ANURIB Advanced National facility for Unstable and Rare Ion Beams

Transcription

1 PRAMANA c Indian Academy of Sciences Vol. 85, No. 3 journal of September 2015 physics pp ANURIB Advanced National facility for Unstable and Rare Ion Beams ARUP BANDYOPADHYAY, V NAIK, S DECHOUDHURY, M MONDAL and A CHAKRABARTI Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata , India Corresponding author. arup@vecc.gov.in DOI: /s ; epublication: 28 August 2015 Abstract. An ISOL post-accelerator type of RIB facility is being developed at Variable Energy Cyclotron Centre (VECC), Kolkata, India. In this scheme, Rare Ion Beams (RIBs) will be produced using light ion beams (p, α) from the K = 130 cyclotron, the RIB of interest will be separated from the other reaction products and accelerated up to about 2 MeV/u using a number of linear accelerators. Recently, a few RIBs have been produced and accelerated using this facility. As an extention of this effort, another RIB facility ANURIB will be developed in a new campus as a green-field project. ANURIB will have two driver accelerators a superconducting electron LINAC to produce n-rich RIBs using photofission route and a 50 MeV proton cyclotron for producing p- rich RIBs. In this paper, the status of the RIB facility in the present campus and future plans with the ANURIB facility will be discussed. Keywords. Rare ion beams; accelerators; ion source; radio frequency quadrupole; LINAC; electron accelerator. PACS Nos c; c; Ej; a 1. Introduction Rare Ion Beams (RIBs) are beams of β-unstable nuclei which provide a number of new experimental opportunities in all fields of accelerator-based research. Just to mention a few, in nuclear physics, RIBs allow production and detailed study of very exotic nuclei, study of nuclei at very high excitation energy and angular momentum and also probing the detailed nature of the nuclear force by extending the study of nuclei from stability line towards the drip lines. In material science, RIBs allow the study of formation and propagation of lattice defects with a very high sensitivity using emission channelling technique and offer new Mössbauer isotopes which can be used for the study of otherwise chemically incompatible structures. In medical research, RIBs open up the possibility of using new radioisotopes for therapy and production of radioisotopes with high specific activity. Pramana J. Phys., Vol. 85, No. 3, September

2 Arup Bandyopadhyay et al RIBs can also provide long-lived radioisotopes for Positron Emission Tomography (PET) imaging. In general, there are two ways of producing RIBs fragment separation method and ISOL post-accelerator method. In fragment separation method, the RIBs are produced using projectile fragmentation reaction at high energy ( 100 MeV/u) and the reaction products are separated using a fragment separator. Highly exotic nuclear species can be produced by this technique having energy close to the primary beam. However, usually, obtaining an absolutely clean beam without sacrificing significant fraction of intensity is a challenge. In ISOL post-acceleration method, generally thick targets are used to maximize the intensity of RIBs the reaction products diffusing out of the target are ionized in an online ion source and mass separated to select the RIB of interest at low energy ( 1 kev/u). The RIBs are then further accelerated to the required energy depending on the nature of the experimental plan using a cyclotron or a series of linear accelerators. If a cyclotron is used, one can avoid a separate mass separator. The ISOL technique provides a clean beam (in terms of impurity) of good quality (emittance and time structure) at comparatively lower energy ( 1 5 MeV/u) but cannot provide very short-lived RIBs because of the delay in the target diffusion and ionization process. VECC has developed an ISOL post-accelerator type of RIB facility at the Bidhan Nagar campus around K = 130 cyclotron as the primary accelerator. Recently, a few RIBs have been produced in this facility and accelerated using Radio Frequency Quadrupole (RFQ) LINAC. During this development, an online electron cyclotron resonance (ECR) ion source, online isotope separator, two RFQ accelerators, four IH LINAC cavities and three RF rebunchers have been developed successfully. This facility will provide RIBs having energy up to about 2 MeV/u. As a natural extention of this effort, another RIB facility ANURIB will be developed in a new campus at Rajarhat as a green-field project. ANURIB will have two driver accelerators: a superconducting electron LINAC to produce RIBs using photofission route and a low energy proton cyclotron will be the primary accelerator for fusion evaporation route of producing RIBs. ANURIB will have both ISOL post-accelerator-type and fragment separator-type RIB facilities and therefore will be able to extract the advantages of both types of production techniques. In this paper, the status of the RIB facility in the Bidhan Nagar campus and future plans with the ANURIB facility at Rajarhat campus will be discussed. 2. The RIB facility at VECC The RIB facility at VECC Bidhan Nagar campus has been the R&D phase for the building blocks of an ISOL post-accelerator-type of RIB facility. The schematic lay-out of the facility is shown in figure 1. The RIB of interest will be produced when light ion (p, α) beams of optimum energy from K = 130 cyclotron fall on a suitable target. Significant progress has been made in different key areas of RIB facility development which will be briefly described in the following sections [1]. During these developments, collaboration with RIKEN, Japan has played an important role. 2.1 Target research The RIB of interest will be produced along with other reaction products when the primary beam falls on a thick target. It is important to use a suitable target compound which can 506 Pramana J. Phys., Vol. 85, No. 3, September 2015

