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For Shielding Applications |
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| It is a well-known fact that X-Rays, unless properly
controlled and directed, are very harmful to the human body and have
even been known to cause death. As a consequence, various materials are
being utilizes for the insulating of the X-Ray tubes. There are two
general methods of securing this protection. One is by the use of
protective plasters, such as barium sulphate plaster, and the other is
the use of sheet lead. |
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| In the use of plaster there is always the danger that
improper mixing will leave parts of the plaster wall pervious to the
rays. It requires the most careful mixing to insure absolute uniformity,
and even then there is no way of telling whether this uniformity has
been obtained. Ideal temperature conditions also must be maintained
during the setting of the plaster. Furthermore, after the plaster is in
place any slight settling of the building, or some other cause, may
result in tiny cracks in the plaster, invisible to the eye, but
sufficient to allow the escape of the rays. Even assuming that the
plaster is properly mixed and applied, etc., it requires a much greater
weight and thickness of plaster than of lead to get the equivalent
protection. |
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| Sheet lead has none of these disadvantages and is
consequently considered the best possible protection. |
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| The Advisory Committee on X-Ray Protection of the
International Congress of Radiology specifically warns against the use
of protective plasters in installations of over 75 kV. |
|
| The thickness of lead required varies with the power
of the X-Ray apparatus. The thickness recommended by the Second
International Congress of Radiology are presented in the accompanying
table: |
|
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Recommended
Thickness of Lead Shielding for X-Ray Rooms
|
|
X-Rays generated by
peak voltages not
exceeding |
Minimum thickness of Lead |
Weight per Square
Foot in Pounds |
|
Millimeters (mm) |
Inches
|
|
75 kV |
1.0
|
0.039
|
2-1/2
|
|
100 kV
|
1.5 |
0.059
|
4
|
|
125 kV
|
2.0
|
0.079
|
5
|
|
150 kV
|
2.5
|
0.098
|
7
|
|
175 kV
|
3.0
|
0.118
|
8
|
|
200 kV
|
4.0
|
0.157
|
10
|
|
225 kV
|
5.0
|
0.197
|
13
|
|
300 kV
|
9.0
|
0.354
|
24
|
|
400 kV
|
15.0
|
0.591
|
38
|
|
500 kV
|
22.0
|
0.369
|
56
|
|
600 kV
|
34.0
|
1.343
|
81
|
|
900 kV
|
51.0
|
2.000
|
120
|
| |
|
Required Lead
Shielding Thickness for Gamma Rays |
|
| The table below, prepared by the National Bureau of
Standards, is frequently used in determining the thickness of lead
needed for shielding from gamma ray sources in laboratories. |
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| To use the table select the column for the energy
required. (Use next higher figure if exact value is not given.) The
entry gives thickness in centimeters of lead for different source
