- Email: [email protected]
A comparison of RIST and ISOLDE tantalum targets and geometries used on-line at ISOLDE
__ __ !iiJ
2sii!i
Nuclear Instruments
and Methods in Physics Research B 126 (1997)121- 124
NOMB
Beam Interactions with Materials&Atoms
ELSEVIER
A comparison of RIST and ISOLDE tantalum targets and geometries used on-line at ISOLDE P.V. Drumm G.R. Murdoch P. Van Duppen C. Thwaites
ill* , J.R.J. Bennett a, C.J. Densham a, W.R. Evans a, M. Holding ‘I, a, A.H. Evenson b, E. Kugler b, J. Lettry b, H. Ravn b, 0. Tengblad b, b, R. Catherall b, 0. Jonsson b, J. Kay ‘, D.D. Warner ‘, M. Harder d, d, J. Honsi ‘, R. Page e, J. Billowes f, S.J. Freeman f, I.S. Grant f, S. Schwebel f, G. Smith f, C. Bishop g, P.M. Walker g ’ CCLRC Ruthrrjkd
Appleton Lahorotory. Chilton. OXI I OQX. (JK b CERN. CH- I2 I I Geneva 23. Swit~erlrnd ’ CCLRC Dureshury Lrrhorutory. Daresbury. l/K d Univer.siry c$Brighton. Brighron. UK e University oj’liverpool. Liverpool L69 3BX. UK f University of Munchester. Munchester Ml3 9PL. UK g University of’ Surrey. Guildfi,rd GU2 5XH. UK
Abstract A comparison is made of the performance of RIST and ISOLDE tantalum targets used on-line at the CERN-ISOLDE isotope separator. The data is taken from a combination of recent measurements at the CERN-PSB and from the former ISOLDE facility at the CERN-SC. Developments in the target geometry have been necessary for the RIST project which aims to design a target capable of dissipating the power developed by an 800 MeV, 100 PA proton beam. These designs are being tested at the ISOLDE-PSB facility and the initial results are reported in comparison with data obtained from typical ISOLDE tantalum-foil targets. The study shows that improvements in the release properties of a number of elements can be made by a judicious choice of target matrix geometry. The target represents a complex system where, in general, modelling alone involves too many parameters to generate a reliable model of the target. From this study, it is clear that further practical work, following simple ideas such as those based on modifications of the target geometry, is necessary in order to obtain progress in the optimisation of the target design.
1. Introduction
2. The RIST target
The RIST project in the UK has designed and built a target capable of dissipating the power developed by an 800 MeV. IO0 p.A beam of protons (- 30 kW). This project, including the target design is discussed elsewhere [I]. but requires that the target geometry is closely controlled to optimise heat transport in the target. This aspect of the RIST target design has been incorporated into an ISOLDE target and tested on-line at the ISOLDE-PSB facility at CERN. The results obtained from this test are compared to observations made on standard ISOLDE tan-
The ISOLDE target consists of a tube 20 cm long and 2 cm in diameter made from tantalum and mounted in a water cooled vacuum chamber [2]. The target is heated ohmically and is surrounded by heat shields to minimise temperature loss through radiation. For a standard ISOLDE tantalum-target, the material is formed as short rolls of thin (20 pm) tantalum foil tied with tantalum wire and inserted along the length of the tube. The tantalum material is to hinder electrical and thermal contact be“dimpled” tween the layers of the roll and to prevent the layers from fusing. The RIST target, designed to operate at high power, needs to have good thermal contact between the target material and the container which is designed to radiate the heat away and is made from an assembly of tantalum discs which are integral to the target container. The RIST target discs have an aperture down their centre
talum
targels.
’ Corresponding
author
0168.583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII sol~~-~8~x~97~olo2~-~
SECTION V. TARGET TECHNIQUES
122
P.V. Drumm et ul./Nucl.
Instr. and Meth. in Phys. Res. B 126 (1997) 121-124
is expected (at least near the centre). The power dissipated by the proton beam does not raise the average temperature of the target since the repetition rate is kept low.