3 Advanced National facility for Unstable and Rare Ion Beams Figure 1. Schematic lay-out of the RIB facility. sustain high temperature without significant change of its physical properties during the RIB production run. It should not have high vapour pressure, it should have the optimum thickness to use the maximum energy window of primary beam for the production of RIB of interest and also, it should have high surface-to-volume ratio to have better diffusion efficiency. A few targets have already been prepared and tested using the primary beam from cyclotron [2]. 2.2 Ion source development The reaction products diffusing out of the target need to be ionized for separation and subsequent acceleration of the RIBs. Unfortunately, there is no unique choice for online ion source which can provide high charge states with reasonable efficiency for all the elements. This problem is being addressed in our facility in two different ways: (a) a two-ion source charge breeder and (b) a multiple thin target gas jet technique coupled to an ECR ion source. In the first method, either a surface ionization source or a permanent magnetfree ECR ion source will produce 1 + ions of the reaction products immediately adjacent to the production target, the ions will pass through a magnetic separator and get decelerated to very low energy ( kev) ensuring that they can be captured and ionized in a high charge state ECR ion source [3,4]. In the second method, multiple thin targets will be used instead of a single thick target and the recoils coming out of the target will be thermalized in an atmosphere of inert gas, transported using gas jet technique and fed to a high charge state ECR ion source after a multiple skimmer stage to pump out the carrier gas. Table 1 shows the important parameters of high charge state online ECR ion source. The ECR ion source has already been used to produce metallic ions using MIVOC (metal ions from volatile compounds) technique in which vapours of an organic compound of the ion of interest is allowed to diffuse into the ion source. Metallic ions have also been produced by using sputtering technique in which the element to be ionized is fed to the ion source very close to the plasma zone and the atoms are sputtered out by the ions of the plasma. It is also possible to keep the feed material at a different potential to enhance the sputtering process. Pramana J. Phys., Vol. 85, No. 3, September

