|Location||History||LayOut of IUAC|
PAST, PRESENT AND FUTURE : A PERSPECTIVE
Research Centres in universities were emphasised in the national policy on education in the late sixties. In recognition of the long felt needs projected by the university community, the concept of the Inter University Centres (IUC) was accepted by the ministry in 1984. Inter-University Accelerator Centre, the first in the families of IUCs, came up in 1984. Two more facilities, IUC for Astronomy and Astrophysics and IUC for DAE Facilities have come up subsequently.
The beginning of the Centre was marked by the joining of the Director Prof. A.P. Patro and the Joint Director Prof. G.K. Mehta in December, 1984. It was decided to purchase a Pelletron accelerator from National Electrostatic Corporation (NEC) with the capability of going up to 15 Million Volts using accelerator tubes of standard geometry. Later, after the installation had commenced, it was decided to change over to the compressed geometry tubes for achieving higher terminal potential.
Construction of the building started in 1986 and was completed in 1989. The commissioning of the Pelletron Accelerator started in August, 1989 and was completed by Dec, 1990. In parallel to installation of the accelerator, the responsibilities for setting up on-line facilities in the beam hall were also taken up. Group discussions were held periodically amongst the core working groups in different fields to decide on focussed areas of research and required facilities. An workshop, to converge on major research programs to be pursued, was organized in Delhi university during Oct, 1987.
On Dec 19, 1988, a small ceremony was held in which Prof. Agwani, Vice Chancellor, JNU, formally handed over IUAC to Prof. Yash Pal, Chairman, UGC. The anniversary of this occasion is celebrated every year around December 19 as our Foundation Day Program.
The main building at Inter-University Accelerator Centre houses the Pelletron Tower, Beam Hall and the Laboratory complex. The 50 meter tall tower is made of heavy concrete for radiation shielding. The accelerator tank of 26.5 m height and 5.5 m diameter is filled with SF6 gas at a pressure of ~ 6-7 atmosphere. The SNICS (Source of Negative Ion by Cesium Sputtering) ion source acts as a source of negative ions that are momentum analyzed by the Injector magnet. For pulsed beam experiments, a drift tube buncher, operating at 4 MHz bunching frequency, was installed by NEC. This has subsequently been replaced by a single gap multi-harmonic buncher, operating at the fundamental frequency of 12 MHz, which was developed in collaboration with the Argonne National lab, USA. The new buncher provides a substantial improvement of bunching efficiency (> 60%) as compared to the old buncher (efficiency ~ 15%). This brings the bunching width to about 1 -2 ns. A second stage buncher, to get ultra-short bunching widths having duration ~ 140 ps, is getting implemented through the use of a superconducting resonating cavity. A Travelling Wave Deflector tube is used to increase the spacing between the bunches, if required by the experiment, by removing the unwanted bunches.
The terminal located at the centre of the tank is raised to a voltage of up to 15 MV by a Pellet charging system. In the original machine, the stability of the voltage was maintained by a corona point voltage grading system. This has now been replaced by a Resistor Voltage Grading System with increased machine stability and the ability to operate between 3- 15 MV without changing gas pressure.
Inside the terminal, the -ve ions are stripped of their electrons by passing through a thin carbon foil. The positive ions so produced are further accelerated towards the ground potential at the exit of the tank and momentum analysed to select the required charge state. One typically obtains an analysed current of 5 - 50 pnA with an energy of 30-200 MeV depending on the ion species.
For very heavy ions, ( A > 50), the life times of the carbon foils used in stripper are limited to a few hours only due to radiation damage. A gas filled canal ( or a combination of gas stripper followed by a foil stripper is used for heavy ions.
The accelerated beam from Pelletron is brought to the beam hall and can be switched to any one of the seven beam lines using the Switching Magnet. Dedicated experimental facilities are located in six of these beam lines for research in focussed areas, with the "zero degree" beam line available for post-acceleration of the beam through the LINAC.
The research activities at the Centre can be grouped in the following general areas :
|Basic Sciences||Applied Sciences||Inter-Disciplinary Areas|
Nuclear Reactions Near Coulomb Barrier
High Spin Spectroscopy
Spectroscopy of Highly Charged Ions
Accelerator Mass Spectroscopy
Interaction of Swift Heavy Ions with Materials
Archeology, Geology, Oceanography etc.
The experimental facilities at the centre were developed with active collaboration and participation from the user community. Initial funding for these facilities came from the University Grants Commission. Other agencies like Department of Science & Technology and BRNS have contributed significantly towards the funding of these facilities.
Gamma Detector Array
The Gamma Detector Array, established in early 90's was the largest facility for the study of gamma spectroscopy in the country, consisting of 12 Compton suppressed HPGe detectors (photo peak efficiency ~ 23%) supplemented by a 14 BGO detector multiplicity filter. It was setup in collaboration with Andhra, Banaras, Bombay, Delhi, Punjab, and MS(Baroda) Universities.
Among the major add-ons to this facility are, recoil distance lifetime measuring equipment, a mini-orange conversion electron spectrometer, a charge particle detector array and an electro-magnet for perturbed angular correlation measurement studies. The whole setup allows experiments on complete spectroscopy, leading to detailed information on nuclear structure.
Heavy Ion Reaction Analyser (HIRA)
HIRA is one of three or four recoil separators in the world. It consists of a series of magnetic and electric dipoles and quadrupoles on a rotating platform. The recoil products produced in a nuclear reaction are first selected according to their E/q in the first electrostatic deflector and then mass analysed in a combination of magnetic and electrostatic dipoles. A unique feature of HIRA is a slot in the anode plate of the first electric dipole, allowing very high beam rejection ratios. HIRA was setup at IUAC in collaboration with the Universities of AMU, Andhra, Bangalore, BHU, Bombay, Calicut, Delhi, Madras, MSU, NEHU, Punjab, and Saurastra.
HIRA has been extensively used for study of heavy ion induced fusion reaction mechanism at and near the fusion barrier. A large NaI gamma detector with plastic veto detectors (HIGRASP) has been used for the study of giant resonance built on excited states. HIRA has also been used in conjunction with GDA detectors, providing mass tagging for gamma spectroscopy and allowing exploration of weak proton-rich reaction channels.
One of the major development work using HIRA has been the production of a low energy Radioactive Ion Beam (RIB) in a nuclear reaction using inverse kinematics. HIRA facility has been used to separate out the reaction products from the direct beam using the excellent momentum resolution offered by the magnetic dipole element of HIRA. The reaction p(7Li,7Be)n has been used to produce a low energy (11-22 MeV) beam of 7Be with better than 99.99% purity and 3 mm diameter spot size ( 5x 104 ions/sec intensity).
The angular distribution of the transfer reaction d(7Be,8B)n at Ecm = 4.5 MeV, has been measured for the extraction of S17, which is of astrophysical interest. Other Radioactive Ion Beams (6He, 8Li, 11C & 17F) are planned in future.
Apart from these two major facilities, the General Purpose Scattering Chamber (GPSC) is extensively used in the studies of Heavy Ion Scattering and transfer reactions, Projectile Breakup and Materials Science. The chamber of 1.5 m diameter and hydrolically controlled lifting mechanism for the lid was setup in collaboration with Bangalore, Gulbarga & Mysore University.
Materials Science experimental facility includes three chambers integrated in the +15° beam line with a base vacuum better than 10-9 torr for various on-line and in-situ measurements. The beam line has a magnetic scanner which can sweep the beam (25 mm in x-direction and 10 mm in y direction) for uniform irradiation of samples. The facilities available are:
High Vacuum Chamber has two movable arms for detector mounting. The various samples are irradiated using a bellow sealed linear movement with provision for cooling the target to LN2 temperature. There is provision for on-line electrical transport and in-situ conduction noise measurement. A Large Area Position Sensitive detector is used for Elastic Recoil Detector Analysis (ERDA). An on-line ionoluminescence and photo-luminescence setup is installed in this chamber.
Ultrahigh vacuum Chamber has been tested to a vacuum of 10-10 torr using a combination of turbo, ion and getter pump. It has a bellow-sealed target ladder operable in the temperature range of 77° to 500° K. RGA is incorporated in this chamber for ion-beam induced desorption. An Ultra High Vacuum (UHV) STM has been installed in this chamber for in-situ surface studies. This is a collaborative project between IUAC and IISC, Bangalore funded by DST.
Goniometer Chamber houses a three-axis goniometer for ion-channeling studies. The sample can be rotated with a precision of 0.01° about the chamber axis and beam axis. In addition, sample can be tilted with a precision of 0.007° and can have translation in all three ( x,y and z) directions. Arrangement for in-situ studies using a Cylindrical Mass Analyser (CMA) is also being planned in the chamber.
