ORAU TEAM

Dose Reconstruction

Project for NIOSH

Oak Ridge Associated Universities I Dade Moeller & Associates I MJW Corporation

Page 1 of 23

Document Title:

Effect of Threshold Energy and Angular 

Response of NTA Film on Missed Neutron 

Dose at the Oak Ridge Y-12 Facility

Document Number: ORAUT-OTIB-0051

Revision: 00

Effective Date: 05/15/2006

Type of Document: OTIB

Supersedes: None

Subject Expert: George D. Kerr

Document Owner

Approval: Signature on File Approval Date: 05/03/2006

William E. Murray, TBD Team Leader

Approval: Signature on File Approval Date: 04/13/2006

John M. Byrne, Task 3 Manager

Concurrence: Signature on File Concurrence Date: 05/12/2006

Kate Kimpan, Project Director

Approval: Signature on File Approval Date: 05/15/2006

James W. Neton, Associate Director for Science

 New Total Rewrite Revision Page Change

FOR DOCUMENTS MARKED AS A TOTAL REWRITE, REVISION, OR PAGE CHANGE, REPLACE THE PRIOR 

REVISION AND DISCARD / DESTROY ALL COPIES OF THE PRIOR REVISION.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 2 of 23

PUBLICATION RECORD

EFFECTIVE 

DATE

REVISION

NUMBER DESCRIPTION

05/15/2006 00 New document to describe the effect of threshold energy and angular 

response of NTA film on missed neutron dose at the Oak Ridge Y-12 

facility. First approved issue. Training required: As determined by 

the Task Manager. Initiated by George D. Kerr.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 3 of 23

TABLE OF CONTENTS

TITLE

SECTION

Acronyms and Abbreviations ...............................................................................................................

PAGE

5

1.0 Introduction ..............................................................................................................................6

2.0 Sources of Neutron Exposure to Workers at Y-12.................................................................... 7

3.0 Threshold Energy for Detection of Fast Neutrons by NTA Film ................................................ 9

4.0 Missed Dose Calculations Assuming a Fixed Threshold Energy ............................................ 11

5.0 Calibration of Fast Neutron Dosimeters Containing NTA films ............................................... 14

6.0 Missed Dose Calculations Without Assuming a Fixed Threshold Energy ............................... 16

7.0 Angular Response of NTA Film.............................................................................................. 17

8.0 Discussion .............................................................................................................................18

References ........................................................................................................................................20

LIST OF TABLES

TABLE TITLE PAGE

2-1 Sources of neutron exposure at Y-12.......................................................................................7

3-1 Literature values of threshold energy for detection of fast neutrons by NTA film .................... 10

8-1 Recommended correction for missed neutron dose due to threshold energy and angular 

response of NTA film dosimeters ........................................................................................... 18
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LIST OF FIGURES

FIGURE TITLE

2-1 Neutron energy spectrum for a 241Am–Be source.....................................................................

PAGE

8

2-2 Neutron energy spectra for various Y-12 sources .................................................................... 9

3-1 Energy response of NTA film exposed on an anthropomorphic phantom to fast neutrons...... 10

4-1 Missed neutron dose from radionuclide sources using (α,n) reaction in beryllium 

calculated as a function of threshold energy of NTA film ....................................................... 11

4-2 Missed neutron dose from unshielded 252Cf fission neutron source calculated as a 

function of threshold energy of NTA film................................................................................. 12

4-3 Missed neutron dose at 86-Inch Cyclotron calculated as a function of threshold energy of 

NTA film.................................................................................................................................12

4-4 Missed neutron dose from shielded radionuclide sources at Health Physics Calibration 

Laboratory calculated as a function of threshold energy of NTA film ...................................... 13

4-5 Missed neutron dose from neutrons emitted by storage containers of highly enriched UF4

and UO3 calculated as a function of threshold energy of NTA film.......................................... 13

4-6 Missed neutron dose versus mean energy of a broad spectrum of neutron energies for 

assumed threshold energy of 700 keV for NTA film................................................................ 14

5-1 Comparison of first collision dose coefficients from NBS Handbook 75 and multicollision 

dose coefficients from NCRP Report 38 and IAEA TRS No. 318............................................ 16

6-1 Missed neutron dose calculated without assuming fixed threshold energy for NTA film ......... 17
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 5 of 23

