Research ArticleGEOLOGY

Boutique neutrons advance 40Ar/39Ar geochronology

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Science Advances  11 Sep 2019:
Vol. 5, no. 9, eaaw5526
DOI: 10.1126/sciadv.aaw5526
  • Fig. 1 Map of the sample holder used for the main HFNG experiment showing determined F values.

    Dark gray fields are the MDP-1 control samples to monitor production rate variations. Transparent holes are not-available (NA) analyses (NA). ps indicates peak suppression; c, accidental contamination. F values are in sample-specific colors for ease of orientation. Figure S1 provides a photograph of the actual sample holder.

  • Fig. 2 40Ar/39Ar age bias as a function of grain geometry for D-D fusion neutron or fission neutron–irradiated minerals.

    Positive age bias of high SAV biotite grains irradiated with fission neutrons is interpreted to represent recoil-related 39Ar loss [data points are partially shown as means and envelope only; (7)]. Negative age bias of a sanidine-plagioclase mixture is interpreted to reflect undercorrection of 36ArCa (12). Biotite grains (7) were irradiated separately, while sanidine fractions [this study and (12)] were irradiated with multiple grains in contact, allowing potential reimplantation and decreasing apparent recoil.

  • Fig. 3 Experimental cross section of the reaction 39K(n,p)39Ar as a function of neutron energy and their comparison with the most recent ENDF evaluation.

    Based on this study, (34) and (14). Error bars in neutron energy represent 67% limits of the neutron energy spectrum; error bars in the cross section represent analytical uncertainty. The strong variability in the cross section offers the possibility to magnify 39Ar production rates by choosing the right neutron energy. ENDF, Evaluated Nuclear Data File.

  • Fig. 4 Irradiation setup of neutron cross section experiments.

    (A) Geometry of samples in the HFNG with respect to the neutron source area where the D+ beamlets impinge on the Ti layer hosting the interstitially implanted D. The neutron energy spectrum each target was exposed to is given in Fig. 5). (B) Setup of the LICORNE experiment and irradiation geometry. Table 1 and table S3 list the irradiation details.

  • Fig. 5 Simulated neutron energy spectra of cross section experiments.

    (A) Neutron energy spectra of the six HFNG irradiation targets calculated using the MCNP. The spectra are a function of the radial distribution of the neutron beam, the energy-angle correlation of the neutron source, the intensity-angle correlation, and the solid angle. Table 1 lists the irradiation details. (B) Simulated neutron energy spectra of the four LICORNE-irradiated samples calculated. The spectra are a function of hydrogen cell target geometry, 7Li energy, and solid angle covered by the sample. Table 1 and table S3 list the irradiation details.

  • Table 1 Summary of irradiations used to determine the cross section (σ) of 39K(n,p)39Ar in the energy range of 1.2 to 3.2 MeV.

    No.TargetNeutron
    source
    Energy
    (MeV)
    Duration
    (hours)
    39K
    (mol)
    Monitor
    reaction
    Monitor σ
    (mb)*
    Fluence
    (cm−2 s−1)
    39Ar
    (mol)
    39K(n,p) σ
    (mb)
    1KBrHFNG2.75 ± 0.026.12.010 ± 0.001 × 10−3115In(n,n′)115mIn345 ± 102.90 ± 0.104 × 10111.03 ± 0.010 × 10−16177 ± 7
    2aKBrLICORNE2.96
    (+0.19/−0.16)
    6.09.6383 ± 0.0048 × 10−3115In(n,n′)115mIn293 ± 122.08 ± 0.075E × 10104.99 ± 0.051 × 10−17249 ± 9
    2bKBrLICORNE1.46
    (+0.15/−0.14)
    9.49.6005 ± 0.0048 × 10−3115In(n,n′)115mIn163.1 ± 6.53.83 ± 0.14 × 10101.61 ± 0.027 × 10−1744 ± 2
    2cKBrLICORNE1.54
    (+0.36/−0.29)
    18.49.3352 ± 0.0046 × 10−3115In(n,n′)115mIn184.4 ± 7.44.72 ± 0.17 × 10101.00 ± 0.029 × 10−1723 ± 1
    2dKBrLICORNE1.94
    (+0.33/−0.28)
    15.29.4607 ± 0.0047 × 10−3115In(n,n′)115mIn213.3 ± 8.52.10 ± 0.076 × 10102.24 ± 0.038 × 10−17112 ± 4
    3bK glassHFNG2.589 ± 0.011210.25.1211 ± 0.0013 × 10−558Ni(n,p)58Co124.7 ± 9.47.55 ± 0.27 × 10124.17 ± 0.049 × 10−17108 ± 4
    3cK glassHFNG2.613 ± 0.024210.21.2366 ± 0.0003 × 10−458Ni(n,p)58Co128.6 ± 9.43.31 ± 0.12 × 10125.30 ± 0.058 × 10−17130 ± 5
    3dK glassHFNG2.542 ± 0.007210.22.0614 ± 0.0005 × 10−458Ni(n,p)58Co116.9 ± 9.75.37 ± 0.19 × 10112.12 ± 0.032 × 10−17191 ± 7
    3eK glassHFNG2.431 ± 0.017210.22.9220 ± 0. 0008 × 10−458Ni(n,p)58Co100.6 ± 7.65.67 ± 0.20 × 10118.49 ± 0.094 × 10−17512 ± 19
    3fK glassHFNG2.382 ± 0.007210.22.0153 ± 0.0005 × 10−458Ni(n,p)58Co94.4 ± 7.51.59 ± 0.057 × 10121.28 ± 0.013 × 10−16399 ± 15

    *Based on the simulated neutron spectra (Fig. 5) and cross section data from (21).

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/9/eaaw5526/DC1

      Fig. S1. Photograph of the sample holder after loading all samples but the <15-μm fraction of MDP-1.

      Fig. S2. Impact and correction of neutron flux fluctuation biasing the determined average neutron fluence.

      Table S1. Results of noble gas mass spectrometric analysis of geological age standards, K-rich glass and CaF2.

      Table S2. Comparison of R values based on literature data cited in the main text and this study.

      Table S3. Irradiation details of LICORNE irradiations.

    • Supplementary Materials

      The PDF file includes:

      • Fig. S1. Photograph of the sample holder after loading all samples but the <15-μm fraction of MDP-1.
      • Fig. S2. Impact and correction of neutron flux fluctuation biasing the determined average neutron fluence.
      • Table S2. Comparison of R values based on literature data cited in the main text and this study.
      • Table S3. Irradiation details of LICORNE irradiations.

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      Other Supplementary Material for this manuscript includes the following:

      • Table S1 (Microsoft Excel format). Results of noble gas mass spectrometric analysis of geological age standards, K-rich glass and CaF2.

      Files in this Data Supplement:

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