The General Purpose Scattering chamber is extensively used for materials science research for depth profiling, on-line hydrogen loss measurements, stopping power measurements and secondary electron emission studies. A time-of-flight setup is planned for electronic sputtering desorption studies.
Other important research areas in the Centre have been in radiobiology and atomic physics. A facility for Accelerator Mass Spectroscopy (AMS) for trace-element analysis is currently under development.
The heavy ions, because of their complex electronic structure, interact with target atoms in a way that are quite different from light ion induced reactions. A major field in IUAC has been the study of ion-atom collisions. The SCORPION facility consists of an elcctrostatic deflector, time-of-flight spectrometer and a parallel plate avalanche counter that allows the coincident detection of individual charge states of the projectile ion and recoil ions after collision. More conventional devices for studying X-rays emitted in such reactions, in the form of Si-Li and low energy Ge detectors (LEPS) are also available. A new setup for beam foil spectroscopy for measuring life times of atomic states by measuring radiation from the excited beam particles after passing through a foil has been installed.
The main interest in radiation biology is in studying the effect of heavy ionising particles, with a large value of Linear Energy Transfer (LET) , on DNA. Since the living cells die in vacuum, an arrangement has been made to bring out the ion beam out in the air through a window foil. There is need of a very low flux (compared to the requirement in nuclear physics and materials science experiments) which has been achieved by scattering the primary beam from a target foil and detecting the small-angle scattered secondary beam. A number of offline facilities like CO2 incubator for cell culture, Gel Electrophoresis equipment for DNA strand damage diagnosis and an UV spectrophotometer to detect biochemical changes in enzymes has been setup at IUAC for research in radiobiology.
IUAC being an inter-University Centre, the main user-base comes from the universities. Currently there are users from 58 universities and 35 colleges. In addition, there is participation from the 5 IITs and 28 research institutions in the country.
For the selections of experiments to be conducted using the Pelletron Accelerator, user workshops are held twice every year ( in July and in December). During the workshop, presentations are made by the users for new proposals asking for beam time at IUAC, which are evaluated by the Accelerator Users Committee and allocation of beam time is made. Currently the maximum number of users are in the field of materials science (~ 65%), although the experiments on nuclear physics continue to get the maximum share of beam time (~ 63%). In order to expand the user base, orientation programs are periodically held at different places, in addition to conducted workshops in focussed areas of research.
During the last ten years of operation of the accelerator, a number of upgradations of the machine components have taken place to improve its performance. The corona-stabilisation voltage distribution inside the tank has been replaced by a resistor-based voltage distribution system that provides a better terminal voltage stability and a wide range of operational terminal voltage (3-15 MV) without the use of shorting rods.
The single harmonic buncher supplied with the accelerator has been replaced by a single gap multi-harmonic buncher for a factor of 4 improvement in the bunched beam current. A spiral cavity pickup unit has been placed down-stream to the accelerator to detect the phase of the beam and lock it to an external lock signal.
The PDP-11 control system supplied by NEC has been replaced by an indigenously developed LINUX based control software. The new system supports multiple control consoles which are connected by ethernet using the Client-Server model. It eliminates the need for serial network through which the remote stations were controlled earlier. Some of the hardware like Assignable Control Knob have been indigenously fabricated. The present software would allow the integration of the Pelletron with the LINAC components which are currently being commissioned. The Data-acquisition system for experiments using Pelletron has been upgraded to support multiple CAMAC crates and a distributed data collection system.
Some of the important developments carried out recently are summarised below:
Low Energy Ion Beam Facility (LEIBF)
For gaining experience in the operation of high charge state positive ions sources, a unique Low Energy Ion Beam Facility (LEIBF) has been developed at NSC. The development and operation of this facility is the first step towards the design and development of a high current injector for the LINAC accelerator under construction.
The ECR-ion source using permanent magnets (NdFeB) is placed on 200 keV high-voltage deck. The NANOGAN source operating at 10 GHz has been supplied by Pantechnick S.A. The high voltage platform and associated beam handling and beam diagnostic facilities like all metal double slits, beam steerers, Faraday cup, Pneumatic valve, isolation transformer, UHV chamber etc have been fabricated indigenously. The facility produces a high current multiply charge state positive ion beam with energy varying from a few tens of keV to a few MeV. All the electronic control devices of the ECR source including high power UHF transmitter placed on the high voltage deck are controlled through optical fibre communication.
