Appendix CRF - Part 3 - Northamptonshire County Council

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Figure 3: Dose points for bulk waste bag
2.5 MicroShield calculation details and uncertainties
Energy deposition to dose rate conversion was performed automatically in MicroShield
using built-in tables of effective dose rate, taken from ICRP-51 [3]. This presents a
series of possible dose rates depending on the assumed irradiation geometry. The
highest biological dose rate is produced assuming anterior-posterior geometry (with the
gamma rays entering a person from the front and exiting through the back), and to be
conservative it was this maximum dose rate that was reported. Dose rates can vary by
approximately 30%, depending on which geometry is assumed.
MicroShield approximates the contribution of scattered radiation to the resulting dose
rate by the use of build-up tables. The dose rate is dependant on which material is
chosen as the dominant scattering medium. In accordance with the MicroShield
manual, the material containing the highest number of gamma ray mean free paths
should be used as the build-up material – hence in these cases, the source was
chosen as build-up material. If the air gap is chosen as the scattering medium, it was
found that the resulting dose rates increased by 6% for 60 Co cases, and increased by
12% for 137 Cs cases, but these results would be over-pessimistic.
MicroShield uses a point-kernel integration technique to determine the dose rate. This
involves splitting the geometry into pieces (kernels). The quadrature order of the
calculation determines the number of kernels used and hence the accuracy of the
approximation; the default quadrature order was used for the reported results. The
order of the calculation was increased by a factor of two in each dimension, and the
contact results only increased by 0.3%, which is well within the range of other sources
of uncertainty in the calculation. Further increases in accuracy produced no change to
the results.
In all cases assessed, the ‘contact’ dose rate point was actually positioned at 1 cm
from the surface, as the method of calculation used by MicroShield is known to become
unstable at distances closer than 1 cm, though this will strongly depend on the
integration order used..
137 Cs is a beta emitter. Its daughter, 137m Ba is the source of the gamma radiation.
Where a source containing 137 Cs was specified, its daughter product 137m Ba was also
included in equilibrium concentration with 137 Cs. Since the half-life of 137m Ba is short
(2.5 minutes), it will almost always be found in equilibrium with its parent radionuclide.
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