Solar proton event damage in space-borne Ge detectors F. Quarati a,Ã , S. Brandenburg b , E.-J. Buis c , P. Dressler d , S. Kraft c , M.-O. Lampert d , R.W. Ostendorf b , Alan Owens a , A. Peacock a,1 , P. Quirin d , D. Quirion d a Advanced Studies and Technology Preparation Division (SCI-PA), ESA/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands b Kernfysisch Versneller Instituut, University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands c Cosine Research BV, Niels Bohrweg 11, 2333 CA Leiden, The Netherlands d CANBERRA Lingolsheim, 1 chemin de la Roseraie, 67834 Tanneries Cedex, France article info Available online 27 May 2009 Keywords: Germanium detectors Radiation damage Gamma-rays abstract By means of annealing it has been possible to fully restore the spectral performance of Ge detector after exposure to simulated Solar Proton Events (SPE) of integrated fluences up to 6  10 10 protons/cm 2 . The required annealing time to achieve the restoration of energy resolution, the ratio FWTM/FWHM and detection efficiency to pre-irradiation values is of the order of 25 weeks. However, about half of this time might be considered sufficient to restore the detector within acceptable operational limits for most applications. The long annealing time required to restore the detection performances opens a question on the actual suitability of Ge detector for inner solar system missions, as BepiColombo and Solar Orbiter, unless enhanced robustness or faster recovering capability can be developed, for instance using specific crystal encapsulation. & 2009 Elsevier B.V. All rights reserved. 1. Introduction The detector is a closed-ended coaxial high-purity Ge detector in the reverse electrode configuration. Negative high-voltage bias is applied to the outer boron implanted p + contact where holes are collected. A schematic of the detector is shown in Fig. 1(left). It has been demonstrated by Pehl et al. [1] that reverse electrode Ge detector are more tolerant to radiation than conventional Ge detectors. For space applications two main sources of radiation damage must be considered: the Galactic Cosmic Rays (GCR) and the Solar Proton Events (SPE). GCR are mainly composed of protons but with a different energy spectrum than SPE. For GCR, protons have energy ranging from 0.1 to 10 GeV and their intensity is approximately constant over time. SPE occur stochastically and their energy spectrum changes from event to event, but rarely exceed 200 MeV [2,3]. Typical proton energy spectral distributions for several large SPE are reported in Fig. 1(right). Two examples of reverse electrode Ge detectors used for space applications were reported by Kurczynsky et al. [4] and Evans et al. [5] for the WIND and Mars Odyssey missions, respectively. These missions operated at distances larger than 1 AU from the sun. The WIND mission did not encounter severe SPE and it was found that the GCR were the main contributors to detector degradation. Mars Odyssey encountered two major SPE amount- ing to an integrated fluence of 8  10 8 protons/cm 2 , which was 80% of the total fluence encountered by the spacecraft (SPE plus GCR). For inner solar system missions, the contribution of SPE has to be scaled by the inverse square of the distance from the sun. It can be assumed that an event as August 1972 (5  10 9 protons/cm 2 for energy 460 MeV, [2]) at the orbit of Mercury can produce a fluence of about 3  10 10 protons/cm 2 . A similar event will make GCR contributions completely negligible. At 1AU, there is a 90% chance in 7 years time to encounter a SPE similar to August 1972 with proton energy exciding 60 MeV and integrated fluence 410 9 protons/cm 2 [3]. BepiColom- bo scheduled for launch in 2014 will spend 6 years on its journey to Mercury and up to 2 years in operation. Therefore such mission will very likely encounter at least one major SPE as large as August 1972, and a radiation tolerance assessment must consider it. 2. Experimental Three n-type reverse electrode Ge detectors have been used for this study—all provided by Canberra, Lingolsheim. Table 1 summarizes dimensions and pre-irradiation detection perfor- mances for the three detectors. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.05.108 Ã Corresponding author. Tel.: +3171565 8530; fax: +3171565 5985. E-mail address: fquarati@rssd.esa.int (F. Quarati). 1 On leave Nuclear Instruments and Methods in Physics Research A 610 (2009) 354–357