Draft White Paper 

SENSITIVITY OF NTA FILM TO NEUTRON SOURCES AT 

MOUND LABORATORY 

Revision 1 

Contract Number 200-2009-28555 

Prepared by 

Robert Anigstein and Dick Olsher 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, Virginia 22182 

Saliant, Inc. 

5579 Catholic Church Road 

Jefferson, Maryland 21755 

July 2010 

Disclaimer 

This document is made available in accordance with the unanimous desire of the Advisory Board on 

Radiation and Worker Health (ABRWH) to maintain all possible openness in its deliberations. However, 

the ABRWH and its contractor, SC&A, caution the reader that at the time of its release, this report is predecisional and has not been reviewed by the Board for factual accuracy or applicability within the 

requirements of 42 CFR 82. This implies that once reviewed by the ABRWH, the Board’s position may 

differ from the report’s conclusions. Thus, the reader should be cautioned that this report is for 

information only and that premature interpretations regarding its conclusions are unwarranted.
Record of Revisions 

Revision 

Number 

Effective 

Date Description of Revision 

0 (Draft) 05/24/2010 Initial issue—distributed on June 4, 2010, following DOE clearance 

1 (Draft) 07/22/2010 Revised correction factors in table 3 and minor editorial changes and format 

corrections. 

Sensitivity of NTA Film–Rev. 1 -ii- July 23, 2010 

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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
SENSITIVITY OF NTA FILM TO NEUTRON SOURCES AT MOUND LABORATORY 

SC&A has performed a set of analyses to determine the relationship between the doses inferred 

from NTA films at the Mound Laboratory and the actual doses to the monitored worker from 

external exposure to neutron radiation. These analyses were performed to resolve some 

questions that were raised at the meeting of the Advisory Board's Work Group on Mound on 

January 5, 2010, regarding the MCNP modeling of neutron spectra by Faust et al. (2009). 

Background 

Dosimeters containing Eastman nuclear track emulsion Type A (NTA) film were used to monitor 

external exposures to neutron radiation of Mound personnel from February 1949 through 

November 1977. Initially, the dental-type film packets were contained in a steel badge copied 

from a design used at ORNL in 1948. ‘This . . . badge, consists of a small steel box measuring 

5/16" x 1 3/8" x 1 7/8".’ One-millimeter-thick cadmium shields covered one half of both the 

front and rear surfaces of the film packets, while the other half was unshielded. This 

configuration was used until September 1, 1968, when a new film badge dosimeter modeled after 

the one then used at the Nevada Test Site was introduced. This “NTS-type” badge used “a ten 

mil [0.25 mm] lead and a plastic insert over the [NTA] film.” (Meyer 1994) 

The energy response of the NTA film was long known to depend on the neutron spectra to which 

the badge was exposed. Mound therefore sought to calibrate the films using sources that were 

similar to the neutron emitters that were being processed on site. Initially, the calibration sources 

were 210Po-Be. Effective January 1, 1963, NTA films of workers in the SM areas were assessed 

using a 238PuF4 calibration source. Meyer (1994, p. 17)1

 states his belief that neutron exposures 

were underestimated since the start of operations in early 1962; however, there is no indication 

that these early doses were corrected. Around January 1963, the average neutron energy in the 

SM Building was measured to be 0.75 MeV. However, we calculated the average energy of a 

bare 238PuF4 source to be 1.3 MeV,2 while that of a 210Po-Be source is 4.7 MeV. Consequently, 

both calibration sources lead to an underestimate of the actual neutron exposures of the workers 

in that location. 

Subsequent to that date, film calibrated using 210Po-Be sources was used as the reference for 

reading film badges at some work locations, notably the T and SW Buildings, while 238PuF4 was 

used as the calibration standard for NTA film from other locations at Mound. Beginning on 

August 9, 1965, “all neutron exposures received in the plant [were to be] estimated on the basis 

of 238PuF4 neutron calibrations.” (Meyer 1994) 

The assumed relationship between neutron flux and dose rate also varied over the years. Starting 

August 1, 1949, a flux of 150 n s-1 cm-2 was assumed to deliver a dose of 300 mrem per 40-h 

work week. The corresponding dose conversion factor is 1.39 × 10-5 mrem per n/cm2. Over the 

next 20 years, the assumed DCF, used to convert the reading of the NTA film to doses to 

 

1

 The pagination refers to the numbers that appear on the bottom of the pages in Meyer’s report of March 1994. This 

report is dispersed throughout seven SRDB pdf files. The pages are not contiguous, and sometimes out of order. 2

 This is confirmed by measurements performed at Mound (Meyers 1994, Vol. 1, p. 302 of PDF file SRDB 3268). 