4 Arup Bandyopadhyay et al Table 1. Parameters of the high charge state online ECR ionsource for the VEC-RIB facility. Parameter Frequency Microwave power (maximum) B ECR B z (Solenoid coils) B r (NdFeB) Solenoid power (maximum) Value 6.4 GHz 3 kw 0.23 T 0.95 T (Inj.), 0.7 T (Ext.) 0.7 T 50 kwa 2.3 Radio frequency quadrupole (RFQ) development The RIB of interest is selected in a magnetic separator downstream of the ion source. The RIB is then accelerated in a series of linear accelerators, the first of which is a RFQ which is an extended rod-type structure that accelerates the ion beams to 98.8 kev/u. The picture of the 3.2 m long RFQ during commissioning is shown in figure 2. The resonant structure is formed by four vanes supported on eight posts on a base plate. Each diagonally opposite pair of vanes is supported by two posts. The basic rf-cell of the above four-rod structure can be described as two coupled λ/4 transmission lines excited in transverse π-mode forming a parallel LC resonant circuit with the vanes as capacitance and posts as inductance. The design specifications and the beam dynamics parameters of the RFQ LINAC for the VEC-RIB facility are listed in table 2 [5,6]. The RFQ vanes and posts were fabricated by the CSIR Central Mechanical Engineering Research Institute (Durgapur, India). 2.4 IH LINACs and beyond After the initial stage of acceleration in the RFQ, the subsequent acceleration of RIBs will be done using IH LINAC cavities which are best suited for the low β and low q/a ions, as they offer quite high shunt impedance at lower frequencies [7]. The transverse focussing will be taken care of by placing quadrupole triplets in between the cavities. The operating Figure 2. The 3.2 m long RFQ during commissioning. 508 Pramana J. Phys., Vol. 85, No. 3, September 2015

5 Advanced National facility for Unstable and Rare Ion Beams Table 2. Important parameters of the RFQ LINAC. Charge-to-mass ratio q/a 1/14 Operating frequency MHz 37.8 Input energy kev/u 1.7 Output energy kev/u 98.8 Length of vanes cm 320 Synchronous phase Total number of cells 146 Intervane voltage kv 53.7 Kilpatrick factor 1.2 Acceptance (design) π-cm-mrad 34 Transmission (<1mA) % 96 Energy width at RFQ exit % ±0.37 frequency of the RFQ, the first acceleration section in our scheme, is 37.8 MHz. This is a typical frequency required for acceleration of low β, low q/a heavy ions and is dictated by the requirement of stability of transverse motion within the RFQ. The LINACs which will follow the RFQ should have the same frequency of RFQ or its integer multiples. We have found that designing the first two cavities at 37.8 MHz and opting for higher harmonic operation thereafter, is a good choice to maximize the energy gain for the same number of cavities. The third cavity has been designed for a frequency of 75.6 MHz. There will be a charge stripper to increase the charge-to-mass ratio of the RIBs from 1/14 to 1/7 at the exit of the third cavity at an energy of kev/u. The fourth and the fifth cavities have been designed for 75.6 MHz and q/a 1/7 and they will accelerate ions to MeV/u. The important parameters of the IH cavities are shown in table 3 the shunt impedance, Q-value and RF power are HFSS calculated values assuming ideal surface conditions. Figure 3 shows IH cavities 1, 3 and 4 at different stages of commissioning. The high-power RF transmitters for the RFQ and the IH LINACs were developed by the Society of Applied Microwave Electronics Engineering and Research, Mumbai, India. Table 3. Important parameters of the IH cavities. Parameter Unit IH-1 IH-2 IH-3 IH-4 IH-5 Frequency MHz q/a 1/14 1/14 1/14 1/7 1/7 T in kev/u T out kev/u Gaps Sync. Phase Cavity length m Accln. gradient MV/m Shunt impedance M /m Q-value RF power kw Pramana J. Phys., Vol. 85, No. 3, September

6 Arup Bandyopadhyay et al Figure 3. IH cavity 1 (a), 3 (b)and4(c) during commissioning. Further acceleration of up to about 2 MeV/u has been planned using superconducting quarter wave resonators (QWRs). There will be two cryostats, each will accommodate four QWRs and a superconducting solenoid at the centre of the cryostat will provide radial focussing. 2.5 Production and acceleration of RIBs The commissioning test of the accelerators are carried out using ions of stable isotopes produced from the high charge state ECR ion source. Stable beams have been successfully accelerated up to the end of IH LINAC-3. A picture of the accelerator hall is shown in figure 4. The RIBs of 14 O(71s), 42 K (12.4 h), 43 K (22.2 h) and 41 Ar (1.8 h) were successfully produced at VECC, using a novel gas-jet recoil transport coupled ECR ionsource technique. The RIB of 14 O was further accelerated through the RFQ to an energy of around 1.4 MeV. Radioactive ion beam of 14 O was produced in one neutron evaporation reaction of proton on nitrogen whereas 42 K, 43 K and 41 Ar were produced from α-particle Figure 4. cavities. The accelerator hall showing RFQ, rebuncher-1 and the first two IH 510 Pramana J. Phys., Vol. 85, No. 3, September 2015