strengths at 1 meter for 8 hours per day to give 50 milliroentgens. Then
add algebraically the correction terms for other working ranges or times
to obtain the shield thickness required. Example: Shield is needed for
handling 500 millicuries of radioactive material emitting 1.8 million
electron volts (Mev) gamma rays at a minimum working distance of 50 cm,
and 4 hours per day. |
|
|
Shield thickness = A(8.60) + B(2.77) +
C(-1.39) = 9.98 cm of lead |
|
| In which A=basic entry, B=correction
for danger range, C=correction for 4 hours per day |
| |
| Energy (Mev)
|
|
Activity
|
0.2
|
0.5
|
0.8
|
1.0
|
1.5
|
2.0
|
2.5
|
3.0
|
4.0
|
|
10 mc
|
-0.14
|
-0.36
|
-0.27
|
-0.11
|
+0.37
|
+0.78
|
+1.15
|
+1.40
|
+1.70
|
|
20 mc
|
-0.09
|
+0.00
|
+0.41
|
+0.76
|
+1.57
|
+2.16
|
+2.63
|
+2.91
|
+3.21
|
|
50 mc
|
-0.01
|
+0.47
|
+1.31
|
+1.90
|
+3.15
|
+4.00
|
+4.57
|
+4.90
|
+5.20
|
|
100 mc
|
+0.06
|
+0.82
|
+1.99
|
+2.77
|
+4.34
|
+5.38
|
+6.05
|
+6.41
|
+6.71
|
|
200 mc
|
+0.10
|
+1.17
|
+2.67
|
+3.63
|
+5.54
|
+6.77
|
+7.52
|
+7.92
|
+8.21
|
|
500 mc
|
+0.17
|
+1.64
|
+3.57
|
+4.78
|
+7.12
|
+8.60
|
+9.47
|
+9.91
|
+10.21
|
|
1 c
|
+0.23
|
+1.99
|
+4.25
|
+5.65
|
+8.31
|
+9.99
|
+10.95
|
+11.41
|
+11.71
|
|
2 c
|
+0.28
|
+2.35
|
+4.93
|
+6.52
|
+9.51
|
+11.37
|
+12.42
|
+12.92
|
+13.22
|
|
5 c
|
+0.36
|
+2.81
|
+5.82
|
+7.66
|
+11.09
|
+13.21
|
+14.37
|
+14.91
|
+15.21
|
|
10 c
|
+0.41
|
+3.17
|
+6.50
|
+8.52
|
+12.28
|
+14.59
|
+15.85
|
+16.42
|
+16.72
|
|
20 c
|
+0.47
|
+3.52
|
+7.18
|
+9.39
|
+13.48
|
+15.98
|
+17.32
|
+17.93
|
+18.23
|
|
50 c
|
+0.54
|
+3.99
|
+8.08
|
+10.54
|
+15.06
|
+17.81
|
+19.27
|
+19.92
|
+20.22
|
|
100 c
|
+0.60
|
+4.34
|
+8.76
|
+11.40
|
+16.25
|
+19.20
|
+20.75
|
+21.43
|
+21.72
|
|
|
Danger
Range
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
|
20 cm
|
+0.26
|
+1.64
|
+3.16
|
+4.02
|
+5.55
|
+6.44
|
+6.85
|
+7.00
|
+7.00
|
|
50 cm
|
+0.11
|
+0.71
|
+1.36
|
+1.73
|
+2.39
|
+2.77
|
+2.95
|
+3.01
|
+3.01
|
|
1 M
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
|
2 M
|
-0.11
|
-0.71
|
-1.36
|
-1.73
|
-2.39
|
-2.77
|
-2.95
|
-3.01
|
-3.01
|
|
5 M
|
-0.26
|
-1.64
|
-3.16
|
-4.02
|
-5.55
|
-6.44
|
-6.85
|
-7.00
|
-7.00
|
|
10 M
|
-0.37
|
-2.35
|
-4.52
|
-5.76
|
-7.94
|
-9.21
|
-9.80
|
-10.01
|
-10.01
|
|
|
Working
time hr/day
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
Plus
|
|
1
|
-0.17
|
-1.06
|
-2.04
|
-2.60
|
-3.59
|
-4.16
|
-4.42
|
-4.52
|
-4.52
|
|
2
|
-0.11
|
-0.71
|
-1.36
|
-1.73
|
-2.39
|
-2.77
|
-2.95
|
-3.01
|
-3.01
|
|
4
|
-0.06
|
-0.35
|
-0.68
|
-0.87
|
-1.20
|
-1.39
|
-1.47
|
-1.51
|
-1.51
|
|
8
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
+0.00
|
|
24
|
+0.09
|
+0.56
|
+1.08
|
+1.37
|
+1.89
|
+2.20
|
+2.34
|
+2.39
|
+2.39
|
| |
| Notes: |
|
| (1) Source activity is quoted in millicuries (mc)
or curies (c), where 1 curie is that amount of radioactive material
that disintegrates at the rate of 3.7 x 1010 disintegrations per
second. However, the table is computed on the further assumption
that each disintegration yields one gamma photon of selected energy.
