tokens: absent in the previous models, – the neutron transfer reactions, consisting in ‘hopping’ (quantum tunnelling) of a neutron from weakly bounded state in one (donor, ‘d’) nucleus to another (acceptor, ‘a’) nucleus. Neutron transfer reactions are well known in nuclear physics for near-barrier and sub-barrier energies (e.g. von Oertzen et al. 1987 ; Zagrebaev 2003 ; Zagrebaev, Samarin Greiner 2007 ; Canto et al. 2015 ; Karpov, Rachkov Samarin 2015 ) and the multinucleon transfer reaction can be used to synthesize previously unexplored superheavy elements (Zagrebaev, Karpov Greiner 2013 ; Wuenschel et al., 2018 ). The specific feature of neutron transfer reactions in NS envelope would be that the main contribution to the neutron transfer rate comes from nuclei located at distance l pk , which can be as large as 100 fm, strongly exceeding nuclei radii. As for thermonuclear reactions, this distance is determined by two competing factors: decrease of the tunnelling probability with increase of internuclear distance and decrease of number of nuclei, which can approach closer, because of Coulomb barrier. In some sense, the neutron transfer reactions are similar to a hopping transition between localized states of electrons, which is the basis of hopping conductivity (see e.g. Gantmakher 2005 ). Note, as shown by Zagrebaev, Samarin Greiner ( 2007 ) neutron transfers can also increase fusion rates due to modification of internucleus potential, but I do not discuss this effect here, limiting myself to neutron transfer reactions only, and, specifically, to order-of-magnitude estimates, which demonstrate that neutron transfer is applicable for astrophysics matters. Namely, I start from calculation of the transfer probability (per unit time) between static nuclei at given distance, following the approximative approach by Bonaccorso, Piccolo Brink ( 1985 ) and Monaco Brink ( 1985 ). Then I estimate the reaction rate λ (number of reactions per one donor nuclei per unit time) for certain plasma conditions by averaging transfer probability over realistic internuclei separation in plasma. A simple approximating expression is suggested. Surely, the model (especially its nuclear part) should be improved, and at the end of the letter I briefly discuss its further refinement, leaving detailed calculations for the future. The neutron transfer rate strongly depends on S n of the donor nuclei, charges Z d and Z a of both donor and acceptor nuclei, temperature T , and electron chemical potential μ e (which determines density ρ for given composition). Typically, neutron transfer is relevant for donor nuclei with S n ≲ 3 MeV, which may exist in envelopes of accreting NS. In particular, it can burnout strongest among URCA pairs – specific pairs of nuclei which can coexist at respective electron chemical potential μ e (density) and cools down the envelope by neutrino emission associated with cycles of beta-captures/decay (see Schatz et al., 2014 ; Meisel et al., 2015 for
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