3. Measurements
Fig. 1. Cross section of the ISOLDE tantalum target roll (a) surrounded by the target container wall. The roll is made from a single strip of tantalum which is coiled and tied with tantalum wire This wire (not shown) allows only point contact between the target container and the foils. Cross section of RIST tantalum target disk as used at ISOLDE (b). A number of modules made from a stack of tantalum foils held by tantalum wire clips are used to fill the container (cl. The clips provide minimal contact between the target material and the container. Under the ion-source, the foils are cut into ‘C’shaped disks, and between each module is a blocking disk to ensure that conduction can only take place down the centre as would be the case in the actual RIST target. The target container is surrounded by heat screens.
to allow products an effusion path to the ion-source. The target itself has too low a resistance to be heated with the normal ISOLDE power supplies, and an altemative arrangement of the foils compatible with the ISOLDE target container has been made, (Fig. 1) and is the RISTgeometry tested at ISOLDE (RIST-I). A second target with thicker foils has also been tested (RIST-II). The ISOLDE target tube is heated by passing an electric current along the length of the target tube, which is surrounded by heat shields. The temperature is calibrated off-line using an optical pyrometer to establish temperatures at various currents. Typically, temperatures of 19002150°C are maintained. Because the heating is external, a roughly uniform temperature within the body of the target RIST
Many elements are produced in the tantalum target, not all of which are able to escape or be ionised. The targets described in this paper all used a hot tungsten surface ion-source. Results from the six tantalum targets run at the ISOLDE-PSB are considered here (Table 1). Isotopes of the alkali metals along with a large number of rare-earth isotopes can be ionised. Observations of a selection of these isotopes are discussed below. Two basic measurements can be made of the target. The first is the rate at which radioactive ions produced in the target are released as a function of time following production, and the second is the total current (yield) that is observed as an ion-beam. Detailed release data have become available for the ISOLDE-PSB machine, and extensive yield information are available for both the SC and PSB machines [2]. The measurements were made by observing the beta decay of the radioactive ions in samples of the beam made on a tape system. Besides mass identification through magnetic separation, further identification was made, when necessary, by observing the half-life of the radioactive decay or, in a limited number of cases, using gamma-ray spectroscopy. The measurements at the ISOLDE-PSB facility are easily made since the proton beam is pulsed and the ion-beam (which is also pulsed) can be sampled at a specific time after the protons impact on the target. By varying the delay time (Td, the time between impact and the measurement), the function p(t) usually used to describe the release of ions from a target matrix can be mapped out. The interval between pulses is judged to be sufficient that the target is exhausted of the products formed by previous pulses.
Table 1 Variation in the release times (in msl of various isotopes RIST and typical ISOLDE targets at different temperatures Element
RIST-I ms PC)
RIST-II ms PC)
ISOLDE ms PC)
Li
150 (1950) 4.5 (2150) 200 (2000) 70 (21501 55 (2000) 600 (2000)
350 ( 1950) 80 (2 150) 150(1950-2140)
900 200 300 200 500 200 600 700 390
Na K Rb cs
660 ( 1950) 50 (2200)
500 ( 1950) 200 (2 150) 400 (1950)
(19501 (2 150) ( 1975) (2 150) (2cOOl ( 1950) (2150) ( 1950) (2150)
from
P.V.
Drumm
et al./Nucl.
Instr.
and Mrth.
in Phys.
Rex
B 126 (1997)
121-124
12.1
1
=o.a
b
EO.6 a 4 o) 0.4
500
0
1000
1500
ea.6
Z 7
2000
0
600
1000
1600
t\ 0
2000
500
0
1500
2000
Td (ms)
Td (ms)
Td (ms)
1000
Fig. 2. Release of 8Li from (a) the ISOLDE and (b) the RIST targets. (a) Shows the range of release times for an ISOLDE target at 1950°C (squares) and 2150°C (circles). The solid curve is a fit to the faster ISOLDE data and is characterised by a 200 ms time constant. (b) Shows release from the RIST-I target at 1950°C with a release time of 200 ms, and this curve is reproduced in (c) along with data measured at 2150°C which have release times of 65 ms (cirlces) and 45 ms (squares).