Sensitivity of NTA Film–Rev. 1 -1- July 23, 2010 

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However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
workers, was changed periodically, based on measurements of the average neutron energies in 

various locations in the plant. The date the change was implemented, the neutron flux 

corresponding to 7.5 mrem/h (i.e., 300 mrem per 40-h work week), and the resulting DCF, are 

listed in table 1. 

Table 1. Neutron Dose Conversion Factor (DCF) Used to Calibrate NTA Film at Mound 

Datea Fluxb

 (n s-1 cm-2) 

DCF 

(mrem per n/cm2) 

8/01/1949 150 1.39e-05 

10/01/1951 75 2.78e-05 

1/01/1955 35 5.95e-05 

1/25/1956 30 6.94e-05 

1/1/1959 55 3.79e-05 

8/09/1963 70 2.98e-05 

12/1/1969 55 3.79e-05 

1/1/1970c 27.5 7.58e-05 

Source: Meyer 1994

a

 Effective date of calibration factor

b

 Neutron flux corresponding to 300 mrem per 40-h work week

c

 All previously reported neutron doses for 1970–1976 were doubled according to 

directive of March 15, 1978

The NTA film calibrations were performed using bare sources. Meyer (1994) cites one instance 

in which a 238PuF4 source was placed inside polyethylene spheres, but this appears to have been 

done for the measurement of neutron energy spectra. However, workers were exposed to 

neutron sources involving various amounts of Benelex shielding (a cellulose laminate). 

Cellulose, whose empirical formula is C6H10O5, has the effect of reducing the neutron flux due 

to its high hydrogen content, but also of shifting the spectrum to lower energies. Since NTA film 

is less sensitive to lower-energy neutrons, the shielding could lead to an underestimate of neutro

doses delivered to workers if these doses were assessed by comparing the films to films 

calibrated with a bare source. 

n 

After development, the NTA films were read under a microscope. Initially, the tracks in each of 

10 static fields of vision were counted—the results were recorded as the average number of 

tracks per field. After March 10, 1967, the film was scanned, still under visual inspection, but 

allowing larger areas to be read. The tracks per field on the calibration film, combined with a 

knowledge of the neutron flux, the exposure duration, and the assumed DCF, were used to 

construct a calibration curve. The doses to individual workers were read from this calibration 

curve, based on the average tracks per field on the worker’s NTA film. 

Method of Analysis

We used MCNPX to simulate the energy-dependent neutron flux from the calibration sources 

used at Mound, as well as the flux at the location of a film badge worn by a worker exposed to 

neutron sources in various exposure geometries. Lehman (1961) proposed the concept of the 

track unit to quantify the response of NTA film packets to neutrons of various energies, and 

Sensitivity of NTA Film–Rev. 1 -2- July 23, 2010 

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presented a model for calculating the response, based on the physical properties of the film 

packets.3

 We applied the Lehman model, incorporating updated neutron cross-sections of 

hydrogen, to compare the tracks per unit exposure observed on the films worn by workers in our 

model geometries to the tracks on the calibration films, and calculated correction factors for 

various combinations of calibration sources and postulated workplace exposures. 

The exposures in our MCNP model were assumed to occur in a room in the shape of a hollow 

cylinder, with a radius of 5 m and a height of 3 m, dimensions which correspond to a typical 

industrial facility. The walls, floors, and ceiling were composed of concrete 1 ft (30.48 cm) 

thick. The source was located along the axis of the cylinder, 131 cm above the floor (a typical 

elevation of a film badge worn on the chest of a worker standing erect). The flux across the 

calibration film was calculated in air at the same elevation, 34.5 cm from the source. This 

distance was the radius of the film badge calibration rack (see “Neutron Film Calibration,” 

Meyer 1994, Vol. 1, p. 255 of PDF file). The simulations included the two types of calibration 

sources used at Mound: 210Po-Be and 238PuF4. 

