TO: Advisory Board on Radiation and Worker Health Work Group on TBD-6000 

FROM: Robert Anigstein and John Mauro, SC&A 

SUBJECT: Review of Skin Doses at GSI 

DATE: December 5, 2013 

Review of Skin Dose Calculations for General Steel Industries 

1 Updated Analysis of Skin Dose 

During the ongoing exchange of information between NIOSH and SC&A regarding the 

assessment of doses to the skin of betatron operators from irradiated steel castings, we reviewed 

and updated our previous analyses, as discussed below. 

We begin with a step-by-step derivation of the doses to the skin from irradiated steel and 

uranium. 

1.1 Original SC&A Skin Dose Model 

As stated by SC&A (2008, pp. 25–26), 

In calculating the concentrations of residual nuclides, we note that the same portion of 

a steel plate could be exposed to the betatron beam several times, both because of 

overlapping shots and because repeated radiographs were performed as flaws were 

repaired and checked. . . . The casting could be returned to the betatron room 5–10 

times before leaving the plant. Given the possibility of four overlapping shots 

irradiating the same area and an average of 71⁄2 examinations, we assumed that the 

HY-80 steel had been exposed for 30 h (4 × 7.5 × 1 h = 30 h). . . . 

Most of the photonuclear interactions in steel, and hence most of the production of 

residual nuclides, would be near the surface of the metal and near the center of the 

betatron x-ray beam. To avoid diluting the activity by averaging over a large volume 

of metal, most of which would have low levels of activation, we restricted the 

analysis to a small, relatively thin disk: a right circular cylinder, 5 cm thick, with a 

radius of 8.255 cm (3.25 in). This radius encompasses the most intense part of the 

beam. The resulting specific activities are thus characteristic of the central, most 

highly exposed portion of the steel. 

The SC&A (2008) analysis of skin doses from photoactivation products in irradiated steel that 

was based on the two scenarios—short and long shots—used a hybrid approach. According to 

SC&A (2008, pp. 37–38), 

About 10% of the radiographs of steel castings were long shots, while the remainder 

were short shots. . . . The long shots included 15 min of setup and one hour of 

exposure, while the short shots took a total of 15 min. Thus, 10 average shots 

included one long shot, which took 75 min, and 9 short shots which took 135 min 

Review of Skin Doses at GSI -1- SC&A –December 5, 2013
 

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.
 
 

 

 

(9 × 15 = 135), for a total of 210 min. As a result, about 36% of the time 

(75 ÷ 210 = 0.36) was spent on long shots and 64% on short shots. 

We will first explain how the two distances of the steel from the betatron target—9 ft for short 

shots and 6 ft for long shots—were addressed in the SC&A (2008) analysis. The exposure 

geometry in our original MCNPX simulation of photoactivation placed the casting 6 ft from the 

point the measuring string was attached to the outside of the betatron—this was based on a 

misunderstanding of the measuring procedure employed at GSI. We later learned from Jack 

Schuetz (former Allis-Chalmers engineer and a NIOSH consultant) that the string was calibrated 

to indicate the distance to the internal betatron target, so that the shots described as 6 ft were 

actually 6 ft (182.88 cm) from the target, not 234.95 cm, as specified in our model. Rather than 

repeat the MCNPX run, we increased the results by applying the inverse square law. Such a 

correction factor essentially contracts the radius of the steel disk to match the angular width of 

the x-ray beam that it intercepts. Since the area and hence the mass (the thickness being held 

constant) of such an adjusted disk varies with the square of the distance, and since the specific 

activities of the residual nuclides are inversely proportional to the mass, such an adjustment 

factor yields a reasonable approximation of the specific activities, the only neglected factor being 

the different amounts of air attenuation over different distances. To evaluate this last effect, we 

observe that the peak energies for photoactivation of the nuclides in the skin dose assessments, 

which are listed in Table 1, are in the range of 15–20 MeV. The linear attenuation coefficient of 

17.5 MeV photons (the midpoint of the energy range) in air is ~0.0021 m-1. Their attenuation in 

52 cm, the difference between the two distances, is ~0.11%, which is entirely negligible in the 

present analysis. 

