Defense Threat Reduction Agency

8725 John J. Kingman Road, MS 6201

Fort Belvoir, VA 22060-6201

TECHNICAL REPORT

DTRA-TR-07-3

Considerations on Estimating 

Upper Bounds of Neutron Doses 

to Military Participants at 

Atmospheric Nuclear Tests

DTRA01-03-C-0064

David C. Kocher

Prepared by:

SENES Oak Ridge, Inc.

Center for Risk Analysis

Oak Ridge, Tennessee

April 2007

Approved for public release; distribution is unlimited.

Defense Threat Reduction Agency

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1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE

Technical report 

3. DATES COVERED (From - To)

30-03-2007 

5a. CONTRACT NUMBER

DTRA01-03-C-0064 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Considerations on Estimating Upper Bounds of Neutron Doses to 

Military Participants at Atmospheric Nuclear Tests 

5c. PROGRAM ELEMENT NUMBER

139D 

5d. PROJECT NUMBER

CS 

6. AUTHOR(S)

5e. TASK NUMBER

AH 

David C. Kocher 

5f. WORK UNIT NUMBER

DH11146 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

Senes Oak Ridge, Inc. REPORT NUMBER

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10. SPONSOR/MONITOR'S ACRONYM(S)

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9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Defense Threat Reduction Agency 

Nuclear Technologies Directorate 

8725 John J. Kingman Road, STOP 6201 

Fort Belvoir, VA 22060-6201 

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AGENCY REPORT NUMBER

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12. DISTRIBUTION AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES

This work was sponsored by the Defense Threat Reduction Agency under RDT&E RMSS Code B 139D K113 CS AH 

111 46 25904 . D

14. ABSTRACT

This report discusses how the Defense Threat Reduction Agency should estimate upper bounds of neutron doses to 

military service members who participated in the atmospheric nuclear weapons testing program. 

15. SUBJECT TERMS

Nuclear Test Personnel Review, Veterans, Atmospheric Nuclear Weapons Testing, Neutron Dosimetry, Uncertainty 

16. SECURITY CLASSIFICATION OF: 17. IMITATION 19a. NAME OF RESPONSIBLE PERSON 

OF BSTRACT 

 L

 A

UU

18. NUMBER 

OF PAGES 

59

a. EPORT R

U

b.

A TRACT BS

U

c. THIS 

P E 

19b. TELEPONE NUMBER (Include area code) 

AG

U

STANDARD FORM 298 (Rev. 8/98)
CON\IERSION TABI,E

Conversion Factors for U.S. Customary to metric 1Sf) units of measurement

MULTIPLY > RY >TOGET

TO GET BY DIVIDE

angsLrcrn

atnrrsphere (norrnal)

bar

barn

British thernral unit (ttrernrcctremical)

ca]orie (therncclrernica])

cal (thernpctrernical/ crt)

curie

degree (angle)

degree Fahrentrcit

eLectron voLt

erg

erg/second

foot

foot-1urnd-force

ga11on (u.s. Iiquid)

inctr

jerk

joule/kilogram (./kg) radiation dose

absorbed

kilotons

kip (1000 lbf)

kip/inctr' (ksi)

ktap

nicron

mil

rnife (international)

ounce

pound-force (lbs avoirdqnis)

pound-force inch

pound-force/jlctr

pound-force/fcriE'

trrcund-force/ilch' (psi)

1rcr.rnd-nass (lbn avoirdr-pois)

trrcund-rnass-foot2 (nnnerrt of inertia)

trrcund-rnass /fcrlE3

rad (radiati-on dose absorbed)

roenEgten

shake

slug

torr (nm Hg, Oo C)

1.000000xE-1-0

1-.01-325 xE+2

L.000 000 xE + 2

1.000000xE-28

1.054350xE+3

4. L84 000

4.l-84 000 x E -2

3.700000x8+1

L.745329x8-2

.Q= (t"f +as9.67)h.8

L.602 19 xE-l-9

1-.000000x8-7

l-.000000x8-7

3.048000x8-L

r-.355 818

3.785412xE-3

2.540000xE-2

1.000000xE+9

r_. 000 000

4.1_83

4.448222x8+3

6.894757xE+3

1.000000x8+2

L.000000xE-6

2.540000xE-5

1.609344xE+3

2.834 952 xE -2

4.448 222

1.129848xE-l1,.751,268x8+2

4.788426x8-2

6.894 757

4.535924x8-1

4.2L4jLLxE-2

1.501 845x8+ll-.000 000 xE - 2

2.5'79 760 xE - 4

1.000000xE-8

1.459390xE+l1,.33322 xE-lneters (m)

kilo trnscal (kPa)

kilo trnscal (kPa)

netef (nf)

joule (,J)

joule (.T)

nega joule/rf (w/ff)

*giga bacguerel (@)

radian (rad)

degree kelvin (K)

joule (,J)

joule (J)

watt (W)

neter (m)

joule (,J)

netet' (nf)

neter (m)

joule (,1)

cray (G!)

terajoules

newton (N)

kilo pascal (]<Pa)

newEon-second/nf (u-s/nf )

neter (m)

seter (m)

neter (m)

kilogram (kg)

neraEon (N)

neraiton-neter (N-m)

newEon/neter (N/m)

kilo pascaf (kPa)

kilo trnscal (kPa)

kilogram (kg)

kilogram-netet' (kg-nf )

kilogram-netet' (kglnf)

**cray (cy)

coulcnb/kil-ogram (C/kg)

second (s)

kilogram (kS)

kilo pascal (kPa)

*The bacquerel

**The cray (cY)

(eq) is the SI

is the SI unit,

unit of radioactiviry; 1 Bq = 1- event/s.

of absorbed radiation.
ABSTRACT

This report addresses a provision in the 2004 Report to Congress by the Departments of

Defense (DoD) and Veterans Affairs (VA) that the Defense Threat Reduction Agency (DTRA)

will investigate the proper degree of uncertainty that should be incorporated in estimated upper

bounds of neutron doses to military participants in the atmospheric nuclear-weapons testing

program to account for uncertainty in the biological effectiveness of neutrons. Analyses

presented in this report led to two main conclusions. First, application of an adjustment factor

of 6 to mean neutron doses that had been calculated by Science Applications International

Corporation (SAIC), as specified in DTRA's 2003 Interim Guidance, is more than suffrcient to

ensure that the resulting upper bounds arc at least upper 95%o credibility limits of doses to any

organ or tissue, in accordance with DoD regulations in 32 CFR Part2l9. Second, now that VA

uses the Interactive RadioEpidemiological Program (IREP) exclusively to evaluate causation of

cancers for the pu{pose of adjudicating claims for compensation and no longer uses a table of

screening doses for specific cancers that were calculated by the Committee on Interagency

Radiation Research and Policy Coordination (CIRRPC), an adjustment factor that incorporates

an uncertainty in the biological effectiveness of neutrons is no longer needed because that

uncertainty is taken into account in IREP. Rather, to provide neutron equivalent doses to organs

or tissues that are suitable for input to IREP, all mean doses and upper bounds calculated

previously by SAIC should be increased by a factor of 2to account for the difference between

the radiation weighting factor (wp) of 20 for fission neutrons recommended by the International

Commission on Radiological Protection (ICRP) and the factor of 13 that SAIC used in most

cases to modify estimates of tissue kerma free-in-air to obtain estimates of organ dose. Mean

lll
neutron doses and upper bounds calculated previously in the case of exposure of certain

participants at Operation PLUMBBOB, Shot DOPPLER at the Nevada Test Site also should be

increased by another factor of 13/11 to correct the modifying factor that SAIC used in that case.

Thus, now that IREP is used to evaluate causation of cancers, upper bounds of neutron

equivalent doses reported by DTRA should account for uncertainty in tissue kerma only, and an

uncertainty in the biological effectiveness of neutrons should not be included.

IV
TABLE OF CONTENTS

SECTION PAGE

ABSTRACT.............. .................. iii

LIST OF FIGURES ......................vi

LIST OF TABLES.. .....................vii

I INTRODUCTION ..........I

2 METHODS OF ESTIMATING NEUTRON DOSES..... .......................5

3 FINDING BY NRC COMMITTEE ON UPPER BOLINDS OF NEUTRON DOSES............17

4 ADJUSTMENT FACTOR IN INTERIM GUIDANCE .............. ........24

5 EFFECT OF ADJUSTMENT FACTOR ON VA's EVALUATIONS OF CAUSATION

oF CANCERS.......... ......................25

6 EVALUATION OF ADEQUACY OF UPPER BOT.JNDS OF NEUTRON DOSES..............30

6.1 Doses to Internal Organs ....................31

6.2 Doses to Skin...... ..............33

6.3 Effect of Revised Estimates of Neutron Fluences ..................34

6.4 Conclusions on Adequacy of Adjustment Factor in Interim Guidance......................35

7 ESTIMATION OF UPPER BOUNDS OF NEUTRON DOSES FOR USE IN IREP..............40

8 SUMMARY AND CONCLUSIONS ................42

9 REFERENCES.......... ......................45

DISTRIBUTION LIST......... .......... DL.l
FIGURE

LIST OF FIGURES

PAGE

Tissue kerma free-in-air per neutron fluence and contributions from different reactions

of neutrons with constituents of tissue vs. neutron energy....... ....................13

Absorbed dose per neutron fluence vs. depth in central region of 30-cm

diameter cylindrical phantom of tissue-equivalent material at incident neutron

energy of 1 MeV.. ..................14

Dose equivalent per neutron fluence vs. depth in central.region of 30-cm

diameter cylindrical phantom of tissue-equivalent material at incident neutron

energy of I MeV.. ..................15

Quality factor (Q) for charged ionizing particles vs. linear energy transfer (Z)

recommended in ICRP Publication 60 and recommended previously in ICRP

Publication 26............. ...........16

Summary of influence of relationship between quality factor (Q) and linear

energy transfer (L) and stopping powers of charged particles on quality factor for

neutrons ...............23

Changes in estimates of neutron kerma free-in-air vs. ground distance from

detonations at Hiroshima and Nagasaki due to change in dosimetry system from

DS86 to DS02.... .....................38

Changes in estimates of absorbed dose to colon from gamma rays and neutrons

vs. ground distance from detonations at Hiroshima and Nagasaki due to change in

dosimetry system from DS86 to DS02 .....39

vl
TABLE

LIST OF TABLES

PAGE

Mean qualrty factors for neutrons recommended in NCRP Report No. 38.....................12

Radiation weighting factors (wfl for neutrons recommended in ICRP

Publication 60............. .............22

Transmission factors for kerma in organs of adult (55-kg) adult Japanese phantom

relative to neutron kerma free-in-air from prompt neutrons produced in detonation at

agasaki and transmission factors weighted by assumed quality factor for neutrons.........37

vll
SECTION 1

INTRODUCTION

This report considers the issue of how the Defense Threat Reduction Agency (DTRA)

should estimate upper bounds of neutron doses to members of military services who participated

in the atmospheric nuclear-weapons testing progftrm when radiation dose assessments include an

assumption that external exposure to neutrons occurred. The need to consider this issue arose as

a consequence of three developments:

[1] A finding by a committee of the National Research Council (NRC, 2003), which

reviewed the dose reconstruction program of DTRA, that upper bounds of doses to

military participants from external exposure to neutrons are always underestimated

because of neglect of the substantial uncertainty in the biological effectiveness of

neutrons relative to high-energy gamma rays (photons);

[2] A provision in DTRA's Interim Guidance (Benavides,2003), whichwas issued in

response to findings by the NRC committee on deficiencies in the dose reconstruction

program, that upper bounds of neutron doses should be obtained by increasing calculated

mean doses bv a factor of 6:

13] A provision in the Report to Congress by the Department of Defense (DoD) and

Department of Veterans Affairs (VA) (Cooper and Klein, 2004) that DTRA will

investigate the proper degree of uncertainty that should be incorporated in estimated

upper bounds of neutron doses to account for uncertainty in the biological effectiveness

of those radiations.