The flux across the film badge worn by the worker was calculated at the same elevation. To 

include a realistic simulation of backscatter from the worker’s body, the flux was calculated just 

in front of the chest of a human body. The body was represented by a mathematical phantom 

generated by BodyBuilder, a commercial computer program from White Rock Science (2004); 

this model is based on the description by Eckerman et al. (1996). The phantom was positioned 

facing the source, at a distance of either 60 or 240 cm from the source. To simulate the effect of 

various thicknesses of shielding, successive simulations modeled a bare source or a source 

surrounded by a spherical volume of water with a radius of 2 to 12 inches (5.08–30.48 cm), in 

2-inch (5.08-cm) increments. The simulations modeled three sources—210Po-Be, 238PuO2, and 

238PuF4—which typify the sources of neutron exposures at Mound. 

The unattenuated neutron spectra generated by these three sources were calculated using the 

computer code Sources-4C (LANL 2002). The output of this code was entered into MCNPX. 

The neutron flux from the various simulations of 210Po-Be and 238PuO2 sources was tallied in 

15-keV-wide bins, while the flux from 238PuF4, spanning a much narrower energy range, was 

tallied in 5-keV-wide bins. The energy range began at 0.4 MeV, the threshold response of the 

NTA film according to the Lehman (1961) model. The upper end was the effective upper limit 

 

3

 Lehman defines the track unit as “the number of tracks per cm2 of emulsion (normalized to the most frequent 

emulsion thickness, 33 μm) resulting from 10,000 neutrons per cm2

 incident normally on the back of the film packet.” It is 

not clear why the concept should be limited to neutron incident on the back of the film packet, since most neutrons would 

strike the film from the front. We have generalized the concept to include neutrons from all directions, and normalizing to 

1 rather than 10,000 neutrons. 

Sensitivity of NTA Film–Rev. 1 -3- July 23, 2010 

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of the spectrum calculated by Sources-4C: 10.87 MeV for 210Po-Be, 11.245 MeV for 238PuO2, 

and 3.745 MeV for 238PuF4.

4

According to Lehman (1961), the tracks produced by the hydrogen nuclei (i.e., protons) in the 

film emulsion, resulting from neutrons impinging on the emulsion, can be expressed as: 

te = track units originating in emulsion per incident neutron

He = atom density of hydrogen in emulsion 

 = 3.5 × 1022 atoms/cm3

H(E) = elastic scattering cross-section for hydrogen for neutrons of energy E (barns) 

(NNDC n/d) 

d = thickness of emulsion 

= 0.0033 cm 

Em = threshold energy of neutron producing detectable track 

= 0.4 MeV 

E = energy of incident neutron (MeV) 

In addition to tracks produced by neutrons colliding with hydrogen nuclei in the film emulsion, 

tracks are also produced by neutrons interacting with hydrogen in the film base and in the 

wrapper. Lehman models this phenomenon using the following equation, which he attributes to 

J. E. Cook: 

tr = track units originating in radiator (film base + paper wrapper) per incident 

 = iator (weighted combination of hydrogen 

nd paper wrapper) 

 = 3.8 × 1022 atoms/cm3 

 

neutron 

Hr atom density of hydrogen in rad

densities in film base a

4

 Our analyses assumed that the oxygen isotopes in the 238PuO2 sources were present in their natural abundances. We 

later noted that, according to Faust et al. (2009), the Mound sources were enriched in 17O and 18O to enhance the neutron 

yield. Our subsequent analysis of the neutron spectrum of 238PuO2 enriched in these isotopes showed that, although the 

absolute neutron yield was greatly increased, the energy distribution was little changed. The average energy of the 

neutrons from the isotopically enriched source is 2.374 MeV, compared to 2.329 MeV for the source with natural 

abundances. Since it is the relative rather than the absolute intensities of the neutrons in the spectrum that is relevant to 

our analysis, this difference would have little effect on our results. 

