- Email: [email protected]
Set-up for systematic measurements of diffusion of atoms from ISOL targets
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 4322–4325 www.elsevier.com/locate/nimb
Set-up for systematic measurements of diffusion of atoms from ISOL targets P. Jardin a,*, M.G. Saint-Laurent a, F. Durantel b, C. Ele´on a, C. Huet-Equilbec a, R. Alve`s Conde´ a, J. Cornell a, G. Gaubert a, J.Y. Pacquet a, F. Pellemoine a, M. Ozille a a
Grand Acce´le´rateur National d’Ions Lourds, 14076 Caen, France b Institut de Physique Nucle´aire d’Orsay, 91406 Orsay, France Available online 6 June 2008
Abstract The design of radioactive ion production systems by the Isotope-Separator-On-Line method requires knowledge of diffusion features of atoms out of solids. Owing to the large number of possible diffusing atoms in target material, it is often difficult to find the right information in the literature and inter-comparisons are often difficult due to differences in the experimental techniques and conditions. The TARGISOL [TARGISOL,] European collaboration aims to study the relevant variables governing the release of radioactive elements from targets, to produce new radioactive ion beams and to build a database which can facilitate the design of radioactive ion beam set-ups. The role of GANIL in this collaboration is to develop and produce new radioactive beams and provide new diffusion coefficient data. For this purpose, GANIL has designed a new system which removes part of the problems of comparing data. The approach is systematic by measurements of release properties from several targets using the same process, the same apparatus and during the same experiment. The set-up and the characterizing tests are described. The principle and results are presented in [C. Ele´on et al., these proceedings]. Ó 2008 Elsevier B.V. All rights reserved. PACS: 29.25. t; 29.38. c; 66.30.Je Keywords: Diffusion; Ar; Graphite; ISOL
1. Introduction The knowledge of the diffusion coefficients of atoms in solids are of first importance in the design of Isotope-Separator-On-Line (ISOL) systems. To make the most of their production in the target, the radioactive atoms have to leave it during a time as short as possible compared to their half-lives. Diffusion properties have already been widely studied but, due to the variety of possible target-element combinations, the major part of the studies has still to be done. In most of the studies, the diffusion coefficient is extracted from experiments involving one target in a spe*
Corresponding author. E-mail address: [email protected] (P. Jardin).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.069
cific set-up. It is often difficult to separate the contribution of the set-up from the contribution of the target itself. It is thus even more difficult to compare the results obtained with different set-ups. To get rid of the differences between experimental conditions, we built a prototype target barrel to allow us to perform systematic on-line measurements of release properties of several targets with the same process, with the same setup and during the same period of experiment. To determine the diffusion coefficient, we measure the current of the specific ions at the exit of a target/ion source system (TISS), either in a continuous mode or as a function of time. The principle of the extraction of the diffusion coefficient from the measurements is given in [2,3]. To fit with assumptions made in the theoretical description, the following prescription has been respected during the design
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325
of the apparatus: (i) the shape of the target is very simple, to allow us to neglect the effusion within its geometry compared to the effusion in the vacuum chamber, (ii) the temperature gradient of the target is low enough for us to consider the diffusion coefficient to be independent of the position of the atoms in the target, (iii) the power of the ion beam implanted in the target is negligible compared to the additional heating power, (iv) the target can be considered to be within the volume of the source, since the conductance between the target chamber and the source chamber is very large, (v) the contribution of the effusion–ionization process to the whole atom-to-ion transformation process can be measured separately and (vi) diffusing elements are noble gases. Elements other than noble gases could be used, but in that case, the theoretical description would have to be rebuilt to take into account the sticking of the element on the chamber surfaces. 2. Experimental set-up The set-up is composed of a target barrel to allow us to change targets on-line during the experiment, an ohmicallyheated oven to set the target to a temperature up to 2100 K and an ECR ion source directly connected to the target chamber (see Fig. 1). The extracted radioactive ion beam (RIB) is mass-selected and captured on the tape-system of an identification station [4]. When a target is placed in the oven, the vacuums of the target barrel chamber and of the ion source are differentially pumped and separated using a metallic cone (conductance 7 10 2 l s 1). 3. The primary beam 35
