gamma rays they do not constitute a direct hazard to life. Their main danger lies in the fact that they make other materials radioactive, and therefore the disintegrated fractions of the exploded bomb, dust in the atmosphere in the vicinity of the explosion and water in the atmosphere become contaminated. If the bomb explodes at a low height then the ground beneath becomes radioactive.
      The resulting radioactive materials, like the nuclear explosion itself, emit alpha, beta and gamma rays and neutrons but of course not to the same degree as the explosion. It is therefore the sequels to the nuclear explosion which constitute the greatest danger, for those who were not directly harmed by the explosion. These sequels are the radioactive cloud and the residual radioactive deposits on the ground. It is from the former that we get the fallout, the very fine radioactive dust which falls to the ground, not only in the area in the immediate vicinity of the explosion, but downwind for many scores of miles. It is not possible in this booklet to be precise about the area of fallout, or the amount, as these depend on the prevailing winds, the size of the weapon and the height it explodes; the higher the explosion the less chance there is of particles being sucked up from the earth's surface, thus reducing the quantity of radioactive materials. During tests every precaution is taken to safeguard the personnel taking part and all persons in the surrounding area.
      The scientific effort will be directed towards the assembly of the weapon, the loading of it into the aircraft and the measuring and recording of the magnitude of the explosion. To carry out these tasks the scientific party is divided into a number of groups, each group. having a specific task.
      Those groups dealing with the weapons are the WEAPON GROUPS, the groups recording and measuring the magnitude and primary effects of the explosions are the MEASUREMENTS GROUPS, and the groups directing, planning and providing services are the SERVICES GROUPS. To avoid any confusion the latter does not mean the Armed Forces.
      The primary task of the WEAPON GROUPS is the preparation and assembly of the explosive and radioactive components of the weapon. When these have been assembled into the weapon, tests are made on it before loading into the aircraft. Further tests are carried out after loading as well as checks on the bomb release mechanism and other installations in the aircraft. Only when everything has been checked and inspected, and has satisfied rigorous test schedules,  is the apparatus handed over to the captain of the aircraft. Two teams of R.A.F. personnel play a large part in this task, under the direction of A.W.R.E. scientists.
      The MEASUREMENTS GROUPS will be providing instruments and apparatus to record and measure the many effects of the explosion. These installations are positioned on the ground, in naval ships and in R.A.F. aircraft. Effects measured and recorded include blast, nuclear and heat radiations, and residual radioactivity. The object of all these measurements is, of course, to find out how well the bomb has worked, that is to find out whether it does what the designers intended it to do. The aircraft dropping the bomb also has a number of instruments to make measurements and records of some of these effects, and after the explosion an aircraft will fly through the cloud collecting samples for testing. It is not permissible here