This will lead to inaccuracies whenever the disintegration is
complex. More accurate calculations can be made by obvious methods
when the disintegration scheme is known. |
|
| (2) The tabulation ignores the increased
effective transmission of shields under wide beam irradiation. |
|
| (3) This form of shielding table is intended to
form a guide to rapid erection of temporary shielding structures in
the laboratory. Where permanent installations of maximum economy are
planned, more detailed calculations by conventional methods are
required. |
|
| The United States Bureau of Standards in
discussing methods of protection says: “Sheet Lead is however the
safest and most permanent protection and should be used wherever
possible. Once installed it is much easier to determine its
efficiency and one has the assurance that it will never become
porous to Roentgen ray radiation”. The principal feature to watch in
effectively shielding X-Ray rooms with sheet lead is to see that no
small parts of any surface, such as a screw, bolt or nail holes with
or without screws, bolts or nails in them, or cracks around the
doors, are left without being completely shut off, or shielded, by
lead. There are a number of ways of satisfactorily lead shielding
X-Ray rooms and the most convenient may be chosen. Some recommended
methods are described in this article. |
|
| Sheet lead of the proper thickness may be applied
to 2” x 4” wooden vertical studs placed flush with the wall with the
long dimension of the sheets laid vertically. The edges of the
sheets are butted together or lapped, and
fastened by means of flat steel bolted or screwed to the studs, the
strip and bolt head being covered by the fold-over of the sheet.
Horizontal joints may be made in the same manner. It is preferable
to solder or “burn” the overlapping parts of the lead to the main
body so that the overlap will be held securely in place. In the
accompanying drawing 4’6” is shown as the maximum span unsupported.
This is a necessary precaution, as otherwise, the lead would have a
tendency to “crawl” under its own weight. Nails and screws for
fastening the lead are not recommended as the unit weight of lead in
relation to its sheer strength is such that there is great danger of
the lead tearing and thus leaving an exposed section through which
the rays may easily penetrate. In laying a floor it is not necessary
to use the steel straps, the lap joint here being quite sufficient. |
|
| The lead protection on the ceiling may be
obtained in two ways. If possible, it is simplest to lay sheet lead
on top of the floor slab above, allowing it to extend out at least
8” in every direction beyond the lead walls below it. Thus, though
there is a gap between the top of the lead wall and the lead on the
floor above, unless the thickness through the ceiling in unusual,
the projecting lead prevents the escape of any direct rays, which
travel in a straight line. |
|
| The other method is to fasten the lead on the
ceiling in a manner similar to that used on the walls except that
the strips should be placed closer together. The lighter the lead
the closer should be the supports. In the case of light lead,
lighter steel straps and bolts or screws may be used. In using this
method the lap should most certainly be sealed. These details of
construction will, of course, be governed by existing conditions.
|
|
|
Whenever pipes or wires penetrate
the lead ling they should be fitted with flanged lead sleeves lapped
over or “burned” or soldered to the sheets on the wall. Switch boxes
or other such appurtenances set through the lead walls should be
backed by much larger sheets of lead applied to the outside of the
wall. |
|
| As lead takes paint very well
the lead lining may be simply painted over and makes a very
presentable appearance. If desired, however, wire lath may be placed
over the lead and held by lead straps over the structural supports
of the lath with the straps “burned” or soldered to the lead shield.
Plaster can then be applied to the lath. |
|
| In the case of doors and
windows the lead should be carried right through to the outside
over the frames, lead-glass used in the windows and the frames of
the doors and windows covered with sheet lead. |
|
| Another method is by the use
of specially constructed furring strips containing slots. Sheet lead
is slipped into the slots and held in place by means of wooden
wedges. The face of the furring strip is lead covered so that no
space is left pervious to the rays. The metal lath is then attached
to the strips and plastered over. |
|
| Alchemy will gladly co-operate
with anyone interested in the design or fabrication of rooms which
are to be proofed against X-Ray. |
| |
|
Suggested Method
of Fastening Sheet Lead in X-Ray Rooms |
| |
|
Please
click here for
Lead-Lined Sheetrock/Plywood |
| |
|
Please
click here for charts of sizes |
| |
| © 2001-2007 Alchemy Castings
(1464012 Ontario Inc.) & Alchemy Art Lead Products (1464013
Ontario Inc.) |
|
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|
563 Kenilworth Ave. N.,
Hamilton, Ontario,
Canada L8H 4T8
Toll-Free: 1-866-312-9084,
Phone: (905) 312-9084,
Fax: (905) 312-9085,
Email
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