To measure the release of Ar out of the target, an Ar beam is implanted in the target after passing through a thin
4323
tantalum window and the ECR plasma. The release is deduced from the radioactive decay yields measured at the exit of the set-up. For stable isotopes, a beam power of some tens of watts would have been necessary to measure the ion current released by the TISS on a Faraday cup, introducing a local temperature gradient. Thus to avoid this gradient, radioactive ions were used, as a very small current of radioactive elements can be detected while using a very low beam power (<0.14 W). The radioactive 35 Ar beam is produced by the ‘‘in-flight fragmentation method” using projectile fragmentation of a 95 MeV/A 36 Ar primary beam in a C foil placed between the solenoids of SISSI [5] and selected by the GANIL alpha spectrometer operating as a recoil separator. The 35Ar was identified by the time-of-flight/energy-loss method using a movable silicon detector located just before the carbon target to be tested. For this identification a ‘‘pepper-pot” intensity attenuator was inserted in the primary beam in order to reduce the 35Ar intensity, to avoid destroying the silicon detector. Then, without the attenuator, the rate of irradiation was determined by a movable plastic detector inserted between two implantation runs of 35Ar. To avoid the irradiation of the container by the beam and to ensure the localization of the beam spot on the target, a 5 cm thick carbon collimator of 10 mm diameter was installed upstream of the target and the plastic scintillator. The beam sent to the targets was mainly 35Ar (with traces of 34Cl ions). The total intensity was lower than 107 pps.
4. The target barrel The target barrel [6] is composed of a wheel including up to 12 targets. By rotation, it is possible to align the chosen target with the axis of the oven (Fig. 2). A linear movement
Jack po position sition Passage forfor Pressure Pressure measurement measurement
window window Pu ing Pumping
Target Target Charger Charger
Fig. 1. Schematic of the GANIL TARGISOL set-up. The oven and the target are placed between the magnets of the source, directly in line with the ECR plasma.
oven oven
Mono1000 Mono1000 source source
Fig. 2. Target barrel connected to the ECR ion source.
4324
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325
with a toothed rack system translates the target to the centre of the oven. 5. The oven The oven (Fig. 3) is a carbon cylinder. Several cuts have been made around the circumference to lengthen the electrical circuit and hence the resistance. The good mechanical rigidity of carbon at high temperature is very important, considering that the oven is placed between two magnetic rings of the ECR source where the magnetic field reaches 3000 G and is thus submitted to a significant Laplace force. Its length is equal to 38 mm and its inner diameter is 20 mm, which has been determined to leave 1.5 mm between the container and the oven during the introduction of the container. The length has been determined to obtain a homogeneous temperature (within 20 K) over the stopping region of the ions in the target. Temperature simulations have been computed using the Systus [7] code. This software takes into account all heat exchanges: radiation, convection and conduction. The maximum heating power of the oven is 2.6 kW. At this power, measurements of the temperature with a thermocouple, as a function of the power, show a maximum of 2284 K. 6. The targets Six targets were made out of graphite (see drawing Fig. 3). As the range of 35Ar impinging in the carbon targets is equal to 1.25 mm, the geometry of the targets, number of slices and thicknesses were determined so as to place the Bragg peak in the middle of a slice. The diameter (13 mm) is much larger than the thickness (0.5 mm or 3 mm) so we can consider the targets to be a foil, as assumed in the analysis. To compare the effect of the grain size on the release efficiency, targets with similar geometries but with different grain sizes of graphite, from 1 lm to 16 lm, were installed in the barrel. The target and its container are positioned in the oven using a cone. The container is made of a 1 mm thick carbon graphite, with 4 lm grain size from Carbone Lorraine. The inner diameter has been fixed at 15 mm to allow for a 2-mm