Sensitivity of NTA Film–Rev. 1 -4- July 23, 2010 

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n = 1.63 (0.4 < E < 2) 

 = 1.77 (2 < E < 10) 

 = 1.81 (E > 10) 

R(E) = range of recoil protons

 = aEn

 a = 0.00177 (0.4 < E < 2) 

 = 0.0016 (2 < E < 10) 

 = 0.00146 (E > 10) 

The total film response can be expressed by the quantity tu = te + tr, as given by equations 1 

and 2. Lehman further notes that equation 2 is strictly valid only if the range of the recoil proton 

does not exceed the thickness of the radiator. According to Meyer (1994), the NTA film packet 

was inserted into the badge behind the film used to detect photon radiation. We will assume that 

the photon film packet was made of similar materials as the NTA packet, except that, as reported 

by Meyer, it contained two films: a sensitive and an insensitive film. The total thickness of the 

radiator in front of the NTA emulsion, comprising the photon film packet and the front wrappers 

of the NTA packet, is estimated to be 0.0751 cm, which corresponds to the maximum range of a 

proton with an energy of 8.8 MeV. The maximum energy of a recoil proton is approximately 

equal to that of the incident neutron. Ninety percent of the unattenuated neutron spectrum of the 

210Po-Be source lies below this energy, as does 99.97% of the 238PuO2 spectrum and all of the 

238PuF4 spectrum. The two equations can therefore be used to estimate the track units registered 

by the NTA film. 

The ambient dose equivalent at a depth of 10 mm, H*(10), per source neutron was calculated at 

the receptor location for each exposure scenario, comprising two distances, seven source 

geometries, and three neutron sources. The doses were calculated by applying the ambient dose 

equivalent per unit neutron fluence, listed by ICRP (1996, Table A.42), to the neutron fluence in 

air computed by MCNPX. (Since the Mound calibration procedures for NTA film were 

performed in air, the phantom was omitted from these simulations.) 

The next step in deriving correction factors for the NTA film readings was to calculate an NTA 

film calibration factor for each combination of source, shield, and position, based on the MCNP 

simulations of the respective exposure geometries and using the track units calculated using the 

Lehman model as a surrogate for the actual track counts. Such a calibration factor is given by 

the following expression: 

fijk = calibration factor for neutrons from source i, attenuated by shield thickness j, at 

position k (mrem per track unit) 

dijk = dose per neutron from source i, attenuated by shield thickness j, at position k, as 

calculated by MCNP (mrem/n) 

Sensitivity of NTA Film–Rev. 1 -5- July 23, 2010 

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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
tijk = track unit per neutron from source i, attenuated by shield thickness j, at position k, 

as calculated by equations 1 and 2, utilizing the results of MCNP simulations 

Next we needed to derive a surrogate for the calibration factors used at Mound, based on the 

Mound calibration procedures and the MCNP-calculated track units to represent the actual track 

counts. 

 Fmnr = derived Mound calibration factor for source m, based on dose conversion factor 

corresponding to flux n, using a calibration rack of radius r

cr = geometrical factor used to calculate neutron flux at radius r from point source at 

center 

 r = 34.5 cm (Meyer 1994) 

D = “tolerance” dose 

= 300 mrem per 40-h week 

kt = factor to convert mrem per 40-h week to mrem/s 

= 40 × 3600 = 144,000 s per work-week 

tm = track units in calibration film from source m at distance r

n= neutron flux n assumed to correspond to “tolerance” dose D (see table 1) 

The correction factors that should be applied to each NTA film for each neutron source in the 

workplace, calibrated against one of two neutron calibration sources used at Mound, assuming 

two locations with reference to the source and various shield thicknesses, are given by the 

following expression: 

Cijkmnr = correction factor for NTA film exposed to neutrons from source i, attenuated by 

shield thickness j, at position k, which had been assessed by reference to 

calibration film exposed to source m in a rack with radius r, using a dose 

conversion factor based on neutron flux n. 

Sensitivity of NTA Film–Rev. 1 -6- July 23, 2010 

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Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
Results of MCNP Analysis

Table 2 lists the correction factors to the NTA films that were evaluated using a calibration curve 

based on a 210Po-Be source, calculated using the methodology described above. The correction 

factors depend on the assumed neutron flux that corresponds to 300 mrem per 40-h work week, 

as listed in Table 1. The factors also depend on the actual neutron source to which the worker 

was exposed, and the amount of hydrogenous material shielding that source (expressed as inches 

of water). The dates span the period from the beginning of neutron dosimetry at Mound until the 

date when NTA films were calibrated exclusively with 238PuF4 sources. Plutonium-239 was first 

present at Mound in 1956, while both 238Pu and 239Pu were used in 1959 and later years.5

 

Consequently, the correction factors for 238PuO2 and 238PuF4 for time periods prior to 1956 are 

displayed with a shaded background—they are included for reference and completeness, but do 

not play a significant role in the dose assessments. Table 3 lists the correction factors to the 

NTA films that were evaluated using a calibration curve based on a 238PuF4 source. The dates 

span the period from the earliest date that 238PuF4 calibration sources were used at Mound until 

the time that the NTA films were replaced with TLDs. 

