An electron test beam at the Frascati Φ-factory offers users a range of options
The Beam Test Facility (BTF) is part of the DAΦNE Φ-factory complex, the most recent of the electron–positron colliders in the long history of the INFN Laboratori Nazionali di Frascati (LNF). The facility features a high-intensity linac that provides electrons and positrons up to 750 MeV and 550 MeV respectively, a damping ring to improve injection efficiency and two main rings designed for the abundant production of K mesons coming from the decay of the Φ resonance at 1.02 GeV (Mazzitelli et al. 2003). The main research goal is to study matter–antimatter asymmetry and the interactions of “s” quarks, but K mesons are also useful tools in nuclear and atomic physics.
Before the high-intensity electron or positron beam pulses produced by the 60 m long linac are injected into the double storage ring, they can be extracted to a transfer line that is dedicated to the calibration and ageing of particle detectors, the characterization and calibration of beam diagnostics, and the study of low-energy electromagnetic interactions (figure 1). Here, the number of particles can be reduced to a single electron or positron per pulse by means of a variable thickness copper target. The particle momentum is then selected, with an accuracy better than 1%, using a dipole magnet and a set of tungsten collimators. The energy range is typically 25–500 MeV, and up to 49 pulses per second can be extracted (20 ms repetition time), with a bunch length of 10 or 1 ns. When not operating in conjunction with the collider, the linac’s maximum beam energy can be raised to 750 MeV (for electrons) and the intensity increased to a maximum of 1010 particles per second, limited by radiation safety.
When operating in low-intensity mode, particles are selected from the secondary showers emerging from the target, so either electron or positron beams can be chosen. The final intensity can easily be tuned (by adjusting the tungsten collimators) over a range of several orders of magnitude – from 104–105 particles per pulse, down to a single particle (Poisson distributed). An optical system of four quadrupoles along the BTF transfer line allows the transverse distribution of the beam to be tuned. A typical beam spot of 2×2 mm2. transverse section (1σ profile), with an angular divergence of about 2 mrad, is produced at 500 MeV in single-particle mode (figure 2). Two different beamlines are available, depending on the configuration of the final dipole magnet in the experimental hall (figure 3). When needed, the full-intensity beam can be extracted to the BTF area by removing the copper target from the beamline.
The commissioning of the transfer line, the two BTF exit lines and all the diagnostic devices needed for a reliable operation of a test-beam facility was completed in autumn 2002. The facility has since hosted tens of groups from all over Europe, who have run a variety of experiments and tests with electron and positron beams.
The applications of the BTF beam – with its intensity and energy range, good spatial and excellent timing properties – are extremely wide ranging. Typical uses of the facility in its single-electron mode of operation include testing the ring-imaging Cherenkov system for the LHCb experiment at CERN’s LHC and using electrons at 500 MeV to make highly accurate measurements of the efficiency of OPAL lead glass used in the NA62 experiment at the SPS. A more unusual investigation concerned the thermoacoustic detection of particles by the type of ultracryogenic resonant antenna used for gravitational-wave detection. Such antennae are sensitive enough to detect the impact of cosmic rays, and indeed the tests could observe the vibration occurring when the full force of the high-intensity BTF beam struck the antenna (figure 4 and 5).
An important upgrade of the BTF line was completed in 2005, with the installation of dedicated devices for the production of a beam of tagged photons. To intercept the BTF beam with a small, but not negligible, probability of emitting a bremsstrahlung photon, an active target made of four layers of single-sided silicon microstrip detector planes can now be inserted just before the last dipole magnet that selects one of the two exit lines. In this case the electron is not transported through the dipole but instead hits the inner wall of the vacuum pipe inside the magnet. Its energy is then detected by a series of silicon microstrip detector modules installed outside the beam-pipe, thus allowing the reconstruction of its bending radius. Combined with the measurement of position and angle in the active target, this yields the energy of the radiated photon with a resolution of 7% in the 200–500 MeV range, at a typical production rate of 0.5 Hz. This photon-tagging system has been used successfully for the calibration of the scientific payload (in particular the tungsten/silicon detector minicalorimeter) of the gamma-ray astronomy satellite AGILE, launched by the Italian Space Agency in summer 2007 (figure 6).
Since March 2007 the duty factor of the facility has been improved from 40% up to 90% of the operation time of DAΦNE, thanks to the installation of a new dedicated pulsed-dipole magnet, designed, in collaboration with Maurizio Incurvati and Claudio Sanelli at INFN-LNF, by CERN (Maccaferri and Chiusano 2006). This is capable of driving any of the 50 linac pulses per second to either the accumulator ring or the BTF transfer line. The BTF, operated by the Frascati Accelerator Division staff, typically provides beam for an average period of 250 days a year.
The BTF facility is already equipped with instrumentation and diagnostics capable of covering the entire energy and intensity range. It is also continuously being improved to satisfy the growing interest of the broad scientific community that it serves. The support and collaboration of the users is crucial for better operation and development of the facility. Many improvements of the BTF diagnostic tools have been introduced in collaboration with hosted groups. The requests and proposals made by the user community are important in pushing exploration towards new operating schemes and new possibilities, including projects for the production of a low-intensity neutron beam and R&D studies for high-precision diagnostics at high intensities, all of which are under way.