 717. 12] P.H. Mokler, H.J. Stein and P. Armbruster, Phys. Rev. Lett. 29 (1972) 827. 131 P. Armbrustcr, in Proc. 2nd Intern. Conf. on Inner Shell Ionization Phenomena, eds. W. Mehlhorn and R. Brenn (Freiburg, 1976) p. 21.
412
14] W.
19 September 1977
Schuh, Institut für Kernphysik, KOin has written the computer program. [5] AR. Sohval, J.P. Delvaile, K. Kalata and H.W. Schnopper, J. Phys. B9 (1976) L 47. [6] S. Hagmann, Thesis K’ôln, 1977 and GSI report Pa-1-77. [7] HO. Lutz, H.J. Stein and C.D. Moak, Phys. Rev. Lett. 28 (1972) 8.