|Reason || ||Details (roll over the word 'show' for each reason).|
Try to think of the reason first.
|The background count causes random fluctuations ||show || |
Radioactive decay is a random process. Therefore there are random changes (up AND down) in the background count (bgc). The system has to allow for these.
|The background count can change from day to day ||show || |
Changes in solar activity can affect the amount of cosmic radiation. Also, changes in the wind direction can change the bgc as natural radioactive sources are carried into the area.
|Sources can be screened by all the scrap on top of them ||show || |
If the radioactive source is right at the bottom of the truck, then a lot of the gamma radiation is absorbed by the scrap on top of it. The amount of radiation that reaches the detector may not be much above background count.
|The truck often reduces the count ||show || |
In some places, a significant amount of radiation comes from the ground and even from the tarmac. When the truck drives into the bay, it actually shields the detector from this natural radiation and reduces the count.