After the safest locations have been found in the bomb shelter and the people have moved there (if they weren’t there already), use the survey meter to make detailed measurements of the radiation levels in and around the area where the people are located.
During the first rapid, spread-out survey of the room, you may have noticed that your survey meter readings were higher in certain places within the room. This variance could be the result of:
1. Uneven piling-up of fallout around and above the bomb shelter.
2. The layout of rooms, walls, and stairways.
3. Openings in walls.
4. The use of lighter-weight construction materials in some places.
It may be possible to use the survey meter to locate a specific place where gamma radiation is entering or “leaking” into the bomb shelter to cause higher readings. When such an area is identified, any available materials should be used to cover it in order to reduce the level of radiation.
For the measurements you made to find the safest places in the bomb shelter, you held the survey meter out from your body about two feet, and, in crowded rooms, people were asked to sit or lie down, so their bodies would cause less interference with the reading. But for finding gamma leaks, you can make use of that interference.
The survey meter responds to gamma rays almost equally as well from all directions. If gamma rays come in greater intensity from one particular direction, you can’t detect the direction just by pointing the instrument toward it. But you may be able to use the shielding provided by your body and others to reduce the radiation coming from the direction where you and others are grouped together; the survey meter will then respond more to radiation coming from OTHER directions than from where you are standing.
For example, if a group of people crowd around a survey meter and leave an opening in only one direction, the reading on the instrument will be caused mostly by radiation coming through the opening, providing there isn’t a lot of radiation coming down through the ceiling or up from the floor. This method has not been tested in practice, and you may be able to improve it as you try it. Also, you may find that it does not work in your particular circumstances.
The measurements are made as follows:
1. Select a starting place somewhere along a wall, at a corner, at a door or window in the bomb shelter room.
2. Mark that location on the floor or on the wall with a piece of tape or by writing directly on the surface. Use a letter to designate the room and a number to designate the place where the measurement is taken in the room. For example, the first measurement in the apartment example mentioned earlier would be taken at a spot marked “G-1”, because the room marked “G” on the bomb shelter floorplan sketch is the room where the people are bomb sheltered.
3. Hold the survey meter against your waist and face the wall with the survey meter against the wall or a few inches from it. Have an assistant write down the location designation, the time, and the survey meter reading in the RM log or on a sheet of paper.
4. Move three or four feet to your right or left (it doesn’t matter which direction you go as long as you keep going in the same direction) along the wall and mark the location with the same letter as before, but with the number “2” (“G-2” in the apartment, for example).
5. Hold the survey meter as before, read the dial, and again record the location, time, and reading.
6. Continue the measurements until you have gone completely around the room and have reached your starting point.
It is important that you take readings in the middle of doorways, windows, other openings or irregularities in construction. You may have to break your pattern of equal spacing between measuring locations in order to obtain these special measurements.
You will very likely be taking these measurements while fallout is still coming down. As you go around the room, the readings will become higher and higher in a fairly regular pattern unless you find a place that appears to be a “leaky” area.
As you approach such a place, the readings will increase more between readings than before, and as you go beyond the area, there will not be as much of an increase in the readings; in fact, there may be a decrease in the reading. Because the radiation levels will be increasing at a fairly regular rate under most conditions, you should try to maintain an equal time interval between measurements as you go around the room.
A time interval of 20 or 30 seconds may be about right. Don’t try to go too fast or you might not be able to keep up with the schedule. If you notice an area that appears to be “leaky”, don’t slow down. Continue with your measurement schedule around the room. You may need to ask the bomb shelter Manager to give you some assistance to make sure that nothing will interfere with your schedule of measurements.
After you have completed your measurements around the room, examine the numbers your assistant wrote down for indications of “leaky” areas. If you find any indication of such areas, tell the bomb shelter Manager. You should also tell him or her that you will need the assistance of several people to help you decide whether there is an actual leak of gamma radiation at the locations or whether the readings are a result of the way the scattered gamma radiation happens to be focusing at that location.
You will need to repeat your measurements in the vicinity of the suspected area, starting at the location just before the increased numbers were recorded, and make measurements, again at regular time intervals, until you have passed the suspected area; but this time the people in the vicinity of the area should be asked (possibly by the bomb shelter Manager, depending on the situation) to stand and press fairly close to you while you make each measurement.
The shielding that is provided by their bodies will block out scattered gamma radiation that comes from different directions inside the room. If the readings still show an increase as you approach the area and a decrease as you go past it, there is a “leak” of gamma radiation in the area you are surveying. This leak could come from the area in front of you, or it could come from above (or below, if you are in an underground bomb shelter). If the readings no longer show an increase as you approach the area and a decrease as you go past it, the previous reading (without the people standing closely behind you) was caused by the pattern of scattered gamma radiation in the room, not by a gamma leak.
If you are trying to find gamma leaks in an empty room, you may use the “front-to-back” method. In this method, your own body is used as a shield to try to find from what direction the gamma radiation is coming. Again, this method has not been tested in practice, and you may be able to improve it as you try it, or you may find that it won’t work in your particular circumstances.