ACRONYMS AND ABBREVIATIONS

CEF Critical Experiments Facility

DOE U.S. Department of Energy

E neutron energy

eV electron-volt

IAEA International Atomic Energy Agency

ICRU International Commission on Radiation Units and Measurements

ISO International Organization for Standardization

keV kiloelectron-volt, 1 thousand electron-volts

MDL minimum detectable limit

MeV megaelectron-volts, 1 million electron-volts 

mm millimeter

mrem millirem

n neutron

NCRP National Council on Radiation Protection and Measurement

NIOSH National Institute for Occupational Safety and Health

NTA nuclear track emulsion, type A 

p proton

PMMA polymethyl methacrylate 

PNL Pacific Northwest Laboratory

TIB technical information bulletin

TRS Technical Reports Series

U.S.C. United States Code

Z atomic number

α alpha particle

§ section
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 6 of 23

1.0 INTRODUCTION

Technical information bulletins (TIBs) are general working documents that provide guidance 

concerning the preparation of dose reconstructions at particular sites or categories of sites. They will 

be revised in the event additional relevant information is obtained. TIBs may be used to assist the 

National Institute for Occupational Safety and Health (NIOSH) in the completion of individual dose 

reconstructions.

In this document the word “facility” is used as a general term for an area, building, or group of 

buildings that served a specific purpose at a site. It does not necessarily connote an “atomic weapons 

employer facility” or a “Department of Energy [DOE] facility” as defined in the Energy Employees 

Occupational Illness Compensation Program Act [EEOICPA; 42 U.S.C. § 7384l(5) and (12)]. 

EEOICPA defines a DOE facility as “any building, structure, or premise, including the grounds upon 

which such building, structure, or premise is located ... in which operations are, or have been, 

conducted by, or on behalf of, the Department of Energy (except for buildings, structures, premises, 

grounds, or operations ... pertaining to the Naval Nuclear Propulsion Program)” [42 U.S.C. § 

7384l(12)]. Accordingly, except for the exclusion for the Naval Nuclear Propulsion Program noted 

above, any facility that performs or performed DOE operations of any nature whatsoever is a DOE 

facility encompassed by EEOICPA.

For employees of DOE or its contractors with cancer, the DOE facility definition only determines 

eligibility for a dose reconstruction, which is a prerequisite to a compensation decision (except for 

members of the Special Exposure Cohort). The compensation decision for cancer claimants is based 

on a section of the statute entitled “Exposure in the Performance of Duty.” That provision [42 U.S.C. § 

7384n(b)] says that an individual with cancer “shall be determined to have sustained that cancer in the 

performance of duty for purposes of the compensation program if, and only if, the cancer ... was at 

least as likely as not related to employment at the facility [where the employee worked], as 

determined in accordance with the POC [probability of causation1

As noted above, the statute includes a definition of a DOE facility that excludes “buildings, structures, 

premises, grounds, or operations covered by Executive Order No. 12344, dated February 1, 1982 (42 

U.S.C. 7158 note), pertaining to the Naval Nuclear Propulsion Program” [42 U.S.C. § 7384l(12)]. 

While this definition contains an exclusion with respect to the Naval Nuclear Propulsion Program, the 

section of EEOICPA that deals with the compensation decision for covered employees with cancer 

[i.e., 42 U.S.C. § 7384n(b), entitled “Exposure in the Performance of Duty”] does not contain such an 

exclusion. Therefore, the statute requires NIOSH to include all occupationally derived radiation 

exposures at the facility in its dose reconstructions for employees at DOE facilities, including radiation 

exposures related to the Naval Nuclear Propulsion Program. As a result, all internal and external 

dosimetry monitoring results are considered valid for use in dose reconstruction. No efforts are made 

to determine the eligibility of any fraction of total measured exposure for inclusion in dose 

reconstruction. NIOSH, however, does not consider the following exposures to be occupationally 

derived:

] guidelines established under 

subsection (c) ...” [42 U.S.C. § 7384n(b)]. Neither the statute nor the probability of causation 

guidelines (nor the dose reconstruction regulation) define “performance of duty” for DOE employees 

with a covered cancer or restrict the “duty” to nuclear weapons work.

• Radiation from naturally occurring radon present in conventional structures

• Radiation from diagnostic X-rays received in the treatment of work-related injuries

 1 The U.S. Department of Labor is ultimately responsible under the EEOICPA for determining the POC.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 7 of 23

The purpose of this TIB is to provide definitive documentation of the effects of threshold energy and 

angular response of nuclear track emulsion, type A (NTA) film on missed neutron dose at the Oak 

Ridge Y-12 Plant. 

