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A target connected to an ECR source for the production of 13N ions
Nuclear Instruments and Methods in Physics Research B 84 (1994) 512-514 North-Holland
NOMB
Beam Interactions with Materials A Atoms
A target connected to an ECR source for the production of 13N ions Th. Delbar, W. Galster, P. Leleux *, P. Lipnik, M. Loiselet and G. Ryckewaert Institut de Physique Nucliaire
and Centre de Recherches du Cyclotron,
Universitk Catholique de Louvain,
Louvain-la-Neuve,
Belgium
Received 27 July 1993 and in revised form 28 October 1993
A target has been designed to efficiently produce and release 13N atoms under bombardment by a high intensity proton beam; the coupling of this target to an electron cyclotron resonance ion source has been performed.
1. Introduction Radioactive ion beams (RIB) are produced in Louvain-la-Neuve by coupling two cyclotrons through an electron cyclotron resonance (ECR) ion source. In this facility, the production cyclotron is a 30 MeV-500 uA proton machine; this has two consequences: (i) the (p,n) reaction is usually the most effective in this energy range, leading to the frequent need for enriched production targets; (ii) the high power which is dissipated in the production target means that an efficient cooling has to be designed, contrary to high-energy/ low-intensity production accelerators, where an additional warming has sometimes to be provided to release the activity. Earlier publications have reported on the general presentation of the facility [l] as well as on previous off- and on-line tests of the 13C target [2] used for the production of 13N ion beams, no conclusion about the best setup being reached at that stage. The purpose of the present paper is to describe the final design of the 13C production target, the efficient on-line extraction of radioactive 13N atoms from it and the coupling to the ECR source. This paper is organized as follows: sections 2 to 4 are devoted to the target material, design and operation, respectively. Section 5 deals with the coupling to the ECR source and the acceleration of 13N beams.
2. Target material Three methods have been applied to obtain a 13C graphite target. In an early stage of the development, the original target material, i.e. carbon powder en-
* Corresponding author. 0168-583X/94/$07.00
riched in 13C up to 99%, was mixed with 15% pitch as a binder. The mixture was then graphitized in an oven at 2100 K to get small pellets of 1 cm length and 1 cm diameter [3]; seven such pellets were then embedded in holes drilled in a 4 cm diameter natural graphite block. In a second stage a cylinder of 3 cm diameter and 1.5 cm length of the same mixture was graphitized at 3000 K in an oven [4], resulting in a more homogeneous target. Finally, enriched carbon powder was compressed and graphitized by the 30 MeV proton beam, leading to the production of a 1.5 cm thick 13C graphite target after a few irradiations, some fresh powder being added each time. No significant difference in the release properties nor in the beam impact resistance was observed between these three targets. The simplest manufacture method, i.e. the third one, appears thus as the most attractive.
3. Target design The target design has to take into account the diffusion properties of the radioactive atoms of interest i e. i3N in our case, as well as the thermal propertie: of the target material, i.e. graphite: the diffusion of 13N is indeed important only above 2200 K [5], while graphite starts sublimating above 2600 K. In this temperature range, thermal conductivity is the most effective process to remove the beam-induced heat. The target is enclosed in a natural graphite container which is itself in thermal contact with an external water-cooled copper jacket (Fig. 1); two interfaces, graphite-graphite and graphite-copper are indeed necessary to keep the target temperature between the mentioned limits. This final design is in fact a combination of the edgecooled and rear-cooled types described in ref. [2].
0 1994 - Elsevier Science B.V. All rights reserved
SSDI 0168-583X(93)E0761-5
Th. Delbar et al. /Nucl.
water
Instr. and Meth. in Phys. Res B 84 (1994) 512-514
513
this activity into to the cyclotron beam pipe, another graphite vacuum window, 0.05 cm thick and 19 cm away from the target, separates the beam pipe from the target region (Fig. 2). This rather large distance is mandatory to protect the vacuum window from too large a radiative heat transfer from the target; the cylinder linking both windows is made of stainless-steel which is also water-cooled. The vacuum window is mounted in a copper frame and is electrically connected to the target to avoid any problem in the volume filled with hot and ionized gas. Graphite collimators are protecting the copper and stainless steel parts against any direct beam impact.
cooling
gas
SUPPLY
tzl
copper graphite
Fig. 1. Detailed view of the 13C target enclosed in a graphite container and surrounded by a water-cooled copper jacket. The 0.05 cm thick target window is set in a copper ring which is then bolted to the copper jacket. Natural nitrogen gas is supplied into the target from the back to release the 13N activity.