DoD regulations in 32 CFR Part 218 specify that upper bounds of neutron doses reported

by DTRA, as well as upper bounds of doses from all other radiation types and routes of external

and internal exposure, should be at least upper 95Yo credibility limits (i.e., upper bounds of 90Yo

credibility intervals of uncertain doses), meaning that upper bounds of doses should exceed
actual doses in at least 95o/of all cases. Methods of estimating upper bounds of doses

are important because VA uses upper bounds reported by DTRA in adjudicating claims for

compensation for cancer or other radiogenic diseases when an evaluation of causation of a

participant's disease is required.

The purpose of this report is to address the provision in the Report to Congress (Cooper

and Klein, 2004) that DTRA will investigate the proper degree of uncertainty that should be

incorporated in estimated upper bounds of doses from external exposure to neutrons to account

for uncerCainty inthe biological effectiveness of those radiations. An important focus of this

investigation is a consideration of the adjustment factor of 6 specified in the Interim Guidance

(Benavides,2003). This report considers whether use of that adjustment factor provided credible

upper bounds of neutron doses, and whether that adjustment factor should continue to be used or

an alternative approach to estimating upper bounds of neutron doses should be adopted.

Topics discussed in this report include the following:

A review of methods and assumptions that were used by Science Applications

Intemational Corporation (SAIC) to estimate mean values and upper bounds of neutron

doses to military participants in the atmospheric testing program;

The basis for the finding by the NRC committee (NRC,2003) that upper bounds of

neutron doses to participants are always underestimated;

The rationale for the adjustment factor of 6 specified in the Interim Guidance (Benavides,

2003) for use in obtaining upper bounds ofneutron doses;

How use of the adjustment factor of 6 by DTRA has affected VA's evaluations of

causation of cancers in adjudicating claims for compensation for those diseases;

An evaluation of whether use of the adjustment factor of 6 provided credible upper

bounds (at least upper 95%o qedibility limits) of neutron doses to specific organs or

tissues of exposed individuals;

An assessment of how mean doses and upper bounds from external exposure to neutrons

can be calculated by DTRA to provide consistency and compatibility with VA's current

approach to evaluating causation ofcancers;
o A sunmary of the results of this investigation and recommendations on how mean

neutron doses and upper bounds should be calculated by DTRA.

Diseases other than cancer that may be caused by exposure to ionizing radiation (e.g., cataract of

the lens of the eye) are not considered in this report. Thus, discussions about the biological

effectiveness of neutrons and approaches used by VA to evaluate causation of diseases apply

only to cancer and not to any other radiogenic disease.

This report is concerned with calculations of biologically significant doses to specific

organs or tissues from exposure to neutrons. It is DTRA's current policy to calculate equivalent

doses (DTRA, 2007) to conform to a recommendation in Publication 60 of the International

Commission on Radiological Protection (ICRP, 1991). In cases of extemal exposure to neutrons,

an equivalent dose is the product ofan average absorbed dose to an organ or tissue due to

neutrons incident on the body surface modified by a radiation weighting factor (wp) to represent

the biological effectiveness of the incident neutrons. For example, a wp of 20 is generally

appropriate in cases ofexposure to spectra offission neutrons.

However, use of the term "equivalent dose" is appropriate only in cases where an average

absorbed dose to an organ or tissue is modified by a wp recommended by ICRP (1991). Thus, it

does not apply to biologically significant doses to organs or tissues from exposure to neutrons

that were calculated by SAIC and reported by DTRA up to the time DTRA's Interim Guidance

(Benavides, 2003) and the Report to Congress (Cooper and Klein, 2004) were issued, in part

because those doses were calculated using modiffing factors that differ from a wp that would

apply to fission neutrons.

Therefore, in this report, the term oodose equivalent" is used to describe biologically

significant doses to organs or tissues from exposure to neutrons that were calculated by SAIC.

This terminology conforms to a recommendation by ICRP (1977) at the time SAIC's

calculations were performed. The term "equivalent dose" is used in this report only in cases

where & wp r€coffimended by ICRP (1991) is used to calculate a biologically significant dose to

an organ or tissue.

The more general term "dose" also is used in some discussions in this report. This term
is used, for example, in introductory discussions in this section, and it also is used in the Report

to Congress (Cooper and Klein, 2004). Whenever the term "dose" by itself is used, reference to

a biologically significant dose to an organ or tissue is implied.
SECTION 2

METHODS OF ESTIMATING NEUTRON DOSES

Methods that were developed by SAIC to estimate doses from external exposure to

neutrons are described by Goetz et aL (1979; 1981; 1985). The last of those reports contains

summary tables of mean neutron dose equivalents that SAIC assigned to participants in

particular military units at specifrc shots at the Nevada Test Site (NTS) and in the Pacific.

In all calculations, SAIC first estimated a quantity referred to as tissue kerma free-in-air

at assumed locations of exposure to neutrons. Kerma is defined as the quotient of dEt, by dm,

where dE1, is the sum of the initial kinetic energies of all charged particles that are liberated by

uncharged ionizing particles (neutrons in this case) in a material of mass dm (an infinitesimal

mass of tissue in air in this case); e.g., see ICRU (1998). Kerma thus has units of absorbedose

and is given in rads by SAIC.2 Tissue kerma free-in-air at assumed locations of exposure was

estimated on the basis of calculations of neutron fluence (i.e., the number of neutrons per unit

cross sectional area) as a function of energy at those locations, cross sections for interactions of

neutrons with constituents of tissue that liberate charged ionizing particles as a function of

energy, and the energy transferred to each charged ionizing particle in each neutron interaction.

Tissue kerma for monoenergetic neutrons of energy up to 14 MeV and contributions from

different interactions that produce charged ionizing particles are shown in Figure 2-1 (NCRP,

l97l; Figure 3). Values of tissue kerma free-in-air used by SAIC were nearly the same as, but

not identical to, those in Figure 2-l (S.D. Egbert, SAIC, personal communication).

SAIC then calculated aneutron dose equivalent (in rem) at assumed locations of

exposure by modifying an estimated tissue kerma free-in-air by a dimensionless quantity that

represents the greater biological effectiveness ofneutrons, relative to high-energy photons, in

inducing stochastic health effects, primarily cancers. This modifying factor was referred to as an

2Tissue kerma free-in-air is not equal to the absorbed dose in an infinitesimal mass of tissue in air

because charged-particle equilibrium is not achieved-i.e., the energy spectrum of charged particles

produced by neutron interactions in air that enter the infinitesimal volume of tissue is not the same as the

energy spectrum of charged particles that are liberated from that volume of tissue by incident neutrons

(e.g., see Shultis and Faw, 1996). 

s
average or effective quality factor or a "rad-rem conversion factor" (Goetz et al., 1979; 1981;

1985). A neutron dose equivalent was initially referred to as a "dose (rem) free-in-air" (Goetz et

al., 1979) and later as an o'equivalent tissue dose" or an "in-trench dose equivalent" in cases of

exposure in trenches at NTS (Goetz et al., 1981) or, more generally, as a "neutron dose (rem)"

(Goetz et al., 1985). For exposure to spectra of fission neutrons that occurred in detonations of

nuclear weapons, SAIC applied a modiffing factor of 13 to tissue kerma free-in-air to account

for the biological effectiveness of neutrons (Goetz et al., l98l; 1985), except in the case of

exposure of certain participants at Operation PLUMBBOB, Shot DOPPLER at NTS, where a

lower modifying factor of 11 discussed later in this section was used (Goetz et al., t979).

Neutron dose equivalents calculated by SAIC, as described above, were used by VA in

adjudicating claims for compensation for cancers in specific organs or tissues. Thus, neutron

dose equivalents that were calculated by modifying estimates of tissue kerma free-in-air by a

factor that represents the biological effectiveness of neutrons were assumed to represent average

dose equivalents to any organ or tissue of an exposed individual.

It is important to emphasize that the quantity used by SAIC to modify an estimate of

tissue kerma free-in-air to obtain an estimate of neutron dose equivalent is not the same as a

neutron quality factor, because the latter is defined as a quantity that modifies an absorbed dose

from neutrons in tissue of humans to obtain a dose equivalent (NCRP, I97l1' ICRP, 1977); i.e., a

quality factor is not a quantity that modifies tissue kerma.3 Because the quantity used by SAIC

to modiff estimates of tissue kerma free-in-air from neutrons is not strictly a quality factor, we

refer to it as a modifying factor in this report. However, even though the biologically significant

dose obtained by applying a modifying factor to tissue kerma free-in-air is not strictly a dose

equivalent, we refer to it as a dose equivalent to be consistent with the terminology that was used

by DTRA in reporting neutron doses to VA.

For neutrons, differences between tissue kerma free-in-air and absorbed dose in tissues of

the body are illustrated by comparing estimates of tissue kerma in Figure 2-1 with estimates of

3SAIC clearly was aware of this distinction, as evidenced, for example, by the occasional use of

the term "rad-rem conversion factor" ratherthan "quality factor" (Goetz et al., 1981) and avoidance ofthe

term "dose equivalent" to describe biologically significant doses from exposure to neutrons that were

calculated on the basis of estimates of tissue kerma free-in-air (Goetz et al., 1979;1981; 1985).

6
absorbed dose from mono-directional neutrons incident on a 30-cm diameter cylindrical phantom

from a direction perpendicular to the axis of the cylinder (NCRP, l97I). For example, at an

energy of 1 MeV, which is near the peak of energy spectra of frssion neutrons (Shleien et al,

1998; Table 7.1), the tissue kerma per neutron fluence in Figure 2-1 is about 2.3 x l}-e rad cm2,

and the absorbed dose in the central region of the cylindrical phantom is given by the curve for

all interactions along the traverse in the beam direction in Figure 2-2 (NCRP,l97l; Figure 15).