2.0 SOURCES OF NEUTRON EXPOSURE TO WORKERS AT Y-12

Table 2-1 lists the most common sources of neutron exposure to Y-12 workers during the period of 

film badge monitoring (ORAUT 2004a, 2005a). With the exception of neutron fission sources at the 

Critical Experiments Facility (CEF), this analysis used two different approaches to calculate the effect 

of the threshold energy of NTA film on the dose measurements for fast neutrons (Sections 4.0 and 

6.0). Information on the effect of threshold energy of NTA film on neutron exposure doses at the CEF

was available from measurements at a number of locations outside and inside the facility during 

extended operations of critical assemblies at relatively high power (ORAUT 2005a). During one 

measurement collection session in January 1960, a critical volume of an enriched uranium solution in 

the West Assembly Room was operated in a steady-state mode to produce 7 × 1012 fissions/s. These 

1960 measurements at six locations in the CEF suggested a range of missed neutron doses for NTA 

film ranging from 42% to 74% with a mean of 55% (ORAUT 2005a).

Table 2-1. Sources of neutron exposure at Y-12.

a

Location Building Source

Assay Laboratory 9203 (Room 8), 9205 Ra–Be, Po–Be 

Chemical Operations 9202, 9206, 9212 UF4, UO3

CEF 9213 Po–Be, Pu–Be 

Fission neutrons 

Electromagnetic Research 9201-2, 9204-3 Po–Be 

86-Inch Cyclotron

Health Physics 9983 (Calibration Laboratory) Po–Be, Am–Be 

Instrument Department 9737 Po–Be 

Nondestructive Assay Laboratory 9720-5 Cf–252 fission neutrons

a. Sources: ORAUT (2004a, 2005a,b).

Most of the neutron sources at Y-12 were small and produced neutrons by alpha particle reactions in 

beryllium. These neutron sources were used in basic research (Buildings 9201-2 and 9204-3), critical 

assembly and reactor research (Building 9213), calibration of radiation detection instruments and 

dosimeters (Buildings 9737 and 9983), and material assay (Buildings 9203 and 9205). The neutron 

energy spectra from all radionuclide sources using the (α, n) reaction in beryllium are similar, and the 

average energy of the neutrons is about 4 MeV (Nachtigall 1967; Kerr, Jones, and Hwang 1978). 

Figure 2-1 shows the neutron energy spectrum used in the calculations of this study for an unshielded 

241Am–Be source from Kluge and Weise (1981; also see Table 4 of IAEA 1990) as fluence per unit 

lethargy. Thus, the area under the curve in any energy range corresponds to the neutron fluence in 

that energy range.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 8 of 23

Figure 2-1. Neutron energy spectrum for a 241Am–Be source 

(Kluge and Weise 1981).

Several other neutron spectra used in the calculations for this study are from spectral measurements 

made by Pacific Northwest Laboratory (PNL 1990; McMahan 1991; BWXT 2001). These spectra are 

from measurements made near an unshielded 252Cf fission neutron source, storage containers of UF4

and UO3 containing highly enriched uranium as feed material for the manufacture of nuclear weapon 

parts, and the storage vault in the calibration laboratory that contained radionuclide sources. These

spectra simulated neutron exposures to Y-12 workers in the Nondestructive Assay Laboratory 

(Building 9720-5), various chemical operations areas throughout the Y-12 facility (Buildings 9202, 

9206, and 9212), and shielded radionuclide sources in the storage vault of the Health Physics 

Calibration Laboratory (Building 9983). Figure 2-2 shows the neutron energy spectra for the above 

PNL measurements (PNL 1990; McMahan 1991; BWXT 2001) as fluence per unit lethargy.

