Technology Archives – Page 116 of 117

Web tool LIGHTs up code

CERN’s Information and Programming Technology (IPT) group has exploited the World Wide Web to make it easier to keep track of masses of computer code. Lifecycle Global Hypertext (LIGHT) was so successful when applied to the ALEPH experiment that it is now being taken up around the world.

The software for a big particle physics experiment can run to tens of thousands of lines of code, organized into hundreds of routines written by dozens of people in many institutions, and often has code from independent program libraries folded in too. It can have a lifetime of over a decade during which it is constantly evolving. No wonder newcomers to an experiment can find it daunting.

The traditional way of finding a way around all this involves the printed word: a bookshelf full of manuals to wade through as well as the source code itself. But help is now at hand thanks to LIGHT, originally developed for CERN’s ALEPH experiment and since adopted by many other experiments around the world.

Paolo Palazzi had the idea for LIGHT in 1993 and it was subsequently developed by members of CERN’s IPT group. The idea was to make all the necessary information available through a Web interface. A click on a subroutine call in a big program, for example, would take you to that routine’s source code, and a click on the code would take you to the documentation. Having built your program from all the different packages available and plugged in your own specific code, you would then be able to submit it through a Web-based template so that your complete analysis could be conducted through the Web.

The ALEPH-specific implementation of LIGHT was ready in 1995, and is now used on all of ALEPH’s main off line software. True to its name, ALEPH LIGHT will manage the software for the lifecycle of the experiment any new code or updates to old routines appearing automatically at the end of hypertext links.

In 1997 the IPT group decided to make LIGHT available generally, and under Alberto Aimar’s co-ordination it was re-engineered and the project was re-organized. Other experiments soon followed ALEPH, with CERN’s DELPHI showing what they thought of the system by giving their customized version the name “Delight”.

Others to follow are ATLAS at CERN, BaBar at Stanford in California, and KLOE at Italy’s Frascati laboratory. With a second version of LIGHT under development, making it easier to add new formats and languages, the product is set to reach a wider range of experiments and projects. The software remains as complex as ever, but with LIGHT all information is just a click away.

Beauty is in the eye of the LHC beholder

news1_11-98 — The diagram shows LHCb in its underground area, surrounding the LHC beam pipe, with the collision point on the left.

The LHCb experiment has been formally approved for CERN’s LHC collider. Unlike the other major LHC detectors (ALICE, ATLAS and CMS) which will completely surround their respective collision points, LHCb will use a large (20 metre) single-arm spectrometer along the beam direction on one side of the collision point. The diagram shows LHCb in its underground area, surrounding the LHC beam pipe, with the collision point on the left. LHCb’s goal is to look for new physics by studying in detail the physics of the Standard Model’s third generation of particles, in particular the “beauty” of “b” quark.

Final magnet installed at Fermilab

On 24 September, US Energy Secretary Bill Richardson put the finishing touches to the installation of the 366th and final 20-ton dipole magnet to steer the beams in Fermilab’s new 150 GeV, 2.25 mile Main Injector Ring.

Construction work on the Main Injector began in 1993 and the new machine should begin operation next year, boosting performance of Fermilab’s Tevatron by increasing its supply of protons and antiprotons. With higher collision rates, the Tevatron, already the world’s highest-energy particle collider, will be able to attack new physics goals.

The Main Injector replaces Fermilab’s original 400 GeV four-mile Main Ring, closed in September 1997 after 25 years of service as the hub of the laboratory’s particle beam system. Many of the Main Ring’s components, including quadrupole focusing magnets, have been taken over for the new Main Injector.

news2_11-98 — US Energy Secretary Bill Richardson helps install the final magnet in Fermilab’s Main Injector Ring. Making sure all is well is Fermilab Director John Peoples (behind) and Main Injector Project Manager Steve Holmes (right).