We increased the specific activities from the long shots by multiplying the specific activities 

derived from the MCNPX simulation by the ratio of the square of the distance used in the 

simulation—234.95 cm—to that of the actual distance—6 ft (182.88 cm); however, in the 

interest of a conservative assessment, we did not decrease the activities for the short shots, which 

were at a distance of 9 ft (274.32 cm) from the target. We then derived an average of the specific 

activities by weighting the activities by the fractions of time devoted to the short and long shots. 

The exposure duration of the betatron operator to irradiated steel, 11.4 min, was an average of 

the setup and handling during short shots (11 min) and long shots (15 min), weighted by the 

relative numbers of short and long shots, 90% and 10%. We assumed that the operator was in 

the immediate proximity of the steel for only one-half of this time. 

1.2 First Revision to Skin Doses from Irradiated Steel 

Anigstein and Olsher (2012) repeated the MCNPX simulation of residual nuclides using the 

MCNPX ver. 2.7.E. We placed the steel disk at a distance of 6 ft (182.88 cm) from the target, 

obviating the need for any adjustment for long shots, and assumed that all the castings were 

exposed at this distance, avoiding the complication of interspersing radiographic exposures of 

different durations and at different distances. Other aspects of the analysis were unchanged. The 

result was intended to be a bounding, claimant-favorable assessment. 

Review of Skin Doses at GSI -2- SC&A – December 5, 2013
 

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.
 
 

 

 

 

c 

Table 1. Residual Nuclides in Steel: Sources of External Exposure to Beta Radiation 

Nuclide Half-life Peak energy a 

(MeV) 

μεt 

b

 (cm2

/g) 

Sp. act.c (Bq/g) 

Shorte

 Longe 

Time-integrated sp. act.d 

(Bq·h/g) 

Short Long 

Fe-53 8.51m 19.6 0.0321 304.43 684.96 36.61 98.18 

Mo-91 15.49m 16.0 0.0311 33.55 75.49 4.84 13.70 

Ni-57 35.6h 17.9 0.0316 20.54 46.20 3.76 11.52 

Mn-56 2.58h 19.6 0.0321 18.57 41.77 3.32 10.10 

Mo-99 65.94h 15.9 0.0310 9.55 21.49 1.75 5.37 

Cr-49 42.3m 19.8 0.0322 4.23 9.52 0.71 2.11 a Energy of peak photonuclear cross-section for production of given nuclide; assumes photoneutrons from 54Fe produced 56Mn by the reaction 55Mn(n,γ)

56Mn 

b Mass attenuation coefficient corresponding to peak energy for HY-80 steel, interpolated from Storm and Israel (1970) 

Specific activity at surface immediately after irradiation 

d Specific activity integrated over exposure duration of betatron operator 

e Short and long shots—see text 

1.3 Final Revision to Skin Doses to Betatron Operators from Irradiated Steel 

Following our review of the NIOSH assessment of doses to the skin of betatron operators from 

photoactivation products in irradiated steel, we performed a more comprehensive analysis of skin 

doses from both the long and short shots. We modeled each scenario (i.e., short shots and long 

shots) separately and apportioned the resulting doses according to the fraction of time taken up 

by both types of shots. 

We began by repeating our simulation of the photoactivation of the HY-80 steel alloy commonly 

produced at GSI, using MCNPX 2.7.0, the latest publicly released version of the code. This 

version also had the advantage of an updated library of photonuclear cross-sections, containing 

an expanded list of nuclides—the input file was modified to explicitly take advantage of these 

data. We ran the code for 2.147 billion histories, very close to the code maximum, to obtain the 

optimum precision in the results. 

