Production and ionization of neutron-rich nuclides

The Research Reactor TRIGA Mainz

The Mainz TRIGA (Training Research Isotope General Atomics) reactor can be operated in a steady-state mode with a maximum power of 100 kW_therm or in the pulsed mode with 30 ms pulse duration (FWHM) at a peak power of 250 MW_therm. Four horizontal beam tubes give access to the strongest neutron flux near the reactor core (1.8x10¹¹ cm^-2s^-1) [1]. Here, a gas-jet system is used for continuous transport of fission products from a fissionable target (U-235, Pu-239 or Cf-249) mounted close to the reactor core, through the biological shield to an ion source on a HV platform.

Fig. 1: The research reactor TRIGA Mainz at the institute of nuclear chemistry

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Gas-jet transport of the fission products

TRIGA-Trap uses an aerosol loaded gas-jet system for fission product transport, see Fig. 2. The fission products are collimated in an aerodynamical lens, ionized in a high-temperature surface ion source, accelerated, mass separated by a 90 deg. dipole magnet, cooled and bunched in a gas-filled radio-frequency quadrupole structure, decelerated in a pulsed drift-tube and transported towards the Penning-trap mass spectrometer [2].

Fig. 2: Detailed sketch of the different stages of TRIGA-Trap. Section A: aerosol-based gas-jet system for fission product transport. Section B: fission product collimation in an aerodynamic lens, ionization and subsequent acceleration. Section C: mass separation by a 90° dipole magnet, cooling and bunching of ions by an RFQ with subsequent deceleration [2].

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High temperature surface ion source

Currently a high temperature surface ion source [2] is available which in connection with a gas jet transport system enables access to certain fission products. For this purpose a 30 kV HV platform has been installed. The cathode is a tantalum ionizer cylinder heated indirectly by electron bombardment from two hot filaments.

The ionizer can be heated rapidly to any temperature up to 2500 °C. The chosen temperature can be kept constant with only minor fluctuations in the range of 4 °C in 24 h. The components of the ion source are long-term stable, thus it can be operated without maintenance for two or more weeks.

Fig. 3: 3D model of the high-temperature surface ion source employed at Triga-Trap [2]. It has been manufactured at MPIK, based on a design of a similar source in operation at JAEA-ISOL facility.

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Mass separator

The mass separator is a 90 deg dipole magnet with a bending radius of 0.5 m and a mass resolution of about 300. The PC controlled DANFYSIK power supply is capable of 250 A current with a stability of 10 ppm, resulting in a B-field intensity of 1.1 T. A stepper motor driven split-pair system is fitted behind the magnet.

Fig. 4: The dipole magnet of the mass separator being positioned.

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Cooler and Buncher

COLETTE (Cooler for Emittance Elimination) is a segmented, radiofrequency quadrupole, which was designed for cooling continuous radioactive beams for injection into the MISTRAL spectrometer at CERN-ISOLDE [4]. The device has been transferred to TRIGA-Trap and it is integrated in the beam-line [5]. COLETTE transforms the continuous 30 keV beam into bunches, and cools the confined ion ensemble by buffer-gas to reduce the phase space volume.

Fig. 5: COLETTE cooler and buncher. The complete assembly has been built up at TRIGA-Trap in Mainz. The RFQ part between the grey insulators is floated at about 30 kV according to the ion energy.

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References

[1]	K. Eberhardt, A. Kronenberg, Kerntechnik 65, 5 (2000).
[2]	J. Grund et al., Nuclear Instrum. Meth. A 972, 164013 (2020)
[3]	M. Liehr et al., Rev. Sci. Instrum. 63, 2541 (1992)
[4]	D. Lunney et al., Nucl. Instrum. Meth. A 598, 379 (2009)
[5]	T. Beyer et al., Appl. Phys. B 114, 129-136 (2014)