At 100 kV, a 1/8-inch thickness of lead should normally be adequate to protect film stored in a room adjacent to the x-ray room if the film is not in the line of the direct beam. At 200 kV, the lead thickness should be increased to 1/4 inch.
With million-volt x-rays, films should be stored beyond the concrete or other protective wall at a distance at least five times farther from the x-ray tube than the area occupied by personnel. The storage period should not exceed the times recommended by the manufacturer.
Medical x-ray films should be stored at approximately 12 times the distance of the personnel from the million-volt x-ray tube, for a total storage period not exceeding two weeks.
In this connection, it should be noted that the shielding requirements for films given in National Bureau of Standards Handbook 76 "Medical X-Ray Protection Up to Three Million Volts" and National Bureau of Standards Handbook 93 "Safety Standard for Non-Medical X-Ray and Sealed Gamma-Ray Sources, Part 1 General" are not adequate to protect the faster types of x-ray films in storage.
Gamma Rays
When radioactive material is not in use, the lead container in which it is stored helps provide protection for film. In many cases, however, the storage container for gamma-ray source will not provide completely satisfactory protection to stored x-ray film. In such cases, to prevent fogging, a sufficient distance should separate the emitter and the stored film. The conditions for the safe storage of x-ray film in the vicinity of gamma-ray emitters are given in Tables VII and VIII.
Table VII: Cobalt 60 Storage Conditions for Film Protection
Source Strength in
Curies
|
1
|
5
|
10
|
25
|
50
|
100
|
Distance from Film Storage in Feet
|
Lead Surrounding Source,
in Inches1
|
|||||
25
|
5.5
|
7.0
|
7.5
|
8.0
|
8.5
|
9.0
|
50
|
4.5
|
6.0
|
6.5
|
7.0
|
7.5
|
8.0
|
100
|
3.5
|
5.0
|
5.5
|
6.0
|
6.5
|
7.0
|
200
|
2.5
|
4.0
|
4.5
|
5.0
|
5.5
|
6.0
|
400
|
1.5
|
3.0
|
3.5
|
4.0
|
4.5
|
5.0
|
Table VIII: Iridium 192 Storage Conditions for Film Protection
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Lead thickness rounded off to nearest half-value thickness.
These show the necessary emitter-film distances and thicknesses of lead that should surround the various gamma-ray emitters to provide protection of stored film. These recommendations allow for a slight but harmless degree of fog on films when stored for the recommended periods.
The lead thicknesses and distances in tables above are considered the minimum tolerable.
To apply the cobalt 60 table to radium, values for source strength should be multiplied by 1.6 to give the grams of radium in a source that will have the same gamma-ray output and hence will require the same lead protection. This table can be extended to larger or smaller source sizes very easily. The half-value layer, in lead, for the gamma rays of radium or cobalt 60 is about 1/2 inch. Therefore, if the source strength is doubled or halved, the lead protection should be increased or decreased by 1/2 inch.
The table can also be adapted to storage times longer than those given in the tabulation. If, for example, film is to be stored in the vicinity of cobalt 60 for twice the recommended time, the protection recommendations for a source twice as large as the actual source should be followed.
Iridium 192 has a high absorption for its own gamma radiation. This means that the external radiation from a large source is lower, per curie of activity, than that from a small source.
Therefore, protection requirements for an iridium 192 source should be based on the radiation output, in terms of roentgens per hour at a known distance. The values of source strength, in curies, are merely a rough guide, and should be used only if the radiation output of the source is unknown. The table above can be extended to sources having higher or lower radiation outputs than those listed. The half-value layer of iridium 192 radiation in lead is about 1/6 inch. Therefore, if the radiation output is doubled or halved, the lead thicknesses should be respectively increased or decreased by 1/6 inch.
Tables VII and VII are based on the storage of a particular amount of radioactive material in a single protective lead container. The problem of protecting film from gamma radiation becomes more complicated when the film is exposed to radiation from several sources, each in its own housing. Assume that a radiography source is stored under the conditions required by Tables VII and VII (for example, a 50-curie cobalt 60 source, in a 6.5-inch lead container 100 feet from the film storage). This combination of lead and distance would adequately protect the film from the gamma radiation for the storage times given in the tables. However, if a second source, identical with the first and in a similar container, is stored alongside the first, the radiation level at the film will be doubled. Obviously, then, if there are several sources in separate containers, the lead protection around each or the distance from the sources to the film must be increased over the values given in the tables.
The simplest method of determining the film protection required for several sources is as follows. Multiply the actual total strength of the source in each container by the number of separate containers. Then use these assumed source strengths to choose lead thicknesses and distances Tables VII and VIII, and apply the values so found for the protection around each of the actual sources. For instance, assume that in a particular radiographic department there are two source containers, both at 100 feet from the film storage area. One container holds 50 curies of cobalt 60 and the other an iridium 192 source whose output is 5 roentgens per hour at 1 metre (5 rhm). Since there are two sources, the 5 curies of cobalt 60 will require the protection needed for a "solitary" 100-curie source, and the iridium 192 source will need the same protection as if a source whose output is 10 rhm were alone irradiating the stored film. The thicknesses of lead needed are shown to be 7.0 inches for the 50 curies of cobalt 60 (Table VII) and 1.7 inches for the iridium 192 whose emission is 5 rhm (Table VIII).
This method of determining the protective requirements when multiple sources must be considered is based on two facts. First, if several sources, say four, simultaneously irradiate stored film, the exposure contributed by each must be only one-quarter that which would be permissible if each source were acting alone--in other words, the gamma-ray attenuation must be increased by a factor of four. Second, any combination of source strength, lead thickness, and distance given in Tables VII and VIII results in the same gamma-ray dose rate--about 0.017 mr per hour--being delivered to the film location. Thus, to determine conditions that would reduce the radiation from a particular source to one-quarter the value on which Tables BII and VIII are based, it is only necessary to use the conditions that are set up for a source four times the actual source strength.
Heat, Humidity, and Fumes
During packaging, most x-ray films are enclosed in a moisture proof container that is hermetically sealed and then boxed. As long as the seal is unbroken, the film is protected against moisture and fumes. Because of the deleterious effect of heat, all films should be stared in a cool, dry place and ordered in such quantities that the supply of film on hand is renewed frequently.
Under no circumstances should opened boxes of film be left in a chemical storage room or in any location where there is leakage of illuminating gas or any other types of gases, or where there is a possibility of contact with formalin vapors, hydrogen sulfide, ammonia, or hydrogen peroxide.
Packages of sheet film should be stored on edge--that is, with the plane of the film vertical. They should not be stacked with the boxes horizontal because the film in the bottom boxes can be damaged by the impact of stacking or by the weight of boxes above. In addition, storing the boxes on edge makes it simpler to rotate the inventory--that is, to use the older films first.