These correction factors do not account for the fading of the latent images on the NTA film 

between the time of exposure and development, which is discussed in a later section of this 

report, nor for the angle of incidence, also discussed later. 

Track Fading

Another factor compromising neutron exposure assessment using NTA film is proton track 

fading. Track fading—the fading of the latent image between the time of exposure and the 

development of the film—depends on both the elapsed time between exposure and development 

and on the energy imparted by the incident neutron to the proton: the lower-energy tracks, 

comprising fewer dots, disappear faster than the higher-energy tracks. Mound dosimetry 

personnel became aware of this phenomenon in 1967. A study of track fading in NTA film 

exposed to a 238PuF4 source, performed in the summer of 1967 and issued as a formal Mound 

report on July 1, 1968, concluded that 33% of the tracks faded after 1 week and 56% after 2 

weeks. We independently analyzed the data presented in that report and derived the following 

expression: 

n(t) = number of tracks after time t (d) 

n0 = number of tracks at t = 0 

λ = track fading decay constant 

 = 0.059888/d 

 

5

 We did not perform an explicit analysis of 239Pu, inasmuch as most references in Meyer 1994 are either to unspecified 

Pu or to 238Pu. However, we did calculate the neutron spectrum from 239PuF4 and found that its average energy is 1.24 

MeV, about 5% less than that of 238PuF4. Thus, the correction factors for 239Pu sources might be somewhat greater than for 238Pu. 

Sensitivity of NTA Film–Rev. 1 -7- July 23, 2010 

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Table 2. Correction Factors to NTA Film Calibrated with 210Po-Be Sources at Mound 

Dates: 8/49-9/51 10/51-12/54 1/55-1/24/56 1/25/56-12/58 1/59-8/8/63 8/9/63-8/8/65 

Flux (n s-1 cm-2)