To protect against target sublimation, the target block is closed by a 0.05 cm thick graphite window to the beam side. The pressure in the enclosed volume is of the order of 10-i Torr. At high temperature this target window is porous, thus allowing the produced activity to leave the target volume. To avoid pumping
4. Target operation The 13C target is bombarded by a 30 MeV 200 uA proton beam which is stopped in the target. The 6 kW beam power is spread homogeneously over the whole target area by 50 Hz wobbling magnets, only a few percent of the beam intensity hitting the graphite collimators. Nitrogen diffuses through the target and the target window and is pumped to the ECR source via a liquid nitrogen trap. Upstream the vacuum window, a vacuum of about 8 x 10m6 Torr was maintained. In order to keep a constant activity transfer, it was found necessary to supply some natural nitrogen to the back of the target (N 0.1 see/h); an exchange process of the form 13N + 14N14N +14N +i3N14N allows thus to obtain the 13N activity as highly stable nitrogen molecules.
Fig. 2. General view of the installation at the end of the beam pipe. The “target” label refers to the upper part of Fig. 1. The activity is pumped through the gas outlet to the ECR source. The vacuum window separates the target low-vacuum region from the beam pipe high-vacuum region.
514
i%. Delbar et al. /Nucl. Instr. and Meth. in Phys. Res B 84 (1994) 512-514
5. The ECR source and beyond
The radioactive gas is ionized in a ECR ion source; off-line tests have shown [6] that an ionization efficiency of 52% for Ne’+ and of 20% for N’ + could be obtained. Those numbers are strongly dependent on the pressure in the source; a mass quadrupolar analysis having revealed the presence of hydrocarbons based on 13C and 12C and of oxides of carbon in the transferred gas, a liquid nitrogen trap was inserted between the target outlet and the ECR source, resulting in a vacuum of the order of 3-4 x lop5 in the source during on-line operation; it should be noticed that CO and N, molecules are not condensed in the liquid nitrogen trap. An ionization efficiency of 8% was obtained with those conditions. In case of the 10 min half-life 13N activity, no noticeable loss due to the transit time or to some kind of sticking was observed. At the exit of the ECR source the ionized atoms are mass-analysed by a 90” magnet and injected into the accelerating cyclotron. A catcher located along that transport line allows to measure the radioactive beam intensity by counting the positrons emitted in the decays. Ions with a given mass-to-charge ratio are injected in the cyclotron, the intensity of the t3N ions being only a few 10e3 of the 13C ions, which originate from the residual gas in the source and from t3C0 molecules not condensed in the trap; this ratio is obtained after baking the target with the 30 MeV proton beam, and it remains fairly constant during the running time. Fortunately, the specific characteristics of the cyclotron as a mass separator [7] provide a clear separation between both species, of which the relative mass difference is
1.8 x 10m4. Intensities of 70 ppA of 13N beams in the lc charge state are routinely obtained.
Acknowledgements
We wish to thank P. Collin and R. Versin for their skillful help in the machining and mounting of the targets. One of us (P. Leleux) is Maitre de Recherches of the National Fund for Scientific Research, Belgium. This paper presents research results of the Belgian programme on Interuniversity Poles of attraction initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming. The scientific responsibility is assumed by the authors.
References [l] D. Darquennes et al., Phys. Rev. C 42 (1990) 804; P. Van Duppen et al., Nucl. Instr. and Meth. B 70 (1992) 393;
P. Van Duppen et al., Nucl. Phys. A 553 (1993) 837~. [2] D. Darquennes et al., Nucl. Instr. and Meth. B 47 (1990) 311.
These pellets were made by Mr. L. Coheur from the CEN/SCK, Mol, Belgium. [4] This graphitization was realized by Mr. Karl Wimmer from the SIGRI Company, Meitingen, Germany. [5] P. Decrock et al., Nucl. Instr. and Meth. B 70 (1992) 182. [3]
[6] P. Decrock et al., Nucl. Instr. and Meth. B 58 (1991) 252. [7] G. Ryckewaert et al., Proc. 13th Int. Cyclotron Conf., Vancouver, Canada, 1992, eds. G. Dutto and M.K. Craddock (World Scientific, 1993) p. 737.