7 Advanced National facility for Unstable and Rare Ion Beams Figure 5. Floor diagram of the RIB beam-line showing the experimental arrangement and positioning of detectors. A typical γ -spectrum at RFQ exit is shown in the inset. reactions on argon gas target. Typical primary beam intensity was around 1 μa on the target. The target chamber was placed inside the cyclotron vault and the radioactive atoms produced in the target were transported 15 m away to the RIB cave (HR cave II) through a 1.4 mm inner diameter tygon capillary. The carrier gas was separated using multiple skimmer stages within the ECR injection chamber and reaction products were stopped on a porous catcher in the ECR ion source. The low-energy RIBs were selected in an isotope separator and further accelerated through the RFQ LINAC to around 100 kev/u. The radioactivities were measured at the detector locations shown in figure 5 using HpGe detectors and a typical γ -ray spectrum measured during the experiment is shown in the inset. The RIB intensities at various measurement locations are listed in table 4 [8]. 3. ANURIB the next phase of RIB development The RIB facility at VECC Bidhan Nagar campus is nearing completion. During this development, reasonable expertise has been developed to start working on a much bigger Table 4. Particle intensities measured at different locations during the experiment. Intensity (particles/s) Before After After After RIB Reaction T 1/2 ECR ECR separator RFQ 14 O 14 N(p, n) 71 s K 40 Ar(α, pn) 12.4 h K 40 Ar(α, p) 22.3 h Pramana J. Phys., Vol. 85, No. 3, September

8 Arup Bandyopadhyay et al facility which can provide more experimental opportunities compared to the present facility. This has prompted us to start working on a facility called Advanced National Facility for Unstable and Rare Ion Beams ANURIB [9,10]. ANURIB is a green-field project that will be developed on the Rajarhat campus of VECC. Financial sanction has been received for the first phase of the project which primarily aims at the preparation of the technical design report for the entire facility and R&D activities on superconducting electron LINAC, target ion source and construction activities of the e-linac and target building. The schematic lay-out of the ANURIB facility and the experimental opportunities at various stages are shown in figure 6. ANURIB will have two primary accelerators for producing RIBs: (1) A 50 MeV, 100 kw superconducting electron LINAC for the production of neutron-rich RIBs using photofission route and (2) a 50 MeV, 100 μa ring cyclotron producing proton-rich RIBs using fusion evaporation reactions. The RIBs will be ionized, mass separated and accelerated. The acceleration scheme will consist of RFQ, IH LINACs and superconducting QWRs (LINAC boosters) giving an energy of around 7 MeV/u. The RIBs will be further accelerated using a ring cyclotron to about 100 MeV/u. The entire acceleration chain will also be used for the acceleration of stable beams from a high current ECR ion source. The facility will provide experimental opportunities during different stages of its commissioning including the spectroscopy of r-process, study of n-rich exotic nuclei, material science research with stable and RIBs at an energy of around 1.5 kev/u to begin with and projectile fragmentation reaction studies with stable and RIBs at an energy of around Figure 6. Schematic lay-out of the ANURIB facility showing experimental opportunities at various phases. 512 Pramana J. Phys., Vol. 85, No. 3, September 2015

