Performance studies and improvements of a Time-of-Flight detector for isochronous mass measurements at the FRS-ESR facility

At GSI Darmstadt the technique of Isochronous Mass Spectrometry (IMS) has been developed for direct mass measurements of exotic nuclides. In this method a cocktail beam of highly-charged ions is produced via projectile fragmentation or fission, separated in the FRagment Separator (FRS) and injected into the Experimental Storage Ring (ESR) operated in an isochronous mode. The mass of the exotic nuclei can be deduced from precise revolution time measurements by a time-of-flight (TOF) detector placed in the ESR. In the detector ions passing a thin foil release secondary electrons, which are transported to two microchannel plate (MCP) detectors in forward and backward directions by electric and magnetic fields. In this work the performance characteristics of the detector were investigated by simulations and by offline and online experiments and significantly improved. In particular the timing performance and the rate capability were measured and enhanced. The detection efficiency improvements developed in previous work were verified and the use of thinner carbon foils to increase the number of turns of the ions in the ring were implemented. This work also forms a basis for the development of a dual detector system for IMS in the collector ring at FAIR. In this work the main contributions to the TOF detector timing such as the transport time of the secondary electrons, the electron transit time through the MCPs and the method of determination of the event time from the MCP signals (event time determination) were analyzed and improved. The timing accuracy of the TOF detector was investigated by coincidence time-of-flight measurements. The timing uncertainty of a single branch of the detector with standard settings was measured in the laboratory with an alpha-source and amounts to sigma(branch)=48 ps. In an online experiment at the ESR using MCPs with 5 µm pore sizes the timing accuracy was measured as sigma(branch)=48 ps with a stable 20^Ne beam and sigma(branch)=45 ps with 238^U fission fragments. Those measurements were performed for the kinetic energy of the secondary electrons (K) equals 700 eV. To improve the transport time of secondary electrons the TOF detector was modified for higher values of electric and magnetic fields. An improved time spread sigma(branch)=37 ps was obtained in the measurements with alpha-particles using MCPs with 10 µm channel diameter for an kinetic energy of 1400 eV of the secondary electrons. The contribution from the transit time through the MCP channels to the time spread was investigated with alpha-particles as a function of different electron yields from the carbon foils. Using a higher thickness of the carbon foil timing is not improved significantly. Therefore, 10 µg/cm^2 is an optimum for the carbon foil thickness in the matter of efficiency and timing. In case of a foil with a Cs-compound on the surface, for which the number of secondary electrons is increased by a factor of 10, the timing was improved to sigma(branch)=27 ps (K=1400 eV). A newly constructed anode design improves the bandwidth of the MCP detector by a factor of 2 leading to a reduction in the width of the MCP signals by a factor of two to an improvement of the rise time by about 20%. The signal shape of the MCP detector influences the determination of the revolution times of the ions in the ring and thus the mass measurement accuracy. Due to the high revolution frequencies of the ions in the ESR (~2 MHz) a high rate capability detector is required. The rate acceptance of the MCP detector was improved in the offline experiments by a factor of 4 due to the larger number of channels of MCPs with 5 µm pore size. At each turn in the ESR the ions pass the foil and lose energy. According to simulations the decrease of the foil thickness by a factor of two allows to double the number of ion revolutions in the ring. To store ions for a longer time in the ESR a thinner carbon foil with a thickness of 10 µg/cm^2 and MCPs with a 5 µm channel diameter were installed in the TOF detector and used for the first time in the online experiments. The results of the experiments measured with 10^Ne^10+ stable beam and 238^U fission fragments were compared to the results of the previous experiments. In the previous experiments a carbon foil with a thickness of 17 µg/cm^2 coated with 10 µg/cm^2 of CsI on both sides, which caused a calculated energy loss of 86 keV (86^As^33+, 386.3 MeV/u) and MCPs with 10 µm pore size were used. For the carbon foil of 10 µg/cm^2 the calculated energy loss is 31 keV, that is a factor of 2.7 less than for the thicker foil. Summing up the results, with thinner carbon foil and higher rate resistance MCPs with 5 µm pore sizes in the TOF detector up to ten times more ion revolutions in the ring were observed. With larger number of turns in the ring one increases the detection efficiency and the mass measurement accuracy

Depth dose measurements in water for 11C and 10C beams with therapy relevant energies

