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Total backscattering of low energy heavy ions from solid surfaces
Nuclear Instruments and Methods 170 (I980) 499-504 © North-Holland Publishing Company
499
TOTAL BACKSCATTERING OF LOW ENERGY HEAVY IONS FROM SOLID SURFACES J. BOTTIGER Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark
O. HOLCK, G. SIDENIUS The Niets Bohr Institute, University of Copenhagen, DK-2100 Copenhagen (~, Demnark
and The 1SOLDE Collaboration, CERN, Geneva, Switzerland
With the application of beams of shortlived radioactive ions from the ISOLDE facility at CERN, Geneva, systematic backscattering measurements have been made possible in broad ranges of particle elements and masses. A detailed description of the experimental principle and apparatus will be presented. Reflection coefficient data from the first series of measurements will be discussed.
1. Introduction Besides being a parameter of considerable interest for the theoretical understanding of the fundamental collision processes, the reflection coefficient for low energy ( 1 - 1 0 0 keV) heavy ions is of practical importance, especially in case of projectiles lighter than the target atoms where reflection coefficients of the order of 10% or higher may be reached. Excluding investigations with ions with masses equal to or lighter than 4He, which form a special group of interest for plasma-wall interaction in fusion research (see for example H. Vernickel [1]), to our knowledge, only one group of systematic experimental measurements of the total reflection coefficient [ 2 - 5 ] exist. Here a radiotracer technique, using long lived isotopes, was applied. The present paper describes a further development o f this technique to allow the use of shortlived isotopes from an on-line isotope separator, the ISOLDE [6] facility at CERN, Geneva, whereby the possible number of projectiles to be studied is greatly increased. A detailed discussion of the experimental results and any detailed comparison with theory [3,4,7] will not be attempted in the present paper, which is concentrated mostly on the experimental technique. A detailed discussion will be presented in subsequent papers.
2. Experimental technique The experimental procedure consists in bombarding a target, surrounded by a collector system, with a radioactive beam, and derives a reflection coefficient from the radioactivity measured on the target and the collector. In fig. 1 a schematic drawing of the main part of the experimental system is shown. The basic parts of the system have previously been used in range measurements in gases [8]. As the ISOLDE facility has to serve several beamlines simultaneously, all beam parameters are optimized for 60 keV ion energy, which is the permanent separator energy setting. Therefore, to perform systematic energy measurements a retardation or postacceleration of the ion beam is required, and the part of the system, where the actual exposure of the target to the ion beam is performed, is placed in an insulated cabinet. From the beam line the ion beam first enter the lens chamber with the retardation or post-acceleration lens. The focal strength of the system is controlled with the lens voltage, U L. The resulting ion energy, Ei, is controlled by the bias voltage, UR, on the scattering chamber, which may be varied between - 6 0 kV and +60 kV, resulting in a possible ion energy range from 120 keV down to a few keV. Ei is measured with a metal film resistor divider chain, R, and a digital voltmeter, DV. XI. SURFACE SCATTERING
J.Bdttiger et aL / Total backscattering
500
1
INSULATINGCABINET
REVOLVING ~.., PE F_L ...P.~ AT....E,.S.,........... # ) ~ ................ • .............. ..D.R,U_M..7......,.~
Fr
H
om ISOLDE~ 11
?,'idN4u~ ~
...........................
.....i'; ~
HOLDER CATCHER~ U / ~ [
]] ~
UXU
8 I ~l~-~l
[
I
=~]
~ 1
°
/
MTeJ .I~
I
[~~'~
RETARDATION" ~4 ~ C A T C H E R - I ~
LENS
~
~L
mu MR
h
M/~ J /
TT~RGET # I
"
~
~I,~I
I~
.~-...~... ,~u~,-~-_, I U IUIN ~ U U f ~ I -
/
R
~
-I '',L2;-';-J
;:, .
. ] .
I
~
L-----I ,,
l]
I,'------4,'i; ii
, :
'*~-- / Pu~P~,q~C~E I
-I"-~ CONTROL TO SEP CORITROL ~q
Fig. 1. Schematic drawing of the experimental apparatus. For details see the text.