The acceleration of the ion-beam is performed by a high gradient acceleration tube. The mass selection is provided by a high resolution (1/800) double focussing 90° analysing magnet. Experimental facilities are being developed to carry out materials science, surface physics and atomic physics studies using low energy ion beams. The 15° port of the analysing magnet is being developed to study atomic physics and cluster physics involving very heavy ions.
Energy Augmentation Program
The maximum energy of ions from the Pelletron ( ~ 200-250 MeV) limits the research program for both nuclear physics and materials science. A superconducting LINAC booster was planned in early 90?s for future augmentation of the Pelletron.
In an electrostatic machine like the Pelletron, the ultimate energy depends on the terminal potential for which there is a definite achievable limit. In a LINAC, instead of a DC beam, alternating voltage of high frequency is applied across gaps through which the beam passes. The phase of the applied voltage between gaps are matched so that the beam is accelerated to higher and higher energies as it passes between the gaps. The frequency of the RF has to be kept high ( ~ 100 MHz) to keep the length of the LINAC to manageable proportions. To keep the total power consumption down, the present generation of LINACs use superconducting resonators using Niobium as the superconducting material at liquid helium temperature (4.2° K) that allow a field gradient of 3-4 MV/meter at a few watts of power level.
Development of the LINAC was carried out in collaboration with Argonne National lab, USA. A superconducting quarter wave coaxial line (QWCL) structure was selected for accelerating heavy ions with particle velocity (v/c) ~ 0.08 . The resonators are formed of superconducting Niobium operating at 4.2° K, well below the critical temperature of Nb. They are jacketed in stainless steel vessels which contain the liquid helium. A stainless steel to Niobium explosively bonded flange provides the welding transition between niobium and stainless steel. The resonators are designed to operate at a nominal frequency of 97 MHz. A novel pneumatic slow tuner in the form of a niobium bellow provides a tuning range of approximately 100 kHz, substantially larger than any working QWCL resonator.
The performance of the first few resonators fabricated in USA was found to be excellent, allowing an operational accelerating field in excess of 4MV/meter. Eight such resonators are to be put in a common cryostat allowing a total accelerating voltage ~ 5 MV per cryostat. Three such cryostats are planned to be put in the beam line, effectively doubling the energy of the ions available form the Pelletron Accelerator.
Because of the changing phase of the accelerating voltage between the gaps in the LINAC cavity, the beam can pass through the LINAC in very short bunches only ( ~ 100-200 ps). The Multi-harmonic buncher acting on the pre-accelerated beam from the ion source can produce bunches of width 1- 2 ns only. For further compressing the accelerated beam from the Tandem, one of the Niobium resonators is used as a superbuncher, compressing the beam down to ~ 150 ps width. Online test of the Superbuncher has already been carried out and the first cryostat is in an advanced state of assembly.
Installation of the remaining two cryostats would require fabrication of more resonators in the country. A superconducting Resonator fabrication Facility (SuRFF) is currently under installation at Inter-University Accelerator Centre. The electron beam welding machine, vacuum furnace, clean room, chemical treatment plant and a High Pressure water rinsing facility have been acquired for this purpose. As per the current schedule, the accelerated beam using the first cryostat only would be available near the end of 2002, and the completion date for all three cryostats is July, 2004.
For the superconducting LINAC project, cryogenics plays an important role to keep the superconducting cavities at liquid helium temperature. The Cryogenic distribution system for LINAC is one of the largest such facilities in India. It has a Helium refrigeration system with 600 W (150 litres/hour) capacity and a closed loop liquid Nitrogen plant of capacity 5000 W. A helium recovery system and a gas purification system have been added to prevent gas loss during a power outage.
Experimental Facilities in New Beam Hall
One of the major objectives of increasing the beam energy is to make new areas of research accessible to the user. In nuclear physics, the primary constraint has been the larger Coulomb barrier for heavy targets that has limited the production of nuclei above A > 200. For medium mass nuclei (A <60), detection of the recoil products and their identification through the measurement of A & Z becomes easier in inverse kinematics where the projectile is heavier than target. Both these mass regions A > 200 and A < 60 in inverse kinematics would become available after LINAC installation. Two major projects, Large Gamma Array (LGA) and Hybrid Recoil Analyser (HYRA) for studying these mass regions for nuclear spectroscopy and reaction dynamics were submitted to DST.
