# The ATLAS facility

 Robert Janssens (2010), Scholarpedia, 5(9):9731. doi:10.4249/scholarpedia.9731 revision #91866 [link to/cite this article]
Post-publication activity

Curator: Robert Janssens

Dr. Robert Janssens accepted the invitation on 6 July 2009 (self-imposed deadline: 6 January 2010).

The Argonne Tandem Linac Accelerator System: ATLAS

## Introduction

Figure 1: Floor plan of ATLAS facility with the most important components of the accelerator and the major experimental equipment identified.

ATLAS (the Argonne Tandem Linac Accelerator System) is the world's first superconducting accelerator for projectiles heavier than the electron. This unique system is a U.S. Department of Energy (DOE) National User Research Facility open to scientists from all over the world. It is located within the Physics Division at Argonne National Laboratory, one of the DOE’s oldest and largest national laboratories for science and engineering research. Argonne is managed by the University of Chicago-Argonne, Limited Liability Company, for the DOE’s Office of Science. ATLAS is one of seven large scientific user facilities located at Argonne.

ATLAS consists of a sequence of machines where each accelerates charged atoms and then feeds the beam into the next section for additional energy gain. The beams are provided by one of two 'injector' accelerators: a 9 million volt (MV) electrostatic tandem Van de Graaff, and a newer 12-MV low-velocity linac coupled to an electron cyclotron resonance (ECR) ion source. The latter system is called the Positive Ion Injector. The beam from one of these injectors is sent on to the 20-MV 'booster' linac, and then finally into the 20-MV 'ATLAS' linac section. High precision heavy-ion beams ranging over all possible elements, from hydrogen to uranium, can be accelerated to energies as high as 20 MeV per nucleon and delivered to one of three target areas. A floor plan of ATLAS can be found in Figure 1.

The heart of ATLAS is the superconducting split-ring resonator ( Figure 2). The first successful test of a niobium split-ring resonator occurred in November 1977 after two and a half years of intensive development. Niobium was chosen as the superconducting surface because it is the best available superconductor for a radio frequency (RF) device. The outer housing of each resonator is made of niobium explosively bonded to copper. The interior components of the resonator are pure niobium in which liquid helium is in direct contact for cooling. The result is an accelerating structure in which a large electric charge flows back and forth 97 million times a second between doughnut-like drift tubes. A potential difference of about 800,000 volts is generated between the tubes and can be used to accelerate a beam of any atomic nucleus. The first acceleration of an ion beam with these resonators was achieved in June 1978. The booster linac with 24 split-ring resonators was completed in 1982, while the first acceleration through the ATLAS section took place in 1985. A second class of resonators, known as 'quarter-wave' resonators, was developed in 1985 and 1986 as part of the Positive Ion Injector facility upgrade which was brought on-line in 1992.

Figure 2: The split-ring resonator, the heart of the ATLAS facility.

In a linac accelerator, ions gain energy in a manner very similar to a person riding a surfboard on an ocean wave. If the speed of the paddling surfer approaches that of the wave, he can "catch it", thereby gaining speed. If another, faster wave comes along, he could catch that one and be accelerated further. The ATLAS linac is constructed with seven different superconducting resonator designs each one creating an electromagnetic surfing wave of a different velocity. In ATLAS a total of 62 of these resonators are arranged so that particles of a given velocity are accelerated and remain synchronous, or in-phase, with the accelerating field. This array of resonators can be tuned to a wide range of effective velocities by adjusting their relative RF phases. The resonators in ATLAS are able to begin accelerating particles which have a velocity of less than 1% the velocity of light, c, and accelerate those particles to velocities up to approximately 0.15 c. Each of the resonators in ATLAS is relatively small and independently controlled. This unusual independent control capability of each resonator makes it possible to adjust the effective speed of the accelerating electromagnetic surf-wave through ATLAS so that any ion, regardless of mass, can be accelerated.