After the retardation lens system a slight correction of the beam position is possible in a four-plate deflector system supplied from the deflector voltage box, LTd. The beam is then collimated by two 1.5 mm diameter apertures before entering the target/catcher system through a 2.5 mm diameter opening, hence any slit-edge scattering should be suppressed. The target/catcher system, described in more details later, is introduced into and removed from the main vacuum system thruogh a vacuum lock system. After having been inserted, the target/catcher system is secured into one out of three openings in a revolvable drum. When the drum is turned 120 ° , the target/ catcher system is moved to a waiting position, behind which is situated a small vacuum lock system, similar to the one shown on the figure behind the exposure position, to which the target]catcher system is brought by a further 120 ° turn of the drum. These two small vacuum lock systems allow the target to be removed or inserted independently of the catcher system and open up the possibility of either in situ cleaning of the target surface, e.g. by plasma sputtering, or even the possibility of vacuum evaporation of a fresh, clean target surface, followed by the exposure to the ion beam with less than 10 s delay.
The drum is turned by a gear motor and locked in position with an electromagnetic latch. The target is electrically insulated from the catcher holder and connected to an electrometer, EM, with a spring slide contact. The electrometer reading and control signals to and from the valve and motor control box, VMC, at the bias voltage potential are transferred to and from ground potential with a light transmission system and connected to a control system which automatically controls pumps and valves in connection with inserting and removing the target and catcher system. The electrometer reading is used to optimize the ion flux onto the target during the exposure. Since this also may require adjustments of the beamline parameters, the electrometer reading, It, is transmitted to the separator control room and displayed on a meter. In fig. 2 is shown a more detailed drawing of the target/catcher system. The targets are normally a metallic layer at least 3000 A thick evaporated onto a polished glass disk 6.5 mm in diameter. The disk is attached to the target holder cylinder by a lock ring with an opening of 5.6 mm in diameter having a 0.2 mm thin edge to minimize any shadowing of the reflected particles. The target holder is fixed to the
501
J. B~ttiger et al. / Total backscattering
CATCHERHOLDER..
CATCHER
I
I I I I
TARGETHOLDER 10mm i
i
! /
Fig. 2. Detailed drawing of the target/catcher system.
catcher holder by a spring lock, so it is possible to remove and insert the target for the mentioned in-situ cleaning or evaporation. One of the reasons for performing these reflection measurements at the ISOLDE facility was the possibility of doing many exposures in a relative short time. Therefore the catcher should either be prefabricated or it should be possible to shape and mount it in the holder within a few minutes. The latter solution was chosen. By use of a mounting rig, from a roll of 0.03 mm thick aluminium foil, 150 mm wide, the shown rectangular catcher bag was folded directly into the catcher holder, the inlet hole punched, the four corners cut off, the holder locked together with the outer cylinder and the upper edges bent in to close the bag around the target holder. This whole procedure is carried out in less than three minutes. After the exposure the catcher holder is opened by removing the outer cylinder, the flaps are folded out and the catcher bag removed. Without touching the inner surface, the bag is flattened and is easily folded into an approximate 7 × 7 X 1.5 mm 3 block. The dotted line shows the outline of the unfolded bag and it is seen that only the absolute necessary material has been used to form the bag. The target and the catcher now have approximately the same size and shape. To further simplify the radiation detection, it was decided only to use
T-counting and again the very small amount of catcher and target material is fortunate, as it reduces any possible difference in "),-ray absorption to a negligible value. The target and catcher are placed in small plastic test tubes and then with a compressed air rabbit system transferred two floors up to the counting station, which is placed in a low-background experimental hall. In fig. 3 is shown a schematic drawing of the counting station. There are two main arguments which call for a counting system where it is possible to follow the activity on the target/catcher pair over several half-lives. The first is that in some cases the decay curve is a combination of several half-lives, with the shape strongly dependent on the delay from the actual exposure to the counting starts, i.e. to perform a proper decay correction, it is necessary to register this decay curve in each case. The second argument is that in most cases a very big difference between the count rates from the target and the catcher exists. So the dead time correction will cause the ratio between the target and the catcher activity to be count rate dependent. To correct for this, it is necessary to follow the activity over at least one decade in count rate, and then extrapolate to zero count rate. By using the design shown, where all the samples XI. SURFACE SCATTERING
J. BqJttiger et al. / Total backscattering
502
SAMPLE POSITION FF
\
SENSOR ~ i SAMPLE- ,tP HOLDER~/ll
HIRADIATION ~[DETECTO~
IMOTOR LOADER
/
:.4
2;
COUNTING
~:~]
) J / : J:"
AOING RE
I
:: /:
[
I 100ram
I
Fig. 3. Schematic drawing of the counter station.
