| New product "Pro-GR" |
| Nippon Electric Glass Co.,
Ltd. has developed the "Pro-GR" lead glass for shielding against
gamma rays in PET treatment. Pro-GR is made of glass materials having
a lead oxide content rate of roughly 70% that is equivalent to ultra-high
lead content block glass for nuclear power facilities. Pro-GR has
a radiation shielding capability higher than that of the conventional
LX-57B. |
We have calculated the effective dose transmission factor of 0.511
MeV gamma ray through our Pro-GR and LX-57B lead glass taking into account
the build-up factor so that Pro-GR and LX-57B lead glass can be used
safely and appropriately as shielding glass in PET treatment facilities.
Primary Penetrated Rays and Scattered
Rays
As shown in the figure below, gamma rays are radiated in all directions
from a subject dosed with medicine in PET treatment, and some of those
gamma rays pass through radiation shielding material (e.g. lead, lead
glass, concrete) as primary penetrated rays that go straight through
the radiation shielding material or as scattered rays that come out
after being scattered and their direction changed inside the radiation
shielding material. As PET uses radiation of such a relatively high
energy as 0.511 MeV, the amount of scattered rays is comparatively large.
Primary Penetrated Rays
and Scattered Rays
Build-up Factor
For this reason, in designing radiation protection with a radiation shielding
material, it is not sufficient to calculate only the primary penetrated
rays in formula (1) below. The effective dose transmission factor must
be calculated taking the scattered rays into account in formula (2) below.
| Formula (1)
: Formula for calculating the primary penetrated ray dose rate |
| II0~e-Κx |
I F Dose rate after penetration@Κ F Linear attenuation
coefficient
I0 F Dose rate at incidence@ F Thickness of radiation
shielding material |
Formula (2) : Formula for calculating the effective dose transmission factor
with scattered rays factored in |
| IB~I0~e-Κx |
B F Build-up factor |
In formula (2), build-up factor (B) expresses the increase in dose caused
by scattered rays. B is always bigger than 1, and its value tends to be
bigger, the lower the density of the radiation shielding material is or
the thicker the radiation shielding material is. The build-up factor varies
depending on the radiation source, and the properties, density and thickness
of the radiation shielding material. In making a shielding calculation
for gamma rays using lead glass, the build-up factor specific to the lead
glass to use must be figured out depending on the radiation source to
shield.
Shielding Performance of Pro-GR
and LX-57B against Gamma Rays
In calculating the shielding performance of Pro-GR and LX-57B lead glass
against 0.511 MeV gamma rays, we adopted Monte Carlo simulation to calculate
the build-up factor of each respective glass, and calculated the effective
dose transmission factor in accordance with the "Manual for Shielding
Calculation of Radiation Facilities, 2000" (issued by the Nuclear Safety
Technology Center). In calculating the lead equivalent of Pro-GR and LX-57B,
we designed the thickness of Pro-GR and LX-57B so that the effective dose
transmission factor is equivalent to the effective dose transmission factor
of lead calculated taking the build-up factor into account as shown on
the following table.
Effective Dose Transmission
Factor of Lead for 0.511 MeV Gamma Rays, and
Thickness of Pro-GR and LX-57B Corresponding to Each Thickness of
Lead
| Lead |
LX-57BAPro-GR |
| Lead Thickness (mm) |
Effective Dose
ransmission Factor (%) |
Glass Thickness (mm)
Corresponding to
Effective Dose
Transmission Factor
of Lead
on Left
|
Recommended LX-57B and
Pro-GR to Use |
| 2@m1.10n |
78.6 |
LX-57B@8.2@m1.19n |
LX-57B@9.0mm |
| 3@m1.14n |
69.0 |
LX-57B@12.0@m1.27n |
LX-57B@14.0mm |
| 5@m1.21n |
52.6 |
Pro-GR@14.2@m1.33n |
Pro-GR@14.7mm |
| 7.5@m1.25n |
36.0 |
Pro-GR@ 21.3 m1.45n |
Pro-GR@21.8mm |
Numbers in blue in brackets [ ] are reference
values, and are the build-up factors of lead,
LX-57B and Pro-GR. |
| Pro-GR Product Data |
| Max. size |
1000 x 1500 mm (42" x 60") |
| Specific gravity |
Min. 5.20 |
| Visible light transmission |
83 % |
"Seeds" or minute air bubbles exist inside Pro-GR as it is made
of special glass materials. This, however, does not adversely
affect its radiation shield performance. |
| ReferenceFEffective
Dose Transmission Factor of X Rays |
In the case of 0.2 MeV X rays, the effective
dose transmission factor of lead of thickness
2 mm, 3 mm, 5 mm, and 7.5 mm is 11.8%, 4.1%, 0.48%, and
0.033%, respectively. |
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