Built as Fermilab’s front-line machine, the Main Ring took on a new role in 1983 as the injector for the superconducting 800900 GeV Tevatron, which operated as a protonantiproton collider and in fixed target mode using proton beams. For the future, the Tevatron is a dedicated protonantiproton collider. As well as feeding the Tevatron collider, the Main Injector will be able to support its own programme of fixed target experiments.

The Main Ring and the Superconducting Tevatron shared the same tunnel. The Main Injector is in a new tunnel which is tangential to the Tevatron.

Marsters of science

Two recent talks at CERN focused on the problems of a voyage to Mars, where novel propulsion techniques are called for to overcome the logistical problems of otherwise having to construct huge space vehicles and for astronauts to spend over a year in space.

On 27 August, 1984 Nobel Physics laureate and former CERN Director General Carlo Rubbia (right) described how his recently completed TARC experiment at CERN suggests how it is possible to confine neutrons as “black body radiation”. Rubbia’s fertile mind sees how surrounding such an “n-Hohlraum” cavity by a micro-layer of americium could quickly go critical, the fission fragments acting as a very-high-energy exhaust, attaining temperatures much higher than those of chemical fuels. Such a motor would also have no moving parts. In this way, a few kilograms of nuclear fuel could be sufficient to power a substantial space vehicle.

The TARC experiment was a neutron study en route to Rubbia’s proposed Energy Amplifier, which would use beams from a particle accelerator to produce spallation neutrons and in turn feed a target-moderator assembly. The TARC study demonstrated how the neutrons could “scavenge” the target-moderator assembly, eating up nuclear waste.

On 3 August NASA astronaut Franklin Chang-Diaz, mission specialist on the recent Space Shuttle flight carrying the Alpha Magnetic Spectrometer , described the 236-hour voyage, which was also the final docking between a Space Shuttle and the Russian Mir space station. Chang-Diaz, an accomplished plasma physicist, is also Director of the Advanced Space Propulsion Laboratory at the Johnson Space Center. He concluded his talk by describing ongoing development work for rocket propulsion systems using magnetically confined high-temperature plasmas, the ultimate objective being to sustain flight to Mars. The picture shows him arriving at Kennedy Space Center for a Terminal Countdown Demonstration Test.

Long hot summer

Smooth running at CERN’s LEP electronpositron collider, now at 189 GeV collision energy (94.5 GeV per beam), is reflected in a fast climbing, integrated luminosity curve. Integrated luminosity is a measure of the number of electronpositron collisions provided for physics, and the 1998 score is already well past that of previous years. LEP runs until early November. Recent performance has yielded luminosities of more than 9×10³¹ per cm² per s and an integrated luminosity of 3 inverse picobarns in a 24-hour period.

MACRO-scope

macro1_11-98 — The MACRO detector in the Italian Gran Sasso underground laboratory. The lower, absorbing, half of the detector is interspersed with streamer tubes and is covered with scintillator and streamer chamber detectors (orange-coloured) to pick up muons emerging from reactions in the inner absorber. Additional information comes from the box-like “roof” of detectors above.

Fully operational since 1994-95, MACRO’s bread-and-butter physics is the detection of cosmic-ray muons, but its ultimate objective is to search for new phenomena and to pick up particles from cosmic sources such as supernovae. In its search for cosmic signals, MACRO is assisted by the EAS-TOP array on the mountain 1400 metres above.

MACRO intercepts particles which pierce the overhead rock shield. 77 metres long, 12 metres across and 9 metres high, the detector is divided lengthways into six modules. The bottom half of the detector is composed of seven layers of crushed rock absorber interspersed with streamer tubes, together with an outer cladding of scintillator and streamer tube detectors and a box-like top layer with scintillator and streamer chamber walls and roof running the length of the detector.

While magnetic monopoles continue to be elusive, a bonanza for MACRO is the study of muons produced by neutrino interactions inside the detector, confirming an intriguing effect seen in other detectors. These studies show a marked difference between the signals due to upward- and downward-moving neutrinos.