When a wave of electromagnetic surf goes by, the ATLAS ion source provides roughly 1,000 to 10,000 ions in a bunch which must 'jump' on the wave for their trip through the linac. Although the ions in each bunch may start out from the same point, they are generally going in slightly different directions. Magnetic fields are used throughout ATLAS to steer these beams of charged particles and force them to go in the desired direction. In the linac these magnetic focusing fields are provided by superconducting solenoids interspersed between the resonators.

Since the first successful acceleration of an ion beam by the prototype ATLAS facility in 1978, ATLAS has been growing and expanding its capability to provide unique and powerful beams for the world-wide nuclear and atomic physics research program. The addition of the Positive Ion Injector was especially noteworthy because it provided the ability to accelerate all ions, including the heaviest ones such as lead, bismuth and uranium. The development of ATLAS has always continued in parallel with the operation of the facility for research. More recent projects, such as the development of radioactive beams, an energy upgrade and an on-going efficiency and intensity upgrade are described below.

The facility serves a national and international user community of more than 500 registered members. The facility is maintained and operated by a staff of roughly 25 technicians, engineers and scientists. In addition, a group of roughly the same size provides support for the experimental program. On average between 40 and 50 experiments take place every year, each requiring from a few days to a week or two to complete. Roughly 300 Users come to the facility yearly to carry out their measurements. The facility operates around the clock, 7 days/week for a total of 5900 hours in 2009, for example. Beam time at ATLAS is granted by the ATLAS Scientific Director based on the advice of a Program Advisory Committee (PAC) consisting of several internationally renowned experts from other institutions. The PAC meets twice a year to evaluate and rank written proposals based on their scientific merit and feasibility. Most research addresses key questions in contemporary nuclear physics, nuclear astrophysics and fundamental interactions as is described below.

## The major scientific goals of ATLAS

At the request of the funding agencies, every national User facility in the U.S. has developed a strategic plan jointly with its community of Users. Such plans are living documents: they get updated when the need arises, usually through workshops and they are available on the facilities’ web sites.

The ATLAS strategic plan is available at http://www.phy.anl.gov/atlas/workshop09/index.html and provides the scientific and strategic vision for ATLAS, the goals for its future capabilities and the expected path forward. It takes into account the highest priorities of nuclear science as expressed in the Long Range Plan elaborated by the Nuclear Science Advisory Committee (NSAC) at the request of the National Science Foundation and the Department of Energy.

The major scientific goals below have been identified for the ATLAS research program.