not being counted are placed in a waiting position in about the same distance from the counter and with space for a rather heavy lead shielding, the increase in background due to a strong source (target) will be nearly independent of where it is situated in the waiting row, i.e. the background only has to be sampled with intervals, normally once for a full revolution of the system. There are 16 sample holders. A position sensor registers the sample holder number, which is being counted. Two of the sample holders are always kept empty and used for the mentioned background registration. This leaves room in the system for 7 target/ catcher pairs. The counter station is automatically controlled, the count interval can be preset in intervals from 10 s to 200 s depending on the activity and also on how fast the exposure rate is. The interval in which the sample is changed is fixed at 10 s. At the same time the number of counts is printed out, together with the sample holder number, the counter sytem revolution number and the time. From another control box placed beside the experimental set-up, is the transfer and loading of the samples automatically controlled. When the sample holder with the number, which has been preset on a "next load number" register, is in position in front of the loader, the new sample will be loaded and the old sample will be forced out and drop down into a con-
tainer below the counting station. For convenience and control of the proper operation of the whole registration system, "the last count number" and the sample number and counting system revolution number are also transferred to and displayed on the control box. In this way it is possible to get information down to the irradiation area on the number of counts on the target and catcher just loaded and immediately know if the exposure conditions have been right and the wanted activity amount obtained.
3. Data evaluation For each pair of target and catcher count numbers a correction has to be performed for the background and the decay of the activity between the counting of the target and catcher activity wherefore the reflection coefficient, R , is found from (N e - Nb) KT
n =
(1)
(NT - Nb) + (Ne - Nb) K r ' where K r = exp(0.69 td/rl/2 ) .
(2)
Art, Arc and Nb are target, catcher and background counts, respectively, rl/2 is the half-life of the activ-
J. B~ttiger et al. / Total backscattering ity and t d is the data time interval found as td =tco + tch ,
503
counter system can be serious. These changes may be caused b y the loading o f a very strong source and the following decay o f the activity. A first order correction of this effect is performed by plotting the background count number as a function of the counter system revolutions and then extrapolate between the points. The estimated uncertainty or standard deviation on the measured reflection coefficients in the 10 -1, 10 -2 and 10 -3 range will be -+10%, +15% and -+35%, respectively. Measured reflection coefficients in the 10 -4 range should at present only be taken as upper limits. Only one type o f targets has been used until now, namely polycrystalline layers of gold, silver, copper and aluminium evaporated onto glass substrates and no target surface effects have at present been investigated.
(3)
where tco and tch are the count and change interval, respectively. The half-life correction is for half-lives of the order of 15 min and td = 0 . 5 rain approximately 2.5%. As mentioned above, in some cases the isotope used as projectile contains an isomeric- or metastable state with a half-life of one or two minutes (shorter half-life states will have disappeared during the target and catcher removal procedure) and then the half-life correction may rise to 1 0 - 2 0 % . But already after the first revolution, the short life time can be neglected. By plotting the activity on the target as a function o f the revolution number it is easily checked that the proper half-life values are used in the correction. The dead-time correction is performed by following the target/catcher pair over several half-lives and then extrapolate to zero count rate as depicted in fig. 4a. Especially in cases o f low reflection coefficients, possible changes in the background between the two background determinations per revolution o f the
4. Results In fig. 4 is presented the reflection coefficient data measured with 60 keV ions. The data are plotted as a
/ , / , K a ~)
10 -1
R
75Br = 84BrV
/
/
/
/
/
Br-Rb /$~, J-.~Na
81Rbo 88Rb i 89Rb •
_;~ ~/
125Cs n 130Cs = 138Cs !
Cs
/-~Ka
i~1/i
2t, N a <1 t ~ ~ . 2 ~ 1.2Ka 0 ~ . . . .
10-2
b
////
.~/I / /'
I
/:~ // ;
II/
10-3.
E= 60 KeV. 1,8_~RA 89 Rb~Ag
60 KeV o
///"f
/j /I // /; 10-~
--
10_1
-
i _
i i i i i
1.?_t. .l
.