The final spectrum used in the calculations of this study was an empirical neutron spectrum that was 

used to simulate exposures to Y-12 workers from stray neutron radiation at the Oak Ridge 86-Inch 

Cyclotron (Building 9201-2). This study used an empirical neutron spectrum because the neutron 

energy spectra in work areas near the cyclotron, like those near other early proton accelerators, are 

not well known. The empirical neutron spectrum used a Maxwellian thermal neutron spectrum at 

energies less than 0.125 eV and a 1/E slowing-down spectrum of neutrons at energies from 0.125 eV 

to 20 MeV, where E is neutron energy. This empirical neutron spectrum appears to give results that 

are consistent with observations by others (IAEA 1988). For example, it was found that most of the 

neutron dose at early accelerators came from neutrons with energies between 0.1 and 10 MeV, and 

our calculations using the above empirical neutron spectrum suggest that almost 80% of the total 

neutron dose in the stray radiation fields of the Oak Ridge 86-Inch Cyclotron was from neutrons with 

energies of more than 0.1 MeV and less than 10 MeV. ORAUT (2005a) contains a detailed 

discussion of the Oak Ridge 86-Inch Cyclotron and its operation.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 9 of 23

Figure 2-2. Neutron energy spectra for various Y-12 sources (PNL 

1990; McMahan 1991; BWXT 2001).

3.0 THRESHOLD ENERGY FOR DETECTION OF FAST NEUTRONS BY NTA FILM

Track counting of fast neutrons in NTA film depends on the hydrogen content of the emulsion and 

surrounding materials of the film packet (Lehman 1961; Moe 1992). Elastic scattering of fast neutrons 

by hydrogen produces proton recoil ions. These recoil ions lose energy to the silver grains in the NTA 

film, which are much finer than those in photographic films used in photon and beta particle dosimetry. 

However, the NTA film contains more of these silver grains than the photon and beta particle films. 

The latent images formed in the NTA film consist of a number of individual silver grains along the path 

of the recoil proton ion. In addition, there is a thermal neutron response, derived from the nitrogen 

content of the NTA film, via the 14N(n,p)14C capture reaction (Tanner et al. 2001). This part of the 

response was suppressed in the Y-12 film dosimeter by track counting in the portion of the NTA film 

that was covered by a cadmium filter with a thickness of approximately 1 mm. The film badge 

dosimeters used at Y-12 from 1950 to 1980 are discussed in more detail in two ORAUT reports 

(2005a,c).

The limitations of the NTA films in neutron dosimetry derive from problems of track identification by a 

reader (Lehman 1961; Tanner et al. 2001). At least 100 keV must be deposited to form a silver grain 

in the emulsion and at least three or four grains are required to define a track because photons 

produce shorter tracks. The problem of track identification becomes even more difficult when the 

background of shorter tracks caused by the photons is intense in comparison to that caused by 

neutrons (Tanner et al. 2001). As a consequence, NTA films have a fast neutron detection threshold 

in the region of approximately 400 keV and an energy-dependent response that rises rapidly from that 

energy. Figure 3-1 shows the energy-dependent response of NTA film exposed on an 

anthropomorphic phantom (Sayed and Piesh 1974; also see Table 3 of IAEA 1990), and Table 3-1 

lists examples of the widely variant values quoted for the fast neutron threshold of NTA films. The fast 

neutron threshold of 1,000 keV in Table 3.1 is an unrealistically high value from a very early study. It 
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 10 of 23

is often cited, however, as a reason for dismissing neutron dose measurements made with NTA films. 

Sections 4.0 and 6.0 discuss the effects of the fast neutron threshold of NTA films on missed neutron 

dose by film badge dosimeters used at Y-12. 

Figure 3-1. Energy response of NTA film exposed on an 

anthropomorphic phantom to fast neutrons (Sayed and Piesh 

1974; IAEA 1990).

Table 3-1. Literature values of threshold 

energy for detection of fast neutrons by NTA 

film.

Threshold 

energy (keV) Reference

400 Lehman 1961, p. 6

400 IAEA 1970, p. 34

400 Gupton 1978, p. 9

400 Tanner et al. 2001, p. 10

500 Kathren, Prevo, and Block 1965

500 Becker 1966

500 Oshion 1973

500 Griffith et al. 1979

500 Parrish 1979, p. 85

500 Moe 1992, pp. 13–14

500 NIOSH 2002, pp. 26–28

500-600 Drew and Thomas 1998, p. 35

700 IAEA 1985, p. 64

700-800 Wilson 1987, p. 2.21

800 Wilson et al. 1990, pp. 3.1–3.2 

1,000 Bemis 1956, p. 489
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 11 of 23

4.0 MISSED DOSE CALCULATIONS ASSUMING A FIXED THRESHOLD ENERGY

Figures 4-1 to 4-5 show the results of missed neutron dose calculations using fixed threshold energies 

for the NTA film of 400, 500, 600, 700, 800, 900, and 1,000 keV. The calculations used Monte Carlo 

sampling and scoring routines from the MCNP computer program (Briesmeister 2000), the energy 

spectra for the neutron sources discussed in Section 2.0, and dose equivalent coefficients from 