1. Understanding the stability and structure of nuclei as many-body systems built of protons and neutrons bound by the strong force. Under this research theme, the following scientific issues have been identified as most urgent by the ATLAS Users:
1. Comparisons of the properties of light nuclei (A<20) with the most modern theories where nuclear structure is calculated from first principles (so-called ab-initio calculations).
2. The study of nuclear structure near the proton drip-line, especially in N=Z nuclei in the 50<A<100 region, and in the direct vicinity of doubly magic 100Sn, in order to explore the role of postulated new pairing correlations.
3. The impact of weak binding on the structural properties of nuclei at the proton drip line and beyond such as shell structure, deformation, and the characteristics of proton radioactivity.
4. The delineation of the structural properties of nuclei with Z>100 as a challenging test of theories describing the structural properties of the heaviest nuclei.
5. The exploration of the properties of neutron-rich nuclei focusing on changes in shell structure, pairing, single-particle strength, new types of collective excitations, and other effects associated with a large neutron excess.
6. The identification of new collective modes and the search for their characteristic spectral signatures throughout the periodic table.
7. The study of the properties of the nuclei at the highest spins and excitation energies, including the exploration of the interplay between collective and single particle degrees of freedom, the search for new nuclear shapes (hyperdeformation) and the study of the dependence of level densities on angular momentum and temperature.
2. Exploring the origin of the chemical elements and their role in shaping the reactions that occur in the high-temperature and explosive events of the cosmos. This aspect of the research program addresses critical issues in contemporary nuclear astrophysics such as: cross section measurements for reactions within the extended CNO cycle, the competition between $$(\alpha,p)$$ and $$(p,\gamma)$$ reactions along the rp-process path, the measurement of reaction cross sections between heavy ions at energies relevant for stellar burning, the identification of waiting point nuclei along the rp-process path, the determination of the end-point of the rp-process path near A~100, the measurement of the mass and decay properties of neutron-rich nuclei close to the r-process path, especially around the N=82 and N=126 waiting points, and the development of the surrogate reaction technique for the determination of reaction yields along the s- and r-process paths.
3. Understanding the dynamics governing interactions between nuclei at energies in the vicinity of the Coulomb barrier. A number of problems have been listed by the ATLAS User community. These are: the study of the hindrance of fusion at extreme sub-barrier energies, especially in systems of relevance for nuclear astrophysics; the impact of nuclear structure (deformation, shell structure, diffuseness) on fusion, especially for reactions leading to Z>100 nuclei; the impact of neutron excess on nuclear reactions in the vicinity of the Coulomb barrier; and the determination of the proton-neutron asymmetry dependence of the surface and volume terms of the nuclear level density.
4. Testing with high accuracy the fundamental symmetries of nature by taking advantage of nuclei with specific properties. This aspect of the research program focuses on the validity of the Standard Model of particle physics and the search for new physics beyond it by studying topics such as: tests of the conserved vector current (CVC) hypothesis and the unitarity of the first row of the Cabibbo-Kobayashi-Maskawa (CKM) matrix from studies of superallowed beta decays; searches for possible extensions of the Standard Model by improving limits on scalar, tensor and right-handed components to the electro-weak interaction; and tests of the nuclear structure inputs leading to large enhancements in sensitivity for an electric dipole moment in octupole-deformed heavy nuclei and critical to the nuclear structure dependent corrections in the determination of coupling constants from nuclear decays.

Smaller scale, complementary research efforts also exploit some of the unique, intrinsic capabilities of ATLAS. Examples of such activities include accelerator research experiments, the irradiation of samples for materials research, or developing accelerator mass spectrometry techniques for applications in environmental studies, oceanography, astrophysics, fundamental interactions, and any other area of basic science where they apply.

The ambitious scientific goals outlined briefly above translate directly into specific requirements for the capabilities of the ATLAS accelerator and for the associated experimental equipment. For example, for ATLAS, parts of the program require high beam intensities (of the order of particle micro-amperes) and energies of the order of 12 MeV/nucleon or somewhat higher. In addition, the production and acceleration of radioactive beams is a priority. On the instrumentation side, the availability of world-class instruments for particle and gamma-ray detection as well as for the measurement of reaction products with high efficiency is always viewed as a high priority.

## ATLAS today and tomorrow

As stated above, ATLAS is the world’s first superconducting linear accelerator for ions. Initial demonstration of RF superconductivity for ion acceleration occurred in late 1978. Since that time, the facility has provided high-quality Coulomb-barrier energy beams of heavy ions for nuclear physics research as well as research in other fields such as solid state physics, atomic physics and the geophysical sciences. The facility has been identified as a National User Facility since 1985. Over that entire period, facility features have continued to be improved as measured in terms of the variety of available beams, beam intensity, and variety of operating modes (acceleration-deceleration mode, radioactive beams, accelerator mass spectroscopy (AMS) techniques). Today the main focus of development is the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) described below and the upgrade of the accelerator system to better provide high intensity, higher energy beams.

