After the announcement is made to the people in the bomb shelter that fallout has begun to come down outside, you (the RM) should use the survey meter to find the places that have the lowest radiation levels. The people in the bomb shelter should be gathered at the locations that are estimated to have the lowest radiation levels.
It should be explained to the people, or at least to the bomb shelter Unit Leaders, that these locations were chosen on the basis of estimates and that places with lower radiation levels might be found by taking readings with the survey meter.
Mark the sketch of the bomb shelter to show the locations where you plan to take readings of the radiation levels. Some of these readings should be taken near walls, posts, or columns upon which you can tape a form showing your readings.
A general survey of radiation levels with the survey meter should be made as soon as possible after the gamma radiation reaches levels that can be detected inside the bomb shelter. Write down the readings, the times the readings were made, and the exact location for each reading so you can find the same spot when you check later. You may wish to mark the floor where you make your measurements and assign a number to each location.
At this time, when you are trying to find the safest places in the bomb shelter as quickly as possible, you should take readings only in those locations where you estimate the lowest radiation levels will be. For example, if you are in a basement bomb shelter you should not take readings on the first floor at this time. If you are in a skyscraper bomb shelter, there is not need to take readings near an outside wall at this time.
The first survey should be spread out to get a general picture of the best bomb shelter areas. Follow-up surveys should then be made to get a detailed picture of radiation levels in the areas where people are finally bomb sheltered.
While fallout is coming down, the radiation levels may be climbing fast. Inside the bomb shelter at the location that you have estimated to be the safest, your survey meter needle may be climbing as fast as one to five smallest divisions on the “X0.1” scale each minute. If you plan to make a detailed comparison between the readings at several locations, the reading at the final location may be quite a lot higher by the time you get to it than it was when you began to take readings.
You will not be able to tell whether the higher reading results from a lower FPF or from an increase in radiation levels at all locations of the bomb shelter. The readings would have to be taken in both places at the same time to show which location had the lowest radiation level. You can only be at one place at one time!
If your bomb shelter has two or more survey meters (most will not) and two or more RM’s, you may work out a simpler method by making readings synchronized by timepieces showing seconds or by the use of two-way radio, telephone, or cell phone communication between the RM’s. The meters should be compared at one location (identical radiation levels) before and after the measurement (the instruments may drift) to make sure they read the same or to compensate for different readings.
You should not wait until the radiation levels stop climbing to make your detailed follow-up measurements, because it might be several hours before the fallout stops coming down. To get a proper comparison of the radiation safety between different locations while the radiation levels are climbing rapidly (due to the buildup of fallout), you will need to use a special method for taking measurements. One of the simplest methods for taking such measurements is the Time-Averaging method described in the following section.
Another method, to be used if no survey meter is available, is to place a dosimeter at each location to be checked. All the dosimeters to be used should be carefully zeroed at approximately the same time before positioning them. You may have to wait several hours before significant differences in the readings are observed, because the smallest division on the dosimeter is 10 R. With the survey meter, you will be able to compare the radiation levels at several locations within just a few minutes by using the time-averaging method.