Juha Äystö
The Nuclear Matter Programme provides full participation of the Finnish teams at CERN in studies of two aspects of nuclear and hadronic matter. These are cold exotic matter with extreme composition of its proton and neutron numbers and hot and dense matter created in relativistic heavy ion collisions. The first project is carried out at the ISOLDE Facility and the second one concentrates on the construction of certain parts of the ALICE detector for the LHC machine as well as on the relevant physics issues. The ISOLDE project has its physics motivation in studies of exotic structures of nuclei, with special emphases on weak interaction phenomena and nuclear astrophysics. The ALICE project aims to study the phase transitions of hadronic matter and possible signatures for a new form of matter, the quark and gluon plasma. The project leaders of these two projects are Docent Ari Jokinen for ISOLDE and Docent Wladyslaw Trzaska for ALICE, both currently at JYFL. In addition, Dr. Markku Oinonen has been acting as the co-ordinator of our bonding project for the inner tracker of ALICE as well as the HIP detector Laboratory at Kumpula.
ALICE/ITS/SSD module assembly
The two largest layers of the ALICE Inner Tracking System (layers 5 & 6) will contain about 2000 Silicon Strip Detector (SSD) modules. The production of these modules is shared between France, Italy and Finland. The assembly of the Finnish share of SSD modules (about one-third of all the modules) will be done in the Detector Laboratory located at the Kumpula campus in Helsinki.
In the past years the entire SSD project has suffered considerable delays caused by the lack of components, redesign of the entire electronics to radiation hard technology and various manufacturing problems at commercial companies. As a result there is no contingency left within the project. It imposes on us very tight planning and demands efficient work. Fortunately, we were able to keep up with the stringent schedule. The first successful assembly of prototype components, still with the semiautomatic bonding machine, was completed in Helsinki in the summer of 2002. Since then, two very significant steps forward have been taken. In the spring of 2003 a fully automatic bonding machine (F&K Delvotec 6400) was installed in the Kumpula clean room and in June 2003 a testing facility to perform detailed electrical tests for the HAL25 front-end chips started taking data. These developments allowed us to assemble and test altogether 44 HAL25 front-end chips within the first half of 2003. This was the first time in the history of the ALICE experiment that a fully automatic bonding scheme for the HAL25 front-end chips had been successfully demonstrated.
By August 2003 the first two SDD modules were assembled in Helsinki. In September 2003 they were tested in-beam at CERN together with 2 other modules manufactured by our colleagues at Strasbourg. The 4 modules were combined to make a mini-tracker. In the run with a 7-GeV proton beam we have evaluated the tracking capabilities of the modules, their spatial resolution, charge matching properties and signal-to-noise ratios (S/N). The results were well in accordance with our expectations. For instance, S/N was around 65 for the P-side and 40 for the N-side of the detectors.
In November 2003, we were ready to check our capability to assemble HAL25 chips with high speed and yield. A set of 122 chips was bonded to assemble 4 hybrids and 2 modules. We were able to ramp-up the production speed to about 35 chips/day with the single-sweep bonding yield of 84%. After the re-bonding phase (to correct the non-connected traces) we could reach the final bonding yield of nearly 100%. The final assembly yield, due to chip-related failures, was 77% and 93% (after re-bonding). Without the skill and the know-how of our Ukrainian collaborators we would not have been able to achieve such excellent results so quickly in Helsinki.
Along with the prototype production for the SSD assembly, our group has addressed the reliability issues. A typical TAB contact is an interface between 5 different materials with different coefficients of thermal expansion. Even at a constant ambient temperature the flow of current through the bonds causes thermal stress leading, in the long run, to possible failures. To test it a set of assembled HAL25 front-end chips was exposed to temperature cycles varying from -20°C to +100°C for a period of 1 month and about 1000 cycles. Only two bonds were destroyed out of 900 monitored. In fact the destroyed bonds were diagnosed as partially broken already at the start of the test. This indicates a very reliable nature of the Al-Al bonds used in the ALICE SSD modules. These tests will continue during the first quarter of 2004. In this way we hope to detect and remove all of the assembly failures and to estimate the lifetime of the ALICE SSD modules under the actual LHC conditions.
All in all, 2003 has been very successful for the ALICE SSD group in Helsinki. Thanks to a lot of hard work and a little good luck we were the first Laboratory to be fully ready for the mass production of the Silicon Strip Detector modules for the ALICE experiment.
ALICE T0 Detector
T0 is one of the smallest sub-detectors in ALICE but it is also one of the few detectors that are absolutely crucial to the operation of the whole experiment. It will give the key trigger- and timing signals, measure on-line vertex position and give rough centrality. Among its many technical challenges are: a total dead time below 25 μs (including digitization and readout), a count rate of up to 10 MHz, a required time resolution better than 50 ps, radiation hardness of up to 500 krad, operation in the 0.5 Tesla magnetic field, compact design, high reliability and maintenance-free operation during the entire lifetime of ALICE.
None of the off-the-shelf products can fulfil these requirements. Practically all of the components and in particular the entire set of front-end electronics must be custom designed and built. We have made a lot of progress in that respect. By the summer of 2003 the first prototypes of all the key electronics modules were ready for in-beam testing at CERN. Although PS beams cannot reproduce the LHC conditions in which T0 will have to operate, a test in the actual accelerator environment (interference from other detectors and devices, real signals passing over the realistic lengths of cables, etc.) was absolutely essential. It was reassuring to realize that all of our components have worked well. For instance, we could reach the time resolution of 37 ps (sigma), our T0 Vertex Module prototype gave a position resolution of 1.3 cm with 98% efficiency and the Mean Timer prototype gave consistent readings to within ±10 ps
An in-depth technical document called TDR (Technical Development Report) is an important milestone marking the end of the main R&D phase of any sub-detector of ALICE. Prior to the approval, several expert groups within ALICE and CERN review the TDR and give feedback to the research groups pointing out possible weak points in the design or scheduling. A joint TDR for all of the front detectors (including T0) is now nearing its completion. Finalizing the technical details, making the necessary Monte Carlo simulations, compiling the results of all the key measurements followed by writing the reports and editing has been only a part of this work. There is also immense challenge in designing the readout and DSC (detector slow control) for T0. We are working hard to complete the TDR in time for the March 2004 LHCC Review.