Report 38 of the National Commission on Radiation Protection and Measurements (NCRP 1971; also 

see Table 2 of IAEA 1990). The missed dose varies significantly with the assumed value for the 

threshold energy of the NTA film, and there is a strong correlation of the missed dose with the mean 

energy of the neutron spectra from the sources as shown in Figure 4-6 for a 700-keV threshold 

energy for the NTA film. The calculated mean energies of the neutron spectra were 525 keV at the 

Calibration Laboratory, 636 keV for the highly enriched UF4 and UO3 storage containers, 1.05 MeV for 

the 86-Inch Cyclotron, 2.48 MeV for the unshielded 252Cf fission neutron source, and 4 MeV for the 

241Am–Be neutron source. Figure 4-6 suggests a mean neutron energy of about 300 to 400 keV for 

neutron spectra at measurement points inside the CEF based on a measured missed neutron dose of 

approximately 55% (see Section 2.0). The data shown in Figures 4-1 to 4-5 can be used to evaluate 

the missed dose for threshold energies other than 700 keV. However, a threshold energy of 700 keV 

appears to give a conservative estimate of the missed dose from NTA film measurements at most 

facilities (see Table 3-1 and Figure 6-1). 

Figure 4-1. Missed neutron dose from 

radionuclide sources using (α,n) reaction in 

beryllium calculated as a function of threshold

energy of NTA film. (The results are from 

calculations for a 241Am–Be source but they are 

applicable to 210Po–Be and 239Pu–Be neutron 

sources.)
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 12 of 23

Figure 4-2. Missed neutron dose from 

unshielded 252Cf fission neutron source 

calculated as a function of threshold energy of 

NTA film.

Figure 4-3. Missed neutron dose at 86-Inch 

Cyclotron calculated as a function of threshold 

energy of NTA film.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 13 of 23

Figure 4-4. Missed neutron dose from shielded 

radionuclide sources at Health Physics Calibration 

Laboratory calculated as a function of threshold 

energy of NTA film.

Figure 4-5. Missed neutron dose from neutrons 

emitted by storage containers of highly enriched 

UF4 and UO3 calculated as a function of threshold 

energy of NTA film.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 14 of 23

Figure 4-6. Missed neutron dose versus mean energy of a broad 

spectrum of neutron energies for assumed threshold energy of 700 keV 

for NTA film.

5.0 CALIBRATION OF FAST NEUTRON DOSIMETERS CONTAINING NTA FILMS

The NTA films were first calibrated at Y-12 using a Po–Be neutron source (Struxness 1949, 1952) and 

later an Am–Be neutron source (McLendon 1963; McRee, West, and McLendon 1965). The 

dosimeters containing the NTA films were exposed free in air (no phantom) to neutrons from the Po–

Be and Am–Be sources for pre-selected times to produce a range of neutron doses similar to those 

normally encountered in the workplace. There have been questions about the validity of calibrations 

using a first collision dose that assumes that the NTA film measures the incident neutron dose on an 

anthropomorphic phantom and the reflected neutron dose from the phantom with equal efficiency 

(Budd 1963; ORAUT 2004b). This assumption is questionable because of the poor response of NTA 

films at energies less than approximately 0.7 MeV (see Figure 3-1) and the lower energies of the 

reflected neutrons from the calibration source.

A calibration study using NTA film badge dosimeters exposed to identical fast neutron fluences (1) 

free in air (no phantom) and (2) with the badges on a moderating phantom was made by Budd (1963) 

to determine the increase in tracks, if any, produced by the reflected neutrons from a Pu–Be source 

with an average neutron energy of about 3.9 MeV and a PuF4 source with an average neutron energy 

of about 1.2 MeV. With the Pu–Be source, there was a 15% increase in track density when the 

dosimeters were exposed on the moderating phantom. With the PuF4 source, which produced tracks 

in the NTA film with an average length that was more similar to those observed in Hanford 

dosimeters, no difference was observed when the dosimeters were backed with the moderator (Budd 

1963; ORAUT 2004b). 