Table 1 Graphite targets tested Carbon supplier
Grain size (lm)
Number of slices thickness
POCO [8] Carbone Lorraine [9] Carbone Lorraine
1 4 16
1 3 mm + 1 0.5 mm 4 0.5 mm 4 0.5 mm
difference in diameter between the container and the target, to allow for effusion of the atoms. The following targets and their containers were tested during the experiment. The sizes of the grains are given by the supplier (see Table 1). 7. The ion source The ion source is a 2.45-GHz singly-charged ECR ion source [10], developed at GANIL. It has been re-designed to match with the constraints of the barrel installation. Its dimensions and the large aperture between the two magnetic rings allow the installation of the oven close to and in direct sight of the plasma. The ionization efficiency depends on the elements to be ionised and on the working conditions. In case of Ar, it is determined by injecting a stable Ar flux into the source through a calibrated leak and by measuring the corresponding current at the exit of the source. The Ar ionization efficiency ranges typically from 20% to 90%, depending on the working conditions and has been measured during the on-line experiment [3]. The atom-to-ion transformation time of the source is determined by injecting short pulses of gas using a fast micro-valve. For an Ar ionization efficiency of 70%, the time is close to 120 ms. The ions being mainly singly-charged (90% of Ar ions are Ar+), the absence of charge-state distribution and of rise-time to produce higher charge-states make the measurements of efficiency and response time easier. Since the behaviour of the ECR ion source can change with the gas present in its chamber, it can be sensitive to the out-gassing coming from the target. Thus the ionization efficiency must be checked from time to take these variations into account during the analysis of the results. Acknowledgements This work has been supported by the EU-RTD project TARGISOL (HPRI-CT-2001-50033) and by the BasseNormandie Re´gion. We would like to thank the technical staff of GANIL, who has been particularly helpful. References
Fig. 3. Drawing of the oven and of the 4 0.5 mm carbon target in the container. In the case shown in the above figure, the 35Ar atoms stop in the middle of the third slice. Dimensions are in mm.
[2] C. Ele´on et al., these proceedings. [3] C. Ele´on, Ph.D. Thesis, University of Caen, France, 2007. [4] S. Kandri Rody, Ph.D. Thesis, Doukkali University of El Jadida, 2000, GANIL T 0004. [5] R. Anne et al., Nucl. Instr. and Meth. B 126 (1997) 279.
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325 [6] F. Durantel, Diploma CNAM, 2005.. [7] Logiciel Systus, ESI Group, Engineering Systems International, PAM – System International.
[8] POCO,. [9] Carbone Lorraine, . [10] P. Jardin et al., Rev. Sci. Instr. 73 (2002) 789.
4325
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 4322–4325 www.elsevier.com/locate/nimb
Set-up for systematic measurements of diffusion of atoms from ISOL targets P. Jardin a,*, M.G. Saint-Laurent a, F. Durantel b, C. Ele´on a, C. Huet-Equilbec a, R. Alve`s Conde´ a, J. Cornell a, G. Gaubert a, J.Y. Pacquet a, F. Pellemoine a, M. Ozille a a
Grand Acce´le´rateur National d’Ions Lourds, 14076 Caen, France b Institut de Physique Nucle´aire d’Orsay, 91406 Orsay, France Available online 6 June 2008
Abstract The design of radioactive ion production systems by the Isotope-Separator-On-Line method requires knowledge of diffusion features of atoms out of solids. Owing to the large number of possible diffusing atoms in target material, it is often difficult to find the right information in the literature and inter-comparisons are often difficult due to differences in the experimental techniques and conditions. The TARGISOL [TARGISOL,
1. Introduction The knowledge of the diffusion coefficients of atoms in solids are of first importance in the design of Isotope-Separator-On-Line (ISOL) systems. To make the most of their production in the target, the radioactive atoms have to leave it during a time as short as possible compared to their half-lives. Diffusion properties have already been widely studied but, due to the variety of possible target-element combinations, the major part of the studies has still to be done. In most of the studies, the diffusion coefficient is extracted from experiments involving one target in a spe*
Corresponding author. E-mail address: [email protected] (P. Jardin).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.069