ALICE Tracking
Production of strange particles in heavy-ion collisions is one of the main research goals of the ALICE experiment. In the central (head-on) collisions of lead nuclei, each accelerated at the LHC to the energy of 1.4 PeV (1.4 x 1015 eV), it should be possible to reach, for the first time since the Big Bang, sufficient densities of quark-gluon plasma where the density of strange quarks would approach that of the light ones. This in turn should result in a boost in the production of strange particles. Unfortunately, positive identification of strange particles is very difficult. It is done, for instance, by the detection of their decay products. These products would not originate from the main vertex (collision point) but from the decay points of the short-lived strange particles, therefore slightly off the main vertex.
One of our achievements was to derive a procedure to reconstruct and identify secondary vertices. Preliminary estimations of the reconstruction efficiency and precision against the realistic background conditions are very encouraging. These methods have been developed and tested on the basis of Monte Carlo simulations using ALIROOT (the ALICE software framework). In order to reconstruct hyperons and identify kaons via their decay topologies we look for their products in the tracking system and find the decay vertices. For this we had to develop a method for finding secondary vertices and kinematical reconstruction of the decay. Coping with the high background (high track density) resulting from the large number of particles expected to originate from Pb-Pb collisions at the LHC energies is the main challenge in our work. Nevertheless, the first results, based on the detailed simulations, indicate the possibility of the successful identification of charged kaons and neutral strange particles using the Time Projection Chamber - the main tracking detector of ALICE.
Assuming that isospin is a good quantum number one expects symmetry between transitions from T=1, Tz=±1 nuclei to the common excited states in the Tz=0 nucleus between them. Such a symmetry test can be performed by comparing the strengths of Gamow-Teller (GT) transitions obtained from (p,n)-type charge-exchange reactions with those obtained from the beta decay. Mass A=58 provides an ideal case for such a comparison, since the same states in 58Cu can be populated in the charge-exchange reaction 58Ni(3He,t)58Cu and in the beta decay of 58Zn. The charge-exchange reaction has been studied in great detail in Osaka and the beta decay of 58Zn was observed by our group for the first time at ISOLDE in 1998, and the decay was revisited in a series of experiments in 2002 and 2003 with an improved experimental set-up. These studies involved high efficiency beta-gamma spectroscopy as well as a dedicated search for proton decay from higher lying excited states in 58Cu. The latter part of the experiment applied the Si-ball array to be discussed below. In 2003 we have analysed the data from the beta-gamma spectroscopy part and the data from beta-delayed proton spectroscopy is under evaluation.
In connection with neutron converter target tests, which aims for the enhanced production of fission fragments compared to spallation products, we performed a spectroscopic study on the beta decay of neutron-rich Cs-isotopes extending to mass A=150. In connection with the same study we also explored decay properties of neutron-rich Fr isotopes up to mass A=232, these were produced by impinging the primary beam directly to a UC-target thus resulting in enhanced spallation production. In 2003 the data of the latter experiment was analysed resulting in an extended decay scheme of the very neutron-rich 232Fr isotope. The data obtained also provides an important addition to yield information on Fr isotopes from the spallation reaction which is of interest for the future European radioactive ion beam facility EURISOL. This project has selected Fr isotopes as one of the key isotopes, since they offer interesting possibilities to study the atomic parity non-conservation which is closely related to lepton-quark interactions through Z0 gauge boson exchange at small momentum transfer.
A rapid proton capture (rp) process is a part of the explosive hydrogen burning occurring in a special astrophysical events, like an accretion in close binary systems. At high temperatures it may continue until the fuel for nuclear burning is exhausted. The typical time scale for the event, known as X-ray burst, is 10-100 s. The rp-process flow proceeds quickly in the neutron-deficient side of the nuclide chart to a mass region A ~70, where it slows down or terminates depending on the nuclear properties. As a continuation to our studies for 72,71,70Kr, we have performed a beta-delayed proton decay study of 69Kr, which gives access to proton unbound states in 69Br. The data from this experiment has been analysed and the results were reported in the RNB6 Conference with results from Monte Carlo calculations for a new design of a thick spallation target at ISOLDE. We have also continued research to find an optimum way to produce a pure beam of 68Se for high-precision mass measurement of this isotope.
Parallel to the ongoing research programme the Finnish group at ISOLDE has initiated a Si-ball R&D detector project and developed new means to produce and manipulate radioactive ions at ISOLDE. The Si-ball project aims for a high-granularity charged particle detector array. The full detector consists of 104 detectors in rhombicuboctahedron geometry made of squares and triangles. The detector provides very high geometrical efficiency close to 100%, wide energy range with a very low energy threshold and high enough granularity for angular distribution measurements between emitted particles. The first half of the Si-ball was successfully commissioned in 2002, and the array was applied in the beta-delayed proton decay study of 58Zn in 2002 and 2003. In 2003 further tests were carried out to characterize each individual detector in terms of leakage current, dead layer and resolution.
Finally, an ion cooler and buncher development has concentrated on the technical design and manufacturing drawings. At the end of 2003, construction of the test bench for the ion cooler with a 60 kV HV platform and test ion source was also started. In addition to the original research project we have initiated a new project for laser spectroscopy of cooled ion beams at ISOLDE.