A number of other studies using a variety of phantoms and neutron sources have been published with

results for the increase of track density in NTA film from reflected neutrons ranging from a low of zero 
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 15 of 23

to a high of 33% (Thomas et al. 1997). To better define the effect of reflected neutrons on track 

density in NTA films exposed on a phantom, the National Physical Laboratory in England made a 

study using two different phantoms [the International Organization for Standardization (ISO) water 

phantom and the International Commission on Radiological Units and Measurements (ICRU)

polymethyl methacrylate (PMMA) phantom], two different neutron sources (252Cf and Am–Be), and 

two different track readers (Thomas et al. 1997). There was a systematic difference between the two 

track readers of about 10%; however, this difference was roughly the same for all irradiation 

conditions and the ratios determined by the two readers typically agreed to within about 3%. The 

subjectivity introduced by manual evaluation of the tracks is part of the dosimeter reading process, 

and the final mean ratios included all data from the two readers (Thomas et al. 1997). The final mean 

ratios suggested that the reflected neutrons add about 5% on average to the track reading of the NTA 

film and that there were no statistically significant differences for NTA film between the responses for 

either the two sources (252Cf and Am–Be) or the two phantoms (ISO or ICRU). The characteristics of 

the ISO water phantom and ICRU PMMA phantom are discussed in ISO (1998) and ICRU (1992), 

respectively. 

For calibrations made without a phantom, one would typically use a first collision dose such as that 

shown by the red circles and red fitted curve in Figure 5-1. This curve was obtained by multiplying the

first collision absorbed dose data in National Bureau of Standards Handbook 75 (NBS 1961) by a 

relative biological effectiveness (or quality factor) of 10 for fast neutrons with energies higher than 

approximately 100 keV. For calibrations made on a phantom, one would typically use the point-wise 

multicollision dose equivalent data from National Council on Radiation Protection and Measurements 

Report 38 (NCRP 1971) shown by the blue circles in Figure 5-1 or the continuous group data for the 

multicollision dose equivalent from Table 2 of the International Atomic Energy Agency’s Technical 

Reports Series (TRS) No. 318 (IAEA 1990) shown by the blue line in Figure 5-1. The study by Budd 

(1963) resulted in a change at the Hanford facility from NTA film calibration using a first collision dose 

(no phantom) to NTA film calibration using a multicollision dose and a phantom. A PuF4 neutron 

source was used in the NTA film calibrations because this best matched the neutron spectra found in 

the workplace at the Hanford facility. This change in the calibration procedure was found to increase 

all fast neutron measurements with NTA films at the Hanford facility by about 38% (Budd 1963) due to 

differences between the first collision and multicollision dose around the 1.2-MeV mean energy of the 

neutrons from the PuF4 source (see Figure 5-1).

It was surprising to find in this study that calibration of NTA film with a Po–Be or Am–Be neutron 

source using a first collision dose and no phantom or a multicollision dose and a phantom would only 

make a difference of a few percent in the calibration factor for the NTA film (i.e., the dose equivalent 

per unit track density in the film) or the recorded neutron dose equivalent to a worker at Y-12. This 

surprising observation resulted from the fact that the first collision and multicollision doses in rem are 

essentially the same due to the cross over of the first collision dose and the multicollision dose near 

the 4-MeV mean energy of the neutrons from either a Po–Be or Am–Be neutron source (see Figure 5-

1) and that the number of tracks in the NTA film from exposure either with or without a phantom vary 

by only 5% (Thomas et al. 1997). Therefore, it is possible to use the energy response of NTA film 

exposed to fast neutrons on a phantom to make a reliable estimate of the missed dose for various 

neutron energy spectra at Y-12. Neutrons from a Po–Be or Am–Be source were used in the 

calibration of NTA film dosimeters at Y-12 because these sources were considered to provide a good 

match to neutron spectra found in the workplace at that facility, particularly the stray neutron field 

about the Oak Ridge 86-Inch Cyclotron (see Section 2.0). The Oak Ridge 86-Inch Cyclotron was the 

primary source for neutron exposure to Y-12 workers during the 1950s and early 1960s (ORAUT

2005a).
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 16 of 23

Figure 5-1. Comparison of first collision dose coefficients from NBS 

Handbook 75 (NBS 1961) and multicollision dose coefficients from

NCRP Report 38 (NCRP 1971) and IAEA TRS No. 318 (IAEA 1990).

