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.

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The Problem

• Work motivated by 10 CFR 835 amendments announced June 2007Instituted newer ICRP recommendations

(ICRP 60, ICRP 68)• External dose from neutrons most

serious problem at acceleratorsNeutron weighting factors (a.k.a.

quality factors) are modified upward

10 CFR 835 now provides no dose per fluence factors

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The Problem• Solution is needed

To understand instrument calibrationsTo perform correct shielding calculationsTo achieve compliance in posting of areas

• This work (also available as a preprint)Surveyed literature Pursued more recent ICRP recommendationsApplied results to 241Am-Be spectraApplied results to accelerator neutron

spectraIgnores DOELAP (for now!)Ignores internal exposure

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Quantities• In the “good old days” :

• connecting absorbed dose D with dose equivalent Hequiv ; D = energy/mass

• Old 10 CFR 835 had table of QF and Hequiv per 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”

(1)equiv oldH QF D

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Table from old 10 CFR

835

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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

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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 connects Hpers(d) with control

of Heff

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Literature Search: Dose per Fluence

• Connect “dose”, H , with fluence, (neutrons cm-2), e.g.;

• Need over wide domain of neutron energy En ; thermal to almost the beam energy

effP H -1Φ (2)

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Literature Search: Dose per Fluence

• Concentrated on Heff , Peff (pSv cm2)

• Side issue with Heff , need “geometries”

ROT = body on axis “the long way”, irradiated from the side (think shish-kabob)

ISO = “isotropic” irradiationNeither are perfect for occupational

setting, both better than the alternatives, fixed orientations

-1eff effP H Φ

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Fits to Heff

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

10 210

10 10

log ( )1 ( log )

(3)1 exp( log ) 1 exp( log )

eff nn

n n

aP E

b c E

d h

f g E j k E

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Fits to Heff

Ferrari et al. (Pelliccioni Group) : > 2000 MeV

Preprint includes tabular values of merged set in preprint.

(4)eff

pnP mE

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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:

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,60 ( ) 5.0 17.0exp ln(2 ) / 6 (5)R n nw E E

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,103 ( ) 2.5 18.2exp ln( ) / 6 , 1 MeV (6)R n n nw E E E

2

,103 ( ) 5.0 17.0exp ln(2 ) / 6 , 1 MeV 50 MeV (7)R n n nw E E E

2

,103 ( ) 2.5 3.25exp ln(0.04 ) / 6 , 50 MeV (8)R n n nw E E E

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Now to Apply This!• 241Am-Be Neutron Spectra

To understand effects of change on instrument calibrations

• Fermilab Accelerator Neutron SpectraAverage 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”

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241Am-Be Neutron Spectra• Lots of spectra around• Difficult to measure, energy of neutrons of in

region of manyenergy-dependent detector efficiencies and

thresholdsresonances

• Source construction can matter• Used 2 spectra of high quality

ISO 8529-1 (2001)DeGuarrini and Malaroda (1971)

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241Am-Be Neutron Spectra

• Calculated Average energiesAverage “weighting factors”Done by numerical integration over

0.1 MeV bins

Used Peff values from equations above

Did same way for Hequiv to get <QFold >

• ICRP 60 and ICRP 103 results were identical; due to limited energy domain

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241Am-Be Neutron Spectra

Does NOT include photons, scattered neutrons, etc.! These all tend to reduce the <QF> .

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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

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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

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Fermilab Accelerator Neutron Spectra

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Conclusions

• Average weighting factor for 241Am-Be is 52% higher with “new” methodology.Instrument calibrations will need alterationMust allow for photons in spectrum!

• For accelerator spectraWeighting 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 occupancyWill need to rethink a few high occupancy areas.

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Contact Information

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