Owing to the favorable depth-dose distribution and the radiobiological properties of heavy ion radiation, ion beam therapy shows an improved success/toxicity ratio compared to conventional radiotherapy. The sharp dose gradients and very high doses in the Bragg peak region, which represent the larger physical advantage of ion beam therapy, make it also extremely sensitive to range uncertainties. The use of beta(+) - radioactive ion beams would be ideal for simultaneous treatment and accurate online range monitoring through PET imaging. Since all the unfragmented primary ions are potentially contributing to the PET signal, these beams offer an improved image quality while preserving the physical and radiobiological advantages of the stable counterparts. The challenging production of radioactive ion beams and the difficulties in reaching high intensities, have discouraged their clinical application. In this context, the project Biomedical Applications of Radioactive ion Beams (BARB) started at GSI (Helmholtzzentrum fur Schwerionenforschung GmbH) with the main goal to assess the technical feasibility and investigate possible advantages of radioactive ion beams on the pre-clinical level. During the first experimental campaign C-11 and C-10 beams were produced and isotopically separated with the FRagment Separator (FRS) at GSI. The beta(+)-radioactive ion beams were produced with a beam purity of 99% for all the beam investigated (except one case where it was 94%) and intensities potentially sufficient to treat a small animal tumors within few minutes of irradiation time, similar to 10(6) particle per spill for the C-10 and similar to 10(7) particle per spill for the C-11 beam, respectively. The impact of different ion optical parameters on the depth dose distribution was studied with a precision water column system. In this work, the measured depth dose distributions are presented together with results from Monte Carlo simulations using the FLUKA software

Studying Gamow-Teller transitions and the assignment of isomeric and ground states at N=50

Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100Sn were performed at the FRS Ion Catcher (FRS-IC) at GSI, Germany. The nuclei were produced by projectile fragmentation of 124Xe, separated in the fragment separator FRS and delivered to the FRS-IC. The masses of 14 ground states and two isomers were measured with relative mass uncertainties down to 1 x 10-7 using the multiple-reflection time-of-flight mass spectrometer of the FRS-IC, including the first direct mass measurements of 98Cd , 97Rh. A new QEC = 5437 +/- 67 keV was obtained for 98Cd, resulting in a summed Gamow-Teller (GT) strength for the five observed transitions (0+ --> 1+) as B(GT) = 2.94+0.32 -0.28. Investigation of this result in state-of-the-art shell model approaches accounting for the first time experimentally observed spectrum of GT transitions points to a perfect agreement for N = 50 isotones. The excitation energy of the long-lived isomeric state in 94Rh was determined for the first time to be 293 +/- 21 keV. This, together with the shell model calculations, allows the level ordering in 94Rh to be understood.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3.Peer reviewe

Radioactive Beams for Image-Guided Particle Therapy : The BARB Experiment at GSI

Several techniques are under development for image-guidance in particle therapy. Positron (β+) emission tomography (PET) is in use since many years, because accelerated ions generate positron-emitting isotopes by nuclear fragmentation in the human body. In heavy ion therapy, a major part of the PET signals is produced by β+-emitters generated via projectile fragmentation. A much higher intensity for the PET signal can be obtained using β+-radioactive beams directly for treatment. This idea has always been hampered by the low intensity of the secondary beams, produced by fragmentation of the primary, stable beams. With the intensity upgrade of the SIS-18 synchrotron and the isotopic separation with the fragment separator FRS in the FAIR-phase-0 in Darmstadt, it is now possible to reach radioactive ion beams with sufficient intensity to treat a tumor in small animals. This was the motivation of the BARB (Biomedical Applications of Radioactive ion Beams) experiment that is ongoing at GSI in Darmstadt. This paper will present the plans and instruments developed by the BARB collaboration for testing the use of radioactive beams in cancer therapy.peerReviewe

Isomeric Lifetime Measurement in the Neutron-rich 189^{189}Ta

International audienceIsomeric states of the neutron-rich isotope 18973Ta116 were populated via fragmentation of a primary beam of 208Pb ions at 1 GeV/u impinging on a 9Be target at GSI, Darmstadt, Germany. The isotopes of interest were separated, identified and delivered to the DESPEC setup. Two isomers were deduced in 189Ta116 and their lifetimes were measured based on the γ-ray time distributions