Globusz^® Publishing 

On 17, 18, and 19 July, all personnel and visitors had to receive permission to approach ground zero from the "Going-in Board." On these three days, 21 groups were authorized to go beyond the Broadway roadblocks. Most of those who sought this authorization were scientists and military support personnel whose job required that they work near ground zero. Except for a group of two military men and three civilians who went to ground zero on 16 and 17 July and a group of two civilians who approached as close as 90 meters on 18 July, the reentry personnel came no closer than 180 meters to ground zero. Of these personnel, the individual who received the highest exposure during the three days was an Army sergeant who received 15 roentgens. During the same period, two civilians received 10 roentgens and 7.5 roentgens, respectively. All other personnel received exposures of 5 roentgens or less (1; 3).

After the "Going-In Board" was disbanded on 19 July, permission to enter the ground zero area had to be obtained from Dr. Bainbridge or one of his deputies. Many scientists entered the ground zero area after 19 July to retrieve instruments or to perform experiments. The population of the TRINITY test site was diminishing, however, as the emphasis shifted to preparing the devices that were to be dropped on Japan (1).

On 23 July, a week after the shot, chain barricades with prominent signs warning against trespassing were placed 910 meters north, south, and west of ground zero. These barricades were supplemented with two concentric circles of red flags 1,830 and 2,740 meters from ground zero. Except during bad weather, the entire ground zero area was visible from the roadblocks. No unauthorized person was ever detected entering the ground zero area (1).

On 10 August, the Broadway roadblocks were removed, and mounted military policemen began patrolling around ground zero at a distance of 730 meters. Each guard was assigned to a daily six-hour shift for a period of two weeks; in the third week, the guard was assigned tasks away from the ground zero area. The mounted guards and their horses wore film badges. No exposure greater than 0.1 roentgen was registered. On 1 September, the mounted patrol moved to a distance of 460 meters from ground zero, just outside a fence installed a week earlier to seal off the area. The same rotating patrol schedule was used. The guards' film badge readings showed an average daily exposure of 0.02 roentgens. The mounted patrol at the fence continued until early 1947 (1).

Between 20 July 1945 and 21 November 1945, 67 groups entered the ground zero area. Most of these parties entered in the month after shot-day. These were the scientists and technicians conducting experiments or retrieving data. By the beginning of September, most of those who entered the ground zero area were invited guests (1).

Also during the period 20 July through 21 November, at least 71 soldiers were at the TRINITY test site. Twenty-five of these men were support personnel who never went within 460 meters of ground zero. The remaining 46 men were technical personnel, laborers who erected the 460-meter fence, or military policemen who served as guides. Eleven of these men, probably members of the fence detail, spent several days at about 460 meters from ground zero. Working three to five hours per day between 9 August and 25 August, they would have been the only group to stay longer than one hour in the ground zero area. Of the remaining personnel who approached within 460 meters from ground zero, 25 spent 15 minutes and ten spent between 30 minutes and one hour in the ground zero area. Only 11 people received exposures of 3 to 5 roentgens between 20 July and 21 November. Most received less than 1 roentgen. After 21 November 1945, no one approached closer than the fence which was 460 meters from ground zero, although about 200 civilian and military personnel worked at or visited the TRINITY site through 1946 (1; 16).

According to dosimetry data, entrance logs, and other records, about 1,000 individuals were at the test site at some time between 16 July 1945 and the end of 1946. This number includes not only the scientists, technicians, and military personnel who were part of Project TRINITY but also many visitors. Some of the scientists took their wives and children on a tour of the area near ground zero, particularly to view the green glass called "trinitite," which covered the crater floor. Trinitite was the product of the detonation's extreme beat, which melted and mixed desert sand, tower steel, and other debris (1; 8; 9; 16).