At this energy, the absorbed dose is greater than tissue kerma free-in-air at depths in the phantom

less than about 3 cm, but is less than tissue kerma at greater depths. Absorbed doses in the

phantom at other energies between 0.025 eV and 14 MeV (NCRP, l97l; Appendix B) give

similar comparisons. At all energies, the maximum absorbed dose from neutrons and secondary

photons produced by neutron interactions in the phantom, which occurs at a depth of about 5 cm

or less and most often at a depth of about I cm, is greater than tissue kerma free-in-air, and the

absorbed dose is less than tissue kerma at greater depths.a

As noted previously, except in the case of exposure of certain participants at Operation

PLUMBBOB, Shot DOPPLER, SAIC assumed that neutron dose equivalents can be calculated

by applying a modifying factor of 13 to estimates of tissue kerma free-in-air. This modifring

factor was based on comparisons of maximum dose equivalents in a 30-cm diameter cylindrical

phantom (NCRP, I97l) with tissue kerma free-in-air over a range of neutron energies of concern

at locations of exposure (S.D. Egbert, SAIC, personal communication). An example calculation

of dose equivalents in the central region of the phantom at an incident neutron energy of 1 MeV

is given in Figure 2-3 (NCRP, l97l; Figure 16). The dose equivalent at any depth in the

phantom was calculated from (a) the absorbed dose from all neutron interactions that produce

charged particles, as given in Figure 2-2, (b) an assumed relationship between the quality factor

olnc.eases in absorbed dose at depths close to the surface of the phantom compared with tissue

kerma free-in-air are due in part to the low attenuation of incident neutrons in transport to those depths

and the multiple collisions of neutrons in finite volumes of tissue that result in a greater averuge energy of

all liberated charged particles per incident neutron compared with the average energy of charged particles

liberated by a single interaction in an infinitesimal mass of tissue. At greater depths in the phantom,

attenuation of the incident neutron fluence is more important than increases in the average energy of all

liberated charged particles per incident neutron, and absorbed dose is less than tissue kerma free-in-air.

The relationship between absorbed dose at any depth and tissue kerma free-in-air also is influenced by

contributions to absorbed dose from secondary photons, which depend on neutron energy.
(Q) for charged ionizing particles and linear energy transfer (I) given by the dashed curve in

Figure 2-4 (ICRP, 1996; Figure 2), (c) the absorbed dose from interactions of secondary photons,

as given in Figure 2-2, and (d) an assumed quality factor for photons of 1.0. The maximum dose

equivalent per neutron fluence at I MeV shown in Figure 2-3 is about 3.2 x 10-8 rem cm2. Given

the tissue kerma at I MeV shown in Figure 2-l,the ratio of the maximum dose equivalent in the

phantom to tissue kerma free-in-air then is about (3.2 x rc-\l(2.3 * l0-e): 14. By examining

ratios of maximum dose equivalent in the phantom to tissue kerma free-in-air as a function of

energy and averaging those ratios over energy spectra of fission neutrons at locations of

exposure,t SAIC concluded that the value 13 was a reasonable representation of that ratio.

SAIC's modifying factor of 13 described above can be compared with mean quality

factors recommended by the National Council on Radiation Protection and Measurements

(NCRP, lgT|)and given in Table 2-1; NCRP's mean quality factors are maximum ratios of dose

equivalent to absorbed dose in a 30-cm diameter cylindrical phantom and were obtained from

calculations such as those shown in Figures 2-2 and 2-3 (NCRP,1.97l; Appendix B). Mean

quality factors calculated by NCRP are less than 13 at all energies.6 However, this difference

does not indicate that the modiffing factor of 13 used by SAIC in most cases is inconsistent with

mean quality factors recommended by NCRP because, as discussed above, SAIC applied its

modifying factor to a tissue kerma free-in-air from neutrons, whereas NCRP's mean quality

factors are intended to be applied to an absorbed dose in tissues of the body. Indeed, SAIC's

modi$ing factor of 13 is consistent with NCRP's mean quality factors in Table 2-1, because

both were derived using the same maximum dose equivalent from neutrons and secondary

photons in a 30-cm diameter cylindrical phantom. As indicated in Table 2-1, NCRP also

recommended that a mean quality factor of 10 was generally suitable for use in radiation

protection when detailed information on energy spectra of neutrons was not available.

5For example, in a case of exposure in a trench at a ground distance of about 2.85 km from a

detonation at NTS, contributions to the calculated fluence were about 10% from thermal (< 0.3 eV)

neutrons, 50Yo from epithermal (0.3 eV-10 keV) neutrons, 15Yo from 0.01-0.63 MeV neutrons, andZ1%o

from the highest energy (> 0.63 MeV) neutrons (Goetz et al., 1979).

6The highest mean quality factor that would be obtained from values in Table 2-1 would not

exceed about 12, and an average over spectra offission neutrons at locations ofexposure would be less

than 11.

8
A modifring factor less than 13 was used by SAIC to calculate neutron dose equivalents

to members of Task Force WARRIOR at Operation PLUMBBOB, Shot DOPPLER (Goetz etal.,

1979); this was the first calculation of neutron doses performed by SAIC. The modifying factor

in that case was derived on the basis of calculations of neutron fluence in different energy groups

at the location of exposure, an estimated tissue kerma free-in-air per neutron fluence for each

energy group obtained from calculations for monoenergetic neutrons similar to calculations in

Figure 2-I, and the energy dependence of the mean quality factor given in Table 2-l (NCRP,

l97l). Goetz et al. (1979) state that a modifying factor of 8.6 applied to the neutron field

immediately above a trench in which members of Task Force WARRIOR were located, and that

the modifuing factor is'oclose to the maximum" at the assumed location of exposure at a depth of

233 ft in a trench. However, the assumed modifying factor atthat depth in a trench was not

stated. On the basis of the ratio of neutron dose equivalent free-in-air of 0.227 rem to tissue

kerma free-in-air of 0.020 rad at a depth of 2.33 ft in a trench given by Goetz et al. (1979), it can

be inferred that the modifying factor used by SAIC in that case is about 1 1.? This inference is

supported by inclusion of a neutron dose equivalent of 0.227 remto members of Task Force

WARRIOR in a summary of doses at all shots at NTS and in the Pacific (Goetz et al., 1985).

There is a slight inconsistency between the modifying factor of 11 used by SAIC in the

case of exposure of members of Task Force WARRIOR at Operation PLUMBBOB, Shot

DOPPLER and the modifying factor of 13 used by SAIC in all other cases. As discussed above,

the value 13 was derived on the basis of ratios of maximum dose equivalent in a 30-cm diameter

cylindrical phantom (NCRP, l97l) to tissue kerma free-in-air. However, the value 11, which

also was used by SAIC to modi$ tissue kerma free-in-air, was derived using mean qualrty

factors that were calculated by NCRP (1971) as ratios of maximum dose equivalent to absorbed

dose in the phantom. Thus, when a modifying factor of I I is applied to estimates of tissue kerma

free-in-air, the resulting dose equivalent is not the same as the maximum dose equivalent in the

phantom that would be calculated by NCRP at the same tissue kerma. Since the maximum

'A modifuing factor of 11 was used only in the case of exposure of members of Task Force

WARRIOR at Shot DOPPLER. A modifuing factor of 13 apparently was used in cases of exposure of

other participants at that shot.
absorbed dose in the phantom is greater than tissue kerma free-in-air at neutron energies of

interest, as discussed above, SAIC's use of a modifuing factor of l l resulted in a slight

underestimate of maximum dose equivalent in the phantom.8

The inconsistency between SAIC's modifuing factor of 11 that was used to estimate

neutron dose equivalents to members of Task Force WARRIOR at Operation PLUMBBOB, Shot

DOPPLER and NCRP's mean quality factors could be addressed by applying a modifying factor

of 13 to the estimated tissue kerma free-in-air of 0.020 rad atthe assumed location of exposure in

a trench (Goetz et al., 1979). Thus, the reported neutron dose equivalent of 0.227 rem in that

case (Goetz et al., 1979;1985) could be increasedtn 0.26 rem. If this change were made, all

neutron dose equivalents calculated by SAIC would correspond to the maximum dose equivalent

in a 30-cm diameter phantom, and SAIC's modifring factor would be consistent with mean

quality factors recommended by NCRP (1971) and given in Table 2-I in all cases.

The discussions in this section may be summarized as follows:

Dose equivalents from external exposure to neutrons were calculated by SAIC by

modiffing estimates of tissue kerma free-in-air at assumed locations of exposure by a

factor (either 13 or 11) to represent the biological effectiveness of neutrons. Neutron

dose equivalents so estimated are assumed to represent average dose equivalents to any

organ or tissue of an exposed individual.

The modifying factor of 13 that was used in most cases is consistent with mean quality

factors recornmended by NCRP (197I) and given in Table 2-1, because both were

derived on the basis of the same maximum dose equivalent in a 30-cm diameter phantom.

The modiffing factor of 11 that was used in one case results in a slight underestimate of

the maximum dose equivalent in a 30-cm diameter phantom. This inconsistency could be

8SAIC's derivation of a modiffing factor of I I in this case was based on the incorrect

assumptions that (a) an average quality factor could be obtained by weighting the energy-dependent

quality factor recommended by NCRP (1971) by the enerry distribution of neutrons in air at the assumed

location of exposure and (b) the maximum dose equivalent in a phantom could be obtained by modiffing

tissue kerma free-in-air by an average quality factor (S.D. Egbert, SAIC, personal communication).

These assumptions were not incorporated in SAIC's subsequent calculations of neutron dose equivalent in

which a modif ing factor of 13 was applied to estimates of tissue kerma free-in-air.

l 0
removed by applying a modi$'ing factor of 13, rather than 11, to the estimated tissue

kerma free-in-air at the location of exposure in that case.