The results of the new analysis listed 37 radionuclides that were produced during the betatron 

irradiation of HY-80 steel. From this list, we identified 27 nuclides that are significant β

emitters. We ranked these 27 radionuclides in descending order of activity integrated over the 

period from 5 s to 11 min or 15 min following irradiation, depending on the scenario. Since the 

first six nuclides in this list accounted for ~99.5% of the total integrated activities, we selected 

these nuclides, which are listed in Table 1, for the assessment of β doses to the skin. These are 

the same six nuclides identified in our prior analyses. 

Employing the same methodology discussed by SC&A (2008, pp. 28–29), we used the total 

activities in the steel that we derived from the new MCNPX simulation to calculate the specific 

activities at the surface by adjusting for the exponential decrease of the x-ray beam over the 5-cm 

Review of Skin Doses at GSI -3- SC&A – December 5, 2013
 

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.
 
 

 

 

thickness of the steel disk, as listed in columns 5–6 in Table 1. The last two columns list the 

specific activity integrated over the setup time in each scenario. The specific activities for the 

long shots were calculated directly from the MCNPX results; those for the short shots were 

scaled by the ratio of the squares of the distances: 62

 ÷ 92 = 0.444. 

Upon reviewing the MCNPX simulations of doses to the skin from the six nuclides listed in 

Table 1, performed in January 2008, we observed several areas for improvement that we 

incorporated in the revised analyses. The more significant of these are described below. The 

energy spectrum of the β rays from 53Fe had been estimated from a graphical representation of 

the spectrum— tabulated data were not available to us at that time. Since then, ICRP (2008) has 

issued a database from which we extracted the spectrum in tabular form. Furthermore, 53Fe and 

91Mo were originally modeled as electron emitters (β−

), when in fact they emit positrons (β+). 

MCNPX uses the same model for the attenuation of electrons and positrons; however, the 

annihilation of positrons is accompanied by the emission of two 511-keV γ rays, which generate 

electrons through photoionization and Compton scatter that can make further contributions to the 

skin dose. We repeated the analyses for the three nuclides—53Fe, 56Mn, and 91Mo—that together 

account for 98%–99% (depending on the scenario) of the dose to the skin of the betatron 

operators. The analyses of all three nuclides utilized the corresponding ICRP (2008) beta 

spectra. 

The NIOSH scenario for the exposure of the betatron operator to irradiated metal specified that 

the worker was at a distance of 1 ft from the metal for one-half of the exposure time and at a 

distance of 1 m for the other half. However, previous simulations of skin doses from 

radionuclides in steel did not include assessments at 1 m. The doses calculated for the skin of 

hands and forearms in contact with the metal, and for other skin at a distance of 1 ft, were 

divided by 2, implying that the dose at 1 m was zero. To correct this oversight, we included 

assessments at 1 m in the new simulations of skin doses from 53Fe, 91Mo, and 56Mn. Based on 

extrapolations from the analyses at other distances, these nuclides are estimated to contribute 

~99% of the dose at 1 m. Given the small contribution of the dose rate at this distance to the 

overall skin dose assessment, omitting the contribution of the other nuclides has a negligible 

impact on the overall uncertainty of this assessment. 

Dose rates were calculated for bare skin in contact with the metal, skin in contact with the metal 

but covered by one layer of clothing, and cloth-covered skin at 1 ft (30.48 cm) and at 1 m from 

the metal. The number of shots during one 8-h shift was calculated by dividing the duration of 

the shift by the total time for one shot—15 or 75 min. The new analyses took advantage of some 

MCNPX features that increased the accuracy of the simulations of electron/positron transport, as 

well as improved geometry that is more representative of the area of exposed skin. 

The results of our analyses are presented in Table 2. 

Review of Skin Doses at GSI -4- SC&A – December 5, 2013
 

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.
 
 

 

 

Table 2. Dose Rates to Skin of Betatron Operators from Beta Radiation from HY-80 Steel 

Distance Shield Dose (mrad/shift) 

Shorta

 Longa Composite 

Contact None 9.35 5.11 7.84 

Contact Cloth 7.98 4.36 6.69 

1 ft Cloth 5.77 3.15 4.83 

1 m Cloth 2.11 1.15 1.76 

a Short and long shots—see text 

The last column in Table 2 lists the composite doses from both scenarios—the time-weighted 

average of the doses from short and long shots. 

