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Nuclear Physics

Lund University

Background

Contemporary nuclear structure studies aim at investigations of atomic nuclei far from the line of stability. Long-standing fundamental questions to be addressed are:
• What are the limits of existence of atomic nuclei?
• How does nuclear shell structure evolve with the number of nucleons?
• Where does the periodic table of elements end?
• What is the origin of the heavy elements?
To reach and investigate the most exotic nuclear species, the future of nuclear physics in Europe foresees the construction of international radioactive ion-beam facilties. One new major accelerator center is FAIR, and the NUSTAR scientific pillar of FAIR receives its scientific justification by and large from the questions posed above. With the FAIR horizon in the years 202x, nuclear physicists throughout Europe prepare for this and related facilities, at optimum by means of physics driven R&D on new or revised production and detection schemes of exotic nuclear species.

One example is the LYCCA device, which became the first NUSTAR device with a FAIR approved Technical Design Report. The main objective of LYCCA is to uniquely characterize exotic nuclei by their mass, A, and charge, Z. The nuclei are secondary reaction products following Coulomb excitation, direct reactions, or fragmentation reactions of relativistic radioactive ion beams. These beams are to be provided by the new Super-FRagment Separator (Super-FRS). They will have energies of 100-300 MeV/u when hitting the secondary target in the HISPEC focus of the NuSTAR Low-Energy Cave. 

The detection scheme of LYCCA-0 within the PRESPEC campaigns, i.e. the HISPEC precursor at the existing GSI facility, is sketched below. A radioactive ion beam (RIB) enters the focal plane experiment area from the left (see also the photograph of PRESPEC), along with A- und Z-identification by detectors related to the roughly 80 meter long GSI Fragment Separator (not shown). The beam passes first a time-of-flight (ToF) start detector - a large plastic scintillator membrane with 32-fold photomultiplier read-out - before it hits the secondary target, where the nuclear reaction of interest is being induced. The reaction products emit gamma-radiation from excited nuclear states, which is detected by an array of high-resolution germanium detectors (not shown) - previously EUROBALL cluster detectors, at present AGATA modules. 

This gamma radiation must be Doppler-corrected and associated with a given nuclear species, and both these tasks serves LYCCA. The former is done by double-sided silicon strip detectors (DSSSD), which provide (x,y) tracking position information both near the target and in the LYCCA chamber. These DSSSD measure also the energy loss,  ΔE, of the heavy-ion, which in conjunction with their total energy, E, provides the proton number, Z, of the reaction products. ΔZ/Z<0.5/100 has been achieved in the first PRESPEC experiments. The LYCCA chamber also hosts a ToF stop detector of the same kind as the ToF start detector. These detectors measure the flight time with a precision of Δt<25 ps (σ<10 ps). Together with the information of the tracked flight path and the total energy measurement in the CsI detector elements, ΔA/A<1.0/100 has already been reached.

LYCCA is a flexible array of detector modules. Each module comprises a set of 9 cesiumiodid (CsI) detectors for the energy measurement of the reaction products and one 32x32 double-sided Si-strip detector (DSSSD) for energy loss and position information. Up to 30 such modules can be hosted by the LYCCA chamber. For the measurement of the time-of-flight between the target position and the LYCCA chamber, a detector prototype made of polycristalline diamond wafers has been tested during the first physics driven LYCCA-0 commissioing phase (2010-2011) by the UK collaborators. During the PreSPEC-AGATA experiment phase (2012 and 2014), this target ToF start signal is being generated by a medium-sized ultra-fast scintillator in conjunction with a 12-fold photomultiplier tube readout, similar to the existing large-membrane ToF detectors.

Since the FAIR project as such is delayed, the LYCCA collaboration decided to maintain the instrument 2017-202x at a dedicated beamline of the 10 MV Tandem accelerator of the University of Cologne. The significantly lower beam energies and thus different nuclear reaction physics led to a reconfiguration from a plain wall to a tunnel (16 LYCCA modules) plus wall (3x3 modules, center empty). Readout electronics and data acquisition, however, resemble the final NUSTAR-LYCCA configuration.

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