Contact Dose Rates from Encapsulated Sources
Edward Waller & James Cleary
Ron Goans
Faculty of Energy Systems and Nuclear Science
REAC/TS
www.nuclear.uoit.ca
orise.orau.gov/reacts
Overview: Significant levels of secondary electron radiation are emitted from the surface of sealed sources. Accurate quantification
of the hazard is essential to accurate contact dose estimation. The contribution of secondary electrons to the total dose
rate is examined and variation from previously accepted values for contact dose rates are presented.
Objective
Methodology
The Monte Carlo radiation transport code MCNPX 2.6e,
was used to create encapsulated source models based
on those reported by Benner (1941) and Quimby,
Marinelli, & Blady, (1939)
• Determine the contribution of secondary electron
radiation in the contact dose rate and compare to
published values for 137Cs, 60Co, 192Ir and 226Ra
tissue eqv.
0.007 cm
Tissue eqv.
backing 1 cm
• Generate revised contact dose rates from Monte Carlo
modeling software and compare this to results
published in NCRP Report No. 40
Source
Original secondary electron
curves (circa 1940s)
wall
material
filter 2
• Forward and backward electron emissions were modeled
and compared to literature (Wilson,1941) with good
agreement.
Gamma Ray Dose Constant at 1 metre
S h-11 MBq
MB -11
mSv
ORNL
MCNPX
%diff
1.07E-04 1.01E-04
6.3
3.69E-04 3.73E-04
1.0
1.63E-04 1.51E-04
7.2
3.13E-04 3.00E-04
4.0
Cs-137
Co-60
Ir-192
Ra-226
Relative Dose From Secondary
Electrons
• Relative contribution of secondary electrons were
modeled and compared to literature (Quimby,
Marinelli, & Blady, 1939) with good agreement
Relation between the atomic number and the
measured dose for emergence and incidence
emissions
1.20E+00
1.9
1.10E+00
1.7
1.00E+00
1.5
9.00E-01
1.3
8.00E-01
1.1
7.00E-01
0.9
6.00E-01
5.00E-01
0.7
0
MCNPX
Quimby
1939
10
20
30
40
50
60
Atomic Number (Z)
70
80
60
80
100
Backward
Emission
Surface Dose Rates
Isotope
0
40
Forward
For
ard
Emission
• Contact dose rates were modeled and found to be a factor
of 3-4 times lower than those published in NCRP 40.
Secondary Electron Emission as a Function
of Z
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
20
Atomic Number of Shield Material
Ratio: Backward Emission with Graphite
• Initially, MCNPX was used to generate gamma
dose rate constants, which were found to be within
10% agreement of published values (Unger &
Trubey, 1982)
Ratio: Forward Emissio
on to Graphite
Results
Isotope
filter 1
90
Cs-137
Co-60
Ir-192
Ra-226
mSv h-1 MBq-1
NCRP40 MCNPX
%diff
8.32
1.83
78
33.6
9.88
71
13.2
2.92
78
21.2
7.67
64
Conclusions
References
[1] Benner, S. (1931). On Secondary β-Rays from the Surface of Radium Containers. Acta
Radiologica [Old Series], 401-412.
[2] Quimby, E. H., Marinelli, L. D., & Blady, J. V. (1939). Secondary Filters in Radium Therapy. San
Francisco: American Radium Society.
[3] Unger, L. M., & Trubey, D. K. (1982). Specific Gamma - Ray Dose Constants for Nuclides
Important to Dosimetry and Radiological Assessment. Oak Ridge: Oak Ridge National Laboratory.
[4] Wilson, C. (1941). The Dependence of the Secondary Electronic Emission Produced by Gamma
Radiation Upon the Direction of the Radiation. London: Physics Department, Westminster Hospital.
It has been found that NCRP40 published contact dose rates
are higher by a factor of 3-4 than those estimated in this work.
The implication is that dose calculations based on NCRP40
values will overestimate dose and lead to underestimated risk
when compared to biological indicators.
The authors wish to acknowledge support from the Natural Sciences and Engineering Research Council (NSERC)
and the University Network of Excellence in Nuclear Engineering (UNENE)