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1. $$\alpha $$-event characterization and rejection in point-contact HPGe detectors

   https://doi.org/10.1140/epjc/s10052-022-10161-y  

   Arnquist, I. J.; Avignone, F. T.; Barabash, A. S.; Barton, C. J.; Bertrand, F. E.; Blalock, E.; Bos, B.; Busch, M.; Buuck, M.; Caldwell, T. S.; et al (March 2022, The European Physical Journal C)

   Abstract P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector’s response to $$\alpha $$ α particles incident on the sensitive passivated and p $$^+$$ + surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the Majorana Demonstrator experiment, a search for neutrinoless double-beta decay ( $$0\nu \beta \beta $$ 0 ν β β ) in $$^{76}$$ 76 Ge. $$\alpha $$ α decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of $$\alpha $$ α identification, reliably identifying $$\alpha $$ α background events on the passivated surface of the detector. We demonstrate effective rejection of all surface $$\alpha $$ α events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the $$0\nu \beta \beta $$ 0 ν β β region of interest window by an order of magnitude in the Majorana Demonstrator   and will be used in the upcoming LEGEND-200 experiment. 

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2. Optical properties of low background PEN structural components for the Legend-200 experiment

   https://doi.org/10.1088/1748-0221/17/09/P09007  

   Manzanillas, L.; Efremenko, Y.; Febbraro, M.; Fischer, F.; Guitart Corominas, M.; Hackett, B.; Leonhardt, A.; Majorovits, B.; Schulz, O. (September 2022, Journal of Instrumentation)

   Abstract Polyethylene Naphthalate (PEN) plastic scintillator has been identified as potential self-vetoing structural material in low-background physics experiments. Radio-pure scintillating components have been produced from PEN using injection compression molding technology. These low-background PEN components will be used as optically active holders to mount the Germanium detectors in the Legend -200 neutrinoless double beta decay experiment. In this paper, we present the measurement of the optical properties of these PEN components. The scintillation light emission spectrum, time constant, attenuation and bulk absorption length as well as light output and light yield are reported. In addition, the surface of these PEN components has been characterized and an estimation of the surface roughness is presented. The light output of the final Legend -200 detector holders has been measured and is reported. These measurements were used to estimate the self-vetoing efficiency of these holders. 

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3. First Results on the Search for Lepton Number Violating Neutrinoless Double-β Decay with the LEGEND-200 Experiment

   https://doi.org/10.1103/25tk-nctn  

   Acharya, H; Ackermann, N; Agostini, M; Alexander, A; Andreoiu, C; Araujo, G R; Avignone, F T; Babicz, M; Bae, W; Bakalyarov, A M; et al (January 2026, Physical Review Letters)

   The LEGEND Collaboration is searching for neutrinoless double-beta ( $0\nu \beta \beta$ ) decay by operating high-purity germanium detectors enriched in ${}^{76}\mathrm{Ge}$ in a low-background liquid argon environment. Building on key technological innovations from the GERmanium Detector Array (GERDA) experiment and the MAJORANA DEMONSTRATOR experiment, LEGEND-200 has performed a first $0\nu \beta \beta$ decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of ${0.5}_{-0.2}^{+0.3}\mathrm{cts}/\left(\mathrm{keV}\mathrm{ton}\mathrm{yr}\right)$ in the $0\nu \beta \beta$ decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of $2.8\times {10}^{26}\mathrm{yr}$ on the half-life of $0\nu \beta \beta$ decay, reveals no evidence for a signal and sets a new observed lower limit at $T_{1/2}^{0\nu }>1.9\times {10}^{26}\mathrm{yr}$ (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range $m_{\beta \beta }<75–200\mathrm{meV}$ , depending on the adopted nuclear matrix element. 

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4. CAGE: An internal source scanning cryostat for HPGe characterization

   https://doi.org/10.1088/1748-0221/21/05/P05028  

   Othman, G; Wiseman, C; Burritt, TH; Detwiler, JA; Held, MP; Henning, R; Mathew, T; Peterson, D; Pettus, W; Song, G; et al (May 2026, Journal of Instrumentation)

   Abstract The success of current and future-generation neutrinoless double beta (0νββ) decay experiments relies on the ability to eliminate or reduce extraneous backgrounds. In addition to constructing experiments using radiopure materials and handling in underground laboratories, it is necessary to understand and reduce known backgrounds in data analysis. The Large Enriched Germanium Experiment for Neutrinoless double beta Decay (LEGEND) is searching for 0νββdecay using⁷⁶Ge-enriched high-purity germanium (HPGe) detectors submerged in an active liquid argon (LAr) veto. A significant background in LEGEND is surface events from shallowly-impinging radiation on detector surfaces. In this paper we introduce the Collimated Alphas, Gammas, and Electrons (CAGE) scanning system, an internal-source scanning vacuum cryostat, designed to perform studies of surface events on sensitive surfaces of HPGe in a surface-lab. CAGE features a collimated radionuclide source inside a movable infrared shield that is able to perform precision scans of detector surfaces by utilizing three independent motor stages for source positioning. This allows detailed studies of pulse shapes as a function of source position and incident angle, where defining features can be extracted and exploited for removing surface backgrounds in data analysis in LEGEND. In this paper, we describe CAGE and demonstrate its performance with a commissioning run with²⁴¹Am. The commissioning run was completed with the source at normal incidence, and we estimate a beam spot precision of 3.1 mm, which includes positioning uncertainties and the beam-spot size. Using the 59.5 keV gamma population from²⁴¹Am, we show that low-energy photon events near the passivated surface feature risetimes that increase with radial distance from the detector center. We suggest a specific metric that can be used to discriminate low-energy gamma backgrounds in LEGEND with the similar characteristics (e.g. contamination from²¹⁰Pb). 

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5. Production and validation of scintillating structural components from low-background Poly(ethylene naphthalate)

   https://doi.org/10.1088/1748-0221/17/01/P01010  

   Efremenko, Y.; Febbraro, M.; Fischer, F.; Guitart Corominas, M.; Gusev, K.; Hackett, B.; Hayward, C.; Hodák, R.; Krause, P.; Majorovits, B.; et al (January 2022, Journal of Instrumentation)

   Abstract Poly Ethylene Naphthalate (PEN) is an industrial polymer plastic which is investigated as a low background, transparent, scintillating and wavelength shifting structural material. PEN scintillates in the blue region and has excellent mechanical properties both at room and cryogenic temperatures. Thus, it is an ideal candidate for active structural components in experiments for the search of rare events like neutrinoless double-beta decay or dark matter recoils. Such optically active structures improve the identification and rejection efficiency of backgrounds events, like this improving the sensitivity of experiments. This paper reports on the production of radiopure and transparent PEN plates These structures can be used to mount germanium detectors operating in cryogenic liquids (LAr, LN). Thus, as first application PEN holders will be used to mount the Ge detectors in the Legend -200 experiment. The whole process from cleaning the raw material to testing the PEN active components under final operational conditions is reported. 

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