The ATLAS facility, shown in Figure 1, presently consists of two injector accelerators providing beams to a 40-MV superconducting linac divided into two sections separated by a 40° beam transport section. The 8.5-MV tandem electrostatic injector, the original facility injector, provides beams from a cesium sputter negative-ion source (SNICS). This injector is most important for stable beams with A<40 as well as long-lived radioactive beams produced at other facilities and then installed in the SNICS source such as 14C, 44Ti, and 56Ni. The second injector, the Positive Ion Linac (PII), began full operation in 1992. The PII was developed to provide ATLAS Users with stable beams of any species from protons to uranium. It is based on a then-totally new ANL concept of low-energy ion acceleration. The PII employs one electron cyclotron resonance (ECR) ion source to provide high charge-state stable-beam ions (note that the charge state of an ion corresponds to the number of electrons removed from it). The second ECR source in Figure 1 is now dedicated to the CARIBU project to charge-breed radioactive ions to high charge states for acceleration in PII and then ATLAS. These ion sources are mounted on high-voltage (up to 300 kV) platforms in order to deliver ions with a velocity of ~0.009c, matching the velocity acceptance of the first independently-phased superconducting resonator of the PII linac.

Today the ATLAS facility makes available to Users:

• Beams of all masses from hydrogen through uranium (See Figure 3),
• Beam energies comparable to internal energies of the nucleus - with maximum energies of 20 MeV/u for light nuclei and 12 MeV/u for the heaviest - all above the Coulomb barrier,
• Beam currents ranging from particle micro-amperes for light heavy-ion projectiles to several hundreds of particle nano-amperes for heavier elements,
• Exceptional beam quality with spot sizes of 1mm diameter or less and with a normalized transverse emittance of approximately 0.2 π mm-mrad and a longitudinal emittance as low as 20 keV-ns,
• Precisely controllable and variable beam energies (from hundreds of keV/nucleon to 20 MeV/nucleon),
• Highly efficient ion sources, suitable for separated isotopes, coupled with high transmission of the beam through the accelerator system,
• Great flexibility in being able to switch beams and/or energies rapidly,
• Excellent energy (10-3 or better) and time (as low as 100 ps, 200 - 400 ps typical) resolutions,
• A 100% duty cycle with beam pulses 82.4 ns apart, corresponding to a frequency of 12.125 MHz; a fast beam sweeper offers the capability of removing individual micro pulses from a delivery rate of 1 in 3 pulses up to long-time period control of the beam based on experiment-derived logic with no timing restriction,
• The ability to produce and accelerate high-quality, short-lived beams with the two-accelerator method and the in-flight technique (see below),
• Very reliable operation with over 5000 hours of beam-on-target for research.

The beams provided by ATLAS for the research program in 2009 are shown in Figure 3 and are representative of ATLAS operations in recent years. A list of beams with energy and intensity levels that are considered standard is provided on the ATLAS website at: http://www.phy.anl.gov/atlas/facility/stable_beams.html.

The ATLAS facility is in a continual state of renewal in order to be prepared to meet the requirements of the User research program. The projects undertaken are decided as a result of priorities set by strategic planning (see above) involving the facility Users. The accelerator staff then develops improvements that address those needs and also plans for the major repairs and improvements needed to maintain the existing capabilities. Historical examples include the Positive Ion Injector, the second ATLAS ECR ion source (ECR-II), the production of short-lived radioactive beams with the in-flight technique and the just completed Energy Upgrade Cryostat (the latter two initiatives are described hereafter). These projects are critical for maintaining the facility at the leading edge of nuclear physics research and often provide support for the development of new technologies beneficial not only to ATLAS, but also to the research community at large.

Figure 3: Beams used by the ATLAS experimental program in 2009 with their respective percentages of usage.

The year 2009 saw the installation of the new Energy Upgrade cryostat. The cryostat ( Figure 4) consists of seven new quarter-wave resonators with a matched velocity of 0.144c. These quarter-wave, two-gap structures have achieved an online average accelerating field of 8.3 MV/m and a total accelerating voltage of 14.5 MV, (2.1 MV/cavity), a world record for cavities in this velocity range. This represents a factor of three performance gain over existing, earlier ATLAS technology. Several new features have been incorporated into both the cavity and cryomodule design. For example, the cavities are designed with provisions to cancel beam steering effects due to the radio frequency (RF) fields. They are housed in a so-called box cryomodule representing the first successful demonstration of separate cavity and insulating vacuum systems for this type of accelerating structure. During fabrication of the cavities, cleaning techniques have been applied to achieve low-particulate surfaces and these have proved essential for reliable long-term high-gradient operation. Operations with beam have been underway since late July 2009 with a variety of ion species.