'0 C/20sec 0,'5 i
i
i
M2/Mt
i
-106
'1"
i=,=~
10
Fig. 4. (a) The reflection coefficient, R, plotted as a function of the target count number, Art; R o is the dead time corrected reflection coefficient. (b) The measured reflection coefficient values for 60 keV fixed energy plotted as a function of the mass ratio M2[MI. The curves are drawn as the best fit to each projectile element or mass region. XI. SURFACE SCATTERING
504
J. B~ttiger et al. / Total backscattering
function o f the mass ratio M:/M1, where M1 and M 2 are the masses o f the projectile and the target atoms, respectively. A few data from the previous measurements [2,3] have also been added to the figure. A complete analysis of the results will be presented in subsequent papers when more results have been obtained. The experimental uncertainty in case of small reflection values is still too large to allow a comparison of isotopes o f the same element with relative big mass differences, but if the data are viewed in subgroups for each projectile element or mass region, then inside the stated uncertainty they will follow the curves drawn to guide the eye. The help and participation of F. Hansen and F. Michaelsen are very much acknowledged, and it is a pleasure to thank J. Chevallier for the production of the targets. The whole ISOLDE staff is also acknowl-
edged for their cooperation and hospitality, during the actual measurements.
References [1] H. Vernickel, Phys. Reports 37, No. 2 (1978) 93. [2] J. B~bttigerand J.A. Davies, Rad. Effects 11 (1971) 61. [3] J. B~ttiger, J.A. Davies, P. Sigmund and K.B. Winterbon, Rad. Effects 11 (1971) 69. [4] J. B~bttiger, H. Wolder J~brgensen and K.B. Winterbon, Rad. Effects 11 (1971) 133. [5] J. B~bttiger and K.B. Winterbon, Rad. Effects 20 (1973) 65. [6] H.L. Ravn, L.C. Carraz, J. Denimal, E. Kugler, M. Skarestad, S. Sundell and L. Westgaard, Nucl. Instr. and Meth. 139 (1976) 267. [7] K.B. Winterbon, Rad. Effects 39 (1978) 31. [8] G. Sidenius, Rad. Effects 38 (1978) 3.
499
TOTAL BACKSCATTERING OF LOW ENERGY HEAVY IONS FROM SOLID SURFACES J. BOTTIGER Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark
O. HOLCK, G. SIDENIUS The Niets Bohr Institute, University of Copenhagen, DK-2100 Copenhagen (~, Demnark
and The 1SOLDE Collaboration, CERN, Geneva, Switzerland
With the application of beams of shortlived radioactive ions from the ISOLDE facility at CERN, Geneva, systematic backscattering measurements have been made possible in broad ranges of particle elements and masses. A detailed description of the experimental principle and apparatus will be presented. Reflection coefficient data from the first series of measurements will be discussed.
1. Introduction Besides being a parameter of considerable interest for the theoretical understanding of the fundamental collision processes, the reflection coefficient for low energy ( 1 - 1 0 0 keV) heavy ions is of practical importance, especially in case of projectiles lighter than the target atoms where reflection coefficients of the order of 10% or higher may be reached. Excluding investigations with ions with masses equal to or lighter than 4He, which form a special group of interest for plasma-wall interaction in fusion research (see for example H. Vernickel [1]), to our knowledge, only one group of systematic experimental measurements of the total reflection coefficient [ 2 - 5 ] exist. Here a radiotracer technique, using long lived isotopes, was applied. The present paper describes a further development o f this technique to allow the use of shortlived isotopes from an on-line isotope separator, the ISOLDE [6] facility at CERN, Geneva, whereby the possible number of projectiles to be studied is greatly increased. A detailed discussion of the experimental results and any detailed comparison with theory [3,4,7] will not be attempted in the present paper, which is concentrated mostly on the experimental technique. A detailed discussion will be presented in subsequent papers.