Multistage decays

On the Earth’s surface, muon-like neutrinos (as distinct to electron-like) mainly result from the decay of particles produced by high-energy cosmic rays hitting nuclei high up in the atmosphere. These reactions produce kaons and pions, which themselves decay to produce muons, which in their turn decay. In these multistage decays, the end result should be that there are twice as many neutrinos producing muons as producing electrons.

However, the detectors see fewer such muons than expected. MACRO, showered by other cosmic muons from above, cannot isolate those downward muons that are due to neutrinos, but does see a clean signal due to muon neutrinos arriving from below, which have passed right through the Earth before hitting the detector.

There are considerably fewer of these upward muons than expected, underlining the suggestion that muon neutrinos “oscillate” on their way through the Earth, changing into other neutrino types which do not produce muons.

Applied superconductivity in Russia

Ever since the pioneering work by P L Kapitza in 1924 (for which he shared the 1978 Nobel Prize), there has been a continual interest in Russia in producing high magnetic fields and putting them to work. Much of this effort has been focused at Moscow’s Kurchatov Institute which also organizes an annual school on applied superconductivity for young engineers and scientists. At this year’s event, Evgeny Krasnoperov reviewed the devices for generating high magnetic fields developed at the Kurchatov Institute.

russia1_11-98 — The Kurchatov Institute’s superconducting-magnet test facility.

In 1939 F Bitter built high-field solenoid-type magnets which could generate stationary fields of 10 T. As early as 1972 the Kurchatov Institute exceeded 25 T by nesting a resistive magnet inside a large superconducting solenoid. Currently under construction is a new hybrid magnet to attain up to 30 T with superconducting coils based on niobiumtitanium and niobiumtin superconductor.

Russia is involved in the International Laboratory for High Magnetic Fields and Low Temperatures in Wroclaw, Poland. Lev Luganskij (Kapitza Institute) described the 30-year history of this laboratory, established by an agreement of the Academies of Sciences of the Soviet Union, German Democratic Republic, Bulgaria, and Poland.

Wroclaw

The Wroclaw laboratory is open to guests for experimental investigations of extreme magnetic fields and low temperatures. At present there are several magnets for stationary fields and one for pulsed fields up to 47 T. Several Bitter-type magnets and superconducting magnets produce a range of stationary fields. The largest Bitter-type magnet generates magnetic fields up to 20 T, its total power exceeding 6 MW. Bitter-type magnets are cooled by a two-circuit water cooling system.

Another important Russian magnetic contribution is tokamaks. In 1950, A D Sakharov and I E Tamm put forward the idea of magnetic confinement of high-temperature plasma and proposed the thermonuclear reactor tokamak concept. Sergey Egorov of the Efremov Institute (St Petersburg) covered the history and progress of these devices. The ultimate outcome is the International Thermonuclear Experimental Reactor (ITER), a fusion device to demonstrate ignition.

The ITER tokamak is currently being developed jointly by Euroatom, Russia, the USA, and Japan. The superconducting components are toroidal and poloidal field coils and a central solenoid, the latter producing a magnetic field up to 13 T at its inner radius. The superconducting coils for the system will require more than 1600 tons of niobiumtin.

In the Bochvar All-Russia Scientific Research Institute of Inorganic Materials (Moscow), niobiumtin wires for ITER have been developed and studied. Production of the first ton of ITER wire was completed in April 1998. Critical current density (non-copper cross-section) exceeds 550 A per sq. mm at 12 T.

A review of fabrication methods of niobiumtitanium and niobiumtin wire was presented by Victor Pantsirnyi. Victor Sytnikov (Cable Institute, Moscow) reported on ITER conductor development. The conductor is of cable-in conduit niobiumtin type with an incoloy alloy 908 external jacket, carrying 46 kA up to 13 T magnetic field. This international collaboration comprises 12 companies in Europe, Russia, Japan and the USA.

Subrata Pradhan (IPR, India) reported on the superconducting magnets for Tokamak SST-1. The superconducting cable for this project was produced in Japan and part of the cable was transported to Moscow in May. Superconducting model coils for the conductor testing will be fabricated and tested by the Kurchatov Institute.