Figure 4: The Energy Upgrade Cryostat installed at ATLAS in 2009.

In many nuclear physics laboratories around the world, there has recently been an increased interest in experiments with short-lived, radioactive nuclei addressing questions in areas of nuclear structure, nuclear astrophysics and fundamental interactions. These beams are also essential to address the scientific objectives outlined above. At ATLAS, the production and subsequent acceleration of radioactive ions is done by one of two techniques: the in-flight method and the CARIBU approach. In the former, a stable beam undergoes nuclear reactions in a production target, and the reaction products of interest are collected, manipulated and separated from the un-reacted primary beam before being used for experiments. Using the production of the short-lived 17F isotope as an example (half life 1.08 m), a beam of an intensity of 2 x 106 17F/s was produced by the d(16O, 17F)n reaction with a 100 pnA primary beam of 16O in a way schematically depicted in Figure 5. The primary 16O beam impinges on a gas cell containing deuterium (d). Because of the kinematics of the reaction, the 17F ions of interest exit the cell in a narrow forward cone. They are then focused and collected into a beam by a 2.2 T superconducting solenoid, before passing through a superconducting RF cavity employed to reduce the energy spread. A bending magnet then separates the 17F ions from the remaining 16O primary beam before it hits the target where the experiment takes place. Recently, a RF beam sweeper has been added to the system in order to improve the purity of the radioactive beam further. Further information on the technique and a list of beams produced in this manner is available at: http://www.phy.anl.gov/atlas/facility/radioactive_beams.html#Beam_prod.

Figure 5: Principle of the in-flight technique for the production of radioactive beams.

In 2005, it was proposed to increase the radioactive beam capabilities of ATLAS by the installation of a new source of ions to provide beams of short-lived neutron-rich isotopes: this is the CARIBU project (http://www.phy.anl.gov/atlas/caribu/index.html). This upgrade will enhance the reach of ATLAS and offer world-unique capabilities to study neutron-rich nuclei. It will also help advance technologies critical for future radioactive beam facilities. In CARIBU (see Figure 1), the neutron-rich isotopes will be obtained from a 1 Curie (Ci), well-shielded 252Cf fission source attached to a large helium gas catcher and Radio Frequency Quadrupole (RFQ) cooler. This arrangement will transform approximately 50% of the fission fragments into a beam of 1+ ions with very low transverse emittance and energy spread. The beam will be mass analyzed by a high resolution (1 part in 20,000) isobar separator. The selected ion species will then be sent either to an experimental area for measurements or to an ECR ion source modified for charge breeding before subsequent acceleration into ATLAS. The source and gas cooler system are installed on a new high voltage platform that allows the ions to gain sufficient velocity for injection into the ATLAS linac and acceleration to energies up to ~15 MeV/u. Optimization of the acceleration process requires weak-beam diagnostics that are also part of the CARIBU project. The project is currently in its commissioning phase.

A series of additional upgrades is currently under way in order to position ATLAS as the nation’s premier stable beam facility for the foreseeable future. These initiatives, referred to as the ATLAS efficiency and intensity upgrade, will expand the stable beam currents available for the research program, increase the beam intensity for neutron rich beams from CARIBU and improve the intensity and purity of the existing in-flight radioactive beams. The project is proposed to take place in a number of phases. The first one is focused on increasing the beam intensity of stable beams by a factor of 10 for Coulomb barrier energies by replacing the first few Positive Ion Injector resonators with a normal conducting, CW, RFQ linac and replacing cryostats of split-ring resonators with a set of newly designed quarter-wave resonators mounted in a cryostat similar in design to that used for the recently completed Energy Upgrade. These activities are funded with the following main objectives: (1) Development and construction of an RFQ to deliver 250 keV/u ion beams with q/A ≥ 1/7; (2) Development of new quarter-wave (QWR) superconducting resonators optimized for high-intensity beams; (3) Construction of a cryomodule with 7 of the new superconducting cavities to replace three Booster cryomodules; (4) Upgrade of the ATLAS liquid helium distribution system. Significant progress has been made since the beginning of the project in the summer of 2009 and, at present, the planned completion date is mid-2013. Additional initiatives will address the goals of improved efficiency and beam currents for CARIBU and in-flight radioactive beams as well as further increases in the intensity of stable beams, especially at energies above the Coulomb barrier.

