High Resolution E-ToF Mass Identification for Heavy Ions with Calorimetric Low Temperature Detectors

The direct in-flight mass determination of nuclides produced in heavy-ion induced reactions is an important task, requested in various fields of heavy-ion physics. It is in many cases mandatory for a unique identification of rare isotopes and an unambiguous interpretation of experimental data. The experimental technique often used for this purpose is a combined energy and time-of-flight (E-ToF) measurement. However, for low energetic heavy ions, ionization based energy detectors suffer from incomplete energy detection due to charge recombination, resulting in pulse-height defect and a relatively poor energy resolution. Therefore, for low energy heavy ions the mass resolution of an E-ToF spectrometer is usually limited by the performance of the energy detector. This problem can be overcome by using calorimetric low temperature detectors (CLTD’s) for the energy measurement. CLTD’s provide, as compared to conventional ionisation detectors, due to their detection principle, substantial advantages in detector performance, such as energy resolution and energy linearity, etc. CLTD’s have been frequently demonstrated to achieve an excellent relative energy resolution of ∆E/E = 1-2×10−3, and a good energy linearity with a complete absence of pulse-heigt defect, in a wide range of ions and energies [1,2]. Thus a combination of CLTD’s as high-resolution energy detectors with ToF detectors provides a detector system for high-resolution mass identification of low energetic heavy ions. Recently a prototype of a E-ToF detector system was setup at GSI and tested with U beams from the UNILAC. The ToF detector consists of two MCP-Chevron detectors, mounted in a relative distance of 1 m, and equipped with 10 and 4 μg/cm carbon converter foils, respectively. The energy was measured by an array of 8 CLTD pixels (Fig. 1) with a total active area of 12×6 mm, operated at 1.5 K. The performance of the E-ToF system was tested with U particles with a broad energy distribution of E = 0.1 1 MeV/u, produced by scattering the beam under small angles from a 20 mg/cm thick Au-Target. With the present time resolution ∆t (FWHM) ≈ 300 ps, which may be improved in future, a mass resolution of ∆m (FWHM) = 1.53(5) amu was obtained (Fig. 2). Possible future applications of such a E-ToF detector system are, after potential improvements in ToF-resolution and active solid angle, the mass identification of superheavy elements, heavy fission products, or reaction products in experiments with radioactive beams. 3 mm