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Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards

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Abstract
Characterizing the neutron-irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast- (Ni) and thermal- (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Rez, Czech Republic. The 58Ni(nf,p)58Co activation reaction and gamma-spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by mass spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of high precision dating and gradient monitoring can be crucial.
Keywords
IMPROVED ACCURACY, CANYON SANIDINE STANDARD, K-40 DECAY CONSTANTS, JOINT DETERMINATION, DATING STANDARDS, INTERCALIBRATION, GEOCHRONOLOGY, RECOIL, AR-40-ASTERISK/K-40, COMPLEX

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MLA
Rutte, Daniel et al. “Radial Fast-neutron Fluence Gradients During Rotating 40Ar/39Ar Sample Irradiation Recorded with Metallic Fluence Monitors and Geological Age Standards.” GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS 16.1 (2015): 336–345. Print.
APA
Rutte, D., Pfänder, J. A., Koleska, M., Jonckheere, R., & Unterricker, S. (2015). Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards. GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, 16(1), 336–345.
Chicago author-date
Rutte, Daniel, Jörg A Pfänder, Michal Koleska, Raymond Jonckheere, and Sepp Unterricker. 2015. “Radial Fast-neutron Fluence Gradients During Rotating 40Ar/39Ar Sample Irradiation Recorded with Metallic Fluence Monitors and Geological Age Standards.” Geochemistry Geophysics Geosystems 16 (1): 336–345.
Chicago author-date (all authors)
Rutte, Daniel, Jörg A Pfänder, Michal Koleska, Raymond Jonckheere, and Sepp Unterricker. 2015. “Radial Fast-neutron Fluence Gradients During Rotating 40Ar/39Ar Sample Irradiation Recorded with Metallic Fluence Monitors and Geological Age Standards.” Geochemistry Geophysics Geosystems 16 (1): 336–345.
Vancouver
1.
Rutte D, Pfänder JA, Koleska M, Jonckheere R, Unterricker S. Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards. GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS. 2015;16(1):336–45.
IEEE
[1]
D. Rutte, J. A. Pfänder, M. Koleska, R. Jonckheere, and S. Unterricker, “Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards,” GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, vol. 16, no. 1, pp. 336–345, 2015.
@article{5895929,
  abstract     = {{Characterizing the neutron-irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated uncertainty of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor, and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast- (Ni) and thermal- (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd shielding in the LVR-15 reactor in Rez, Czech Republic. The 58Ni(nf,p)58Co activation reaction and gamma-spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow one to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by mass spectrometric 40Ar/39Ar measurements of the geological age standards. Radial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corresponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not always suffice to meet the needs of high precision dating and gradient monitoring can be crucial.}},
  author       = {{Rutte, Daniel and Pfänder, Jörg A and Koleska, Michal and Jonckheere, Raymond and Unterricker, Sepp}},
  issn         = {{1525-2027}},
  journal      = {{GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS}},
  keywords     = {{IMPROVED ACCURACY,CANYON SANIDINE STANDARD,K-40 DECAY CONSTANTS,JOINT DETERMINATION,DATING STANDARDS,INTERCALIBRATION,GEOCHRONOLOGY,RECOIL,AR-40-ASTERISK/K-40,COMPLEX}},
  language     = {{eng}},
  number       = {{1}},
  pages        = {{336--345}},
  title        = {{Radial fast-neutron fluence gradients during rotating 40Ar/39Ar sample irradiation recorded with metallic fluence monitors and geological age standards}},
  url          = {{http://dx.doi.org/10.1002/2014GC005611}},
  volume       = {{16}},
  year         = {{2015}},
}

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