## Experimental Equipment at ATLAS

ATLAS is the home of world-class instrumentation housed in a number of experimental areas. Most of this equipment has been developed by large collaborations of ATLAS Users and is being maintained and continuously upgraded jointly by the Users and ATLAS support staff for the benefit of the research community. Some of the more noteworthy instruments are described hereafter.

### Gammasphere

This instrument is at the present time the world's most powerful gamma-ray spectrometer. The device was built by a consortium of scientists from U.S. national laboratories and universities. The project was coordinated by scientists at Lawrence Berkeley National Laboratory, where the device was first assembled and brought on-line. The core of Gammasphere is an array of 110 Compton-suppressed, high-purity, large-volume germanium detectors assembled in a spherical shell around the target. (Each of the modules consists of a Germanium detector surrounded by a bismuth germinate (BGO) shield for the suppression of scattered photons.) Gammasphere is especially well suited for the collection of gamma-ray data following the fusion of heavy ions, i.e., situations where a large number of photons are emitted in rapid succession from a nucleus. Its superior resolving power also provides the ability to pull events associated with very weak reaction channels out of the “background” from the more copiously produced nuclei. A photograph of Gammasphere is given in Figure 6. Gammasphere first moved from Lawrence Berkeley National Laboratory to ATLAS in the fall of 1997 and returned to Argonne again in 2003. Further information is available at http://www.phy.anl.gov/gammasphere/index.html.

Figure 6: Gammasphere and the FMA.

#### Auxiliary detectors for Gammasphere

The addition of auxiliary detectors can improve the capabilities of Gammasphere significantly for certain applications. Most of these devices have been built by interested Users. The microball, assembled by the Washington University group in St Louis, is an array of CsI detectors for light charged particle detection. Good particle identification results in excellent channel selection and the ability of improving Doppler shift corrections. In some applications, parts of microball have been replaced by position-sensitive Si strip detectors. The Washington University group is also responsible for: (a) the neutron array, consisting of up to 30 tapered hexagonal detectors that can replace the Gammasphere modules of the 6 most forward rings of the spectrometer; and (b) HERCULES, a high-efficiency evaporation-residue counter with 64 thin fast-plastic scintillators, arranged in four rings covering an angular range of 3–19 degrees at 31 cm from the target, where residues are well separated from elastic scattering, fission events, and other undesirable reaction debris by time-of-flight and pulse-height analysis.

The position-sensitive heavy ion detector CHICO was developed specifically for use with Gammasphere by the group of the University of Rochester and is now maintained by the Rochester-Livermore Nat. Lab. collaboration. It consists of a set of position-sensitive parallel-plate avalanche counters for the study of fission, Coulomb excitation, transfer and other binary reactions in a conical array and covers 67% of the $$4\pi$$ solid angle around the target.

The ICE Ball spectrometer, originally developed at the University of Pittsburgh and maintained by the University of Richmond group, consists of six mini-orange electron spectrometers for the detection of conversion electrons. The devices fit within the space of the Gammasphere target chamber. Electrons originating at the target position are transported by the toroidal magnetic field onto cooled Si(Li) detectors that measure their energy with high resolution. The transmission efficiency of the mini-orange spectrometers can easily be tuned to the desired energy region by adjusting the strength and arrangement (or simply the number) of the permanent magnets.