When neutron doses to military participants in the atmospheric testing program are

estimated, DTRA reports mean doses and upper bounds to VA. Prior to the Interim Guidance

(Benavides,2003), SAIC estimated uncertainties in mean dose equivalents calculated as

described above by taking into account uncertainties in estimates of tissue kerma free-in-air only;

an uncertainty in the biological effectiveness of neutrons was not taken into account.n For

example, in estimating neutron dose equivalents from exposure in trenches at NTS, sources of

uncertainty in estimating tissue kerma that were taken into account include (a) uncertainties in

neutron output of a weapon, including uncertainties in the number of neutrons, their energy

spectrum, and their angular distribution as it deviates from radial symmetry, @) uncertainty in

calculations of neutron transport from the burst point to a location above a trench, (c) uncertainty

in the downward-directed fraction of the neutron field above a trench, and (d) uncertainty in the

location of an individual in a trench (Goetz et al., 1979; T98T). In cases of exposure in trenches

at NTS, estimated upper 95o/o qedibility limits of tissue kerma at specific shots exceed mean

values by a factor between 1.5 and 2.2 (Goetz et al.,1979;1981); i.e., estimated uncertainties in

mean neutron dose equivalents in those cases were about a factor of 2 on average.to

9SAIC did recognize, however, that there is substantial uncertainty in radiobiological data that

were used by NCRP (1971\ in developing recommended mean quality factors (Goetz et al., 1979).

loGoetz et al. (1985) does not give estimated uncertainties in calculated mean neutron dose

equivalents in the Pacific. However, neutron doses in the Pacific generally were low compared with

doses at NTS.

l l
Table L. Mean quality factors for neutrons recommended in NCRP Report No. 38"

Neutron energy

(MeV)

2.5 x10-8 (thermal)

1 x10-7

I *10{

1 *10-s

1 x10{

Mean qualrty

factorb

2

2

2

2

2

2

2.5

7.5

1 1

1 1

9

Neutron energy

(MeV)

5

7

l 0

I4

20

40

60

I x102

2 xI02

3 x102

'4 

xl02

Mean qu{ity

factorb

1x10-3

"See NCRP (197 I), T able 2.

bMaximum values of mean quality factor-i.e., ratios of dose equivalent to absorbed dose-at

any depth in central region of a 30-cm diameter cylindrical phantom. When sufficiently detailed

information on neutron energies at locations where absorbed dose is delivered is not available, mean

quality factor of 10 should be assumed for purposes of radiation protection.

I xl02

1 x10-1

5 x10-r

I

2.5

8

7

6.5

7.5

8

7

5.5

4

3.5

3.5

3.5

t2
CONTRIBUTIONS TO KERMA OF TISSUE

CURVE REACTPN

A ALL

MRNCLE

ALL CI.IARGED

I

c

o

E

F

G'

Ht

2-t5 (OCEPTd .:.;:!.i i:l: ALL (|\p' R€acIto|G PRolu{s .:iii. ,, ALLtrdR€ACTlOfSt ALPHAS :| |i,':

PARTICLES

}IYDROGEN ELASTC PFOToI{S

ELASNC OXYGEN IONS

ELASNC CARBof{ K]NS

ELASTIC NITROGEN |q{S

ALL llON-ELASllC |oilS t{AVlNO ITTASSES i :

'KERMI ("rg g-trr19l r

o-4 rds ro

NEUTRON ENERGY (MEV)

7

q

(\l

e()

cn

lD

lrl ()

z

tr,

f

J

l!

z

o(r

F

f

lr,

z

(l

lr,

:a

|o-e

rds

|op

to-t'

p4

tlJ

D

ct>

a

tr ro.

Figure 1. Tissue kerma free-in-air per neutron fluence and contributions from different reactions

of neutrons with constituents of tissue vs. neutron energy (NCRP, l97l; Figure 3). T

obtain kerma per neutron fluence in units of rad cm', divide values in units of erg cm'

gtuy too.

l 3
lo-8

5

\

T

lttl tttl

RAVERSE 2 -I TO

BEAM DIRECTION

t\-

\

!A

:r

+ l

-rF i

I

.g -(

*

t l

PHANTOM

t l

cei,rreA I

o 2 4 6 I rO t2 t4 16 18 2c22242628 3 0

PENETRATION DISTANCE (cml

Figure 2. Absorbed dose per neutron fluence vs. depth in central region of 30-cm diameter

cylindrical phantom of tissue-equivalent material at incident neutron energy of I MeV

(NCRP, l97I; Figure l5).

5

2

- o

ro5

2

tde

2

tdlo

2

rc-lo

2

|o-ll

T4

ABSORBED DOSE D FROM

o NEUTRON TNTERACTTONS (ta if a/Dn> O,ll

A PHOTON |NTERACTTONS (ts il a/Do6>e,tl :

o ALL INTERACTIONS#

TRAVERSE I.BEAM DIRECNON
ro-?

5

2

to-8

5

o-e

5

I

to- fo

5

2

|o-8

IrL

TRAVERSE 2_ -__*_

I. TO BEAM UREcT;ofr.

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PHAN

t l

lTOwCENTEIr l

o 2 4 6 I tO t? t4 t6 t8 eo?2?:4 262830

PENETRATION DTSTANCE (cm)

Figure 3. Dose equivalent per neutron fluence vs. depth in central region of 30-cm diameter

cylindrical phantom of tissue-equivalent material at incident neutron energy of I

MeV Q'{CRP ' l97l; Figure 16)'

l 5

(\|

E

o

e

c,

.>

Lr, ()

z,

t&f

f

|.'-

z,

og

F

:)

lrj

z

z,

Lr.l

J

<

5

o

til

lJ.l (t

o

cl ?

to-e

5

2

to- lo

DOSE EQUIVALENT DE FROM

o NEUTRON tNTERAfitONS &cr if a/Dgn> o,f l

a Pt{OTON INTERACTIONS (*cr tf slDEoh"O.t}

gALL INTERACTIONSM

TRAVER$E I - BEAM DIRECTON
30

25

9* 9rs

3

6ro

100 t0r tf 1d

L (kcv/ru)

Figure 4. Quality factor (Q) tor charged ionizing particles vs. linear energy transfer (I)

recornmended in ICRP Publication 60 (ICRP, 1991) and recommended previously in

ICRP Publication 26 (ICRP, 1977); see also ICRP (1996;2003).

*-.-* Ct$Knphtihriu25

c[r) ICRPh&rin 60

t6
SECTION 3

FINDING BY NRC COMMITTEE ON UPPER

BOUNDS OF NEUTRON DOSES

As noted in Section l, an NRC committee found that upper bounds of neutron doses to

military participants in the atmospheric testing program reported by DTRA are always

underestimated because of neglect of uncertainty in the biological effectiveness of neutrons

relative to high-energy photons (NRC, 2003). The committee found that the degree of

underestimation of credible upper bounds of neutron doses was a factor of about 3-5, depending

on the neutron quality factor (i.e., modifying factor) that had been used by SAIC.

In considering the NRC committee's finding, it is importanto note that the committee

assumed that the factor used by SAIC to modify estimates of tissue kerma free-in-air to obtain

estimates of dose equivalent was either 13 or 8.5 (NRC, 2003). The higher value is as stated by

Goetz et al. (1981; 1985), and it applies in most cases. However, as discussed in the previous

section, the lower value (which should have been 8.6) applied above a trench in which members

of Task Force WARzuOR at Operation PLUMBBOB, Shot DOPPLER were located, but the

modifuing factor at locations of exposure in the trench used by SAIC in that case is about 11

(Goetz et al., 1979). Thus, the committee erred in its assumption about the modifying factor

used by SAIC in that case.

The NRC committee's finding was based mainly on an evaluation of data on the relative

biological effectiveness (RBE) of fission neutrons and their uncertainties by Kocher et al. (2002);

see also Kocher et al. (2005). On the basis of estimates of RBE in studies of life-shortening,

which was due primarily to cancers, and induction of specific cancers in mice, Kocher et al.

(2002) developed a probability distribution of an uncertain radiation effectiveness factor (REF)tt

for induction of solid tumors in humans by fission neutrons at high doses and high dose rates.

rrThe term "radiation effectiveness factor" (REF) was introduced to distinguish a quantity that

represents biological effectiveness for purposes of estimating cancer risks and probability of causation of

cancers in identified individuals from (a) the quality factor and radiation weighting factor used in

radiation protection and (b) an RBE, which strictly applies only to results of a specific radiobiological

study under controlled conditions (Kocher et a1.,2002).
That probability distribution has an upper 95Yo credibility limit of about 25, which is about a

factor of 3 above the median REF at high acute doses of 7 .7 . By applying a dose and dose-rate

effectiveness factor (DDREF) for photons of 2 (ICRP, 1991) to the REF at high acute doses, the

committee considered that the median and upper 95Vo credibility limit of REF for induction of

solid tumors by fission neutrons at low doses are about 15 and 50, respectively. The committee

thus assumed that an upper 95o/o credibility limit of REF for fission neutrons is a factor of about

4 or 6 higher than the modifring factor of 13 or 8.5, respectively, that the committee assumed

was used by SAIC. The probability distribution of REF for fission neutrons developed by

Kocher et al. (2002) is assumed to apply at energies of 0.1-2 MeV. By considering that military

participants at atmospheric tests also were exposed to substantial fluences of neutrons of lower

energies (see footno te 4) thathave substantially lower REFs than 0.1-2 MeV neutrons (Kocher et

a1.,2002), consistent with the energy dependence of the mean quality factor in Table 2-l,the

NRC committee reduced the assumed degree of underestimation of upper bounds of neutron

dose equivalents calculated by SAIC to a factor of about 3-5 (NRC, 2003). The lower end of this

range applied when a modiffing factor of 13 was used by SAIC, and the upper end applied to the

presumed modifying factor of 8.5 in the case of exposure of certain participants at Operation

PLUMBBOB, Shot DOPPLER.

The following points about the NRC committee's evaluation should be noted. First, the

committee assumed that the degree of underestimation of credible upper bounds of neutron dose

equivalents of 3-5 applies to all organs or tissues of an exposed individual. This assumption is

consistent with VA's use of neutron dose equivalents calculated by SAIC and reported by DTRA

in adjudicating claims for compensation for cancers in specific organs or tissues.

Second, the committee's finding that upper bounds of neutron dose equivalents are

always underestimated by a factor of about 3-5 was based only on a consideration of uncertainty

in the biological effectiveness of neutrons (i.e., an upper 95Yo credibility limit of an REF for

neutrons compared with modifying factors used by SAIC), and uncertainties in tissue kerma

estimated by SAIC were not taken into account. Thus, in effect, the committee assumed that

uncertainties in estimates of tissue kerma are small compared with the uncertainty in biological

effectiveness. As noted in the previous section, SAIC's estimates of upper 95oh credibility limits

t 8
of tissue kerma in trenches at NTS exceed mean values by a factor between 1.5 and 2.2. The

effect of combining the two uncertainties is considered in Section 4.

Third, the committee's finding was based on its evaluation of REFs for solid tumors only,

and REFs for leukemia were not considered. Although REFs for induction of leukemia by

fission neutrons tend to be less than REFs for solid tumors, the upper 95Yo credibility limit of

REF for fission neutrons and leukemia at low doses developed by Kocher et al. (2002) differs

from the upper 95%o credibility limit of REF for solid tumors at low doses of about 50 noted

above by less than llYo. Thus, the committee's finding could be applied to all cancer types.