NOMB
Beam Interactions with Materials A Atoms
A target connected to an ECR source for the production of 13N ions Th. Delbar, W. Galster, P. Leleux *, P. Lipnik, M. Loiselet and G. Ryckewaert Institut de Physique Nucliaire
and Centre de Recherches du Cyclotron,
Universitk Catholique de Louvain,
Louvain-la-Neuve,
Belgium
Received 27 July 1993 and in revised form 28 October 1993
A target has been designed to efficiently produce and release 13N atoms under bombardment by a high intensity proton beam; the coupling of this target to an electron cyclotron resonance ion source has been performed.
1. Introduction Radioactive ion beams (RIB) are produced in Louvain-la-Neuve by coupling two cyclotrons through an electron cyclotron resonance (ECR) ion source. In this facility, the production cyclotron is a 30 MeV-500 uA proton machine; this has two consequences: (i) the (p,n) reaction is usually the most effective in this energy range, leading to the frequent need for enriched production targets; (ii) the high power which is dissipated in the production target means that an efficient cooling has to be designed, contrary to high-energy/ low-intensity production accelerators, where an additional warming has sometimes to be provided to release the activity. Earlier publications have reported on the general presentation of the facility [l] as well as on previous off- and on-line tests of the 13C target [2] used for the production of 13N ion beams, no conclusion about the best setup being reached at that stage. The purpose of the present paper is to describe the final design of the 13C production target, the efficient on-line extraction of radioactive 13N atoms from it and the coupling to the ECR source. This paper is organized as follows: sections 2 to 4 are devoted to the target material, design and operation, respectively. Section 5 deals with the coupling to the ECR source and the acceleration of 13N beams.
2. Target material Three methods have been applied to obtain a 13C graphite target. In an early stage of the development, the original target material, i.e. carbon powder en-
* Corresponding author. 0168-583X/94/$07.00
riched in 13C up to 99%, was mixed with 15% pitch as a binder. The mixture was then graphitized in an oven at 2100 K to get small pellets of 1 cm length and 1 cm diameter [3]; seven such pellets were then embedded in holes drilled in a 4 cm diameter natural graphite block. In a second stage a cylinder of 3 cm diameter and 1.5 cm length of the same mixture was graphitized at 3000 K in an oven [4], resulting in a more homogeneous target. Finally, enriched carbon powder was compressed and graphitized by the 30 MeV proton beam, leading to the production of a 1.5 cm thick 13C graphite target after a few irradiations, some fresh powder being added each time. No significant difference in the release properties nor in the beam impact resistance was observed between these three targets. The simplest manufacture method, i.e. the third one, appears thus as the most attractive.
3. Target design The target design has to take into account the diffusion properties of the radioactive atoms of interest i e. i3N in our case, as well as the thermal propertie: of the target material, i.e. graphite: the diffusion of 13N is indeed important only above 2200 K [5], while graphite starts sublimating above 2600 K. In this temperature range, thermal conductivity is the most effective process to remove the beam-induced heat. The target is enclosed in a natural graphite container which is itself in thermal contact with an external water-cooled copper jacket (Fig. 1); two interfaces, graphite-graphite and graphite-copper are indeed necessary to keep the target temperature between the mentioned limits. This final design is in fact a combination of the edgecooled and rear-cooled types described in ref. [2].
0 1994 - Elsevier Science B.V. All rights reserved
SSDI 0168-583X(93)E0761-5
Th. Delbar et al. /Nucl.
water
Instr. and Meth. in Phys. Res B 84 (1994) 512-514
513
this activity into to the cyclotron beam pipe, another graphite vacuum window, 0.05 cm thick and 19 cm away from the target, separates the beam pipe from the target region (Fig. 2). This rather large distance is mandatory to protect the vacuum window from too large a radiative heat transfer from the target; the cylinder linking both windows is made of stainless-steel which is also water-cooled. The vacuum window is mounted in a copper frame and is electrically connected to the target to avoid any problem in the volume filled with hot and ionized gas. Graphite collimators are protecting the copper and stainless steel parts against any direct beam impact.
cooling
gas
SUPPLY
tzl
copper graphite
Fig. 1. Detailed view of the 13C target enclosed in a graphite container and surrounded by a water-cooled copper jacket. The 0.05 cm thick target window is set in a copper ring which is then bolted to the copper jacket. Natural nitrogen gas is supplied into the target from the back to release the 13N activity.