Alexey Dudarev of the Kurchatov Institute reported on a 6 T superconducting wiggler. This three-pole wiggler was built at the Kurchatov Institute and successfully tested in the Chinese National Synchrotron Radiation Laboratory (800 MeV Hefei storage ring) in March. Its magnetic field is generated by three pairs of racetrack niobiumtitanium windings. The wiggler, with a short magnetic field period of 187 mm, enables an electron storage ring to provide a wider spectrum of synchrotron radiation.

Nikolai Chernoplekov, the Director of the Institute for Solid State Physics and Superconductivity, concluded the school with a review of several problems vital to the future of this field. The advent of warm superconductors has inspired new interest, with the promise of new (and as yet unknown) applications, or even a revolution in the traditional applications of superconductors.

High vacuum

Particles circulating in a particle accelerator near the speed of light cover enormous distances. In synchrotrons, where beams are only held for a few seconds at the most, the tube containing the particles has to be evacuated down to at least 10^–6 mbar. In storage rings, where the particles have to circulate for hours and sometimes days, pressures need to be taken down still further, to about 10^–12 mbar, to minimize beam-gas collisions and maximize machine output.

The development of light bulbs and electronic tubes earlier this century provided the first big boost to vacuum technology, and further impetus came in the 1950s where the development of the first big particle accelerators and of space simulation chambers called for industrial participation in both prototype development and actual construction.

In this work, the need for ‘clean’ vacuum free of hydrocarbons pushed the performance of oil and mercury diffusion pumps. Later, the more exacting demands of storage rings led to a swing to turbomolecular and sputter ion pumps. The first major series of turbopumps from Pfeiffer (Germany) was installed in CERN’s PS proton synchrotron and ISR Intersecting Storage Rings, while sputter ion pumps, requiring little maintenance, with no moving parts and with almost infinite lifetime, have become the preferred pump for the high reliability large systems needed for particle physics.

Ion pumps initially had difficulties both at high pressures and in handling chemically inert gases (helium, argon, etc.) but continuous ‘pressure’ from the accelerator sector led to improved performance, such as in the Leybold (Germany) differential diode developed for CERN’s SPS Super Proton Synchrotron and the Varian (Italy) Starcell triode ion pump, developed from the stringent rare gas pumping specifications for the 27 km LEP storage ring at CERN.

Particle accelerators underlined the importance of surface cleanliness in reducing desorption. Pioneering work at the ISR on the famous ‘pressure bump instability’ led to increased understanding of particle-surface interactions, and the curing techniques have benefited other vacuum applications (plasma fusion devices, thin film technology, microelectronics,…).

Experience from particle physics has also stressed the importance of ‘outgassing’ from the myriad of materials in contact with the vacuum system. Now a wide range of materials (stainless steel, ceramics, ferrites, aluminium, alloys, plastics,…) has been specially studied for high vacuum applications, while in situ bakeout at high temperatures is standard practice.

A special problem in accelerators, typically using a narrow but very long vacuum pipe, is that of conductance – ensuring a uniform vacuum with grouped pumping systems. One approach is to reduce the space between pumps, or even have one long continuous pump.

For LEP, a non-evaporable getter (NEG) pump was designed in close collaboration with industry (SAES Getters, Italy). Although getters had been used for 30 years to evacuate lamps and electronic tubes, the 27 km LEP tube provided a new challenge.

Built to handle protons, CERN’s seven kilometre SPS proton synchrotron faced new problems when called on to store antiprotons in CERN’s antiproton project, launched ten years ago. Flashed titanium sublimation getter pumps for the SPS and for the AC and LEAR antiproton rings were designed to meet CERN specifications (Vacuum Generators, UK, and Balzers, Switzerland), and improvements to such sublimators have gone on to be available commercially from industry.

