HR: 1330h
AN: V32C-1042    [PDF]
TI: Neutron Irradiation Using A Deuteron-Deuteron (D-D) Fusion Source: Benefits for $^{40}$Ar/$^{39}$Ar Dating
AU: * Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: * Renne, P R
EM: prenne@bgc.org
AF: Univ. California, Dept. Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Knight, K B
EM: kbk@uclink.berkeley.edu
AF: Univ. California, Dept. Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Nomade, S
EM: snomade@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: Nomade, S
EM: snomade@bgc.org
AF: Univ. California, Dept. Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Leung, K
EM: knleung@lbl.gov
AF: Lawrence Berkeley National Lab., Accelerator and Fusion Research Div., 1 Cyclotron Rd., Berkeley, CA 94720 United States
AU: Leung, K
EM: knleung@lbl.gov
AF: Univ. California, Dept. Nuclear Engineering, Berkeley, CA 94720 United States
AU: Lou, T
EM: lou@nuc.berkeley.edu
AF: Lawrence Berkeley National Lab., Accelerator and Fusion Research Div., 1 Cyclotron Rd., Berkeley, CA 94720 United States
AU: Lou, T
EM: lou@nuc.berkeley.edu
AF: Univ. California, Dept. Nuclear Engineering, Berkeley, CA 94720 United States
AB: Irradiation of samples for $^{40}$Ar/$^{39}$Ar dating, traditionally achieved in a fission reactor, carries the necessary evils of recoil, interference corrections, radiological concerns, and lengthy turnaround time. Alternative high-flux neutron sources with appropriate energies (1-6 MeV) to drive the $^{39}$K(n,p)$^{39}$Ar reaction have been little explored. The deuteron-deuteron (D-D) fusion reaction D(d,n)$^{3}$He, which can produce essentially monoenergetic 2.45 MeV neutrons, has been achievable for decades albeit with neutron fluxes far too low to be of interest for $^{40}$Ar/$^{39}$Ar dating. D-D neutrons at 2.45 MeV are in many ways ideal for $^{40}$Ar/$^{39}$Ar dating and recent developments in fusion research foreshadow the realization of adequately high neutron fluxes. A toroidal geometry plasma source with axial, self-loading target and central sample placement appears especially promising. Modeling argon isotope production from K, Ca, and Cl shows reductions in all unwanted reaction rates relative to a fission spectrum. Production of $^{36}$Ar and $^{38}$Ar from Ca and $^{37}$Ar from K are negligible from 2.45 MeV neutrons. The production ratios $^{40}$Ar/$^{39}$Ar and $^{38}$Ar/$^{39}$Ar from K and $^{39}$Ar/$^{37}$Ar from Ca are reduced by two, four, and one order(s) of magnitude respectively compared with their production rates in a fission spectrum, reducing these conventionally important interference corrections to the point of negligibility. Additionally, the $^{24}$Al(n,$\alpha$)$^{24}$Na reaction, which has important radiological consequences due to the $\beta$$^{-}$ activity of $^{24}$Na, is essentially eliminated, allowing safer/quicker handling of samples irradiated in Al containers. Eliminating higher energy neutrons, unavoidable in a fission spectrum, also reduces the recoil energy, hence recoil distance, of $^{39}$Ar and $^{37}$Ar atoms produced chiefly from K and Ca respectively. With fission spectrum neutrons, $^{39}$Ar recoil severely limits the applicability of the $^{40}$Ar/$^{39}$Ar method to fine-grained materials such as clay minerals. Modeling of recoil effects using the SRIM ion transport code reveals significant benefits of D-D neutrons in reducing recoil effects. For example, in 1 micron diameter illite crystals, fission spectrum neutrons produce $^{39}$Ar with a mean recoil energy of 177 KeV, resulting in $\sim$17% loss of $^{39}$Ar by displacement from the crystals. In contrast, D-D neutrons produce $^{39}$Ar with a mean recoil energy of 30 KeV, resulting in only $\sim$3% loss. Thus, irradiation by D-D neutrons will significantly broaden the scope of problems uniquely addressable by $^{40}$Ar/$^{39}$Ar methods.
DE: 0932 Radioactivity methods
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting