High-intensity cyclotrons for radioisotope production and accelerator driven systems

High-intensity cyclotrons for radioisotope production and accelerator driven systems

Nuclear Physics A 701 (2002) 100c–103c www.elsevier.com/locate/npe High-intensity cyclotrons for radioisotope production and accelerator driven syste...

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Nuclear Physics A 701 (2002) 100c–103c www.elsevier.com/locate/npe

High-intensity cyclotrons for radioisotope production and accelerator driven systems Y. Jongen, D. Vandeplassche, W. Kleeven, W. Beeckman, S. Zaremba, G. Lannoye, F. Stichelbaut ∗ Ion Beam Applications S.A. (IBA), Chemin du Cyclotron, 3, B-1348 Louvain-La-Neuve, Belgium

Abstract IBA recently proposed a new method to extract high-intensity positive ion beams from a cyclotron based on the concept of auto-extraction. We review the design of a 14 MeV, multi-milliampere cyclotron using this new technology. IBA is also involved in the design of the accelerator system foreseen to drive the MYRRHA facility, a multipurpose neutron source developed jointly by SCKCEN and IBA.  2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Since 1991, IBA has built and installed 16 CYCLONE 18+, a 18 MeV proton cyclotron specifically designed for the production of 103 Pd. This radioisotope is produced by a (p, n) reaction on a 103 Rh internal target. Due to the relatively low production yield, the cyclotron is designed to deliver a high current of 2 mA on target. IBA is now in the process of building a 14 MeV, multi-milliampere proton cyclotron for radioisotope production. This cyclotron described in Section 2 uses the concept of self-extraction to extract positive ions from the machine without making use of an electrostatic deflector. Another area where IBA is deeply involved is the design of high-intensity high-energy cyclotrons used for Accelerator Driven Systems (ADS). IBA is collaborating with SCKCEN (Mol, Belgium) on the MYRRHA project, a spallation neutron source described in Section 3.

* Corresponding author.

0375-9474/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 1 ) 0 1 5 5 5 - X

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2. The self-extraction cyclotron As it is well known, the critical point in a cyclotron is the extraction process, which needs to be highly efficient in order to obtain high-intensity beams. For positive ion extraction, the common method is to use an electrostatic deflector to modify the ions orbit in their last turn. This technique requires well separated turns and very good beam quality to reach high extraction efficiency. An alternative extraction method can be used when H− ions are accelerated. At extraction radius, the beam passes through a thin carbon foil wherein the negative ions are stripped from their electrons and are converted into positive ions. This extraction system is much simpler than the previous one and allows one to obtain extraction efficiency close to 100%. However, due to limitations on the use of high magnetic fields, these H− cyclotrons become huge once high energies are required. Recently, IBA has proposed a new method to extract high currents of positive ions from a cyclotron [1]. This new method, called the auto-extraction, will allow to reach close to 100% extraction efficiency without using extraction elements that could be damaged by high currents. The concept of auto-extraction relies on the following basic principles. In an isochronous cyclotron, the average field increases with radius to compensate the relativistic mass increase of the accelerated particles. Close to the pole edge, it becomes impossible to maintain an isochronous radial field profile, resulting in a phase lag of the accelerated particle with respect to the accelerating voltage on the dee. The acceleration stops when this phase lag reaches 90◦ , this point defining the limit of acceleration. At an other radius, the field index   dB r0 n=− B(r0 ) dr r0 reaches the value −1. This radius corresponds to the limit of radial focusing beyond which the ions escape the influence of the magnetic field. This limit of self-extraction is usually found at a larger radius than the limit of acceleration but, when the magnet gap at extraction become very small (like smaller than 20 times the radius gain per turn at extraction), the limit of self-extraction is reached before the limit of acceleration. The beam then escapes spontaneously the magnetic field when the pole edge is reached. In 1998, IBA started the design and construction of a high-current proton cyclotron to demonstrate the validity of this new extraction method. The energy of 14 MeV was selected for this prototype as this is the preferred energy for production of the radioisotope 103 Pd used in brachytherapy. Also, the new method requires a large turn separation at extraction, which is easier to achieve at lower energy. The design of this self-extraction cyclotron is illustrated in Fig. 1, showing a cut through the median plane. The magnet is characterized by a non-uniform hill gap decreasing from 36 mm close to the center to 15 mm at extraction. It is divided into 4 sectors, the one guiding the extracted beam containing a specially shaped groove. This groove is cut along the trajectory of the extracted orbit and is the passive equivalent of a septum magnet. The beam is generated by a cold cathode PIG source located at the center. It is normally accelerated from the central region on a well-centered orbit. At the last internal turn, the beam encounters two radially opposed Sm–Co permanent magnets that generate a ±0.1 Tesla field bump, extending