Physics demands for high reliability and efficiency have also brought in their wake much new vacuum hardware (seals, flanges, joints, all-metal valves, etc.).

This article was adapted from text in CERN Courier vol. 28, November 1988, pp15–17

How Borexino will stare at the Sun

Layout of Borexino — Layout of the Borexino solar neutrino experiment, contained in a cylindrical stainless steel tank 18 m in diameter and 18 m high in the centre.

The energy-dependent deficit of the measured solar neutrino flux compared to the predictions of the Standard Solar Model is often called the Solar Neutrino Problem. In recent years, this problem has become a paradox because the more recent experimental results (from Gallex and Sage), together with the older data (from Homestake and Kamiokande) and the measurement of the solar luminosity indicate a severe suppression of the solar neutrino flux from beryllium-7 reactions, pushing it lower than the neutrino flux from boron-8, which is a product of the reaction involving beryllium-7.

New neutrino physics, in the guise of oscillations, with neutrinos changing their “flavour” (electron-, muon-, or tau-) as they fly through space (September 1998), can fix these problems, which cannot be explained by changing the Standard Solar Model. In addition to this basic vacuum oscillation scenario, additional effects matter oscillations are possible due to charged current interactions between neutrinos and the electrons of matter, if the material (the Sun) traversed by the neutrinos is dense.

Measuring beryllium-7 neutrinos

One of the most crucial problems is the behaviour of the beryllium-7 solar neutrinos, whose energy range is below 1 MeV (two monochromatic lines at 384 and 862 keV; the second has by far the higher flux). In addition to measuring the total flux of beryllium-7 neutrinos, time variations of the flux should provide evidence of oscillations, independent of the Solar Standard Model.

Previous experiments (Homestake, Gallex, Sage) able to measure solar neutrinos at low energy were radiochemical experiments, with at most one event per day. A solar neutrino experiment detecting low-energy events in real time at high rate, was needed. Fulfilling the need is Borexino, located underground in the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The main neutrino reaction detected by Borexino is neutrinoelectron elastic scattering, and the main problem dictating the design is natural radioactivity.

Borexino is an unsegmented detector, based on a liquid scintillator, and conceived as a sequence of concentric shells; the more internal the shell, the higher its radio-purity. At least two metres of pure water provides the first shield against gammas and neutrons from the surrounding rock. The water, some 2300 m³ in total, is contained in a cylindrical stainless steel tank 18 m in diameter and 18 m high in the centre (shown blue in figure 1). An internal stainless sphere, 13.7 m in diameter, supports 2400 phototubes and divides the external water from an internal buffer liquid. 200 outward-pointing phototubes fixed on the external walls of the sphere provide a muon veto. 2200 phototubes are mounted on the internal walls, 1800 of them coupled to optical concentrators. The total optical coverage of the sensitive volume is about 30%.

The Pseudocumene buffer liquid (yellow in figure 1) assures another 2.6 m of shielding against rock emanation and against gammas produced by the phototubes, optical concentrators and related materials (mu-metal, sealing etc). Finally, the liquid scintillator is contained in a thin, transparent sheet of nylon, 8.5 m in diameter and 300 m³ in volume. The densities of the scintillator and the buffer liquid are practically the same to assure negligible buoyancy of the nylon vessel. Using an inner fiducial volume of 100 m³, 6 m in diameter (orange in figure 1) provides a further 1.25 m of shielding against emanation from the nylon walls and gammas from the liquid buffer. A further nylon balloon is installed between the stainless steel sphere and the inner vessel as a barrier against radon emanation from the various materials inserted in the sphere.