cific set-up. It is often difficult to separate the contribution of the set-up from the contribution of the target itself. It is thus even more difficult to compare the results obtained with different set-ups. To get rid of the differences between experimental conditions, we built a prototype target barrel to allow us to perform systematic on-line measurements of release properties of several targets with the same process, with the same setup and during the same period of experiment. To determine the diffusion coefficient, we measure the current of the specific ions at the exit of a target/ion source system (TISS), either in a continuous mode or as a function of time. The principle of the extraction of the diffusion coefficient from the measurements is given in [2,3]. To fit with assumptions made in the theoretical description, the following prescription has been respected during the design
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325
of the apparatus: (i) the shape of the target is very simple, to allow us to neglect the effusion within its geometry compared to the effusion in the vacuum chamber, (ii) the temperature gradient of the target is low enough for us to consider the diffusion coefficient to be independent of the position of the atoms in the target, (iii) the power of the ion beam implanted in the target is negligible compared to the additional heating power, (iv) the target can be considered to be within the volume of the source, since the conductance between the target chamber and the source chamber is very large, (v) the contribution of the effusion–ionization process to the whole atom-to-ion transformation process can be measured separately and (vi) diffusing elements are noble gases. Elements other than noble gases could be used, but in that case, the theoretical description would have to be rebuilt to take into account the sticking of the element on the chamber surfaces. 2. Experimental set-up The set-up is composed of a target barrel to allow us to change targets on-line during the experiment, an ohmicallyheated oven to set the target to a temperature up to 2100 K and an ECR ion source directly connected to the target chamber (see Fig. 1). The extracted radioactive ion beam (RIB) is mass-selected and captured on the tape-system of an identification station [4]. When a target is placed in the oven, the vacuums of the target barrel chamber and of the ion source are differentially pumped and separated using a metallic cone (conductance 7 10 2 l s 1). 3. The primary beam 35
To measure the release of Ar out of the target, an Ar beam is implanted in the target after passing through a thin
4323
tantalum window and the ECR plasma. The release is deduced from the radioactive decay yields measured at the exit of the set-up. For stable isotopes, a beam power of some tens of watts would have been necessary to measure the ion current released by the TISS on a Faraday cup, introducing a local temperature gradient. Thus to avoid this gradient, radioactive ions were used, as a very small current of radioactive elements can be detected while using a very low beam power (<0.14 W). The radioactive 35 Ar beam is produced by the ‘‘in-flight fragmentation method” using projectile fragmentation of a 95 MeV/A 36 Ar primary beam in a C foil placed between the solenoids of SISSI [5] and selected by the GANIL alpha spectrometer operating as a recoil separator. The 35Ar was identified by the time-of-flight/energy-loss method using a movable silicon detector located just before the carbon target to be tested. For this identification a ‘‘pepper-pot” intensity attenuator was inserted in the primary beam in order to reduce the 35Ar intensity, to avoid destroying the silicon detector. Then, without the attenuator, the rate of irradiation was determined by a movable plastic detector inserted between two implantation runs of 35Ar. To avoid the irradiation of the container by the beam and to ensure the localization of the beam spot on the target, a 5 cm thick carbon collimator of 10 mm diameter was installed upstream of the target and the plastic scintillator. The beam sent to the targets was mainly 35Ar (with traces of 34Cl ions). The total intensity was lower than 107 pps.
4. The target barrel The target barrel [6] is composed of a wheel including up to 12 targets. By rotation, it is possible to align the chosen target with the axis of the oven (Fig. 2). A linear movement
Jack po position sition Passage forfor Pressure Pressure measurement measurement
window window Pu ing Pumping
Target Target Charger Charger
Fig. 1. Schematic of the GANIL TARGISOL set-up. The oven and the target are placed between the magnets of the source, directly in line with the ECR plasma.
oven oven
Mono1000 Mono1000 source source
Fig. 2. Target barrel connected to the ECR ion source.