The function ments I(r)
p(t)
is then
of the ion-beam
current,
= I, e-“‘p(
deduced
from
“slow’‘-fall time. The latter is used to account for a relatively slow release (*‘tail”) following a sharp rise in the ion-beam (“spike”) associated with the proton beam impact. The association of any physical significance directly to these parameters should be made with care, but they are a useful aid in comparing target performance. Errors are not quoted with the measurements presented since the measurements are rather variable in nature (the system is not tightly controlled and changes with time). Further, the measurements are taken at varying times during the use of the target for “production-running”. Consequently, the conditions of the target from one run to another are often difficult to reproduce. The release of Li
the measure-
t).
The release data shown in the figures are the raw measured data (without correction for half-life, and only the background counts have been removed). The curves are generated from a shown with the data [e -*‘p(r)] simple parameterisation of p(r) which is then integrated over the sample collection time. Yield data can easily be obtained by integration of the release curve. The function p(r) is here described by three exponentials giving the characteristics of a “rise’‘-time, a “fall’‘-time and a
1
=o.a ii
r;o.a ;
50.6 P 5 ‘@0.4
e.6
0
_~.
0
0
2000 Td (ms)
4000
0
1000
._-
2000
Td (ms)
3000
0 ! 0
1000
2000
3000
Td (ms)
Fig. 3. Release of 25Na from (a) the ISOLDE and (b) the RIST targets. The data in (a) were taken at 1950°C and have a release time of t s. The RlST data in (b) at this temperature have a release time of 250 ms, while those in (c) measured at 2150°C are. characterised by a release of 100 ms (squares) and 70 ms (circles).
SECTION V. TARGET TECHNIQUES
P.V. Drumm et al,/Nucl.
124
and Na release general to other
Instr. und Meth.in Phys. Rex B 126 (1997) 121-124
is discussed in detail as an illustration of the of the RIST and ISOLDE targets, although the observations and conclusions are likely to extend isotopes which are directly produced by the beam.
> 2 compared to the ISOLDE roll target at similar temperatures.
5. Conclusions 4. Results and discussion 4.1. Lithium The release of Li is most easily measured for 8Li (t ,,* - 0.8 s). Fig. 2a shows the range in release seen for an ISOLDE roll target: at a temperature of 1950°C the release is characterised by a 600 ms fall time (squares). A more recent result (circles) shows, at a temperature of 2150°C. a faster release of 200 ms. Results from the RIST-I target are shown in Fig. 2b and c ranging in temperature from 1950 (b) to 2 150°C (c), where the release varies from 200 ms in (b) to 65 ms (circles) and 45 ms (squares) in (c). The curve in (c) is taken from Fig. 2b, and indicates the increase of speed with temperature. The RIST target for Li is considerably faster than the ISOLDE target at the same temperature, by as much as a factor of 4. Despite the increase in release speed of the RIST target over the ISOLDE target, significant gains in the yields of short lived isotopes have yet to be observed for the RIST target.
4.2. Sodium The release of 25Na (t,,, - 59 s) is shown in Fig. 3a) for an ISOLDE target, and in (b) and (c) for the RIST target. Release times vary for the ISOLDE target from typically 1s (at 1950°C) as shown in Fig. 3a, down to 200 ms, and for the RIST target from 250 ms at 1950°C (Fig. 3b), down to 70 ms at 2150°C as shown in Fig. 3c. The curve shown in Fig. 3c is the fit obtained for the data in Fig. 3b. The RIST target for Na is faster by a factor of
A summary of the variation seen in the RIST and ISOLDE release curves is given in Table 1. It is quite clear from the given release data that temperature is very important in the release of the elements. In addition to temperature, the geometry of the RIST target has proved to be successful in increasing the release speed of ions by a significant factor (X 4 for the case of Li, and X 2 for Na). The RIST-II target which had thicker foils and a decreased effusion path shows results that are slower that for RIST-I, but still generally faster than for the roll target, indicating that more investigation is required to separate the effects of diffusion and effusion. The link between fast release and increased yield seems to be somewhat elusive, and it must be acknowledged that other factors (target fatigue, cold spots) play an undetermined part. The RIST target demonstrates that an ordered system has a significant effect on the release speed of the targets. The targets represent a complex system where modelling involves many unknown parameters (diffusion and sticking data being scant). It is clear that further practical work, following simple ideas - such as in the target geometry design - can be useful in order to obtain progress in target design.
References [I] J.R. Bennett, these Proceedings (EMIS-13). Nucl. Instr. and Meth. B 126 (1997) 105. [2] H.J. Kluge. ed., in: ISOLDE Users Guide, CERN-86-05 ( 1986).