a

: 150 75 35 30 55 70 210Po-Be 

H2O shield (in) Worker position: 60 cm 

0 2.8 1.4 0.7 0.6 1.0 1.3 

2 3.2 1.6 0.8 0.6 1.2 1.5 

4 3.4 1.7 0.8 0.7 1.2 1.6 

6 3.4 1.7 0.8 0.7 1.2 1.6 

8 3.4 1.7 0.8 0.7 1.2 1.6 

10 3.4 1.7 0.8 0.7 1.2 1.6 

12 3.3 1.7 0.8 0.7 1.2 1.5 

Observer position: 240 cm 

0 3.7 1.8 0.9 0.7 1.3 1.7 

2 4.2 2.1 1.0 0.8 1.5 2.0 

4 4.4 2.2 1.0 0.9 1.6 2.1 

6 4.4 2.2 1.0 0.9 1.6 2.1 

8 4.5 2.2 1.0 0.9 1.6 2.1 

10 4.4 2.2 1.0 0.9 1.6 2.1 

12 4.5 2.3 1.1 0.9 1.7 2.1 238PuO2

Worker position: 60 cm 

0 4.2 2.1 1.0 0.8 1.6 2.0 

2 4.9 2.5 1.1 1.0 1.8 2.3 

4 5.1 2.5 1.2 1.0 1.9 2.4 

6 5.1 2.6 1.2 1.0 1.9 2.4 

8 5.2 2.6 1.2 1.1 1.9 2.4 

10 5.3 2.6 1.2 1.1 1.9 2.5 

12 5.2 2.6 1.2 1.0 1.9 2.4 

Observer position: 240 cm 

0 5.3 2.7 1.2 1.1 1.9 2.5 

2 6.4 3.2 1.5 1.3 2.3 3.0 

4 6.7 3.3 1.6 1.3 2.4 3.1 

6 6.6 3.3 1.5 1.3 2.4 3.1 

8 6.9 3.4 1.6 1.4 2.5 3.2 

10 6.7 3.4 1.6 1.3 2.5 3.1 

12 5.8 2.9 1.3 1.2 2.1 2.7 238PuF4

Worker position: 60 cm 

0 5.7 2.8 1.3 1.1 2.1 2.6 

2 6.6 3.3 1.5 1.3 2.4 3.1 

4 6.9 3.5 1.6 1.4 2.5 3.2 

6 6.8 3.4 1.6 1.4 2.5 3.2 

8 6.8 3.4 1.6 1.4 2.5 3.2 

10 6.5 3.3 1.5 1.3 2.4 3.0 

12 6.2 3.1 1.4 1.2 2.3 2.9 

Observer position: 240 cm 

0 7.2 3.6 1.7 1.4 2.7 3.4 

2 8.6 4.3 2.0 1.7 3.1 4.0 

4 9.4 4.7 2.2 1.9 3.5 4.4 

6 9.4 4.7 2.2 1.9 3.4 4.4 

8 10.1 5.0 2.4 2.0 3.7 4.7 

10 9.6 4.8 2.2 1.9 3.5 4.5 

12 8.5 4.2 2.0 1.7 3.1 3.9

a

 Neutron flux equal to 300 mrem per 40-h work week 

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Table 3. Correction Factors to NTA Film Calibrated with 238PuF4 Sources at Mound 

Dates: 1/63–8/8/63 8/9/63–11/30/68 12/1/68–12-31-69 1970-77 

Flux (n s-1 cm-2)

a

: 55 70 55 27.5 

238PuO2

H2O shield (in) Worker position: 60 cm 

0 0.8 1.0 0.8 0.4 

2 0.9 1.1 0.9 0.4 

4 0.9 1.1 0.9 0.5 

6 0.9 1.2 0.9 0.5 

8 0.9 1.2 0.9 0.5 

10 0.9 1.2 0.9 0.5 

12 0.9 1.2 0.9 0.5 

Observer position: 240 cm 

0 0.9 1.2 0.9 0.5 

2 1.1 1.4 1.1 0.6 

4 1.2 1.5 1.2 0.6 

6 1.2 1.5 1.2 0.6 

8 1.2 1.5 1.2 0.6 

10 1.2 1.5 1.2 0.6 

12 1.0 1.3 1.0 0.5 

238PuF4

Worker position: 60 cm 

0 1.0 1.3 1.0 0.5 

2 1.2 1.5 1.2 0.6 

4 1.2 1.6 1.2 0.6 

6 1.2 1.5 1.2 0.6 

8 1.2 1.5 1.2 0.6 

10 1.2 1.5 1.2 0.6 

12 1.1 1.4 1.1 0.5 

Observer position: 240 cm 

0 1.3 1.6 1.3 0.6 

2 1.5 1.9 1.5 0.8 

4 1.7 2.1 1.7 0.8 

6 1.7 2.1 1.7 0.8 

8 1.7 2.2 1.7 0.9 

10 1.7 2.2 1.7 0.8 

12 1.5 1.9 1.5 0.7 

a

 Neutron flux equal to 300 mrem per 40-h work week. 

Based on this derivation, we predict fading of 34% after 1 week and 87% after 2 weeks, a minor 

discrepancy with the Mound report. The square of the product-moment (Pearson) correlation 

coefficient derived from our analysis, R2

 = 0.985, indicates an excellent fit of equation 6 to the 

11 data points. 

No action was taken on this finding by Mound until July 15, 1968, when the dosimetry 

supervisor directed that all weekly and visitor neutron film badges processed after that date were 

to be corrected for 33% fading, and 2-week badges for 56% fading. Based on an earlier track 

Sensitivity of NTA Film–Rev. 1 -9- July 23, 2010 

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fading study, which is not further documented, 4-week badges were also to be corrected for 56% 

fading. 

During the summer of 1968, a second study was performed of track fading in NTA film exposed 

to an attenuated 238PuO2 source. The aim of this study was to determine an energy dependence 

of track fading. That study, which was never issued as a formal Mound report, produced 

inconclusive results. 

An undated, unsigned document included in Meyer 1994, and assumed by Meyer to postdate the 

studies discussed above, reports another study of track fading in NTA film exposed to a 238PuF4

source. In this study, the films were exposed on a daily basis on each weekday for periods of 1, 

2, 3, or 4 weeks, and processed the following Monday. Such an experiment more closely 

simulated the actual exposures of films worn by workers than the study in the July 1, 1968, 

report, in which each film was subjected to a single exposure. The report concludes that, on the 

basis of five data points—the baseline exposure with no delay from the exposure time to 

development plus the four points corresponding to delays of 1–4 weeks—the films faded an 

average of 9% per week. Our independent analysis of the data presented in this report yields a 

fading rate of 9.55%, a minor discrepancy. 