2. Experimental technique The experimental procedure consists in bombarding a target, surrounded by a collector system, with a radioactive beam, and derives a reflection coefficient from the radioactivity measured on the target and the collector. In fig. 1 a schematic drawing of the main part of the experimental system is shown. The basic parts of the system have previously been used in range measurements in gases [8]. As the ISOLDE facility has to serve several beamlines simultaneously, all beam parameters are optimized for 60 keV ion energy, which is the permanent separator energy setting. Therefore, to perform systematic energy measurements a retardation or postacceleration of the ion beam is required, and the part of the system, where the actual exposure of the target to the ion beam is performed, is placed in an insulated cabinet. From the beam line the ion beam first enter the lens chamber with the retardation or post-acceleration lens. The focal strength of the system is controlled with the lens voltage, U L. The resulting ion energy, Ei, is controlled by the bias voltage, UR, on the scattering chamber, which may be varied between - 6 0 kV and +60 kV, resulting in a possible ion energy range from 120 keV down to a few keV. Ei is measured with a metal film resistor divider chain, R, and a digital voltmeter, DV. XI. SURFACE SCATTERING
J.Bdttiger et aL / Total backscattering
500
1
INSULATINGCABINET
REVOLVING ~.., PE F_L ...P.~ AT....E,.S.,........... # ) ~ ................ • .............. ..D.R,U_M..7......,.~
Fr
H
om ISOLDE~ 11
?,'idN4u~ ~
...........................
.....i'; ~
HOLDER CATCHER~ U / ~ [
]] ~
UXU
8 I ~l~-~l
[
I
=~]
~ 1
°
/
MTeJ .I~
I
[~~'~
RETARDATION" ~4 ~ C A T C H E R - I ~
LENS
~
~L
mu MR
h
M/~ J /
TT~RGET # I
"
~
~I,~I
I~
.~-...~... ,~u~,-~-_, I U IUIN ~ U U f ~ I -
/
R
~
-I '',L2;-';-J
;:, .
. ] .
I
~
L-----I ,,
l]
I,'------4,'i; ii
, :
'*~-- / Pu~P~,q~C~E I
-I"-~ CONTROL TO SEP CORITROL ~q
Fig. 1. Schematic drawing of the experimental apparatus. For details see the text.
After the retardation lens system a slight correction of the beam position is possible in a four-plate deflector system supplied from the deflector voltage box, LTd. The beam is then collimated by two 1.5 mm diameter apertures before entering the target/catcher system through a 2.5 mm diameter opening, hence any slit-edge scattering should be suppressed. The target/catcher system, described in more details later, is introduced into and removed from the main vacuum system thruogh a vacuum lock system. After having been inserted, the target/catcher system is secured into one out of three openings in a revolvable drum. When the drum is turned 120 ° , the target/ catcher system is moved to a waiting position, behind which is situated a small vacuum lock system, similar to the one shown on the figure behind the exposure position, to which the target]catcher system is brought by a further 120 ° turn of the drum. These two small vacuum lock systems allow the target to be removed or inserted independently of the catcher system and open up the possibility of either in situ cleaning of the target surface, e.g. by plasma sputtering, or even the possibility of vacuum evaporation of a fresh, clean target surface, followed by the exposure to the ion beam with less than 10 s delay.
The drum is turned by a gear motor and locked in position with an electromagnetic latch. The target is electrically insulated from the catcher holder and connected to an electrometer, EM, with a spring slide contact. The electrometer reading and control signals to and from the valve and motor control box, VMC, at the bias voltage potential are transferred to and from ground potential with a light transmission system and connected to a control system which automatically controls pumps and valves in connection with inserting and removing the target and catcher system. The electrometer reading is used to optimize the ion flux onto the target during the exposure. Since this also may require adjustments of the beamline parameters, the electrometer reading, It, is transmitted to the separator control room and displayed on a meter. In fig. 2 is shown a more detailed drawing of the target/catcher system. The targets are normally a metallic layer at least 3000 A thick evaporated onto a polished glass disk 6.5 mm in diameter. The disk is attached to the target holder cylinder by a lock ring with an opening of 5.6 mm in diameter having a 0.2 mm thin edge to minimize any shadowing of the reflected particles. The target holder is fixed to the
501
J. B~ttiger et al. / Total backscattering
CATCHERHOLDER..