2sii!i
Nuclear Instruments
and Methods in Physics Research B 126 (1997)121- 124
NOMB
Beam Interactions with Materials&Atoms
ELSEVIER
A comparison of RIST and ISOLDE tantalum targets and geometries used on-line at ISOLDE P.V. Drumm G.R. Murdoch P. Van Duppen C. Thwaites
ill* , J.R.J. Bennett a, C.J. Densham a, W.R. Evans a, M. Holding ‘I, a, A.H. Evenson b, E. Kugler b, J. Lettry b, H. Ravn b, 0. Tengblad b, b, R. Catherall b, 0. Jonsson b, J. Kay ‘, D.D. Warner ‘, M. Harder d, d, J. Honsi ‘, R. Page e, J. Billowes f, S.J. Freeman f, I.S. Grant f, S. Schwebel f, G. Smith f, C. Bishop g, P.M. Walker g ’ CCLRC Ruthrrjkd
Appleton Lahorotory. Chilton. OXI I OQX. (JK b CERN. CH- I2 I I Geneva 23. Swit~erlrnd ’ CCLRC Dureshury Lrrhorutory. Daresbury. l/K d Univer.siry c$Brighton. Brighron. UK e University oj’liverpool. Liverpool L69 3BX. UK f University of Munchester. Munchester Ml3 9PL. UK g University of’ Surrey. Guildfi,rd GU2 5XH. UK
Abstract A comparison is made of the performance of RIST and ISOLDE tantalum targets used on-line at the CERN-ISOLDE isotope separator. The data is taken from a combination of recent measurements at the CERN-PSB and from the former ISOLDE facility at the CERN-SC. Developments in the target geometry have been necessary for the RIST project which aims to design a target capable of dissipating the power developed by an 800 MeV, 100 PA proton beam. These designs are being tested at the ISOLDE-PSB facility and the initial results are reported in comparison with data obtained from typical ISOLDE tantalum-foil targets. The study shows that improvements in the release properties of a number of elements can be made by a judicious choice of target matrix geometry. The target represents a complex system where, in general, modelling alone involves too many parameters to generate a reliable model of the target. From this study, it is clear that further practical work, following simple ideas such as those based on modifications of the target geometry, is necessary in order to obtain progress in the optimisation of the target design.
1. Introduction
2. The RIST target
The RIST project in the UK has designed and built a target capable of dissipating the power developed by an 800 MeV. IO0 p.A beam of protons (- 30 kW). This project, including the target design is discussed elsewhere [I]. but requires that the target geometry is closely controlled to optimise heat transport in the target. This aspect of the RIST target design has been incorporated into an ISOLDE target and tested on-line at the ISOLDE-PSB facility at CERN. The results obtained from this test are compared to observations made on standard ISOLDE tan-
The ISOLDE target consists of a tube 20 cm long and 2 cm in diameter made from tantalum and mounted in a water cooled vacuum chamber [2]. The target is heated ohmically and is surrounded by heat shields to minimise temperature loss through radiation. For a standard ISOLDE tantalum-target, the material is formed as short rolls of thin (20 pm) tantalum foil tied with tantalum wire and inserted along the length of the tube. The tantalum material is to hinder electrical and thermal contact be“dimpled” tween the layers of the roll and to prevent the layers from fusing. The RIST target, designed to operate at high power, needs to have good thermal contact between the target material and the container which is designed to radiate the heat away and is made from an assembly of tantalum discs which are integral to the target container. The RIST target discs have an aperture down their centre
talum
targels.
’ Corresponding
author
0168.583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII sol~~-~8~x~97~olo2~-~
SECTION V. TARGET TECHNIQUES
122
P.V. Drumm et ul./Nucl.
Instr. and Meth. in Phys. Res. B 126 (1997) 121-124
is expected (at least near the centre). The power dissipated by the proton beam does not raise the average temperature of the target since the repetition rate is kept low.
