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J . Donald Cossairt, Ph.D., C.H.P. Kamran Vaziri, Ph.D. Fermi National Accelerator Laboratory

Neutron Dose Per Fluence and Weighting Factors for Use at High Energy Accelerators (Submitted to Health Physics ). J . Donald Cossairt, Ph.D., C.H.P. Kamran Vaziri, Ph.D. Fermi National Accelerator Laboratory Batavia, IL

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J . Donald Cossairt, Ph.D., C.H.P. Kamran Vaziri, Ph.D. Fermi National Accelerator Laboratory

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  1. Neutron Dose Per Fluence and Weighting Factors for Use at High Energy Accelerators(Submitted to Health Physics) J. Donald Cossairt, Ph.D., C.H.P. Kamran Vaziri, Ph.D. Fermi National Accelerator Laboratory Batavia, IL Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.

  2. The Problem • Work motivated by 10 CFR 835 amendments announced June 2007 • Instituted newer ICRP recommendations (ICRP 60, ICRP 68) • External dose from neutrons most serious problem at accelerators • Neutron weighting factors (a.k.a. quality factors) are modified upward • 10 CFR 835 now provides no dose per fluence factors

  3. The Problem • Solution is needed • To understand instrument calibrations • To perform correct shielding calculations • To achieve compliance in posting of areas • This work (also available as a preprint) • Surveyed literature • Pursued more recent ICRP recommendations • Applied results to 241Am-Be spectra • Applied results to accelerator neutron spectra • Ignores DOELAP (for now!) • Ignores internal exposure

  4. Quantities • In the “good old days” : • connecting absorbed dose Dwith dose equivalent Hequiv; D = energy/mass • Old 10 CFR 835 had table of QF and Hequivper fluence values • Identical to that in NCRP Report 38 • With unit conversion, was identical to 10 CFR 20. • Still have the form of above Equation, with several different dosimetry quantities replacing “H”

  5. Table from old 10 CFR 835

  6. Quantities (“nutshell” definitions) Protection Quantities(theoretical, used for limits and standards) • Equivalent dose = absorbed dose in an organ weighted by radiation weighting (a.k.a “quality”) factor; Eequiv • Effective dose = Sum of equivalent doses over the whole body weighted by organ weighting factors (includes both internal and external dose, ignored internal here); Heff

  7. Quantities (“nutshell” definitions) Operational Quantities(measurable, perhaps, used to demonstrate compliance) • Ambient dose equivalent = dose equivalent on a radius of the “ICRU” sphere (30 cm diam, 1 g cm-1 of “tissue equivalent material)at depth d;Hamb(d) • Personal dose equivalent = equivalent dose in soft tissue measured at depth d (d= 1 cm for whole body); Hpers(d) 10 CFR 835 specifically connectsHpers(d) with control ofHeff

  8. Literature Search: Dose per Fluence • Connect “dose”, H, with fluence, F (neutrons cm-2), e.g.; • Need over wide domain of neutron energyEn; thermal to almost the beam energy

  9. Literature Search: Dose per Fluence • Concentrated on Heff, Peff(pSv cm2) • Side issue withHeff , need“geometries” • ROT = body on axis “the long way”, irradiated from the side (think shish-kabob) • ISO = “isotropic” irradiation • Neither are perfect for occupational setting, both better than the alternatives, fixed orientations

  10. Fits toHeff • Sutton-Ferenci (Hertel Group) : 10-9 to 2000 MeV

  11. Fits toHeff Ferrari et al. (Pelliccioni Group) : > 2000 MeV Preprint includes tabular values of merged set in preprint.

  12. Literature Search: Weighting Factors • “Old” values from 10 CFR 835, NCRP 38, etc.; QFold [Note from ICRP: we can still use the terms “quality factor”, or “effective quality factor”!] • ICRP 60 (1990) gives: • ICRP 103 (2007) gives:

  13. Now to Apply This! • 241Am-Be Neutron Spectra • To understand effects of change on instrument calibrations • Fermilab Accelerator Neutron Spectra • Average QF might change, need to reset preset values in “Chipmunk” ion chamber monitors; also “Snoopies”, “REM-500s”. • Background information for possible policy changes, some related to “occupancy”

  14. 241Am-Be Neutron Spectra • Lots of spectra around • Difficult to measure, energy of neutrons of in region of many • energy-dependent detector efficiencies and thresholds • resonances • Source construction can matter • Used 2 spectra of high quality • ISO 8529-1 (2001) • DeGuarrini and Malaroda (1971)

  15. 241Am-Be Neutron Spectra • Calculated • Average energies • Average “weighting factors” • Done by numerical integration over 0.1 MeV bins • Used Peffvalues from equations above • Did same way for Hequivto get <QFold > • ICRP 60 and ICRP 103 results were identical; due to limited energy domain

  16. 241Am-Be Neutron Spectra Does NOT include photons, scattered neutrons, etc.! These all tend to reduce the <QF> .

  17. Fermilab Accelerator Neutron Spectra • Reanalyzed 9 neutron spectra measured at Fermilab (lettered A-I) • Compared “old” with ICRP 60 and ICRP 103 schemes • Details of spectra described in the preprint and some elsewhere • All were inferred from Bonner Spheres

  18. Fermilab Accelerator Neutron Spectra • Extracted neutrons per bin from “lethargy” plots • Applied ICRP 60, ICRP 103, and “old” weighting factors bin-by-bin • Found average values of weighting factors for each spectra • Obtained “cumulative” plots

  19. Fermilab Accelerator Neutron Spectra

  20. Conclusions • Average weighting factor for 241Am-Be is 52% higher with “new” methodology. • Instrument calibrations will need alteration • Must allow for photons in spectrum! • For accelerator spectra • Weighting factor increased from 4.8 to 6.1 using ICRP 103, relatively small change. • No average weighting factor > 10 with ICRP 103. • Most posted areas are minimal occupancy • Will need to rethink a few high occupancy areas.

  21. Contact Information • Phone: 630-840-3465 • Email: cossairt@fnal.gov

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