Monitoring the Site

The cloud drifted to the northeast, and higher gamma readings due to fallout were encountered in this direction.

Continuing The First Atomic Bomb,
our selection from Project Trinity 1945-1946 by Carl Maag and by Steve Rohrer published in 1982. The selection is presented in nine installments for 5-minute daily reading. For works benefiting from the latest research see the “More information” section at the bottom of these pages.

Previously in The First Atomic Bomb.

Time: July 16, 1945
Place: Old McDonald Ranch House, New Mexico

The chief monitor arrived at Guard Post 2 at about 0550 hours and found the post empty. He then continued five kilometers north along Broadway to the foxholes from which the military police had watched the detonation. There he found the guards, the five radiological safety monitors assigned to the evacuation detachment, and the Commanding Officer of the evacuation detachment. [1; 18]

The military policemen refused to return to Guard Post 2, insisting that they had received orders over their two-way radio from the Base Commander to evacuate their post and head for San Antonio, New Mexico, a town 28 kilometers northwest of the Guard Post. The Base Commander had noted that portions of the cloud were heading northwestward and, fearing that fallout from the cloud would contaminate Guard Post 2, had ordered the military police to evacuate. The chief monitor, however, had found no significant radiation levels anywhere along the northern part of Broadway nor around Guard Post 2. The Base Commander, after being contacted by the chief monitor, drove to the foxholes and ordered the guards to return to their post. This was the only unplanned incident during the onsite monitoring. [1]

After Guard Post 2 was reoccupied, the chief monitor returned to the roadblock at the intersection of Broadway and the North Shelter Road. The north shelter monitor informed the chief monitor of the sudden evacuation of the north shelter, whereupon the chief monitor surveyed the north shelter area and found intensities of only 0.01 and 0.02 roentgens per hour (R/h). The chief monitor then contacted the south shelter and informed Dr. Bainbridge that the north shelter region was safe for those who needed to return, that Broadway was safe from the Base Camp to Guard Post 2, and that Guard Post 2 was now manned so that personnel leaving for LASL could be checked out. [1]

The chief monitor then returned to the south shelter and assembled the monitors from the three roadblocks and Guard Post 4 to prepare for entrance into the ground zero area. The time was about 0815 hours. The military police at the roadblocks were given radiation meters to survey the adjoining area. Broadway from the south shelter to Guard Post 2 was remonitored occasionally to reassure the military police that there was no radiation problem. Monitors also surveyed the Base Camp for 24 hours after the detonation. No radiation above background levels was detected there. [1]

The following brief description of the radiological environment in the TRINITY test area is based primarily on the results of the remote gamma recorders situated in the test area and on results of the road surveys conducted after the detonation. [1]

Within about 1,400 meters of ground zero (except to the north), radiation intensities between 0.2 and 1.3 R/h were detected during the first few minutes after the detonation. These readings decreased to less than 0.1 R/h within a few hours. At greater distances to the east, south, and west, radiation levels above background were not detected. [1]

The cloud drifted to the northeast, and higher gamma readings due to fallout were encountered in this direction. About five minutes after the detonation, a reading of 3 R/h was recorded 1,400 meters north of ground zero. Several minutes later, the intensity there had increased to greater than 7 R/h, and it continued to increase for several more minutes. Gamma detectors 9,150 meters north of ground zero, however, recorded no radiation above background levels. This indicated that the cloud had passed over or near the 1,400-meter area and only partially over the 9,150-meter area where the north shelter was located. Subsequent ground surveys of this area found no gamma intensities higher than 0.02 R/h. [1]

Gamma radiation levels at and around ground zero were much higher than in other onsite areas because of induced activity in the soil. Twenty-four hours after the detonation, the gamma intensity at ground zero was estimated to be 600 to 700 R/h. This estimate was based on data provided by the tank crew that drove to ground zero to obtain soil samples. The intensity decreased to about 2 R/h at 725 meters from ground zero. Gamma intensities of 0.1 R/h or more were confined within a circular area extending about 1,100 meters from ground zero (except in areas of fallout). One week after the shot, the gamma intensity at ground zero was about 45 R/h. After 30 days, intensities at ground zero had decreased to 15 R/h, and intensities of 0.1 R/h or more were not encountered beyond about 365 meters from ground zero. Gamma intensities of 3 to 10 R/h were found at ground zero three months after the detonation. [1; 19]

3.3 Offsite Monitoring Group

Four two-man teams and one five-man team supervised by the chief offsite monitor constituted the Offsite Monitoring Group. Before the detonation, the four two-man teams established monitoring posts in towns outside the test area. These towns were Nogal, Roswell, Fort Sumner, and Socorro, all in New Mexico. The five-man team remained at Guard Post 2 to assist in evacuation of nearby residences if the TRINITY cloud drifted in that direction. These residences, the Fite house and the homes in the town of Tokay, were 24 and 32 kilometers northwest of ground zero, respectively. Since the cloud drifted to the northeast, evacuation was not required. All offsite monitoring teams were in radio or telephone contact with personnel at the Base Camp. [11]

Offsite monitoring teams in areas northeast of ground zero encountered gamma readings ranging from 1.5 to 15 R/h two to four hours after the detonation. Three hours after the detonation, surveys taken in Bingham, New Mexico (located 30 kilometers northeast of ground zero) found gamma intensities of about 1.5 R/h. Radiation readings at the town of White, nine kilometers southeast of Bingham, were 6.5 R/h three hours after the detonation and 2.5 R/h two hours later. Another team monitoring in a canyon 11 kilometers east of Bingham found a gamma intensity of about 15 R/h. Five hours later, the intensity had decreased to 3.8 R/h. It was estimated that peak intensities of gamma radiation from fallout on shot-day were about 7 R/h at an occupied ranch house in this canyon area. [1; 11; 19]

SOURCES CITED ON THIS PAGE

[1. Aebersold, Paul. July 16th Nuclear Explosion-Safety and Monitoring of Personnel (U). Los Alamos Scientific Laboratory, Atomic Energy Commission. Los Alamos, NM.: LASL. LA-616. January 9, 1947. 170 Pages.]

[11. Hoffman, J. G. {Extracts from “Health Physics Report on Radioactive Contamination throughout New Mexico Following the Nuclear Explosion, Part A–Physics.”} Los Alamos Scientific Laboratory, Manhattan Engineer District. {Los Alamos, NM.} {1945.} 31 Pages.]

[18. Warren, S. L., COL, USA; Hempelmann, L. H., M.D. Extracts from: Personal Notes, Subject: Events in Camp Immediately Following Shot–July 16, 1945. 1945. 2 Pages.]

[19. Weisskopf, V.; Hoffman, J.; Aebersold, Paul; Hempelmann, L. H. Memorandum for George Kistiakowsky, Subject: Measurement of Blast, Radiation, Heat and Light and Radioactivity at Trinity. {Los Alamos, NM.} 5 September 1945. 2 Pages.]

More information here and here, and below.