Fourth, the committee's estimate of an upper 95Yo credibility limit of REF for fission

neutrons and solid tumors at low doses, which was obtained from a probability distribution of

REF at high acute doses (Kocher et al., 2002) by assuming a DDREF for photons of 2 (ICRP,

l99l), incorporates an assumption that there is no uncertainty in DDREF. However, DDREF

also is uncertain. For most solid tumors, DDREF has been described by a probability

distribution that ranges from 0.5 to 5.0 and has a mean value of 1.8 (Land et al., 2003). When

that probability distribution is used to adjust the REF for fission neutrons and solid tumors at

high acute doses, the median REF at low doses is reduced by about 20%o compared with the

value obtained by assuming a DDREF of 2, but the upper 95%o uedibility limit is reduced by less

than2Yo. A similar result is obtained when a somewhat different probability distribution of

DDREF for breast and thyroid cancer (Land et al., 2003) is assumed. Thus, neglect of an

uncertainty in DDREF does not have a significant effect on the committee's finding.

Finally, as noted earlier in this section, the NRC committee erred in assuming that SAIC

used a modiffing factor of 8.5 in the case of exposure of certain participants at Operation

PLUMBBOB, Shot DOPPLER. If the modifying factor of I I that was actually used by SAIC

had been assumed instead, the NRC committee probably would have concluded that the degree

of underestimation of upper bounds of neutron dose equivalents that results from neglect of an

uncertainty in biological effectiveness should be no more than a factor of 50/1 | : 4.5 in that

l 9
case, and should not exceed a factor of about 4 when the neutron spectrum at locations of

exposure and the energy dependence of REF are taken into account.l2

The NRC committee's consideration of credible upper bounds of neutron doses also was

influenced by recommendations in Publication 60 of the Intemational Commission on

Radiological Protection (ICRP, 1991) on radiation weighting factors (wps) for neutrons, which

also represent their biological effectiveness and are given in Table 3-1 and Figure 3-1. Those

recommendations are consistent with previous recommendations, which were developed

subsequento SAIC's calculations of neutron dose equivalents, that mean quality factors for

neutrons in Table 2-1 should be increased by a factor of 2 (ICRP, 1985; NCRP, 1987).13 For

example, the wp of 20 at energies of 0.1-2 MeV recommended by ICRP (1991), which is

generally appropriate for fission neutrons, is a factor of 2 higher than the mean quality factor of

10 that had been recommended previously for general use in radiation protection (see Table 2-1).

Thus, since the modifying factor of 13 used by SAIC in most cases is consistent with mean

quality factors in Table 2-1, as discussed in Section 2, ICRP's recommendation on & wn for

fission neutrons implies that mean neutron dose equivalents calculated by SAIC should be

increased by a factor of 2. Recommended increases in the biological effectiveness of neutrons

were based mainly on estimates of RBE at low doses in studies of various stochastic endpoints

that were reviewed by expert groups (ICRU, 1986; NCRP, 1990), including RBEs that were used

by Kocher et al. (2002) to develop probability distributions of REFs.

In response to data on RBE which indicated that the biological effectiveness of neutrons

at low doses should be increased by a factor of 2, ICRP (1991) also modified the relationship

between the quality factor (Q) tor charged ionizing particles and linear energy transfer (I) given

by the dashed curve in Figure 2-4. That relationship had been used by NCRP (1971) to calculate

12We also note that the NRC committee apparently did not recognize that modiffing factors used

by SAIC are not the same as mean quality factors recommended by NCRP (1971), and that the modiffing

factor of 11 that was used by SAIC in one case resulted from an incorrect application of NCRP's mean

quality factors to an estimate of tissue kerma free-in-air. "Radiation weighting factors replace mean quality factors used previously for purposes of

radiation protection. Radiation weighting factors also modif estimates of average absorbed dose in

specific organs or tissues (ICRP, l99l). However, radiation weighting factors for any radiation type

differ from mean quality factors in that the former are determined by the energies of radiations incident

on the body or emitted by radionuclides in the body, whereas the latter are determined by the energies of

radiations at locations in the bodv where absorbed dose is delivered.

20
neutron dose equivalents in a 30-cm diameter cylindrical phantom and, thus, is incorporated in

modiffing factors used by SAIC. The Q-L relationship currently recommended by ICRP (1991;

1996;2003) is given by the solid curve in Figure 2-4. As indicated in Figure 3-1 (ICRP, t996;

Figure 3), use of the revised Q-L relationship, as well as newer data on stopping powers of

charged particles, results in a higher maximum biological effectiveness of neutrons at energies

near 0.5 MeV and a more pronounced energy dependence than were obtained by using the

previous Q-L relationship and previous stopping-power data.

2 l
Table 2. Radiation weighting factors (wp) for neutrons

recommended in ICRP Publication 60 (ICRP, 1991)

Neutron energy Radiation weighting factor (wp)

5

1 0

20

1 0

5

< 10 keV

10-100 keV

0.1-2 MeV

2-20 MeY

> 20 MeV

Note: When calculation of radiation weighting factors for neutrons requires a continuous function, the following

approximation can be used (ICRP, l99l):

wn = 5 * rz.*pf:!n(25)f I ,

L6.l

where En is the neutron energy in MeV. There is no intent to imply any biological meaning to this

relationship; it is simply a tool for calculation.

22
w*'rpprorimrtion

r ll.(l0yDf(10), g(t) rnd L.oldhe-

: l!'(l0yDr(10), Q(l) ncw L old

. flr(10)/D'(10), Cl(L) urd I, old

- w*-rppmximation

,t\

f r

,t/\

:::::::rt:tr$ii

r0{ to-t to-7 to{ r03 t0-. t0.l ro-2 l0.r rip lot rd

Hsutroa clcrSy tMcV)

Figure 5. Summary of influence of relationship between quality factor (Q) artd linear energy

transfer (Z) and stopping powers of charged particles on quality factor for neutrons

(ICRP, 1996; Figure 3); new and old Q-L relationships are given in Figure 2-4. Data

points are ratios of ambient dose equivalent and absorbed dose at depth of I cm in

tissue, and solid and broken curyes are recommendations on radiation weighting

factors (wns) for neutrons given in Table 3-1 (ICRP, 1991). Scale on left vertical axis

applies to all data points and cuwes, except scale on right vertical axis applies to data

points obtained using old Q-L relationship and stopping-power data.

25 7 0

la

-20

o

(,

€

€e

Frs t

'

€o

€

rqslo

o

':

6

g

d

23
SECTION 4

ADJUSTMENT FACTOR IN INTERIM GUIDANCE

Following the NRC committee's [eview of the dose reconstruction program of DTRA

(NRC, 2003), DTRA issued Interim Gui{ance (Benavides,2003) on changes to be made in

methods of dose reconstruction to addre$s various findings by the committee. In response to the

finding that upper bounds of neutron dosps are always underestimated by a factor of about 3-5,

the Interim Guidance specified that mea4 values of neutron dose equivalents that had been

calculated by SAIC, as described in Section 2, should be increased by a factor of 6 to obtain

upper bounds (i.e., at least upper 95% ufdibility limits) in all cases.

A basis for the adjustment factor pf 6 is not discussed in the Interim Guidance. However,

we believe it is reasonable to presume thht it was obtained by combining the higher uncertainty

factor of 5 to represent uncertainty in the biological effectiveness (REF) of fission neutrons,

which applied when the modifuing factor used by SAIC was assumed by the NRC committee to

be 8.5, with a factor that represents uncertainty in estimates of tissue kerma free-in-air. As noted

in Section 2,the uncertainty in tissue kerma estimated by SAIC is a factor of about 1.5 to 2.2 in

cases of exposure in trenches at NTS. By combining uncertainty factors (i.e., ratios of upper

95% credibility limits to median values) of 5 and 2.2, an overall uncertainty factor of about 6 is

obtained,la in agreement with the adjustment factor in the Interim Guidance.

Given the presumption that the adjustment factor of 6 specified in the Interim Guidance

was based on an effoneous assumption by the NRC committee (NRC,2003) that SAIC used a

modifuing factor of 8.5 in the case of exposure of certain participants at Operation PLUMBBOB,

Shot DOPPLER, use of that adjustment factor should have been more than sufficient to provide

upper bounds of neutron doses that are at least upper 95o/o credibility limits. An analysis to

address this issue in more detail is presented in Section 6.

laln this report, an uncertainty in the product of two uncertain parameters is estimated by

assuming that the parameters are uncorrelated and are described by lognormal probability distributions

that are defined by assumed ratios of upper 95o/o credibility limits to median values.
SECTION 5

EFFECT OF ADJUSTMENT FACTOR ON VA's EVALUATIONS OF

CAUSATION OF CANCERS

When an evaluation of causation is required in adjudicating claims for compensation for

cancers experienced by military participants in the atmospheric testing program, VA uses upper

bounds of dose equivalents from all radiation types and routes of exposure reported by DTRA in

those evaluations; VA does not use reported mean doses. Use of upper bounds of doses in

evaluating causation of cancers is in accordance with VA regulations in 38 CFR 3.31 1(a), which

specify that whenever a range of doses is reported, the highest level of the reported dose range

will be presumed.

When the Interim Guidance (Benavides,2003) and Report to Congress (Cooper and

Klein, 2004) were issued, VA was evaluating causation of cancers in two different ways. This

section discusses how use of the adjustment factor of 6 to estimate upper bounds of neutron

doses, as specified in the Interim Guidance, affected the two ways of evaluating causation of

cancers when a neutron dose was reported by DTRA.

In the approach to evaluating causation of certain cancers that VA had been using for

many years, an estimated upper bound of the total dose equivalent from all radiation types and

routes of exposure combined was compared with a so-called screening dose for the cancer type

of concern in a claim for compensation. Screening doses for various cancer types were given in

a table that was developed by the Committee on Interagency Radiation Research and Policy

Coordination (CIRRPC, 1988; Table 3); the CIRRPC table is reproduced in Table III.E.3 of the

NRC (2003) report. Screening doses corresponded to an estimated probability of causation (PC)

of cancer of 50% at the upper 99%o credibility limit. They were calculated by taking into account

various sources of uncertainty in estimating cancer risk and PC, including uncertainties in

epidemiological data on risks of specific types of cancer associated with given doses of low-LET

radiation and assumed extrapolations of observed risks to lower doses (CIRRPC, 1988). If an

estimated upper bound of the total dose equivalent to a participant equaled or exceeded the

25
screening dose that applied to the participant's cancer type, age at time of exposure, and time

since exposure when cancer was diagnosed in cases of leukemia, VA normally concluded that it

is at least as likely as not that the participant's cancer was caused by radiation exposure and

compensation was awarded.