Nuclei as secret agents

ISOLDE is a source of radioactive-particle beams with applications ranging from nuclear medicine to astrophysics. Recent ISOLDE experiments have opened up a new approach to examining the interface between ultra-thin magnetic and non-magnetic layers. The ultra-high vacuum ASPIC (Apparatus for Surface Physics and Interfaces at CERN) chamber takes keV ions from the 1 GeV proton beam bombardment of an ISOLDE ion source target, evaporates them at eV energies onto carefully cleaned surfaces, and studies interactions at the micro-eV scale. Each experiment covers 15 orders of magnitude in energy to study interactions at an atomic scale. Early results have produced some surprises.

isolde1-10-98 — Spying on atoms the Perturbed Angular Correlation (PAC) spectroscopy technique uses probe nuclei with electric and magnetic moments and which decay by emitting two gamma rays nanoseconds apart. This allows PAC nuclei to “spy” on neighbouring atoms. The figure shows a model of Ni(001)c(16×2)Pd with a radioactive Cd-111 impurity at a bridge site.

The technique used at ISOLDE is called Perturbed Angular Correlation (PAC) spectroscopy. It relies on probe nuclei with two properties: they must have electric and magnetic moments, and they must decay by emitting two gamma rays nanoseconds apart. This combination of characteristics allows PAC nuclei to spy on neighbouring atoms after being “parachuted” onto surfaces, like secret agents. The interaction of the PAC nucleus’s electric moment with the electric field gradients on and around the surface allows its precise position to be determined. The coupling between the magnetic moment of the PAC nucleus and the magnetic field it feels either applied or due to surrounding nuclei perturbs the correlation between the two gamma rays, allowing information to be gleaned at the atomic scale. This coupling, known as the magnetic hyperfine interaction, gives rise to magnetic hyperfine fields which are often very large.

Surprising result

For example, the magnetic hyperfine field of selenium atoms in bulk nickel is 15 Tesla. The first ISOLDE experiment conducted with the ASPIC chamber was designed to investigate whether this was also true for the selenium atoms deposited as relatively loosely bound atoms, called adatoms, on a surface of ferromagnetic nickel. The result came as a surprise, showing that the field is in fact much lower for selenium as an adatom at around 1 Tesla. This result prompted a theoretical study which revealed a completely different magnetic behaviour for impurities as adatoms compared to impurities in bulk, in agreement with the experiment.

Because hyperfine interactions are of very short range from atom to atom the PAC technique gives resolution to a single atomic layer. Moreover, because the gamma rays have long range, it allows deeply embedded atomic monolayers in a system to be investigated, something which has not before been possible. Furthermore, the technique is extremely sensitive. Only one “spy” atom is needed per thousand atoms on the surface or interface under investigation. Impurities at such a low concentration do not influence the intrinsic properties of magnetic thin layer systems, making PAC atoms the perfect spies.

ASPIC experiments capitalized on these properties by introducing PAC atoms into single atomic layers to investigate the interactions between layers. An isotope of cadmium was incorporated as an impurity into a single layer of palladium on a nickel single crystal. A sample of about 1 square centimetre was used, corresponding to 10¹⁵ atoms per layer with 10¹² PAC spies incorporated. Results showed that the nickel induces ferromagnetism in the palladium. Further studies showed that this induced magnetism continued through to a second layer, though with much reduced strength. This observation led ISOLDE researchers to ask what would happen deep in palladium if the palladium was covered by ferromagnetic nickel. Seven atomic layers of palladium were deposited on a palladium crystal, after parachuting the PAC probe atoms on to the crystal surface. These were then covered with two atomic layers of nickel. The result was again surprising, showing that in this case paramagnetic palladium was converted into a superparamagnetic material.

Another curious finding is that isolated cadmium probes choose specific sites, called bridge sites, in a palladium monolayer on nickel. This raises the question of whether a larger number of cadmium atoms occupying all bridge sites would break up the structure of the palladium into cells of sub-nanometre scale. If they do, the consequences for the electronics industry could be profound; state-of-the-art today is nanometre-scale devices.

The application of PAC probe atoms to the study of local magnetic properties in ultra-thin layer systems is in its infancy. But these results from ASPIC clearly indicate the power of the technique to provide detailed information about surfaces and interfaces with single-atom precision.

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Multistage decays

Wroclaw

Measuring beryllium-7 neutrinos

Surprising result