6.0 MISSED DOSE CALCULATIONS WITHOUT ASSUMING A FIXED THRESHOLD 

ENERGY

Calculations of the missed neutron dose without assuming a fixed threshold energy for NTA films 

were made using the following equation:

Missed Dose (S ) Dose Equivalent (S ) - NTA Tracks (S ) Dose Equivalent (S )

NTA Tracks (S ) X X X

C

C

=

where Sx designates calculations using the neutron spectra for the source of interest and Sc

designates calculations using the neutron spectra for the source used to calibrate the NTA film 

(typically 210Po–Be or 241Am–Be). The NTA films were usually calibrated in air (no phantom), but the 

results of this study show that there is no significant statistical difference in response between NTA 

film calibrated in air or on a phantom using either a 210Po–Be or 241Am–Be source (see Section 5.0). 

Therefore, it is possible to calculate both “Dose Equivalent” terms using the dose coefficients from 

NCRP Report 38 (NCRP 1971; also see Table 2 of IAEA 1990) and both NTA Tracks terms in the 

above equation using the NTA response coefficients from Sayed and Piesch (1974; also see Table 3 

of IAEA 1990). 

The Missed Dose (Sx) in percent was calculated using the following equation:

Percent Missed Dose (Sx) = 100 1 - NTA Tracks (Sx)

Dose Equivalent (Sx)

Dose Equivalent (Sc)

NTA Tracks (Sc)





 




Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 17 of 23

Figure 6-1 shows the results of MCNP calculations for the sources discussed in Section 2.0 without 

assuming a fixed threshold energy for the NTA films. This figure suggests a mean neutron energy of 

about 400 keV for the neutron spectra at measurement points inside the CEF based on a measured 

missed neutron dose of approximately 55% (see Section 2.0). Figures 4-6 and 6-1 show the results 

of the calculations, which suggest that it is possible to make reliable estimates of neutron dose to 

Y-12 workers by adjusting their recorded neutron doses using the missed dose values in this 

document. The missed dose values in Figure 6-1 are recommended for use in neutron dose 

reconstruction for Y-12 workers during the film badge dosimetry period starting in 1950 and ending in 

1979 (ORAUT 2005a,b,c). These missed dose values are as follows: 21% for the unshielded 252Cf 

fission neutron source, 28% for the 86-Inch Cyclotron, 51% for the shielded radionuclide sources at 

the Health Physics Calibration Laboratory, and 54% for the highly enriched UF4 and UO3 storage 

containers. Based on these data, dose reconstructors should apply a correction of [100/(100 –

Percent Missed Dose)] to all recorded NTA film dosimeter results for Y-12 workers.

Figure 6-1. Missed neutron dose calculated without assuming a fixed 

threshold energy for NTA film.

7.0 ANGULAR RESPONSE OF NTA FILM

NTA film response is dependent on the angle of incidence of the neutrons (IAEA 1970). The angular 

dependence is due to several factors including the backscattering of neutrons from the body of the 

worker wearing the dosimeter, the geometry of the film, and that the proton recoil ions generated in 

the hydrogen rich material surrounding the film produce a large portion of the tracks in the film. The 

geometry of the film suggests that an angular dependence would be likely (i.e., the response of the 

film would vary for different angles of incidence in relation to the plane of the film). Neutrons at nearly

parallel incidence on the NTA film pack produce proton recoil ions that have a greater probability of 

escape from the film emulsion before producing observable tracks than do proton recoil ions from 

neutrons at nearly normal incidence on the film pack. The problem of angular dependence is further 

complicated because a large fraction of the proton tracks in the emulsion are produced by proton 

recoil ions generated in the hydrogenous material of the wrapper for the NTA film pack.
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 18 of 23

The angular dependence of NTA film has been studied experimentally by Cheka (1954), Lehman 

(1961), Piesch (1963; also see Figure 3.8 in IAEA 1970) and Kathren, Prevo, and Block (1965). In 

most workplace environments, neutrons will be incident on the NTA film dosimeter at widely variant

angles, and dose reconstructors will underestimate a worker’s neutron dose if the neutrons from a 

calibration source were perpendicularly incident on the NTA film dosimeter (i.e., the normal practice 

for calibration of NTA film dosimeters). Therefore, Kathren, Prevo, and Block (1965) suggested that 

multiplication of tracks produced by perpendicularly incident neutrons from a calibration source by a 

factor 0.75 would eliminate the underestimation of the dose equivalent to workers in such an 

environment. This is equivalent to multiplying the recorded dose equivalent from a worker’s NTA film 

dosimeter by a factor of 1.3. Dose reconstructors should apply this correction to all recorded NTA film 

dosimeter results for Y-12 workers.