1.4 Skin Doses to Betatron Operators from Irradiated Uranium 

We next repeated the MCNPX analyses of the generation of residual nuclides from the betatron 

radiography of uranium slices. The simulation that was the basis of our previous assessments of 

skin doses from irradiated uranium had originally been performed using MCNPX ver. 2.6.E. In 

that version of the code, the generation of residual nuclides was not specific to the specific 

region of interest in the metal being irradiated, requiring the analyst to use his judgement to 

determine which nuclides were in the metal. The new version of the code identifies the nuclides 

in the user-specified region (i.e., the uranium slice). In the process, we changed the thickness of 

the uranium disk from 0.5 cm to 5 cm. A 5-cm thickness of uranium absorbs ~99.5% of 

13.1 MeV photons—the center of the peak energy of the total photonuclear cross-section of 

238U—as compared to an absorption of ~42% in 0.5 cm of the metal. The greater thickness 

specified in the simulation represents a more realistic scenario, and also leads to greater 

efficiency in the MCNPX analysis. Other enhancements were specifying uranium as an 

electrical conductor, and air as a gas. These lead to minor changes in the interactions of these 

materials with incident radiation and thus yield a more realistic analysis. As before, we found 

that the two principal β-emitting radionuclides produced by photoactivation were 237U and 239U. 

The revised specific activities are presented in Table 3. 

We next performed an MCNPX simulation of skin doses from β rays emitted from a uranium 

slice. The exposures were to the side as well as to the front face of the cylindrical slab. The 

radiation source in the exposures at the side of the metal comprised the short-lived β-emitting 

progeny of 238U and 235U, enhanced by the Putzier effect. The β-ray spectrum is thus that of 

these progenies in secular equilibrium with the parent nuclides, which have the relative 

abundance of naturally-occurring uranium; however, the results of the assessments are multiplied 

by a factor of 12.5 to account for the enhancement of these progenies at the surface of the 

uranium ingot during casting. Because the activities of these β-emitting progenies would far 

exceed the activities of the photoactivation products, whose concentrations exponentially 

decreased over the 4-inch thickness of the slab, the latter nuclides were not incorporated in the β

spectrum from the cylindrical surface (i.e., at the side of the slice). 

Review of Skin Doses at GSI -5- SC&A – December 5, 2013
 

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.
 
 

 

 

Table 3. Residual Nuclides in Uranium: Sources of External Exposure to Beta Radiation 

Peak a μεt

b Sp. act.c TWA sp. act.d 

Nuclide Half-life energy (cm2

/g) (Bq/g) (Bq/g) (MeV) 

U-237 6.75 d 19.9 19.9 13.1 0.0559 

U-239 23.45 m 9,718 7,830 

a Energy of peak of total photonuclear cross-section of 238U 

b Mass attenuation coefficient corresponding to peak energy for uranium, interpolated from Storm and 

Israel (1970) 

c Specific activity at surface immediately after irradiation, assuming an exponential profile over a 5-cm 

thickness 

d Time-weighted average: specific activity averaged over exposure duration of betatron operator 

The simulation of skin doses from the front face of the uranium slice utilized a combined β

spectrum comprising the short-lived uranium progenies as well as the β spectra of 237U and 239U. 

The relative intensities of the β emissions from the latter isotopes were derived from the timeweighted average specific activities listed in Table 3. The simulation used the same tallies and 

exposure geometry as the analyses of the β-emitting nuclides in steel, described previously. The 

calculated skin doses from exposure to a uranium slice are presented in Table 4. 

The final results of the skin dose assessment are listed in Table 5. The annual number of shifts 

devoted to the handling of uranium was based on the revised hours of uranium handling listed by 

Anigstein and Mauro (2013, Table 1). The remainder of the shifts were assumed to be devoted 

to the radiography of steel. 