In order to make a meaningful comparison with the July 1, 1968, report, we used the results of 

our analysis of the data in the latter report to simulate the fading of films worn by workers during 

the normal work-week. We assumed that the films were exposed to a uniform neutron flux for 8 

hours, Monday–Friday, during the day shift, for either 1, 2, or 4 weeks, and that they were 

developed on the Monday following the monitoring period at the start of the same shift. This is 

consistent with the film badge monitoring procedures described in Meyer (1994, Vol. 1). The 

fading of the films was calculated from the following expression: 

f(τ) = fraction of tracks faded during monitoring period τ

n = number of shifts during monitoring period τ

ti = time from beginning of i-th shift until film is processed 

ti-1⁄3 = time from end of i-th shift until film is processed 

The factor 3 accounts for the length of the shift as a fraction of a 24-h day. The decay constant λ

is derived in equation 6. Applying equation 7, we derived fading fractions of 25% for 1-week 

films, 38% for 2-week films, and 55% for 4-week films. This is far higher than the fading 

measured in the later study. 

In November 1968, Mound began to expose calibration films daily over 1–4-week periods and 

used these films to calibrate personnel dosimeters worn during corresponding monitoring periods 

in order to correct for the fading of the films worn by the workers. 

Sensitivity of NTA Film–Rev. 1 -10- July 23, 2010 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
Angular Dependence

Kathren (1965) discusses the angular dependence of NTA film, and concludes that the number of 

tracks in the film exposed to a calibration source should be multiplied by 0.75 to account for the 

random angles of incidence on the film worn by the monitored personnel for neutrons of energy 

above 3.0 MeV, which would lead to an overestimate of the doses from neutrons, En < 3 MeV. 

Such a correction would thus produce a claimant-favorable dose estimate. 

Conclusions and Recommendations

The results of the MCNP analysis show that correction factors due to the differences between the 

neutron spectra experienced by the workers and the spectra used to calibrate the film, as well as 

the varying neutron flux DCFs employed at Mound during different periods of operation, span a 

range of 0.6–4.7. Since one cannot know the neutron spectrum to which a worker was exposed, 

we recommend that the highest correction factors for each time period be applied. For periods 

from August 1949 until January 24, 1956, the correction factors listed in Table 2 for the 

“observer“ exposed to 210Po-Be shielded by 8 inches of water should be applied to neutron doses 

during each period listed at the top of this table. From January 24, 1956, until August 8, 1965, 

the correction factors listed in Table 2 for the “observer“ exposed to 238PuF4 shielded by 8 inches 

of water should be used. From August 8, 1965, until the end of 1977, the correction factors 

listed in Table 3 for the “observer“ exposed to 238PuF4 shielded by 8 inches of water should be 

applied to neutron doses during each period listed at the top of this table. 

Given the range of results of the track fading studies and our subsequent analysis, we 

recommend that the most claimant-favorable correction factors be applied to the neutron doses 

prior to July 15, 1968. The Mound procedure to correct future NTA film reports—applying a 

correction of 33% to 1-week films, and 56% to 2-week and 4-week films—is consistent with the 

assumption that the entire dose is received at the beginning of the 1-week or 2-week monitoring 

periods, but that it is delivered uniformly during each workday to workers on a 4-week 

monitoring schedule. This suggests that the study cited in the dosimetry supervisor’s July 15, 

1968, memo may have involved exposing films daily for a 4-week period, with results that were 

in complete agreement with our analysis of the July 1, 1968, report. The corrections that were 

applied by Mound following July 15, 1968, should be applied retrospectively to earlier neutron 

doses measured at Mound. These corrections are in addition to the correction factors derived 

from Tables 2 and 3. 

urce 

y 

The track fading study which forms the basis of the correction factors used a bare 238PuF4

source, with neutrons normally incident on the film. The average neutron energy of this so

is 1.3 MeV. As noted by Cusimano (1963), fading depends on the track length. A high-energ

neutron produces a longer track than a neutron of lower energy. Since tracks of fewer than four 

grains are attributable to background and are not counted, a high-energy track can lose a larger 

fraction of its grains due to fading and still be counted, while if a four-grain track loses only one 

grain, it will not be counted. Measurements of neutron energies in radiation controlled areas at 

Mound were performed at various times. In January 1963, the time that a 238PuF4 source was 

first used for film calibration, the average neutron energy in the SM Building was measured to be 