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Fig. 1. Schematic view of the self-extraction cyclotron.

azimuthally over 8 cm and rising radially in less than 1 cm. The strong radial oscillation resulting from these kickers creates a beam separation larger than 1 cm at extraction point, enough to bring the beam on a trajectory clearly escaping the cyclotron magnetic field. According to simulation the extraction efficiency should be around 90%. The RF amplifier is sized to be able to accelerate up to 140 kW of beam, corresponding to a maximum beam intensity at extraction of 10 mA. 3. The MYRRHA project Since 1997, the SCK-CEN and IBA have been studying together the development of the MYRRHA project [2], an ADS aimed at providing protons and neutrons for various R&D topics. The main missions of MYRRHA are: • the study of the transmutation in realistic conditions of minor actinides (MA such as 237 Np, 241 Am, 242 Cm, . . .) and long lived fission products (LLFP such as 95 Zr,99 Tc, 129 I,135 Cs, . . .); • the testing of reactor fuel and materials in high neutron fluxes; • the production of medical radioisotopes like 99 Mo or 125 I; • the study of various aspects of ADS technology. The MYRRHA facility would represent a preliminary step towards the construction of a full scale ADS to be used as waste transmutation and/or energy amplifier. It would consist of three main elements:

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1. An acceleration system able to deliver a proton beam of 2.7 mA at energy of 375 MeV into a spallation neutron source. This would correspond to a beam power of 1 MW. 2. A neutron source made of a Pb–Bi eutectic with a windowless design. 3. A subcritical core (0.85 < keff < 0.95) acting as an amplifier of the primary spallation source. This subcritical assembly would contain a fast neutron spectrum zone and a thermal spectrum zone, with neutron fluxes Φ>0.1 MeV > 1015 n/(cm2 s) and Φth > 2 × 1015 n/(cm2 s), respectively. The fast neutron zone will be used to perform MA transmutation studies and ADS fuel studies while radioisotope production, LLFD transmutation studies and researches on light water reactor fuels will be conducted in the thermal zone. The thermal power generated in the subcritical assembly would be between 20 and 30 MWth . IBA is conducting preliminary design studies of the accelerator system needed for MYRRHA. In view of the high energies required, the choice was made to use positive ion acceleration in a separate sector cyclotron. This cyclotron would contain four straight sectors of 45◦ and four acceleration cavities. That design would be able to accelerate protons up to 390 MeV before reaching the beam stability limits. The beam injection would be made at 2 MeV using a high-current DC injector like the Dynamitron accelerator. The beam extraction would be performed using a classical electrostatic deflector in a valley without RF followed by a septum magnet in the following valley. The possibility to accelerate the beam from 2 MeV to 375 MeV using a single cyclotron is attractive from the point of view of simplicity, reliability and cost but a two-stage accelerator complex is also considered as an option. A first cyclotron would accelerate protons up to 80 MeV and inject them into a second cyclotron to boost the beam energy to 375 MeV.

References [1] Y. Jongen, D. Vandeplassche, P. Cohilis, High Intensity Cyclotrons for Radioisotope Production, or the Comeback of the Positive Ion, in: Proc. 14th Int. Conf. on Cyclotrons and their Applications, Cape Town, South Africa, World Scientific, 1995, p. 115. [2] K. Van Tichelen et al., MYRRHA project, a windowless ADS design, in: 3rd Int. Conf. on Accelerator Driven Transmutation Technologies and Applications (ADDTA’99), Praha, Czech Republic, June 7–11, 1999.