When VA used the CIRRPC table to evaluate causation of cancers, estimated upper

bounds of neutron dose equivalents needed to take into account an uncertainty in the biological

effectiveness of those radiations, as well as an uncertainty in the estimated tissue kerma, because

uncertainties in the biological effectiveness of different radiation types were not incorporated in

screening doses in the CIRRPC table.15 Thus, since uncertainties in neutron doses estimated by

SAIC represented uncertainties in tissue kerma only, use of an adjustment factor that accounts

for an uncertainty in biological effectiveness, as specified in the 2003 Interim Guidance, was

required when the CIRRPC table was used, in order to ensure that credible upper bounds (at least

upper 95Yo credibility limits) of neutron doses would be used in evaluating causation of cancers.

In late 2003, VA began to use software called the Interactive RadioEpidemiological

Program (IREP) (Land et al., 2003) as an alternative to screening doses in the CIRRPC table in

evaluating causation of cancers (VHA/OPHEH,2003;Mather and Otchin,2004).16 In contrast to

VA's use of the CIRRPC table, which did not involve an estimation of PC, IREP calculates a

probability distribution of an uncertain PC for a cancer type of concern that is associated with

given doses, ages at times of exposure, and age at the time of diagnosis of cancer by taking into

account several sources of uncertainty. VA then used the upper 99Yo credibility limit of PC for

the cancer type of concern that was calculated by IREP in adjudicating claims for compensation,

to be consistent with the assumption used in developing screening doses in the CIRRPC table.

When a cancer type of concem in a claim was included in the CIRRPC table, VA based its

l5Screening doses in the CIRRPC table were based on estimates of cancer risks and PC associated

with given doses of low-LET radiation, principally high-energy photons (NIH, 1985). Thus, use of those

screening doses in cases of exposure to high-LET radiations (e.g., neutrons and alpha particles) required

assumptions about the biological effectiveness of those radiations.

l6Versions of IREP that have been used by VA were developed for use by The National Institute

for Occupational Safety and Health (NIOSH, 2002) inthe compensation program for energy workers.

The current version of NIOSH-IREP is available at http://www.niosh-irep.com/irep niosh.
medical opinion on causation on the approach (the CIRRPC table or IREP) that was more

favorable to the claimant.rT

An important difference between IREP and the CIRRPC table is that, instead of using a

total dose from all radiation types and routes of exposure combined as required by the CIRRPC

table, it is intended that doses will be entered into IREP by radiation type. Specifically,

equivalent doses from photons, electrons, neutrons, and alpha particles that are calculated using

radiation weighting factors (wns) recommended by ICRP (1991) should be entered separately.18

IREP accounts for uncertainties in the biological effectiveness of different radiation types

relative to high-energy (> 250 keV) photons by incorporating probability distributions of REFs

developed by Kocher et al. (2002), including the probability distributions of REF for fission

neutrons discussed in Section 3. When a neutron equivalent dose and the associated energy

range of neutrons are entered into IREP, the equivalent dose is divided by the wn that IREP

assumes was used to estimate equivalent dose to obtain the corresponding absorbed dose. IREP

then modifies the absorbed dose by the probability distribution of REF that applies to the

specified neutron energy range and cancer type of concern (either solid tumors or leukemia).re

The result is a probability distribution of a biologically significant dose that is used in IREP to

calculate a probability distribution of PC. In cases of exposure to fission neutrons or neutrons of

unspecified energy, an energy range of 0.1-2 MeV should be assumed, in which case IREP

assumes that a wn of 20 was used to calculate equivalent doses (see Table 3-1).

rTUse of screening doses in the CIRRPC table usually was more favorable, except possibly in

cases of cancer of the colon or female breast. However, only 12 cancers were included in the CIRRPC

table, whereas IREP calculates PC for 32 cancers, including cancers of the skin and prostate, which are

the most common in claims for compensation by military participants but are not included in the CIRRPC

table. Therefore, in practice, VA used IREP to evaluate causation of cancer in most cases.

r8ln calculating PC for cancers in military participants at atmospheric tests, equivalent doses from

photons and electrons can be combined and entered into IREP as the equivalent dose from high-energy

(> 250 keV) photons or higher-energy (>15 keV) elecfions. However, equivalent doses from neutrons

and alpha particles should always be entered separately into IREP to ensure that uncertainties in the

biological effectiveness of those radiations are taken into account properly in estimating PC.

tnIREP incorporates probability distributions of REFs for neutrons that apply to energy ranges

used by ICRP (1991) to specift the step-function representation of wp given in Table 3-1.

27
When VA evaluated causation of cancers using IREP, upper bounds of doses from each

radiation type reported by DTRA were entered as point (constant) values without uncertainty.

This way of using IREP, which is in accordance with VA regulations, as noted above, should

result in a higher estimate of an upper 99o/o credibility limit of PC than would be obtained by

using probability distributions of uncertain doses and, thus, is favorable to claimants.2o

The foregoing discussions indicate that the effect of using the adjustment factor of 6

specified in the 2003 Interim Guidance to estimate upper bounds of neutron doses differed

depending on whether VA used the CIRRPC table or IREP to evaluate causation of cancers.

When the CIRRPC table was used, application of an adjustment factor to mean neutron doses

that had been calculated by SAIC was necessary to ensure that credible upper bounds of doses

were used in evaluating causation of cancers, because an adjustment factor was the only means

of accounting for an uncertainty in the biological effectiveness (REF) of neutrons.

When VA used IREP, however, an uncertainty in REF for neutrons was taken into

accountwice, once by means of the adjustment factor specified in the 2003 Interim Guidance,

which takes into accounthe upper 95Yo credibility limit of an REF for fission neutrons relative

to modifring factors used by SAIC, and again by means of the probability distribution of REF

for fission neutrons incorporated in IREP. This double-counting of uncertainty in REF, which

was favorable to claimants, resulted in an upper 95%o qedibility limit of a biologically significant

dose from exposure to neutrons that is higher than needed by a factor given by the ratio of the

upper 95Yo credibility limit of REF for fission neutrons at low doses of about 50 incorporated in

IREP to the wn of 20 that IREP assumes was used to estimate equivalent doses from fission

neutrons, or a factor of about 2.5.21

In April 2005, VA discontinued use of the CIRRPC table and adopted a policy of using

IREP exclusively to evaluate causation of cancers (Mansfield, 2005). With that change in

policy, the adjustment factor that should be applied to mean neutron doses calculated by SAIC to

20In addition to constant doses, various probability distributions of doses (lognormal, normal,

triangular, logtriangular, uniform, or loguniform) can be entered into IREP.

2tlt is importantto emphasizethatan overestimation of doseby a some factor does not result in an

overestimation of PC by the same factor, because PC is a nonlinear function of dose (D, rem) given by

[ERR/rem x D]/[(ERR/rem x D) * 1], where ERR"/rem is the excess relative risk per rem for a

participant's cancer type, ages at times of exposure, and age at the time of diagnosis.

28
obtain credible upper bounds should take into account (a) an uncertainty in tissue kerma only and

(b) an increase in mean doses by a factor of 2 to represent the ratio of the wn of 20 that IREP

assumes was used to estimate equivalent doses from fission neutrons to the modifuing factor of

11 or 13 used by SAIC. The adjustment factor should no longer include an uncertainty in the

biological effectiveness of fission neutrons, because that uncertainty is incorporated in IREP.

For example, if the uncertainty in tissue kerma is a factor of 2.2, as assumed in the previous

section, the adjustment factor that should be applied to mean doses calculated by SAIC would be

2.2 x 2, or about 4,rather than 6 as specifred in the 2003 Interim Guidance.

29
SECTION 6

EVALUATION OF ADEQUACY OF UPPER

BOUNDS OF NEUTRON DOSES

An important concern in this investigation is whether upper bounds of neutron doses that

were obtained by applying the adjustment factor of 6 specified in the Interim Guidance

(Benavides,2003) to mean doses that had been calculated by SAIC were adequate for the

purpose of evaluating causation of cancers. As discussed in the previous section, this issue is

particularly important in cases where VA evaluated causation of cancers using screening doses in

the CIRRPC table. Upper bounds obtained by using the adjustment factor of 6 would be

adequate ifthey are at least upper 95Yo credibility limits of doses to any organ or tissue from

external exposure to neutrons. In addressing this issue, it is importanto recognize that mean

quality factors and radiation weighting factors for neutrons recommended by NCRP and ICRP

and given in Tables 2-I and 3-1, respectively, are intended to be used for purposes of radiation

protection (control of exposures), rather than for purposes of estimating risks and PC of specific

cancers in identified individuals. That is, it is intended that use of radiation protection quantities

would not result in substantial underestimates ofdose to specific organs or tissues of concern.

In evaluating the adequacy of upper bounds of neutron doses that were obtained using the

adjustment factor of 6, we consider doses to skin separately from doses to internal organs. The

latter lie well below the body surface, whereas radiosensitive tissues of the skin lie at an average

depth of about 0.07 mm (ICRP, 1977). We also consider the potential importance of recent

changes in evaluated cross sections for interactions of neutrons with constituents of air as they

affect estimates of neutron fluences at assumed locations of exposure of military participants in

the atmospheric testing program.

30
6.1 Doses to Internal Organs

A comparison of doses to intemal organs from exposure to fission neutrons with doses

calculated by SAIC can be made on the basis of organ-specific transmission factors in an adult

(55-kg) Japanese phantom from exposure to prompt neutrons produced in the detonations at

Hiroshima and Nagasaki (Kaul et al., lgll)and an assumed qualrty factor for neutrons. A

transmission factor is defined as the ratio of an organ kerma to tissue kerma free-in-air from

neutrons. Transmission factors were calculated for neutrons and secondary photons produced by

interactions of neutrons in tissues of the body. Calculated transmission factors essentially give

absorbed doses to specific organs from neutrons or secondary photons relative to tissue kerma

free-in-air for an assumed spectrum of incident neutrons.

Organ-specific transmission factors at a ground distance of 1.5 km from the detonation at

Nagasaki that were calculated by assuming a standing phantom facing the detonation (Kaul et al.,

1987; Tables 74 and78) are given in the middle two columns of Table 6-1. Transmission factors

in the last column were calculated by weighting the transmission factor for neutrons by a quality

factor of l l and adding the transmission factor for secondary photons; a quality factor of 11 is

the maximum value in Table 2-l and is the same as the lower value of the modiffing factor that

SAIC applied to tissue kerma free-in-air. Weighted transmission factors in Table 6-1 thus

provide dose equivalents from neutrons and secondary photons combined that can be compared

with the dose equivalent that was calculated by SAIC by applying a modiffing factor of 11 to an

estimate of tissue kerma ftee-in-air.22

We frnd that the neutron dose equivalent that was calculated by SAIC using a modiffing

factor of 1l exceeds dose equivalents to specific organs in the adult phantom by a factor that

ranges from 1118.6:1.3 in the female breasto 1112.5:4.4 in the pancreas. The average of

these ratios in all organs listed in Table 6-1 is 2.2. All such ratios are l8%o higher when dose

equivalents were calculated by SAIC using a modifuing factor of 13, as has most often been the

case, and the same quality factor of 11 is used to calculate weighted transmission factors in the

22In estimating dose equivalents to specific organs of Japanese atomic-bomb survivors, a quality

factor of 10 for neutrons has often been used; e.g., see Roesch (1987).

