Application of deuteron-deuteron (d-d) fusion neutrons to 40 ar/39/ar geochronology - eScholarship

ARTICLE IN PRESS Applied Radiation and Isotopes 62 (2005) 25–32 Application of deuteron–deuteron (D–D) fusion neutrons to Ar/ 39 Ar geochronology Paul R. Renne a,b, *, Kim B. Knight b , Se bastien Nomade a,b , Ka-Ngo Leung c,d , Tak-Pui Lou c,d Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA c Division of Accelerator and Fusion Research, Lawrence Berkeley Laboratory, One Cyclotron Road, Berkeley, CA 94720, USA d Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA b a Received 15 March 2004; received in revised form 10 June 2004; accepted 11 June 2004 Abstract Neutron irradiation of samples for 40 Ar/ 39 Ar dating in a 235 U fission reactor requires error-producing corrections for the argon isotopes created from Ca, K, and, to a lesser extent, Cl. The fission spectrum includes neutrons with energies above 2–3 MeV, which are not optimal for the 39 K(n,p) 39 Ar reaction. These higher-energy neutrons are responsible for the largest recoil displacements, which may introduce age artifacts in the case of fine-grained samples. Both interference corrections and recoil displacements would be significantly reduced by irradiation with 2.45 MeV neutrons, which are produced by the deuteron–deuteron (D–D) fusion reaction 2 H(d,n) 3 He. A new generation of D–D reactors should yield sufficiently high neutron fluxes (>10 12 n cm A2 s A1 ) to be useful for 40 Ar/ 39 Ar dating. Modeling indicates that irradiation with D–D neutrons would result in scientific benefits of improved accuracy and broader applicability to fine-grained materials. In addition, radiological safety would be improved, while both maintenance and operational costs would be reduced. Thus, development of high-flux D–D fusion reactors is a worthy goal for 40 Ar/ 39 Ar geochronology. r 2004 Elsevier Ltd. All rights reserved. Keywords: Deuteron; Fusion; Neutron; Argon; Geochronology Introduction The 40 Ar/ 39 Ar dating method (Merrihue and Turner, 1966) is the most widely applicable geochronometer available for measuring geologic time. It is unique among radioisotopic dating methods in being applicable over time scales ranging from that of the early solar system (4.5 billion years ago) to that of Homo sapiens, extending even into the recorded history (Renne et al., 1997). The method is based on the electron-capture decay of 40 K to 40 Ar. It utilizes neutron activation of K to 39 Ar via the 39 K(n,p) 39 Ar reaction as a proxy for *Corresponding author. Tel.: +1-510-644-9200; fax: +1- E-mail address: prenne@bgc.org (P.R. Renne). the parent isotope 40 K in light of the general invariance of 39 K/K in nature (Garner et al., 1975; Humayun and Clayton, 1995). This approach makes possible mass- spectrometric measurements of the parent proxy ( 39 Ar) and the daughter ( 40 Ar) from the same sample. The neutron activation is conventionally achieved by irra- diating samples in the core of a 235 U fission reactor, where they are bombarded by neutrons of a broad spectrum of energies. The 39 K(n,p) 39 Ar cross section increases sharply between 1.5 and 3.0 MeV, and, in a typical fission reactor, approximately 50% of the 39 Ar nuclides are produced by neutrons with energies greater than 3 MeV (Fig. 1). The pioneers of 40 Ar/ 39 Ar dating (e.g., Turner, 1971) recognized that, in geological samples, the fission- spectrum neutrons produce, along with the desired 0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.06.004