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