3 l
adult phantom. Similar results would be obtained if transmission factors that apply to prompt

neutrons produced in the detonation at Hiroshima (Kaul et al., 1987) were used.

If an upper 95% credibility limit of an REF for induction of solid tumors at low doses of

50 discussed in Section 3 were used to represent the biological effectiveness of neutrons, rather

than the lower modifying factor of 11 used by SAIC and the quality factor of 11 used to calculate

weighted transmission factors in Table 6-1, ratios of dose equivalents calculated by SAIC to

organ-specific dose equivalents in the adult phantom would differ little from the ratios

summarized above, mainly because weighted transmission factors in Table 6-l arc dominated by

the contribution from neutrons in all cases and, thus, would be nearly proportional to an assumed

biological effectiveness of neutrons. That is, the ratios summarized above also apply reasonably

well at upper 95o/o credibility limits of the two dose equivalents. The biggest change would

occur in the ratio of dose equivalents in the pancreas, which would increase from 4.4 to 4.8.

Our analysis of neutron dose equivalents calculated by SAIC compared with weighted

transmission factors in specific organs in an adult phantom, as described above, is subject to

several limitations. For example: (a) energy and angular distributions of neutrons above ground

at a ground distance of 1.5 km from the detonation at Nagasaki differ from distributions at

greater distances in trenches at NTS;23 (b) transmission factors in Table 6-l apply to a standing

phantom facing the detonation and, thus, differ from values that would apply to an individual

crouching in a trench; and (c) transmission factors in most participants at atmospheric tests

would be lower than values in a 55-kg phantom, due to their greater body mass. Nonetheless, we

believe it is reasonable to conclude that estimates of tissue kerma free-in-air modified by an

appropriate factor to account for the biological effectiveness ofneutrons should provide

overestimates of doses to intemal organs of participants in all cases. However, our analysis also

suggests that the degree of overestimation of doses to internal organs that lie relatively close to

the body surface could be less than a factor of2.

23The closest ground distance of observers at NTS was about 1.8 km, and other exposures on the

ground at NTS occurred at distances greater thanZ km (Goetz et al., 1981).
6.2 Doses to Skin

In contrasto doses to internal organs discussed above, neutron dose equivalents

calculated by SAIC probably do not overestimate doses to skin by a substantial amount. This

conclusion is based on the following considerations. First, at an average depth of skin below the

body surface of 0.07 mm (ICRP,1977), the transmission factor for spectra of fission neutrons

should be close to 1.0; i.e., kerma atthatdepth should differ little from tissue kerma free-in-air.2a

Second, when the variable angles of incidence of neutrons at the body surface and ranges of

recoil protons that are produced by scattering of neutrons by nuclei of hydrogen atoms, which are

less than about 0.4 mm (Shleien et al., 1998; Figure 5.8.1), are taken into account, the absorbed

dose to skin from neutron interactions should be only slightly less than tissue kerma free-in-air,

perhaps by about 10o .25 Third, as illustrated by calculations in Figure 2-2,the absorbed dose to

skin from interactions of secondary photons should be insignificant at incident neutron energies

above about 10 keV (NCRP, I97I; Appendix B).

Thus, estimates of tissue kerma free-in-air modified by an appropriate factor to account

for the biological effectiveness of neutrons should provide a reasonable representation of doses

to skin. Discussions on the NRC committee's evaluation of SAIC's calculations of neutron

doses (see Section 3) and uncertainties in tissue kerma free-in-air estimated by SAIC (see

Section 2) indicate that when an adjustment factor of 6 was applied to mean doses, as specified

in the 2003 Interim Guidance, the resulting upper bounds probably were about 50% higher than

needed to represent upper 95o/o qedibility limits of doses to skin when a modiffing factor of 13

2aAn implicit assumption in these discussions is that neutrons are incident on the body at locations

of skin cancers, rather than transported through a substantial thickness oftissue prior to inadiating skin at

those locations. Attenuation of incident neutrons by transport through the body could greatly reduce dose

equivalents to skin relative to tissue kerma free-in-air (e.g., see Figure 2-3).

25Recoil protons from elastic scattering by hydrogen nuclei are the most important charged

particles produced by neutron interactions in tissue at energies above 50 eV (see Figure 2-1), and the

maximum range of recoil protons of about 0.4 mm applies to backscattering of 6 MeV neutrons, which is

about the highest energy of neutrons to which military participants at atmospheric tests were exposed

(S.D. Egbert, SAIC, personal communication). Absorbed dose from neutron interactions would be the

same as tissue kerma free-in-air if charged-particle equilibrium were achieved at a depth of 0.07 mm

below the body surface, but absorbed dose is less than tissue kerma free-in-air when charged-particle

equilibrium is not achieved; e.g., see Shultis and Faw (1996).
was used by SAIC. However, the upper bound probably was no more than about 2|%ohigher

than needed in the one case where a modiffing factor of 11 was used.

6.3 Effect of Revised Estimates of Neutron Fluences

Neutron fluences at assumed locations of exposure of military participants at atmospheric

tests that were calculated by SAIC incorporate assumptions about cross sections for interactions

of neutrons with constituents of air during transport from a burst point. Subsequento SAIC's

calculations, evaluated cross sections for inelastic scattering of neutrons from nuclei of nitrogen

and oxygen atoms at energies greater than about 2 MeV were increased (S.D. Egbert, SAIC,

personal communication). As a result, SAIC probably ovgrestimated fluences of higher-energy

neutrons at locations of exposure and, thus, neutron doses to participants.

The potential importance of increases in cross sections for inelastic scattering of neutrons

in air is suggested by recent revisions of dosimetry data at Hiroshima and Nagasaki. As shown

in Figure 6-1, tissue kerma free-in-air from neutrons is reduced at ground distances from the

detonation at Hiroshima greater than2 km and is reduced at all distances from the detonation at

Nagasaki compared with previous calculations (Cullings and Fujita, 2003); the reduction in

tissue kerma atNagasaki is about 30% at2 km. Similarly, as shown in Figure 6-2, absorbed

doses to the colon from neutrons ar€ reduced at all distances at both sites (Preston et a1.,2004);

the reductions at 2htn ne about 20Vo at Hiroshima and 40%o at Nagasaki. Reductions in tissue

kerma free-in-air and absorbed dose to the colon from neutrons increase as the ground distance

increases beyond 2V,n at both sites, due to the more rapid attenuation of lower-energy neutrons

and, thus, the greater proportion of higher-energy neutrons in fluences at greater distances.

If we assume that reductions in tissue kerma free-in-air and absorbed dose to the colon at

Nagasaki are representative of reductions that would apply to detonations at NTS and in the

34
Pacific,26 we would conclude that neutron doses calculated by SAIC probably overestirpate doses

to all organs and tissues, including skin, by at least 30Yo in all cases, and that larger reductions

could apply at distances from a detonation greater than2 km. However, reductions in dosimetry

data atNagasaki indicated in Figures 6-l and 6-2 should be interpreted with caution when factors

other than increases in cross sections for neutron interactions in air also contributed to the

reductions (Cullings and Fujita, 2003) and the importance of higher-energy neutrons produced in

detonations at NTS and in the Pacific could be different than in the detonation at Nagasaki. In

addition, reductions in neutron fluences due to increases in cross sections for inelastic scattering

from nitrogen and oxygen nuclei could have been less at NTS than in Japan, due to the lower

density of the atmosphere at higher elevations and the lower absolute humidity. Therefore,

definitive conclusions about the effect of increased cross sections on calculated neutron doses to

military participants at atmospheric tests would require further consideration of the applicability

of revised dosimetry data atNagasaki to detonations at NTS and in the Pacific.

A more rigorous approach to addressing the importance of increases in neutron cross

sections in air and resulting reductions in fluences of higher-energy neutrons would require

additional calculations of neutron transport in air to assumed locations of exposure of military

participants at each atmospheric test. In this approach, mean neutron doses and upper bounds

calculated previously by SAIC could be reduced by the same factor at any location, but the

reductions would depend on distance from a detonation and the neutron spectrum in each case.

6.4 Conclusions on Adequacy of Adjustment Factor in Interim Guidance

The foregoing discussions indicate that use of the adjustment factor of 6 specified in the

Interim Guidance (Benavides ,2003)to obtain upper bounds of neutron doses from mean doses

that were calculated by SAIC is more than sufficient to ensure that those upper bounds are at

least upper 95o/o credibility limits of neutron doses to any organ or tissue of military participants

at atmospheric tests. This conclusion is based on several considerations, including: (a) the

'uThis assumption takes into account that the Hiroshima bomb did not resemble weapons

detonated in the atmospheric testing program.

3 5
erroneous assumption by the NRC committee (NRC,2003) about the modiffing factor that SAIC

applied to an estimate of tissue kerma free-in-air from neutrons to estimate dose equivalent in a

particular case of exposure at Operation PLUMBBOB, Shot DOPPLER; (b) the conclusions that

doses to all intemal organs should be overestimated and that doses to skin should not be

underestimated; and (c) the conclusion that tissue kerma free-in-air and neutron dose equivalents

at locations of exposure of military participants at atmospheric tests were always overestimated

by SAIC as a result of an overestimation of fluences of higher-energy neutrons in all cases.

The conclusion that use of the adjustment factor of 6 specified in the 2003 Interim

Guidance is more than sufficient to given credible upper bounds (at least upper 95Yo cledibility

limits) of neutron doses is important mainly in cases where VA used screening doses in the

CIRRPC table (CIRRPC, 1988) to evaluate causation of cancers. In those cases, upper bounds of

neutron doses reported by DTRA needed to account for uncertainty in the biological

effectiveness of neutrons. However, now that VA no longer uses the CIRRPC table but uses

IREP exclusively to evaluate causation of cancers (Mansfield,2005), an adjustment factor that

includes an uncertainty in the biological effectiveness of neutrons is no longer needed, because

an uncertainty in REF for fission neutrons is incorporated in IREP.