We are glad to inform the user community that these two projects have been approved by DST. LGA would now be implemented as part of the Indian National Gamma Array (INGA) composed of 24 Compton-suppressed Clover detectors. DST would initially provide funding for six detectors, and the rest would be obtained by pooling from other research institutions in the country (TIFR, IUC-DAEF, SINP and VECC). The first such collaborative experiment using eight Clover detectors has been undertaken at TIFR during May-July, 2001.
The HYRA facility consists of a split dipole which can operate in two different modes depending on whether it is run under vacuum or is filled with a low density gas. In the vacuum mode, used exclusively for inverse kinematics, the recoil products are separated from the beam-like particles using the dipole as a momentum filter. The selected components are mass-analyzed in an ED-MD combination and are detected at the end of the ~ 10 m long arm with a modest solid angle of ~ 10 msr.
For very heavy recoil products, filling the chamber with a low pressure gas causes an equilibration of charge states and allows an efficient detection of recoil products with a very large solid angle (~ 40 msr). Since all the charge states present in the recoil products are focused to a single point in the focal plane, gas-filled mode allows for a very high detection efficiency (> 50%). This is specially important for the study of the formation and decay of trans-Uranium nuclei where the fusion cross-section drops drastically due to the strong fission competition.
In the field of materials science, NSC is augmenting the ion-beam facilities to provide the user community with ions of energies ranging from a few keV to hundreds of MeV and mass ranging from 1 to 200. To exploit the various online/ in-situ facilities, the following thrust areas of research in materials science have been identified:
Looking Towards the Future
Apart from the limitation of beam energy from the Pelletron Accelerator, the available charge state is limited by the stripping process at the terminal voltage. Although in principle one can obtain a higher charge state after stripping the accelerated beam from the Pelletron, the intensity of the beam is reduced significantly due to the charge state fragmentation at each stripping process.
With the feasibility of acceleration of low energy ions through RF cavities, the general trend world-wide has been to replace electrostatic accelerators by high intensity high charge states positive ions from ECR ion sources. Since the maximum energy available from such ion-sources is ~ 10 keV/A, further acceleration using RFQ or low-beta Quarter wave LINAC structures is required to match the velocity of the high charge state ion beam with the energy acceptance of the LINAC booster (b ~ 0.08 for NSC LINAC).
The immediate future plan of Inter_University Accelerator Centre is to develop an ECR-based high current ion-source along with an intermediate acceleration stage in order to inject into the LINAC under construction. Apart from higher current ( ~ 10 or higher for heavy ions as compared to Pelletron), the higher charge state would also allow an increase in the ultimate energy achievable with the accelerator. The present thinking is to commission an ECR source operated using high Tc superconductor coils which would be mounted on a high voltage deck ( 200 kV) . In parallel to the development of the ECR ion source, RFQ accelerator and Low beta resonator cavities will also be developed, which will form part of the high current injector.
Once the High Current Injector starts to operate along with the superconducting linear accelerator modules, the Pelletron will become a totally separate and independent accelerator. This can be used for the production of low-energy radioactive ion beams for research in areas of nuclear astrophysics. In addition, there are plans to use the Pelletron for Accelerator Mass Spectroscopy (AMS) with immense applications in wide areas such as environmental sciences, archaeology, geology, etc. Besides this new area, Pelletron will continue to provide heavy ion beams of lower energies needed for other disciplines e.g. materials science.
Looking at future possibilities, the centre is progressing in a direction which can lead to adding storage rings for heavy ion beams to remain internationally competitive. In addition, opportunity exists in undertaking development of a radioactive Ion beam Facility. A stand alone Electron LINAC (of ~ 50 MeV energy) can produce neutron rich nuclei by photo-fission. This can, after mass selection, be fed to the ECR source for charge boosting, and accelerated to 10 MeV/A energies using the infrastructure of the high current ion-source. During the 10th Plan period, development work on a thick target fission source and a very high resolution isobar separator is planned to develop expertise in handing RIB. This could be part of the national plan on Radioactive Ion beam involving other institutions and organisations. The future indeed looks very bright at this stage !
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