Two plungers for lifetime measurements with the recoil-distance technique have been developed by the groups at the University of Notre Dame and Yale University. For the detection of X rays and low-energy gamma rays, a set of 5 germanium Low-Energy Photon Spectrometers (LEPS) detectors is available. These detectors can replace Gammasphere modules at any location around the target.

While at ATLAS, Gammasphere is most often located in front of the Fragment Mass Analyzer (FMA). In that sense, the FMA can also be viewed as an auxiliary detector. It is described below. Further information on auxiliary detectors is available from the Gammasphere web site: http://www.phy.anl.gov/gammasphere/index.html.

### The Fragment Mass Analyzer (FMA)

The FMA is a recoil mass spectrometer used to separate nuclear reaction products from the primary heavy ion beam and to disperse them by mass/charge (M/q) at the focal plane of the instrument. The FMA features wide acceptances in particle energy and M/q, as well as high mass resolution. Magnetic and electric fields (up to 500 kV across a 10 cm gap) are used to guide the desired reaction products and focus them onto detectors at the focal plane. A number of detection systems for the FMA focal plane have been developed for specific applications. The dispersion in M/q at the focal plane is most often measured by a parallel grid avalanche counter (which also provides an energy loss signal), but a set of position-sensitive channel plate detectors is available as well for high count rate applications. These detectors are often backed by sets of gas counters, the most elaborate being a PGAC-TIC-PGAC-TIC-PGAC-IC combination for the detection of very small fusion-evaporation cross sections. Here PGAC stands for an x–y position-sensitive parallel grid avalanche counter, TIC for a transmission ionization chamber and IC for a large volume multi-anode ionization chamber. Another much used focal plane system involves a double-sided silicon strip detector in which reaction productions are implanted and spatially correlated with their subsequent decay by the emission of a charged particle (proton, alpha or beta radioactivity). A box arrangement of these detectors is available as well and can be surrounded by the X array. This is a gamma-ray array optimized for radioactive decay studies, where high efficiency and good energy resolution are paramount. The system consists of large, closely-packed, unshielded, germanium clover detectors integrated into a self-contained mobile unit that can also be located at other locations at ATLAS, e.g. the Beta Paul Ion Trap (BPT), and CARIBU. Finally, a moving tape collector is available for measurements of long-lived radioactivity. Further information on the FMA and its equipment is available at: http://www.phy.anl.gov/fma/index.html.

### The Helical Orbit Spectrometer (HELIOS)

A new concept has been developed by a collaboration of scientists at Western Michigan University, the University of Manchester (United Kingdom) and Argonne National Laboratory for investigations of nuclear reactions with radioactive beams. It is ideally adapted to the `inverse kinematics' regime necessary for nuclear structure studies with exotic nuclei, an area of great current interest and a focus of the upcoming CARIBU research program. The concept is based on a superconducting solenoidal spectrometer ( Figure 7) with a uniform axial field. The target and detector are both on the solenoid axis in the field with the reaction products being bent back to the axis where their energy and position of impact are measured. These quantities translate into the desired information of excitation energy and center-of-mass angle. Such a device has a number of attractive features: greatly improved effective resolution, large solid angle, compact detectors and electronics, and easy particle identification. It is well suited to experiments that probe the structure of the exotic nuclei that are currently of high interest: single-nucleon transfer reactions, pair transfer, inelastic scattering, or knockout reactions. The device consists of a 0.9 m bore, and 2.0 m long 3 Tesla superconducting solenoid (i.e., MRI magnet). At present, a set of position-sensitive Si detectors from a previous project is being used for the detection of light ejectiles (protons, alpha particles or deuterons), but a complete array of modern Si detectors and associated read-out electronics is under development. A recoil detector system to measure the ‘beam-like’ reaction products is also being installed. Additional information can be found at http://www.phy.anl.gov/atlas/helios/index.html.

Figure 7: Schematic cross section of the HELIOS spectrometer.