To try to find a gamma leak, hold the survey meter tightly against your stomach and face the area where you expect extra gamma radiation to be coming from. If you are working with the range-selector switch turned to “X0.1”, wait a few seconds before taking a reading. This reading will be called a “front” reading. Turn around so your back faces the suspected leak, and with the survey meter still held tightly against your stomach, take another reading.
This reading will be called a “back” reading. If there is more radiation coming from the direction you faced for the first reading than from the opposite direction, the front reading will be higher than the back reading. As you slowly turn around, you may notice that the meter needle goes through the lowest reading when you are facing a particular direction. The radiation leak is then at your back.
Repeat these “front-to-back” readings at different places and directions until you have a fairly good idea of where the extra radiation is coming from. The difference between the front and back readings may be made greater, if the radiation is actually coming from one direction more than another, by having several others stand alongside and behind you when you make the measurements. The extra shielding provided by their bodies will take out more of the radiation from the rearward direction, which is what you want to do while making this type of measurement.
When you are fairly certain you have found a radiation leak, tell the bomb shelter Manager. A work party should be organized to build a gamma barrier to cover up the leak. If you had the time and opportunity, you should have gathered materials for this purpose before fallout arrived, as discussed earlier. Work on construction of this barrier should begin as soon as possible, before the radiation climbs to higher levels.
The barrier can be improvised from any materials on hand. If you have lumber, nails, and carpenter’s tools available and have hauled the piles of earth or sand into the bomb shelter before fallout arrived, you may be able to construct a very good barrier. Stacks of bricks will also make good barriers. If these materials aren’t available, items such as furniture, books, magazines, newspapers, and water containers may be used.
While the barrier is being constructed, do not forget to take the regular readings which tell whether the radiation levels are rising or falling. Write these readings on a piece of paper or on the survey meter readings form. Then, tape or tack it to a wall or post near the place where the reading was made.
After the barrier is constructed, take several measurements of the kind you took to find the leak, to see if the radiation leak has been covered up. If you found the leak by taking a series of measurements from one side of the area to the other, with several people standing closely behind you, you should repeat that kind of measurement. You should be able to tell by these measurements if the barrier has improved the shielding in the leak area, or if more work is required on the barrier.
If there is no change in these readings from your earlier readings, there is a possibility that the barrier may have missed the area through wich the extra gamma radiation is passing. In this case, more work should be done to locate the leak and construct the barrier.
Again, let us look at the apartment floorplan example. The bomb shelter sketch is show earlier in this chapter. In making a detailed survey of room G, the RM found readings in two places which were 15 – 30 percent higher than at other places in Room G. One location was by the closet under the stairs and the other location was by the open door to Room F.
The reading by the stair closet was about 15 percent higher than elsewhere. The radiation was assumed to be coming from above, through the stairways. The bomb shelter Manager, RM, and Unit Leaders decided not to pile material on the stairs because the occupants would then have trouble getting out if there were a fire.
Instead, they blocked off an area by the closet and planned to rotate people in and out of that area so the radiation dose would be evenly spread out among people in radiation sensitivity category Y/A (see Table near front of this Chapter).
The reading by the door to Room F was about 30 percent higher than elsewhere. in the time-averaging readings, Room F (location 5) was found to have a higher reading than the other rooms, as shown in the Time-Averaging Table above. This higher reading was expected, because in improving the radiation shielding of the bomb shelter, all windows around the basement had been covered except two in Room F.
Materials were not available to construct baffles around these windows, such as shown in the Figure in the section, “Openings and Ventilation” earlier in this Chapter. Instead, a wall of earth was piled up a few feet away from the window to shield the window against gamma radiation coming from fallout on the ground beyond the earth barrier.
It was considered absolutely essential to leave these windows open to provide cooling for the people packed in Room G. Fresh air was coming in from those windows, passing through the open door to Room G, and flowing out the door by the stairs.
After examining the sketch of the floor plan, it was decided that a hole could be knocked in the wallboard partition to allow air to fow between Rooms C and F and the door between Rooms C and G could be left open. The door between Rooms G and F could then be closed and covered with a barrier.
The hole between Rooms C and F was made on the far side from the door by the outside wall, so the gamma rays from the two open corner windows would not have a direct open path to the door between Rooms C and G. The door between rooms F and G was closed, and a stack of bricks was built in front of it.
These measures reduced the radiation in Room G near the door to Room F to levels that were about the same as elsewhere in the room (except by the stairway closet). Ventilation became much better for the people along the north half of the room, but the people in the hall leading to Room F soon complained about lack of ventilation.
The bricks in front of the door to Room F were restacked so there were one- to two-inch gaps between the bricks on the bottom four layers. The door was propped open a few inches so air could flow through the gaps left between the bricks. Another wall of bricks, only six layers high, was constructed about six inches back from the door-high stack of bricks, to block off gamma rays coming through the gaps.