36
Table 3. Transmission factors for kerma in organs of adult (55-kg) adult Japanese phantom

relative to neutron kerma free-in-air from prompt neutrons produced in detonation

at Nagasaki and transmission factors weighted by assumed quality factor for

neutronsu

Organ

Transmission factor

(organ kerma/tissue kerma free-in-air)b

Transmission factor

weighted by Q: ll

Neutrons Secondary photons for neutronst

Active marrow

Bladder

Bone

Brain

Breasts (female)

Eyes

Intestinal tract

Liver

Lungs

Pancreas

Stomach

Testes

Thyroid

0.34

0.30

0.40

0.42

0.77

0.75

0.23

0.34

0.36

0.21

0.37

0.51

0.57

0.18

0.21

0.18

0.15

0.13

0.r2

0.24

0.19

0.t7

0.22

0.20

0.18

0.18

3.9

3.5

4.6

4.8

8.6

8.4

2.8

3.9

4.1

2.5

4.3

5.8

6.5

"Transmission factors for kerma obtained from Tables 74 and 78 of Kaul et al. (1987) apply to

standing phantom facing detonation at ground distance of 1.5 km.

bo.gatt kerma for neutrons and secondary photons produced by neutron interactions in tissue is

essentially the same as absorbed dose from those radiations.

bCalculated as TFn , ^+ TF, where TF is transmission factor for neutrons or photons and

quality factor for neutrons (gJ of 11 is lower of two values used by SAIC to modiff estimates of tissue

kerma free-in-air to obtain dose equivalents.
Neutron Free-in-Air Kerma

{n

t3l c

6

JG

{}

r|J

g

p

g

(&

s50* fg00 tw 2m0

Ground Fistanse tm)

Figure 6. Changes in estimates of neutron kerma free-in-air vs. ground distance from

detonations at Hiroshima and Nagasaki due to change in dosimetry system from

DS86 to DS02 (Cullings and Fujita, 2003).

3 8
o

s

a

o 0

E

o

s '10

E

{t

? .20

tt

d

c

* ,30

a

0,

40

1000 1500

Distunce tohyPocenter (m)

Figure 7. Changes in estimates of absorbed dose to colon from gamma rays and neutrons vs.

ground distance from detonations at Hiroshima (left panel) and Nagasaki (right panel)

due to change in dosimetry system from DS86 to DS02 (Preston et al., 2004).

39
SECTION 7

ESTIMATION OF UPPER BOUNDS OF NEUTRON DOSES

FOR USE IN IREP

Now that VA uses IREP exclusively to evaluate causation of cancers in military

participants at atmospheric tests and it is no longer necessary to account for an uncertainty in the

biological effectiveness of neutrons in applying an adjustment factor to mean neutron doses

calculated by SAIC, DTRA could use one of two approaches to estimate upper bounds of

neutron doses to be reported to VA. As noted in Section 5, upper bounds of neutron doses

should be entered into IREP separately from upper bounds of doses from other radiation types.

The first approach is based on an assumption that mean neutron doses that were

calculated previously by SAIC, as described in Section 2, would not be revised; i.e., DTRA

would continue to report neutron doses that were calculated using a modifying factor of 11 or 13,

except we also assume that the dose that was calculated using a modifuing factor of 11 in one

case would be increased to a value that corresponds to a modiffing factor of 13 to correct an

inconsistency between the modifying factor of 11 and NCRP's mean quality factors. On the

basis of these assumptions, upper bounds of neutron doses estimated by SAIC would need to

incorporate (a) an uncertainty in tissue kerma only and (b) an increase that accounts for the

difference between the radiation weighting factor (wp) of 20 that IREP assumes was used to

calculate equivalent doses from fission neutrons and the modiffing factor of 13 used by SAIC.

Since the modifying factor of 13 is consistent with NCRP's mean quality factors inTable2-t,

upper bounds of neutron doses estimated by SAIC that take into account uncertainties in tissue

kerma only would need to be increased by a factor of 2to obtain upper bounds of equivalent

doses suitable for input to IREP.

In the second approach, neutron doses calculated previously by SAIC would need to be

revised to incorporate a wp of 20, rather than a modiffing factor of 11 or 13, to be consistent

with the recommendation on the biological effectiveness of fission neutrons (ICRP, 1991) and

the assumption in IREP. If we assume that the neutron dose that was calculated using a

40
modifying factor of I I would be increased to give a value that corresponds to a modifying factor

of 13, SAIC would need to increase mean neutron doses calculated previously (Goetz etal.,

1985) by a factor of 2. The resulting doses would be mean equivalent doses. With these

increases in mean neutron doses, SAIC could revert to the approach that was used to estimate

upper bounds of neutron doses prior to implementation of the provision in the 2003 Interim

Guidance on use of an adjustment factor of 6. That is, upper bounds of neutron equivalent doses

could be estimated by accounting for uncertainties in estimates of tissue kerma only, and no

further adjustment would be needed. Thus, in this approach, means and upper bounds of neutron

doses calculated previously by SAIC both would be increased by a factor of 2 to obtain means

and upper bounds ofequivalent doses.

Thus, in either approach, upper bounds of neutron doses calculated previously by SAIC

would be increased by a factor of 2, but only in the second approach would mean neutron doses

calculated previously also be increased. Again, an uncertainty in biological effectiveness (REF)

of fission neutrons would not need to be taken into account in estimating upper bounds, because

that uncertainty is incorporated in IREP.

Of the two approaches to revising neutron doses calculated previously by SAIC, we

believe that the second approach is preferable because means and upper bounds both would be

calculated using a wp of 20 for fission neutrons, as recommended by ICRP and assumed in IREP.

Thus, both would be equivalent doses. This approach would be particularly advantageous if VA

revised its regulations to allow the use of ranges of doses in evaluating causation of cancers.

With this change, neutron doses could be entered into IREP as probability distributions (see

footnote 19), but only if means and upper bounds both were calculated using a wn of 20. In

addition, the interpretation of upper bounds of neutron doses would be more transparent when

they represent uncertainties in tissue kerma only and do not incorporate an additional adjustment

to account for an increase in biological effectiveness compared with the modifring factors used

previously by SAIC. Finally, an approach of estimating means and upper bounds of neutron

equivalent doses using a ws of 20 is preferable if additional calculations for participants whose

neutron doses were not estimated previously were needed.

4 1
SECTION 8

SUMMARY AND CONCLUSIONS

This report has considered approaches to estimating upper bounds of neutron doses to

veterans of military services who participated in the atmospheric nuclear-weapons testing

program. An important focus of this report is an investigation of the adequacy of the adjustment

factor of 6 that was specified in the Interim Guidance (Benavides,2003) for use in estimating

upper bounds of neutron doses from mean doses calculated by SAIC (Goetz et al., 1979;1981;

1985). That adjustment factor was developed to take into account an uncertainty in the

biological effectiveness of fission neutrons that was not incorporated in upper bounds of neutron

doses calculated previously by SAIC. An investigation of the adequacy of the adjustment factor

of 6 involves an assessment ofwhether upper bounds of neutron doses obtained using that

adjustment factor are at least upper 95Yo credibility limits of doses to any organ or tissue.

On the basis of an evaluation of the adjustment factor specified in the 2003 Interim

Guidance and assumptions that were used by SAIC to calculate mean neutron doses and upper

bounds, we concluded that applying an adjustment factor of 6 should be more than sufficient to

ensure that the resulting upper bounds would be at least upper 95o/o credibility limits of doses to

any organ or tissue, including skin. This conclusion is particularly important in cases where VA

evaluated causation of cancers in participants at atmospheric tests using screening doses in the

CIRRPC table (CIRRPC, 1988).

However, now that VA no longer uses the CIRRPC table but uses IREP exclusively to

evaluate causation of cancers (Mansfield, 2005), an adjustment of mean neutron doses calculated

by SAIC by a factor that accounts for an uncertainty in biological effectiveness, as specified in

the 2003 Interim Guidance, is no longer needed, because an uncertainty in the biological

effectiveness (REF) of fission neutrons is incorporated in IREP. Therefore, a different approach

can be used to estimate upper bounds of neutron doses to be reported to VA.

On the basis of analyses presented in this report, we recommend that mean neutron doses

and upper bounds calculated previously by SAIC (Goetz et al., 1979; 1981; 1985) both should be

42
revised as summarized below to provide doses suitable for input to IREP.

. The mean dose equivalent and upper bound that were calculated by applying a modifying

factor of 11 to an estimate of tissue kerma free-in-air in the case of exposure of certain

participants at Operation PLUMBBOB, Shot DOPPLER (Goetz et al., 1979) both should

be increased to give values that are consistent with the use of a modifying factor of 13 in

all other cases (Goetz et a1.,1981; 1985). The modi$ing factor of 11 was based on an

incorrect application of mean quality factors recommended by NCRP (1971) and given in

Table 2-l to an estimate of tissue kerma.

o All mean dose equivalents and upper bounds that were calculated by applying a

modi$ing factor of l3 to an estimate of tissue kerma free-in-air, including revised

estimates in the case of exposure of certain participants at Operation PLUMBBOB, Shot

DOPPLER as recommended above, should be increased by a factor of 2 to account for

the difference between the radiation weighting factor (wn) of 20 recommended by ICRP

(1991) and the modifying factor of l3 used by SAIC. The resulting doses would be

means and upper bounds of neutron equivalent doses that are suitable for input to IREP.

These upper bounds would take into account an uncertainty in tissue kerma only, as was

the case prior to use of the adjustment factor specified in the 2003 Interim Guidance. The

recommended increase in means and upper bounds calculated previously by a factor of 2

is based on the consideration that SAIC's modifying factor of 13 is consistent with the

mean quality factor of 10 for neutrons that was used in radiation protection (NCRP, l97l;

ICRP, 1977) prior to its replacement by awpof 20.

We also recommend that any additional calculations of neutron doses to participants that might

be performed should use o l/p of 20 to obtain means and upper bounds of equivalent doses.

At the present time, VA uses only the upper bounds of neutron doses reported by DTRA

in evaluating causation of cancers. We have shown that an increase in upper bounds calculated

previously by SAIC by a factor of 2, as recommended above, should provide appropriate upper

bounds of equivalent doses for input to IREP, meaning that upper bounds of biologically
significant doses from neutrons that are calculated in IREP by taking into account the uncertainty

in biological effectiveness (REF) incorporated in IREP should exceed upper 95% credibility

limits of doses to any organ or tissue of exposed individuals, including skin. This conclusion is

based on data and analyses which indicate that (a) doses to intemal organs should be

overestimated due to neglect of the attenuation of incident neutrons in transport through tissues

of the body and (b) fluences of important higher-energy neutrons and, therefore, tissue kerma at

locations of exposure of military participants were overestimated by SAIC in all cases.

Acknowledgement

The many important contributions from S.D. Egbert of SAIC to discussions in this report

are gratefully acknowledged. His knowledge and historical insights were invaluable.

44
SECTION 9

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49
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