### The Canadian Penning Trap (CPT) and associated equipment

This is an online Penning trap system used for mass measurements of high accuracy on short-lived isotopes. For most applications, production of the isotopes of interest is done with beams from ATLAS impinging on a rotating target wheel and the reaction products are focused by a superconducting solenoid into a gas catcher (containing high-purity helium gas) where they are slowed down and thermalized. Guided by the gas flow and a combination of RF and DC fields, the 1+ ions are subsequently extracted from the catcher and guided to a radio frequency quadrupole ion trap (Paul trap) with the help of a RFQ. They accumulate in the Paul trap until being transferred to the precision Penning trap. For some applications, the 1+ ions can also be sent to another, lower resolution, Penning trap serving as an isobar separator. The measurement occurs in a Penning trap installed inside a 6 Tesla superconducting magnet. An ion of mass m and charge q is confined by the superposition of magnetic and electric fields. The central, or ring, electrode of the Penning trap is divided into quadrants to enable the application of azimuthally oscillating quadrupole potentials superimposed on the static trapping potential. The influence of a quadrupole excitation results in a resonance at the cyclotron frequency, $$\omega_c = qB/m\ ,$$ where B represents the strength of the magnetic field. After the excitation frequency has been applied, the ions are ejected from the trap. The radial energy gained from the excitation is converted into axial energy by their passage through the magnetic field gradient outside the trap and the resonant frequency $$\omega_c$$ is obtained by determining the position of the minimum of the time of flight spectrum. At the present time, the CPT has been moved near CARIBU for a program of mass measurement on neutron rich fission fragments. The systems in place for the production and preparation can also be used to send the ions into another Paul trap dedicated to weak interaction studies.

An atom trap (ATTA, see http://www.phy.anl.gov/mep/atta/index.html) has also been installed at ATLAS in earlier years. This trap was used for the first, high-precision measurement of the charge radius of 6He. It is presently being converted off-line into an apparatus capable of measuring beta-neutrino angular correlations in order to search for physics beyond the standard model.

Only the major, most recent equipment installed at ATLAS is described in some detail above. Other available instrumentation includes two Enge split pole spectrographs with the capability of operating in the gas-filled mode where the charge states of the reaction products are equilibrated to an average one, a multi-purpose large scattering chamber and a number of specialized particle and gamma-ray detectors. In addition, a number of general purpose beam lines are available for the installation of equipment brought in by Users for specific measurements.

ATLAS is also equipped with a state-of-the-art target laboratory. For more information see http://www.phy.anl.gov/targetlab/index.html.

## ATLAS and the accelerators of the future

Because of its pioneering role in the use of RF superconductivity for low velocity beams, ATLAS has been a source of continuing accelerator research and development. Argonne scientists pioneered the practical application of RF superconductivity for low velocity beams to develop the ATLAS facility. This technology is now in widespread use around the world. Recent achievements include: new classes of superconducting cavities such as spoke cavities, new tools for accelerator design, new construction techniques and new instruments for beam diagnostics.

At present, there is a focus on the development of new technologies for the construction and fabrication of superconducting cavities. These are the construction and testing of cavities with atomic layer deposition, optimization of the electro-polishing procedures and development of a second sound system for the monitoring of cavity performance. In addition, the R&D team develops highly scalable beam dynamics simulation codes able to take advantage of parallel computing. The group pursues the development of the “model driven accelerator” concept that can be applied to the design, commissioning and operation of large scale accelerator facilities. Another aspect of the Accelerator R&D effort of particular benefit to the nuclear science community is the support of the accelerator design and simulation code TRACK. This code incorporates many of the features particularly relevant to nuclear physics accelerators (multi-charge state acceleration, dynamics of slow and fast moving ions, space charge effects for heavy ions, etc.). It has become the tool of choice for many national and foreign laboratories because it is able to perform both design optimizations and end-to-end simulations of an accelerator concept.

## Acknowledgements

Work at ATLAS is supported in part by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357.

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