4324
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325
with a toothed rack system translates the target to the centre of the oven. 5. The oven The oven (Fig. 3) is a carbon cylinder. Several cuts have been made around the circumference to lengthen the electrical circuit and hence the resistance. The good mechanical rigidity of carbon at high temperature is very important, considering that the oven is placed between two magnetic rings of the ECR source where the magnetic field reaches 3000 G and is thus submitted to a significant Laplace force. Its length is equal to 38 mm and its inner diameter is 20 mm, which has been determined to leave 1.5 mm between the container and the oven during the introduction of the container. The length has been determined to obtain a homogeneous temperature (within 20 K) over the stopping region of the ions in the target. Temperature simulations have been computed using the Systus [7] code. This software takes into account all heat exchanges: radiation, convection and conduction. The maximum heating power of the oven is 2.6 kW. At this power, measurements of the temperature with a thermocouple, as a function of the power, show a maximum of 2284 K. 6. The targets Six targets were made out of graphite (see drawing Fig. 3). As the range of 35Ar impinging in the carbon targets is equal to 1.25 mm, the geometry of the targets, number of slices and thicknesses were determined so as to place the Bragg peak in the middle of a slice. The diameter (13 mm) is much larger than the thickness (0.5 mm or 3 mm) so we can consider the targets to be a foil, as assumed in the analysis. To compare the effect of the grain size on the release efficiency, targets with similar geometries but with different grain sizes of graphite, from 1 lm to 16 lm, were installed in the barrel. The target and its container are positioned in the oven using a cone. The container is made of a 1 mm thick carbon graphite, with 4 lm grain size from Carbone Lorraine. The inner diameter has been fixed at 15 mm to allow for a 2-mm
Table 1 Graphite targets tested Carbon supplier
Grain size (lm)
Number of slices thickness
POCO [8] Carbone Lorraine [9] Carbone Lorraine
1 4 16
1 3 mm + 1 0.5 mm 4 0.5 mm 4 0.5 mm
difference in diameter between the container and the target, to allow for effusion of the atoms. The following targets and their containers were tested during the experiment. The sizes of the grains are given by the supplier (see Table 1). 7. The ion source The ion source is a 2.45-GHz singly-charged ECR ion source [10], developed at GANIL. It has been re-designed to match with the constraints of the barrel installation. Its dimensions and the large aperture between the two magnetic rings allow the installation of the oven close to and in direct sight of the plasma. The ionization efficiency depends on the elements to be ionised and on the working conditions. In case of Ar, it is determined by injecting a stable Ar flux into the source through a calibrated leak and by measuring the corresponding current at the exit of the source. The Ar ionization efficiency ranges typically from 20% to 90%, depending on the working conditions and has been measured during the on-line experiment [3]. The atom-to-ion transformation time of the source is determined by injecting short pulses of gas using a fast micro-valve. For an Ar ionization efficiency of 70%, the time is close to 120 ms. The ions being mainly singly-charged (90% of Ar ions are Ar+), the absence of charge-state distribution and of rise-time to produce higher charge-states make the measurements of efficiency and response time easier. Since the behaviour of the ECR ion source can change with the gas present in its chamber, it can be sensitive to the out-gassing coming from the target. Thus the ionization efficiency must be checked from time to take these variations into account during the analysis of the results. Acknowledgements This work has been supported by the EU-RTD project TARGISOL (HPRI-CT-2001-50033) and by the BasseNormandie Re´gion. We would like to thank the technical staff of GANIL, who has been particularly helpful. References
Fig. 3. Drawing of the oven and of the 4 0.5 mm carbon target in the container. In the case shown in the above figure, the 35Ar atoms stop in the middle of the third slice. Dimensions are in mm.
[2] C. Ele´on et al., these proceedings. [3] C. Ele´on, Ph.D. Thesis, University of Caen, France, 2007. [4] S. Kandri Rody, Ph.D. Thesis, Doukkali University of El Jadida, 2000, GANIL T 0004. [5] R. Anne et al., Nucl. Instr. and Meth. B 126 (1997) 279.
P. Jardin et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4322–4325 [6] F. Durantel, Diploma CNAM, 2005.
[8] POCO,
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