9 Advanced National facility for Unstable and Rare Ion Beams Figure 7. The front end of the e-linac driver. 100 MeV/u. The electron accelerator will also pave the way for two more interesting research areas using positron and neutron beams. The technical design report will be prepared within a couple of years that will contain the baseline design of the components. Reasonable progress has already been made in the design and development of superconducting e-linac [11,12]. A glimpse of these developments are given in the following section. 3.1 Electron LINAC RIBs can be produced using photofission reaction with 238 U using an electron LINAC. The electron beam can be stopped using a Ta converter and the Bremstrahlung photons can be used for the RIB production or the target itself can be used as the converter. Figure 8. The CCM has been designed and ready for fabrication. Pramana J. Phys., Vol. 85, No. 3, September

10 Arup Bandyopadhyay et al Figure 9. The mechanical model of the ICM without the cryostat and some of its components during fabrication. The photofission reaction of 238 U is dominated by GDR channel and the peak is around 15 MeV. At around 45 MeV electron energy, the number of electrons that can contribute to the GDR channel saturates and therefore a 50 MeV 2 ma e-linac accelerator has been selected. The 50 MeV energy will be reached using five Tesla type nine cell cavities operated at 1.3 GHz at 2 K temperature. The first cavity will be within the injector cryomodule and there will be two more acceleration cryomodules each containing two nine-cell cavities. The front end of the e-linac is shown in figure 7. A 100 kv, 10 ma DC thermoionic gun will be used as an electron source and the beam will be pulsed at 650 MHz using a gridded cathode. A room-temperature buncher will bunch the electron beam at 1.3 GHz followed by two single-cell elliptical cavities within the capture cryomodule (CCM) for matching the beam to the accepance of the first nine cell cavity placed within the injector cryomodule (ICM). The CCM has been designed (figure 8) and the fabrication will be taken up shortly. The ICM is being developed in collaboration with TRIUMF, Canada. Figure 9 shows the mechanical model without the cryostat tank (left) and some of the components during fabrication (right). ICM has been fabricated and beam tests are planned. 4. Conclusion The development of an ISOL post-accelerator type of RIB facility at the VECC Bidhan Nagar campus is nearing completion. A test run has been successfully completed to produce, ionize and accelerate the RIBs through RFQ. Stable beams have been accelerated till the end of the third IH LINAC cavity. Accelerator components up to about 1 MeV/u will be ready for commissioning soon. QWRs will be added to further augment the energy to 2 MeV/u. Another RIB facility, ANURIB, has been given financial sanction. ANURIB facility will be producing a large number of RIBs as well as high-intensity stable beams up to 100 MeV/u. 514 Pramana J. Phys., Vol. 85, No. 3, September 2015

11 References Advanced National facility for Unstable and Rare Ion Beams [1] Alok Chakrabarti et al, Curr. Sci. 108, 22 (2015) [2] D Bhowmick et al, Nucl. Instrum. Methods A 539, 54 (2005) [3] V Banerjee et al, Nucl. Instrum. Methods A 447, 345 (2000) [4] D Naik et al, Nucl. Instrum. Methods A 547, 270 (2005) [5] A Chakrabarti et al, Nucl. Instrum. Methods A 535, 599 (2004) [6] S Dechoudhury et al, Rev. Sci. Instrum. 81, (2010) [7] A Bandyopadhyay et al, Proc. Linear Accl. Conf. (Victoria, BC, Canada, 103, 2008) [8] V Naik et al, Rev. Sci. Instrum. 84, (2013) [9] Alok Chakrabarti, Nucl. Instrum. Methods B 261, 1018 (2007) [10] Alok Chakrabarti et al, Nucl. Instrum. Methods B 317, 253 (2013) [11] V Naik et al, Proc. Linear Accl. Conf. (Tsukuba, Japan, 727, 2010) [12] A Chakrabarti et al, Proc. Linear Accl. Conf. (Tel-Aviv, Israel, 225, 2012) Pramana J. Phys., Vol. 85, No. 3, September