8.0 DISCUSSION

Table 8-1 summarizes correction factors for missed neutron dose due to the threshold energy and 

angular response of the NTA film dosimeters used at Y-12. Dose reconstructors should apply these 

factors to all recorded quarterly NTA film results for Y-12 workers to ensure that the estimated neutron 

doses are claimant-favorable. These factors should also be applied to missed neutron dose from 

recording a zero for a worker when the NTA film dosimeter indicated a quarterly neutron exposure 

less than the minimum detectable limit (MDL) of approximately 50 mrem (ORAUT 2005a,b). To 

assign missed neutron dose for measurements below the MDL for the NTA film dosimeters, the dose 

reconstructor must first determine if a person worked near neutrons and the relevant category of 

neutrons (see Table 8-1). The best approach is to look for the work location and whether the worker 

or others in the badge reporting group (or department) were assigned any neutron dose equivalent. If 

no neutron dose was assigned to the worker or to coworkers for several quarters, the dose 

reconstructor should assume that the worker was not exposed to neutrons, so there would be no 

missed neutron dose.

Table 8-1. Recommended correction for missed neutron dose due to threshold energy and angular 

response of NTA film.

Neutron sourcea

Correction factor for 

threshold energy

Correction factor for 

angular response

Total correction 

factor

Unshielded radionuclide sources 1.0 1.3 1.3

Cf-252 fission neutron source 1.3 1.3 1.7

86-Inch Cyclotron 1.4 1.3 1.8

Shielded radionuclide sourcesb 2.0 1.3 2.7

Enriched uranium storage containers 2.2 1.3 2.9

CEFc 2.2 1.3 2.9

a. See Section 2.0 and Table 2-1.

b. Health Physics Calibration Laboratory.

c. Normally inhabited locations inside CEF.

If a worker was exposed in an area with a highly degraded energy spectrum, most of the neutron dose 

might not be detected by the NTA film. However, exposure to highly degraded spectra of neutrons at 

Y-12 was the exception rather than the rule, and missed neutron dose is limited because of a very low 

potential for neutron exposure at Y-12. For example, the interaction of alpha particles from uranium 

with the nuclei of fluorine or other low-atomic number (Z) atoms generates neutrons with energies of 

approximately 2 MeV (DOE 2004). The potential for neutron exposure depends on both the total 

activity of the uranium (which is a function of enrichment) and the chemical compound in question (the 

mixing of uranium and fluorine or other low-Z atoms). In general, the exposure potential of workers to 

neutrons generated by (α, n) reactions is low unless their work assignments require them to spend 

significant time near storage or process areas for large quantities of highly enriched uranium 
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 19 of 23

compounds (DOE 2004). Therefore, the neutron fields of concern in chemical processing areas at 

Y-12 were not highly degraded in energy and are adequately defined by the PNL measurements near 

UF4 and UO3 storage containers at Y-12 (see Figure 2-2).

The techniques used here to investigate missed neutron dose due to the threshold energy and 

angular response of NTA film dosimeters were applied to neutron sources in use at Y-12 during the 

film badge dosimetry period from 1948 to 1980. These techniques appear to have useful application 

in investigations of the missed dose from neutron sources at other U.S. Department of Energy (DOE)

sites of interest. However, the application of techniques used here to investigate missed neutron 

dose might not be useful at some facilities, particularly exposures of a worker to a highly moderated 

neutron fission spectra from nuclear reactors. For example, a PNL measurement near a K-reactor at 

the Savannah Ridge Site indicated a neutron energy spectrum with a mean energy of only 170 keV 

(ORAUT 2005d). Based on the data in Figure 4-6, approximately 85% of the neutron dose would 

have been missed by a dosimeter containing NTA film. Based on the data in Figure 6-1, an NTA film

dosimeter calibrated with a Pu–Be or Am–Be source would have missed essentially 100% of the 

neutron dose from the K-reactor. In such cases the dose reconstructor should rely on an alternative 

technique such as the use of a neutron-to-gamma dose ratio for exposures at that DOE facility and a 

film badge estimate of the worker’s dose from gamma rays to make a reliable estimate of the worker’s 

dose from neutrons (ORAUT 2005d).
Document No. ORAUT-OTIB-0051 Revision No. 00 Effective Date: 05/15/2006 Page 20 of 23

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