To protect against target sublimation, the target block is closed by a 0.05 cm thick graphite window to the beam side. The pressure in the enclosed volume is of the order of 10-i Torr. At high temperature this target window is porous, thus allowing the produced activity to leave the target volume. To avoid pumping
4. Target operation The 13C target is bombarded by a 30 MeV 200 uA proton beam which is stopped in the target. The 6 kW beam power is spread homogeneously over the whole target area by 50 Hz wobbling magnets, only a few percent of the beam intensity hitting the graphite collimators. Nitrogen diffuses through the target and the target window and is pumped to the ECR source via a liquid nitrogen trap. Upstream the vacuum window, a vacuum of about 8 x 10m6 Torr was maintained. In order to keep a constant activity transfer, it was found necessary to supply some natural nitrogen to the back of the target (N 0.1 see/h); an exchange process of the form 13N + 14N14N +14N +i3N14N allows thus to obtain the 13N activity as highly stable nitrogen molecules.
Fig. 2. General view of the installation at the end of the beam pipe. The “target” label refers to the upper part of Fig. 1. The activity is pumped through the gas outlet to the ECR source. The vacuum window separates the target low-vacuum region from the beam pipe high-vacuum region.
514
i%. Delbar et al. /Nucl. Instr. and Meth. in Phys. Res B 84 (1994) 512-514
5. The ECR source and beyond
The radioactive gas is ionized in a ECR ion source; off-line tests have shown [6] that an ionization efficiency of 52% for Ne’+ and of 20% for N’ + could be obtained. Those numbers are strongly dependent on the pressure in the source; a mass quadrupolar analysis having revealed the presence of hydrocarbons based on 13C and 12C and of oxides of carbon in the transferred gas, a liquid nitrogen trap was inserted between the target outlet and the ECR source, resulting in a vacuum of the order of 3-4 x lop5 in the source during on-line operation; it should be noticed that CO and N, molecules are not condensed in the liquid nitrogen trap. An ionization efficiency of 8% was obtained with those conditions. In case of the 10 min half-life 13N activity, no noticeable loss due to the transit time or to some kind of sticking was observed. At the exit of the ECR source the ionized atoms are mass-analysed by a 90” magnet and injected into the accelerating cyclotron. A catcher located along that transport line allows to measure the radioactive beam intensity by counting the positrons emitted in the decays. Ions with a given mass-to-charge ratio are injected in the cyclotron, the intensity of the t3N ions being only a few 10e3 of the 13C ions, which originate from the residual gas in the source and from t3C0 molecules not condensed in the trap; this ratio is obtained after baking the target with the 30 MeV proton beam, and it remains fairly constant during the running time. Fortunately, the specific characteristics of the cyclotron as a mass separator [7] provide a clear separation between both species, of which the relative mass difference is
1.8 x 10m4. Intensities of 70 ppA of 13N beams in the lc charge state are routinely obtained.
Acknowledgements
We wish to thank P. Collin and R. Versin for their skillful help in the machining and mounting of the targets. One of us (P. Leleux) is Maitre de Recherches of the National Fund for Scientific Research, Belgium. This paper presents research results of the Belgian programme on Interuniversity Poles of attraction initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming. The scientific responsibility is assumed by the authors.
References [l] D. Darquennes et al., Phys. Rev. C 42 (1990) 804; P. Van Duppen et al., Nucl. Instr. and Meth. B 70 (1992) 393;
P. Van Duppen et al., Nucl. Phys. A 553 (1993) 837~. [2] D. Darquennes et al., Nucl. Instr. and Meth. B 47 (1990) 311.
These pellets were made by Mr. L. Coheur from the CEN/SCK, Mol, Belgium. [4] This graphitization was realized by Mr. Karl Wimmer from the SIGRI Company, Meitingen, Germany. [5] P. Decrock et al., Nucl. Instr. and Meth. B 70 (1992) 182. [3]
[6] P. Decrock et al., Nucl. Instr. and Meth. B 58 (1991) 252. [7] G. Ryckewaert et al., Proc. 13th Int. Cyclotron Conf., Vancouver, Canada, 1992, eds. G. Dutto and M.K. Craddock (World Scientific, 1993) p. 737.