0.75 MeV. According to an unsigned memo dated 11/26/69: Repeated energy evaluations in 

radiation control areas have indicated that personnel are subject to exposure to neutrons with an 

Sensitivity of NTA Film–Rev. 1 -11- July 23, 2010 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
average energy of approximately 0.9 MeV (Meyer 1994, Vol. 8, p. 10). In January 1974, 

measurements in the R Building showed an average neutron energy range of 0.72–2.4 MeV, with 

a building average of 1.01 MeV. In February 1974, measurements in the PP Building showed an 

average neutron energy range of 0.5–1.2 MeV, with a building average of 0.8 MeV. Thus, the 

fading corrections, which are based on a 1.3 MeV spectrum, could underestimate the neutron 

doses to most workers in areas with lower-energy spectra. We have no sound basis for 

determining a fading correction under these circumstances. 

The practice of exposing calibration films over the same time periods that the workers were 

monitored would have been an accurate means of correcting for track fading, provided: 

(1) Personnel exposures were uniform during each workday during the monitoring period. 

(2) Neutron spectra in the workplace were the same as the spectrum of the calibration 

sources. 

We do not know if item 1 constitutes a valid assumption. It can be argued that, on average, the 

exposures over a period of one year would be uniformly distributed over the film badge 

monitoring period, provided that there were no systematic deviations: e.g., certain duties 

involving higher-than-normal exposures were always performed at the beginning of the week. 

With regard to item 2, the average energies of neutrons in the workplace during this period were 

definitely lower than that of the calibration source, so the tracks on the personnel badge might 

have faded faster than those on the calibration sources. 

In summary, we believe that the NTA film badge records can be used to reconstruct doses to 

workers who wore these badges, provided that appropriate correction factors are applied. We 

have some remaining concerns over the fading of films exposed to low-energy neutrons and 

believe that this issue merits further study. 

Sensitivity of NTA Film–Rev. 1 -12- July 23, 2010 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 
Sensitivity of NTA Film–Rev. 1 -13- July 23, 2010 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

References

Cusimano, J.P. 1963. “Latent Image Fading in Personal Neutron Monitoring Film at the 

NRTS,” IDO-12031. Idaho Falls, Idaho: U.S. Atomic Energy Commission, Idaho Operations 

Office. [SRDB Ref ID: 8135] 

Eckerman, K. F., M. Cristy, and J. C. Ryman. 1996. “The ORNL Mathematical Phantom 

Series.” http://homer.hsr.ornl.gov/VLab/mird2.pdf

Faust, L., R. Morris, and B. Smith. 2009. “Neutron Dose Reconstruction at Mound.” 

International Commission on Radiological Protection (ICRP). 1996. “Conversion Coefficients 

for use in Radiological Protection against External Radiation,” ICRP Publication 74. Annals of 

the ICRP, 26 (3/4). Tarrytown, New York: Elsevier Science, Inc. 

Kathren, R. L., C. T. Prevo, and S. Block. 1965. “Angular Dependence of Eastman Type A 

(NTA) Personnel Monitoring Film.” Health Physics 11, 1067-1069. 

Lehman, R. L. 1961. “Energy Response and Physical Properties of NTA Personnel Neutron 

Dosimeter Nuclear Track Film.” UCRL-9513. Berkeley, California: Lawrence Berkeley 

National Laboratory. http://www.escholarship.org/uc/item/8v94979t.pdf. 

Los Alamos National Laboratory (LANL) and Texas A&M University. 2002. SOURCES-4C: 

Code System For Calculating alpha,n; Spontaneous Fission; and Delayed Neutron Sources and 

Spectra, RSICC Code Package CCC- 661. Oak Ridge, Tennessee: Oak Ridge National 

Laboratory. 

Meyer, H. E. 1994. “History of Personnel External Dosimetry Program at the Dayton Project 

and Mound Laboratory -1946 to 1993.” (Vols 1–8). Miamisburg, Ohio: EG&G Mound Applied 

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National Nuclear Data Center (NNDC) n/d. “Interpreted ENDF file for sigma-ei.in” 

http://www.nndc.bnl.gov/sigma/getInterpreted.jsp?evalid=4208&mf=3&mt=2. 

White Rock Science. 2004. BodyBuilder (Version 1.30) [Computer software and manual]. Los 

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