CATCHER
I
I I I I
TARGETHOLDER 10mm i
i
! /
Fig. 2. Detailed drawing of the target/catcher system.
catcher holder by a spring lock, so it is possible to remove and insert the target for the mentioned in-situ cleaning or evaporation. One of the reasons for performing these reflection measurements at the ISOLDE facility was the possibility of doing many exposures in a relative short time. Therefore the catcher should either be prefabricated or it should be possible to shape and mount it in the holder within a few minutes. The latter solution was chosen. By use of a mounting rig, from a roll of 0.03 mm thick aluminium foil, 150 mm wide, the shown rectangular catcher bag was folded directly into the catcher holder, the inlet hole punched, the four corners cut off, the holder locked together with the outer cylinder and the upper edges bent in to close the bag around the target holder. This whole procedure is carried out in less than three minutes. After the exposure the catcher holder is opened by removing the outer cylinder, the flaps are folded out and the catcher bag removed. Without touching the inner surface, the bag is flattened and is easily folded into an approximate 7 × 7 X 1.5 mm 3 block. The dotted line shows the outline of the unfolded bag and it is seen that only the absolute necessary material has been used to form the bag. The target and the catcher now have approximately the same size and shape. To further simplify the radiation detection, it was decided only to use
T-counting and again the very small amount of catcher and target material is fortunate, as it reduces any possible difference in "),-ray absorption to a negligible value. The target and catcher are placed in small plastic test tubes and then with a compressed air rabbit system transferred two floors up to the counting station, which is placed in a low-background experimental hall. In fig. 3 is shown a schematic drawing of the counting station. There are two main arguments which call for a counting system where it is possible to follow the activity on the target/catcher pair over several half-lives. The first is that in some cases the decay curve is a combination of several half-lives, with the shape strongly dependent on the delay from the actual exposure to the counting starts, i.e. to perform a proper decay correction, it is necessary to register this decay curve in each case. The second argument is that in most cases a very big difference between the count rates from the target and the catcher exists. So the dead time correction will cause the ratio between the target and the catcher activity to be count rate dependent. To correct for this, it is necessary to follow the activity over at least one decade in count rate, and then extrapolate to zero count rate. By using the design shown, where all the samples XI. SURFACE SCATTERING
J. BqJttiger et al. / Total backscattering
502
SAMPLE POSITION FF
\
SENSOR ~ i SAMPLE- ,tP HOLDER~/ll
HIRADIATION ~[DETECTO~
IMOTOR LOADER
/
:.4
2;
COUNTING
~:~]
) J / : J:"
AOING RE
I
:: /:
[
I 100ram
I
Fig. 3. Schematic drawing of the counter station.
not being counted are placed in a waiting position in about the same distance from the counter and with space for a rather heavy lead shielding, the increase in background due to a strong source (target) will be nearly independent of where it is situated in the waiting row, i.e. the background only has to be sampled with intervals, normally once for a full revolution of the system. There are 16 sample holders. A position sensor registers the sample holder number, which is being counted. Two of the sample holders are always kept empty and used for the mentioned background registration. This leaves room in the system for 7 target/ catcher pairs. The counter station is automatically controlled, the count interval can be preset in intervals from 10 s to 200 s depending on the activity and also on how fast the exposure rate is. The interval in which the sample is changed is fixed at 10 s. At the same time the number of counts is printed out, together with the sample holder number, the counter sytem revolution number and the time. From another control box placed beside the experimental set-up, is the transfer and loading of the samples automatically controlled. When the sample holder with the number, which has been preset on a "next load number" register, is in position in front of the loader, the new sample will be loaded and the old sample will be forced out and drop down into a con-
tainer below the counting station. For convenience and control of the proper operation of the whole registration system, "the last count number" and the sample number and counting system revolution number are also transferred to and displayed on the control box. In this way it is possible to get information down to the irradiation area on the number of counts on the target and catcher just loaded and immediately know if the exposure conditions have been right and the wanted activity amount obtained.
3. Data evaluation For each pair of target and catcher count numbers a correction has to be performed for the background and the decay of the activity between the counting of the target and catcher activity wherefore the reflection coefficient, R , is found from (N e - Nb) KT
n =
(1)
(NT - Nb) + (Ne - Nb) K r ' where K r = exp(0.69 td/rl/2 ) .
