An apparatus for Detection of Iinternal Conversion Electrons

Recoil ion sources can provide free atoms or ions of hard to obtain nuclides. These daughter nuclides emerge from a thin film source of their mother nuclide, usually prepared by the molecular plating (MP) method, and can then be then extracted and analysed offline [1-2]. The daughter nuclide emission rate, or recoil efficiency, is a key performance indicator of the recoil ion sources. Source requests usually have specific criteria and assessment of its performance is necessary so the source production method, such as MP, can be evaluated and the plating parameters changed if needed to reach a more optimum performance. Sometimes quantification of the daughter nuclide emission rate by conventional methods can be challenging due to e.g., long half-lives. Hence, a new method for evaluation of recoil ion source recoil efficiencies was needed. For this, the Detection of Internal Conversion Electrons (DICE) setup was been built and commissioned at JGU.

The DICE setup (fig. 1) comprises a vacuum chamber which houses a two-stage multi-channel plate (MCP) stack (DET40 MCP detector with timing anode, RoentDek Handels GmbH) and can be used to count individual particles such as electrons, photons and ions with respect to time. An insulated source holder is directly in front of the stack and is connected to an external rod which allows manipulation of the source-to-MCP distance by up to 25 mm. The holder accommodates samples of up to 22 mm radius. The source can be biased up to ±2 kV using a high voltage power supply to propel the particles towards the MCP stack. Impulses detected from the stack are decoupled, amplified and discriminated. The signal is then sent to a field-programmable gate array (FPGA) board, which computes the data and displays them in the chosen format on the computer.

Fig. 1. The Detection of Internal Conversion Electrons (DICE) apparatus, adapted for the U-235m study. Schematic/©: L. E. Reed / Univ. Mainz

First studies focused on U-235m, the second lowest known ultra-low-lying state (76.8 eV) which has a half-life of 26 min and decays by conversion electron (CE) emission to the U-235 ground state [3-4]. It can be obtained by producing thin layer Pu-239 recoil ion sources emitting 235mU, with the isomer being populated in the 239Pu alpha decay with a summed branching of 99.8%. Classic quantification of the recoil efficiency was not possible because the U-235 ground state has too large a half-life (7·108 a) to be observed via alpha spectrometry, but it would be possible to measure the number of CEs emitted during the U-235m deexcitation to the ground state, which would then be proportional to the daughter nuclide emission rate and hence the recoil efficiency using DICE. Recoiling U-235m from thin Pu-239 sources produced my MP (fig. 2) were caught under vacuum on catcher foil. The foil can be then mounted in the holder inside DICE and the CEs per time quanta counted. Preliminary results show that CEs from U-235m deexcitation were detected and the characteristic half-life of U-235m was observed.

Fig. 2 Pictures of Pu-239 recoil ion sources produced by MP for the U-235m study. Picture/©: L. E. Reed / Univ. Mainz

We offer topics related to DICE for research modules, B.Sc. and M.Sc. theses. Proposed topics include but are not limited to, adaption of DICE to enable in vacuum recoil collection and MCP measurement, recoil efficiency of recoil ion sources, and study of the rarely observed effect in chemistry, in which the half-life of U-235m, depending on the chemical environment can vary by up to several percent [5].

[1] R. Haas et al., RCA 108 (2020) 923
[2] W. Parker and R. Falk, NIM 16 (1962) 355
[3] F. Ponce et at., Phys. Rev. C 97 (2018) 054310
[4] Y. Shigekawa et al., Phys. Rev. C 98 (2018) 014306
[5] M. Neve de Mevergnies, Phys. Rev. Lett. 29 (1972) 1188