In some tests collimator systems have been used to record gamma or neutron radiation. Exact positioning is a necessity, There is a declining height system of towers and of concrete walls extending from the tower to an underground recording station, Each © tower or wall supports a heavy mass with several holes in it. These holes are aligned so that there is direct line-of-sight from the atomic device to the underground recarding equipment, The ' holes provide clear paths for gamma radiation or neutrons, with heavy shields insuring that gamma or neutrons from regions outside the line of sight will not reach the detectors underground. Large underground bunkers or blockhouses for recording instru- ments have been built close to ground zero in several firing areas. These massive concrete and steel units are topped with a thick mound of earth, the surface of which is stabilized by an asphalt coating. Depending on their nature and the type of equipment used, these blockhouses cost from $100,000 to $600,000, They are built to withstand all effects of detonations, Their initial cost is high, but they may be used for many test operations. The underground bunkers not only protect the instruments against blast, but also against radiation. Without shielding, the intense radiation fields which accompany the detonation would immediately fog all film, ionize the gasses in the electronic tubes and cause other severe damage putting the equipment out of order, Underground bunkers at NTS are used to record blast, heat, neutron or gamma radiation, or for taking photographs, but they vary considerably in design. While data from an experiment may be recorded in a few millionths of a second, many months of work go into constructing and equipping a bunker, The scientists responsible for setting up the equipment work for months in home laboratories and fabricating plants before working the clock around for weeks or months to install it in the bunker. Working with them at NTS are construction and electrical contractor personnel. Final calibration cof instruments, checking circuits, testing of signal strengths, time signal relays, and electrical power behavior are performed during the week immediately preceding a detonation, at Prior to the shot, hundreds of switches for the recording instruments are pre-set, then the bunker is evacuated with no person inside at shot time. Heavy lead-lined doors like the bulkhead doors of a large warship are closed and sealed, When the massive outer door swings shut the bunker is ready to receive and record the data from the assortment of instruments above ground -instruments which may be vaporized in the instant of detonation, : - 34 - cee. i LAM \ (\