3. Measurements
Fig. 1. Cross section of the ISOLDE tantalum target roll (a) surrounded by the target container wall. The roll is made from a single strip of tantalum which is coiled and tied with tantalum wire This wire (not shown) allows only point contact between the target container and the foils. Cross section of RIST tantalum target disk as used at ISOLDE (b). A number of modules made from a stack of tantalum foils held by tantalum wire clips are used to fill the container (cl. The clips provide minimal contact between the target material and the container. Under the ion-source, the foils are cut into ‘C’shaped disks, and between each module is a blocking disk to ensure that conduction can only take place down the centre as would be the case in the actual RIST target. The target container is surrounded by heat screens.
to allow products an effusion path to the ion-source. The target itself has too low a resistance to be heated with the normal ISOLDE power supplies, and an altemative arrangement of the foils compatible with the ISOLDE target container has been made, (Fig. 1) and is the RISTgeometry tested at ISOLDE (RIST-I). A second target with thicker foils has also been tested (RIST-II). The ISOLDE target tube is heated by passing an electric current along the length of the target tube, which is surrounded by heat shields. The temperature is calibrated off-line using an optical pyrometer to establish temperatures at various currents. Typically, temperatures of 19002150°C are maintained. Because the heating is external, a roughly uniform temperature within the body of the target RIST
Many elements are produced in the tantalum target, not all of which are able to escape or be ionised. The targets described in this paper all used a hot tungsten surface ion-source. Results from the six tantalum targets run at the ISOLDE-PSB are considered here (Table 1). Isotopes of the alkali metals along with a large number of rare-earth isotopes can be ionised. Observations of a selection of these isotopes are discussed below. Two basic measurements can be made of the target. The first is the rate at which radioactive ions produced in the target are released as a function of time following production, and the second is the total current (yield) that is observed as an ion-beam. Detailed release data have become available for the ISOLDE-PSB machine, and extensive yield information are available for both the SC and PSB machines [2]. The measurements were made by observing the beta decay of the radioactive ions in samples of the beam made on a tape system. Besides mass identification through magnetic separation, further identification was made, when necessary, by observing the half-life of the radioactive decay or, in a limited number of cases, using gamma-ray spectroscopy. The measurements at the ISOLDE-PSB facility are easily made since the proton beam is pulsed and the ion-beam (which is also pulsed) can be sampled at a specific time after the protons impact on the target. By varying the delay time (Td, the time between impact and the measurement), the function p(t) usually used to describe the release of ions from a target matrix can be mapped out. The interval between pulses is judged to be sufficient that the target is exhausted of the products formed by previous pulses.
Table 1 Variation in the release times (in msl of various isotopes RIST and typical ISOLDE targets at different temperatures Element
RIST-I ms PC)
RIST-II ms PC)
ISOLDE ms PC)
Li
150 (1950) 4.5 (2150) 200 (2000) 70 (21501 55 (2000) 600 (2000)
350 ( 1950) 80 (2 150) 150(1950-2140)
900 200 300 200 500 200 600 700 390
Na K Rb cs
660 ( 1950) 50 (2200)
500 ( 1950) 200 (2 150) 400 (1950)
(19501 (2 150) ( 1975) (2 150) (2cOOl ( 1950) (2150) ( 1950) (2150)
from
P.V.
Drumm
et al./Nucl.
Instr.
and Mrth.
in Phys.
Rex
B 126 (1997)
121-124
12.1
1
=o.a
b
EO.6 a 4 o) 0.4
500
0
1000
1500
ea.6
Z 7
2000
0
600
1000
1600
t\ 0
2000
500
0
1500
2000
Td (ms)
Td (ms)
Td (ms)
1000
Fig. 2. Release of 8Li from (a) the ISOLDE and (b) the RIST targets. (a) Shows the range of release times for an ISOLDE target at 1950°C (squares) and 2150°C (circles). The solid curve is a fit to the faster ISOLDE data and is characterised by a 200 ms time constant. (b) Shows release from the RIST-I target at 1950°C with a release time of 200 ms, and this curve is reproduced in (c) along with data measured at 2150°C which have release times of 65 ms (cirlces) and 45 ms (squares).
The function ments I(r)
p(t)
is then
of the ion-beam
current,
= I, e-“‘p(
deduced
from
“slow’‘-fall time. The latter is used to account for a relatively slow release (*‘tail”) following a sharp rise in the ion-beam (“spike”) associated with the proton beam impact. The association of any physical significance directly to these parameters should be made with care, but they are a useful aid in comparing target performance. Errors are not quoted with the measurements presented since the measurements are rather variable in nature (the system is not tightly controlled and changes with time). Further, the measurements are taken at varying times during the use of the target for “production-running”. Consequently, the conditions of the target from one run to another are often difficult to reproduce. The release of Li
the measure-
t).