2 Conclusions 

In the course of the continuing exchange of information between SC&A and NIOSH, most 

outstanding differences in the calculation of β doses to the skin of betatron operators appear to 

have been resolved. The latest exchange of spreadsheet calculations shows complete agreement 

on the methodology of calculating the doses. Assuming that NIOSH adopts or replicates out 

MCNPX simulations, and that the uranium work hours differences are resolved, SC&A and 

NIOSH will be in agreement regarding the limiting doses to the skin from β radiation, which are 

those experienced by a betatron operator. 

Review of Skin Doses at GSI -6- SC&A – December 5, 2013
 

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.
 
 

 

 

 

Table 4. Dose Rates to Skin from External Exposure to Beta Radiation from Uranium Slice 

Dose Rates to Skin Location Orientation (mrad/h) 

Front 119 

Contact Side 1,428 

Average 774 

Front 95 

Cloth Side 1,094 

Average 594 

Front 23 

1 ft Side 65 

Average 44 

Front 2.5 

1 m Side 8.9 

Average 5.7 

Average for bare skin 389.6 

Average for other skin 24.8 

Table 5. Annual Doses to Skin of Betatron Operators from Beta Radiation (rads) 

Year Annual number of shifts Hands and forearms Other skin 

Uranium Steel Uranium Steel Total Uranium Steel Total 

1952-1957 54.7 351.6 31.96 2.76 34.72 2.03 1.70 3.73 

1958 46.5 359.8 27.17 2.82 29.99 1.73 1.74 3.46 

1959-1960 42.2 364.1 24.66 2.85 27.51 1.57 1.76 3.33 

1961 48.4 357.8 28.31 2.80 31.11 1.80 1.73 3.53 

1962 35.2 371.1 20.55 2.91 23.45 1.31 1.79 3.10 

1963 9.6 396.7 5.59 3.11 8.70 0.36 1.92 2.27 

1964 3.5 402.7 2.05 3.16 5.21 0.13 1.95 2.08 

1965 2.6 403.7 1.50 3.16 4.66 0.10 1.95 2.05 

1966a 0.8 202.3 0.47 1.59 2.06 0.03 0.98 1.01 

a

 During contract period January 1–June 30 

Review of Skin Doses at GSI -7- SC&A – December 5, 2013 

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 

Anigstein, R., and J. Mauro. 2013. “SC&A Response to NIOSH White Paper: ‘Summary Dose 

Estimates for GSI’.” Memo to Advisory Board on Radiation and Worker Health Work Group on 

TBD-6000. http://www.cdc.gov/niosh/ocas/pdfs/abrwh/scarpts/sca-gsisde-100613.pdf 

Anigstein, R., and R. H. Olsher. 2012. “Response to ‘Battelle-TBD-6000 Appendix BB General 

Steel Industries: Dose Estimates for Betatron Operations.’” 

http://www.cdc.gov/niosh/ocas/pdfs/abrwh/scarpts/sca-gsibeta-r0.pdf 

International Commission on Radiological Protection (ICRP). 2008. “Nuclear Decay Data for 

Dosimetric Calculations,” ICRP Publication 107. Annals of the ICRP 38, (3). Tarrytown, NY: 

Elsevier Science, Inc. 

S. Cohen & Associates (SC&A). 2008. “Review of ‘Site Profiles for Atomic Weapons 

Employers That Worked Uranium and Thorium Metals - Appendix BB: General Steel 

Industries,’ Battelle-TBD-6000, Appendix BB, Rev. 0” (March 17, 2008). 

http://www.cdc.gov/niosh/ocas/pdfs/abrwh/scarpts/sca-gsisp-r1.pdf 

Storm, E., and H. I. Israel. 1970. “Photon Cross Sections from 1 keV to 100 MeV for Elements Z 

= 1 to Z = 100.” Nuclear Data Tables, A7, 565-681. 

Review of Skin Doses at GSI -8- SC&A – December 5, 2013
 

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.