Dosimetric Quantities and Neutron Spectra Outside the Shielding of Electron Accelerators Page: 3 of 5
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2.4. Ambient Dose Equivalent or Effective Dose?
Effective Dose is the legally limiting quantity.
Operational quantities are allowed in practice, if they
provide an overestimation of the protection quantity.
However, above about 40 MeV Ambient Dose
Equivalent is not conservative with respect to the
maximum ("worst") value of Effective Dose (Fig. 7)
-
E 1---
1oI -. I A.": 101- I~ ,.v
Figure 7. Comparison of H*(10) (blue curve) and E(worst) (red)
The calculated ratio between H*(10) and Effective Dose
E is shown in Fig. 8. Ambient Dose Equivalent is found to
underestimate Effective Dose by more than 10% in most
cases (1 and 10 GeV electron beam, concrete shielding
thicker than 1 m).1.1
1.r:e I kk 2
Figure 8. Ratio of calculated H*(10) and E(worst) as a function of
electron energy and shielding thickness
It can be seen that H*(10) is always conservative in the
case of a 100 MeV electron beam, it underestimates E for
shielding thicknesses larger than 60 cm in the case of 1 and
10 GeV, and the maximum underestimation is about 15%.
On the other hand, the usual properties of protection and
operational dosimetric quantities are not applicable to
neutrons: both kinds of quantities can neither be measured
nor be calculated directly, both can be derived from
calculated fluences by means of conversion coefficients,
and both can be measured with some approximation using a
calibrated instrument with a suitable energy response (the
Andersson-Braun response can be adjusted to fit either
H*(10) or E).
In addition, for neutrons with energies larger than a few
tens of MeV, which constitute an important fraction of theradiation fields at high energy accelerators, the operational
quantity H*(10) is not conservative with respect to the
protection quantity E. Therefore: why use H*(10), an
approximation which is not even conservative, when with
the same techniques one can obtain directly E? E(worst) is
a much better choice, since it is always conservative by
definition.
2.5. Attenuation length
In the literature, different values have been reported for
the attenuation length at equilibrium of neutron dose at
about 90 in electron accelerator shielding. Table 1 shows
some of the reported values as a function of electron
energy.
Table 1. Values of attenuation lengths reported in the literature .
Ref. Electron energy (GeV) k (g/cm2)
7 0.027 28
8 0.4 35
9 0.75 96
10 3.0 110
11 6.3 91
12 10 105
13 15 104
14 20 120
15 20 120
16 28.7 116
17 30 94
18 100 115
Because the attenuation at equilibrium is governed by
neutrons of similar energies, it is generally believed that the
equilibrium attenuation length should be equal to that
found at proton accelerators, which has been evaluated to
be 117 2 g/cm2 [19].~E
f/
Is
SiekI Ihknr 9n
Figure 9. Neutron dose attenuation length
electron energy and shielding thickness1CC1 V"c'
2a
as a function ofHowever, early calculations seemed to indicate that the
equilibrium attenuation length at electron accelerators
would be much shorter, and a "conservative" value of 100
g/cm2 was recommended by Tesch in 1988 [20], based on1030
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Fasso, Alberto; Lab, /SLAC /Jefferson; Liu, James C.; Rokni, Sayed H & /SLAC. Dosimetric Quantities and Neutron Spectra Outside the Shielding of Electron Accelerators, article, October 8, 2012; United States. (https://digital.library.unt.edu/ark:/67531/metadc846542/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.