The release data shown in the figures are the raw measured data (without correction for half-life, and only the background counts have been removed). The curves are generated from a shown with the data [e -*‘p(r)] simple parameterisation of p(r) which is then integrated over the sample collection time. Yield data can easily be obtained by integration of the release curve. The function p(r) is here described by three exponentials giving the characteristics of a “rise’‘-time, a “fall’‘-time and a
1
=o.a ii
r;o.a ;
50.6 P 5 ‘@0.4
e.6
0
_~.
0
0
2000 Td (ms)
4000
0
1000
._-
2000
Td (ms)
3000
0 ! 0
1000
2000
3000
Td (ms)
Fig. 3. Release of 25Na from (a) the ISOLDE and (b) the RIST targets. The data in (a) were taken at 1950°C and have a release time of t s. The RlST data in (b) at this temperature have a release time of 250 ms, while those in (c) measured at 2150°C are. characterised by a release of 100 ms (squares) and 70 ms (circles).
SECTION V. TARGET TECHNIQUES
P.V. Drumm et al,/Nucl.
124
and Na release general to other
Instr. und Meth.in Phys. Rex B 126 (1997) 121-124
is discussed in detail as an illustration of the of the RIST and ISOLDE targets, although the observations and conclusions are likely to extend isotopes which are directly produced by the beam.
> 2 compared to the ISOLDE roll target at similar temperatures.
5. Conclusions 4. Results and discussion 4.1. Lithium The release of Li is most easily measured for 8Li (t ,,* - 0.8 s). Fig. 2a shows the range in release seen for an ISOLDE roll target: at a temperature of 1950°C the release is characterised by a 600 ms fall time (squares). A more recent result (circles) shows, at a temperature of 2150°C. a faster release of 200 ms. Results from the RIST-I target are shown in Fig. 2b and c ranging in temperature from 1950 (b) to 2 150°C (c), where the release varies from 200 ms in (b) to 65 ms (circles) and 45 ms (squares) in (c). The curve in (c) is taken from Fig. 2b, and indicates the increase of speed with temperature. The RIST target for Li is considerably faster than the ISOLDE target at the same temperature, by as much as a factor of 4. Despite the increase in release speed of the RIST target over the ISOLDE target, significant gains in the yields of short lived isotopes have yet to be observed for the RIST target.
4.2. Sodium The release of 25Na (t,,, - 59 s) is shown in Fig. 3a) for an ISOLDE target, and in (b) and (c) for the RIST target. Release times vary for the ISOLDE target from typically 1s (at 1950°C) as shown in Fig. 3a, down to 200 ms, and for the RIST target from 250 ms at 1950°C (Fig. 3b), down to 70 ms at 2150°C as shown in Fig. 3c. The curve shown in Fig. 3c is the fit obtained for the data in Fig. 3b. The RIST target for Na is faster by a factor of
A summary of the variation seen in the RIST and ISOLDE release curves is given in Table 1. It is quite clear from the given release data that temperature is very important in the release of the elements. In addition to temperature, the geometry of the RIST target has proved to be successful in increasing the release speed of ions by a significant factor (X 4 for the case of Li, and X 2 for Na). The RIST-II target which had thicker foils and a decreased effusion path shows results that are slower that for RIST-I, but still generally faster than for the roll target, indicating that more investigation is required to separate the effects of diffusion and effusion. The link between fast release and increased yield seems to be somewhat elusive, and it must be acknowledged that other factors (target fatigue, cold spots) play an undetermined part. The RIST target demonstrates that an ordered system has a significant effect on the release speed of the targets. The targets represent a complex system where modelling involves many unknown parameters (diffusion and sticking data being scant). It is clear that further practical work, following simple ideas - such as in the target geometry design - can be useful in order to obtain progress in target design.
References [I] J.R. Bennett, these Proceedings (EMIS-13). Nucl. Instr. and Meth. B 126 (1997) 105. [2] H.J. Kluge. ed., in: ISOLDE Users Guide, CERN-86-05 ( 1986).