(2)
Art, Arc and Nb are target, catcher and background counts, respectively, rl/2 is the half-life of the activ-
J. B~ttiger et al. / Total backscattering ity and t d is the data time interval found as td =tco + tch ,
503
counter system can be serious. These changes may be caused b y the loading o f a very strong source and the following decay o f the activity. A first order correction of this effect is performed by plotting the background count number as a function of the counter system revolutions and then extrapolate between the points. The estimated uncertainty or standard deviation on the measured reflection coefficients in the 10 -1, 10 -2 and 10 -3 range will be -+10%, +15% and -+35%, respectively. Measured reflection coefficients in the 10 -4 range should at present only be taken as upper limits. Only one type o f targets has been used until now, namely polycrystalline layers of gold, silver, copper and aluminium evaporated onto glass substrates and no target surface effects have at present been investigated.
(3)
where tco and tch are the count and change interval, respectively. The half-life correction is for half-lives of the order of 15 min and td = 0 . 5 rain approximately 2.5%. As mentioned above, in some cases the isotope used as projectile contains an isomeric- or metastable state with a half-life of one or two minutes (shorter half-life states will have disappeared during the target and catcher removal procedure) and then the half-life correction may rise to 1 0 - 2 0 % . But already after the first revolution, the short life time can be neglected. By plotting the activity on the target as a function o f the revolution number it is easily checked that the proper half-life values are used in the correction. The dead-time correction is performed by following the target/catcher pair over several half-lives and then extrapolate to zero count rate as depicted in fig. 4a. Especially in cases o f low reflection coefficients, possible changes in the background between the two background determinations per revolution o f the
4. Results In fig. 4 is presented the reflection coefficient data measured with 60 keV ions. The data are plotted as a
/ , / , K a ~)
10 -1
R
75Br = 84BrV
/
/
/
/
/
Br-Rb /$~, J-.~Na
81Rbo 88Rb i 89Rb •
_;~ ~/
125Cs n 130Cs = 138Cs !
Cs
/-~Ka
i~1/i
2t, N a <1 t ~ ~ . 2 ~ 1.2Ka 0 ~ . . . .
10-2
b
////
.~/I / /'
I
/:~ // ;
II/
10-3.
E= 60 KeV. 1,8_~RA 89 Rb~Ag
60 KeV o
///"f
/j /I // /; 10-~
--
10_1
-
i _
i i i i i
1.?_t. .l
.
'0 C/20sec 0,'5 i
i
i
M2/Mt
i
-106
'1"
i=,=~
10
Fig. 4. (a) The reflection coefficient, R, plotted as a function of the target count number, Art; R o is the dead time corrected reflection coefficient. (b) The measured reflection coefficient values for 60 keV fixed energy plotted as a function of the mass ratio M2[MI. The curves are drawn as the best fit to each projectile element or mass region. XI. SURFACE SCATTERING
504
J. B~ttiger et al. / Total backscattering
function o f the mass ratio M:/M1, where M1 and M 2 are the masses o f the projectile and the target atoms, respectively. A few data from the previous measurements [2,3] have also been added to the figure. A complete analysis of the results will be presented in subsequent papers when more results have been obtained. The experimental uncertainty in case of small reflection values is still too large to allow a comparison of isotopes o f the same element with relative big mass differences, but if the data are viewed in subgroups for each projectile element or mass region, then inside the stated uncertainty they will follow the curves drawn to guide the eye. The help and participation of F. Hansen and F. Michaelsen are very much acknowledged, and it is a pleasure to thank J. Chevallier for the production of the targets. The whole ISOLDE staff is also acknowl-
edged for their cooperation and hospitality, during the actual measurements.
References [1] H. Vernickel, Phys. Reports 37, No. 2 (1978) 93. [2] J. B~bttigerand J.A. Davies, Rad. Effects 11 (1971) 61. [3] J. B~ttiger, J.A. Davies, P. Sigmund and K.B. Winterbon, Rad. Effects 11 (1971) 69. [4] J. B~bttiger, H. Wolder J~brgensen and K.B. Winterbon, Rad. Effects 11 (1971) 133. [5] J. B~bttiger and K.B. Winterbon, Rad. Effects 20 (1973) 65. [6] H.L. Ravn, L.C. Carraz, J. Denimal, E. Kugler, M. Skarestad, S. Sundell and L. Westgaard, Nucl. Instr. and Meth. 139 (1976) 267. [7] K.B. Winterbon, Rad. Effects 39 (1978) 31. [8] G. Sidenius, Rad. Effects 38 (1978) 3.