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Direct measurement of the ionization quenching factor of nuclear recoils in germanium in the keV energy range


A direct measurement of the ionization quenching factor for nuclear recoils in germanium was performed at the PTB Ion Accelerator Facility (PIAF) by the Max‑Planck‑Institut für Kernphysik (MPIK in collaboration with PTB Department Neutron Radiation. The energy dependence of the quenching factor was determined through the irradiation of a thin high‑purity germanium target with monoenergetic neutrons in a kinematically constrained measurement. The results, crucial for the interpretation of the CONUS (COherent Neutrino nUcleus Scattering) experiment led by MPIK.

The behavior of particles travelling through matter varies depending on the particle type, their energy, and the medium. In most cases, however, some amount of ionization is produced as result of direct or indirect interactions with atomic electrons and nuclei. Several particle detector technologies take advantage of this process, for example by collecting the produced charged particles (electrons and positive ions), and producing measurable electric signals. All interaction processes that do not produce the same amount as the ionization as the particles used for the detector calibration (typically photons and electrons) are generally referred to as “quenching”.

For germanium‑based detectors, the ionization quenching factor is defined as the ratio of the ionization energy generated by nuclear recoils over the one generated by electron recoils of the same energy. This quantity is crucial for understanding the behavior of High‑Purity Germanium (HPGe) detectors operating in the keV range, as for example those used in low‑mass dark matter searches and other fundamental physics experiments.

The Max‑Planck‑Institut für Kernphysik (MPIK) is currently running CONUS (COherent Neutrino nUcleus Scattering), an experiment aimed at the detection of coherent neutrino‑nucleus scattering on germanium nuclei [1,2], in which they operate four sub‑keV sensitive HPGe detectors [3]. In 2022, they carried out a measurement at the PTB Ion Accelerator Facility (PIAF) in collaboration with PTB Department Neutron Radiation, for a direct determination of the energy dependence of the germanium quenching factor.

A thin HPGe detector was irradiated with monoenergetic neutrons, while it was operated in coincidence with an array of liquid organic scintillator (neutron detectors). This allowed for the simultaneous measurement of the energy of the recoil germanium nuclei and the collided neutrons, so to perform a kinematically‑constrained experiment. The quenching factor was determined by comparing these results with an accurate energy calibration performed with photon sources. The results are reported in: A. Bonhomme et al, Eur. Phys. J. C (2022) 82:815,


[1]    https://www.mpi-hd.mpg.de/lin/research_conus.en.html

[2]    C Buck et al, J. Phys.: Conf. Ser. 1342 (2020) 012094,

[3]    H. Bonet et al, Eur. Phys. J. C 81 (2021) 267,

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